United States
Environmental Protection
Agency
Office of Air Qualrty
Planning and Standards
Research Triangle Park NC 27711
EPA-453/B-93-020
April 1993
Air
Municipal Waste Combustor
Operator Training Program
Course Manual
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MUNICIPAL WASTE COMBUSTOR
OPERATOR TRAINING PROGRAM
COURSE MANUAL
Prepared for:
U. S. Environmental Protection Agency
Industrial Studies Branch/BSD
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
April 13, 1993
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NOTICE
This manual is part of a model state training program which addresses the training needs
of municipal waste combustor (MWC) operators. This manual generally describes the 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 municipal waste combustors.
Proper operation of combustion equipment is the responsibility of the owner and
operating organization. Therefore, owners of municipal waste combustors 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 Manual was prepared for the Industrial Studies Branch, Emission Standards
Division, U. S. Environmental Protection Agency. It was prepared in accordance with USEPA
Contract Number 68-CO-0094, Work Assignment Number 7. Partial support was also provided
by the University of Virginia through its Sesquicentennial Associates Program.
The contents of this report are reproduced as received from the contractor. The opinions,
findings and conclusions expressed are those of the authors and not necessarily those of the
U.S. Environmental Protection Agency. Any mention of product names does not constitute an
endorsement by the U. S. Environmental Protection Agency.
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AVAILABILITY
This document is issued by the Office of Air Quality Planning and Standards of the U.
S. Environmental Protection Agency. It was developed as part of a set of training materials to
assist operators of municipal waste combustors 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 municipal waste combustors. 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 Course Manual
Introduction. Any duplication of this material, in whole or in part, may constitute a violation
of the copyright laws, and unauthorized use could result hi criminal prosecution and/or civil
liabilities.
The recommended procedure for mass duplication of the course materials 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 Manual Introduction xi
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 Requirements 1-3
2. Environmental Concerns and Regulations 2-1
Public Concerns & Historic Issues 2-1
Solid Waste Regulation 2-3
Air Pollution Regulation 2-4
Operator's Role in Public Relations 2-8
3. Municipal Solid Waste Treatment 3-1
Integrated Waste Management 3-1
Landfill Requirements 3-4
Municipal Waste Combustors 3-5
RDF Fuel Processing 3-7
4. Characterization of MSW Fuels 4-1
Sources and Types of Solid Wastes 4-1
Composition & Generation Rates 4-5
Characterization of Waste Fuel Properties 4-6
Heating Values & Ultimate Analyses 4-7
111
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Page
5. Combustion Principles I: Complete Reactions 5-1
Basic Combustion Concepts 5-1
Stoichiometric Air-Fuel Mixtures 5-5
Chemical Reaction Equations 5-7
Balancing Stoichiometry Reaction Equation 5-11
Balancing Excess Air Reaction Equation 5-13
6. Municipal Waste Combustors 6-1
Evolution of MWC Designs 6-2
Mass Burn: Refractory Wall, Excess-Air 6-4
Mass Burn: Waterwall, Excess-Air 6-5
Mass Burn: Rotary Waterwall 6-8
Modular Mass Burn: Starved-Air/Controlled-Air 6-9
RFD Units 6-12
7. Combustion Principles n: Thermochemistry 7-1
Higher & Lower Heating Values 7-1
Capacity and Operating Load 7-2
Ignition & Volatilization Temperatures 7-3
Combustion Temperature Control & Heat Sinks 7-5
Starved-Air and Excess-Air Considerations 7-6
8. MSW Handling Equipment 8-1
Materials Flow Path 8-1
Undesirable MSW Components 8-2
Variable MSW Fuel Considerations 8-3
Receiving and Feeding Equipment 8-4
Grate Burning Concepts and Equipment 8-7
Ash Removal & Disposal 8-13
iv
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Page
9. Combustion Principles HI: Reaction Processes 9-1
Important Reaction Characteristics 9-1
Multiple Reactions & Incomplete Combustion 9-2
Oxidation & Reduction Reactions 9-4
Flame Types 9-5
Bed Burning Processes 9-6
Char and Carbon Monoxide Reactions 9-9
10. Design & Operation of Combustion Equipment 10-1
Charging Methods: Direct Bed & Suspension 10-1
Two-Stage & Excess Air Combustion 10-2
Waste-Heat and Integral Waterwall Boilers 10-3
Boiler Component Equipment 10-7
General Operational Considerations 10-10
Excess-Air and Starved-Air Operational Considerations 10-12
11. Design & Operation of Gas Flow Equipment 11-1
Air & Flue Gas Flow Path 11-1
Centrifugal Fans & Dampers 11-4
Draft 11-7
Dew Point 11-8
Slag & Soot Deposits 11-9
12. NSPS & EG: Good Combustion Practice 12-1
Pollutant Emission Groups and Surrogates 12-1
NSPS/EG Emission Limits 12-2
Carbon Monoxide Limits & Relationships 12-3
Typical Unit Combustion Indicator Ranges 12-6
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Page
13. Instrumentation I: General Measurements 13-1
Purposes of Instrumentation 13-1
Temperature Equivalences & Measurements 13-2
Pressure Gages 13-5
Flow Meters 13-8
Weight Scales 13-12
14. Instrumentation n: Continuous Emission Monitoring 14-1
Typical Parameters Monitored 14-1
Extractive & In-situ CEMs 14-2
In-Situ Measurement Concepts 14-3
Extractive Measurement Concepts 14-6
Gas Analyzer Maintenance Procedures 14-13
Bias Checks 14-14
15. Air Pollution I: Introduction 15-1
Fuel Dependent Emissions 15-1
Combustion Dependent Emissions 15-2
Smoke and Particulates 15-3
Gas Concentrations 15-4
Correcting Concentrations for Standard Dilutions 15-6
Combustion Efficiency 15-10
Excess Air 15-11
16. Air Pollution n: Products of Incomplete Combustion 16-1
Surrogates 16-1
Dioxins and Furans 16-2
Formation of MWC Organics 16-3
Annual Testing for Dioxins/Furans 16-5
vi
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Page
17. Air Pollution HI: Nitrogen Oxides 17-1
Sources and Nitrogen Oxide Compounds 17-1
Environmental Concerns 17-2
Fuel NOx and Thermal NOx Formation 17-3
Combustion Analysis for Fuel NOx Formation 17-5
Combustion Modifications 17-7
18. Air Pollution IV: Metals and Ash 18-1
Characterization of MWC Metals 18-1
Metal Pathways in MWCs 18-3
Control Strategy for Metal Air Pollutants 18-4
Groundwater Contamination by Toxic Metals 18-5
Ash Testing, Ash Treatment 18-7
19. Flue Gas Control I: Paniculate Matter (PM) 19-1
Combustion System Partitioning of Solid Residues 19-1
Particle Entrainment Factors 19-2
Fabric Filtration Concepts 19-4
Classes & Operational Features of Fabric Filters 19-5
ESP Collection Process 19-9
Factors Affecting ESP Performance 19-12
Venturi Scrubber Design & Operation 19-15
20. Flue Gas Control H: Acid Gas Removal 20-1
Spray Dryer Absorber Systems 20-1
Spray Dryer Operational Considerations 20-4
Dry Sorbent Injections Systems 20-6
Packed Bed Wet Scrubbers 20-8
Wet Scrubber Applications 20-9
vii
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Page
21. Flue Gas Control HI: NOx Control 21-1
Combustion Modifications 21-1
Reburning with Natural Gas 21-2
Selective Non-Catalytic Reaction Systems 21-3
Operational Considerations with Thermal De-NOx & Urea 21-4
Selective Catalytic Reaction Systems 21-7
22. Automatic Control Systems 22-1
Types of Automatic Control Systems 22-2
Automatic Control System Elements 22-3
Gas-Side and Water-Side Control Parameters 22-4
Single, Two & Three Element Controllers 22-5
Micro-processor Based Control Systems
Control System Applications 22-9
23. Control Room Operations 23-1
Operator Control Functions 23-1
Control Room Communications 23-2
Panel Mounted Instruments 23-2
Graphic Screen Displays 23-3
Operator Initiated Changes 23-5
24. Operating Practices 24-1
Responsibilities & Functions 24-1
Potential Major Hazards 24-3
Standard Operating Procedures 24-4
Combustion, Boiler, Water Treatment Controls 24-5
Combustion System Start-Up & Shut-Down 24-10
APCD System Start-Up & Shut-Down 24-13
viii
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Page
25. Troubleshooting of Combustion Upsets 25-1
Typical Combustion Upsets 25-1
Upsets Associated with Fuel Problems 25-4
Combustion Air Upsets 25-5
Combustion Temperature Upsets 25-7
Furnace Draft Upsets 25-8
Temperature & Draft Upsets
26. Special System Considerations I: Water Treatment 26-1
Boiler Water Impurities & Problems 26-1
Water Treatment System Components 26-4
Deaeration 26-5
Water Softeners & Condensate Purification 26-8
Blowdown 26-9
Indicators of Water Quality 26-12
27. Special System Considerations n: FJectrical Theory 27-1
Basic Electrical Parameters 27-1
Ohms Law 27-4
Electrical Power (DC & AC) 27-4
Apparent Power, Reactive Power, Power Factor 27-6
Transformer Principles 27-7
3-Phase Fundamentals 27-8
Circuit Breakers, Rectifiers, Inverters 27-9
28. Special System Considerations HI: Turbine Generator 28-1
Energy Recovery and Conversion Options 28-1
Impulse Steam Turbine Features 28-3
Reactive Steam Turbine Features 28-4
ix
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Steam Turbine Generator Equipment Configurations
Turbine Generator Operation
Generator Synchronization with Utility Grid
Abnormal Turbine Generator Conditions
Page
28-5
28-8
28-10
28-11
29. Risk Management I: Preventive Maintenance
Potential Economic Losses
Features of Preventive Maintenance
In-Service Maintenance
Outage Maintenance Planning
29-1
29-2
29-3
29-5
29-6
30. Risk Management n: Safety
MWC System Safety Hazards
Standard Safety Considerations
Personal Protection Equipment
Symptoms of Illness
30-1
30-1
30-3
30-4
30-4
Appendix
Glossary, Acronyms, and Symbols
Appendix-1
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COURSE MANUAL INTRODUCTION
This manual was developed for the U. S. Environmental Protection Agency (USEPA) in
support of improving the air pollution control practices at municipal waste combustors (MWCs).
The USEPA is required to develop a model state training and certification program for solid
waste incinerator operators under Title m, Section 129 of the Clean Air Act Amendments of
1990. The manual is an integral part of the model state MWC operator training and certification
program. As such, state and regional air pollution control agencies are encouraged to develop
training programs which make use of this manual.
TRAINING PROGRAM GOAL
The primary goal of the training program is to provide an adequate level of understanding
to MWC operators to successfully complete the requirements of the ASME QRO Standard for
provisional certification as resource recovery facility operators.
The training program focuses on the knowledge required by operators for understanding
the basis for proper operation and maintenance of municipal waste combustors. 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. Trainees are encouraged to comment and ask questions during the training
program. Such discussion will both increase the utility of the program and make it more
interesting.
The program was 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.
TRAINING PROGRAM INTENDED AUDIENCE
The training program concentrates on the range of MWC units covered by the ASME
Standard for Qualifications and Certification of Resource Recovery Facility Operators (ASME
QRO-1-1989). This includes unit sizes from capacities as small as 24 tons/day up through the
regional waste-to-energy plants which may have capacities greater than 4,000 tons/day.
Therefore, the course focuses on the special training needs of operators of the larger sizes of
MWC units, which typically have continuous ash removal systems and an intermittent or
continuous waste feeding system. This course does not focus on the training needs of operators
of small batch-fired incinerators.
XI
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Other persons who are expected to be trainees in this program include MWC operating
and management staff members, technical managers, mechanics and maintenance personnel,
instrument and control technicians, general engineers and design engineers.
In addition, regulatory officials, particularly those involved in permit review, are
expected to find this program both informative and useful.
PROGRAM AGENDA
The training program is designed for a five-day sequence of learning units. This manual
follows the sequence of the recommended agenda which is found in the Municipal Waste
Combustor Operator Training Program. Instructor's Guide. However, the course can depart
from the recommended agenda to accommodate the special scheduling needs of the speakers.
MANUAL ORGANIZATION
The manual presents information in the subject areas addressed in the ASME Examination
for Provisional Certification as Chief Facility Operators and Shift Supervisors. Additional
information about qualifications may be obtained from a review of the ASME Standard. The
manual will also be useful in state and/or private entity training programs which are conducted
under equivalent state standards for operator training and certification.
The sequence of topics was selected to reinforce the integration of the basic or
fundamental aspects with the more familiar applied materials. Generally, a unit of fundamental
information is followed by an applications unit. For instance, units on combustion chemistry
are interspersed with units on equipment design and operation.
The manual begins with an introduction of the training program and its relationship to
the operator certification process. The program considers the operator's role in the regulatory
environment and in public relations.
This manual focuses on the technical and operational aspects of good combustion
practices in MWC units. The characteristics of municipal solid waste (MSW), its fuel
properties, and the influence of waste processing are presented. These are followed by learning
units on combustion principles and MWC equipment features. Next comes a sequence on good
combustion practices, air pollution control, instrumentation, and flue gas treatment. The training
program concludes with consideration of automatic control theory, control systems, trouble
shooting, special system considerations, and risk management.
Xll
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COURSE LIMITATIONS
To the extent possible, this manual was written in a manner consistent with USEPA
policy regarding municipal waste combustors 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.
Xlll
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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 municipal solid waste combustors (MWCs). In
addition, the amendments make the operation of such a unit unlawful unless each
person having control over the processes affecting its emissions has satisfactorily
completed an appropriate training program.
New Source Performance Standards2 (NSPS) and Emission Guidelines3 (EG)
became effective on February 11, 1991. The NSPS and EG are applicable to MWC
units with capacities of 225 Mg/day (250 tons/day) or greater. Operators of MWC
units must satisfy federal training and certification requirements. These
requirements are generally administered by the various states. Some states have
adopted regulations requiring ASME certification of operators, whereas other states
have developed their own operator certification programs.
Slide 1-2
MUNICIPAL WASTE COMBUSTOR
OPERATOR TRAINING PROGRAM
Goal: Adequate Understanding to Pass
ASME General Examination for
Provisional Certification
The primary goal of this operator training program is to provide an adequate
level of understanding to enable MWC operators to complete the requirements of the
ASME Standard for provisional certification as resource recovery facility operators
(ASME QRO^l-1989).4 The actual testing required for the ASME Certification is
administered separately by the ASME and is not included in this training program.
1-1
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Slide 1-3
MUNICIPAL WASTE COMBUSTOR
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 municipal waste combustors.
Particular emphasis is placed on the various technical and operational aspects of
combustion which are important for environmental control.
Slide 1-4
COURSE MANUAL ORGANIZATION
1 Introduction
2,12 Environmental Concerns & Regulations
3-4 Characteristics of MSW
5-9 Combustion Principles
6-11 MWC Equipment Features
13,14 Instrumentation
15-21 Air Pollution Control
22-23 Automatic Control
24-25 Operating Practices & Upsets
26-28 Special System Considerations
29-30 Risk Management
The course manual begins with an introduction to the training program and its
relationship to the operator certification process. Next, the operator's role in the
regulatory environment and in public relations is discussed. This is followed by
consideration of municipal solid waste (MSW) characteristics, its fuel properties, and
the influence of solid waste processing.
Learning units are presented on combustion principles and on MWC equipment
design and operational features. Next is a sequence of learning units on good
combustion practice, instrumentation, air pollution control, and flue gas treatment.
The training program concludes with consideration of automatic control theory,
control systems, trouble shooting, special system considerations and risk
management.
1-2
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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 General Examination for Provisional Certification, although they are designed
to focus on the same subject areas. The ASME QRO Provisional Certification
Examination, the Pre-Test, and the Post-Test are in the form of multiple-choice
questions.4
Slide 1-6
ASME PROVISIONAL CERTIFICATION
PROVISIONAL CERTIFICATION REQUIREMENTS
High School Diploma or Equivalent
Five Years of Acceptable Experience
Pass General Examination
The qualifications for ASME QRO Provisional Certification4 are the same for
both shift supervisors and chief facility operators. To obtain provisional certification,
the applicant must pass the General Examination. The examination may be retaken
whenever it is offered.
Up to two of the five years of acceptable experience can be met by applicants
who have completed a baccalaureate degree with a major in a physical science or
engineering or who have completed 60 credits of course work in advanced math,
chemistry, fluid dynamics, thermodynamics, material science, combustion, and/or
environmental, mechanical, civil, chemical or electrical engineering.
1-3
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Slide 1-7
ASME PROVISIONAL CERTIFICATION EXAMINATION
General Examination (Written)
Solid Waste Management (25%)
Theory (25%)
Operations (50%)
The ASME General Examination for Provisional Certification is a written,
multiple-choice, closed-book test, which is administered as a national examination.
The examination will cover solid waste management and regulations, theory, and
operations, with the questions distributed as indicated by the percentages listed.
Slide 1-8
ASME GENERAL EXAMINATION SUBJECT AREAS9
Panl
25% of examination
Solid waste collection,
transfer, and management,
covering the following:
• Municipal solid waste
composition
• Collection techniques
• Seasonal and industrial
impact on the character
of refuse
• Ash disposal
«Landfills
° Composting
• Environmental public
relations
e Environmental regulation
and requirements
Part 2
25% of examination
Theory, covering the
following:
• Combustion
e Chemistry
«Thermodynamics
• Material science
• Mechanical and electrical
operation and technology
• Air pollution control
technology
e Air emission stack
monitoring
Pan3
50% of examination
Operation of a resource
recovery facility, covering
the following:
• Material handling
equipment
• Boiler operations
• Generator and turbine
operations
• Ash T*aT"fljpg and
disposal operations
• General operations and
maintenance
procedures and
techniques
• Worker safety
• Control room
operations
• Continuous emissions
monitors and their
calibration
Courtesy of ASME Codes & Standards, Printed with Permission
The subject areas of the General Examination are listed in the ASME Standard
for Qualifications and Certification of Resource Recovery Facility Operators (ASME
QRO-1-1989).4'5 Specific information about the examination application procedures
can be obtained by writing the ASME, Codes & Standards, United Engineering
Center, 345 East 47th Street, New York, NY 10017-2392.
1-4
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Slide 1-9
LEARNING UNITS IN ASME EXAM AREAS
Part 1, Solid Waste Management (25%)
Learning Units: 2, 3, 4, 12
Part 2, Theory & Technology (25%)
Learning Units: 5, 7, 9, 13/ 15, 16
17, 20, 21, 22, 27
Part 3, Operations (50%)
Learning Units: 6, 8, 10, 11, 12
14, 15, 18, 23, 24
25, 26, 28, 29, 30
The current training program focuses on the subjects to be covered in the
ASME General 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 either component or
system designs, which are dependent upon MSW properties, regulations, and
theoretical considerations.
Slide 1-10
ASME CERTIFICATION EXAMINATIONS
OPERATOR CERTIFICATION
Operator Examination (Oral)
Site-Specific Equipment
Operations and Maintenance
Procedures & Regulations
The Operator Examination is a site-specific, oral examination administered by
three outside examiners. The Shift Supervisors and Chief Facility operators will be
tested on many of the same topics in equipment design, operation, maintenance and
procedures.
Some questions in the Operator Examination will depend upon whether the
applicant is applying for certification as a Shift Supervisor or a Chief Facility
Operator. The Shift Supervisor's examination will include special emphasis on
operational aspects, while the Chief Facility Operator's examination will include
sections on overall unit operation, maintenance, and performance; job duties and
responsibilities; and formulation of operational policies and procedures.
1-5
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Slide 1-11
ASME OPERATOR CERTIFICATION QUALIFICATIONS
SHIFT SUPERVISOR
CHIEF FACILITY OPERATOR
Hold a Valid Provisional Certification
Have 6 Months Acceptable Experience as
Shift Supervisor or
Chief Facility Operator
Pass a Site-Specific Operator Exam
The procedures for obtaining ASME Operator Certification are approximately
the same for both Shift Supervisors and Chief Facility Operators. A person's
experience as either a Shift Supervisor or Chief Facility Operator may be an indicator
of which certification is appropriate.
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. 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. U. S. Environmental Protection Agency, "Emission Guidelines; Municipal Waste
Combustors," Federal Register. Vol. 56, No. 28. February 11, 1991, pp. 5514-
5527.
4. "Standard for the Qualification and Certification of Resource Recovery Facility
Operators," ASME QRO-1-1989, American Society of Mechanical Engineers,
New York, March 31, 1990.
5. "Memorandum on Provisional Certification Examination," Addressed to
Individuals Interested in QRO Provisional Certification, by Alan Bagner,
Director of Accreditation and Certification, ASME, 345 East 47th Street, New
York, NY 10017, December 26, 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
Toxic and Carcinogenic Air Pollution
Ground Water Contamination
The public attitude about solid waste management has been illustrated by the
phrase "out of sight, out of mind". Our national life-style has been portrayed as one
of consumerism in a throw-away economy. Planned obsolescence often appears to be
a basic assumption in the manufacturing of consumer goods. For example, people
in the urban areas of the United States generate about twice as much waste as those
in comparable European societies.
The public relations aspects of solid waste management have become crucial,
as the public has expressed genuine health and environmental concerns related to
toxic and carcinogenic gaseous emissions and ground water contamination from solid
waste disposal products.
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, as
illustrated above. There are many examples where one or more of these have been
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
PUBLIC RELATIONS PHENOMENA
Basis for Public's Mistrust
Impact of Past "Acceptable Practices"
Concern About Waste Disposal Costs
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 increase in waste disposal costs,
which are brought about by expensive technological solutions, environmental
requirements, and political realities. Environmentally responsible solid waste
management has become increasingly costly. For example, federally mandated
standards of performance for new and existing municipal waste incinerators are
estimated to cost an additional $10.00 per ton of refuse processed.1 The public is
generally willing to pay for proper disposal of its own waste, but may well resist
paying for disposal of the wastes of others.
Slide 2-4
HISTORIC LANDFILL ISSUES
Closed Dumps
Regulated Sanitary Landfills
Ground Water Contamination
Superfund Clean-Up
Public issues have shifted over the years as new information has become
available. Regulations have evolved which forced the closing of dumps and
established standards for sanitary landfills. In 1989, it was estimated that over
30,000 landfills in the nation had been closed and that many of the existing landfills
would be closed in the next 15 years.2 Landfill closures occur because of capacity
limitations, inadequate provisions for ground water protection, and environmental
problems associated with earlier disposal practices.
Landfills containing significant amounts of hazardous waste materials may be
designated for Superfund clean-up activities. Superfund remediation projects have
very considerable costs.
2-2
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Slide 2-5
FEDERAL SOLID WASTE LAWS & REGULATIONS
Resource Conservation and Recovery Act, RCRA
Subtitle C: Hazardous Waste Regulation
Manifest System
Hazardous Waste Incineration Standards
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 administered by the
USEPA, Office of Solid Waste.
RCRA is probably best known for its "manifest" system, which requires a
"cradle-to-grave11 documentation of the movement of hazardous materials from their
manufacture until ultimate disposal. Mandatory reductions in the production of
selected hazardous wastes and design standards for hazardous waste disposal sites
have been established through RCRA. Under RCRA Subtitle D, standards have been
established for the design and operation of 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 2-6
HISTORIC INCINERATION ISSUES
Smoke & Odor From Incinerators
Toxic Emissions
Ground Water Contamination From Ash
Early concerns about incinerators focused on smoke, odor, and carbon monoxide
emissions. These were addressed by state and local regulations. The first federal
standard for incinerators was the 1971 New Source Performance Standard (NSPS)
which applied to units burning greater than 50 tons/day, limiting their particulate
emissions to a maximum of 0.08 grains per dry standard cubic foot (gr/dscf).4
In recent years the public has become increasingly concerned about acid rain,
toxic air pollutant emissions, and proper ash disposal procedures.
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Slide 2-7
INCINERATOR AIR POLLUTANTS
Particulate Matter (PM)
Carbon Monoxide
Nitrogen Oxides
MWC Acid Gases
Hydrogen Chloride & Sulfur Dioxide
MWC Organics
Dioxins, Furans & Other Organics
MWC Metals
Lead, Cadmium, Mercury & Other Metals
Currently regulated air pollutants from incinerators are listed above.
Additional federal regulations for lead, cadmium, and mercury emissions from MWCs
are currently under development.
Slide 2-8
FEDERAL AIR POLLUTION LAWS & REGULATIONS
Clean Air Act, CAA
State Implementation Plans
State Rules and.Regulations Must be at
Least as Strict as the Federal Guidelines
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 U. S. Environmental Protection Agency (USEPA), Office of Air Quality
Planning and Standards, administers federal air quality regulations. A provision of
the CAA requires the USEPA to take into consideration the full range of economic
consequences of its 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 be approved by the USEPA. State implementation plans
must include regulations which are at least as strict as the applicable federal
standards.
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Slide 2-9
NATIONAL AMBIENT AIR QUALITY STANDARDS (NAAQS)
Criteria Air Pollutants (emitted by sources)
Secondary Air Pollutants (formed indirectly) .
Non-Attainment Areas
Prevention of Significant Deterioration, PSD
National Ambient Air Quality Standards (NAAQS) have been established by the
administrator of the USEPA to define levels of air quality which protect public health
and welfare. NAAQS have been established for both criteria and secondary air
pollutants.
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, with less contamination than
that specified in the NAAQS. PSD rules have required facilities to apply the "Best
Available Control Technology" (BACT).6
PSD rules have been specifically applied to MWC units built after June 1,1975
if their capacity was greater than 250 tons/day.5 Therefore, as the technology has
advanced, the degree of emissions control required for new units has considerably
exceeded that of the previously mentioned NSPS of 1971.
Slide 2-10
CRITERIA POLLUTANTS
Particulate Matter (PM)
Sulfur Dioxide
Carbon Monoxide
Nitrogen Dioxide
Lead
Ozone
Criteria pollutants are those 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 of emissions of criteria pollutants are generally based on health and
welfare effects. For instance, sulfur dioxide emissions from identified sources are
regulated to assure that the ambient level concentrations do not exceed the NAAQS.
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Slide 2-11
NATIONAL EMISSION STANDARDS FOR
HAZARDOUS AIR POLLUTANTS (NESHAP)
Identify Toxic Air Pollutant Emissions
Set Maximum Emission Limits
Apply Equally to New & Existing Units
One section of the CAA establishes the procedures for regulating hazardous air
pollutants. If the Administrator of USEPA establishes that a particular of 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 2-12
CLEAN AIR ACT AMENDMENTS OF 1990
New Units: New Source Performance Standards
Existing Units: Emission Guidelines
The CAAA requires the EPA to adopt standards for both new and existing
MWCs after taking into consideration the Maximum Available Control Technology
(MACT). Federal standards for new and modified stationary sources are known as
New Source Performance Standards (NSPS). In February 1991, the USEPA updated
their earlier NSPS for municipal waste combustor (MWC) units. These now apply to
units having capacities greater than 250 tons/day.6 One example of the regulatory
changes from 1971 is that the PM limit went from 0.08 to 0.015 grains/dscf.
Likewise, federal standards for existing units, known as Emission Guidelines
(EG), were published for existing MWC units with capacities greater than 250
tons/day.7 The NSPS and EG include requirements which are technology-based,
rather than health-based. Technology-based emission limits are based on control
available through the application of the Best Demonstrated Technology (BDT).
The NSPS and EG require continuous emission monitoring systems (GEMS),
the use of Good Combustion Practice (GCP), as well as annual stack tests to
demonstrate compliance with particulate matter, dioxin/furan, and acid gas emission
limits. In addition to the previously discussed operator certification requirements,
a site-specific training manual must be maintained and updated annually and used
by all staff associated with MWC unit operations.
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Slide 2-13
PUBLIC RELATIONS IN WASTE MANAGEMENT
Problems Which Are "Owned" Can Be Solved
Public Must Be Informed
Environmental Controls Are Available
Method of Payment Required
Good public relations are crucial to achieving environmentally responsible
waste management. The public generally must "own the problem" before it will
develop 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. To reach the goal of environmentally responsible waste management, the
public will have to accept and support methods for paying the associated costs.
Slide 2-14
PUBLIC RELATIONS POSITIVES
Good Signs
Clean Air Act Amendments of 1990
Recycling
Waste Minimization
Conservation and Renewable Energy
There are good indications that the public is ready to do its part. For example,
after years of debate, the Clean Air Act Amendments of 1990 were passed. More
stringent air pollution controls are now being required, in spite of their expense.
A second good sign is that recycling appears to be capturing the public's
attention. In the 1980s, over half the aluminum cans produced were recycled.8 In
1988, a national goal was established to recycle 25% of the MSW generated,9 with
some states proposing to recycle up to 50%.10 Of course, a basic life-style change is
required to move from our throw-away economy to one that emphasizes creating less
waste and recycling. In addition, recycling programs tend to be labor intensive and
expensive, with the secondary materials markets underdeveloped and unstable.
As a parallel, it is interesting to note that national energy consumption did not
grow from 1973 to 1986,11 although the population grew. Energy conservation
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occurred primarily because of energy price increases. However, from 1986 to 1991
prices dropped and consumption increased.
Slide 2-15
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 (chief facility operator, shift supervisors) is required
as a method of protecting against operations which threaten the health and welfare
of both the public and employees.
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 assure
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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REFERENCES
1. Lloyd Compton, "Assessing the Costs of EPA's New Regulations for Existing
WTE Facilities," Solid Waste and Power. June 1991, pp. 18-26.
2. 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, pp. 121-133.
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. 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.
6. 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.
7. U. S. Environmental Protection Agency, "Emission Guidelines; Municipal Waste
Combustors," Federal Register. Vol. 56, No. 28. February 11, 1991, pp. 5514-
5527.
8. Jerry Powell, "Recycling in the '80s: How are We Doing?" Resource Recycling.
Vol. 8, No. 2, May/June 1989, pp. 30-31.
9. U. S. Environmental Protection Agency, "The Solid Waste Dilemma: An Agenda
for Action," EPA/530-SW-89-019, 1989.
10. David Spencer and Jerry Powell, "Recycling," Integrated Solid Waste
Management. Frank Kreith, editor, Genium Publishing Corporation,
Schenectady, NY, 1990, pp. 59-98.
11- Energy Conservation Trends. Understanding the Factors That Affect
Conservation Gains in the U.S. Economy. U. S. Department of Energy,
DOE/PE-0092, September 1989, pp. 2-16.
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3. MUNICIPAL SOLID WASTE TREATMENT
Slide 3-1
INTEGRATED SOLID WASTE MANAGEMENT
Source Reduction
Recycle and Reuse
Incinerate
Landfill
The available components of an integrated solid waste management program
include source reduction, recycling, incineration, and landfill disposal. The set of
disposal methods selected will vary depending on the size of population, availability
of sites, and political acceptance.
Slide 3-2
SOURCE REDUCTION - WASTE MINIMIZATION
REDUCE QUANTITY
Improve Efficiency
Improve Product Life
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.1
Examples of quantity reduction include the use of returnable bottles and the
purchase of bulk items which require less packaging material. Waste minimization
relates to socioeconomic value issues, such as life-style (consumption), product life,
and efficiency. Although elements of source reduction are advocated by many
members of the public, high rates of consumption and a throw-away economy are
often considered basic to our way of life.
Source reduction also includes the substitution of less toxic and
environmentally preferred materials in manufacturing products. Substitution may
be achieved through governmental regulations on manufacturing, such as for
asbestos, PCB oils, and pesticides. Other substitutions can be achieved through
consumer market forces, such as reduced use of styrofoam cups. In general, the use
of cadmium and other toxic materials in ink is being phased out, so that such
hazardous waste will not result from the recycling of paper.
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Although it is not common in solid waste management, source reduction could
be stimulated by requiring the cost of disposal to be included in the purchase price.
Many communities now require the payment of a disposal fee for each tire sold, as
the tire is presumed to eventually be discarded as waste.
Slide 3-3
RECYCLING
Positive Public Perception
Separation of Reusable Products
Raw Material Markets
Conserve Natural Resources
Reduce Environmental Impact
Extend Landfill Life
Recycling is perhaps the most positively accepted component in waste
management. Source separation at the curb-side or drop-off centers and on-site
mixed waste mechanical techniques are used to segregate raw materials such as
aluminum, ferrous metals, newsprint, and glass. These are sold either to primary
manufacturers or in secondary materials markets. The costs of recycling programs
are off-set by materials sales. However, the materials markets traditionally fluctuate
and may become marginal, with the "soft" newsprint market being a recent example.
Sales to primary manufacturers can result in reduced mining activities and
energy consumption, thereby conserving natural resources and reducing the
environmental impact. However, care must be taken in the reclamation effort, as
undesirable and/or hazardous waste streams may be created. An example is the de-
inking operations for newsprint which may produce toxic contaminants if cadmium
is used in the ink.
An important credit for recycling is the extension of landfill life due to the
direct reduction in disposal requirements.
Slide 3-4
COMPOSTING
Aerobic Decomposition (with Oxygen)
Biological Microorganisms Required
Produces Carbon Dioxide & Moisture
Anaerobic (Without Oxygen) Decomposition
Produces Methane
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Composting is another important area of recycling MSW.2 Composting of grass
clippings, leaves, and other yard wastes can be accomplished adjacent to where they
are produced or at a composting center. Yard wastes may be banned from a landfill
because they use valuable capacity. In addition, yard wastes are undesirable in MSW
fuel because of high moisture and low heating value.
Most of the early composting facilities in the United States were characterized
by their limited capital investment, mechanical processing, and control of the
biological process. Aerobic (with air) composting in windrows or long piles uses
traditional biological decomposition of organic materials through the action of
microorganisms in the presence of oxygen. Carbon dioxide and moisture is produced.
Systems using anaerobic decomposition (without air) can be designed to provide
faster decomposition at higher temperatures. Anaerobic decomposition requires a
closed vessel to control the methane gas and odors which are produced.
The trend is toward regional facilities which are "under roof," provide
mechanical processing, and control the decomposition process. This increases the
decomposition rates, minimizes odors and kills pathogens and weed seeds.3 Normal
composting requires aeration, mixing, adequate time and temperature and moisture
control.
Composting of organic materials such as paper products and waste from
restuarants and food processing industries is often accomplished in closed vessels.4
If composting materials include sewage sludge, special concerns include the
environmental impact of its potential heavy metals content.
Slide 3-5
COMPOST MARKET REQUIREMENTS
Process Requirements
Pre-Processing
Post-Processing
Market Development
The compost from MSW and sewage sludge will be effectively free of viral,
bacterial, and parasitic pathogens if the process maintains temperatures above 130°
F for several days, as required under RCRA Subtitle D.5
Mechanical pre-processing can be accomplished with shredders and screens (for
size control and removal of glass and metals). Post-processing may be required to
remove additional glass, rocks, and plastic to meet market product specifications.
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The market value of compost is that of a humus-type soil conditioner, with
minimal value as a fertilizer. Compost can improve the soil's ability to retain water
and aid in aeration by decreasing soil's crusting tendency.
Slide 3-6
LANDFILL REQUIREMENTS UNDER RCRA
Containment System
Cap System
Bottom Liner
Leachate Collection & Treatment
Ground Water Monitoring
Gas Monitoring & Collection
There are various requirements for a modern sanitary landfill under RCRA,
Subtitle D.5 The particular concern is to prevent the leaching of heavy metals from
the wastes into the ground water and to control the emissions of methane and other
landfill gases which are formed from waste decomposition.
Systems for containment and liquid and gas collection are required. Also,
ground water wells and gas probes are installed for monitoring at selected locations.
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 be piped to a combustion device for power production.
Slide 3-7
MODERN LANDFILL SYSTEM
Groundwvier
~ Monitoring
WeU
Bottom
Liner(Clay)
\ \
Luchatt Collection System Groundwtter
Monitoring -
Well
• Groundwiicr Row
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Upon initial construction of a landfill, the entire bottom is covered with a liner,
which 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.
Slide 3-8
MONOFILL
Special Landfill
Hazardous Waste: Concentrations
Below Specified Limits
MWC Ash
HWI Ash
Hazardous Waste
Chemical Waste
A monofill is a special landfill which accepts only a single type of waste.
Examples of such waste types would include ash from MWC units, ash from
hazardous waste incinerators, and certain low-level hazardous and chemical wastes.
Monofills would have special design, operating, and monitoring requirements in
addition to those of modern sanitary landfills.
Issues related to the disposal of MWC ash in sanitary landfills and monofills
are related to their heavy metals content and leaching characteristics. These issues
will be presented in Learning Unit 18.
Slide 3-9
MUNICIPAL WASTE COMBUSTORS, MWC
Incineration
Volume Reduction
Waste-to-Energy Resource Recovery
Volume Reduction & Energy
In the past decade, MWC technology has shifted from incinerators to waste-to-
energy resource recovery units.7 Although MSW is still considered to be garbage,
which should be disposed of safely and economically, it can also be considered as a
renewable energy resource. Waste-to-energy MWC units produce important revenue
3-5
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associated with the sale of either steam or electricity. However, MSW has a
relatively low value as a fuel.
In the late 1960's there were some ill-fated ideas about selling garbage to the
power plants. From an economic view-point, the value of MSW fuel is positive only
if the costs of alternate disposal techniques are taken into account. Consequently,
MSW may be considered to have increased value where landfill capacities are limited
and transportation costs to landfills are high.
Combustion of MSW does not eliminate the need for landfills, although it does
reduce the amount of such residues sent to the landfill. The ash residues have
around 25% of the weight and 10% of the volume of the original MSW.8'9
Slide 3-10
MUNICIPAL WASTE COMBUSTORS
Mass Burning Units
Refuse-Derived Fuel (RDF) Units
Mass burning technology of mixed MSW is often adopted where the projected
earnings from selling raw materials recoverable from MSW do not support the
increased processing costs. Recovery of materials such as aluminum, ferrous metals,
and glass have economic value which depends upon market conditions. Because
metals, glass, and masonry materials have essentially no heating value, they are
generally considered as undesirable components in MSW fuel. However, mass burn
system designs are able to compensate for the presence of such non-combustibles.
Refuse-derived fuel (RDF) technology makes use of various combinations of
front-end processing to remove a reasonable amount of the non-combustibles and
materials with limited heating value, such as metals, glass and yard wastes. The
RDF fuel is also more homogeneous in size and composition than raw MSW.10
RDF generally has less moisture than MSW, because of the drying associated
with frictional heating during shredding and drying during conveying. "Fluff RDF"
generally has heating values which are about 25% larger than that of its MSW. This
is because of its lower ash (inorganic) and moisture contents.
RDF is generally considered to be a better fuel than MSW, because its heating
value is increased over that of raw MSW. However, RDF generally burns with a
lower combustion efficiency than raw MSW, based on carbon monoxide emissions.
This issue will be further discussed in Learning Unit 12.
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Slide 3-11
FRONT-END PROCESSING
Elimination of Undesirable Materials
Size Reduction
Pre-Combustion Materials Recovery
for Refuse-Derived Fuel, RDF
Front-end processing includes on-site fuel modification and materials recovery
from MSW before it is burned. At a minimum, its purpose is to eliminate certain
undesirable materials from the waste stream for both mass burning or RDF units.
Gas tanks must be removed as they could explode and damage the combustion
equipment. Bulky white-goods and metal objects (e.g., water tanks and barrels) have
no heating value and are basically un-treatable. Such bulky items would generally
interrupt the air supply to the fuel.
Some materials which are undesirable at one unit may be acceptable in a unit
of a different design. For instance, a car tire or mattress is often unacceptable at
modular, mass burn units, but may be acceptable at larger mass burn units. Some
facilities exclude tires because they are a source of S02 emissions.
Slide 3-12
RDF PROCESSING EQUIPMENT
Flail Mill Shredder
Trommel Screen
Magnetic Separator
Eddy Current Separator
Hammer Mill Shredder
Disk Screen or Air Classifier
RDF is generally produced in a materials recovery facility (MRF). Waste
processing equipment at a particular site may include various conveyors, shredders,
screens, and separators.
Various classifications of RDF are listed in the next learning unit. "Shred-and-
burn" RDF systems often produce a "coarse RDF" which features size reduction
without materials recovery. Coarse RDF is less disruptive of the air flow through the
fuel bed than would be the mass burning of its original MSW.
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Slide 3-13
RDF PROCESSING EQUIPMENT
11
Receiving
& Storage
Area
Magnet
Ferrous Material
Secondary
Shredder
F
•recipitator or Sc
Fabric Filter
wvv
: rubber
V
Heat
Recovery
o»-
o
Boiler
Metering
Bin
-•JAsh System \<
From Joseph G. Singer, Combustion Fossil Power. 4th Edition, 1991,
reprinted with permission of Combustion Engineering, Inc.
An example arrangement of waste processing equipment components for the
production of RDF is illustrated above.
After the primary shredding process, materials may be removed by screening
and separation devices.10 The resulting higher quality RDF is called "fluff RDF." Its
production includes metals separation, removal of fines (sand and glass grit, grass,
etc.) and size reduction to about one-third that of coarse RDF.
Magnetic separator designs provide electromagnets which cause the removal
of the ferrous metals from the waste stream. Eddy current units use special
electrical equipment which causes aluminum to be removed as it passes over the unit.
Some facilities will have staff who hand-pick aluminum cans and other items
from the shredded material on a conveyor.
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Slide 3-14
HAMMER MILL SHREDDER
Drive
Motor
Ballistic
Rejection
Hammers
Neck
Section
Discharge
From J. D. Blue et al., "Waste Fuels: Their Preparation, Handling, and
Firing," Standard Handbook of Power Plant Engineering. Thomas C.
Elliott, editor, McGraw Hill Book Co., NY, 1989, reprinted with
permission.
A hammer mill shredder can reduce most material items to below 2.5 inch
sizes. By contrast, flail mills are designed to tear open plastic bags and to provide
moderate size reduction. In some facilities, flail mills are the primary shredder and
hammer mills are the secondary shredder.
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Slide 3-15
TROMMEL ROTARY SCREEN
From J. D. Blue et al., "Waste Fuels: Their Preparation, Handling, and
Firing," Standard Handbook of Power Plant Engineering. Thomas C.
Elliott, editor, McGraw Hill Book Co., NY, 1989, reprinted with
permission.
Trommel and disk screens have special abilities to sort materials into two or
more size categories, which can tend to segregate some materials according to
compositions. For instance, very small sized materials such as grass and grit can be
separated from streams which have larger sizes and higher combustibles content.
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Slide 3-16
AIR CLASSIFIER
From J. D. Blue et al., "Waste Fuels: Their Preparation, Handling, and
Firing," Standard Handbook of Power Plant Engineering. Thomas C.
Elliott, editor, McGraw Hill Book Co., NY, 1989, reprinted with
permission.
Air classifiers were developed to remove small dense particles such as glass
and grit from the larger and less dense combustible material, called fluff. The fluff
is more easily entrained in high velocity air, and the denser grit falls-out. Many
facilities use disc screens to achieve such size reductions.
Slide 3-17
POST COMBUSTION PROCESSING
Ferrous Metal Extraction from Ash
Many mass burn units use magnets to recover ferrous metals from the bottom
ash, after the combustion has been completed.
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REFERENCES
1. 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.
2. Elliot Epstein and Todd Williams, "Solid Waste Composting Gains New
Credence," Solid Waste and Power. June, 1988, pp. 275-278.
3. Jeffrey K. Turner, "MSW Composting, 1990s-Style," Solid Waste and Power.
January/February, 1992, pp. 18-25.
4. R. A..Denison and J. Ruston, Editors, Recycling & Incineration. Evaluating the
Choices. Environmental Defense Fund, Island Press, Washington, DC, 1990,
pp. 84-89.
5. Resource Conservation and Recovery Act, Part 40, CFR Part 257, Subtitle D.
6. 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.
7. Scott Siddens, "A Decade of Innovations in WTE Incineration," Solid Waste and
Power. April, 1990, pp. 16-23.
8. Jeffrey L. Hahn, "Managing Ash—Closing in on Policy Decisions," Solid Waste
& Power. August 1989, pp. 12-18.
9. Richard A. Denison and John Ruston, Recycling & Incineration. Evaluating the
Choices. Environmental Defense Fund, Island Press, Washington, DC, 1990,
p. 9.
10. Victor Brown and Henry Hefty, "Aiming for High-Spec RDF," Solid Waste and
Power. April, 1990, pp. 38-44.
11. Joseph G. Singer, Combustion. Fossil Power. 4th Edition, Combustion
Engineering, Inc., Windsor, CT, 1991, p. 8-22.
12. J. D. Blue et al., "Waste Fuels: Their Preparation, Handling, and Firing,"
Standard Handbook of Power Plant Engineering. Thomas C. Elliott, editor,
McGraw Hill Book Co., NY, 1989, pp. 3-145 to 3-146.
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4. CHARACTERIZATION OF MSW FUELS
Slide 4-1
SOLID WASTE ACRONYMS
HWI Hazardous Waste Incinerator
MRF Materials Recovery Facility
MSW Municipal Solid Waste
MWC Municipal Waste Combustor
MWI Medical Waste Incinerator
RDF Refuse-Derived Fuel
The acronyms listed above are often used in discussions of waste management.
Their distinctive differences are important in regulatory and operational
considerations.
Slide 4-2
CHARACTERIZATION OF WASTE COMPOSITION
Source
Type
Material Constituents
Ultimate Analysis (Element by Weight)
Proximate Analysis (Group by Weight)
MSW can be characterized by its source, type, constituents, elemental or
ultimate analysis, and proximate analysis. These distinguish MSW and provide
useful information about its properties.
Slide 4-3
MUNICIPAL SOLID WASTE SOURCES
Household waste
Commercial (Retail)
Institutional
Specific Items
Municipal Solid Waste (MSW) is composed of the discards from household,
commercial and institutional sources.1 Since MSW includes both unprocessed and
processed wastes from these sources, MSW includes wastes which have undergone
source separation and on-site processing to recover useful materials and form RDF.
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Household wastes are the materials discarded by single and multifamily
dwellings, including motels and hotels as well as both permanent and temporary
residential units.
Commercial (or retail) materials include those discarded from stores, offices,
restaurants, warehouses, and the non-manufacturing activities in industry.
Institutional wastes include the materials discarded by schools, hospitals, and
government facilities.
Although local definitions may vary, the NSPS definition of MSW includes:
motor vehicle maintenance parts which are limited to batteries, tires and used motor
oil.
Slide 4-4
MUNICIPAL SOLID HASTE EXCLUDES
Industrial Process Waste
Segregated Medical Waste
Hazardous Waste
Specific Items
Municipal solid waste (MSW), as defined in the NSPS,1 excludes segregated
medical waste, industrial process waste, and hazardous waste. MSW is further
defined to exclude mixtures of medical waste and medical waste discards (MSW from
hospitals) which contain less than 30 percent by weight of medical waste discards.
Some MWCs are permitted to burn segregated medical waste, if MSW represents at
least 70% of the charge.
The federal definition of MSW excludes certain specific materials such as
sewage, construction and demolition debris, industrial process and manufacturing
waste, motor vehicles and engine blocks.
Slide 4-5
MEDICAL WASTE SOURCES
Human & Animal Diagnosis
Human & Animal Treatment
Human & Animal Immunization
Processing of Biologicals
4-2
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Medical waste is the solid waste associated with the diagnosis, treatment and
immunization of humans and animals, including the solid waste from research,
production and testing of biologicals.1 The term biologicals refers to vaccines made
from living organisms.
Segregated medical wastes are wastes from the above sources which are
required to be transported in special packages with labels and markings.2 Such
waste is often referred to as "red bag" waste, although it may be contained in a
specially marked corrugated paste-board box. In general, operators should know
whether their permit allows any labelled medical waste to be burned at their facility.
Slide 4-6
REGULATED MEDICAL WASTES3
Heterogeneous Mixture of Materials Capable
of Producing Infectious Diseases in Humans
1. Cultures & Stocks of Infectious Agents
& Associated Biologicals (incl. Vaccines)
2. Human Pathological wastes (Human Tissues,
Organs, Body Parts, Body Fluids)
3. Blood & Blood Products
4. Sharps (Needles, Syringes, Scalpel Blades,
Pipettes, Broken Glass)
5. Contaminated Animal Carcasses & Body Parts
6. Isolation Wastes
7. Unused Sharps
The terms regulated medical wastes, controlled medical waste, and segregated
medical wastes are often used interchangeably to identify the above types of
infectious materials which are included in segregated medical waste.3'4*5
Slide 4-7
HAZARDOUS WASTE CONSTITUTES DANGER TO
Public Health
Welfare
4-3
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Hazardous wastes are those which may constitute a danger to public health and
welfare of the environment.2 To be hazardous, the substance must contain a
hazardous component which is above an established threshold concentration.
Hazardous waste is not part of MSW, as it is regulated under the Resource
Conservation and Recovery Act (RCRA).
Slide 4-8
HAZARDOUS WASTE
Oils
Flammable organics
Toxic metals & solvents
Explosives
Salts, Acids, Bases
Hazardous wastes are grouped as: waste oils and chlorinated oils; flammable
wastes and synthetic organics; toxic metals, etchants, pickling and plating wastes;
explosives, reactive metals and compounds; and salts, acids, and bases.6
Slide 4-9
INCINERATOR INSTITUTE OF AMERICA CLASSIFICATIONS7
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 € Industrial Solid Byproduct Waste
rubber, plastic, wood wastes
Type 7 Municipal Sewage Sludge Wastes
residue from processing of raw sludge
4-4
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Classification standards for solid waste were established by the Incinerator
Institute of America (IIA) in 1968.7 Average MSW could be approximated as a
combination of IIA type 1 and type 2 waste. The IIA classification standard is
generally considered to be out-of-date, although it is included in some local
regulations. As discussed later, the American Society of Testing Materials has
identified seven types of MSW fuels which are based on the composition, waste
particle sizes, and the type of waste processing.
i
The IIA standard illustrates the fact that heating value is inversely related to
fuel moisture. Moisture acts as a heat sink, requiring energy for evaporation, rather
than contributing to heat release. Incombustibles, likewise, do not add any heating
value to the fuel.
Therefore the major determining factors in the heating value of MSW are the
moisture and ash contents. The other materials-such as plastic, textile, and paper--
provide a relatively high amount of heat release, which is comparable to wood. The
improvement in heating value associated with solid waste processing will be
illustrated in discussions about RDF at the end of this learning unit.
Slide 4-10
MSW COMPOSITION/ GENERATION8'9
Paper and cardboard
Yard wastes
Metals
Glass
Plastics
Food wastes
Wood
Rubber and leather
Textiles
Miscellaneous
Total
Weight
Percent
40.0
17.6
8.5
7.0
8.0
7.3
3.6
2.6
2.2
3.2
100.0
Million
tons/yr
71.8
31.6
15.3
12.5
14.4
13.2
6.5
4.6
3.9
5.8
179.6
The national average composition percentages and generation totals for MSW
for 1988 in the United States are presented above.8-9 The average person generates
about 3.2 Ib/day of MSW plus 0.4 Ib/day of recovered materials.10
The compositions of MSW vary considerably depending on the time and the
local conditions. The average metals and glass contents presented above may be
over-estimated, due to national recycling efforts which have grown since the
projections were made. In general, more waste is produced in urban regions and in
affluent regions, however these same areas tend to emphasize recycling. Less wastes
are generally produced in the winter than in other seasons, partly because of reduced
purchasing activities and less yard wastes being generated.
4-5
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Slide 4-11
ESTIMATE OF DAILY MSW FOR A REGION
Example Population:
Per Capita Production:
Daily Amount Produced:
200,000 persons
3.2 Ib/day-person
640,000 Ib/day or
320 tons/day
By multiplying 3.2 Ib/day-person by the appropriate population, one can
estimate the MSW produced in a region per day. Since there are 2,000 Ib/ton, the
daily tonnage produced can be established by dividing the daily pounds by 2,000.
Slide 4-12
AVE. ULTIMATE ANALYSIS11'" As Received
Percent
Element
Carbon
Hydrogen
Oxygen
Nitrogen
Chlorine
Sulfur
Inorganics (ash)
Moisture
Total
by Weight
25.6
3.4
20.3
0.5
0.5
0.2
24.3
25.2
100.0
Dry Basis
Percent
by Weight
34.2
4.5
27.1
0.7
0.7
0.2
32.6
100.0
The ultimate analysis presents the distribution of total weight among the
various chemical elements plus the moisture and ash (mineral incombustibles).
Knowledge of the weight distribution among the elements can be used to estimate the
amount of various products of combustion. For instance, uncontrolled sulfur dioxide
and hydrogen chloride emissions can be estimated by using the ultimate analysis.
The inorganic constituents (incombustible ash) and moisture are also reported
in the ultimate analysis. Note that ash and moisture make up about half the total
weight of average MSW. The ash can be analyzed further to determine its
constituents, such as silicon, iron, calcium oxide or other metal compounds.
Because of the variability of MSW composition, special techniques are required
to obtain a representative sample. As indicated in the MSW example, the analysis
can be based on the total weight of the sample "as received" in the laboratory.
Alternately, a "dry basis" analysis of the sample can be presented, whereby the
sample is dried in an oven before the elemental composition is determined.
4-6
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Slide 4-13
EXAMPLE OF PROXIMATE ANALYSIS OF RDF
13
Moisture
Ash
Volatile Matter
Fixed Carbon
Total
Yr. Average
Percentage
by Weight
26.6
21.7
43.6
8.1
100.0
Range During Year
Minimum Maximum
Value
42.2
34.5
60.4
Value
2.3
10.8
34.9
0.0
21.6
The proximate analysis provides relative information about the burning
mechanisms of the fuel. It presents the volatile matter, which is the fractional
weight of the fuel which will burn as a gas, and the fixed carbon, which will burn as
a solid. As in the ultimate analysis, the proximate analysis also indicates the amount
of moisture which must be evaporated before the fuel will burn and the ash content.
One should note that both MSW and RDF are high volatility fuels, which burn
primarily as a gas. However, a sufficient residence time will be required to obtain
a high fraction of carbon burn-out from the ash.
The need for special design features to accommodate the fuel properties is
indicated by the proximate analysis. As an example, because MSW and RDF are wet
fuels, provisions for drying are required. Also, as both fuels have high volatility,
relatively large furnace volumes will be required for the combustion of gases.
Slide 4-14
COMPARISON OF MSW AND COAL VALUES14'15
Higher Heating Value (Btu/lb)
MSW 2,000 - 7,700
Bituminous Coal 9,000 - 13,500
Fuel Oil 18,000 - 20,000
Normal Fuel Size
MSW
Pulverized Coal
Stoker Coal
MSW
Bituminous Coal
Powder - 6 ft
Fine Powder
1/32 in. -1.2 in,
Ash Fusion Temperature (°F)
1,300 - 1,600
2,100 - 2,500
4-7
-------
Because so many operators have experiences with conventional fossil fuels, it
may be helpful to illustrate some of the comparable fuel characteristics. These
property variabilities affect the design and operational features of combustion
equipment. Also* some MWC installations co-fire some other type of fuel than MSW.
For instance, combustion control is reasonably easy to obtain with fuel oils and
natural gas, because they burn with near uniform fuel properties. Of course, there
are source and/or grade dependent variations.
Coal properties may vary depending on its rank and the coal seam from which
it is mined. Major variation in coal properties relates to its handling and exposure
to the weather. Various applications attempt to moderate these variations, for
instance, by grinding the coal to a fine powder before its firing in pulverized coal
units.
Ash fusion temperatures represent temperatures at which liquid ash particles
will begin to solidify. Ash fusion is a problem with MWCs because temperatures
below the ash fusion temperature are often found on the fuel bed and heat exchange
surfaces, causing the formation of clinkers.
Ash fusion temperatures are often reported as the "ash softening temperature"
(the temperature at which a cone shaped ash sample will fuse into a hemispherical
shape).15 The fusion temperature depends upon whether reducing or oxidizing
combustion conditions are present.
Slide 4-15
MSW FUEL VARIABILITY
Wet, Dry
Large Pieces, Small Particles
Combustibles, Incombustibles
Uniformity of Composition
A distinguishing feature of MSW fuel relative to conventional fuels is that of
fuel variability. Material composition of refuse received at a local facility will also
vary depending on the socioeconomic character of its neighborhood or source.
The moisture content and composition of MSW will vary considerably
throughout the year and with the day of week, season, and climate. For instance,
yard wastes may be delivered primarily just after the weekends in the summer and
fall. Local precipitation generally adds moisture to the waste, unless covered
containers are uniformly used.
4-8
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The particle sizes may vary from that of a mattress or sheet of plywood to a
dust particle. Therefore, combustion can only approach uniformity if there is
appropriate materials segregation, pre-processing, and mixing.
Some components of waste have unique burning characteristics. Wet waste,
such as yard wastes and food wastes, must be dried as the first stage in the burning
process. Other materials, such as plastic bags, will volatilize quickly upon exposure
to hot combustion gases. Newsprint will burn readily if hot air is delivered to its
surface. However, newsprint burns poorly if air is restricted, as in the example of a
telephone book.
Incombustible materials, such as metal objects, can restrict air flow to
combustible materials.
Slide 4-16
EXAMPLE OF MSW COMPOSITION13
Paper and cardboard
Miscellaneous
Glass
Natural organic s
wood
Metals
Plastics
Textiles
Tar
Total
MSW
Percent
46.6
18.9
9.5
6.6
6.4
6.4
3.2
1.7
0.7
100.0
RDF
Percent
78.8
6.6
1.4
1.5
4.3
0.7
5.1
1.6
0.0
100.0
Solid waste processing involves various combinations and configurations of
equipment.16'17'18 Therefore, RDF compositions will depend on the particular
situation.
The example above illustrates the material constituents of the MSW before and
after processing into RDF at the Ames, Iowa facility.13 As shown in the next slide,
the example RDF has a lower moisture and ash content than the parent MSW. The
processing facility included shredding of the MSW for improved size uniformity and
materials separation through screening and magnetic recovery techniques.
Representative properties of RDF and MSW have been reported with RDF
having a much lower moisture content but a slightly higher ash content than MSW.15
This difference illustrates the variability of MSW. Properties at any site will be
influenced by the source, factors of recycling and the design and operational features
of the materials recovery facility.
4-9
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Slide 4-17
EXAMPLE OF ULTIMATE
Component :
Carbon
Hydrogen
Oxygen
Nitrogen
Chlorine
Sulfur
Inorganics (ash)
Moisture
Total
ANALYSES MSW
As Received
Percent
by Weight
22.2
5.4
33.3
0.3
0.2
0.2
16.4
22.0
100.0
RDF
As Received
Percent
by Weight
30.0
6.0
37.2
0.2
0.2
0.2
7.8
18.4
100.0
Slide 4-18
EXAMPLE OF ULTIMATE ANALYSIS MSW
Dry Basis
Percent
Component:
Carbon
Hydrogen
Oxygen
Nitrogen
Chlorine
Sulfur
Inorganics (ash)
Total
by Weight
28.5
6.9
42.7
0.4
0.2
0.3
21.0
100.0
RDF
Dry Basis
Percent
by Weight
36.7
7.4
45.6
0.3
0.2
0.2
9.6
100.0
The corresponding "as received" and dry-basis ultimate analyses for the
previous MSW and RDF samples from Ames, Iowa are presented above. The "as
received" MSW sample was obtained after the primary shredder, so it is probably
dryer than the average of that delivered because of frictional heating in the shredder.
The "dry basis" analysis was calculated from the "as received" analysis by dividing
each component's percentage by (1.0 - moisture fraction).
The heating value was increased about 25% (from 4,830 Btu/lb for the MSW to
6,110 Btu/lb for the RDF).13 This is because the front-end processing effectively cut
the inorganic composition (glass & metals) to half that of the MSW feed. The carbon,
hydrogen, and oxygen contents in RDF were all increased, relative to raw MSW.
Also, RDF has better size uniformity.
4-10
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Slide 4-19
CLASS RDF BOILER FUEL DESCRIPTION19
1 Raw MSW fuel. Used as a fuel in the
as-discarded form; Oversized bulky
waste items have been removed.
2 Coarse RDF. Processed to a course size
with or without ferrous metal separation,
95% passing thru a 6-inch mesh screen.
3 Prepared RDF. Processed to remove 90%
of ferrous metal, glass, and other
inorganics, sized with 99% passing thru
a 6-inch square mesh screen.
4 Recovery Prepared RDF. Equivalent to
Class 3, but with aluminum, other non-
ferrous & glass removed for market sales.
5 Fluff RDF. Shredded; metals, glass and
other inorganics removed, sized for 95%
passing thru a 2-inch square mesh screen.
6. Densified RDF. Combustibles compressed
or densified into pellets, slugs,
cubettes, briquettes, etc.
Various MSW fuel classification systems have been used to distinguish the
processing operations used and the physical size features of RDF. In the above
classification, the power industry has identified the forms of RDF which can be used
as boiler fuels.19
4-11
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Slide 4-20
ASTM FUEL CLASSIFICATIONS"
RDF-l Municipal solid waste used as a fuel
(MSW) in as-discarded form, without oversized
bulky wastes
RDF-2 MSW processed to coarse particle size,
(c-RDF) with or without ferrous metal separation,
95% passing a 6-inch mesh screen
RDF-3 Shredded fuel derived from MSW, with
(f-RDF) processing to remove metal, glass and
other inorganics, 95% passing a 2-inch
square mesh screen (Fluff RDF)
RDF-4 Combustible-waste fraction processed
(p-RDF) into a powdered form, 95% passing a
10-mesh (.035-inch) screen
RDF-5 Combustible-waste fraction densified
(d-RDF) or compressed into the form of pellets,
slugs, cubettes, briquettes, etc.
RDF-6 Combustible-waste fraction processed
into a liquid fuel
RDF-7 Combustible-waste fraction processed
into a gaseous fuel
Likewise the American Society of Testing Materials has established a general
RDF classification scheme which also includes liquid and gaseous fuel types.20
4-12
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REFERENCES
1. 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.
2. Louis Theodore, Air Pollution Control and Waste Incineration for Hospitals and
Other Medical Facilities. Van Nostrand Reinhold, New York, 1990, pp. 228-229.
3. U. S. Environmental Protection Agency, "Standards for the Tracking and
Management of Medical Waste: Interim Final Rule and Request for
Comments," Federal Register 54:12373, 12374, Washington, D.C.March 24,
1989.
4. "EPA Guide for Infectious Waste Management," EPA/530-SW-86-014, May
1986, p. ix.
5. "Infectious Waste Management Regulations," Commonwealth of Virginia,
Department of Waste Management, VR 672-40-01, May 2, 1990, p 3-4, 3-5.
6. 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.
7. "Incinerator Standards," Incinerator Institute of America, NY, Nov. 1968.
8. 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.
9. William F. Davidson, "MSW Composition," MWC Operator Training Overview
Course Manual. Edited by John Eppich, ASME Professional Development
Program, West Palm Beach, FL, February 24-26, 1992, Lesson II.C, pp. 1-28.
10. William Franklin and Marjorie Franklin, "Solid Waste Characteristics,"
Integrated Solid Waste Management. Frank Kreith, editor, Genium Publishing
Corporation, Schenectady, NY, 1990, pp. 33.
11. 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, pp. 3-3, 3-4.
4-13
-------
12. Ronald E. Myers, "Municipal Waste Combustors - Background Information for
Proposed Guidelines for Existing Facilities." U.S. Environmental Protection
Agency, EPA-450/3-89-27e, August 1989, p. 2-5.
13. D. E. Fiscus et al., "Study of Existing RDF-Cofiring 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.
14. Municipal Waste Incinerator Field Inspection Notebook. U.S. Environmental
Protection Agency, EPA-340/1-88-007, July 1988, pp. 12-15.
15. Joseph G. Singer, Combustion Fossil Power. 4th Edition, Combustion
Engineering, Inc., Windsor, CT, 1991, pp. 2-13, 2-31, 3-5 and 8-16.
16. Stuart H. Russell, "Solid Waste Preprocessing: The Return of an Alternative to
Mass Burning," Proceedings of the 1988 Waste Processing Conference. ASME,
Philadelphia, May 1988, pp. 73-76.
17. Mark C. Turner et al., "First Year of Operation at the SEMASS WTE Facility,"
Proceedings of the 1990 Waste Processing Conference. ASME, Long Beach, CA,
June 1990, pp. 1-7.
18. Gary L. Boley, "Startup and Operations of the Mid-Connecticut Resource
Recovery Project," Proceedings of the 1990 Waste Processing Conference.
ASME, Long Beach, CA, June 1990, pp. 21-29.
19. J. D. Blue et al., "Waste Fuels: Their Preparation, Handling, and Firing,"
Standard Handbook of Power Plant Engineering. Thomas C. Elliott, editor,
McGraw Hill Book Co., NY, 1989, pp. 3-139, 3-140.
20. "Classification of Refuse Derived Fuels," American Society of Testing and
Materials.
4-14
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5. COMBUSTION PRINCIPLES I: BASIC COMBUSTION
Slide 5-1
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.
•\
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 15, where the concepts of dilution and
the techniques for correcting air pollutant concentrations to a standard basis will be
presented.
Slide 5-2
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.
A fuel is a substance which experiences 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).
5-1
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Slide 5-3
COMBUSTIBLE SUBSTANCES
Organic - Hydrocarbons
Paper, Wood, Plastic
Fossil Fuels
Renewable Fuels
MSW is composed of combustible materials, incombustible (ash) materials, and
water (moisture).
The combustible fraction of MSW is primarily composed of 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).
Hydrocarbons are 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).
Fossil fuels, like coal and oil, were formed over many thousands of years from
organic materials.
Renewable fuels are distinguished by their short production time. MSW is
often considered to be a renewable fuel, because of its high paper, cardboard, yard
waste and wood composition.
Slide 5-4
INCOMBUSTIBLE SUBSTANCES
Inorganic
Metals
Glass, Sand, Ceramics, Concrete
Inorganic materials are those which have no hydrocarbons in their composition.
Examples of such inorganic materials include metal cans, glass bottles and ceramic
materials.
5-2
-------
The inorganic materials in MSW are generally considered to be non-
combustible. However, some inorganic materials 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 MSW.
Other elements in MSW 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.
Slide 5-5
EXAMPLE ULTIMATE ANALYSES
AS
Component :
Carbon
Hydrogen
Oxygen
Nitrogen
Chlorine
Sulfur
Inorganics (ash)
Moisture
Total
MSW
Received
Percent
by Weight
22.2
5.4
33.3
0.3
0.2
0.2
16.4
22.0
100.0
RDF
As Received
Percent
by Weight
30.0
6.0
37.2
0.2
0.2
0.2
7.8
18.4
100.0
The example ultimate analyses of MSW and RDF was provided in the previous
learning unit in Slide 4-17. This was actual data from the Ames Resource Recovery
Facility.
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. Such ultimate analyses can be used in
general calculations related to the combustion air requirements, as will be
demonstrated later.
Note that the inorganic matter (ash) is the non-combustible fuel fraction which
is primarily collected as the bottom ash and fly ash. That which is not collected is
emitted up the stack into the atmosphere.
The moisture in the fuel will generally evaporate in a drying stage as the fuel
is heated in the MWC. Fuel moisture acts primarily as a heat sink, requiring energy
for its evaporation. Therefore, fuel moisture reduces the maximum combustion
temperature but passes through the combustion process otherwise unchanged.
5-3
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Slide 5-6
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 5-7
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 can be assumed to be a mixture of oxygen and nitrogen, with the other
constituents being small enough to be neglected. If two gases are mixed together at
constant temperature and pressure, their volume after mixing will be the sum of their
individual initial volumes. Therefore, if 0.21 cubic feet of pure oxygen is mixed with
0.79 cubic feet of nitrogen, one cubic foot of air will be obtained.
5-4
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Slide 5-8
DEFINITION OF A POUND-MOLE
w
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 5-9
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
5-5
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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 5-10
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 if chlorine is present.
For simplicity, we will assume that the nitrogen in the fuel is converted to
molecular nitrogen, and the nitrogen in the air remains as molecular nitrogen.
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 5-11
PRODUCTS OF INCOMPLETE COMBUSTION
*
*
*
Carbon Monoxide
Dioxins
Furans
Combustion is generally not perfect, 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 16.
5-6
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Slide 5-12
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 be discussed in Learning Units 17 and 21.
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 Unit 18.
Slide 5-13
CHEMICAL REACTION EQUATION
Carbon: C + O2 --> COa
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).
5-7
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Slide 5-14
BALANCED CHEMICAL REACTION EQUATIONS
COMBUSTION IN OXYGEN
Carbon: C + O2 —> COa
Hydrogen: 2 H2 + 02 —> 2 H2O
Sulfur: S + O2 --> S02
Chlorine: H2 + 2 Cl --> 2 HC1
The reaction equations listed above represent the complete combustion
reactions of the major constituents in MSW. 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 5-15
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.
5-8
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Slide 5-16
EXAMPLE OF BALANCING A COMBUSTION EQUATION
Methane, CH4/ with Stoichiometric Oxygen
CH4 + 2 O2 —> C0a + 2 H2O
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 5-17
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.
5-9
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Slide 5-18
EXAMPLE OF BALANCING A COMBUSTION EQUATION
Methane, CH4/ with Stoichiometric Air
CH4 + 2 O2 + 7.52 Na —>
C0a + 2 H20 + 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 5-19
EQUIVALENT MOLECULAR FORM OF MSW
C1.,5H5.402.0,N.02C1.006S.00< +1.22 H20
It can be shown that 100 pounds of the example of MSW presented in Slide 5-5
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 1.22 molecules of
water, which corresponds to 22 pounds of water (from the ultimate analysis) divided
by 18, the molecular weight of water.
Slide 5-20
THEORETICAL COMBUSTION OF MSW IN AIR
Ci..A.«Oa.eiII.MCl.0..S.00,+ 1.22 H20+ 2.165 O2+ 8.14 N2 —
1.85 CO2 + 3.92 H2O + 8.15 N2 + 0.006 HC1 + 0.006 SO2
5-10
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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 each element are
conserved (same number on the left-hand side and the right-hand side).
Slide 5-21
MASS ANALYSIS
REACTANTS
I1. 22 Xo2 °'
2.165 Oa
8.14 N2
Total
OF STOICHIOMETRIC FUEL & AIR
Moles
sS.oos 1-0
1.22
2.165
8.14
Molecular Wt
Ib/mole
61.6
18
32
28
MIXTURE
Mass
Ib
61.6
22.0
69.3
227.9
380.8
The combustion equations can be used to find the air-to-fuel ratio, as follows.
The molar weight of the "equivalent molecule" of dry MSW can be found by summing
the product of the number of atoms of each element by its atomic weight. This gives:
(1.85x12) + (5.4x1) + (2.08x16) + (.02x14) + (.006x35) + (.006x32) = 61.6 Ib/mole. The
corresponding weight of ash and water in the MSW was 16.4 and 22 Ibs, respectively,
for a total weight of 100 Ibs.
Likewise, the weight of the air is found to be (2.165x32)+(8.14x28) = 69.3 +
227.9 = 297.2 Ib-air. This corresponds to a stoichiometric air-to-fuel ratio of about 3.0
Ib-air/lb-MSW. At 100% excess air, about 6.0 Ib-air/lb-MSW would be required.
Slide 5-22
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)
5-11
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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.
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.
Slide 5-23
METHANE COMBUSTION IN THEORETICAL AIR:
CH4 + 2 02 +7.52 N2 -->
CO2 + 2 H2O + 7.52 N2
METHANE COMBUSTION IN EXCESS AIR:
CH4 + (1+EA)(2) O2 + (1+EA) (7.52) N2 -->
CO2 + 2 H20 + (1+EA) (7.52) N2 + (EA) (2) 02
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 5-24
METHANE COMBUSTION, 20 PERCENT EXCESS AIR
CH4 + 2.4 02 + 9.024 N2 — >
C0
2 H20 + 9.024 N2
0.4 O
5-12
-------
Consider the example of methane combustion under conditions of 20 percent
excess air, as illustrated in Slide 5-24. If 0.2 is substituted for EA in the previous
slide, the result will be as indicated.
If the techniques illustrated in Slide 5-21 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 5-25
PRODUCT
PRODUCTS
CO,
H2O
03
Na
Total
Dry Gas
Total
GAS ANALYSIS
Moles
1.0
2.0
0.4
9.024
12.424
10.424
, METHANE
-------
6. MUNICIPAL WASTE COMBUSTORS
Slide 6-1
ORGANIZATIONAL STRUCTURES
BUILDER OWNER OPERATOR
Vendor
Vendor
Vendor
Vendor
Public
Public
3rd Party
Vendor
Public
Private/Vendor
Private/Vendor
Vendor
Various organizational structures have been developed for MWC units. A
manufacturer or technology vendor will hold the exclusive rights to produce systems
based on certain equipment patents. Therefore, the vendor generally establishes the
design and builds the MWC unit. Of course, an architect and engineering firm and
a general contractor may set specifications for construction of the building and unit.
The early MWC units were owned and operated by public authorities, such as
trash disposal agencies. A manufacturer would erect the unit and operate it through
a shake-down period. Unit operations would then be turned over to the agency,
which would select staff and operate the unit.1 When unit modifications were
required, the equipment vendor or some other contractor might provide advice and/or
do the work.
Another public ownership option would be for the agency to contract with a
private service organization to operate the unit. Of course, the service organization
could be owned by the vendor.
Another option includes having so called "third-party investors" be the owners
of the unit. They would contract with public agencies to receive MSW and with a
service organization to operate the unit. The service organization could be an
independent private company or a vendor-owned service company. This has the
advantage of shifting some of the risk-taking from the public to the investors and
service organizations. Of course, the contract establishing the pricing structure for
cost recovery will have to balance the interest of the public with those of the private
parties.
Vendor ownership with operations by a vendor-owned service company is an
increasingly attractive organizational structure.2 The public gains because the
expertise of the vendor is available throughout the unit's life. The vendor may have
resources which are not available to other organizations. The vendor gains by
creating additional revenue and by assuring that the operation of equipment is
consistent with its design. Also, information developed through operations can be
incorporated into the new designs.
6-1
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Slide 6-2
EVOLUTION OF MWC DESIGNS,
Single Chamber, Flue-Fed
Multiple Chamber
Refractory Wall Incineration
Mass Burn Waste-to-Energy
Modular
RDF Waste-to-Energy
There has been a significant evolution in the design of municipal waste
combustor equipment.
Slide 6-3
SINGLE CHAMBER FLUE-FED INCINERATOR3
Uadnfitc air pun
Flue-fed, single chamber incinerators were widely used in apartment houses
in urban areas in the 1940s and 1950s. Their performance was hampered by the lack
of skilled operators. Various design fixes, for example roof-top afterburners, were
initiated to reduce smoke.3
6-2
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Slide 6-4
MULTIPLE CHAMBER INCINERATOR STANDARDS4
1. Sack 6. Flame port
i. Secondary air pom 7. Ignition «*•••*»*
3. Aih pit cleanout doon 8. Orcrfire air pans
4. Grata 9. Mixing chamber
5. Charging door 10. Go
doon
11.
12. Uaderfire air pom
IS. Curtain wall port
14. Damprr
15. G« boraen
Standards for small, refractory, multiple chamber incinerators were adopted
in the 1960s in Los Angeles County, California.4 Standardized sizes and geometries
and provisions for damper controls and auxiliary fuel burners to control combustion
chamber temperatures were included. These standards were developed for small
batch fired operations, including pathological waste incinerators.
Slide 6-5
REFRACTORY-WALL, MASS BURN
High Excess Air
High Gas Velocities
Particle Entrainment
Smoke
Shut Down in Late-1970s
During the 1940s and 1950s, many refractory-walled, mass burn incinerators
were built in the United States. These units are called mass burn units because
almost all of the mass picked up by garbage trucks goes into the incinerator. A
survey taken in 1965 reported that there were 299 major incinerators operating in
the United States.5 More than 200 of them had been closed by 1979.6
6-3
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Slide 6-6
REFRACTORY WALL INCINERATOR7
Combustion gases are only moderately cooled by heat transfer to the refractory
walls, which provide only incidental heat extraction from the adjacent flaming gases.
The temperature of a refractory wall is controlled primarily by regulating the excess
air during combustion, with such air acting as a heat sink. Large amounts of excess
air are required to keep temperatures low enough to prevent damage to the
refractory.
High velocities and poor mixing in the combustion chamber of such refractory
wall incinerators has resulted in ash entrainment and smoke emissions.
With the development of smoke ordinances and the Clean Air Act legislation,
many municipal incinerators were shut down. For instance, by the mid-1970s about
80 incinerators in New York state were either shut down or required to undergo
significant system modifications.8
6-4
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Slide 6-7
WATERWALL MASS BURN
Waste-to-Energy
European Designs
ESP for Particulate Control
The birth of incineration was in Nottingham, England in 1874, and by 1921
there were more than 200 plants in Great Britain, many producing steam and electric
power.9 Two full scale steam generating plants were built in New York in 1906.
However, the waste-to-energy technology became inactive within a few years, because
of the auxiliary fuel requirements, poor designs and unskilled operations.
Slide 6-8
EUROPEAN WATERWALL INCINERATOR DESIGNS
10
From ASME Journal of Engineering for Power, printed
with permission.
In the late 1960s, European technology for grate designs and waterwall
incinerators began to be adopted in the United States.11 A number of different grate
designs were developed, including those which vibrate, reciprocate, or translate to
agitate the fuel bed. These will be discussed further in Learning Unit 8.
6-5
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Although steam was produced, large condensers were often provided so that
the steam could be recirculated as feedwater when a customer did not need steam.
Slide 6-9
WATERWALL DESIGN CONCEPT12
MmbnntBar
Wall Tube
Courtesy of Babcock and Wilcox
The term waterwall or membrane wall relates to the integral boiler design
concepts used in most power boilers. Waterwall units have multiple tubes in the
form of metal enclosures, which surrounds the ball of flaming combustion gases. The
tubes are filled with flowing water, which extracts energy from the adjacent
combustion gases.
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.
6-6
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Slide 6-10
REFRACTORY COATED WATERWALLS1
Membrane Bar
Metal Lagging
Insulation
Wall Tubes—i
'—Cylindrical Studs
Refractory -
Courtesy of Babcock and Wilcox
Waterwall MWCs with exposed waterwall surfaces have experienced metal
wastage from corrosion and erosion. Such metal losses have caused tube failures
which require units to be shut down for repair.
Most MWC 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.13 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 6-11
ROTARY WATERWALL MASS BURN
Mass Burn or RDF Fired
Rotary Waterwall Section
Fixed Waterwall Section
The rotary waterwall technology is an interesting variation on the design of a
waterwall MWC unit. The characterizing feature is that the primary grate has rotary
action which tumbles the fuel bed. The rotary action is similar to the rotary kiln
incinerators, which are used in the cement kilns and hazardous waste incinerators.
6-7
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Slide 6-12
ROTARY WATERWALL UNIT2
Steam
Drum
Radiant
Section
Ring
Header
Branch
Pipe
Ram
Feeding Header
System
Windbox
Convection
—•— Section
Water
Drum
Economizer
Forced
Circulating
Pump
Residue
Conveyer
Courtesy of Westinghouse Electric Corporation
In a rotary waterwall combustor, waste materials are exposed to combustion
air which also passes through the small air holes in the combustor barrel. As the
combustor turns, the tumbling action stirs the waste materials and exposes them to
air. The rotary barrel is inclined at a modest angle (e.g., 6 degrees), so that gravity
forces help direct the tumbling MSW residues through the unit.
The rotary waterwall combustor has unique piping features which allow water
to circulate through the grate and extract thermal energy before being directed to the
fixed waterwalls in the furnace area.
Conventional water-cooled grates are provided in a secondary burn-out
chamber, which receives the ash from the rotary combustor and holds it in residence
for final carbon burn-out before being delivered to the ash pit.
6-8
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Slide 6-13
NODULAR MASS BURN
Factory Manufactured
Refractory-Wall
Controlled-Air, Starved-Air
Low Velocity in Primary
Low Particulate Entrainment
Solids Retention for Burn-Out
Modular refractory-wall incinerators were developed in the mid-1960s.14 These
factory manufactured units gained popularity because of their reduced participate
and smoke emissions, without requiring stack gas cleaning equipment. Because
standard models were marketed, many installations were permitted based upon the
testing of comparable units. Competition from such units stimulated improved
combustion equipment and controls in larger MWC units.
Controlled air gets its name from the use of fans to deliver air to strategic
locations. Primary combustion conditions can be either rich or lean, starved-air or
excess air.
Starved-air is a phrase used to characterize units which maintain sub-
stoichiometric or fuel rich combustion conditions in the primary chamber. Starved-air
units are also characterized two-stage combustion units: fuel rich in the primary
chamber and lean in the secondary. Therefore, the stack gases leave under excess
air conditions.
6-9
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Slide 6-14
MODULAR CONTROLLED-AIR OR STARVBD-AIR UNIT"
MODULAR INCltCRATOR
AUK
Courtesy of Joy Energy Systems, Incorporated
Auxiliary fuel burners in the primary chamber are used initially to raise
chamber temperatures for enhanced gas volatization.
Under normal conditions, the limited air flow in the primary chamber results
in temperatures which are hot enough to drive off the volatiles. A mixture of
partially oxidized volatile gases enters the secondary chamber and mixes with
additional air for completion of the combustion process.
The amount of ash entrainment is controlled by design features which restrict
the air velocities in the primary chamber.
6-10
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Slide 6-15
MODULAR INCINERATOR WITH BNERGY RECOVERY
14
1. Automatic Feed System
2. Primary Chamoer
3. Transfer Rams
4. Secondary Chamoer
5. Steam Generator
6. Steam Separator
7. Energy Duct
8. Emissions Control System
9. Exhaust Stack
10. Emergency By-Pass
11. WM Ash Sump
12. Ash Conveyor
Courtesy of Consumat Systems, Incorporated
Many modular incinerator units function as waste-to-energy units. Energy is
extracted in waste heat or recovery 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.
6-11
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Approximately 140 new MWC units were constructed in the USA during the
1970s and 1980s,2 with about half of the units being modular refractory units with
recovery boilers and half being the larger waste-to-energy units.16 These units were
subject to the 1971 NSPS for incinerators having sizes greater than 50 tons/day.
During this time period, MWCs generally used electrostatic precipitators to meet the
particulate control standards.11
The NSPS and EG promulgated for MWCs in 1991 set standards for new and
existing MWC units which require new controls for acid gases, products of incomplete
combustion, and particulates. Units subject to these regulations typically require dry
scrubbers and either fabric filters or electrostatic precipitators.
Slide 6-16
RDF UNITS
Waste Processing of RDF
Utility Pulverized Coal Units
Suspension Firing
Spreader Stoker Units
Suspension & Grate Burning
Co-Firing with Coal
Refuse derived fuel preparation systems and combustion technologies were
developed in the mid-1970s.17 Initially, refuse was prepared for suspension firing in
utility boilers burning pulverized coal.
Later modifications included the co-firing of RDF along with coal in spreader
stoker units. Today, RDF may be exclusively fired in spreader stoker units, although
the option of co-firing with coal is often provided.18'19
The main advantage of RDF combustion relative to mass burning of MSW is
that the RDF has a higher heating value and has more nearly uniform physical
properties.
6-12
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Slide 6-17
RDF COMBUSTION UNIT20
Boiler
Enclosed RDF
Feed Convey
RDF Metering
and Feed System
Gas Scrubber
ABB CES
Travelling
Grate Stoker
Zone Control
of Underfire Air
Ash Conveyor (s)
Courtesy of Combustion Engineering, Incorporated
The features of combustion of RDF can be compared to burning coal in a
spreader stoker fired unit. Both coarse and fluff refuse derived fuels (c-RDF and f-
RDF, respectively) are less dense than coal and tends to be formed as chips which are
more easily entrained. A greater fraction of RDF typically burns in suspension,
although the residence time may be inadequate for full carbon burnout. Therefore
RDF units will have greater soot and particulate loadings than mass-burn systems.
The ash fusion temperature for MSW and RDF is generally lower than that of
coal. Special grate designs have been successful in reducing freezing of molten ash
on the grates which would otherwise plug the siftings and under-grate air passages.13
Clinker formation on the grates is generally a concern when switching from coal
firing to RDF firing.
In addition, RDF has a lower heating value than coal and requires less
theoretical oxygen for complete combustion. Therefore, switching from burning RDF
to coal requires some combustion system adjustments.
6-13
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REFERENCES
1. Christopher Hocker, "Waste-to-Energy Development: Who's Doing What and
Why?" Solid Waste'and Power. August 1989, pp. 12-19.
2. Scott Siddens, "A Decade of Innovation in WTE Incineration/' Solid Waste and
Power. April 1990, pp. 16-23.
3. J. A. Danielson, Air Pollution Engineering Manual. AP-40, Second Edition, U.
S. Environmental Protection Agency, May 1973, p. 472.
4. J. E. Williamson et al., "Multiple Chamber Incinerator Design Standards for
Los Angeles County," Los Angeles County Air Pollution Control District,
October 1960.
5. R. J. Alvarez, "Study of Conversion of Solid Waste to Energy in North
America," Proceedings of 1976 ASME National Waste Processing Conference.
Boston, May 1976, pp. 163-174.
6. R. J. Alvarez, "Status of Incineration and Generation of Energy from
Processing of MSW," Proceedings of 1980 ASME National Waste Processing
Conference. New York, May 1980, pp. 5-26.
7. Municipal Incineration. A Review of Literature. AP-79, U. S. Environmental
Protection Agency, 1971, p. 29.
8. "Energy Recovery from Existing Municipal Incinerators," A Final Report to
New York Power Authority, prepared by Chesner Engineering, PC and Black
and Veatch Engineers, Nov. 1984, pp S-l to 3-46.
9. R. C. Corey, Principles and Practices of Incineration. Wiley-Interscience, 1969,
pp. 2-4.
10. Georg Stabenow, "Results of Stack Emissions Tests at the New Chicago
Northwest Incinerator," ASME J. Engineering for Power, pp. 137-141, July
1973.
11. 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.
12. Steam. Its Generation and Use. 39th Edition, Babcock and Wilcox, New York,
1978, p. 16-3.
6-14
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13. Joseph G. Singer, Combustion Fossil Power. 4th Edition, Combustion
Engineering, Inc., Windsor, CT, 1991, pp. 8-16 to 8-26 and 12-22.
14. "Integrated Waste Services, Information Summary," Consumat Systems, Inc.,
Richmond, Virginia, Undated Brochure.
15. "Controlled Air Incineration," Joy Energy Systems, Inc., Charlotte, NC,
Undated Brochure.
16. Municipal Waste Combustion Study. Report to Congress." U.S. Environmental
Protection Agency, EPA-530-SW-87-021-a, May 1987, pp. vi-viii.
17. Floyd Hasselriis, Thermal Systems for Conversion of Municipal Solid Waste,
Vol. 4: Burning Refuse-Derived Fuels in Boilers: A Technology Status Report.
Report ANL/CNSV-TM-120, Vol. 4, Argonne National Laboratory, March 1983,
pp. 1-166.
18. Daniel F. Moats, J. Mathews, and K. C. O'Brien, "A Performance Update for
the Columbus Project," Proceedings of 1988 ASME National Waste Processing
Conference. Philadelphia, PA, May 1988, pp. 181-189.
19. Gary L. Boley, "Startup and Operations of the Mid-Connecticut Resource
Recovery Project," Proceedings of 1990 ASME National Waste Processing
Conference. Long Beach, CA, June 1990, pp. 21-29.
20. "Prepared Fuel Steam Generation System," ABB Resource Recovery Systems,
Windsor, Connecticut, Undated Pamphlet.
6-15
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7. COMBUSTION PRINCIPLES II: THERMOCHEMISTRY
Slide 7-1
THERMOCHEMICAL CONCEPTS
Heating Values & Load
Ignition Temperatures
Combustion Temperatures
Temperature Control Methods
This learning unit introduces the thermochemical concepts of combustion which
have relevance to plant operations. Included are: fuel properties, combustion energy
release rates, heat sinks, combustion temperature and temperature control.
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 produce 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 and the HHV will be the same. The HHV of the example MSW of Slide 4-17
was given as 4,830 Btu/lb; its corresponding LHV was computed to be 4,100 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, but some of those used in Europe are based on LHV.
7-1
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Slide 7-3
HEATING VALUES OP SELECTED FUELS1' 2'3
FUEL
Methane
Fuel Oil, #6
Coal, PA Bitum.
Coal, W? Subbitum.
Wood, White Pine
Wood, White Oak
Lignite, ND
MSW, Ames, IA
RDF, Ames, IA
MSW, Ames, IA
Wood, Fresh Cut
HHV
Btu/lb
23,875
18,300
13,800
9,345
8,900
8,810
7,255
6,372
6,110
4,830
4,450
BASIS
Dry
As Received
As Received
As Received
Kiln Dried
Kiln Dried
As Received
Dry
As Received
As Received
As Received
MOISTURE
%
0.0
0.7
1.5
25.0
8.0
8.0
37.0
0.0
6.5
24.2
50.0
The HHVs of various representative fuel samples are presented above. Higher
heating values 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 60°F and atmospheric pressure.
One should note that the moisture content of RDF is less than that of MSW
because some drying occurs during its preparation. The amount of metals recovery
in RDF processing affects not only the ash content but also the heating value.
Slide 7-4
UNIT RATED CAPACITY
MSW Charging Rate
tons/day
Ib/day
Ib/hour
UNIT OPERATING LOAD
Gross Energy Input
Btu/hour
The rated capacity of a MWC unit is generally stated in terms of a nominal
MSW charging rate, which is generally given in tons/day. By contrast, the phrase
"operating load" generally corresponds to the overall energy input, which is based on
the fuel charging rate and its HHV. Load is generally presented in units of Btu/hour.
7-2
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Slide 7-5
UNIT OPERATING LOAD =
FUEL CHARGING RATE X HHV
Example: 500 tons/day Unit
4,500 Btu/lb HHV
UNIT OPERATING LOAD =
500 tons /day x 2,000 Ib/ton x
4,500 Btu/lb x 1 day/24 hours
UNIT OPERATING LOAD
188,000,000 Btu/hr
There is a direct relationship between the fuel charging rate, the HHV of the
fuel, and the overall rate of energy input. The charging rate times the HHV equals
the load. Of course, the units must be properly canceled, as in the above example.
Combustion chambers are generally designed for a given gross energy input
rate. Therefore, as the fuel composition and its HHV changes, the charging rate
under maximum load conditions can change accordingly. For instance, if fuel enters
with a larger heating value, the charging rate should be reduced accordingly.
Slide 7-6
IGNITION TEMPERATURES5
Material
Sulfur
Charcoal
Gasoline
Acetylene
Fixed Carbon
Hydrogen
Methane
Carbon Monoxide
Benzene
Phase at 60°F
& 14.7 psia
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 defined
as the temperature at which rapid combustion in air ignites automatically and
becomes self-sustaining.
7-3
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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). 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
EXAMPLE OF RDF PROXIMATE ANALYSIS
Percentage
by Weight
Moisture 26.6
Ash 21.7
Volatile Matter 43.6
Fixed Carbon 8.1
Total
100.0
Our understanding of combustion can be aided by referring to the parameters
of the proximate analysis. The moisture fraction is the water which vaporizes as the
fuel is heated above ambient conditions. One can assume that the fuel is essentially
dry by the time it is heated to approximately 220° F.5
Volatile hydrocarbon materials are gaseous materials which burn much like
natural gas. These gases are evolved as the MSW is Heated. Volatilization begins
as a straight distillation process. Light hydrocarbons are distilled 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 distillation process is generally completed by
the time the fuel temperature reaches around 1,750° F.6
Fixed carbon is the solid combustible fraction of the fuel. It undergoes burning
on the surface. Solid carbon combustion can occur as the volatile gases are being
evolved, since the ignition temperature of carbon is as low as 765° F.
7-4
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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 a
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 other materials. As such, adiabatic conditions require that
there be no heat loss to the surroundings. Of course, the hot combustion gases
produced under adiabatic conditions could be directed into regions where heat
transfer could occur. Adiabatic combustion conditions can be approximated in an
idealized refractory furnace, with no 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 furnace structure and the heat exchange surfaces. Therefore, controlling
furnace temperature is important from the standpoint of protecting the combustion
unit.
Slide 7-9
COMBUSTION TEMPERATURE CONTROL
Fuel Modulation
Heat Transfer to Surroundings
Heat Sink Materials
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 extraction to the surroundings can be somewhat regulated.
For instance, if the flow of feedwater increases in a furnace with an integral boiler,
the heat transfer to the waterwalls will increase and the combustion temperatures
will drop.
7-5
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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. They may be considered as either undesirable or desirable. The water
in MSW is generally considered to be an undesirable heat sink material, 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. This method is used particularly in the secondary combustion chamber of a
modular incinerator. Under excess air conditions, 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.6
This method is often used in power plants and in some MWCs 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
process, thereby decreasing the local fuel-to-air ratio.7 With lower temperatures, the
velocity of the gases are also reduced.
7-6
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Water can be sprayed onto a fresh charge of waste to slow down the rate of
volatilization of plastics. Otherwise, a burst of volatiles would produce a fuel-rich
mixture which might not burn completely, resulting in opacity problems.
Water is also sometimes sprayed onto the charging ram to prevent fires in the
charging hopper.8
Slide 7-12
STARVED-AIR UNITS
Two Stage Combustion
Lower Velocities in Primary
Primary Chamber: Gasifier
More Primary Air
Higher Primary 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, distilling the volatile gases and mixing them with
less air than would be required for stoichiometric conditions. The process of
distillation of the volatile components is sometimes considered pyrolysis. Although
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.
Adequate provisions for supplying air and 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.
7-7
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Slide 7-13
EXCESS AIR COMBUSTION
Excess Air - Heat Sink
More Excess Air
Temperature Reduction
Excess air combustion is in contrast to starved-air combustion. 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, as will be discussed later, with under-grate and over-fire air and other
considerations.
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. Steam. Its Generation and Use. 39th Edition, Babcock and Wilcox, New York,
1978, pp. 6-6.
5. Joseph G. Singer, Combustion. Fossil Power Systems. 3rd Edition, Combustion
Engineering, Inc., Windsor, CT, 1981, p. D-5.
6. "Nitrogen Oxide Control For Stationary Combustion Sources," EPA/625-5-
86/020, U. S. Environmental Protection Agency, July 1986.
7. 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.
8. 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.
7-8
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8. MSW HANDLING EQUIPMENT
Slide 8-1
SOLID MATERIALS FLOW PATH
1. Weight Scales
2. Tipping Floor, MSW Storage Pit
3. Front-End Processing Equipment
4. Charge Hopper, Feeder Device
5. Combustion Chamber Grate
6. Ash & Fly Ash Collection Devices
7. Ash Removal System
8. Ash Disposal at Landfill/Mono fill
The various elements of the solid materials flow path are listed above. Note
that these elements will take different forms at different MWC facilities. For
example, the full scale front-end processing equipment for RDF will not be found at
a mass burn facility, and the grate in large mass burn units may be replaced by a
hearth with a ram device in modular incinerators.
Slide 8-2
SCALE OPERATOR FUNCTIONS
1. Restrict Delivery to Facility
2. Basis for Tipping Fees
3. Processed Waste
4. Unprocessed Wastes
5. Ash
6. Recovered Materials
Note that the weight scale operation has various important management
functions. Certainly, the weight scale provides the basis for tipping fee collections as
well as a record of the source and weight of material processed by the MWC facility.
The scale operator may have the duty of restricting the delivery of materials
to those which can be appropriately processed by the MWC facility. During periods
of shut-down, the scale operator can provide information about the location of
alternative disposal sites.
The operators will typically weigh the materials being transported "off site."
These can include the ash which is sent to monofills and/or landfills, undesirable
materials which are removed from the waste stream before combustion, and recovered
materials delivered to recycle and reprocessing operations.
8-1
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Slide 8-3
UNACCEPTABLE AND/OR UNDESIRABLE MATERIALS
1. Not Permitted - Hazardous, etc.
2. Cause Damage - Explosion, Breakage
3. Restrict Operations - Blockage
4. Incombustible
Permit restrictions may include provisions which prevent specified materials
from being processed in MWC units. Examples would include hazardous wastes,
radioactive wastes, and "red bag" medical waste. Other materials may include
construction debris and industrial wastes.
Automotive and rechargeable consumer batteries are undesirable incineration
materials because they are sources of some heavy metals emissions. Even with
effective gas cleaning, it would be much better if batteries were recycled or
alternately disposed, since incineration does not destroy the heavy metals.
Among the materials which can cause damage to the unit are the pressurized
gas canister, cans of gasoline, and metal drums of waste oils, solvents, gasoline, and
unknown liquids. These items can lead to explosions in the combustion chamber.
Some very dry materials, like fine sander dust, are capable of burning violently.
Heavy objects like automotive engine blocks can cause damage to the grates
and conveyors. Metal cables and pipes can cause blockages of conveyors and may
damage the grates. Large bulky items such as construction, demolition, and land
clearing debris (e.g., masonry materials and tree stumps) and industrial wastes (e.g.,
rolls of plastic and carpet and miscellaneous chemical materials) are generally
considered undesirable.
Metals and glass are examples of non-combustible materials which, although
detrimental to equipment operations, are often charged into MWC units. Although
some metals are basically unmodified by the combustion process, lead melts at
around 620°F and aluminum melts at around 1,200°F. These temperatures are below
the typical active fuel-bed combustion temperatures.1 Lead contributes to the metal
deposits on heat exchanger surfaces. Melted aluminum can solidify in the air-entry
holes in the grate, restricting air flow through the fuel bed and creating uneven
burning. Routine maintenance may be required to remove such deposits.
Glass melts at around 2,000° F, a common temperature level in combustion
chambers.1 Much of the clinkering on the grate is related to the melting and
solidification of glass.
8-2
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Slide 8-4
ISSUES OF FUEL VARIABILITY
1. Fuel Size
2. Heating Value
3. Volatility
4. Fuel Moisture
5. Ash (incombustibles)
Bulky materials which are combustible include tires, mattresses, and wood
furniture. Their large sizes would restrict the air flow to other combustible materials.
The heating values of component materials have considerable variability. For
instance, plastics have almost twice the heating value of paper. Plastic materials
may appear to undergo pyrolysis instantaneously upon being delivered into the
combustion chamber, whereas other materials may require a considerable drying
period before their combustible volatiles are evolved.
Some materials have a very low heating value because of their high moisture
content. For example, yard wastes have little value as a fuel, although they are
organic materials which can be effectively reduced in MWC units.
Slide 8-5
OPERATING STRATEGIES FOR FUEL VARIABILITY
1. Source Separation
2. Front End Process
3. Mix Wet and Dry Wastes
4. Compensate Through Equipment Design
Source separation techniques in recycling programs can reduce the amount of
glass and metals in the waste charged into MWC units, thereby improving the fuel.
Bottle bills have been able to significantly reduce the glass and aluminum in the
MSW. Some communities limit the acceptance of garbage to those items which can
fit inside a standard container. Separate disposal procedures can be adopted for
undesirable, oversized and/or non-combustible materials, such as waste oils, batteries,
metals and "white goods" (water tanks, stoves, refrigerators).
The front-end waste processing techniques, presented in Learning Unit 3, can
be used to remove undesirable components and produce RDF fuel. RDF processing
facilities may have picking stations adjacent to a conveyor to remove undesirable
8-3
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materials and direct them to alternate disposal sites. TV monitors can help operators
review the waste. Manual controls can be used to stop the conveyor when
inappropriate items are observed.
Front-end processing at mass burn units focuses on the removal of materials
which are untreatable or inappropriate for combustion. Operators of front-end
loaders and cranes generally remove such materials from the tipping floor.
The mixing of MSW before the combustion process is one of the most important
MWC operations. Overhead crane and front-end loader operators should mix wet and
dry materials before they are placed in the charge hopper or feed system. This makes
the fuel more nearly homogeneous, reducing swings in combustion heat release rate.
MWCs are designed to compensate for the expected diversity of fuel size and
material composition. However, most units will have reduced performance associated
with changes in fuel moisture, air flow restrictions, and fuel-bed disturbances.
Specific equipment design features will vary by unit size and local conditions.
Slide 8-6
RECEIVING AND FEEDING EQUIPMENT
GENERAL:
Receiving Area (Tipping Floor)
Storage Pit or Area
MODULAR MASS BURN UNITS:
Front Loader
Hydraulic Ram Feed System
LARGER MASS BURN UNITS:
Overhead Crane & Grapple
Gravity-Fed Charge Hopper
RDF UNITS:
Conveyors & Processing Equipment
Gravity-Fed Charge Hopper
Air Swept Distributor
Modular MWC units have automatic feed systems which include a hatch door,
hydraulic transfer rams, and a moveable fire door. The system is sequentially
operated by a timer. The hatch opens and a charge is placed in the transfer device;
the hatch closes, acting as an air lock; the fire door opens; the ram delivers the
charge into the combustion chamber; the ram is retracted; and the fire door closes.
8-4
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Front loader operators at modular units can generally control the amount of
MSW in each charge. The control board operator can establish the dwell time
between each charging cycle. A water spray may be used to cool the ram and/or fire
doors, thereby preventing the ignition of the MSW in the feed system.
Slide 8-7
GRAVITY FED CHARGE HOPPER3
Courtesy of Detroit Stoker Company.
Large MWC units generally have gravity-fed charge hoppers which deliver fuel
to the combustion grates on a continuous basis. The crane operators may be required
to weigh each grapple of waste that is delivered to the charge hopper. An operator
variable is the height of MSW kept in the charging hopper, with higher levels
resulting in a denser material on the fuel bed and more mass being charged.
Charge hoppers often include provisions for charging rams, resistance doors,
and/or vibrating feed mechanisms. The fresh MSW may act as an air lock between
the inside of the combustion chamber and storage area. The control room operator
may be able to control the operation of charging rams or other feeder mechanisms,
if they are available.
The front-end processing of RDF was discussed in Learning Unit 3. Note that
the receiving and temporary storage area can be either a flat tipping floor or a
storage pit.
8-5
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Slide 8-8
AIR SWEPT DISTRIBUTOR SYSTEM FOR RDF
BALANCED
DAMPER
DISTRIBUTOR
SPOUT
ROTARY
AIR DAMPER
Courtesy of Detroit Stoker Company
The delivery of RDF into the combustion chamber is typically very similar to
that used in spreader stoker coal and wood feed systems.3 The example shown above
delivers RDF into the combustion chamber for suspension burning in the overfire air.
Most of the fixed carbon is allowed to achieve burnout on a travelling grate at the
bottom of the unit.
Slide 8-9
FUNCTIONS OF GRATES & HEARTH
1. Support MSW During Drying
2. Support MSW During Volatilization
3. Distribute Under-Grate Air
4. Stir, Tumble and Mix Wastes
5. Support MSW During Burn-Out
6. Deliver Bottom Ash to Ash Pit
The grate and hearth have the multiple functions indicated above. Radiant
heating from the combustion of gases and appropriate rates of air flow must be
supplied for the grates/hearth to function properly.
8-6
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Slide 8-10
GRATE BURNING COMBUSTION CONCEPT4
RtfuM input
ignition starts
Qnti«Sy*t*m
residue
The four principle activities which MSW undergoes in the fuel-bed on a grate
are: drying, devolatilization, ignition of volatiles and burning of fixed carbon.4
The sketch shows an initial region of unignited refuse. As the MSW is heated,
it first undergoes a drying process. The drying process is driven by the heating of the
fuel bed by radiant energy from the combustion regions. The moisture is transferred
to the under-grate air which diffuses upward through the fuel-bed. Of course, if the
air is pre-heated, the drying will occur more quickly.
Subsequent heating of the fuel bed results in the distillation or gasification of
the volatile matter. The process proceeds with different gaseous materials being
evolved as the bed temperature is increased.
Ignition begins on the top of the fuel bed when the gaseous materials are
heated above ignition temperatures. The burning proceeds downward, with the rate
being limited by the delivery of under-grate air. Fuel bed agitation is also important
because it increases the porosity of the fuel bed. The increased porosity allows more
air to mix with the volatiles, increasing the combustion intensity. This, in turn,
causes greater rates of heat transfer and distillation of volatiles.
8-7
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After the volatilization process has been completed, additional surface burning
of the fixed carbon can occur until the burn-out process is completed and the ash is
delivered to the ash pit.
Other aspects of bed combustion will be presented in the next learning unit.
Slide 8-11
GRATE DESIGNS
1. Reciprocating Stoker Grate
2. Reversed Reciprocating Grate
3. Rocking Grate
4. Vibrating Grate
5. Roller Grate
6. Travelling Chain Grate
7. Refractory Lined Rotary Kiln Grate
8. Rotating Waterwall Grate
A major design consideration for grates is the stirring, tumbling, and mixing
of waste, so as to expose fresh MSW surface area to the air. If such actions are
inadequate, pieces of waste can stick together, forming an impermeable mat of
unburned material. Such unburned material may pass through the combustion
chamber and be observed in the bottom ash.
Air-flow passageways in the grates are designed to slowly diffuse under-grate
air through the MSW in the fire bed. If the air flow is too great, entrainment of the
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 will lead to both lean and rich air/fuel gaseous mixtures being evolved along the
fuel bed. This may be compensated for in the unit*s over-air delivery system design.
8-8
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Slide 8-12
RECIPROCATING STOKER ORATE5
Another purpose of the under-grate air is to cool the grate. Mechanical damage
to an overheated grate can include the spot welding of adjacent metal parts. Some
grates have design provisions for water cooling with passage-ways through the grate.
The rotary waterwall MWC unit is such an example.
Slide 8-13
REVERSED RECIPROCATING GRATE6
FlxM Potoit Hvott
From Proceedings of the 1978 ASME Solid Waste Processing
Conference, printed with permission.
8-9
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Slide 8-14
Slide 8-15
OSCILLATING OR ROCKER GRATE7
BARREL OR ROLLER GRATE5
8-10
-------
Slide 8-16
TRAVELLING GRATE*
RDF
Chute
Air Supply
Courtesy of Detroit Stoker Company
8-11
-------
Slide 8-17
Slide 8-18
ROTATING WATBRWALL COMBUSTOR3'9
rrt»
Courtesy of Westinghouse Electric Corporation
GRATE MALFUNCTIONS
1. Overheating (Thermal Stresses)
2. Corrosion, Erosion
3. Blockage
4. Hydraulic System Problems
5. Deposits from Molten Metal
6. Breakage by Heavy Objects
Malfunctions can also disable the normal movements of grates. These can be
due to overheating (thermal stresses), materials changes, blockages, and hydraulic
8-12
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and/or mechanical system problems. A large bulky item falling on a hot grate can
cause mechanical damage, and thermal stresses can cause materials to fail.
Slide 8-19
ASH REMOVAL LOCATIONS
1. Grate Sittings
2. Bottom Ash
3. Boiler Ash
4. Fly Ash
Ash or combustion residue material is typically collected from the four above-
mentioned areas in waterwall MWC units.10 Modular MWC units will have their ash
collected as bottom ash, boiler ash, and fly ash. Modular units typically do not have
provisions for grate siftings, as an impermeable refractory hearth is used instead.
Slide 8-20
REMOVAL OF GRATE SIFTINGS & BOTTOM ASH
Water-Filled Quench Tank
Submerged Drag Chain Conveyor
Ram Type Ash Discharger
Grizzly Scalper
Belt or Vibrating Conveyor
Magnetic Separator
The grate siftings have their sizes limited by the openings and crevices in the
grate, typically less than 0.5 inch.
By contrast, bottom ash contains both fine fly ash, unburned pieces of
combustible materials like wood and plastic, and the non-combustible materials like
metal cans, mattress springs, car wheels, broken glass, and pieces of concrete.
Both grate siftings and bottom ash are delivered into a quench tank located
under the combustion chamber. In modular units, the quench tank is at the end of
the combustion chamber, and material is delivered there through the action of
hydraulic ram devices.
Because of the size diversity in bottom ash, its removal devices must be robust
in design. A grizzly scalper is sometimes used to remove oversized items from the
conveyor system. Magnetic separation is often used for ferrous metal recovery from
mass burn MWC units.
8-13
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Slide 8-21
REMOVAL OF BOILER ASH & FLY ASH
Collection Hopper
Automatically Operated Air-Locks
Screw or Dry Drag Chain Conveyor
Pneumatic Conveyor
Bucket Elevator
Boiler ash is the particulate material which is deposited in hoppers as the
combustion gases change directions in the convection sections of the boiler. This
material is approximately the size of sand, generally passing a 30-mesh screen.
Fly ash is the fine particulate which is collected in the air pollution control
device. Fly ash typically includes both the fine residue from the combustion process
and the lime or other scrubber reagent materials which are used for acid gas control.
Boiler and fly ash removal from collection hoppers is accomplished with the use
of automatically operated air-locks which are designed to prevent air leakage from
the outside into the flue gas. Extraction and removal of ash from collection hoppers
is accomplished with various devices, including pneumatically operated double gate
valves, screw conveyors, pneumatic extraction, dry drag chain conveyors, and bucket
elevators.
Slide 8-22
MOTOR OPERATED ROTARY AIR-LOCK11
'Ash Hopper
Rotary Valve Air Lock
8-14
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Slide 8-23
ASH REMOVAL SYSTEMS
1. Continuous Operation
2. Intermittent Operation
3. Batch Operation
Boiler ash, fly ash and bottom ash removal systems may be operated
continuously or intermittently, depending upon the design. Because the quench tank
and collection hoppers have substantial capacity, continuous ash removal may not be
required.
Slide 8-24
ISSUES REGARDING ASH DISPOSAL IN LANDFILLS
Environmental Impact
Landfill or Monofill
Leachate Effect on Groundwater
Heavy Metals Concentrations
Fugitive Emissions
Landfills have been used for ash disposal for many years. However, there have
been some very important issues raised about the environmental impact of such
disposal practices.
Currently some agencies have regulations requiring ash to be disposed in
monofills, which are special landfills which receive only ash, whereas others allow for
co-disposal with mixed wastes in modern landfills.12
Monofills may have the same requirements as modern landfills, but organic
materials are kept out. This would reduce the tendency for the organics to
decompose and make acids which would increase the rate of leaching.
Modern 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, some have indicated
that the fugitive emissions may be the major public health risk. These can be
controlled by proper design and operational considerations.
8-15
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REFERENCES
1. Elizabeth A. Bretz, "Energy from Wastes," Power. March 1990, S-l, S-2.
2. Municipal Waste Combustors: Background Information for Proposed Guidelines
for Existing Facilities. U.S. Environmental Protection Agency, EPA-450/3-89-
27e, August 1989, pp. 5-41 and 9-7.
3. Joseph G. Singer; Combustion Fossil Power. 4th Edition, Combustion
Engineering, Inc., Windsor, CT, 1991, p. 12-20.
4. 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, p. 26.
5. "Field and Enforcement Guide, Combustion and Incineration Sources," U.S.
Environmental Protection Agency, APTD-1449.
6. Georg Stabenow, "Design Criteria to Achieve Industrial Power Plant Reliability
in Solid Waste Processing Plants With Energy Recovery," Proceedings of the
1978 ASME Solid Waste Processing Conference. Chicago, pp. 427-446, May
1978.
7. Miro Dvirka, "Direct Co-Burning of Unprepared Municipal Solid Waste and
Sludge," Proceedings of the 1982 ASME Solid Waste Processing Conference.
New York, p. 114, 1982.
8. J. D. Blue et al., "Waste Fuels: Their Preparation, Handling, and Firing,"
Standard Handbook of Power Plant Engineering. Thomas C. Elliott, editor,
McGraw Hill Book Co., NY, 1989, pp. 3-134.
9. 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. 5-52.
10. Manzoor Alam and David J. Schlotthauer, "Planning for Successful Ash
Handling," Solid Waste & Power. August 1990, pp. 30-36.
11. "Control Techniques for Particulate Emissions from Stationary Sources,"
Volume 1, EPA 450/3-8 l-005a, U. S. Environmental Protection Agency,
September 1982.
12. Marc J. Roggoff, "The Ash Debate: States Provide Solutions," Solid Waste &
Power. October 1991, pp. 12-18.
8-16
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9. COMBUSTION PRINCIPLES III: REACTION PROCESSES
Slide 9-1
COMBUSTION REACTION PROCESSES
Oxidation & Reduction
Incomplete Combustion
Reaction Rate Variables
Flame Phenomena
Bed-Burning
Gasification
Oxidation of Carbon Monoxide
This learning unit considers the above applications of combustion reaction
processes, which have special relevance to MWC plant operations.
Slide 9-2
IMPORTANT REACTION CHARACTERISTICS
1. Multiple Reactions Occur in Combustion
2. Reactions May Not 60 to Completion
3. Reactions Are Somewhat Reversible
4. Reaction Rates Increase with Temperature
5. Reactions Are Influenced by Concentrations
6. Reaction Are Limited by Mixing
7. Compositions Vary with Temperature
The completeness of combustion depends on the operating conditions in the
MWC unit. 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
MSW 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.
9-1
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Slide 9-3
REACTIONS OF CARBON AND HYDROGEN IN OXYGEN
C + O2 --- > C02
H2 + 0.5 Oa --- > H3O
0.5 O2 --- > O
C + O --- > CO
2 H2 + O2 --- > 2 H2O
2 H2O --- > 2 OH + H2
CO + 2 OH --- > CO2
HO
The idealized concepts of combustion were introduced in Learning Unit 5. At
that time, we simplified our analysis by assuming that our reactions went 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. These and other reactions may occur
simultaneously, including many which do not go to completion.
Slide 9-4
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, the reason for carbon monoxide
emissions is 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 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 17.
9-2
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Slide 9-5
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 have a final mixture of H2, CO, OH, O, CO2 and H2O.
The products of incomplete combustion from MSW include carbon monoxide
(CO) and other reaction products such as MWC organics, including dioxins and
furans. These will be considered in more detail in Learning Unit 16.
Slide 9-6
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
A number of the operational problems which can contribute to the formation
of products of incomplete combustion are listed above.
Slide 9-7
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.
Every reaction is reversible, to some extent. Therefore, the forward rate is
used to signify the production of product gases, whereas the reversed reaction rate
relates to the dissociation of product gases back to their original form.
9-3
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Slide 9-8
OXIDATION AND REDUCTION REACTIONS
Lean Mixture - Oxidizing Atmosphere
Oxidation Reaction
Converts Reactants to Products
Rich Mixture - Reducing Atmosphere
Reduction Reaction
Converts Products to React ants
The knowledge of oxidation/reduction phenomena has important applications
in MWC 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.
Fireside corrosion of metal surfaces may be caused by cycling between lean
mixture and fuel-rich mixture conditions. Such oxidation/reduction cycling is
probably due to variable fuel properties. 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 or water tubes, thus
destroying the protective metal oxide layer. The unstable chemistry of the wall
surface can lead to metal wastage. Various design changes are available to help
reduce this problem.
The knowledge of the relative reaction rates of oxygen with nitrogen and fuel
has been utilized to develop techniques for nitric oxide control. In particular, a
reducing atmosphere can be created by injecting a gaseous fuel. This can cause the
gas to be oxidized and the nitric oxide to be reduced back to nitrogen gas.
Slide 9-9
REACTION RATE DEPENDS UPON
Temperature
Mixture Concentrations
Stirring Process (Turbulence)
9-4
-------
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 5,
the theoretical 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 9-10
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 MSW 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.
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.
9-5
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Slide 9-11
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 or heat exchanger, the gas temperatures will fall and the
combustion process will essentially stop. When this happens, a local puff of smoke
and/or soot deposit will be formed.
Slide 9-12
BED BURNING PROCESSES
Diffusion Limited Combustion
Volatile Gases
Fixed Carbon
Diffusion Limited Flame
MSW is a highly variable fuel, composed of incombustible materials, volatile
materials which burn as a gas, and fixed carbon (solid) which burns on its surface.
Although the actual burning process proceeds through many steps, only the general
characteristics will be described.
9-6
-------
The actual 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 MSW 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 9-13
IDEALIZED FUEL BED REGIONS1
^ 2.4
H
u.
1.6
UJ
t-
<
ff
O
U
O
CD
ui o.e
u
z
Q r-
0
Location of
— Ignition Front
1 I I I I
.Location of Top of Bad
I _
Dapth of Accumulated
Inert —
Active Burning
Dapth
1OOO 2OOO 30OO
TIME (SEC)
4OOO
Many MWCs accommodate these combustion features by initially providing a
thick fuel bed which slowly moves down the grate. The grate has provisions which
enable the diffusion of air through the fuel bed.
The slide 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
grate (or the distance down the grate) increases.
The illustration, as well as that of slide 8-10, emphasize that ignition starts on
the top of the .fuel bed surface. An ignition front moves downward through the fuel
bed, acting as a diffusion flame which moves toward the available oxygen in the
under-grate air supply. Note that, in the illustration, the ignition front required
almost an hour to get to the grate (indicated by 0 ft above the grate).1
9-7
-------
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 its being dropped into
the ash collection pit.
The location of the active burning region is controlled by the supply of under-
grate air and grate agitation. Movement of the grate can also expose fresh MSW
surfaces, which stimulates the heat transfer, volatilization, and the fuel-oxygen
mixing processes of combustion.
If an operator were to increase the under-grate air supply, it would cause the
burning process to become more intense and lead to additional evolution of volatiles.
Slide 9-14
BASIC BED-BURNING PROCESS
Gaseous Products Leaving Fuel Bed
H2O, CO,, CO, Methane, Hydrogen
Solid Products:
Char (Fixed Carbon)
Solid Residues
Inorganic Materials (Ash)
The MSW fuel bed can be characterized as a gasifier. The active burning
region uses up all the available oxygen. As other volatile gases are evolved, they will
migrate into the combustion chamber region above the fuel bed. Many of these
volatiles will have been heated above their ignition temperatures in the fuel bed, but
will lack the oxygen for combustion.
Thus, an oxygen-starved mixture of hydrocarbons and products of complete and
incomplete combustion will be formed. The gases flowing from the top of the fuel bed
have been found to contain significant levels of methane (hydrocarbon), hydrogen, and
carbon monoxide, but are essentially depleted of oxygen.1 These gases will migrate
upward and leave behind the ash and some of the char (carbon).
Therefore, the products of incomplete combustion in the flue gas will tend to
be increased when the under-grate air is increased.
However, combustion controls generally provide a simultaneous addition of
overfire air along with under-grate air. The overfire air is directed into the
combustible gases to supply the required oxygen and to increase their mixing.
Therefore, the over-bed combustion zone acts as a second stage of combustion.
9-8
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Slide 9-15
REACTIONS
C H
C H
C H
h Oa —
K HaO --
h C0a —
WITH CHAR:
> C0a
-> CO + Ha
-> 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
endothermic (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 9-16
DESTRUCTION OF CARBON MONOXIDE
CO
CO
CO
+ OH -
+ 2 OH
+ 0 -
--> H + C02
> C0a + HaO
--> C0a
(1)
(2)
(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 C02 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 excess air was added to the combustion gases, the temperature
could be lowered 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.
9-9
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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.
9-10
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10. DESIGN AND OPERATION OF COMBUSTION EQUIPMENT
Slide 10-1
MWC DESIGN OPTIONS
1. Fuel Processing
2. Charging Method
3. Stoichiometric Design
4. Chamber Hall Construction
5. Energy Recovery Design
There are five overall design options which can distinguish the combustion
features of a MWC unit. These are listed above.
As discussed in Learning Units 3 and 4, MWCs are generally distinguished as
being either mass burn or RDF units.
Slide 10-2
CHARGING METHOD
Direct Bed
Suspens ion-Fired
Air-Swept Spreader
As was discussed in Learning Unit 8, the most common method of delivering
raw MSW into the combustion chamber is direct bed charging. The MSW is charged
onto the grates or hearth where it goes through a sequence of drying, volatilization,
ignition, burning, and burn-out processes.
Suspension firing is the major method used in firing pulverized coal. The coal
is ground to a fine powder and then is blown into the hot combustion chamber where
it burns in a manner similar to fuel oil. Early attempts at pulverized-type suspension
firing of RDF were unsuccessful because of the incomplete burning of the large pieces
of RDF. Therefore, suspension burning designs generally have provisions for a
travelling grate which can provide an appropriate residence time for the burning of
the large pieces of RDF.
Spreader stokers were originally designed to deliver chunks of coal into the
over-bed combustion zone, with the small particles burning in suspension and the
larger pieces settling onto a fuel bed. The firing of RDF in a spreader stoker unit is
very similar. Generally, an air-swept distributer blows RDF into the combustion zone
where it is rapidly exposed to hot gases. Drying, volatilization, ignition, and burning
appear to occur almost simultaneously. The larger pieces of RDF fall upon the
combustion bed, where additional volatilization and carbon burnout occurs.
10-1
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Slide 10-3
STOICHIOMETRIC DESIGNS
Excess-Air
Starved-Air (Two-Stage)
There are two categories of stoichiometric air designs in MWC systems-
excess-air and starved-air. The method of construction is also a distinguishing factor.
Large units are generally erected through on-site construction and use excess-air
designs which feature burning of MSW on grates.
Excess-air, waterwall units have burning on the grates and in the over-bed
furnace region above the grates. A relatively high supply of underfire air causes
intense combustion on the grates, which aids in carbon burnout and increases the
amount of material throughput.
The gases in the over-bed region of an excess-air unit may have local
stoichiometries which vary considerably. -Such variations are due to both the fuel
composition and different gas compositions leaving the drying, volatilization, and
combustion regions of the grates. Mixing of the gases is enhanced by the combustion
chamber geometry and the over-fire air designs.
About 40 percent of the total air is supplied as over-fire air. Combustion is
typically completed in an overall mixture having somewhere between 70 and 100
percent excess air.1
Slide 10-4
STARVED-AIR, TWO-STAGE COMBUSTION UNIT2
To Bator
-FosaH Fuel Burner
/-Primary Chamber
Feed Ram
AthSurnp
Ach Transfer Rama
Air Tuba
A* Discharge Ram
AahChuM
A«h Quench
Courtesy of Consumat Systems, Incorporated
10-2
-------
Smaller units utilize two combustion chambers which provide for the
application of two-stage combustion concepts. These units are often assembled on-site
from modular components which are manufactured in a factory.
The primary chamber is designed to operate with somewhere around 40
percent of the air theoretically required for complete combustion.1 The partial
combustion of the volatile gases and fixed carbon is designed to heat the primary
chamber gases enough to drive the drying and volatilization processes. Thus, the
primary chamber acts as a gasifier.
The secondary chamber receives the fuel-rich mixture and mixes in the
secondary air. Secondary air is added so that the overall air supply is from 80 to 100
percent excess air.1 Additional energy release can be supplied by an auxiliary burner,
whose purpose is to maintain secondary combustion temperature requirements.
Slide 10-5
ENERGY RECOVERY DESIGNS
Fire-Tube Boiler
Waste-Heat Boiler
Integral Boiler
There are three different types of boilers which are widely used in industry.
Fire-tube boilers are the package boiler units which are often used for
residential, commercial and light industrial applications. They generally burn
natural gas or fuel oil, and are characterized by the combustion gases passing inside
tubes, which are surrounded by water. They are generally not used in MWC
applications.
Integral boilers designs are those which make use of waterwalls. These are
widely used by industries and utilities for the production of steam and electrical
energy. The efficiency of conversion of fuel energy (higher heating value) to steam
energy varies with the fuel and design application. Example values under baseload
conditions are: 88% for pulverized bituminous coal, 75% for RDF, and 70% for MSW.3
Additional consideration of integral boilers will be included in the following discussion
of waterwall units.
10-3
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Slide 10-6
WASTE-HEAT BOILER4
A special add-on heat exchanger uses hot flue gas to evaporate water and
produce steam. Such boilers are often called waste-heat boilers, since otherwise the
energy in the hot flue gases would be wasted. Heat exchange is by convection from
the combustion gases which exit the final combustion chamber. As such, recovery
boilers are not exposed to gas temperatures as high as those of integral boilers.
Slide 10-7
CHAMBER WALL CONSTRUCTION
Refractory-Wal1
Waterwall
Combustion chamber designs are characterized by their provisions for heat
exchange. Walls can be insulated against heat loss or can provide for energy
extraction. Refractory walls insulate the combustion gases against heat loss and
reflect radiant energy back to the combustion zone and onto the fuel bed. Early
refractory-wall incinerators used considerable amounts of excess air to lower
combustion temperatures, so as to prevent significant refractory damage.
Modern starved-air refractory-wall units have their secondary chamber
temperatures controlled either by limiting the rate of air and/or fuel supplied or by
using flue gas recirculation as an inert heat sink material. Gas temperatures in the
secondary chambers of modular starved-air units are generally limited to around
2,200°F to prevent damage to the refractory materials.
10-4
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Slide 10-8
GRATE /ROTARY KILN REFRACTORY-WALL COMBUSTOR5
A different design of a refractory-wall incinerator is illustrated above.
Although the unit has a rotary combustor, it has many different features from the
rotary waterwall units, discussed in Learning Units 6 and 8.
The refractory unit has provisions for drying and ignition of the MSW on a
grate, diverting part of the gas flow to a by-pass duct, obtaining final burn-out in a
refractory-lined rotary kiln, and using a waste-heat boiler for energy utilization.6
Slide 10-9
WATERWALL FURNACE ENCLOSURE7
Courtesy of Detroit Stoker Company
10-5
-------
Water-walls are designed to provide for direct heat exchange from the
combustion chamber. The heat extraction helps to limit the combustion
temperatures. Therefore, less excess air is used than would be required in the
refractory-wall incinerators.
The operating temperatures of the waterwalls are generally established by the
saturation conditions associated with the boiler pressure. In the early 1960s many
waste-to-energy units had waterwall saturation temperatures as high as 500°F (690
psig boiler pressure), with superheat temperatures as high as 875°F.8 However, many
of these units experienced significant waterwall and superheater metal wastage
problems, associated with either corrosion and/or erosion.
Slide 10-10
DOUBLE-PASS RADIANT SECTION WATERWALL UNIT3
From Joseph G. Singer, Combustion Fossil Power. 4th Edition, 1991,
reprinted with permission of Combustion Engineering, Inc.
Waterwalls are typically covered with a special refractory material or nickel-
based alloy, particularly in the regions below the overfire air ports. The trend has
been to provide waterwall protection and reduce the gas velocities and temperatures.
Typical boiler saturation temperatures can range from 500 to 550°F (650 to 900 psig
boiler pressure)3 and superheater temperatures are generally limited to around 900°F.
10-6
-------
The above slide illustrates the design of a furnace with a double-pass radiant
heat exchanger. The unit is designed to extract more energy in the radiant section,
so as to limit the flue gas temperatures and control superheater metal wastage.
Slide 10-11
INTEGRAL BOILER*
An integral boiler uses the combustion chamber as an integral part of the
boiler. The combustion chamber or furnace is also called the radiant boiler section,
as radiation heat transfer is its primary mechanism of heat exchange. 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.
Slide 10-12
BOILER COMPONENT EQUIPMENT
Radiant Section
Feedwater Heating
Evaporation
Convective Section
Superheater
Evaporator
Economizer
Feed-Water Heater
The walls of the furnace heat the feedwater to boiling conditions and provide
energy for the evaporation process.
10-7
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After the combustion gases leave the radiant waterwall section, they pass into
the convective section of the boiler. The gases, which are also called flue gases, now
pass over the exterior of tubes which contain steam, liquid water, or both. Among the
possible boiler components which can be found in convection sections are the
evaporator or steam generator, superheater, reheater and economizer.
Boiling occurs in the tubes of the evaporator or steam generator. The
evaporator tubes contain steam (water vapor) and liquid water at the saturation
temperature. Steam is separated from boiling water in the steam drum, which is
generally located at the top of the convection section.
The superheater heats water vapor to temperatures above saturation values,
as may be required by the steam turbine or other steam application. A reheater is
basically another superheater, which is often found in utility power plants. After
steam leaves the high pressure turbine, it can be reheated before it is delivered to the
intermediate pressure turbine. Most MWC units do not have reheaters.
The economizer is a heat exchanger in which the feedwater is heated with
energy from warm/hot flue gas. The economizer is generally located in the convective
section, downstream of the superheater. Prior to this stage, the flue gases have been
considerably cooled in the evaporator and superheater, so that additional steam
production is not feasible. However, the feedwater temperature is relatively cool so
that heat transfer can occur, thereby increasing the overall unit efficiency.
Another heat exchanger, often located in the convection section, is an air
preheater. Although at this point the flue gases are too cool to effectively heat
feedwater, they are hot enough to preheat the combustion air. Other MWC facilities
may use steam for air preheating. Air preheating is important because heated air
will aid the MWC drying process and make combustion easier to control.
Slide 10-13
FEEDWATER HEATING
Economizer: Energy from Flue Gas
Feedwater Heaters: Energy from Steam
Closed Feedwater Heater
Shell & Tube Heat Exchanger
Open Feedwater Heater
Deaerating Heater
Many MWC units have feedwater heaters which use steam extracted from the
turbine to heat the feedwater. Feedwater heaters improve the overall efficiency of
the unit. Most feedwater heaters are called "closed feedwater heaters" because the
10-8
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steam and feedwater are on opposite sides of a "shell and tube" heat exchanger.
Typically, steam condenses on the outside of the tubes (shell side), and the feedwater
passes through the inside of the tubes (tube side).
Open feedwater heaters, such as the deaerating heater described in Learning
Unit 26, have the extracted steam condense and mix with the feedwater.
Slide 10-14
GENERIC TYPES OF COMBUSTION EQUIPMENT
Excess-Air Unit
Mass Burn or RDF
Waterwall and Rotary Waterwall
Integral Boiler
Starved-Air (Controlled-Air) Unit
Mass Burn
Refractory-wall (Modular)
Waste-Heat Boiler
Two generic types of combustion units identified above will be used in order
to consider the major operational features which are most often encountered in
current practice. The terms excess-air and starved-air will be used to identify the
generic types because they relate to the contrasting features of combustion
stoichiometry. The commonly used names are "waterwaU" units and "modular" or
"controlled air" units.
Each vendor will have unique design and operational features, but general
operational considerations can be referenced to one of these generic designs.
Slide 10-15
GENERIC COMBUSTION COMPARISONS
Excess-Air Unit
Gasification & Combustion in Fuel Bed
Complete Combustion in Furnace
Relatively High Gas Velocities
Relatively High Particle Bntrainment
Good Carbon Burn-Out of Residue
Starved-Air Unit
Gasification in Primary Chamber
Relatively Low Gas Velocities
Relatively Low Particle Bntrainment
Acceptable Carbon Burn-Out of Residue
10-9
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Because of the differences in the stoichiometric designs of excess-air and
starved-air units, their gas velocities are very different. Starved-air units provide
less air so that the intensity of combustion and the resulting gas velocities and
temperatures are lower than in excess-air units.
This provides a major operational advantage for starved-air units in that they
have lower particle entrainment than excess-air units. The disadvantage, however,
is that there is considerably more carbon left in the combustion residues. The
condition would be much worse if it were not for the fact that the solids' residence
time in starved-air units is much greater than for excess-air units.
Slide 10-16
OPERATIONAL CONSIDERATIONS
Steady Combustion Temperatures
Steady Energy/Steam Production
Steady Heating of the Fuel Bed
Steady Mixing
Constant Residence Time
In general, the operation of MWC units is improved by maintaining constant
and uniform combustion temperatures. But this goal is difficult to achieve due to the
diversity of MSW constituent materials and their moisture content. If combustion
temperatures vary greatly, there will be corresponding variations in the rates of
steam production, MSW gasification, combustion air requirements, mixing conditions,
gas velocities, and residence time in the combustion chamber.
Nevertheless, if steady combustion temperatures can be achieved, unit
operations will be improved. Steady combustion temperatures lead to steady steam
production and steady heating of the fuel bed. Steady fuel bed heating leads to
reduced variations in the rates of drying and gasification and in the associated air
requirements for mixing and residence times.
10-10
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Slide 10-17
THEORETICAL COMBUSTION EQUILIBRIUM TEMPERATURES
MSW WITH DIFFERENT MOISTURE LEVELS
MPERATURE, DEG F
nj ^j
0-
/
///
v//
I/'
v
r
y//
/x/
V
0.4 0.6
x^*=f:3\
^ I "x^
/
^^
^^
^
^^r
^
!
^-^
^S^
— i i | — i ^j
i
^«-0%water -*- 10% water -*- 20% water
-e- 30% Water -M- 40% water
I j j j
0'.8 1 ^2. 1.4 1'.6 1'.B
STOICHIOMETRIC RATIO
i
2
This figure is presented to provide insight about how MSW will burn under
different air supply conditions. Theoretical combustion temperatures are plotted as
a function of relative amount of air supplied. Various moisture contents in MSW
were selected, with the higher curves corresponding to dryer MSW. The
temperatures in the slide are maximum values which correspond to combustion
without heat loss.
The stoichiometric ratio is defined as the air-to-fuel ratio divided by the
stoichiometric air-to-fuel ratio. As indicated in Learning Unit 5, the stoichiometric
.ir-fuel ratio for MSW is about 3 Ib-air/lb-fuel. Therefore a unit operating with 100
percent excess air would supply about 8 Ib-air/lb-fuel and have 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, as the air
supply is increased the operating conditions will shift to the right.
It may be interesting to note that an example MSW was assumed and the
NASA computer model CET859 used for this illustration. The model included
provisions for simulating the dissociation of water and other compounds which occurs
at high temperatures. This influence gave rise to the unexpected result of the peak
temperatures, occurring under sub-stoichiometric conditions. If complete combustion
had been assumed and the adiabatic flame temperatures displayed, they would reach
their maximums with a stoichiometric ratio of 1.0.
10-11
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Slide 10-18
OPERATIONAL CONSIDERATIONS
EXCESS-AIR CONDITIONS
Increase in Secondary Air Supply:
Decrease in Furnace Temperature
Increase in Fuel Charging Rate:
Increase in Furnace Temperature
STARVED-AIR CONDITIONS
Increase in Primary Air Supply:
Increase in Primary Gas Temperature
Increase in Fuel Charging Rate:
Decrease in Primary Gas Temperature
The previous slide 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 over-fire air and seeing lower combustion temperatures.
The same logic shows that, under excess air conditions, an increase in the fuel
supply rate (decrease the stoichiometric ratio) will increase the temperatures. This
is consistent with grate agitation causing an increase in combustion temperatures.
By contrast, the primary chamber of a starved-air unit is designed to operate
with about 50 percent of the air theoretically required for complete combustion. This
corresponds to a 0.5 stoichiometric ratio. If the primary chamber air supply is
increased (stoichiometric ratio increased) the primary chamber gas temperatures will
increase. This will cause an increase in the volatilization of gases, so that there will
be a corresponding increase in the supply of fuel 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.
10-12
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Slide 10-19
STARVED-AIR UNIT OPERATIONAL CONSIDERATIONS
PRIMARY AUXILIARY FUEL BURNER
Preheat Refractory
Initiate Ignition
Increase Gas Temperature
Increases the Volatilization Rate
SECONDARY AUXILIARY FUEL BURNER
Preheat Refractory
Increase 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 can be used to preheat the
primary chamber and to ignite a fuel charge. It can also maTTitqin adequate gas and
refractory temperatures to control the volatilization process.
The auxiliary fuel burner in the secondary chamber is used to assure that the
secondary chamber temperatures are adequate. By doing so, the chance of a unit's
smoking and emitting products of incomplete combustion are reduced.
Slide 10-20
EXCESS-AIR WATERWALL UNIT OPERATIONS
HEAT TRANSFER
From the Gas Side
To the Water/Steam Side
Many of the operational considerations of MWC units involve maintaining
proper combustion conditions and meeting load demands. Meeting a variable steam
demand is generally difficult with MSW fuel because of its variable properties and
the difficulty in maintaining uniform combustion conditions.
For waterwall units, the operation of the combustion system is closely related
to the boiler operation. Heat transfer ties the combustion conditions to the energy
recovery system. 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.
10-13
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Slide 10-21
EXCESS-AIR WATERWALL UNIT OPERATIONS
TO MEET INCREASED STEAM DEMAND:
Increase Grate Agitation & Dnder-Fire Air
Increases Fuel Supply & Burning Rate
Increases Gas Temperatures & Heat Transfer
Reduce Over-Fire Air (Overall Excess Air)
Excess air can be used to control the combustion temperatures, since it is a
readily available inert heat sink material. Excess-air units have the ability to
independently regulate the underfire and over-fire supplies of air.
If there is an increased demand for steam, the combustion gas temperature will
need to be increased to produce the required radiant heating to the waterwalls of the
boiler. This will generally be achieved by increasing the combustion intensity
through increasing the rate of burning. Increased agitation of the burning section
grates and a corresponding increase in under-grate air will increase the bed
temperature, volatilization rate and burning rate.
The gas temperatures will also tend to increase if the overall stoichiometric
ratio (excess air) is decreased (increased fuel burning rate). This condition will
generally cause increased carbon monoxide emissions.
Similarly, a drop in load will need to be controlled by a corresponding drop in
the combustion gas temperatures, so that the radiant energy extraction and rate of
steam production will be reduced. This may be accomplished with an increase in
over-fire air and a decrease in fuel supply.
Slide 10-22
EXCESS-AIR WATERWALL UNIT OPERATIONS
INCREASED FUEL MOISTURE
Gas Temperature Will Drop
Gas Temperature Can Be Restored
Reduce Air Supply (Excess air)
Increase Fuel Supply (Grate Agitation)
If an increase in the moisture content of the fuel occurs, the extra moisture will
act as a heat sink and cause the gas temperatures to drop. The combustion gas
temperatures may be restored by reducing the over-fire air supply and increasing the
grate agitation.
10-14
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However, there are limits on the amount of swing which can be tolerated in the
air supply or excess air. Control of excess air and the maintenance of combustion
stoichiometry are often used to control nitrogen oxide emissions. A design option is
to provide flue gas recirculation rather than excess air as the inert heat sink
material. These design and control features will be discussed in Learning Units 16,
21, and 22.
Slide 10-23
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 in MWCs has been a serious
operational problem. It generally occurs as a result of corrosion or erosion.
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. Many design options with reduced gas temperatures and
local gas velocities are available.
The metal wastage from 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.
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.3
Corrosion conditions can also be caused by chlorine and sulfur reactions which
appear to catalytically occur within the fly ash deposits on metal surfaces,3 such as
in the superheater. These problems are particularly severe if metal temperatures are
above 900 °F.
10-15
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REFERENCES
1. 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. 3-6, 7-8.
2. "Integrated Waste Services, Information Summary," Consumat Systems, Inc.,
Richmond, Virginia, Undated Brochure.
3. Joseph G. Singer, Combustion Fossil Power. 4th Edition, Combustion
Engineering, Inc, Windsor, CT, 1991, pp. 6-13, 8-16 to 8-18.
4. 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.
5. Hazardous Materials Design Criteria. U.S. Environmental Protection Agency,
EPA-600/2-79-198, October 1979.
6. Municipal Waste Combustors: Background Information for Proposed Guidelines
for Existing Facilities. U.S. Environmental Protection Agency, EPA-450/3-89-
27e, August 1989, pp. 4-84 to 4-118.
7. J. H. Pohl and L. P. Nelson, "Research Required to Generate Power from
Municipal Solid Waste," Report to Southern California Edison Company,
Rosemead, CA, Submitted by Energy and Environmental Research
Corporation, February, 1985, p. 6-47.
8. 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. 5, 149.
9. Sanford Gordon and Bonnie J. McBride, "Computer Program for Calculation
of Complex Chemical Equilibrium Compositions," NASA SP-273, March 1976.
10-16
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11. DESIGN AND OPERATION OF GAS FLOW EQUIPMENT
Slide 11-1
TYPICAL AIR & FLUE GAS FLOW PATH
1. Forced Draft Fan
2. Air Preheater (Air Side)
3. Dnder-Fire Air
Over-Fire Air
4. Furnace (Radiant Section)
5. Convective Section Heat Exchangers
Superheater
Evaporator (Boiler)
Economizer
. Air Preheater (Flue Gas Side)
6. Air Pollution Control Devices (APCDs)
Scrubber (Wet or Dry)
Fabric Filter (Baghouse) or
Electrostatic Precipitator (ESP)
7. Induced Draft Fan
8. Stack
A representative sequence of equipment which comprises the air and flue gas
flow path is illustrated above. Note that the air preheater is listed twice.
Combustion air, after leaving the forced draft fan, is sent through the air preheater.
Later the flue gases pass through the air preheater and provide energy to the
combustion air.
As was indicated in the previous learning unit, some MWCs have steam
operated air preheaters, whereas others may not have air preheaters.
Early MSW incinerators operated without fans like open fireplaces. They
achieved the delivery of air through the phenomena of natural draft. A column of hot
air in a chimney is less dense than the adjacent air. Therefore, natural circulation
causes the hot air to rise and be replaced by cooler air.
Natural draft systems have gas velocities which are hard to control, as they are
directly related to combustion temperatures. A change in operating conditions, such
as the entrance of wet MSW, can cause a reduced combustion temperature and after
a time delay, the air supply will respond to the cooler gas temperatures. Natural
draft systems are also very difficult to control during unit start-up.
Although a tall stack will provide some natural draft, modern MWC units use
both forced draft and induced draft fans. These are required to deliver a controlled
amount of air flow and achieve proper mixing, which are required for complete
combustion.
11-1
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Slide 11-2
AIR & FLUE GAS FLOW PATH OF A RDF UNIT1
Travelling
Grate Stoker
Zone Control
of Underfln Air
\
Air Prtfcater
Courtesy of Combustion Engineering, Incorporated
Both forced draft and induced draft fans are typically used in MWC
applications. If the fan is upstream of the furnace, it is called a forced draft fan. A
forced draft fan can be considered as one that drives air into the furnace. Forced
draft fans are attached to ducts which lead to plenum chambers and distribution
passageways into the furnace. Depending upon the design, more than one forced
draft fan can be used to supply the under-fire and over-fire air.
Induced draft fans act to draw flue gases out of the furnace, past the convective
heat exchangers and air pollution control devices (APCDs). Induced draft fans are
located upstream of the stack, so they also effectively force flue gases up the stack.
If an induced draft fan is not used, the furnace will generally have to operate with
a positive pressure. Such furnaces would be called pressurized.
Balanced draft systems are those with both forced draft and induced draft fans
in the air and flue gas flow path. In general, the forced draft fan provides for the
pressure drop associated with moving air into the furnace, and the induced draft fan
provides for the flue gas pressure drop from the furnace to the stack exit. The
pressure inside the furnace of a balanced draft system is generally slightly below
atmospheric pressure.
Note that in the RDF illustration, the forced draft fan was shown drawing
fresh air from the air preheater and blowing it through separate ducts into the under-
fire and over-fire regions of the furnace. Air preheaters are often used in RDF units.
Some mass burning units do not use air preheaters because of the more intense
burning in the fuel bed which requires that cooler air be supplied to cool the grates.
11-2
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Slide 11-3
AIR & FLUE GAS FLOW PATH OF A MASS BORN UNIT2
SuptrMaur
\
induced Don f «ni
Draft Ftnt
The mass burn unit sketch is typical of many MWC units. Although not shown
explicitly in the sketch, most obtain combustion air from the MSW storage pit area
in order to control odors.
After the combustion process is completed, the flue gases pass across the
various tubes which make up the boiler, superheater, and economizer, and air
preheater. Flue gases then flow into the APCDs. Most of the frictional losses are
associated with moving the gases through the heat exchangers in the convective
section and the flow restrictions of the APCDs (e.g., fabric filters). The pressure
drops along the flow path must be overcome by the induced draft fan, which is
typically located near the base of the stack.
11-3
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Slide 11-4
CENTRIFUGAL FAN WITH INLET VANE DAMPERS3
Single-
Width
Rotor
Inlet Vane Control
Backward Curved
Blades
Courtesy of Babcock and Wilcox
Centrifugal fans, such as that illustrated in the slide, are most often used for
both forced draft and induced draft fans in power plant operations.
There are a number of different fan designs available, including centrifugal and
axial fans. Centrifugal fans can have blade designs with forward curved, radial or
backward curved impellers. The design of the fan determines its performance
characteristics, which are provided by the manufacturer as a fan curve. Some fans
are capable of delivering a fairly large range of flow rates with a modest drop off in
pressure, whereas others have their delivery pressure vary considerably with flow
rate.
Fans with backward curved blades tend to have high energy efficiencies and
are able to produce a flow which is not highly dependent upon the delivered static
pressure.
Axial fans are characterized by a window fan in which the air flows along the
direction of the axis of the fan. Axial fans are much less often used than centrifugal
fans. Some axial fans are controlled by variable pitch fan blades, which can lead to
improved fan efficiencies at low flow rates.
11-4
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Slide 11-5
METHODS OF CONTROLLING AIR FLOW
1. Variable Speed Fan
2. Damper in Duct
3. Variable Inlet Vane Damper
Air flow through centrifugal fans can be controlled by modulating the speed of
the fan, the position of a duct damper or the angle of a variable inlet vane damper.
Automatic controls are typically used to automatically increase or decrease the
control variable and thereby deliver the desired flow rate.
Variable speed fans have some advantages in terms of high fan efficiency, but
have some limitations in terms of performance under modulated flow conditions.
Variable operation can be obtained by using variable speed motors or steam turbine
drives. In addition, a fixed speed motor could be fitted with a mechanical or magnetic
coupling device which turns the fan at different speeds for air flow regulation.
Variable speed motors are fairly expensive, and variable drive units may have
undesirable frictional losses.
A duct damper is a variable restriction which is placed in the duct. Dampers
are often used to control the delivery of under-fire air to the various sections of the
grate. Note that because under-fire flow through the grate sections is divided into
parallel paths, a changed damper setting in one section which restricts air flow will
cause some additional flow in the other sections.
Slide 11-6
VARIABLE INLET VANES FOR CONTROLLING SWIRL4
Air Row
-Inlet Vanes
From Joseph G. Singer, Combustion Fossil Power. 4th Edition, 1991,
reprinted with permission of Combustion Engineering, Inc.
11-5
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Many forced centrifugal fans are primarily controlled by varying the blade
angle on the inlet dampers. This change acts to modify the swirl or aerodynamic
behavior of the flow. In addition, it restricts the effective open area at the inlet.
In general, fans must be designed to meet both the flow and pressure drop
requirements of the air and flue gas flow path. The fen must provide the pressure
to overcome the frictional forces associated with moving the gas through its flow path.
Generally, as the flow rate increases the fan energy requirements go up considerably.
Also, the frictional forces are increased as gas temperature increases.
Slide 11-7
RELATIVE FAN PERFORMANCE CHARACTERISTICS4
Specified Conditions
Static
Efficiency
.85%
Vanes Wide
Open
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170
Percent Volume
From Joseph G. Singer, Combustion Fossil Power. 4th Edition, 1991,
reprinted with permission of Combustion Engineering, Inc.
The designers who select fans must know the required pressure drop associated
with the air and flue gas system. This is generally plotted as a "system curve," which
indicates the amount of pressure drop required to deliver various amounts of flow.
In the slide, an example system curve is plotted with the origin at the lower left
corner. The nonlinear curve illustrates the greater static pressure rise required for
flow increases near the rated flow (100%). System curves are typically plotted on a
manufacturer's fan curve. Designers can correct system curves to accommodate
anticipated changes in gas density or gas temperature by the application of the
general fan laws.5
A family of "fan curves" is shown above, going from left to right with a concave
downward shape. Each fan curve indicates the performance associated with a
specified vane angle. As the vanes close down from 90° the flow will be restricted, as
indicated by a lower intersection with the system curve. The fan curve also indicates
that the fan efficiency deteriorates fairly rapidly (from 87% to 40%) as the vanes are
closed from the wide open (100% flow) to a restricted flow (70% of design flow rate).
11-6
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Slide 11-8
DRAFT
Negative Pressure (Vacuum)
Measured in Inches of water
Must be Maintained in Furnace
Draft is defined as the difference between atmospheric pressure and the
absolute pressure at a point. Furnace draft indicates the amount of vacuum on the
inside of a combustion chamber. Likewise, draft can provide useful information about
the suction capability of a fan. Draft is generally measured with an inclined
manometer or other instrument, with typical units being inches of water column.
A modest amount of furnace draft (furnace under slight vacuum) is the general
design condition of MWC units, with values typically being between 0.05 and 0.5
inches of water. If the furnace pressure becomes greater than atmospheric, hot
combustion gases will leak out through cracks and openings. If this occurs, the
surrounding work environment may accumulate fly ash. Personal injury may also
occur, and the unit could sustain structural damage.
Draft is routinely monitored in a control room to assure safe operating
conditions, particularly in the event that a hatch needs to be opened for inspecting
the furnace wall or combustion conditions. If the combustion unit swings to a
positive pressure when a hatch is open, hot combustion gases could suddenly burst
through the hatch and cause injury.
A unit which is operated with considerably more draft than specified in its
design will probably have higher gas velocities, less mixing, and less complete
combustion than under normal operating conditions.
The restoration of normal draft may be accommodated through use of a damper
or a change in fan operation. Either using a more restrictive damper setting on the
exit side of the furnace or opening the damper on the air supply side could be
considered to restore proper draft conditions. Automatic draft controls are included
in most pneumatic and electronic combustion control systems, as discussed in
Learning Unit 22.
11-7
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Slide 11-9
DEW POINT
Threshold Condensation Temperature
Typically Value is Around 300° F
Fuel Moisture Dependent
Ambient Air Moisture Dependent
Water Spray Dependent
Dew point is the temperature at which condensation begins to occur as the
mixture of gases is cooled. Dew points are often in the range of 225 to 300° F,6 which
compares with the typical values 280 to 320° F for coal firing.4.
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.
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.
11-8
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Slide 11-10
SLAG AND SOOT DEPOSITS
Slag on Combustion Chamber Walls
Soot on Heat Exchanger Surfaces
A routine problem with all power plants is the control of slag and soot deposits.
Slag deposits on waterwall surfaces are caused by deposits of liquid fly ash which
solidify on the relatively cool wall surface. In fossil-fuel fired power plants, the
composition is primarily silicon dioxide, a common inorganic material.
The slag composition in MWC units also includes glass. Significant slag
deposits can occur if particle entrainment is high, fly ash composition is unfavorable,
and gas temperatures are very high. Physical removal of deposited slag can be
accomplished during outage periods for scheduled maintenance. In some units with
considerable slagging problems, some breaking off of slag deposits can occur during
system operations.
Soot is the name given to the solid deposits which routinely accumulate on the
convective heat exchange surfaces. Soot acts as an insulating material, restricting
the useful energy extraction by the heat exchanger. The soot is generally deposited
as a solid, so it is fairly easy to dislodge.
Soot removal is often obtained by routinely operating either pressurized steam
or air operated soot blowers. High pressure jet should not directly impact the
surfaces, however, as they would effectively "sand blast" the surfaces and cause metal
wastage and tube failures.
Many MWC units now use mechanical rappers or acoustic horns. These cause
the metal surfaces to vibrate enough to effectively remove soot deposits, without the
metal wastage problems caused by soot blowers.
Soot should be removed before it accumulates enough to significantly limit unit
performance. The condition of accumulated soot on a heat exchanger can be
monitored by measuring the change of temperatures across the gas side of the unit.
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.
Sequential soot removal of the various heat exchanger sections is typically
provided. This allows the entrained soot to be at a low enough concentration so that
the air pollution collection device can handle the extra partieulate loading without
causing an opacity violation.
11-9
-------
REFERENCES
1. "Prepared Fuel Steam Generation System," ABB Resource Recovery Systems,
Windsor, Connecticut, Undated Pamphlet.
2. 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, p. 86.
3. Steam. Its Generation and Use. 39th Edition, Babcock and Wilcox, New York,
1978, p. 17-6.
4. Joseph G. Singer, Combustion Fossil Power, 4th Edition, Combustion
Engineering, Inc, Windsor, CT, 1991, pp. 14-12 to 14-15,14-36.
5. George Clifford, Modern Heating. Ventilating, and Air Conditioning. Prentice
Hall, Englewood Cliffs, NJ, 1990, pp. 535-536.
6. Municipal Waste Incinerator Field Inspection Notebook. U.S. Environmental
Protection Agency, EPA-340/1-88-007, July 1988, p. 169.
11-10
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12. NSPS: GOOD COMBUSTION PRACTICES
Slide 12-1
NEW UNITS:
EXISTING:
NEW SOURCE PERFORMANCE STANDARDS
EMISSION GUIDELINES
Emission Limitations
Good Combustion Practices
Continuous Monitoring Systems, CEMs
The NSPS and EG set federal limits for maximum allowable air pollutant
emissions and require that good combustion practices (GCP) be followed.1>a GCP
requires the continuous monitoring of emissions and combustion conditions. The
measurement principles and the operation, calibration and maintenance requirements
for continuous monitoring systems (CEMs) are presented in Learning Unit 14.
Slide 12-2
TECHNOLOGY-BASED EMISSION GROUPS
MWC Metals
MWC Organics
MWC Acid Gases
The federal NSPS and EG include standards which are technology-based,
rather than based on health effects. Technology-based emission limits are based on
the best demonstrated control technology.
The regulated pollutant, MWC emissions, is categorized into three sub-classes
of emissions from MWC units by the NSPS.1 MWC metals include the pure metals,
organometallic compounds, and inorganic metal compounds emitted in the exhaust
stack. Such metals of concern include: mercury, lead, cadmium, chromium, nickel,
and beryllium. These are commonly referred to as "heavy metals".
MWC organics include the products of incomplete combustion (PICs) which are
organic compounds. These include dioxins and furans which are carcinogenic
materials. Federal regulations are based on the total of tetra-through octa-
chlorinated dibenzo-p-dioxins and dibenzofurans.
MWC add gases include sulfur dioxide and hydrogen chloride. Such acid gases
are controlled by acid gas control equipment presented in Learning Unit 20.
12-1
-------
Slide 12-3
SURROGATES
For MWC Metals:
* Particulate Matter, PM
* Opacity
For MWC Organics:
* Dioxin/Furan (PCDD/PCDF)
* Carbon Monoxide
For MWC Acid Gases:
* Sulfur Dioxide
* Hydrogen Chloride
Surrogates are substitutes which are used because the emissions of interest
can not be directly measured. Values of surrogates are directly related to the
emissions of concern. The NSPS and EG specify surrogates for MWC metals, MWC
organics and MWC acid gases.
Slide 12-4
NSPS/EG Emission
Emission
MWC Organics
(PCDD/PCDF)
ng/dscm . .
MWC Metals (PM)
mg/dscm . .
Sulfur Dioxide
% Removal . .
ppm-volume . .
Hydrogen Chloride
% Removal . .
ppm-volume . .
Nitrogen Oxides
ppm-volume . .
* Note: Limit for
New
Unit
>250 tpd
. . 30
. . 34
. . 80
. . 30
. . 95
. . 25
. . 180
RDF Stoker
limits1'2
Existing
Unit
>250 tpd
125*
69
50
30
50
25
NA
Unit is 250
Existing
Facility
>1100 tpd
60
34
70
30
90
25
NA
mg/dscm
12-2
-------
For the annual stack test, particulate matter (PM) is used as the surrogate for
the MWC metals, with the exception of mercury. During unit operations opacity is
the continuously monitored surrogate for MWC metals. The opacity limit is generally
10 percent, based on a six-minute averaging period.1
Sulfur dioxide and hydrogen chloride are the surrogates for MWC acid gases.
They can be monitored continuously. The control of SO2 and HC1 can be satisfied by
either limiting the emissions to the ppm levels indicated or by removing the indicated
percentage of the upstream gas. All concentrations are to be corrected to 7% oxygen
and standard conditions at 68°F and 14.7 psia.
The federal NSPS and EG do not require monitoring of HC1, but HC1 control
is to be demonstrated during the annual performance test. The federal EG do not
set standards for NOx emissions from existing units and do not require monitoring
of NOx. However, some state regulations require both HC1 and NOx monitoring at
existing as well as new units.
Slide 12-5
NSPS /EG Carbon Monoxide
Type of MWC Unit
Modular
Mass Burn Waterwall
Mass Burn Refractory
Mass Burn Rotary Waterwall
RDF Stoker
Coal /RDF Co-Fired
Bubbling Fluidized Bed
Circulating Fluidized Bed
Limits,
New
50
100
100
100
150
150
100
100
ppm1'2
Existing
50
100
100
250
200
150
100
100
Dioxins and furans are the surrogate for MWC organics, as they can be
measured directly during an annual stack test. During unit operations, carbon
monoxide, opacity, load, and APCD inlet temperature are used as its continuous
monitoring surrogate.
Carbon monoxide emission limits depend upon the type of unit, which is
illustrative of a technology-based standard. Carbon monoxide is also a toxic material,
but the above emission limits are more restrictive than would be required on the
basis of the health effects of carbon monoxide alone. The above CO limits are based
on a 4-hour averaging period, except for RDF stokers and mass burn rotary waterwall
units, whose limits are based on a 24-hour average.
12-3
-------
Slide 12-6
GENERAL COMBUSTION SYSTEM CO - Oa RELATIONSHIP3
A - INSUFFICIENT AIR
B - APPROPRIATE OPERATING REGION
C - "COLD BURNING"
0 3 6 » W
OXYGEN CONCENTRATION
For each combustion system, there is a general relationship between the
emissions of carbon monoxide and the amount of excess air.3 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.
Slide 12-7
EXAMPLE CO - Oa RELATIONSHIP FOR RDF UNIT4
800
~ 600
400-
200-
I
8
6 a
o2e%)
10 12
14
12-4
-------
GEMS instrument readings can be plotted to show the relationship between
CO and O2. Slide 12-7 was obtained at an RDF unit. Note that the CO
concentrations corrected to 7% oxygen,4 using computational techniques which will
be developed in Learning Unit 15.
Slide 12-8
PARAMETERS MONITORED FOR GCP
Carbon Monoxide
Opacity: Not to Exceed 10%
Load: Not to Exceed 110% of Load
of Most Recent Dioxin Test
Temperature of Flue Gas into APCD
Not to Exceed by 30°F
That of Most Recent Dioxin Test
The NSPS and EG set the operating requirements for good combustion
practices (GCP) to assure that MWC organic emissions are acceptable. Good
combustion practices are satisfied by MWC units which operate under specified
limitations for carbon monoxide, opacity, load, and flue gas temperature at the
entrance of the air pollution control device (APCD) which removes particulates.
The load limit is keyed to the annual stack test for dioxins. There is evidence
that dioxin and furan emissions tend to increase if units are overloaded. This
condition may be related to the air supply operating at full capacity. By increasing
the load, more fuel-rich pockets of gas can pass through the combustion chamber
without complete combustion.
The load limit is monitored as steam flow rate at waste-to-energy units. At the
smaller MWC units or refractory MWC units which do not have provisions for energy
recovery, a load limit is not required. This is primarily because of the difficulty in
monitoring load or heat release in such units.
The temperature requirement at the entrance of the particulate air pollution
control device (APCD) is based on evidence that dioxin and furan emissions are
catalytically formed on the surface of fly ash. Such catalytic reactions increase with
temperature and are also time dependent. If the collected fly ash is held at too high
a temperature in the APCD, dioxin and furan emissions have been demonstrated to
increase. Maintaining a temperature limit is more practical than controlling the
retention time of fly ash in the APCD.
12-5
-------
Slide 12-9
COMBUSTION CONDITION INDICATORS
Opacity
Temperature
Furnace or Primary & Secondary
Flue Gas Entering APCD
Draft
Carbon Monoxide
Carbon Dioxide
Oxygen
Steam Flow Rate (Load)
Combustion conditions can be evaluated through use of a number of
measurable parameters. The specific set of OEMs will vary from one unit to another.
Small units tend to have fewer monitored parameters than larger units would have.
For example, flue gas monitors for carbon dioxide, carbon monoxide, and/or
oxygen can be used to provide relative indication of the amount of excess air in the
combustible mixture. Some small combustion units will measure only carbon dioxide,
whereas the larger units typically will be required to monitor oxygen.
The oxygen level can also be used as a control signal in an oxygen trim control
unit which controls air flow to a desired amount of excess air. Such units should be
able to provide adequate air for combustion without either causing a significant
increase of pollutant emissions or impairing the unit's operating efficiency.
Slide 12-10
MODULAR UNIT COMBUSTION
PARAMETER
Opacity, %
Primary Temp . , F
Secondary Temp . , F
Draft, in w.g.
APCD inlet Temp., F
Oxygen, %
Carbon Monoxide, ppm
INDICATOR
LOW
0
1,200
1,800
0.05
6
0
RANGES3
HIGH
10
1,400
2,200
0.15
450
12
50
The above range of parameters are often indicators of acceptable combustion
conditions in modular starved-air MWC units. For example, a modular starved-air
12-6
-------
unit operating with 15 percent flue gas oxygen would have too much excess air.
Unless very dry MSW was available, auxiliary burners would be required to achieve
the recommended temperatures under such excess air conditions.
Slide 12-11
MASS BURN WATERWALL COMBUSTION
PARAMETER
Opacity, %
Furnace Temp, at
Mixed Height,
APCD Inlet Temp.,
Oxygen, %
Carbon Monoxide,
Fully-
°F
r °F
ppm
LOW
0
1,800
6
0
RANGES3
HIGH
10
2,000
450
12
100
Slide 12-12
RDF WATERWALL COMBUSTOR RANGES3
PARAMETER
Opacity, %
Furnace Temp, at
Mixed Height,
APCD inlet Temp.,
Oxygen, %
Carbon Monoxide,
Fully -
°F
, °F
PPm
LOW
0
1,800
3
0
HIGH
10
2,000
450
9
150
Mass burn and RDF fired MWC units have somewhat similar combustion
operating ranges. Note that RDF units typically uses less excess air (oxygen).
12-7
-------
REFERENCES
1. 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.
2. U. S. Environmental Protection Agency, "Emission Guidelines; Municipal
Waste Combustors," Federal Register. Vol. 56, No. 28. February 11, 1991, pp.
5514-5527.
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, 3-13, and
7-8.
4. James D. Kilgroe et al., "Combustion Control of Organic Emissions from
Municipal Waste Combustors," Combustion Science and Technology. Vol 74,
1990, p. 237.
12-8
-------
13. INSTRUMENTATION I: GENERAL MEASUREMENTS
Slide 13-1
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(CEMS)
through use of a continuous data telemetry system.
13-1
-------
Slide 13-2
GENERAL MEASUREMENTS
1. Temperature
2. Pressure
3. Flow Rate (Velocity)
4. Weight
The basic instrumental measurement techniques used in MWC 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 14, and the use of such measurements in
automatic control systems will be presented in Learning Unit 22.
Slide 13-3
TEMPERATURE EQUIVALENTS
°C = (5/9) (°F - 32)
°F = (9/5) °C + 32
°K (Kelvin) = °C + 273.15
°R (Rankin) = °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 13-4
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
Acoustic Temperature Probe - Speed of Sound
Temperature Paint - Change of Color
13-2
-------
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 the phenomena, which some materials exhibit,
whereby their color is dependent upon their temperature. An object coated with such
a paint will change colors upon being heated above a certain temperature range.
Slide 13-5
THERMOCOUPLE TEMPERATURE MEASUREMENT DEVICE1
Mill! voltmeter
(cold junction-'
compensation)
xlron
£2
t4
•
L *» Lead wire
d^5
XCold
junction
Hot
junction
^-Const antan
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 because of their high temperature and rapid
response capabilities. When the ends of two different metals, such as iron and
constantan, are joined together, a small electrical voltage is produced which is
proportional to the "hot junction" temperature. Combinations of other metals can be
selected depending upon the junction temperature range and the desired accuracy.
13-3
-------
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.
Thermocouples can be obtained in protective sheathes which protects the hot
junctions from corrosion and/or oxidation. In MWC applications, they may routinely
fail after extended exposure to combustion gases or rapidly changing temperatures.
Boiler furnace temperature is commonly measured with a modified
thermocouple device called a water-cooled suction pyrometer. Due to the large
radiation heat loss from a hot junction to the surrounding cold wall, an unshielded
thermocouple will measure temperatures several hundred degrees lower than the
actual gas temperatures, particularly if gas temperatures are above 1,500°F. To
minimize the loss, the hot junction is shielded and flue gas is rapidly aspirated
through the measuring device, so that heat transfer from the hot gas to the
thermocouple is greatly increased. The entire probe is generally water-cooled to
protect the probe and to prevent it from sagging when it is inserted into the furnace.
Slide 13-6
ACOUSTIC TEMPERATURE PROBE APPLICATION3
(Typical Arrangement for 6 Port Mapping)
Processor
Furnace
d
A
---N-i -r--*
V
Wave-Guide
Transducer
Units
13-4
-------
Acoustic temperature probes employ the effect of the speed of sound being
influenced by the temperature and composition of the gas through which it passes.
Once calibrated, it can measure an average temperature of the gas in the path of
sound travel. The acoustic probe usually consists of a microphone and a receiver
arranged on opposite sides of the furnace. The acoustic temperature probe technology
is still being developed, but when fully operational, it promises to be a very useful
device for mapping the temperature patterns of a boiler.
Slide 13-7
MANOMETER PRESSURE MEASUREMENTS3
P,
3
- 4
- 3
- 2
- 1
1 -
2-
Fluid 1
Low density
\&
Fluid 2
High density
U-Tube Manometer
Single-Leg Manometer
From Edgar E. Ambrosius et al., Mechanical Measurement and
Instrumentation. Ronald Press, New York, 1966, printed with
permission.
Pressure is the force per unit area exerted by a fluid, which is typically
indicated in units of [Ib/in2]. Pressure is generally measured with a gage relative to
the atmospheric pressure. Therefore, absolute pressure, which can be indicated as
"psia", is equal to gage pressure, "psig", plus atmospheric pressure, "patm".
Traditionally, the gage pressure is the positive amount of absolute pressure above
atmospheric conditions, whereas the vacuum pressure is used to indicate the negative
gage pressure associated with an absolute pressure below atmospheric pressure.
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
13-5
-------
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, du 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.
Slide 13-8
BOURDON TUBE GAGE1
Bourdon tube
Scole
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.
There are various designs of these gages, but the operating principle is that flattened
and curved tubes will change their curvature as internal pressure increases. As one
end of the tube is fixed, the displacement of the other end is transmitted through
mechanical linkages to a pointer. Pressure readings are proportional to the pointer
displacement, so that, after calibration, the units make reproducible measurements.
13-6
-------
Slide 13-9
LVDT DIFFERENTIAL PRESSURE CELL4
t
Primary
I
PreMtire
portp,
L
M
Secondary
J
.Pressure
portp.
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 particularly 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 LVOT 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.
Slide 13-10
PRESSURE TRANSMITTER
Low Voltage Electrical and
Low Pressure Pneumatic Signals
Easy to Transmit
Safety Considerations
13-7
-------
The previously described Bourdon gages may be used to measure high
pressures of pumps and other equipment.
However, safety and operational considerations dictate 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 13-11
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 & 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.
13-8
-------
Slide 13-12
PITOT STATIC TUBE1
•Static opening
^—Impact
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.
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.
The major disadvantage of the pitot tube is that the velocity indicated is the
local velocity at a point, rather than the average fluid velocity which is generally
more useful for combustion control.
A slight modification of the pitot tube device is used as part of the standard
EPA Method 5 stack sampling equipment.
13-9
-------
Slide 13-13
ORIFICE PLATE - PRESSURE DIFFERENCE1
Flange
Orifice plate
t ^?
Upstream
tap
Vena contracta
Downstream tap
From Robert T. Carry 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).
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.
13-10
-------
Slide 13-14
PROPELLER TYPE FLOWMETER1
..... '
Bearing,. 1 p
i I
Propeller-""}"'" «|
I \3/
Magnetic Jp
sensing element-' 1
I
i
1
1 *
J
1
i
1 *
1
1
Amplifier — * R
ecorder
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.
Slide 13-15
ROTAMETER1
Inlet
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.
13-11
-------
Rotameters operate on the principle of balancing of 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.
Slide 13-16
EQUAL ARM BALANCE3
Mu
From Edgar E. Ambrosius et aL, Mechanical Measurement and
Instrumentation. Ronald Press, New York, 1966, printed with
permission.
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.
13-12
-------
Slide 13-17
PLATFORM SCALE LEVER SYSTEM3
Adjustable
counterpoise
for balance
purposes
1 7 3 6 i 9 10
W'pon
weights
"P\athm
8T"
(5F*2
From Edgar E. Ambrosius et al., Mechanical Measurement and
Instrumentataon. 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.
REFERENCES
1.
2.
3.
4.
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, pp. 16-9 to 16-16.
"Gas Temperature Measurement by Acoustic Pyrometer," Boilerwatch Model
31AP-H, Scientific Engineering Instruments, Inc., Sparks, NV.
Edgar E. Ambrosius et al., Mechanical Measurement and Instrumentation.
Ronald Press, New York, 1966, pp. 252-255, 360-361.
* -±
J. P. Holman, Experimental Methods for Engineers. MCGraw Hill Book
Company, New York, Fifth Edition, 1989, p. 213.
13-13
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14. INSTRUMENTATION II: CONTINUOUS EMISSION MONITORING
Slide 14-1
CONTINUOUS EMISSION MONITORING SYSTEMS
1. Temperature
2. Fluid Flow Rate (Velocity)
3. Opacity
4. Concentrations of Gases
Learning Unit 13 presented various types of general instrumentation which
can be used to provide combustion units operating and control information. Included
were the continuous emission monitoring system (GEMS) instruments for fluid flow
and temperature typically required at MWC units.
This learning unit presents the design and operational features of instruments
which are used to measure stack opacity and the concentrations of selected gases.
Slide 14-2
TYPICAL CEMS USED AT MWC UNITS
1. Temperature of Gas Entering APCD
2. Steam Flow Rate (Load)
3. Opacity
4. Carbon Dioxide
5. Oxygen
6. Carbon Monoxide
7. Sulfur Dioxide
8. Nitrogen Oxides
9. Hydrogen Chloride
CEMS requirements depend upon the regulatory agency, unit size, and
whether the unit is considered to be new or existing. In addition to those listed
above, CEMS for total hydrocarbon, ammonia, stack gas flow rate and/or pH may be
required.
The NSPS for new MWC units require CEMS for inlet flue gas temperature at
the APCD, steam flow rate (load), opacity, nitrogen oxides, carbon monoxide, and
sulfur dioxide. Although not specifically listed, CEMS for oxygen may be required so
that concentrations can be corrected to the 7% oxygen standard. Alternatively, some
regulations are based on concentrations corrected to the 12% carbon dioxide standard.
14-1
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Monitoring of load is required by the NSPS only if the unit recovers energy.
However, continuous emission rate monitors (CERMs) are currently being developed
to measure flow and provide readings of emission rates rather than concentrations.
Federal regulations applicable to GEMS in MWCs have been published.1"2
These include daily calibration drift tests, accuracy tests, accuracy audits, and gas
cylinder audits. 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 14-3
CATEGORIES OF GEMS
In-situ:
Stack Mounted Analyzer
Extractive:
Sample Flows to Remote Analyzer
Monitoring systems are categorized as either in-situ GEMS or extractive GEMS
according to the location of the gas analyzer.3
The extractive systems remove a continuous sample gas stream using probes,
gas conditioning equipment, and tubing for transporting the sample to a remote
analyzer.
The sensors and analyzers for in-situ analyzers are either mounted within or
adjacent to the gas stream. The typical in-situ instrument will utilize an energy
(light) source which is directed in a beam across the stack to a detector. The detector
can produce an electrical signal whose strength is proportional to the energy received.
The desired measurement can be based on the reduction of energy associated with
gas absorption or particle scattering.
The physical measurement concepts of dispersive and non-dispersive absorption
devices used to measure gas concentration will be presented later.
14-2
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Slide 14-4
SINGLE-PASS OPACITY IN-SITU GEMS
Collimating
lens
\
Collimating
tens
Detector
iiiiiiiiiiiiiiiiiiiiiiiiiiliiiiiiiiiiiiiiiiiiiiiiiimiiiiiiiiiiiiiiiiiiiM-
AC11FPI .'•• : : : EHinK Y
ACROSS-STACK
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
"•asses through stack gases. The visible light attenuation by smoke is primarily due
to the scattering of light by small particulates.
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.3 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.
14-3
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Slide 14-5
DOUBLE-PASS TRANSMISSOMRTER GEMS
Collimating
lens $r ,o
CK*
V
Reflecting
mirror
Light
source
Beam
splitter
• '. 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
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.
14-4
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Slide 14-6
IN-SITU MONOCHROMATOR FOR GAS CONCENTRATION4
UGHT I I
SOURCE |PATHLENGTH|
r-i_l—-I
U
BLOWER
MIRROR PHOTODETECTOR
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 participate 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.
14-5
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Slide 14-7
EXTRACTION TYPE OF 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 GEMS 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 participates 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 14-8
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.
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
14-6
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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 14-9
ABSORPTION SPECTROSCOPY
Dispersive Absorption
Differential Absorption
Nondispersive Absorption
Gas Filter Correlation Method
Absorption spectroscopy is used in both in-situ and extractive GEMS
applications to determine the presence and concentration of specific gases. These
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 energy 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.
14-7
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Slide 14-10
DIFFERENTIAL ABSORPTION EXTRACTIVE GEMS4
SwwuansparertMirT
Measuring (Bum Spttttr) •
•XTL
Cal
Bectrenics
Lanp
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 14-6. 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 H20. 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.
14-8
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Slide 14-11
NONDISPERSIVE INFRARED EXTRACTIVE GEMS4
Beam
Chopper
Saraor
Infrared
Source
Nondispersive infrared analyzers are used extensively for measuring the
concentrations of CO, CO2, NO, and S02. 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.
14-9
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Slide 14-12
NONDISPERSIVE GAS FILTER CORRELATION GEMS5
LIGHT
SOURCE
STACK
BEAM
ALTERNATOR
\ NEUTRAL FILTER DETECTOR
GAS-FILTER
CORRELATION
CELL
Another nondispersive concept is used in the gas filter correlation method.
.-ifter 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 he 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 14-13
OTHER ANALYTICAL TECHNIQUES
Chemi lumine s cence
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.
14-10
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Slide 14-14
CHEMILUMINESCBNCE ANALYZER4
Row Conn* Samptoln
Light is emitted by the chemilnminescent 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 NO2 to NO converter efficiency test is generally established by injecting a
known concentration of NO2 into the supply line. These tests are typically run for 30
minutes to determine any degradation in the ozone driven NO2 to NO conversion
process.
14-11
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Slide 14-15
BLECTROCATALYTIC ANALYZER4
POfOUS
Electrocatalytic analyzers are widely used for in-situ measurements of the
concentration of oxygen in a flue gas stream.
The electrocatalytic analyzer operates as a high temperature fuel cell,
producing an electrical current which is related to the oxygen concentration.
The basis of the process occurs in a special porous ceramic material made of
zirconium oxide. When it is heated to around 1,550 °F, the unit will catalytically
produce oxygen ions.5 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 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.
14-12
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Slide 14-16
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 CEMS includes routine calibration. Calibration of the
individual instruments can be achieved through the use of standard calibrated optical
filters or through the use of zero and span gases, depending upon the design of the
instrument.5 Span gases must be analyzed periodically to assure their validity.6
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 undergoing changes in concentration. CEMS are normally zeroed and
spanned at least once a day, with some units 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.
14-13
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Slide 14-17
PROVISIONS FOR DELIVERY SYSTEM BIAS CHECKS6
POW-A-
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.
A number of new instrumentation initiatives are currently under development.
One includes the possibility of making site-specific correlation tests between GEMS
opacity readings and various emissions which are difficult to measure. If such
correlations are sufficiently valid, the opacity monitor could provide important
indications of unit operations.
14-14
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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. 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.
4. James Jahnke and G. J. Aldina, Handbook. Continuous Air Pollution Source
Monitoring Systems. Technology Transfer, EPA 625/6-79-005, June 1979.
5. 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.
6. John Richards, "Municipal Waste Incinerator Air Pollution Control Inspection
Course," Submitted to U. S. Environmental Protection Agency, June 1991.
14-15
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15. AIR POLLUTION I: INTRODUCTION
Slide 15-1
COMBUSTION SOURCE AIR POLLUTANTS
Fuel Dependent
Combustion Quality Dependent
APCD 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 15-2
FUEL DEPENDENT AIR POLLUTANTS
Acid Gases
Sulfur Oxides
Hydrogen Chloride
Nitrogen Oxides (Fuel NOx)
Metals (Heavy Metals)
Lead
Cadmium
Mercury
Carbon Dioxide
Fuel dependent air pollutant emissions generally can be controlled by either
changing the fuel mixture before combustion or by removing the contaminant from
the flue gas after combustion. For instance, chemically bound nitrogen (fuel nitrogen)
in MSW produces undesirable emissions of NOx. These emissions will be reduced if
yard wastes (which are high in nitrogen content) are removed from the MSW before
combustion. Stack gas cleaning also is an important emission control option.
MSW typically contains modest amounts of sulfur, chlorine, fluorine, and other
halogen elements. During combustion, these elements will form acid gases. Acid
gases create emission problems and tend to cause fire-side corrosion of the metal heat
exchanger surfaces. Acid gas emissions from large MWC units are limited through
requiring the best demonstrated control technology. Allowable emission limits are
considerably below those levels allowable in conventional fuel fired power plants.
15-1
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MSW contains trace concentrations of heavy metals such as cadmium, lead,
and mercury. Upon being heated 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. By cooling flue gases adequately, many of the metal
vapors and compounds will condense on the fly ash and be removed by the APCD.
Carbon dioxide is not generally considered to be an air pollutant. It is a
naturally occurring compound which participates in the carbon cycle of organic
growth and decay. However, atmospheric carbon dioxide does act as a greenhouse
gas. 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 create global warming.
Global warming issues relate to the potential effect of increased atmospheric
temperatures on weather, and in particular on polar ice melting, reduced rainfall, and
increased cloud cover. There is general agreement that carbon dioxide emissions
have increased as a result of the combustion of fossil fuels. The reduction or
redistribution of rainfall could cause major agricultural problems.
MSW is generally considered to be a renewable energy source since it is
primarily composed of both organic and waste materials. MSW buried in landfills
will eventually decay and produce methane and carbon dioxide. Therefore, energy
recovery from MSW can help to both conserve fossil fuels and reduce the emission of
the methane, which is an important greenhouse gas.
Slide 15-3
COMBUSTION DEPENDENT AIR POLLUTANTS
Products of Incomplete Combustion (PIC)
Smoke
Particulates
Carbon Monoxide
Volatile Organic Hydrocarbons
MWC Organics
Dioxins & Furans
Nitrogen Oxides
A number of air pollutants can be formed from incomplete combustion of
organic materials. Carbon monoxide, volatile organic hydrocarbons, MWC organics
(including dioxins & furans) and nitrogen oxides will be discussed in subsequent
learning units.
Smoke is composed of small particulates which have the effect of obscuring the
transmission of light (increasing opacity). The particles, which include solid and
condensed liquid materials, actually cause the opacity by scattering light.
15-2
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Slide 15-4
SMOKE & PARTICOLATES
Black Smoke
Carbon in Particulates
Particulates
Removed by APCDs
White Smoke
Condensed Hydrocarbon Gases
Ammonium Chloride
Water Droplets (Not Smoke)
Blue Smoke
Ammonium Sulfate
Brown Smoke
Nitrogen Oxides
Black smoke is flue gas which contains unburned carbon particles. Although
the major constituents of the particulate 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.
Particulate emissions are controlled by APCDs, as will be discussed in
Learning Unit 19. 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 there.
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 scatter light very well. 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 can result from reactions of sulfur oxides with the ammonia or
urea used for NOx control. The reaction produces gaseous ammonium sulfate which
will condense in the atmosphere, so this smoke will appear as a detached plume.
Brown smoke can be caused by NOx emissions and/or particulates. Formation
and control of NOx emissions will be discussed in Learning Units 17 and 21.
15-3
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Slide 15-5
APCD TEMPERATURE DEPENDENT AIR POLLUTANTS
MWC Organics (Dioxins/Furans)
Metal Vapors (Mercury)
The temperature of the flue gas as it enters the particulate collection device
is very important.2 Fly ash collected in APCDs has been found to act catalytically in
the formation of dioxins and furans. These reactions are temperature dependent, so
that cooling the gas before it enters the APCD will limit dioxin and furan formation.
Mercury vapor and other heavy metal vapors in the flue gas will condense or
be adsorbed onto the surfaces of participates as the gas cools. A fabric filter can act
as a sieve to remove such condensate materials. The APCD temperatures are
generally maintained above the dew point to avoid acid gas condensation problems.
Slide 15-6
GAS CONCENTRATIONS:
MOLECULAR FRACTIONS
MOLE FRACTIONS
In Learning Unit 5, we considered complete combustion under stoichiometric
and excess air conditions. We now wish to consider gas concentrations, which may
be fractions of total molecules or mole fractions. The balanced chemical reaction
equation can be used to obtain ideal gas concentrations. A mole fraction for a gas can
be obtained by dividing the moles of the gas by the total moles in the mixture.
Slide 15-7
IDEALIZED
Ci.a5Hs.40,.
1.85 CO3 •»
Product
Gas
C02
H20
N2
HC1
SO2
Total
(STOICHIOMETRIC
oeN.o2Cl.oo6S.oo«+ !•
• 3.92 H20 + 8.15
Wet Gas
Moles
1.85
3.92
8.15
0.006
0.006
13.932
, COMPLETE)
COMBUSTION OF MSW
22 H20+ 2.165 O2+ 8.14 N2 -->
N2 + 0.006
Dry Gas
Moles
1.85
8.15
0.006
0.006
10.012
HC1 + 0.006
Dry Gas
Mole %
18.48
81.4
0.06
0.06
100.00
S02
15-4
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A complete combustion reaction equation for burning
stoichiometric air conditions was presented in Slide 5-24.
MSW under
The product gases for this idealized condition are composed of 1.85 moles of
C02, 3.92 moles of water vapor, 8.15 moles of N2, 0.006 moles of SO2, and 0.006 moles
of HC1. If the water vapor is neglected, the product gas analysis can be performed
on a dry basis. Therefore, there will be 10.012 moles of dry gas.
Slide 15-8
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 concentrations 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 MSW, the dry gas
mole fraction of CO2 is 1.85/10.012 = 0.185, which can be expressed as 18.5 percent.
The mole fraction of both HC1 and S02 in the dry product gases are 0.006/10.012 =
0.0006, which can also be expressed as 0.06 percent or 600 parts-per-million (ppm).
Slide 15-9
GAS CONCENTRATIONS AT STANDARD DILUTION
Example: CO Concentration Limit
50 ppm at 7% O2 on a
Dry Gas Basis
15-5
-------
Gas concentration limits are expressed at standard dilutions in order to
prevent the 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. The early nitrogen oxide data for coal and oil combustion was often presented
with 3% O2 as the basis. The NSPS for MWC units uses 7% Oa as its basis, with
alternative provisions for making corrections to a 12 C02 basis.
Slide 15-10
EQUATION FOR CONVERTING TO 7% OXYGEN2
Assume CO. is the Measured Dry Gas CO
Expressed as a ppm or %
Oa. is the Measured Dry Gas O2
Expressed as a Percentage
CO «? 7% O2 )
CO. x (21 - 7)/(21 -
CO. x (14)/(21 - Oj.)
As indicated in the slide, gas concentrations can be corrected to 7% oxygen by
multiplying the measured concentration by (21 - 7) and dividing by (21 - O^). 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 gas concentration conversion equation is based on
theoretical gas mixture considerations, including the assumption that air is 21%
oxygen on a volumetric basis.2
Slide 15-11
PRODUCT
Gas
C02
H20
oa
N2
CO
Total
GAS ANALYSIS
Wet Gas
Moles
1.0
2.0
0.4
9.024
0.001
12.425
, METHANE (
Dry Gas
Moles
1.0
0.4
9.024
0.001
10.425
a 20% EA
Dry Gas
Mole %
9.59
3.84
86.56
0.01
100.00
15-6
-------
To illustrate corrections of concentrations from the measured values to a
standard basis, let us consider the combustion of methane gas at 20% excess air as
presented in Slide 5-29. Note that for illustrative purposes, 0.001 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 0.4/10.425, 0.0384 or 3.84 percent on a dry gas basis. This is fairly
typical of operating numbers for commercial and industrial gas-fired equipment.
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 either the moles of oxygen or
the total moles in the mixture.
Slide 15-12
CONVERSION OF GAS CONCENTRATIONS TO 7% OXYGEN
Let: COm =100 ppm
°2* = 3.84% (dry gas)
CO «? 7% Oa) = CO, x (21 - 7)/(21 - O^ )
= 100 x (14)/(21 - 3.84)
= 81.6 ppm
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 81.6 ppm when corrected to 7% oxygen.
Slide 15-13
CONVERSION OF PARTICULATES TO 7% OXYGEN
Let: PM. = 0.035 gr/dscf (Particulate Matter)
°2» = 3.84% (Measured Dry Gas O2 )
PM «? 7% Oa) = PM. x (21 - 7)/(21 - Oj. )
= 0.035 x (14)7(21 - 3.84)
= 0.0286 gr/dscf & 7% Oa
15-7
-------
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/dscfl or [mg/dscm].
Slide 15-14
EQUATION .FOR CONVERTING TO 12% CARBON DIOXIDE2
Assume CO.
is the Measured Dry Gas CO
Expressed as a ppm or %
is the Measured Dry Gas CO2
Expressed as a Percentage
CO «? 12% C0a ) = CO. x (12/COa.)
Gas concentrations (e.g., CO in the above slide) can also be corrected to 12%
C02 by multiplying the measured concentration by the ratio of 12% divided by the
measured percentage of CO2. 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.
The logic of the equation is consistent with the fact that if the measured CO2
were less than 12%, the actual volume of mixture would be larger than that
corresponding to 12% CO2, so that the actual gas concentration would be less than
the standard concentration. Therefore, the gas concentration is adjusted by
multiplying by 12 divided by the measured percent of CO2.
Slide 15-15
EXAMPLE CONVERSION TO 12% CARBON DIOXIDE
Let:
CO.
COj.
= 100 ppm
= 9.59% (dry gas)
CO «? 12% CO2 ) = CO. x (12/COj.)
= 100 x (12/9.59)
=125 ppm
15-8
-------
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% C02 yields 125 ppm of carbon monoxide
when corrected to 12% carbon dioxide.
Slide 15-16
CONVERSION OP [gr/dscf ] TO [mg/dscm]
Basic Identities:
1 pound [Ib] = 7,000 grains [gr]
1 pound [Ib] = 453.6 grains [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]
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/dscf] 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/dscf] 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.
15-9
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Slide 15-17
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/dscf] = 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/dscf]
units into [mg/dscm] units, the quantity would need to be multiplied by (2,290
[mg/dscm]/l [gr/dscf]).
Slide 15-18
EQUATION FOR COMBUSTION EFFICIENCY
(BASED ON CARBON COMBUSTION TO CO2)
C.E.(%) = (100% x COj.) / (COa. + CO.)
or
C.E.(%) = 100% x (1 - (CO. / (COa. + 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. The equations
are often referred to as a carbon combustion efficiency, but they actually measure the
carbon monoxide combustion efficiency.
15-10
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Slide 15-19
EXAMPLE COMBUSTION EFFICIENCY CALCULATION
Let COa, be 9.59 Percent
CO. be 0.01 Percent (100 ppm)
C.E.(%) = (100% x COaii)/(C02m + CO.)
= (100% x 9.59)7(9.59 + 0.01)
= 99.9%
The above slide provides an example calculation of combustion efficiency. Note
that carbon monoxide is generally measured as ppm and will require conversion to
a percentage basis.
Slide 15-20
DETERMINATION OF EXCESS AIR2
FROM DRY GAS ANALYSIS
Assume C02, is the Percent Dry Gas CO,
CO, is the Percent Dry Gas CO
O
Therefore Nto = 100 -
And EA =
2, is the Percent Dry Gas O2
CO, + Oa,)
- 0.5 CO,)/(.264 Na, - Oa, + 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.
Next, the excess air expressed as a percentage can be found using the indicated
equation, which is derived using theoretical considerations.2 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 15-11, the CO value of 100 ppm corresponds to 0.01%, which
is much smaller than the other values used in the equation.
15-11
-------
Slide 15-21
EXAMPLE DETERMINING EXCESS AIR
Let CO_ = 9.59%
= 0.01%
= 3.84%
CO
Therefore
And EA =
= 100 - (CO^ + COm + Oj.)
100 - (9.59 + 0.01 + 3.84) = 86.56
- 0.5 CO.)/ (.264 Nj. - O^ + 0.5 CO,)
EA = (3.84 - 0.005)/(.264 x 86.56 - 3.84 + 0.005)
EA = 0.20 --> 20%
The basic excess air calculation process is illustrated using the previous
example of methane gas combustion. The above calculation for excess air gives the
value of 20 percent. We know that this calculation is correct, since our original
numbers were based on 20% EA.
REFERENCES
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.
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.
15-12
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16. AIR POLLUTION II: PRODUCTS OF INCOMPLETE COMBUSTION
Slide 16-1
PRODUCTS OF INCOMPLETE COMBUSTION (PICs)
Smoke & Particulate Matter
Carbon Monoxide
MWC Organics (Dioxins & Furans)
Volatile Organic Hydrocarbons
There are potentially a large number of chemical compounds which can be
formed from incomplete combustion. Visual smoke emissions, as discussed in
Learning Unit 15, are an indication of incomplete combustion.
The combustion conditions which lead to products of incomplete combustion
were discussed in Learning Unit 9. These conditions include non-uniform and low
temperature conditions, poor mixing of oxygen and volatile gases in the fuel-bed and
the over-bed regions, inadequate air, and/or too much air which cools the gases.
Carbon monoxide is the most easily measured toxic product of incomplete
combustion. Trace amounts of many organic compounds have been found in the flue
gas of MWCs. These include both volatile organic hydrocarbons (VOHC) and semi-
volatile hydrocarbons. Compounds of interest include chlorobenzenes, chlorophenols,
polycyclic aromatic hydrocarbons (PAH), and polychlorinated biphenyls (PCBs).
Slide 16-2
SURROGATES FOR MWC ORGANIC EMISSIONS
Routine Operations: Carbon Monoxide
Annual Stack Test: Total Dioxins/Furans
The magnitude of carbon monoxide concentrations in the flue gas is an
important indicator of combustion quality. In general, if carbon monoxide levels are
high it has been demonstrated that the MWC organic emissions will be unacceptably
high. The current regulatory trend is to limit carbon monoxide emissions as the
surrogate for the MWC organic emissions during routine operations.
Considerable research has been performed to develop knowledge about the
formation and control of trace organic emissions from incinerators.1'2'3'4'5 Much of it
has focused upon dioxins and furans, which are potentially carcinogenic materials.
16-1
-------
Dioxins and furans are representative of the other organic compounds in that
control techniques for dioxins/furans will also control the other organics. The total
of the tetra- through octa- isomers of dioxin and furan emissions are used in federal
regulations as the surrogate for MWC organics in annual stack tests.
Slide 16-3
DIOXINS/FURANS (CDD/CDF)
Dioxins (CDD)
Polychlorinated Dibenzo-p-dioxins
Furans (CDF)
Polychlorinated Dibenzofurans
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 16-4
DIAGRAMS OF DIOXIN AND FURAN STRUCTURES1
Cl
Example Dioxin
C!
Cl Cl
Example Furan
Dioxins and furans are a complex group of chemical compounds of carbon,
hydrogen and chlorine. Chemists refer to both groups as "ring compounds." Dioxins
and furans come in a variety of possible molecular configurations and have different
toxicity features.
16-2
-------
For instance, the number of chlorine atoms in the dioxin/furan group of
molecules can vary from one to eight (with four each shown in the illustration). Total
CDD/CDF are defined by the NSPS and EG as the total tetra- through octa-
chlorinated dibenzo-p-dioxins and dibenzonirans. Such molecules will have from four
to eight chlorine atoms.
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 16-5
CONDITIONS WHICH CONTROL DIOXINS/FURANS
COMBUSTION ZONE
Adequate Temperature & Mixing
FLY ASH COLLECTION DEVICE
Low Temperature
There are two different regions where operating conditions determine the
dioxin and furan emissions.2 Reduced emissions will come from the combustion zone
if the complete combustion conditions of adequate temperature and mixing are
achieved. Additional formation of dioxins and furans can occur under relatively high
temperatures in particulate collection devices (ESPs, fabric filters).
Slide 16-6
FORMATION OF MWC ORGANICS
COMBUSTION ZONE
Relatively Low Combustion Temperatures
Poor Mixing - Pockets of Rich Mixtures
High Particulate Loadings
Operating Above Unit Capacity
A considerable fraction of the dioxin/furan emissions are related to the
incomplete combustion in the combustion zone.3 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 combustion chamber.
16-3
-------
Some chlorinated compounds undergo partial combustion reactions. For
example, the incomplete combustion of complex chlorophenol and PCB molecules can
lead to the formation of dioxins and furans. Another process is the burning of organic
material in the presence of chlorine compounds. For instance, vegetable matter,
wood, and lignite coal can form dioxins and furans when burned in the presence of
chlorinated compounds, such as hydrogen chloride.3
High particulate loading is associated with high velocity particle entrainment
in the fuel bed and combustion chamber. 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, leading to their
cooling before the combustion process is completed. Under such conditions, one would
expect increased carbon content in the fly ash, as well as increased dioxin/furan
emissions.
Slide 16-7
FORMATION OF MWC ORGANICS
APCD: ESP or Fabric Filter
Catalytic Formation on Fly Ash
High Operating Temperatures (450° F)
Low Carbon Loadings in Stack Gas
More Dioxin/Furan Emissions
Less Retained in Collected Fly Ash
Another way in which dioxins and furans are formed is through catalytic
reactions downstream of the combustor. These reactions occur on the surface of fly
ash. Fly ash is composed of many inorganic and metal materials (e.g., copper) which
are known to act as catalysts.
Some dioxins and furans can be formed on the surface of fly ash when it is held
for a relatively long time at a high enough operating temperature in the ESP and/or
fabric filter. Considerable increases have been measured5 when the operating
temperatures are increased from around 350° to 600° F.
Therefore, regulations limit the APCD inlet temperature to a small amount
higher (30° F) than was present during the most recent successful annual stack test6'7.
One operating constraint in selecting the APCD operating temperature is that it
should be above the acid gas dew point to avoid condensation and corrosion problems.
There is a partitioning of dioxin and furan emissions between that which is
emitted up the stack and that which is retained in the collected ash. As the carbon
loading goes down, it appears that more of the dioxin/furan is emitted in the stack
gas, whereas when carbon loadings go up, more is retained on the collected fly ash.
16-4
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Slide 16-8
ANNUAL TEST FOR DIOXINS/FURANS
Stack Test: EPA Method 23
Total Dioxins/Furans
Gaseous & Solid
Although dioxins and furans are non-volatile, they may be emitted in either a
gaseous form or as absorbed onto solid surfaces, such as fly ash. The amount of each
isomer can be obtained by a standard stack test, using EPA Method 23. Under the
NSPS method, the total MWC organics are determined by adding the mass of all the
tetra- through octa- isomers of dioxins and furans.
Slide 16-9
REGULATORY BASIS FOR EMISSIONS LIMITS
NSPS: Total Mass of All Dioxins and Furans
Some States: Toxic Equivalent Limitation
Determine Mass of Bach Isomer
Toxicity Level Assigned to Each Isomer
Multiply Masses by Levels to Obtain Total
Some states use a toxic equivalent limitation instead of the total mass of
dioxins/furans. In the toxic equivalency 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.
16-5
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REFERENCES
1. "Municipal Waste Combustion Study, Report to Congress," U.S. Environmental
Protection Agency, EPA-530-SW-87-021-a, June 1987, pp. 42-61.
2. 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.
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. 4-1 to 4-8.
4. "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.
5. 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.
6. 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.
7. U. S. Environmental Protection Agency, "Emission Guidelines; Municipal
Waste Combustors," Federal Register. Vol. 56, No. 28. February 11, 1991, pp.
5514-5527.
16-6
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17. AIR POLLUTION III: NITROGEN OXIDES
Slide 17-1
SOURCES OF NITROGEN OXIDES
Mobile Combustion Sources
Automobiles, Trucks
Stationary Combustion Sources
Power Plants, Heaters
Natural Combustion Sources
Forest Fires, Volcanos
Non-Combustion Sources
Nitric Acid Manufacturing
Nitrogen oxides are emitted from almost all combustion sources, including
stationary sources such as power plants, mobile sources such as automobiles, and
natural sources such as forest fires. Non-combustion sources include those associated
with the manufacture and use of nitric acid.
Slide 17-2
NITROGEN OXIDES
Nitric Oxide (NO)
Nitrogen Dioxide (NO,)
Nitrous Oxide (N30)
Nitrogen Trioxide (NaO3)
Nitrogen Pentoxide (N2OS)
There are a number of different oxides of nitrogen listed above. Nitrogen
oxides are essential to the nitrogen cycle in nature. Nitrogen dioxide (N02) can be
converted to nitric acid in the atmosphere, which under normal circumstances reacts
to form nitrates which return to the earth as either dry deposition or precipitation
(rain and snow). Nita-ates are an important natural fertilizer for organic growth.
For regulatory purposes, nitrogen oxides (NOx) are composed of nitric oxide
(NO) and nitrogen dioxide (N02), the two major combustion related oxides of nitrogen.
NO is the dominant molecular form produced during combustion. It undergoes slow
oxidation to N02, with most of the conversion occurring in atmospheric air. As
described in Learning Unit 14, NOx emission measuring instruments have provisions
for measuring NO and N02.
17-1
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Nitrous oxide is commonly known as laughing gas. It is a greenhouse gas
which reacts in the upper atmosphere (stratosphere) to form nitric oxide which
subsequently depletes ozone.1 Nitrous oxide is generally not included as part of
NOx, because conventional NOx instruments do not measure it.
Nitrogen trioxide and nitrogen pentoxide are found in very small, trace
quantities.
Slide 17-3
ENVIRONMENTAL CONCERNS ABOUT NOx
Acid Rain
Damage to Structures
Damage to Water Quality & Fish Life
Sudden Release of Acids
Photochemical Smog
Impairs Human Health, Respiration
Stunts Growth of Vegetation
Oxidizes Materials
A significant fraction of the NOx emitted from stationary combustion sources
can result in either the formation of acid rain and/or photochemical smog.
Environmental concerns about acid rain can relate to the damage done to
structures, plants and fish-life both near and far from the acid's emission sources.
The problems are worsened by the sudden release of acid materials, such as occurs
during the melting of accumulated snow. The first rain after a drought generally is
much more acidic than normal due to the scrubbing action of rain water on the
atmosphere.
Photochemical smog is the brownish colored air, first identified in the 1940s
in the Los Angeles air basin. Sunlight causes the dissociation of nitrogen dioxide,
which leads to a series of chemical reactions with hydrocarbons and other gases.
Photochemical smog is often trapped by an atmospheric inversion,2 which prevents
its dilution with fresh air. Smog is particularly observable by looking through the
horizontal layers of such stratified air.
The high oxidant levels associated with smog can impair human health,
particularly for those individuals with respiratory diseases. Other measurable effects
of smog are the stunting of growth of vegetation, the discoloration of fabrics, the
cracking of rubber, the deterioration of concrete structures, and the corrosion of
metals.
17-2
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Slide 17-4
GENERALIZED PHOTOCHEMICAL REACTION EQUATIONS2
NO2 + Solar Energy --- > NO + O
O + Oa --- > O3
O3 + NO — > NO, + Oa
O + CH --- > Stable Products + Radicals
O +
---- > Stable Products + Radicals
Radicals + C^Hy --- > Stable Products + Radicals
Radicals + NO — > Radicals + N0a
Radicals + N02 --- > Stable Products
Radicals + Radicals --- > Stable Products
The formation of photochemical smog in atmospheric air is associated with the
special ability of NO2 molecules to absorb ultraviolet solar energy which causes the
molecule to dissociate to NO and atomic oxygen (0). These atoms of oxygen are very
unstable, reacting readily with almost any molecule with which they collide. This
reactivity is characterized by high oxidant levels which cause environmental concerns.
Photochemical smog formation is a transient process which varies with
sunlight, atmospheric mixing conditions, and the emissions of NOx, hydrocarbons and
other products of combustion.
Slide 17-5
FORMATION OP NOx - CONVENTIONAL POWER PLANTS
FUEL NOx
Combustion of Chemically-Bound
Nitrogen in the Fuel with Oxygen
THERMAL NOx
High Temperature Reaction of
Oxygen and Nitrogen from Air
Depending upon the combustion conditions, NOx can be produced by either the
oxidation of the nitrogen in the fuel or the high temperature "thermal fixation" of
molecular nitrogen from air.3
Fuel NOx designates the NO formed by oxidation of the chemically-bounded
nitrogen in the fuel. During the devolatilization process, most of the fuel nitrogen
will be released as N2, HCN, NO and NH3, with a modest fraction remaining with the
char.4 The stoichiometric and mixing conditions will determine the fractions of the
HCN and NH3 which is oxidized to NO. Conversions of fuel nitrogen to NO range
from around 5% at starved-air conditions to 50% under well-mixed conditions.
17-3
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Thermal NOx designates the NO formed through the mechanism of high
temperature oxidation of nitrogen from the air. A fuel with a low fraction of
chemically-bound nitrogen, such as natural gas, forms mainly thermal NOx. Most
MWC designs have much lower peak combustion temperatures than conventional
fossil fuel units. Therefore, very little thermal NOx is formed by MWC units.
As will be discussed in Learning Unit 21, in addition to flue gas treatment, the
applicable combustion modification techniques for NOx control depend upon the
mechanism of formation. Controlling stoichiometric conditions, which limits the
formation of fuel NOx, will be effective in MWCs. By contrast, thermal NOx
techniques which reduce peak gas temperatures are not expected to be effective.
Slide 17-6
IMPACT OF TEMPERATURE AND FUEL NITROGEN
ON NOx EMISSIONS4
T(°F)
3UO 2813 2509 2310 2112 1941
30X EXCESS AIR
T- 0.5 SEC.
0.45 0.50 0.55 0.60 0.65 0.70
103/T(K"1)
Theoretical studies have demonstrated that most of the NO formed from
burning MSW is fuel NOx.
In the case illustrated above, a mixture of MSW volatiles is burned with 30%
excess air in a combustion chamber with a half second residence time. When the
MSW does not contain nitrogen, the illustration shows that NO formation is strongly
dependent upon temperature, but is very small (<10 ppm) in the 2,300 - 2,500°F peak
operating temperature range of MWC units.
17-4
-------
By contrast, when the fuel contains 0.5% fuel-bound nitrogen, the NO
theoretical concentrations are in the range of 100 ppm, a value which has the same
order of magnitude as the typical NO measurements at MWC units. Note also, in
this MWC operating range the NO formation is almost independent of temperature.
Slide 17-7
IDEALIZED REACTION EQUATION FOR MSW TO PRODUCE
MAXIMUM FUEL NOx WITH 50% EXCESS AIR
.006 + 1.22 H20 + 1.5a O2 + 5.64a N2 —
b CO2 + c H20 + d N2 + e HC1 + f SO2 + g NO +.5 a O2
This analysis assumes that all the fuel nitrogen goes to NO and that the excess
air level is 50%. Also, we assume that all the Cl goes to HC1.
The influence of the 50% excess air assumption is illustrated above with 1.5(a)
moles of oxygen and 5.64(a) or 1.5 x 3.76(a) moles of nitrogen in the combustion air.
Likewise, the excess 0.5(a) moles of oxygen appear on the product side of the
equation.
Slide 17-8
CONSERVATION EQUATIONS
Carbon :
Hydrogen :
Oxygen :
Sulfur:
Chlorine :
Fuel N:
Air N:
1.85 = b
5.4 + 2(1.22) « 2c + e
2.08 + 1.22 + 3a » 2b + c + 2f + g + a
0.006 = f
0.006 = e
0.02 = g
5.64{2a) = 2d
The respective equations for the conservation of the atoms for each element are
presented above. Note that all the fuel nitrogen goes to make g moles of NO, and
that the nitrogen in the air simply passes through unchanged.
17-5
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Slide 17-9
SOLUTION OF CONSERVATION EQUATIONS:
a=2.175; b=1.85; c=3.92; d=12.26
e=0.006; f=0.006; g=0.02
The corresponding solution of the equations is illustrated above.
Slide 17-10
IDEALIZED COMBUSTION OF MSW TO PRODUCE MAXIMUM
FUEL NOx WITH 50% EXCESS AIR
C1.85HS.4°a
+ 12.26
+
Product
Gas
CO,
H2O
N2
HC1
SO3
NO
02
Total
. 08". 03^^. 006^.006'*'
N2 > 1.85
0.006 HC1 + 0
Wet Gas
Moles
1.85
3.92
12.26
0.006
0.006
0.02
1.09
19.152
1.22 H30+ 3.262 O2
CO2 + 3.92 H2O + 12
.006 SO2 + 0.02 NO
Dry Gas
Moles
1.85
12.26
0.006
0.006
0.02
1.09
15.232
.26 N2
+ 1.09 O2
Dry Gas
Mole %
12.04
80.49
0.04
0.04
0.13
7.16
100.00
The dry gas analysis for the idealized combustion example is illustrated above.
Note that the NO is 0.13 percent of the dry gas, which corresponds to 1,300 ppm.
Because NOx emissions are often around 200 ppm, this example confirms that there
is more than enough fuel nitrogen in MSW to account for the NOx formation.
Note that in actual combustion systems, the maximum theoretical amount of
fuel NOx is not obtained. There are some indications that around 20% of the fuel
nitrogen can be expected to form NOx in MWC units. Therefore, about 260 ppm
would be expected to be formed. This approximation is generally consistent with the
stack gas concentration measurements of many operating MWC unite.
As a point of reference, the NSPS for large MWC unite limits NOx emissions
to 180 ppm. Therefore, new unite must have design provisions which either limit the
formation of fuel NOx or cause its dissociation.
17-6
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Slide 17-11
COMBUSTION MODIFICATIONS FOR FUEL NOx
Two-Stage Combustion
Excess Air - Stoichiometric Control
Controlled Mixing - Low NOx Burners
Fuel NOx may be controlled through the modification of local combustion
stoichiometries. If combustion occurs under sub-stoichiometric (starved air)
conditions, there will not be enough oxygen for complete combustion of the hydrogen
and carbon. Reactions favor hydrogen and carbon oxidation over that of nitrogen, so
the oxygen could be depleted before significant NOx is formed. The nitrogen atoms
released under starved air combustion conditions will generally tend to form
molecular nitrogen.
A subsequent addition of excess air will be required to provide for the oxidation
of carbon monoxide and other products of incomplete combustion before the gases
leave the combustion zone.
As discussed in Learning Unit 9, the fuel bed of a grate-burning system acts
as a gasifier. If more uniform bed conditions were possible, it may be that the fuel
bed could also act as a low NOx burner. However, the fuel variability leads to
oscillations in local stoichiometry and control of stoichiometry is very difficult.
Starved-air incinerators have consistent performance features which produce
sub-stoichiometric conditions in the primary chamber. This makes the primary
chamber function as a low-NOx burner.
Slide 17-12
COMBUSTION MODIFICATIONS FOR THERMAL NOx
THERMAL NOx
Not a Significant Source of MWC NOx
Thermal NOx Control Techniques
Limit Peak Combustion Temperatures
Heat Sinks (Flue Gas, Steam)
Control Mixing to Reduced Hot Spots
Control Stoichiometry
17-7
-------
Although thermal NOx is not an important mechanism for NOx formation in
MWCs, it is important in many conventional combustion systems. Thermal NOx can
be controlled by limiting the peak combustion temperatures through such techniques
as flue gas recirculation, steam injection, low NOx burners, and excess air control.
Inert flue gas and/or steam can be mixed with the combustion gases and act
as heat sink materials which reduce combustion gas temperatures. Flue gas
recirculation is often used to control thermal NOx emissions from natural gas fired
power plants. Steam injection is a traditional means of NOx control in gas turbines.
Flue gas recirculation is used in some modular starved air incinerators as a
method of controlling primary chamber gas temperatures. Because of the starved-air
condition, its use will have limited impact on their NOx emissions.
The low NOx burners attempt to eliminate "hot spots" in the flame where very
intense combustion occurs. Low NOx burners can also be used to control local
stoichiometry.
Controlled stoichiometry is used to limit the amount of oxygen available for
oxidizing nitrogen, as in the case of fuel NOx. Controlling excess air and/or the
stoichiometry under peak combustion temperature conditions may also control gas
temperatures. Depending on whether conditions are fuel rich or lean, the combustion
temperatures will increase or decrease upon an increase in the air supply.
Slide 17-13
FLUE GAS CONTROL OF NOx
Catalytic and Non-Catalytic
Reducing Agent Injection
Reducing agents can be injected into the combustion gases to create a reducing
atmosphere. Under such conditions, the reaction kinetics will cause the NO to be
dissociated as the reagent is oxidized. Molecules of water and nitrogen are formed
from the decomposition process.
Reducing atmospheres can be obtained by injecting ammonia and urea into the
combustion gases. Reagent injection must be properly controlled to assure that the
right amount of agent is injected, the temperature where the injection occurs is
appropriate, and the reagent does not slip through the reaction and become a
significant pollutant emission. Some reagents require the use of a catalyst surface,
whereas others do not. The details of such flue gas treatment techniques will be
presented in Learning Unit 21.
17-8
-------
REFERENCES
1. H. Whalely et al, "Control of Acid Rain Precursors," Emissions from
Combustion Processes: Origin. Measurement. Control. Raymond Clement and
Ron Kagel, editors, Lewis Publishers, Chelsea, Michigan, 1990, pp. 315-316.
2. Philip A. Leighton, Photochemistry of Air Pollution. Academic Press, New
York, 1961, pp. 1-5, 254-278.
3. "Nitrogen Oxide Control For Stationary Combustion Sources," EPA/625-5-
86/020, U. S. Environmental Protection Agency, July 1986, pp. 1-3.
4. 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. 4-8 to 4-11.
17-9
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18. AIR POLLUTION IV: METALS AND ASH
Slide 18-1
METAL COMPOSITION IN MSW
Example Composition of MSW: 6.4% Metals
16.4% Inorganic (Ash)
Major Toxic Metals:
Other Trace Metals:
Lead, Cadmium, Mercury
Antimony, Arsenic, Barium
Beryllium, Chromium, Nickel
Silver, Thallium
The example MSW presented in Learning Unit 4 had a metals content of 6.4%
and an inorganic (ash) content of 16.4%. The difference between these two numbers
relates primarily to silica and oxygen. Much of the ash is in the form of an oxide, so
the oxygen contributes to its weight. As illustrated below, the metallic constituents
contribute about 60% of the total weight of ash, with oxygen providing the balance.
Slide 18-2
EXAMPLE OF METALLIC CONSTITUENTS IN ASH1
Silicon
Iron
Calcium
Sodium
Aluminum
Titanium
Manganese
Potassium
Zinc
Lead
Copper
Molybdenum
Barium
Chromium
Selenium
Arsenic
Cadmium
Mercury
Silver
30. %
10.
8.
6.
3.
0.7
0.6
0.4
0.3
0.2
0.1
0.1
0.05
0.02
0.004
0.003
0.003
0.0006
0.0006
The metal contents in ash vary considerably depending on the sources and the
degree of materials separation utilized. Silicon is listed here as a metal but it is
generally in the form of sand and glass.
18-1
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Ferrous metals in ash include iron and the various steel alloys. The sodium
is generally in the form of a salt. Aluminum is a significant component. Other non-
ferrous metals are found to have small but important trace contents.
Slide 18-3
COMMON TERMS WHICH CHARACTERIZE METALS
TOXIC METALS
Threat to Human Health
HEAVY METALS
High Molecular Weight
TRACE METALS
Found in Low Concentrations
Terms such as "toxic metals," "heavy metals," and "trace metals" are often used
to characterize metals which are of concern as air and water pollutants. Toxic metals
are so characterized because of their threat to human health. Heavy metals are those
with relatively high molecular weights. A metal is characterized as a trace metal if
it is found in very small concentrations.
The above terms for toxic metals are often used interchangeably because the
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 a national
ambient air quality standard (1.5 micrograms/cubic meter) established to protect
public health and welfare.
Slide 18-4
NSPS: MWC METALS
Metals and Metal Compounds Emitted
in Exhaust Gases from MWC Units
Particulate Matter (Solid and Liquid)
Vapors (Gas)
The NSPS has defined MWC metal emissions as those metals and metal
compounds which are emitted in the exhaust gases of MWC units. MWC metals
include the emissions of metals found in both particulate and vapor forms.
18-2
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Slide 18-5
METAL PATHWAYS IN MWCs
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
Metals which enter the combustion zone as MSW can follow different paths,
depending on their chemical characteristics, the combustion temperatures, melting
points, and vapor pressures.
Many of the metals in MSW have high melting temperatures and are non-
volatile. Upon heating in the combustion zone, metals can react to form oxides,
sulfides and chlorides. If local starved-air conditions exist, some metal compounds
can be reduced back to a pure metal, but they generally leave the combustion
chamber in an oxide form.
These metal compounds are generally collected in solid form. They are the
major constituents of the residual (bottom) ash and grate siftings. They are also the
major constituents of the fly ash which is collected in the heat recovery regions and
by flue gas cleaning equipment.
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 grate, wall, or ash. As flue gas cooling occurs, vapors
of volatile metals may be adsorbed on the fly ash or may condense in the form of
small liquid metal droplets. A fraction of the metals in MSW are emitted by the unit
in gaseous (vapor) form in the exhaust gas.
Slide 18-6
TOXIC METALS AS AIR POLLUTANTS
Particulates
Gases (Vapors)
18-3
-------
The primary environmental concern about metals relates to their toxicity as
air pollutants. Metals can be emitted in either particulate form or as vapors in the
stack gases.2 With the exception of mercury, most of the toxic metals are retained
in solid form as part of the MWC fly ash and bottom ash residues.
Slide 18-7
TOXIC METALS WITH LARGEST CONCENTRATIONS
Lead, Mercury and Cadmium
Lead - Particulate
Mercury - Particulate and Vapor
Cadmium - Particulate
The toxic metals with the largest concentrations in the flue gas before the
APCDs are lead, mercury and cadmium. Lead and cadmium are generally deposited
(condensed, adsorbed) on the surface of fly ash. Their concentration is higher in the
smaller particulates of fly ash, because the transfer process is limited by surface area,
and the smaller sized particles have considerably more surface area per unit weight.
*
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 is inversely related to vapor pressure, a considerable fraction
of mercury does not condense as the flue gases are cooled before leaving the stack.
Mercury is emitted both in vapor and particulate forms.
Slide 18-8
CONTROL STRATEGY FOR METAL AIR POLLUTANTS
Provide for Condensation and Adsorption
by Controlling APCD Temperature
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 can be collected if the APCD is adequate.
18-4
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Slide 18-9
SURROGATES
For MWC Metals (Except Mercury)
Particulate Matter, PM
Opacity
As discussed in Learning Unit 12, surrogates are specified to indicate the
adequacy of controls for MWC metals. Particulate matter (PM) is the surrogate for
MWC metals, other than mercury, during an annual stack test. Opacity is the
surrogate during continuous operations.
Slide 18-10
HEAVY METALS - OPERATIONAL CONCERNS
Procedures to Prevent Exposure
Special Equipment (Suits, Aspirators)
Personal Monitors
MWC plants operate under Occupational Safety and Health Act (OSHA)
requirements which include special procedures designed to limit a worker's exposure
to heavy 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.
Slide 18-11
TOXIC METALS AS GROUND WATER POLLUTANTS
Organic Decomposition to Form Acids
Acid Extraction of Heavy Metals from Ash
Leakage of Leachate into Ground Water
Another major environmental concern about toxic metals relates to ash disposal
and the long-term possibility of polluting the ground water. Depending upon the
applicable regulations, combustion residues can be disposed either alone in a single
composite-lined monofill or with mixed waste in a modern landfill having a double
composite-liner.1
18-5
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Acids, which are formed by organic decomposition in landfills, can leach the
heavy metals from the ash and form a liquid called leachate. Ground water pollution
occurs when the leachate leaks into the ground water.
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.1 Although, the metal
concentrations in monofills are greater than in sanitary landfills, the metals are
generally retained better in ash than in MSW disposed in a sanitary landfill.
Slide 18-12
IS MWC 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 MWC ash residue
should be classified as a hazardous or non-hazardous material.
The USEPA presently has no formal statement on MWC ash. However, many
states have developed their own answer to this question. The definitions and
requirements for testing and disposal varying from state to state.3
Amendments to RCRA, which were passed in 1984, have provisions which
define MSW ash as being non-hazardous, subject to some limitations3. However,
Section 3001 of RCRA specifically classifies a waste material as being hazardous if
it fails tests for ignitability, reactivity, corrosivity, or toxicity.
MSW ash can generally pass the ignitability, reactivity and corrosivity tests.
The typical carbon burn-out of ash prevents it from being ignited, and the reactivity
is controlled by keeping radioactive materials out of MSW. The corrosivity test
considers the pH of the material, and the pH of MSW ash is generally within the
acceptable limits of 7.0 to 12.5. However, the testing for toxicity has presented
problems.
18-6
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Slide 18-13
LABORATORY PROCEDURES FOR TOXICS
EP - Extraction Procedure Toxicity Test
(an early procedure)
TCLP - Toxicity Characteristic Leaching
Procedure (EPA 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.4 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 Test (EP). Regulatory thresholds, based on the EP test, have been
established for 8 metals, 4 pesticides and 2 herbicides.4
The Toxicity Characteristic Leaching Procedure (TCLP)5 is a newer regulatory
test, which has been proposed as a replacement for the EP test.4 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.3
Ash sampling requires complicated procedures because of the variations in the
MSW constituents. Bottom ash represents around 90% 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).
Because of the large number of fly ash particles in the flue gas, they provide
a considerable amount of surface area for absorption of heavy metals. Fly ash
concentrations of heavy metals are generally greater than those of bottom ash. When
fly ash has been tested alone by the TCLP procedure, it sometimes exceeds the
regulatory thresholds for lead and cadmium.
18-7
-------
By contrast, bottom ash and combined mixtures of bottom ash and fly ash often
pass TCLP tests. Combining the fly ash with bottom ash is a common practice in
regions where the practice is not prohibited by regulations.
The regulatory requirements for ash disposal are not currently uniform, and
vary from state to state. Some states3 have adopted requirements which specify the
testing of ash through the use of EPA Method 13127 or the EPA Method 3050.8 These
tests have some contrasting features, so that one or the other may better simulate
landfill or monofill conditions.
Slide 18-14
MWC ASH TREATMENT & UTILIZATION
Treatment Before Disposal
Chemical Extraction
Chemical Additives
Compaction
vitrification
Create Useful End-Products
Road-Bed Aggregate
Landfill Cover
Ash/Concrete Blocks
There are a number of ash treatment and utilization strategies currently in use
and under development.
Ferrous metal separation from the bottom ash residues can be accomplished
by the use of magnetic separation and screening techniques. This not only reduces
the amount of ash, but can also provide additional revenue at mass burn facilities.
Various treatment strategies are designed to reduce the amount of leaching of
toxic metals from the ash.9 One possibility, which is very expensive, is to use
chemical extraction in which the ash is treated with an acid to extract the metals
prior to either disposal or utilization.
The ash can be treated with additives which bind the metals in a stable form
that is effectively impermeable to acids. 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 occurs.
18-8
-------
Reported end-product applications include the use of ash as a low-grade road
bed aggregate and as a landfill cover. However, ash treatment may be required for
such applications to prevent the previously described leaching problems.
A higher quality end-product can be formed as an ash/concrete block or
building material. The carbon content of the ash may have to meet certain
specifications, however, to assure the formation of a stable aggregate or building
block.
A considerable research effort 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.10 The product formed is expected to be acceptable for use as
a light-weight aggregate material suitable for construction projects.
18-9
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REFERENCES
1. Jeffrey L. Hahn, "Managing Ash-Closing in on Policy Decisions," Solid Waste
& Power. August 1989, pp. 12-18.
2. 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.
3. Marc J. Roggoff, "The Ash Debate: States Provide Solutions," Solid Waste &
Power. October 1991, pp. 12-18.
4. 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.
5. "Toxicity Characteristic Leaching Procedure," Method 1311, U. S.
Environmental Protection Agency, SW-846, 1985.
6. 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.
7. "Synthetic Precipitation Leach Test for Soils," Method 1312, U. S.
Environmental Protection Agency, SW-846, 1985.
8. "Acid Digestion of Sediments, Sludges, & Soils," Method 3050, U. S.
Environmental Protection Agency, SW-846, 1985.
9. "Treatment and Use: The Future of Ash Management?" Solid Waste & Power.
October 1991, pp. 20-28.
10. 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.
18-10
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19. FLUE GAS CONTROL I: PARTICULATE MATTER (PM)
Slide 19-1
PARTITIONING OF SOLID RESIDUES1'2
COMBUSTION SYSTEM
Pulverized Coal
RDF - Spreader
Mass Burn - Grate
Modular Starved-Air
EXAMPLE VALUES, %
BOTTOM ASH FLY ASH
30
25
90
98
70
75
10
2
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 particulate entrainment is greatest with
suspension-firing systems. Spreader stokers burning RDF have combined suspension
and bed combustion features. Spreader stoker units also have high entrainment of
particulates (fly ash) in the gases leaving the radiant zone.1
Some mass burning systems which use grates have been reported to collect in
the range of 75% of the solid residue as bottom ash.1 Other grate-fired facilities with
grates have indicated as much as 85 to 95% of the solids collected as bottom ash.2
Variations are dependent upon the metal and other inorganic composition of MSW.
By contrast, the modular starved-air units attempt to minimize the
entrainment in the primary chamber, so that only a very small fraction of their solid
residues are collected as fly ash.
The inorganic material in MSW makes up a substantial fraction of the ash
residues from MWC units, with unburned carbon often being less than two percent
of the solid residues. Dry sorbent injection (DSI) and spray dry absorber (SDA)
systems provide additional solid residues which are collected as fly ash.
Slide 19-2
INDICATORS OF PARTICULATE COLLECTION
1. Visible Emissions
2. Opacity GEMS
3. APCD Inlet Gas Temperature
4. Stack Test Results
19-1
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There are various indicators of particulate emissions escaping an air pollution
control device (APCD). A visual observation of the plume opacity is the most obvious.
High opacity is an indication that something is wrong with either the design or
operation of either the combustor or the air pollution control device.
The opacity GEMS provides an instantaneous reading which can be reviewed
conveniently by the operator. It can provide readings with greater precision and
without the subjective features of visual observations. In addition, a continuous
record of stack opacity is typically maintained. These records can be examined to
provide an indication of the history of unit operations.
One requirement of GCP is to operate with the flue gas at the inlet of the
particulate removing APCD being below a temperature limit. Continuous monitoring
of this operating temperature is generally required. The inlet temperature should be
compared with that of the most recent annual stack test for dioxin/furan emissions.
Temperatures more than 30 °F above those test conditions would indicate a violation
of federal NSPS and EG requirements for applicable MWC units.
Flue gas temperature conditions at the APCD inlet must be maintained to limit
the catalytic formation of dioxin/furan compounds on the surfaces of collected ash and
to assure adequate condensation of the MWC organics and toxic metals.
An annual stack test of particulate matter is required by federal standards and
guidelines to assure that both the MWC organic emissions and MWC metal emissions
are adequately controlled.
Slide 19-3
PARTICLE ENTRAINMENT FACTORS
1. Particle Size, Shape & Density
2. Fuel Charging Method
3. Underfire Air Velocity
4. Fuel Burning Rate
5. Primary Zone Velocity
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
large piece of wood tends to remain on the fuel bed.
Some MWC combustion designs provide charging or grate agitation methods
which result in considerable entrainment of burning fuel and ash particles. These
include the RDF systems which use air-swept distributors for suspension burning and
rotary waterwall units which have a constant tumbling bed agitation.
19-2
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Entrainment can be reduced by limiting the velocity through the fuel bed and
limiting fuel bed agitation. Modular starved air units have low particulate loadings
because less air is supplied to the primary chamber and there is limited fuel bed
agitation. Additional combustion air is delivered at the entrance to the secondary
chamber. The reduced fuel bed agitation which reduces gaseous particulate loadings
unfortunately also reduces carbon burnout in the bottom ash. This is partially
compensated for by allowing a relatively long residence time in the burn-out section
of the primary chamber.
Other mass burn units direct substantial amounts of underfire air through the
burning fuel bed. This causes considerable entrainment of particulates. However,
the relatively high velocities and temperatures in the primary combustion region can
also provide for good mixing and carbon burn-out.
The sizes of the entrained particles are reduced as the combustibles are either
evolved or burned on the surface. Their residue can be either removed as boiler ash
or as fly ash. Some boiler ash will be deposited on the heat exchanger surfaces and
removed during soot blowing. Boiler ash includes the particulates which are large
enough to be collected by inertial separation as the flue gases change directions
through the convective section of the boiler. Those particulates which are entrained
in the flue gas and pass out of the boiler make up the fly ash loading at the APCD.
Slide 19-4
TYPES OF PARTICULATE APCDS
1. Fabric Filters
2. Electrostatic Precipitators
3. Venturi Scrubbers
4. Mechanical Collectors
This learning unit focuses on fabric filters (FFs) and electrostatic precipitators
(ESPs). They are the primary air pollution control devices (APCDs) which are
currently used to meet the particulate matter collection requirements. Venturi wet
scrubbers, which have been used for particulate control applications at earlier MWC
units, are also introduced.
As will be discussed in Learning Unit 20, both dry and wet scrubbing systems
have been used in combined particulate and acid gas control systems.
Dry scrubbing systems produce a dry waste product, and the flue gas which
exits is not- saturated with water. Spray dryers absorbers (SDA) and dry sorbent
injection (DSI) systems are primarily designed for absorbing acid gases, leading to the
19-3
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production of calcium based salts. These dry products and particulates are
subsequently collected by fabric filters or ESPs. Dry scrubber systems influence the
particulate collection performance by changing the flue gas temperature, total
particulate load, and fly ash properties.
Venturi and other wet scrubbing systems produce a wet waste (sludge) and a
flue gas saturated with water. Such systems are seldom used in MWC units in the
United States, because of their high operating and maintenance costs associated with
providing energy for the pressure drop, treating the sludge, and controlling corrosion
and erosion.
Mechanical collectors, such as cyclonic and other inertial separation devices,
are effectively limited to large size particulate removal applications, such as for boiler
ash collection. Mechanical collectors are not presented in this training course because
they have limited influence in meeting the particulate emission control requirements
at MWC applications.
Slide 19-5
FABRIC FILTER COLLECTION MECHANISMS
1. Inertial Impaction
2. Direct Interception
3. Diffusion
4. Electrostatic Attraction
Fabric filters collect particles are through a combination of inertial impaction,
direct interception, diffusion, and electrostatic attraction.3 Inertial impaction is
caused by the inability of large particles to change directions and turn past objects
such as single fabric fibers. Upon impact, particulates are deposited on the fibers.
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
by direct interception by the filter cake.
Diffusion is the method whereby sub-micron (sub-micrometer) sized
particulates 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.
19-4
-------
Electrostatic forces can affect collection because of the differences in electrical
charge between the particulates and the filter.
Slide 19-6
FABRIC FILTER DESIGN FACTOR
AIR-TO-CLOTH RATIO
Total Air Flew/Filter Surface Area
Average Velocity Through Filter
Until recently ESP systems were the dominant particulate collectors at MWCs.
Fabric filter systems, which for many years have been used in industrial applications,
are now widely used in MWCs to collect the dry reaction products and particulate
matter and as a secondary reactor for dry scrubbing. ESPs are also continuing to be
used for particulate matter collection.
The collection mechanism of fabric filters is very similar to that of household
vacuum cleaners. A fan sucks air through the bag which cleans air by filtration.
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/fi^). In general, greater filter surface areas result in lower
velocities and require lower pressure drops to force the flue gas through the filter.
Slide 19-7
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 and made from natural fibers like wool, or synthetic
fibers like fiber glass. Each material generally has maximum recommended
operating temperatures and known chemical resistance properties.
19-5
-------
Operating pressure drops in fabric filters range from 1.5 to 5.0 inches w.c.
Generally, fabric filter systems are equipped with automatic controls, which will
initiate a cleaning cycle when the pressure drop across the fabric filter exceeds a
particular value. Fabric filter cleaning can also be accomplished on a regular timed
basis.
Slide 19-8
PULSE-JET FABRIC FILTER4
Dirty Air Inlet and Ditfuser .
ToCfcan Air Outlet
and Exhauster
Homing
Tubular Rtir Bags
Dirty Air Plenum
B««y Valve Air lock
Courtesy of George A. Rolfes Company
Pulse-jet fabric filters are often used in MWC applications, probably because
they tend to require the least capital cost of the three designs. Particulates are
collected on the exterior surfaces of vertical bags. A cage inside each bag prevents
it from collapsing. The top of each bag is attached to the tube sheet of the clean air
plenum, and the bottom is closed.
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 participate cake material.
19-6
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Most of the agglomerated particulate matter falls into a collection hopper, with
a modest amount being re-entrained and returning to the fabric. If the fabric is
cleaned too well, the collection efficiency will be poor until the filter cake is restored.
The bags should not be cleaned too often, as cleaning can damage the bags. Operators
are generally able to modify the pulse sequence for a pulse-jet fabric filter.
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 filters 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 19-9
REVERSE-AIR FABRIC FILTER5
REVERSE-GAS
DUCT
OUTLET
VALVE
REVERSE-GAS
VALVE (OPEN)
PURGE AIR
DUCT
BYPASS
VALVE
DIRTY FLUE
GAS INLET
CLEAN FLUE
GAS OUTLET
ACCESS
DOORS
RINGED
FILTER BAGS
INLE
VALVE
UBESHEET
Courtesy of ABB Flakt, Inc., printed with permission.
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.
19-7
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Slide 19-10
SHAKER-TYPE FABRIC FILTER5
00IMHN VALVE
•• amir
flYFASS
VALVE
RJBGWQ
mtNTCNAKE
RSfOSAL
VALVE
THIMBU
Courtesy of ABB Flakt, Inc., printed with permission.
The shaker-type fabric filter system uses a mechanical shaking device 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.
Slide 19-11
INDICATORS OF FABRIC FILTER PERFORMANCE
Opacity
Pressure Drop
The major indicators of the performance of fabric filter systems are the records
of stack opacity and the pressure drop. Fabric filters have vendor specified operating
limits for maximum and minimum pressure drop. If the pressure drop gets too high,
it may affect the combustion by not allowing the specified draft conditions. If the
pressure drop is too low, the collection efficiency would be expected to be lower.
19-8
-------
A ruptured bag is a typical problem which could be indicated by both above
normal opacity and reduced pressure drop.
Slide 19-12
ELECTROSTATIC PRBCIPITATOR*
dcsapi
Particulate matter (PM) can be removed very effectively from MWC flue gas
streams by ESPs, such as are illustrated above.
Slide 19-13
ESP DESIGN COMPONENTS
High Voltage Equipment
Step-Up Transformer
High Voltage Rectifier
Shell Enclosure for Support & Insulation
Vertical Wires - Discharge Electrodes Wires
Vertical Plates - Collection Electrodes
Multiple Horizontal Gas Flow Paths
Rappers
Hoppers
19-9
-------
Step-up transformers and high voltage rectifiers are generally found on the roof
of ESPs. This electrical equipment is 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.7
The shell enclosure is designed to provide a stable support structure for the
various components and to prevent electrical shorts. Insulation is provided to prevent
condensation associated with the cooling of metal surfaces to the dew point.
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.
The.length-to-height ratio of an ESP is an important design factor. If the ratio
is too high, the flue gas may be channelized rather than flow through the ESP with
a uniform velocity. Appropriately long geometries and low velocities are used to
provide the required residence times for collection.
Slide 19-14
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.
Electric fields are also 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.
19-10
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Slide 19-15
CORONA & COLLECTOR PLATE CONFIGURATION*
BiCDOK
mtnfENUUTT
smr
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 by the electrical
field toward the plate.
An electrical current is caused by the flow of electrons from the discharge
electrode to the collection plates. If the participate loading is very high, a significant
current will result as will localized sparking and field instability. This can be
controlled by limiting the discharge voltage.
The participate concentration density is decreased as collection occurs and the
flue gas flows through the ESP. Therefore, a corresponding increase in discharge
voltage can be provided to the downstream electrodes to improve the overall collection
efficiency. Separate transformer-rectifier sets can be used to supply the higher
voltages at the downstream locations.
ESP designs provide for multiple opportunities for the particulate charging and
collection. Many ESPs used on MWCs have three electric fields (or stages) in series.
Particulates which pass one stage uncharged may be charged and collected in a
subsequent stage.
Also, some of the particulates initially collected are re-entrained because of
plate cleaning, electrical field instability, vibrations or turbulent gas flow. Generally,
these can be collected in the next downstream section of the ESP.
19-11
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Slide 19-16
ESP PARTICUliATB 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 partieulate 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.
Some ESP systems use a wet removal process in which an intermittent or
continuous streams of water is sprayed onto the collected dust and flows down the
plates into a sump.
Slide 19-17
FACTORS AFFECTING ESP PERFORMANCE
1. Particle Size Distribution
2. Specific Collection Area
Area/Gas Flow Rate
3. Gas Stream Properties
Velocity
4. Ash Resistivity
Temperature
Moisture
Composition (Carbon)
ESPs are very effective for large particulates. However, the collection
efficiency of smaller particles is reduced. The particle sizes between 0.2 and 0.4
micrometers are especially difficult to collect. These are also very important in ternis
of meeting current emission requirements.
19-12
-------
The specific collection area (SCA) is a general design parameter which
establishes the amount of collector area required for a given gas flow rate. It is
defined as the collector plate area divided by the flow rate. The specific collection
area generally establishes an upper limit on the overall ESP collector efficiency.
The local gas velocity in the ESP is another variable which influences
performance. As the gas velocity increases, the amount of time for particle migration
to the collection plates is reduced and re-entrainment is increased. An increase in
flue gas velocity is expected to occur with an increase in excess air and in unit load.
In addition, the ESP collection efficiency decreases as the flue gas temperature
increases because of the corresponding increase in velocity. Controlling the load
(combustion rate) is the general method of assuring that gas velocities are maintained
below maximum limits.
Ash resistivity is a parameter which influences the flow of current through the
collected ash. Ash resistivity is reduced with an increase in either the flue gas
temperature or the moisture, sulfur, or carbon content of the ash. If the electrical
resistivity of the ash is too low, the charge will be drained away quickly and the
particulates can be re-entrained in the flue gas. Since the resistivity of MWC ash is
relatively low, re-entrainment is a typical problem for MWC units.
Some ESP applications may provide an injection of a low concentration of a gas
as a "fly ash conditioning" agent. If the ash resistivity is too high, a modest injection
of gaseous sulfur trioxide (e.g., 10 ppm) or ammonia will cause the ash resistivity to
be reduced, resulting in better removal of collected particulates from collector plates.
By contrast, if the ash's resistivity is too high, sparking and back corona
problems can occur which will impair collection -efficiency by reducing the charging
of particles. In addition the PM on the plate may be difficult to remove.
Slide 19-18
ESP MAINTENANCE & OPERATIONAL FEATURES
1. Discharge Electrode Voltage
Automatic Controls
Transformer-Rectifier Data
2. Electrical Component Failure
3. Rapper Operation
4. Air Leakage
Excessive Temperature Drop
Corrosion of Metals
Fugitive Dust
5. Start-up and Shut Down
Heating; Purge Air
19-13
-------
ESP designs which have been adequately designed and maintained have been
demonstrated to adequately control the PM from MWC units.
Automatic circuits are used to control the voltage delivered to the discharge
electrode and the resulting electric field conditions within the ESP. In general as
soon as sparking occurs, the automatic voltage controller shuts down the power to the
discharge electrode for a few milliseconds. Then the electrode's voltage is restored
to approximately its pre-spark value.
Transformer-rectifier sets have gages which are used to monitor the ESFs
electrical data.9'10 The data include the primary and secondary voltages and currents
associated with each field or set of fields. The primary data are associated with the
supply side of the transformer, and secondary data are associated with the high
voltage d.c. electricity supplied to the discharge electrodes. The secondary voltages
and currents will vary with time as the automatic controls attempt to operate at
near-sparking conditions for optimum charging and collection efficiency.
Abnormal transformer-rectifier data will signal the existence of problems
caused by failures in either electrical or mechanical components. Electrical leakages
or short circuits can be caused by broken insulators or by the accumulation of water
and solids on the charged surfaces. Proper alignment of the discharge electrodes
relative to collector plates is crucial for efficient ESP operation. Poor alignment could
result from initial construction errors, insulator fatigue or failure, and thermal
stresses associated with either improper thermal insulation, hopper fly ash fires, or
air infiltration.
Entrainment can be caused by rapping too often or with too much force.
Operators can adjust the rapping frequency and severity through the control system.
ESPs which operate at below atmospheric pressure can suffer from the leakage
of ambient air through cracks into the unit. An excessive drop in temperature from
the inlet to the exit of the ESP would be an indication of air leakage. The local
cooling of surfaces below the acid dew points can be caused by air leakage. The
resulting metal corrosion can weaken the metal structures and enlarge cracks leading
to greater leakage.
Fugitive dust emissions are associated with leakage of ESPs which operate at
above atmospheric pressure.
During unit start-up and shutdown, the ESP will typically swing through the
dew points. An auxiliary system for heating could be used to maintain ESP
temperatures above the dew points. Purging the ESP of any combustion gases upon
shutdown may help control acid corrosion. However, excessive purging with ambient
air could cool the ESP metal surfaces enough to cause condensation of moisture from
the ambient air.
19-14
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Slide 19-19
VBNTURI SCRUBBER11
DIRTY FLUE GAS
SPRAY
LIQUID INLET
VENTURI THROA'
CYCLONIC MIST
EUHIHATOR
• l!
Venturi scrubbers are capable of high particle collection efficiencies, however
a high pressure drop is required. Hydrochloric acid and sulfur dioxide can also be
removed by a venturi scrubber with a suitable reagent.
A venturi nozzle has a restrictive throat area which creates a region of high
velocity flue gas. Water droplets can be injected into the throat area, as illustrated
in the slide. Another approach is to deliver a film of water at the entrance of the
converging section of the nozzle, with water droplets being formed by the action of
fluid friction in the throat region.
Variable throat designs are often used in order to have the desired scrubbing
action over a range of flue gas flow rates. Such designs maintain the same throat
velocities regardless of the flow rate.
Particulates in the high velocity flue gas collide with and are absorbed by the
water droplets. The droplets are typically removed from the flue gas by inertial,
gravity and/or centrifugal forces in either a flooded elbow section at the end of the
diverging section or in a cyclonic mist eliminator or packed bed separator located
downstream.
19-15
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Slide 19-20
KEY VENTURI SCRUBBER CONTROL VARIABLES
Pressure Drop
Liquid/Gas Flow Rate Ratio
Scrubber pH
The two primary control variables are pressure drop and the liquid-to-gas ratio.
Although high particulate collection efficiencies may be obtained by venturi
scrubbers, a very high pressure drop is required. Industrial applications often call
for a pressure drop of from 20 to 60 inches of water column. A considerable amount
of operating cost is associated with the energy required to force the entire flue gas
stream through such high pressure drops.
Most venturi scrubber applications are operated by an induced draft fan (fan
located downstream). It effectively pulls a vacuum which sucks the flue gases
through the scrubber and separator. The fan can either be operated by a variable-
speed motor or by a fixed speed motor with a damper for flow control.
Generally, greater particulate removal efficiency occurs with more scrubbing
liquid provided to the throat of the venturi.
If the venturi is used only for PM control, pH is used only as a sensor for
potential corrosion problems. However, if the venturi is for acid gas removal, pH is
an important control parameter. In acid removal systems, the liquid is a caustic
solution which neutralizes the acids. Such systems recirculate the solution, with a
blowdown stream bled-offto remove particulates and gases. Fresh make-up solution
is added to compensate for the lost liquid, with the amount of caustic being regulated
so as to maintain the pH at 7.0 or slightly less.
Slide 19-21
DISADVANTAGES OF VENTURI SCRUBBERS
High Energy Requirements, Pressure Drop
Liquid Waste Residue
Corrosion and Erosion
The disadvantages of venturi scrubbers include the high energy requirement
associated with operating at high particulate collection efficiencies, the fact that
liquid residues must be processed for clean-up, and the associated corrosion and
erosion of metal surfaces.
19-16
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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. 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.
4. Control Techniques for Particulate Emissions from Stationary Sources, Volume
1, U. S. Environmental Protection Agency, EPA-450/3-81-005a, September
1982.
5. Illustrations of Reverse-Gas-Cleaned Baghouse and Shake/Deflate-Cleaned
Baghouse, ABB Environmental Systems, ABB Flakt, Inc., April 1992.
6- APTI Course SL412B. Electrostatic Precipitator Plan Review-Self
Instructional Guidebook. U. S. Environmental Protection Agency, EPA-450/2-
82-019, July 1983.
7. 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.
8. PEI Associates, Operation and Maintenance Manual for Electrostatic
Precipitators. EPA-625/1-85-017, September 1984.
9. John Richards, "Municipal Waste Incinerator Air Pollution Control Inspection
Course," Submitted to U. S. Environmental Protection Agency, June 1991, p.
6-13.
10- Municipal Waste Incinerator Field Inspection Notebook. U.S. Environmental
Protection Agency, EPA-340/1-88-007, July 1988, p. 150.
11. J. Joseph and David Beachler, APTI Course SL412C, Wet Scrubber Plan
Review-Self Instructional Guidebook. U. S. Environmental Protection Agency,
EPA-450/2-82-020, March 1984.
19-17
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20. FLUE GAS CONTROL II: ACID GAS REMOVAL
Slide 20-1
ACID GAS REMOVAL TECHNIQUES
Dry Scrubbers
Spray Dry Absorber
Dry Sorbent Injection
Wet Scrubber - Packed Tower
Dry scrubber systems get their name from the scrubbing action of materials
that absorb chemicals and from the removal of the reaction products and particulate
matter as dry materials. Sorbent materials can be delivered into the flue gas stream
as either a liquid or dry material. The sorbents are partially converted into chemical
products by reacting with absorbed gases.
The reaction products and residual reagent are typically collected along with
the fly ash by ESPs or fabric filters. Dry scrubber systems are usually considered to
be comprised of acid gas neutralizing components plus the PM removal components.
The particulate collection performance is influenced by the flue gas temperature, total
particulate loading, and ash properties.
Slide 20-2
BEST DEMONSTRATED TECHNOLOGY
New MWC Units
Good Combustion Practices
Spray Dry Absorber &
Fabric Filter
Large Existing Plants
Good Combustion Practices
Dry Sorbent Injection & ESP
Very Large Existing Plants
Good Combustion Practices
Spray Dry Scrubber & ESP (or FF)
Various combinations of scrubbing equipment and reagents can be used for
acid gas control. However, the NSPS and EG require that emissions be at least
equivalent to that attainable by the use of the best demonstrated control technology
(BDT) for MWC organics, MWC metals, and MWC acid gases.
20-1
-------
The USEPA has established that at new units larger than 250 tpd the BDT for
controlling MWC acid gas emissions includes spray dry absorbers with fabric filters
(SDA/FF) along with good combustion practices. The primary acid gas collection
occurs in the spray absorber. However, additional collection occurs as the gases pass
through the fabric filter and react with the dry chemicals in the filter cake.
Dry sorbent injection systems with ESPs (DSI/ESP) are the BDT for existing
units larger than 250 tons/day at "large existing plants" (from 250 to 1,100 tons/day
capacity).
Spray dry absorbers with ESPs (SDA/ESP) are identified as best demonstrated
technology for controlling acid gas emissions from existing units larger than 250
tons/day at "very large existing plants" (greater than 1,100 tons/day capacity).
Therefore, units with an ESP should be able to meet its acid gases requirements
adequately with the addition of a spray dry absorber unit.
Slide 20-3
SPRAY DRYER ABSORPTION PROCESS1'2
1. UME FEEDER
2. UMESLAKER
3. FEEDTANK
4. HEAD TANK
S. SPRAY ABSORBER
6. DUST COLLECTOR
7. STACK
DRY WASTE
20-2
-------
Spray dry absorbers are designed to remove acid gases. The typical slurry
material is formed by blending pebble lime (CaO) with water to form wet calcium
hydroxide (hydrated lime), an alkaline, sorbent material. The blending tank system,
which is called a slaker, produces a liquid slurry which can range from 5 to 20
percent by weight of solids.3 Screens are used to assure that the pebbles of lime
remain in the tank and do not clog the slurry delivery piping.
«.
The slurry is raised to the range of 165 to 190°F by the chemical energy
released in the slaking process where the calcium hydroxide is produced. To prevent
slurry solidification as a cement-type material, mixing the solution and maintaining
its temperature to at least around 140°F are required.
Slide 20-4
SPRAY DRYER ATOMIZER & REACTION CHAMBER
Slurry Atomized to Fine Droplets
High Speed Rotary Atomizer
High Pressure Air Atomizer
Reaction Chamber Provides Residence Time for
Acid Absorption on the Slurry Droplets
Slurry Droplets are Dried by Hot Flue Gas
Flue Gases are Cooled by Evaporation
The slurry is typically pumped to either a high speed rotary atomizer (e.g.,
10,000 to 17,000 rpm) or to an air atomizer nozzle (e.g., 70 to 90 psig). Typically, a
cone-shaped spray of small liquid droplets (e.g., 70 to 200 microns in diameter) is
produced.3 The flue gas flows through the spray, creating opportunities for collisions
of gas molecules with droplets of sorbent material.
A reactor vessel which encloses the atomizer provides a period of residence
time (e.g., up to 10 seconds4) for moisture evaporation from the slurry and for acid
gas absorption on the surface of liquid droplets or solid particles. The typical reactor
vessel length to diameter ratios are smaller for rotary atomizers than for air
atomizers because of the spray shape.
20-3
-------
Slide 20-5
SPRAY DRYER & FABRIC FILTER5
nn
Gas Scrubber
Courtesy of ABB Combustion Engineering, Incorporated
The acid gas absorption by calcium hydroxide (hydrated lime) is followed by the
chemical reactions which produce calcium chloride and calcium sulfate. These
reactions can occur in slurry droplets, on dried particles, and on the filter cake of the
fabric filter.
Slide 20-6
SPRAY DRYER OPERATIONAL CONSIDERATIONS
1. Slurry Flow Rate
Exit Acid Gas Concentration
2. Adequate Drying of Slurry Droplets
Atomizer Maintenance
3. Overall Drying Conditions
Exit Dry Bulb Temperature
Exit Wet Bulb Temperature
Exit Dry Bulb-Wet Bulb Difference
Inlet-Exit Dry Bulb Difference
4. Slurry Water Content
Exit Dry Bulb Temperature
5. Air Leakage Prevention
6. Maintenance of Hopper Temperatures
20-4
-------
The slurry flow rate is designed to meet a desired reaction stoichiometric ratio.
This stoichiometric ratio is defined as the ratio of calcium supplied in the lime slurry
feed to the theoretical amount required to completely react with the SO2 and HC1 in
the flue gas. Because of imperfect mixing of acid gases and sorbent, stoichiometric
ratios have been reported to range from 1.2 to 1.33 up to as much as 2.2 and 3.0.1
Systems operating with the higher stoichiometric ranges are able to achieve
S02 and HC1 removal in excess of that required by the NSPS (see slide 12-4). As
higher stoichiometric ratios are used, the amount of un-reacted sorbent goes up
disproportionately. Various SDA/FF systems have been reported to remove over 90%
of the hydrogen chloride gas and 85% of the sulfur dioxide.6 An example MWC unit
equipped with a SDA/FF system had 99% hydrogen chloride and above 93% sulfur
dioxide removal, when operated at stoichiometric ratios from 2.4 to 3.0.1 Some
SDA/FF systems are able to remove above 99% of the heavy metals (nickel, cadmium,
chromium and lead) and pilot scale tests have shown dioxin/furan emission reductions
in excess of 99%.7
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.7
The sorbent content can be adjusted to control acid gas removal, while dilution
water in the slurry can be used to control gas temperatures leaving the SDA.
Automatic control systems, which sense both downstream S02 concentration and
temperature, can be used to control the slurry flow to the atomizer. Another control
strategy is to set the feed rate appropriately for the maximum flue gas flow rate and
acid gas concentration anticipated.
Slide 20-7
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, and ash hopper and removal system
Plugging. These problems are often of particular concern during the start-up of the
system.
20-5
-------
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.
The water content in the slurry will also influence the amount of drying.
Slaker operations can be modified to control the water content in response to exit flue
gas temperature, although the time response for this control is slow.
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.
Slide 20-8
DRY SORBENT INJECTION SYSTEM7
Own*
Courtesy of ABB Flakt, Inc., printed with permission
20-6
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Dry sorbent injection (DSI) systems deliver a stream of dry hydrated lime
(calcium hydroxide) powder into the flue gas. 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 is 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 SDA
systems. 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.
An automatic control system is generally provided to regulate the delivery of
the sorbent from the storage and into the air stream. The storage bin may be
equipped with a shaker and screw auger feeder and rotary airlock to control the
delivery rate and prevent clumping of the sorbent.
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.3
Many DSI/FF systems for refractory wall MWC units require provisions for
heat exchangers or evaporative cooling of the flue gas before the sorbent is added.
The cooling chamber would be located upstream of the lime injection point, as
illustrated above.
Most water-wall MWC units are able to obtain adequate gas temperature
control from the upstream heat exchangers, so water sprays are not typically used in
these applications.
Most system designs provide for a mixer or reactor vessel to be located between
the injection point and the fabric filter.
Slide 20-9
DRY SORBENT OPERATIONAL PROBLEMS
Ash Removal from Collection Hopper
Air Impactors
Vibrators
Hopper Heaters & Insulation
Maintenance of Air Seals
20-7
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The main 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 leads to air channeling and incomplete removal.
Some combination of air impactors, hopper vibrators, hopper heaters, adequate
insulation, and proper maintenance of seals to prevent air leakage is required to
assist the removal process.
Slide 20-10
PACKED BED WET SCRUBBER*
EXHAUST
SHELL
DIRTY EXHAUST
MIST ELIMINATOR
LIQUID SPRAYS
PACKING
Wet scrubber equipment combinations have been considered for removing both
acid gases and particulates. A typical combination would be a venturi scrubber,
which primarily achieves particulate removal, and a packed bed scrubber supplied
with a sodium hydroxide solution for acid gas removal. Venturi scrubbers may
operate entirely with water for PM removal or with a caustic solution for both acid
gas and PM control. Packed towers are provided with various packing materials
which can provide greater liquid surface areas for enhanced acid gas removal.
20-8
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Slide 20-11
WET SCRUBBER APPLICATIONS
Advantages
Handles Gases & Particulates
Disadvantages
May Not be Able to Meet Standards
High Pressure Drop (Energy Cost)
Liojiid Residue Produced
Corrosion and Erosion of Metals
Wet scrubbing systems (e.g., venturi and packed bed) are not currently used
in the United States for MWC applications. It is unclear if such systems can meet
the federal PM emission limits in the NSPS and EG. In addition, concerns exist
about wet scrubbers' abilities to remove and control dioxin/furan and metal emissions.
In general, dry systems are found to operate with higher collection efficiencies
and more reliably and economically.
Wet scrubbing systems have high maintenance and operating costs associated
with treating the liquid waste, controlling erosion, and providing energy for the
pressure drop. A contaminated wet sludge is a waste stream from a wet scrubber.
It has generally higher disposal cost than the dry residues of fabric filter and ESP
equipment.
Considerable erosion and corrosion of metal surfaces can occur with wet
scrubbing systems. 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 and caustic reactions, particularly on metal surfaces which have
undergone erosion.
20-9
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REFERENCES
1. Theodore G. Brna, "Cleaning of Flue Gases from Waste Combustors,"
Combustion Science and Technology. Vol. 74, 1990, pp. 83-98.
2. T. G. Brna and C. B. Sedman, "Waste Incineration and Emission Control
Technologies," International Congress on Hazardous Materials Management,
Chattanooga, TN, 1987.
3. Richards Engineering, "Municipal Waste Incinerator Field Inspection
Notebook," U. S. Environmental Protection Agency, EPA-340/1-88-007, July
1988, pp. 45-55.
4. William Ellison, "Flue-Gas Desulfiirization," Standard Handbook of Power
Plant Engineering. Thomas C. Elliott, editor, McGraw Hill Book Co., NY, 1989,
pp. 4.95-4.115.
5. "Prepared Fuel Steam Generation System," ABB Resource Recovery Systems,
Windsor, Connecticut, Undated Pamphlet.
6. C. David Gaige and Richard T. Halil, Jr., "Clearing the Air About Municipal
Waste Combustors," Solid Waste & Power, January/February 1992, pp. 12-17.
7. Robert G. Mclnnes, "Spray Dryers and Fabric Filters: State of the Art," Solid
Waste & Power. April 1990, pp. 24-30.
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.
20-10
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21. FLUE GAS CONTROL TECHNOLOGY III: NOx CONTROL
Slide 21-1
POSSIBLE NOx CONTROL TECHNIQUES FOR MWCs
COMBUSTION MODIFICATION
Combustion with Limited Excess Air
Two-Stage Combustion Design
Three-Stage Combustion Design
FLUE GAS TREATMENT
Selective Non Catalytic Reduction (SNCR)
Selective Catalytic Reduction (SCR)
Most of the NOx from MWC units is produced as fuel NOx, although MSW has
a fairly low nitrogen content. The techniques listed above for controlling fuel NOx
emissions are presented in this learning unit.
Slide 21-2
COMBUSTION MODIFICATION FOR FUEL NOx CONTROL
1. Combustion with Limited Excess Air
A full range of actual combustion conditions is simultaneously formed along the
grate and within the combustion region above the grate. This is caused by the
variations in the local delivery of under-fire and over-fire air and the fuel's variable
drying, distillation, and combustion characteristics. Consequently, although the
combustion can be characterized as occurring with excess air, a wide range of
stoichiometric combustion conditions actually are found.
If limited excess air conditions are maintained in the early stages of
combustion, some degree of NOx control would be expected. As lower levels of oxygen
are delivered, lower fractions of the bound fuel-nitrogen will be converted to NOx.
Although this results in some NOx control, there is concern about the pockets of fuel-
rich mixtures which potentially pass through the combustion zone and produce
products of incomplete combustion, including CO and MWC organic emissions.
Therefore, in order to obtain acceptable combustion in MWCs an adequate
overall level of excess air must be delivered and the corresponding level of NOx
control is limited.
21-1
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Slide 21-3
COMBUSTION MODIFICATION FOR FUEL NOx CONTROL
2. Two-Stage Combustion in Starved-Air Units
Sub-Stoichiometric Primary Combustion
Excess Air Secondary Combustion
Two-stage combustion is an integral feature of the design of modular starved-
air and controlled-air incinerators. The primary chambers of starved-air units can
be thought of as low NOx burners. These systems can typically meet the NSPS NOx
limits without the need for flue gas treatment.
Modular starved-air units achieve sub-stoichiometric conditions in the primary
chamber by limiting the air to about 40% of that theoretically required for complete
combustion.1 The pyrolysis process which occurs converts most of the fuel nitrogen
to molecular nitrogen. This nitrogen does not react and form NOx in the secondary
chamber because temperatures are below those required for producing thermal NOx.
Controlled-air incinerators are very similar to starved-air units, except that the
requirement of maintaining sub-stoichiometric conditions in the primary chamber
may be relaxed.
Slide 21-4
POSSIBLE NOx CONTROL TECHNIQUES FOR MWCs
3. Three-Stage Combustion Design
Gas Reburning
Controlled Mixing - Low NOx Burner
Gas reburning is a three-stage NOx control technique which has been applied
in various fossil fuel power plants. Conventional gas reburning techniques are not
generally used as a control technique for MWC NOx.
In gas reburning, the first stage of combustion is under excess air conditions.
The second stage occurs downstream where an auxiliary fuel is injected into the
combustion product gases. The auxiliary fuel is typically natural gas or some other
low nitrogen content fuel. Reburning is generally designed to occur in the radiant
section of the furnace, so that auxiliary fuel is part of the overall energy input.
Reburning creates a reducing atmosphere with about 90 percent of the
theoretically required air.2 The reburn fuel and NO are primarily converted to CO,
N2, and H2O. A third combustion stage is required after reburning in order to convert
the CO to C02.
21-2
-------
It is possible for reburning to occur with the use of a conventional auxiliary
fuel burner. However, auxiliary fuel burners typically operate under excess air
conditions and are designed to increase combustion gas temperatures. Modifications
would be required to produce the required reducing atmosphere and the subsequent
downstream addition of air for complete combustion.
Low-NOx burners for conventional fossil fuels feature controlling the mixing
of the fuel and air, so as to limit the delivery of oxygen to the fuel and reduce
combustion intensity (hot spots).
Rotary waterwall combustors appear to have some features of a low NOx
burner, in that they have been able to meet the NSPS NOx limit of 180 ppm without
the need for flue gas treatment.3'4
The rotary waterwall combustors have a different gas flow geometry from other
grate burning MWC systems. As the product gases flow down the rotating chamber
toward the exit, they mix with the volatile and other gases from the downstream
rotary grate sections. Undoubtedly there are reactions of the combustion product
gases with the volatile gases. These appear to result in a form of reburning, where
the volatile MSW gas is the reburn fuel. Although excess air conditions are found in
the gases leaving the rotary chamber, local conditions in the tumbling fuel bed may
approximate the reburning conditions.
Slide 21-5
FLUE GAS NOx CONTROL
Selective Non-Catalytic Reduction (SNCR)
Selective Catalytic Reduction (SCR)
The selective non-catalytic reduction (SNCR) and selective catalytic reduction
(SCR) techniques have some important similarities to gas reburning. Materials are
injected into the product gases to create a reducing atmosphere for the dissociation
of NO into molecular nitrogen, water and carbon monoxide.
The contrasting feature is that gas reburning provides an additional fuel input
into the furnace combustion zone, whereas the SNCR and SCR reagents are generally
injected after the combustion zone.
21-3
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Slide 21-6
BEST DEMONSTRATED CONTROL TECHNOLOGY
Selective Non-Catalytic Reduction (SNCR)
Reagents: Ammonia, Urea, Other Compounds
NOx emission limits are included in the federal New Source Performance
Standards for new MWC units. The USEPA has established that selective non-
catalytic reduction (SNCR) is the Best Demonstrated Technology for NOx control for
large MWCs.
The operation of SCNR systems requires the injection of a reagent material
which can react with nitrogen oxide (NO) to produce nitrogen gas (N2). A number of
reagent materials can be used. Ammonia and urea are the most widely used
reagents. The process using ammonia for SCNR is generally known as Thermal de-
NOx, which is patented by EXXON. The process using urea for SNCR is patented
by the Electric Power Research Institute. Chemicals such as hydrazine hydrate,
methanol, ammonium sulfate and other compounds are also effective SNCR reagents.
Slide 21-7
SNCR PERFORMANCE FACTORS
Reagent Selection
Temperature Region: 1,600° - 1,800°F
CO Concentration
Residence Time
Reagent Injection Rate Keyed to NO
Gas Mixing Efficiency
The major operational factors which influence the performance of SNCR are
listed above. A properly designed SNCR system is required to have controls which
take into consideration the influences of these factors.
The most important constraint is the temperature of the flue gases into which
the reagent is injected.6'6 Although the process will often work in the design
temperature window from 1,600° to 1,800°F, the temperature at which the process
works best will vary depending upon which reagent is used and on the specific
features of the application.
21-4
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For instance, a complicating factor is that as the concentration of CO increases,
there is a shift of the effective range for the reduction reaction to lower temperatures.
Because of the variability of the combustion conditions, the practical limit of NO
reduction is considerably less than would occur under ideally controlled mixture
conditions.
Under steady conditions, the relative rate of reagent injection must be set to
obtain the amount of NO reduction desired. However, the fuel burning rates and
combustion gas temperatures will vary as the fuel properties change. Corresponding
variations will occur in residence times and NO concentrations. Therefore, the
injection rate must be metered in response to demand.
In addition, mixing of reagent and combustion gases must be appropriately
controlled. The usual application includes a heater for the reagent and a carrier gas
stream to enhance injection of the reagent into the furnace.
Either steam or compressed air can be used as the carrier gas. In some
applications the reagent is mixed with recirculated flue gas before being injected into
the flue gas stream. An air compressor or blower can be used to deliver the mixture
to various damper controlled nozzles which will control the injection location and to
regulate its delivery and mixing rate.
Slide 21-8
COMPETING REACTIONS OF AMMONIA
Reduction:
NH3 + NO + 0.25 02 > N2 + 1.5 H2O
Oxidation (Flue Gas too Hot):
NH3 + 1.25 Oa > NO + 1.5 H2O
No Reaction (Cool Flue Gas, Ammonia Slip)
NH3 > NH3
Operators should be aware that there are three possible reactions which can
occur when reagents are injected into the flue gas. The reduction reaction, oxidation
reaction, or neither type of reaction may dominate, depending upon the combustion
gas temperature and local mixture conditions.
Ammonia will be used to illustrate the three types of reactions because it
undergoes a fairly simple set of chemical reactions. The first reaction in Slide 21-8
is the reduction reaction, in which the ammonia causes the NO to be reduced back
to molecular nitrogen. As noted below, this reduction process generally occurs if the
gases are in the right temperature range.
21-5
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However, if the gas temperatures are too high (e.g. above 2,000 °F), the
ammonia will undergo an oxidation process, forming additional NO.
If the gas is too cool, the ammonia will simply flow out with the combustion
gases, or "slip" through (or break through) without being reacted or influencing
nitrogen oxide.6
If excessive ammonia is injected, some of it may react with HC1 or sulfur
compounds, forming ammonium chloride or ammonium sulfate. These two products
cause the formation of a white or blue smoke, respectively, as discussed in Learning
Unit 15.
Slide 21-9
CHEMICAL DECOMPOSITION OF UREA, CO(NH2),
CO(NHj)a > NH3 + HNCO (Iso-cynauric acid)
Upon being injected into the furnace, urea decomposes to ammonia and iso-
cyanuric acid (HNCO). The ammonia fraction performs in much the same way as in
the thermal de-NOx process. The iso-cyanuric acid can also react with NO to cause
its reduction.
Slide 21-10
SELECTIVE NON-CATALYTIC REDUCTION (SNCR)
Operational Problems
Furnace Temperature Variations
Spacial and Temporal Variations
NO Increases if T > 2,000 °F
Ammonia Slip - Can React to Form
Ammonium Chloride & White Smoke
If too much reagent is injected or if the mixing or temperature levels are
inappropriate, some of the reagent can slip through unreacted.6 This reagent can
subsequently react into such compounds as ammonium chloride, ammonium sulfide,
21-6
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or ammonium bisulfide. These materials may cause plume opacity and/or corrosion
problems. White smoke from ammonium chloride is often observed in the plume from
a MWC unit using this control technology.
If hydrogen gas is blended with the ammonia, the operating window of gas
temperatures can be expanded to include lower temperatures.5 Although, this
technique is not commonly used in MWC applications, it could help compensate for
the flue gas variability found at MWC units.
Slide 21-11
SELECTIVE CATALYTIC REDUCTION (SCR)
Reagent: Ammonia
In the selective catalytic reduction (SCR) process, ammonia is injected into
properly selected locations of the flue gas path where a catalyst bed is installed.
Many metals such as copper, iron, chromium, nickel, molybdenum, vanadium and
cobalt can be used as the catalyst material. In general, catalyst systems have high
capital costs.
The major problem with catalyst systems is that the gas must be cleaned
before the catalyst is used. If any dirty gas were to come in contact with the catalyst,
its surface would become fouled by the particulate and/or condensable materials. The
conversion performance would be considerably decreased by fouling of the catalysts.
Therefore, a designer would have to provide for flue gas cleaning and for
controlling the formation of dioxins/furans on the fly ash, before the gases enter the
SCR process. Assuming that a spray dry absorber and fabric filter system is used,
the flue gas would be cooled to somewhere around 400°F. This temperature is below
the optimum operating temperature range for the catalysts, which generally range
from 530° to 800°F.6
Therefore, the flue gas would have to be reheated, requiring either a
considerable expenditure for auxiliary fuel or the application of a air-to-air heat
exchangers. Either of these would require increased capital and operating costs.
Because of the associated costs, SCR has not been widely adopted for NOx control at
MWC installations.
21-7
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REFERENCES
1. 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. 4.8 to 4-14 and 7-1
to 7-14.
2. Craig A. Penterson et al., "Natural Gas Reburning Technology for NOx
Reduction from MSW Combustion Systems," Proceedings of 1990 ASME
National Waste Processing Conference. Long Beach, CA, June 1990, pp. 185-
191.
3. David S. Beachler and Nancy M. Hirko, "Air Emission Test Results from the
Dutchess County Resource Recovery Facility," Proceedings of 1990 ASME
National Waste Processing Conference. Long Beach, CA, June 1990, pp. 405-
416.
4. David S. Beachler and Nancy M. Hirko, "Nitrogen Oxide Emission Rates from
Waste-to-Energy Plants Using Westinghouse O'Conner (Rotary) Combustors,"
Proceedings of 1990 ASME National Waste Processing Conference. Long Beach,
CA, June 1990, pp. 235-247.
5. Michael Medock, "An Overview of Non-Catalytic NOx Control," Solid Waste
and Power, February 1990, pp. 46-51.
6. Ramon Li et al., "Identifying and Controlling WTE Stack Emissions," Solid
Waste and Power. February 1990, pp. 16-22.
21-8
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22. AUTOMATIC CONTROL SYSTEMS
Slide 22-1
MWC SYSTEMS REQUIRING CONTROL
1. Crane Operation
2. Combustion Control System
3. Ash Handling System
4. Flue Gas Cleaning System
5. Turbine-Generator
6. Feedwater Demineralizer Plant
7. Boiler Feedwater & Condensate
8. Motor Controllers
9. Cooling Water
Many important systems or processes within MWC units, such as those listed
above, require some appropriate combination of manual and automatic controls to
achieve stable and safe unit operations.1
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.
For example, the operator of an overhead crane will control the movement of
the crane, the location of each grapple load, and where it is to be deposited.
However, the pressure exerted by the grapple, the height the grapple load is lifted,
and its claw movements are generally automatically controlled. In addition, the
crane often has provisions for a load cell and a microprocessor-based data processing
unit to automatically weigh the load before it is deposited in the charging hopper.
The operator may be required to manually record the weight of each grapple load.
Slide 22-2
AUTOMATIC CONTROLS SYSTEM FUNCTIONS
1. Modulating Control
2. Sequential Control Logic
3. Process Monitoring
Automatic controls provide for the automatic modulation of control variables,
the control of sequences of logical events, and the monitoring of the operational status
of various processes and/or systems. Modulating controls use variable equipment
settings for operation under various load conditions.
22-1
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Sequential logic controllers are used to assure safe and appropriate operations
during transient conditions such as 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 indicating lights, alarm annunciators,
instruments and recorders for selected physical parameters (e.g., fluid flow rates,
temperatures, pressures, and gas concentrations). Monitors can 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.
Slide 22-3
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
either dedicated pneumatic controllers, hard-wired electronic analog controllers or
programmable logic controllers (PLCs).
Slide 22-4
PNEUMATIC CONTROL LOOP: FLOW CONTROL
SIGNAL TO
CONTROLLER
SIGNAL TO
FINAL CONTROL
ELEMENT
CONTROLLER
TRANSMITTER
SENSING LINE
SENSOR
PROCESS LINE
FINAL CONTROL
ELEMENT
22-2
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Dedicated pneumatic systems, such as shown in the above slide, and hard-
wired controllers have been widely used in combustion units.
Microprocessor-based distributed control systems with PLCs are most often
used in new MWC applications. 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.
The various PLCs are then tied together by a data highway into a computer
network. 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 to operate with a malfunctioning controller or sensor. The
operator is notified of the malfunction by light indicators and alarm annunciators.
Slide 22-5
AUTOMATIC CONTROL SYSTEM ELEMENTS
1. Manipulated Variable (Parameter)
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 manipulated variable is the control parameter being
regulated. It is generally measured by a sensor and transducer device which
produces an electrical feedback signal for comparison with the set point signal.
A controller produces a 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.
22-3
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Slide 22-6
GAS-SIDE 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 combustion equipment vendor will have
a unique combustion control system which probably considers at least four of the
above parameters.
Slide 22-7
WATER-SIDE CONTROL PARAMETERS
1. Steam Temperature
2. Steam Pressure
3. Steam Flow Rate
4. Drum Level
5. Feedwater Flow Rate
The water-side parameters listed above may be used as control parameters for
combustion control systems. In addition, pH is an important water treatment control
parameter for maintaining optimum heat transfer and system performance.
Slide 22-8
FINAL CONTROL ELEMENTS
1. Grate Speed/Ram Speed
2. Timer Delay Period (Dwell Time)
3. Valve Position
4. Damper Position
5. Motor/Fan/Pump/Turbine Speed
Variable Speed Drive
The final control elements listed above may be used by control systems to
achieve the desired equipment operational changes.
22-4
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Slide 22-9
SINGLE-ELEMENT CONTROL SYSTEM: DRAFT2
Adaptation of a Figure of the Instrument Society of America
A furnace 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 furnace. The pressure
transducer (PT) measures furnace 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 22-10
SINGLE-ELEMENT CONTROL SYSTEM: DRUM LEVEL2
Steam drum
w«ter level
Courtesy of the Instrument Society of America
22-5
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The drum level requirement is that the level of liquid in the boiler be
maintained, so that the boiler neither runs dry nor delivers liquid into the
superheater coils.
A single-element drum level controller (LC) would 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 an increase in the 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 flow 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.
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 22-11
TWO-ELEMENT CONTROL SYSTEM: DRUM LEVEL3
Courtesy of the Instrument Society of America
22-6
-------
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 maintains
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.
Slide 22-12
THREE-ELEMENT CONTROL SYSTEM: DRUM LEVEL2
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
routinely used for many years in industrial- and utility-sized combustion equipment.
It is particularly applicable in modern microprocessor-based PLC control systems.
22-7
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Slide 22-13
MICRO-PROCESSOR BASED CONTROL SYSTEM
The above slide illustrates the features of a microprocessor-based combustion
control system. The input variables include both steam pressure and flow as well .as
air flow, flue gas temperature, oxygen content and opacity.
Because of the increased computational ability, computational algorithms can
be used to take into account additional special features. The features of variable
gain, differential control and integral control are design features which will not be
considered further in this training program.
The illustration does indicate that steam pressure would be the major indicator
of load and the corresponding demand for fuel and air flow. However, the combustion
restraints require that air flow be modified to take into consideration the flue gas
conditions as well as the corresponding steam flow rate.
Slide 22-14
TRIM CONTROL FEATURES
1. Oxygen Trim Control
2. Flue Gas APCD Temperature Control
22-8
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A trim control system uses special microprocessor-based control logic to modify
combustion conditions so as to maintain some required operating condition. The
normal control units would allow air and fuel to change in response to demand, but
the trim control unite 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 level in the flue gas. The primary motivation for such control
systems could be either to control NOx emissions or to reduce the energy loss
associated with excessive amounts of flue gas.
A different trim control unit could limit the flue gas temperature at the
entrance to the APCD as a dioxin/furan control requirement. This control could be
achieved through delivering heat sink materials such as excess air or water sprays
into the flue gas after combustion.
Slide 22-15
CONTROL SYSTEM COMPARISONS
Conventional Fuels
Gas & Fuel Oil
Coal
Municipal Solid Waste
The control systems for conventional fuels are capable of creating a steady
combustion condition which can respond to transient demands for load. In general
natural gas is the easiest conventional fuel to burn and to control. Oil is also fairly
easy to control. Coal presents a more difficult control problem because of the specific
fuel properties of volatility and fixed carbon. For this reason, coal combustion units
may be base-loaded, if natural gas or oil-fired unite are available to meet the swing
loads.
MSW is a considerably more variable fuel than the conventional fossil fuels.
The moisture content and other combustion properties can vary widely from one
grapple load to the next, in spite of the mixing which is done before charging into the
combustion unit.
The MWC combustion control system is required to accommodate the widely
varying fuel properties. This is partially achieved by using more excess air than
would be typically used for conventional fuels. It is also achieved by using longer
residence times for the solid fuel on the grates.
22-9
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Slide 22-16
WATERWALL MWC CONTROL FEATURES
Base Load
Steady Combustion Temperatures
A very important combustion control method is to limit the demand driven load
transients. MWC combustion units are typically designed as base load units, so that
the combustion environment is as steady as possible.
Such control systems can establish a priority of either constant combustion
temperature or constant load. Under either case, air delivery will need to only
respond to variable fuel properties and not primarily to transients in the combustion
chamber thermal environment caused by changing steam flow rates and the
corresponding changes in waterwall temperatures.
Of course, overall increases in combustion intensity will result from increasing
both the fuel volatization and its mixing with the air. Therefore, grate agitation and
charging ram operation will be controlled as well as the air supply.
Each MWC vendor may have its own unique control system features. However,
the major control variable is generally either under-fire or over-fire air flow. As was
illustrated earlier, the PLC features of modern control systems allow other variables
to be used to trim the air supply to achieve desired combustion and exit gas
conditions.
Slide 22-17
STARVED-AIR UNIT CONTROL SYSTEMS
Two-Stage Combustion Design
Steady Combustion Temperatures
Low Primary Air Flow
Long Solids Residence Time
Air Controlled in the Secondary
Starved-air units achieve combustion control by separately regulating the air
flow rates and the corresponding fuel/air stoichiometries in the primary and
secondary combustion chambers. The primary chamber is maintained under sub-
stoichiometric conditions.
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The control strategy attempts to limit the fuel volatilization rate by controlling
the primary chamber temperature. However, a variable rate of evolution of volatile
matter will occur because of fuel characteristics. This is in spite of the fact that
primary temperatures are limited so that the rate of volatilization is fairly slow and
the solid fuel residence time is fairly long.
As was presented in Learning Unit 10, an increase in air supply under sub-
stoichiometric conditions will increase the equilibrium temperature. Therefore, the
main controller response is to regulate the delivery of air so as to obtain desired
primary chamber sub-stoichiometric conditions which in turn regulate the primary
chamber temperature.
Additional control features can regulate the dwell time between delivering new
fuel charges and the amount of fuel bed agitation.
Slide 22-18
CONTROL SYSTEM INTERLOCKS
CEMS Operational Requirement
High Carbon Monoxide
Auxiliary Burner Flame Sensor
Fan Running during Pre-Ignition Purge
Many control systems are required to have system interlocks to prevent
operation under upset conditions. For example, the ram feeders for the waste
charging system may be prevented from delivering waste if the CEMS is not
operating properly. Some units have provisions to shut down the feeders in the event
of a high carbon monoxide reading. Other provisions in the control system design
should anticipate this condition and regulate the delivery of air or start up an
auxiliary burner.
Some interlock systems are provided to assure the safe operation of the
equipment. During start-up operations, a tinier 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 would
disable the start-up cycle.
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.
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REFERENCES
1. George R. Kotynek and Calvin L. Hartman, "Using State-of-the-Art Electronic
Controls for Controlling Boiler/Furnaces in Refuse-to-Energy Plants,"
Proceedings of 1990 ASME National Waste Processing Conference. Long Beach,
CA, June 1990, pp. 123-133.
2. Reprinted by permission. Copyright® Instrument Society of America 1988.
From "Boiler Feedwater and Steam - Controlling for Safety and Efficiency,"
Videotape from ISA's Boiler Control Series.
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23. CONTROL ROOM OPERATIONS
Slide 23-1
OPERATOR CONTROL FUNCTIONS
1. Monitor System Operations
2. Evaluate Conditions
3. Institute Appropriate Changes
Operators provide critical functions of monitoring system operations,
evaluating current conditions and trends and instituting appropriate changes. These
functions are required even though the unit's automatic control systems may have
many designed features which provide for automatic corrections of undesirable
operating conditions.
Slide 23-2
OPERATING SYSTEMS
MSW Handling
Combustion
Boiler & Feedwater
Power Generation
APCD & Ash Removal
Electrical Service
Water Treatment
Cooling Water
Fire Protection
The major unit operations centered in the control room will typically include
the MSW handling, combustion, boiler and feedwater, power generation, APCD and
ash removal systems. Many auxiliary systems are also operated from the control
room, including the unit electrical service, water treatment, cooling water, lube oil,
and fire protection systems.
Some of the above mentioned systems may have their control functions
operated through remote control rooms or stations. Remote operations may have the
advantage of allowing auxiliary operators to be located closer to the equipment, which
should facilitate prompt equipment inspections and trouble shooting.
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Slide 23-3
CONTROL ROOM COMMUNICATIONS
Operator/Unit Interface
Receive Operating Information
Transmit Instructions
The control room is the location of the central communications system where
the operators receive information about the unit's operational status and transmit
instructions to the equipment or other operating personnel.1 The communications
interface is sometimes called the control hoard or control panel.
The control room is analogous in function to the cockpit of an airplane where
the pilot and other officers receive information and perform unit operations.
Information about status of system operations is obtained from the available
instruments, indicator lights, annunciators and alarms.
Communications with auxiliary operators and support staff may be crucial in
developing the basis for the operator's decisions. Valuable information is often
obtained by dispatching individuals to confirm instrument readings and to check for
abnormal equipment symptoms such as material blockages, leakages, vibrations,
noise and surface temperatures. Personal conversations in the control room are
augmented through the use of telephones, intercoms and radio devices. Operators
must not only give clear instructions, they must also listen carefully to understand
what others are saying.
Slide 23-4
PANEL MOUNTED INSTRUMENTS
Analog Displays
Digital Displays
Status Indicator Lights
Annunciators
Alarms
Television Monitors
Recording Devices
Circular Charts
Strip Charts
Panel mounted instruments may include analog and digital instrument read-
outs, status indicator lights, annunciators, alarms, TV monitors and recording
devices.2
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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.
TV monitors can provide visual information about unit operations such as
MSW handling, combustion conditions, ash removal and visual stack emissions.
Recording devices include circular and strip chart recorders. These can provide
a continuous record of the values of selected parameters such as opacity, steam flow,
steam pressure, combustion temperatures, gaseous emissions, etc.
Slide 23-5
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.1 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 by touching the graphic screen.
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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 23-6
COMBUSTION SYSTEM MONITORS
Opacity
Carbon Monoxide
Oxygen
Acid Gas Concentrations
Air & Flue Gas Temperatures
Television Monitors
A number of instruments which are typically used as combustion monitors are
listed above. Most of these were previously presented in Learning Units 13 and 14.
Slide 23-7
BOILER & FEEDWATER MONITORS
Steam Pressure & Temperature
Steam Flow Rates
Water Pressure & Temperature
Feedwater Flow Rates
Feedwater pH & Conductivity
Typical boiler and feedwater monitors are listed above. Principles and
techniques of water treatment will be presented in Learning Unit 26.
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Slide 23-8
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 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.
Other operator initiated changes include unit start-up and shutdown
operations, which will be described in Learning Unit 24.
Operator's instructions include requesting verification of operating conditions.
Operators also have the tasks of assuring that routine maintenance is performed and
that proper requests are made for major maintenance, which will be described in
Learning Unit 29.
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REFERENCES
1. J. A. Moore, "The Man/Machine Interface," Standard Handbook of Power Plant
Engineering. Thomas C. Elliott, editor, McGraw Hill Book Co., NY, 1989, pp.
6.125-6.145.
2. 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.
23-6
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24. OPERATING PRACTICES
Slide 24-1
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 MWC units,
including start-up and shutdown operations. Additional consideration will be given
in subsequent learning units to operator practices associated with combustion system
upsets, water treatment, turbine/generator systems, maintenance and safety.
The primary responsibility of the operator is to operate the plant in a safe and
efficient manner.1*2 This will assure the protection of the plant personnel and reduce
the potential damage to the equipment and surrounding environment. Once these
are assured, the optimization of equipment performance will require a balance
between the desire for operation at a maximum production rate and the minimization
of operating cost.
Slide 24-2
OPERATOR JOB FUNCTIONS
Automatic Control System Manager
Equipment Operator
What is Happening?
Why?
What are the Options?
What are the Consequences?
The operator is considerably more than an automatic control system manager.3
Automatic control systems are designed to aid operators in making routine responses
to system variables. Such systems are designed to account for the complex
interactions between the MWC system components and monitors. Control systems
generally operate very rapidly and have safety features included in their logic.
Operators are often required to take manual control of the equipment. This
can occur because the control logic does not cover a particular situation or because
a sensor or control system component has failed.
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Operators must be able to know what is happening in the unit, why it is
happening, what can be done, and the consequences of the various actions.
Slide 24-3
TYPICAL WALK-DOWN CHECK-LIST
1. Fuel Charging & Pit Operations
2. Fuel Bed Uniformity
3. Fuel Bed Clinkering
4. Slag Deposits on Water-walls
5. Equipment Noise/Overheating
6. Ash Leaks, Blockages, Conditions
7. Pumps, Fans & Dampers
8. Water & Oil Leaks (Valve Packing)
9. Safety Valve Leaks
10. Soot-Blowers (Confirm Operation)
11. Hydraulic Systems (Temp., Pressure)
Operators can identify some aspects of unit performance by maintaining and
reviewing a comprehensive log sheet of component equipment operating conditions.
A daily log sheet can be used to tabulate selected operating conditions. For
instance the specific components' pressures, temperatures, flow meter readings,
indicator positions and levels may be recorded as well as an indication of whether the
equipment is operating or not.4
Such log sheets are generally maintained at specified time intervals (e.g., every
four hours or at the beginning of each shift).
Slide 24-4
OPERATOR REQUIREMENTS
1. Know the System Characteristics
2. Assess the Operating Conditions
3. Identify Potential Modifications
4. Make Timely Decisions
5. Establish Proper Procedures
6. Keep Proper Records
The operators are required to understand the special system design features,
which include the operating limits of the component equipment and the important
24-2
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influences of one component on another.5 Operators must conduct operations within
the limitations of the site-specific equipment. Of course, the operational and control
features of the equipment will vary from plant to plant.
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 and drawings found in plant documentation
manuals.
Operators must assess the operating conditions through the surveillance of
both the instruments and the equipment. Operators must also judge the importance
of deviations from normal operating conditions.
When an upset condition is detected, 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, it is probable that the operating
conditions will deteriorate. Inappropriate actions could cause the system to operate
outside its design limits, leading to an unsafe condition, unit trip (shutdown) and/or
a violation of the applicable regulations.
Slide 24-5
POTENTIAL MAJOR HAZARDS
1. Loss of Water
2. Explosive Mixture of Fuel/Air
3. High Pressure Steam Pipe Rupture
As has been the case for generations, the two most dangerous conditions in
power plants are the loss of water in the boiler and the existence of explosive
mixtures in the furnace.
Drum water level control is important because the water is the major provision
for removal of heat from combustion. The drum level represents the balancing of the
inflow and outflow of water from the unit. Therefore, a drum level problem can
result in problems with the control system, a failure in the feedwater system,
excessive steam flow or steam leaks.
If an operating boiler loses water, overheating will occur in the boiler drums,
pipes and waterwall regions. This will cause increased thermal stress in the metal
members, which can result in cracks and tube failures. The extent of the damage can
be controlled by restoring the water flow and/or reducing the level of combustion.
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If the lost water is due to tube failures, the unit will generally need to be taken
off-line for tube replacement or plugging. The steam or water escaping from a tube
leak could result in the cutting of adjacent tubes and/or the overheating of other
sections of the boiler circuit.6 In addition, thermal stress problems may be increased
by the inflow of large quantities of relatively cool feedwater.
Explosive mixtures are avoided by purging the unit with air prior to lighting
the burners. A minimum air flow should be maintained at all times after the purge
process. The combustion chambers should be pre-heated prior to the charging of
MSW. This improves initial combustion and reduces the possibility of the quenching
of combustible gases, which could lead to the accumulation of explosive gas mixtures.
The third hazard is a steam pipe rupture. Steam pipes may rupture under the
uncontrolled conditions of either high thermal stress or water hammer. Water
hammer occurs when slugs of liquid water and steam flow together in a steam pipe.5
General protection against excessive pressures in pipes is provided by the installation
of safety valves, which should be regularly tested.
Slide 24-6
STANDARD OPERATING PROCEDURES
1. Safe Practices & Systems
2. Emergency Procedures
3. General Operations
4. Routine & Major Maintenance
5. Start-Up 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 safety 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.
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 arrangements for routine inspections,
and preventive maintenance and the scheduling of major maintenance.
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Slide 24-7
POLLUTANTS INFLUENCED BY OPERATIONS
1. Air Pollutants
Smoke
Particulates
Gases
2. Waste-Water Discharge
3. Odor
4. Noise
Operators have responsibilities for maintaining operations within all relevant
permit conditions. These include both air pollutant emissions and waste-water
discharges. Operators are responsible for operating the combustion, heat recovery,
and APCD systems so as to control the smoke, particulates, CO, NOx, organics, acids
and heavy metal emissions from the unit. In addition, operators have responsibilities
for limiting the nuisances of odor and noise in the neighborhood.
Slide 24-8
NORMAL OPERATING SYSTEM CONTROLS
1. Combustion
2. Boiler
3. Boiler Water Treatment
4. Air Pollution Control Devices
Operators are responsible for controlling all major and auxiliary systems found
in MWC units. This learning unit, however, focuses on the combustion and boiler
systems.
Slide 24-9
COMBUSTION CONTROL
Air and Fuel Transients
Operator Activities
Review System Performance
Improve Equipment Setting
24-5
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Operators will need to continually review the combustion conditions by
monitoring the gas temperatures in the fully mixed region (above the overfire air jets)
or the primary and secondary combustion chambers. In addition, the steam pressure,
temperature and flow rate, as well as smoke and all GEMS for flue gas
concentrations should be monitored.
As discussed in Learning Unit 22, 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 MSW 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 grate.
Slide 24-10
GRATE BURNING OPERATOR CONTROL
1. Underfire Air to Each Zone
Damper Controls
Supply Air Pressure
Draft
2. Fuel Bed
Waste Feed Rate
Bed Thickness & Uniformity
Bed Agitation
3. Overfire Air Supply Pressure
The grate burning system is considered to have a drying zone, a pyrolysis zone,
a burning zone and a burn-out zone. Ideally, the active burning region will be
appropriately located relative to the main furnace geometry. The distribution of
underfire air will affect the location of the burning region and the rate of waste burn-
out. The ash should appear to be completely burned as it falls from the grate.
Combustion control is generally achieved by modulating air supply, grate
agitation, and fuel delivery. Operators may exercise control on the underfire (or
under-grate) air using dampers which are designed to maintain the desired air flow
distribution and draft conditions.
In many systems, the overfire and underfire air supplies are coupled
parameters, so that adjustments of a damper for one will influence the other.
Operators must know how these system characteristics will affect unit performance.
The solid MSW must be properly moved through each zone by grate actions so
that the bed thickness and bed uniformity are maintained. A non-uniform air supply
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can be caused by the plugging of air nozzles or by the influence of bed thickness on
air distribution. Thicker bed regions have more resistance to air flow than thin bed
regions, so the thick beds get less than the desired air flow and the thin beds get too
much. The channelizing of air leads to local hot spots of intense combustion, causing
glass melting and clinker formation. Clinkers then act to restrict the air flow.
Operators can improve the uniformity of bed combustion conditions by
maintaining the integrity of the MSW mixing and feeding and by controlling the
underfire air distribution and grate conditions.
The waste feed rate will determine the total heat input and must be matched
to the air flow rate. However, waste feed has a fairly slow response time, whereas
changes in underfire can have an immediate impact upon the overall combustion
conditions. The unique response time characteristics of the unit must be considered
in the adjustments of combustion control systems.
The overfire air supply must be appropriately operated to provide the mixing
of combustible gases and oxygen for complete combustion. Overfire air nozzles are
generally designed to provide adequate penetration and mixing in the gaseous region
above the grates. Operators can maintain the air plenum pressures, damper settings
and fan performance to assure that mixing is adequate.
Slide 24-11
BOILER CONTROL
Drum Level
Load
Steam Temperatures
Feedwater Conditions
Operator Activities
Review System Performance
Make Furnace Observations
Soot Blowing (Automatic/Manual)
Detect Tube Failures
Drum level controllers and the control features of conventional fossil fuel and
MWC-fired power boilers were introduced in Learning Unit 22.
MWC combustion performance is greatly improved when the operating load
(steam flow) is as steady as possible. Conventional power plants are designed to meet
transient demands, but MSW fuels have such large variability in fuel properties that
meeting transient demands is difficult. Most MWC units, however, have control
systems which can meet the anticipated fluctuations in steam demand.
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The temperature of the steam delivered from the unit is dependent upon a
number of variables such as the following: steam flow, steam pressure, cleanliness
of the radiant and convective heat absorbing surfaces, level of excess air, air
preheating, and general flame conditions, as well as the combustion control system.
In general, changes in the fuel charging rate occur too slowly to be primary elements
in temperature control. Most plants use steam or water attemperators to achieve
steam temperature control.
Operators are able to maintain appropriate steam temperature conditions by
reviewing performance results and making regular furnace observations to check the
condition of the deposits on the heat exchange surfaces. Although rapping or soot
blowing may be performed automatically, supplemental manual operations may be
able to improve steam temperature control and combustion system performance.3
Operators will want to compare the feedwater and steam flow rates. An
increased feedwater rate which is not in response to an increased steam supply could
be an indication of a boiler tube failure or other steam system leakage. Because of
the pressure associated with such leakage, adjacent steam pipes may be cut by the
leaking steam. In addition, steam tube failures can release steam into the
combustion zone, lowering the flue gas temperature and causing a deterioration in
the combustion conditions. A significant increase in carbon monoxide could also be
caused by a boiler tube failure.
If a tube failure is confirmed by other evidence, such as a visual observation
or hearing the steam leak, the normal operator response is to take the unit off-line
as soon as possible to inspect and repair or plug the ruptured tube(s).
Slide 24-12
BOILER WATER TREATMENT
Oxygen & Dissolved Gases
Carbonates
Acidic or Alkali Conditions
Operator Activities
Monitor Conditions
Chemical Treatment
Slowdown
Boiler water treatment is introduced in this learning unit and will be covered
in greater depth in Learning Unit 26.
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Operators must pay particular attention to the conditions of the water in the
boiler. Water contains various impurities which can cause corrosion and pitting of
metal as well as scale build-up. If left untreated, these conditions can lead to metal
failure and the formation of blisters, bags and/or burned out heat exchange surfaces.2
Oxygen, carbon dioxide and other dissolved gases can cause corrosion and
pitting. The gases released from the water in the deaerating feedwater heater are
typically purged by venting. 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.
To avoid scale formation, calcium carbonate and magnesium carbonate in the
water supply must be treated and removed. These compounds generally enter by the
leaking of raw water through cracks in the condenser. The carbonates can be treated
by the addition of an alkali substance, such as sodium hydroxide or caustic soda,
which converts the carbonates to a precipitate. Note that high alkali levels can cause
caustic embrittlement of metals.
The alkali level in the feedwater can be monitored using a pH meter, which
can also indicate any acidic condition. The monitoring of pH in both boiler water and
in feedwater is very important because significant damage due to scale build-up
and/or acid corrosion can occur in a short period of time.3 The pH levels in boiler
water5 should range between 8.0 and 9.5. The pH number of 7.0 indicates a neutral
condition, with larger values indicating alkaline (basic) conditions and values below
7.0 being acidic.
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 the steam (pure water) leaves the impurities
behind.
The dissolved solids in the boiler water must be removed by blowdown, a
process where a portion of the water is discharged from the boiler and replaced by
treated make-up water. Such impurities could cause foaming or the trapping of
steam bubbles below the water surface in the steam drum. Foaming also can cause
the carryover of slugs of water into the steam lines, which create water hammer and
the rupture of steam headers and steam lines and damage to steam turbines.
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Slide 24-13
COMBUSTION SYSTEM START-UP
1. Prepare Boiler For Ignition
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
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 the manufacturer's
recommended procedures.
During the initial system start-up, steam leaks can be detected by conducting
a full-scale hydrostatic test. This test includes filling up the boiler with water and
pressurizing the boiler section to 50% above its rated pressure.2
Special gas-side cleaning, boiler chemical cleaning and/or static testing with
at the system pressure may be required, depending on the conditions. In addition,
the routine testing of various components, such as fans, pumps and safety valves,
may be required. Inspection includes using the proper procedures for clearing safety
tags from breakers and on hatch doors.
Valves should be correctly positioned for start-up according to the
manufacturer's standard operating procedures. This generally includes provisions for
blocking off and draining appropriate portions of the steam-circuit headers. The
superheater and reheater vents should be opened so that any residual moisture will
be boiled off.
Prior to starting up or restarting a unit, the boiler should be inspected. For
instance, during an annual outage, ultrasonic testing (UT) is generally performed to
identify the locations where the tube thicknesses has been substantially reduced due
to corrosion and erosion.
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.
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Slide 24-14
COMBUSTION SYSTEM START-UP
2. Warm Up Boiler
Purge Air & Ignite Burner
Maintain Minimum Air Flow
Vent Air from Drum & Headers
Limit Thermal Stresses
Vent Steam from Economizer
Boil-Out the Superheater
Prior to auxiliary fuel ignition, the gases in the furnace 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.
The auxiliary fuel burner generally has an ignition sensor which will trip the
fuel flow if the flame is extinguished. A purge period will again be required before
attempting to relight the burner.
During warm-up, there will be little or no steam flow through the superheater.
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 from thermal stresses, the gas temperatures leaving
the furnace should be limited as recommended by the manufacturer. The
temperature rise profile is generally specified by the manufacturer, with heat-up
values typically limited to around 200 °F per hour.3 When the manufacturer-
specified operating conditions are met, steam flow to the superheater and other parts
of the steam circuit can be initiated.
Slide 24-15
COMBUSTION SYSTEM START-UP
3. Begin to Charge MSW
Ignition
Enable Automatic Controls
Monitor Auxiliary Systems
24-11
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The charging of MSW can only begin after the unit has been brought up to the
required temperature conditions.
The sequence of events associated with increasing the level of underfire air,
overfire air, and MSW charging will vary from unit to unit. This condition may be
controlled manually until the unit is up to load before switching on the automatic
combustion control system. Some units may have automatic combustion controls
which operate through the start-up process.
Slide 24-16
COMBUSTION SYSTEM UNIT SHUTDOWN
Stop Feeding Waste into Unit
Burn the Fuel on the Grate
Operate Auxiliary Burners as Necessary
Allow Steam Pressure to Decay
Limit the Cool Down Rate
Maintain APCD Temperatures
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 MSW charging
and maintaining temperatures through the use of auxiliary fuel firing equipment.
The ash handling equipment should remain operational until after the ash from the
bed material has cleared the grate.
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 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 modular, refractory-wall unit include locking
out the loader, maintaining chamber temperatures with the auxiliary burners, and
continuing blower operation until after residue burn-out has been obtained.
Removal of solid residues from the grate or hearth areas are typically
accomplished by operation of the movable grates or ram devices. To avoid being
burned, personnel should enter the grate or hearth area only after assurance that the
ash has been removed or sufficiently cooled below the bed surface.
24-12
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Slide 24-17
APCD SYSTEM START-UP, SHUTDOWN, UPSET
Water Freeze Protection
Preheat Fabric Filter
Dew Point Controls
Flue Gas Redirection upon Bag Rupture
Controls to Prevent Slurry Solidification
High Temperature Protection of Bags
APCD systems have a number of unique features associated with their
operation under normal, start-up, shutdown, and upset conditions. Potential
problems range from preventing the freezing of liquids to the prevention of acid gas
corrosion of metal surfaces and the protection of fabric filters against blinding.
Depending upon the design of the freeze protection application, operators may
be required to monitor the system to assure that it is operating properly or to be
assured that the protection system has been enabled.
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 the potential solidification problems.
Fabric filters (FF) can become "blinded" by small particulates becoming lodged
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 in the process pass through the dew point. Condensation of
moisture on the fabric can cause particulates to solidify, resulting in blinding.
Provisions to avoid blinding include using the auxiliary fuel pre-heat cycle 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 go up. 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.
24-13
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If the flue gas temperature were to rise above the maximum operating range
of the fabric filters, some protection feature will be required to avoid melting the
fabric. Many systems will initiate an upstream water spray into the flue gas to
provide evaporative cooling.
Another form of temperature protection would be to open the damper to a by-
pass duct. Of course, the operator will need to determine what caused the upset to
occur and take corrective measures.
APCD systems can have special heaters (electrical or auxiliary fuel fired) to
maintain temperatures above the dew point. Other designs will emphasize thermal
insulation and damper controls to prevent moist gases entering the unit during a
shutdown.
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,
177-293.
3. Joseph G. Singer, Combustion, Fossil Power Systems, 3rd Edition, Combustion
Engineering, Inc., Windsor, CT, 1981, pp. 20-1 to 20-31.
4 PEI Associates, Inc., Combustion Source Inspection Module. Student Reference
Manual. Submitted to U.S. Environmental Protection Agency, September 1990,
pp. 242-245, 272-291.
«
5. Steam. Its Generation and Use. 39th Edition, Babcock and Wilcox, New York,
1978, pp. 34-1 to 34-14.
24-14
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25. TROUBLESHOOTING OF COMBUSTION UPSETS
Slide 25-1
TYPICAL COMBUSTION UPSETS
1. MSW/RDF Charging System
2. Grates
3. Combustion Air Supply
4. Waterwalls/Tubes
5. Ash Handling
6. Power Failures/Excursions
The troubleshooting of combustion upsets generally focuses on corrective
actions associated with the component equipment listed above.
Components may malfunction because of a number of reasons, including:
normal wear, operation outside of design conditions, improper lubrication, blockages,
and electrical failures (unit kicked off line, frequency excursions).
Upsets may also be caused by operator error, such as by reacting to a situation
before thinking of all the ramifications of the action or by reacting too slowly to an
emergency situation.
Slide 25-2
INDICATORS OF COMBUSTION QUALITY
1. Opacity
2. Carbon Monoxide
3. Temperature (Furnace & APCD)
4. Oxygen
5. Visual Appearance of Fire
6. Total Hydrocarbon
7. Furnace Draft
8. Air Supply Pressures
A number of instruments can be used to indicate the quality of the combustion
conditions. Typically, opacity is used to indicate combustion and/or APCD system
performance. Combustion quality is primarily indicated by carbon monoxide and
temperature monitors.
In addition, furnace draft, supply air pressure, and oxygen monitors can
provide important information about combustion conditions.
25-1
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Slide 25-3
PERSONAL COMBUSTION OBSERVATIONS
Combustion Conditions
Bottom Ash
Personal visual observations of the fuel bed and in the above-bed regions can
provide valuable information as to the combustion conditions. Viewing the
combustion region through observation ports or with the aid of TV monitors can
provide information about the movement of the burning zones, intensity of
combustion, bed thickness and uniformity, clinkers and slag deposits, and the
presence of undesirable oversized materials which were included in the feed.
Sparklers in the flue gas convection passes indicate excessive entrainment.
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. The ash should be gray in color and have no recognizable combustible
components, such as paper materials.
In addition to visual observations, operators will detect some component
malfunctions by hearing abnormal equipment noises and sensing vibrations. For
example, a high pressure steam leak associated with a tube failure may be heard.
Slide 25-4
FUEL PREPARATION & HANDLING
1. Wide Swings in Fuel Properties
Fuel Moisture: Mix Wet & Dry MSW
2. Feed Hopper/Conveyor - Bridging
Maintain Proper Charging Level
Redirect Undesirable Materials
3. Grapple/Loader Breakdown
4. Pit Fire
Charge Into Unit
Extinguish with Water/CO2
As discussed in Learning Unit Number 8, a number of general conditions must
be maintained to avoid combustion upsets associated with the fuel preparation and
handling system. Problems associated with wide swings in fuel properties can be
avoided by mixing the waste in the pit before it is charged into the combustor.1
25-2
-------
Inspection, removal, and redirection of undesirable materials can prevent some
of the problems of system blockages and explosions. If a conveyor or hopper blockage
occurs, the standard remedy should be consistent with the design of the unit and
should include consideration for the safety of personnel.
A jammed charge hopper is the major cause for an upset condition in the
combustor. Avoiding overloading should help prevent the bridging and jamming of
the charge hopper.1 Maintaining the appropriate level of MSW in the charge hopper
is generally required for obtaining a uniform distribution of fuel on the fuel bed.
Hopper fires can be caused by improper combustor operations which allow
combustion to move from the fuel bed into the feed hopper. To avoid hopper fires,
feeder systems are required to maintain a proper air-lock condition. Such design
provisions can include a water-cooled guillotine or resistance door which maintains
the proper level of compaction of the charge in the hopper.
The maintenance of the applicable grapples and cranes or loaders is important
to continuous operations. Inspection and routine maintenance of crane/grapple units
will include routine consideration of cable or hydraulic system replacement.
Pit fires can be caused by breakage of chemical bottles, spontaneous
combustion, and smoldering ashes from fireplaces and barbecues. Ashes have good
insulating properties and may be hot enough to cause ignition for hours after being
discarded. Pit fires can be difficult to put out, with the operator's response dependent
upon the situation. Remedies include placing the smoldering material into the charge
hopper and extinguishing the materials with water2 or other extinguishing material.
Pit fires are not a typical problem in RDF plants because the MSW is normally
processed into RDF fairly soon after it is received. However, RDF processing plants
have their own problems, including the potential for serious shredder explosions.
Design features will determine the tendency of a grapple to be damaged by
exposure to burning materials. The hydraulic lines of hydraulic grapples may be
destroyed by hot materials, whereas cable-operated grapples may be undamaged by
such activity.
25-3
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Slide 25-5
2
3
UPSETS ASSOCIATED WITH FUEL PROBLEMS
Improper Feed Rate
Too High - Excessive Gas Temperatures
- High Steam Production
- Poor Burn-out of the Ash
Too Low - Insufficient Fuel
- Low Combustion Temperatures
- Low Steam Production
Improper Fuel Bed Thickness
Too High - Improper Air, Poor Burn-Out
Too Low - Entrainment
Sudden Change in Fuel Properties
High Moisture - Reduced Temperatures
High Volatiles - Increased Temperatures
High Inorganics - Reduced Temperatures
In most MWC systems, the feed rate is controlled by the steam demand. To
obtain increased energy production, the energy release rate must be increased.
Agitation of the burning grate and the delivery of underfire air to the burning grate
zone are the major parameters used to control the energy release rate.3
Slide 25-6
UPSETS ASSOCIATED WITH FUEL PROBLEMS
REMEDIES:
Regulate Grate Agitation
Regulate Underfire Air Supply
Regulate Charging Rate
Change MSW Mixing Conditions
Modify Trim Control System Settings
The production of steam actually responds fairly slowly to changes in the fuel
charging system. This is because of the relatively long residence time on the grate,
which is caused by the need for fuel drying and heating before the new fuel
contributes to the combustion process. In fact, a new charge of MSW fuel can be
thought of as a temporary heat sink.
However, if the fuel has a high plastic content, it could cause a burst of
volatiles to be emitted. High rates of volatilization of plastic bags can be a special
problem if large quantities are charged without mixing with other waste.
25-4
-------
Trim control units are required to fine-tune the air delivery system to control
combustion gas temperatures and carbon monoxide. An oxygen trim control system
has been typically provided, although carbon monoxide trim systems are available
with modern microprocessor-based combustion control systems.
A sudden charge of wet MSW can upset the combustion conditions by acting
as a heat sink. In general, this upset is responded to by increasing the underfire air
supply and reducing the fuel charging rate. Another technique is to reduce the fuel
bed thickness so that better drying can occur.
Of course, the crane operator can be requested to provide better mixing of the
MSW. Mixing is not generally a problem in RDF systems.
Slide 25-7
COMBUSTION AIR UPSETS
Underfire Air Supply
Low Pressure - Inadequate Oxygen
High Pressure - Excessive Entrainment
Poor Distribution (Front/Rear)
Overfire Air Supply
Low Pressure - Inadequate Mixing
High Pressure - Excessive Gas Cooling
Poor Distribution, Mixing
Fuel Bed Thickness, Clinkers
Too Thick - Delayed Burning
Too Thin - Particulate Entrainment
Clinkers - Prevents Air Flow
Air Intrusion from Feed Hoppers
Inadequate control of combustion air will generally be detected through
excessive carbon monoxide and/or smoke emissions. Abnormal combustion
temperatures could also indicate problems with the air supply and distribution. Air
control problems include those caused by fan and damper failures. Clinkers are
formed because the temperatures often exceed the ash fusion temperature.
25-5
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Slide 25-8
COMBUSTION AIR UPSETS
REMEDIES:
Check Draft Gage Readings
Adjust Fan Controls/Dampers
Modify Fuel Charging Rate
Remove Clinkers
Operators should be cautioned that control of combustion air upsets will
depend upon the site-specific design features of the air supply and the combustion
systems. The comments presented below are designed to be general in nature, and
as such will not apply to every situation.
Poor combustion air conditions can result from improper operation of the
control system or a fan/damper failure. Such conditions are generally detected by
observing the draft and air pressure gages. Fan dampers may have pneumatic
actuators which mechanically provide the adjustments. A pneumatic system leak can
cause a deteriorated response. Many fan applications include both electric and steam
turbine drives, so shutdown will not be caused by a single energy source failure.
If the fuel bed is too thick, the air flow through the bed will be reduced. If the
bed is too thin, the air flow will be excessive. In either case, improper mixing
conditions can occur. The regions of increased air flow will tend to burn more
intensely and have a tendency for particle entrainment. Compensation for variations
in fuel bed thickness may be obtained by modifying the underfire air flow rate and
its distribution. An obvious response would be to modify the fuel charging rate.
Usually, both the fuel charging rate and the underfire air flow rate are adjusted
together to obtain an optimal bed thickness.
Clinkers on the fuel bed will interrupt the normal air flow and cause
inadequate mixing and combustion. The related ash fusion temperatures are
generally lower under reducing conditions than under oxidizing conditions.4 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 boiler to be shut down for removing the clinker from the grates (sometimes with
a jackhammer).4
Excessive carbon monoxide and/or smoke can also be caused by fuel property
variations. For instance, when a charge of wet MSW is delivered, incomplete
combustion may occur due to reduced gas temperatures.
25-6
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Slide 25-9
COMBUSTION TEMPERATURE UPSETS
1. High Temperature in Upper Region
Refractory or Metal Damage
Excessive Slagging
Remedy:
Increase Overall Air Supply
Reduce the Underfire Air Supply
Reduce the Feed Rate
Abnormal combustion temperatures, carbon monoxide, and smoke can be
caused by improper air delivery and distribution.
In many excess-air units, the general control system is designed to modulate
the secondary air to control combustion temperature. Since excess air is a heat sink,
adding more air will decrease the overall combustion temperatures. However, an
increase of the underfire air will generally tend to increase the rate of production of
volatiles and the intensity of combustion in grate-burning systems.3
Combustion temperatures are typically indicated through the use of shielded
thermocouples which measure the temperature of the adjacent flowing gases.1 The
thermocouples should be shielded or placed in a location where they cannot "see" the
flames of the radiant section. In general multiple thermocouples are provided, so
that a malfunction of one unit will not cause an upset condition. Broken
thermocouples can be routinely replaced. Slag deposits on thermocouples will act as
insulation, causing the thermocouples to give low readings.
Slagging conditions on the heat exchanger surfaces can reduce heat transfer
to steam and cause an increase in flue gas temperatures leaving the convective
section.
Slide 25-10
COMBUSTION TEMPERATURE UPSETS
2. Low Temperature in Upper Region
Inadequate Combustion
Inadequate Energy Production
Remedy:
Increase Underfire Air Supply
Decrease Overfire Air Supply
Increase the Feed Rate
Increase Auxiliary Fuel Burning
25-7
-------
Low combustion gas temperatures can be caused by low fuel feed rate and by
the delivery of low quality fuels. Fuels having excessive moisture or inorganic
content will have reduced heating values.
MSW/RDF feeder and auxiliary fuel burner control systems can be used to help
maintain combustion gas temperatures. Auxiliary burners are generally provided for
pre-heating the combustion chambers prior to the introduction of waste by the
charging systems.
Some combustion units use flue gas recirculation as a heat sink to control
temperatures. If flue gas is recirculated, the automatic controls will control the
combustion temperatures by regulating the damper positions to modulate the amount
of recirculated flue gas.
Slide 25-11
FURNACE DRAFT CONDITION UPSETS
1. Excessive Draft
High Velocities and Poor Mixing
Excessive Particulate Entrainment
2. Inadequate Draft
Low Velocities and Pressure
Transients, Puffing
3. Operation with Positive Pressure
Exterior Fly Ash Accumulation
Gases/Smoke Leaking Out of Furnace
Combustion Quenching
Pollutant Exposure to Personnel
Damage to Furnace Structure
Torching - Flames Down Thru Grates
Damage to Grates & Air System
In addition to the reading of draft gages, a general indication of improper draft
control would be smoke emissions from cracks or openings in the combustion chamber
walls and/or the unexpected build-up of fly ash around the unit's exterior.
Transients in the gas velocities are associated with improper fan/damper and/or
fuel charging system operations. These conditions can lead to poor mixing and cause
products of incomplete combustion to be formed.
Operating a furnace at an improper draft or at a pressure greater than
ambient can cause problems with the combustion unit. The recommended amount
of draft is designed to assure that appropriate gas velocities are maintained. If
25-8
-------
pressure changes occur, gases can not only change their speed but also their
direction.
Puffing is the condition which occurs when the combustion chamber pressure
becomes positive and gases reverse their directions. Torching is the name of the
condition where the combustible gases flow downward through the grate, causing
damage to the grate, structural members, and underfire air supply system.
Slide 25-12
FURNACE DRAFT CONDITION UPSETS
REMEDY
Balance Forced Draft Fan/Dampers
and Induced Draft Fan/Dampers
The remedy for improper draft conditions is to restore the controlled operation
of the forced draft and induced draft fans and the associated dampers.
Combustion problems are also expected when the unit operates above design
capacity. Many operators will try to operate their unit at the upper levels of its rated
capacity. This could be a problem if the fans are unable to supply adequate air. Fan
performance may be compromised if the fans are not well maintained or if unplanned
air resistance occurs in the dampers or ducts.
25-9
-------
REFERENCES
1. Municipal Waste Combustion Systems Operations and Maintenance Study."
U.S. Environmental Protection Agency, EPA-340/1-87-002, June 1987, pp. 4-18.
2. Joseph G. Singer, Combustion. Fossil Power Systems. 3rd Edition, Combustion
Engineering, Inc., Windsor, CT, 1981, pp. 20-1 to 20-31.
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. 8-1 to 8-25.
4. PEI Associates, Inc., Combustion Source Inspection Module. Student Reference
Manual. Submitted to U.S. Environmental Protection Agency, September 1990,
pp. 242-245, 272-291.
25-10
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26. SPECIAL SYSTEM CONSIDERATIONS I: WATER TREATMENT
Slide 26-1
IMPURITIES OF RAW WATER
Composition Varies with Source
Chemical Wastes
Organic Wastes & Bacteria
Oxygen & Dissolved Gases
Dissolved or Suspended Minerals
Suspended Solids
Water treatment has many complex aspects which are developed in various
references, including 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 treatment are also important operator responsibilities.
Water impurities include those items listed above. 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, and the chloride content of the leakage will often vary with the tides.
Slide 26-2
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: CV, NO3' and S04~ are formed.
26-1
-------
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 (CaC03, limestone), magnesium carbonate
(MgCO3), and sodium chloride (table salt).2
Slide 26-3
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 salts, such as calcium sulfate,
calcium carbonate and magnesium carbonate, have reduced solubility as the
temperature increases. This leads to the important phenomena of deaeration and
scale formation which will be discussed below.
Slide 26-4
BOILER WATER PROBLEMS
Corrosion of Metal Tubes
Scale BuiId-Up Inside Tubes
Contamination of Steam:
Deposits in Tubes & Turbine
Operators must pay particular attention to the conditions of the water in the
boiler. Boiler water contains various impurities which can cause corrosion and
pitting of metal, as well as scale build-up and contamination of the steam. If left
untreated, these conditions can lead to boiler tube failures and damage to steam
turbines.
26-2
-------
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 26-5
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 metal.1**16
As indicated in Slide 26-5, 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 requirements of the particular MWC unit.7
26-3
-------
Slide 26-6
WATER TREATMENT FOR A STEAM GENERATOR
MAKE-UP WATER
STEAM
i
CLARIFIER
SOFTENER
DEAERATOR
VENT
STORAGE TANK
PUMP
FEEDWATER
BOILER
SLOWDOWN
TURBINE
OR
CUSTOMER
SUPERHEATED STEAM
ELECTRICITY
LOW PRESSURE STEAM
CONDENSER
CONDENSATE
PUMP
PURIFICATION
26-4
-------
The condensate, boiler feedwater, and steam circuit generally follows the flow
path indicated in the slide. Note, however, that the actual plumbing system can be
considerably more complicated than indicated in Slide 26-6 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 extracted at an intermediate
pressure from the turbine to heat the condensate and make-up water mixture enough
to control the amount of dissolved gases. In addition to water treatment benefits, the
system thermodynamic efficiency also is increased by this feedwater heating
operation in that the latent energy which would otherwise be lost to the condenser
is delivered to the feedwater.
Slide 26-6 indicates that a condensate purification system can be located
downstream of the condenser. Depending upon the application, condensate
purification may be necessary, particularly if steam is reclaimed from an industrial
application. Such condensate conditioning systems may take different forms,
including that of a water softener or condensate polishing (demineralizer) system.
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 26-7
BOILER WATER PROBLEMS & REMEDIES
1. Dissolved Gases
Metal Corrosion & Pitting
Remedy:
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.3
26-5
-------
Gases are removed by heating water enough to cause the gases to be vaporized
in a deaerating feedwater heater. The operating principle is that the solubility of
oxygen decreases significantly as the water temperature increases.
In addition, chemical deaeration can be achieved by the addition of chemicals
(scavengers) such as sodium sulfite or hydrazine.7 In MWC boilers, 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 26-8.
TRAY-TYPE DEAERATING FEEDWATER HEATER3
STEAM
INLET
HATER INLET
SPRAY
TRAY. SECTION
TO BOILER FEED PUHP
Courtesy of Boiler Efficiency Institute,
The deaerating heater is an "open feedwater heater" which is used to remove
dissolved gases.4 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.
26-6
-------
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 26-9
DEAERATING FEEDWATER HEATER & FLASH TANK4
•Boiler
Vent
Makeup
Water-
Internal
Overflow
-Internal
Overflow
Line
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 turbine,
or it can come from the boiler blow-down after it has passed through the flash tank.
In general, the extraction steam pressure will vary with the design. However,
as larger pressures are specified, the corresponding feedwater temperatures and
vapor pressures are increased.4 To avoid cavitation problems (pump becoming steam-
bound), the design must provide for the net positive suction head (NPSH)
requirements of the pump. Cavitation can be avoided if the height of the deaerator
storage tank above 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 boiler4. In the flash tank,
the contaminants from the blowdown are concentrated in the waste water and pure
steam is delivered to heat the feedwater.
26-7
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Slide 26-10
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.
These minerals are typically found as calcium carbonate and magnesium
carbonate. Such carbonates (or salts) are generally detected by testing for hardness,4
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.4 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.3 Note that high alkalinity levels can cause deposits which weaken
metal and lead to cracking, called caustic embrittlement.
Demineralizers are used in some MWC 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).4
26-8
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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.4 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 26-11
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 these impurities are not removed, their concentration will build up in the
boiler water. High concentrations of impurities 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.
The level of such impurities may be indicated by high alkalinity, the total
dissolved solids, and suspended solids which should be kept below the recommended
limits that depend upon the unit's design (boiler pressure).3'7
26-9
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Slide 26-12
INDICATORS OF WATER QUALITY
1. pH - indicates Acidic/Alkali Quality
<7: Acidic; 7: Neutral; >7: Basic
2. Conductivity of Steam & Feedwater
MicroSiemens/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 indicating instruments
which can provide an indication of water quality. The most common are the pH and
conductivity meters.
A pH meter monitors the acidic or basic characteristics of a solution. The
optimum pH levels in boiler water will depend upon the application.9 A pH number
of 7 indicates a neutral condition, with larger values indicating basic (alkaline)
conditions and smaller values indicating acidic conditions.
The pH in the condensate is generally lower than that in the drum water. The
pH of the boiler water at one typical MWC with 100% condensate return ranges from
9.3 to 10.4, and that in the condensate ranges 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.
The typical conductivity instrument provides direct reading in microsiemens/cm
or microhms/cm (Siemens and ohms are equivalent).11 A typical boiler water
conductivity range is from 75 to 100 microhms/cm, whereas the feedwater range is
from 10 to 15 microhms/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.
26-10
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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.4
The example hardness, silica, and iron numbers which follow were measured
at a MWC unit which has 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.
26-11
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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. Joseph G. Singer, Combustion. Fossil Power Systems. 4th Edition, Combustion
Engineering, Inc., Windsor, CT, 1991, pp. 20-1 to 20-45.
4. Frederick M. Steingress and Harold J. Frost, Stationary Engineering.
American Technical Publishers, Inc., Homewood, IL, 1991, pp. 83-87,277-293.
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 SL428A. 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.
26-12
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27. SPECIAL SYSTEM CONSIDERATIONS II: ELECTRICAL THEORY
Slide 27-1
BASIC ELECTRICITY
Ohms Law
DC vs. AC Current
Electrical Phases
Power
Transformer
Rectifier
A basic understanding of electrical principles is necessary for informed
operation, maintenance, and troubleshooting.1 This learning unit will present the
basic principles of electricity. It will focus on the basic knowledge required for the
understanding of transformers, rectifiers, and electric generators.
This learning unit will generally follow the development of electrical principles
presented in the book, Stationary Engineering, by Steingress and Frost.1
Slide 27-2
ELECTRICITY & CURRENT
Electricity
Flow of Electrons
Direct Current: DC
Steady Flow of Electrons
Current
Rate of Electron Flow
We will begin by denning electricity and some of its related properties.
Initially, we will focus on steady "direct current" (DC), such as that associated with
battery powered systems.
Current is the flow of electrons. When we say, "electric current or electricity
can flow through a wire," we mean that electrons can flow through the wire. The
electrons of steady DC current flow in one direction.
Current is measured in amperes or amps. Current can be thought of as the
rate of electron flow, where one ampere is a very large number of electrons per
second.2
27-1
-------
Slide 27-3
ELECTRICITY - FLUID FLOW ANALOGY
Parameter Electricity Fluids
Flow Rate
Driving Force
Electron Flow/Current
(amps)
Fluid Flow
(gpm)
Electrical Potential Pressure
or Voltage Difference Difference
(volts) (psi)
We will use the fluid flow analogy to help our understanding of electricity.3
Current is analogous to fluid flow rate, which can be expressed in gallons per minute
(gpm).
We know that fluid flow is caused by a pressure difference, such as that
developed by a pump. The electrical driving force is called the electrical potential or
voltage difference. It causes current to flow through a conductor. Electrical potential
is measured in volts and is analogous to pressure, which can be measured in pounds
per square inch (psi).
Slide 27-4
VOLTAGE OSCILLATIONS OF ALTERNATING CURRENT
270 360 450
Osctatton, Degrees
630 720
27-2
-------
The voltage of alternating current (AC) electricity periodically changes, as
illustrated above. AC is the traditional form of commercial electricity which is
produced by most steam turbine generators and is delivered to customers through
interconnecting electrical transmissions lines (the grid). The voltages for standard
A.C. electricity in the United States oscillates at 60 cycles per second (60 Hz), where
each cycle would correspond to an oscillation occurring over 360 degrees, as
illustrated above.
Because of the oscillations, the actual time-average AC voltages and the
corresponding AC currents have values of zero. However, conventional AC
instruments measure representative voltages and currents. The instruments are
designed to give "root-mean-square" values, which are the average of the square of
the values over time. It may be interesting to note that the oscillating features of AC
voltages and currents can be measured by oscilloscopes.
Slide 27-5
OTHER BASIC ELECTRICAL PARAMETERS
1. Conductor - Material Which Permits
Electrons to Flow
2. Resistance - Measures Opposition to Flow
3. Ohm - Unit of Electrical Resistance
4. Insulator - Material with High Resistance
5. Circuit - The Path of Electrical Current
From a Source Through Various
Conductors and Devices
Conductors are materials with properties that permit the flow of electrons.
Materials are generally ranked according to their relative resistance to electrical flow.
Electrical resistance is generally measured in units of ohms.
The resistances of conductors depend upon chemical composition, size
(diameter), and length. Glass objects generally have enough electrical resistance to
be called insulators, whereas metal wires are called conductors. Large diameter
(small gage) copper wires have much lower electrical resistance than thin copper
wires. Electrical conductors must be properly selected so that they will not overheat
and fail (melt) under high current conditions. The current carrying capacity of wire
conductors is dependent upon size (gage) and electrical insulation design.
An electrical circuit is composed of at least one electrical source, various
conductors (or resistors) and/or other electrical devices (e.g., motors) which are
connected together. A battery or some other electrical source is required to provide
the voltage which will cause electricity to flow through a circuit.
27-3
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Slide 27-6
OHMS LAW
Voltage = Current x Resistance
E = I x R
or
I = E
R
Ohms law establishes the basic relationship of steady electrical flow. The
electrical potential (measured in volts) is equal to the current (measured in amps)
times the resistance (measured in ohms). Ohms law uses "E" as the standard symbol
for the voltage or electrical potential. The symbol for current is "I", and the symbol
for resistance is "R".
If any two of these variables are known, the third can be found from Ohms law
by using simple algebra.
Ohms law can be conveniently used in steady applications which involve both
large and small electrical quantities. Many power applications involve high voltages,
measured in kilovolts (thousand volts) or megavolts (million volts). By contrast,
instruments often produce small voltage signals which are measured in millivolts
(one-thousandth of a volt) or current signals which are measured in milliamperes
(one-thousandth of an amp). It is recommended that conversions be made to the
standard units of volts, amps, and ohms before Ohms law is used for calculations.
Slide 27-7
ELECTRICAL POWER
Watt - Unit of Electrical Power
The basic unit of electrical power is a Watt. In power generation, we often are
concerned with large quantities of power which can be measured in kilowatts (kW,
thousand Watts) or megawatts (MW, million Watts). As described later, the
alternative units often used in AC electric generation applications are volt-amps (VA),
kilovolt-amps (kVA), and megavolt-amps (MVA).
27-4
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Slide 27-8
DC ELECTRICAL POWER
Power = Voltage x Current
P = E x I
or
or
(I x R) x I
Iax R
Electrical power is generally defined as the product of the voltage times
current. The electrical power unit is a Watt which has dimensions equivalent to a
volt-amp (VA). Other formulas listed above were obtained using Ohms law
substitutions.
The power relationships can be applied to the example of the resistance
heating of a conductor. If the resistance of the conductor and the voltage across it
can be calculated or measured, the rate of resistance heating can be determined.
Slide 27-9
AC VOLTAGE AND CURRENT RELATIONSHIPS
(EXAMPLE WITH CURRENT LAGGING)
Voltage --Current
I & ' " lio'' "SB"
OocRaton, Degrees
27-5
-------
Note that the AC current is generally not exactly "in phase" with the voltage.
In the example illustrated above, the current is shown to be lagging the voltage by
15 degrees. This is because the horizontal axis is basically time, with the current
reaching its peak value a short time after the voltage peaks.
Slide 27-10
Power
or
or
AC ELECTRICAL POWER
= Voltage x Current x Power Factor
= E X I X COS ©
= (IxR)xIx cos 0
= I2 X R X COS 0
= Ex ( E ) x cos ©
R
= E2 x cos 0
R
The previously developed DC power equations require modification for AC
power, as indicated above.
Slide 27-11
AC ELECTRICAL POWER
Apparent Power is Current times Voltage
• apparent
= E X I, [KVA]
Power Factor:
Power Factor = cos 0 = P/P.pp.r«nt
Reactive Power is Imaginary Power
Pr..ctiv. = E x I x sin 0, [KVAR]
The "power factor" is the correction factor required because the current and
voltage generally are slightly out of phase with each other. If steady current and
voltage readings are obtained across a component of an AC circuit, they may be
multiplied together to yield the "apparent power."
27-6
-------
In electric power generation, the apparent power is often expressed in MVA
units, as it represents an upper production limit. The power factor is defined as the
ratio of the "real power" divided by the "apparent power." The conventional
representation of a power factor is "cos 0," where 0 represents the phase angle
difference between the voltage and current.* For the example where the phase angle
difference was 15 degrees, the power factor would be 0.96.
"Reactive power" is imaginary power which is defined as being equal to the
"apparent power" times "sin 0." For the example of a 15 degree phase angle, the
reactive power is 0.26 times the voltage-current product. "Reactive power" can be
expressed in units of VARS (volt-amperes reactive)5, KVARs (Mlovolt-amperes
reactive), or MVARs (Megavolt-amperes reactive).
MWC units often sell electric power to utilities who stipulate in the contract
the purchase of "real power" (MW). Utilities will typically include a penalty in the
contract if adequate VARs are not produced. VARs requirements are equivalent to
requiring the generated energy to have a particular power factor. An example
contract specifies a 0.9 "lagging" power factor (current lagging the voltage).
The VARs output can be controlled by varying the field excitation of the
generator. The generator can be "overexcited" to produce VARs which can flow to the
plant's motors, providing their excitation and reducing the VARs drawn from the
utility system. Many generators make use of special power-factor controllers which
work in conjunction with the voltage regulator. Also, power-factor drawn by the in-
plant motors from the utility grid can be controlled by using special capacitors.5
Slide 27-12
TRANSFORMER WINDING SCHEMATIC1
Coils
440V
I
Secondary Coil
Primary Coil
Step-Down Transformer
From F. M. Steingress and H. J. Frost, Stationary Engineering.
American Technical Publishers, Inc., Homewood, IL, 1991, printed with
Permission.
27-7
-------
Slide 27-12 illustrates the basic wiring of a transformer, which uses the
phenomenon of electrical induction. Transformers are designed to increase or
decrease voltage in AC circuits. A step-up transformer delivers a higher voltage from
the secondary coil than is received by the primary coil, and a step-down transformer
will deliver a reduced voltage.
The fractional increase or decrease in voltage of a transformer is proportional
to the ratio of the windings of wire in the primary and secondary coils. If there are
twice as many windings oh the primary coil as on the secondary coil, the transformer
will function as a step-down transformer with the resulting voltage at one-half that
of the supply voltage, as illustrated in the slide.
Transformers are essential elements in the connection between the utility grid,
MWC electric generator, and in-plant distribution system. However, the wiring of the
transformers and their protection systems are much more complex than indicated
above.
Design considerations include the wiring of transformers (using either the wye-
delta, delta-wye, wye-wye, or delta-delta configurations) with their special grounding
provisions.5 These electrical design features are beyond the scope of this training
program.
Slide 27-13
SCHEMATIC OF 3-PHASE ELECTRIC CURRENT
Oacteflon, Debate
27-8
-------
Operators should be aware that the utilities transport 3-phase, high voltage,
AC power. Therefore, the MWC units which generate electricity for sale to utilities
produce 3-phase power. Three-phase generation uses three primary conductors which
carry AC currents whose cycles are timed in a regular offset pattern, as illustrated
above. Many circuit designs will use four conductors in order to accommodate a
ground requirement.
The electrical system design is required to accommodate both the utility and
plant requirements. The voltages and voltage differences between the conductors will
depend upon the wiring design. Also, the grounding, transformer connections, voltage
regulation, and circuit breaker systems will be dependent upon the special design
features which are selected.5
The design, wiring, and operational features of 3-phase motors will be left to
the electrical system designers and in-plant training programs.
Slide 27-14
CIRCUIT BREAKER:
Controls the Flow of Electricity
RECTIFIER:
Converts AC Electricity to DC
INVERTOR:
Converts DC Electricity to AC
Special electrical power equipment such as circuit breakers, rectifiers, and
inverters are commonly used in MWC units.
Circuit breakers6 are designed to provide the switching required in electrical
generation and electrical service. Some circuit breakers are provided primarily to
protect the utility grid, whereas others function as a local safety device. The utility
can trip the main circuit breaker as part of the normal and correct functioning of the
network protective system.
Circuit breakers are switches that are mechanically closed after a heavy spring
is compressed and held in place by a latch. Circuit breakers are designed to open
automatically when an external tripping signal is received. The signal may signify
that the current flow on the circuit is too high, the voltage is too high or too low, or
that some fault condition has occurred.
27-9
-------
The signals are often actuated by the action of a relay. For example, a
directional relay is designed to prevent the reversed flow of electricity, such as the
case when a generator receives grid power which causes it to act as a motor.
A rectifier is an electronic device which receives an AC electrical supply and
produces DC electricity. Rectifiers are used to produce the DC electricity which is
used to produce the electrical fields required for particle collection in electrostatic
precipitators. Rectifiers can also be used for charging the batteries which store
energy for emergency power supply systems.
Inverters are used to convert the DC electricity into AC electricity. Emergency
power supply systems are generally designed around storage batteries and inverters.
These are required because AC power is generally required to drive almost all the
electric motors and controls used in MWC units.
REFERENCES
1. Frederick M. Steingress and Harold J. Frost, Stationary Engineering.
American Technical Publishers, Inc., Homewood, IL, 1991, pp. 301-328.
2. Basic Electricity. Field Manual, FM 55-506-1, Department of the Army,
Washington, DC, 22 April 1977, p. 32.
3. Electricity. Theory and Fundamentals. Student Workbook 52C10, US Army
Engineer School, Fort Belvoir, VA, June 1979, pp. 5-7.
4. W. A. La Pierre and B. M. Jones, "Electrical and Electronics Engineering,"
Mark's Standard Handbook for Mechanical Engineers. Eighth Edition, Edited
by T. Baumeister, et al, McGraw Hill Book Company, NY, 1978, pp. 15-1 to
15-6.
5. John Reason, "Electrical Interconnections," Standard Handbook of Power Plant
Engineering. Thomas C. Elliott, editor, McGraw Hill Book Co., NY, 1989, pp.
5.3-5.24.
27-10
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28. SPECIAL SYSTEM CONSIDERATIONS III: TURBINE GENERATOR
Slide 28-1
ENERGY RECOVERY/CONVERSION OPTIONS
Produce and Sell Steam
Produce and Sell Both Steam & Electricity
Produce and Sell Electricity
Modern MWC facilities are equipped with a variety of systems for converting
heat energy in the waste into a usable and sellable form. A growing portion of new,
large MWCs are being equipped with steam turbines and electrical generators. Such
systems allow either a portion or all of the thermal energy generated in the boiler to
be converted to electrical power.
Three basic system configuration options considered in the initial design of the
facility include a MWC unit which (1) sells only steam, (2) sells both steam and
electricity, and (3) sells only electricity.
The design selection will obviously depend on local economic considerations
(e.g., the sale price of steam and electricity) and the general demand characteristics
of a steam customer. As covered in previous learning units, optimal MWC
performance is achieved when the combustor is operated at its design load on a
nearly continuous basis. Many steam customers will have a varying demand, which
implies that either the boiler load is modulated or that process steam be dumped.
Several facilities have been designed to serve steam customers but have elected
to account for varying steam demand by adding turbine generator sets. In this
manner, the combustor is operated at nearly constant conditions and the amount of
electricity generation is set by the difference between steam customer demand and
the total energy generation by the MWC boiler.
The third option provides for all of the heat release from the boiler to be
converted to electricity and sold to a customer such as the local electric power utility.
This system design option generally permits the MWC boiler to be operated at its
design load capacity on a continuous basis.
28-1
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Slide 28-2
TURBINE GENERATOR SYSTEM CONFIGURATIONS
Steam Turbine
Electrical Generator
Condenser, Hotwell, & Air Ejector
Condensate Pump & Heater
Deaerator
Feedwater Pumps & Heaters
A typical turbine generator set will consist of a variety of major components
and system ancillary components. Steam from the boiler first passes through a
throttle valve which modulates the flow rate and pressure of steam being delivered
into the steam turbine.
Slide 28-3
STEAM TURBINE TYPES & FEATURES
Impulse Steam Turbine
Reaction Steam Turbine
Impulse-Reaction Steam Turbine
Multiple Stages
Conversion of Thermal Energy
Production of Mechanical Energy
Three types of turbines are commonly used: the impulse, reaction and impulse-
reaction turbines. Their designs have many similarities, as described below.
Steam turbines consist of many stages of airfoil blades which convert energy
in the steam to rotating shaft power. Steam pressure and temperature decrease from
stage to stage in a steam turbine.
As the pressure is decreased, the density drops which causes the steam to
expand and occupy more volume. This results in the progressive increases in turbine
wheel diameter and blade length as steam moves through each successive stage.
28-2
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Slide 28-4
IMPULSE TURBINE BLADE CONFIGURATION & FLOW PARAMETERS1
Fixed Blades
Revolving Blades
Revolving Blades
Second-stage
Node
Second-stage
Revolving
Blades
Initial
Steam Pressure
I I
Initial
Steam Velocity
—
\ ^ I
Exit
Steam Pressure
I I
Exit
Steam Velocity
Tine
From F. M. Steingress and H. J. Frost, Stationary Engineering. American
Technical Publishers, Inc., Homewood, IL, 1991, printed with Permission.
Impulse steam turbines use the impact of high velocity steam to create a force
acting on a blade mounted on a wheel. As illustrated in the above slide, impulse
turbines have multiple stages consisting of a nozzle, two rows of rotating airfoil
blades and one row of stationary blades. The system is designed so that most of the
pressure drops occur through the nozzles rather than the blades. Although the
pressure within a blade section is nearly constant, the velocity drops-ofF considerably.
The rotating blades are attached to disk wheels which are mated to the rotor
shaft while the fixed blades, or stators, are attached to the turbine casing. Expansion
of the steam through -the turbine causes the shaft to rotate and converts the high
pressure thermal energy of steam into mechanical energy (shaft work). The
stationary blades act mainly to change the direction of the steam flow, so the
optimum angle of steam flow exists as the steam enters the second set of rotating
blades.
28-3
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Slide 28-5
REACTION TURBINE CONFIGURATION & FLOW PARAMETERS1
Revolving Blades
Fixed Blades
Initial
Steam Pressure
Fixed Blades
Revolving Blades
Exit
Steam Pressure
Initial
Steam Velocity
\
Exit
Steam Velocity
Time
From F. M. Steingress and H. J. Frost, Stationary Engineering. American
Technical Publishers, Inc., Homewood, IL, 1991, printed with Permission.
Reaction steam turbines make use of reaction forces produced by a flow of
steam through the turbine blades. The difference in the momentum of the flow
entering and leaving the rotating blades causes a mechanical force on the blades
which is transferred to the shaft. The high velocity steam gives up its kinetic energy
(velocity) as it flows through the revolving blades.
The reaction turbine uses fixed blades to act as nozzles. As illustrated above,
steam enters the first set of fixed blades and has its velocity increased as its pressure
drops. The fixed blades also change the direction of the steam flow so it enters the
rotating blades at the optimum angle. The process then repeats itself at succeeding
stages.
Impulse-reaction turbines combine impulse and reaction blading. There are
several stages of impulse blades in the high pressure end of the steam turbine and
reaction blades in the low pressure end of the turbine.
28-4
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Steam turbines used in large MWC installations will typically have many
stages. For example, the turbine generator at the Mid Connecticut RDF plant is
rated at 45 MW and has 21 turbine stages. In typical MWC applications, the
electrical generator shaft is directly coupled to the turbine shaft, so that the
generator rotates at the same speed as the turbine.
The amount of power generated by the turbine is proportional to the flow rate
of steam through the turbine and the steam pressure drop across the turbine. By
modulating the main throttle valve, the total flow of steam and the steam pressure
at the first stage nozzle can be adjusted to provide the required rotor speed at the
desired power output level.
Slide 28-6
STEAM GENERATOR EQUIPMENT & FLOW SCHEMATIC2
The above slide illustrates the typical arrangement for the steam side of a
boiler and turbine generator set and its auxiliary components. As pressure and
temperature drbp through the turbine, the steam loses its superheat and approaches
conditions where it will condense.
28-5
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Slide 28-7
STEAM CONDENSER SCHEMATIC1
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.
Outlet flow from the turbine is directed into a condenser, which 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. A minimum level of water
must be maintained in the hotwell at all times.
The condensate pump extracts the water from the hotwell and directs it to a
device known as a deaerator. The purpose of the deaerator is to remove dissolved
gases from the water before it is pumped to the boiler. To accomplish that objective,
the condensate is heated to its saturation point where dissolved gases will boil off.
Released gases are then vented to the atmosphere. The deaerator operates at a
pressure ranging from around 5 to 75 psig, depending on the particular design of the
plant.
The boiler feedwater pump increases the pressure of deaerated water to the
full boiler operating level. Also at this stage, the feedwater is preheated before it
enters the boiler economizer. As illustrated in the previous slide, heat for operation
of the deaerator and the feedwater heaters is often supplied by extracting steam from
various stages of the turbine.
28-6
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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.
Slide 28-8
AC GENERATOR1
Frame
Rotor
Stator
Slip Rings
Fan
Stator Leads
From F. M. Steingress and H. J. Frost, Stationary Engineering,
American Technical Publishers, Inc., Homewood, IL, 1991, printed with
Permission.
In addition to the steam cycle described above, other key ancillary components
of the turbine/generator are the oil heating and cooling systems and the turning gear.
The shaft in the turbine and generator rotates on bearings which are bathed
in oil. The oil is provided at a pressure (by a lift pump) high enough to maintain an
oil film which carries the load of the rotor. To avoid serious bearing friction, the oil
must continually flow through the system at a proper viscosity. Under no load or cold
start conditions, the viscosity considerations require oil heating. Under full load
operation, oil coolers are used to maintain viscosity by rejecting heat. Because of the
critical nature of the oil flow, turbine generators will be supplied with a redundant
set of pumps and will generally have an emergency system operated by an alternate
power supply, such as DC powered pumps with a battery pack.
28-7
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Once the turbine generator set is placed into operation, the shafts should be
kept in continual rotation, even when the unit'is down and no power is being
generated. The rotor is kept turning to maintain the turbine and generator shaft
alignment and to minimize vibrations in the shaft. The turning gear is a motor
driver which provides rotation of the main shaft, generally less than 10 rpm.
Typically,. a centrifugal clutch mechanism disengages the turning gear whenever
steam flow through the turbine causes the rotor to turn at higher speeds.
The purpose of the slip rings, shown in Slide 28-8, is to transmit DC electric
energy as excitation current to the rotor. The stator leads are connected to switching
devices which transmit the three-phase AC energy from the coils on the stator to the
utility grid.
Slide 28-9
TURBINE GENERATOR OPERATION
Cold Start
Synchronization
Shut-Down
Each turbine generator set will be provided with a detailed set of operating
instructions developed by the vendor.
In large installations, there will typically be an area operator at the turbine
generator as well as remote control from the main control room. In the main control
room, the operator will usually have a major panel devoted to instrumentation and
control of the turbine generator set. Redundant measurements and controls may be
provided for the area operator.
It is critically important that the control room and area operators remain in
close contact especially during system start-up and shut-down.
During start-up, the generator will provide no load to the turbine and the
turbine will be relatively cool (or cold). Bringing the system from that state to full
operational status is a critical responsibility of the operator.
All system components such as condensate pumps, deaerator, feedwater pumps,
feedwater heaters, control valves, recirculation valves, vent valves, and emergency
systems must be sequentially brought to operational status or standby status. All
liquid supply system components must also be at their proper level and automatic
control systems operational.
28-8
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A key responsibility of the operator is to bring the turbine generator to its
operational state at a rate specified by the manufacturer. During the start-up cycle,
the turbine metal temperature will rise to the temperature of the steam supplied by
the boiler. That process can cause severe thermal stress to build within the turbine
and will cause a differential expansion of the casing and internal components. From
a cool start, manufacturers generally require that the casing be warmed at a rate of
about 100 *F per hour and at all times the temperature difference across any metal
wall should not exceed about 150 *F. Procedures associated with preheating of the
steam supply lines and assuring proper operation of emergency and auxiliary systems
may cause the time required for the start-up process to extend over more than a
single operating shift.
In addition to metal temperature concerns, care must be exercised to prevent
the turbine rotor from accelerating too rapidly. After coming off the turning gear, the
rotor is often limited to an acceleration of about 100 rpm/minute until a mid-range
speed is reached. Then a hold period is often required to prewann the rotor and
casing. Typical mid-range rotor speed will be on the order of 1000 rpm, and a hold
time on the order of a half hour or more is not unusual. The manufacturer will also
specify the acceleration rate for the turbine rotor which will assure safe operation
through the rotor's critical speed points. Compliance with the start-up cycle is vitally
important.
The generator rotor will begin to turn as the turbine rotor begins to turn. Fine
adjustments of the rotor speed and excitation will be required to match the generator
electrical voltage and phase characteristics with those of the utility grid. The proper
rotational speed will typically be 3600 rpm to provide 60 Hz, three phase power
output, although some generators are designed to operate at 1800 and 1200 rpm.
Generation of electricity by the system requires that an array of electrical
subsystems be in operation. Provisions will be made for an excitation system and for
adjusting the generator side voltage. During start-up, careful adjustment of
generator side voltage may be required to prevent possible feedback to the generator
end when the generator is synchronized.
Picking up load on the generator must be accomplished in a gradual fashion
and coordinated with adjustments on the steam flow side of the turbine generator.
The rate at which load can be added will depend upon the turbine starting conditions.
For colder starts, the load should be added at a very gradual rate (about 0.5% of
rated capacity per minute). For hot starts, load can be added at a much faster rate
(about 2 to 3 % of rated capacity per minute is typical).
Unit shut-down must rigorously follow a schedule of events which will basically
be a reverse of the start-up procedures. The operator is responsible for assuring that
the sequencing of events in the shut-down schedule are appropriately fulfilled.
28-9
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The preceding material only addresses the broad general features of that
operation. It is the responsibility of the operator to become familiar with the detailed
instructions provided by the turbine generator manufacturer. These instructions
should also be incorporated into a detailed operating procedures manual which is
specific to the overall MWC facility configuration and characteristics.
Slide 28-10
GENERATOR SYNCHRONIZATION WITH UTILITY GRID3
SYNCHRONIZING
LAMPS
L3
ABC
A-C
BUSES
INCOMING
GENERATOR
The synchronizing process requires that the three phases of electricity
generated exactly match with the three phases of the AC buses of the utility grid
before the breaker switch is closed.
The three phases are indicated in the above sketch as "A," "B" and "C." For
the wiring diagram shown, the first synchronizing light (LI) will be off and the other
two will be on when the three phases of electricity are appropriately oriented.3
28-10
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Slide 28-11
TURBINE GENERATOR SYNCHRONIZATION
Synchroscope: Phase Angle Meter
Clockwise Rotation
Counterclockwise Rotation
Stationary Indicator
Indicator Pointing Upward
A synchroscope is a phase angle meter which is provided to indicate any
frequency mismatch and the difference in the phase angle between that of one leg of
the generated power and the corresponding leg of the grid.4 The operator uses
information from the synchroscope to adjust turbine speed and excitation energy,
thereby synchronizing the generator to the grid before closing the switch gear.
If the synchroscope indicator rotates counterclockwise, it is an indication that
the frequency of the generated energy ("incoming" to the grid) is greater than that of
the grid.4 A clockwise rotation would indicate a lower generator frequency than that
of the grid.
A stationary synchroscope pointer indicates that the frequencies are matched.
The angular displacement of the pointer is an indication of the phase difference
between the incoming and grid energy. The indicator is stationary and points directly
upward when the phase of the incoming energy is matched to that of the grid.4
Slide 28-12
TURBINE GENERATOR ABNORMAL CONDITIONS
Water Induction
Excessive Vibration
High Bearing Temperatures
High Back-Pressure
Speed Control
A number of potential abnormal turbine generator operating conditions are
listed above.
Water induction can be caused by the flow of relatively cold vapor or liquid
water from the feedwater heater through the steam extraction line and into the
turbine casing.5 Water induction can cause severe thermal stress problems which can
cause distortion of the casing and destruction of turbine blades. It can be controlled
by installing automatic block valves and proper feedwater heater maintenance.
28-11
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Excessive vibration of the turbine-generator may be an indication of potential
problems, which can cause serious damage to the unit. Many modern turbine-
generators are instrumented to "trip" if pre-set vibration limits are reached.
Likewise, high bearing temperatures can be caused by an inadequate cooling
water flow rate, excessive cooling water temperature, or by the lack of lubrication.
Most modern turbine-generator sets have bearing temperature controllers which will
"trip" the unit if excessive temperatures occur. If this happens, the turbine
manufacturer should be contacted for assistance.
It is possible for the generator to act as a synchronous motor and drive the
turbine. This phenomena, which is called "motoring," may occur during low load
operations, such as during the turbine generator start-up and shutdown.5 The
turbine blades can become overheated and damaged due to the lack of steam flow,
which normally controls the blade temperatures. Modern systems generally
incorporate reverse power relays to automatically prevent this occurrence.
High steam turbine back-pressure or low vacuum in the condenser can be
caused by inadequacies in the cooling water system. High back-pressure can result
in overheating of the low pressure blading if the problem is not corrected.
Temperature sensors or vacuum gages can be set to trip the turbine automatically.
Problems with speed control can be attributed to the turbine governor, voltage
regulator or mechanical problems in the control system. Most units now incorporate
mechanical or electrical over-speed limit controls.
REFERENCES
1. Frederick M. Steingress and Harold J. Frost, Stationary Engineering.
American Technical Publishers, Inc., Homewood, IL, 1991, pp. 227-275.
2. Kenneth Wark, Jr., Thermodynamics. Fifth Edition, McGraw Hill Book
Company, New York, 1988, p. 739.
3. Basic Electricity, Field Manual, FM 55-506-1, Department of The Army,
Washington, DC, April 1977, pp. 299-304.
4 Joe Kaiser, Electrical Power. Motors. Controls, Generators. Transformers.
Goodheart-Willcox Company, Inc., South Holland, Illinois, 1991, pp. 166-186.
5. George Woodward, "Plant Operations," MWC Operator Training Overview
Course Manual. Edited by John Eppich, ASME Professional Development
Program, West Palm Beach, FL, February 24-26, 1992, Lesson V.K, pp. 1-17.
28-12
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29. RISK MANAGEMENT I: PREVENTIVE MAINTENANCE
Slide 29-1
RISK MANAGEMENT PRINCIPLES
1. Achieve a Fair Return
2. Minimize Potential for Losses
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 also minimizes the potential
for financial losses.
This learning unit considers the general aspects of preventive maintenance as
a part of a risk management program. The safety aspects of risk management will
be addressed in Learning Unit Number 30.
Slide 29-2
ASPECTS OF RISK MANAGEMENT
1. Insurance Against
Production & Casualty Losses
2. Evaluation of Current Conditions
3. Evaluation of Probability
4. Consideration of Economics
5. Consideration of Intangibles
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 unit 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 conditions on-site, the operating experiences at other similar
facilities, and the general experiences of the industry.
A broader view of risk management includes the considerations of the
29-1
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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 various economic
aspects of production and loss. Since operators are generally considered to be an
important part of management, they have responsibilities to be open to staff
concerns about values and costs, as well as suggestions for system improvement.
Slide 29-3
POTENTIAL ECONOMIC LOSSES
1. Cost of Preventive Maintenance
2. Personal Injury
3. Equipment Repair/Replacement
4. Lost Revenue - Tipping Fees
5. Lost Revenue - Energy Sales
6. Extra Landfill Costs
7. Extra Transportation Costs
8. Fines - Regulatory Violations
9. Contractual Noncompliance Losses
General economic considerations include balancing applicable costs with the
revenue associated with unit operations. Operating costs can be determined from
financial records, as can the capital costs associated with equipment.
The economic aspects of risk management 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.
Slide 29-4
OPERATOR RESPONSIBILITIES
1. Safety
2. Production (System Operations)
3. Preventive Maintenance
4. Corrective 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.
29-2
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A combustion unit must be properly operated and maintained so that it can
perform reliably, efficiently and safely over its expected life. Consequently, there are
considerable economic risks associated with an improper maintenance program.
Preventive maintenance (PM) consists of planned maintenance actions
performed to prevent equipment breakdown. Corrective maintenance consists of the
repairing of equipment that has failed or malfunctioned.2
Operators also have responsibilities for record keeping and communications.
Operators help address these issues in the development of policies and standard
operating procedures. For instance, many modern facilities now 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.2
Slide 29-5
GOALS OF PREVENTIVE MAINTENANCE
1. Minimize Total Operating Costs
2. Enhance Equipment Life
3. Assure Equipment Reliability
4. Restore Unit Performance
5. Minimize Down-Time
A preventive management concern is to minimize the total costs while
preserving the plant's capital investment. Excessive maintenance can represent a
significant cost. Priorities must be set in an attempt to balance the economic
consequences associated with either acting now or deferring maintenance.
"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 both the direct losses associated with
making the repairs, and lost production and potential costs associated with safety
hazards.
Preventive maintenance is performed to assure that the unit can operate
safely, efficiently, and reliably3. The general goals are particularly important in
MWC units because of the increased complexity associated with adequately
controlling the combustion of a fuel with such variable properties.
29-3
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The old maintenance expression, "If it ain't broke, don't fix it," has generally
evolved into "Fix it just before it breaks.2" To some extent, the performance of all
equipment tends to deteriorate with operating time. Therefore, a preventive
maintenance program is designed to assure that productive equipment life is
preserved. Obviously, good operating practices must include many routine activities,
such as maintaining proper boiler water conditions to avoid tube failures and boiler
problems and maintaining the lubricating conditions for rotational equipment like
compressors, fans, and turbines.
Preventive maintenance is also designed to restore performance efficiencies.
Routine operations, such as soot blowing, are important operations which restore
system efficiency.
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, lube oil sample analysis,
ultrasonic testing, infrared imaging and meggering (e.g., measuring electrical
resistance of insulation) of electric motors and wiring.2
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 29-6
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 specified
lubrication requirements and limits operating conditions (temperatures, pressures,
loads, etc.).
29-4
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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 bad bearing can be replaced as a short-term solution.
However, determining and eliminating the cause of the failure, such as correcting an
imbalanced rotor or preventing a corrosion condition, can lead to a long-term solution
to the problem.
Communication with other individuals is also an important aspect of operators'
duties. Discussions and group meetings with auxiliary operators, 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. Knowledge of both the equipment design features and maintenance records
will be important in the decision. 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 29-7
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.4 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.
29-5
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Examples of in-service maintenance may include repairing cranes and leaking
valve seals and replacing thermocouples, GEMS instruments, and control sensors.
Slide 29-8
OUTAGE MAINTENANCE
I. Make & update an Outage Plan
2. Arrange for Materials/Services
3. Make Detailed Inspections
4. Revise Plans as Necessary
5. Follow Proper Procedures
6. Inspect Upon Conclusion
The annual boiler outage is a standard inspection requirement associated with
operating a pressurized boiler. The inspection and repair must be performed in
accordance with applicable requirements. In general, the ASME Boiler Code relates
to pressure vessel requirements and the National Boiler Inspection Code (NBIC)
covers problems of inspections and repairs to boilers and auxiliary equipment that
are not otherwise covered by the ASME Code3. In particular, the NBIC Code relates
to authorized inspectors and to the proper welding procedures and welder
qualifications.
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. 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.
3. Steam. Its Generation and Use. 39th Edition, Babcock and Wilcox, New York,
1978, pp. 36-1 to 36-20.
4. Joseph G. Singer, Combustion. Fossil Power Systems. 3rd Edition, Combustion
Engineering, Inc., Windsor, CT, 1981, pp. 22-1 to 22-23.
29-6
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30. RISK MANAGEMENT II: SAFETY
Slide 30-1
GENERAL HEALTH AND SAFETY
1. Recognition of Hazards
2. Consequences of Exposures
3. Standard Safety Procedures
4. Personal Protection Equipment
As mentioned in the previous learning unit, safety of people and equipment are
prime considerations1 in risk management. Management has the responsibility for
training employees about the health and safety aspects of their jobs. Management
also has the responsibility for enforcing safe practices.
All staff members should be able to recognize potential hazards and know the
consequences of such exposures. They should also know the recommended safety
procedures and be able to use safety equipment.
Operators should be aware of the significant safety risks associated with
operating staff who perform their duties while having certain symptoms of illness.
Slide 30-2
MAJOR HAZARDS OF OPERATIONAL SYSTEMS
1. Water Side Explosions
Due to Loss of Water
2. Gas Side Explosions
Due to Explosive Mixtures
The MWC combustion system operates at high temperatures so that losses of
water to the boiler and/or water-cooled heat exchangers can cause steam pressures
to build up, leading to serious explosions.1 The ASME boiler code requires designs
to include pressure relief valves on the boiler system to prevent such explosions.2
Specific code provisions include the number, type, installation and testing features
of such safety valves.3 Water-cooled heat exchangers may also have such protection.
Explosions can also result from the buildup of combustible gases, such as when
ignition is lost by an auxiliary burner.1 Modern combustion control systems are
designed to prevent such hazards by tripping the fuel flow until an adequate purge
condition has been obtained.
30-1
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Slide 30-3
OTHER MWC SYSTEM SAFETY HAZARDS
1. Exposure to MSW
2. Pit Fires & Explosions
3. Combustion & Boiler Systems
4. Removal of Blockages
5. Observation Hatches/Hopper Doors
6. Operations in Confined Spaces
All operating personnel must be aware that MWC units have potential safety
hazards associated with exposure to MSW. When MSW is received, it contains
components which can cause falls, cuts and diseases. Therefore, personnel should
avoid personal contact with MSW.
Serious burns can occur if individuals contact steam pipes, valves and other hot
metal objects. The improper opening of high pressure steam vents and/or the rupture
of steam pipes can cause severe scalding.
MSW often contains items which could cause pit fires and/or explosions in the
combustion unit. Although flammable liquids and explosives should never be fed into
the unit4, these problems can be limited by system design features and care during
charging.
Generally, observation hatches into the furnace should be opened only after
taking precautions against exposure to furnace pressure pulsations. Materials can
blow through an observation hatch as a result of sootblowing, tube failure or aerosol
can explosions on the fuel bed.5 Therefore, individuals should not stand directly in
front of open furnace ports or doors. Design provisions for safety include delivering
aspirating air to the observation hatch and providing a protective transparent cover.6
Open-ended pipes should not be used for removing slag from walls, as the hollow
pipes can become very hot and can direct hot gases onto the handler.6
Operators should also be aware of the potential for eye damage associated with
intense thermal radiation. Tinted goggles or other eye protection should be used.5
Hot, solid materials can flow out of open hopper doors and cause serious burns
and injuries. Hot ash can remain hot for a long time due to its insulating properties.6
Therefore, one should never step on fly ash.
In addition, it is potentially hazardous to enter confined spaces such as the
equipment cavities of the furnace, steam drum, and baghouse or ESP. Hazards here
30-2
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arise from poor ventilation, space being cramped, limited lighting and exposure to
accumulated pollutants and/or dust.
Confined spaces should be entered only after they have been properly cooled
and ventilated. Appropriate doors and valves should be locked or tagged. Explosion
proof lights and properly grounded electrical extension cords should be used.5
Slide 30-4
Standard Safety Considerations
Electrical Shock
Exposure to Corrosives
Noise & Vibration
Exposure to Rotary Equipment
Awkward Access
Movement of Heavy Objects
Welding and Metal Forming
Fire Hazards
Standard industrial safety considerations include those associated with
electrical shocks, noise, vibrating equipment, exposure to hot metal surfaces, exposure
to rotating equipment, awkward access to equipment, movement of heavy objects,
welding, exposure to corrosives and fire hazards.
For example, the electrical and steam service to rotary equipment should be
"locked out" and "tagged out" before servicing. In addition, it may be appropriate to
have the shaft blocked 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 industrial chemicals. Important data sheets for
MWC 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 features of exposures on humans and animals.
Fire safety is another major concern, particularly as pit fires can be very
serious. Provisions are generally made in the charging device to limit the penetration
of flames from the combustion chamber into the charge hopper. These can include
the use of flame scanners which actuate water sprays6 and water-cooled doors to
reduce temperatures to below that required for ignition.
30-3
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Slide 30-5
PERSONAL PROTECTION EQUIPMENT
1. Ear Protection
2. Heavy Gloves
3. Hard Hat
4. Respirator
5. Goggles
€. ' Safety Shoes
7. Proper Clothing
Standard industrial safety procedures relate to the use of personal protective
equipment, such as that listed above.6'6
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 also be
worn because some synthetic fibers can melt when exposed to hot equipment.6
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.6
Special care should be taken to avoid chemical burns resulting from skin
contact with strong alkalis or acids.
Slide 30-6
SYMPTOMS OF ILLNESS
1. Headaches
2. Lightheadedness
3. Dizziness
4. Nausea
5. Loss of Coordination
6. Difficulty in Breathing
7. Chest Pains
8. Exhaustion
The above symptoms of illness may result from heat stress, inhalation
problems and/or a variety of non-occupational related conditions.6 An impaired
worker is a threat to the overall safety of the unit as well as to fellow personnel.
30-4
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REFERENCES
1. Steam. Its Generation and Use. 39th Edition, Babcock and Wilcox, New York,
1978, pp. 34-8.
2. Frederick M. Steingress and Harold J. Frost, Stationary Engineering.
American Technical Publishers, Inc., Homewood, IL, 1991, pp. 71-82.
3. E. B. Woodruff, H. B. Lammers and Thomas F. Lanuners, Steam Plant
Operations. Fifth Edition, McGraw-Hill Book Company, New York, 1984, pp.
223-230.
4. Louis Theodore, Air Pollution Control and Waste Incineration for Hospitals
and Other Medical Facilities. Van Nostrand Reinhold, New York, 1990, pp.
364-367.
5. Joseph G. Singer, Combustion. Fossil Power Systems. 4th Edition, Combustion
Engineering, Inc., Windsor, CT, 1991, pp. 21-1 to 21-34.
6. John Richards, Municipal Waste Incinerator Air Pollution Control Inspector
Course. Prepared by Entropy Environmentalists, Submitted to U. S.
Environmental Protection Agency, June 1991.
30-5
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Absorption
Acid Deposition
AC
Activated Charcoal
Adsorption
Agglomeration
Air Pollutant
APCD
APPENDIX: GLOSSARY, ACRONYMS, AND SYMBOLS1
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. Generally, they fall into two
different groups (1) those emitted directly by
identifiable 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 incorporated
into the exhaust gas stream of an incinerator to
reduce emissions to the atomosphere of solid or
gaseous pollutants. Such add-on devices include
scrubbers (wet & dry) and fabric filters.
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-1
-------
APTI
Air Pollution Training Institute, USEPA
Air Quality Criteria
Air Quality Standards
ASME
ASTM
Atmosphere, atm
Attainment Area
AWMA
BACT
Baghouse Filter
BDT
Biologicals
Bottle Bill
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
4
American Society of Testing and Materials
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.
Legislation which requires a returnable deposit on
beer and soft drink containers and provides for retail
Appendix-2
-------
stores or other redemption centers.
Bottom Ash
Btu
Burn Cycle
Burn Rate
By-product
C
CAA
CAAA
Cadmium (Cd)
Cap
Carbon Dioxide, CO2
Carbon Monoxide, CO
Carcinogen
Catalyst
Incombustible material remaining after combustion
has been completed.
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.
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.
A colorless, odorless, poisonous gas which is produced
by incomplete combustion of carbon.
Any substance that can cause or contribute to the
production of cancer.
A device that enhances the rate of a specified
chemical reaction.
Appendix-3
-------
CDD
CDF
GEM
CEMS
CERCLA
CFM
CFR
Charge Rate
Chlorinated Hydrocarbon
CO
C02
Combustion
Combustion Product
Compost
Controlled Air
Contaminant
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 (ftVmin)
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.
A mixture of garbage and degradable trash with soil
in which certain bacteria in the soil break down the
garbage and trash into an organic soil conditioner or
fertilizer.
Combustion units which control the air flow rate to
attain the desired rate of combustion.
Any physical, chemical, biological, or radiological
substance or matter that has an adverse affect on air,
Appendix-4
-------
water, or soil.
Coolant
Cooling Tower
Corrosion
Criteria Pollutants
DAS
DB
DC
Decibel (dB)
Desulfurization
DI
Dioxin
DS
dscf
dscm
E
EER
Effluent
A liquid or gas used to remove the heat generated by
power production, industrial, and/or mechanical
processes.
A 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.
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 (Dry Bulb Temperature)
Direct Current (Electricity)
A unit of sound measurement. The loudness of sound
doubles for every increase of ten decibels.
Removal of sulfur from fossil fuels to reduce pollution.
Dry 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
Energy and Environmental Research Corporation
Wastewater-treated or untreated~that flows out of a
Appendix-5
-------
EDF
Emission
Emission Standard
Endothermic
EP
EPA
EPRI
ESP
Excess-Air
Excess Air
Exothermic
Fabric Filter
FBC
FCE
treatment plant, sewer, or industrial system.
Environmental Defense Fund
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.
Extraction Procedure
U. S. Environmental Protection Agency
Electric Power Research Institute
Electrostatic Precipitator: An air pollution control
device that removes particulates 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.
Combustion units operationg with fuel/air mixtures
having greater than stoichiometric quantities of air.
The additional air supplied above the stoichiometric
quantities required for complete combustions, 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).
Chemical reactions where chemical energy is
transferred as heat to the surroundings.
Fahrenheit (Degrees)
A cloth device that catches dust particles.
Fluidized Bed Combustion
Final Control Element
Appendix-6
-------
FC
FD
FE
FF
FGD
FID
Fixed Carbon
Flue Gas
Fly Ash
FR
FT
ft3
Fuel Bed
Fugitive Emissions
FY
g
GC
GC/MS
GCP
Generator
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.
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.
Fiscal Year
Gram
Gas Chromatograph
Gas Chromatograph/Mass Spectrograph
Good Combustion Practice
(1) A facility that emits pollutants into the air or
Appendix-7
-------
GLC
Greenhouse Effect
GW
GWM
H2O
Halogen
Hammer-mill
HAP
HAZMAT
HC
HC1
HDPE
Hearth
Heat Release
Heating Value
Heavy Metals
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.
Ground Water
Ground Water Monitoring
Water
Any of a group of five chemically-related nonmetallic
elements that includes bromine, fluorine, chlorine,
iodine, and astatine.
A high-speed machine that uses hammers and cutters
to crush, grind, chip or shred solid wastes.
Hazardous Air Pollutant
Hazardous Material
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
Appendix-8
-------
HHV
HON
HP
hr
HVAC
HW
HWI
I
IAP
ID
ID
IIA
I/M
in. we
Incineration
Incinerator
Infectious Agent
and tend to accumulate in the food chain.
Higher Heating Value
Hazardous Organic (NESHAP)
Horsepower
Hour
Heating, Ventilating, and Air Conditioning (System)
Hazardous Waste
Hazardous Waste Incinerator
Current (Electrical)
Indoor Air Pollution
Inside Diameter
Induced Draft
Incinerator Institute of America
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.
Ion Exchange Treatment A common water softening method that removes some
Appendix-9
-------
IPM
IR
kg
kW
KWH
LAER
Landfill
Ib
LC
LC
LCRS
LD
LDCRS
Leachate
Lead (Pb)
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
(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
Liquid Chromatography
Logic Controller (Level Controller)
Leachate Collection and Removal System
Land Disposal
Leachate Detection, Collection, and Removal System
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.
Appendix-10
-------
LEL
Lower Explosive Limit
LFL
LHV
LIMB
Limestone Scrubbing
Liner
LNG
LPG
LT
m3
MACT
MD
Medical Waste
Mercury
Methane
Lower Flammability Limit
Lower Heating Value
Limestone-Injection, Multi-Stage Burner
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
leachate form leaking from a landfill. Liner materials
include plastic and dense clay.
Liquified Natural Gas
Liquified Petroleum Gas
Level Transducer
cubic meter
Maximum Acheivable Control Technology
Mail Drop
Any waste, including solid, semi-solid, or liquid
material, which is generated in the diagnosis,
treatment, or immunization of human beings or
animals, in research pertaining thereto, or in the
production or testing of biologicals. This includes
both regulated and non-regulated waste materials.
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.
Appendix-11
-------
MIS
MMT
Moisture
Monitoring
Monofill
MP
MRF
MS
MSDS
MSW
MT
MW
MW
MWC
MWI
Management Information System
Million Metric Ton(s)
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
siting considerations and design provisions for a cap,
leachate containment, monitoring, and treatment
system.
Melting Point
Material Recovery Facility
Mass Spectrometry
Material Safety Data Sheet
Municipal Solid Waste
Metric Ton(s) (1,000 Kilograms)
Megawatt(s)
Molecular Weight
Municipal Waste Combustor
Medical Waste Incinerator: All equipment related to
the medical waste incineration process, including the
feeder to the incinerator, the incinerator, the gas
cleaning equipment, the residue management
equipment, control and monitoring equipment,, and
any boiler or heat exchanger equipment that utilizes
waste heat from the incinerator.
Appendix-12
-------
MWTA
N2
NAA
NAAQS
Natural Gas
NDIR
NESCAUM
NESHAPS
New Source
ng
ng/dscm
NIOSH
NO
N02
NOx
NKDC
NSPS
NSWMA
NTIS
OD
Medical Waste Tracking Act
Nitrogen
Nonattainment Area
National Ambient Air Quality Standards
A natural fuel containing primarily methane and
ethane that occurs in certain geologic formations.
Nondispersive Infrared Analysis
Northeast States for Coordinated Air Use
Management
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
National Resources Defence Council
New Source Performance Standards
National Solid Waste Management Association
National Technical Information Service
Outside Diameter
Appendix-13
-------
O&M
Opacity
Organic
OSHA
OSW
Overfire Air
Packed Tower
PAH
PAN
Particulate Matter
Particulate Emission
Particulate Loading
Pathogen
Pathogenic Waste
Pathological Waste
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 fumes found in the flue gas emissions carried
into the atmosphere.
The mass of parti culate emissions, generally
expressed in mass per unit volume of the air.
Those organisms (e.g., bacteria and viruses) which are
capable of causing disease.
Waste materials that contain organisms capable of
causing an infectious disease.
Waste materials that consisting of anatomical parts
Appendix-14
-------
such as body parts and blood. Also waste materials
relating to the study of the nature of disease.
Personal Computer
PC Pulverized Coal
PCB 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.
PCDD Polychlorinated Dibenzo-p-dioxins
PCDF Polychlorinated Dibenzofurans
PEL Personal Exposure Limit
Permit An authorization, license, or equivalent control
document issued by an approving agency to
implement the requirements of an environmental
regulation.
pH A measure of acidity or alkalinity of a liquid or solid
material.
PHC Principal Hazardous Constituent
PIC Products of Incomplete Combustion
PLC Programmable Logic Controller
PM Particulate Matter
PM Preventive Maintenance
PM-10 Particulate Matter, Sized Less Than 10 Micrometers
PMR Pollutant Mass Rate
POM Polycyclic Organic Matter
ppb Parts per Billion
PPE Personal Protective Equipment
Appendix-15
-------
ppm
ppmdv
ppmv
ppm-weight
ppt
Precursor
Proximate Analysis
PSD
psi
psia
psig
PT
PVC
Pyrolysis
QA
QA/QC
QC
QMO Standard
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.
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
Appendix-16
-------
Incinerator Operators
QRO Standard
Quench Tank
R
RACT
RCRA
R&D
RDF
Refuse Reclamation
Resource Recovery
Retention Time
Risk Assessment
RPM
RTF
SARA
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
Conversion of solid waste into useful products.
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
Appendix-17
-------
1986
SCFM
SCR
Scrubber
SD
SDA
sec
SI
SIP
Sludge
Slurry
Smog
Smoke
SNCR
S02
SOC
SOCMI
Soot
SOP
SP
Standard Cubic Feet per Minute
Selective Catalytic Reduction
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 any of a number of air or
water treatment processes.
A watery mixture of insoluble matter that results
from some pollution control technique.
Air pollution associated with oxidants.
4
Particles suspended in gases after incomplete
combustion of materials.
Selective Non-Catalytic Reduction
Sulfur Dioxide: A heavy, pungent colorless gaseous
air pollutant formed by oxidation of sulfur.
Synthetic Organic Chemicals
Synthetic Organic Chemicals Manufacturing Industry
Carbon dust formed by incomplete combustion.
Standard Operating Procedure
Set Point
Appendix-18
-------
Stack
STALAPCO
Starved Air
Starved-Air
Stationary Source
Stoiehiometric
STP
Stuff and Burn
SWANA
SWDA
TCDD
TCDF
TCLP
TDS
THC
TLV
Toxic
TPD
TPY
A chimney or smokestack; a vertical pipe that
discharges used gases.
State and Territorial Air Pollution Control Officials
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
Dioxin (Tetrachlorodibenzo-p-dioxin)
Furan (Tetrachlorodibenzofurans)
Toxicity Characteristic Leachate Procedure
Total Dissolved Solids
Total Hydrocarbons
Threshold Limit Value
Materials which have the effect of a poison and as
such present an unreasonable risk of injury to health
or to the environment.
Tons per Day
Tons per Year
Appendix-19
-------
TSP
TSS
UEL
UFL
Underfire Air
UV
USEPA
Vapor
Vaporization
VE
Virulence
voc
Volatile Matter
VOST
VP
Wastewater
WB
WTE
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 or openings in the grate.
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
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 disease-evoking power of a microorganism 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 (Wet Bulb Temperature)
Waste-to-Energy
Appendix-20
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-453/B-93-020
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Municipal Waste Combustor Operator Training Program
Course Manual
5. REPORT DATE
April 1993
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S) ~ "
J. Taylor Beard, W. Steven Lanier, and Suh Y. Lee
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Energy & Environmental Research Corporation
18 Mason Lane
Irvine, CA 92718
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-CO-0094
12. SPONSORING AGENCY NAME AND ADDRESS
DAA for Office of Air Quality Planning and Standards
Office of Air and Radiation
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
200 /04
15. SUPPLEMENTARY NOTES
Michael G. Johnston, EPA/Office of Air Quality Planning and Standards
16. ABSTRACT
The Course Manual, along with the Instructor's Guide (EPA-453/B-93-021).
constitute a model State training program to address the training needs of
municipal waste combustor (MWC) operators. The training program focuses on
the knowledge required by operators for understanding the basis for proper
operation and maintenance of MWC's with particular emphasis on the aspects of
combustion which are important for environmental control. The training
program includes general introductory material relative to municipal solid
waste (MSW) treatment and MSW as a fuel. The bulk of the program addresses
the principles of good combustion. The potential sources of air pollution
emissions and their control are discussed. Instrumentation, automatic control
systems, control room operations and practices, and the troubleshooting of
upsets are presented. Special system considerations are included: water
treatment, electrical theory, and turbines and generators. Finally, risk
management procedures such as preventive maintenance and safety considerations
are addressed.
The training program fulfills the requirements of the Clean Air Act of
1990, as amended, for the development of a model State training program.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
Municipal Waste Combustors
Operator Training
Incinerators
Air Pollution Control
13B
18. DISTRIBUTION STATEMENT
Release Unlimited
19. SECURITY CLASS (This Report)
Unclassified
393
20. SECURITY CLASS (Thispage!
Unclassified
22. PRICE
EPA Form 2220-1 (R«x. 4-77) PREVIOUS EDITION is OBSOLETE
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