United States Office of Air Quality EPA-453/B-93-018
Environmental Protection Planning and Standards April 1993
Agency Research Triangle Park NC 27711
Air ~
& EPA Medical Waste Incinerator
Operator Training Program
Course Handbook
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EPA-453/B-93-018
o
MEDICAL WASTE INCINERATOR
OPERATOR TRAINING PROGRAM
COURSE HANDBOOK
U. S. Environmental Protection Agency
Industrial Studies Branch/BSD
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
April 1, 1993
U.S. Environm"rf-' Drotection Agency
Region 5, Lib:. •. ,< , 12J)
77 West Jacks-. ..•: ward, 12th Floor
Chicago, 1L 60604-3590
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NOTICE
This Course Handbook is part of a model state training program which addresses the
training needs of medical waste incinerator (MWI) operators. Included are generic equipment
design features, combustion control relationships, and operating and maintenance procedures
which are designed to be consistent with the purposes of the Clean Air Act Amendments of
1990.
This training program is not designed to replace the site-specific, on-the-job training
programs which are crucial to proper operation and maintenance of medical waste incinerators.
Proper operation of combustion equipment is the responsibility of the owner and
operating organization. Therefore, owners of medical waste incinerators and organizations
operating such facilities will continue to be responsible for employee training in the operation
and maintenance of their specific equipment.
DISCLAIMER
This Course Handbook was prepared by the Industrial Studies Branch, Emission
Standards Division, U. S. Environmental Protection Agency (USEPA). It was prepared in
accordance with USEPA Contract Number 68-CO-0094, Work Assignment Number 8. Partial
support was also provided by the University of Virginia through its Sesquicentennial Associates
Program.
Any mention of product names does not constitute an endorsement by the U. S.
Environmental Protection Agency.
The U. S. Environmental Protection Agency expressly disclaim any liability for any
personal injuries, death, property damage, or economic loss arising from any actions taken in
reliance upon this Handbook or any training program, seminar, short course, or other
presentation based on this Course Handbook.
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AVAILABILITY
This Course Handbook and the accompanying Instructors' Guide are issued by the Office
of Air Quality Planning and Standards of the U. S. Environmental Protection Agency. These
training materials were developed to assist operators of medical waste incinerators in becoming
certified as required by the federal and state regulatory agencies.
Individual copies of this publication are available to state regulatory agencies and other
organizations providing training of operators of medical waste incinerators. Copies may be
obtained from the Air Pollution Training Institute (APTI), USEPA, MD-17, Research Triangle
Park, NC 27711. Others may obtain copies, for a fee, from the National Technical Information
Service, 5825 Port Royal Road, Springfield, VA 22161.
Although this government publication is not copyrighted, it does contain some
copyrighted materials. Permission has been received by the authors to use the copyrighted
material for the original intended purpose as described in the section titled Handbook
Introduction. Any duplication of this material, in whole or in part, may constitute a violation
of the copyright laws, and unauthorized use could result in criminal prosecution and/or civil
liabilities.
The recommended procedure for mass duplication of the Course Handbook is as follows:
Permission to use this material in total may be obtained from the APTI, provided the
cover sheet is retained in its present form. Permission to use part of this material may
also be obtained from the APTI, provided that the APTI and the authors are properly
acknowledged.
11
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TABLE OF CONTENTS
Page
Notice and Disclaimer i
Availability ii
Course Handbook Introduction x
Learning Unit
1. Introduction 1-1
Regulatory Requirements: Training/Certification 1-1
Course Overview 1-2
Purpose of Pre-Test and Post-Test 1-3
ASME Certification Procedures 1-4
2. Environmental Concerns and Regulations 2-1
Public Relations & Public Concerns 2-1
Operator's Role in Public Relations 2-4
Solid Waste Laws & Regulations 2-6
Air Pollution Laws & Regulations 2-7
Operating Permit Requirements 2-10
Occupational Health & Safety Act 2-10
3. Characterization of Medical Waste 3-1
Waste Mixture Characterizations 3-1
Definition of Medical Waste 3-2
Sources & Generation Factors 3-7
Compositions & Fuel Properties 3-9
Heavy Metals & Ash Characteristics 3-12
4. Medical Waste Safety, Handling and Treatment 4-1
General Health and Safety 4-1
Standard Safety Concerns 4-4
111
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Page
Personal Protection Equipment 4-5
Waste Management: Source Reduction, Recycling 4-6
Packaging Requirements 4-8
Scale Operations & Record Keeping 4-10
Segregation of Unacceptable Wastes 4-11
Storage 4-12
Alternative Treatment Techniques 4-12
Autoclave, Microwave & Chemical Treatment 4-13
Landfill Disposal of Ash 4-16
5. Medical Waste Incinerators 5-1
MWI Designs & Applications 5-1
Multiple Chamber, Excess-Air 5-3
Modular Controlled-Air 5-4
Air & Flue Gas Flow Path 5-5
Rotary Kiln 5-9
Recovery Boilers & Waterwall Units 5-10
Feeding Equipment & Charging Strategies 5-14
Fuel-Bed Combustion 5-17
Hearth Designs & Ash Removal 5-18
6. Combustion Principles I: Complete Combustion 6-1
Basic Combustion Concepts 6-1
Combustible & Incombustible Substances 6-2
Stoichiometric Considerations: Excess-Air & Starved-Air 6-5
Complete & Incomplete Combustion Products 6-6
Chemical Reaction Equations 6-7
Theoretical Combustion of Medical Waste 6-10
Combustion Under Excess Air Conditions 6-11
IV
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Page
7. Combustion Principles II: Thermochemistry 7-1
Heating Values 7-1
Capacity and Operating Load 7-3
Ignition & Volatilization Temperatures 7-4
Combustion Temperature Control 7-6
Stoichiometric Considerations 7-8
8. Combustion Principles ffl: Reaction Processes 8-1
Multiple Reactions 8-2
Incomplete Combustion 8-3
Reaction Rates 8-3
Oxidation & Reduction Reactions 8-4
Diffusion Limited Combustion 8-6
Fuel-Bed Combustion Processes 8-7
Theoretical Combustion Temperatures 8-9
Moisture and Stoichiometric Operational Relationships 8-10
Char and Carbon Monoxide Reactions 8-12
9. Combustion System Design & Control 9-1
Design & Operational Considerations 9-1
Starved-Air & Excess-Air System Comparisons 9-2
Combustion Chamber Temperatures 9-4
Automatic Control Applications & Concepts 9-6
Centrifugal Fans & Air Flow Control 9-10
Draft & Draft Control 9-12
Combustion Control Comparisons 9-14
Starved-Air and Excess-Air Unit Operations 9-16
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Page
10. Air Pollution Formation 10-1
Waste & Operations Dependent Emissions 10-1
Products of Incomplete Combustion, Smoke & CO 10-3
Dioxins/Furans 10-5
Gas Concentrations & Correction for Standard Dilutions 10-8
Conversion of [gr/dscf] to [mg/dscm] 10-13
Calculations of Combustion Efficiency 10-15
Calculations of Excess Air 10-16
11. Instrumentation I: General Measurements 11-1
Purposes of Instrumentation 11-1
Temperature Conversions & Measurements (Thermocouples) 11 -2
Pressure Measurements: Manometers & Gages 11-4
Flow Measurements: Pitot, Orifice, Rotameter 11 -8
Weight Scales 11-11
12. Instrumentation II: Continuous Emission Monitoring 12-1
Parameters Monitored & Typical Ranges 12-1
Extractive & In-situ Measurement Concepts 12-2
Sample Conditioning & Special Operating Concerns 12-6
Dispersive and Non-Dispersive Methods 12-7
Routine Calibration & Bias Checks 12-13
13. Incinerator Operations and Upsets 13-1
Operator Responsibilities 13-1
Operator Communications, Monitors & Logs 13-3
Check-Lists of Operating Systems 13-6
Potential Combustion Hazard: Explosion 13-7
Standard Operating Procedures 13-8
VI
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Page
Operational Sequences: Loading, Start-Up & Shutdown 13-10
Typical Combustion Upsets and Remedies 13-13
14. Maintenance: Corrective & Preventive 14-1
Risk Management & Economic Losses 14-1
Operator Responsibilities 14-2
Goals & Features of Maintenance 14-3
In-Service & Outage Maintenance 14-5
15. Flue Gas Cleaning I: Paniculate Matter 15-1
Particle Sizes & Entrainment Factors 15-1
Particle Formation 15-2
Influences of Particulates on Dioxin/Furan Formation 15-3
Venturi & Wet Scrubbers 15-5
Fabric Filters 15-9
Electrostatic Precipitators 15-13
16. Flue Gas Cleaning Et: Acid Gas Removal 16-1
Caustic Solutions & Wet Scrubbing Systems 16-1
Dry Sorbent Injection (DSI/FF) 16-6
Spray Dry Absorber (SDA/FF) 16-7
17. Toxic Metal Characteristics and Emissions Control 17-1
Toxic Metals in Medical Waste 17-1
Changes in Metals During Incineration 17-1
Example Metal Compositions 17-3
Toxic Metal Air Pollutants 17-4
Control by Adsorption, Condensation, Activated Carbon 17-5
Ash Testing & Groundwater Contamination 17-7
Vll
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Page
18. APCD Performance & System Control Features 18-1
Performance Indicators 18-1
Stack Testing Methods 18-2
APCD Control Considerations 18-4
Dew Point Considerations 18-5
Acid Gas Wet Scrubbing Operations 18-6
Venturi Scrubber Control Variables 18-7
Fabric Filter Performance Variables 18-8
Dry Scrubber & Spray Dryer Operational Features 18-10
19. APCD Operational and Safety Considerations 19-1
Methods for Detecting APCD Upsets 19-1
Monitoring & Control Concerns 19-2
APCD Start-up and Shutdown 19-3
By-Pass Stack Operations 19-4
Routine Operational Concerns 19-4
Effect of Upsets on Other Systems 19-5
Wet Scrubber Operational Upsets 19-7
Fabric Filter Operational Upsets 19-8
APCD Safety Hazards 19-9
20. Boilers and Other Heat Recovery Equipment 20-1
Heat Recovery Equipment 20-1
Heat Exchangers 20-2
Boiler Applications 20-4
Fire-Tube Boilers 20-6
Water-Tube Boilers & Economizers 20-8
Waterwall Boilers 20-11
Condensers 20-14
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21. Boiler Energy Principles
Types of Energy in MWI Boilers
Categories of Thermal Energy
Steam Characteristics
Energy Conversion Examples & Heat Transfer
Energy Conservation (Energy Balance)
Features of Steam Production
Page
21-1
21-1
21-3
21-4
21-6
21-8
21-11
22. Boiler Water Treatment
Boiler Water Impurities & Problems
Influence of Scale on Temperatures
Boiler Water Treatment Systems
Deaeration, Chemical Treatment & Slowdown
Indicators of Water Quality
22-1
22-1
22-3
22-4
22-5
22-10
23. Boiler Operational, Control & Safety Considerations
Operating Responsibilities
Water-Side Control Parameters
Potential Major Hazards
Single, Two and Three Element Control Systems
Standard Operating Procedures
Inspection Check-List
Boiler Operational Activities
Boiler Start-Up and Shutdown
Excess-Air Waterwall Unit Operations
23-1
23-1
23-1
23-2
23-4
23-7
23-7
23-8
23-11
23-12
Appendices
A.
B.
Glossary, Acronyms, and Symbols
ASME QMO Examination Guidelines
Appendix A-l
Appendix B-l
IX
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COURSE HANDBOOK INTRODUCTION
This Course Handbook was developed by the U. S. Environmental Protection Agency
(USEPA) in support of improving the air pollution control practices at medical waste incinerators
(MWIs). The USEPA was required to develop a model state training and certification program
for MWI operators under Title m, Section 129 of the Clean Air Act of 1990.
The training materials are composed of this Course Handbook and an Instructor's Guide
which includes the masters for the projection transparencies. State and regional air pollution
control agencies are encouraged to use these training materials in their training and certification
programs. The materials will also be useful in private entity training programs.
TRAINING PROGRAM GOAL
The primary goal of the training program is to provide an adequate level of understanding
to MWI operators to successfully complete the Operator Certification Examination of the ASME
Standard for Qualifications and Certification of Medical Waste Incinerator Operators (ASME
QMO-1). Certification through the ASME or an equivalent state-approved program will likely
be required when the USEPA promulgates standards for new MWIs and emission guidelines for
existing MWIs.
The training program focuses on the knowledge required by operators for understanding
the basis for proper operation and maintenance of medical waste incinerators in minimizing air
pollutant emissions. Particular emphasis is placed on the various aspects of combustion which
are important for environmental control. Fundamental information is related to applications and
to the operator's own work experiences.
Participants are encouraged to make comments and ask questions throughout the program,
as such discussion will help establish a creative environment for the course.
The program is designed to augment the normal site-specific, on-the-job, and supervised
self-study training programs which are typically provided by the vendor, owner, or operating
company. The program is not a substitute for such hands-on operator training programs.
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TRAINING PROGRAM INTENDED AUDIENCE
The program provides training for operators and operator supervisors of all sizes of MWI
units. The training focuses on the examination topics listed in the ASME QMO-1 Standard.
Operators of large regional MWIs may also wish to consider the USEPA Municipal Waste
Combustor Operator Training Program.
Other persons who are expected to participate in this operator training program include:
MWI operating and management staff members, general engineers, design engineers, technical
managers, mechanics, instrument and control technicians, and other maintenance personnel.
Regulatory officials, particularly those involved in permit review and inspections, are
expected to participate in this training program.
PROGRAM AGENDA
The training program is designed around a four-day sequence of learning units. This
manual follows the sequence of the recommended agenda which is found in the Medical Waste
Incinerator Operator Training Program, Instructor's Guide. However, the course can depart
from the recommended agenda to accommodate the special scheduling needs of the speakers.
COURSE HANDBOOK ORGANIZATION
The Course Handbook presents information in the subject areas addressed in the ASME
Examination for Certification as Operators and Operator Supervisors. Additional information
about qualifications may be obtained from a review of the ASME Standard.
The course is divided into three parts, which generally correspond to the three areas of
the ASME MWI certification examination. The sequence of topics was selected to reinforce the
integration of the basic aspects with the operational aspects. Each part considers the relevant
automatic control systems, trouble shooting, and preventive maintenance.
Part I begins by introducing the relation of the training program to the certification
process, the regulatory aspects of medical waste disposal, and the operator's role in public
relations. However, the emphasis in Part I is on the consideration of the combustion principles,
MWI equipment design and operational aspects, air pollution formation, and emissions
monitoring systems.
Part n focuses on air pollution control devices. The design and operational aspects of
air pollution control devices (APCDs) for particulates, acid gases, and heavy metals are
considered.
XI
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Part HI addresses the topic of heat recovery systems. The basic concepts of heat transfer
and thermodynamics are presented first. The design and operational features of heat recovery
systems (boilers) are then considered.
COURSE LIMITATIONS
To the extent possible, this Course Handbook was written hi a manner consistent with
USEPA policy regarding medical waste incinerators and the demonstrated features of good
combustion practice.
Detailed administrative and legal aspects of unit operations are not emphasized in the
program because the regulations under which units operate will vary with location and time.
Operators are urged to obtain specific regulatory information and permit requirements from the
owner/operator organization.
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1. INTRODUCTION
Slide 1-1
CLEAN AIR ACT AMENDMENTS
(CAAA) OF 1990
Develop Training & Certification
Require Operators to be Trained
Publish New Source Performance Stds
& Emission Guidelines
Regulate Through State Plans
The Clean Air Act Amendments (CAAA) of 19901 call for the U. S.
Environmental Protection Agency (USEPA) to develop a model state training and
certification program for operators of medical waste incinerator (MWI) units. The
amendments also state that the operation of such a unit would be prohibited unless
each person having control over the processes affecting its emissions has satisfactorily
completed an appropriate training program. This requirement will go into effect
within three years after promulgation of standards for MWIs by the USEPA.
The USEPA is currently considering various regulatory alternatives for
inclusion in the New Source Performance Standards (NSPS) and Emission Guidelines
(EG) for MWIs. The specific dates when operators of MWI units will be required to
satisfy federal training and certification requirements will be establish when the
NSPS and EG are promulgated.
Operator training and certification requirements are administered by the
various states through their state plans for achieving air quality standards. Some
states have already adopted or plan to adopt regulations requiring ASME certification
of MWI operators, whereas other states have developed or plan to develop their own
MWI operator certification programs.
Slide 1-2
MEDICAL WASTE INCINERATOR
OPERATOR TRAINING PROGRAM
Goal: Adequate Understanding to Pass
All Parts of the ASME
Certification Examination
1-1
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The primary goal of this operator training program is to provide an adequate
level of understanding to enable MWI operators to complete the examination
requirements of the ASME Standard for certification as medical waste incinerator
operators (ASME QMO-1).2 The actual testing required for the ASME Certification
is administered separately by the ASME and is not included in this training program.
Slide 1-3
MEDICAL WASTE INCINERATOR
OPERATOR TRAINING PROGRAM
Focus: Basis for Equipment Operation
and Maintenance
Basis for Good Combustion Practice
and Environmental Control
The training focuses on the knowledge required by operators to understand the
basis for proper operation and maintenance of medical waste incinerators in
minimizing air pollutant emissions. Particular emphasis is placed on the various
technical and operational aspects of combustion which are important for
environmental control.
Slide 1-4
Unit
COURSE HANDBOOK ORGANIZATION
Topic
Part I, Incineration & Monitoring
1 Introduction
2,10 Environmental Concerns & Regulations
3,4 Medical Waste Properties & Handling
5-10 Incinerator Equipment & Comb\istion
11,12 Instrumentation
13,14 System Operation & Maintenance
Part II, Air Pollution Control Devices
15-17 Control of Pollutants
18,19 System Operation & Maintenance
Part III, Heat Recovery Systems
20,21 Basic Concepts in Boiler Systems
22,23 Boiler Design & Operational Features
1-2
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The course is divided into three parts, which generally correspond to the three
areas of the ASME MWI certification examination.3
Part I introduces the regulatory aspects of medical waste management and
disposal. It emphasizes the combustion principles, MWI equipment design and
operational aspects, air pollution formation, and emissions monitoring systems.
Part II focuses on air pollution control devices. The design and operational
aspects of air pollution control devices (APCDs) for particulates, acid gases, and
heavy metals are considered.
Part III addresses the topic of heat recovery systems. The basic concepts of
heat transfer and thermodynamics are presented first. The design and operational
features of heat recovery systems (boilers) are then considered.
Each part of the training program considers the relevant aspects of automatic
control systems, trouble shooting, and preventive maintenance.
Slide 1-5
TRAINING PROGRAM TESTING
Pre-Test
Post-Test
Same Form and Difficulty
Measures Training Effectiveness
The Pre-Test is a measure of knowledge at the beginning of the training
program, and the Post-Test measures it at completion. The Pre-Test and Post-Test
are designed to be of approximately the same level of difficulty. Questions are based
on the material presented in the course manual. The tests are to be taken under
"closed book, time limited" testing conditions.
Pre-Test scores are expected to vary widely due to variations in individual
educational background and experience. A Pre-Test score is also an indication of the
need for training. The level of improvement in grades on the Post-Test over those of
the Pre-Test is a measure of the learning which has occurred.
The Pre-Test and Post-Test are not necessarily equivalent in difficulty to the
ASME MWI Certification Examination, although they are designed to focus on the
same subject areas. The ASME QMO Certification Examination, the Pre-Test, and
the Post-Test are in the form of multiple-choice questions.
1-3
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Slide 1-6
ASME MWI CERTIFICATION REQUIREMENTS
* High School Diploma or Equivalent
* Six Months of Acceptable Experience
* Demonstration of Operational Abilities
* Passing of MWI Certification Examination
The proposed qualifications for ASME QMO Certification2 are similar for
Operators, Operator Supervisors, and Special Operators. However, Operators and
Operator Supervisors will take different examinations which reflect the differences
in their duties.
Slide 1-7
ASME OPERATOR CLASSIFICATIONS
Operator
Class A: MWI with APCD & Heat Recovery
Class B: MWI with Heat Recovery System
Class C: MWI with APCD System
Class D: MWI without APCD or Heat Recovery
Operator Supervisor
Class A: MWI with APCD & Heat Recovery
Class B: MWI with Heat Recovery System
Class C: MWI with APCD System
Class D: MWI without APCD or Heat Recovery
Special Operator
Class A: MWI with APCD & Heat Recovery
Class A certification is the highest certification for Operators and Operator
Supervisors. With this certification, an Operator or Operator Supervisor can operate
MWI units which have any combination of MWI incinerators,, air pollution control
devices (APCDs), and heat recovery systems.
Class B certification is the minimum certification for Operators and Operator
Supervisors who operate MWI units which have heat recovery systems but do not
have an air pollution control device.
Class C certification is minimum certification for Operators and Operator
Supervisors who operate MWI units which have air pollution control devices but do
not have a heat recovery system.
1-4
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Class D certification is the lowest certification offered for Operators and
Operator Supervisors. With this certification, an Operator or Operator Supervisor
can operate only those MWI units which have neither an air pollution control device
nor a heat recovery system.
Special Operator Certification is available to the technical and operating staff
of manufacturing organizations as well as regulatory officials and other industry
representatives. Such individuals may have job functions which require operational
skills and making operational decisions. The Special Operator Certification is only
available for Class A certification.
Slide 1-8
ASME MWI CERTIFICATION EXAMINATION
Part I, Incineration & Monitoring
Basic Principles (30%)
E
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Additional information about the scheduling of the examinations and the
examination application procedures can be obtained from the ASME. Their address
is: ASME, Codes & Standards, United Engineering Center, 345 East 47th Street,
New York, NY 10017-2392. Their telephone number is (212) 605-3381, and their FAX
number is (212) 605-8713.
The current training program focuses on the subjects to be covered in the
ASME MWI Examination. Although many of the learning units include overlapping
subject areas, their primary focus can be distributed as indicated above. In general,
the operational aspects of equipment are based on component and system designs,
which are dependent upon the basic features such as medical waste properties,
regulations, and theoretical considerations.
REFERENCES
1. Clean Air Act Amendments of 1990 and Conference Report to Accompany S;
1630, Report 101-952, U. S. Government Printing Office, October 26, 1990.
2. "Proposed QMO-1 Standard for the Qualification and Certification of Resource
Recovery Facility Operators, American Society of Mechanical Engineers, New
York, Draft Version, November 21, 1991.
1-6
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2. ENVIRONMENTAL CONCERNS AND REGULATIONS
Slide 2-1
PUBLIC RELATIONS IN WASTE MANAGEMENT
* Out of Sight, Out of Mind
* Concern About Health & Environment
* Support for Recycling
The public attitude about waste management has often been illustrated by the
phrase "out of sight, out of mind." Roadside litter is a particular case in point.
However, the public is able to express concern when it feels its health and
environment are threatened. The public's enthusiastic advocacy of recycling is a
public relations positive.
Slide 2-2
ACRONYMS
NIMBY
YIMBY
BANANA
NIMTO
Not in My Back Yard
Yes, in My Back Yard
Build Absolutely Nothing Anywhere
Near Anybody
Not in My Term of Office
Public concerns are often caricatured through the use of acronyms, such as
those listed above. There are many examples where acronyms have been used in
discussions related to the siting of new public service facilities, such as waste
management facilities, highways, airports, and power lines.
The NIMBY concern focuses on the expected adverse impact of a particular
proposal on individual property and environmental values. By contrast, the YIMBY
advocacy is based upon the assumptions that the operation will have proper quality
assurance controls and that financial rewards will be available to the host
community.
The BANANA acronym reflects the reality that almost every location is in
someone's back yard. The NIMTO acronym characterizes the disowning of a problem
by a public official who assures the public that the proposed solution can be avoided,
at least for the time being.
2-1
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Slide 2-3
SOLID WASTE INCINERATION PUBLIC RELATIONS
Concern About Health
Ground Water Contamination from Ash
Toxic Air Pollutant Emissions
Pathogens: Transmission of Disease
Hepatitis B Virus (HBV)
Human Immunodeficiency virus (HIV)
The public relations aspects of medical waste management and of incinerator
operations can become crucial when the public has genuine health and environmental
concerns. Adequate management of medical waste first became a major focus of
public attention when medical waste and other debris washed ashore on the East
Coast in the summer of 1988.1>2
The public is often concerned about the potential for toxic emissions from
medical waste incinerators (MWIs). Toxic materials are poisons which can present
an unreasonable risk of injury to health or to the environment. Toxic air emissions
include carbon monoxide, toxic organics (e.g., dioxins) and heavy metals (e.g., lead
and cadmium) emissions. Other air pollutants of concern include acid gases (e.g.,
hydrogen chloride) which can lead to the formation of acid rain.
The removal of acid gases from flue gases will be discussed in Learning Unit
16. The concern about contamination of the ground water from leaching of toxic
constituents of MWI ash after disposal in a monofill or landfill will be discussed
further in Learning Units 4 and 17.
Medical waste, as defined in Learning Unit 3, represents about 29c of the
municipal solid waste (MSW) generated in the United States.1'2 Medical waste has
many component materials which are found in MSW, except that some are
contaminated by exposure to pathogens. Pathogens are those organisms (e.g.,
bacteria and viruses) which are capable of causing infection or disease. Potentially
infectious waste represents a modest fraction (e.g., 15%) of the medical waste from
hospital and other medical waste producing facilities.2
In general, landfills have refused to accept untreated medical waste as part of
municipal solid waste because of the potential for exposure to infectious agents.
Many states have laws and regulations that require treatment of medical waste
before it can be landfilled.
Incinerators, when properly operated and maintained, provide a waste
destruction method which destroys the infectious agents in medical waste. The
combustion conditions required for good combustion are able to destroy infectious
2-2
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agents, such as the hepatitis-type B virus (HBV), human immunodeficiency virus
(HIV, which leads to the acquired immune deficiency syndrome, AIDS) and other
blood borne pathogens.2 Incineration generally satisfies the public concern about
improper disposal of medical waste. In addition, incineration produces solid residues
which are unrecognizable as medical waste and have much less mass and volume.
Slide 2-4
PUBLIC RELATIONS PHENOMENA
Basis for Public's Mistrust:
* Impact of Past "Acceptable Practices"
* Concern about Toxic Emissions
* Potential for Traffic Accidents
Public opposition to new facilities may relate to its mistrust of the proposers
and the scientific establishment. The basis for public mistrust may relate to the
examples of certain facilities which previously were presumed to have been operated
acceptably but were later found to produce toxic emissions, endangering the
environment and public health. Outrage may be expressed by those who feel their
legitimate concern has been discounted.
The public may also be concerned about the trucking or importing of medical
waste. In general, the public is willing to support the appropriate treatment of
locally produced medical waste but has concern about transporting medical waste
from distant locations to a local treatment facility.
Slide 2-5
PUBLIC RELATIONS IN WASTE MANAGEMENT
Problems Which Are "Owned" Can Be Solved
Public Must Be Informed
Environmental Controls Are Available
Good public relations are crucial to achieving environmentally responsible
medical waste management. The public generally must "own the problem" before it
develops the desire to solve it. The public and its trusted representatives must be
fully informed about the magnitude of the problem and the trade-offs between risks
and costs. The public needs to obtain clear information rather than misinformation.
Although advocates generally conclude that environmentally acceptable solutions are
available, public and political support is required for implementing them.
2-3
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Slide 2-6
OPERATOR'S ROLE IN PUBLIC RELATIONS
Operators Must
Be Trustworthy
Be Certified as Being Qualified
Know What Is Expected
Demonstrate Willingness to
* Execute Responsibilities
* File Reports
* Communicate
* Assure Safety
Certification of operators (operator and operator supervisors) is required as a
method of protecting the health and welfare of both the public and employees against
poor operations which would pose a threat.
The operator's primary responsibility in public relations is to be trustworthy.
Sanctions may be imposed on certified operators who fail to make accurate reports
and/or to operate units properly. If management or the public concludes that an
operator's integrity has been compromised, the operator's job may be in jeopardy.
Management will generally fire an untrustworthy person rather than risk exposure
to equipment failures, accidents, downtime, sanctions, fines, and lawsuits.
In addition to being qualified and certified, the operator must assure that the
unit operates appropriately as required by the permit or regulations. Although the
unit may have a control system which is designed to automatically correct unit
operations and is "fail-safe," operators are required to understand the control systems
so they can anticipate and make appropriate changes in control settings. The
operator must be able to supervise those who assist in operating the unit.
Where violations of standards occur, the operator is required to file timely
reports about the type, severity, duration, and cause of the violation.
The operator must communicate effectively with both staff and upper
management. Questions must be asked, answers listened to, changes advocated, and
decisions made that assure the proper operation and maintenance of the equipment.
Such actions can help conserve the capital investment of equipment and help ensure
the productivity of the unit. The operator, as a representative of management, has
responsibility for assuring that safe operating procedures are developed and followed
and that personal safety equipment is available and used.
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Slide 2-7
FEDERAL SOLID WASTE LAWS & REGULATIONS
Resource Conservation and Recovery Act, RCRA
Subtitle C: Hazardous Waste Regulation
Manifest System
Subtitle D: Solid Waste Regulation
Sanitary Landfill Standards
Federal solid waste legislation began in 1965 and has evolved as the Resource
Conservation and Recovery Act of 19843 (RCRA). RCRA is the major federal statute
addressing management of hazardous, municipal, industrial and other types of solid
waste, including medical waste. A reauthorization of RCRA with possible new
applications to medical waste management may occur.
RCRA is administered by the USEPA, Office of Solid Waste. Under RCRA, the
USEPA has the authority to regulate the handling, storage, treatment, transportation
and disposal of solid wastes.1 (Incidentally, medical waste is no longer regulated as
a separate RCRA Subtitle J category, as the MWTA expired in 1991.)
Subtitle C of RCRA sets the legal basis for design and operational standards
for hazardous waste disposal sites and for mandatory reductions in the production
of selected hazardous wastes. RCRA is probably best known for its "manifest"
system, which requires a "cradle-to-grave" documentation of the movement of
hazardous materials from their manufacture until ultimate disposal.
Subtitle D of RCRA sets the basis for the design and operational standards for
sanitary landfills.
Legislation related to RCRA includes the Comprehensive Environmental
Response Compensation and Liability Act of 1980 (CERCLA) and the Superfund
Amendment and Reauthorization Act of 1986 (SARA). These acts established the
procedures for selecting hazardous waste sites, remediating such sites, and recovering
the costs from responsible parties.
Slide 2-8
HISTORIC INCINERATION ISSUES
Smoke & Odor From Incinerators
Toxic Emissions
Ground Water Contamination From Ash
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Early concerns about incinerators focused on smoke, odor, and carbon monoxide
emissions. These have been or are being addressed by state and federal regulations.
Slide 2-9
FEDERAL AIR POLLUTION LAWS & REGULATIONS
Clean Air Act, CAA
State Implementation Plans
State Rules and Regulations Must be at
Least as Strict as the Applicable Federal
Requirements and Approved by the USEPA
Federal air pollution legislation began with the passage of the Air Pollution
Control Act of 1955.4 Since then, revisions and amendments to the law have occurred
seven times, with the most recent being the passage of the Clean Air Act (CAA)
Amendments of 1990.
The USEPA, Office of Air Quality Planning and Standards, administers federal
air quality regulations. In general, a regulation is first publicly proposed. After a
review period, it may be modified and then promulgated. The USEPA generally
specifies the federal requirements, with enforcement provided by state agencies.
Each state develops its own plan for implementing air quality control, which must he
approved by the USEPA. State plans must include regulations which are at least as
strict as the applicable federal requirements.
Slide 2-10
CLEAN AIR ACT REGULATIONS
New Source Performance Standards, NSPS
Federal standards for new and modified stationary sources of air pollutants are
known as New Source Performance Standards (NSPS). The first federal NSPS for
incinerators was promulgated in 1971. It applied to new units sized to burn greater
than 50 tons/day.4
In February 1991, the USEPA updated the NSPS for new municipal waste
combustor (MWC) units and also promulgated Emission Guidelines (EG) for MWCs.
Both the 1991 NSPS and EG apply to units having capacities greater than 250
2-6
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tons/day.5 These require new controls for acid gases, products of incomplete
combustion, and particulates. Because of these size limitations, most MWIs were not
regulated under this NSPS.
Obviously, the federal, state and local regulations are revised from time to
time. At the current time the USEPA is developing NSPS and EG for medical waste
incinerators, as mandated in the Clean Air Act Amendments of 1990.
Slide 2-11
OTHER CLEAN AIR ACT REGULATIONS
National Ambient Air Quality Standards, NAAQS
Criteria Air Pollutants
Secondary Air Pollutants
Non-Attainment Areas
Prevention of Significant Deterioration, PSD
As required by the Clean Air Act, the National Ambient Air Quality Standards
(NAAQS) were established by the USEPA to define levels of air quality which protect
public health and welfare. NAAQS have been established for both criteria pollutants
and for secondary air pollutants which are formed in the atmosphere (e.g., ozone).
Slide 2-12
CRITERIA POLLUTANTS
Particulate Matter (PM)
Sulfur Dioxide
Carbon Monoxide
Nitrogen Dioxide
Lead
Ozone
Criteria pollutants are emitted air pollutants which have appropriately
documented health effects for which a NAAQS has been established. Secondary
criteria air pollutants are those formed indirectly in the atmosphere (e.g., ozone)
rather than being directly emitted.
Regulation by the states of emissions of criteria pollutants are generally based
on health and welfare effects. For instance, sources of sulfur dioxide emissions are
regulated to assure that the ambient level concentrations do not exceed the NAAQS.
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Non-Attainment areas are those which exceed the NAAQS for a particular
pollutant, such as ozone or sulfur dioxide.
Prevention of Significant Deterioration (PSD) measures have been established
to help maintain the quality of air in selected regions which have less contamination
than that specified in the NAAQS. PSD rules have required new facilities to apply
the "Best Available Control Technology" (BACT).6
Slide 2-13
NATIONAL EMISSION STANDARDS FOR
HAZARDOUS AIR POLLUTANTS (NESHAP)
Identify Toxic Air Pollutants
Set Maximum Emission Limits
Apply Equally to New & Existing Units
The CAA establishes the procedures for regulating hazardous air pollutants.
If the Administrator of USEPA establishes that a particular emission is toxic,
National Emission Standards for Hazardous Air Pollutants (NESHAPs) regulations
are required to be established to protect health. The maximum concentrations
allowable under NESHAPs are the same for both new and existing units.
Slide 2-14
CLEAN AIR ACT AMENDMENTS OF 1990, CAAA
New Units: New Source Performance Standards
Existing Units: Emission Guidelines
In recent years the public has become increasingly concerned about public
health issues, acid rain, toxic air pollutant emissions, and proper ash disposal
procedures.
The Clean Air Act Amendments of 1990 require the USEPA to adopt federal
control requirements, for both new and existing MWIs, which are equivalent to that
of the Maximum Achievable Control Technology (MACT). Therefore, the NSPS and
EG being developed for MWIs will be based upon technology rather than health
effects. Thus, the emission limits will be based on the control levels possible through
the application of MACT.
2-8
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Currently, the new regulations for air pollution control and emissions
monitoring have not been announced. However, one can anticipate that the federal
standards may correspond to some of the more restrictive state regulations.
Slide 2-15
MEDICAL WASTE TRACKING ACT
Established a Medical Waste Tracking System
Defined Regulated Medical Waste
Imposed Record Keeping Requirements
Imposed Penalties for Non-Compliance
Initiated Research on Risks & Exposures
In 1988, the U. S. Congress passed the Medical Waste Tracking Act (MWTA)
which required the USEPA to develop a two-year demonstration program of
comprehensive medical waste management, including a cradle-to-grave tracking
system for medical waste. Participating in the MWTA demonstration program were:
Connecticut, New Jersey, New York, Rhode Island, and Puerto Rico.7
The MWTA also required the USEPA to study issues related to medical waste,
including health risk assessment, available and potentially available treatment
techniques (including incineration).
The USEPA defined the waste to be tracked and established specific
regulations for medical waste segregation, packaging, and labeling.7 Definitions of
regulated medical waste and medical waste (waste generated by hospitals and other
medical facilities) were established. These definitions will be discussed in greater
detail in the next learning unit.
The MWTA provided regulations and imposed fines on sources which violated
these regulations. Tracking provisions included a "tracking form" closed loop record
keeping system for auditing purposes which is similar to the manifest system used
for hazardous waste.
The two-year demonstration program expired in 1991. However, many of its
provisions have been adopted by various state regulatory agencies. There are
currently 49 states that have some sort of regulations which define medical waste,
and 15 have some form of chain of custody for waste accountability. In addition, the
federal RCRA reauthorization legislation, currently under debate, may establish
specific medical waste requirements.8
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Slide 2-16
OPERATING PERMIT REQUIREMENTS
Air Pollution Monitoring & Reporting
Waste Processing Records
Ash Sampling and Testing Requirements
Waste Water Permit Requirements
OSHA Accident Reporting Requirements
There are a number of permits which are typically required for the operation
of a medical waste incinerator. Individual permits are typically required from the
state control agencies which regulate air pollution, solid waste disposal, and water
pollution.
The permits will generally state the required conditions under which the unit
must operate. For instance, the permit from the air pollution control agency may
stipulate the allowable capacity and emissions concentrations. In addition, the
permit may list the required combustion temperatures, monitoring equipment,
instrument calibration, and stack testing.
The requirements for waste processing records may include an indication of the
source, content, weight, date of receipt, and date of incineration for each box of
medical waste processed. The weight of the ash delivered for disposal may be
required, as well as special sampling and testing procedures followed to determine
the heavy metals composition, carbon composition, and leaching characteristics.
In addition, accident reports must be filed for specified types of accidents.
These may be in the form of quarterly composite reports which are routinely filed
with the OSHA office. Certain serious accidents will require the notification of the
OSHA office within a short time period (e.g., within one day of the event).
Slide 2-17
OCCUPATIONAL SAFETY AND HEALTH ACT, OSHA
Safety Standards to Protect Employees
Inspections Requirements & Penalties
Accident Reporting Requirements
The Occupational and Safety Health Act (OSHA) mandated that the U.S.
Department of Labor establish safety regulations and procedures which essentially
require employers to provide workplaces that are free of recognized hazards that are
2-10
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likely to cause death or serious physical harm to employees.9 The workplace hazards
may include those associated with electrical shock, corrosives, rotary equipment,
noise, and others which are discussed in Learning Unit 4.
"Process Hazards Analyses" (or Job Safety Analysis) and "human-factor
analyses" are among the management procedures which OSHA has developed to
reduce the risks associated with potential workplace hazards.9 In general, each
facility is required to perform their own assessment of hazards and to develop
standard operating procedures which provide protection against the occurrence of the
identified hazards.
Slide 2-18
OSHA BLOODBORNE PATHOGEN STANDARD
Exposure Control Plan
Exposure Determination
Schedule & Methods of Compliance
Procedures for Evaluating an Incident
Information, Training and Record Keeping
Engineering and Work Practice Controls
Personal Protective Equipment
Hepatitis B Vaccination
Labels and Signs
The OSHA bloodborne pathogens standard10, which became effective on March
6, 1992, has particular relevance to MWI operators. Standard safety procedures are
required to be developed and followed. These including developing a written
"exposure control plan" at each facility to identify the potential activities where
exposures to blood can occur and to develop preventive measures.
In general, compliance requires the implementation of "engineering controls"
and "work practice controls" in a manner consistent with universal precautions,
which will be discussed in Learning Unit 3. Also included are procedures for the
appropriate use of personal protective equipment to protect workers against
hazardous material and equipment exposures.
Inspections by state or federal OSHA will typically include checking on the
status of the safety management plans, procedures, reporting practices and training
activities. Significant fines are typically assessed for improper safety management.
General OSHA regulations require employee training programs which are
beyond the scope of this training course. This course will, however, present the
general features of medical waste handling practices in Learning Unit 4. Waste
handling practices must be consistent with OSHA requirements.
2-11
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REFERENCES
1 "Issues in Medical Waste Management, Background Paper," OTA-BP-0-49, U.
S. Congress, Office of Technology Assessment, October 1988.
2. Finding the Rx for Managing Medical Waste", OTA-O-459, U. S. Congress,
Office of Technology Assessment, September 1990, pp. iii, 2, 11, 16.
3. Terry Grogan, "The Environmental Protection Agency's Municipal Solid Waste
Program: Current Action and Future Plans, Integrated Solid Waste
Management. Frank Kreith, editor, Genium Publishing Corporation,
Schenectady, NY, 1990, pp. 155-164.
4. Kenneth Wark and C. F. Warner, Air Pollution. Its Origin and Control. 2nd
Edition, Harper & Row, Publishers, NY 1981, pp. 41-61.
5. U. S. Environmental Protection Agency, "Standards of Performance for New
Stationary Sources; Municipal Waste Combustors" and "Emission Guidelines;
Municipal Waste Combustors," Federal Register. Vol. 56, No. 28. February 11,
1991, pp. 5488-5527.
6. C. David Gaige and Richard T. Halil, Jr., "Clearing the Air About Municipal
Waste Combustors," Solid Waste and Power. January/February 1992, pp. 12-
17.
7. "EPA says Tracking Program Has Come a Long Way...," Infectious Waste
News, Vol 5, No. 26, December 24, 1990.
8. "Senate Medical Waste Bill Introduced..," Infectious Waste News, Vol 7, No.
2, January 20, 1992.
9. Matthey L. Kuryla and Stephen C. Yahay, "New Safety Rules Add to Plant
Manager's Worries; OSHA's Process-Safety Standards Expand Management
Responsibilities," Chemical Engineering, June 1992, pp. 153-160.
10. "Occupational Exposure to Bloodborne Pathogens, Final Rule," Federal
Register. 29 CFR Pate 1910.1030, Occupation Safety and Health
Administration, December 6, 1991, pp. 64175-64182.
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3. CHARACTERIZATION OF MEDICAL WASTE
Slide 3-1
CHARACTERIZATION OF WASTE MIXTURES
Type
Source
Material Constituents
Characteristic Features
Waste materials can be defined by their general type, source, constituent
materials, and characteristic features. Among the major types of waste mixtures are
medical waste, municipal solid waste, refuse derived fuel, industrial waste, hazardous
waste, and radioactive (nuclear) waste. Refuse derived fuel is formed by shredding
municipal solid waste and separating portions of the incombustibles.
Municipal solid waste (MSW) is the discarded garbage or trash from
institutional sources (e.g., hospitals), residential and commercial (business) sources.
Medical waste is generally considered to be part of municipal solid waste, but it may
be considered separately for regulatory purposes.
Hazardous wastes are those which pose a substantial present or potential
hazard to human health and to the welfare of the environment.1 To be hazardous,
the substance must contain a hazardous component which is above a specified
threshold concentration. Hazardous waste packaging, handling, and disposal are
regulated under the Resource Conservation and Recovery Act (RCRA).
Slide 3-2
SOLID WASTE ACRONYMS
BMW Biomedical Waste
HWI Hazardous Waste Incinerator
LLRW Low-Level Radioactive Waste
MSW Municipal Solid Waste
MWC Municipal Waste Combustor
MWI Medical Waste Incinerator
RDF Refuse Derived Fuel
RMW Regulated Medical Waste
The acronyms listed above are often used in discussions of waste management.
Their distinctive differences are important in regulatory and operational
considerations.
3-1
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Slide 3-3
MEDICAL WASTE TERMS
Biohazardous Waste
Biological Waste
Biomedical Waste
Cytotoxic Waste
Hospital Waste
Infectious Waste
Low Level Radioactive Waste
Medical Waste
Microbiological Waste
Pathogenic Waste
Pathological Waste
Potentially Infectious Waste
Red Bag Waste
Regulated Medical Waste
Segregated Medical Waste
Sharps
Special Medical Waste
Listed above are various terms which have been used to characterize medical
waste, based upon it source, constituent materials, and characteristic features.
This course will emphasize medical waste which is a heterogeneous mixture
that may include many of the above types of waste. The composition varies with the
applicable regulatory definition, as well as the waste segregation and waste
management practices. Medical waste is segregated for special handling and disposal
through the use of special containers such as red bags, plastic-lined corrugated boxes,
and sharps receptacles for disposal.
The course will not emphasize other wastes such as hazardous wastes,
chemical waste, radioactive waste, sewage and waste water, each of which has unique
treatment, disposal and regulatory requirements.
Cytotoxic wastes include specific pharmaceutical chemicals which are used in
chemotherapy for treating cancer. There are more than 50 cytotoxic drugs approved
by the Federal Drug Administration; seven of these are listed as hazardous by the
USEPA. In bulk quantities, these chemicals require handling as hazardous waste.
In general, most facilities handle all cytotoxic drugs as if they were hazardous. In
trace quantities, cytotoxic drugs may be disposed of in medical waste incinerators.
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Low-level radioactive wastes (LLRW) are generated by diagnostic, treatment,
and research applications of nuclear medicine. Many of the radioactive waste
materials have a short half-life, so after they have decayed to nonradioactive forms,
they can be disposed of routinely with other medical waste. The disposal of many
low-level radioactive wastes are regulated by the Nuclear Regulatory Commission
(NRC). Incineration of some radioactive wastes requires a special permit issued by
the NRC.
Slide 3-4
DEFINITION OF MEDICAL WASTE
Solid Waste Generated in the:
Diagnosis, Treatment and Immunization
of Humans or Animals
Research Related to Diagnosis,
Treatment and Immunization
Production and Testing of Biologicals
Medical waste, as defined in both the Medical Waste Tracking Act and the
federal New Source Performance Standards for municipal waste combustors,2 includes
any solid waste which is generated in the diagnosis, treatment, or immunization of
human beings or animals, in research related to diagnosis, treatment or
immunization, or in the production or testing of biological s (vaccines made from living
organisms).
The definitions of medical waste, infectious waste and regulated medical waste
varies between agencies, both at the state and federal levels. For purposes of this
training program, medical waste will be considered to include a heterogeneous
mixture of wastes, including that characterized as infectious waste (Slide 3-5) and
defined under the MWTA as regulated medical waste (Slide 3-7).
Other general solid waste (e.g., plastic materials, paper products, food wastes,
glass and metal containers) can be disposed of as municipal solid waste if it is not
contaminated by infectious agents. However, if general solid waste is placed in
medical waste containers it will be identified and handled as medical waste.
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Slide 3-5
INFECTIOUS WASTE
CDC:
All Blood and Body Fluids from
All Patients
USEPA: Waste Capable of Producing an
Infectious Disease
In 1987, the Centers for Disease Control recommended the "universal
precaution" because of the concern about the transmission of AIDS. The universal
precaution initially recommended that the blood and body fluids from all patients be
considered as infected with HIV (human immune-deficiency virus).3'4
In 1986, the USEPA defined infectious waste, more narrowly, as the waste
which is capable of producing an infectious disease.5 Infectious diseases are
communicable sicknesses or maladies which are produced by pathogenic organisms
(bacteria and viruses).
Slide 3-6
FACTORS NECESSARY FOR DISEASE DEVELOPMENT
1. Presence of a Pathogen
2. Sufficient Virulence
3. Sufficient Dose
4. Portal of Entry (Contact)
5. Susceptible Host
Five factors are necessary for the development of infection or disease,4 and the
absence of any one disrupts the development. The waste must contain pathogens of
sufficient virulence (degree of pathogenicity) and dose (quantity). The pathogens
must also have a "portal of entry" into a susceptible host. In other words, they must
be able to penetrate the natural barriers of the host, such as skin (e.g., though
mucous membranes or a puncture wound or cut).
Pathogens or infectious agents are the micro-organisms which are capable of
reproduction (self-replication) and can produce a harmful effect on other organisms.6
Medical waste incinerator operational staff are generally protected against the
transmission of infection or disease by limiting the factor of portal of entry. Exposure
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to pathogens is limited by isolating the wastes which may contain pathogens inside
special boxes or packages. The use of proper packaging, protective clothing and
washing procedures limits the potential for creating a portal of entry.
Slide 3-7
REGULATED MEDICAL WASTES7
Class Description
1 Cultures & Stocks of Infectious Agents
& Associated Biologicals (incl. Vaccines)
2 Pathological Wastes (Human Tissues, Organs
Body Parts, Body Fluids)
3 Human Blood & Blood Products
4 Unused Sharps (Needles, Syringes, Scalpel
Blades, Pipettes, Broken Glass)
5 Animal Wastes (Carcasses & Body Parts)
6 Isolation Wastes
7 Unused Sharps (Discarded)
Regulated medical waste (RMW) has been variously defined through a list of
seven to ten classes of constituent materials. The US Department of Transportation
currently uses the definition with the seven classes listed in slide 3-7.
Regulated medical waste, as defined by the MWTA, is a heterogeneous mixture
of the above listed classes of materials potentially capable of producing infection or
diseases in humans. The terms regulated medical waste, controlled medical waste
and infectious waste are often used interchangeably.8 Regulated medical waste is a
term which generally refers to mixtures of the classes of infectious waste listed above.
Isolation waste, Class 6, includes the biological waste and discarded materials
contaminated with blood and/or excretions from humans and animals that are
isolated to protect others from certain highly communicable and rare diseases, not
including HIV and HBV (hepatitis B virus).
Medical and laboratory equipment and implements (e.g., glass bottles and
plastic tubes), bedding, and personal protective equipment which have come into
3-5
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contact with or have been contaminated by any of the above classes of wastes may
be included in regulated medical waste by state regulations or individual facilities.
Regulated medical waste is not listed as hazardous by the USEPA. However,
some hazardous wastes may be found in the medical waste stream, such as some of
the chemical wastes from chemotherapy treatments. However, if the quantities are
small enough (according to the regulations of RCRA Subtitle C), these materials can
be disposed of in MWI units along with-other medical waste.
Slide 3-8
BIOLOGICAL HAZARD SYMBOL9
NOTE: Symbol It ftu
-------
Slide 3-9
SOURCES OF REGULATED
Type of Facility
Hospitals
Long Term Health Care
Physicians' Offices
Clinics
Laboratories
Dentists' Offices
Veterinarians
Funeral Homes
Blood Banks
Totals
MEDICAL
Tons/Yr
359,000
29,600
26,400
15,500
15,400
7,600
4,600
3,900
2,400
465,000
WASTE10
Percentage
77.1
6.4
5.7
3.6
3.3
1.6
1.0
0.8
0.5
100.0
Hospitals produce about 77 percent of the regulated medical waste generated
in the United States.10
Slide 3-10
HOSPITAL WASTE GENERATION FACTORS, (lb/bed/day)"
GENERAL INFECTIOUS
0.28
Georgetown Univ. Hosp.12
N.Y. Dept. of Health12
Rutala13
Cross14
Minnesota AQD15
21.6
20
17 - 23
24.8
4.0
2.4
3.2
1.2
The above hospital waste generation factors are based on the number of beds
in the hospitals. The high degree of variability in the above waste generation factors
is based on variations in waste management practices, with waste segregation
practices being the main factor in determining the amount classified as infectious
medical waste. Obviously, the patient occupancy rate influences the amount of waste
produced, with the average national bed occupancy rate being about 62%.16
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Various estimates have been made of the general waste produced by medical
facilities in a typical year in the United States. The general waste from hospitals in
the United States can be estimated at about 4 million tons/year (assuming 21
Ib/bed/day and 1.1 million hospital beds). The total from all medical facilities would
be about 5 million tons/year, assuming that hospitals provide 77% of the general
waste. This corresponds to 2.5 percent of the 180 million tons/year of MSW.17
Slide 3-11
FACTORS INFLUENCING THE AMOUNT OF
SEGREGATED MEDICAL WASTE
1. Type of Facility
2. Procedures Performed & Care Provided
3. Waste Management Practices
4. Applicable Regulations
The amount of medical waste which is segregated for special handling varies
considerably between and among facility types. For example, hospital operating
rooms generate more medical waste than other less intensive health care facilities.
Additionally, the waste management practices of a facility and the applicable
state and local regulations also influence the amount of segregated medical waste.
A medical unit whose policies are driven by liability concerns from the "universal
precautions" of the Centers for Disease Control may tend to classify large fractions
of their waste as infectious.
Other units may be able to exercise more restraint in placing items in the
medical waste receptacles. A number of medical units are implementing various
forms of waste minimization practices which reduce the quantity of waste classified
for handling as segregated medical waste.
A recent survey of hospital and health care facilities in St. Paul, Minnesota
found that the infectious waste varied from 0.90 to 3.27 Ib/patient/day, with an
average of 1.8 Ib/patient/day.18
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Slide 3-12
SEGREGATED MEDICAL WASTE COMPOSITION
Plastics
Paper
Rubber
Textiles
Glass
Body Fluids
Metals
The medical waste stream is composed of many of the same constituents as
municipal solid waste. The material composition varies from batch to batch,
depending upon the source and constituents.
Generally, medical waste contains a much higher percentage of plastics. Also,
the moisture content may vary widely depending primarily upon the components in
the waste load (i.e., pathological waste, blood, body fluids).
Slide 3-13
MOISTURE & HEATING VALUES OF SELECTED WASTES
WASTE MIXTURES MOISTURE HEATING VALUE
(Percent) (Btu/lb)
Medical Waste (Dry)19 9.0 9,240
Refuse Derived Fuel20 18.4 6,110
Medical Waste (Wet)22 37.3 5,290
Municipal Solid Waste20 24.2 4,830
Pathological Waste21
Plastics19
Alcohol, Disinfect.19
Swabs, Absorbents19
Gauze, Pads, Swabs
Garments, Paper19
Bedding, Shavings,
Paper, Fecal Matter19
Fluids, Residuals19
Human Anatomical19
Infected Animals19
85.0
0
0
0
0
20
80
70
60
- 1
- 0.2
- 30
- 30
- 46
- 100
- 90
- 90
1,000
13,860
10,980
5,600
5,600
4,000
0
800
900
- 20,000
- 14,000
- 12,000
- 12,000
- 8,100
- 2,000
- 3,600
- 6,400
3-9
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A characteristic feature of medical waste is its high degree of fuel property
variations. The variable moisture and plastics contents, which are influenced by the
contents or source of the waste, cause the heating values and burning characteristics
to vary considerably.
Plastic bags will volatilize quickly upon being charged into a hot combustion
chamber. Paper will burn readily if hot air is delivered to its surface, however, it
burns poorly if air is restricted, as in the example of a telephone book. Pathological
waste is organic material which is very high in moisture content. Generally, its
combustion requires the provisions for auxiliary fuel burning.
Slide 3-14
INCINERATOR INSTITUTE OF AMERICA CLASSIFICATIONS21
Type 0 Trash with 8,500 Btu/lb
10% moisture, 5% incombustible
Type 1 Rubbish with 6,500 Btu/lb
25% moisture, 10% incombustible
Type 2 Refuse with 4,300 Btu/lb
50% moisture, 7% incombustible
Type 3 Garbage with 2,500 Btu/lb
70% moisture, 5% Incombustible
Type 4 Human & Animal Parts, with 1,000 Btu/lb
85% moisture, 5% incombustible
Type 5 Industrial By-Product Wastes which are
gaseous, liquid, & semi-liquid
Type 6 Industrial Solid Byproduct Waste
rubber, plastic, wood wastes
Type 7 Municipal Sewage Sludge Wastes
residue from processing of raw sludge
Classification standards for solid waste were established by the Incinerator
Institute of America (IIA) in 1968.21
The IIA standard illustrates the fact that as the moisture in the waste
increases, the heating value is decreased. Moisture is incombustible and acts as a
heat sink because it requires energy for evaporation. Other incombustibles, such as
glass and metals, do not add any heating value to the fuel. Therefore the major
determining factors in the heating value of solid waste are the moisture and
incombustibles contents.
On the basis of its heating value, most medical waste can be approximated as
a type 0 waste by the IIA. Much of its heating value comes from its high plastic and
cellulose (paper & fabric) contents.
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Pathological waste, which is composed of human tissues, organs, body parts,
and body fluids, is clearly a type IV IIA waste. Pathological waste has a high
moisture content, averaging around 85%. The destruction of pathological waste in
an incinerator generally requires the burning of auxiliary fuel.
Slide 3-15
MEDICAL WASTE ULTIMATE
Element
Carbon
Hydrogen
Oxygen
Nitrogen
Chlorine
Sulfur
Inorganics (ash)
Moisture
Total
ANALYSIS
WET"
SAMPLE
Percent
by Weight
36.98
5.21
8.56
0.08
1.76
0.01
10.14
37.26
100.00
EXAMPLES
DRY"
SAMPLE
Percent
by Weight
51.1
6.2
21.3
0.5
4.1
0.2
7.6
9.0
100.0
The ultimate analysis of a fuel provides information about its chemical
composition. Specifically, it gives the fraction of weight represented by the various
chemical elements, moisture and ash (incombustibles).
The ultimate analyses of medical waste from two different sources are shown
above. Note that, although the samples have about the same ash contents, the "wet"
sample contains about four times as much moisture as the "dry" sample. The
corresponding heating values, which were previously listed in Slide 3-13, are
dependent upon the moisture content.
As will be demonstrated in Learning Unit 6, the ultimate analysis can be used
to estimate the products of combustion, including pollutant concentrations such as
hydrogen chloride and sulfur dioxide.
3-11
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Slide 3-16
COMMON TERMS WHICH CHARACTERIZE METALS
TOXIC METALS
Threat to Human Health
HEAVY METALS
High Molecular Weight
TRACE METALS
Found in Low Concentrations
The inorganic (ash) fraction is primarily composed of silicon oxides (glass) and
metal oxides. Upon incineration, some metals compounds will be formed which have
undesirable health affects. Both air pollution and groundwater contamination are
possible. Groundwater can absorb the metals which are leached from the ash
disposed in landfills.
"Toxic metals," "heavy metals," and "trace metals" are terms which are used
to characterize the metals of concern. Toxic metal is a term which is used to
characterize the potential threat to human health. Heavy metals are those having
relatively high molecular weights, and a trace metal has a small concentration.
These three terms are often used interchangeably because toxic metals are
those of high molecular weight which are often found in small concentrations. For
instance, lead is a toxic, trace, and heavy metal with an ambient air quality standard
(1.5 micrograms/cubic meter) which is based on public health and welfare
considerations.
Slide 3-17
POSSIBLE SOURCES OF HEAVY METALS IN MEDICAL WASTE
ITEM
Batteries
Autoclave Bag
Red Bag
Sharps Container
Rubber Cap
Sharps Tray
Syringe
Urine Container
METAL OF CONCERN
Lead, Mercury, Cadmium, Nickel
Lead, Chromium
Lead, Chromium
Lead, Chromium
Arsenic, Cadmium, Lead, Chromium
Lead, Chromium
Arsenic, Cadmium, Lead, Chromium
Cadmium, Lead, Chromium
3-12
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Some of the items in medical waste which may contain heavy metals are listed
in Slide 3-17. In addition to batteries, which should be disposed of separately, many
plastic items are formed with heavy metals in their dyes and plasticizers. Note that
because of the desire to minimize emissions of heavy metals, facilities may specify
"low metal bags" and other plastic materials which are colored with organic dyes.
Incineration of wastes containing heavy metals would be expected to result in
some distribution of the metals between the bottom ash, fly ash, and as airborne
emissions. Metals that vaporize in the primary combustion chamber (e.g., mercury,
cadmium, lead, and arsenic) may be converted to metal oxides or metal chlorides.23
Slide 3-18
AVERAGE COMPOSITIONS
OF MEDICAL
WASTE
Component UNIT A UNIT B
Carbon (Percent) :
Total Dioxin/Furan
(micrograms/kg) : 1,
Metals (mg/kg) :
Arsenic
Barium 3,
Cadmium
Hexavalent Chromium
Total Chromium
Copper 15,
Lead
Manganese
Mercury
Nickel
Selenium
Silver
Tin
1.8
450.
3.8
810. 2
2.7
5.0
28.2
360. 102
66.1
54.1
0.1
43.2
0.1
2.4
42.5
1.7
10.4
1.6
,130.
3.1
7.4
29.5
,700.
187.5
73.2
0.3
18.6
0.2
4.6
52.2
ASH"
UNIT C
1.8
25.3
0.7
1,640.
1.5
6.3
13.5
2,300.
76.5
17.9
0.2
9.7
0.1
0.6
70.3
The carbon, dioxin, and metals composition in the bottom ash from small
medical waste incinerators is illustrated above.24 Note that the numbers for metal
concentrations are expressed as fractions of the total ash in units of mg/kg
(milligrams per kilogram). These numbers are also equivalent to weight fractions
which could be expressed "ppm-weight" units.
3-13
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In this study, the medical waste burned was primarily hospital operating room,
delivery room and laboratory medical waste. The ash was judged to be non-
hazardous and acceptable for disposal in a landfill.
The units were each located on-site at a hospital. Unit A was a modern
modular, controlled-air incinerator, and Units B and C were older multiple-chamber,
excess-air incinerators. The units were operated with intermittent, manual charging.
Although the three units were nominally rated at capacities around 100 Ib/hr,
charges weighing from 10 to 30 pounds were fed into the units at half hour intervals
for the charging times ranging from 2 to 16 hours.
Slide 3-19
EXAMPLES OF COMBUSTIBLES IN ASH22'25'26'27
UNIT D UNIT E UNIT F
BOTTOM ASH
Carbon, % 10.4 3-16 6.2
L-O-I, % 15.3 7-20 19.5
BAGHOUSE ASH
Carbon, % 4-5
L-O-I, % 15-20
The amount of carbonaceous materials remaining in bottom ash is dependent
upon combustion conditions. Carbon may be determined by standard analytical
methods, with the values indicated above and in the previous slide ranging from
around 1.0% up to 16%.
Alternately, the "loss-on-ignition" provides a relative indication of the
combustibles in ash. Loss-on-ignition tests provide for measuring the weight loss
associated with heating a sample of ash to 1,450 °F in the presence of an oxidizer.28
Continuous operating units often have higher carbon contents in bottom ash
than intermittently-fired units. The continuously operated units have a relatively
short residence time for the solids in the incinerator. Intermittently fired units use
a burn-down period which provides for much better carbon burn-out.
Note that Unit D is a continuously operated, pulsed hearth incinerator rated
at 1,500 Ib/hr. Unit E is an intermittently-fired, controlled-air unit, rated at 650
Ib/hr, with a lime injection and baghouse system. Unit F is a single, batch-burn
incinerator rated at 750 Ib/batch.
3-14
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REFERENCES
1. L. Theodore, APTI Course 502, Hazardous Waste Incineration, Instructor's
Guide. U. S. Environmental Protection Agency, Air Pollution Training
Institute, Research Triangle Park, NC, March 7, 1986, p. 16-2.
2. U. S. Environmental Protection Agency, "Standards of Performance for New
Stationary Sources: Municipal Waste Combustors", Federal Register, Vol. 56,
No. 28. February 11, 1991, pp. 5488-5514.
3. "Issues in Medical Waste Management, Background Paper," OTA-BP-0-49, U.
S. Congress, Office of Technology Assessment, October 1988, pp. 3-6.
4. Finding the Rx for Managing Medical Waste", OTA-O-459, U. S. Congress,
Office of Technology Assessment, September 1990, pp. 9-18.
5. "Guide for Infectious Waste Management," EPA/530-SW-86-014, U. S.
Environmental Protection Agency, Washington, DC, May 1986.
6. "Getting a Handle on Defining Infectious and Biohazardous Wastes," Infectious
Waste News, Volume 7, No. 3, February 3, 1992.
7. U. S. Department of Transportation, Code of Federal Register 49 CFR Parts
173.134, Appendix G.
8. Louis Theodore, Air Pollution Control and Waste Incineration for Hospitals
and Other Medical Facilities, Van Nostrand Reinhold, New York, 1990, pp.
228-229.
9. "Infectious Waste Management Regulations," Commonwealth of Virginia,
Department of Waste Management, VR 672-40-01, May 2, 1990, p 3-4, 3-5.
10. "Medical Waste Management in the United States: First Interim Report to
Congress," U. S. Environmental Protection Agency, EPA-530-SW-90-051A, May
1990, p. 1-5.
11. R. G. Barton, et al., "State-Of-The-Art Assessment of Medical Waste Thermal
Treatment," Report to Risk Reduction Engineering Laboratory, USEPA, and
California Air Resources Board, June 15, 1990, pp. 27-36.
12. "EPA Releases Estimates on Infectious Wastes Generation for This Week's
Meeting," Infectious Waste News. Vol. 2, Number 24, November 17, 1988.
13. W. Rutala, "Management of Infectious Waste by United States Hospitals,"
3-15
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13. W. Rutala, "Management of Infectious Waste by United States Hospitals,"
Proceedings of the Twenty-Eighth ICAAC. Los Angeles, 1988.
14. Frank L. Cross, "The Case For Regional Incineration of Hospital Waste,"
Proceedings of the National Workshops on Hospital Waste Incineration and
Sterilization, U.S. Environmental Protection Agency, OAQPS, San Francisco,
CA, May 1988.
15. American Hospital Association, Hospital Statistics. Chicago, IL, 1987.
16. Paul Kaldjian, Characterization of Municipal Solid Waste in the United States:
1990 Update. U. S. Environmental Protection Agency, EPA-530-SW-90-042,
June 1990, pp. ES-7, 10.
17. "Hospital Waste Generation Study," Minnesota Pollution Control Agency, Air
Quality Division, June 1990.
18. "Minnesota Hospitals Study Alternatives; Evaluate Waste Stream
Composition," Infectious Waste News. Vol. 7, Number 2, January 20, 1992.
19. J. J. Santolevi and R. L. Kratz, "Medical Waste Incineration Requirements for
System Design Modifications," Proceedings of the Third National Symposium
on Infectious Waste Management: Incinerator Retrofit for Hospitals and
Industry. Chicago, IL, April 1989.
20. D. E. Fiscus et al., "Study of Existing RDF Co-Firing Experiences. Vol. 2:
Appendixes to Phase I Final Report." Report ANL/CNSV-TM-134, Vol. 2,
Argonne National Laboratory, October 1983, pp. C-l-13, C-l-14.
21. "Incinerator Standards," Incinerator Institute of America, NY, Nov. 1968.
22. Glen England et al., "Michigan Hospital Incinerator Emissions Test Program,
Borgess Medical Center Incinerator," Final Report, EPA Contract No. 68-03-
3365, submitted by Energy and Environmental Research Corp. to Michigan
Public Service Commission and the U. S. Environmental Protection Agency,
April 15, 1991, pp. 2-66 to 2-71 and 2-89 to 2-97.
23. "Medical Waste Incinerators-Background Information for Proposed Standards
and Guidelines: Process Description/Baseline Emissions Reporting for New
and Existing Facilities," Draft Report by the U. S. Environmental Protection
Agency, Sept. 30, 1991, p. 7.
3-16
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24. Peter Torkelson, "Report on the Assessment of Operations and Emissions of
On-Site Medical Waste Incinerators," Minnesota Pollution Control Agency,
December 1991, Appendix F, Part 2.m through Part 2.p.
25. Glen England et al., "Michigan Hospital Incinerator Emissions Test Program,
University of Michigan Medical Center Incinerator," Final Draft Report, EPA
Contract No. 68-03-3365, submitted by Energy and Environmental Research
Corp. to Michigan Public Service Commission and the U. S. Environmental
Protection Agency, April 15, 1991, pp. 2-49 to 2-54.
26. Radian Corporation, "Medical Waste Incineration, Emission Test Report,
Volume 1, Jordan Hospital," Report Number DCN: 90-275-026-25-0, EPA
Contract No. 68-D-90054, Submitted to the U. S. Environmental Protection
Agency, February 1992, pp. 2-73, 2-74.
27. Robert Barton et al., "Michigan Hospital Incinerator Emissions Test Program,"
Final Report, Volume 1: Project Summary Report, EPA Contract No. 68-03-
3365, submitted by Energy and Environmental Research Corp. to Michigan
Public Service Commission and the U. S. Environmental Protection Agency,
September 6, 1991, pp. 73 to 75.
28. John Richards, "Municipal Waste Incinerator Air Pollution Control Inspection
Course," Submitted to U. S. Environmental Protection Agency, June 1991, p.
5-17.
3-17
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4. MEDICAL WASTE SAFETY, HANDLING & TREATMENT
Slide 4-1
GENERAL HEALTH AND SAFETY
1. Training About Health & Safety
2. Personal Protection Equipment
3. Recognition of Hazards
4. Consequences of Exposures
The safety of people and equipment is an important operator responsibility.
Management has the responsibility for training employees about the health and
safety aspects of their jobs and for providing a safe workplace, including personal
protective equipment (PPE). Management also has the responsibility for enforcing
safe practices.
All staff members should be able to recognize potential hazards and know the
possible consequences. They should also know the recommended safety procedures
and be able to use safety equipment.
Slide 4-2
POSSIBLE HAZARDS IN OPERATIONAL SYSTEMS
1. Exposure to Infectious Agents:
Needle Sticks: AIDS & Hepatitis
Medical Waste Spills
Inhalation of Particulates
2. Combustion System Explosions:
Ignition of Explosive Mixtures
3. Boiler System Explosions:
Loss of Water, Tube Failures
MWI systems use combustion to chemically destroy the infectious agents found
in medical waste. The conditions required for good combustion are able to destroy
infectious agents, such as the hepatitis-type B virus (HBV), the human immuno-
deficiency virus (HIV) and other blood borne pathogens.1 A potential hazard for
operators is being accidentally stuck by a contaminated needle which has been
improperly packaged.
4-1
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The exposure to infectious agents is minimized by the use of safe packaging
and handling procedures, including the use of personal protective equipment (PPE).
In the event of a spill, proper containment, absorption and removal practices are
required. Exposure can occur from both contacting medical waste and breathing
contaminated air.
Explosions can be caused by the ignition of combustible gases, such as may
accumulate after a loss-of-flame by an auxiliary burner. Modern combustion control
systems are designed to prevent such hazards by stopping the fuel flow upon loss of
flame and assuring an adequate purge before re-ignition is attempted.2
Incinerator explosions can be caused by burning large quantities of volatile
solvents or gas cylinders. Large quantities of flammable liquids should never be fed
into MWI units,6 although small, quantities of such fluids, when mixed with other
medical waste, can be burned. The occurrences of explosions can be minimized by
effective management of the waste stream, including training and communications
about the risks. Effective record keeping may be able to identify the source of the
explosions.
Serious boiler explosions can occur when temperatures and pressures increase
due to a loss of water in the boiler and/or water-cooled heat exchangers.2 To prevent
such explosions, the ASME boiler code requires designs to include pressure relief
valves on the boiler system and water-cooled heat exchangers.3 Specific code
provisions include the number, type, installation and testing features of such safety
valves.4
Slide 4-3
OTHER MWI SYSTEM SAFETY HAZARDS
Skin Burns from Contacting Hot Objects
Eye Damage from Viewing Flames
Fire Hazards
Inhalation of Fugitive Dust
Confined Space Hazards
Cuts Associated with Removing Blockages
Serious burns can occur if individuals touch hot metal objects and materials
such as the incinerator charging doors, duct surfaces and ash. Refractory materials,
bottom ash and fly ash can remain hot for a long time due to their insulating
properties.5 Therefore, one should never step onto ash.
4-2
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To avoid skin burns, proper equipment and personal protective equipment
should be used. For instance, open-ended pipes should not be used for removing slag
deposits or clinkers, because under combustion conditions the hollow pipes can
become very hot and may direct hot gases onto the individual.7
To minimize the potential eye damage from viewing flames from combustion
and welding torches, tinted goggles or other eye protection should be used.7
Fires are potentially associated with upsets in waste charging and ash removal.
A fire in the charge hopper may be caused by ignited material (e.g., plastics) adhering
to the charging ram. Most MWIs have fire doors which limit the penetration of
flames into the charge hopper from the combustion chamber. Water sprays may be
used to cool the charging ram and extinguish the burning of any attached materials.
Some systems provide flame scanners which automatically turn on water sprays.5
To avoid ignition from sparks, medical waste should be stored away from the ash
removal area.
It is hazardous to enter confined spaces such as the secondary combustion
chamber and baghouse without proper precautions. Hazards may arise from poor
ventilation, cramped space, limited lighting and accumulated pollutants and/or toxic
dust. Confined spaces should be entered only after they have been properly cooled
and ventilated or when wearing a proper respirator. Entrance doors and appropriate
valves should be locked or tagged. Explosion proof lights and properly grounded
electrical extension cords should be used.6
Fugitive dust characterizes particulates which are not captured by a collection
system. Breathing air which is contaminated by smoke (e.g., from hopper fires or
incinerator discharges), dusts (e.g., from ash removal) and caustic fumes (e.g., from
scrubbing system upsets) may cause serious lung impairment.
The opening of observation hatches, ash hoppers and charging doors can result
in exposure to an unexpected blast of hot gases with toxic dust. The gases may rush
out of the unit because of combustion instabilities (known as puffing) or other positive
pressure conditions. At a minimum, precautions against such exposures include
standing to the side of the port or door and using protective equipment for the face
and eyes. Some systems may have safety provisions which deliver aspirating air to
the observation hatch or provide a protective transparent cover.7
When removing blockages in the charging, ash removal, and caustic scrubbing
systems, special precautions should be taken, including the use of proper mechanical
equipment and personal protective equipment. The blockage may include materials
which can cause cuts, burns or other injuries.
4-3
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Slide 4-4
OTHER STANDARD SAFETY CONCERNS
Electrical Shock
Corrosives
Rotary Equipment
Noise
Awkward Access
Movement of Heavy Objects
Standard industrial safety considerations include the engineering and
administrative controls associated with reducing the potential hazards associated
with electrical shocks, corrosives, rotating equipment, awkward access to equipment,
and movement of heavy objects. These include establishing maximum noise limits
in the workplace and procedures for protecting against hearing loss.
Before servicing rotary equipment the electrical current should be interrupted,
using "locked out" and "tagged out" procedures. In addition, it may be appropriate
to secure shafts to prevent rotation, which could otherwise cause injuries such as
mashed, cut, or severed fingers.
Operators should make use of material safety data sheets which describe the
standard safety information about various chemicals. Important data sheets for MWI
operators would include those for chemicals used in water treatment and scrubber
systems, solvents, refractories, and paints. These data sheets provide information
about the need for special handling and personal protective equipment (e.g.,
respirators). They also include health information, such as the toxic or carcinogenic
effects of exposure on humans and animals.
Slide 4-5
PERSONAL PROTECTION EQUIPMENT
1. Hearing Protection
2. Heavy Gloves
3. Hard Hat
4. Respirator
5. Goggles
6. Safety Shoes
7. Proper Clothing
4-4
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Standard safety procedures relate to the use of personal protective equipment,
such as that listed above.5'7 In addition to the above listed personal safety protection
equipment, persons should wear proper clothing so that there are no loose fitting
parts which could become tangled in rotating equipment. Natural fiber work clothes
should be worn because some synthetic fibers can melt when exposed to hot surfaces.7
Also care should be exercised when walking to avoid bumps, slips and falls.
Improper ladders and unsecured scaffolds should not be used. Care should be used
to assure that objects are placed securely to avoid damage due to falling.7
Special care should be taken to avoid chemical burns resulting from skin
contact with strong alkalis or acids.
Slide 4-6
SYMPTOMS OF ILLNESS
1. Headaches
2. Lightheadedness
3. Dizziness
4. Nausea
5. Loss of Coordination
6. Difficulty in Breathing
7. Chest Pains
8. Exhaustion
Operators should be aware that there are significant safety risks associated
with staff who perform their duties while having the symptoms of illness. An
impaired worker is a threat to the overall safety of the unit as well as to coworkers.
The above symptoms of illness may result from heat stress, inhalation problems
and/or a variety of both occupational and non-occupational related conditions.5
Slide 4-7
MEDICAL WASTE MATERIALS FLOW PATH
1. Generation at its Source
2. Collection & Packaging
3. Transportation
4. Receiving & Weight Scales Operation
5. Medical Waste Storage
6. Treatment (Incinerator, etc.)
7. Residue Removal & Disposal
4-5
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Medical waste handling practices are designed to protect workers from
exposures to infectious agents, risks of needle-stick injuries, medical waste spills, and
inhalation of reentrained particulates. In addition, they are designed to protect the
general public and the environment from the consequences of unhealthy exposures
to air pollution and water pollution.
Medical waste handling includes the management of medical waste along its
path from its generation at its source (e.g., hospital) through the activities of
collection, packaging, transportation, storage, treatment and disposal.
As discussed in Learning Unit 2, many medical waste management procedures
are regulated under the OSHA, Department of Transportation, Clean Air Act
Amendments, and RCRA, as well as state and local programs. In addition, insurance
companies and accrediting associations may require certain procedures to improve
safety.
Therefore, the specific waste handling features will take different forms
depending upon the applicable federal and state medical waste regulations, the type
of treatment, whether the treatment facility is on-site or off-site, and the standard
operating procedures of the operating unit.
Slide 4-8
WASTE MANAGEMENT AT THE SOURCE
Source Reduction
Recycling and Reuse
Packaging
Treatment (Decontamination)
Transportation
An integrated solid waste management program generally includes the
consideration of source reduction, recycling and reuse, treatment, and waste disposal
in some type of landfill. Management of medical waste at the source (e.g., hospital)
may include source reduction, recycling and reuse, waste segregation, packaging,
storage, transportation, treatment and disposal.
The treatment methods selected at a particular site will vary depending upon
the amount and character of the medical waste, availability of treatment facilities,
and economic, political and environmental considerations. Incineration and
treatment by steam autoclaves are the traditional medical waste treatment options,
with other methods such as microwaving and chemical treatment gaining in
popularity.
4-6
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Slide 4-9
SOURCE REDUCTION - WASTE MINIMIZATION
REDUCE QUANTITY
Reusable versus Throwaway
Packaging Materials
REDUCE TOXICITY
Material Substitution
Source reduction or waste minimization focuses on process changes which
reduce both the amount and toxicity of materials entering the waste stream.8 Waste
minimization includes the substitution of items which can be reused after treatment
in the place of throwaway items. The purchase of items which require less packaging
material is also a means of reducing the total waste stream. This practice must,
however, be evaluated on a case by case basis.
The application of some medical waste minimization concepts are often in
tension with the "universal precautions" of the Centers for Disease Control because
of the extra waste created by the packaging.9 However, the high costs of waste
disposal are leading many medical units to re-evaluate their medical waste
management programs.
Plastic bags of infectious waste are often transported from their sources to on-
site treatment facilities in laundry-type carts. By using such reusable carts, the labor
and materials required for packing in cardboard boxes has been eliminated.10
However, regulations may effectively require the use of cardboard boxes as
secondary containers for the transport of medical waste to off-site treatment facilities.
The box may represent as much as 20% of the total waste received at the incinerator.
The substitution of less toxic and environmentally preferred materials may
require management to undertake both an inventory of current materials and the
advantages and disadvantages of potential substitutes. Substitute materials could
include those which are lower in chlorine and heavy metal (e.g., cadmium & lead).
As mentioned in the previous learning unit, specific items of medical waste
may have a significant heavy metals content. Sharps containers may represent a
large percentage of the segregated medical waste of some facilities. Reusable sharps
containers are used in some facilities as a method of waste minimization; however,
special precautions are required to avoid needle punctures.
4-7
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Slide 4-10
RECYCLING
Has Public Support
Reduces the Quantity of Waste
Conserves Natural Resources
Reduces Environmental Impact
May Reduce Disposal Costs
Recycling is generally considered to be the most positively accepted component
in waste management. Source separation provides segregation of reusable products
and recyclable materials, such as paper products, aluminum, ferrous metals and
glass, as they are discarded.
Revenue from recycled materials may partially off-set the cost of the recycling
program. Generally, the recycling of cardboard and aluminum cans will be
economically attractive. Often the recycling of many other non-patient wastes, such
as high quality paper, glass and newsprint, will create economic savings only as a
result of reducing the charges for waste disposal.11 One should be aware that the
materials markets have traditionally fluctuated and may become marginal, with the
"soft" newsprint market being a recent example.
Slide 4-11
PACKAGING OF SHARPS
Plastic Sharps Container
* Disposable or Reusable
* Puncture and Leak Resistant
* Closable
* Label or Biohazard Symbol
Medical waste packaging practices are designed to protect workers from
exposures to infectious agents. Packaging is subject to the applicable federal, state
and local regulations. In addition, the medical facilities and medical waste disposal
organizations have established additional operational and safety procedures.
Sharps, such as needles, scalpels and syringes, may cause accidental
puncturing or cutting of an individual's skin. Such events are rare, but pose a threat
to worker safety by creating a "portal of entry" for pathogens. Therefore, sharps are
often required to be segregated from other infectious waste. They are packaged in
rigid, puncture resistant containers, which are often constructed of molded plastic.
4-8
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Slide 4-12
PACKAGING FOR ON-SITE TREATMENT
Plastic Bags: Single or Double
Plastic Containers: Sealed Lids
Colors: Red, Orange, Blue, White,
Labels: Biohazard Stickers
Biohazard Symbol
Open Carts for On-Site Transport
Clear
Packing, storage and transportation practices are regulated at the state level
in most states. Packaging requirements for medical waste which will be treated on-
site are, generally, less severe than those for off-site treatment. On-site packaging
requirements are now enforceable under the OSHA Bloodborne Pathogens Rule.12
The primary container for medical waste is generally either plastic bags or
rigid plastic containers. Glassware and liquids that are not disposed in the sanitary
sewer are commonly placed in rigid, break-resistant containers. Some fluids which
are suctioned during medical treatment may flow directly into rigid plastic disposable
containers.13 Small quantities of liquids from laboratory specimens (e.g., blood vials)
are often poured directly into plastic bags.
Although the waste is often called "red bag waste," various colors are used,
such as red, orange, blue, white and clear. Depending upon the applicable
regulations or standard operating practices, the primary containers may be labeled
with a biohazard symbol or sticker which indicates its contents.
Reusable carts are typically used as secondary containers for on-site treatment.
They provide temporary storage as well as transport to the incinerator or other
treatment device. Although open carts are often used, carts with scalable lids will
provide greater security.
Slide 4-13
PACKAGING FOR OFF-SITE TREATMENT MUST BE:
*
*
*
Rigid
Leak-Resistant
Impervious to Moisture
Strong to Prevent Bursting & Tearing
Sealed to Prevent Leakage
Puncture-Resistant for Sharps
Break-Resistant, Lids/Stoppers for Liquids
4-9
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Regulated medical waste which is to be transported for off-site treatment (e.g.,
incineration) will require packaging in containers that satisfy the regulations of the
Department of Transportation (DOT).14 The packaging must be rigid, leak resistant,
impervious to moisture, and sufficiently strong to prevent tearing or bursting under
normal handling.
The DOT generally requires the use of both primary and secondary containers.
The primary containers were described above. The typical secondary container is a
corrugated box which protects the inner bags (primary containers) from tearing.
Many applications use double boxing and double plastic liners. Some applications
may allow reusable bins, drums, or carts with scalable lids. Steam cleaning or
disinfection of reusable containers is often required before reuse.
The DOT regulations require that containers pass a "drop-test" to verify their
resistance to bursting and a leak proofness test.15 DOT regulations may also limit
the use of bulk containers (including those with scalable lids) for transporting
medical waste to off-site disposal facilities.
Treated medical waste is often disposed of as MSW.13 For aesthetic and public
relations reasons, many regulations require the waste accepted at landfills to be
unrecognizable as medical waste. In such instances, treated medical waste must
have its physical character changed, generally either by shredding or incineration.
Slide 4-14
RECEIVING & SCALE OPERATIONS FUNCTIONS
1. Establish Performance Records
2. Satisfy Record Keeping Requirements
3. Quantify Ash Removal
4. Restrict Delivery to Facility
The receiving and weight scale operations can provide important management
information about a unit's performance. However, the weighing and record keeping
function may not be required for on-site treatment facilities.
For off-site treatment, the manifest record keeping or shipping paper
requirements will depend upon the applicable regulations. Of course, the weight
scale operations provide the basis for the collection of processing fees at off-site
commercial facilities.
4-10
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The operators of off-site treatment facilities may have the responsibility for
sampling of ash for leach testing purposes and for establishing records of the weight
of the ash which is transported off-site (for disposal at monofills and/or landfills).
Slide 4-15
UNACCEPTABLE AND/OR UNDESIRABLE MATERIALS
1. Not Permitted - Hazardous, Radioactive
2. Cause Damage - Explosive Chemicals
The operator generally has the duty of restricting the delivery of materials to
those which can be appropriately processed. Permit restrictions will include
provisions which prevent specified materials from being processed by MWIs.
Radioactive waste is unacceptable if its radioactivity is above a threshold level.
Geiger counters are standard instruments which can detect the presence and strength
of radioactive materials.
Another typical example of unacceptable waste would be large quantities of
cytotoxic (chemotherapy) wastes. As discussed in Learning Unit 3, seven of these
chemicals are considered to be hazardous by the USEPA. To protect workers against
exposure, incinerator operators may restrict the acceptance of other than trace
amounts of chemotherapy wastes.
Among the materials which could cause damage to an incinerator are large
containers of solvents and unknown liquids. A large load of solvents could possibly
lead to an explosion in the combustion chamber. A large quantity of water or other
solutions could cause quenching of the normal combustion process. Large quantities
of plastic, although very combustible, could also lead to unstable combustion, smoking
conditions and excessive air pollutant emissions.
Metals and glass are incombustible materials which, although somewhat
detrimental to the equipment, are often included in the waste charge. Some metals
are basically unmodified by the combustion process. Others such as lead and
aluminum melt at temperatures (around 620°F and 1,200° F, respectively) which are
below the fuel-bed combustion temperatures. Lead is found in the metal deposits on
heat exchanger surfaces. Melted aluminum can solidify in the air-entry holes in the
hearth or grate, restricting air flow through the fuel-bed and creating uneven
burning. Glass, which melts at around 2,000° F, contributes to the clinkering.
4-11
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Slide 4-16
STORAGE
Locate Indoors or Outdoors
Buildings, Dumpsters, Tractor Trailers
Limit Access to Authorized Persons (Locked)
Keep Animals Away
Protect Against Water, Rain, wind Damage
Maintain in Non-Putrescent State
Refrigeration for Pathological Waste
Provisions for storage vary considerably with the location and capacity of the
source and treatment facility. The storage requirements are generally more severe
in urban areas than in rural areas and also more severe at large, commercial off-site
treatment facilities than at small, on-site facilities.
To reduce the possibility of exposure to the infectious waste, the integrity of
the packages must be protected against damage by the weather, unauthorized people
and animals. Storage inside buildings adjacent to the treatment facility may provide
easy accessibility during inclement weather. Outside storage includes the use of
locked tractor trailers or trucks in which the waste was transported. This type of
storage can reduce the amount of handling required and decrease the chance for
spills.
Regulations often stipulate that waste must be stored in a non-putrescent state
(without foul odor from decomposition). Storage times are not always regulated.
Therefore, storage times may vary from less than an hour up to as long as a month.13
Refrigerated storage at less than 45°F is commonly required if the medical waste is
to be stored for longer than a few days. Pathological waste may require refrigeration
even if the storage times are fairly short. Many medical waste transport trucks have
refrigeration units which allow for hauling over a long distance without creating
odors.
Slide 4-17
MEDICAL WASTE TREATMENT TECHNIQUES
1. Incineration
2. Autoclave (Steam Sterilization)
3. Microwave Irradiation Treatment
4. Chemical Treatment
5. Thermal Inactivation
6. Irradiation
4-12
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In addition to incineration, which will be discussed in the next learning unit,
there are a number of waste treatment techniques which are used to treat medical
waste. These alternative techniques are generally effective for microbiological waste,
but some are not effective for pathological waste.
Slide 4-18
AUTOCLAVE FOR MEDICAL WASTE
i<
Courtesy of AMSCO, International, Inc., Erie, PA
Steam sterilization in autoclaves is a treatment which combines the exposure
of moisture, heat and pressure to inactivate microorganisms. Steam autoclaves are
constructed as metal chambers which are designed to accommodate the operating
temperatures and pressures. The size of the devices range from benchtop models for
decontaminating medical equipment to commercial-sized models which can treat more
than a ton of medical waste per cycle.
4-13
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The operating factors that influence the effectiveness of steam autoclave
treatment of medical waste are the factors of temperature, duration, and steam
penetration of the waste. Generally, steam autoclaves operate most effectively when
the temperature measured at the center of the waste load approaches 250 °F and
there is adequate steam penetration through the waste.
Body parts and contaminated animal carcasses are excluded from treatment
by steam autoclaves because of their inadequate steam penetration. Radioactive,
hazardous and cytotoxic wastes are also inappropriate for treatment by steam
autoclaves.
Slide 4-19
MICROWAVE MEDICAL WASTE UNIT17
1. Automatic Feeding Assembly
2. Hopper Ud
3. Waste Receiving Hopper
4. Shredder
5. Inspection Window
6. Auger Motor
7. Main Auger Conveyor
8. Microwave Generator (Seven Used)
9. Resonance Chamber (Seven Used)
10. Temperature Sensor
11. Steam injection Line
12. Steam Generator
13. Discharge Conveyor Auger
14. Forced Draft Fan
©
yy
I Jt T T TfT/T T T T T
Courtesy of ABB Sanitec, Inc., Wayne, NJ
Microwave irradiation treatment of medical waste has recently become
commercial in the United States.18 In the typical unit, the waste is automatically
loaded into a closed hopper which feeds a shredder. An initial destruction phase
includes injecting steam into the hopper and feeding the waste into a grinding device
where it is shredded to small (e.g., 1-inch sized chips). The waste is then sprayed
with steam to increase the moisture content.
4-14
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The chipped waste is delivered by an auger to a series of microwave units. The
waste is exposed to microwave irradiation which heats the waste to between 205 °F
and 212 °F, causing the decontamination.13 After being maintained at these
conditions for an adequate period of time, the waste is generally discharged into a
dumpster. The entire process takes from one and a half hours to two hours. The
material leaving the unit is not recognizable as medical waste, so it can be landfilled
or used as a fuel.
Microwave treatment units can treat most infectious waste with the exception
of cytotoxic, hazardous, or radioactive wastes. Contaminated animal carcasses, body
parts, human organs, and large metal items may also be unsuitable for treatment by
microwave irradiation.
Slide 4-20
CHEMICAL TREATMENT UNIT WITH SHREDDER1
Negative
M*pa finer-. tyetem blower —i
Chlorine
"™
(oj
Courtesy of Medical Safe Tec, Inc., Indianapolis, IN
Various chemical oxidants, such as sodium hypochlorite (chlorine bleach) or
solutions of chlorine dioxide in water, have been used to treat medical waste. The
effectiveness of chemical treatment depends upon the characteristic of the chemical
antimicrobial agent, the concentration of the active ingredient, the contact time of the
chemical with the waste, and the characteristics of the waste being treated. An
antimicrobial agent can be described as a chemical substance which destroys disease-
causing pathogens or harmful microorganisms.
Some applications include immersing the waste into baths of the heated
solution before it is shredded.21
4-15
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Most medical waste items are suitable for treatment by chemical antimicrobial
agents, with the exception of body parts and contaminated animal carcasses which
are excluded from treatment for aesthetic reasons. Radioactive, hazardous, and
cytotoxic wastes are also inappropriate for treatment by this means.
The disinfecting and sterilizing features of antimicrobial agents may need to
be tested and officially registered (e.g., under the Federal Insecticide, Fungicide and
Rodenticide Act, Administered by the USEPA Office of Pollution Prevention and Toxic
Substances). Antimicrobial agents may be used alone or in combination with
shredding devices or encapsulation agents.
Many treatment technologies provide for a reduction in the waste volume
through some form of shredding.20 When the shredding process occurs before the
treatment, there is a potential for an environmental release of infectious agents, so
either a filtration or other effective control system may be required.
Other techniques for medical waste treatment use ultraviolet energy, radio
frequency radiation, and plasma arc devices to inactivate the infectious agents.12
After treatment, the options include incineration or compaction with direct
disposal in a landfill. Some systems compress the waste to extract the liquids, which
may require special treatment before being disposed of in sanitary sewers.
Slide 4-21
ISSUES REGARDING ASH DISPOSAL IN LANDFILLS
Environmental Impact
Landfill or Monofill
Leachate Effect on Groundwater
Heavy Metals Concentrations
Fugitive Emissions
Until fairly recently, landfills were used for the co-disposal of solid waste and
ash. However, important issues have been raised about the environmental impact
of such disposal practices, which can lead to the contamination of the groundwater.
Currently some agencies have regulations requiring ash to be disposed of in
monofills, which are special landfills which receive only ash, whereas others allow for
co-disposal with mixed wastes in modern landfills.22 Monofills may have the same
requirements as modern landfills, but organic materials are kept out. This reduces
the tendency for the organics to decompose and create acids which would increase the
rate of leaching.
4-16
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Modern landfills and monofills require leachate monitoring and treatment, as
well as groundwater monitoring. It is reasonable to assume that a modern, well-
operated monofill can operate without any significant health risk.
In fact, fugitive dust emissions at the landfill may be a major public health
risk.23 Fugative dust can be controlled by use of wetting agents and other operational
considerations.
Slide 4-22
MODERN LANDFILL SYSTEM
Slide 4-23
LANDFILL REQUIREMENTS UNDER RCRA
Containment System
Cap System
Bottom Liner
Leachate Collection & Treatment
Groundwater Monitoring
Gas Monitoring & Collection
4-17
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There are various requirements for a modern sanitary landfill under RCRA,
Subtitle D.24 The particular concern is to prevent leaching of heavy metals from the
wastes into the groundwater and to control the emissions of methane and other
landfill gases formed from waste decomposition.
Systems for containment and liquid and gas collection are required. Also,
groundwater wells and gas probes are installed for monitoring at selected locations.
Upon initial construction of a landfill, the entire bottom is covered with a liner.
This liner may be formed from both clay and synthetic materials such as high-density
polyethylene. Next, a leachate collection system is installed to capture and remove
liquids, including those from precipitation and those formed by decomposition of
waste. The top of the landfill is covered with a cap to prevent the infiltration of
water from precipitation into the landfill and to control landfill gas emissions.25
An on-site leachate treatment plant will probably be located at larger landfills.
Also, such sites may have a gas extraction system which recovers methane. The
methane can then be piped to a combustion device for power production.
Slide 4-24
*
*
*
MONOFILL
Hazardous Waste: Concentrations
Below Specified Limits
Chemical Waste
HWI Ash
MWI Ash
A monofill is a special landfill which accepts only a single type of waste.
Examples of such waste types include ash from MWI units, ash from municipal and
hazardous waste incinerators, and certain low-level hazardous and chemical wastes.
Monofills have special design, operating, and monitoring requirements in addition to
those of modern sanitary landfills.
Controversy related to the disposal of ash in sanitary landfills and monofills
is related to their heavy metals content and leaching characteristics. These issues
will be presented in Learning Unit 17.
4-18
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REFERENCES
1. Thomas Naber, "Infectious Wastes: The Questions Remain," Waste Age. May
1989, pp. 89-94.
2. Steam. Its Generation and Use. 39th Edition, Babcock and Wilcox, New York,
1978, pp. 34-8.
3. Frederick M. Steingress and Harold J. Frost, Stationary Engineering.
American Technical Publishers, Inc., Homewood, IL, 1991, pp. 71-82.
4. E. B. Woodruff, H. B. Lammers and Thomas F. Lammers, Steam Plant
Operations. Fifth Edition, McGraw-Hill Book Company, New York, 1984, pp.
223-230.
5. John Richards, Municipal Waste Incinerator Air Pollution Control Inspector
Course. Prepared by Entropy Environmentalists, Submitted to U. S.
Environmental Protection Agency, June 1991.
6. Louis Theodore, Air Pollution Control and Waste Incineration for Hospitals
and Other Medical Facilities. Van Nostrand Reinhold, New York, 1990, pp.
364-367.
7. Joseph G. Singer, Combustion, Fossil Power. 4th Edition, Combustion
Engineering, Inc., Windsor, CT, 1991, pp. 21-1 to 21-34.
8. Harry M. Freeman, "Source Reduction as an Option for Municipal Waste
Management," Integrated Solid Waste Management. Frank Kreith, editor,
Genium Publishing Corporation, Schenectady, NY, 1990, pp. 39-58.
9. "Issues in Medical Waste Management, Background Paper," OTA-BP-O-49, U.
S. Congress, Office of Technology Assessment, October 1988, pp. 20-25.
10. "Different Systems, Products Aimed at Reducing Packaging," Infectious Waste
News. Volume 6, No. 26, December 23, 1991.
11. "Finding the Rx for Managing Medical Waste," OTA-0-459, U. S. Congress,
Office of Technology Assessment, September 1990, pp. 19-40.
12. Occupation Safety and Health Administration, "Occupational Exposure to
Bloodborn Pathogens, Final Rule," Federal Register. December 6, 1991, pp
64175-64182.
4-19
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13. "Medical Waste Management in the United States: First Interim Report to
Congress", U. S. Environmental Protection Agency, EPA-530-SW-90-051A, May
1990, pp. 6-1 to 6-13.
14. "Performance Oriented Packaging Standards, Revisions and Responses to
Petitions for Reconsideration, Final Rule," Federal Register. 49 CFR 193.197,
U.S. Department of Transportation, December 20, 1991.
15. "DOT: No Bulk Containers of Medwaste Under HM-181," Infectious Waste
News. Volume 7, No. 6, March 16, 1992.
16. Courtesy of AMSCO International, Inc., Erie, PA.
17. Redrawn from an illustration provided courtesy of ABB Sanitec, Inc., a
subsidiary of ABB Environmental Services, Inc.
18. "University of Pittsburgh Gets Microwave Unit; Technology Gaining More
Regulatory Acceptance," Infectious WasteNews. Vol. 6, No. 24, Nov. 25,1991.
19. Courtesy of Medical Safe Tec, Inc., Indianapolis, IN.
20. "R. I. Company Introduces New Chemical Treatment System," Infectious Waste
News. Volume 7, No. 4, February 17, 1992.
21. "Company Gives WVTJ $200,000 Grant to Test Medwaste System," Infectious
Waste News. Volume 6, No. 21, October 14, 1991.
22. Marc J. Roggoff, "The Ash Debate: States Provide Solutions," Solid Waste &
Power. October 1991, pp. 12-18.
23. David B. Sussman, "Municipal Waste Combustion Ash: Testing Methods,
Constituents and Potential Uses," Proceedings. International Conference on
Municipal Waste Combustion. Volume 2, April 11-14,1989, Hollywood, Florida,
pp. 118-13 to 118-25.
24. Resource Conservation and Recovery Act, Part 40, CFR Part 257, Subtitle D.
25. Donald A Wallgren, "Modern Landfill Technology: The Cornerstone of an
Integrated Solid Waste Management Program," Integrated Solid Waste
Management, Frank Kreith, editor, Genium Publishing Corporation,
Schenectady, NY, 1990, p. 129.
4-20
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5. MEDICAL WASTE INCINERATORS
Slide 5-1
PURPOSE OF MEDICAL WASTE INCINERATION
Decontamination
Destruction by Oxidation for:
Mass Reduction
Volume Reduction
Making Waste Unrecognizable
MWIs are treatment devices which use combustion processes to decontaminate
or sterilize medical waste and, at the same time, render the waste un-recognizable,
and reduce the mass and volume of the material which must be landfilled.
Slide 5-2
EXISTING MEDICAL WASTE INCINERATORS
Multiple Chamber, Excess-Air, Refractory Wall
Single Chamber, Excess-Air, Refractory Wall
Modular, Controlled-Air, Refractory Wall
Rotary Kiln, Excess-Air, Refractory Wall
Multistage Combustion, Waterwall
A number of designs have been used for MWIs. The dominance of multiple
chamber (excess-air, refractory wall) incinerators in the 1960s went through a
dramatic shift to controlled-air (starved-air) units in the 1970s and 1980s as a result
of combustion system innovations and environmental control requirements.
Many of the smaller units are manually charged and do not have air pollution
control devices (APCDs). New systems options include improved combustion control
systems, larger unit sizes, automation of the waste charging, air pollution control
devices, and continuous emissions monitoring systems (CEMs). Some of the larger
MWIs also have waste-heat steam boilers which recover energy from the hot
combustion gases.
Controlled-air incinerators will be the focus of this training program because
they represent the majority of current MWI applications. Multiple chamber and
other excess-air MWIs will be discussed for general information and as a frame of
reference. Although a number of multiple chamber units exist, they are generally
smaller sized and older units, which are not currently being produced.
5-1
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Slide 5-3
EXISTING MWIS WITH CAPACITY ABOVE 50 LB/HR1
TYPE OP DESIGN NUMBER OF UNITS
Batch Controlled-Air & Pathological
Hospitals 115
Intermittent Duty Controlled-Air
Hospitals 775
Laboratories & Research Units 73
Nursing Homes 39
Animal Shelters & Veterinaries 10
Intermittent Duty Pathological
Hospitals 158
Animal Shelters & Veterinaries 86
Laboratories & Research Units 50
Nursing Homes 14
Continuous Duty Controlled-Air
Hospitals 62
Commercial 39
Municipal Waste Combustore 17
Laboratories & Research Units 4
Rotary Kiln, Excess-Air
Hospitals 10
Municipal Waste Combustore 2
Commercial 2
Total 1,456
The above slide is provided to show the diversity of MWI designs and
applications.1 Manufacturers have traditionally sold incinerators to hospitals,
medical facilities, research institutions, pharmaceutical companies and waste disposal
companies. The numbers exclude MWIs operated at below 50 Ib/hr, so many of the
small retort multiple chamber incinerators are not listed. Pathological waste
incinerators include both those with controlled air and multiple chamber designs.
Waterwall MWIs are not listed separately but are included as controlled-air MWCs.
Incinerators with controlled-air design concepts can be operated in a single
batch mode, intermittent duty or continuous duty. Batch operated units burn single
batches of waste on a daily burn cycle; intermittent duty units operate on a daily
burn cycle with intermittent feeding of small charges of waste; and continuous duty
units operate 24 hours per day, with periodic ash removal as required.
5-2
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Slide 5-4
MULTIPLE CHAMBER EXCESS-AIR UNITS
Refractory Wall Designs
High Excess Air & Gas Velocities
Particle Entrainment
Smoke
The multiple chamber incinerators, which were primarily built in the 1950s
and 1960s, are characterized by the use of high excess air and by being site-built
refractory-wall incinerators. They are called "excess-air" units because they operate
with excess air in both the primary and secondary combustion chambers. The two
basic configurations are the "retort" and "in-line" multiple chamber incinerators.
In the 1960s and early 1970s, many of the multiple-chamber, excess-air
incinerators were shut down due to their inability to meet air pollution regulations.
They were generally replaced by modular controlled-air incinerators which had fans
for controlling the air supply and burners to control combustion temperatures.
However, many multiple chamber incinerators are currently in use as MWIs
at hospitals.2 Some have been permitted, basically, as unregulated sources because
they are fairly small (e.g., 100 Ib/hr) and produce limited smoke emissions. Many
units have operated under the single requirement that opacity limits be met.
Slide 5-5
MULTIPLE CHAMBER "RETORT" INCINERATOR1
Charging
Door
Secondary
Air Potts
Ignition Chamb«r
Htarth
Secondary
Bunwr Port
Miring
Chamber
fira
Undirhtanrt
Port
5-3
-------
The smaller multiple chamber "retort" incinerators are characterized by
multiple changes of direction in the gas path, including directing the combustion
gases under the hearth to provide heating of the hearth.
Slide 5-6
MULTIPLE CHAMBER "IN-LINE" INCINERATOR2
Source: Cross/Tessitore & Associates, P.A., Orlando, FL
The combustion gases are only slightly cooled by heat transfer to the refractory
walls. The temperature of the refractory wall is controlled by providing enough
excess air to act as a heat sink and keep temperatures low enough to prevent damage
to the refractory. This extra air typically results in high velocities in the primary or
ignition chamber, which causes high particulate entrainment and smoke emissions.
Slide 5-7
MODULAR CONTROLLED-AIR UNITS
Factory Manufactured
Refractory-Wall
Starved-Air (in Primary Chamber)
Low Velocity in Primary
Low Particulate Entrainment
5-4
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Modular, controlled-air incinerators were developed in the mid-1960s.3 These
manufactured units gained popularity because they had relatively low particulate and
smoke emissions, without requiring stack gas cleaning equipment. Relatively low
emissions resulted from the low velocities in the primary chamber and improved
combustion in the secondary chambers controlled by auxiliary fuel burners.
Controlled-air units get their name from the concept of controlling the
combustion conditions by regulating the delivery of combustion air to strategic
locations using fans and dampers. Starved-air is a name which characterizes units
which operate with less than the theoretically required air (fuel-rich combustion) in
the primary chamber. Starved-air units operate with two-stage combustion: fuel-rich
in the primary chamber and lean in the secondary. The flue gases leave under
excess-air conditions.
Most controlled-air units have primary combustion chambers operating under
starved-air conditions, although some units are designed to operate with air/fuel
mixtures at approximately stoichiometric conditions. Stoichiometric considerations
will be discussed in Learning Units 7, 8 and 9.
Since the 1970s, the trend has been toward controlled-air incineration,
although some multiple chamber incinerators have been equipped with add-on air
pollution control devices. Many locations now require incinerators to operate with
removal of as much as 99.8% of the hydrogen chloride (HC1) emissions. Controls on
some units must meet particulate emission limits as low as 0.01 gr/dscf at 12% CO2
and flue gas dioxin emissions below 0.1 ng/dscm.*
Slide 5-8
TYPICAL AIR & FLUE GAS FLOW PATH
1. Forced Draft Fans
2. Underfire Air Ducts & Nozzles
3. Primary Combustion Chamber
4. Flame Port Secondary Air
5. Secondary Fuel Burner
6. Recovery Boiler
7. Air Pollution Control Devices (APCDs)
8. Induced Draft Fan
9. Stack
Equipment which comprises the air and flue gas flow path for a large MWI is
illustrated above. As discussed in Learning Unit 9, the primary difference between
forced draft and induced fans is that forced draft fans push air into the MWI and
induced draft fans are positioned downstream of the incinerator and effectively pull
air out of the unit.
5-5
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Slide 5-9
MANUALLY CHARGED MODULAR CONTROLLBD-AIR UNIT*
Wall
Air Incuctor fling
Charging Dosr
Ash Clean
Out Door
Source: Cross/Tessitore & Associates, P.A., Orlando, FL
Slide 5-10
CONTROLLED-AIR UNIT WITH A PNEUMATIC LOADER5
STACX CAP
STACK
WATER SPRAY
Source: Shenandoah Manufacturing Company, Harrisonburg, VA
5-6
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Slide 5-11
MODULAR CONTROLLED-AIR OR STARVED-AIR UNIT*
JBSil*- SRMK SCREEN
DRAFT CONTROL
DAMPER ASSEMBLY
PYROLYTIC
INCINERATOR I
ASSEMBLY I
RETENTION CHAMBER
THERMOCOUPLE
HEAT RECOVERY
OR SYNCHHOFIRE
BOILER ASSEMBLY
Courtesy of Clever Brooks, Division of Aqua-Chem, Inc., Milwaukee, WI
Some of the smaller controlled-air units designed for manual batch charging
have been replaced by intermittently charged units. These units generally have
provisions for automated charging and improved combustion control systems.
Intermittently charged units operate on a daily burn cycle, with intermittent feeding
and burning followed by a period for burning out the waste residues.
*
Auxiliary fuel burners in the primary ("pyrolytic") chamber are used initially
to preheat the refractory and maintain an adequate chamber temperature. Under
normal conditions, the air flow in the primary chamber is limited to maintain
temperatures which are high enough to drive off the volatiles, but not so hot as to
damage the refractory. The evolution of volatile gases will be increased if the
primary chamber temperatures are raised, for instance by increasing the underfire
air supply or by burning auxiliary fuel.
5-7
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The partially oxidized volatile gases from the primary chamber flow into the
secondary chamber, where they are mixed with additional air for completion of the
combustion process. Auxiliary fuel burners in the secondary chamber may be needed
to maintain the secondary chamber temperature requirements.
Slide 5-12
CONTINUOUSLY OPERATED CONTROLLED-AIR UNIT7
MODULAR INCirCRATCR
Courtesy of Joy Energy Systems, Incorporated
Continuously operated incinerators are typically large controlled-air units.
They are charged on a 24-hour per day basis and have automatic systems for bottom
ash removal. Hydraulic rams, or some form of hearth movement, agitates the
burning waste and gradually moves it toward the ash collection or quench area. Ash
removal systems generally operate on an intermittent basis, although some
continuously operating ash removal systems have been used.
5-8
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Slide 5-13
ROTARY KILN INCINERATORS
Ram or Auger Feeding System
Refractory Lined Rotary chamber
Excess-Air Combustion
Tumbling Action In Primary
Secondary Combustion Chamber
Rotary kiln combustion units have been widely used in the fertilizer, cement,
and aggregate industries. In addition, they have been used extensively in hazardous
waste incineration applications. Their applications for medical waste disposal has
been limited by the cost considerations associated with the heavy rotating equipment.
Slide 5-14
ROTARY KILN INCINERATOR SYSTEM9
APCD
Induced
Draft Pan
Secondary
Combustion
Chamber
Heal Recovery
System
Continuous
Ash Removal
Auxiliary Burner
Hydraulic
Power Pack
Auger-Slier
Feeder
Courtesy of Industronics, South Windsor, Connecticut
5-9
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Medical waste is fed automatically into the primary combustion chamber from
a hopper. Either an auger system (as illustrated above) or a ram driven charging
device can be used.
The characteristic feature of a rotary kiln incinerator is that the primary
chamber rotates and tumbles the waste as it burns. The rotary kiln is oriented on a
small incline, so that the tumbling action causes the migration of the waste toward
the ash hopper as it burns.8
Rotary kiln units have many similar features with other incinerators, such as
secondary combustion chambers and provisions for air pollution control devices and
waste-heat boilers.
Slide 5-15
COMBUSTION CHAMBER WALL CONSIDERATIONS
REFRACTORY WALL
Thermal Insulation
Protection Against Thermal Damage
WATERWALL
Radiant Energy Extraction
Refractory walls are designed to insulate the combustion gases against heat
loss and to reflect radiant energy back to the combustion zone to enhance the
combustion processes.
In general, the refractory must be maintained below a given temperature to
avoid mechanical damage (refractory melting, cracking, fracturing). Some refractory
damage can be anticipated for units operating with a daily heat-up, cool-down cycle.
If the high temperature limits are exceeded, the refractory damage may be excessive.
The multiple chamber, refractory wall incinerators built in the 1950s and
1960s were typically designed to use large amounts of excess air to control
combustion temperatures, thereby limiting refractory damage. Many modern
refractory wall incinerators use starved-air techniques to restricting combustion
chamber temperatures.
5-10
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Slide 5-16
MODULAR INCINERATOR WITH ENERGY RECOVERY8
BY-f*SS STACK
ANDVW.VE
DUAL UPPER CHAMBER
COOLING
SYSTEM HEAT
EXCHANGER
LOWER CHAMBER
ASH
CONVEYOR
WASTE CART
DUMPER
LOADER
HOT GAS
DUCT
LD.FAN
Courtesy of Consumat Systems, Incorporated
Many modular incinerator units are able to function as waste-to-energy units.
Energy is extracted in waste-heat boilers, which are located downstream of where the
gases leave the secondary chamber. These boilers function basically as air-to-liquid
heat exchangers. Water inside of vertical tubes is converted to steam, which is
collected and distributed for commercial purposes.
Waste-heat boilers generally extract a smaller fraction of the fuel energy than
is possible in waterwall units. This is because waterwall units operate with high
temperature, radiant heat extraction in the primary chamber, whereas modular units
recover energy from lower temperature flue gases.
5-11
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Waste-heat boilers and heat exchangers (e.g., air preheaters and economizers)
are generally required to be protected against overheating. If a system upset
occurred so that the metal temperatures were heated above their design point, the
unit could be irreparably damaged. Temperature protection is generally provided
through the use of fresh air supply dampers and by-pass or dump stacks.
Slide 5-17
WATERWALL INCINERATION
Tubes of Water: Membrane Wall
Waste-to-Energy
Radiant Energy Extraction
The term waterwall or membrane wall relates to the boiler design concepts
used in most power boilers. Waterwall units have multiple tubes in the form of metal
enclosures, which surround the combustion gases. The tubes are filled with flowing
water, which extracts energy from the adjacent combustion gases.
Slide 5-18
WATERWALL DESIGN CONCEPT*
Courtesy of Babcock and Wilcox
5-12
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The region containing the burning gases is called the radiant boiler or radiant
furnace section. The overall unit is often called a boiler, integral boiler or steam
generator-terms which are often used interchangeably.
Many of the early waterwall incinerators burning waste fuels experienced
metal wastage from corrosion and erosion. Such metal losses have caused tube
failures which require units to be shut down for repair. Most waterwall units are
now using either a highly conductive refractory material (e.g., silicon carbide) or
nickel-based alloy overlays on the boiler tubes (e.g., inconel 625), at least in the lower
furnace region.10 These keep the corrosive combustion gases away from the tubes.
These barriers also act as insulation, which helps to achieve a more uniform heat
extraction rate.
Slide 5-19
WATERWALL MWIs
Waterwall in Primary Chamber
Stoichiometric Air Supply
Special Moveable Hearth
Special Air Nozzle System
Multistage Combustion
Auxiliary Fuel Burner
Special Air Injector Designs
Waterwall technology is used in many municipal waste combustor designs.
From a combustion standpoint, many of these large units are capable of burning
medical waste. There is, however, concern about the possibility of contaminated
sharps or other infectious waste passing through or getting lodged in the air passage-
ways of the grates. Many MWCs have permit restrictions which prohibit the burning
of medical waste.
Waterwall MWIs are used in a limited number of hospital applications.11 The
design includes a number of innovations such as a cable-suspended hearth (pulsed
hearth), which moves in such a way that cause the solid waste to migrate toward the
ash hopper.
Many of the same hearth design and combustion system concepts are also used
in MWIs which are designed with refractory walls.11
5-13
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Slide 5-20
WATERWALL INCINERATOR11
IM*I Cap
Stac*
ID. Fin
Courtesy of Basic Environmental Engineering, Inc., Glen Ellyn, IL
Other innovations include the use of special air injectors (thermal exciters) in
the combustion regions downstream of the primary combustion chamber and
secondary burner. These air injectors, in effect, cause multiple stages of mixing of
air and products of combustion which can lead to complete combustion.
Slide 5-21
FEEDING EQUIPMENT
Manual Batch Charging
Manual Intermittent Charging
Automatic Ram Feeders
Intermittent Charging
Transfer Rams: Bed Agitation
Continuous Auger System
5-14
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Modular MWI units are designed to operate with both manual (batch and
intermittent) and automatic feeding systems.
Batch charging literally means that a single charge of waste is loaded into the
incinerator and burned completely before the ash is removed and the unit operating
cycle is repeated. Such units are sometimes characterized by the phrase: "stuff and
burn."
Slide 5-22
INTERMITTENT WASTE FEEDING8
WASTE LOAD INTO HOPPER
START
FIRE DOOR OPENS
STEP1
STEP 2
RAM REVERSES TO CLEAR
FIRE DOOR
STEPS
FIRE DOOR CLOSES
STEP 4
RAM COMES FORWARD
RAM RETURNS TO START
STEPS
Most MWIs are operated in an intermittent charging mode, either manually
or with an automatic feeding system. This typical system will receive a fresh charge
of waste at selected time intervals (e.g., every 6 to 10 minutes).
Many intermittently operated systems have an interlock device can be provided
to prevent the charging door from opening until the secondary chamber has been
preheated adequately. At the end of the cycle, the charging door may be locked to
prevent it from opening until the unit has properly cooled down for ash removal.
Automatic charging systems may provide for manually loading each charge into
the charge hopper. After the hopper lid closes, the fire door opens, the ram pushes
the charge into the MWI, the ram reverses, the fire door closes, and then the hopper
lid opens to receive the next load.
5-15
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The operator may be able to select and mix the medical waste and control the
amount of material (weight) placed in the hopper for each charge. Water sprays may
be used to cool the ram and/or fire doors as a precaution against the ignition of the
waste in the feed system.
Slide 5-23
ISSUES OF FUEL VARIABILITY
1. Fuel Size
2. Heating Value
3. Volatility
4. Fuel Moisture
5. Ash (incombustibles)
Bulky boxes of medical waste are routinely charged into MWIs, even though
their size and the variable properties of their contents can lead to poor combustion
conditions. For example, the heating value of plastics is high, about twice that of
paper. By contrast, pathological waste has a very low or negligible heating value
because of its moisture content.
Plastic materials are so highly volatile that they may appear to vaporize
instantaneously upon being delivered into the combustion chamber. This can lead to
inappropriate air/fuel mixtures which result in smoke and other emissions.
However, pathological waste requires a considerable drying period before its
organic volatiles are evolved. A large charge of pathological waste will require a
considerable quantity of auxiliary fuel to keep the combustion zone temperatures high
enough for adequate destruction of the organic materials.
The glass and metal portions of the sharps are incombustible. As such, they
contribute no thermal energy to the combustion process.
Slide 5-24
OPERATING STRATEGIES FOR FUEL VARIABILITY
1. Mix Wet and Dry Wastes
2. Mix Heavy Boxes with Light Boxes
3. Compensate Through Equipment Design
Medical waste is a highly variable material with large variations in plastic and
moisture contents. Mixing is very desirable for controlling combustion.
5-16
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The packaging, which is intended to prevent exposure, restricts the operator's
knowledge of the contents and the ability to perform mixing. In general, the heavier
boxes contain materials which burn slowly (e.g., pathological waste) and light boxes
contain more materials which burn rapidly (e.g., plastic and paper products). For this
reason, the mixing of heavy boxes and light boxes in the charge hopper is much better
than a charge of either all light or all heavy boxes.
However, there is a limit to the effectiveness of mixing. Therefore, the
combustion system designs must be able to accommodate the variable fuel
characteristics and attempt to reduce swings in combustion conditions.
Slide 5-25
Refuse input
BED BURNING COMBUSTION CONCEPT1
ignition starts
Grata System
residue
The four principle activities which waste undergoes in the fuel-bed on a grate
are: drying, volatilization, ignition of volatiles and burning of fixed carbon.12
The sketch illustrates an idealized sequence of processes. First the unignited
waste undergoes a drying process. The drying process is driven by radiant and
convective heat transfer from the combustion regions. The moisture is transferred
to the underfire air which diffuses upward through the fuel-bed.
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Subsequent heating of the waste results in the volatilization or gasification of
the volatile matter. The process proceeds with different gaseous materials being
evolved as the waste's temperature is increased.
Ignition begins on the outer surface of the waste when the gaseous materials
are heated above ignition temperatures. The burning proceeds downward through
the fuel bed, with the rate being limited by the delivery of underfire air.
Fuel bed agitation (by transfer rams or mechanisms) is important in larger
systems because agitation increases the porosity of the fuel bed which allows air to
mix with the volatiles. This increases the combustion intensity and temperature,
which increases the evolution of volatiles.
After the volatilization process has been completed, bed agitation can expose
additional fixed carbon surfaces for burning. After the burn-out process is completed,
the ash will be delivered to the ash pit or removed after cooling.
Slide 5-26
HEARTH DESIGNS
1. Single Fixed Hearth
2. Multiple Stepped Fixed Hearths
3. Moveable Fixed Hearths
4. Rotary Kiln Hearths
A major design consideration for hearth is to provide the desired combustion
air and the residence time required for the evaporation of moisture, volatilization of
combustible gases, and burning of fixed carbon. The underfire air is generally
supplied through air nozzles formed in the hearth.
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Slide 5-27
FIXED HEARTH WITH INTERNAL TRANSFER RAMS8
Courtesy of Consumat Systems, Inc., Richmond, VA
Transfer rams are often used to provide a stirring or tumbling action which
exposes fresh surface area of waste to the under-fire air. Without such agitation,
pieces of waste can stick together and form an impermeable mat of unburned waste.
A tumbling action is also created in MWIs having multiple hearths, when
waste materials are pushed to the end of one hearth and drop onto the next hearth.
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Slide 5-28
PULSED HEARTH8
Outer
"Frame
Hearth
Suspension
Cables (External
to Furnace)
Furnace Wall
Water Seal
Water
Trough
Courtesy of Basic Environmental Engineering, Inc., Glen Ellyn, IL
As an alternative to internal transfer rams, moveable hearths can be used to
provide agitation of the fuel bed for exposing fresh waste surface area to the air.
Most hearth designs provide some form of air-flow passageways through the
hearth so that the under-grate air can slowly diffuse through the waste in the fuel
bed. If the air flow is too great, entrainment of the ash or bed material will occur,
leading to increased fouling and particulate carry-over.
If the fuel bed is of uneven depth or has some material constituents which pack
differently, the flow of air through the bed will not be uniform. Air flows along the
path of least resistance. Under such conditions, the mixing of air and volatile fuel
gases can lead to both lean and rich air/fuel gaseous mixtures being evolved along the
fuel bed. This is generally compensated for in the unit's secondary air delivery
system design.
5-20
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Slide 5-29
BOTTOM ASH REMOVAL SYSTEM OPTIONS
* Manual Ash Removal
Batch & Intermittent Charged Units
Removal of Dry Residues
* Mechanical Ash Removal
Continuous Duty Units
Dry Ash Removal Systems
Wet Ash Removal Systems
Automatic or Semi-Automatic Operations
Manual removal of bottom ash is typical of smaller batch or intermittent
charged units which operate on a daily burn cycle. After appropriate burn-down and
cool down periods, the ash clean-out door is opened and some form of a rake, hoe or
shovel is used to extract the ash. Personal protective equipment such as heavy
gloves, protective shoes and filter type respirators are typically used.
The safety considerations of manual ash removal included assuring that the
ash and incinerator surfaces have cooled adequately, so that the potential for thermal
injury is reduced. Workers should be aware that ash is a good insulator and that,
although th6 surface may appear to be cool, hot spots can be found within
accumulated ash.
Care should be exercised to avoid inhaling fugitive dust (ash particles) which
become airborne from the disturbance the ash during removal. The dust may be
contaminated with heavy metals (e.g., lead) or organic compounds (e.g., dioxin).
The typical schedule for manual removal of ash is to remove the ash from the
previous day's burn as the first activity in the morning. Many units will use some
method to create a draft condition which will entrain any dust from the ash removal,
thereby removing the dust so that it does not contaminate the air being breathed by
the workers. The draft can be caused by the secondary air fans or by starting the
secondary burners, which also initiates the preheating of the secondary combustion
chamber refractory.
Mechanical bottom ash removal systems are used in continuous operating
MWIs. For example, sequentially operating rams can push the burning waste
through the incinerator, onto the final hearth and then ash chute into an ash pit.
If the system is designed for dry ash removal, an ash gate with a proper seal
will be used to prevent air leakage into the incinerator. A typical dry removal
installation will have the ash accumulate on top of the ash gate, with a periodic
5-21
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opening of the gate to drop the ash into the ash hopper or container. The hopper will
require provisions which seal it to the incinerator to prevent a surge of air leakage
into the incinerator when the gate is opened.
Dry ash removal from an ash hopper can be accomplished by automatic or
semi-automatic operation of drag conveyors or auger devices. Dry removal systems
often make use of water sprays to prevent fugitive dust problems.
Slide 5-30
ASH DISCHARGE RAM, ASH CHUTE AND ASH QUENCH3
To Boiler
Fossil Fuel Burner
Primary Chamber
Feed Ram
Ash Sump
Ash Transfer Rams
Air Tube
Ash Discharge Ram
Ash Chute
Ash Quench
7
Courtesy of Consumat Systems, Incorporated
Water quench tanks, located at the ash discharge location, are commonly used
in continuous operating MWIs. The water cools the ash and creates a slurry. The
water also acts as a seal that prevents uncontrolled air from entering through the
back end of the incinerator.
Wet removal of the bottom ash accumulated at the bottom of the ash quench
can be accomplished by automatic or semi-automatic methods. The ash removal
system are generally operated on an intermittent basis, with the residues being
removed after a period of accumulation. The removal systems generally are designed
with drag chain or backhoe device which pushes the wet solids out of the sump,
drains some of the water back into the sump and delivers the wet solids to a hopper.
5-22
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REFERENCES
1. "Medical Waste Incinerators -- Background Information for Proposed
Standards and Guidelines: "Process Description Report for New and Existing
Facilities," U. S. Environmental Protection Agency, Draft Report, April 30,
1992.
2. Peter Torkelson, "Report on the Assessment of Operations and Emissions of
On-Site Medical Waste Incinerators," Minnesota Pollution Control Agency,
December 1991, pp. 1-6.
3. "Integrated Waste Services, Information Summary," Consumat Systems, Inc,
Richmond, Virginia, Undated Brochure.
4. "Emission Control Systems for Incinerators," Report Number TR-89-900239,
Andersen 2000, Inc., Peachtree City, Georgia, February 1989, pp. 4 and 5.
5. Shenandoah Manufacturing Company, Inc., Harrisonburg, VA, Undated
Brochure.
6. "Operation, Maintenance and Parts Manual, Pyrolytic Incinerator," Publication
Number CBK-6826 9/88, Cleaver Brooks Division of Aqua-Chem, Inc.,
Milwaukee, Wisconsin, p. 1-3.
7. "Controlled Air Incineration," Joy Energy Systems, Inc., Charlotte, NC,
Undated Brochure.
8. R. G. Barton, et al., "State-Of-The-Art Assessment of Medical Waste Thermal
Treatment," Report to Risk Reduction Engineering Laboratory, USEPA, and
California Air Resources Board, June 15, 1990, pp. 37-80.
9. Steam, Its Generation and Use. 39th Edition, Babcock and Wilcox, New York,
1978, p. 16-3.
10. Joseph G. Singer, Combustion Fossil Power. 4th Edition, Combustion
Engineering, Inc., Windsor, CT, 1991, pp. 8-16 to 8-26 and 12-22.
11. Basic Environmental Engineering, Inc., Glen Ellyn, IL, Undated Brochure,
Received: April 1992.
12. W. D. Turner, Thermal Systems for Conversion of Municipal Solid Waste, Vol.
2: Mass Burning of Solid Waste in Large-Scale Combustors: A Technology
Status Report. Report ANL/CNSV-TM-120, Vol. 2, Argonne National
Laboratory, December 1982, pp. 43-168.
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6. COMBUSTION PRINCIPLES I: COMPLETE COMBUSTION
Slide 6-1
BASIC COMBUSTION CONCEPTS
Fuel and Air Characteristics
Products of Complete Combustion
Complete Combustion Reactions
Excess Air Considerations
Unlike combustion system design engineers, operators are seldom called upon
to make combustion calculations. However, knowledge of the basic concepts of
complete combustion will aid operators in understanding the complex combustion
features of their unit operations, particularly those associated with forming air
pollutants.
Example calculations are presented in this learning unit to help operators
understand some of the basic combustion concepts. Many of these concepts will be
referred to in later learning units. For instance, the combustion calculations of this
learning unit will be applied in Learning Unit 10, where the concepts of dilution and
the techniques for correcting air pollutant concentrations to a standard basis will be
presented.
Slide 6-2
COMBUSTION: CHEMICAL REACTION
Rapid Oxidation (Fuel & Oxygen)
Heat and Light Given Off '
Products of Combustion
* Oxides
* Other Compounds
Combustion is the rapid oxidation of combustible material which converts its
constituent elements into various oxides or compounds. The oxidation is rapid in
combustion, as opposed to the slow oxidation of the rusting of metal. Combustion is
accompanied by the release of substantial energy in the form of heat and light.
Fuels are substances which experience a rapid chemical reaction when
adequately heated in the presence of oxygen. Both fuel and oxygen are required for
the combustion process. The relative amounts of oxygen and fuel can vary widely,
resulting in differing combustion characteristics of temperature and combustion
products (emissions).
6-1
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Slide 6-3
COMBUSTIBLE SUBSTANCES
Hydrocarbons, Organic Materials
Paper & Wood (Cellulose)
Fossil Fuels
Plastics
Medical waste is composed of combustible materials, incombustible (ash)
materials, and water (moisture). The combustible fraction includes the organic
materials, which are defined as the compounds of carbon. The phrase "organic
material" is used because many of these compounds were formed from living plants
or animals (organs). Fossil fuels, like coal and oil, were formed over many thousands
of years from organic materials.
Hydrocarbons are combustible compounds which have both carbon and
hydrogen as elemental constituents. Paper and wood are composed mainly of
cellulose, a chemical material containing carbon, hydrogen and oxygen. Cellulose and
plastic materials are considered as both organic and hydrocarbon materials because
of their carbon and hydrogen compositions. Note that all hydrocarbons are organic
compounds, but not all organic compounds are hydrocarbons (e.g., carbon monoxide
is an organic material, not a hydrocarbon).
Slide 6-4
INCOMBUSTIBLE SUBSTANCES
Inorganic Materials
Metals
Glass, Sand, Ceramics, Concrete
Inorganic materials are those which have no hydrocarbons in their composition.
Examples include metal cans, glass bottles and ceramic materials.
Although most inorganic materials are considered incombustible, some may
oxidize, forming oxides such as iron oxide and aluminum oxide. Of course, some
metals, such as sodium and phosphorus, are capable of rapid oxidation (as is evident
in fireworks). However, these are not typical constituents in medical waste.
Other elements in medical waste which cause considerable concern are the
halogens: chlorine, fluorine and bromine. Although the halogen concentrations are
fairly small, their impact in forming acid gases is very important.
6-2
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Slide 6-5
MEDICAL WASTE ULTIMATE
Element
Carbon
Hydrogen
Oxygen
Nitrogen
Chlorine
Sulfur
Inorganics (ash)
Moisture
Total
ANALYSIS
"WET"
SAMPLE
Percent
by Weight
36.98
5.21
8.56
0.08
1.76
0.01
10.14
37.26
100.00
EXAMPLES
"DRY"
SAMPLE
Percent
by Weight
51.1
6.2
21.3
0.5
4.1
0.2
7.6
9.0
100.0
The example ultimate analyses of two different samples of medical waste was
provided earlier in Slide 3-16. The ultimate analysis provides the percentage of the
total weight that is associated with the elements of carbon, hydrogen, oxygen,
nitrogen, chlorine and sulfur, as well as the ash and moisture.
Note that the inorganic matter (ash) is the incombustible waste fraction which
can be collected as the bottom ash and fly ash, or it can be emitted up the stack into
the atmosphere. Bottom ash and fly ash also contain some unburned combustible
materials, generally as compounds of carbon.
The moisture in the waste will generally evaporate in a drying stage as the
waste is heated in the incinerator. The moisture in waste acts primarily as a heat
sink, requiring energy for its evaporation. Therefore, moisture reduces the maximum
combustion temperature but passes through the combustion process otherwise
unchanged.
In spite of considerable variations between different samples of medical waste,
calculations based upon ultimate analysis data can be useful in developing a better
understanding of combustion processes. The ultimate analysis of the "dry" sample
will be used later in general calculations to determine the combustion air
requirements.
6-3
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Slide 6-6
ATOMIC STRUCTURE OF MATTER
Atoms
Molecules of One Element
Molecular Compounds
Mixtures
"String Compounds"
A general discussion of the atomic structure of material is presented to aid in
the understanding of combustion calculations. All matter is composed of elements in
the form of atoms, and each element has its own unique atomic structure and atomic
mass or weight.
Materials are found in a number of different forms. Some materials are in the
form of single atoms of an element, such as gaseous helium. Other materials are in
the form of molecules with multiple atoms of a single element, such as oxygen which
has two oxygen atoms.
Other materials are in the form of molecular compounds of various elements,
such as carbon dioxide, which is composed of a carbon atom and two oxygen atoms.
Many materials are actually mixtures of atoms and molecules (e.g., air).
Other materials are formed from complex "string compounds" which do not
have a single molecular composition (e.g., paper or cellulose).
Slide 6-7
AIR
Mixture of Oxygen and Nitrogen
Oxygen - 21% by volume
Nitrogen - 79% by volume
3.76 moles of nitrogen per mole
of oxygen in air
Air is basically a mixture of oxygen and nitrogen, with the concentrations of
the other constituents small enough to be neglected. On a volumetric basis, air is
approximated as 21% oxygen and 79% nitrogen.
6-4
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Slide 6-8
DEFINITION OF A POUND-MOLE
Mass or Weight of Gas Equal to Its
Molecular Weight in Pounds
A Unique Number of Molecules,
Regardless of the Gas
379 Cubic Feet of Gas at Standard
Conditions, Regardless of Gas
A mole of a particular gas has mass or weight equal to the molecular weight
expressed in the system of units being used. For instance, a Ib-mole of oxygen weighs
approximately 32 pounds (Ib) because the molecular weight of oxygen is
approximately 32. Other systems of units deal with kilogram-moles and gram-moles.
For our considerations, a mole will be considered to be a Ib-mole.
The number of molecules in a Ib-mole is a unique and very large number. It
does not change from gas to gas. Thus, of the total molecules in a cubic foot of air,
79% of the molecules will be nitrogen and 21% will be oxygen.
The ratio of volume of nitrogen to the volume of oxygen is .797.21 or 3.76. This
ratio is equivalent to the ratio of the molecules of nitrogen to the molecules of oxygen,
as well as the number of moles of nitrogen per mole of oxygen in air. Therefore, the
air required to get one mole of oxygen will contain 3.76 (.797.21) moles of nitrogen.
This air will contain a total of 4.76 moles (1.0 moles oxygen + 3.76 moles of nitrogen).
At the standard conditions of 60°F and atmospheric pressure, a Ib-mole of any
gas occupies 379 cubic feet. Therefore, if one knows the number of moles, the
corresponding volume which the moles occupy at standard conditions is easily
calculated. This information is used by combustion system designers in sizing the
fans, combustion chambers and ducts.
Slide 6-9
STOICHIOMETRIC (THEORETICAL)
AIR-FUEL MIXTURE
Fuel Completely Burned
Oxygen Completely Consumed
Products of Complete Combustion
Are Formed
The minimum amount of oxygen required for the complete combustion of a
particular fuel is called theoretical or "stoichiometric oxygen." It varies from fuel to
6-5
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fuel. A "stoichiometric mixture" of a particular fuel and air (oxygen) is one in which
all the fuel and oxygen will be consumed under complete combustion conditions.
If combustion goes to completion under "stoichiometric conditions," there will
not be any molecules of oxygen (uncombined) in the flue gas (products of combustion).
However, if excess air is provided, oxygen will be found in the flue gas.
Slide 6-10
PRODUCTS OF COMPLETE COMBUSTION
Carbon Dioxide
Water (vapor)
Sulfur Dioxide
Hydrogen Chloride (acid)
Nitrogen (molecular)
Oxygen (molecular)
*
*
Under complete combustion conditions, each combustible element in the fuel
will generally form its own unique combustion product. Carbon forms carbon dioxide;
sulfur forms sulfur dioxide; chlorine forms hydrogen chloride; and hydrogen forms
water, except for that small amount required to convert chlorine to HC1.
For simplicity of presentation, we will assume that all the nitrogen in the fuel
is converted to molecular nitrogen, and the nitrogen in air remains in molecular form.
If complete combustion occurs under stoichiometric conditions, all the oxygen
in the fuel and the theoretical oxygen from the supply air will presumably be
consumed. Therefore, there will be no oxygen in the product gases.
For complete combustion under excess air conditions, any excess oxygen will
be assumed to flow directly into the products of combustion as molecular oxygen.
Slide 6-11
PRODUCTS OF INCOMPLETE COMBUSTION
* Carbon Monoxide
* Dioxins
* Furans
Combustion is generally imperfect, leading to the formation of products of
incomplete combustion (PICs), such as carbon monoxide, dioxins, and furans. Their
control will be discussed in Learning Unit 10.
6-6
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Slide 6-12
OTHER COMBUSTION PRODUCTS
*
*
*
*
Nitrogen Oxides
Metal Vapors
Metal Oxides
Metal Chlorides
The nitrogen in the fuel can also form nitric oxide and other nitrogen
compounds during combustion. These will also be discussed in Learning Unit 10.
The inorganic fraction of medical waste may contain heavy metals. These
metals can remain unchanged or can oxidize or vaporize during the combustion
process. The formation of metal oxides and vapors and their collection by air
pollution control devices will be discussed in Learning Units 15 and 17.
Slide 6-13
CHEMICAL REACTION EQUATION
Carbon: C + 02 --> C02
A chemical reaction equation uses special procedures to represent the
combustion of reactants into products of combustion. Symbols are used to represent
the substances participating in the reaction. Those symbols on the left of the arrow
represent the reactants, and those on the right represent products of combustion.
The arrow indicates that the reaction goes from the reactants to products.
In simplest form, a capital letter represents a single atom of a particular
element. A letter symbol followed by a subscripted number indicates a molecule
composed of the given number of atoms of the element. Two adjacent capital letter
symbols indicate a molecule of a compound formed from atoms of two elements.
Numbers placed in front of letter symbols indicate the number of molecules or atoms.
The number 1 is assumed when there is no number in front of the symbol.
From the above illustration, carbon (C) and oxygen (02) react to produce carbon
dioxide (CO2). One atom of carbon reacts with one molecule of oxygen (two oxygen
atoms) to form one molecule of carbon dioxide (an atom of carbon and two atoms of
oxygen).
6-7
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The chemical reaction equation also indicates that one mole of carbon reacts
with one mole of oxygen to form one mole of carbon dioxide, where a mole is a unique,
large number of molecules.
Slide 6-14
BALANCED CHEMICAL REACTION EQUATIONS
COMBUSTION IN OXYGEN
Carbon: C + 02 —> C02
Hydrogen: 2 H2 + O2 —> 2 H2O
Sulfur: S + 02 —> S02
Chlorine: H2 + 2 Cl --> 2 HC1
The reaction, equations listed above represent the complete combustion
reactions of the major constituents in medical waste. Note that the letter symbol for
chlorine is Cl. The symbol for hydrogen chloride or hydrochloric acid is HC1.
In actual practice, a small fraction of the S, C and Cl will not react, but will
remain in the ash. Generally, greater than 95% will react as indicated above.
Slide 6-15
BALANCED CHEMICAL REACTION EQUATIONS
Each Type of Atom Is Conserved
Each Element's Mass Is Conserved
Total Mass Conserved
The Number of Molecules is Not Conserved
A chemical reaction equation is called a balanced equation, since the number
of atoms of each element is the same on both sides of the equation. Mass is also
conserved, with the total mass of the reactants equal to the mass of the products of
combustion. This is because the number of atoms of each element is the same on
both sides of the reaction equation and the mass of each atom is unchanged.
One should note that although the atoms and the mass are conserved, the
number of molecules of reactants is not necessarily the same as the number of
molecules of products.
6-8
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Slide 6-16
EXAMPLE OF BALANCING A COMBUSTION EQUATION
Methane, CH4, with Stoichiometric Oxygen
CH4 + 2 Os --> COj + 2 HaO
The balancing of a chemical equation for complete combustion in pure oxygen
will be demonstrated. Assume that one molecule of methane, which is the main
constituent in natural gas, is to be completely burned with Stoichiometric oxygen.
The methane molecule is composed of one atom of carbon and four atoms of
hydrogen. Because carbon and hydrogen elements are in methane, one knows that
carbon dioxide and water will be the products of complete combustion.
The above slide illustrates the fact that the atoms of carbon, hydrogen, and
oxygen are each conserved in the reaction process. For example, the same number
of carbon atoms (1) are found on the left-hand side (reactants) and the right-hand
side (products). Likewise, the atoms of hydrogen and oxygen are conserved.
Slide 6-17
COMBUSTION REACTIONS IN AIR
3.76 moles of nitrogen in air
per mole of oxygen
The previous example was simplified by assuming that combustion occurred
in pure oxygen. However, the oxygen is obtained from air which is essentially a
mixture of oxygen and nitrogen. Approximately 79% of the air molecules are nitrogen
and 21% are oxygen.
Therefore, each molecule of oxygen obtained from air also has 3.76 (.797.21)
moles of nitrogen which go along for the ride.
In our example, 2 molecules of oxygen were required, so, 2 x 3.76 or 7.52 moles
of nitrogen will need to be in the combustion air.
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Slide 6-18
EXAMPLE OF BALANCING A COMBUSTION EQUATION
Methane, CH4, with Stoichiometric Air
CH4 + 2 02 +7.52 N2 -->
C0a + 2 H30 + 7.52 N2
Because oxygen is obtained from air, the complete combustion reaction
equation must be modified as indicated above. The nitrogen in the air must be added
as both a reactant and product of reaction. The nitrogen is assumed to pass through
the reaction unchanged, although it is heated.
Slide 6-19
EQUIVALENT MOLECULAR FORM OF MEDICAL WASTE
C4.2«H6.201.33N.04C1.117S.OOS +0.5 H20
It can be shown that 100 pounds of the "dry sample" of medical waste
presented in Slide 6-5 has the equivalent molecular form illustrated above.
The atoms of each element in the "equivalent molecule" correspond to the
percentage of the element in the ultimate analysis divided by its atomic weight. Note
that the moisture is considered separately, since its form is unchanged as it passes
through the combustion process.
Each "equivalent molecule" of this sample waste will have 0.5 molecules of
water, which corresponds to 9 pounds of water (from the ultimate analysis) divided
by 18, the molecular weight of water).
Slide 6-20
THEORETICAL COMBUSTION OF MEDICAL WASTE IN AIR
C4.MH6.801.,,N.e4Cl.117S.OQi + 0.5 H20 + 5.12 O2+ 19.26 N2 —
4.26 C02 + 3.54 H2O + 19.28 N2 + 0.117 HC1 + 0.006 SO2
6-10
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Because carbon, hydrogen, chlorine and sulfur elements are in the waste, one
knows that carbon dioxide, water, hydrogen chloride and sulfur dioxide will be in the
products of complete combustion. Also, the product gases will include nitrogen in
molecular form. Note that nitrogen oxides are ignored in this example.
As in the previous example, it can be shown that atoms of the each element are
conserved (same number on the left-hand side and the right-hand side).
Slide 6-21
MASS ANALYSIS
REACTANTS
^4 . 26"6 . 2^1 .33". 04^-1 . 117
0.5 H20
5.12 02
19.26 Ns
Total
OF STOICHIOMETRIC FUEL & AIR
Moles
S.006 1-0
0.50
5.12
19.26
Molecular wt
Ib/mole
83.4
18
32
28
MIXTURE
Mass
lb
83.4
9.0
163.8
539.3
795.5
Balanced combustion equations can be used to find air-to-fuel ratios, as follows.
The molar weight of the "equivalent molecule" of dry waste can be found by summing
the product of the number of atoms of each element by its atomic weight. This gives:
(4.26x12) + (6.2x1) + (1.33x16) + (.04x14) + (.117x35) + (.006x32) = 83.4 Ib/mole. The
corresponding weight of ash and water in the waste were 7.6 and 9.0 Ibs, respectively,
for a total weight of 100 Ibs.
Likewise, the weight of the air is found to be (5.12x32) + (19.26x28) = 163.8 +
539.3 = 703.1 Ib-air for the 100 lb-waste. The stoichiometric air-to-fuel ratio is about
7.0 Ib-air/lb-waste, and at 100% excess air, 14 Ib-air/lb-waste would be required.
Slide 6-22
EXCESS AIR
Air in Excess of Theoretical
Fraction: Extra/Theoretical
Symbol: EA
Total Supply Air is
(1+EA) x (Theoretical Air)
Oxygen in Flue Gas is
EA x (Theoretical Oxygen)
6-11
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Excess air is required in combustion to compensate for the inability to obtain
perfect mixing of the fuel with the theoretical oxygen. Excess air can be expressed
as the fraction of the extra air divided by the theoretical air or its equivalent
percentage.
The expressions of "percent excess air" and "percent flue gas oxygen"
(sometimes called excess oxygen) are often used in describing combustion conditions.
Because these provide an indication of the average fuel and oxygen relationship,
control systems often use these variables to achieve an optimum operating condition.
Slide 6-23
METHANE COMBUSTION IN THEORETICAL AIR:
CH4 + 2 O2 +7.52 N2 -->
C02 + 2 H2O + 7.52 N2
METHANE COMBUSTION IN EXCESS AIR:
CH4 + (1+EA)(2) 02 + (1+EA)(7.52) N2 --
C0
2 H2O + (1+EA)(7.52)
(EA)(2) 02
In chemical reaction equations, the symbol for the excess air fraction is EA.
The total air supplied is (1+EA) times the theoretical air requirements. Because the
theoretical oxygen is presumed to be fully consumed in forming combustion products,
the corresponding un-reacted oxygen in the flue gas is EA times the theoretical
oxygen. This will be demonstrated in the following example of methane combustion.
The-complete combustion equations for methane under theoretical and excess
air conditions are contrasted in the above slide. Note that the moles of carbon
dioxide and water vapor produced do not change. However, since oxygen is present
and nitrogen increases, the mole fractions of the product gas constituents will change.
Slide 6-24
METHANE COMBUSTION, 20 PERCENT EXCESS AIR:
CH< + 2.4 O2 + 9.024 N2 -->
C02 + 2 H20 + 9.024 N2 + 0.4 O2
6-12
-------
Consider the example of methane combustion under conditions of 20 percent
excess air, as illustrated in Slide 6-24. Note that 0.2 was substituted for EA in the
previous slide, with the results as indicated.
If the techniques illustrated in Slide 6-21 were used, it could be shown that
when methane burns under stoichiometric conditions, the air-to-fuel ratio is 17.2.
Also, the corresponding air-to-fuel ratio for 20% excess air will be 20.6.
Slide 6-25
PRODUCT
PRODUCTS
C02
H20
02
N2
Total
Dry Gas
Total
GAS ANALYSIS
Moles
1.0
2.0
0.4
9.024
12.424
10.424
, METHANE @
Molar Wt.
lb/mole
44.0
18.0
32.0
28.0
20% EA
MASS
lb
44.0
36.0
12.8
252.7
345.5
309.5
The basic concepts of gas concentrations in flue gas mixtures will be presented
in Learning Unit 10. The flue gas mixtures illustrated in Slides 6-20 and 6-25 will
be used to illustrate these concepts.
As will be discussed in Learning Unit 14, many instruments report gaseous
concentrations on a dry gas basis. A dry gas is obtained by physically absorbing the
water vapor out of the mixture with a desiccant material.
For the example in Slide 6-25, note that the oxygen in the flue gas would be
about 4% on a dry gas basis and the carbon dioxide would be about 10%. These are
fairly typical operating numbers for commercial and industrial gas-fired equipment.
6-13
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7. COMBUSTION PRINCIPLES II: THERMOCHEMISTRY
Slide 7-1
THERMOCHEMICAL CONCEPTS
Heating values & Load
Ignition Temperatures
Combustion Temperatures
Temperature Control Methods
In this learning unit operators are introduced to the thermochemical concepts
of combustion energy which have relevance to incinerator operations. These include:
fuel properties, combustion energy release rates, combustion temperatures, heat sinks
and combustion temperature control concepts.
Slide 7-2
HEATING VALUES
Higher Heating Value (HHV)
Bomb Calorimeter
Water Formed is Condensed
Lower Heating Value (LHV)
Computed from HHV
Assumes Water Formed is Vapor
The higher heating value (HHV) is denned as the maximum combustion energy
released by a fuel per unit mass.1 Heating values for solid fuels can be determined
with a laboratory device called a bomb calorimeter. The procedure includes weighing
the fuel sample, burning it completely, and measuring the energy absorbed by an
adjacent heat sink. Since the combustion products are cooled back down to room
temperature, all the water vapor in the product gases will be condensed. The HHV
is obtained by dividing the energy gain of the heat sink by the mass of the sample.
The lower heating value (LHV) is the same as HHV except that the
vaporization energy associated with the water in the product gases must be
subtracted.
If one is able to calculate or measure the amount of water in the product gases
and look up the heat of vaporization per unit mass, the LHV can be computed. Of
course, if no water is formed in combustion (e.g., as in burning pure carbon) the LHV
7-1
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and the HHV will be the same. The HHV of an example medical waste was given in
Slide 3-14 as 9,240 Btu/lb; its corresponding LHV was computed to be 8,570 Btu/lb.
Because the water produced by combustion generally leaves the unit in the
stack gases as water vapor, the LHV is often used to represent an upper limit for
energy recovery. Most combustion unit capacity standards in the United States are
based on HHV.
Slide 7-3
HEATING VALUES OF SELECTED FUELS1
FUEL
Methane
Fuel Oil, #2
Fuel Oil, #6
Coal, PA Bitum.
Coal, WY Subbitum.
Medical Waste
Wood, White Pine
Wood, White Oak
Lignite, ND
RDF, Ames, IA
MSW, Ames, IA
Wood, Fresh Cut
BASIS
Dry
As Received
As Received
As Received
As Received
As Received
Kiln Dried
Kiln Dried
As Received
As Received
As Received
As Received
MOISTURE
%
0.0
0.0
0.7
1.5
25.0
9.0
8.0
8.0
37.0
6.5
24 ..2
50, .0
2,3,4
HHV
Btu/lb
23,875
19,430
18,300
13,800
9,345
9,240
8,900
8,810
7,255
6,110
4,830
4,450
The HHVs of various representative fuel samples are presented above. Values
for HHV can be reported either on a "dry" basis or on the basis of the total sample
"as received" in the laboratory. The HHV is most often reported on an "as received"
basis because the moisture influences its combustion. Note that the methane value
corresponds to gaseous methane at GOT and atmospheric pressure.
One should note that many items in MSW are included in medical waste, but
that medical waste generally has a lower moisture content, lower inorganic content,
higher plastics content and higher heating value than municipal solid waste.
7-2
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Slide 7-4
INCINERATOR CAPACITY
OVERALL OPERATING LOAD
* Btu/hour (Energy Input)
WASTE CHARGING CAPACITY OR RATE
* Ib/day or tons/day
* Ib/hour
Most MWIs are designed around an operating load which is typically expressed
in units of Btu/hour. The phrase "operating load" is generally used to quantify the
rate of energy input from medical waste. Sometimes, the term "overall operating
load" is used to refer to the total energy input from burning both medical waste and
auxiliary fuel.
The "waste charging capacity" of an incinerator is typically rated in terms of
a nominal waste burning rate, (e.g., 500 pounds/hour). Alternatively, some units are
rated on the basis of a daily charge rate (pounds burned/day) for a batch charged
operation or for an intermittent charging unit operating with a standard feeding
period (e.g., 10-hour).
Many units have limits in their operating permits which are based upon a
maximum waste charging rate, expressed as Ib/hr. This charging rate is typically
based on a manufacturer's specified "operating load," which is based upon a "design"
heating value for the waste (e.g., 9,000 Btu/lb).
Therefore, it can be argued that the waste charging rate should vary depending
upon the heating value of the waste. For example, as the heating value decreases,
the charging rate should be increased.
The general experience is, however, that as the heating value of the waste
decreases, the waste burns poorly. Therefore, instead of increasing the waste
charging rate, the auxiliary fuel burning rate should be increased.
Medical waste incinerators are designed to have auxiliary burners respond to
changes in operating conditions. For example, when wet waste is charged, the heat
input from the waste will be decreased. In this case, the combustion chamber
temperatures will decrease unless the auxiliary fuel is increased. When dry waste
with a larger heating value is burned, its charging rate should be reduced and the
auxiliary fuel firing rate, if applicable, should be reduced.
7-3
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Slide 7-5
EXAMPLE CALCULATION OF ACTUAL CHARGING RATE
Example: 150 Ibs/hr Unit (Design Capacity)
8,500 Btu/lb HHV (Design Basis)
9,240 Btu/lb HHV (Actual)
OPERATING LOAD
CHARGTING RATE
CHARGING RATE x HHV (Design)
150 Ibs/hr x 8,500 Btu/lb
1,275,000 Btu/hr
OPERATING LOAD x HHV (Actual)
1,275,000 Btu/hr/ 9,240 Btu/lb
138 Ibs/hr
The above slide illustrates the relationship between the charging rate and the
operating load. The approximate charging rate Gb/hr) corresponds to the quotient of
the operating load or rate of energy input (Btu/lb) from burning waste divided by the
higher heating value, HHV, of the waste (Btu/lb).
Slide 7-6
IGNITION
TEMPERATURES5
Material Phase at 60°F
& 14.7 psia
Sulfur
Charcoal
Gasoline
Acetylene
Fixed Carbon
Hydrogen
Methane
Carbon Monoxide
Benzene
Solid
Solid
Liquid
Gas
Solid
Gas
Gas
Gas
Liquid
Ignition
Temp . , °F
470
650
663-702
589-825
765-1115
1065-1095
1170-1380
1130-1215
1335
Ignition (or auto-ignition) temperatures are presented as general indications
of the temperatures required for combustion. Ignition temperature may be denned
as the temperature at which rapid combustion in air ignites automatically and
becomes self-sustaining.
7-4
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As illustrated above, ignition temperatures of volatile hydrocarbons vary from
acetylene at about 700°F to methane, carbon monoxide and benzene at around 1,200°F
or higher. The fixed carbon remains in a solid form and has ignition temperatures
as low as 765°F.
Ignition temperatures of paper and plastic materials are not provided because
a large fraction of these materials burn as volatile matter, composed of a number of
different hydrocarbon compounds.
Note, that actual combustion processes are driven by complex reactions
involving presence of radicals (e.g., OH, 0, H). Therefore, the combustion processes
can be self-sustained at temperatures below those listed in Slide 7-6, In addition,
combustion depends upon the enclosure design, air-fuel mixture, air velocity and the
source of ignition.
Slide 7-7
COMPONENTS IN PROXIMATE ANALYSIS
* Moisture
* Volatile Matter
* Fixed Carbon
* Ash
Consideration of the "proximate analysis" can aid in understanding combustion.
It includes "moisture," which is the fraction which vaporizes as the medical waste is
heated. One can assume that the waste is dry by the time it is heated to 220° F.6
Volatile matter is the combustible gaseous material which is evolved as the
waste is heated further. Volatilization begins as a distillation process. Light
hydrocarbons are given off first, and heavier hydrocarbons are evolved as the
temperature increases. Some heavy or residual fuel hydrocarbon materials will
undergo thermal and/or catalytic cracking before being evolved as new hydrocarbon
gases. The vaporization process is generally completed by the time the waste
temperature reaches around 1,750° F.6
Fixed carbon is the solid combustible fraction of the waste. It undergoes
burning on the surface. Fixed carbon combustion can occur as the volatile gases are
being evolved, since the ignition temperature of carbon is as low as 765° F.
Ash is the noncombustible solid fraction of the waste. It is primarily composed
of the metals and glass.
7-5
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Slide 7-8
ADIABATIC COMBUSTION CONDITIONS
Energy Release From Combustion
No External Heat Losses
Heats Combustion Product Gases
Vaporizes Moisture
Adiabatic combustion conditions are idealized conditions which can serve as an
aid for evaluating combustion phenomena. An adiabatic combustion condition is one
where all the combustion energy is used either to heat the products of combustion or
to vaporize moisture and volatile materials. Adiabatic conditions occur when the
combustion zone is perfectly insulated and there is no heat loss to the surroundings.
Near adiabatic combustion conditions can occur in refractory wall incinerators, which
have relatively low heat loss.
In general, the maximum adiabatic temperatures correspond to complete
burning of stoichiometric air/fuel mixtures. Such conditions are not generally
realized in actual combustion conditions and they may not even be desirable. For
instance, the maximum adiabatic temperatures would generally be high enough to
damage the refractory materials and the incinerator structure.
Slide 7-9
COMBUSTION TEMPERATURE CONTROL
Fuel Modulation
Heat Transfer to Surroundings
Heat Sink Materials
Controlling chamber temperature is an important method of protecting the
combustion unit. Combustion temperatures can be controlled by modulating the
delivery of fuel and air, assuming that the heat losses are relatively constant in time.
The rate of heat loss for refractory wall incinerators is fairly small and fairly
constant. The rate of heat extraction to the waterwalls of such integral boiler
furnaces can be varied to some degree. For instance, if the flow of feedwater
increases in such a unit, the heat transfer to the waterwalls will increase and the
combustion temperatures will drop.
7-6
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Slide 7-10
HEAT SINK MATERIALS
Water in Fuel
Nitrogen
Excess Air
Flue Gas
Water Sprays
Heat sink materials have the effect of lowering the effective combustion
temperature. Heat sinks can have both desirable and undesirable features.
However, the water in medical waste is generally considered to be undesirable, since
all the water must be vaporized before combustion can occur.
Temperature control can also be established by the delivery of various heat
sink materials. Excess air is the most common heat sink available for operator
control. As will be discussed in Learning Unit 8 and 9, the controlling the air supply
is a common method used for controlling temperatures in incinerators. For instance,
the secondary combustion chamber operates under excess air conditions, so that an
increase of excess air can be used to reduce combustion gas temperatures and protect
the refractory.
The nitrogen in air is a less obvious heat sink material because the combustion
temperatures would be considerably higher if pure oxygen were used rather than air.
Combustion temperatures can also be controlled by flue gas recirculation.7
This method is often used in power plants because the extra flue gas acts as a heat
sink material.
Slide 7-11
WATER SPRAYS
Reduce Fuel-to-Air Ratio
Reduce Temperature
Reduce Velocity
Reduce Opacity
Reduce Fires in the Charge Hopper
Water sprays are used as a heat sink in the primary chambers of modular
controlled-air incinerators. The lower temperature will retard the volatilization
7-7
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process, thereby decreasing the local fuel-to-air ratio.8 With lower temperatures, the
velocity of the gases are also reduced.
Water can be sprayed onto a fresh charge to act as a heat sink and slow down
the rate of volatilization of plastics. Otherwise, a surge of volatiles may be formed,
producing a fuel-rich mixture which could overload the secondary chamber, resulting
in smoke and excessive pollutant emissions.
Water is also typically sprayed onto the face of the charging ram or fire-door
to prevent fires in the charging hopper.9
Slide 7-12
STARVED-AIR UNITS
Two Stage Combustion
Primary Chamber: Gasifier
Low Velocities: Low Entrainment:
Increased Air: Higher Temperatures
Secondary Chamber: Excess Air Combustion
Starved-air is a phrase used to describe a particular form of two-stage
combustion which occurs in modular incinerators. Under starved-air conditions, the
primary chamber acts as a gasifier, vaporizing the volatile gases and mixing them
with less air than would be required for stoichiometric conditions. The process of
vaporization of the volatile components is sometimes considered pyrolysis. Although
some combustion does occur in the primary chamber, there is not enough air for
complete combustion of the volatile gases.
The secondary chambers of modular units are designed to mix additional air
with the partially reacted gaseous mixture (fuel) from the primary chamber.
Provisions for supplying air and adequately mixing it with the fuel are required to
obtain complete combustion in the secondary chamber.
Starved-air incinerators were initially developed because of their inherent
feature of low particulate emissions. If more air were to be added in the primary
chamber, the rate of combustion would increase, raising the gas temperatures and
thereby increasing the rate of gas volatilization. The increased air supply and gas
temperatures would cause the velocities to increase, which would increase the amount
of particle entrainment.
7-8
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Slide 7-13
EXCESS AIR COMBUSTION
Excess Air - Heat Sink
More Excess Air
Temperature Reduction
Excess air combustion behaves differently from starved-air combustion, as will
be discussed in Learning Units 8 and 9.
For example, in an excess-air unit, an increase in the delivery of air generally
results in a decrease in the combustion temperature.
However, the combustion phenomena is much more complicated. The
influences of fuel bed agitation and of changing the under-fire and over-fire air
supplies will be discussed in Learning Unit 9.
7-9
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REFERENCES
1. Joseph G. Singer, Combustion Fossil Power. 4th Edition, Combustion
Engineering, Inc., Windsor, CT, 1991, pp. 2-13 to 2-27.
2. Steam. Its Generation and Use. 39th Edition, Babcock and Wilcox, New York,
1978, pp. 5-8 to 5-17 and 6-2.
3. D. E. Fiscus, et al., Study of Existing RDF Co-Firing Experiences. Vol. 2:
Appendixes to Phase I Final Report. Report ANL/CNSV-TM-134, Vol. 2,
Argonne National Laboratory, October 1983, pp. C-l-14, C-l-15, & C-9-14.
4. J. J. Santolevi and R. L. Kratz, "Medical Waste Incineration Requirements for
System Design Modifications," Proceedings of the Third National Symposium
on Infectious Waste Management: Incinerator Retrofit for Hospitals and
Industry. Chicago, IL, April 1989.
5. Steam. Its Generation and Use. 39th Edition, Babcock and Wilcox, New York,
1978, pp. 6-6.
6. Joseph G. Singer, Combustion, Fossil Power Systems. 3rd Edition, Combustion
Engineering, Inc., Windsor, CT, 1981, p. D-5.
7. "Nitrogen Oxide Control For Stationary Combustion Sources," EPA/625-5-
86/020, U. S. Environmental Protection Agency, July 1986.
8. W. R. Seeker, W. S. Lanier and M. P. Heap, Municipal Waste Combustion
Study, Combustion Control of Organic Emissions, U.S. Environmental
Protection Agency, EPA-530-SW-87-021-C, May 1987, p. 7-9.
9. Handbook of Operation and Maintenance of Hospital Medical Waste
Incinerators. EPA/625/6-89/024, U. S. Environmental Protection Agency,
January 1990, pp. 16-17.
7-10
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8. COMBUSTION PRINCIPLES III: REACTION PROCESSES
Slide 8-1
COMBUSTION REACTION PROCESSES
Oxidation & Reduction
Incomplete Combustion
Reaction Rate Variables
Flame Phenomena
Bed-Burning
Volatilization
Oxidation of Carbon Monoxide
This learning unit considers the above applications of combustion reaction
processes, which have special relevance to incinerator operations.
Slide 8-2
IMPORTANT REACTION CHARACTERISTICS
1. Multiple Reactions Occur in Combustion
2. Reactions May Not Go to Completion if
Temperature, Time & Mixing Are Inadequate
3. Reactions Are Somewhat Reversible
4. Reaction Rates Increase with Temperature
5. Reactions Are Influenced by Concentrations
6. Reactions Are Limited by Mixing
7. Gaseous Compositions Vary with Temperature
The completeness of combustion depends on the operating conditions in the
incinerator. These conditions could be evaluated by using the analytical concepts of
reaction kinetics. However, the details of reaction kinetics are beyond the scope of
this training program.
Nevertheless, insight about some of the subtle aspects of the combustion of
medical waste can be gained by considering factors that influence reaction rates. For
example, the features of reaction rates can help explain the formation and destruction
of carbon monoxide.
The above list summarizes reaction characteristics which can help operators
understand how certain variables influence combustion.
8-1
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Slide 8-3
REACTIONS OF CARBON AND HYDROGEN IN OXYGEN
C + O2 > CO2
H2 + 0.5 02 > H2O
0.5 02 > O
C + O > CO
2 H2 + O2 > 2 H20
2 H2O > 2 OH + H2
CO + 2 OH > CO2 + H2O
The idealized concepts of combustion were introduced in Learning Unit 6. In
that section, we simplified our analysis by assuming that all the reactions go to
completion. As indicated in the first two equations listed above, carbon and oxygen
can react to form carbon dioxide, and hydrogen and oxygen can react to form water.
However, actual combustion reactions are much more complex, as indicated by
the other equations listed above. Multiple reactions may occur simultaneously,
including many which do not go to completion.
Slide 8-4
CONSEQUENCE OF MULTIPLE REACTIONS
Not all Reactions Can Go To Completion
Some Components May Be Depleted
The actual burning of hydrocarbons is a multi-step process. For instance, when
carbon is burned, carbon monoxide is formed first. The carbon monoxide is then
generally oxidized to carbon dioxide. Therefore, carbon monoxide is emitted because
of incomplete combustion. The destruction of carbon monoxide will be discussed
further at the end of this learning unit.
As another example, the nitrogen in the air can react with oxygen if the
temperature is high enough. However, the burning of hydrocarbon (fuel) occurs much
faster than the burning of nitrogen. Therefore, if only enough oxygen is available to
burn the fuel, it will be primarily consumed by the fuel. Under such conditions, the
oxygen will be essentially depleted and very little will be available for oxidizing the
nitrogen, even if the temperatures are very high.
The subject of limiting excess air as a method of controlling the formation of
nitrogen oxides will be discussed in more detail in Learning Unit 10.
8-2
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Slide 8-5
PRODUCTS OF INCOMPLETE COMBUSTION
Carbon Monoxide
Dioxins and Furans
When considering the possibility of incomplete combustion, we might expect
various gases to be present in the final mixture. For instance, a simple mixture of
hydrocarbon and air could react and form a mixture of H2, CO, OH, O, CO2 and H20.
The products of incomplete combustion also include other reaction products, such as
organic compounds of dioxins and furans. These will be considered in more detail in
Learning Unit 10.
Slide 8-6
REASONS FOR INCOMPLETE COMBUSTION
1. Variable Fuel Properties
2. Irregular Fuel Feeding Characteristics
3. Inadequate Air Supply
4. Improper Distribution of Air
5. Incomplete Mixing of Oxygen & Fuel
6. Inadequate Temperature
7. Premature Cooling of Combustible Gases
(Inadequate Time)
A number of operational problems which can contribute to the formation of
products of incomplete combustion are listed above.
Slide 8-7
REACTION RATES
Rate of Chemical Change
Forward Reaction (Production)
Reversed Reaction (Dissociation)
A reaction rate is the rate of chemical change, which might be thought of as
the number of new combustion product molecules formed per second.
Because every reaction has reversible aspects, the phrase "forward reaction
rate" is used to signify the production of product gases, whereas the "reversed
reaction rate" quantifies the dissociation of product gases back to their original form.
8-3
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Slide 8-8
OXIDATION AND REDUCTION REACTIONS
Lean Mixture - Oxidizing Atmosphere
Oxidation Reaction
Converts Reactants to Products
Rich Mixture - Reducing Atmosphere
Reduction Reaction
Converts Products to Reactants
The knowledge of oxidation/reduction phenomena has important applications
in unit operations.
Oxidizing atmospheres are characterized by fuel-lean mixtures. Lean mixtures
have more air in the mixture than would occur under stoichiometric conditions.
Reducing atmospheres are characterized by fuel-rich mixtures. A rich mixture
has more fuel in a given amount of air than would occur in a stoichiometric mixture.
There are a number of combustion situations where reducing atmospheres can cause
dissociation to occur.
MWIs with waterwalls experience fireside corrosion of metal surfaces, which
may be caused by the cycling between lean mixture and fuel-rich mixture conditions.
Oxidation/reduction cycling can occur because of the transient volatilization
characteristics of medical waste, particularly the plastics. Under lean mixture
(oxidizing) conditions, the metal walls are oxidized. However, under rich (reducing)
conditions, oxygen is extracted from the exposed metal oxide on the waterwalls, thus
destroying the protective metal oxide layer. The fireside corrosion problem is avoided
in most MWIs through the use of refractory wall surfaces.
The reaction rates of oxygen with fuel, relative to that of oxygen with nitrogen,
have been utilized in developing control techniques for nitric oxide emissions. In
particular, in the reburning process a reducing atmosphere is created by the injection
of natural gas. As the natural gas burns, it takes the oxygen atoms away from the
nitric oxide which allows the nitrogen atoms to form gaseous nitrogen.
8-4
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Slide 8-9
REACTION RATE DEPENDS UPON
Temperature
Mixture Concentrations
Stirring Process (Turbulence)
Reaction rates increase with temperature because molecular vibrations
increase as the temperature increases. As the vibrations increase, the frequency of
molecular collisions increases and the molecular binding forces are weakened. When
molecules collide at elevated temperatures, the chances of the collision causing the
formation of a new compound (combustion product) are increased.
The rates of reaction are limited by the mixing of fuel molecules and oxygen
molecules. As the mixing is improved, the rate of reaction increases.
Mixing can be thought of as the process of creating the desired constituent
concentrations in the mixture. For instance, the concentration of oxygen needed
varies with the type of fuel and its concentration. As indicated in Learning Unit 6,
the theoretical stoichiometric mixture (or air-to-fuel ratio) varies from fuel to fuel.
In addition, mixing can be thought of as a stirring or blending process which
enhances the rate of collision of oxygen and fuel molecules. This second aspect of
mixing is sometimes referred to as turbulence.
Slide 8-10
PRE-MIXED GASEOUS FUEL COMBUSTION
Blue Flame Combustion:
Natural Gas in an Appliance
There are two different types of combustion phenomena which are indicated
by the observable blue and yellow flames. Understanding flame phenomena can lead
to insight about waste combustion.
Gas stoves feature blue flame combustion, which is produced by burning a
mixture of natural gas and air. Blue flame or pre-mixed gas combustion is generally
considered to be clean burning, because black deposits are not formed on adjacent
surfaces.
8-5
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After the mixture is heated, the combustion process begins with the oxidation
of carbon to carbon monoxide. With appropriate combustion system design and
operating conditions, the carbon monoxide can be subsequently oxidized to carbon
dioxide.
If the combustion of pre-mixed flames is incomplete, carbon monoxide will be
emitted. This can occur when the flames are cooled rapidly and/or if inadequate
oxygen is available.
Slide 8-11
DIFFUSION-LIMITED COMBUSTION
Yellow/Orange Flame Combustion:
Inadequately Pre-Mixed Air & Fuel
Dark Flame Tips
Black Deposits on Adjacent Surfaces
If the air supply of a blue flame gas burner is constricted, the flame will be
transformed into a yellow flame. Yellow flames are called diffusion flames because
the oxygen for combustion must be diffused from the air adjacent to the fuel.
In yellow flame combustion, the hydrocarbon gases get hot enough to break
their molecular bonds, releasing hydrogen gas which burns readily. Since the oxygen
supply is limited, many of the carbon molecules get hot enough to glow with their
characteristic yellow color, before they are oxidized.
Such flames are often observed to be long, luminous, yellow flames, whose tips
will become dark as the flames cool. The diffusion process occurs relatively slowly,
so large combustion chambers may be required to accommodate the long flames.
Black soot is an observable incomplete combustion product associated with
diffusion flames. Such black deposits which occur on adjacent surfaces are associated
with premature flame cooling or quenching. If a diffusion flame impinges on the wall
of the combustion chamber, the gas temperatures will fall and the combustion process
will essentially stop. When this happens, smoke and/or soot deposits will be formed.
8-6
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Slide 8-12
MEDICAL WASTE BURNING CHARACTERISTICS
Volatiles Burn as Gases
Fixed Carbon Burns as a Solid
Diffusion Limited Flame
Medical waste is a highly variable fuel, composed of incombustible materials,
volatile materials which burn as a gas, and solid, fixed carbon which burns on its
surface. Although the actual burning process proceeds through many steps, only the
general characteristics will be described.
The combustion process occurs with the collision of adequately heated fuel and
oxygen molecules. Therefore, molecules which are too cool will not burn. Also, those
fuel materials which are trapped inside a piece of solid waste cannot be burned until
they are either evolved as a gas or become exposed to collisions on the surface.
The volatile gases may burn in the fuel-bed if they are adequately heated by
the adjacent combustion gases and mixed with the oxygen. These volatile gases will
burn with a yellow flame, characteristic of diffusion limited combustion.
Slide 8-13
IDEALIZED FUEL-BED REGIONS1
a.
UJ
v-
cr
O
UJ
UJ
u
z
1.6
Location of
— Ignition Front
0.8 -
T I I I I
,Location of Top of Bad
Daptn of Accumulated
Inart -
Active Burning
Depth
I
I
1OOO 2OOO
TIME (SEC)
3000
4OOO
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MWIs with provisions for continuous waste charging provide for the bed
burning processes by initially establishing a thick fuel-bed which slowly moves down
the hearth. The hearth has provisions for supplying underfire air, which can diffuse
through the fuel-bed to dry the waste and provide for combustion.
Slide 8-13 illustrates an idealized fuel-bed, which becomes thinner as the
organic matter is consumed. This is plotted as a function of the residence time on the
hearth (or the distance from the charging door).
The illustration emphasizes that ignition starts on the top of the fuel-bed
surface and moves downward through the fuel-bed. Burning occurs with a diffusion
flame moving toward the air supplied from below the fuel-bed. In the illustration,
the ignition front requires an hour to reach the hearth (indicated by 0 ft above the
hearth).1 The solid material can burn in the primary chamber for a much longer time
in batch and intermittently charged units which operate on a daily burn cycle.
Adjacent to the ignition front is an active burning region which moves down
through the fuel-bed. The bed-burning process continues by consuming organic
matter deeper and deeper into the bed, leaving a layer of ash of increasing thickness.
Ideally, the burnout of the fuel will be completed just prior to being dropped into the
ash collection pit.
The location of the active burning region is controlled by the supply of
underfire air and fuel-bed agitation. Agitation exposes fresh waste surfaces to the
air flowing through the bed, which stimulates the heat transfer, volatilization, and
the fuel-oxygen mixing processes of combustion.
When an operator increases the underfire air supply, it causes the burning
process to become more intense, the temperature to rise and the volatiles to have a
higher rate of evolution.
Slide 8-14
PRIMARY CHAMBER BURNING PROCESSES
Gaseous Products Partially Oxidize to:
H2O, C02, CO, Methane, Hydrogen
Solids Materials Burn in Bed:
Char (Fixed Carbon)
Solid Residues Accumulate on Hearth:
Inorganic Materials (Ash)
The combustion in the primary combustion chamber for batch charged and
intermittently charged controlled-air MWIs has significant differences from the
theoretical bed burning concept described above. For instance, the solid waste does
8-8
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not actually move through the primary chamber, but is delivered onto the top of the
burning bed and the residues accumulate on the hearth. When a fresh charge is
delivered onto the top of the burning residues, the drying, volatilization and bed
burning processes appear to proceed almost simultaneous from the outside of the
package inward. Combustion in the primary zone occurs in a cycle associated with
the delivery of the charge and its partial burn-out before the next charge is delivered.
However, the primary chamber can be characterized as a gasifier or pyrolysis
unit because the supply of primary air is controlled and the active burning region
uses up all the available oxygen. As volatile gases are evolved they will be heated
above their ignition temperatures but the restricted oxygen supply will prevent
complete combustion from occurring in the primary chamber.
Thus, oxygen-starved mixtures of volatile hydrocarbons and products of
complete and incomplete combustion will be formed. The gases flowing at the top of
the fuel-bed contain significant levels of methane (hydrocarbon), hydrogen, and
carbon monoxide, but are depleted of oxygen.1 These gases will migrate upward and
leave behind the ash and some of the char (fixed carbon).
The stoichiometry of the gaseous mixtures leaving the fuel-bed vary with
location and time. Differences in the fuel properties of the waste, the cyclic
variations in fuel-bed temperatures, and the rates of volatilization causes variations
in the gaseous mixture leaving the primary chamber.
Slide 8-15
THEORETICAL COMBUSTION EQUILIBRIUM TEMPERATURES
WITH DIFFERENT FUEL MOISTURE LEVELS
0.8 i 1.2 1.4 1.6
STOICHIOMETRIC RATIO
8-9
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Slide 8-15 illustrates how fuel moisture and air supply influence the burning
of medical waste. The theoretical combustion temperature presented are maximum
values which correspond to burning without heat loss. The temperatures are
dependent upon both the amount of air supplied and the moisture content in the
waste.
As expected, the highest curve corresponds to dry medical waste. The next
curve (10% moisture) corresponds to an average medical waste. Note that because
pathological waste has such high moisture (e.g., 85%), it will not generally burn
unless heating is provided by from some other source.
The stoichiometric ratio is defined as the air-to-fuel ratio divided by the
stoichiometric air-to-fuel ratio. As indicated in Learning Unit 6, the stoichiometric
air-fuel ratio for medical waste is about 7 Ib-air/lb-fuel. Therefore a unit operating
with 100 percent excess air would supply about 14 Ib-air/lb-fuel, which corresponds
to a stoichiometric ratio of 2.0.
Generally, an increase in the air supply will directly result in an increase in
the stoichiometric ratio, unless the fuel supply rate is changed. Therefore, the
operating conditions will shift to the right when the air supply is increased.
If the underfire (primary) air supply of a starved-air unit is increased
considerably, the primary chamber can operate under approximately stoichiometric
conditions. This could cause thermal damage the refractory. In addition, higher
velocities (shorter residence times) would be found in the combustion chambers, so
that some gases would probably pass through the unit without mixing with oxygen
and burning completely.
Slide 8-15 was obtained by making assumptions about the composition of the
medical waste and inputing them into the NASA computer model CET85.2 The model
has provisions which simulate the dissociation of water and other compounds at high
temperatures. This influence gave rise to the unexpected result of the peak
temperatures occurring under sub-stoichiometric conditions. If complete combustion
were to have been assumed, the maximum flame temperatures would be reached at
stoichiometric ratios of 1.0.
8-10
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Slide 8-16
OPERATIONAL RELATIONSHIPS
STARVED-AIR CONDITIONS
Increase in Primary Air Supply:
Increase in Primary Gas Temperature
Increase in Waste Fuel Charging Rate:
Decrease in Primary Gas Temperature
EXCESS-AIR CONDITIONS
Increase in Secondary Air Supply:
Decrease in Incinerator Temperature
Increase in Waste Fuel Charging Rate:
Increase in Incinerator Temperature
Slide 8-15 showed that the operating conditions will influence how the
temperature will change as the air supply changes. An increase in air supply can
cause the combustion temperatures to either increase or decrease. For instance,
under excess air conditions, an increase in the air supply (increase in stoichiometric
ratio) will result in lower combustion temperatures. This is consistent with the
experience of adding more secondary air and seeing lower flue gas temperatures.
The primary chamber of a starved-air unit is designed to operate with about
50 percent of the air required for complete combustion. This corresponds to a 0.5
stoichiometric ratio. If the primary air supply is increased (stoichiometric ratio
increased) the primary chamber temperature will increase. This will cause increased
volatilization and an increased fuel supply to the secondary chamber.
Alternately, when the fuel delivery is increased (e.g., by fuel-bed agitation) and
the air supply is unchanged, the stoichiometric ratio in the primary chamber will be
decreased, causing a decrease in primary chamber gas temperatures.
The same logic shows that, for primary chambers operating under excess air
conditions, an increase in the fuel supply rate (decrease in stoichiometric ratio) will
increase the temperatures. This is consistent with increased fuel-bed agitation
causing an increase in combustion temperatures in such units.
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Slide 8-17
REACTIONS WITH CHAR:
C
C
C
+ oa -
+ HaO
+ CO2
— > CO,
— -> CO + Hj
> 2 CO
(1)
(2)
(3)
The above indicated processes are useful in burning carbon (char) on the fuel-
bed. The first is the general carbon oxidation reaction. The other two are
endothennic (energy extracting) reactions, which occur under high temperature
combustion conditions. These reactions occur on the surfaces of the solid materials
in the fuel-bed, producing carbon monoxide and hydrogen gas.
The fixed carbon on the fuel-bed burns with a yellow/orange glow. The
migration of oxygen and other gases to the surface is limited by the diffusion process.
Slide 8-18
DESTRUCTION OF CARBON MONOXIDE
CO + OH > H + C02 (1)
CO + 2 OH ---> C02 + HjO (2)
CO + O ---> C02 (3)
The conversion of carbon to carbon monoxide is a most important process.
Carbon monoxide is a very stable molecule which is not easily oxidized. It will not
readily react with molecular oxygen, even at high temperatures. The dominant
reaction paths for converting CO to CO2 is shown above. Thus, CO will react with
the OH radical and atomic oxygen, which are associated with high temperature
dissociation of water and molecular oxygen.
If too much secondary air is added to the combustion gases, the temperature
could be decreased enough to reduce the OH radical production. The lower
temperature would also reduce the rate of CO and OH reactions.
Actually, the combustion processes which occur are far more complex than has
been described. Other reactions, including the depletion of some of the important
constituents, can take place. For instance, chlorine reacts readily with hydrogen
which makes less hydrogen gas and OH radicals available for completion of the
combustion process.
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REFERENCES
1. G. C. Williams et al., "Design and Control of Incinerators," Final Report to
Office of Research and Monitoring, U. S. Environmental Protection Agency,
Grant Number EC-00330-03, 1974.
2. Sanford Gordon and Bonnie J. McBride, "Computer Program for Calculation
of Complex Chemical Equilibrium Compositions," NASA SP-273, March 1976.
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9. COMBUSTION SYSTEM DESIGN & CONTROL
Slide 9-1
MWI DESIGN CONSIDERATIONS
1. Charging Method
2. Stoichiometric Design
3. Combustion Chamber Wall
4. Combustion Control
5. Air Pollution Control
6. Energy Recovery
There are a number of interconnected design considerations which establish
the general operational features of MWI units. The features of manual or automatic
charging and the different types of feeding equipment were presented in Learning
Unit 5. The design and operational aspects of air pollution control and energy
recovery equipment will be presented in subsequent learning units.
This learning unit will emphasize the interrelated operational aspects of
combustion chamber design, Stoichiometric design and combustion control systems.
Slide 9-2
OPERATIONAL CONSIDERATIONS
Steady Combustion Temperatures
* Steady Heating of the Fuel-Bed
* Controlled Evolution of Volatile Gases
* Steady Combustion Air Requirements
* Constant Velocities & Residence Times
Near-Continuous Waste Feeding
In general, the operation of MWI units can be improved and emissions
minimized by maintaining appropriate combustion temperatures. The primary
chamber temperature must be maintained above a lower limit to assure good burnout
of volatile gases and below an upper limit to protect the refractory.
Unit operations are improved by the maintenance of steady temperatures in
the primary chamber. The gas temperature in the primary chamber drives the heat
transfer, waste volatilization and combustion processes. As the fuel-bed is heated,
volatile gases are evolved from the waste. The lower molecular weight (light)
hydrocarbons are evolved at relatively low temperatures, and the heavier volatiles are
evolved at higher temperatures.
9-1
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Operation with intermittent charging of large batches of medical waste makes
the maintenance of steady combustion temperatures more difficult. Combustion is
often improved with smaller charges occurring at shorter intervals instead of larger
charges less frequently. Improvement may be obtained under continuous feed
operations, such as is often provided by an auger screw feeder in a rotary kiln.
However, this feature is not available for most MWI operations.
Operating with steady combustion temperatures will contribute to lowering the
fluctuations in velocity and residence times, as well as reducing the variations in
volatilization rates and the combustion air requirements for burning the volatiles.
Slide 9-3
GENERIC CATEGORIES OP MWI INCINERATORS
Starved-Air Type
Modular Controlled-Air, Refractory Wall
Excess-Air Type
Multiple Chamber, Refractory wall
Rotary Kiln, Refractory Wall
Waterwall (integral Boiler)
The two basic categories of MWIs, as introduced in Learning Unit 5, are
starved-air and excess-air. These terms have been used to describe the contrasting
combustion and stoichiometric features of the air/fuel mixtures, particularly in the
primary chambers. These generic terms will be used to aid in considering the major
operational features which are encountered most often in current MWI practices.
The starved-air units are commonly known as controlled-air modular units.
The term controlled-air also relates to the use of fans to control the delivery and
distribution of combustion air. Therefore, some units operating with stoichiometric
air or excess-air in the primary chamber can also be called controlled-air units.
Among the incinerators characterized as excess-air \inits are the multiple
chamber, rotary kiln and waterwall incinerators. Although they have important
design differences, the combustion-related considerations are very similar. One
should note that some rotary kiln incinerators used for hazardous waste destruction
operate under air-starved conditions with temperatures high enough to cause the
residues to be removed as a molten slag.
9-2
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Slide 9-4
GENERIC COMBUSTION COMPARISONS
Starved-Air Unit
Volatilization in Primary Chamber
Incomplete Combustion in Primary
Relatively Low Gas Velocities
Relatively Low Particle Entrainment
Acceptable Carbon Bum-Out of Residue
Excess-Air Unit
Volatilization & Combustion in Fuel-Bed
Adequate Air for Complete Combustion
Relatively High Gas Velocities
Relatively High Particle Entrainment
Good Carbon Burn-Out of Residue
Most modular, controlled-air MWIs are operated as starved-air units. Such
units operate with sub-stoichiometric conditions (insufficient air for complete
combustion) in the primary chamber. Because incomplete combustion occurs, the
primary chamber is sometimes referred to as a "pyrolysis" chamber or "gasifier".
Differences in the relative amounts of air supplied to the primary chambers of
starved-air and excess-air incinerators lead to different gas velocities. Starved-air
units provide less air, which results in lower combustion intensities, temperatures
and gas velocities than in excess-air units. Starved-air units, therefore, have lower
particle entrainment.
Under excess-air conditions, burning occurs both in the fuel-bed and in the
region above the fuel-bed. A relatively high supply of underfire air causes intense
combustion in the fuel-bed, which aids in carbon burnout and increases the rate of
production of volatile gases. The high underfire air also causes particulate matter
to be entrained in the flue gas.
Large excess-air units have provisions for overfire air to be mixed with the
volatile gases. The resulting mixtures are generally around 50 to 100% excess air.
This mixing process is designed to provide adequate oxygen at a high enough
temperature for complete burning. Generally, a modest quantity of carbon monoxide
(e.g., < 50 ppm) is emitted with the flue gases, indicating some incomplete
combustion.
The primary chambers of excess-air units operate with more than the
theoretical amount of air. Excess air can be used to control the primary chamber
temperatures. For example, the older multiple chamber, refractory wall incinerators
were designed to operate with very high amounts of excess-air.
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Both starved-air and excess-air units, when operated in batch and intermittent
duty with long burndown periods, produce ash residues with low carbon (organic
material) content, as illustrated in Slide 3-19. Continuously operated starved-air
units generally produce ash residues with relatively large carbon content. The burn-
out can be improved by providing relatively large amounts of underfire air to the final
burn-out hearth section.
Slide 9-5
EXAMPLE MWI COMBUSTION TEMPERATURES
CHAMBER/SYSTEM FEATURE
Primary/Batch Operation
Primary/Intermittent Duty
Upper Limit
Primary/Continuous Duty
at Charging Zone
Secondary Chamber
Upper Limit
TEMPERATURE RANGES
1,000 to 2,000 °F
1,200 to 1,600 °F
1,800 °F
1,400 to 1,800 °F
1,600 °F
1,800 to 2,000 °F
2,200 to 2,300 °F
Most modern, starved-air refractory-wall units control the primary chamber
temperatures by regulating the supply of underfire air. Although auxiliary fuel
burners could be used to increase primary temperatures, their main purpose is
generally to preheat the combustion chamber and to provide for waste ignition.
The primary temperatures must be low enough to limit the volatilization rate
so that the secondary combustion can be adequately controlled. The temperatures
are generally controlled by limiting the underfire air or by using water sprays.
Depending upon the unit, the primary combustion chamber temperatures can
be designed to operate within the range of 1,000 and 2,000 °F, with continuously
charged units typically operating with an upper limit of 1,800 "F.1
The secondary chambers of modular starved-air units are operated under
excess-air conditions. The gases in the secondary chamber are generally maintained
at around 1,800 to 2,000 °F to assure complete combustion of the volatile gases and
to meet the regulatory requirements. An upper temperature limit of 2,200 to 2,300
°F is set to prevent damage to the refractory materials. The secondary air supply is
used as the heat sink to limit the secondary chamber temperatures. Of course, the
secondary temperatures can be maintained above a minimum limit by using auxiliary
fuel burners.
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Slide 9-6
STARVED-AIR, TWO-STAGE COMBUSTION UNIT2
To Boiler
ol
Fossil Fuel Burner
Secondary Chamber
o
f- Primary Chamber
Feed Ram
Ash Sump
Ash Transfer Rams
Air Tube
Ash Discharge Ram
Courtesy of Consumat Systems, Incorporated
Most MWI units utilize two combustion chambers which provide for the
application of two-stage or multi-stage combustion concepts. These units are often
assembled on-site from modular components which are manufactured in a factory.
The primary chamber acts as a gasifier, as it operates with less than the
theoretical air required for complete combustion (e.g., approximately 40%).3 The
partial combustion of the volatile gases is designed to heat the primary chamber
gases high enough to drive the drying and volatilization processes. Increases in the
primary chamber temperatures will increase the delivery rate of volatile gases to the
secondary chamber.
The volatiles are generally the major fuel input to the secondary chamber. To
achieve good burning, the secondary air must be supplied and mixed in proportion
to the fuel supply. Flame port or downstream air injection systems are designed to
provide the secondary air, which raises the overall air supply.
The overall excess air varies widely depending upon both equipment and
operation (e.g, continuous burning, burndown, cool-down). Values from 100 to 135%
are representative values during steady operations, with higher values (e.g., 240%)
during burndown. Units operating at the lower overall excess air levels will require
less auxiliary fuel to maintain the secondary combustion temperature requirements.
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Slide 9-7
AUTOHATIC CONTROL APPLICATIONS
1. Medical Waste Feeding System
2. Combustion Control System
3. Ash Removal System
4. Flue Gas Cleaning System
5. Boiler System
6. Water Treatment Systems
Many of the sub-systems of MWI units, such as those listed above, require a
combination of manual and/or automatic controls to achieve the desired operations.4
Manual controls include the simple on/off and variable switches which are used by
operators to power electrical equipment and to position valves and dampers.
Slide 9-8
AUTOMATIC CONTROLS SYSTEM FUNCTIONS
1. Modulating Control
2. Sequential Control Logic
3. Process Monitoring
In general, the combustion quality and the safety of unit operations are
improved by the appropriate application of automatic controls. Control systems are
designed to monitor the operational status of various processes and automatically
change equipment settings in response to variations from the desired condition.
Modulating controls use variable equipment settings in response to variable
operational conditions, whereas on-off controls provide only a single operating
response.
Automatic controls may be used to establish a sequence of logical events. Such
sequential logic controllers may set appropriate time periods between events to
assure safe and appropriate operations during repetitive, transient conditions.
Examples of sequential controller applications include start-up, materials charging,
soot blowing, ash removal and shut-down operations.
Monitoring of the operational status of various systems or processes can be
accomplished with the aid of panel-mounted devices. These include indicating lights,
alarm annunciators, and instruments and recorders for selected physical parameters
(e.g., temperatures, pressures, gas concentrations and fluid flow rates). Monitors can
9-6
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provide a warning of potential upset conditions, as well as associated data which may
be used to evaluate the possible causes and remedies for such conditions.
In addition, automatic control systems can improve the working conditions by
automatically performing repetitive and tedious activities. For example, the operator
may be required to weigh and record selected information for each box (container)
just before the medical waste is loaded into a MWI. The total weight charged each
hour may need to be controlled to assure that the feed rate does not exceed that
allowable by the permit. Weighing, recording and summing activities can be
performed by a microprocessor system with an optical scanner and automatic scales,
similar to the typical grocery store's check-out equipment.
Slide 9-9
TYPES OF AUTOMATIC CONTROL SYSTEMS
1. Pneumatic
2. Hard-wire Electronic Analog
3. Programmable Logic Controllers
Microprocessor-Based
Distributive Control Systems
Modulation of a particular system variable can be obtained through use of
dedicated pneumatic controllers, hard-wired electronic analog controllers, or
programmable logic controllers (PLCs).
A distributive control system is one where a number of micro-computers or
PLCs are distributed around the unit to provide local control of specific equipment.
Microprocessor-based distributed control systems with various PLCs may be used to
control complex systems, such as for automated cart dumping and charging, air
pollution control and energy recovery.
A computer network can be formed by connecting the various PLCs together.
This allows the control and sensor signals to be accessed from throughout the system.
The system also allows for self-checking and redundancy, so that a unit may be able
to continue operation with a malfunctioning controller or sensor. The operator is
notified of the malfunction by light indicators and alarm annunciators.
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Slide 9-10
PNEUMATIC CONTROL LOOP: FLOW CONTROL
SIGNAL TO
CONTROLLER
SIGNAL TO
FINAL CONTROL
ELEMENT
CONTROLLER
TRANSMITTER
SENSING LINE
SENSOR
PROCESS LINE
FINAL CONTROL
ELEMENT
Dedicated pneumatic systems, such as shown in the above slide, and hard-
wired controllers are widely used in smaller MWI units.
Slide 9-11
AUTOMATIC CONTROL SYSTEM ELEMENTS
1. Process (Manipulated) Variable
2. Measuring Device (Transducer)
3. Feedback Signal
4. Set Point (SP)
5. Controller
6. Actuating Signal
7. Final Control Element (FCE)
8. Status Indicator
The typical elements of a control system are listed above. An electronic-based
control system will be assumed in the following presentation, although pneumatic
control systems use almost the same principles.
A simple control system measures the difference between a feedback signal and
its set point (SP) or reference value to determine the appropriate change in
equipment operation. The process variable is the parameter which can be
manipulated or controlled. It is measured by a sensor and transducer device which
produces an electrical feedback signal for comparison with the set point signal.
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A controller produces an actuating signal which is sent to a final control
element (FCE) to modify the operation of the equipment. The status indicators could
include indicating lights, alarm annunciators, and instrument recorders and readout
devices.
Control systems are designed to respond to transient operating conditions by
automatically correcting the equipment settings. Under various system disturbances,
operators will need to make modifications such as: changing the set point, changing
the controller gain (bias), or switching to the manual mode to control the equipment.
Slide 9-12
GAS-8IDE CONTROL PARAMETERS
1. Air Flow Rate
2. Opacity
3. Oxygen Content
4. Carbon Monoxide
5. Draft
6. Combustion Temperature
7. Flue Gas Temperature at APCD
The gas-side parameters listed above may be used as control parameters in
automatic combustion control systems. Each incinerator equipment vendor will have
a unique combustion control system which considers some combination of the above
parameters.
Slide 9-13
FINAL CONTROL ELEMENTS
1. Damper Position
2. Fan and Pump Drives
3. Motor Controller
4. Auxiliary Fuel Valve Position
5. Ram
The final control element parameters listed above may be used by control
systems to achieve the desired equipment operational changes. For example, the
motor controllers on the induced draft fan adjacent to an APCD can be very
important for controlling the incinerator draft. Also, continuous duty MWIs provide
for agitation of the fuel-bed through the use of ram devices, which are controlled by
selecting the length of the stroke (by setting limit switches) and the stroke frequency,
or dwell time, between strokes.
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Slide 9-14
CENTRIFUGAL FAN5
Scroll tide
Outlet
Scroll
(bousing, volute)
Rim
(shroud, wheel ring,
retaining ring,
wheel rim)
Impeller
(wheel, rotor)
Forward curved
Backward curved
Radial
Centrifugal fans, such as that illustrated above, are most often used in MWI
applications. Axial fans, with fan blades resembling a propeller, may also be used.
Centrifugal fans can have blade designs with forward curved, radial or
backward curved impellers. The impeller determines its performance characteristics.
Fans with backward curved blades are often used, as they tend to have the high
efficiencies and produce a flow which is fairly independent of static pressure.
Modern MWIs use both forced draft and induced draft fans to control the air
flow and achieve the mixing required for complete combustion. Fans located
upstream of the incinerators are called forced draft fans because they force the air
into the incinerator. Forced draft fans are often mounted on ducts or plenum
chambers which deliver air to one or more distribution points. Depending upon the
design, more than one forced draft fan can be used. In some applications, separate
fans supply the underfire air and the secondary air.
Induced draft fans provide the flue gas pressure drop required to draw flue
gases out of the incinerator and past the air pollution control devices (APCDs) and
recovery boiler heat exchangers. Excessive frictional losses may be associated with
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moving the flue gas through the flow restrictions (e.g., scrubber with pluggage
problems, blinded fabric filters). Induced draft fans are located upstream of the
stack, so they also effectively force flue gases up the stack.
Slide 9-15
METHODS OF CONTROLLING AIR FLOW
1. Variable Speed Fan
2. Damper in Duct
3. Variable Inlet Vane
Damper
The air flow rate delivered by centrifugal fans can be automatically controlled
by changing fan speed with a motor controller or by changing the position of a duct
damper or inlet vane damper. Variable speed fans, although fairly expensive, operate
with high fan efficiencies and are sometimes used in MWI applications.
A duct damper is a variable restriction placed in the duct. Dampers are used
to control the delivery of air to the various sections of the hearth or other locations.
Note that when parallel flow paths are provided, changing a damper setting to
restrict the air flow in one section may cause additional flow in another section.
Slide 9-16
VARIABLE INLET VANES FOR CONTROLLING SWIRL*
Air Flow
-Inlet Vanes
Courtesy of Combustion Engineering, Incorporated
Centrifugal fans are often controlled by varying the blade angle on the inlet
dampers. This change acts to modify the swirl or aerodynamic behavior of the flow
and to restrict the effective open area at the inlet.
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Slide 9-17
DRAFT
Negative Pressure (Vacuum)
Measured in Inches of Water
Maintained Inside Incinerators
Draft is defined as the difference between atmospheric pressure and absolute
pressure. Draft can be routinely monitored by draft gages, with typical units being
inches of water column. The operation of pressure gages will be presented in
Learning Unit 11.
Primary chamber draft indicates the amount of vacuum on the inside of a
combustion chamber. Generally, the pressure inside the incinerator is maintained
slightly below atmospheric pressure (under slight vacuum). Average operating values
will typically range from 0.05 to 0.15 inches of water.
If the incinerator pressure becomes greater than atmospheric, hot combustion
gases will leak out through cracks and openings and the unit could sustain structural
damage. If gases leak out, the surrounding work environment may accumulate fly
ash. If the unit suddenly swings to a positive pressure when a hatch is open, hot
combustion gases could burst through the hatch and cause injury.
A unit which is operated with considerably more draft than specified by the
manufacturer will tend to have higher gas velocities. Excessive "tramp" air will be
sucked into the incinerator through cracks and openings. Excessive tramp air causes
poor mixing, uneven temperatures and loss of control of the combustion process. This
can be a significant problem during the charging process, when the fire door at the
charge hopper is opened.
The restoration of normal draft maybe accommodated through use of a damper
or a change in fan operation. Using a more restrictive damper setting on the exit
side of the incinerator or opening the damper on the air supply side could be
considered to restore proper draft conditions. Control systems which automatically
maintain the draft conditions can be provided as part of the overall combustion
control system.
9-12
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Slide 9-18
SINGLE ELEMENT CONTROL SYSTEM: DRAFT7
SP
i
r>
4
CM
ink
Adaptation of a Figure of the Instmment Society of America
An incinerator draft controller is an example of a simple, single-element control
system which generally operates very well. An internal damper or flow control device
can be used to restrict the flow of gases through the incinerator. The pressure
transducer (PT) measures incinerator pressure above atmospheric conditions. The
controller compares the value with the set point (SP) and then sends a signal to a
final control element (FCE).
Slide 9-19
TRIM CONTROL FEATURES
1. Oxygen Trim Control
2. Flue Gas Temperature Control
Some MWIs use trim control systems with special microprocessor-based control
logic to modify combustion conditions so as to maintain some required operating
condition. The normal control units would change the air flow or auxiliary fuel firing
in response to a measured temperature, but the trim control units would fine-tune
the air delivery to maintain certain desired conditions.
For instance, an oxygen trim control system can be used to control the level of
excess air or oxygen in the flue gas. The motivation for such control systems could
be either to assure that enough oxygen is available for burning carbon monoxide or
to limit the oxygen available as a technique for controlling NOx emissions.
9-13
-------
Another trim control unit could limit the flue gas temperature at the entrance
to the APCD. Such temperature control may be important for protecting the APCD
against thermal damage, as well as acting as a method for controlling the catalytic
formation of dioxin/furan compounds (as discussed in the next learning unit).
Temperature can be controlled through the operation of special heat exchangers, or
by delivering heat sink materials (e.g., excess air, water sprays) into the flue gas.
Slide 9-20
COMBUSTION CONTROL SYSTEM COMPARISONS
Conventional Fuels
Gas, Fuel Oil & Coal
Medical Waste
The combustion control systems for conventional fuel-burning units are
generally designed to maintain the required combustion quality under transient
demands for energy output. Natural gas and oil have uniform properties which make
them easy to control. Coal is more difficult because it burns both as a gas (volatile
matter) and as fixed carbon.
A very important combustion control concept is to consider that MWI units are
primarily incinerators rather than power plants. Since MWIs are primarily waste
burning devices, the combustion environment is able to focus on accommodating the
variable fuel properties rather than having to respond to changes in load demand.
The properties of medical waste are considerably more variable than those of
conventional fossil fuels. As illustrated in Slide 3-13, the plastic and moisture
contents can vary widely from one charge to the next. MWI combustion systems use
much larger excess air levels than comparably sized conventional combustion
systems. Also, medical waste is typically retained in the combustion zone for much
longer residence times than conventional fuels.
Slide 9-21
STARVED-AIR UNIT CONTROL VARIABLES
Primary & Secondary Temperatures
Underfire Air Flow in Primary
Charging Method and Bed Agitation
Secondary Air Flow
Auxiliary Fuel Burning
Solids Residence Time
9-14
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Combustion control systems for starved-air units generally have features which
attempt to provide steady combustion conditions in each chamber. Ideally, this can
be obtained by maintaining constant combustion temperatures.
As discussed in Learning Unit 8, an increase in the underfire air supply will
increase the combustion temperatures. Modulation of the underfire air is the main
control variable used to attain the desired primary chamber temperature. This
temperature is also directly influenced by fuel-bed agitation (e.g., ram stroke and
frequency).
Additional control variables for the primary chamber include the time period
between the charging and the size of the charge. Less frequent charging will result
in less inflow of tramp air during the charging process. However, smaller charges
cause less disturbance of the bed-burning process.
The secondary chamber control variables include those related to the
production of volatiles (underfire air, fuel-bed agitation, primary chamber
temperature), the secondary air supply and the rate of auxiliary fuel burning.
Some MWIs limit the amount of excess air during steady operations and are
able to achieve secondary temperature requirements without burning auxiliary fuel.
Other units will operate with higher flue gas oxygen levels (excess air) and may
require auxiliary fuel to maintain secondary combustion temperatures.
The primary chamber temperature is a good indication of the delivery of
volatile gases to the secondary combustion chamber. Plastic bags, used in waste
packaging, will tend to volatilize upon being charged into an incinerator. To avoid
overloading the secondary chamber with volatile matter, most MWIs control the
production of volatiles by controlling the primary combustion temperature. Some
MWI systems have logic which retards the delivery of primary air during the
charging process. This limits the primary chamber temperature, thereby retarding
the surge of volatiles from the plastic materials.
An additional method of controlling secondary chamber combustion would be
to control the secondary air delivery rate based on the primary chamber temperature.
However, thermocouple sensitivity and fan response rates may be too slow to deliver
the air required for adequate control, particularly during a surge of volatiles.
For units which operate on a daily burn cycle, the residence time of the solid
waste residues in the primary chamber is designed to be fairly long (e.g., 8 hours).
This is designed to provide for good carbon burnout. Systems which operate
continuously (24 hr/day), have much shorter residence times before the residues are
delivered to an ash quench. Some continuous systems compensate for the relatively
shorter residence times by providing an increased air supply to the hearth in the final
hearth (burn-out section).
9-15
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Slide 9-22
STARVED-AIR UNIT OPERATIONS
PRIMARY AUXILIARY FUEL BURNER
Preheat Refractory
Initiate Ignition
Increase Gas Temperature
* Increases Volatilization
SECONDARY AUXILIARY FUEL BURNER
Preheat Refractory
Maintain Secondary Gas Temperature
* Reduces Smoking
* Reduces Incomplete Combustion
Starved-air units make use of auxiliary fuel burners in both the primary and
the secondary combustion chambers. The primary burner is used to preheat the
primary chamber and to ignite the waste. It can also be used, to maintain adequate
gas temperatures to drive the volatilization process (e.g., for disposing of pathological
waste) and to assure good combustion during the burn-down period.
Auxiliary fuel burners are also used to preheat the refractory in the secondary
chamber. They are also used to maintain secondary temperatures during the steady
burning and burn-down operations, thereby controlling the emission of smoke and
products of incomplete combustion.
Slide 9-23
CONTROL SYSTEM INTERLOCKS
High Primary Temperatures
High Secondary Temperature
Low Secondary Temperature
Low Draft
Burner Flame Outage
Fan Failure during Purge
High Flue Gas Temperatures
High APCD Pressure Drop
Stop Underfire Air
Lock-Out Feeder
Lock-Out Feeder
Open Bypass Stack
Stop Auxiliary Fuel
Stop Auxiliary Fuel
Open Bypass Stack
Open Bypass Stack
Many control systems have system interlocks which prevent the operation of
certain component equipment under upset conditions. The waste feeder may be
locked out if monitors indicate that the system is not operating properly (e.g., high
primary or secondary temperature, low secondary temperature).
9-16
-------
For example, an electromagnetic lock may prevent the opening of the charging
door if the secondary chamber is either below 1,800 °F or above 2,200 to 2,300 °F.
Some interlock systems are provided to assure the safe operation of the
equipment. During start-up operations, a timer generally prevents the flow of fuel
to the auxiliary burner until after an adequate time period for purging the
combustion chamber with fresh air. If the fans fail to operate, an interlock could be
provided to disable the start-up cycle. Also, after auxiliary burner start-up, a flame
detector must sense the burning of the fuel, or else the burner's fuel flow will be
automatically stopped.
Other interlock systems may protect the APCD or boiler equipment by
automatically allowing the flue gas to flow up a by-pass stack in the event of high
flue gas temperatures (e.g., over 500 °F). Such systems are generally designed to be
"fail-safe," so that in the event of an electric power outage, the flue gas is directed to
the by-pass stack.
Slide 9-24
EXCESS-AIR UNIT OPERATIONS
To Obtain Increased Combustion Temperatures:
Increase Bed Agitation & Underfire Air
Increases the Burning Rate
Reduce Overall Excess Air (Overfire Air)
Controlling the overall amount of excess air is a traditional technique for
controlling final combustion gas temperatures in excess-air units. Excess air is a
readily available heat sink material. Most waterwall excess-air units can
independently regulate the underfire and overfire air supplies.
As with starved-air units, increased combustion intensity is achieved in excess-
air units by increasing the underfire air and the amount of bed agitation.
However, the gas temperatures are primarily controlled by the overfire air supply
which established the overall stoichiometric ratio (excess air). The gas temperature
will increase as the overfire air is decreased, but if adequate mixing does not occur,
carbon monoxide emissions may increase.
9-17
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Slide 9-25
EXCESS-AIR UNIT OPERATIONS
Increased Fuel Moisture:
Gas Temperature Will Drop
Gas Temperature Can Be Restored by:
* Increasing Bed Agitation
* Reducing Overfire Air (Excess Air)
For all incinerators, if the moisture content of the fuel increases, the eytra
moisture will act as a heat sink and cause the gas temperatures to drop. The
combustion gas temperatures can be restored by increasing the fuel-bed agitation and
by reducing the overfire air.
9-18
-------
REFERENCES
1. Handbook, Operation and Maintenance of Hospital Medical Waste
Incinerators. U.S. Environmental Protection Agency, EPA/625/6-89/-024,
January 1990, p. 36-40.
2. "Integrated Waste Services, Information Summary," Consumat Systems, Inc,
Richmond, Virginia, Undated Brochure.
3. W. R. Seeker, W. S. Lanier, and M. P. Heap, Municipal Waste Combustion
Study. Combustion Control of Organic Emissions, U.S. Environmental
Protection Agency, EPA-530-SW-87-021-C, June 1987, pp. 1-7 to 1-9.
4. George R. Kotynek and Calvin L. Hartman, "Using State-of-the-Art Electronic
Controls for Controlling Boiler/Incinerators in Refuse-to-Energy Plants,"
Proceedings of 1990 ASME National Waste Processing Conference, Long Beach,
CA, June 1990, pp. 123-133.
5. Gerald T. Joseph and David S. Beachler, "APTI Course 415, Control of Gaseous
Emissions, Student Manual," Report Number EPA 450/2-81-005, U. S.
Environmental Protection Agency, Research Triangle Park, NC, December
1981, p. 9-26.
6. Joseph G. Singer, Combustion Fossil Power. 4th Edition, Combustion
Engineering, Inc. Windsor, CT, 1991, p. 14-14.
7. Reprinted by permission. Copyright by Instrument Society of America 1988.
From "Boiler Feedwater and Steam - Controlling for Safety and Efficiency,"
Videotape from ISA's Boiler Control Series.
9-19
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10. AIR POLLUTION FORMATION
Slide 10-1
COMBUSTION SOURCE AIR POLLUTANTS
Waste Dependent
Combustion Quality Dependent
Baghouse or ESP Temperature Dependent
There are a number of different types of air pollutants which are emitted from
combustion sources. In general, their formation is dependent upon the composition
of the fuel, combustion quality, and temperature of the flue gas as it enters the
particulate collection device.
Slide 10-2
WASTE DEPENDENT AIR POLLUTANTS
Acid Gases
Hydrogen Chloride
Sulfur Oxides
Nitrogen Oxides (Fuel NOx)
Metals (Heavy Metals)
Lead, Cadmium, Mercury
Carbon Dioxide
Waste dependent air pollutant emissions generally can be controlled by either
changing the waste mixture before combustion or by removing the contaminant from
the flue gas after combustion.
For instance, the chlorine and lead contents in medical waste produce
undesirable emissions of HC1 (hydrogen chloride) and lead particulates. These
emissions would be reduced if medical supplies containing less polyvinylchloride
(PVC) plastics were used and if batteries (which are a major source of lead) were
removed from medical waste before combustion.
As discussed in Learning Unit 3, medical waste contains a very high
percentage of plastics (e.g., 20 to 65%) and the chlorine content as high as 4%.
During combustion, the chlorine will form an acid gas, HC1, which creates emission
problems and tends to cause corrosion of the metal surfaces of the flue gas ducts and
recovery boiler. Acid gas emissions are often controlled by flue gas scrubbers.
10-1
-------
Medical waste contains trace concentrations of heavy metals such as lead,
chromium, cadmium and mercury. Upon heating during combustion, mercury will
tend to vaporize and react to form oxides or chlorides. Other metals will vaporize,
react or remain unchanged in the solid residue.
Carbon dioxide is not generally considered to be an air pollutant. It occurs
naturally from the carbon cycle of organic growth and decay. However, atmospheric
carbon dioxide acts as a greenhouse gas and its emissions have increased as a result
of increased burning of fossil fuels. Carbon dioxide gas is transparent to solar energy
but opaque to the long wavelength infrared energy emitted from the earth to the sky.
Therefore, it may effectively trap radiant energy in the earth's atmosphere and lead
to global warming.
Slide 10-3
APCD TEMPERATURE DEPENDENT AIR POLLUTANTS
Metal Vapors (Mercury)
Trace Organics (Dioxins/Furans)
Mercury vapor and other heavy metal vapors in the flue gas can be condensed
and adsorbed onto the surfaces of particulates. The condensation-adsorption process
will be improved if the temperature of the flue gas entering the particulate collection
device is cooled to 400°F or lower. The adsorbed metals can be removed with the fly
ash in the APCD.1 Fabric filters are often used to act as a sieve for removing
particulate materials.
The fly ash in the APCD can act catalytically to form dioxins and furans.
These reactions are controlled by cooling the flue gas (e.g., 400°F) before entering the
APCD.
The APCD temperatures for dry removal systems are generally maintained
above 350 °F to avoid corrosion problems associated with condensation. This
temperature compares with the highest reasonably expected acid dew point for flue
gas of around 300 °F and the highest dew point for moisture of around 175 °F.
Wet scrubbing systems, however, typically cool flue gases to between 150 and
185 °F. A final water washing process can minimize the entrainment of scrubbing
solution in the flue gas, to help prevent corrosion associated with the condensation
which may occur downstream.
10-2
-------
Slide 10-4
PRODUCTS OP INCOMPLETE COMBUSTION (PIC)
Smoke & Particulates
Carbon Monoxide
Trace Organics: Dioxins & Furans
Volatile Organic Hydrocarbons
Inadequate combustion conditions can lead to formation of the PICs listed
above. Such conditions include: low and non-uniform combustion temperatures; poor
mixing of oxygen and volatile gases; inadequate air (oxygen); too much air (heat sink);
and cooling of gases before combustion is completed (inadequate time).
Slide 10-5
SMOKE & PARTICULATES
Black Smoke
Carbon in Particulates
Brown Smoke
Nitrogen Oxides & Particulates
White Smoke
Condensed Hydrocarbon Gases
Ammonium Chloride
Water Droplets (Not Smoke)
Blue Smoke
Condensed Hydrocarbon Gases
Ammonium Sulfate
Visible smoke is often used as an indicator of excessive particulate emissions.
Smoke is composed of small particulates which have the effect of obscuring the
transmission of light (increasing opacity). The particles include solid and condensed
liquid materials which scatter light.
Particulate emissions are controlled by APCDs, as discussed in Learning Unit
15. The very small particulates are the most difficult to collect and are of most
concern because they are able to pass into the lungs of humans and become trapped.
Black smoke indicates high particulate emissions in flue gas. Although the
major constituents of black smoke are inorganic materials, the unburned carbon and
carbonaceous materials are responsible for the black color. Improving the combustion
conditions will generally reduce the smoke emissions.
10-3
-------
Brown smoke can be caused by NOx emissions and/or particulates.
White smoke can be formed by the condensation of unburned hydrocarbon
gases. It can also result from the reaction of ammonia with HC1 in the flue gas,
which forms ammonium chloride. Vapors often are condensed when cooled by the air,
forming small droplets which are in the size range to scatter light. Such smoke is
visually detached from the stack because a period of cooling is required for droplet
formation.
Condensed water vapor, such as the emission from a scrubber stack, has the
appearance of white smoke. Such droplets will tend to re-vaporize after a fairly short
period of time in the atmosphere, unless the ambient relative humidity is very high.
Blue smoke, like white smoke, can be formed by the condensation of unburned
hydrocarbon gases. Blue smoke can also result from reactions of sulfur oxides with
the ammonia or urea, which may be injected into the flue gas to control NOx. The
reaction produces ammonium sulfate which can condense in the atmosphere.
Slide 10-6
GENERAL COMBUSTION SYSTEM CO - O2 RELATIONSHIP2
oz
i—o
XH-i
oce
oc-j
ccz
ceo
A - INSUFFICIENT AIR
8 - APPROPRIATE OPERATING REGION
C - "COLO BURNING"
3 6 9 12 IS
OXYGEN CONCENTRATION
For each combustion system, there is a general relationship between the
emissions of carbon monoxide and the amount of excess air.2 The excess air is
generally indicated by oxygen measurements in the flue gas. The recommended
operating practice is to maintain the oxygen level in the zone where the carbon
monoxide is minimized.
10-4
-------
Slide 10-7
DIOXINS/FDRANS (CDD/CDF)
Dioxins (CDD)
Polychlorinated Dibenzo-p-dioxins
Furans (CDF)
Polychlorinated Dibenzofurans
A large number of chemical compounds can be formed during incomplete
combustion in MWI operations. Smoke, carbon monoxide and trace organics are of
particular concern. The trace organic compounds include both volatile organic
hydrocarbons (VOHC) and semi-volatile hydrocarbons. Compounds of interest include
chlorobenzenes, chlorophenols, polycyclic aromatic hydrocarbons (PAH), and
Polychlorinated biphenyls (PCBs).
Dioxin is the name given to polychlorinated dibenzo-p-dioxins (CDD), and furan
is the name of polychlorinated dibenzofuran (CDF) compounds. Dioxins and furans
are non-volatile, chlorinated organic compounds from the PAH group.
Slide 10-8
DIAGRAMS OF DIOXIN AND FURAN STRUCTURES3
cr ^^^ o
Example Dioxin
Cl CJ
Example Furan
Dioxins and furans are a complex group of chemical compounds of carbon,
hydrogen and chlorine. Both groups are referred to as "ring compounds."
10-5
-------
The individual compounds are found in a variety of possible molecular
configurations and have different toxicity features. For instance, the number of
chlorine atoms in the dioxin/furan molecules can vary from one to eight (with four
each shown in the illustration). In addition, the chlorine atoms can be attached at
different positions, leading to different "isomers".
The dioxin illustrated is the "2,3,7,8 isomer," which is the most toxic of all
dioxins.
Slide 10-9
CONDITIONS FOR DIOXIN/FURAN FORMATION
COMBUSTION ZONE
Relatively Low Combustion Temperatures
Poor Mixing - Pockets of Rich Mixtures
High Particulate Loadings
Operating Above Unit Capacity
FLY ASH COLLECTION DEVICE
Catalytic Formation on Fly Ash
High Operating Temperatures (450° F)
A great deal of research has been performed to develop knowledge about the
formation and control of trace organic emissions from incinerators.2'3'4'5'6 Much of it
has focused upon dioxins and furans, which are potentially carcinogenic materials.
Dioxin/furan emissions can be formed under incomplete! combustion conditions
in the combustion zone.2 Low temperatures and inadequate mixing can lead to high
dioxin and furan emissions. Poor mixing can result in pockets of fuel-rich mixtures
which do not get the oxygen required for complete combustion until after they leave
the secondary combustion chamber.
Some chlorinated compounds, such as complex chlorophenol and PCB
molecules, undergo incomplete combustion and lead to the formation of dioxins and
furans. Another process which can form dioxins and furans is the burning of organic
material in the presence of hydrogen chloride.2
When a unit is operated above its rated capacity, higher gas velocities and poor
mixing generally occur. Under such conditions, the solids have less residence time
in the combustion zone and cool before the combustion process is completed. If this
occurs, increased amounts of unburned carbon in the fly ash and dioxin/furan
formation would be expected.
10-6
-------
As previously mentioned, dioxins and furans can be formed by catalytic
reactions on the surface of fly ash, downstream of the incinerator. Fly ash is
composed of many inorganic and metal materials (e.g., copper) which are known to
act as catalysts. Formation requires that the surface of fly ash be held in the APCDs
(e.g., fabric filter) for a relatively long period of time at a reasonably high
temperature. Considerable increases have been measured6 when the operating
temperatures were increased from around 350° to 600° F.
Therefore, regulations limiting the APCD inlet temperature have been issued7'8.
One operating constraint is that the APCD temperature should be above the acid gas
dew point to avoid condensation and corrosion problems.
There is a partitioning of dioxins and furans between those which are emitted
up the stack and those retained in the collected ash. As the carbon loading goes
down, it appears that more dioxins and furans are emitted in the stack gas, whereas
when the carbon loadings go up, more are retained on the collected fly ash.
Slide 10-10
TEST FOR DIOXINS/FDRANS
Stack Test: EPA Method 23
Although dioxins and furans are considered to be non-volatile, they may be
emitted in either a gaseous form or as absorbed onto solid surfaces, such as fly ash.
Some regulations may require MWIs to be tested to determine the amount of each
isomer of dioxins/furans using a standard EPA Method 23 stack test.
Slide 10-11
REGULATORY BASIS FOR EMISSIONS LIMITS
Federal: Total Mass of All Dioxins and Furans
Some States: Toxic Equivalent Limitation
Determine Mass of Each Isomer
Toxicity Level Assigned to Each Isomer
Multiply Masses by Levels to Obtain Total
Federal regulations are based on total CDD/CDF, which is defined as the total
tetra- through octa-chlorinated dibenzo-p-dioxins and dibenzofurans, which have from
four to eight chlorine atoms.
10-7
-------
Some states use a toxic equivalent limitation instead of the total mass of
dioxins/furans. In the toxicity equivalent method, a toxicity value is assigned to each
isomer. The overall toxic equivalent is determined by summing the products of the
mass fraction of each isomer times its toxicity value.
Slide 10-12
GAS CONCENTRATIONS:
MOLECULAR FRACTIONS
MOLE FRACTIONS
In Learning Unit 6, we considered the combustion of medical waste. We will
now consider the concentrations of the various gases in the mixture of products of
combustion.
Gas concentrations are the fractions of total molecules or mole fractions, which
can be found by dividing the moles of the gas by the total moles in the mixture. Gas
concentrations can be determined from balanced chemical reaction equations.
Slide 10-13
COMPLETE
C4.26H6.2°1.33N.
Product
Gas
C02
H20
N2
HC1
S02
Total
COMBUSTION
OF MEDICAL WASTE WITH 125% EA
,5C1.117S.006 + 0.5 H20 + 11.52
--> 4.26
+ 0.
Wet Gas
Moles
4.26
3.54
43.38
0.117
0.006
6.40
57.703
O2+ 43.36 N2
C02 + 3.54 H20 + 43.38 N2
117 HC1 + 0.006
Dry Gas
Moles
4.26
43.38
0.117
0.006
6.40
54.163
SO2 + 6.40 O2
Dry Gas
Mole %
7.87
80.09
0.22
0.01
11.81
100.00
A complete combustion reaction equation for burning medical waste with
stoichiometric air was presented in Slide 6-20. The products of combustion with
125% excess air are as shown in Slide 10-13 (using the techniques of Slide 6-23).
10-8
-------
The idealized product gases for 125% excess air are composed of: 4.26 moles
of CO2, 3.54 moles of water vapor, 43.38 moles of N2, 0.117 moles of HC1, 0.006 moles
of SO2 and 6.4 molse of O2. If the water vapor is neglected, the product gas analysis
can be performed on a dry basis. Therefore, there will be 54.163 moles of dry gas.
Slide 10-14
EQUIVALENCE OF GAS CONCENTRATIONS
Mole Fraction X 100 —> Percentage
Mole Fraction X 1,000,000 --> ppm
Percentage X 10,000 --> ppm
Mole fractions are volume fractions. Such fractions can be converted to either
mole percentages or parts-per-million (ppm or ppmv) by the simple mathematics of
moving the decimal point.
Literally, a percentage is a part-per-hundred, whereas a part-per-million (ppm)
is defined with one million in the denominator instead of one hundred. A mole
fraction can be converted to a ppm basis by moving the decimal six places to the
right.
Conversions from a percentage to a ppm basis involve four orders of magnitude
or moving the decimal four places to the right. Correspondingly, the 1 ppm is
equivalent to 0.0001 percent, with the decimal going four places to the left.
Using the previous example of stoichiometric combustion of medical waste, the
dry gas mole fraction of C02 is 4.26/54.163 = 0.0787. This can be multiplied by 100
to yield 7.87%. Note that this C02 value is typically of those measured under steady
operations in the exhaust from a secondary combustion chamber.
The mole fraction of HC1 in the dry product gases is 0.117/54.163 = 0.0022,
which is equivalent to 0.22 percent or 2,200 parts-per-million (ppm). Note that this
number is above that typically measured in the flue gas upstream of the scrubber
because a high chlorine content was measured in the "dry sample" of medical waste.
10-9
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Slide 10-15
GAS CONCENTRATIONS AT STANDARD DILUTION
Example: CO Concentration Limit
50 ppm at 7% 02 on a
Dry Gas Basis
Gas concentration limits are expressed at standard dilutions in order to
prevent dilution from being a method for meeting the concentration requirements.
Emission standards used over the years have been referenced to different
dilution bases. For instance, both 12% C02 and 50% excess air have been widely
used. Recent incinerator regulations have used 7% O2 as the basis.
Slide 10-16
EQUATION FOR CONVERTING TO 7% OXYGEN9
Assume COB is the Measured Dry Gas CO
Expressed as a ppm or %
O2m is the Measured Dry Gas O2
Expressed as a Percentage
CO (@ 7% 02 ) = COB x (21 - 7)/(21 - O2m)
= C0m x (14)/(21 - 02m)
Gas concentrations (e.g., CO in the above slide) can be corrected to 7% oxygen
by multiplying the measured concentration by (21 - 7) and dividing by (21 - 02m). The
variables with a subscript "m" are the measured dry gas concentrations. If the
available data is obtained from in situ instruments, the values will be on a wet basis.
Corrections to a dry basis are made by dividing by (1.0 - moisture fraction).
The derivation of the above conversion equation is based on theoretical mixture
considerations, including the assumption that air is 21% oxygen by volume.9
10-10
-------
Slide 10-17
PRODUCT
Product
Gas
C02
H20
N2
HC1
S02
O2
CO
Total
GAS ANALYSIS,
Wet Gas
Moles
4.26
3.54
43.38
0.117
0.006
6.40
0.005
57.708
MEDICAL WASTE
Dry Gas
Moles
4.26
43.38
0.117
0.006
6.40
0.005
54.168
& 125% EA
Dry Gas
Mole %
7.86
80.09
0.22
0.01
11.81
0.01
100.00
To illustrate corrections of concentrations from the measured values to a
standard basis, let us consider the example of combustion of medical waste at 125%
excess air as presented in Slide 10-13. Note that for illustrative purposes, 0.005
moles of CO have been arbitrarily added to the product gases.
The flue gas analysis is illustrated above, with the oxygen content in the dry
gases being 6.4/54.168,0.118 or 11.8 percent on a dry gas basis. This is fairly typical
of operating numbers for MWIs when operated under steady conditions.
Let us now consider the incomplete combustion indicated in the above gas
analysis, with 100 ppm CO measured on a dry flue gas basis. Note that 100 ppm
corresponds to a mole fraction of 0.0001 or 0.01 percent. Therefore, the mole fraction
of CO is so small that it will not significantly change the moles of oxygen or the total
moles in the mixture.
Slide 10-18
CONVERSION OF GAS CONCENTRATIONS TO 7% OXYGEN
Let: COB = 100 ppm
02m = 11.8% (dry gas)
CO (& 7% 02) = CO. x (21 - 7)/(21 - O2m )
= 100 x (14)/(21 - 11.8)
= 152 ppm & 7% 02
10-11
-------
The correction of the CO concentration to the standard dilution rate of 7% O2
is a straightforward calculation. In the above example, the measured 100 ppm of
carbon monoxide became 152 ppm when corrected to 1% oxygen.
Slide 10-19
CONVERSION OP PARTICULATES TO 7% OXYGEN
Let: PM. = 0.035 gr/dscf (Particulate Matter)
O2> = 11.8% (Measured Dry Gas O2 )
PM (@ 7% 02) = PM. x (21 - 7)/(21 - O21B )
= 0.035 x (14)/(21 - 11.8)
= 0.0533 gr/dscf & 7% O2
Particulate concentrations which are presented on a mass per unit volume
basis can be corrected by the same equation as before. Particulate loadings are a
case in point, regardless of whether they are measured in [gr/dscf] or [mg/dscm].
Slide 10-20
EQUATION FOR CONVERTING TO 12% CARBON DIOXIDE9
Assume CO.
CO
2m
is the Measured Dry Gas CO
Expressed as a ppm or %
is the Measured Dry Gas CO2
Expressed as a Percentage
CO (@ 12% C02 ) = COB x (12/CO2m)
Gas concentrations (e.g., CO in the above slide) can also be corrected to 12%
CO2 by multiplying the measured concentration by the ratio of 12% divided by the
measured percentage of C02. As before, the variables with a subscript "m" are the
measured dry gas concentrations. If the gas concentration data were to be obtained
using in situ instruments, the values will be on a wet basis, so that corrections to a
dry basis would require dividing original concentrations by (1.0 - moisture fraction).
The derivation of the above conversion equation is based on theoretical mixture
considerations, including the assumption that the percentage of CO is much less than
the percentage of C02, which is valid for most combustion product gas samples.
10-12
-------
The equation is consistent with the fact that if the measured CO2 were less
than 12%, the mixture volume would be larger than that at 12% CO2, so that the gas
concentration would be less than the standard at 12% CO2. Therefore, the correction
is obtained by multiplying by 12 dividing by the measured percent of CO2.
Slide 10-21
EXAMPLE CONVERSION TO 12% CARBON DIOXIDE9
Let: CO. = 100 ppm
COa, « 7.86% (dry gas)
CO <@ 12% C0a ) = CO, x (12/C02m) « 100 x (12/7.86)
» 153 ppm & 12% C02
Consider the previous example of incomplete combustion with 100 ppm CO (or
0.01 percent) measured on a dry flue gas basis. The correction of the CO
concentration to the standard dilution of 12% CO2 yields 153 ppm of carbon monoxide
when corrected to 12% carbon dioxide. Note that for medical waste, corrections to
12% C02 give approximately the same values as corrections to 7% O2.
Slide 10-22
CONVERSION OF [gr/dscf] TO [mg/dscm]
Basic Identities:
1 pound [Ib] = 7,000 grains [gr]
1 pound [Ib] ^ 453.6 grams [g]
1 gram [g] = 1,000 milligrams [mg]
1 foot [ft] = 0.3048 meters [m]
For Dry Gases at Standard Conditions:
1 dry standard cubic foot = 1 [dscf]
1 dry standard cubic meter = 1 [dscm]
1 cubic ft [dscf] = 0.0283 cubic meters [dscm]
So That:
1 [gr/dscf] «
1 [gr/dscf] x (1 lb/7000 gr) x (454 g/lb)
x (1000 mg/g) x (1 dscf/O.0283 dscm)
Therefore:
1 [gr/dscf] = 2,290 [mg/dscm]
10-13
-------
Many of the current regulations are in System International (SI) Units. For
example, the NSPS standard for MWC metals is given in terms of particulate matter
measured in [mg/dscm]. Other particulate standards are expressed in [gr/dscfj units,
where there are 7,000 grains in a pound. The abbreviation, dscf, refers to dry
standard cubic foot, and dscm refers to dry standard cubic meter.
The factor for converting expressions in units of [gr/dscfj to units of [mg/dscm]
is presented above. Conversion factors are developed using various identities and
simple multiplication and division. The theoretical basis for conversion factors
relates to the fact that the quantity obtained by dividing one side of an identity by
the other can be treated as unity. For example, an expression can be multiplied by
(1 lb/7000 gr) without changing its value.
Slide 10-23
EXAMPLE APPLICATION OF THE CONVERSION FACTOR
Factor: 1 [gr/dscf] = 2,290 [mg/dscm]
Given: 34 [mg/dscm]
Therefore: 34 [mg/dscm] =
34 [mg/dscm] x (1 [gr/dscf]/2,290 [mg/dscm])
34 [mg/dscm] = 0.015 [gr/dscf]
The application of the conversion factor, 1 [gr/dscfj = 2,290 [mg/dscm], is
demonstrated in the above slide. The given particulate concentration of 34 [mg/dscm]
is shown to be equivalent to a concentration of 0.015 [gr/dscf].
Note also, if it were desired to convert a given quantity expressed in [gr/dscfj
units into [mg/dscm] units, the quantity would need to be multiplied by (2,290
[mg/dscmyi [gr/dscf]).
10-14
-------
Slide 10-24
EQUATION FOR COMBUSTION EFFICIENCY
(BASED ON CARBON COMBUSTION TO COa)
C.E.(%) = (100% x CO2m) / (COj, + CO.)
or
C.E.(%)'= 100% x (1 - (CO. / (C02M + CO,))
Many state regulations require a minimum combustion efficiency to avoid a
fine and mandatory shut-down. There are two equivalent forms of the combustion
efficiency equation, both of which are illustrated in the above slide. Both equations
are referred to as the carbon combustion efficiency, but they actually measure the
carbon monoxide combustion efficiency.
Slide 10-25
EXAMPLE COMBUSTION EFFICIENCY CALCULATION
Let CO2n be 7.86 Percent
C0m be 0.01 Percent (100 ppm)
= (100% x C02a)/(C02in + C0m)
= (100% x 7.86)7(7.86 + 0.01)
= 99.9%
The above slide provides an example calculation of combustion efficiency. Note
that CO is generally measured as ppm, requiring conversion to a percentage. Values
of combustion efficiency greater than 99.9% are typical of most MWIs.
10-15
-------
Slide 10-26
DETERMINATION OF EXCESS AIR
FROM DRY GAS ANALYSIS
Ass\ime CO2, is the Percent Dry Gas CO2
COm is the Percent Dry Gas CO
Oa, is the Percent Dry Gas O2
Therefore Nj. • 100 - (COj,'* CO, + Oj,)
And EA = (02, - 0.5 COm)/(.264 N2, - O2m + 0.5 CO.)
The average amount of excess air can be determined from a set of dry gas
measurements for oxygen, carbon dioxide, and carbon monoxide. The procedure
includes the assumption that the gas concentrations are expressed as a percentage
and that sulfur oxides, nitrogen oxides, and hydrogen chlorine are small enough to
be neglected. Therefore, the nitrogen in the dry product gas can be determined by
subtracting each of the other percentages from 100 percent.
The percentage of excess air can be found using the indicated equation, which
is derived using theoretical considerations.9 Note that COm is included in the above
equations. However, when CO is expressed on a percentage basis, it generally has
a trivial influence on the excess air calculation. In the example of Slide 10-11, the
CO value of 100 ppm corresponds to 0.01%, which is much smaller than the other
values used in the equation.
Slide 10-27
EXAMPLE DETERMINING EXCESS AIR
Let C02tt = 7.86%
com
'2m
0.01%
11.8%
Therefore N2, = 100 - (CO2, + CO, + O2J
N2, « 100 - (7.86 + 0.01 + 11.8) = 80.33
And EA = (02m - 0.5 CO,)/(.264 N2, - O2m + 0.5 CO,)
EA = (11.8 - 0.005)/(.264 x 80.33 - 11.8 + 0.005)
EA = 1.25 --> 125% Excess Air
10-16
-------
The basic excess air calculation process is illustrated using the previous
example of medical waste combustion. The above calculation for excess air gives the
value of 125 percent. We know that this calculation is correct, since our original
numbers (from Slide 10-17) were based on 125% EA.
Values of excess air in the stack gas of a MWI vary widely. Variation depends
upon the combustion conditions. During active burning, most controlled-air MWIs
operate with from around 100% to 150%. This range is also applicable for waterwall
excess air units.
In addition, more excess air is typically used during the burn-down and cool-
down periods at the end of the burn-down cycle. For example, data indicating excess
air values as high as 900% have been reported for a single batch-fired, excess-air
multiple chamber unit.10
10-17
-------
REFERENCES
1. P. J. Schindler and L. P. Nelson, Municipal Waste Combustion Assessment:
Technical Basis for Good Combustion Practice. U. S. Environmental Protection
Agency, EPA-600/8-89-063, August 1989.
2. W. R. Seeker, W. S. Lanier, and M. P. Heap, Municipal Waste Combustion
Study, Combustion Control of Organic Emissions. U.S. Environmental
Protection Agency, EPA-530-SW-87-021-C, June 1987, pp. 4-1 to 4-8.
3. "Municipal Waste Combustion Study, Report to Congress," U.S. Environmental
Protection Agency, EPA-530-SW-87-021-a, June 1987, pp. 42-61.
4. J. D. Kilgroe, W. S. Lanier, and T. R. Von Alten, "Montgomery County South
Incinerator Test Project: Formation, Emission and Control of Organic
Pollutants," presented to Second International Conference on Municipal Waste
Combustion, Tampa, Florida, April 16-19, 1991.
5. "National Incinerator Testing and Evaluation Program, Mass Burning
Technology, Quebec City," Volume II, Main Report, Prepared by Lavalin Inc.,
for Environment Canada, Conservation and Protection, December 1987, pp. 79-
287.
6. H. Vogg, J. Metzger, and L. Stieglitz, "Recent Findings on the Formation and
Decomposition of PCDD/PCDF in Solid Municipal Waste Incineration",
Proceedings of Emissions of Trace Organics From Municipal Waste
Incinerators. Specialized Seminar, Part 1, Sec. 2, Copenhagen, January 1987.
7. U. S. Environmental Protection Agency, "Standards of Performance for New
Stationary Sources; Municipal Waste Combustors", Federal Register, Vol. 56,
No. 28. February 11, 1991, pp. 5488-5514.
8. U. S. Environmental Protection Agency, "Emission Guidelines; Municipal
Waste Combustors," Federal Register. Vol. 56, No. 28. February 11, 1991, pp.
5514-5527.
9. J. T. Beard, F. A. lachetta, and L. U. Lilleleht, APTI Course 427. Combustion
Evaluation. Student Manual. U. S. Environmental Protection Agency, EPA-
450/2-80-063, February 1980, pp. 5-4 to 5-21.
10. Peter Torkelson, "Report on the Assessment of Operations and Emissions of
On-Site Medical Waste Incinerators," Minnesota Pollution Control Agency,
December 1991, pp. 1-6.
10-18
-------
11. INSTRUMENTATION I: GENERAL MEASUREMENTS
Slide 11-1
PURPOSE OF INSTRUMENTATION
1. Supervision of Operations
2. Automatic Control Signals
3. Management Data
4. Pollutant Emissions Surveillance
Instruments are installed in combustion systems to provide data about the
state of the unit's operating conditions. Such measurable information is vital to the
operator who must evaluate various aspects of unit performance, including the
identification of problem areas. Operators are routinely required to change manual
and/or automatic control system settings, consistent with accepted operational
procedures.
Predictions about the effect of possible operational changes can be aided by
examining the magnitudes of current data and past records. Operational parameter
data from various instruments may be required to develop a logical and consistent
picture of what is happening and what should be done.
Instruments also provide information used to drive automatic control systems.
Both the traditional pneumatic control systems and modern microcomputer-based
control systems are designed around the use of control signals from sensors or
instruments.
Access to selected operating data is required by corporate management.
Recorded data is used by management in determining unit production performance,
the need for equipment maintenance, and measures of cost accountability. For
example, flue gas temperature data can be used to estimate the degree of boiler
fouling and to establish proper boiler cleaning intervals.
In addition to written reports, some environmental control agencies require
access to the readings of selected continuous emission monitoring systems (GEMS)
through use of a continuous data telemetry system.
11-1
-------
Slide 11-2
GENERAL MEASUREMENTS
1. Temperature
2. Pressure
3. Flow Rate (Velocity)
4. Weight
The basic instrumental measurement techniques used in medical waste unit
operations are presented in this learning unit. The focus will be on the equipment
operating principles used in general measurements.
The measurement of gas concentrations by continuous emission monitoring
devices will be presented in Learning Unit 12.
Slide 11-3
TEMPERATURE EQUIVALENTS
°C = (5/9) (°F - 32)
°F = (9/5) °C + 32
°K (Kelvin) = °C + 273.15
°R (Rankine) = °F + 459.67
Temperatures can be expressed in different units: Fahrenheit, Celsius,
Rankine, and Kelvin. The conversion factors for these units are given above.
Slide 11-4
TEMPERATURE MEASUREMENTS
Thermometer - Expansion of a Liquid
Dial Thermometer - Expansion of Metals
Thermocouple - Thermoelectric Potential
Thermistor/RTD - Electrical Resistance
Infrared Temperature Probe - Infrared Energy
Optical Pyrometer - Infrared Energy
Temperature Paint - Change of Color
11-2
-------
The mercury-in-glass thermometer has been commonly used for many years.
As the temperature increases, the mercury in the bulb expands and rises in a thin
capillary calibrated stem. Mercury thermometers can range from -30 to +900° F.
Thermistors are often used for low temperature applications. Thermistors are
made from semiconductor materials whose electrical resistance varies inversely with
temperature, and RTDs are made from conductors whose resistance increases with
temperature. Appropriate probes and read-out devices are available which directly
indicate temperatures. The devices usually include circuits for automatic calibration.
Infrared temperature probes and optical pyrometers utilize the characteristic
dependence of infrared energy on the temperature of the surface. These are often
used to indicate the effective temperature of hot surfaces such as refractory, molten
metal, and fuel beds. Temperature paints make use of an optical phenomena
whereby some materials undergo a color change which is dependent upon their
temperature. An object coated with such a paint will change colors upon being
heated above a certain temperature range.
Slide 11-5
THERMOCOUPLE TEMPERATURE MEASUREMENT DEVICE1
Millivoltmeter
(cold junction^
compensation)
/Iron
^<*r|Tr
Ł^j
^ >• Lead wire
C=^
^Cold
junction
Hot
junction
v.Constantan
From Robert T. Corry et al., "Instruments and Control", Mark's
Standard Handbook for Mechanical Engineers. Eighth Edition, Edited
by T. Baumeister, et al., McGraw Hill Book Company, NY, 1978,
reprinted with permission.
Thermocouples are commonly used for measuring combustion chamber
temperatures. When the ends of two different metals are joined together, a small
electrical voltage can be produced which is proportional to the "hot junction"
temperature. Combinations of various metals can be selected depending upon the
junction temperature range and the desired accuracy. Type J (iron-constantan), type
K (chromel-alumel) and type T (copper-constantan) thermocouples are often used.
11-3
-------
The thermocouple's voltage can be measured by a special millivoltmeter in a
read-out instrument. A typical read-out device provides an electrical circuit which
requires the opposite ends of each thermocouple wire to be attached to special "lead
wires" (extension wires) which are connected to the "cold junctions" or reference
junctions of the read-out unit. The "cold junction" temperatures must be properly
compensated in the unit's circuit to determine the thermocouple's temperature.
Combustion temperatures are typically indicated through the use of
thermocouples which measure the temperature of the adjacent flowing gases.
Thermocouples can be obtained in protective sheathes which protects the hot
junctions from corrosion and/or oxidation. Thermocouples routinely fail after
extended exposure to combustion conditions or rapidly changing temperatures.
Broken thermocouples can be routinely replaced. Slag deposits on thermocouples will
act as insulation, causing the thermocouples to give low readings.
Thermocouple readings can be in error due to radiant heat gains or losses
which cause the thermocouple to actually be at a higher or lower temperature than
the gas it is trying to measure. Thermocouples which can "see" a hot flame or a cold
region (e.g., the convection section of a heat exchanger) will give erroneous readings.
Shielded thermocouples should be used in such applications. The depth of the
thermocouple penetration into the chamber also influences its accuracy, as short
thermocouples will have larger errors due to conduction cooling by the refractory.
Slide 11-6
PRESSURE MEASUREMENTS
Manometers - Height of a Column of Liquid
D-Tube/ Single-Leg, Inclined
Bourdon Tube Gages - Bending of a Curved Tube
Mechanical/Electrical Devices
Diaphragm Gages
Bellows Gages
Differential Pressure (DP) Cells
Linear Variable Dif. Transformers (LVDTs)
Pressure is the force per unit area exerted by a fluid, which is typically
indicated in units of lb/in2 or psi. Pressure is generally measured with a gage relative
to the atmospheric pressure. Therefore, the value of absolute pressure (indicated as
"psia") is equal to gage pressure ("psig") plus atmospheric pressure ("patm").
Traditionally, the "gage pressure" is the positive amount of pressure above
atmospheric conditions, whereas the negative gage pressure is a vacuum pressure
measured below atmospheric pressure.
11-4
-------
Slide 11-7
MANOMETER PRESSURE MEASUREMENTS2
P* P,
a
h
. 3
- 4
- 3
- 2
- 1
--0-
1 -
2-j
p
> Fluid 1
^Low density
ct*
f Fluid 2
High density
10
9
8
7
6
5
4
3 ^1
i-djh*
•••••
U-Tube Manometer Single-Leg Manometer
From Edgar E. Ambrosius et al., Mechanical Measurement and
Instrumentation. Ronald Press, New York, 1966, printed with
permission.
The simplest pressure measuring instrument is the U-tube differential
manometer. The U-tube manometer directly indicates the difference between a
pressure in two low density fluid regions as the vertical height of a column of high
density, indicator liquid. When pressure differences are measured, one leg of the
manometer will have more liquid than the other, and the height difference is referred
to as the "height of the column."
Note that water is the liquid most often used in power plant pressure
measurements. The corresponding pressure difference is expressed as "inches of water
column", which can be abbreviated as "in. we." Although mercury has often been
used in the past, its use is discouraged because of concern about spillage.
Single-leg and inclined manometers operate on the same basis as a U-tube
unit, but the construction is such that all the height differential occurs in one tube.
This is particularly true if the diameter of the well, d1} is much greater than the
diameter of the leg, d2. An inclined manometer operates as a single-leg manometer
with an extra horizontal displacement that provides for greater accuracy.
11-5
-------
Slide 11-8
BOURDON TUBE CAGE1
Bourbon tube
Scale
Pointer
Cose
From Robert T. Corry et al., "Instruments and Control", Mark's
Standard Handbook for Mechanical Engineers. Eighth Edition, Edited
by T. Baumeister, et al., McGraw Hill Book Company, NY, 1978,
reprinted with permission.
A Bourdon tube pressure gage is the most common pressure measuring device.
The operating principle of the Bourdon gage is that a flattened and curved tube will
change its curvature as internal pressure increases. One end of the tube is fixed, so
the displacement of the other end is transmitted through mechanical linkages to a
pointer. Pressure readings are proportional to the pointer displacement.
Bourdon gages are often used to measure high pressures of pumps and other
equipment. Generally, these applications have the gage located on the equipment.
If Bourdon gages on a central control panel were to be used for sensing the pressure
at a remote location, high pressure transmission lines would be required from the
equipment to the control panel.
Slide 11-9
PRESSURE TRANSMITTER
Low Voltage Electrical and
Low Pressure Pneumatic Signals
Easy to Transmit
Safety Considerations
11-6
-------
Safety and operational considerations in many applications require that either
a low pressure or low voltage device be used to transmit high pressure signals to
control panels.
Low pressure pneumatic devices were very popular in the past, but today's
trend is to use low voltage electrical pressure transducers to transmit an analog
signal to a control panel's read-out device.
Slide 11-10
LVDT DIFFERENTIAL PRESSURE CELL3
From J. P. Holman, Experimental Methods for Engineers. McGraw Hill
Book Company, New York, Fifth Edition, 1989, printed with permission.
Differential pressure (DP) cells, diaphragms and bellows gages are very similar
in their basic measurement concept and have design options of producing mechanical
analog readings or electrical signals. Diaphragm and bellows devices will expand
upon an increase in pressure in much the same way as the elastic metal tube of the
Bourdon gage.
Mechanisms can be used to translate the linear expansion into an indicator
displacement. These instruments are often applied in low pressure applications.
Properly selected springs are required to restore the devices to a zero reading for
atmospheric pressure or no pressure difference.
A DP cell may use a linear variable differential transformer (LVDT) circuit, as
shown in the illustration. The LVDT provides an electrical signal which is
proportional to the diaphragm displacement. Other units may have a capacitance
pick-up designed to provide a linear signal of pressure variations.
11-7
-------
Slide 11-11
MEASUREMENT OF FLUID FLOW
MEASURING DEVICE
Pitot Static Tube
Orifice Plate
Venturi
Propeller-Type
Rotameter
APPLICATION
Combustion Air Flow
High Steam & Water Flow
(Large Pressure Drop)
High Steam & Water Flow
(Small Pressure Drop)
Medium Air 6 Water Flow
Low Water Flow
There are a number of different methods available for measuring the flow of
fluids. The instruments generally measure some disturbance to the fluid which is
proportional to the velocity.
Slide 11-12
PITOT STATIC TUBE1
r— Static opening
- J.
opening
Manometer •
From Robert T. Cony et al., "Instruments and Control", Mark's
Standard Handbook for Mechanical Engineers. Eighth Edition, Edited
by T. Baumeister, et al., McGraw Hill Book Company, NY, 1978,
reprinted with permission.
11-8
-------
The pitot static tube is a device which uses a manometer to measure the
difference between the dynamic pressure and the static pressure at a point. Fluid
flow theory can be used to establish that this pressure difference is directly related
to the fluid velocity at the point of measurement.
A slight modification of the pitot tube device is used as part of the standard
EPA Method 5 stack sampling equipment. One should note that pitot tubes are used
to measure the instantaneous velocity at a point, rather than the average fluid
velocity in a duct, which would generally be more useful.
Slide 11-13
ORIFICE PLATE - PRESSURE DIFFERENCE1
Flange
Orifice plate
Upstream
tap
•Vena contracta
Downstream tap
From Robert T. Corry et al., "Instruments and Control", Mark's
Standard Handbook for Mechanical Engineers. Eighth Edition, Edited
by T. Baumeister, et al., McGraw Hill Book Company, NY, 1978,
reprinted with permission.
Average flow rates can be measured with orifice plates, flow nozzles, and
venturi tubes. Each of these will cause a disturbance in the flow which can be
measured as a pressure drop between upstream and downstream locations. As the
flow increases, the pressure drop increases.
A differential manometer or pressure transducer can be used to provide the
flow rate, which can be indicated as a velocity (e.g., ft/sec), volumetric flow rate (e.g.,
cu-ft/min), or mass flow rate (e.g., Ib/hr).
11-9
-------
Some form of calibration is generally required because the pressure differences
for a given flow will depend upon the installation as well as the type of flow
restriction. Steam flow is often measured with orifice plates designed to conform to
an ASME design standard. When specified measurement locations are provided,
standard calibration curves and correction factor relationships may be used.
Slide 11-14
PROPELLER TYPE FLOWMETER1
Magnetic 5?
sensing elemenK
From Robert T. Corry et al., "Instruments and Control", Mark's
Standard Handbook for Mechanical Engineers. Eighth Edition, Edited
by T. Baumeister, et al., McGraw Hill Book Company, NY, 1978,
reprinted with permission.
A propeller mounted in the flow will spin faster as the velocity increases.
Flowmeters using this principle can measure the turning speed either through a
mechanical linkage or magnetic sensor coupled with a counter or tachometer.
Turbine flow meters operate on this principle and provide accurate measurements.
Rotameters, illustrated in the following slide, operate on the principle of
balancing gravitational forces with drag forces. The weight of a float placed in a
tapered tube is balanced by the drag force of the fluid on the float. Fluid is directed
upward in a tapered tube, which causes the velocity to decrease as height increases.
Since the weight of the float is fixed, the drag forces will cause the float to stabilize
at a point where the velocity is appropriate. If the bulk fluid velocity is increased,
the float will move upward and the height of the float will be proportional to the
velocity.
11-10
-------
Slide 11-15
ROTAMETER1
Net
Rotameter
tube
Metering
float
Scale
From Robert T. Corry et al., "Instruments and Control", Mark's
Standard Handbook for Mechanical Engineers. Eighth Edition, Edited
by T. Baumeister, et al., McGraw Hill Book Company, NY, 1978,
reprinted with permission.
Slide 11-16
EQUAL ARM BALANCE2
From Edgar E. Ambrosius et al., Mechanical Measurement and
Instrumentation. Ronald Press, New York, 1966, printed with
permission.
11-11
-------
Weight scale equipment falls into two major categories: balances and force-
deflection devices. The simplest form of scales is the equal-arm balance, which is
commonly found in a laboratory. In this example, an unknown weight, Mu, is
determined by adding up the various standard weights, Ms, which are required to
make the balance indicator point to zero.
Slide 11-17
]
r-i/^"
<&=
n
Adjustable '
for balance j
purposes
<
c
PLATFORM SCALE LEVER SYSTEM2
Uh* * k - H
-b- • /, ^ .
^^•w ^ — ^v v
~5 CH ^/^"rT"0
=1 0 1 i 3 4/ 6 7 8 9 10 y F
^ Beam^
W2 non !
weights ..
f Wj Wj
2 2
W, I
| \ ts- Platform
/
From Edgar E. Ambrosius et al., Mechanical Measurement and
Instrumentation, Ronald Press, New York, 1966, printed with
permission.
Large weight scales are based on a modification of the principles used in
the equal arm balance scales. The difference is that linkages are used so that
mechanical advantages allow for the use of proportionality, rather than direct weight
balancing.
11-12
-------
REFERENCES
1. Robert T. Corry et al., "Instruments and Control", Mark's Standard Handbook
for Mechanical Engineers. Eighth Edition, Edited by T. Baumeister, et al.,
McGraw Hill Book Company, NY, 1978, pp. 16-9 to 16-16.
2. Edgar E. Ambrosius et al., Mechanical Measurement and Instrumentation.
Ronald Press, New York, 1966, pp. 252-255, 360-361.
3. J. P. Holman, Experimental Methods for Engineers. McGraw Hill Book
Company, New York, Fifth Edition, 1989, p. 213.
11-13
-------
12. INSTRUMENTATION II: CONTINUOUS EMISSION MONITORING
Slide 12-1
CONTINUOUS EMISSION MONITORING SYSTEMS
Temperature
* Primary & Secondary Chambers
* Temperature of Flue Gas into APCD
Fluid Flow Rate (Velocity)
Opacity
Concentrations of Gases
* Carbon Monoxide
* Hydrogen Chloride, Sulfur Dioxide
* Oxygen/ Carbon Dioxide
This learning unit presents the design and operational features of continuous
emission monitoring systems, CEMS, which are used to measure stack opacity and
the concentrations of selected gases. Learning Unit 11 presented various types of
general instrumentation which can be used to provide operating and control
information. Included were the monitoring instruments for fluid flow and
temperature typically required at MWIs.
CEMS requirements depend upon the regulatory agency, unit size, and
whether the unit is new or existing. At the time of this writing, the NSPS and EG
for medical waste incinerators had not yet been promulgated.
Federal regulations applicable to CEMS include daily calibration drift tests,
accuracy tests, accuracy audits, and gas cylinder audits.1'2 Relative Accuracy Test
Audits (RATA) are required each year, using the applicable Performance Specification
Tests (PST) procedures. Up to three quarterly Relative Accuracy Audits (RAA) are
also required each year.
Slide 12-2
INDICATORS OF COMBUSTION CONDITIONS
Opacity
Carbon Monoxide
Carbon Dioxide
Oxygen
Primary Chamber Temperature
Secondary Chamber Temperature
Draft in Primary Chamber
12-1
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Combustion conditions can be evaluated through use of a number of
measurable parameters. The specific set of GEMS will vary with the applicable
regulations and from one unit to another. Small units tend to have fewer monitored
parameters than larger units. As discussed in Learning Unit 9, various combustion
parameters will also be used as process variables in automatic control systems.
Gas concentrations are typically measured at some point in the flue gas
stream, downstream of the secondary combustion chamber. Combustion chamber
temperatures are often measured with thermocouples at a location selected to
represent the average, rather than peak combustion temperatures. Draft is listed as
an indicator of combustion conditions since systems with improper draft often produce
excessive emissions due to poor combustion quality.
Slide 12-3
TYPICAL MWI COMBUSTION INDICATOR RANGES
PARAMETER
Opacity, %
Primary Temp.,°F
Secondary Temp . , °F
Upper Limit, °F
Draft, in w.c.
with Door Open, in w.c.
Baghouse inlet Temp . , °F
Oxygen, %
Carbon Monoxide, ppm
LOW
0
1,000
1,800
2,200
0.03
300
11
0
HIGH
10
2,000
2,200
2,300
0.15
0.00
450
15
100
The range of parameters listed above are typical values for acceptable
combustion conditions in starved-air MWI units.3 For example, a starved-air unit
operating steadily with more than 15 percent oxygen in the flue gas would probably
be using too much excess air.
Slide 12-4
CATEGORIES OF CEMS
In-situ:
Stack Mounted Analyzer
Extractive:
Sample Flows to Remote Analyzer
12-2
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Monitoring systems are categorized as either in-situ CEMS or extractive GEMS
according to the location of the gas analyzer.*
The in-situ systems have the sensors and analyzers mounted in or adjacent to
the gas stream. The typical in-situ analyzers utilize an energy (light) source which
is directed in a beam across the stack to a detector. The desired measurement is
often based on the reduction of energy, associated with either gas absorption or
particle scattering. The detector typically produces an electrical signal whose
strength is proportional to the energy received.
The extractive systems remove a continuous sample using probes, gas
conditioning equipment, and tubing which transports the sample to a remote
analyzer.
The measurement concepts of dispersive and non-dispersive absorption devices,
which are used in both in-situ and extractive devices used to measure gas
concentration, will be presented later in this Learning Unit.
Slide 12-5
SINGLE-PASS OPACITY IN-SITU CEMS
Cotlimating
tens
\
Collimating
lens
( BMiiitiitiiiiiiiiitiiiiiiiiliiiiiimiitiiiiiiiiiuiiiiiiiiiiiiiiniiiiii
Detector
mm.
Rotary blower
From J. A. Moore, "Key Measurements in Power Plants," Standard
Handbook of Power Plant Engineering. Thomas C. Elliott, editor,
McGraw Hill Book Co., NY, 1989, printed with permission.
Opacity is defined as the amount of attenuation of a visible light beam as it
passes through stack gases. The visible light attenuation by smoke is primarily due
to the scattering of light by small particulates.
12-3
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Opacity GEMS are the simplest of the in-situ monitoring devices. Continuous
opacity measurements are made at a fixed line through the flue gas as it passes up
the stack.
The single pass transmissometer incorporates a light source, collimating and
focusing lens, and a detector.4 A linear detector is required to measure the intensity
of light transmitted through the stack. The ratio of the detector signal to that at zero
opacity is the stack transmittance. The opacity is equivalent to 1.0 minus the
transmittance. This fraction is generally expressed as a percentage.
Before such devices were developed, visual opacity measurements were made
by individual smoke readers who observed the flue gas plume leaving the top of the
stack. Legally binding visual opacity observations can be made by smoke readers
who are certified through standard visual emissions examinations. However, visual
opacity observations have limited precision and require a subjective accommodation
for different atmospheric conditions.
Slide 12-6
DOUBLE-PASS TRANSMISSOMETER CEMS
Call/mating
lens
Light
source
Beam
splitter
Reflecting
mirror
I^IHtnH1MIIIHIMIIHIIinilHIIMIHtllllltllftlll'll'>lll*(l"lttllimt|tlltMIIHMIIIHIIIItlHHHmillllll|
YlllllllllllllllllllimilllllilllimillUlltllllllllll'llllllilltimillllllllNHIIIIIIIIII IIIHIIrmillll I
ACROSS-STACK
\
Rotary blower
J. A. Moore, "Key Measurements in Power Plants," Standard Handbook
of Power Plant Engineering. Thomas C. Elliott, editor, McGraw Hill
Book Co., NY, 1989,
Double-pass transmissometer systems have various advantages. A chopper is
used to pulse the light passing through the stack. The light is reflected back across
12-4
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the stack and onto the detector. A controlled reference beam is also directed onto the
detector. The alternating signal from the detector at the chopper frequency provides
a measurement which is proportional to transmittance and insensitive to ambient
light. Calibration at zero and span values is obtained through the use of standard
filters which can be inserted into the path of the beam.
Although fans are provided to limit the accumulation of deposits on the optical
surfaces, routine maintenance includes cleaning of such surfaces and recalibration.
Slide 12-7
IN-SITO MONOCHROMATOR FOR GAS CONCENTRATION5
LIGHT
SOURCE
PHOTODETECTOR
BLOWER
CHOPPER
A cross-stack in-situ device for measuring for gas concentration is illustrated
above. Gas concentration measurements differ from opacity in that opacity uses the
entire visible light spectrum, whereas only selected wave lengths are used for gas
concentrations. Such measurements may use a monochromator system as illustrated
in Slide 12-7. Measurements are based on the concept of absorption spectroscopy in
which each gas absorbs infrared energy at characteristic wave lengths or regions in
the spectrum. The diffraction grating works like a prism to separate the beam of
light into wavelengths of interest.
Other in-situ monitors make use of probes which are inserted into the stack.
Such probes are designed to operate as a double-pass gas absorption cell. They are
surrounded by a ceramic or stainless steel filter which allows gases to diffuse into the
cell but prevents particulate contamination of internal optics. Typical probe lengths
range from 5 cm to 1 m. Calibration is obtained by using solenoid valves which can
allow either a span gas of known concentration or fresh air to purge the probe.
12-5
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Slide 12-8
EXTRACTION TYPE OP GAS ANALYZER
Extraction of Gas Sample by a Probe
Removal of Particulates
Removal or Compensation for Water
Transport to Remote Detector/Analyzer
Conversion from Wet Basis to Dry Basis
Extractive CEMS use probes to remove a continuous gas sample. Multiple
probes can be used to provide for the spatial averaging.
The collected gas sample is directed through special conditioning equipment
before being delivered to a remote analyzer. Otherwise, the particulates and
condensate could cause blockages in the pipes and/or chemical changes which would
impair the validity of the measurement. Conditioning generally includes filtration to
remove particulate and a refrigeration drying, dilution, or heating system which
eliminates acid and water condensation.
Many extractive systems provide measurements on a dry gas basis, whereas
in-situ systems typically make measurements on a wet basis. Concentrations on a
wet basis are smaller than the corresponding dry basis values. The concentrations
measured on a wet basis (in-situ insturments) may be converted to a dry basis by
dividing by (1.0 minus the moisture fraction in the flue gas).
Slide 12-9
WATER REMOVAL OR COMPENSATION SYSTEMS
1. Desiccant
2. Refrigeration
3. Dilution
4. Heating of Sample Line
The selection of the method for preventing water condensation depends upon
the type of component to be analyzed. For example, the refrigeration drying system
is not suitable for measuring HC1 because condensed water absorbs HC1, resulting
in an incorrect measurement by the analyzer. Therefore, HC1 sampling systems
typically use either a heat traced line (e.g., electrical heaters) to keep the gas
temperature above the dew point or a dilution type extraction probe. Heated sample
lines are also used to prevent the condensation of hydrocarbon gas when total
hydrocarbons are measured.
12-6
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The dilution probe system uses a supply of dry air to dilute the sampling gas
by a constant and known ratio (e.g., 100). A large amount of dilution is provided to
assure that the dew point of the diluted sample is low enough to prevent water
condensation. The diluted sample stream is analyzed by ambient level analyzers.
Since no moisture is removed from the sample, the measurements are on a wet basis.
After leaving the conditioner, the sample flows through a pipe to a conveniently
located analyzer. A problem with extractive systems is related to the time delay
which occurs between the taking of the sample and its arrival at the analyzer. If
proper procedures are not followed, chemical changes can occur between the time
when the sample is removed from the gas stream and when it is analyzed. Also, the
ducting system is vulnerable to plugging and leakage.
Slide 12-10
ABSORPTION SPECTROSCOPY
Dispersive Absorption
Differential Absorption
Nondispersive Absorption
Gas Filter Correlation Method
Absorption spectroscopy is used in both in-situ and extractive GEMS to
determine the presence and concentration of specific gases. The analytical techniques
are based on the fact that gases absorb energy at characteristic wavelength regions
of the energy spectrum. Some gases such as water vapor and carbon dioxide have
broad absorption bands, whereas other gases absorb in a narrow wavelength region.
The characteristic absorption wavelength for a gas is most often in the infrared
portion of the spectrum. However, some gases have characteristic absorption
wavelengths in visible and ultraviolet regions. Corresponding instruments are
commonly referred to as infrared analyzers and ultraviolet analyzers.
The influence of energy at all the other wavelengths can be averted by using
either non-dispersive or dispersive analyzers. Dispersive devices such as gratings or
prisms can be used to separate the energy into a beam having a single wavelength.
The wavelength can be properly selected to measure the concentration of the gas of
interest.
A traditional dispersive system would have an optical arrangement with a
sample gas cell and a reference gas cell which alternately receive the beam of light
before it is eventually focused onto the detector. The detector signal will be
dependent upon the amount of energy absorption by the gases in the respective cells.
The concentration of the gas in the sample can be obtained after calibration.
12-7
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Slide 12-11
DIFFERENTIAL ABSORPTION EXTRACTIVE CEMS5
Semtranspareni Mirer caitoraon SanvteCeU
Measuring (Beam Sputter)-\ RjUf SO2/NO,
Phototube.
Lamp
Recorder
Differential absorption measurements are alternatives to the traditional
dispersive energy absorption concept. A grating or special filter can be used to
disperse the energy into two beams having different wavelengths but equal
intensities. An application using a grating was illustrated in Slide 12-7. The above
slide illustrates an application using a filter with two different detectors.
One beam is selected at the characteristic wavelength of the gas in question
and the other wavelength is selected for a region where the gas is transparent. A
measurement of the concentration can be obtained from a detector signal which is
proportional to the ratio of transmitted energy at the two wavelengths.
A differential absorption in-situ monitor has been developed to allow a single
instrument to sequentially measure up to three of the following gases: SO2, NOx, CO,
CO2, and/or H2O. The design includes a filter wheel which sequentially selects the
appropriate wavelengths for detection of each applicable gas. Special provisions in
the instrument can compensate for background electrical noise, temperature and
pressure influences, interference from other gases, and the non-linearity of the
detector.
12-8
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Slide 12-12
NONDISPERSIVE INFRARED EXTRACTIVE GEMS6
Sensor
Infrared
Source
Nondispersive infrared analyzers are used extensively for measuring the
concentrations of CO, C02, NO, and SO2. Nondispersive ultraviolet analyzers can be
used for the direct measurement of N02. Nondispersive infrared and ultraviolet
analyzers operate on the same principles, although the filtering devices and detectors
are different.
Nondispersive analyzers make use of a sample cell containing the flue gas, a
reference cell which does not have any of the specific gas, and a detector. The
detector is designed with two regions which are filled with a significant concentration
of the specific gas. The two regions of the detector are separated by a diaphragm.
The energy absorbed by gas in the section of the detector adjacent to the
sample cell is less than that of the section adjacent to the reference cell because of
energy absorption by the gas in the sample cell. The different levels of absorbed
energy cause the diaphragm to oscillate.
By alternately passing a beam of energy through the sample and reference
cells, the position of the diaphragm will oscillate. Such changes can be measured by
a microphone-type sensor. The signal can be calibrated to give measurements of the
concentration of the specific gas.
12-9
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Slide 12-13
NONDISPERSIVE GAS FILTER CORRELATION GEMS'
LIGHT
SOURCE
STACK
o
BLOWER
BEAM
ALTERNATOR
\ NEUTRAL FILTER OETECTOR
GAS-FILTER
CORRELATION
CELL
ELECTRONICS
Another nondispersive concept is used in the gas filter correlation method.
After passing through the stack, the beam is split. One beam passes through a
neutral filter and the other passes through a gas filter cell containing a significant
quantity of the specific gas. The neutral filter is selected so that the detector signals
are equal when the specific gas is not found in the stack. When the specific gas is
in the stack, the energy transmitted through the neutral filter will be decreased, but
the energy passing through the gas filter cell will not change. A detector can
measure this energy difference.
Calibration is often performed by inserting a gas-filter cell of known
concentration into the optical path between the light source and the unit.
Slide 12-14
OTHER ANALYTICAL TECHNIQUES
Chemiluminescence
Electrocatalytic
There are various other analytical techniques which are available for use in
determining gas concentrations. Chemiluminescence is often used for NOx
measurements, and oxygen is often measured using a electrocatalytic analyzer.
12-10
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Slide 12-15
CHEMILDMINESCBNCE ANALYZER5
Light is emitted by the chemiluminescent reaction which occurs when nitric
oxide molecules react with ozone. An ozone generator uses oxygen to make ozone,
which is then mixed with nitric oxide in a reaction chamber. If adequate ozone is
available, the light given off will be proportional to the concentration of nitric oxide.
Chemiluminescence analyzers typically use a photomultiplier tube to measure the
light given off by the reaction.
In order for the instrument to measure nitrogen dioxide, a catalytic reduction
of nitrogen dioxide to nitric oxide is first required. One molecule of nitric oxide is
produced for each molecule of nitrogen dioxide, so the light signal will be proportional
to the total of nitrogen dioxide and nitric oxide in the original gas sample. The
amount of nitrogen dioxide can be obtained by subtracting the results from runs with
the catalytic converter operational from those with it not operational.
An N02 to NO converter efficiency test is generally established by injecting a
known concentration of N02 into the supply line. These tests are typically run for 30
minutes to determine any degradation in the ozone driven N02 to NO conversion
process.
12-11
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Slide 12-16
BLECTROCATALYTIC ANALYZER*
- e
Electrocatalytic analyzers are widely used for in-situ measurements of the
concentration of oxygen in a flue gas stream. The analyzer operates as a high
temperature fuel cell, producing an electrical current which is related to the oxygen
concentration.
A key component of the analyzer is special porous ceramic material made of
zirconium oxide. When it is heated to around 1,550 °F, the unit will catalytically
produce oxygen ions.6 Oxygen ions are oxygen atoms which have absorbed an
electron.
Air is supplied to one side of the ceramic and flue gas to the other. Oxygen
atoms in the air will diffuse through the ceramic material because of the difference
in oxygen concentration between the flue gas sample and ambient air. In the above
slide, the flue gas sample is on the left and ambient air is on the right. Upon
arriving at the electrode on the flue-gas side of the cell, the oxygen ions give up their
electrons to an electrode.
The resulting current produced is used for measuring oxygen concentrations.
The current is proportional to the concentration difference between air and flue gas
and is inversely proportional to the flue gas concentration.
12-12
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Slide 12-17
GAS ANALYZER MAINTENANCE PROCEDURES
Routine Calibration
Zero Gas or Filter
Span Gas or Filter
Delivery System Bias Checks
Probe Blockage
Probe Leaks
Electrical Circuit Problems
Component Replacement
The maintenance of GEMS includes routine calibration. Calibration of the
individual instruments can be achieved through the use of standard calibrated optical
filters or of zero and span gases, depending upon the design of the instrument.6 Span
gases must be analyzed periodically to assure their validity.7
The calibration procedure for each instrument is dependent upon its design.
Generally, an extraction CEMS must be zeroed and spanned using special bottled
gases of known concentrations. Nitrogen is normally selected as a zero gas, and it
must be free of moisture and oil. Span gases are selected to cover the concentration
ranges for the instruments being calibrated. They should have a reasonable shelf
life without changing concentration. CEMS are normally zeroed and spanned at least
once a day, with some systems performing the function automatically.
The blockage of gas sampling probes and lines can be a serious problem,
particularly if the flue gas contains significant fly ash or hydrated lime (e.g., from dry
scrubber) concentrations. Filters which are used to remove particulates must be
cleaned or replaced routinely. If the temperature of the moisture trap is not properly
regulated, the condensate may freeze, forming a blockage of the sampling stream.
Other problems related to the condensation of moisture and acid gases can
occur. These liquids can absorb gases and cause chemical reactions which lead to
scale build-up and corrosion.
Leakage of gases into tubes transporting collected samples to analyzers can
occur if the pipe joints are not properly sealed. Vibration and thermal expansion can
cause leakage to occur. Because the sample line is generally below atmospheric
pressure, leakage will be into the sampling line. Such leakage will result in sample
dilution and gas concentration readings which are too low.
Operating instruments under excessive temperature and vibration conditions
can be a particularly difficult problem for the optical and electronic components.
Because such problems may be more severe for in-situ instruments, special design
considerations are required.
12-13
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Slide 12-18
PROVISIONS FOR DELIVERY SYSTEM BIAS CHECKS7
From John Richards, "Municipal Waste Incinerator Air Pollution Control
Inspection Course," Submitted to U. S. Environmental Protection Agency
by Entropy Environmentalists, Inc., June 1991, printed with permission.
A delivery system bias check is routinely performed to confirm that changes
have not occurred as a result of the gases flowing through the conditioning and
delivery system. A continuous flow of "span gas" is inserted into the delivery system
as close as possible to the entrance probe. The instrument is then operated for a
continuous time period to determine if the instrument readings change.
An instrumentation preventive maintenance program can make use of the bias
check records. This should help prevent unplanned instrument outages, which under
some regulations could be considered the same as an emissions violation.
12-14
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REFERENCES
1. Title 40--Protection of Environment. Code of Federal Regulations Part 60.
Subpart Ea. "Standards of Performance for Municipal Waste Combustors,"
Office of the Federal Register, National Archives and Records Administration,
Washington, DC, pp. 325-328, July 1, 1991.
2. Title 40-Protection of Environment. Code of Federal Regulations Part 60,
Appendix B. "Performance Specifications," Office of the Federal Register,
National Archives and Records Administration, Washington, DC, pp. 1099-
1128, July 1, 1991.
3. W. R. Seeker, W. S. Lanier, and M. P. Heap, Municipal Waste Combustion
Study. Combustion Control of Organic Emissions, U.S. Environmental
Protection Agency, EPA-530-SW-87-021-C, June 1987, p. 1-7 to 1-9.
4. J. A. Moore, "Key Measurements in Power Plants," Standard Handbook of
Power Plant Engineering, Thomas C. Elliott, editor, McGraw Hill Book Co.,
NY, 1989, p. 6-61.
5. James Jahnke and G. J. Aldina, Handbook. Continuous Air Pollution Source
Monitoring Systems. Technology Transfer, EPA 625/6-79-005, June 1979.
6. Robert Holloway, W. S. Lanier, and S. B. Robinson, "Alternative Approaches
to Real-Time Continuous Measurement for Combustion Efficiency of Hazardous
Waste Incinerators," Contract 68-03-3365, Work Assignment 03 Report to U.
S. Environmental Protection Agency, Office of Solid Waste and Emergency
Response, Submitted by Energy and Environmental Research Corporation,
March 25, 1987.
7. John Richards, "Municipal Waste Incinerator Air Pollution Control Inspection
Course," Submitted to U. S. Environmental Protection Agency, June 1991.
12-15
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13. INCINERATOR OPERATIONS AND UPSETS
Slide 13-1
OPERATING RESPONSIBILITIES
1. Maintain Safety of People
2. Maintain Safety of Equipment
3. Operate within Legal Regulations
4. Optimize Equipment Performance
This learning unit presents the general operating features of MWI units,
including start-up and shutdown operations and operational responses to upset
conditions.
The primary responsibility of the operator is to operate the incinerator in an
efficient manner that protects the safety of personnel, reduces the potential damage
to the equipment, and is consistent with applicable regulations.1'2
Since incinerators are designed to perform at or below certain physical
operating limits, the operation must be consistent with these requirements.
Operation above the design limits may cause accelerated deterioration of the
equipment, increased maintenance costs, reduced unit availability, and increased
pollutant emissions.
Slide 13-2
OPERATOR JOB FUNCTIONS
Scale Operator and Waste Handler
Combustion System Operator
Automatic Control System Manager
Safety Officer
APCD/Boiler Operator
Maintenance Technician
Communicator/Recorder
Depending upon the complexity of the MWI system, the operator's job can have
many dimensions, including those of a combustion system operator, scale operator,
waste handler, automatic control system manager, safety officer, APCD operator,
boiler operator and maintenance technician. In addition, the operator has the task
of maintaining the records of unit operations and communicating clearly with other
individuals, including operators for other shifts, supervisors, assistants, and
maintenance personnel.
13-1
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Slide 13-3
OPERATOR REQUIREMENTS
1. Know the System Characteristics
2 Know the Emergency Procedures
3. Assess the Operating Conditions
What is Happening? Why?
4. Identify Potential Modifications
What are the Options?
What are the Consequences?
5. Make Timely Decisions
6. Establish Proper Procedures
7. Keep Proper Records
MWI Operators and Operator Supervisors provide critical functions in system
operations, including evaluating the current conditions. Operators are required to
know what is happening in the unit and why it is happening. Operators must assess
the operating conditions through the surveillance of both instruments and equipment.
Operators must also judge the importance of deviations from normal operating
conditions. Operators also have the tasks of assuring that routine maintenance is
performed and that proper requests are made for major maintenance.
These functions are required even though the unit's automatic control systems
may have features which provide for the correction of certain undesirable operating
conditions. Automatic control systems are designed to aid operators in making
routine responses to selected system variables.3 Control systems generally operate
very rapidly and have safety features included in their logic.
When an upset condition is detected, the operator must evaluate its severity,
consider possible remedies, know the emergency procedures, and make a proper
response. Operator responses are typically transmitted to the equipment through
manual manipulations which override the control system. Manual manipulations
may be required when the control logic does not cover a particular situation or when
a sensor or control system component has failed.
The unit may be restored to proper operating conditions by changing the
control system set points. Operator initiated control signals can be sent to specific
equipment or controllers through microcomputer keyboards, touch-screens, and/or
panel-mounted controller knobs and switches.
Before instituting changes, the operators are required to know what options
are available and the probable consequences. The operator must understand the
influences of one component on another and the operational and control features of
the unit-specific equipment.
13-2
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Operator decisions must be made in a timely manner. The operator must be
prepared to respond to all upset and emergency situations which may arise. If
decisions are delayed and/or corrective actions taken at inappropriate times, the
operating conditions will probably deteriorate. This could cause the system to operate
in an unsafe condition, outside its design limits, and produce increased emissions
which may result in a violation of the permit.
Operators gain their knowledge through on-the-job experiences of operating
and inspecting the equipment, discussions with knowledgeable personnel, and by
reviewing the unit operations manuals and training manuals.
Slide 13-4
OPERATING SYSTEMS
Medical Waste Handling
Combustion
Ash Removal
APCD
Boiler & Feedwater
Electrical Service
Fire Protection
The major unit operations may include medical waste handling, combustion,
ash removal, APCD, boiler and feedwater systems. The auxiliary systems often
include the unit electrical, hydraulic and/or pneumatic fluid, cooling water, and fire
protection systems.
Slide 13-5
OPERATOR COMMUNICATIONS
Operator/Unit Interface
Receive Operating Information
Transmit Instructions
The operators receive information about the unit's operational status and
transmit instructions to the equipment or to maintenance technicians.
Communications with operating staff may be crucial in developing the basis for the
operator's decisions. Operators must not only give clear instructions, they must also
listen carefully to understand what others are saying.
13-3
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Information about the status of system operations is obtained from the
available instruments, indicator lights, annunciators and alarms. An abnormal signal
from an instrument or indicator can be caused by an upset condition or by the need
for replacing a sensor (e.g., thermocouple) or re-calibrating an instrument.
Normal operations can often be restored after checking for abnormal equipment
symptoms such as material blockages, leakages, vibrations, noise, and surface
temperatures and making appropriate changes.
Slide 13-6
PANEL-MOUNTED INSTRUMENTS
Analog Displays
Digital Displays
Status Indicator Lights
Annunciators
Alarms
Recording Devices
Circular Charts
Strip Charts
Panel-mounted instruments may include analog and digital instrument read-
outs, status indicator lights, annunciators, alarms and recording devices.4
Status lights indicate whether the equipment is operating or off-line.
Annunciators are the back-lighted messages which light up to indicate
operating status, such as when a given system parameter is outside its normal
operating limits. The annunciator light is shut off automatically when its parameter
returns to a normal operating value.
Audible alarms are additionally used to alert operators to the existence of
selected upset conditions. After an alarm has been sounded, it may be shut off by the
operator to avoid being a continued distraction.
Recording devices include circular and strip chart recorders. These can provide
a continuous record of the values of selected parameters such as primary and
secondary combustion chamber temperatures, opacity, steam flow, steam pressure,
gaseous emissions, etc.
13-4
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Slide 13-7
GRAPHIC SCREEN DISPLAYS
Alpha/Numeric
Menus, Lists, Warnings
Two-Dimensional Equipment
Schematic with Data
Individual Component
Groups of Equipment
Overview of Performance
Trends of Selected Data
Some sophisticated installations will have terminal units which provide access
to the unit's microprocessor-based distributed control system. Information can be
obtained from graphic screens which can display data in a number of forms.5 The
software is generally menu-driven and includes other alphanumeric displays which
provide messages and warnings.
Two-dimensional displays can provide current operating data arranged logically
on a graphical sketch of the equipment, system, process, or instrument. Detailed
displays of individual instruments and controllers can be obtained with indicators of
current and maximum readings as well as the set point. An interactive graphics
feature may allow the changing of set points from this display.
Group displays typically indicate a series of bar graphs indicating levels of
operations of selected groups of controllers along with their set points.
Overview displays are generally designed to provide indications of equipment
operations in such a way that abnormal conditions are obvious from deviations in the
pattern of displays.
Trend displays can provide a graph of the continuous record of a selected
variable over a period of time. Displays of multiple parameters on the same graph
can be enhanced by the use of different colors for each parameter.
Slide 13-8
COMBUSTION SYSTEM MONITORS
Combustion Temperatures
Opacity
Carbon Monoxide
Oxygen
Acid Gas Concentrations
13-5
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A number of instruments used as combustion and pollutant emissions monitors
are listed above. These were previously described in Learning Units 11 and 12.
Slide 13-9
TYPICAL CHECK-LIST OF OPERATING SYSTEMS
Waste Loader
Pneumatic and/or Hydraulic System
Incinerator Chamber Fuel-Bed & Ash
Incinerator Chamber Refractory
Underfire Air Supply Nozzles
Forced Draft and Induced Draft Fans
Dampers
Auxiliary Fuel Burner
Motors, Motor Controllers/ Belt Drives
Water Spray Nozzle
Cooling Water System
Ash Removal System
Emergency By-Pass Stack Cap or Damper
Safety System Interlocks
*
*
*
*
Log sheets are used to establish a record of unit operating conditions. The log
sheets are site specific, in that the component equipment and system features will
vary with the unit's design and its complexity. Log sheets are designed to be
maintained at specified time intervals (e.g., every charge, every hour, four hours, or
each shift).
Examples of information which could be recorded frequently (i.e., each charge)
to indicate whether the unit is being operated within proper and/or permitted
conditions include charge time, charge weight, primary chamber temperature,
secondary chamber temperature, draft, and APCD inlet temperature,
Examples of information which could be recorded hourly include indications
about whether the above listed systems are operating properly, or are in need of
adjustment, removal of a blockage, or repair. Examples of measured parameters
include the equipment's operating pressures (e.g., air supply, primary chamber draft,
hydraulic system, air compressor, water pump), temperatures (e.g., combustion, flue
gas, cooling water), meter readings (e.g., cooling water flow), and levels (e.g, water
level in ash quench tank). Of course, some components can be inspected only when
the unit has cooled down (e.g., refractory condition and water spray nozzle condition).
Good operating practices often include maintaining a daily log of unusual
events, in which the operator records the time and nature of each unusual event, as
13-6
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well as the action taken. This includes the observations of unusual odors, noises,
vibrations and the sensing of equipment being overheated.
Component equipment performances can be evaluated by considering the
trends in operating conditions indicated on the log sheet. This review may provide
an indication of the need for equipment maintenance or operational changes.
Slide 13-10
POTENTIAL COMBUSTION HAZARD
Explosive Mixture of Fuel/Air
As has been the case for generations, the most dangerous condition in an
incinerator is the existence of explosive mixtures in the combustion chamber.
Explosive mixtures are routinely avoided by purging the unit with air prior to
lighting of burners. A minimum air flow should be maintained at all times after the
purge process. In addition, the combustion chambers should be pre-heated prior to
the charging of medical waste to improve the initial combustion and reduce the
possibilities of explosive gas mixtures accumulating in the unit.
Slide 13-11
STANDARD OPERATING PROCEDURES
1. Safe Practices & Systems
2. Emergency Procedures
3. General Operations
4. Routine & Major Maintenance
5. Start-Dp and Shutdown
6. Testing and Calibration
Operator actions should generally be consistent with established plant
procedures, such as in those areas listed above. For instance, operators must
promptly respond to potentially hazardous conditions to reduce the possibility of
injury to personnel or damage to equipment. Particular diligence is required during
start-up and shutdown, as these transient operations may provide opportunities for
uncontrolled conditions.
13-7
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The operators typically participate in the development and refinement of
standard operating procedures. In addition, the operator must consider measures
which will maintain or improve the equipment's ability to perform continuously and
efficiently. These measures include making routine inspections, implementing
preventive maintenance and arranging for major maintenance.
Slide 13-12
POLLOTANTS INFLUENCED BY OPERATIONS
1. Air Pollutants
* Smoke
* Particulates
* Gases
2. Waste-Water Discharge
3. Odor
4. Noise
Operators are responsible for maintaining operations within all relevant permit
conditions. These conditions include both air pollutant emissions, solid waste
residues, and waste-water discharges. Operators are responsible for operating the
combustion, APCD and heat recovery systems to control the smoke,, particulates, CO,
organics, acids and heavy metal emissions from the unit. In addition, operators must
limit the nuisances of odor and noise in the neighborhood.
Slide 13-13
COMBUSTION CONTROL
Air and Fuel Transients
Operator Activities
Review System Performance
Improve Equipment Setting
Operators are responsible for controlling all major and auxiliary systems found
in MWI units. This learning unit, however, focuses on combustion systems.
Operators will need to continually review the combustion conditions by
monitoring the gas temperatures in the primary and secondary combustion chambers,
as well as smoke and all monitored flue gas concentrations.
13-8
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As discussed in Learning Unit 9, the combustion control systems will vary
depending upon design concepts and individual unit features. However, all systems
are designed to follow a desired combustion profile as the materials pass through the
unit. This profile includes the medical waste being dried and heated, the evolution
of the various gases, ignition of the combustible gases within or above the bed, and
the burning of the fixed carbon on the hearth.
Slide 13-14
CONTINUOUS BURNING ON A HEARTH
1. Underfire Air
Damper Controls
Supply Air Pressure
Draft
2. Fuel Bed
Waste Feed Size & Frequency
Bed Agitation
The hearths of large, continuously operated MWIs are considered to have a
drying zone, a pyrolysis zone, a burning zone and a burn-out zone. Ideally, the bed
thickness decreases as the waste moves down the hearth, with the most active
burning region located in the middle of the primary chamber.
Combustion is improved when air diffuses uniformly through all the waste.
Non-uniform air flow can be caused by the plugging of air nozzles or the influence of
bed thickness on air distribution. Thicker bed regions have more resistance to air
flow than thin bed regions, so the thin bed regions get too much air and thick beds
get too little. This channelization of air leads to local hot spots of intense combustion,
causing glass melting and clinker formation, which further restricts the air flow.
Some continuous burning MWIs have underfire air controls which enhance the
final burn-out of fixed carbon before the residue drops into the ash quench tank.
Combustion control in the primary chamber can be achieved by modulating the
air supply, hearth agitation, waste delivery, and water sprays. MWI systems
typically use dampers to maintain the desired air flow distribution and draft
conditions. Operators must know how changes in damper positions will affect unit
performance.
The waste feed rate will determine the total heat input and must be matched
to the air flow rate. Changes in underfire air will cause an immediate impact upon
the overall combustion conditions. Because of the volatile character of the plastics
13-9
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(e.g., plastic bags), the waste feed may have a dramatic response on combustion
conditions.
The secondary air supply must be appropriately operated to accomplish the
mixing of combustible gases and oxygen for complete combustion. Secondary air
nozzles are generally designed to provide air penetration and mixing in the flame
port, the duct between the primary and secondary chambers. Operators may need
to maintain the air pressure, damper mechanisms, seals, and fan performance to
assure that this mixing is adequate.
Slide 13-15
SEQUENCE OF BATCH OPERATIONS
Remove Ash from Previous Burn
Load Batch Charge into Incinerator
Close and Lock the Charge Door
Operate the Fans to Purge Gases
Preheat Secondary Chamber
Ignite Waste in Primary Chamber
Burn-Down, Cool-Down, Shut-Down
MWI systems which operate with single batch and intermittent duty charging
undergo a different sequence of combustion processes.
Batch operations typically begin with the removal of ash from the previous
burn cycle. Note that an adequate burn-down period (e.g., 12 hours) is prescribed to
obtain ash with a low carbon content. Additional cool-down and shut-down periods
are prescribed to help avoid thermal injuries during ash removal.
The sequence of operations for a batch charged unit varies somewhat from
manufacturer to manufacturer. In general, after ash removal, the batch is loaded
into the unit. Many batch charged units have temperature controlled interlocks
which prevent the charge door from opening after the auxiliary fuel burners have
ignited and the unit has an elevated temperature.
The operating sequence, as indicated in Slide 13-15, includes operating the fans
for a minute or so to purge the potentially explosive mixture of gases from the unit.
After the purge is completed, the auxiliary fuel burner in the secondary chamber can
be ignited to preheat the secondary chamber.
In general, the batch of waste is ignited by the primary burner only after the
secondary chamber adequately preheated (e.g., to 1,800 °F). The secondary chamber
refractory is preheated to provide an adequate temperature for burning the volatile
gases which will evolve from the primary chamber upon ignition of the waste.
13-10
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Slide 13-16
INTERMITTENT DUTY CHARGING
Charge Delivered at:
6 to 15 Minute Intervals
Charge Size Correspondingly Set at:
10% to 25% of Hourly Capacity
The time between the delivery of each intermittent batch can be designed to
allow the evolution of most of the volatile material from a charge before another is
delivered. The residue from each charge accumulates in the bottom of the primary
chamber and remains there for an extended burn-down period (e.g., 8 hours) to
accommodate the diffusion of air required for the combustion of fixed carbon.
If the time period between each charge is too short or the size of the charge too
large, the primary combustion chamber may be hot enough to cause an abnormal
surge in volatiles from the new charge. The surge of volatiles can overload the
secondary chamber, leading to a surge in emissions of smoke and pollutants.
One strategy to reduce the surge of volatiles is to reduce the underfire air prior
to and during the charging process, which reduces the primary chamber temperature.
Another operational strategy is to suppress the temperature by spraying water onto
the charge of waste as it is delivered into the primary chamber.
In general, intermittent charged units perform best if the wastes are mixed
and the unit is not operated above its rated capacity. Of course, feeding waste in
excess of the unit's rated capacity can lead to overloading the secondary chamber and
increasing pollutant emissions.
It is generally recommended to use small charges that are delivered frequently,
as in the case of charging every six minutes (10 times per hour) with charges
weighing not more than 10% of the hourly design capacity. However, some systems
have interlocks which prevent charging more frequently than every 15 minutes. In
this case the charges should weigh no more than 25% of the rated hourly capacity.
13-11
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Slide 13-17
INTERMITTENT DOTY START-UP/SHUTDOWN
Remove the Ash Residue After
A Cool-Down Period
Preheat Refractory in Secondary
Preheat Refractory in Primary
Deliver & Ignite the First Charge
Charge Intermittently
Implement the Burn-Down Operation
Implement the Cool-Down Operation
The first step in the start-up operation is to remove the ash residue from the
previous burn-cycle. This removal should occur only after an adequate cool-down
period, so that the refractory will be cooled considerably and the operators will be
protected against injury associated with contacting either hot ash or hot surfaces.
Because the ash may contain heavy metals, operators should use air filter-type
masks when exposed to ash dust. An alternate strategy is to use the natural draft,
from the operation of the secondary chamber burners, to suck the fugitive dust up the
stack and away from the operators. The ash may be wetted with a water spray to
control fugitive dust. However, water should not be sprayed onto hot refractory
materials or high temperature ash, because steam would form and entrain dust.
The sequence of events associated with start-up may vary from unit to unit.
Start-up may be controlled manually during the first charging cycle before control is
switched to the automatic combustion control system. Some units may have
automatic combustion controls which operate throughout the start-up process. Many
control systems have interlocks which prevent charging until various conditions are
met (e.g., minimum primary temperature, secondary temperature, adequate time
since last charge, hopper door of loader is closed).
The auxiliary fuel burners are used to pre-heat the secondary combustion
chamber. The manufacturer's recommended procedures during the pre-heating period
should be followed. For instance, the preheat period may be specified (e.g., 1 hour).
Auxiliary fuel burners generally have ignition sensors which, will trip the fuel
flow if the flame is extinguished. There is also a minimum amount of air flow which
must be supplied at all times (except after the cool down period) to prevent the
accumulation of explosive gaseous mixtures. Sometimes preheating causes smoke to
leak from the primary chamber, an indication that the draft conditions are
inadequate.
13-12
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Slide 13-18
COMBUSTION SYSTEM UNIT SHUTDOWN
Stop Feeding Waste into Unit
Burn the Waste on the Hearth
Operate Auxiliary Burners as Necessary
Maintain Underfire Air Supply
Maintain APCD Temperatures
The procedures and time requirements for shutting down a MWI depend upon
the design of the unit and the nature of the shutdown. Shutdowns for intermittent
operating units can occur on a daily schedule or emergency shutdowns can occur.
The "burn-down" procedures provide for maintaining air flow and temperatures
throughout the period by the use of the blower and auxiliary fuel firing equipment.
The purpose of the procedures is to burn the combustible content of the residues,
while minimizing the pollutant emissions.
After the loader has been "locked out," the blower and auxiliary fuel burning
operations are often controlled to operate at appropriate levels by a timer until the
residues have been adequately burned. In general, the length of the burn-down
period and the temperature levels maintained by burning auxiliary fuel should be
based on manufacturer's recommendations. Visible observation of the ash can
indicate if the length of the burn-down period shoud be increased to improve burnout
of the waste. After the auxiliary fuel firing stops, the blowers generally continue to
operate during the "cool-down" period. The "shut-down" period follows with the
burners and fans generally turned off.
Removal of solid residues from the hearth areas of batch charged and
intermittently charged units is typically accomplished at the end of the "shutdown
period." Manual removal is performed with special ash rakes or shovels. To avoid
being burned, personnel should enter the hearth area only after assurance that either
the ash has been removed or the ash and refractory are sufficiently cooled.
Automatic ash removal may accomplished by operation of the ram devices.
Slide 13-19
TYPICAL COMBUSTION UPSETS
Waste Charging System
Combustion Air Supply
Ash Handling
Power Failures/Excursions
13-13
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The troubleshooting of combustion upsets generally focuses on corrective
actions associated with component equipment of the systems listed above.
Malfunctions can occur for a number of reasons, including normal wear, operation
outside of design conditions, improper lubrication, blockages, and electrical failures.
Upsets may also be caused by operator error, such as reacting to a situation
before thinking through all the ramifications of the action or reacting too slowly to
an emergency situation.
Slide 13-20
INDICATORS OF COMBUSTION QUALITY
1. Opacity
2. Carbon Monoxide
3. Temperature (Secondary)
4. Oxygen
5. Visual Appearance of Fire
6. Color of Bottom Ash
Visual observations and a number of instruments can be used to indicate the
quality of the combustion conditions. Typically, the continuous temperature and
opacity monitors are used to indicate combustion and/or APCD system performance.
Combustion quality for a larger MWI may also be indicated by a carbon monoxide
monitor. Incinerator draft and oxygen monitors can provide additional information
about combustion conditions.
Personal visual observations of the fuel bed can provide valuable insight as to
the combustion conditions. Viewing the combustion region through observation ports
can provide information about the location of the burning zones, intensity of
combustion, bed thickness and uniformity, clinkers, and the presence of undesirable
oversized materials which were included in the feed.
Slide 13-21
HIGH CARBON CONTENT IN BOTTOM ASH
Caused by
Insufficient Underfire Air
Inade
-------
down period during which the carbon content of the bottom ash is reduced.
Therefore, an inadequate burn-down period or operation with temperatures that are
too low could cause a high carbon content in the bottom ash.
Continuous duty starved-air MWIs may produce excessive carbon content in
the bottom ash because of insufficient underfire air during the final burning of the
residues on the hearth. The residence time for solid residues on the hearth of
continuous duty units is much shorter than the burn-down periods typical of single
batch and intermittent duty MWIs.
Information about combustion quality can also be obtained by observing the
ash color and consistency and by laboratory testing of the ash to determine its carbon
content. When removed, the ash will be gray if completely burned. Ash with a high
carbon content will be black, and recognizable pieces of unburned material may be
observed. Testing and sample results for carbon content in ash and "loss-on-ignition"
were discussed in Learning Unit 3.
Slide 13-22
UPSETS ASSOCIATED WITH WASTE FEEDING PROBLEMS
1. Improper Feed Rate
Too High - Excessive Smoke
- Poor Burn-out of the Ash
Too Low - Insufficient Fuel
- Low Combustion Temperatures
2. Non-Uniform Fuel Bed Thickness
Too High - Restricted Air & Burn-Out
Too Low - Clinkering
3. Sudden Change in Fuel Properties
High Moisture - Reduces Primary Temp.
High Volatiles - Incomplete Combustion
The waste feed rate and the time between waste loadings are controlled by the
operator. These parameters influence not only the primary combustion temperature
but also the delivery of volatiles (fuel) to the secondary chamber. An improper feed
rate can result in either overloading the secondary chamber or in excessive auxiliary
fuel consumption, required to maintain secondary chamber temperatures.
Of course, the delivery of underfire air to the hearth and fuel bed agitation are
two major parameters used for controlling the energy release rate and primary zone
temperature. However, some MWI units are designed to operate with a constant
underfire air flow and without agitation of the fuel bed.
13-15
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Slide 13-23
REMEDIES TO WASTE UPSETS
Regulate Underfire Air Supply
Regulate Charging Rate
Mix Wastes to Reduced Surges
As discussed earlier in this learning unit, volatile gases from plastic materials
may be evolved in a transient surge, just after the waste is fed into the primary
chamber. When this surge of volatiles reaches the secondary chamber it can deplete
the available oxygen and result in excessive pollutant emissions. The volatile surge
may be less of a problem if the waste is packaged in cardboard containers instead of
plastic bags. Of course, wet waste causes a combustion temperature reduction which
slows down the drying and the volatilization process.
Many incinerators have interlock systems which shut off the primary burner
and/or the under-fire air supply prior to and during charging.6 This helps to reduce
the intensity of combustion and to control the surge of volatiles and maintain draft.
Many intermittent duty units use water sprays (a heat sink) to limit the
primary combustion temperatures and the rate of volatilization. The nozzles will
require protection against the combustion zone temperatures, often using a forced air
heat exchanger built into the nozzle housing.
Excessive carbon monoxide and/or smoke can also be caused by fuel property
variations. For instance, when a charge of wet medical waste is delivered, incomplete
combustion may occur due to reduced gas temperatures. This upset may be corrected
by increasing the underfire air supply, fuel bed agitation or auxiliary fuel burning
rate.
Combustion problems are also expected when the unit is charged above design
capacity. Units operated above the rated capacity experience higher gas velocities,
shorter residence times, greater particulate entrainment and increased smoke
opacity. Part of the problem may be associated with the fans being unable to supply
adequate air. Fan performance may be compromised if the fans are not well
maintained or if flow restrictions occur in the dampers or ducts.
The observation of unburned materials in the bottom ash is generally an
indication that either the fuel-bed agitation, the underfire air in the burnout regions
of the hearth, or the residence time for the solids on the hearth is inadequate for
complete combustion.
13-16
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Slide 13-24
COMBUSTION AIR UPSETS
Dnderfire Air Supply
Low Supply - Inadequate Oxygen
High Supply - Excessive Entrainment
Poor Distribution (Channeling)
Secondary Air Supply
Low Supply - Inadequate Mixing
High Supply - Excessive Gas Cooling
Poor Distribution, Mixing
Tramp Air Intrusion Through Poor Seals
Overloading the secondary chamber can occur when the air supply and mixing
is inadequate for complete combustion. This is would be indicated by smoke and
carbon monoxide and by high primary and secondary chamber temperatures. This
typically occurs with high gas velocities which would cause less uniform mixing and
shorter residence time in the secondary chamber.
Abnormal combustion temperatures can also indicate problems with the air
supply and distribution. Uncontrolled air flow into the primary combustion chamber
("tramp" air), caused by poor seals around the charging door, can upset combustion
by causing poor mixing and by reducing the uniformity of combustion stoichiometry.
Slide 13-25
REMEDIES FOR COMBUSTION AIR UPSETS
*
*
*
Check Draft Gage Readings
Remove Blockages in Air Nozzles
Adjust Fan Controls/Dampers
Remove Clinkers
Modify Trim Control System Settings
Operators should be cautioned that control of combustion air upsets will
depend upon the design features of the air supply and the combustion systems. The
comments presented below are designed to be general in nature, and may not apply
to every situation.
Poor primary air conditions can result from blockages of the underfire air
nozzles, a fan/damper failure, or an improperly operating control system. Such
conditions are generally detected by monitoring the draft gages. The damper systems
13-17
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often have mechanical linkages which are adjustable and may slip, causing a
deteriorated response. Poorly fitting damper seals can also allow air to flow through
a closed damper.
In regions where the fuel bed is too thick, the air flow through the bed will be
reduced. Uneven fuel beds can cause improper mixing conditions. Where the bed is
too thin, the air flow will be excessive. The regions of increased air flow will tend to
burn more intensely and entrain more particulates. Compensation for variations in
fuel bed thickness may include cleaning the underfire air nozzles or modifying the
underfire air flow rate and its distribution.
Clinkers can be formed when the fuel bed temperatures exceed the ash fusion
temperature. When clinkers are on the fuel bed, the normal air flow will be
interrupted, causing inadequate mixing and combustion. Once the clinkers are
formed, air flow will be restricted and the conditions will tend to deteriorate. It is
possible to turn the entire fuel bed into a fused mass that requires the unit to be shut
down to remove the clinker from the hearth.5
Trim control units may be available to fine-tune the air delivery system
through the use of sensors for flue gas oxygen or carbon monoxide. An oxygen trim
control system is often provided for larger MWIs.
Slide 13-26
COMBUSTION TEMPERATURE UPSETS
1. High Secondary Chamber Temperatures
Refractory Damage
Remedy:
Reduce Auxiliary Fuel Firing
Increase Overall Air Supply
Reduce Waste Charging Rate
Reduce the Underfire Air Supply
Abnormal combustion temperatures, carbon monoxide, and smoke can be
caused by improper air delivery. An increase in the secondary chamber temperature
often results from the previously described surge of volatiles from the primary
chamber. It may also be caused by excessive burning of auxiliary fuel.
Secondary combustion chamber temperatures may be controlled by modulating
the delivery of secondary air. Since the excess air is a heat sink, adding more air will
decrease the overall combustion temperatures.
13-18
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Decreasing the underfire air supply is an important method of reducing the
primary temperature and volatilization rate, which will lower the secondary chamber
temperatures due to the corresponding reduction in volatiles.
Slide 13-27
COMBUSTION TEMPERATURE UPSETS
2. Low Temperature in Secondary
Inadequate Combustion
Production of Pollutants
Remedy:
Decrease Secondary Air Supply
Increase Underfire Air Supply
Increase Auxiliary Fuel Burning
Low secondary chamber temperatures can be caused by a low fuel feed rate or
by the delivery of high moisture fuels which have reduced heating values.
Auxiliary fuel burner control systems are typically used to help maintain
secondary combustion gas temperatures. The same auxiliary burners are also used
for pre-heating the combustion chambers prior to the introduction of waste into the
incinerator.
Some combustion units use flue gas recirculation as a heat sink to control
temperatures. An automatic control system can regulate the damper positions to
modulate the amount of recirculated flue gas for temperature control.
Slide 13-28
INCINERATOR DRAFT CONDITION UPSETS
1. Excessive Draft
High Velocities and Poor Mixing
Excessive Particulate Entrainment
Excessive Tramp Air
2. Inadequate Draft
Low Velocities
Pressure Transients/Puffing
3. Operation with Positive Pressure
Accumulation of Fly Ash Outside Unit
Gases/Smoke Leaking Out of Chamber
* Combustion Quenching
* Pollutant Exposure to Personnel
* Damage to MWI Structure
13-19
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In addition to reading draft gages, a general indicator of improper draft control
would be smoke emissions from cracks or openings in the combustion chamber walls
and/or the build-up of fly ash around the unit's exterior. Puffing is the condition
which occurs when the combustion chamber pressure becomes positive and gases flow
out of the cracks in the incinerator rather than leak inward.
Poor draft conditions can be associated with improper operation of fans or
dampers. Improper draft may result from changes in the air pollution control device.
For instance, a change in the pressure drop in a scrubber or fabric filter system can
influence the draft conditions in the incinerator.
Operating an incinerator with an improper draft or at a pressure greater than
ambient can cause poor mixing and an increase in the concentrations of products of
incomplete combustion. The recommended amount of draft is designed to assure that
appropriate gas velocities are maintained.
The remedy for improper draft is to restore the operation of the forced draft
and induced draft fans and the associated dampers. Damper linkages and seals
should be inspected regularly and adjustments or replacements made as required.
13-20
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REFERENCES
1. E. B. Woodruff, H. B. Lammers and Thomas F. Lammers, Steam Plant
Operations. Fifth Edition, McGraw-Hill Book Company, New York, 1984, pp.
259-265.
2. Frederick M. Steingress and Harold J. Frost, Stationary Engineering.
American Technical Publishers, Inc., Homewood, IL, 1991, pp. 87, 181-192.
3. Joseph G. Singer, Combustion. Fossil Power Systems. 3rd Edition, Combustion
Engineering, Inc., Windsor, CT, 1981, pp. 20-1 to 20-31.
4. J. A. Moore, "Key Systems and Components," Standard Handbook of Power
Plant Engineering. Thomas C. Elliott, editor, McGraw Hill Book Co., NY, 1989,
pp. 6.95-6.123.
5. PEI Associates, Inc., Combustion Source Inspection Module. Student Reference
Manual. Submitted to U.S. Environmental Protection Agency, September 1990,
pp. 242-245, 272-291.
6. Louis Theodore, Air Pollution Control and Waste Incineration for Hospitals
and Other Medical Facilities. Van Nostrand Reinhold, New York, 1990, pp.
364-367.
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14. MAINTENANCE: CORRECTIVE & PREVENTIVE
Slide 14-1
CORRECTIVE & PREVENTIVE MAINTENANCE
1. Risk Management
2. Efficient & Reliable Operation
This learning unit considers the general aspects of corrective maintenance and
preventive maintenance. Such activities are elements in a general risk management
program.
Risk management is a relatively new management term which focuses on the
financial consequences of various operational activities.1 It includes balancing the
financial return from unit operations with the total operating expenses. In general,
management plans are designed to achieve an optimum production which provides
a fair return to the public, owners, and/or investors and minimizes the potential for
financial losses.
Slide 14-2
*
*
*
*
*
ASPECTS OF RISK MANAGEMENT
Insurance Against Production &
Casualty Losses
Evaluation of Current Conditions
Evaluation of Probability ,
Consideration of Economics
Consideration of Intangibles
OSHA Regulatory Requirements
Risk management can be thought of as an effective insurance program. In
some industries, insurance policies are available to provide protection against revenue
losses associated with equipment outages, as well as casualty and liability losses. In
general, losses will occur if equipment is not properly operated and maintained.
The achievement of balanced risk management is not easy, as it requires an
evaluation of the probability of events occurring and a consideration of the value or
cost of their consequences.1 Judgments, of course, are aided by taking into account
an analysis of the facility conditions, the operating experiences at other similar
facilities, and the general experiences of the industry.
14-1
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A broader view of risk management includes consideration of the intangible
aspects of good public and personnel relations, as well as the economic aspects of
production and loss. Operator supervisors are an important part of management,
with responsibilities related to system performance improvement and cost control.
OSHA mandated safety controls essentially requires employers to provide
workplaces that are free of recognized hazards that are likely to cause death or
serious physical harm to employees.2 Job-safety analyses and human-factor analyses
are designed to reduce the risks associated with hazards which may occur in the
workplace.
The OSHA bloodborne pathogens standard3 requires the development of an
"exposure control plan" to identify where exposures to blood can occur and to develop
preventive measures. In general, compliance requires the implementation of
"engineering controls" and "work practice controls" in a manner consistent with
universal precautions.
Slide 14-3
*
*
*
*
*
*
*
*
*
*
POTENTIAL ECONOMIC LOSSES
Cost of Preventive Maintenance
Personal Injury to Employees
Injury to Visitors & the Public
Equipment Repair/Replacement
Lost Revenue - Treatment Fees
Lost Revenue - Energy Sales
Alternative Disposal Costs
Extra Transportation Costs
Pines - Regulatory Violations
Contractual Noncompliance Losses
General economic considerations include balancing applicable costs with the
revenue associated with incinerator operations. Operating costs can be determined
from financial records, as well as the capital costs or the annual fixed costs associated
with equipment.
The economic aspects of risk management, as listed above, include: the costs
of the preventive maintenance program; consideration of losses associated with injury
and repair; lost revenue associated with equipment outages; and fines for regulatory
violations.
Risk management also includes consideration of the potential liability
associated with an accident involving a staff member and the liability associated with
allowing outside people to enter the working areas.
14-2
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In addition to the waste packaging and handling measures, risk management
also includes consideration of the control, cleaning and disposal of contaminated
personal items, such as gloves and work clothes. Practices should prevent the spread
of contamination to areas outside the workplace.
For instance, one MWI site requires that workers, upon entering the facility,
shower and put on company provided clean work clothes. Upon leaving they are
required to shower and change back into their personal clothes.
OSHA regulations emphasize hand washing so that, prior to leaving the main
incinerator area, employees may be instructed wash their hands with anti-septic or
gennicidal soaps.
Slide 14-4
OPERATOR RESPONSIBILITIES
1. Safety
2. Production (System Operations)
3. Corrective Maintenance
4. Preventive Maintenance
5. Record Keeping & Communications
Operators participate in risk management through their decision-making on
issues of safety, operation, corrective maintenance and preventive maintenance.
Operators have the responsibility for assuring that the equipment is both properly
operated and maintained, so that it can perform reliably, efficiently, and safely over
its expected life. There are considerable economic risks associated with poor
operations and with improper maintenance.
Corrective maintenance consists of the repairing of equipment that has failed
or malfunctioned.4 Examples include the replacing of a thermocouple which has been
destroyed by thermal shocks and a fan rotor which has eroded due to its exposure to
corrosives.
Preventive maintenance consists of planned maintenance actions performed to
prevent equipment breakdown. Examples of preventive maintenance include the
lubrication of moving parts (e.g., motors, pulleys, wheel bearings) and the filling in
of small cracks in the refractory (e.g., with poundable refractory). Cracks in the
refractory will tend to grow with time, due to thermal cycling and the associated
thermal stresses, and if not maintained can lead to a major refractory replacement.
Operators also have the management responsibilities of record keeping and
communications. Operators generally participate in the development of policies and
14-3
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standard operating procedures. Large incinerator facilities may utilize computerized
maintenance systems which aid supervisory personnel in their tasks of overseeing
and planning the maintenance effort, evaluating its effectiveness, and controlling its
costs.4
Slide 14-5
GOALS OF PREVENTIVE MAINTENANCE
1. Minimize Total Operating Costs
2. Enhance Equipment Life
3. Assure Equipment Reliability
4. Assure Regulatory Compliance
5. Restore Unit Performance
6. Minimize Down-Time
A preventive management concern is to minimize the total costs while
preserving the plant's capital investment. Preventive maintenance is performed to
assure that the unit can operate safely, efficiently, and reliably5. The general goals
are particularly important in MWI units because of the increased complexity
associated with adequately controlling the combustion of such a variable waste fuel.
"A stitch in time saves nine" is still a valid expression. As maintenance is
deferred, operational problems will generally worsen and unit down-time will be
increased. The financial consequences include the costs associated with making the
repairs, lost revenue from reduced production, increased cost for alternative disposal
and treatment, and potential costs associated with safety hazards.
Excessive maintenance can also represent a significant cost. Priorities must
be set in an attempt to balance the economic consequences associated with either
acting now or deferring maintenance.
The old maintenance expression, "If it ain't broke, don't fix it," has generally
evolved into "Fix it just before it breaks."* To some extent, the performance of all
equipment tends to deteriorate with operating time. Preventive maintenance
programs are designed to restore the performance efficiencies of the equipment.
Each incinerator manufacturer provides a maintenance schedule that should
be closely followed for specific incinerators. Generally, the regular routine inspections
that an operator should perform on a daily basis include:
14-4
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• calibration checks of stack gas monitors,
• observation of exhaust stack for visible emissions,
• inspection of door seals for closeness of fit (gaps should be reported to
maintenance department), and
• inspection and cleanout of air ports.
Predictive maintenance is the part of preventive maintenance which tries to
identify potential problems. Routine inspection of equipment and instrument
readings, as well as a review of the performance and maintenance records, can lead
to the identification of equipment problems. Predictive maintenance also includes the
use of special instruments for vibrational analysis, ultrasonic testing, infrared
imaging and meggering (e.g., measuring electrical resistance of insulation) of electric
motors and wiring.4
Whether specific maintenance can be performed while the unit is in service or
during an outage will be determined by the severity of the conditions and the design
of the equipment. The scheduling of major (annual) outages requires planning,
setting priorities, and arranging for special inspectors and appropriate maintenance
personnel, supplies, replacement parts and repair equipment.
Slide 14-6
FEATURES OF A MAINTENANCE PROGRAM
1. Review Vendor Recommendations
2. Identification of Problems
3. Evaluation of Options
4. Communication & Planning
5. Implementation
A maintenance program generally begins with the review of the equipment
design features and an implementation of the manufacturers' recommended
maintenance procedures. Special attention should be given to the lubrication
requirements and limits on operating conditions (e.g., temperatures and loads).
An important aspect of unit operations is to identify problems and solve them
before they become unmanageable. Problem evaluation begins with an analysis of the
current status and an attempt to identify problems and evaluate the causes.
Repairing the symptom of a problem may be easier than solving the real
problem. For instance, a flue gas duct that becomes thin and develops leaks can be
replaced. However, determining that dew point corrosion problems exist and
eliminating the corrosive condition may provide a long-term solution to the problem.
14-5
-------
Communication with other individuals is also an important aspect of operators'
duties. Discussions and group meetings with operating staff, maintenance staff,
vendor representatives, engineers and/or designers may be required for planning a
proper solution to a special maintenance problem.
Cooperative discussions may be in order before deciding whether to take the
equipment off-line for repair or to delay the maintenance until the next scheduled
outage. Consideration of both the equipment design features and maintenance
records will be important in such decisions. Nevertheless, operators are called upon
to make timely judgments about taking a unit out of service because of various
equipment upsets and safety considerations.
The implementation of maintenance must be scheduled with the operator. A
proper lock-out/tag-out program should be used for equipment that poses an
electrocution or mechanical injury hazard. Lock-out often refers to the use of
padlocks to lock circuit breakers in the "off" position. It also refers to the use
of mechanical means to secure equipment (e.g., prevent rotation) for personnel
safety. Tags are used to identify who is in charge of the maintenance activity.
Slide 14-7
IN-SERVICE MAINTENANCE
1. Follow Recommended Procedures
2. Know Special Design Features
3. Know Operational Relationships
In-service preventive maintenance includes the routine operations which are
designed to maintain the equipment according to recommended procedures.6 The
equipment's special design and operational features will have to be known, including
its relationship to other elements of the unit and the possibility of switching to
alternate equipment.
Examples of in-service maintenance may include repairing leaking valve seals
and replacing GEMS instruments, thermocouples and other sensors.
14-6
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Slide 14-8
OUTAGE MAINTENANCE
1. Make & Update an Outage Plan
2. Arrange for Materials/Services
3. Make Detailed Inspections
4. Revise Plans as Necessary
5. Follow Proper Procedures
€. Inspect Upon Conclusion
An annual incinerator outage may be an operating permit requirement. The
inspection and repair must be performed in accordance with applicable requirements.
14-7
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REFERENCES
1. James L. Riggs and Thomas M. West, Essentials of Engineering Economics.
Second Edition, McGraw Hill Book Company, New York, 1986, pp. 438-464.
2. Matthey L. Kuryla and Stephen C. Yahay, "New Safety Rules Add to Plant
Manager's Worries; OSHA's Process-Safety Standards Expand Management
Responsibilities," Chemical Engineering. June 1992, pp. 153-160.
3. "Occupational Exposure to Bloodborn Pathogens, Final Rule," Federal Register.
29 CFR Pate 1910.1030, Occupation Safety and Health Administration,
December 6, 1991, pp. 64175-64182.
4. Mike Shatynski, "Plant Maintenance," MWC Operator Training Overview
Course Manual. Edited by John Eppich, ASME Professional Development
Program, West Palm Beach, FL, February 24-26,1992, Lesson IV.L, pp. 1-15.
5. Steam. Its Generation and Use. 39th Edition, Babcock and Wilcox, New York,
1978, pp. 36-1 to 36-20.
6. Joseph G. Singer, Combustion. Fossil Power Systems. 3rd Edition, Combustion
Engineering, Inc., Windsor, CT, 1981, pp. 22-1 to 22-23.
14-8
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15. FLUE GAS CLEANING I: PARTICULATE MATTER
Slide 15-1
PARTITIONING OF SOLID RESIDUES1'2
FUEL/EQUIPMENT TYPE
EXAMPLE VALUES, %
BOTTOM ASH FLY ASH
MWI Modular Starved-Air
MSW Mass Burn - Grate
Pulverized Coal
RDF - Spreader
98
90
30
25
2
10
70
75
The above slide illustrates the influence of fuel type and combustion equipment
design on the distribution of the solid combustion residues between the bottom ash
and fly ash entrained in the flue gases.
The total quantity of ash is dependent upon the amount of incombustibles in
the waste being burned. Inorganic materials represent approximately 10% of the
mass of medical waste.
The low velocity design features of modular starved-air MWIs help to minimize
particulate entrainment. Around 2% of the solid residues become fly ash, with
around 98% collected in the bottom ash.
Slide 15-2
PARTICLE ENTRAINMENT FACTORS
1. Particle Size, Shape & Density
2. Underfire Air Velocity
3. Waste Charging Method
4. Fuel Burning Rate
Entrainment of particulate matter is influenced by particle size, shape and
density. For instance, a small piece of paper is fairly easily entrained, whereas a
similar sized metal object will tend to remain on the fuel bed.
Particulate entrainment in the flue gas is minimized by limiting the gas
velocity through the fuel bed. This technique allows modular starved-air units to
have lower particulate loadings than other incinerators. The influences of air supply,
waste charging method and burning rate were presented in Learning Unit 13.
15-1
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Slide 15-3
s
c
cd
CO
00
on
TO
-
05
3
I
o
MWI PARTICLE SIZE DISTRIBUTIONS3'4'5
At Secondary Chamber Exits
Andersen MWI
Borgess MWI
U. of Michigan
2 4 6 8 10 12 14 16 18 20
Particle Sizes, Micrometers
Particulate size distributions from three MWIs are shown above. Typically,
lore than half of the participates are smaller than one micrometer in diameter.
Slide 15-4
MAJOR TYPES OF PARTICULATES6
PARTICULATE
Refractory Oxides
Inorganic Salts
Volatile Elements
Heavy Metals
Trace Organics
PARTICLE SIZE
(Micrometers)
Greater than 0
Less than 0.5
Less than 0.3
Less than 1.0
Less than 0.5
8
There are at least five major sources of particulate materials (fly ash). The
three largest sources are the refractory oxides, inorganic salts, and volatile elements.
In addition, heavy metals and unburned organic materials ("trace organics") present
particular environmental concerns because of their potential toxicity.
15-2
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Refractory oxides are the most common residues of waste combustion, including
the clay filler materials in plastics and paper, soils, paint and dye pigments and non-
volatile metals.6 Examples are the oxides of calcium, magnesium and metals, such
as iron, aluminum and titanium. These are formed as relatively large sized
particulates (e.g., above 0.8 micrometers) with high densities. Although most of these
oxides are found in the bottom ash, they are also entrained in the flue gas.
The second most common source of particulate emissions are the water soluble
inorganic salts (e.g., sodium chloride, potassium chloride and calcium chloride).6 They
are vaporized during the drying and combustion process and are condensed as the
flue gases are cooled. These inorganic salts form small particulates (e.g., less than
0.5 micrometers).
A third major source of particulates is the group of volatile elements, such as
phosphorus, sulfur and silicon.6 These are easily converted into submicron oxide
aerosols (e.g., less than 0.3 micrometers) and are referred to as "fume sized
particulates." Because these particles are so small, they are difficult to remove from
the flue gas. The oxide aerosols are difficult to remove by wet scrubbing, which relies
on impaction of particulates onto droplets for removal.
Volatile metals, such as mercury, zinc, cadmium and lead, are a major
environmental concern. As the flue gases are cooled, the volatile metals can be
condensed, adsorbed, or chemisorbed onto the surfaces of fly ash or can be emitted
in gaseous form.
Although found in much lower concentrations, emissions of trace organic
compounds are also a major environmental concern. Most of the organic materials
are burned under proper combustion conditions, but a small fraction may be emitted
in the flue gas. The "blue smoke" which can be seen in the exhaust from poorly
operating incinerators is probably caused by condensed organics.6
Particle sizes are generally increased as the flue gases are cooled. This is due
to a combination of particle agglomeration and the condensation and adsorption of
volatile substances onto fly ash.
Slide 15-5
INFLUENCES OF PARTICULATES ON DIOXIN/FURAN
1. Catalytic Formation on Ash Surfaces
2. Adsorption by Carbon in Fly Ash
Low Fly Ash Loading and Carbon Content
3. Activated Carbon Injection
15-3
-------
The catalytic formation of dioxin/furan and other trace organic compounds on
the surface of fly ash was discussed in Learning Unit 10. After formation, these
organic compounds can be either emitted in the gas phase or retained on the fly ash.
Generally, MWIs have higher concentrations of dioxin/furan in the stack gases
than MWCs. The reasons relate to the features of adsorption of dioxin/furan
compounds by carbon.7 MWIs also produce less fly ash with smaller particle sizes
and lower carbon contents than MWC systems. The lower fly ash carbon contents
and lower particulate loadings appear to cause more of the dioxin/furan of MWIs to
be emitted as a gas rather than be adsorbed and retained with the ash.7
Injection of activated carbon powder into the flue gas can cause a considerable
fraction of trace organics to be absorbed (chemisorbed) onto the carbon.
Subsequently, the carbon can be collected along with the fly ash in an APCD. This
control concept is receiving considerable interest as an emission control method for
both gaseous dioxin/furan and volatile heavy metals (e.g., mercury).
Slide 15-6
PARTICULATE COLLECTION EQUIPMENT
1. Venturi Scrubbers
2. Fabric Filters
3. Electrostatic Precipitators
4. Mechanical Collectors
Cyclone Separators
Gravimetric Settling Chambers
The collection of particulates from MWIs is made difficult by the small
diameter of the particles and high temperature and corrosive nature of the gases.
However, venturi scrubbers and fabric filters are capable of routinely and
reliably meeting the current particulate matter collection requirements. The
operating principles of venturi scrubbers and fabric filters will be emphasized in this
learning unit.
Although most MWIs with particulate control equipment use venturi scrubbers ~
or fabric filters, electrostatic precipitators (ESPs) can also used. The description of
the operating principles of ESP will follow the discussion of venturi scrubbers and
fabric filters.
Mechanical APCDs, such as cyclonic separators and gravimetric settling
devices, are seldom used in MWI applications. Cyclonic devices use centrifugal forces
to separate large and dense particles from gas streams. Centrifugal force concepts
are used in mist eliminators which remove water droplets after wet scrubbing.
15-4
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Settling chambers make use of the force of gravity to remove very large
particles from slowly moving gas streams. Large ducts, such as the secondary
chamber of a controlled-air incinerator, can act as a settling chamber. The large
particles which are removed have little influence in meeting the regulatory
requirements. These particles, which accumulate on the lower refractory surfaces,
are generally removed during routine maintenance to repair the refractory.
Slide 15-7
VENTURI SCRUBBER WITH WATER SPRAY NOZZLES8
DIRTY FLUE GAS
CYCLONIC MIST
ELIMINATOR
SPRAY NOZZLES
LIQUID INLET
VENTURI THROAT-
Venturi scrubbers are the most commonly used particulate APCDs at MWIs.
They use a relatively high amount of energy and produce a wet sludge which contains
the collected particles. The energy requirements are proportional to the pressure
drop across the venturi, with typical values ranging from 20 to 60 in. we. The flue
gas leaving the venturi is generally saturated with water. Venturi scrubbers can be
integrated into systems for both particulate collection and acid gas removal.
Venturi scrubbers remove particles through the mechanism of impaction.
First, the gases are accelerated to a high velocity as they pass through the converging
section of the venturi. Water droplets are then injected into the throat area, creating
a uniform cloud of droplets. Impaction occurs as the fast moving particulates in the
gas stream collide with and stick on the relatively slow moving water droplets.
15-5
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Slide 15-8
VENTURI SCRUBBER WITH WATER FILM ATOMI.ZATION8
Liquid
Some venturi scrubbers introduce a film of water at the entrance of the
converging section and form droplets by the interaction of frictional forces.
Slide 15-9
VENTURI SCRUBBER WITH VARIABLE THROAT AREA9
THIMBLE' INLET
SCRUBBING UOOOB
TO THROAT
(K)% Of UOUIO)
ALTERNATE OR SUPPLEMENTAL
NOZZLE LOCATION FOR VEHV
HKSH TEMPERATURE GASES
TANGENTIAL UOJID INLETS
(40* OF LIQUID.
CONVERGING INLET WETTED
WtTH TANGENTIAL LIQUID
SCRUBBING UOUOB
TO THROAT
THROAT INSERT
THHOATCROSS SECTION
VARIES WITH INSERT
POSITION
EXPANDER SSCTICN
WETTED ELBOW-FILLS
wrrwuQuiD
ORi'juc OR MECHANICAL
ADJUSTMENT FOR THROAT
Courtesy of Andersen 2000, Inc.
15-6
-------
Other venturi designs have provisions for a variable throat area. Slide 15-9
shows a throat insert whose position can be changed to produce a range of throat
areas. This allows the venturi to maintain the same throat velocities regardless of
changes in the gas flow rate.
Such provisions will modulate the scrubbing action to obtain the required
collection performance over a range of flue gas flow rates. Such systems have the
disadvantage of greater factional losses associated with gas flow past the throat
insert.
Slide 15-10
FLOODED ELBOW AND CYCLONIC SEPARATOR FEATURES8
Cyclonic
separator
Flooded
elbow
The water droplets and their captured particulates can be removed from the
flue gas by application of various designs involving inertial, gravity and/or centrifugal
forces. Two of these design concepts are illustrated in the flooded elbow section at
the end of the diverging section and in the cyclonic mist eliminator.
Combined systems for acid gas control typically use a packed bed separator
with some form of a mist eliminator located at the exit.
15-7
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Slide 15-11
IMPINGEMENT PLATE SCRUBBER
dean |M
10
Mist elisuamtor
Impingement plate
Other low energy wet scrubbers, designed primarily for particulate removal
applications, include impingement plate scrubbers and sieve plate scrubbers.
These make use of a series of plates having a large number of holes. Water
cascades over the plates, and the holes act as orifices, creating high velocity jets of
flue gas which atomizes the water.10 The atomization process causes the particulates
to impact the liquid and be removed from the gas stream. Often, the sizes of the
holes are decreased in the direction of the flow, causing higher velocities and smaller
particles to be removed as the gas penetrates the scrubber.
Impingement plate scrubbers, as illustrated above, have an impaction plate
located above the holes to enhance the separation process.
One should note that these low energy scrubbers are very good for collecting
large particles, but their efficiencies are limited for small particles. Also, these wet
scrubbers can remove acid gases when used with a proper caustic solution.
15-8
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Slide 15-12
FABRIC FILTER COLLECTION MECHANISMS
1. Inertial Impaction
2. Direct Interception
3. Diffusion
4. Electrostatic Attraction
Fabric filter systems use a low energy technique for removing particulates from
gaseous streams, with pressure drops in the range of 1 to 5 in. we. The collection by
fabric filters is similar to that of a household vacuum cleaner. A fan sucks flue gas
through a fabric material (bag) which efficiently removes particulates of all sizes (e.g.,
98%).
Fabric filters are applied at MWIs either exclusively for particulate control or
as part of a combined system for removal of both particulates and acid gases. A
typical combined system is the dry sorbent injection (DSD system, where hydrated
lime (a calcium-based material) is injected into the flue gas to absorb acid gases. The
fly ash and the dry sorbent materials are subsequently collected by fabric filters.
Particles are collected by a combination of inertia! separation, direct
interception, diffusion, and electrostatic attraction.11 Inertial separation results from
the inability of large particles to change directions and turn past obstructions such
as single fabric fibers. Therefore, the particles impact the fibers, become trapped,
and, thus, are removed from the flue gas.
Direct interception is the sieving action which occurs when the size of the
particle is greater than that of the passageways through the filter. This is the
primary collection method of the filter cake, which is formed by the accumulation of
particulates on the fabric fibers. Most of the moderate sized particulates are collected
through direct interception by the filter cake.
Diffusion is the method whereby sub-micron particles (particle diameters less
than a micrometer) are collected. Diffusion relates to the random motion of such
particulates which allows collisions to occur in all directions, even though the gas
stream is primarily moving in one direction. Sub-micron particles are primarily
collected within the filter cake.
Electrostatic forces can affect collection because of the differences in electrical
charge between the particulates and the filter.
15-9
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Slide 15-13
CLASSES OF FABRIC FILTER SYSTEMS
1. Pulse-Jet
2. Reverse-Air
3. Shaker
Fabric filters may be classified, according to the method used for cleaning the
bags, as either pulse-jet, reverse-air, or shaker type units. The filter can be in the
form of woven or felt fabrics, which are made from natural fibers like wool or
synthetic fiber like as fiber glass. Each material generally has maximum
recommended operating temperatures and known chemical resistance properties.
Slide 15-14
PULSE-JET FABRIC FILTER
12
ClMn Air Plenum^. •
Blow Pipe
TeOran Ak Outlet
and Exhauster
Dirty Air Plenum
notary Valve Air Ux»
Courtesy of George A. Rolfes Company
Pulse-jet fabric filters systems are often used in MWI applications, probably
because they tend to require the least capital cost of the three designs. Particulates
are collected on the exterior of the vertical bags, and a cage inside each bag prevents
15-10
-------
it from collapsing. The top of each bag (open end) is attached to a tube sheet.
Cleaning is initiated by directing a burst of high pressure air through a nozzle
and down the inside of the bag. The pressure wave causes the bag to flex and
dislodge the particulate cake material. Operators are generally able to modify the
pulse sequence for a pulse-jet fabric filter.
Most of the agglomerated particulate matter falls into a collection hopper, with
a modest amount being re-entrained and subsequently removed by the fabric. If the
fabric is cleaned too well, the collection efficiency will be poor until the filter cake is
restored. Cleaning can also damage the bags.
Cleaning in some pulse-jet applications occurs without having to remove the
unit from service. The pressure front generally takes less than 0.2 seconds to move
down the bag, so system operation can be quickly restored. Of course, there is some
inefficiency of collection during this period.
Other pulse-jet filter are modularized to accommodate being taken off-line for
cleaning. Gas flow can be directed to adjacent compartments so that air cleaning
requirements are met while a particular compartment is off-line.
Slide 15-15
REVERSE-AIR FABRIC FILTER10
dean
15-11
-------
The reverse-air fabric filter system is designed to collect particulates on the
inside of the bags. Cleaning of this type system requires that a section be removed
from service. A relatively low pressure supply of clean air is directed back through
the filter, causing it to collapse and dislodge the collected particulate matter.
Slide 15-16
SHAKER-TYPE FABRIC FILTER13
MF1A1KMVM.VC
ASH
DCS ro SAL
MAHKXO
NAINTtKANCr
TMIM1U
Courtesy of ABB Flakt, Inc., printed with permission.
The shaker-type fabric filter system uses a mechanical shaker for cleaning the
bags. As with the reverse-air system, the particulates are collected on the inside of
the bags. The shaker-type units have the advantage of using mechanical linkages
rather than a high pressure air supply or a special set of dampers and air ducts.
15-12
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Slide 15-17
ELECTROSTATIC PRECIPITATOR
10
Di>chargt
electrodes
* Collection
electrode*
Hoppen
Although seldom used with MWIs, ESPs are often used to remove particulate
matter from MWC flue gas streams. A typical ESP is illustrated above.
Slide 15-18
ELECTROSTATIC COLLECTION PROCESS
High Voltage lonization of Molecules
Corona & Electric Fields Created
Charges Transferred to Particulates
Migration of Particulates to Plates
Removal of Particulates
The electrostatic collection process is based upon the attractive forces between
particles with different electrical charges. Particles are given an electrical charge
and then migrate out of the gas stream under the force of an electric field.
The charging is achieved by discharge electrodes which are charged with a
negative-polarity direct current voltage. The voltage must be high enough to ionize
the adjacent gas molecules and create a visible corona.
15-13
-------
Electric fields are created between the discharge electrodes and the collection
plates, which are electrically grounded. The electric field causes the negatively
charged gas molecules to migrate away from the corona and move toward the
grounded plates.
Slide 15-19
CORONA & COLLECTOR PLATE CONFIGURATION
•14
COLLECTOR
ElECTSODE AT
POSITIVE POLARITY
(i/ ELECTRICAL CHARGED
| "ELD PARTICLE
DISCHARGE ELECTRODE
AT NEGATIVE POLARITY
HIGH VOLTAGE
CURRENT SUPPLY
UNCHARGED
PARTICLES
PARTICLES ATTRACTED
TO COLLECTOR ELECTRODE
MO FORHIRt OUST LAYER
Particles in the contaminated gas stream will collide with the charged
molecules flowing across the stream. As collisions occur, the particles absorb
electrons and obtain a negative charge. The particles are then pulled toward the
plate by the electrical field.
Many ESPs use a series of separately controlled electric fields (or stages) to
provide multiple opportunities for the charging and collection. Particles which pass
through one stage may be charged and collected in a downstream stage. Also, some
of the collected particles will be re-entrained because of plate cleaning, electrical field
instability, vibrations or turbulent gas flow disturbances. These can be collected in
the next downstream section of the ESP.
15-14
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Slide 15-20
ESP DESIGN COMPONENTS
Step-Up Transformer
High Voltage Rectifier
Shell Enclosure for Support & Insulation
Vertical Wires - Discharge Electrode Wires
Vertical Plates - Collection Electrodes
Rappers
Hoppers
Step-up transformers and high voltage rectifiers are used to convert alternating
current (a.c.) electricity into high voltage direct current (d.c.). Depending upon the
design, the voltage can range from 20,000 to 100,000 d.c. volts.15
ESPs contain a number of vertical plates which act as collection electrodes.
The plates form parallel gas flow paths which are horizontally oriented. Discharge
electrodes are suspended between the plates. The electrodes can be either vertical
wires or supported frames which are less prone to movement.
Slide 15-21
ESP PARTICULATE REMOVAL
Charged Particle Adheres to Plate
Dry Removal - Mechanical Rappers
Wet Removal - Water Sprays
Delivery to the Hopper
Under normal conditions, the particles have fairly high electrical resistance
and will only be partially discharged when they arrive at the collector plate. Since
opposite charges attract, the particles will continue to adhere to the plate and to
other collected PM as the charge is slowly leaked to ground. The charge of the
collected particulate is generally maintained by the arrival of additional particles.
Layers of dry collected particulates are removed through the mechanical action
of rappers or vibrators. Rappers are often actuated by pneumatic or solenoid devices.
Some of these devices operate upon the release of a magnetic field or through a series
of rotating cams or hammers. The dislodged PM is designed to fall into collection
hoppers from which it is subsequently removed.
15-15
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REFERENCES
1. David A. Tillman et al., Incineration of Municipal and Hazardous Solid Wastes.
Academic Press, Inc, New York, 1989, pp. 67, 85, 98, 143.
2. David B. Sussman, "Municipal Waste Combustion Ash: Testing Methods,
Constituents, and Potential Uses," Proceedings of the International Conference
on Municipal Waste Combustion. Volume 2, Sponsored by U. S. Environmental
Protection Agency and Environment Canada, Hollywood, FL, 1989, pp. 11B-13
to 11B-25.
3. "Emission Control Systems for Incinerators," Report Number TR-89-900239,
Andersen 2000, Inc., Peachtree City, Georgia, February 1989, pp. 2 and 3.
4. W. S. Lanier and T. R. von Alten, "Investigation into the Discrepancy Between
MWI and MWC CDD/CDF Emissions," 1992 Incineration Conference (The
Eleventh Annual International Symposium on Thermal Treatment
Technologies), Albuquerque, New Mexico, May 11-14, 1992.
5. Jack Brady, "Submicron Aerosol Generation and Aerosol Emission Control in
Infectious Waste Incinerators," Presented at the Second International
Conference on Municipal Waste Combustion, Tampa, Florida, April 16-19,
1991.
6. Glen England et al., "Michigan Hospital Incinerator Emissions Test Program,
University of Michigan Medical Center Incinerator," Final Draft Report, EPA
Contract No. 68-03-3365, Prepared by Energy and Environmental Research
Corporation jointly for the Michigan Public Service Commission and the U. S.
Environmental Protection Agency, April 25, 1991, p. 2-19.
7. Glen England et al., "Michigan Hospital Incinerator Emissions Test Program,
Borgess Medical Center Incinerator," Final Draft Report, EPA Contract No.
68-03-3365, Prepared by Energy and Environmental Research Corporation
jointly for the Michigan Public Service Commission and the U. S.
Environmental Protection Agency, April 15, 1991, p. 2-19.
8. J. Joseph and David Beachler, APTI Course SI:412C. Wet Scrubber Plan
Review-Self Instructional Guidebook. U. S. Environmental Protection Agency,
EPA-450/2-82-020, March 1984.
9. J. Brady, "Economically and Operationally Attractive Incinerator Emission
Controls," Undated Report from Andersen 2000, Inc., Peachtree City, Georgia,
p. 21.
15-16
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10. David S. Beachler and James A. Jahnke, APTI Course 413. Control of
Particulate Emissions, Student Manual. U. S. Environmental Protection
Agency, EPA-450/2-80-066, October 1981, pp. 7-1, 8-23, 9-24, 9-25.
11. David S. Beachler and M. M. Peterson, APTI Course SL412A. Baghouse Plan
Review—Self Instructional Guidebook. U. S. Environmental Protection Agency,
EPA-450/2-82-005, April 1982.
12. Control Techniques for Particulate Emissions from Stationary Sources. Volume
1, U. S. Environmental Protection Agency, EPA-450/3-81-005a, September
1982.
13. Illustration of Shake/Deflate-Cleaned Baghouse, ABB Environmental Systems,
ABB Flakt, Inc., April 1992.
14. PEI Associates, Operation and Maintenance Manual for Electrostatic
Precipitators. EPA-625/1-85-017, September 1984.
15. Municipal Waste Combustion Study. Flue Gas Cleaning Technology. U. S.
Environmental Protection Agency, EPA/530-SW-87-021d, June 1987, pp 2-1 to
2-19.
15-17
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16. FLUE GAS CLEANING II: ACID GAS REMOVAL
Slide 16-1
ACID GAS REMOVAL TECHNOLOGY
Wet Scrubbers - Venturi
- Packed-Bed
- Impingement Scrubber
Dry Scrubbers - Dry Sorbent Injection
- Spray Dry Absorber
The USEPA is currently developing regulations which may require control of
acid gas emissions from MWIs. Typically, the USEPA requires control systems which
are at least equivalent to that obtained through the application of the best
demonstrated control technology (BDT). To date, the designation of BDT for MWI
units has not yet been established by the USEPA.
Various wet, dry and combination scrubbing processes can be used for acid gas
control.1 In general, the cost of the reagent is an important part of the scrubber's
annual costs. Sodium-based liquid reagents are more expensive than lime based dry
reagents, although their operational requirements are generally less demanding.
Slide 16-2
EXAMPLE ACID GAS NEUTRALIZATION REACTIONS
Caustic Soda: NaOH + HC1 --> NaCl + H2O
Soda Ash: Na2CO3 + 2HC1 --> 2NaCl + H2O + C02
Sodium Bicarbonate: NaHCO3 + HC1 --> NaCl + H2O + CO2
Slaked Lime: Ca(OH)2 + 2 HC1 --> CaCl2 + 2 H2O
An aqueous (water) solution of a selected caustic (alkali) material can be used
to collect and neutralize acid gases. Example alkali materials include sodium
hydroxide, NaOH (caustic soda, liquid), sodium carbonate, Na2CO3 (soda ash, dry),
sodium bicarbonate, NaHC03 (bicarbonate of soda), and lime-based solutions of
calcium hydroxide, Ca(OH)2 (slaked lime).2 Reactions between the acids and
scrubbing agents neutralize the acids and form salts which either are soluble in water
or will precipitate out of solution.
16-1
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Wet scrubbing systems can use various types of equipment, including high
energy venturi scrubbers and low energy packed-bed, baffle plate, or impingement
plate scrubbing systems.
Dry scrubber systems get their name from the scrubbing action of materials
that absorb acids and from the removal of the reaction products as dry materials.
Dry scrubber systems are usually comprised of acid gas neutralizing equipment and
particle removal equipment.
Slide 16-3
PACKED-BED WET SCRUBBER1
dean gu
Mist eliminator
Absorption of acid gases into liquid droplets is enhanced by providing large
liquid contact areas, good mixing of the gas and liquid, and sufficient contact time for
the absorption process to occur.
Packed-beds provide enhanced wetted surface areas for the absorption of acid
gases. Designers can select from among a number of different types of packing
materials and element geometries. Each type of element will have a characteristic
amount of surface area augmentation with a corresponding pressure drop.
16-2
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Slide 16-4
COMBINED SYSTEM: VENTURI AND PACKED-BED2
Courtesy of Andersen 2000 Inc.
Slide 16-5
COMBINED SYSTEM: VENTURI/BAFFLE PLATE SCRUBBER2
BLENCH SECTIW
uauiu onouMc
Courtesy of Andersen 2000 Inc.
16-3
-------
'r
Wet scrubber equipment combinations have been considered for removing both
acid gases and particulates. A typical combination, as illustrated in the slide above,
would be a venturi scrubber, which is used primarily for particulate removal, and a
packed bed scrubber supplied with a sodium hydroxide solution for acid gas removal.
Lime-based caustic scrubbing solutions, such as calcium hydroxide (slaked
lime), cause the formation of salts which are not soluble in water. These salts will
form precipitates which may cause plugging of passageways. Therefore, lime-based
caustic materials are generally not used in packed-beds. However, lime-based
scrubbing liquids are routinely used in baffle type scrubbers.
Combined systems typically use a venturi for removing particulates and a
packed bed or other low energy wet scrubber for acid gas removal. In fact, both acid
gas and particulate matter are removed in each part of the combined system.
Water, which is often used as the scrubbing liquid in the venturi, will also
absorb HC1. If a caustic scrubbing solution is used in the venturi, both particulate
and some acid gas removal can be provided. Using a caustic solution in the venturi
will also tend to minimize the corrosion of the equipment. Of course, caustic
solutions are used in low energy wet scrubbers for acid gas removal.
Slide 16-6
WET SCRUBBER ACID GAS APPLICATIONS
Advantage
Moderate Pressure Drop
No Special Start-Up Problems
Disadvantages
Corrosion and Erosion
Liquid Residue Produced
There is a considerable amount of operating experience using wet scrubbing
systems for the removal of acid gases in MWI applications. Typical applications
which are designed only for acid gas removal will require a pressure drop of 1 to 5
in, we., approximately equivalent to that of a fabric filter.
As discussed in Learning Unit 15, pressure drops as high as 60 in. we. are
often required when venturi scrubbers are used to remove particulates.
Another advantage of wet scrubbing systems is that there are no special start-
up problems, generally, a switch is turned on and the liquid begins to circulate
through the system.
16-4
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Considerable corrosion of metal surfaces can occur with wet scrubbing systems.
If the scrubbed gases are cooled below the dew point, acidic or caustic droplets may
be formed on the duct walls and cause significant metal wastage. Routine treatment
of the liquid streams is generally required for controlling pH and minimizing the
corrosion of the equipment exposed to the liquids.
Both erosion and corrosion can occur in high energy venturi scrubbers, where
gases are often accelerated up to 400 ft/sec. The entrained particulates can cause
serious erosion, essentially sand-blasting metal surfaces. Corrosion is associated with
the imperfect neutralization of the absorbed gases, leading to acidic and caustic
reactions, particularly on metal surfaces which have undergone erosion.
With proper pH control, many applications have been able to discharge the
liquid waste into a sanitary sewer. However, some sewer authorities are disallowing
or restricting such disposal because of concern about contamination of the water
supply. Special treatment may be required before disposal of liquid residues is
allowed.
Slide 16-7
WET SCRUBBER SYSTEM WITH SPRAY DRYER3
FROM
INCINERATOR
DRY WASTE
Courtesy of AirPol, Inc., printed with permission.
It is possible to combine wet scrubbing with a spray dryer and remove the
residue as a dry material. Systems with this feature, called "zero liquid discharge
16-5
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systems," avoid the waste water treatment and disposal problems associated with
liquid waste streams. Systems, such as that illustrated above will be found to
produce dry residues which are characterized by the very small diameter particles of
salt or calcium chloride in a mixture with fly ash.
The flue gas can be used to evaporate the scrubber blow-down (liquid waste).
Some systems use a special filter system to extract the suspended solids before the
liquid waste is sent to the spray dryer. Systems, such as that illustrated above, will
be found to produce dry residues which are characterized by the very small diameter
particles of salt or calcium chloride.
The physical absorption and chemical reaction processes of the scrubber are
generally improved with lower flue gas temperatures, so the cooling of the hot flue
gases in the spray dryer is desirable. Note that the use of a waste heat recovery
boiler will, typically, cool the flue gases enough to prevent the direct application of
this evaporative cooling strategy.
Slide 16-8
DRY SORBENT INJECTION SYSTEM4
Courtesy of ABB Flakt, Inc., printed with permission
Dry sorbent injection (DSI) systems are increasingly being used in MWI
applications. Typically, powdered hydrated lime (a calcium-based material) is
injected into the flue gas after it leaves the secondary chamber. The sorbent
materials absorb the acid gases and are partially converted into new chemical
products (calcium-based salts). The dry reaction products and residual sorbent
materials are typically collected along with the fly ash by fabric filters.
16-6
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DSI systems may deliver a stream of powdered hydrated lime (calcium
hydroxide) or sodium bicarbonate into the flue gas. Hydrated lime is the most
common sorbent materials, and it has been found to be a widely available at a
moderate cost. The powdered sorbent can be delivered directly into the flue gas by
a blower.
Hydrated lime, which has been ground to the consistency of talcum powder, is
commercially available and often used as the sorbent. Magnesium oxide and sodium
bicarbonate are alternative sorbent materials which can be used for acid gases.
Activated carbon injection is a potential aid for enhancing the adsorption of mercury
and dioxin/furan emissions.
The stoichiometric ratios for DSI systems are generally above those of spray
dry absorber systems, which is shown below. Designers will consider the economic
trade-off associated with the maintenance and operational costs of operating the
slaker and atomizer versus the extra cost of dry sorbent materials.
Slide 16-9
SPRAY DRYER ABSORPTION PROCESS5''
LUtE FEEDER
LIME SLAKER
3. FEEDTANK
4. HEAD TANK
ft. SPRAY ABSORBER
B. DUST COLLECTOR
7. STACK
DRY WASTE
Spray dryer absorption (SDA) systems achieve acid gas absorption by spraying
an aqueous slurry of calcium hydroxide (hydrated lime) into the flue gas. SDA
systems are fairly complicated from both a mechanical and thermal process control
standpoint. These systems are routinely used in MWC applications, but are currently
used on only a few MWI applications.
16-7
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The typical slurry is generally formed by blending pebble lime (CaO) with
water to form an aqueous solution of calcium hydroxide (hydrated lime). The
blending tank system, which is called a slaker, produces a liquid slurry which
typically ranges from 5 to 20 percent by weight of solids.7
The slurry temperature is raised to the range of 165 to 190°F by the chemical
energy released by the slaking process which produces the calcium hydroxide. Mixing
of the alkali, sorbent solution and maintaining its temperature to at least around
140°F are required to prevent slurry solidification as a cement-type material.
Slide 16-10
SPRAY DRYER ABSORBER OPERATIONS
* Slurry Atomized to Pine Droplets
* Reaction Chamber Provides Residence Time for
Acid Absorption on the Slurry Droplets
* Slurry Droplets are Dried by Hot Flxie Gas
* Flue Gases are Cooled by Evaporation
* Particulates Collected by a Fabric Filter
The slurry is typically distributed into the flue gas stream by either a high
speed rotary atomizer (e.g., 10,000 to 17,000 rpm) or to an air atomizer nozzle.
Typically, a cone-shaped spray of small liquid droplets is produced.7 The flue gas
flows through the spray, creating opportunities for collisions of gas molecules with
droplets of sorbent material.
After being absorbed by the droplets, the acids chemically react with the
reagent, producing calcium chloride and calcium sulfate. These reactions can occur
in slurry droplets or on dried particles.
A reactor vessel which encloses the atomizer provides a period of residence
time (e.g., up to 10 seconds8) for moisture evaporation from the slurry and for acid
gas absorption on the surface of liquid droplets or solid particles.
The latent heat, which is required to vaporize the water of the slurry, is
provided by the hot combustion gases. Consequently, the gases leaving the reactor
vessel can be cooled to temperatures as low as 250°F.
The hydrated lime collected on the "cake" of the fabric filter also serves to
enhance the acid gas removal process.
16-8
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REFERENCES
1. David S. Beachler and James A. Jahnke, APT! Course 413. Control of
Particulate Emissions, Student Manual. U. S. Environmental Protection
Agency, EPA-450/2-80-066, October 1981, p. 9-54.
2. "Emission Control Systems for Incinerators," Report Number TR-89-900239,
Andersen 2000 Inc., Peachtree City, Georgia, February 1989, pp. 2, 3, 15, and
16.
3. R. G. Barton, et al., "State-Of-The-Art Assessment of Medical Waste Thermal
Treatment," Report to Risk Reduction Engineering Laboratory, USEPA, and
California Air Resources Board, June 15, 1990, pp. 97-108.
4. Robert G. Mclnnes, "Spray Dryers and Fabric Filters: State of the Art," Solid
Waste & Power. April 1990, pp. 24-30.
5. Theodore G. Brna, "Cleaning of Flue Gases from Waste Combustors",
Combustion Science and Technology. Vol. 74, 1990, pp. 83-98.
6. T. G. Brna and C. B. Sedman, "Waste Incineration and Emission Control
Technologies," International Congress on Hazardous Materials Management,
Chattanooga, TN, 1987.
7. Richards Engineering, "Municipal Waste Incinerator Field Inspection
Notebook," U. S. Environmental Protection Agency, EPA-340/1-88-007, July
1988, pp. 45-55.
8. William Ellison, "Flue-Gas Desulfurization," Standard Handbook of Power
Plant Engineering. Thomas C. Elliott, editor, McGraw Hill Book Co., NY, 1989,
pp. 4.95-4.115.
16-9
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17. TOXIC METAL CHARACTERISTICS AND EMISSIONS CONTROL
Slide 17-1
METALS IN MEDICAL WASTE
Medical Waste Composition:
Average of 8 to 10% Inorganic (Ash)
Up to 4% Metals
Non-Toxic Metals:
Major Toxic Metals:
Other Toxic Metals:
Iron, Aluminum
Lead, Cadmium, Mercury
Antimony, Arsenic, Barium
Beryllium, Chromium, Nickel
Silver, Selenium, Thallium
The average medical waste compositions presented in Slide 3-16 had inorganic
(ash) contents in the range of 8 to 10%. The ash is generally considered to be
incombustible, mostly in the form of silicon oxides (glass) and metal oxides. Glass
contents have been found to vary considerably, with some laboratories producing
medical waste with 35% glass. Metals generally average less than 4% of the mass
of medical waste, as previously illustrated in Slide 3-13.
The toxic metals includes four carcinogenic metals: arsenic, cadmium,
beryllium and chromium. This concern is reflected in leaching characteristics tests.
Source separation and source reduction, as discussed in Learning Unit 4, are
major methods of controlling the emissions of toxic metals. For instance, keeping
consumer batteries out of the medical waste stream will reduce the potential
emissions of lead, cadmium and mercury.
Slide 17-2
CHANGES IN METALS DURING INCINERATION
High Melting Point (Non-Volatile) Metals:
Form Oxides, Chlorides, Sulfides
Remain in the Solid Residue (Ash)
Low Melting Point (Volatile) Metals:
Form Liquids which Solidify when Cooled
Form Vapors which Condense when Cooled, Are
Adsorbed onto Fly Ash, or Remain As Vapor
17-1
-------
Metals which enter the combustion zone can undergo different physical and
chemical transformations, depending on their chemical characteristics, the
combustion temperatures, melting points, and vapor pressures.
Many of the metals have high melting temperatures and are non-volatile.
Upon heating in the combustion zone, these metals can react to form oxides, sulfides
and chlorides. Under local starved-air conditions, some metal compounds can be
reduced back to a pure metal, but most non-volatile metals will leave in the
combustion chamber in the form of solid oxide particles.
Volatile metals, such as lead and aluminum, may be melted and partially
vaporized during the combustion process. Liquid metals may be deposited and
solidified on the relatively cold surfaces of the hearth, refractory liners or particles.
Slide 17-3
TOXIC METAL PATHWAYS INTO ENVIRONMENT
1. Wet Scrubber Liquids - Waste Water
2. Bottom Ash - Leachate from Landfill
3. Collected Fly Ash - Leachate from Landfill
4. Stack Emissions - Gases & Particles
The above slide lists four pathways of exposure through which toxic metals are
released from medical waste into the environment. The primary concern is that the
heavy metals may cause toxic exposures to humans, through contamination of plant
and animal tissues which make up the aquatic and terrestrial food-chain or through
inhalation.
Wet scrubbing systems will produce liquid wastes (blow-down) that contain
various suspended (undissolved) solids, including salts of heavy metals such as lead
and cadmium. These liquid wastes may be contaminated enough to require dilution
or waste water treatment before they can be disposed in sanitary sewers.
17-2
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Slide 17-4
EXAMPLE COMPOSITIONS OF METALS
COMPONENT (mg/kg)
Arsenic
Cadmium
Chromium
Iron
Lead
Manganese
Mercury
Nickel
Total Metals
UNIT D
11.7
5.6
180
6,200 15
< 5.6
340.2 1
< 0.02
55
6,798
IN BOTTOM
UNIT E
13.9
5.1
120
,000
180
,000
0.2
41
16,360
ASH1'2'3
UNIT
5
46
685
901
< 9
86
F
.4
.4
.8
.8
Bottom ash represents approximately 98% of the ash residues from MWIs.
Heavy metals can be leached (extracted by acids) from ash in landfills, forming
ground water contamination. The metal contents in bottom ash vary considerably
depending on the sources and the degree of materials separation utilized.
The above slide presents the metals contents of bottom ash from representative
MWIs. Similar results from three small MWIs were previously presented in Slide 3-
19. Note that the Unit D incinerator is a continuously operated MWI (1,500 Ib/hr)
with a variable venturi scrubber and a waste-heat boiler. Unit E is an intermittent
duty (650 Ib/hr), controlled-air unit with a lime injection and baghouse system. The
Unit F incinerator is a single, batch-burn unit (750 Ib/batch) equipped with a
baghouse and packed bed scrubber.
Slide 17-5
COMPARISON OF BOTTOM ASH & BAGHOUSE
COMPONENT (mg/kg)
Arsenic
Cadmium
Chromium
Iron
Lead
Manganese
Mercury
Nickel
Total Metals
BOTTOM ASH
13.9
5.1
120
15,000
180
1,000
0.2
41
16,360
ASH METALS1
BAGHOUSE ASH
12.5
94
55
1,200
870
49
18
12
2,311
17-3
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As shown in the above slide, the small particles (baghouse ash) will contain
higher concentrations of some heavy metals than the larger particles (bottom ash).
This phenomena is associated with the condensation of volatile metals as the
flue gas cools. Surface condensation is a process which is limited by the area
available. Smaller sized particles have more surface area per unit weight, and most
of the particles formed by MWIs have submicron sizes. Therefore, smaller particles
often have higher concentrations of volatile metals, such as lead and cadmium.
Condensation can also result in the formation of submicron sized liquid
droplets.
Slide 17-6
MAJOR TOXIC METAL AIR POLLUTANTS
Lead - Particulate
Cadmium - Particulate
Mercury - Particulate and Vapor
The primary environmental concern about metals relates to their toxicity as
air pollutants. The toxic metals with the largest concentrations in the flue gas
leaving the secondary combustion chamber are lead, mercury and cadmium.
Metals can be emitted from the stack as solid or liquid particulate or in vapor
(gaseous) form, depending upon the local thermal conditions and the metal.4 If the
flue gas is adequately cooling, most of the volatile metal vapors will either be
condensed in the form of small liquid metal droplets or adsorbed onto the surfaces of
particulates.
Lead and cadmium are examples of volatile metals that are generally deposited
(condensed, adsorbed) on the surface of fly ash. Their concentration is often higher
in the smaller particulates of fly ash (baghouse ash), as indicated in Slide 17-5.
The volatile toxic metal of greatest concern is mercury. It is vaporized during
combustion and has relatively high vapor pressures at flue gas temperatures.
Because condensation becomes less likely as the vapor pressure rises, a considerable
fraction of mercury will not condense before leaving the stack. Therefore, mercury
is emitted both in vapor and particulate forms.
Most of the toxic metals can be routinely collected in solid form as part of the
fly ash and bottom ash residues.
17-4
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Slide 17-7
CONTROL STRATEGY FOR METAL AIR POLLUTANTS
* Provide for Condensation and Adsorption
by Controlling APCD Temperatures
* Inject Activated Carbon Powder for
Enhanced Adsorption
* Collect Metals as Particulates
The metal emissions control strategy is to provide for adequate condensation
and adsorption of metal vapors onto the particulates as the gases cool so that metals
can be collected by the particulate control equipment. Some vapors will condense
and form small diameter liquid droplets which may be collected by a high efficiency
APCD.
Injection of activated carbon powder into the flue gas can cause a considerable
fraction of mercury vapor to be absorbed (chemisorbed) onto the charcoal.
Subsequently, the carbon can be collected along with the fly ash in an APCD. This
control concept is receiving considerable interest as a control method for both mercury
and gaseous dioxin/furan emissions.
Slide 17-8
TOXIC METALS AS GROUND WATER POLLUTANTS
Organic Decomposition to Form Acids
Acid Extraction of Heavy Metals from Ash
Ground Water Contamination by Heavy Metals
Ash disposal in a landfill creates a concern about the long-term possibility of
polluting the ground water with toxic metals leached from the ash. Acids, which are
can be formed by organic decomposition in landfills, can leach the heavy metals from
the ash and form a liquid called leachate. Ground water pollution will occur when
the leachate leaks into the ground water.
Depending upon the applicable regulations, bottom ash and fly ash can be
disposed either alone in a single composite-lined monofill or with mixed waste in a
modern landfill having a double composite-liner and leachate monitoring system.6
17-5
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Ground water contamination from landfills and monofills can be controlled by
proper construction, verification and treatment. Modern landfills and monofills must
have the liners, caps and leachate monitoring, collecting, and treating systems
specified by the RCRA regulations.
A monofill, which is restricted to receive ash, will have very little organic
material which can form acids. Therefore, the leachate from a monofill is
predominantly inorganic, with salts and metal compounds.5
Slide 17-9
IS ASH HAZARDOUS OR NON-HAZARDOUS?
Answer Varies from State to State:
Regulatory Definitions
Toxicity Test Requirements
Ash Sampling Procedures
Currently, an unresolved national question is whether ash residue should be
classified as a hazardous or non-hazardous material.
The USEPA presently has no formal statement on the hazardous waste aspects
of ash. However, many states have developed their own answer to this question, with
the definitions and requirements for testing and disposal varying from state to state.6
Amendments to RCRA, which passed in 1984, have provisions which define
solid waste ash as being non-hazardous, subject to some limitations6. However,
Section 3001 of RCRA specifically classifies a waste material as being hazardous if
it fails tests for ignitability, reactivity, corrosiveness, or toxicity.
Ash can generally pass the ignitability, reactivity and corrosiveness tests. The
typical carbon burn-out of ash prevents it from being ignited, and the reactivity is
controlled by separating and treating separately those medical waste materials
containing radioactive materials. The corrosion test considers the pH of the material,
and the pH of medical waste ash is generally within the acceptable limits (from 7.0
to 12.5).
17-6
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Slide 17-10
LABORATORY PROCEDURES FOR TOXICS
EP - Extraction Procedure Toxicity Test
(an early procedure)
TCLP - Toxic Characteristic Leaching
Procedure (Method 1311)
EPA Method 1312 - Synthetic Precipitation
Leach Test for Soils
EPA Method 3050 - Acid Digestion of
Sediments, Sludges & Soils
Testing of the leaching characteristics of ash is used to determine if an
improperly managed waste poses an unacceptable risk to groundwater and therefore
should be managed as a hazardous waste.7 This testing of ash samples is typically
required before ash can be disposed in a sanitary landfill or monofill.
The initial USEPA method for determining ash toxicity is the Extraction
Procedure Toxicity Characteristics Test (EP). Regulatory thresholds, based on the
EP test, have been established for 8 metals, 4 pesticides and 2 herbicides.7
The Toxic Characteristic Leaching Procedure (TCLP) is a newer regulatory test,
which has been proposed as a replacement for the EP test.7 This test method is
designed to simulate the leaching of toxic constituents in a co-disposal sanitary
landfill and to measure the toxicity of such leachate. The TCLP involves passing
waste material through a sieve, adding various extraction fluids, and agitating the
mixture. The mixture is filtered and the leachate is analyzed for 40 constituents. If
any of the constituent's concentrations exceed their specified limit, the waste is
classified as hazardous.6
Ash sampling requires complicated procedures because of the variations in the
constituents. Bottom ash represents around 98% of the ash formed. Bottom ash is
composed mainly of glass, metal and other inorganic compounds from paper, plastic,
rubber, food wastes, and other products (e.g., clays in papers, stabilizers in plastics,
pigments in printing inks, salts in vegetables).8 Fly ash is formed from entrained
inorganic materials and the adsorption of volatiles during the flue gas cooling process.
Samples of bottom ash and mixtures of bottom ash and fly ash often pass
TCLP tests. Because the fly ash particles contain greater concentrations of heavy
metals, the results may exceed the regulatory thresholds for lead and cadmium when
tested alone by the TCLP procedure.6
17-7
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Some regulations allow mixing of bottom ash and fly ash prior to conducting
a leachate test, but other regulations require separate testing of bottom ash and fly
ash.9 If the ash fails a leach test, alternate treatment and disposal options will have
to be considered.
The regulatory requirements are not currently uniform, and vary from state
to state. Some states6 have adopted requirements which specify the testing of ash
through the use of EPA Method 131210 or the EPA Method 3050.11 These tests have
some contrasting features, so that one or the other may better simulate their landfill
or monofill conditions.
Slide 17-11
ASH TREATMENT BEFORE DISPOSAL
Chemical Extraction
Chemical Additives
Compaction
Vitrification
There are a number of ash treatment strategies currently in use, including
treatment strategies which are designed to reduce the amount ofleaching of toxic
metals from the ash and disposal in a hazardous landfill.12
Chemical extraction techniques can be used to treat the ash with an acid to
remove the metals prior to disposal. Alternatively, the ash can be treated with
additives which bind the metals in a stable and impermeable form.
Physical compaction of ash, combined with the injection of mixtures of water
and either lime or portland cement, can reduce the effective amount of exposed
surface area so that less leaching will occur.
Research is under way to develop an optimum method of encapsulating ash in
such a way as to permanently fix the metals within glass beads and, therefore, be
highly resistant to ground water leaching. This process is called "ash vitrification"
and provides for the melting of ash into a slag in special high temperature furnaces.13
The product formed is expected to be acceptable for use as a light-weight aggregate
material suitable for construction projects.
17-8
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REFERENCES
1. Glen England et al., "Michigan Hospital Incinerator Emissions Test Program,
Borgess Medical Center Incinerator," Final Report, EPA Contract No. 68-03-
3365, submitted by Energy and Environmental Research Corp. to Michigan
Public Service Commission and the USEPA, April 15, 1991, pp. 2-70 to 2-71,
2-81.
2. Glen England et al., "Michigan Hospital Incinerator Emissions Test Program,
University of Michigan Medical Center Incinerator," Final Draft Report, EPA
Contract No. 68-03-3365, Prepared by Energy and Environmental Research
Corporation jointly for the Michigan Public Service Commission and the U. S.
Environmental Protection Agency, April 25, 1991, p. 2-54.
3. Radian Corporation, "Medical Waste Incineration, Emission Test Report,
Volume 1, Jordan Hospital," Report Number DCN: 90-275-026-25-0, EPA
Contract No. 68-D-90054, Submitted to the U. S. Environmental Protection
Agency, February 1992, pp. 2-53, 2-54.
4. Robert G. Barton et al., "Fate of Metals in Waste Combustion Systems,"
Combustion Science and Technology. Vol. 74, Number 1-6,1990, pp. 327-343.
5. Jeffrey L. Hahn, "Managing Ash--Closing in on Policy Decisions," Solid Waste
& Power. August 1989, pp. 12-18.
6. Marc J. Roggoff, "The Ash Debate: States Provide Solutions," Solid Waste &
Power. October 1991, pp. 12-18.
7. R. Mark Bricka et al., "A Comparative Evaluation of Two Extraction
Procedures: The TCLP and The EP," U. S. Environmental Protection Agency,
EPA/600-S2-91/049, March 1992.
8. David B. Sussman, "Municipal Waste Combustion Ash: Testing Methods,
Constituents and Potential Uses," Proceedings, International Conference on
Municipal Waste Combustion. Volume 2, April 11-14,1989, Hollywood, Florida,
pp. 118-13 to 118-25.
9. R. A. Denison and J. Ruston, Editors, Recycling & Incineration. Evaluating the
Choices. Island Press, Washington, DC, 1990, pp. 177-198.
10. "Synthetic Precipitation Leach Test for Soils," Method 1312, U. S.
Environmental Protection Agency, SW-846, 1985.
17-9
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11. "Acid Digestion of Sediments, Sludges, & Soils," Method 3050, U. S.
Environmental Protection Agency, SW-846, 1985.
12. "Treatment and Use: The Future of Ash Management?" Solid Waste & Power,
October 1991, pp. 20-28.
13. G. L. Harlow, et al., "Ash Vitrification--A Technology Ready for Transfer,"
Proceedings of the 1990 ASME National Waste Processing Conference. Long
Beach, CA, June 3-6, 1990, pp. 143-150.
17-10
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18. APCD PERFORMANCE & SYSTEM CONTROL FEATURES
Slide 18-1
APCD PERFORMANCE INDICATORS
Operational Indicators
Opacity or Visual Emissions
Temperatures
Pressure Drops
Gas Concentrations
Stack Tests
Particulate Emissions
Dioxin/Furan Emissions
This learning unit presents both the APCD performance evaluation features
and the operating APCD features used to control particulate and gaseous pollutant
emissions.
A visual observation of high plume opacity is the most obvious indicator that
something is wrong with the MWI operation and/or APCD. Opacity CEMS are more
useful because their readings are reliable and can be continuously recorded.
Temperatures, pressure drops and concentrations of pollutant emissions (e.g., HC1)
are other indicators of APCD performance.
Slide 18-2
EXAMPLE ACID GAS REMOVAL EFFICIENCIES1'2
Equipment HC1 SO2
Wet Scrubbers
Dry Scrubbers
95 - 99.9
90 - 99
90 - 99
60 - 85
In general, wet scrubbing systems are capable of achieving very high acid gas
collection efficiencies (e.g., 95 to 99.9% of HC1).1 Efficiencies are influenced by
pressure drop, flue gas temperature, and the amount of scrubbing liquid delivered.
Dry sorbent injection system vendors have reported HC1 removal efficiencies
ranging from 90 to 99%.l The higher removal efficiencies may require that the
sorbent stoichiometric ratio (ratio of actual to theoretical requirement) to be as high
as 4:1 or an extended retention time be provided in a reaction chamber.
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Spray dryer absorber (SDA) systems have been reported to remove 90 to 95%
of the hydrogen chloride gas and 60 to 85% of the sulfur dioxide.1 One unit was
reported to achieve 99 percent hydrogen chloride and 93 to 98% sulfur dioxide
removal.2
Slide 18-3
TYPICAL CEMS USED AT MWI UNITS
1. Combustion Temperatures
2. Opacity
3. Carbon Dioxide
4. Oxygen
5. Carbon Monoxide
6. Sulfur Dioxide
7. Hydrogen Chloride
CEMS requirements will depend upon the applicable regulation or permit
condition, unit size, and whether the unit is considered to be new or existing.
Typically, the permits will specify the variables which must be monitored and
the corresponding maximum concentrations or operational limits. In addition to
those listed above, CEMS for total hydrocarbon, and/or pH may be required.
Slide 18-4
STACK TESTING METHODS
Particulates:
Hydrogen Chloride:
Multi-Metals:
Dioxin/Furan:
Methods 5 & 17
Method 26
Method 29
Method 23
Stack testing is performed to determine emissions of selected pollutants such
as particulates, acid gases, trace organics and trace metals, and to verify the ability
of the equipment to meet emission limitations. Stack testing is typically performed
during initial start-up of new systems. MWI regulations may require annual stack
testing, although this requirement is often limited to units above a given capacity.
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Slide 18-5
PARTICLE SAMPLING TRAIN, METHOD S3
PitotTube
-Temperature Sensor
^Thermometer
Filter Holder
Check Valve
\
Ice Bath Impingers
By-Pass
, Valve
Probe
"7
Reverse-Type
Pilot Tube
Stack Wall
•Air-Tight Pump
Dry-Gas Meter
Courtesy of ABB Combustion Engineering, Inc.
The standard USEPA Method 5 sampling train and stack testing procedure are
used to determine particulate concentrations.2 The Method 5 sampling train is
illustrated in Slide 18-5. If particle size distributions are desired, an in-stack filter
(impactor) can be used, as in Method 17.
Most stack sampling methods employ isokinetic sampling, which requires that
the sampling velocity in the probe be approximately equal to stack velocity. This type
of sampling assures that the particle concentrations are not substantially changed by
the sample extraction process. Stack velocities are measured with an S-type pitot
tube and inclined manometer, and sampling probe velocities are measured using an
orifice plate.
Method 5 equipment includes a filter enclosed in a heated enclosure to remove
most of the particles larger than 0.3 micrometers in diameter. The gas then flows to
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a series of impingers immersed in an ice bath, which removes the smaller
particulates and dries the sample.4 A gas meter measures the gas flow rate, which
can be corrected to standard conditions. After analysis of the collected particulates,
concentrations are often reported in units of grains per dry standard cubic foot
(gr/dscf), corrected to 7% oxygen.
Dioxin/furan concentrations can be obtained through the use of USEPA Method
23.6 This sampling train is similar to that of Method 5, but a special condenser, resin
module, specified impinger conditions and analytical procedures must be used.
Other test methods can be used to determine acid gas scrubbing efficiency. For
example, hydrogen chloride concentrations upstream and downstream of the scrubber
can be determined using the equipment and procedures of USEPA Method 26.
Slide 18-6
APCD CONTROL CONSIDERATIONS
Flue Gas Temperature
Flue Gas Flow Rate
Pollutant Concentration
Pressure Drop
Reagent Flow
PH
Thermal Protection
Dew Point
The control features in the above slide illustrate a wide range of operational
considerations which may be associated with APCD control equipment. APCDs are
required to accommodate the variable flue gas rates and pollutant concentrations
produced by incinerators.
The specific control features will vary with technology of the site-specific
equipment. Many of the smaller MWIs have APCDs that are designed for steady,
base load operating conditions, with simple "on-off' controls.
By contrast, "feedback control" is often used on larger units to modulate system
operation. For example, a water spray with a regulated flow rate can assure that flue
gas temperatures are kept below a maximum "set point" to protect a fabric filter.
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Slide 18-7
DEW POINT CONSIDERATIONS
Threshold Condensation Temperature
Typically Values Range from 225 to 300° F
Dependent Upon The Mixture's
Moisture and Acid Concentrations
Dew point is the temperature at which condensation begins to occur as the
mixture of gases is cooled. Dew points are dependent upon the concentrations of
moisture and acid gases, including those formed by combustion and the moisture from
the waste and ambient air. Dew points are also influenced by evaporation from water
sprays and scrubber liquids. Acid dew points are often in the range of 225 to 300° F.6
If acid gases were not present, the maximum expected dew point values for flue gas
from a MWI would be around 175 °F.
When flue gas is cooled to its dew point, condensation will begin. Generally,
water will condense and absorb acid gases. These acid solutions can cause corrosion
of metal ducts and hoppers and other materials. Corrosion can lead to holes in metal
enclosures, allowing ambient air to leak into the unit. If this occurs, the system
performance will generally deteriorate and the cool air will lead to additional
condensation and corrosion.
Acidic droplets can cause deterioration and blinding of fabric filters. If lime
scrubbing systems are used, condensate formed on the collected filter cake can cause
reactions which lead to blinding of fabric filters (material solidification within fabric
filters which prevents normal gas flow). Removal of such solids is very difficult, so
fabric filter replacement may be the only remedy.
Dew point problems are particularly troublesome during unit start-up and
shut-down, when the unit temperatures change between ambient and operating
conditions. Intermittently operated units require special care, as they are cooled to
ambient temperatures each day.
Standard procedures include maintaining flue gas operating temperatures
above the dew point range to avoid corrosion problems. Also, they often call for the
burning of auxiliary fuel until the acidic gases have been purged.
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Slide 18-8
ACID GAS WET SCRUBBER OPERATIONS
Absorption and Chemical Reactions
Recirculation of Caustic Solution
Removal of Salts
Maintenance of Scrubber pH
Control Inlet Temperature
Provide Required Pressure Drop
Wet scrubbing equipment is designed to optimize the absorption process by
mixing the gases with caustic (alkali) solution through the use of enhanced contact
areas and sufficient contact time. The mixing of the caustic and flue gases is often
achieved by providing countercurrent flow through a bed of packing elements in
packed-beds (packed-towers). The packing elements provide a considerable amount
of "wetted surface area" for the absorption process in a relatively small volume.
Absorption of acid gases is followed by chemical reactions which produce salts
that are subsequently removed from the scrubbing solution. In general, the scrubbing
solution is recirculated through the scrubber so that the chemical utilization is nearly
100%. Of course, some unreacted caustic materials are removed in the "scrubber
blowdown" which is performed to remove the salts and particulate matter.
Depending upon the caustic, the salts may be soluble or insoluble. Insoluble
salts will precipitate (settle) or be filtered from the solution. Soluble salts may
require subsequent waste water treatment for removal. Some state regulations do
not allow the discharge of scrubber blowdown into the sanitary sewer. For such
application a spray dryer is often used, as discussed in Learning Unit 16.
The primary control variable is pH, which provides an indication of the
corrosive nature of the scrubber solution. A pH of 7.0 (neutral) or less (slightly
acidic) is most often desired.7 The pH is typically maintained by balancing the
caustic reagents in the "make-up" solution. In general, the "make-up" requires the
caustic materials to maintain the chemical balance and the water to compensate for
evaporative losses and scrubber blowdown.
Wet scrubbers can develop serious corrosion problems if the solutions become
either too acidic or too basic. Such problems can be addressed either by maintaining
pH or by constructing scrubbers, ducts, and associated equipment from corrosion
resistant materials.
Because of corrosion problems, special acid resistant materials, such as high
nickel alloys, are often required for construction of scrubber equipment.7 Stainless
steel and low nickel alloys will sustain significant corrosion unless special acid
18-6
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resistant liners are provided to prevent the contact of the acids with the metal.
Hastelloy-C is a type of high nickel steel alloys that, although very expensive, are
very resistant to corrosion. Non-metal materials, such as fiberglass reinforced plastic
(FRP), teflon, polypropylene and PVC, can be used for some relatively low
temperature applications in scrubber equipment. However, they may be damaged by
exposure to flue gas at excessive temperatures.
The solubility of gases in liquids varies with the gas and scrubbing agent. For
example, HC1 is much more soluble than S02 in water and most caustic solutions.
Also, the solubility of gases in liquids is decreased as the temperature increases. For
this reason, water sprays or heat exchangers are often used to cool the incoming
exhaust, thereby increasing the collection efficiency.
The pressure drops of acid gas scrubbers are comparable to those of fabric filter
systems. However, the pressure drop is maintained at a near constant value during
operations, with some variations occurring in response to changes in gas flow rates.
Slide 18-9
VENTURI SCRUBBER CONTROL VARIABLES
Pressure Drop
Liquid/Gas Flow Rate Ratio
Scrubber pH
The control variables which influence the performance of venturi scrubbers are
the pressure drop, liquid-to-gas ratio and pH.
Although greater particulate removal efficiency occurs with more scrubbing
liquid, pressure drop is the main control variable. The pressure drop is typically
produced by the operation of a high performance induced draft fan. The fan
effectively pulls a vacuum, which sucks the flue gases through the scrubber. The
operating cost for the fan's electricity increases as the required pressure drop
increases.
High collection efficiencies in MWI applications often require pressure drops
as high as 60 in. we. The high pressure drop is needed to produce the high flue gas
velocity for the impaction of the submicron particulates onto the droplets.
Most venturi scrubbers are used for controlling particulates, and water is the
scrubbing liquid. Water absorbs hydrochloric acid (HC1) fairly readily, so the
scrubber water can become very acidic. Therefore, pH is monitored to identify
potential corrosion problems and to control the water treatment.
18-7
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Slide 18-10
DISADVANTAGES OF VENTURI SCRUBBERS
High Energy Requirements,
Liquid Waste Residue
Corrosion and Erosion
Pressure Drop
The main disadvantage of venturi scrubbers is the high energy requirement
associated with operating at high efficiency collection for the small particulates of
MWIs. Another disadvantage is associated with treatment of liquid residues, which
may be required before their disposal.
Erosion is particularly a problem with the high energy venturi scrubbers,
where gases are often accelerated up to 400 ft/sec. The entrained particulates can
cause serious erosion, essentially sand-blasting metal surfaces.
Corrosion is associated with the imperfect neutralization of the absorbed gases,
leading to acidic or caustic reactions, particularly on metal surfaces which have
undergone erosion.
Slide 18-11
FABRIC FILTER DESIGN FACTOR
AIR-TO-CLOTH RATIO
Total Air Flow/Filter Surface Area
Average Velocity Through Filter
Maximum Operating Temperature
A standard design factor is the air-to-cloth ratio, which is the total air flow rate
divided by the nominal filter surface area. This factor is equivalent to the average
velocity of gas passing through the filter, with typical design values ranging from 1.5
to 4.5 ft/min (or acfm/ft2). In general, greater filter surface areas result in lower
velocities and require lower pressure drops to force the flue gas through the filter.
Fabric filters are designed to operate with specified temperature limits to
prevent the melting or deterioration of the fabric.
The flue gas temperature and volumetric flow rates can be reduced by using
some form of heat exchange, such as an air heater, flue gas reheater or waste heat
recovery boiler. Some MWIs use water sprays to control flue gas temperatures.
18-8
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Volumetric flow rates change in direct proportion to changes in the absolute
temperature. Thus, as the temperature increases, the flow rate and average velocity
through the fabric increase correspondingly. This increase will typically be
accompanied by an increase in the required pressure drop.
Slide 18-12
FABRIC FILTER OPERATIONAL PROBLEMS
Low Pressure Drop:
* Holes in Bags
* Over Cleaning
High Pressure Drop:
* Blinding
* Under Cleaning
Fabric Deterioration or Fires:
* Improper Flue Gas Cooling
* Surge of Burning Flue Gas
Corrosion:
* Improper Insulation
* Leaking Gaskets
* Improper Temperature Control
* Improper Air Dryer Operation
The main indicators of the performance of fabric filter systems are stack
opacity and the pressure drop measured across the fabric filter. Vendors typically
recommend operating with pressure drop ranging between 1.0 to 5.0 inches w.c.
Large fabric filter systems are often equipped with automatic controls, which
initiate a cleaning cycle when the pressure drop across the fabric exceeds a set value.
Fabric filter cleaning can be accomplished on a regular timed basis. The fabric filters
of some units, which operate on a daily cycle, are cleaned once a day, during the cool-
down period.
After the fabric is cleaned, the pressure drop is reduced for a period of time
until the filter cake can be built-up. During this time period, the collection
efficiencies are reduced.
A low pressure drop can result from holes in bags (bag rupture) which allows
air move through the unit without being filtered. Over cleaning can also cause a low
pressure drop. Over cleaning removes an excessive amount of the particulates
collected within the fabric which would otherwise act as part of the filter medium.
18-9
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Maintaining the pressure drops below the maximum limit can help limit the
energy costs associated with fan operation. A high pressure drop can be caused by
the blinding of fabric filter. A high pressure drop can also affect combustion by
restricting the air flow rate and upsetting the draft conditions in the combustion
chambers.
A malfunctioning flue gas cooling system or by-pass stack damper can cause
the melting or deterioration of the fabric. In addition, fires can be caused by a surge
of high temperature or burning flue gas which ignites the carbonaceous materials
collected on the fabric filter.
To avoid fabric filter blinding and corrosion problems, it is important that the
baghouse system be maintained above the dew point. Blinding of the fabric occurs
when particulates fill the pores within the fabric, overly restricting the gas flow.
Blinding is often caused by reactions between condensate and the sorbent particules.
Corrosion in fabric filter systems can be caused by a number of potential
problems which are associated with operating with metal surfaces below the dew
point. Improper insulation can allow metal structure and wall surfaces which are
exposed to flue gas to be cooled below the dew point, even if the unit is located inside
a building. Leaking gaskets (e.g., inspection doors) can allow cool ambient air to leak
into the baghouse, causing cooling of metal surfaces. Of course, systems using water
sprays for cooling flue gas must maintain temperatures above the dew point.
It is also important that dry air be used for cleaning fabric filters. Various
types of dryers are available, including systems using either desiccant materials or
refrigeration. Operators of fabric filter systems must routinely confirm that the dryer
is operating properly.
Slide 18-13
DRY SORBENT OPERATIONAL FEATURES
Sorbent Material Delivery Rate
Mixing of Sorbent Powder with Flue Gas
Optimum Temperature for Sorbent Reaction
Dew Point Considerations
Dry sorbent injection (DSI) systems generally have an automatic control
system to regulate the delivery of the sorbent from the storage and to flue gas
stream. The rate of delivery is set in response to the concentration of acid gases,
either upstream or downstream of the DSI system.
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The ratio of the amount of reagent to the theoretical amount required for acid
neutralization is known as the stoichiometric ratio. Control systems, in effect,
regulate the reagent flow rate to maintain a desired stoichiometric ratio.
In addition to the theoretical aspects, the storage and delivery systems require
that a proper amount of air be used to carry the sorbent into the flue gas and that
the sorbent be prevented from forming clumps of accumulated material. The delivery
rate is often controlled by actions of a shaker and screw auger feeder in the storage
bin and a rotary airlock in the delivery system. Most systems also provide a large
mixing chamber or reactor vessel between the injection point and the fabric filter.
The optimum temperature for the dry adsorption of acid gases is between 250°
and 350°F, with some recommendations urging operation at the lower temperature.6
As discussed in the fabric filter section, the flue gas temperature can be
reduced by using some form of heat exchange, such as an air heater, flue gas reheater
or waste heat recovery boiler.
Many DSI/FF systems have provisions for evaporative cooling of the flue gas
before the sorbent is added. The evaporative cooling chamber is located upstream of
the lime injection point, as previously illustrated in Slide 16-6. If such systems are
used, the flue gas temperature must be controlled above the dew point to prevent
corrosion problems.
Other systems use an air-to-air heat exchanger to cool the flue gas before the
sorbent is injected. Heat exchangers can be used to preheat combustion air or to
reheat clean flue gas. Such systems will cool the flue gas without adding moisture.
Slide 18-14
DRY SORBENT OPERATIONAL PROBLEMS
Ash Removal from Collection Hopper
* Air Impactors
* Sonic Horns
* Vibrators
* Hopper Heaters & Insulation
* Maintenance of Air Seals
Another operational problem with dry sorbent systems is the removal of the
final product from the ash collection hopper. Although the material is light, it has
cohesive properties that lead to material accumulation on the hopper walls. Removal
by pneumatic suction can lead to air channeling and incomplete removal.
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A combination of equipment can be used to assist the removal process,
including air impactors, sonic horns, hopper vibrators, hopper heaters, adequate
insulation, and proper maintenance of seals to prevent air leakage.
Slide 18-15
SPRAY DRYER OPERATIONAL CONSIDERATIONS
* Slurry Flow Rate
Acid Gas Concentration
* Adequate Drying of Slurry Droplets
Atomizer Maintenance
* Overall Drying Conditions
Exit Dry Bulb Temperature
Exit Wet Bulb Temperature
Inlet-Exit Dry Bulb Difference
* Slurry Water Content
Exit Dry Bulb Temperature
* Air Leakage Prevention
* Maintenance of Hopper Temperatures
The acid gas removal efficiencies can be controlled by regulating the slurry flow
rates. This is because increasing the flow rate enhances collection. However, the
amount of unreacted sorbent goes up disproportionately.
SDA systems are also able to remove large fractions of the heavy metals
(nickel, cadmium, chromium and lead) and dioxin/furan emissions.8
Example gas temperatures into and out of the spray dryer are 400°F and 250°F,
respectively. In general, higher temperatures would help keep the absorber product
dry, but the acid gas removal is more effective at the lower temperatures.1
The dilution water in the slurry can be used to control gas temperatures
leaving the SDA. Automatic control systems, which sense both downstream SO2
concentration and temperature, can be used to control the slurry flow to the atomizer.
Another control strategy is to maintain a steady feed rate appropriate for the
maximum flue gas flow rate and acid gas concentration anticipated.
18-12
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Slide 18-16
SPRAY DRYER OPERATIONAL PROBLEMS
1. Slurry Droplets Sticking on Wall
2. Liquid Carryover
3. Caking of Solids on Fabric Filter
4. Ash Hopper & Removal System Plugging
A number of problems have been experienced with spray dryer systems. Large
slurry droplets from the atomizer can adhere to the vessel wall and create blockages
and poor operations. Other problems include liquid carryover from the absorber
vessel, caking of solids on the fabric filter, ash hopper and removal system plugging.
These problems are often of particular concern during the start-up of the system.
It is important for the slurry droplets to be dried adequately before impacting
the surface of the reactor vessel. If drying is inadequate, a wet powder can build up
on downstream surfaces. If the drying is too fast, acid absorption will be reduced.
A requirement for good drying is to have the atomizer produce uniform droplets
of appropriate sizes. Atomizers can be removed from service for cleaning. However,
this may require a system outage, unless multiple atomizers are available to permit
system operation during the servicing of a single atomizer.
Even if the droplet sizes are appropriate, the overall drying process must be
controlled. The difference between the dry bulb and the wet bulb temperature at the
reactor vessel exit can be used as an indication of the relative humidity and overall
drying conditions. Some control systems monitor the wet and dry bulb temperatures
and modify the flow rate of slurry accordingly.
Hopper heaters and adequate insulation are important techniques for reducing
ash handling problems. In addition, seals must be maintained to prevent air leakage
into the system, as ambient air can cause cooling and introduce excessive moisture.
Seal leaks can cause serious problems for fabric filters and ash hoppers because
calcium chloride absorbs water readily and becomes a putty or paste material.
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REFERENCES
1. "Medical Waste Incinerators - Background Information for Proposed
Standards and Guidelines: Control Technology Report for New and Existing
Facilities," U. S. Environmental Protection Agency, Draft Report, September
30, 1991, pp. 57, 87, 92, 93.
2. Theodore G. Brna, "Cleaning of Flue Gases from Waste Combustors,"
Combustion Science and Technology. Vol. 74, 1990, pp. 83-98.
3. Joseph G. Singer, Combustion Fossil Power. 4th Edition, Combustion
Engineering, Inc., Windsor, CT, 1991, pp. 22-13 to 23-20.
4. Title 40--Protection of Environment, Code of Federal Regulations Part 60,.
Appendix A. "Test Methods, Method 5," Office of the Federal Register,
National Archives and Records Administration, Washington, DC, pp. 742-777,
July 1, 1991.
5. Title 40~Protection of Environment. Code of Federal Regulations Part 60.
Appendix A. "Test Methods, Method 23," Office of the Federal Register,
National Archives and Records Administration, Washington, DC, pp. 1035-
1048, July 1, 1991.
6. Richards Engineering, "Municipal Waste Incinerator Field Inspection
Notebook," U. S. Environmental Protection Agency, EPA-340/1-88-007, July
1988, pp. 45-55.
7. "Emission Control Systems for Incinerators," Report Number TR-89-900239,
Andersen 2000, Inc., Peachtree City, Georgia, February 1989, pp. 14 and 18.
8. C. David Gaige and Richard T. Halil, Jr., "Clearing the Air About Municipal
Waste Combustors," Solid Waste & Power. January/February 1992, pp. 12-17.
9. Robert G. Mclnnes, "Spray Dryers and Fabric Filters: State of the Art," Solid
Waste & Power. April 1990, pp. 24-30.
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19. APCD OPERATIONAL AND SAFETY CONSIDERATIONS
Slide 19-1
OPERATOR REQUIREMENTS
1. Know the System Characteristics
2. Assess the Operating Conditions
3. Identify Potential Modifications
4. Make Timely Decisions
5. Keep Proper Records
As discussed in Learning Unit 13, Operators and Operator Supervisors must
know the operational characteristics of the APCD equipment and be able to assess
current conditions. Operators should also be very familiar with the manufacturer's
operations manual and established standard operating procedures.
When an upset condition is detected, operators must evaluate it, consider
possible remedies, and make a reasoned response. Intelligent choices require the
operators to know the control features of the equipment and understand the probable
consequences, such as the influences of one component on another.
Operator decisions must be made in a timely manner because the operating
conditions may deteriorate if unconnected. Delays may result in damage to the
system, a safety hazard or a permit violation.
Appropriate record keeping is also important. Significant operational upsets
should be noted, including operating conditions, changes made and system responses.
Slide 19-2
METHODS FOR DETECTING APCD UPSETS
1. Observe Visual Emissions
2. Review instrument Readings
3. Inspect APCD Equipment
4. Listen for Abnormal Sounds
5, Feel Unusual Vibrations/Hot Surfaces
6. Smell Unusual Odors
Operators assess operating conditions through surveillance of both instruments
and equipment. Routine visual inspections are made, taking into account any
irregular conditions sensed through hearing, smelling and touching.
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Slide 19-3
EXAMPLE UNCONTROLLED
PARAMETER
Opacity, %
Primary Temp . , F
Secondary Temp . , F
Draft, in w.g.
O2, % Dry Basis
COa, % Dry Basis
CO, ppm at 7% Oj
HC1, ppm at 7% Oj
S03, ppm at 7% 02
PM, gr/dscf at 7% 02
EMISSIONS AND PARAMETERS
LOW
0
1,000
1,800
0.03
11
5
0
HIGH
10
2,000
2,200
0.15
15
10
100
EXAMPLE
13
935
207
0.211
The above example set of parameters corresponds to the uncontrolled emissions
of a controlled-air MWI unit operated under steady operating conditions.1'2'3 Note
that the example unit will typically require some form of APCD, depending upon the
applicable regulations.
Example standards can vary with time and location, from regulations only
requiring the passing of an opacity limit to those which must meet specific regulatory
limits for particulates, carbon monoxide, hydrogen chloride, sulfur dioxide,
dioxin/furan and toxic metals.
Slide 19-4
MONITORING & CONTROL CONCERNS
Emissions Exceedance
Instrument Malfunction
Controller Malfunction
Operators must be aware of the various factors influencing emission, so that
proper corrective measures can be implemented as the emissions approach the upper
allowable limit. For instance, collection efficiencies are often a function of APCD
pressure drop and/or the rate of reagent delivery.
High CEMS readings can be caused by an upset condition in the incinerator,
APCD, ancillary equipment or CEMS. Faults in the CEMS can result from the
failure of an instrument or component (e.g., gas dryer or leak in the sampling line).
As discussed in Learning Unit 12, routine calibration and bias checks are performed
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to correct instrument readings and to detect the malfunctions in GEMS. Operators
need to know the standard procedures for instrument calibration and for obtaining
a replacement for a faulty instrument.
High pollutant emissions can result from a malfunctioning controller. Many
of the controllers have actuators which can be visually inspected to confirm their
operation (e.g., fan damper linkage). The positions of linkages should be compared
with the normal ranges of positions during operations and at shutdown. Many
controllers can be switched from the automatic to manual operating mode verify that
the actuator is working properly.
Slide 19-5
APCD START-UP & SHUTDOWN
* Manufacturer's Recommendations
* Sequence of Operations
* Preheating
* Maintaining Temperatures
The routine start-up and shutdown of an APCD should be performed in
accordance with the manufacturer's recommendations. Potential problems associated
with the start-up and shutdown of APCDs range from the freezing of liquids to
corrosion of metal surfaces and blinding of fabric filters.
The start-up of a scrubber includes initiating the blending of the scrubber fluid
and the control of its delivery rate relative to the flue gas flow, temperature and acid
gas concentrations. The pH must be monitored and controlled during start-up and
throughout the operations because the scrubbing process causes changes in pH.
Baghouses and ducts should be preheated to above the dew point before the
incineration of medical waste begins. Baghouses and ducts should also be continually
operated above the dew point until after the burn-down process has been completed.
This will minimize acid droplet condensation which can damage the fabric filters and
cause metal corrosion.
The gases from the incinerator preheating process are generally used to
preheat the fabric filter. Some APCD systems may have special heaters (electrical
or auxiliary fuel fired) to maintain temperatures above the dew point. Other designs
will emphasize thermal insulation to limit the cool down temperature and damper
controls to prevent moist gases from entering the unit during a shutdown.
Preheating of APCDs also limits the thermal stresses and bending of metal
objects which can occur during rapid temperature changes and cause cracks to form.
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Slide 19-6
BY-PASS STACK OPERATIONS
* APCD Start-up & Shutdown
* Baghouse & Scrubber Protection
* "Pail Safe" Provisions
Some regulations may allow the short-term release of uncontrolled emissions
through a by-pass stack (also called a dump stack or emergency relief stack) during
start-up and/or upset conditions. The by-pass stack is generally equipped with an
automatically operated cap or barometric damper device which opens in the event of
a fan failure (electrical service disconnection). This will allow combustion to continue
under natural draft conditions with the flue gas flowing up the by-pass stack.
An automatic opening valve in the duct to the by-pass stack is often provided
as an emergency control. This valve is opened if the baghouse or scrubber gas
temperature control system fails. Normally, the temperature of the gases entering
the APCDs is controlled through the use of a water spray, heat exchanger (waste heat
recovery boilers) and/or damper controlled ambient air mixing. These are designed
to limit the potential damage to APCD from thermal stresses, warping, and physical
changes such as the burning or melting of fabric and/or liner surfaces.
Slide 19-7
ROUTINE OPERATIONAL CONCERNS
* Leakage through Seals & Cracks
* Material Blockages
* Freeze Protection
Damaged seals and containment systems can cause liquids and gases to leak.
Hydraulic fluid can leak from the high pressure regions (e.g., pumps, piston actuator
devices, hoses). Mechanical seals around bearings and other moving objects may
need to be routinely adjusted and replaced as necessary.
Air leaks can occur through improper seals (e.g., charging device, ash hopper)
or cracks in the incinerator, APCD or ducts. Seals can be damaged by wear from
mechanical objects and through excessive temperature excursions. If applicable, the
water levels of water seals between the incinerator and ash quench tank should be
checked to assure a proper water seal.
19-4
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Smoke emissions and ash accumulation outside of an APCD often indicate that
flue gas and pollutants are leaking under positive pressure conditions. In addition
to worn seals, cracks can be caused by thermal stress and corrosion of the metal
surfaces. Leakage of air into APCDs can occur under draft conditions, leading to
localized cooling and additional corrosive action. Cracks and corrosion damage to the
metal ducts and enclosures may require extensive maintenance or replacement.
Many operational headaches are caused by blockages in delivery and removal
systems. Blockages in the fly ash removal system and the bridging of powdered feed
material in hoppers require routine preventive and corrective actions. Different
applications of thermal insulation, auxiliary heating and vibration devices have been
found to minimize blockages.
An operator's concern is not only to remove the blockage as soon as possible,
but also to do it in a safe manner. The blockage removal should be planned so as to
limit the possibility of caustic and thermal burning, as well as cuts and other
personal injuries. Care should be taken when opening hatch doors, as operators may
receive a burst of dust if the unit is under pressure.
It is important that freeze protection be anticipated, in part because MWIs are
required to operate in all kinds of weather. In addition to the winter freezing
problems of water, various reagents require special heating to minimize their
solidification throughout the year. Depending upon the design of the freeze
protection, operators may be required to monitor the system to assure that it is
operating properly.
A special type of freeze problem is the solidification of the slurry in the SDA's
delivery system. If the slurry is cooled below normal hot water temperatures (140°F),
there is a chance that the liquid will solidify. This is particularly a problem when the
flow stops. SDA designs generally include provisions for heating (electric resistance
or steam tracing) and recirculating the flow. If maintenance is required, such as the
replacement of a spray nozzle, the procedures should be accomplished in a planned
manner so as to minimize potential solidification problems.
Slide 19-8
EFFECT OF UPSETS ON OTHER SYSTEMS
* Incinerator's Upset on APCD
Increased Emissions
Baghouse or Liner Fire
* APCD's Upset on Incinerator
Puffing (Draft/Air Upsets)
19-5
-------
Generally, APCDs are installed downstream of the incinerator to clean up the
emissions. However, an incinerator upset can create problems in the operation of the
APCD, and an APCD upset can create problems with the operation of the incinerator.
Incineration upsets could be caused by either the quantity (e.g., overloading)
or the composition of the material charged (e.g., load of dry plastic bags).
Overloading can lead to an abnormal surge of flue gases with excessive emissions of
products of incomplete combustion, acid gases and particles. Excessive stack
emissions will result if the concentrations are beyond the collection and removal
capability of the APCD.
In addition, many incinerator regulations limit the maximum temperature at
the entrance of the APCD. This limit is based on the catalytic formation of dioxin
and furan compounds on the surface of fly ash. Catalytic reactions increase with
temperature, so maintaining a maximum limit at the APCD entrance is a practical
method for controlling dioxin and furan emissions.
Another potentially serious impact of the incinerator operation would be to
cause a baghouse fire. Baghouse fires can be ignited by sparks carried over from the
incinerator combustion chambers. Although the carbon content of fly ash is rather
low, combustion could occur if the flue gases are hot enough, since adequate oxygen
for combustion is available from the flue gas. Baghouse fires are generally prevented
by cooling the flue gas, so they are more probable during the failure of a water spray
(or flue gas heat exchanger).
Some metal surfaces have liners which can burn or be destroyed if their
operating temperature limit is exceeded. These include liners made from rubber,
polypropylene, PVC and fiberglass reinforced vinylester.
Likewise, an APCD upset can be experienced within the incinerator. For
example, changes in the scrubber or baghouse pressure drop can upset the draft
conditions and air supply to the combustion chambers. This phenomena is observed
as puffing, where the incinerator pressure becomes positive and produces smoke
which leaks through the charging door seals and other openings.
Routine cleaning of the fabric filter modules can result in cycling of the position
of the damper on the blower and, consequently, cycling of pressure in the combustion
chambers. Changes in the pressure drop across a venturi scrubber may produce large
changes in the combustion chamber pressures. Therefore, many venturi control
systems attempt to maintain a constant venturi pressure drop through the use of
variable venturi controls.
19-6
-------
Slide 19-9
WET SCRUBBER SYSTEM UPSETS
Flue Gas Cooling System
Water Spray Nozzle
Thermocouple
Control System
Pump
Fan or Damper
Caustic Holding Tank
Level Control Valve
Corrosion
Scrubber systems must be protected against exposures to high temperature flue
gas which can cause deterioration of the liners and packing elements. Also, the
absorption of gases into liquids is dependent upon solubility, which generally
decreases as temperatures rise.
Therefore, a water spray nozzle failure can reduce the evaporative cooling,
causing increased gas temperatures and increased emissions. Thermocouples are
typically used as the control system sensor, with a broken thermocouple leading to
a system upset.
A pump failure can, obviously, interrupt the flow of reagent to the scrubber.
Likewise, an induced draft fan can fail to provide the pressure drop required for the
normal operation of a scrubber. Such upsets can be associated with a malfunctioning
automatic control system or a deteriorated fan impeller, motor drive or damper.
The caustic holding and blending tanks, piping, nozzles, and overflow area
must be operated and maintained in accordance with design specifications. Caustic
systems may require that the water content in the solution be maintained and heated
to prevent precipitation (freezing).
Also, if the caustic material is not properly blended in the scrubbing solution,
the pH can vary from the normal operating range. A malfunctioning level control
(float) valve in the caustic blend tank can cause reduced flow of caustic, resulting in
acidic conditions downstream of the scrubber and a "low pH" alarm. High and low
water valves, if provided, must be maintained to assure the proper mixture level is
maintained.
Corrosion problems can occur if the scrubbing solution becomes either too
acidic or to basic. Blending problems may result from faulty controllers, blockages
and improper mixing temperatures.
19-7
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Slide 19-10
FABRIC FILTER OPERATIONAL UPSETS
* Fabric Filter Blinding
* Fabric Filter Rupture
* Thermal Protection of Bags
* Water Spray Nozzle Failure
Fabric filters (FF) can become "blinded" by the lodging of small particulates
within the fabric. During start-up, the potential for blinding is worse than normal
due to the fact that the FF must generally be heated from ambient to operating
temperatures, and pass through the dew point in the process. Condensation of
moisture on the fabric can cause particulates to solidify, resulting in blinding.
Provisions to avoid blinding include using the auxiliary fuel to preheat the
APCD system before flue gas from waste combustion is introduced. Other provisions
may allow for the use of a by-pass duct for a limited time period during start-up.
In the event of a rupture in a fabric filter bag, the baghouse pressure will fall
and the opacity will rise. An automatic control system with pressure and/or opacity
sensors may redirect the flue gas to an adjacent fabric filter module. Alternately, this
function may be performed manually by an operator. Of course, care must be taken
during bag replacement to avoid human exposure to hazardous conditions.
If the flue gas temperature were to rise above the maximum operating range
of the fabric filters, some protection feature would required to avoid melting the
fabric. Many systems will initiate an upstream water spray into the flue gas to
provide evaporative cooling. If the evaporative cooling fails, the damper to a by-pass
duct and stack will generally open as an automatic form of temperature protection.
The water sprays are designed to operate as needed, but spray nozzles may
experience corrosion, erosion and the accumulation of soot deposits. These deposits
could change the spray pattern and cause the formation of larger drops of water
which are less likely to vaporize. In addition, nozzles which are cooled for thermal
protection may have increased corrosion due to being cooled below the dew point.
A malfunctioning water spray nozzles in the duct above the secondary chamber
can cause a series of problems. For instance, if the spray nozzle control valve were
to leak, it could cause not only moisture and corrosion problems in the APCD system,
but also refractory damage in the duct and secondary chamber.
19-8
-------
Slide 19-11
*
*
*
*
*
APCD SAFETY HAZARDS
Skin Puncture/Cuts
Thermal Injury
Chemical Burns
Confined Space Suffocation
Inhalation of Toxic Dust
The safety aspects of medical waste handling were discussed in Learning Unit
4. However, there are several special safety concerns associated with the operation
of APCDs. These include injuries associated with skin punctures, cuts, thermal burns
and caustic burns. The possibility of these injuries is increased when handling fly
ash and caustic reagents, such as during the removal of blockage material.
Many of the APCD surfaces are above the normal 140°F temperature level of
hot water used in residential applications. When surfaces above this temperature are
contacted by human skin, burns can be expected. Therefore, appropriate personal
protective equipment (gloves, etc.) should be used.
Scrubbing systems often make use of caustic materials in either a liquid or dry
form. Chemical burns can be caused by exposure to caustic and acidic liquids. Hot,
acidic solutions are often produced by wet scrubbing operations, such as the use of
water in a venturi scrubber. In addition to contacting liquids, a person with wet or
sweaty skin can be burned by contact with dry caustic powder.
Maintenance operations within APCDs can require potential exposure to
suffocating conditions in confined spaces. For example, the replacement of ruptured
fabric filter bags may require entry into the confined space of the baghouse. Proper
precautions are required, including wearing a properly fitting and operating
respirator.
Fly ash can flow out of an open baghouse hopper door and cause burn injuries
and inhalation exposure to dusts that are potentially contaminated with heavy
metals. Doors should be opened carefully, with operators using personal protective
equipment to protect against thermal injury and inhalation.
19-9
-------
Slide 19-12
OPERATIONAL CONCERNS ABOUT TOXIC METALS
Procedures to Prevent Exposure
Special Equipment (Suits, Aspirators)
Personal Monitors
MWI facilities operate under Occupational Safety and Health Act (OSHA)
requirements which include special procedures designed to limit a worker's exposure
to toxic metals. The safety procedures at many plants include provisions for the
monitoring of heavy metals (lead, cadmium and mercury), blood testing, and the use
of respirators or special suits in contaminated and confined areas.
REFERENCES
1. Handbook. Operation and Maintenance of Hospital Medical Waste
Incinerators. U.S. Environmental Protection Agency, EPA/625/6-89/-024,
January 1990, p. 36-40.
2. "Emission Control Systems for Incinerators," Report Number TR-89-900239,
Andersen 2000, Inc., Peachtree City, Georgia, February 1989, p. 5.
3. W. R. Seeker, W. S. Lanier and M. P. Heap, Municipal Waste Combustion
Study, Combustion Control of Organic Emissions. U.S. Environmental
Protection Agency, EPA-530-SW-87-021-C, May 1987, p. 1-9.
19-10
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20. BOILERS AND OTHER HEAT RECOVERY EQUIPMENT
Slide 20-1
HEAT RECOVERY SYSTEM FEATURES
Raise Revenue through Steam Sales
Reduce Auxiliary Fuel Use
Reduce Electrical Energy Use
Lower Flue Gas Temperatures
Heat recovery equipment, such as waste-heat boilers and heat exchangers, can
be used on MWIs to extract useful energy from the hot flue gas. Heat recovery
equipment may improve the overall unit economy by producing revenue from selling
"process" steam or by reducing the electrical energy or auxiliary fuel requirements.
In many applications, such as at a hospital, the steam which is produced passes
through a throttle valve and then flows into the main steam line. However, the
economic benefits associated with producing steam may not justify the capital and
operating costs, particularly in small, off-site MWI facilities.
Heat recovery can be obtained by preheating the combustion air, which will
improve the combustion process and reduce the need for auxiliary fuel. Alternately,
the hot flue gas can be used to reheat the cool flue gas leaving a scrubber as a
method of increasing plume buoyancy and reducing the energy required by a fan.
Extracting energy from the flue gas can have the advantage of reducing its
temperature to a range which is more appropriate for APCD operations. This cooling
is achieved without adding water to the flue gas (e.g., with a spray dryer or scrubber),
which would increase its moisture content. Moisture addition would increase the flue
gas dew point, potentially creating condensation and corrosion problems. Some MWI
units use a boiler solely to cool the flue gas, with the steam being delivered to a
condenser and subsequently returned as feedwater.
Slide 20-2
MWI HEAT RECOVERY EQUIPMENT
Air-to-Air Heat Exchangers
Waste-Heat Boilers
Integral Waterwall Boilers
20-1
-------
The selection of a waste-heat boiler or heat exchanger will depend on economic
considerations. These include the cost of equipment, operating costs, and the
potential revenue generated by selling "process steam" or the credits associated with
reduced energy consumption.
Part of this consideration includes matching potential steam production
characteristics with the steam customer's demand. Most potential steam customers
have demand characteristics which are independent of MWI operations, so the
customer may also need an alternative source of steam. Generally, some of the steam
produced by the MWI facility will not be needed by the customer. Therefore,
provisions will be required for "dumping" the steam into the atmosphere or a
condenser.
The operation of recuperative heat exchangers is less complex than boilers
because boiler safety issues are avoided, as well as the need for boiler maintenance
and chemical treatment of the water.
Slide 20-3
RECUPERATIVE, SHELL-AMD-TUBE HEAT EXCHANGER1
To stack
exhaust
20-2
-------
Slide 20-3 illustrates a shell-and-tu.be heat exchanger designed for an air-to-air
application. Air-to-air heat exchangers are often called "recuperative heat
exchangers."
Slide 20-4
HEAT EXCHANGER FOR REHEATING FLUE GAS1
The above slide is a schematic illustration of an application in which hot flue
gas is used to reheat the cool flue gas. In this case, the hot flue gas would flow from
the secondary chamber to the heat exchanger and then to the APCD, such as a
scrubber. After leaving the scrubber, the flue gas would be reheated to increase
plume buoyancy and dispersion, thereby reducing the electrical energy associated
with operating the fans.
In this example, the hot flue gas flows through the tubes and the cool flue gas
flows around the tubes. An alternative design, with the hot flue gas flowing over the
tubes and the cool flue gas inside the tubes, would also work.
20-3
-------
Slide 20-5
BOILER DESIGNS FOR MWI APPLICATIONS
Unfired Waste-Heat Boiler Designs:
Fire-Tube Convection Unit or
Water-Tube Convection Unit
* Hot Water Boiler
* Saturated Steam Boiler
Integral Boiler Designs:
Waterwall Radiant Section
Water-Tube Convection Sections
* Saturated Steam Boiler
* Superheat Boiler
Fire-tube, water-tube and integral boilers are widely used in industry and have
been adapted for MWI operations. This Learning Unit presents descriptions of both
the fire-tube and water-tube types of waste-heat boilers, as well as the integral
boilers found in MWI applications. The special safety, control and operational
features will be presented in Learning Unit 23.
Waste-heat boilers are an add-on heat exchangers which produce steam or hot
water. Note that a waste-heat boiler is sometimes referred to as a heat recovery
steam generator (HRSG).2
Convection heat transfer is obtained from the hot combustion gases leaving the
final combustion chamber. Waste-heat boilers are generally designed to receive flue
gas at temperatures below 2,200 °F. This is considerably below the flame
temperatures (e.g., 3,000 °F) found in the radiant section of integral boilers which
operate with limited excess air.
Waste-heat boilers and heat exchangers (e.g., air preheaters and economizers)
are generally required to be protected against overheating. If a system upset
occurred (e.g., feed water flow restriction) so that the metal temperatures were heated
above their design point, the unit could be irreparably damaged. Temperature
protection is generally provided through the use of by-pass stacks or provisions for
mixing the flue gas with either fresh air or recirculated flue gas from downstream of
the heat exchanger.
Automatically controlled thermal protection systems generally require the
dampers which "fail safe" in a position that directs the flue gas away from the heat
exchanger in the event of a power failure.
20-4
-------
Slide 20-6
MWI AND WASTE-HEAT BOILER3
Most MWI boilers are waste-heat boilers which produce relatively low pressure
saturated steam (e.g, 150 psig, 365 °F).
The steam produced is called "process steam" if it is transported to a customer,
either on-site or nearby. Examples of steam users include hospital cafeterias,
laundries, and autoclaves, as well as space heating and cooling operations.
20-5
-------
Slide 20-7
CONVENTIONAL FIRE-TUBE BOILER4
Water
Heat
exchanger
tubes
Combustion
chamber
Conventional fire-tube boilers (or Scotch-marine type boilers) are typical of the
package boilers used for residential, commercial and light industrial applications.
Fire-tube boilers are characterized by the hot combustion gases flowing through a
group of parallel tubes immersed in a tank of water. The flue gases flow inside the
tubes, and boiling occurs as bubbles of vapor are formed on the exterior surfaces of
the submerged tubes. The bubbles rise due to their buoyancy, and steam is collected
in the region above the water surface at the top of the tank.
The tank is referred to as a "pressure vessel" because it must be designed to
meet the ASME Boiler and Pressure Vessel Code requirements.6 Feedwater flows
into the bottom of the tank. Saturated steam can be removed by a pipe located above
the water level in the tank. If the unit is a hot water boiler (heater), the water will
be pressurized so that vapor will not be formed. In this case, hot water can be
removed from the top of the tank, as in a conventional residential hot water heater.
20-6
-------
Fire-tube boilers are generally limited to low pressure applications, because the
structural design and safety requirements become more severe as pressure is
increased.5 Generally, fire-tube boilers are designed to produce saturated steam at
pressures below 250 psig (or saturation temperature below 405° F).6
Slide 20-8
FIRE-TUBE WASTE-HEAT BOILER SCHEMATIC
Hot
Flue
Gas
Liquid/Vapor
Interface
t ft
Feedwater
Saturated
Steam
Waste-heat boilers are also known as "unfired boilers." The primary difference
between waste-heat boilers and conventional boilers is that the fuel burning process
inside the conventional boiler vessel has been replaced by the delivery of hot flue gas
from the incinerator.
The above slide illustrates the geometry and flow paths of a fire-tube waste-
heat boiler. Flue gas flows inside the tubes (on the "tube side") and makes two
passes down the length of the unit. A relatively large quantity of Iquid water is
contained on the "shell side" of the unit. Therefore, the illustrated waste-heat
recovery boiler could be referred to as a "double pass" shell and tube heat exchanger.7
Steam, formed as vapor bubbles on the surface of the tubes, rises to the top of
the liquid. Saturated steam is collected above the liquid level and flows out the top.
Feedwater flows in from below to maintain the water level in the boiler.
20-7
-------
Slide 20-9
WASTE-HEAT BOILER WITH HORIZONTAL WATER TUBES2
AVA
—— Economizer Section
"""V Evaporator Sections
— Superheater Section
Courtesy of ABB Combustion Engineering, Inc.
The above slide illustrates a water-tube waste-heat boiler with horizontal water
tubes. This unit requires a pump to provide circulation of the water through the
tubes. The slide also illustrates the possibility of including both an economizer and
superheater, as well as an evaporator, in the boiler housing.
Water-tube boilers are typically used for applications requiring higher
pressures and temperatures than fire-tube boilers. An example water-tube boiler for
a MWI application produces high pressure water at 600 psig.8
20-8
-------
Slide 20-10
WATER-TUBE WASTE-HEAT BOILER AMD ECONOMIZER9
iiiiiiii
iiii!
!! Ill
illlllll
Courtesy of Deltak Corporation, Minneapolis, Minnesota
Water-tube waste-heat boilers can also resemble the traditional vertical-tube
geometry of the convective sections of integral boilers. In such designs, mixtures of
liquid water and vapor bubbles circulate inside tubes, with separation of the steam
from the liquid occurring in the steam drum.
The above slide illustrates a waste-heat boiler designed for a 50 ton/day MWI.
The unit is designed to produce saturated steam, with the flue gas making multiple
passes through the unit. An economizer is also located downstream of the evaporator.
20-9
-------
Slide 20-11
ECONOMIZER4
Outlet header
Flue gas
Water inlet
An economizer is another type of water-tube heat exchanger which is often
used in large installations. Like fire-tube and water-tube boilers, economizers are
air-to-water heat exchangers.
Economizers transfer energy from the flue gas to the feedwater prior to its
delivery into the evaporator section of the boiler. As shown in Slide 20-10, the
economizer is generally located downstream of the evaporator.
Prior to arriving at the economizer, the flue gas is generally too cool to produce
additional steam. However, the feedwater temperature is cool enough so that
effective heat transfer can occur from the flue gas to the feedwater.
The water flows inside the tubes. If the particulate loading of the flue gas is
relatively low, the "air-side" of the heat exchanger can be equipped with finned
surfaces, as illustrated in the above slide. The extra surfaces enhance the heat
transfer from the flue gas. However, finned surfaces are not generally provided if the
particulate loading is expected to be high because the fins accumulate particulate
deposits which are difficult to remove.
20-10
-------
Slide 20-12
WATERWALL FURNACE ENCLOSURE
10
Courtesy of Basic Environmental Engineering, Inc.
Integral boilers are widely used by industries and utilities for the production
of steam and electrical energy. They are also used in some medical waste
incineration applications.
Waterwall or membrane wall enclosures are designed to provide radiant heat
exchange from the burning process in the combustion chamber to the waterwalls.
The water in the waterwalls is first heated from the feedwater temperature to the
saturation temperature. After the saturation temperature is reached, evaporation of
water will begin with the production of vapor bubbles on the waterwalls. As with
other water-tube designs, the steam is removed from the mixture in a steam drum.
The operating temperatures of the waterwalls are generally limited by the
saturation conditions associated with the boiler pressure. Many large MWC units use
integral waterwall boilers to produce superheat steam at fairly high pressures (e.g.,
600 psi). The steam can be sold commercially, used internally or used to drive steam
turbines for the production of electricity.
20-11
-------
Slide 20-13
WATERWALL INCINERATOR
11
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tUMl. V4 JTMC
THBUH. norms*
MJBUUAKT me.
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ITAW *4
>TMe *4 KOW DUCT
•TAW M HI amma
WAIW nw coxtcnm MU>
nut
Courtesy of Basic Environmental Engineering, Inc., Glen Ellyn, IL
Some MWI waterwall systems provide flue gas recirculatiori as a heat sink to
control the gas temperatures to around 1400° F, low enough to avoid slag formation10
and high temperature chlorine corrosion problems on the heat exchanger surfaces.
The typical economic trade-off is that as the gas temperatures are reduced, the boiler
operates with reduced thermal efficiencies.
Slide 20-14
INTEGRAL BOILER SECTIONS
Radiant Section
Feedwater Heating
Evaporation
Convective Section
Evaporator
Superheater
Economizer
20-12
-------
Integral boiler designs provide energy extraction in two separate sections which
are characterized by either radiant or convection heat transfer. Waterwalls surround
the combustion zone, which is also called the radiant section, since the heat transfer
there is dominated by radiation. High temperature luminous gases, glowing with
their characteristic yellow color, emit infrared energy which is partially absorbed by
the waterwalls and partially reflected onto the fuel bed. The waterwalls heat the
feedwater to boiling conditions and provide energy for the evaporation process.
After the combustion gases leave the radiant waterwall section, they pass into
the convective section of the boiler. The flue gases pass over the exterior of tubes
which, depending upon the application, will contain steam vapor, liquid water, or
mixtures of liquid and vapor. Boiler components in the convection section can include
the evaporator, superheater and the economizer (feedwater heater).
Boiling occurs in the tubes of the evaporator. The evaporator tubes contain
steam (water vapor) and liquid water at the saturation temperature. Steam is
separated from boiling water in a steam drum, which is generally located at the top
of the evaporator section.
The superheater heats water vapor to temperatures above saturation values,
as may be required by a steam turbine or other steam application. However, most
MWI units do not have superheaters.
An air preheater is another heat exchanger which can be located in the
convection section. Although at this point the flue gas is too cool to effectively heat
feedwater, it is hot enough to preheat the combustion air. Steam can also be used
for air preheating. Air preheating can be important because heated air will aid the
waste drying process, making the combustion easier to control.
20-13
-------
Slide 20-15
STEAM CONDENSER SCHEMATIC15
L
Exhaust Steam Inlet
Condenser
Tubes /-Cooling Water
/ Outlet
Baffle
Cooling Water
Inlet
From F. M. Steingress and H. J. Frost, Stationary Engineering.
American Technical Publishers, Inc., Homewood, IL, 1991, printed with
Permission.
Water which is transported to a customer as "process steam" may either be
replaced as make-up feedwater, or the condensate may be returned. As an
alternative, MWI boiler installations can include a condenser to convert the waste
steam back to liquid water. The condenser is basically a series of cooled tubes that
cause the steam to condense. Water then drips from the condenser tubes into a lower
chamber known as a hotwell.
The condensate pump extracts the water from the hotwell and directs it to a
device known as a deaerator, which will be described in Learning Unit 22. The
deaerator pressure depends upon the particular design of the unit. A feedwater
pump increases the pressure of water to the full boiler operating level. Because the
functions of the feedwater pump are so critical to unit operations, a parallel set of
pumps is generally installed. This allows switching between the pumps to provide
for maintenance, without having to take the boiler off-line.
20-14
-------
REFERENCES
1. Gerald T. Joseph and David S. Beachler, APTI Course 415. Control of Gaseous
Emissions. Student Manual. U. S. Environmental Protection Agency, EPA-
450/2-81-005, December 1981, pp. 3-32 to 3-37.
2. Joseph G. Singer, Combustion Fossil Power. 4th Edition, Combustion
Engineering, Inc, Windsor, CT, 1991, p. 8-32.
3. S. E. Sawell and T. W. Constable, "NITEP: Assessment of Contaminant
Leachability from MSW Incinerator Ash," Proceedings of an International
Workshop on Municipal Waste Incineration. Sponsored by Environment
Canada, Montreal, Quebec, October 1-2, 1987, p. 335-336.
4. David S. Beachler, APTI Course SI-428A. Introduction to Boiler Operation.
Self-instructional Guidebook. U. S. Environmental Protection Agency, EPA-
450/2-84-010, December 1984, pp. 1-3 to 1-13.
5. "ASME Boiler and Pressure Vessel Code, An American National Standard,"
American Society of Mechanical Engineers, Boiler and Pressure Vessel
Committee, New York, NY, 1983.
6. Power Magazine Editors, "Steam Generators," Standard Handbook of Power
Plant Engineering. Thomas C. Elliott, editor, McGraw Hill Book Co., NY, 1989,
pp. 1.76 to 1.86.
7. Albert Thurmann, Fundamentals of Energy Engineering. Prentice-Hall, Inc.,
Englewood Cliffs, NJ, 1984, pp. 126-186.
8. "Integrated Waste Services, Information Summary," Consumat Systems, Inc,
Richmond, Virginia, Undated Brochure.
9. Deltak Corporation, Minneapolis, MN, Unpublished report, 1992.
10. "Basic Environmental Engineering, Inc.," Basic Environmental Engineering,
Inc., Glen Ellyn, IL, Undated Brochure, Received: April 1992.
11. "Hazardous Waste Incineration," Basic Environmental Engineering, Inc., Glen
Ellyn, IL, Undated Brochure, Received: April 1992.
12. Frederick M. Steingress and Harold J. Frost, Stationary Engineering.
American Technical Publishers, Inc., Homewood, IL, 1991, pp. 227-275.
20-15
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21. BOILER ENERGY PRINCIPLES
Slide 21-1
BOILER ENERGY PRINCIPLES
Types of Energy
Units of Energy
Steam Characteristics
Energy Conversion
Heat Transfer
This Learning Unit presents the general energy principles needed to
understand boiler operations. These include some of the basic concepts of
thermodynamics, which are presented to aid in understanding steam characteristics
and the operation boilers. The general features of energy, energy conversion, heat
transfer and steam characteristics will be presented in order to emphasize the
practical aspects of boiler operations.
Slide 21-2
TYPES OF ENERGY IN MWI BOILERS
Chemical - Heat of Combustion
Thermal - Sensible Heat & Latent Heat
Mechanical - Work Related to Force x Distance
Potential - Elevation Related Energy Storage
Kinetic - Motion Related Energy Storage
Electrical - Electrical Power
A number of different types of energy are listed above. Of particular interest
is the chemical energy of a fuel which is released by combustion in an incinerator.
Chemical energy is released in the combustion process. Combustion converts
the chemical energy to thermal energy, which is directly absorbed by the products of
combustion.
Thermal energy can be stored in two general forms, sensible heat and latent
heat. The amount of sensible energy can be directly indicated by a thermometer. A
large fraction of the chemical energy released in burning is absorbed by the
combustion gases as sensible energy. The latent heat of vaporization, which is the
energy required to drive the evaporation process, is stored in the water vapor of the
combustion products, as will be discussed below.
21-1
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Mechanical energy or work can be observed as the result of a force acting
through a distance.
Potential energy is energy which is stored at some higher level. An elevated
weight has potential energy, relative to its rest position at the lower level. For
example, a wagon on top of a hill is said to possess potential energy.
Kinetic energy is energy associated with motion or velocity. An example of
kinetic energy is the energy stored in a rapidly moving car. Molecules also store
kinetic energy, with their increasingly rapid vibrations as temperatures increase.
Rotational energy is another form of stored kinetic energy, as in the case of a fly
wheel.
Slide 21-3
*
*
*
*
ENERGY UNITS
BtU
Calorie (kilogram-calorie)
Joule
Kilojoule
There are a number of different energy units corresponding to the different
systems of units. The standard energy unit for the Engineering English System is
the Btu (British thermal unit). A Btu is the amount of energy required to raise the
temperature of 1 pound of water 1 °F.
Another energy units is the kilogram-calorie of the metric system of units. The
kilogram-calorie is the energy unit commonly used in nutrition and called a "calorie."
A kilogram-calorie is the amount of energy required to raise the temperature of 1
kilogram of water 1 °C. One Btu is equivalent to 0.252 kilogram-calories.
The joule is the standard energy unit in the SI (International System) of units.
One Btu is equivalent to 1,055 joules or 1.055 kilojoules.
21-2
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Slide 21-4
CATEGORIES OF THERMAL ENERGY
Sensible Heat
Latent Heat of vaporization
Heat Content
Enthalpy
Sensible energy can be stored in gases, liquids and solids. For example, the
water leaving an economizer has more sensible heat than the feedwater entering
because the water leaving is at a higher temperature. The sensible heat gain by the
water is provided by flue gas, whose temperature drops as its sensible energy is
extracted.
The boiling temperature increases with pressure. For example, a hot water
boiler is typically pressurized (e.g., 500 psia) so that hot water can be produced at
temperatures (e.g., 425 °F) below the boiling point (e.g., 467 °F) without creating
water vapor or steam.
Latent heat (latent heat of vaporization) is the energy required to evaporate
a liquid. In the evaporation process, the liquid absorbs the latent heat and is
transformed into a vapor. The vapor retains the latent heat until it condenses, gives
up its latent heat, and returns to a liquid form. The evaporation of water in a boiler
occurs with energy being extracted from the hot flue gas. In a condenser, the latent
heat of the steam is transferred to the cooling water or cooling air, depending upon
the design.
Water vapor (steam) contains both sensible heat and latent heat. The overall
heat content of a substance is the sum of its sensible heat and latent heat (if any).
Therefore, the heat content of steam includes both the sensible heat associated with
its temperature and the latent heat associated with its phase change from a liquid.
The term "enthalpy" is often used for the overall heat content of particular
material. Enthalpy values are often available in tabular form for common materials
(e.g., steam, nitrogen, carbon dioxide, air, etc.). The enthalpy values are provided as
relative values, based upon the assumption that enthalpy is zero at a selected set of
standard conditions of temperature and pressure.
21-3
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Slide 21-5
0
HEAT CONTENT OF LIQUID WATER & WATER VAPOR
VARIES WITH TEMPERATURE & PRESSURE
500
300-
200-
100-
psia
15 psia
i r
0 200 400 600 800 1000 1200
100 300 500 700 900 1100 1300
Heat Content (Enthalpy), Btu/lb
In the above slide, liquid water is indicated by the sloping line on the left. As
water is heated, its energy content goes up linearly with temperature.
When water at atmospheric pressure (approximately 15 psia) reaches 212 °F
it begins to boil. The energy content of the boiling mixture is represented by a
horizontal line, with the saturated liquid represented on the left end of the line and
saturated vapor on the right.
As water vapor is heated above the saturation conditions, its energy content
increases as its temperature rises. This is indicated by the sloping lines on the right
of the slide.
Note also that, as the pressure is increased, the boiling point is increased (e.g.,
354 °F at 140 psia).
21-4
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Slide 21-6
Steam Characteristics
Saturated Liquid (Water)
Saturated Vapor (Dry Steam)
Mixture of. Liquid & Vapor
Quality (Vapor Fraction)
Superheat Steam
Water enters the boiler as a liquid, is converted to a mixture of liquid and
vapor in the boiling process, and leaves the boiler as a vapor. Liquid water is first
heated up to the boiling point or saturation temperature. Liquid water at the boiling
point is often called "saturated liquid."
After the boiling point is reached, the evaporation process begins with the
formation of vapor bubbles. As heating continues, more vapor bubbles are formed,
with the liquid/vapor mixture remaining at the saturated temperature. Vapor
bubbles rise through the mixture and are subsequently collected at the top of the
boiler.
Mixtures of liquid and vapor are often called "saturated steam," "wet steam,"
or "steam." Water vapor is also commonly called "steam," but a better expression for
water vapor is "dry saturated steam."
The term "quality" is used to characterize the mixture of saturated steam.
Quality is the fraction of the vapor/liquid mixture which is water vapor. Therefore,
steam with a quality of 99% would be composed of 99% water vapor and 1% liquid
water. Dry steam would have a quality of 100%, and saturated liquid would have a
quality of 0%.
Superheat steam is particularly appropriate for steam turbine applications, but
is not generally required for "process steam" typical of MWI boiler applications.
However, operators should be aware that superheat steam is water vapor that
has been heated above the saturation temperature (boiling point). As such, superheat
steam has no liquid water content. The superheating process occurs in special heat
exchangers called superheaters. Superheaters have multiple tubes through which
water vapor flows, with hot flue gas passing over the outside surfaces of the tubes to
provide the heating.
21-5
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Slide 21-7
ENERGY CONVERSION EXAMPLES
Chemical Energy to Thermal Energy
Mechanical Energy to Potential Energy
Potential Energy to Kinetic Energy
Thermal Energy to Work of Automobile
Thermal Energy to Rotate a Turbine
Rotational Energy to Create Electricity
Energy can often be converted from one form to another. As discussed in
Learning Units 7 and 9, the chemical energy of fuel is converted by combustion into
thermal energy of hot combustion gases. This conversion efficiency ranges from 85
to 95%, depending upon the heat losses and competeness of combustion.
An example of mechanical energy being converted into potential energy would
be the action of pushing a wagon up a hill. When the wagon reaches the top of the
hill, it contains potential energy associated with its elevation. The conversion process
may not be very efficient, since any work done against friction will be lost.
When the wagon conies back downhill, some of its potential energy will be
converted into kinetic energy and some will be lost to frictional heating.
Thermal energy can be converted to mechanical energy through the use of an
engine, such as the piston engine in a car. In this case, combustion provides thermal
energy (hot, high pressure gases) which is converted to mechanical energy by the
work of the pressure forces on a piston. The linear motion of the piston is transferred
to a crank shaft, creating rotary motion which is subsequently transferred to the
drive shaft, wheels and tires, where mechanical work can be produced. Electrical
energy can be produced if a steam turbine and electric generator are provided.
In waste-heat boilers, the thermal energy of hot flue gas is transferred in the
production of steam. For an example boiler with an inlet at 2,000 °F and an exit gas
at 500 °F, the thermal conversion efficiency would be approximately 75%.
Slide 21-8
MODES OF HEAT TRANSFER
* Conduction
* Convection
* Radiation
21-6
-------
The three primary modes of heat transfer are conduction, convection and
radiation. Other forms of heat transfer, such as boiling and condensation, are
generally considered to be special cases of convection heat transfer.
Conduction heat transfer is generally responsible for energy loss through the
refractory materials and metal enclosure of the incinerator. Conduction occurs as a
result of hot molecules of a substance directly transferring energy to the adjacent
molecules. Although conduction is the major mode of heat transfer within solids, it
can also occur in liquids and gases.
Slide 21-9
CONVECTION HEAT TRANSFER
Natural or Free Convection
Forced Convection
Boiling
Condensation
Convection is the transfer of energy associated with the motion of a fluid,
either in gaseous or liquid form. In convective, the hot molecules carry and transfer
energy as they move. The fluid temperatures will be changed as hot molecules mix
with cooler adjacent molecules and as fluid molecules contact surfaces.
Natural convection or free convection is the heat transfer resulting from the
movement caused by the buoyant forces associated with temperature and density
differences within the fluid. Free convection occurs in downcomers and risers of
water-tube boilers where the density of the liquid vapor mixture in the risers is less
dense than in the downcomers. Free convection also occurs on the water side of fire-
tube boilers as the vapor bubbles rise to the surface in the top of the tank.
Forced convection occurs when the fluid motion results from a mechanical
device, such as a fan or pump. Forced convection is the primary mode of heat
transfer from hot flue gases to the metal tubes of both fire-tube and water-tube
waste-heat boilers.
Boiling is a special form of natural convection in which vapor bubbles are
formed. The bubbles naturally move away from the surface because their density is
lower than the liquid's density.
When a vapor condenses, it gives up its latent heat to the surroundings. A
surface exposed directly to steam may be scalded (or burned) because of the high rate
of heat transfer associated with condensation and the transfer of both the sensible
and latent heat from the steam.
21-7
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Slide 21-10
RADIATION HEAT TRANSFER
Electromagnetic Radiation
Solar Energy Example
* Ultraviolet
* Visible
* Infrared
Heat Transfer to Waterwalls
Radiant heat transfer results from electromagnetic radiation, which is
characterized as heat transfer which does not require a material medium for its
transport. Radiant heat transfer is primarily dependent upon the temperature of the
emitting and receiving surfaces. Solar energy is the most obvious example of radiant
heat transfer in that it is transported from the sun to the earth through space.
Radiant energy can be transmitted over a wide spectrum of wavelengths,
including the ultraviolet, visible and infrared wavelengths. Ultraviolet (short
wavelength) energy from the sun is generally associated with sunburning of skin. A
large fraction of solar energy is in the visible wavelengths, giving the sun its
yellow/orange color.
In an incinerator, radiant heat or "infrared heat" is transmitted directly from
the flames to the fuel bed, leading to the heating and drying of the waste and the
devolatilization process. The refractory walls reflect radiant energy to the fuel bed,
as well as provide thermal insulation to reduce conduction losses.
In integral boilers, the waterwalls receive radiant heat from the flames, hot
combustion gases and glowing embers on the fuel bed. A considerable fraction (e.g.,
40%) of the combustion energy can be extracted by radiant heat transfer, with the
fraction varying with the temperatures of the combustion gases and waterwall.
Slide 21-11
ENERGY CONSERVATION FOR A SYSTEM
Total Energy Input Rate
Must Equal
Total Energy Leaving Rate
A basic concept is that energy can be neither created nor destroyed, although
it can be converted, degraded and transferred to alternate forms. Therefore, the
energy input to a system must equal the energy leaving the system.
21-8
-------
When considering the MWI as the system, the rate of energy input is the total
of the combustion energy of the medical waste and of the auxiliary fiiel burned. This
total rate of energy input can be found by multiplying the delivery rate (Ib/hr) of each
times the respective heating value (Btu/lb).
The total rate of energy leaving the MWI system is associated with the change
in heat content of the flue gas relative to that of ambient air, the heat transfer from
the system, and the heating value of the residual ash.
Slide 21-12
ENERGY LEAVING AN INCINERATOR
Sensible Heat Gain by Combustion Gases
Latent Heat Gain by Combustion Gases
Water formed in Combustion
Water from Fuel Moisture
Water from Spray Nozzles
Moisture in Combustion Air
Heat Loss through the Enclosure
Energy Loss from Incomplete Combustion
The heat content of the flue gas depends upon its temperature as it leaves the
system and its composition. It is possible to separately estimate the sensible heat
associated with the flue gas and the latent heat associated with the flue gas moisture.
The flue gas moisture can include the water formed in combustion, the
moisture of the fuel, the water sprayed into the incinerator for temperature control,
and the moisture from the combustion air.
The heat loss from the system includes the conduction loss from the incinerator
to the surroundings. Generally, this heat loss can be estimated as a fraction of the
heat input (e.g., 5%). An additional energy loss is associated with the energy content
of the unburned ash residues leaving the system, which can be estimated using the
ash's carbon content and the heating value for carbon.
Slide 21-13
HEAT BALANCE ON A HEAT EXCHANGER
Energy Extracted from the Hot Fluid
Must Equal
Energy Gained by the Heated Fluid
21-9
-------
The heat balance of a heat exchanger (e.g., air preheaters, economizers,
feed-water heaters, waste-heat boilers and condensers) simply equates the energy
extracted from hot fluid to the energy gained by the cooler fluid. It is often
reasonable to assume that the exchanger is perfectly insulated and has no heat loss.
Slide 21-14
AIR PREHBATBR IN A CODNTBRFLOW CONFIGURATION
fnn
Temperature, F
" i r-i I 1.1 1 i «• I
W-^
Ifc—
3- —
uo.o
Preheated
I
•^ P"
I
•<*•
'__ ^
i fr»
•
-«— Flue Gas
— e— Combustion Air
1
\
i
i
0.2 0.4 0.6 0.53 1.0
Relative Position Down The Unit
Cooled Flue Gas
r~+
Combustion Air
4
I
^
M
I
* —
Ambient Air
__ ^~
Hot Flue Gas
21-10
-------
Heat exchangers can be designed in a counterflow configuration to provide for
optimum heat transfer between the hot and cold fluids, as illustrated in Slide 21-14.
Alternatively, as shown in Slide 20-11, crossflow heat exchangers with
extended surfaces may be used to provide a high rate of heat transfer in a relatively
small space.
Slide 21-15
FEATURES OF STEAM PRODUCTION IN BOILERS
Raise Water Temperature to Saturation
Vaporize Water (Make Steam)
Heat the Vapor to Form Superheated Steam
Boilers can have three different heat gain requirements, which are typically
delivered in separate regions or sections of the boiler. The first requirement is to
heat the liquid water to its saturation temperature. The saturation temperature
depends upon the pressure of the boiler, which is generally controlled by balancing
the operating conditions of the feedwater pump and main steam or throttle valve.
Heating of the water can occur in an economizer, a feedwater heater (heated by
steam), the bottom region of the fire-tube boiler's tank, and/or a special section of
boiler.
As soon as the saturation temperature is reached, any additional heat transfer
will result in the production of water vapor or steam bubbles. During the boiling
process, the temperature and pressure of the water are maintained at saturation
conditions.
Boilers must have some form of steam separation in order to deliver essentially
dry steam to the main steam line. The simplest form is separation by gravity, which
is typical of a fire-tube boiler system. In this case, steam is removed from the top of
the boiler, above the liquid-vapor interface. The liquid is considerably more dense
than the vapor, so it tends to remain in the liquid-vapor mixture.
Water-tube boilers have steam drums with special design features (e.g., baffles,
turbo separators, and screens) to separate the vapor from the vapor/liquid mixture.
As most MWI boilers are of the fire-tube design, the various design features of the
steam drum will not be discussed further.
21-11
-------
22. BOILER WATER TREATMENT
Slide 22-1
IMPURITIES OF RAW WATER
Composition Varies with Source
Chemical Wastes
Organic Wastes & Bacteria
Oxygen & Dissolved Gases
Dissolved or Suspended Minerals
Suspended Solids
Raw water often has dissolved solid impurities which cannot be removed by
conventional settling, clarification and filtering techniques. These impurities are
concentrated in boiler water because they are not vaporized in the boiling process.
Water treatment has many complex aspects which are developed in various
references, such as those listed at the end of this learning unit. The emphasis of this
learning unit will be on summarizing boiler water treatment features. Cooling water
and wastewater treatments are also important operator responsibilities.
Impurities generally get into boiler water systems through the raw water
supply and through the leakage of the condenser's cooling water into the condensed
steam (condensate) which is recirculated into the boiler as feedwater. For example,
in applications where coastal water is used for condenser cooling water, salt water
(chloride) can leak into the condensate. Since the salinity of coastal water changes
with the tides, the chloride content of the leakage will also vary with the tides.
Slide 22-2
CHEMICAL COMPOUNDS
Acids: Hydrogen Ions in Solution
Bases: Metal-Hydroxyl ions in Solution
Salts: Compounds of Acids & Bases
When materials dissolve in water, they dissociate into ions. The positively
charged ions are called cations and the negatively charged ions are called anions.1
The chemical compounds found in water are classified as acids, bases (alkalis)
and salts. Examples of acids include hydrogen chloride, nitric acid and sulfuric acid.
When acids are dissolved in water, hydrogen ions (H*) and negatively charged anions
such as: Cl", N03' and SO4~ are formed.
22-1
-------
Bases are those liquid solutions characterized by the presence of metal ions
(e.g., Ca**) and hydroxyl ions (e.g., OH"). These may be referred to as basic, alkali,
and caustic solutions. Examples of basic materials include water solutions such as
calcium hydroxide or hydrated lime which are formed when lime (calcium oxide)
reacts with water.
Salts are formed when acids and bases react together. Examples of salts are
calcium sulfate, calcium carbonate (CaCO3, limestone), magnesium carbonate
(MgCO3), and sodium chloride (table salt).2
Slide 22-3
BOILER WATER IMPURITIES
1. Dissolved Gases
2. Dissolved Minerals - Hardness
3. Dissolved and Suspended Solids
The major impurities in boiler water can be grouped as: dissolved gases,
dissolved minerals, and dissolved and suspended solids.
Many materials have increased solubility in water as the temperature
increases. However, oxygen, carbon dioxide and many salts (e.g., calcium sulfate,
calcium carbonate & magnesium carbonate) have their solubility reduced as the
temperature increases. This leads to the important phenomena of deaeration and
scale formation, which will be discussed below.
Slide 22-4
BOILER WATER PROBLEMS
Corrosion & Pitting of Metal
Scale Build-Dp Inside Tubes
Operators of MWI systems with boilers must pay particular attention to the
conditions of the boiler water. Water contains various impurities which can cause
corrosion and pitting of metal as well as scale build-up. Oxygen, carbon dioxide and
other dissolved gases can cause corrosion and pitting which can lead to metal failure
and the formation of blisters, bags and/or burned out heat exchange surfaces.3
Dissolved gases are generally removed from the water in the deaerating feedwater
heater, which will be discussed later.
22-2
-------
Scale formation is most difficult in the high temperature regions of the boiler,
due to the decreased solubility which results in precipitation of scale materials. The
typical compounds found in scale deposits include calcium carbonate, iron oxide,
alumina, calcium phosphate, magnesium hydroxide, magnesium silicate, and silica.1
Slide 22-5
Influence of Scale on Metal Temperatures
Scale
Tube Without
Scale
Tube With
Scale
The tube failure mechanisms include the long-term, repetitive formation of
scale which acts as insulation and causes increased metal temperatures and thermal
expansion of the
As indicated in Slide 22-5, the outside metal temperature (Tmo) is increased
considerably as a result of the insulating properties of built-up scale.
When the scale cracks off, the metals are suddenly cooled and undergo
contraction. This process results in the formation of bulges or blisters which
eventually lead to tube failures. Another mode of tube failure is associated with
rapid accumulation of scale deposits which causes overheating to the point that the
metal undergoes plastic flow and ruptures.6
The water treatment at each site must be appropriate for the water conditions
of the individual sources of contamination and the operating steam pressure and
temperature of the particular boiler.7
22-3
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Slide 22-6
WATER TREATMENT FOR A STEAM GENERATOR
MAKE-UP WATER
STEAM
i
CLARIFIER
SOFTENER
DEAERATOR
VENT
STORAGE TANK
I
PUMP
1
FEEOWATER
BOILER
SLOWDOWN
I
TURBINE
OR
CUSTOMER
SUPERHEATED STEAM
ELECTRICITY
1
LOW PRESSURE STEAM
CONDENSER
CONDENSATE
PUMP
PURIFICATION
22-4
-------
The condensate, boiler feedwater, and steam circuit generally follows the flow
path indicated in Slide 22-6. Note, however, that the actual system can be
considerably more complicated than indicated in order to accommodate all the
features of water treatment.
The clarifier is a conventional element of a water treatment system which is
used to remove suspended matter from the raw water. Removal can be accomplished
through the application of coagulation, flocculation and sedimentation concepts.1 The
particles suspended in the water have electrical charges which cause the particles to
repel each other. Coagulation works by neutralizing these charges; flocculation works
to cause the materials to agglomerate; and sedimentation is the physical settling
process.
Softening and deaeration will be introduced below. However, note that the
deaerator acts as a feedwater heater, using steam to heat the condensate and make-
up water mixture enough to control the amount of dissolved gases.
Slide 22-6 indicates that a condensate purification system can be located
downstream of the condenser. Condensate purification can include condensate
polishing (demineralizer) and water softener systems which effectively remove
particulates and dissolved or ionized impurities. Although uncommon for waste-heat
boilers, condensate purification may be used where condensate is reclaimed from an
industrial process and where seawater is used as the condenser coolant.
Although not indicated in the sketch, various chemical additives or conditioners
may be added to the water in the deaerator storage tank, boiler, or main steam
supply line to achieve the desired changes in water or steam properties.
Slide 22-7
BOILER HATER PROBLEMS & REMEDIES
1. Dissolved Gases Cause
Metal Corrosion & Pitting
Remedies:
Deaeration
Chemical Scavengers
Oxygen, carbon dioxide and other dissolved gases can cause corrosion and
pitting. For example, oxygen can react with the protective iron oxide (magnetite) on
metal surfaces to form a non-protective oxide (hematite) which allows pitting to
occur.7 This oxidation can be controlled by reducing the amount of dissolved oxygen
in the water and creating a slightly basic water solution.4
22-5
-------
Gases are removed by heating water enough to cause the gases to he vaporized
in a deaerating feedwater heater. The operating principle is thait the solubility of
oxygen decreases significantly as the water temperature increases. To be most
effective, the temperature must be as high as possible, although not so high as to
cause the feedwater pump to become steambound.
In addition, chemical deaeration can be achieved by the addition of chemicals
(scavengers) such as sodium sulfite or hydrazine.7 Sodium sulfite is often added to
the water in the deaerator storage tank. It reacts with the traces of oxygen leaving
the deaerator, and the residual sodium sulfite is maintained in the boiler to remove
any un-reacted oxygen.
Hydrazine works well as an oxygen scavenger in high pressure boilers (where
sodium sulfite breaks down and becomes corrosive), but hydrazine can cause
undesirable health problems.7
Slide 22-8
TRAY -TYPE DBAERATING FEEDWATER HEATER2
INLET ~U— 1
^1
\
^— ?S"r-^
frT^S^Srh
i
>'J5^ — •"VI1
^ HTP =^, WATER IKLET
yi ; --
-------
The deaerating heater is an "open feedwater heater" which is used to remove
dissolved gases.3 The gases are forced out of solution by heating the water with
steam. In general, the heating is provided by steam. The steam is extracted from
the turbine at an intermediate pressure between the condenser pressure and the
throttle pressure. Deaerator pressures are often found between 5 and 75 psig.
The gases released by heating can be purged by opening a vent valve at the
top, since the unit is pressurized. The water collected in the storage tank becomes
boiler feedwater.
Slide 22-9
DEAERATING FEEDWATER HEATER & FLASH TANK3
-Boiler
Vert
Continuous
Slowdown
Hashing
Steam
Makeup
Water
Rash Tank
Feedwater Heater
From Frederick M. Steingress and Harold J. Frost, Stationary
Engineering. American Technical Publishers, Inc., Homewood, IL, 1991,
printed with permission.
The steam for the deaerating heater can be extracted from the steam line, or
it can come from the boiler blow-down after it has passed through the flash tank. In
general, the steam pressure will vary with the design. However, as larger pressures
are specified, the corresponding feedwater temperatures and vapor pressures are
increased.3
To avoid cavitation problems (pump becoming steam-bound), the design must
provide for the net positive suction head (NPSH) requirements of the feedwater
pump. Cavitation can be avoided if the height of the deaerator storage tank above
22-7
-------
the feedwater pump is adequate to compensate for the frictional losses in the supply
pipe, the losses of the fittings, and the fluid's vapor pressure.
The flash tank is a special evaporative heat exchanger which heats feedwater
and also evaporates part of the blowdown water from the boiler3. In the flash tank,
the contaminants from the blowdown are concentrated in the waste water and pure
steam is delivered to heat the feedwater.
Slide 22-10
BOILER WATER PROBLEMS & REMEDIES
2. Dissolved Minerals - Hardness
Increase Metal Corrosion
Form Scale & Sludge
Remedies:
Water Softeners
Condensate Purification
Minerals dissolve in water as ions that carry an electrical charge.8 The ions
can be indicated by measurements of conductivity and hardness. Minerals often
enter from the water supply or as cooling water which leaks through seals and
connections in the condenser. Minerals such as calcium and magnesium must be
treated and removed to avoid scale formation.
A general approach is to treat the water by the addition of an alkali substance,
such as sodium hydroxide or caustic soda, which converts the calcium and magnesium
carbonates to a precipitate. These minerals are typically found as calcium carbonate
and magnesium carbonate. Such carbonates (or salts) are generally detected by
testing for hardness,3 which relates to the ability of water to dissolve soap,2
Mineral salts can be treated by the addition of an alkali substance, such as
sodium hydroxide (caustic soda) and calcium oxide (lime), which converts the
carbonates to a precipitate.3 The precipitates can be removed by filtration. Other
methods include conversion to a calcium phosphate and magnesium phosphate
sludge, which does not adhere to metal surfaces.
The monitoring of pH in both boiler water and in feedwater is very important
because significant damage due to scale build-up and corrosion can occur in a short
period of time.4 Note that high alkalinity levels can cause deposits which weaken
metal and lead to cracking, called caustic embrittlement.
22-8
-------
Demineralizers are used in some boiler operations to remove ionized mineral
salts (hardness).1 Cations (pronounced as "cat-ions") such as calcium, magnesium,
and sodium can be removed in a hydrogen cation exchanger. Anions (pronounced as
"an-ions") such as bicarbonates, sulfates, chlorides, and soluble silica are removed in
the anion exchanger.
A sodium zeolite ion-exchange water softening process is one of the simplest
methods of removing hardness. The zeolite can be thought to act like a sponge
material that gives up its sodium ions in exchange for calcium carbonate, magnesium
carbonate, and ferrous oxide. When the zeolite has been exhausted of sodium ions,
it can be regenerated by flushing with common table salt (sodium chloride).3
Synthetic cation and anion exchange resins, which can be regenerated, are
often used. Demineralizers can involve two-vessel systems as well as mixed bed
units.3 Generally, mixed bed units are more difficult to operate and not as efficient
as systems with separate vessels for the cation and anion resins.
Slide 22-11
BOILER WATER PROBLEMS & REMEDIES
3. Dissolved & Suspended Solids
Causes Carry-over of Impurities
Damages Superheater, Valves, Turbine
Remedy:
Boiler Water Slowdown
Impurities such as dissolved and suspended solids in the water are removed
by blowdown, a process where a portion of the water is discharged from the boiler
and replaced by treated make-up water. Note that these impurities are non-volatile,
so they will not evaporate and leave the boiler with the steam.
If dissolved and suspended solids are not removed, their concentration will
build up in the boiler water. High concentrations will cause foaming, which is the
trapping of steam bubbles below the water surface in the steam drum.
Foaming can cause the carry-over of slugs of liquid water with dissolved and
suspended solids into the steam lines. The solids may be deposited in the
superheater tubes, valves, and steam turbine, resulting in considerable damage and
loss of production.6 For instance, high concentrations of silica can cause severe
turbine blade fouling.
22-9
-------
Slide 22-12
INDICATORS OF WATER QUALITY
1. pH - indicates Acidle/Alkali Quality
<7: Acidic; 7: Neutral; >7: Basic
2. Conductivity of Steam & Feedwater
Micros iemens/cm
3. Total Dissolved Solids in Boiler Water
Microsiemens/cm
4. Alkalinity
Equivalent Calcium Carbonate, ppm
5. Hardness - Ability to Dissolve Soap
Calcium & Magnesium Salts, ppm
6. Silica - Silicon Dioxide, ppm
There are a number of laboratory tests and continuous monitoring instruments
which can provide an indication of water quality. The most common are the pH and
conductivity meters. The level of water impurities may also be indicated by the total
dissolved solids, alkalinity, hardness and silica. Each should be kept below the
manufacturer's recommended limits that depend upon the unit's design (boiler
pressure).4'7
A pH meter monitors the acidic or basic (alkali) characteristics of a solution.
A pH number of 7 indicates a neutral condition, with larger values indicating basic
(alkaline) conditions and smaller values indicating acidic conditions.
The optimum pH levels in boiler water will depend upon the application9 and
whether the water under consideration is condensate, feedwater or boiler water. The
monitoring of pH is very important because significant damage due to scale build-up
and/or acid corrosion can occur in a short period of time.* The pH values in the
feedwater and condensate are generally lower than the boiler drum water. The pH
of the boiler water at one MWC unit with 100% condensate return ranges from 9.3
to 10.4, and the condensate ranged from 9.0 to 9.5.10 Feedwater pH often ranges
between 8.0 and 9.5.9
Electrical conductance is the reciprocal of electrical resistance. Conductance
relates to the flow of electrons through the water, with more impurities causing
electrons to flow more easily. Conductivity meters measure the reciprocal of electrical
resistance of water, with low conductance and high resistance being an indication of
pure water. High conductance would therefore be an indication of a high level of
impurities. Conductivity meters can be mounted on the steam and feedwater tubes
to provide a continuous reading.
22-10
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The typical conductivity instrument provides direct reading in microsiemens/cm
or micromhos/cm (Siemens and mhos are equivalent).11 A typical boiler water
conductivity range is from 75 to 100 micromhos/cm, whereas the feedwater range is
from 10 to 15 micromhos/cm.10
The meter for total suspended solids operates exactly as a conductivity meter.
It is used to measure the quality of water leaving the boiler in the blowdown.
Alkalinity, hardness and silica can also be measured as an equivalent
concentration of calcium carbonate, calcium and magnesium salts, and silicon dioxide,
respectively. Silica monitors are typically mounted on the main steam line to the
turbine, giving measurements in mass-based parts per million (ppm). For instance,
1 ppm of silica would indicate that a pound of silicon dioxide would be found in a
million pounds of water.3
The example hardness, silica, and iron numbers which follow were measured
at a boiler with 100% condensate return.10 The hardness in the feedwater was
limited to 0.1 ppm. The measured silica in the boiler water was around 3 ppm, with
the silica in the feedwater limited to 0.1 ppm. Similarly, the iron in the boiler water
was around 1 ppm, and the corresponding iron in the feedwater was 0.02 ppm.
22-11
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REFERENCES
1. Frank N. Kemmer, Nalco Water Handbook. The Nalco Chemical Company,
Second Edition, published by McGraw Hill Book Company, 1988, pp. 3.3 to
3.15,12.24 to 12.45, 39.1 to 39-66.
2. David F. Dyer and Glennon Maples, Boiler Efficiency Improvement. Boiler
Efficiency Institute, Auburn, AL, 1981, pp. 8.1-8.32.
3. Frederick M. Steingress and Harold J. Frost, Stationary Engineering.
American Technical Publishers, Inc., Homewood, IL, 1991, pp. 83-87,277-293.
4. Joseph G. Singer, Combustion. Fossil Power Systems. 4th Edition, Combustion
Engineering, Inc., Windsor, CT, 1991, pp. 20-1 to 20-45.
5. E. B. Woodruff, H. B. Lammers and Thomas F. Lammers, Steam Plant
Operations. Fifth Edition, McGraw-Hill Book Company, New York, 1984, pp.
264-270.
6. Betz Handbook of Industrial Water Conditioning. Eighth Edition, Betz
Laboratories, Inc., Trevose, PA, 1980.
7. James A. Baumbach, "Key Treatment Chemicals," Standard Handbook of
Power Plant Engineering. Thomas C. Elliott, editor, McGraw Hill Book Co.,
NY, 1989, pp. 4.217-4.237.
8. David S. Beachler, APTI Course SI:428A. Introduction to Boiler Operation. U.
S. Environmental Protection Agency, EPA 450/2-84-010,1984, pp. 4-13 to 4-16.
9. Steam. Its Generation and Use. 39th Edition, Babcock and Wilcox, New York,
1978, pp. 34-10 to 34-18.
10. Personal Communication, Vicki Barnhart, York Resource Energy Systems, Inc.,
York, PA.
11. J. A. Moore, "Pollution Instrumentation," Standard Handbook of Power Plant
Engineering. Thomas C. Elliott, editor, McGraw Hill Book Co., NY, 1989, pp.
6.45-6.57.
22-12
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23. BOILER SAFETY, CONTROL & OPERATIONAL FEATURES
Slide 23-1
OPERATING RESPONSIBILITIES
1. Maintain Safety of People
2. Maintain Safety of Equipment
3. Optimize Equipment Performance
Learning Units 13 and 19 indicated that the primary responsibility of operators
is to conduct operations in a safe and efficient manner. For boilers, these concerns
include the protection of the operating personnel, the reduction of potential damage
to the boiler, and the optimization of steam production.
Operators must understand general characteristics of the boiler system, the
component equipment and the important influences of one component on another.1
Of course, the operational limitations and control features vary from boiler to boiler.
Operators gain much of their knowledge through on-the-job experiences of
operating and inspecting the equipment. Additional knowledge comes from
discussing issues with knowledgeable personnel, studying the site-specific training
manuals, and reviewing the instructions found in their MWI unit's operations
manual.
The operator must assess boiler operating conditions through surveillance of
the instruments, the boiler and the ancillary equipment. Operators must also judge
the importance of deviations from normal operating conditions.
When an unsatisfactory condition is found, the possible remedies must be
considered and a decision made in a timely manner. The operator must be prepared
to respond to all situations which may arise. If appropriate decisions are delayed
and/or corrective actions taken at inappropriate times, operating conditions will
probably deteriorate. The boiler could operate outside its design limits, for example,
leading to an unsafe condition and/or automatic opening of safety valves.
Slide 23-2
WATER-SIDE CONTROL PARAMETERS
Steam Flow Rate
Feedwater Flow Rate
Steam Pressure
Steam Temperature
Drum Level
23-1
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The water-side parameters listed in Slide 23-2 may be used as control
parameters for the boiler's control system. In large MWC systems, these control
parameters could also be used in the combustion control system, but this is seldom
the case in MWI systems.
Steam production rates (flow rates) in recovery boilers are dependent upon flue
gas temperatures, which vary with the operating conditions in the incinerator. As
discussed in Learning Unit 8, flue gas temperatures are dependent upon the feed
rate, moisture content and the amount of excess air. A variation in the flue gas
temperature will cause corresponding changes in steam production. Therefore, steady
production generally occurs only if the incinerator operation is steady.
The pressure on the water-side of the boiler is controlled by modulating the
feedwater flow rate (feedwater pump) and the rate of steam flow (supply valve). The
boiler control system attempts to maintain a fairly constant pressure on the water-
side of the boiler. However, if the rate of steam flow is controlled by the steam
customer, the pressure will tend to drop as steam flow increases and increase as the
flow is restricted.
Steam temperature, which is an important control variable in boilers producing
superheat steam, is seldom used as a primary control variable in MWI boiler
applications. This is because the boiling temperature is normally fixed by the
pressure. However, steam temperatures above a set point could be used as a signal
to open the damper to the relief stack or open a vent, allowing ambient air to mix
with and control the flue gas temperatures.
Slide 23-3
POTENTIAL MAJOR HAZARDS
1. Explosive Mixture of Fuel/Air
2. High Pressure Steam Pipe Rupture
3. Loss of Water
As has been the case for generations, the three most dangerous conditions in
boilers are fire-side explosions associated with the existence of explosive mixtures,
steam pipe ruptures associated with high pressure, and water-side explosions
associated with the loss of water.
Explosive mixtures are avoided by purging the gas-side of the boiler and APCD
with air prior to lighting the burners. A minimum air flow should be maintained at
all times after the purge process. Preheating of the secondary chamber improves the
combustion and reduces the possibility of the quenching of combustible gases, which
could lead to the accumulation of explosive gas mixtures in the APCD and boiler.
23-2
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Exposure to pressurized steam from vents and/or the rupture of steam pipes
can cause severe scalding. General protection against excessive pressures in pipes
is provided by the installation of safety valves, which should be regularly tested.
The potential for damage should be controlled by redirecting the flue gas to an
emergency by-pass stack, in the event that a high metal temperatures are sensed.
Steam pipes may rupture under the uncontrolled conditions of water hammer.
Water hammer occurs when slugs of liquid water and steam flow together in a steam
pipe.1 Liquid water is routinely removed from steam pipes by the use of steam traps.
Slide 23-4
LOSS OF WATER LEVEL SCENARIO
1. Boiler Runs Dry
'2. Heat Extraction is Interrupted
3. Metal Overheats
4. Water Supply Causes Thermal Stresses
5. Tubes or Pressure Vessel Ruptures
Maintaining the water level in the boiler is important because the boiling of
water is the major mode of heat removal. Under normal conditions, the tube
temperatures are very near the water's boiling point.
A water-side boiler explosion can be caused by an interruption in the boiler
water supply. If the boiling process drys out the boiler, the metal will be overheated
to approximately the flue gas temperature, which may result in metal weakness.
Metals have reduced pressure carrying capacity as their temperatures increase.
Thermal stress problems may be increased by the inflow of large quantities of
relatively cool feedwater. Boiler explosions may result from high stresses which are
caused by adding water to a boiler whose tubes have overheated during a period of
low level of water (no water in the sight glass).
The level of water in the boiler must be maintained so that the boiler neither
"runs dry" nor delivers liquid into the main steam line. The water level represents
the balancing of the inflow and outflow of water from the unit.
A water level problem can result from a malfunctioning control system, a
failure in the feedwater system, excessive steam flow or steam leaks. The water level
in the boiler is controlled by modulating the feedwater flow rate (or feedwater pump
pressure) and the rate of steam flow (supply valve).
If the lost water is due to water-tube failures, the unit will generally need to
be taken off-line for tube replacement or plugging. The steam or water escaping from
23-3
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a tube leak could result in the cutting of adjacent tubes and/or the overheating of
other sections of the boiler circuit.1
The water level of a fire-tube boiler does not change very rapidly, so a single
element control system, as described below, is often used. By contrast, the water
level in the steam drum of a water-tube boiler often requires a two-element or three-
element automatic controls system to avoid either producing liquid carry-over (low
quality steam) or interrupting the circulation process.
Slide 23-5
SINGLE-ELEMENT CONTROL SYSTEM: DRUM LEVEL2
StMflidnim
mtwivwl
SP
Feedwater
LC
-L
FCE
Boiiw
Courtesy of the Instrument Society of America
A single-element drum level controller (LC) is designed to receive a signal from
a level transducer (LT) for comparison with the set point. Single-element drum level
controllers, however, produce unstable drum level conditions because they are
incapable of considering logic based on the phenomena of "shrink and swell."
When steam flow increases (e.g., throttle valve is opened), the boiler steam
pressure drops. The reduced pressure causes the mixture of vapor and water in the
boiler to expand. This phenomenon, called "swell," results in a higher level of water
in the boiler drum. A single-element controller would sense this act and reduce the
feedwater flow into the boiler.
However, a simple mass balance tells us that as steam How goes up the
feedwater flow should be increased. Incidentally, increasing the flow of relatively cold
feedwater into the boiler will also decrease the average temperature and increase the
mixture density in the boiler, which could somewhat offset the "swell" condition.
23-4
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The alternate problem is that of "shrink," which occurs during a reduction of
steam flow. Shrink occurs with an increase of steam pressure and density and a
decrease in drum level.
Slide 23-6
TWO-ELEMENT CONTROL SYSTEM: DRUM LEVEL3
Courtesy of the Instrument Society of America
Two-element controllers improve the stability of the drum level by sensing the
rate of steam flow as well as the drum level. A signal from a flow transducer (FT)
produces an actuating signal from a flow controller (FC) which is compared with the
actuating signal from the level controller.
For instance, under an increasing steam flow condition, the flow controller
produces a positive signal while the level controller produces a negative signal. The
two signals are added, producing a null output signal which temporarily maintain the
feedwater flow rate into the boiler. After a period of time, the pressure will stabilize
and the drum level will drop, causing the flow to be appropriately increased.
23-5
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Slide 23-7
THREE-ELEMENT CONTROL SYSTEM: DRUM LEVEL3
Courtesy of the Instrument Society of America
A three-element control or cascade control system makes use of the output
from one control loop to provide the set point for another. In the example, the steam
flow and boiler water level are used to correct the set point for the feedwater flow
controller.
Although cascade control logic may appear to be complicated, it has been used
for many years in industrial-sized combustion equipment. It is particularly
applicable in modern microprocessor-based PLC (programmable logic controller)
control systems. These systems use computational techniques which can take into
account special features of variable gain, differential control and integral control.
Because these specific control features are beyond the scope of this training program,
they will not be considered further.
Under rapid operational transients, such as pressure and water level changes,
some liquid water may become entrained with the vapor and delivered into the main
steam line. This is generally not a serious problem for "process steam." By contrast,
liquid carryover would be highly undesirable if the steam were to be used for driving
a high speed steam turbine.
A modest amount of liquid water will condense in the main steam line as a
result of heat loss to the surroundings. Although steam lines are generally insulated
to minimize condensation, they are also equipped with steam traps which remove
liquid water.
23-6
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Slide 23-8
BOILER PERFORMANCE INDICATORS
*
*
*
Steam Pressure
Steam Temperature
Steam Flow Rate
Steam pressure and flow rate are closely watched performance indicators for
waste-heat boilers. Because "saturated steam" is generally produced, steam
temperature measurements will correspond to the steam pressure at boiling
(saturation) conditions. Temperatures below saturation values will be observed
during start-up and shutdown.
Slide 23-9
STANDARD OPERATING PROCEDURES
1. Safe Practices & Systems
2. General Operations
3. Start-Up and Shutdown
4. Routine & Major Maintenance
5. Emergency Procedures
Operator actions should generally be consistent with established procedures,
such as in those areas listed above. For instance, operators must promptly respond
to potentially hazardous safety conditions to reduce the possibility of injury to
personnel or damage to the boiler. Particular diligence is required during start-up
and shutdown, as these transient operations may provide opportunities for
uncontrolled conditions.
Slide 23-10
BOILER INSPECTION CHECK-LIST
1. Boiler Operating Conditions
2. Feedwater & Condenser System
3. Water & Oil Leaks (Valve Packing)
4. Steam Leaks (Safety Valve, Inspection Ports)
5. Equipment Noise
6. Tube Conditions
7. Soot-Blowers (Confirm Operation)
23-7
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The operator must consider measures which will maintain or improve the
boiler's ability to perform continuously and efficiently. This includes conducting
routine inspections and making arrangements for preventive and corrective
maintenance.
Operators can identify most of the important aspects of boiler performance by
maintaining and reviewing comprehensive log sheets of boiler equipment operating
conditions. Such log sheets are generally maintained at specified time intervals (e.g.,
at the beginning of each shift).
A daily log sheet is typically used to tabulate the operating conditions of the
boiler systems, including its various component equipment. Log sheets can record
whether the component is operating or not, with a listing of relevant pressures,
temperatures, flow meter readings, indicator positions and levels.3
Slide 23-11
BOILER OPERATIONAL ACTIVITIES
* Detect & Repair Tube Failures
* Monitor Steam Production
* Review Fluid Temperatures
* Remove Soot from Boiler Tubes
Generally, steam tube failures can be detected by the sound of the leaking
steam. An increased feedwater rate without an increased steam flow is another
indication of a boiler tube failure or other steam system leakage. Tube failures will
cause additional moisture in the flue gas, increasing the possibility of gas dew point
problems downstream and a visible "steam plume."
The failure of one tube can cause a jet of pressurized steam which can damage
adjacent tubes. Therefore, the standard operational procedure is to take the unit off-
line as soon as possible in order to inspect and repair or plug the ruptured tube(s).
Boiler performance will be reduced by the insulating properties of soot deposits
on the fire-side and scale build-up on the water-side of the boiler tubes. Reduced
boiler performance is primarily monitored as the steam flow rate. The amount of
energy extracted in a waste-heat boiler is also proportional to the difference between
the entering and exiting flue gas temperatures.
The condition of the boiler heat exchanger surface will not directly influence
the boiling temperature, as it is established by the pressure. However, scale deposits
inside water-tubes and soot or fly ash deposits inside fire-tubes can restrict flow.
23-8
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Slide 23-12
SOOT DEPOSITS
Soot on Heat Exchanger Surfaces
Soot deposits in recovery boilers and other heat exchangers (e.g., air preheater
and economizer) will accumulate over time and cause reduced heat transfer. Soot is
the name given to the solid deposits which form on the fire-side (flue gas side) of heat
exchange surfaces.
The accumulated soot on the boiler or heat exchanger surface may be detected
by monitoring the pressure drop across the flue gas-side of the unit. Alternately, the
relative amount of deposits can be determined from a review of the temperature
change records of the fluids passing through the heat exchangers.
For instance, consider the flue gas which normally has a 150 °F temperature
drop as it passes through a heat exchanger. If the temperature drop deteriorates to
75 °F, there would be reason to suspect that soot deposits had caused a 50% reduction
in the energy extraction. Soot should be removed before it accumulates enough to
significantly limit unit performance.
Soot is generally deposited as a solid, so it is fairly easy to dislodge. In some
high temperature flue gas applications, the deposited materials may initially be in
a liquid form with the deposits subsequently freezing (solidifying) and fusing on the
heat exchanger. Such deposits are generally difficult to remove, requiring a manual
scraping effort.
Some systems provide for soot removal as often as once each shift. Soot is
removed in other systems on an as needed basis. Soot removal can be accomplished
by using "soot blowers" or by manually cleaning the heat exchanger surfaces.
Automatic or manually controlled soot blowing systems can be used on fire-tube
boilers, water-tube boilers and economizers. Soot blowers operate by directing
pressurized steam or air in the vicinity of the surfaces to dislodge the soot. The high
pressure jets should not directly impact the surfaces, however, as they can effectively
"sand blast" the surfaces and cause metal wastage and tube failures. Likewise, if
steam soot blowing is used, the steam should be passed through a steam trap to
remove liquid water before being delivered to the soot blowing nozzles.
Manual cleaning of fire-tube boilers may include "rodding" of tubes, which is
performed as needed when the unit is off-line. Manual cleaning of soot from water-
tubes generally requires the use of steel brushes.
23-9
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Slide 23-13
OPERATOR-INITIATED CHANGES
Transmit Direct Signals
Motor, Pumps, Switches
Transmit Signals to Controllers
Modify Set Points
Initiate Start-Up or Shutdown
Request Maintenance
When a boiler system disturbance is observed, the operator must evaluate the
severity of the condition, consider possible remedies, and make a proper response.
Operator responses include transmitting instructions to the equipment through
manual manipulations which override or modify the settings of the control system.
The unit may be restored to proper operating conditions by changing the
control system set points or by switching to the manual mode to make specific
operational equipment modifications. Operator-initiated control signals can be sent
to either specific equipment or controllers through microcomputer keyboards, touch-
screens, and/or panel-mounted controller knobs and switches.
Operator's instructions include requesting verification of operating conditions.
Operators also have the tasks of assuring that routine maintenance is performed and
proper requests are made for major maintenance.
Slide 23-14
PREPARATION FOR BOILER START-DP
Inspect Boiler
Test Components:
Fans, Pumps, Safety Valves
Clean Gas-Side of Boiler
Chemically Clean Water-Side
Fill Boiler with Water
Static Test Boiler at Pressure
Adjust Control System Settings
Prior to starting or restarting a unit, the boiler should be inspected. In
addition, the routine testing of various components, such as damper, damper seals,
fans, pumps and safety valves, may be needed. Inspection includes using the proper
procedures for clearing safety tags from breakers and on hatch doors.
23-10
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The start-up procedure depends upon how long the boiler has been off-line and
the existing conditions within the boiler (hot water, cold water, or empty). The
standard operating procedures should be followed, including correctly positioning the
valves and dampers for start-up according to the manufacturer's recommended
procedures.
Gas-side cleaning and water-side chemical cleaning may be required, depending
on the conditions. During an annual outage, ultrasonic testing (UT) may be
performed to identify the locations where the tube thicknesses has been substantially
reduced due to corrosion and erosion.
During the initial system start-up, steam leaks can be detected by conducting
a hydrostatic test. Water should be added to the boiler up to the proper level on the
glass water level indicator. The water should be warm enough to prevent
condensation on the gas-side of the metal surfaces. Hydrostatic testing can include
pressurizing the boiler section up to 50% above its rated pressure.4 Typically, a full-
scale hydrostatic test is only performed during initial start-up.
Slide 23-15
BOILER START-UP
Purge Air & Ignite Burner
Maintain Minimum Air Flow
Vent Air from Drum & Headers
Limit Thermal Stresses
Vent Steam from Economizer
Enable Automatic Controls
Monitor Auxiliary Systems
Prior to igniting the auxiliary fuel of the MWI, the gases in the boiler are
generally purged with fresh air for five minutes. The auxiliary fuel burners are then
used to heat the combustion chamber, with some minimum air flow supplied at all
times to avoid explosive conditions.
During warm-up, there will be little or no steam flow through the main steam
line. Special care will be required to vent air and steam and to prevent the
occurrence of water hammer and other unsafe conditions.
To protect boiler metals against thermal stress problems, the flue gas
temperatures entering the waste-heat boiler should be controlled as recommended by
the manufacturer. The temperature rise profile is generally specified by the
23-11
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manufacturer, with heat-up values typically limited to around 200° F per hour.5
When the manufacturer-specified operating conditions are met, steam flow to the
steam circuit can be initiated.
Slide 23-16
BOILER & COMBUSTION SYSTEM SHUTDOWN
Stop Feeding Waste into MWI
Burn the Waste in the Primary Chamber
Operate Auxiliary Burners as Necessary
Let the Steam Pressure Decay
Limit the Cool Down Rate
The procedures and time requirements for shutting down a boiler depend upon
the design of the unit and the nature of the shutdown. Shutting down a boiler can
occur as a result of an emergency, or as a planned shut down from normal operations
to either hot stand-by or cold conditions.
The general shutdown procedures provide for terminating the waste charging
and maintaining temperatures through the use of auxiliary fuel firing equipment.
Depending upon how quickly the unit is to be shut down, the boiler pressure
can be allowed to decay naturally (e.g., a bottled-up unit) or the fans can be operated
to accelerate the boiler's cooling process. Normally, the boiler will not be drained
unless the boiler or steam drum is to be entered. A special wet lay-up procedure
must be initiated if the unit is to be down for more than a week.
The shutdown procedures for a waste-heat boiler include locking out the loader,
maintaining combustion chamber temperatures with the auxiliary burners, and
continuing blower operation until after residue burn-out has been obtained.
An emergency shutdown would generally call for some provisions to stop the
delivery of hot flue gas to the waste-heat boiler, such as through the operation of a
by-pass stack. Some emergency procedures require that the induced draft fan be
stopped, under the assumption that reduced combustion will occur under conditions
of natural draft.
23-12
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Slide 23-17
EXCESS-AIR WATERWALL UNIT OPERATIONS
HEAT TRANSFER
From the Gas Side
To the Water/Steam Side
Many of the operational considerations of MWI waterwall boilers involve
maintaining proper combustion conditions. The operation of the combustion system
is related to the boiler operation by heat transfer. An upset in combustion will
change the steam system. Similarly, a change in the steam demand will cause the
combustion system to change. Also, a drop in feedwater temperature will cause
greater radiant heat transfer to the waterwalls, which will reduce the combustion gas
temperatures.
Slide 23-18
METAL WASTAGE IN EXCESS-AIR UNITS
Erosion (High Temperature)
Temperature Control
Velocity Control
Rapping Rather than Soot Blowing
Corrosion
Oxidation/Reduction Oscillations
Chlorine (HC1) Reactions
Metal Reactions
Metal wastage of heat exchange surfaces, through either corrosion or erosion,
can be a serious operational problem. Erosion is the wastage of metal caused by the
impact of particulates on metal surfaces. Erosion is particularly problematic under
high temperature, velocity, and entrainment conditions. Design options with reduced
gas temperatures and local gas velocities are available.
Corrosion is generally aggravated by the variability of fuel properties, which
causes local oscillations between oxidizing to reducing conditions in the regions above
the fuel bed and adjacent to the metal surfaces. Under oxidizing conditions, iron
oxides will build up on the metal walls. Under reducing conditions, however, the
oxygen will be extracted from the wall, exposing pure metal.
23-13
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Therefore, an important consideration in corrosion control is the improvement
of the combustion process. Proper overfire air supply and mixing will aid the
completion of oxidation and, thereby, reduce the potential for corrosive attack of the
heat exchange surfaces.5
Corrosion conditions can also be caused by chlorine and sulfur reactions which
appear to catalytically occur within the fly ash deposits on metal surfaces,5 such as
in the superheater. These problems are particularly severe if metal temperatures are
above 900 °F.
REFERENCES
1. Steam. Its Generation and Use. 39th Edition, Babcock and Wilcox, New York,
1978, pp. 34-1 to 34-14.
2. "Boiler Feedwater and Steam - Controlling for Safety and Efficiency,"
Videotape from ISA's Boiler Control Series, Reprinted by permission,
Copyright by Instrument Society of America, 1988.
3. PEI Associates, Inc., Combustion Source Inspection Module, Student Reference
Manual". Submitted to U.S. Environmental Protection Agency, September
1990, pp. 242-245, 272-291.
4. Frederick M. Steingress and Harold J. Frost, Stationary Engineering.
American Technical Publishers, Inc., Homewood, IL, 1991,, pp. 87, 181-192,
277-293.
5. Joseph G. Singer, Combustion, Fossil Power Systems. 3rd Edition, Combustion
Engineering, Inc, Windsor, CT, 1981, pp. 6-13,8-16 to 8-18, 20-1 to 20-31.
23-14
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APPENDIX: GLOSSARY, ACRONYMS, AND SYMBOLS1
Absorption
Acid Deposition
AC
Activated Carbon
i
Adsorption
Agglomeration
Air Pollutant
APCD
APTI
The passage of one substance into or through another,
e.g., an operation in which soluble gases are dissolved
into a liquid.
Acid Rain: A complex chemical and atmospheric
phenomena that occurs when emissions of sulfur and
nitrogen compounds and other substances are
transformed by chemical processes in the atmosphere
and then deposited on earth in wet or dry forms. The
dry forms are acidic gases or particulates.
Alternating Current (Electricity)
A highly adsorbent form of carbon used to remove
odors and toxic substances from liquid or gaseous
emissions.
Adhesion of molecules of gas, liquid, or dissolved
solids to a surface, such as occurs with the use of
activated carbon.
The process by which particulates grow larger by
collisions or contact with other particulates.
Any substance in air which could, if in high enough
concentration, harm humans, other animals,
vegetation, or material. Air pollutants include two
different groups (1) those emitted directly by sources
and (2) those produced in the atmosphere by
interaction between two or more primary pollutants or
normal atmospheric constituents.
Air Pollution Control Device: Equipment used to
reduce solid or gaseous pollutants emissions to the
atmosphere from the exhaust gas stream. Such add-on
devices include scrubbers and fabric filters.
Air Pollution Training Institute, USEPA
1 The source for many of the items is the: "Glossary of Environmental Terms
and Acronym List," Publication Number 19K-1002, USEPA, Office of
Communications and Public Affairs, Washington, DC 20460, December 1989.
Appendix A-l
-------
Air Quality Criteria
Air Quality Standards
ASME
ASTM
Atomic Weight
Atmosphere, atm
Attainment Area
AWMA
BACT
Baghouse Filter
BDT
Biologicals
Blood
The levels of pollution and lengths of exposure above
which adverse health and welfare effects may occur.
The maximum levels of pollutants prescribed by
regulations that may not be exceeded during a
specified time (National Ambient Air Quality
Standards).
American Society of Mechanical Engineers
American Society of Testing and Materials
The average weight of an atom of an element.
A standard unit of pressure representing the pressure
exerted by a 29.92-inch column of mercury at sea
level.
An area considered to have air quality as good as or
better than the national ambient air quality
standards. The area may satisfy attainment for one
or more pollutants and a nonattainment area for
others.
Air and Waste Management Association
Best Available Control Technology: An emission
limitation based on the maximum degree of emission
reduction which (considering energy, environmental,
and economic impacts, and other costs) is achievable
through application of production processes and
available methods, systems and techniques. *
Large fabric filter bag, usually made of glass fibers,
used to control the emission of particulates.
Best Demonstrated Technology
Preparations made from organisms or from the
products of their metabolism, intended for use in
diagnosing, immunizing or treating humans or
animals, or in research pertaining thereto.
Human blood, human blood components and products
made from human blood.
Appendix A-2
-------
Bloodborne Pathogens
Bottom Ash
Btu
Burn Cycle
Burn-Down
Burn Rate
By-product
C
CAA
CAAA
Cadmium (Cd)
Cap
Carbon Dioxide, CO2
Pathogenic microorganisms that are present in human
blood and can cause disease in humans. These
pathogens include hepatitis B virus (HBV) and human
immunodeficiency virus (HIV).
Ash residues (unburned and incombustible materials)
remaining after completion of the bed-burning
process.
British Thermal Unit
An operating period (e.g., 24 hours) which includes at
least one of each of the following MWI system
operations, unit preheat, initiation of waste charging,
waste destruction, burn-down (if applicable), unit
shut-down, and ash removal.
The burning period which occurs after the feeding has
ceased and continues until the residues have been
adequately burned so that the secondary fuel burners
are shut off.
Total quantity of waste burned, usually expresses in
Ib/hr.
Material, other than the principal product, that is
generated as a consequence of an industrial process.
Celsius (Degrees)
Clean Air Act
Clean Air Act Amendments
A heavy metal element that accumulates in the
environment.
A layer of clay or other highly impermeable material
installed over the top of a closed landfill to prevent
entry of rainwater and minimize production of
leachate.
A colorless, odorless, non-poisonous gas which is
produced by carbon combustion.
Appendix A-3
-------
Carbon Monoxide, CO
Carcinogen
Catalyst
CDD
CDF
CEM
GEMS
CERCLA
CFM
CFR
Charge Rate
Chlorinated Hydrocarbon
CO
C02
Combustion
Combustion Product
Controlled Air
A colorless, odorless, poisonous gas which is produced
by incomplete combustion of carbon.
A substance or agent that can cause or contribute to
the production of cancer (abnormal tissues or tumors).
A device that enhances the rate of a specified
chemical reaction.
Chlorinated Dibenzo-p-dioxin (All of the isomers of the
tetra-, penta-, hexachloro, dibenzo-p-dioxins)
Chlorinated Dibenzofuran (All of the isomers of the
tetra-, penta-, hexachloro, dibenzo-p-dioxins)
Continuous Emission Monitor
Continuous Emission Monitoring System
Comprehensive Environmental Response,
Compensation, and Liability Act of 1980
Cubic Feet per Minute (ft3/min)
Code of Federal Regulations
Quantity of waste material loaded into a combustion
unit, usually expressed in Ib/hr.
A group of chemicals composed of chlorine, hydrogen,
and carbon elements. They include the persistent,
broad-spectrum insecticides that linger in the
environment and accumulate in the food chain.
Carbon Monoxide
Carbon Dioxide
Rapid oxidation or burning which is accompanied by
the release of energy in the form of heat and light.
Substances produced during the burning or oxidation
of a material.
Combustion units which control the air flow rate to
Appendix A-4
-------
attain the desired rate of combustion.
Contaminant
Contaminated
Contaminated Sharps
Coolant
Cooling Tower
Corrosion
Corrosive
Criteria Pollutants
DAS
DB
DC
Decontamination
Any physical, chemical, biological, or radiological
substance or matter that has an adverse affect on air,
water, or soil.
The presence or the reasonably anticipated presence
of blood or other potentially infectious materials on an
item or surface.
Any contaminated object that can penetrate the skin
including needles, scalpels, broken glass, and broken
capillary tubes.
A liquid or gas used to remove the heat generated by
power production, industrial, and/or mechanical
processes.
A heat exchanger structure designed to remove heat
from water.
The dissolving and wearing away of metal caused by
chemical reactions, such as between water and the
pipes in which water is contained, chemicals touching
metal surfaces, or contact between two metals.
A liquid or solid that causes visible destruction or
irreversible alterations to human skin tissue at the
site of contact or, in the case of leakage from its
packaging, a liquid that has a severe corrosion rate on
steel.
Primary emitted pollutants identified by the USEPA
under the Clean Air Act for which ambient air quality
standards have been set to protect human health and
welfare.
Data Acquisition System
Dry Bulb Temperature, the air temperature measured
by a standard thermocouple or thermometer.
Direct Current (Electricity)
The use of physical or chemical means to remove,
Appendix A-5
-------
Desulfurization
Dew Point
DSI
Dioxin
DS
dscf
dscm
E
Effluent
Emission
Emission Standard
Endothermic
Engineered Controls
EP
EPA
inactivate, or destroy bloodborne pathogens on a
surface or item to the point where they are no longer
capable of transmitting the infectious agent and the
surface is safe for handling.
Removal of sulfur from fossil fuels to reduce pollution.
Temperature at which condensation begins to occur as
a mixture of gases is cooled. Dew points are often in
the range of 225 to 300° F, dependent upon the
concentrations of moisture and acid gases.
Dry Sorbent Injection
Any of a family of potentially toxic compounds known
chemically as dibenzo-p-dioxins.
Dry Scrubber
Dry Standard Cubic Feet
Dry Standard Cubic Meter
Electrical Potential, Voltage
Wastewater—treated or untreated—that flows out of a
treatment plant, sewer, or industrial system.
Pollution discharged into the atmosphere from
smokestacks and other vents.
The maximum amount of air polluting discharge
legally allowed from a single source.
Chemical reactions that absorb energy from the
surroundings.
Control measures, such as the use of sharps disposal
containers, self-sheathing needles, handwashing
facilities, and safety hoods, that isolate or remove the
bloodborne pathogens hazard from the workplace.
Extraction Procedure
U. S. Environmental Protection Agency
Appendix A-6
-------
ESP
Exceedance
Excess Air
Excess-Air (Unit)
Exothermic
Fabric Filter
FCE
FC
FD
FE
FF
FGD
FID
Fixed Carbon
Electrostatic Precipitator: An air pollution control
device that removes participates from a gas stream by
imparting a electrical charge to the particulates which
causes them to be deposited and adhere to metal
plates in the unit.
Violation of air pollution control regulations by
exceeding allowable limits or concentration levels.
The additional air supplied above the stoichiometric
quantities required for complete combustion, often
expressed as a percentage of the stoichiometric air
(e.g., Air supplied at 50% excess air corresponds to 1.5
times the quantity of stoichiometric air).
Incinerators or combustion units operating with
fuel/air mixtures having greater than stoichiometric
quantities of air.
Chemical reactions where chemical energy is
transferred as heat to the surroundings.
Fahrenheit (Degrees)
A cloth device (baghouse) that catches dust particles.
Final Control Element
Flow Controller
Forced Draft
Fugitive Emissions
Fabric Filter
Flue Gas Desulfurization: A technology which uses a
sorbent, usually lime or limestone, to remove sulfur
dioxide from the gases produced by burning fuels.
Flame lonization Detector
The combustible, non-volatile portion of the fuel's
composition.
Appendix A-7
-------
Flue Gas
Fly Ash
FR
FT
ft3
Fuel Bed
Fugitive Emissions
Fume
S
GC
GC/MS
GCP
Generator
GLC
Greenhouse Effect
H2O
Halogen
A mixture of products of combustion and air
constituting the exhaust of a combustion process.
Non-combustible residual particles removed from the
combustion chamber by flue gas.
Federal Register
Flow Transducer
Cubic Feet
The layer of waste material (fuel) undergoing the
combustion process on a hearth or grate.
Emissions not caught by a capture system.
Submicron sized particulates formed by condensation
of a vapor, including some volatile metals and their
oxides. Fumes can agglomerate, forming larger
particles.
Gram
Gas Chromatograph
Gas Chromatograph/Mass Spectrograph
Good Combustion Practice
(1) A facility that emits pollutants into the air or
releases hazardous waste into water or soil.
(2) A facility that produces electricity.
Gas Liquid Chromatograph
The warming of the Earth's atmosphere caused by a
build-up of carbon dioxide and other trace gases. This
build-up allows light from the sun's rays to heat the
Earth without a counterbalancing loss of heat.
Water
Any of a group of five chemically-related nonmetallic
elements that includes bromine, fluorine, chlorine,
Appendix A-8
-------
iodine, and astatine.
Hammermill
HAP
HAZMAT
HBV
HC
HC1
HDPE
Hearth
Heat Release
Heating Value
Heavy Metals
HHV
HIV
HON
HP
hr
HW
HWI
A high-speed machine that uses hammers and cutters
to crush, grind, chip or shred solid wastes.
Hazardous Air Pollutant
Hazardous Material
Hepatitis B virus
Hydrocarbon
Hydrogen Chloride
High Density Polyethylene
Refractory or cast iron surface with orifices allowing
underfire air to enter the fuel bed
The total energy released from combustion, can be
expressed in either Btu/lb or Btu/hr.
The total energy released from combustion, usually
expressed in either Btu/lb.
Metallic elements with high atomic weights, e.g.,
mercury, chromium, cadmium, arsenic, and lead.
They can damage living things at low concentrations
and tend to accumulate in the food chain.
Higher Heating Value
Human immunodeficiency virus
Hazardous Organic (NESHAP)
Horsepower
Hour
Hazardous Waste
Hazardous Waste Incinerator
Appendix A-9
-------
I
IAP
ID
ID
IIA
I/M
in. we
Incineration
Incinerator
Infectious Agent
Ion Exchange Treatment
IPM
IR
kg
kW
KWH
LAER
Current (Electrical)
Indoor Air Pollution
Inside Diameter
Induced Draft
Incinerator Institute of America (No longer active)
Inspection/Maintenance
Inches of water column, a differential pressure
indicated by the height of a water column, often
measured by a manometer.
A treatment technology involving destruction of waste
by controlled burning at high temperatures, to remove
the water and reduce the remaining residues to a non-
burnable residue which can be properly disposed.
A furnace designed for burning of certain types of
solid, liquid, or gaseous materials.
Any microorganism that is capable of producing
infection or disease and may adversely impact human
health.
A common water softening method that removes some
organic and other materials by adding calcium oxide
or calcium hydroxide to increase the pH to a level
where the metals will precipitate out.
Inhalable Particulate Matter
Infrared
Kilogram
Kilowatt
Kilowatt Hour
Lowest Achievable Emission Rate
Appendix A-10
-------
Landfill
Ib
LC
Leachate
Lead (Pb)
LEL
LFL
LHV
Limestone Scrubbing
Liner
LT
m3
MACT
MD
Medical Waste
(1) Sanitary landfills are land disposal sites for non-
hazardous solid wastes. The waste is spread in layers
and compacted. A cover material is added each day.
(2) Chemical landfills are disposal sites for hazardous
wastes. They are selected and designed to minimize
the release of hazardous substances into the
environment.
Pound
Logic Controller (Level Controller)
A liquid that results from water collecting
contaminants as it trickles through wastes,
particularly, from waste disposed in landfill. Leaching
may result in hazardous substances entering surface
water, ground water, or soil.
A heavy metal that can be hazardous to health if
large enough amounts are breathed or swallowed.
Lower Explosive Limit
Lower Flammability Limit
Lower Heating Value
Process in which sulfur gases in flue gas are passed
through a limestone and water solution to remove
sulfur before the gases reach the atmosphere.
A relatively impermeable barrier designed to prevent
the passage of moisture. Liner materials include the
plastic and dense clay used to prevent leachate from
leaking from a landfill.
Level Transducer
cubic meter
Maximum Achievable Control Technology
Mail Drop
Any solid waste material which is generated in the
Appendix A-11
-------
Mercury
Methane
Microorganism
MIS
Moisture
Monitoring
Monofill
MP
MS
MSDS
MSW
MT
MW
diagnosis, treatment, or immunization of human
beings or animals, in research pertaining thereto, or
in the production or testing of biologicals. The term
does not include any hazardous waste or any
household waste.
A heavy metal that can accumulate in the
environment and can be highly toxic if large enough
amounts are breathed or swallowed.
A colorless, nonpoisonous flammable gas which can be
created by anaerobic decomposition of organic
compounds.
Examples: viruses, bacteria, fungi.
Management Information System
The water content in the fuel composition (fuel
moisture) or in flue gas (flue gas moisture), which is
derived from fuel moisture, combustion reactions, and
moisture in air.
Periodic or continuous surveillance of testing to
determine the level of compliance with statutory
requirements and/or pollutant levels.
A special landfill which receives only a single waste
material. Federal RCRA requirements require special
citing considerations and design provisions for a cap,
leachate containment, monitoring, arid treatment
system.
Melting Point
Mass Spectrometry
Material Safety Data Sheet
Municipal Solid Waste
Metric Ton(s) (1,000 Kilograms)
Molecular Weight
Appendix A-12
-------
MWC
MWI
MWTA
N2
N/A
NAA
NAAQS
Natural Gas
NDIR
NESHAPS
New Source
ng
ng/dscm
NIOSH
NO
N02
NOx
NSPS
Municipal Waste Combustor
Medical Waste Incinerator and all equipment related
to the incineration process, including the waste
feeder, incinerator, gas cleaning, residue removal,
control and monitoring equipment, and any boiler or
heat exchanger equipment that extracts waste heat.
Medical Waste Tracking Act
Nitrogen
Not Applicable (Available)
Nonattainment Area
National Ambient Air Quality Standards
A natural fuel containing primarily methane and
ethane that occurs in certain geologic formations.
Nondispersive Infrared Analysis
National Emissions Standards for Hazardous Air
Pollutants
Any stationary source which is built or modified after
publication of final or proposed regulations that
prescribe a standard of performance which is intended
to apply to that type of emission source.
Nanogram(s), 10"9 grams
Nanogram(s) per Dry Standard Cubic Meter
National Institute of Occupational Safety and Health
Nitric Oxide
Nitrogen Dioxide
Nitrogen Oxides
New Source Performance Standards
Appendix A-13
-------
NSWMA
NTIS
Occupational Exposure
OD
O&M
Opacity
Organic
OSHA
OSW
Overfire Air
Packed Tower
PAH
PAN
Particulate Matter
Particulate Emission
National Solid Waste Management Association
National Technical Information Service
A reasonably anticipated skin, eye, mucous
membrane, or parenteral contact with blood or other
potentially infectious materials that may result from
the performance of an employee's duties.
Outside Diameter
Operations and Maintenance
The amount of light obscured by particulate pollution
in a gas stream. Opacity is used as an indicator of
emissions of particulates and organic products of
incomplete combustion.
In chemistry, any compound containing carbon.
Generally refers to compounds derived from living
organisms.
Occupational Safety and Health Act (Administration)
Office of Solid Waste, USEPA
Air forced into the region above the fuel bed of an
incinerator to provide the oxygen required for
complete combustion.
A pollution control device that forces dirty gases
through a tower fitted with packing materials while
liquid is sprayed over the packing. The liquid is
selected to enhance the pollutants being either
dissolved or chemically reacted.
Polycyclic Aromatic Hydrocarbon
Peroxyacetyl Nitrate
Fine liquid or solid particles such as dust, smoke,
mist, or fumes.
Fine liquid or solid particles such as dust, smoke,
mist, or fames found in the flue gas emissions carried
Appendix A-14
-------
into the atmosphere.
Particulate Loading
Pathogen
Pathogenic Waste
Pathological Waste
PC
PCB
PCDD
PCDF
PEL
Permit
pH
PHC
PIC
PLC
PM
PM-10
The mass of particulate emissions, generally
expressed in mass per unit volume of the air.
Those microorganisms (e.g., bacteria, viruses, fungi)
which are capable of causing disease.
Waste materials that contain organisms capable of
causing an infectious disease.
Waste materials that consisting of anatomical parts
such as body parts and blood. Also waste materials
relating to the study of the nature of disease.
Personal Computer
Polychlorinated Biphenyls: A group of toxic,
persistent chemicals which have been used in
transformers and capacitors for insulating purposes.
Further sales and new uses were banned in 1979.
Polychlorinated Dibenzo-p-dioxins
Polychlorinated Dibenzofurans
Personal Exposure Limit
An authorization, license, or equivalent control
document issued by an approving agency to
implement the requirements of an environmental
regulation.
A measure of acidity or alkalinity of a liquid or solid
material.
Principal Hazardous Constituent
Products of Incomplete Combustion
Programmable Logic Controller
Particulate Matter
Particulate Matter, Sized Less Than 10 Micrometers
Appendix A-15
-------
PMR
POM
ppb
PPE
ppm
ppmdv
ppmv
ppm-weight
ppt
Precursor
Proximate Analysis
PSD
psi
psia
psig
PT
PVC
Pyrolysis
Pollutant Mass Rate
Polycyclic Organic Matter
Parts per Billion
Personal Protective Equipment, specialized clothing or
equipment for protection against a hazard. General
work clothes (e.g., uniforms, pants, shirts or blouses)
are not intended to function as protection against a
hazard and are not considered to he personal
protective equipment.
Parts per Million (generally on a volumetric basis)
Parts per Million on a Dry, Volumetric Basis
Parts per Million on a Volumetric Basis
A measure of mass concentration which is equivalent
to both micrograms/gram and milligrams/kilogram.
Parts per Trillion
A compound that "precedes" or leads to the formation
of a particular compound of interest.
The fuel's composition expressed fractionally as
volatile matter, fixed carbon, moisture, and non-
combustible (ash).
Prevention of Significant Deterioration
Pounds per Square Inch (Pressure)
Pounds per Square Inch Absolute (Pressure)
Pounds per Square Inch Gage (Pressure)
Pressure Transducer
Polyvinyl Chloride
Chemical decomposition of a organic materials under
conditions of high temperature and limited oxygen.
Appendix A-16
-------
QA
QA/QC
QC
QMO Standard
QRO Standard
Quench Tank
R
RACT
RCRA
R&D
RDF
Refractory
Regulated Waste
Quality Assurance
Quality Assurance/Quality Control: A system of
procedures, checks, audits, and corrective actions to
ensure that all EPA required sampling, monitoring
and reporting activities are of the highest achievable
quality.
Quality Control
ASME Standard QMO-1, Standard for the
Qualification and Certification of Medical Waste
Incinerator Operators
ASME Standard QMO-1-1989, Standard for the
Qualification and Certification of Resource Recovery
Facility Operators
A water-filled tank used to cool incinerator ash
residues.
Resistance (Electrical)
Reasonably Available Control Technology: The lowest
emission limitation that a particular source is capable
of meeting by application of control technology that is
reasonably available, technically feasible, and
economically feasible.
Resource Conservation and Recovery Act
Research and Development
Refuse-Derived Fuel
An inorganic incinerator liner material (e.g., ceramic,
fire brick) which acts as an insulator and radiant
energy reflector. Its physical properties are fairly
stable at high temperatures.
Liquid or semi-liquid blood or other potentially
infectious materials; contaminated items that would
release blood or other potentially infectious material
in a liquid or semi-liquid state if compressed; items
that are caked with dried blood or other potentially
Appendix A-17
-------
Resource Recovery
Residence Time
Risk Assessment
RPM
RTF
SARA
SCFM
Scrubber
SD
SDA
sec
SI
SIP
Sludge
infectious material and are capable of releasing these
materials during handling; contaminated sharps; and
pathological and microbiological wastes containing
blood and other potentially infectious materials.
The process of obtaining matter or energy from
materials formerly discarded.
Amount of time materials are maintained under high
temperature combustion conditions (e.g., the time
volatile matter is retained in secondary chamber or
the time solids are retained in primary chamber).
The qualitative and quantitative evaluation performed
in an effort to define the risk. Example risks include
the potential for economic loss and the environmental
risks to human health and/or the environment caused
by the presence or potential presence or use of
pollutants.
Revolutions per Minute
Research Triangle Park, NC
Superfund Amendments and Reauthorization Act of
1986
Standard Cubic Feet per Minute
An air pollution control device that uses a spray of
water, liquid solutions, or dry materials in a process
to remove pollutants from a flue gas.
Standard Deviation
Spray Dryer Absorber
Second
System International (Units)
State Implementation Plan
The semi-solid residue from air or water treatment
processes.
Appendix A-18
-------
Slurry
Smog
Smoke
SO2
Soot
SOP
SP
Stack
STALAPCO
Standard Conditions
Starved Air
Starved-Air (Unit)
Stationary Source
Stoichiometric Air
STP
Stuff and Burn
SWANA
SWDA
A watery mixture of insoluble matter that results
from some pollution control technique.
Air pollution associated with oxidants.
Particles suspended in gases after incomplete
combustion of materials.
Sulfur Dioxide: A heavy, pungent colorless gaseous
air pollutant formed by oxidation of sulfur.
Carbon dust formed by incomplete combustion.
Standard Operating Procedure
Set Point
A chimney or smokestack; a vertical pipe that
discharges used gases.
State and Territorial Air Pollution Control Officials
Temperature of 60°F and Atmospheric Pressure.
The characteristic combustion condition where
burning occurs with less than Stoichiometric air.
Incinerators which are designed for the primary
chamber to operate under sub-stoichiometric
conditions.
A fixed, non-moving producer of pollution, mainly
facilities using combustion processes.
The theoretical air required for complete combustion.
Standard Temperature and Pressure
A characterization applicable to units which are
operated in a batch charging mode.
Solid Waste Association of North America
Solid Waste Disposal Act
Appendix A-19
-------
TCDD
TCDF
TCLP
TDS
THC
TLV
Toxic
TPD
TPY
TSP
TSS
UEL
UFL
Underfire Air
Universal Precautions
UV
USEPA
Dioxin (Tetrachlorodibenzo-p-dioxin)
Furan (Tetrachlorodibenzofurans)
Toxicity Characteristic Leachate Procedure
Total Dissolved Solids
Total Hydrocarbons
Threshold Limit Value
Any substance which can cause acute or chronic
injury to the human body or which is suspected of
being able to cause an unreasonable risk of disease or
injury to health or to the environment.
Tons per Day
Tons per Year
Total Suspended Particulates
Total Suspended Solids
Upper Explosive Limit
Upper Flammability Limit
Combustion air which enters the fuel bed from orifices
in the hearth or openings in the grate.
A approach to infection control, developed by the
Centers for Disease Control, in which all human blood
and certain human body fluids are treated as if
known to be infected with HIV, HBV, and other
bloodborne pathogens.
Ultraviolet rays: Radiation emitted from the sun that
can be useful or potentially harmful. UV rays from
some parts of the spectrum enhance plant life.
Human exposure to UV rays can cause sun burns and
skin cancer.
U. S. Environmental Protection Agency
Appendix A-20
-------
Vapor
Vaporization
VE
Virulence
VOC
Volatile Matter
VOST
VP
Wastewater
WB
WTE
The gaseous phase of substances that are liquid or
solid at atmospheric temperature and pressure.
The change of a substance from a liquid to a gas.
Visual Emissions
The ability of a microorganism to cause disease in a
given host.
Volatile Organic Compounds
The combustible portion of the fuel which is evolved
as gaseous matter upon the application of heat.
Volatile Organic Sampling Train
Vapor Pressure
The spent or used water (from a process) which
contains dissolved or suspended matter.
Wet Bulb Temperature, the equilibrium temperature
of a measurement device which is supplied with a film
of water which evaporates and cools the device below
the corresponding dry bulb temperature.
Waste-to-E nergy
Appendix A-21
------- |