EPA/625/6-89/024
January 1990
Handbook
Operation and Maintenance of
Hospital Medical Waste Incinerators
Office of Air Quality Planning and Standards
and
Air and Energy Engineering Research Laboratory
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
Center for Environmental Research Information
Office of Research and Development
U. S. Environmental Protection Agency
Cincinnati, OH 45268
Printed on Recycled Paper
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Disclaimer
This document generally describes the proper operation of a hospital waste incinerator. It
is based on EPA's review and assessment of various scientific and technical sources. The
EPA does not represent that this document comprehensively sets forth procedures for
incinerator operation or that it describes applicable legal requirements, which vary
according to an incinerator's location. Proper operation of an incinerator is the
responsibility of the owner and operator.
Mention of trade names or commercial products does not constitute endorsement or
recommendation for use.
11
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Preface
The hospital medical waste incinerator program was funded as a project of EPA's Control
Technology Center (CTC). The CTC was established by EPA's Office of Research and
Development (ORD) and Office of Air Quality Planning and Standards (OAQPS) to
provide technical assistance to State and local air pollution control agencies. Three levels
of assistance can be accessed through the CTC. First, a CTC HOTLINE has been
established to provide telephone assistance on matters related to air pollution control
technology. Second, more in-depth engineering assistance can be provided when
appropriate. Third, the CTC can provide technical guidance through publication of
technical guidance documents, development of personal computer software, and
presentation of workshops on control technology matters.
The technical guidance projects, such as this one, focus on topics of national or regional
interest that are identified through contact wth State and local agencies. In this case, the
CTC became interested in developing a training course for operators of hospital waste
incinerators through a request by the State of Maryland. This document was prepared to
be used as the basis for development of the training materials. The document also is
intended as a technical guide for use by Federal, State, and local agency personnel,
hospital waste management personnel, and hospital incinerator operators.
This document provides information on the operation and maintenance (O&M)
procedures that should be practiced on hospital waste incinerators and associated air
pollution control equipment to minimize air emissions. This document provides only a
general overview of proper O&M procedures with the intention of identifying good
operating practices. Operators of hospital waste incinerators should have O&M manuals
from the manufacturer which provide specific O&M instructions for their equipment.
This document should be viewed as a supplement to the manufacturer's O&M
recommendations.
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Table of Contents
Disclaimer ii
Preface iii
Figures viii
Tables ix
Acknowledgments x
1 Introduction 1
2 Hospital Incineration Systems 3
2.1 Introduction 3
2.2 Fundamental Concepts Related to Hospital Waste Incineration 3
2.2.1 Pathogen Destruction 4
2.2.2 Principles of Combustion 4
2.3 Hospital Waste Characteristics 9
2.4 Types of Hospital Waste Incinerator Systems 10
2.4.1 Introduction 10
2.4.2 Multiple-Chamber Incinerators 11
2.4.3 Controlled-Air Incinerators 14
2.4.4 Rotary Kilns 18
2.4.5 ^Auxiliary Equipment 20
2.5 References for Chapter 2 20
3 Air Pollution Control 23
3.1 Introduction 23
3.2 Pollutant Formation and Generation 23
3.3 Control Strategies 24
3.3.1 Controlling Feed Material 24
3.3.2 Combustion Control 24
3.3.3 Add-On Air Pollution Control Systems 24
3.4 References for Chapter 3 32
4 Operation 35
4.1 General Objectives 35
4.2 Incinerator Key Operating Parameters 36
4.2.1 Introduction 36
4.2.2 Controlled-Air Incinerators 36
4.2.3 Multiple-Chamber Incinerators 39
4.2.4 Rotary Kiln Incinerators 40
4.3 Waste Feed Handling 41
4.3.1 Proper Waste Handling 41
4.3.2 Restricted Wastes 43
4.4 Incinerator Operation, Control, and Monitoring 43
4.4.1 Batch Feed Controlled-Air Incinerator 43
4.4.2 Intermittent-Duty, Controlled-Air Incinerators 48
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Table of Contents (continued)
4.4.3 Continuous-Duty, Controlled-Air Incinerators 50
4.4.4 Multiple-Chamber Incinerators 51
4.5 Add-On Air Pollution Control Systems 52
4.5.1 Wet Scrubbers 53
4.5.2 Fabric Filters ''.'.'.'.'.'. 55
4.5.3 Spray Dryers 57
4.5.4 Dry Injection 53
4.6 References for Chapter 4 59
5 Maintenance 61
5.1 Hospital Waste Incinerators 61
5.1.1 Hourly/Daily Maintenance 61
5.1.2 Weekly/Biweekly Maintenance 63
5.1.3 Monthly/Semiannual Maintenance 63
5.2 Wet Scrubbers 64
5.2.1 Daily/Weekly Maintenance 64
5.2.2 Monthly/Semiannual Maintenance '. 64
5.3 Maintenance of Fabric Filters 65
5.3.1 Daily Inspection/Maintenance 66
5.3.2 Weekly Inspection/Maintenance 66
5.3.3 Monthly/Quarterly Inspection/Maintenance 66
5.3.4 Semiannual/Annual Inspection/Maintenance 67
5.4 References for Chapter 5 67
6 Control and Monitoring Instrumentation 69
6.1 Operating Parameters that Should be Monitored 69
6.2 Typical Instrumentation 69
6.2.1 Temperature Sensors 69
6.2.2 Pressure 70
6.2.3 Oxygen Concentration 71
6.2.4 Carbon Monoxide 72
6.2.5 Opacity '.. " 73
6.2.6 Charge Rate 74
6.2.7 Scrubber Liquor pH 75
6.3 References for Chapter 6 75
7 Operational Problems and Solutions 77
7.1 Operational Problems and Solutions Associated with
Hospital Waste Incinerators 77
7.1.1 Excessive Stack Emissions - Controlled-Air Units 77
7.1.2 Excessive Stack Emissions - Multiple-Chamber Units 78
7.1.3 Leakage of Smoke From Primary Chamber 78
7.1.4 Excessive Auxiliary Fuel Usage 79
7.1.5 Incomplete Burnout-Poor Ash Quality 79
7.2 Operational Problems and Solutions Associated with
Wet Scrubbers 80
7.2.1 Corrosion 80
7.2.2 Scaling ' 80
7.2.3 Erosion 81
7.3 Operational Problems and Solutions Associated with
Fabric Filters 81
_
VI
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Table of Contents (continued)
7.3.1 Opacity 81
7.3.2 Pressure Drop 82
7.4 References for Chapter 7 83
8 Recordkeeping 85
8.1 Manufacturer's Specifications and Literature 85
8.2 Compliance Emission Test Records 85
8.3 Operating Records 85
8.4 Maintenance Records 86
8.4.1 Retrieval of Records 87
8.5 References for Chapter 8 87
9 Safety Guidelines .• 89
9.1 Prevention of Infection During Waste Handling 89
9.2 Equipment Safety Procedures 89
9.3 Fire Safety 90
9.4 References for Chapter 9 91
10 Glossary 93
10.1 References for Chapter 10 97
VII
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Figures
2-1. Major components of an incineration system 4
2-2. Relationship of temperature to excess air 7
2-3. In-line multiple-chamber incinerator with grate 12
2-4. Retort multiple-chamber incinerator for pathological wastes 13
2-5. Schematic of a controlled-air incinerator 15
2-6. Schematic of a single batch controlled-air incinerator 15
2-7. Example intermittent-duty, controlled-air incinerator 16
2-8. Hopper/ram mechanical waste feed system 17
2-9. Incinerator with step hearths and automatic ash removal 18
2-10. Rotary kiln with auger feed 19
2-11. Incinerator with waste heat boiler and bypass stack 19
3-1. Impaction 25
3-2. Spray venturi with circular throat 26
3-3. Spray venturi with rectangular throat 26
3-4. Countercurrent packed tower absorber 27
3-5. Countercurrent-flow spray tower 28
3-6. Pulse jet baghouse 29
3-7. Components of a spray dryer absorber system (semiwet process) 31
3-8. Components of a dry injection absorption system (dry process) 31
6-1. Schematic of an extractive monitoring system 73
6-2. Typical transmissometer installation for measuring opacity 74
Vlll
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Tables
2-1. Stoichiometric Oxygen Requirements and Combustion Product Yields 6
2-2. Examples of Infectious Waste .' 9
2-3. Characterization of Hospital Waste H
2-4. Characterization of Hospital Waste H
2-5. Classification of Hospital Incinerators 12
3-1. Control Strategies for Air Pollutants from Hospital Waste Incineration 25
4-1. Key Incinerator Operating Parameters and Recommended
Operating Range: Controlled-Air Incinerator 37
4-2. Key Incinerator Operating Parameters and Recommended
Operating Range: Multiple-Chamber Incinerator 40
4-3. Example Timed Control Cycle for Batch Mode Incinerator 47
4-4. Wet Scrubber Performance Parameters for Hospital Waste Incinerators ... 54
4-5. Key Operating Parameters for Fabric Filter Control Systems 55
5-1. Typical Maintenance Inspection/Cleaning/Lubrication Schedule
for a Hospital Waste Incinerator 62
5-2. Typical Maintenance Inspection/Cleaning/Lubrication Schedule
for a Wet Scrubber • 65
5-3. Typical Maintenance Inspection/Cleaning/Lubrication Schedule
for a Fabric Filter System 67
6-1. Thermocouple Types 70
6-2. Performance Specifications for Opacity Monitors 74
8-1. Recommended Operating Parameters that Should be Included in
Operating Logs for Incinerators, Wet Scrubbers, Fabric Filters,
and Continuous Emission Monitors 86
IX
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Acknowledgments
This document was prepared by Midwest Research Institute located in Gary, North
Carolina. Principal authors were Roy Neulicht, Mark Turner, Dennis Wallace, and Stacy
Smith. Participating on the project team for EPA were Ken Durkee and James Eddinger
of the Office of Air Quality Planning and Standards, Charles Masser of the Air and
Energy Engineering Research Laboratory, James Topsale of Region III, Charles Pratt of
the Air Pollution Training Institute, and Justice Manning of the Center for
Environmental Research Information. Also participating on the project team were Carl
York and William Paul of the Maryland Air Management Administration.
The review of this document by representatives of the National Solid Waste Management
Association Waste Combustion Equipment Council (NSWMA - WCEC) and the Ontario
Ministry of the Environment is acknowledged and appreciated.
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Chapter 1
Introduction
The main objective of this document is to identify the
operation and maintenance (O&M) procedures that
should be practiced on hospital waste incinerators
and associated air pollution control equipment to
minimize air emissions. Proper O&M, in addition to
reducing air emissions, improves equipment
reliability and performance, prolongs equipment life,
and helps to ensure proper ash burnout. This
document provides only general guidance on proper
O&M procedures with the intention of identifying
good operating practices. The operator of a hospital
waste incinerator should have O&M manuals from
the manufacturer which provide specific O&M
instructions for their equipment. This document does
not provide such specific instructions, and is not
intended to do so. The document is intended as a
technical guide for use by Federal, State, and local
agency personnel, hospital waste management
personnel, and hospital incinerator operators.
The concern about disposal of infectious wastes
generated by hospitals is increasing rapidly due to
the fear of the spread of viruses such as acquired
immune deficiency syndrome (AIDS) and hepatitis B,
as well as the concern about exposure to toxic metals
and organics. Incineration continues to be an
attractive infectious waste disposal option for
hospitals encountering high disposal costs, refusal of
their waste at treatment and disposal facilities, and
tighter regulation. Proper incineration sterilizes
pathogenic waste, reduces waste volumes by over 90
percent, and, in some cases, may provide economic
benefits through waste heat recovery. Onsite
incineration is an attractive option because it reduces
handling and transportation of the wastes and
because waste heat recovery can have economic
advantages for the facility. The need for disposal of
infectious wastes and problems associated with such
disposal also has spurred the interest in special
commercial incineration facilities. In either case, the
operating facility must address the problems of air
emissions and ash disposal from the incinerator.
Hospital waste incinerators may emit a number of
pollutants depending on the waste being incinerated.
These pollutants include: particulate matter, acid
gases, toxic metals, toxic organic compounds, carbon
monoxide, sulfur oxides, nitrogen oxides, and
pathogens and viruses. Proper operation of the
incinerator will reduce the emissions of most of these
pollutants. Air pollution control devices are available
to further control these pollutants.
In response to public concern about hospital
incineration, several States recently have enacted or
are in the process of enacting regulations which
govern the incineration of general hospital wastes
and infectious wastes, specifically. These regulations
specify emission limits for hospital incinerators, and
frequently they also address operating practices
related to waste handling and charging, combustor
operations, ash characteristics, and ash handling
practices. Some of the operating practices that have
been specified in these regulations include:
1. Limits on characteristics of wastes charged to the
incinerator;
2. Specific requirements for waste packaging and
waste charging practices;
3. Combustor temperature limits;
4. Ash burnout levels; and
5. Ash handling and disposal practices.
The combustion efficiency of a hospital waste
incinerator and the pollutant removal efficiency of its
associated air pollution control equipment ultimately
is affected by equipment O&M. Regardless of how
well the equipment is designed, poor O&M practices
will lead to the deterioration of components and a
resultant decrease in both combustion quality and
pollutant removal efficiency. In addition, O&M
practices affect equipment reliability, on-line
availability, continuous compliance with emission
limits and operating practice standards, and
regulatory agency/source relations. Lack of timely
and-proper O&M leads to a gradual deterioration in
the equipment, which in turn increases the
probability of equipment failure and decreases both
the reliability and on-line availability of the
equipment. Frequent violations of emission limits or
operating the facility outside the limits of operating
practices established by regulations can result in
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public complaints, increased frequency of inspection,
potential fines for noncompliance, and in some cases,
mandatory shutdown until emission problems are
solved. Good O&M practices are essential to safe,
reliable operation of the facility.
This document summarizes technical information
related to the proper operation and maintenance of
hospital waste incinerators and associated air
pollution control systems. Chapter 2 presents
background information on hospital waste
incineration systems including a summary of
combustion principles and descriptions of the types of
incinerators typically used for hospital waste.
Chapter 3 presents background information on add-
on air pollution control systems for hospital waste
incinerators. Chapter 4 identifies key operating
parameters and good operating practices for hospital
waste incineration and air pollution control systems.
Operating parameters which can be monitored and/or
automatically controlled also are discussed in
Chapter 4. Chapter 5 provides general guidance on
the maintenance of incinerators and air pollution
control systems. Chapter 6 describes instrumentation
which can be used for the control and monitoring of
the key operating parameters. Chapter 7 identifies
some common operational problems associated with
hospital waste incinerators and air pollution control
systems; the possible causes and solutions to the
problems are discussed. Chapter 8 discusses
recordkeeping procedures which can assist in
operating and maintaining an incineration system.
Chapter 9 provides general safety guidelines, and
Chapter 10 presents a glossary of terms.
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Chapter 2
Hospital Incineration Systems
2.1 Introduction
Incineration is the process by which combustible
materials are burned, producing combustion gases
and noncombustible residue and ash. The product
combustion gases are vented directly to the
atmosphere or to the atmosphere after treatment in
an air pollution control device. The noncombustible
ash residue is removed from the incinerator system
and is disposed of in a landfill. Incineration provides
the advantage of greatly reducing the mass and
volume of the waste. This reduction substantially
reduces transportation and disposal costs. For
infectious hospital wastes, another major objective of
the incineration process is the destruction of
infectious organisms (pathogens) that may exist in
the waste. The destruction of the pathogens is caused
by their exposure to the high temperatures which
exist within the incinerator. Incineration of hospital
wastes also is attractive aesthetically because it
destroys organic components of the waste that the
community often finds objectionable when wastes are
disposed of in landfills.
Two additional objectives achievable through proper
operation of hospital waste incinerators are
minimizing organic content in the solid residue and
controlling atmospheric emissions to acceptable
levels. Generally, tight control on organics in the ash,
i.e., good burnout, promotes waste reduction and
pathogen destruction. Reduction of atmospheric
emissions of constituents that are potentially
harmful to human health and the environment is a
prerequisite to acceptance of hospital incineration as
a feasible disposal alternative by the community.
The overall purpose of this technical document is to
present information on the operation of hospital
waste incineration systems that can contribute to
achieving these objectives. This chapter provides
background information on the incineration process
that will enhance the usefulness of the remainder of
the document. It presents information on basic
incineration principles and processes and integrates
this information to promote an understanding of
hospital waste incineration as an overall system.
The hospital waste incineration process can be
separated into the following steps:
1. Waste preparation;
2. Waste charging;
3. Waste combustion;
4. Treatment of the combustion gases, (i.e., add-on
air pollution control); and
5. Residue ash handling.
Waste heat recovery also may be included as a part of
the incinerator system. The incineration process and
the major subsystems of an incineration system are
depicted in Figure 2-1. An incinerator operates as a
system in which all of the process steps mentioned
above are interrelated. For example, the charging
procedures implemented by the operator will affect
how well the wastes burn in the combustion chamber
and, consequently, the ash quality Proper operation
of a system requires an understanding of the
principles of operation of each of the components and
of the interrelationship of those components for a
particular system configuration.
The remainder of this chapter presents background
information on hospital incineration processes in
three parts. Section 2.2 presents fundamental
concepts on pathogen destruction and combustion
principles. Section 2.3 describes hospital waste
characteristics and briefly addresses how these
characteristics affect incineration. Section 2.4
describes incineration system components and the
different incinerator systems that are used to treat
hospital wastes.
2.2 Fundamental Concepts Related to
Hospital Waste Incineration
An understanding of how an incinerator operates as a
system requires familiarity with some basic scientific
and engineering principles. This section provides a
brief discussion of the key principles related to two
basic areas - pathogen destruction and combustion
chemistry/physics. The discussions are quite
abbreviated and are designed to provide the reader
with the basic information needed to understand the
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Control and
Monitoring
Waste
Waste
Charge
System
Incinerator
To Atmosphere
Stack
To Atmosphere
I
Stack
Ash
Removal
System
1 Waste '
"! Heat f
j_ Boiler J
Air |
Pollution i--
Control [
System i
Figure 2-1. Major components of an incineration system.
remainder of this document; they do not provide
complete coverage of the subject areas.
Those readers desiring more information on these
subjects should consult References 1 through 3 on
pathogen destruction and References 4 through 8 on
combustion.
2.2.1 Pathogen Destruction
The primary objective of hospital waste incineration
is the destruction of pathogens in infectious wastes.
The U. S. Environmental Protection Agency (EPA)
has defined infectious waste as "waste capable of
producing an infectious disease For a waste to be
infectious, it must contain pathogens with sufficient
virulence and quantity so that exposure to the waste
by & susceptible host could result in an infectious
disease."1 Some examples of hospital wastes which
may be considered to be infectious are: i
1.
2.
3.
4.
5.
6.
Microbiological laboratory wastes including
cultures and equipment which has come in
contact with cultures of infectious agents;
Blood and blood products (such as serum,
plasma);
Sharps, including needles, laboratory glass
wastes, and glass pipets;
Surgical, autopsy, and obstetrical wastes which
have had contact with patient blood or body
fluids;
Wastes which have had contact with
communicable disease isolation wastes;
Human and animal tissue containing pathogens
with sufficient virulence and quantity so that
exposure to the waste by a susceptible human
host could result in an infectious disease; and
_
Ash
7. Dialysis unit wastes; i.e., wastes that were in
contact with the blood of patients undergoing
hemodialysis.
The pathogens in infectious waste can be destroyed
by the high temperatures achieved in hospital
incinerators. Data on the incinerator conditions
required to destroy the universe of pathogens that are
present in infectious waste are quite limited, but they
do indicate that temperature and time of exposure
are important. Emissions of microorganisms from the
incinerator could be attributed to insufficient
retention time and temperature as a result of the
following conditions:
1. Initial charging of the incinerator before
operating temperatures are achieved;
2. Failure to preheat the refractory lining;
3. Temperature fluctuations caused by intermittent
4.
use;
Exceeding design airflow rates, thereby reducing
the retention time;
5. Charging beyond incinerator capacity; and
6. Excessive moisture content of the waste.
Other factors such as the type of refractory lining, the
positioning and number of burners, and the precision
of temperature controlling devices also can have a
significant bearing on the effectiveness of
sterilization.2,3 The destruction of microorganisms in
the incinerator ash also depends on temperature and
time exposure.
2.2.2 Principles of Combustion^
In principle, combustion of hospital waste is a
chemical process that is equivalent to combustion of
fossil fuels for energy recovery. It is a chemical
reaction that involves rapid oxidation of the organic
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substances in the waste and auxiliary fuels. This
violent reaction releases energy in the form of heat
and light and converts the organic materials to an
oxidized form. Some of the basic principles related to
the combustion process are discussed in the following
subsections. Specifically, the basic chemical
reactions, stoichiometric air requirements,
thermochemical relations, and volumetric flows are
discussed in the first four subsections. In the last
subsection, the combustion process is discussed in
general terms, and the role of waste characteristics in
that process is outlined.
2.2.2.1 Chemical Reactions.
The organic portion of hospital waste consists
primarily of carbon (C), hydrogen (H), and oxygen
(O). These elements are involved in the reactions that
generate most of the energy and the bulk of the
combustion gas products that are released during
incineration. Other elements found to a lesser degree
are metals, sulfur (S), nitrogen (N), and chlorine (Cl).
Although these elements are of lesser importance to
the main "combustion reaction," they play an
important role in potential air pollution problems.
The reaction chemistry in the combustion zone is
quite complex and involves a wide variety of organic
compounds and free radical species. However, a
general understanding of combustion can be achieved
by treating these reactions simply as a combination
of carbon and hydrogen in the organic material with
the oxygen in the combustion air and organic
material. These simplified reactions can be
represented by the chemical equations:
C + O2-»CO2 + Heat (2-1)
2H2 + O2 -> 2H2O + Heat (2-2)
When complete combustion occurs, carbon and
hydrogen combine with the oxygen of the combustion
air to form carbon dioxide (CC>2) and water vapor
(H2O), respectively, as shown above. If incomplete
combustion occurs, carbon monoxide (CO) also will be
formed.
Available thermodynamic equilibrium data and
bench-scale test data indicate that organic chlorine
which enters the combustion chamber reacts almost
completely to form hydrogen chloride (HC1) and
elemental chlorine (C12). Unless the system has very
low H:C1 ratios in the feed, almost no C12 is formed.8
The HC1 and any C12 that is formed leave the
combustion chamber in vapor phase.
Sulfur that is chemically bound in organic materials
making up the hospital waste is oxidized during the
combustion process to form sulfur dioxide (SO2) at a
rate directly proportional to the sulfur content of the
waste. Some SO2 may react with alkaline reagents
also present in the waste or ash. However, the
amount of SO2 involved in such reactions is expected
to be negligible due to the high HC1 content of the
flue gas. Because it is a stronger acid than SO2, HC1
will react more quickly with available alkaline
compounds than SO2 and will tie up the alkaline
compounds before they have a chance to react with
SO2. Consequently, essentially all organic sulfur
present in the waste will leave the combustion
chamber as vapor phase SO2.
Nitrogen enters the combustion chamber as a
component of the waste and in the combustion air. It
can react in the combustion chamber to produce
nitrogen oxides (NOX). The NOX is formed by one of
two general mechanisms. "Thermal NOX" is the
result of the reaction between molecular nitrogen and
molecular oxygen, both of which enter the
combustion zone in the combustion air. "Fuel NOX"
results from the oxidation of nitrogen which enters
the combustion zone chemically bound within the
fuel structure. The rate of thermal NOX formation is
sensitive to the flame temperature. The detailed
mechanisms of fuel NOX formation are not well
understood, but the formation is not extremely
sensitive to temperature.9
2.2.2.2 Stoichiometric Combustion Air.
The theoretical amount of oxygen required for
complete combustion is known as the stoichiometric
or theoretical oxygen and is determined by the nature
and the quantity of the combustible material to be
burned. Combustion oxygen is usually obtained from
atmospheric air. The additional oxygen (or air) over
and above the stoichiometric amount is called "excess
air."
The overall chemical composition (ultimate analysis)
of the waste/fuel mixture can be used to calculate the
mass-based stoichiometric oxygen requirements with
the factors in Table 2-1. Volumetric oxygen require-
ments can be estimated with the following equation:
Qo = M0 x K (2-3)
where:
Q0 = volumetric flow of O2 (scm/h)
M0 = Mass flow of O2(kg/h)
K = 0.2404 scm O2/kg O2 at 20°C and 1 atm.
Total volumetric air requirements can be estimated
by multiplying volumetric oxygen requirements by 5.
2.2.2.3 Thermochemical Relations.
Thermochemical calculations are used to estimate
heat release and heat transfer associated with
combustion. These calculations permit determination
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Tabla 2-1. Stoichlometrfc Oxygen Requirements and
Combustion Product Yields10
Elemental
Waste
Component
C
H2
02
N2
H20
CI2
F2
Br2
k
S
P
Air N2
Stoichiometric
Oxygen
Requirement
2.67 Ib/Ib C
8.0 Ib/Ib H2
-1.0lb/lb O2
-
-
-0.23 Ib/Ib CI2
-0.42lb/lb F2
-
-
1.0lb/lbS
1.29 Ib/Ib P
-
Combustion Product Yield
3.67 Ib C02/lb C
9.0 Ib H2O/lb H2
1.0 Ib N2/lb N2
1.0 Ib H2O/lb H2O
1.03 lbHCI/lbCI2
-0.25 Ib H2O/lbCI2
1.05lbHF/lb F2
-0.47 Ib H20/lb F2
1.0lbBr2/Ib Br2
I.0lbl2/lbl2
2.0 Ib SO2/lb S
2.29 Ib P2O5/lb P
3.31 Ib N2/lb ( 02)sloioh
Stotchtometric air requirement = 4.3ix(O2)stoich.
of the energy released by the combustion process and
assessment of the transfer of the energy to the
environment. Thermochemical calculations involve
determination of fuel (or waste) heating values, heat
contents of entering and leaving streams, and any
other heat losses. A definition of terms that will be
helpful for understanding these calculations is
presented below.
Heat of combustion. Heat energy evolved from the
union of a combustible substance with oxygen to form
COg and H2O (and SO2) as the end products, with
both the reactants starting and the products ending
at the same conditions, usually 20°C (68°F) and 1 atm
(29.92 in Hg).
Gross or higher heating value - HVc or HHV. The
quantity of heat evolved as determined by a
calorimeter where the combustion products are
cooled to 16°C (60°F) and all water vapor is condensed
to liquid. Usually expressed in terms of kcal/g or
kcal/m3 (Btu/lb or Btu/scf).
Net or lower heating value - HVff or LHV. Similar to
the higher heating value except that the water
produced by the combustion is not condensed but is
retained as vapor at 18°C (60°F). Expressed in the
same units as the gross heating value.
Enthalpy or heat content. Total heat content,
expressed in kcal/g (Btu/lb), above a standard
reference condition.
Sensible heat. Heat, the addition or removal of which
results in a change of temperature.
Latent heat. Heat associated with a change of phase,
e.g., from liquid to vapor (vaporization) or from liquid
to solid (fusion), without a change in temperature.
Usually expressed as kcal/g (Btu/lb).
Available heat. The quantity of heat available for
intended (useful) purposes. The difference between
the gross heat input to a combustion chamber and all
the losses.
For steady-state operations:
Heat in (sensible + HHV)
= Heat out (sensible + latent + available)
(2-4)
Heat is liberated in the combustion process at the
rate of 7.8 kcal/g of carbon burned and 34 kcal/g of
hydrogen burned (14,100 Btu per pound of carbon
burned and 61,000 Btu per pound of hydrogen
burned). Alternatively, heat is liberated from the
combustion process at a rate that is equal to the
product of the mass flow rate of the waste (and
auxiliary fuel) and the higher heating value of waste
(and_fuel). A major latent "heat loss" is the energy
required to evaporate moisture in the waste and
vaporize organic constituents. The energy required to
raise the temperature of the evaporated moisture and
any excess air to combustion chamber temperatures
is a part of the sensible "heat loss."
Maximum combustion temperatures are attained at
Stoichiometric conditions. As the amount of excess air
is increased above the Stoichiometric point, the
combustion temperature is lowered because energy is
used to heat the combustion air from ambient
temperature to the combustion chamber temper-
ature. The greater the volume of the excess air, the
greater the "heat loss" due to raising the air
temperature. As the amount of excess air is
decreased, the combustion temperature increases
until it becomes maximum at the Stoichiometric
point. Below the Stoichiometric point, the tem-
perature decreases because complete combustion has
not occurred. Since the complete combustion reaction
(which is exothermic) has not occurred, the
maximum heat is not generated. A graphical
representation of the relationship between combus-
tion temperature and excess air level is shown in
Figure 2-2.
As the excess-air level increases, the volume of
oxygen supplied to the incinerator that is not reacted
increases, resulting in a higher oxygen concentration
in the effluent gas stream. Since the total amount by
weight (kg) of carbon dioxide generated during
combustion is based on Stoichiometric ratios, it
remains constant for a specific quantity of carbon in
the waste. However, as the excess-air level increases,
the concentration of carbon dioxide in the total
combustion gas stream decreases due to dilution from
the excess air (oxygen and nitrogen). The oxygen
concentration and carbon dioxide concentration of the
effluent gas stream are useful indicators of the
-------
Temperature
Deficient Air
Percent Excess Air
Figure 2-2. Relationship of temperature to excess air.7
combustion excess air levels and are useful for
monitoring the combustion process.
2.2.2.4 Volumetric Gas Flows.
Gas flows through a combustion system and air
pollution control system are a major consideration in
the design and operation of those systems. Three key
principles that are important to understanding the
operation of those systems are the relationship of
volumetric flow to gas residence time, the volume
occupied by compounds (either organic constituents
or water) vaporized in the combustion chamber, and
the relationship of gas volume to temperature and
pressure.
Complete destruction of pathogens and complete
combustion of organic constituents require exposure
of the materials to high temperatures for minimum
residence times. Some State regulations now require
a minimum residence time in the secondary
combustion chambers, and operators must comply
with these requirements. The residence time is a
function of chamber volume and volumetric flow rate
and can be estimated (plug flow assumed) with the
following equations:
60V (2-5)
where:
t = residence time, s
V
Qgas
Qair
QH2O
= combustion chamber volume, m3
= combustion gas flow rate at combustion
chamber conditions, m3/min
= combustion airflow rate at combustion
chamber conditions, m3/min
= release rate of free moisture from the
waste at combustion chamber conditions,
m3/min
These equations assume complete turbulent flow in
the combustion chamber and that the volume of
combustion gas is approximately equal to the volume
of combustion air.
As waste is exposed to high temperatures in the
combustion chamber, some materials in that waste
are volatilized into the vapor or gas phase. Two issues
of particular concern are the volume of water vapor
produced by the moisture in the waste and the
volume of gas produced by highly volatile organic
materials such as solvents that evaporate almost
instantly as they are exposed to high temperatures.
The volume of gas produced from a given mass of
material can be calculated from the equation:
M. (2-7)
V. = 24.04- l
MW.
where:
= volume of compound i at standard
conditions, scm
-------
MJ = mass of compound i, kg
MW, = molecular weight of compound i, g/g mole
This equation estimates volumes at standard
conditions, which are defined to be 20°C (68°F) and 1
atm (29.92 in. Hg). The paragraphs below describe
how volumes can be estimated for different
temperatures and pressures.
The volume that a gas occupies, or the volumetric
flow rate of a gas, is dependent on the temperature
and pressure of the gas. Estimates of gas volume or
gas volumetric flow can be modified to reflect
different temperatures or pressures with the
equations:
(2-8)
(2-9)
T P
Ar
Q2 =
where:
V{ = volume at condition i, m3
Tf = temperature at condition i, K
(where K = 273+°C)
Pf = absolute pressure at condition i, atm
Qi = volumetric flow at condition i, m3/min
2.2.2.5 The Combustion Process.
The goal of the combustion process is complete
combustion of the organic constituents in the waste
feed. Complete combustion requires sufficient air in
the combustion chamber, sufficient temperatures in
the combustion bed and combustion gas, sufficient
time over which materials are exposed to a
temperature profile, and mixing that assures good
contact of the waste/fuel with the combustion air.
Each organic substance in hospital waste has a
characteristic minimum ignition temperature that
must be attained or exceeded, in the presence of
oxygen, for combustion to occur. Above the ignition
temperature, heat is generated at a higher rate than
it is lost to the surroundings, and the elevated
temperatures necessary for sustained combustion are
maintained.
The residence time of a constituent in the high-
temperature region should exceed the time required
for the combustion of that constituent to take place.
Residence time requirements establish constraints
on the size and shape of the furnace for a desired
firing rate. Because the reaction rate increases with
increasing temperature, a shorter residence time will
be required for combustion at higher temperatures if
good combustion conditions are present.
Adequate oxygen supplies and turbulence and the
resultant mixing of organic materials and oxygen are
also essential for efficient combustion. Inadequate
mixing of combustible gases and air in the furnace
can lead to emissions of incomplete combustion
products, even from an otherwise properly sized unit
with sufficient oxygen. Turbulence will speed up the
evaporation of liquid fuels for combustion in the
vapor phase. In the combustion of waste solids,
turbulence will help to break up the layer of
combustion products formed around a burning
particle of waste. Under nonturbulent conditions, the
combustion rate is slowed because this layer of
combustion products decreases the amount of oxygen
that contacts the surface of the particle.
For hospital waste incinerators, the distribution of
combustion air between the primary and secondary
chambers and the methods used to inject that air to
the chamber are key operating considerations. The
distribution of air between the two chambers affects
the amount of oxygen that is available for reaction in
each chamber. (See the discussion of stoichiometry
above.) The method of injection of the air to the two
chambers is one of the factors that controls
turbulence and mixing.
The chemical and physical characteristics of the
waste stream also have significant effects on the
combustion process and on the composition of the
effluents from the process. Generally, wastes can be
characterized by a "proximate analysis," which is a
laboratory determination of four waste
characteristics - volatile matter, fixed carbon,
moisture, and ash or noncombustibles, The
paragraphs below define these four parameters and
describe their effects on the combustion process.
Volatile matter is that portion of the waste which can
be liberated (i.e., vaporized) with the application of
heat only (i.e., no chemical reaction occurs). Actual
combustion of volatile matter is a gas-phase reaction
(i.e., the combustion occurs after the material has
been vaporized). Key factors that must be considered
when volatile matter is introduced to the combustor
are the rate at which the material is vaporized and
the ability of the combustor to provide adequate
volume and airflow for that release.
Fixed carbon is the nonvolatile carbon portion of the
waste and must be burned at higher temperatures
and at increased duration of exposure to the
combustion air. The combustion of fixed carbon is a
solid-phase reaction. An incinerator should be
-------
operated with a solids residence time that is
sufficient for fixed carbon combustion.
Moisture, which is evaporated from the waste as
temperatures of the waste are raised in the
combustion chamber, passes through the incinerator,
unchanged, as water vapor. Two major impacts that
this moisture has on the combustion system are
increasing volumetric flow and reducing residence
time of the combustion gases. It also absorbs energy
and reduces the temperature in the combustor.
Unlike organic constituents, inorganic constituents,
specifically metals, are not "destroyed" during the
combustion process. Rather, they are distributed or
partitioned among the incinerator effluent streams.
Metals constituents can leave the combustion
chamber as bottom ash or in the combustion gas. The
relative distribution of the metals between these
streams is based on such factors as the chemical form
of the metals charged to the combustor, the localized
reaction atmosphere in the combustion chamber,
localized chamber temperatures, and localized
chamber airflows. Metals that leave the chamber as
bottom ash ultimately can reach a land disposal site
or can be lost to the atmosphere as fugitive emissions.
Metals can leave the combustion chamber in the gas
stream either as entrained particulate matter or as a
metal vapor. If the metals emissions from an
incinerator are of concern, they can be reduced by
using an add-on air pollution control system. The
efficiency of such a control system will depend on the
control system design and operating characteristics
and the physical characteristics of the metals,
particularly the solid/vapor distribution and the size
distribution of the metals that are entrained as
particles.
2.3 Hospital Waste Characteristics
Hospital wastes are heterogeneous mixtures of
general refuse, laboratory and pharmaceutical
chemicals and containers, and pathological wastes.
All of these wastes may contain potentially infectious
wastes. In some cases, the wastes fired to hospital
incinerators also may contain wastes classified as
hazardous under the Resource Conservation and
Recovery Act (RCRA) or low-level radioactive waste.
General refuse from hospitals is similar to generic
wastes from residences and institutions and includes
artificial linens, paper, flowers, food, cans, diapers,
and plastic cups. Laboratory and pharmaceutical
chemicals can include alcohols; disinfectants;
antineoplastic agents; and heavy metals, such as
mercury. Infectious wastes include isolation wastes
(refuse associated with isolation patients); cultures
and stocks of infectious agents and associated
biologicals; human blood and blood products;
pathological wastes; contaminated sharps; and
contaminated animal carcasses, body parts, and
bedding.1 Examples of wastes defined as infectious
are presented in Table 2-2.1 in the U.S., infectious
wastes are required to be discarded in orange or red
plastic bags or containers. Often these "red bag"
wastes may contain noncontaminated general refuse
discarded along with the infectious waste.
Table 2-2. Examples of Infectious Waste1
Waste category
Examples3
Isolation wastes
Cultures and stocks of
infectious agents and
associated biologicals
Human blood and blood
products
Pathological waste
Contaminated sharps
Contaminated animal
carcasses, body parts,
and bedding
« Wastes from patients with diseases
considered communicable and
requiring isolation
« Refer to Centers for Disease
Control, Guidelines for Isolation
Precautions in Hospitals, July 1983
• Specimens from medical and
pathology laboratories
« Cultures and stocks of infectious
agents from clinical, research, and
industrial laboratories; disposable
culture dishes, and devices used to
transfer, inoculate and mix cultures
• Wastes from production of
biologicals
* Discarded live and attenuated
vaccines
• Waste blood, serum, plasma, and
blood products
• Tissues, organs, body parts, blood,
and body fluids removed during
surgery, autopsy, and biopsy
« Contaminated hypodermic needles,
syringes, scalpel blades, pasteur
pipettes, and broken glass
• Contaminated animal carcasses,
body parts, and bedding of animals
that were intentionally exposed to
pathogens
a These materials are examples of wastes covered by each category.
The categories are not limited to these materials.
Incinerators are thermal systems; they are designed
to operate at a certain heat input (kcal/h or Btu/h).
Under the most efficient operating conditions, the
heat input rate would be constant at or near the
maximum design rate. Furthermore, all of the heat
input would come from the waste, with little or no
need for auxiliary fuel. The waste characteristics,
i.e., heat content and moisture content of the waste
being charged to the incinerator, will affect the
operator's ability to maintain good combustion
conditions in the incinerator without the use of
auxiliary fuel.
The chemical and physical characteristics of the
different waste materials that are fired to hospital
waste incinerators vary widely. A study of hospitals
in Ontario provided information on the heating
value, bulk density, and moisture content of different
waste materials. The results from this study are
-------
presented in Table 2-3 (SI units) and Table 2-4
(English units).2
Hospital waste can vary considerably in composition
and consequently in heat content, moisture content,
and bulk density. Hospital waste can vary in Btu
content from a low value of about 3,400 kJ/kg (1,500
Btu/lb) (primarily low-Btu, high-moisture
anatomical waste) to 45,000 kJ/kg (20,000 Btu/lb)
(low-moisture, high-heat content plastics such as
polyethylene). Because of the potential for a wide
range in waste characteristics and the impact of
varying waste characteristics on incinerator
performance, large volumes of wastes with unusually
high or low Btu or moisture contents should be
identified so that charging procedures and rates to
the incinerator can be adjusted accordingly as
described in Chapter 4.
The chemical composition of the waste materials also
may affect pollutant emissions. Wastes containing
metals and plastics are of particular concern. Metals
which vaporize at the primary combustion chamber
temperature (e.g., mercury) may become metal oxides
with particle size distributions primarily in the size
range of 1 um or less. These small particles may
become easily entrained and exhausted with the
combustion gases with limited capture by
conventional air pollution control equipment.
Halogenated plastics, such as polyvinyl chloride, will
produce acid gases such as HC1. The presence of the
chlorinated waste also may contribute to the
formation of toxic polycyclic organic material such as
dioxins and furans under poor operating conditions.
Some plastics such as polyethylene and polystyrene
do not contain significant amounts of halogens and
can be incinerated efficiently without major concern
for acid gas or toxic pollutant formation. However,
the high heating value of these and other plastic
materials can cause excessively high temperatures in
the primary combustion chamber. The potential for
refractory damage, slagging, and clinker formation
increases unless charging rates are adjusted or the
plastics are mixed with other wastes of lower heat
content.
2.4 Types of Hospital Waste Incinerator
Systems
2.4.1 Introduct!on7,ii,i2
The terminology used to describe hospital waste
incinerators that has evolved over the years is quite
varied. Multiple names have been used for the same
basic types of incinerators, and much of the
terminology does not enhance precise definitions that
can be used to define good O&M practices. However,
most incinerators have been grouped historically into
one of three types - "multiple-chamber," "controlled-
air," and "rotary kiln."
Most incineration systems installed before the early
1960's were "multiple-chamber" systems designed
and constructed according to Incinerator Institute of
America (IIA) standards. The multiple-chamber
incinerator has two or more combustion chambers.
These "multiple-chamber" systems are designed to
operate at high excess-air levels and, hence, are often
referred to as "excess-air" incinerators. Multiple-
chamber, excess-air incinerators are still found at
many hospitals. Many of the multiple-chamber
incinerators were designed specifically for
pathological wastes and are still being used for that
purpose. Note that although the term "multiple-
chamber" incinerator typically is used to describe
this type of excess-air incinerator, the typical
controlled-air and rotary kiln units also contain
multiple chambers.
The incineration technology that has been installed
most extensively for hospital wastes over the last 15
years generally has been "controlled-air"
incineration. This technology is also called "starved-
air" combustion, "modular" combustion, and
"pyrolytic" combustion. The systems are called
"controlled-air" or "starved-air" because they operate
with two chambers in series and the first chamber
operates at substoichiometric conditions. Similar
modular "controlled-air" units which operate with
excess-air levels in the primary chamber are also
manufactured and sold for combustion of municipal
solid waste (MSW); however, these units apparently
are not widely used for hospital waste incineration.
Rotary kiln-type incineration systems have been
widely used for hazardous waste incineration in the
U.S. The rotary kiln has two combustion chambers.
The primary chamber is a horizontal rotating kiln
that typically operates with excess air. However,
some manufacturers now have rotary kilns designed
to operate with a substoichiometric atmosphere in
the kiln; these kilns use special seals and air
injection schemes. 13 The exhaust gases exit the kiln
to a fixed secondary chamber. Rotary kiln
incineration technology is being applied to hospital
waste incineration at a few locations in the U.S. and
Canada.11
This historical grouping of incineration types is of
some assistance in characterizing how hospital waste
incinerators operate, but it is limited because it does
not address the complete combustion "system" and
how the incinerator is operated. Three important
factors which help to characterize the hospital waste
incinerator system and its operation are (1) the air
distribution to the combustion chambers, (2) the
mode of operation and method of moving waste
through the system, and (3) the method of ash
removal. For hospital waste incinerators, air
distribution can be classified based on whether the
primary chamber operates under substoichiometric
(starved) or excess-air conditions. The mode of
_
10
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Table 2-3. Characterization of Hospital Waste2 (Metric)
Component description
Human anatomical
Plastics
Swabs, absorbants
Alcohol, disinfectants
Animal infected anatomical
Glass
Beddings, shavings, paper, fecal
matter
Gauze, pads, swabs, garments,
paper, cellulose
Plastics, PVC, syringes
Sharps, needles
Fluids, residuals
HHV dry basis,
kJ/kg
18,600-27,900
32,500-46,500
18,600-27,900
25,500-32,500
20,900-37,100
0
18,600-20,900
18,600-27,900
22,500-46,500
140
0-23,200
Bulk density as
fired, kg/m3
800-1,200
80-2,300
80-1,000
800-1,000
500-1,300
2,800-3,600
320-730
80-1,000
80-2,300
7,200-8,000
990-1,010
Moisture content
of component,
weight %
70-90
0-1
0-30
0-0:2
60-90
0
10-50
0-30
0-1
0-1
80-100
Heat value as fired,
kJ/g
1,860-8,370
32,300-46,500
13,000-27,900
25,500-32,500
2,090-14,900
0
9,300-18,800
13,000-27,900
22,300-46,500
140
0-4,640
Table 2-4. Characterization of Hospital Waste2 (English)
Component description
Human anatomical
Plastics
Swabs, absorbants
Alcohol, disinfectants
Animal infected anatomical
Glass
Beddings, shavings, paper, fecal
matter
Gauze, pads, swabs, garments,
paper, cellulose
Plastics, PVC, syringes
Sharps, needles
Fluids, residuals
HHV dry basis,
Btu/lb
8,000-12,000
14,000-20,000
8,000-12,000
11,000-14,000
9,000-16,000
0
8,000-9,000
8,000-12,000
9,700-20,000
60
0-10,000
Bulk density as
fired, Ibffis
50-75
5-144
5-62
48-62
30-80
175-225
20-45
5-62
5-144
450-500
62-63
Moisture content
of component,
weight %
70-90
0-1
0-30
0-0.2
60-90
0
10-50
0-30
0-1
0-1
80-100
Heat value as fired,
Btu/lb
800-3,600
13,900-20,000
5,600-12,000
11,000-14,000
900-6,400
0
4,000-8,100
5,600-12,000
9,600-20,000
60
0-2,000
operation can be single batch, intermittent duty, or
continuous duty. Ash is removed on a batch or a
semicontinuous basis. Characteristics of the major
types of incinerators that are likely to be found at
U.S. hospitals with respect to these three factors
described above are listed in Table 2-5. The
remainder of this section describes the types of
incinerators as classified in Table 2-5.
2.4.2 Multiple-Chamber Incinerators™
Two traditional designs that are used for multiple
chamber incinerators are the "in-line hearth" and
"retort" hearth. The in-line hearth design is depicted
in Figure 2-3. For in-line hearth incinerators,
combustion gases flow straight through the
incinerator, with turns in the vertical direction only
(as depicted by the arrows in Figure 2-3). Depicted in
Figure 2-4 is the retort hearth multiple-chamber
incinerator. In the retort hearth design, the
combustion gases turn in the vertical direction
(upward and downward) as in the in-line incinerator,
but also turn sideways as they flow through the
incinerator. Because the secondary chamber is
adjacent to the primary chamber (they share a wall)
and the gases turn in the shape of a U, the design of
the incinerator is more compact. In-line incinerators
perform better at capacities greater than 750 Ib/h.
The retort design performs more efficiently than the
in-line design at capacities less than 750 Ib/h. The
retort design is the most common design used in
hospital waste applications.
Multiple-chamber incinerators may have fixed
hearths or grates or a combination of the two in the
primary chamber. The use of grates for a system
incinerating infectious waste is not recommended
because liquids, sharps, and small, partially
11
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Table 2-5. Classification of Hospital Incinerators
Type of incinerator Air supply3
Waste feed
Ash removal
Multiple chamber Excess
Batch/controlled air Starved
Intermittent duty controlled air Starved
Continuous duty controlled air Starved
Rotary kiln Excess
Manual or mechanical batch feed; single Batch at end of
or multiple batches per burn burn
Batch (manual or mechanical); 1 batch Batch at end of
per burn burn
Manual or mechanical batch feed; Batch at end of
multiple batches per burn burn
Mechanical continuous or multiple batch Intermittently or
fee^ continuously during
burn
Mechanical semicontinuous or continuous Continuous
feed
a Indicates whether primary chamber operates at below (starved) or above (excess) stoichiometric air levels.
Grates
Charging Door
with Overfire
Air Port
Ignition
Chamber/Flame
Port
Curtain Wall
Secondary
Combustion
Chamber
Secondary
Air Port
Cleanout Doors with-
Undergrate Air Port
Location of
Secondary
Burner
Mixing
Chamber
Curtain
Wall Port
Figure 2-3. In-line multiple-chamber incinerator with grate.14
combusted items can fall through the grates prior to
complete combustion or sterilization.
Multiple-chamber incinerators frequently are
designed and used specifically for incinerating
pathological ("Type 4" anatomical) wastes.
Pathological waste has a high moisture content and
may contain liquids; consequently, a pathological
incinerator always will be designed with a fixed
hearth. A raised "lip" at the door often is designed
into the hearth to prevent liquids from spilling out
the door during charging. Because the heating value
of pathological waste is low and is not sufficient to
sustain combustion, the auxiliary burner(s) provided
in the primary chamber of pathological incinerators
are designed for continuous operation and with
sufficient capacity to provide the total heat input
required.
2.4.2.1 Principle of Combustion and Air
Distribution
Combustion in the multiple-chamber incinerator
occurs in two (or more) combustion chambers. Both
chambers are operated with excess air (thus these
units often are referred to as "excess-air"
incinerators). Ignition of the waste, volatilization of
moisture, vaporization of volatile matter, and
combustion of the fixed carbon (solid-phase
combustion) occur in the primary chamber. The
combustion gases containing the volatiles exit the
primary chamber through a flame port into a mixing
_
12
-------
Charging
Door
Stack
Ignition Chamber
Hearth
Secondary
Combustion
Chamber
Flame
, Port
Secondary
Air Ports
Secondary
Burner
Port
Mixing
Chamber
First
Underhearth
Port
Mixing Chamber
Flame Port
Cleanout
Doors
Charging
Door
Hearth
Primary
Burner
Port
Second
Underhearth
Port
Figure 2-4.
Retort multiple-chamber incinerator for
pathological wastes.14
chamber and then pass into the secondary
combustion chamber. Secondary air is added into the
flame port and is mixed with the combustion gas in
the mixing chamber. A secondary burner is provided
in the mixing chamber to maintain adequate
temperatures for complete combustion as the gases
pass into and through the secondary combustion
chamber.
The incinerator is designed for surface combustion of
the waste which is achieved by predominant use of
overfire combustion air and limiting the amount of
underfire air in the primary chamber. Multiple-
chamber, excess-air incinerators operate with an
overall excess-air range of 300 to 600 percent.14 In
older units, combustion air typically was provided by
natural draft via manually adjusted dampers and air
in-leakage through doors. Newer multiple-chamber
incinerators often use forced-draft combustion air
blowers to provide the combustion air to the
combustion chambers.15
2.4.2.2 Mode of Operation
Multiple-chamber incinerators typically are designed
for single batch or intermittent-duty operation. That
is, this type of incinerator typically does not have an
automatic, continuous ash removal system which
would make continuous operation possible.
Consequently, the incinerator must be shut down at
routine intervals (for example, daily) for ash
removal, and the incinerator is operated
"intermittently."
2.4.2.3 Waste Feed Charging Systems
Waste feed charging to multiple-chamber units is
typically done manually. The waste is loaded into the
primary chamber through the open charging door.
Mechanical charging systems, such as hopper/ram-
feed systems also may be used. 15 (Hopper/ram-feed
systems are discussed in the next section.)
2.4.2.4 Ash Removal Systems
For the typical multiple-chamber incinerator, ash is
removed manually after the incinerator is shut down.
The charging and/or ash cleanout doors are opened,
and either a rake or shovel is used to remove the ash.
2.4.2.5 Use of Multiple-Chamber Incinerators for
Incinerating Hospital Wastes
The use of multiple-chamber, excess-air incinerators
for incineration of redbag wastes has several
drawbacks. First, operating in the surface-
combustion excess-air mode in the primary chamber
results in entrainment of flyash which can cause
excessive particulate matter emissions. Second, since
the incinerator is designed to operate with the
primary chamber in an excess-air mode, the
combustion air levels and the combustion rate within
the primary chamber are not easily controlled.
Consequently, the incinerator control system may
not provide a sufficient level of control to assure
complete combustion when waste composition and
volatile content of the waste fluctuates over a wide
range. Red bag wastes are not homogeneous and may
vary widely in volatile content and moisture content.
Third, operating with high levels of excess air is less
energy efficient because it requires auxiliary fuel
usage to maintain secondary combustion chamber
temperatures.
Multiple-chamber, excess-air incinerators are better
suited to incineration of pathological wastes than red
bag wastes because the consequences of the above-
mentioned drawbacks are not as severe for
pathological wastes.2 The volatile content of
13
-------
pathological waste is low, and in general, the waste
composition is not highly variable. The primary
burner provides most of the heat input and the
incinerator operates in a steady, constant mode with
a steady, consistent combustion air input and excess-
air level.
In some cases, older multiple-chamber incinerators
may be upgraded to include more modern technology.
Recently, some older multiple-chamber units have
been retrofitted so that the incinerator operates with
substoichiometric air levels in the primary chamber;
in essence, these units have been converted into the
controlled-air units discussed in the next section. 15
2.4.3 Controlled-Air Incinerators
2.4.3,1 Principle of Controlled-Air Incinerations
The principle of controlled-air incineration involves
sequential combustion operations carried out in two
separate chambers. Figure 2-5 is a simplified
schematic of an incinerator that operates on the
controlled-air principle. The primary chamber
(sometimes referred to as the ignition chamber)
accepts the waste, and the combustion process is
begun in a below stoichiometric oxygen atmosphere.
The amount of combustion air to the primary
chamber is strictly regulated (controlled). The
combustion air usually is fed to the system as
underfire air. Three processes occur in the primary
chamber. First, the moisture in the waste is
volatilized. Second, the volatile fraction of the waste
is vaporized, and the volatile gases are directed to the
secondary chamber. Third, the fixed carbon
remaining in the waste is combusted. The
combustion gases containing the volatile combustible
materials from the primary chamber are directed to
the secondary chamber (sometimes referred to as the
"combustion chamber"). There, the combustion air is
regulated to provide an excess-air combustion
condition and is introduced to the chamber in such a
manner as to produce turbulence and promote good
mixing of the combustion gases and combustion air.
This gas/air mixture is burned, usually at high
temperatures. The burning of the combustion gases
under conditions of high temperature, excess oxygen,
and turbulence promotes complete combustion.
Combustion control for a controlled-air incinerator is
usually based on the temperature of the primary
(ignition) and secondary (combustion) chambers.
Thermocouples within each chamber are used to
monitor temperatures continuously; the combustion
air rate to each chamber is adjusted to maintain the
desired temperatures. Systems operating under
controlled-air principles have varied degrees of
combustion air control. In many systems, the
primary and secondary combustion air systems are
automatically and continuously regulated or
"modulated" to maintain the desired combustion
chamber temperatures despite varying waste
composition and characteristics (e.g., moisture
content, volatile content, Btu value).H In other
systems (particularly batch or intermittent-duty
systems), the combustion air level control is
simplified and consists of switching the combustion
air rate from a "high" to a "low" level setting when
temperature setpoints are reached or at preset time
intervals.
The controlled-air technique has several advantages.
Limiting air in the primary chamber to below
stoichiometric conditions prevents rapid combustion
and allows a quiescent condition to exist within the
chamber. This quiescent condition minimizes the
entrainment of particulate matter in the combustion
gases, which ultimately are emitted to the
atmosphere. High temperatures can be maintained
in a turbulent condition with excess oxygen in the
secondary chamber to assure complete combustion of
the volatilized gases emitted from the primary
chamber. The temperature of the secondary chamber
can be maintained in the desired range (hot enough
for complete combustion but not hot enough to cause
refractory damage) by separately controlling the
excess-air level in the secondary chamber; as the
excess-air level is increased, the temperature
decreases. Second, control of the primary chamber
combustion air to below stoichiometric levels
maintains primary chamber temperatures below the
melting and fusion temperatures of most metals,
glass, and other noncombustibles, thereby
minimizing slagging and clinker formation.
For the controlled-air incinerator, the capacity of the
secondary combustion chamber dictates (i.e., limits)
the burning rate; the combustion chamber must have
adequate volume to accept and completely oxidize all
the volatile gases generated in the primary chamber
and to maintain sufficient combustion air so that
excess oxygen is available.
2.4.3.2 Batchi'Controlled-Air Incinerators"
In this type of unit, the incinerator is charged with a
single "batch" of waste, the waste is incinerated, the
incinerator is cooled, and the ash residue is removed;
the cycle is then repeated. Incinerators designed for
this type of operation range in capacity from about 50
to 500 Ib/h. In the smaller sizes, the combustion
chambers are often vertically oriented with the
primary and secondary chambers combined within a
single casing. Figure 2-6 is a schematic of a
controlled-air incinerator intended for batch
operation. 17 This unit's combustion chambers are
rectangular in design and are contained within the
same casing.
Batch/controlled-air units can be loaded manually or
mechanically. For the smaller units up to about 300
Ib/h, manual waste feed charging typically is used.
14
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Combustion Gases
rt
Waste Feed
Auxiliary
Ignition
Burner
Secondary Chamber
Volatile Content is Burned Under
Excess Air Conditions
Primary Chamber
(Starved Air Condition)
Volatiles and Moisture
I, i i
Main Burner for Maintaining Minimum
Combustion Temperature
Main Flameport Air
Ash and Non-combustibles
Controlled Undertire Air for
Burning "Fixed Carbon"
Figure 2-5. Schematic of a controlled-air incinerator.16
Waste
Charge
Door
I i
OfanL- I T*
Stackl J
Barometric yf
Damper 1 i
Oxygen
': Control
Primary
Blower
Figure 2-6. Schematic of a single batch controlled-air incinerator.17
Air
"Secondary Blower
15
-------
Manual ^loading involves having the operator load the
waste directly to the primary chamber without any
mechanical assistance. For a batch-type unit, one
loading cycle per day is used. The incinerator is
manually loaded; the incinerator is sealed; and the
incineration cycle is then continued through
burndown, cooldown, and ash removal without any
additional charging. Ash is removed manually at the
end of the cycle by raking or shoveling the ash from
the primary chamber.
2.4.3.3 Intermittent-Duty, Controlled-Air
Incinerators
The charging procedures of an incinerator that could
operate in single batch mode often are varied to
include multiple charges (batches) during the 12- to
14-hour operating period before final burndown is
initiated. These intermittent-duty units typically
operate in the 50 to 1,000 Ib/h range. The
intermittent charging procedure allows the daily
charge to the incinerator to be divided into a number
of smaller charges that can be introduced over the
combustion cycle. Consequently, a more uniform gas
stream is fed to the secondary chamber. Figure 2-7 is
a schematic of a controlled-air incinerator which is
intended for intermittent-duty operation. This unit
has a vertically oriented primary chamber followed
by a horizontal combustion chamber. This unit,
although not shown with a mechanical feeder, can be
fitted with a hopper/ram assembly.
A typical daily operating cycle for a controlled-air,
intermittent- duty-type incinerator is as follows:
Operating step
1. Cleanout of ash
from previous day
15 to 30 minutes
2. Preheat of
incinerator 15 to
60 minutes
3. Burndown
4, Cooldown
Typical duration
15 to 30 minutes
15 to 60 minutes
2 to 4 hours
5 to 8 hours
For jintermittent-duty operation, the daily waste
loading cycle of the incinerator is limited to about an
8- to 14-hour period. The waste loading period is
limited by the amount of ash the primary chamber
can physically hold prior to shutting down the unit
for ash removal. The remainder of the 24-hour period
is required for burndown of the ash, cooldown, ash
cleanout, and preheat.
For smaller units, the waste often is fed manually.
For units in the 300 to 500 Ib/h range, mechanical
waste feed systems often are employed, and for units
Stack
Charging
Door
Secondary Chamber
Secondary Burner
Primary and Secondary
Combustion Blower
Primary Chamber
Ignition Burner
Figure 2-7.
Example intermittent-duty, controlled-air
incinerator.18
above 500 Ib/h, mechanical waste feed systems
typically are employed. The typical mechanical waste
feed system is a hopper ram assembly. Figure 2-8 is a
schematic of a typical hopper ram assembly.20 In a
mechanical hopper/ram feed system, waste is
manually placed into a charging hopper, and the
hopper cover is closed. A fire door isolating the
hopper from the incinerator opens, and the ram
moves forward to push the waste into the incinerator.
The ram reverses to a location behind the fire door.
After the fire door closes, the ram retracts to the
starting position and is ready to accept another
charge. A water spray to quench the ram face as it
retracts typically is provided. The entire charging
sequence normally is timed and controlled by an
automatic sequence. The cycle can be started
manually by the operator or, in some systems, the
cycle is automatically started on a predetermined
basis.
Mechanical loading systems have several
advantages. First, they provide added safety to the
operating personnel by preventing heat, flames, and
combustion products from escaping the incinerator
during charging. Second, they limit ambient air
infiltration into the incinerator. This assists in
controlling the combustion rate by strictly
controlling the quantity of available combustion air.
Third, they facilitate charging the incinerator with
smaller batches of waste at regulated time
intervals.11
With intermittent-duty incinerators, ash removal is
a limiting factor for the incinerator operations. As
with the single batch-operated units, the ash is
removed at regular intervals (typically daily) after
the incinerator has gone through a cooldown cycle.
The ash usually is manually removed by raking
and/or shoveling from the primary chamber.
16
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Hydraulic Fire *j
Door Actuator | |
Hopper Cover
Hydraulic
Ram
Actuator
Charging ~Ram Fac8
Ram Quench
Spray
Waste
Charging
Hopper
Primary
Combustion
Chamber
Fire Door
Enclosure
Furnace
Opening
Figure 2-8. Hopper/ram mechanical waste feed system.2
2.4.3.4 Continuous-Duty, Controlled-Air
Incinerators.
Controlled-air units intended for continuous
operation are available in the 500 to 3,000 Ib/h
operating range. Continuous-duty, controlled-air
units operate according to the controlled-air
principles of the systems described earlier. However,
continuous operation requires a mechanism for
automatically removing ash from the incinerator
hearth. The ash must be moved across the hearth,
collected, and removed from the combustion chamber.
Continuous-duty units typically have mechanical
waste feeding systems. For large continuous-duty
units, the charging sequence may be fully automatic.
The incinerator can be automatically charged with
relatively small batches (in relation to the primary
chamber capacity) at frequent, regulated time
intervals. The use of frequent, small charges
promotes relatively stable combustion conditions and
approximates steady-state operation. For large
systems, the mechanical charging system may
include waste loading devices such as cart dumpers,
which automatically lift and dump the contents of
carts that are used to collect and contain the waste,
into the charge hoppers. Use of these loading devices
reduces the operator's need to handle infectious waste
and, consequently, further improves worker safety.
For smaller units, the mechanical waste feed
charging ram is sometimes used to move the ash
across the hearth. As a new load is pushed into the
incinerator, the previous load is pushed forward.
Each subsequent load has the same effect of moving
the waste across the hearth. The waste should be
fully reduced to ash by the time it reaches the end of
the hearth. For larger systems, one or more special
transfer rams are provided to move the waste across
the hearth. A continuous-duty, controlled-air
incinerator with a stepped hearth and multiple ash
transfer rams is depicted in Figure 2-9.21 The use of
the stepped hearth promotes "mixing" the ash bed as
the ash is moved from hearth to hearth and, conse-
quently, promotes improved solid-phase combustion.
Typically, when the ash reaches the end of the
hearth, it drops off ibhe end of the hearth into a
discharge chute. One of two methods for collecting
ash is usually used. The ash can discharge directly
into an ash container positioned within an air-sealed
chamber or sealed directly to the discharge chute.
When the container is full, it is removed from the
chamber and replaced with an empty ash container.
The second method is for the ash to discharge into a
water pit. The water bath quenches the ash, and it
also forms an air seal with the incinerator. A
mechanical device, either a rake or a conveyor, is
used to remove the ash from the quench pit
intermittently or continuously. The excess water is
allowed to drain from the ash as it is removed from
the pit, and the wetted ash is discharged into a
collection container. 11
17
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To Boiler
Fossil Fuel Burner
r— Primary Chamber
Feed Ram
Ash Sump
Ash Transfer Rams
Air Tube
Ash Discharge Ram
Ash Chute
Ash Quench
Figure 2-9. Incinerator with step hearths and automatic ash removal.2°
2.4.4 Rotary Kilns
A rotary kiln also utilizes two-stage combustion and
has two combustion chambers. Figure 2-10 is a
simplified schematic of a rotary kiln. The primary
combustion chamber is a rotating cylindrical
chamber which is slightly inclined from the
horizontal plane; hence, the name "rotary kiln." The
secondary chamber often is cylindrical in shape and
oriented horizontally much like the secondary
chambers described for controlled-air incinerators, or
it may be box-like as depicted in Figure 2-10.
2.4.4.1 Principle of Operation.
The rotating kiln is inclined at an angle determined
during design of the system. Waste is fed to the
higher end of the kiln by a mechanical feed system.
Typically, combustion air is provided to the kiln such
that an excess-air atmosphere exists. However, some
manufacturers now have rotary kilns designed to
operate with a substoichiometric atmosphere in the
kiln; these kilns use special seals and air injection
schemes. Running the kiln substoichiometrically
decreases kiln sizes required and reduces auxiliary
fuel usage in the secondary chamber. Inside the kiln,
moisture and volatiles are vaporized from the waste,
and the waste is ignited. An auxiliary burner
provided in the kiln maintains the desired
combustion temperature if sufficient heat input is not
available from the waste. As the kiln rotates, the
solids are tumbled within the kiln and slowly move
down the incline toward the discharge end. The
turbulence of the waste within the kiln provides
exposure of the solid waste to the combustion air.
Combustion of the solids occurs within the kiln, and
the residue ash is discharged from the end of the kiln
into an ash removal system.
The volatile gases pass into the secondary chamber
where combustion of the gases is completed. A
secondary burner is used to maintain the secondary
chamber temperature, and secondary combustion air
is added to the chamber as necessary to maintain the
desired excess-air level.
2.4.4.2 Mode of Operation.
Since the solid waste continuously moves down the
length of the rotating kiln, the incineration system is
designed to operate in a continuous mode with a
semicontinuous or continuous waste feed input.
Consequently, a rotary kiln typically has a mechan-
ical waste feed system and a system for continuous
ash removal.
18
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Rotating Primary
Oxidation Chamber
High Temp. Secondary
1 Oxidation Chamber
,Baghouse
Waste Heat
Boiler
Auger-Sizer
Stack
, -A,
'
<\sh
*,'
^
\
N
By-Pass Gate
Hot Water /
—
I 1
)
\
'.'
^ *
V
—
jC\
X
1 A
Stack
Exhaust Fan
Figure 2-10. Rotary kiln with auger feed.21
Bypass
Shutoff
Valve
Bypass
Stack
Gas Flow
Waste Heat
Boiler
Incinerator
Figure 2-11. Incinerator with waste heat boiler and bypass stack.
Stack
ID
Fan
Damper
19
-------
2.4.4.3 Charging System.
The waste feed system must provide waste to the kiln
in a continuous or semicontinuous manner. One
manufacturer that provides rotary kiln incinerators
for hospital waste applications uses an auger-feeder
system to feed the waste continuously to the kiln.22
Waste is fed to the feed hopper, and the auger-feeder
continuously discharges the waste from the bottom of
the hopper to the kiln.
The hopper/ram feed system of the type previously
described also has been used for feeding rotary kilns
(particularly in hazardous waste incineration
applications).
2.4.4.4 Ash Removal.
As the kiln rotates, the ash is continuously
discharged from the end of the kiln. A system for
collecting the ash and continuously or
semicontinuously removing the ash is required.
Automated ash removal systems such as those
previously described for the continuous-duty,
controlled-air incinerators are used.
2.4.5 Auxiliary Equipment
2.4.5.T Waste Heat Boilers.
Incinerator manufacturers often provide waste heat
boilers as an option to their incineration units. Waste
heat boilers are used in conjunction with a hospital
waste incinerator to generate steam or hot water for
use in the hospital. The combustion gases from the
incinerator pass through the waste heat boiler prior
to being emitted to the atmosphere via the stack. Use
of a waste heat boiler requires that an induced draft
fan be added to the system. Furthermore, incin-
erators equipped with waste heat boilers have a
system for diverting the combustion gases directly to
the atmosphere and bypassing the boiler. This bypass
system is required for safety (for example, to avoid
excessive pressures in the incinerator should the fan
cease operation) and for normal operation if demand
for waste heat is low (so the boiler can be taken off
line). Typical systems include either a second
"bypass" stack before the waste heat boiler or a
breeching directly connecting the incinerator to the
stack and bypassing the boiler. Figure 2-11 is a
schematic of a controlled-air unit with waste heat
recovery. 16
2.4.5.2 Auxiliary Waste Liquid Injection.
Incinerators also may include the capability to inject
liquid wastes into the primary chamber. Generally
liquid waste incineration is accomplished through
either an atomizing nozzle or burner assembly in the
primary chamber. Wastes such as used solvents can
be readily incinerated via liquid injection.
2.5 References for Chapter 2
I. U.S. Environmental Protection Agency. EPA
Guide for Infectious Waste Management,
EPA/530-SW-86-014. (NTIS PB86-199130). U. S.
EPA Office of Solid Waste. May 1986.
2. Ontario Ministry of the Environment.
Incinerator Design and Operating Criteria,
Volume II-Biomedical Waste Incineration.
October 1986.
3. Barbeito, M. S. and M. Shapiro. Microbiological
Safety Evaluation of a Solid and Liquid
Pathological Incinerator. Journal of Medical
Primatology. pp. 264-273.1977.
4 Beard, J. T., F. A. lachetta, and L. V. Lillelehet.
APTI Course 427, Combustion Evaluation,
Student Manual, EPA 450/2-80-063. U. S. EPA
Air Pollution Training Institute. February 1980.
5. Beachler, D. S. APTI Course SI:428A,
Introduction to Boiler Operation, Self
Instructional Guidebook, EPA 450/2-84-010.
U. S. EPA. December 1984.
6. Brunner, C. R. Incineration Systems. Van
Nostrand Reinhold. 1984.
7. McRee, R. "Operation and Maintenance of
Controlled Air Incinerators." Undated.
8. Chang, D., R. Mournighan, and G. Hoffman.
Thermodynamic Analysis of Post Flame
Reactions Applied to Waste Combustion. Land
Disposal, Remedial Action, Incineration, and
Treatment of Hazardous Waste: Proceedings of
the Thirteenth Annual Research Symposium.
EPA/600-9-87-015. (NTIS PB87-233151).
9. U. S. Environmental Protection Agency.
Municipal Waste Combustion Study: Combustion
Control. EPA 530-SW-87-021C. (NTIS PB 87-
206090). June 1987. pp. 4-8 - 4-11.
10. Bonner, T., B. Desai, J. Fullenkamp, T. Huges, E.
Kennedy, R. McCormick, J. Peters, and D.
Zanders. Draft Engineering Handbook for
Hazardous Waste Incineration. EPA Contract
No. 68-03-2550. November 1980.
11. Doucet, L. C. Controlled Air Incineration:
Design, Procurement and Operational
Considerations. Prepared for the American
Society of Hospital Engineering, Technical
Document No. 55872. January 1986.
20
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12. U. S. Environmental Protection Agency. Hospital
Waste Combustion Study: Data Gathering Phase.
EPA 450/3-88-017. December 1988.
13. Letter from K. Wright, John Zink Company, to J.
Eddinger, EPA. January 25,1989.
14. Air Pollution Control District of Los Angeles
County. Air Pollution Engineering Manual, 2nd
Edition AP-40. (NTIS PB 225132) U. S. EPA.
May 1973.
15. Personal conversation with Larry Doucet, Doucet
& Mainka Consulting Engineers. November 28,
1988.
16. Ecolaire Combustion Products, Inc., Technical
Article: Principles of Controlled Air Incineration.
Undated.
17. Ashworth R. Batch Incinerators - Count Them
In; Technical Paper Prepared for the National
Symposium of Infectious Waste. Washington,
D.C. May 1988.
.18. Ecolaire Combustion Products, Inc., Technical
Data Sheet for E Series Incinerator. Undated.
19. Doucet, L. Fire Prevention Handbook, National
Fire Protection Association, Waste Handling
Systems and Equipment. Chapter 12, p. 116.
20. Source Category Survey: Industrial Incinerators.
EPA 450/3-80-013 (NTIS PB 80-193303). U. S.
Environmental Protection Agency. May 1980.
21. Technical Data Form: Consertherm Systems,
Industronics, Inc. Undated.
21
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Chapter 3
Air Pollution Control
3.1 Introduction
Hospital incinerators are potentially significant
sources of air pollutants. Pollutants of concern from
hospital incinerators include particulate matter;
toxic metals; toxic organics; carbon monoxide (CO);
and the acid gases, hydrogen chloride (HC1), sulfur
dioxide (SO2), and nitrous oxides (NOX). Emission
rates of these pollutants can be limited by removing
pollutant generating materials from the waste feed
material, proper operation of the incinerator, and
add-on pollution control equipment. The objectives of
this Chapter are to describe the factors affecting
formation and generation of the pollutants and to
identify and discuss control strategies that can be
used to control the pollutant, will describe The factors
affecting pollutant formation and generation,
including the effects of waste feed composition, are
described in Section 3.2. Air pollution control
strategies are discussed in Section 3.3. Electrostatic
precipitators are not discussed in this chapter
because they are not commonly used for air pollution
control on "local" hospital incinerators. They are
more likely to be used for large regional-type
facilities, possibly in conjunction with a dry-
scrubbing system. Information on their operation and
maintenance can be found in reference 2 of Chapter 8.
3.2 Pollutant Formation and Generation
The pollutants of concern from hospital incinerators
either exist in the waste feed material or are formed
in the combustion process.
Particulate matter. Particulate matter emissions
from the combustion of hospital wastes are
determined by three factors: (1) suspension of
noncombustible materials, (2) incomplete combustion
of combustible materials, and (3) condensation of
vaporous materials. The ash content of the waste feed
material is a measure of the noncombustible portion
of the waste feed and represents those materials
which will not burn under any conditions in the
incinerator. Emissions of noncombustible materials
result from the suspension or entrainment of ash by
the combustion air added to the primary chamber of
the incinerator. The more air added, the more likely
that noncombustibles will become entrained.
Particulate emissions from incomplete combustion of
combustible materials result from improper
combustion control of the incinerator. Condensation
of vaporous materials results from noncombustible
substances that volatilize at primary combustion
chamber temperatures with subsequent cooling in
the flue gas. These materials usually condense on the
surface of other fine particles.
Toxic metals. Particulate metal emissions are
dependent on the metals content of the feed material.
Metals may exist in the waste as parts of. discarded
instruments or utensils, in plastics, paper, pigments,
and inks, or as discarded heavy metals used in
laboratories. Many metals are converted to oxides
during combustion and are emitted primarily as
submicron to micron size particles. Metals that
volatilize at primary combustion chamber
temperatures may selectively condense on small,
difficult to control particles in the incinerator flue
gas. Metals generally thought to exhibit fine-particle
enrichment are arsenic (As), cadmium (Cd),
chromium (Cr), manganese (Mn), nickel (Ni),
molybdenum (Mo), lead (Pb), antimony (Sb),
selenium (Se), vanadium (V), and zinc (Zn).1
Toxic organics. Organic material found in the waste
feed material theoretically can be completely
combusted to form water (H^O) and carbon dioxide
(CO2). However, incomplete combustion will result in
emissions of organics found in the waste feed and in
the generation of new organic species from complex
chemical reactions occurring in the combustion
process. When chlorine is available in the form of
PVC plastic materials, these organics can include
highly toxic chlorinated organics, such as dioxins and
furans. Combustion conditions that favor increased
particulate matter emissions due to incomplete
combustion also favor increased organic emissions.
Carbon monoxide (CO). As noted above, complete
combustion of organic material will result in the
formation of H2O and CO2- The concentration of CO
in the incinerator exhaust gas stream is an indicator
of the combustion efficiency of the unit. The
formation of CO is dictated by the oxygen
concentration in the incinerator, the degree of mixing
of the fuel and air, and the temperature of the gases.
23
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Carbon monoxide is an intermediate product of the
reaction between carbonaceous fuels and oxygen.
Combustion conditions that result in incomplete
combustion will produce CO as well as particulate
matter and organics.
Acid gases. The principal acid gas of concern from
hospital incinerators is HC1. The determining factor
in HC1 formation and emission is the availability of
chlorine in the feed material. In the presence of
available hydrogen, as would exist in highly organic
hospital wastes, most of the available chlorine will be
converted to HC1. Sulfur dioxide generation is
similar to HC1; most of the sulfur in the wastes will
be converted to SO-2 during combustion regardless of
incinerator design or operation.
Nitrogen enters the combustion chamber as a
component of the waste and in the combustion air. It
can react in the combustion chamber to produce NOX.
3.3 Control Strategies
Formation and generation of all of the pollutants
discussed above are dependent on either the
availability of certain materials in the waste feed or
on the efficiency of the combustion process. Removing
problem materials from the waste feed and
improving combustion efficiency are, therefore, two
control strategies for reducing pollutant emissions. If
pollutants are formed and generated, add-on
pollution control equipment represents a third
control strategy for emissions reduction. Table 3-1
presents the control strategies that can be effective
for each of the pollutants of concern.
3.3.1 Controlling Feed Material
Controlling feed material consists of either
establishing procedures that eliminate the use of
certain materials, specifying segregation of certain
wastes at the point of origin, or removing problem
materials from the waste material prior to
incineration. Obviously, eliminating the use of
certain materials or establishing segregation
procedures are more feasible. The substitution of
nonchlorinated plastics (e.g., polyethylene) for PVC,
where possible, would reduce the chlorine input to
the incinerator. Materials that could be segregated
from waste to be incinerated include noncombustible
fine dust and powders that could contribute to
particulate matter emissions, heavy metals from
dental clinics or laboratories, PVC plastics that
contribute to HC1 formation, and other chlorine- or
sulfur-containing materials.
3.3.2 Combustion Control
Complete combustion of combustible material
requires adequate temperatures, excess air,
turbulence, mixing, and retention time. Because of
the variability in hospital waste with respect to
heating values, moisture contents, etc., incinerator
operating parameters should be varied with the
variations in the waste to maximize combustion. In
general, higher temperatures, excess air rates,
mixing, and retention time result in improved
combustion and lower emissions of particulate
matter, organics, and CO. However, as these factors
are increased there is an economic penalty in
auxiliary fuel costs and energy loss. In addition,
combustion control has little or no effect on emissions
of HC1 and SO2. In both cases, most of the chlorine
and sulfur will be converted to HC1 and SO2 under
the entire range of combustion conditions which
normally occur in a hospital incinerator. As opposed
to the other pollutants, both NOX and toxic metal
emissions can actually be increased by combustion
adjustments that increase temperatures and excess
air rates. Therefore, the waste feed composition
should be considered before making adjustments to
control combustion.
3.3.3 Add-On Air Pollution Control Systems
Add-on air pollution control systems (APC's) used on
hospital incinerators are usually wet scrubbers,
fabric filters, or dry scrubbers. Wet scrubbers, in
their various designs, are used to remove particulate
matter as well as HC1 and SO2J fabric filters are used
to remove particulate matter; dry scrubbers are used
to remove HC1 and SO2-
3.3.3.1 Wet Sceubbers
Venturi, spray tower, and packed-bed scrubbers are
the most common types of wet scrubber systems used
on hospital incinerators. Venturi scrubbers are used
primarily for particulate matter control and packed-
bed scrubbers are used primarily for acid gas control.
However, both types of systems achieve some degree
of control for both particulate matter and acid gases.
A third scrubber type is spray towers. Spray towers
are used to remove particulate matter; however, they
are relatively inefficient for the removal of fine
particulate matter. These units are not effective for
controlling particulate emissions from controlled-air
incinerators and are used exclusively on multiple-
chamber units which can emit significant quantities
of large particles. There are other scrubber types with
potential application to hospital waste incinerators
including (but not limited to) the wet ionizing
scrubber, the collision scrubber, and the Hydro-
sonic® steam ejector scrubber.3>4 Hydro-sonic®
scrubber uses supersonic steam ejector drives to
accelerate the injected water droplets and provide
mixing. The Hydro-sonic® scrubbers are reported to
offer high efficiency on fine particulate emissions at
relatively low energy costs where there is a source of
waste heat for steam production. Use of these
scrubbers is not widespread on hospital waste
incinerators. Detailed operation and maintenance
24
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Table 3-1. Control Strategies for Air Pollutants from Hospital Waste Incinerators
Control strategy Add-on pollution control equipment
Pollutants
Controlling Combustion Venturi Packed-bed Dry
feed material control scrubber3 scrubber15 Fabric filter scrubber
Particulate matter
Toxic metals
Toxic organics
Carbon monoxide
Hydrogen chloride
Sulfur dioxide
Nitrous oxides
X
X
X
X
X
X
X
X
X
X
X
c
c
C
c
C
c
X
X
X
X
c
X
X
aVenturi scrubber with water as the scrubbing media.
bPacked-bed scrubber utilizing an alkaline sorbent
cWill achieve some limited control but not designed for high-efficiency collection.
information for these units controlling hospital waste
incinerators is not readily available, and they will
not be discussed in this document.
Wet scrubbing principles? Wet scrubbers capture
relatively small dust particles with large liquid
droplets. Droplets are produced by injecting liquid at
high pressure through specially designed nozzles, by
aspirating the particle-laden gas stream through a
liquid pool, or by submerging a whirling rotor in a
liquid pool. These droplets collect particles by using
two primary collection mechanisms—impaction and
diffusion.
In a wet scrubbing system, dust particles will tend to
follow the streamlines of the exhaust stream.
However, when liquid droplets are introduced into
the exhaust stream, particles cannot always follow
these streamlines as they diverge around the droplet
(Figure 3-1). The particle's mass causes it to break
away from the streamlines and impact on the droplet.
Impaction is the predominant collection mechanism
for scrubbers having gas stream velocities greater
than 0.3 meters per second (m/s) (1 ft/s). Most
scrubbers operate with gas stream velocities well
above 0.3 m/s. Therefore, at these velocities, particles
having diameters greater than 1.0 micrometer (um)
are collected by this mechanism.
Very small particles (less than 0.1 um in diameter)
experience random movement in an exhaust stream.
These particles are so tiny that they are bumped by
gas molecules as they move in the exhaust stream.
This bumping, or bombardment, causes them to move
first one way and then another in a random manner,
i.e., diffuse, through the gas. This irregular motion
can cause the particles to collide with a droplet and be
collected.
The rate of diffusion depends on relative velocity,
particle diameter, and liquid-droplet diameter. As
with impaction, collection due to diffusion increases
Gas Streamlines
Particle
Droplet
Figure 3-1. lmpaction.5
with an increase in relative velocity (liquid- or gas-
phase input) and a decrease in liquid-droplet size.
However, collection by diffusion increases as particle
size decreases. This mechanism enables certain
scrubbers to remove the very tiny particles
effectively.
The process of dissolving gaseous pollutants in a
liquid is referred to as absorption. Absorption is a
mass-transfer operation in which mass is transferred
as a result of a concentration difference. Absorption
continues as long as a concentration differential
exists between the liquid and the gas from which the
contaminant is being removed. In absorption,
equilibrium depends on the solubility of the pollutant
in the liquid.
To remove a gaseous pollutant by absorption, the
exhaust stream must be passed through (brought in
contact with) a liquid. Three steps are involved in
absorption. In the first step, the gaseous pollutant
diffuses from the bulk area of the gas phase to the
gas-liquid interface. In the second step, the gas moves
(transfers) across the interface to the liquid phase.
25
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This step occurs extremely rapidly once the gas
molecules (pollutant) arrive at the interface area. In
the third step, the gas molecule(s) diffuses into the
bulk area of the liquid, thus making room for
additional gas molecules to be absorbed. The rate of
absorption (mass transfer of the pollutant from the
gas phase to the liquid phase) depends on the
diffusion rates of the pollutant in the gas phase (first
step) and in the liquid phase (third step).
To enhance gas diffusion, and, therefore, absorption:
1. Provide a large interfacial contact area between
the gas and liquid phases;
2. Provide good mixing of the gas and liquid phases
(turbulence); and
3. Allow sufficient residence, or contact, time
between the phases for absorption to occur.
Venturi scrubbers. A venturi scrubber consists of a
liquid sprayed upstream from a vessel containing a
converging and diverging cross-sectional area as
illustrated in Figure 3-2. The portion of the venturi
which has the minimal cross-sectional area and
consequently the maximum gas velocity is commonly
referred to as the throat. The throat can be circular as
shown in Figure 3-2 or rectangular as shown in
Figure 3-3. As the gas stream approaches the venturi
throat, the gas velocity and turbulence increases.
Liquid droplets serve as the collection media and can
be created by two different methods. The most
common method is to allow the shearing action of the
high gas velocity in the throat to atomize the liquid
into droplets. The other method is to use spray
nozzles to atomize the liquid by supplying high
pressure liquid through small orifices.
To attain a high collection efficiency, venturi
scrubbers need to achieve gas velocities in the throat
in the range of 10,000 to 40,000 feet per minute.
These high gas velocities atomize the water droplets
and create the relative velocity differential between
the gas and the droplets to effect particle-droplet
collision. The effectiveness of a venturi scrubber is
related to the square of the particle diameter and the
difference in velocities of the liquor droplets and the
particles.
The performance of a venturi scrubber is strongly
affected by the size distribution of the particulate
matter. For particles greater than 1 to 2 um in
diameter, impaction is so effective that penetration is
quite low. However, penetration of smaller particles,
such as the particles in the 0.1 to 0.5 um range is very
high. Unfortunately, small particle size distribution
is typical for fuel combustion sources including
hospital waste incinerators and results from the
— Converging Section
— Throat
— Diverging Section
Figure 3-2. Spray venturi with circular throat.6
Figure 3-3. Spray venturi with rectangular throat.7
26
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condensation of partially combusted organic
compounds and metallic vapors.
The particulate matter collection efficiency in a
venturi scrubber system increases as the static
pressure drop increases. The static pressure drop is a
measure of the total amount of energy used in the
scrubber to accelerate the gas stream and to atomize
the liquor droplets. The pressure drop across the
venturi is a function of the gas velocity and liquid/gas
ratio and in practice acts as a surrogate measure for
gas velocity. The Calvert equation can be used to
predict the pressure drop for a given throat velocity.
The Calvert equation is:8
AP = (5x10-5) V2(L/G)
(3-1)
where
AP =
v •—
pressure drop, in. w.c. (inches of water
column)
the gas velocity in the venturi throat, ft/s
(feet per second), and
L/G = the liquid-to-gas ratio, gal/Macf (gallons per
thousand actual cubic feet).
The equation implies that pressure drop is equal to
the power required to accelerate the liquid to the gas
velocity. The Calvert equation predicts pressure drop
reasonably well for the range of L/G ratios from 5 to
12 gal/Macf. At L/G ratios above 12, measured
pressure drops are normally about 80 percent of the
value predicted by the Calvert equation. In practice,
an inadequate liquid supply to cover the venturi
throat completely has resulted from L/G ratios less
than 3 gal/Macf.
Other variables that are important to venturi
scrubber performance are the liquid surface tension
and liquid turbidity. If surface tension is too high,
some small particles which impact on the water
droplet will "bounce" off and not be captured. High
surface tension also has an adverse effect on droplet
formation. High liquid turbidity, or high suspended
solids content, will cause erosion and abrasion of the
venturi section and ultimately lead to reduced
performance of the system.
A list of the major components of commercial
scrubber systems follows:
1. Venturi section;
2. Spray nozzles;
3. Liquor treatment equipment;
4. Gas stream demister;
5. Liquor recirculation tanks, pumps, and piping;
6. Alkaline addition equipment;
7. Fans, dampers, and bypass stacks; and
8. Controllers for venturi throat area, caustic feed,
makeup water, and emergency water quench for
temperature excursions.
Packed-bed scrubbers. A packed-bed scrubber
generally is used for acid gas removal. The large
liquor-to-gas surface area created as the liquor
gradually passes over the packing material favors
gas diffusion and absorption. Packed-bed scrubbers
are not effective as stand-alone scrubbers for
collection of fine particulate matter (less than
2.5 um) since the gas velocity through the bed(s) is
relatively low. However, packed beds are effective for
the removal of particle-laden droplets or charged
particles when used as a downstream collector behind
a venturi or electrostatically-enhanced wet scrubber.
Packed beds can be either vertical or horizontal.
Figure 3-4 illustrates a vertically oriented scrubber.
Regardless of the orientation of the bed, the liquor is
sprayed from the top and flows downward across the
bed. Proper liquor distribution is important for
efficient removal of gases.
Mist Eliminator
Liquid Sprays
Packing
Figure 3-4. Countercurrent packed tower absorbed
Absorption is the primary means of collection of acid
gases in packed-bed scrubbers. The effectiveness of
absorption in packed beds is related to the uniformity
of the gas velocity distribution, the surface area of the
packing material, the amount and uniform
distribution of scrubber liquid, and the pH and
turbidity of the scrubbing liquid.
27
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Gas absorption is affected by the extensive liquid
surface contacted by the gas stream as the liquid
flows downward over the packing material. A variety
of available packing materials offer a large exposed
surface area to facilitate contact with and absorption
of acid gases. The packing materials range in size
from 0.5 to 3 in. and are randomly oriented in the bed.
Typically, sodium hydroxide (NaOH), or occasionally
sodium carbonate (NaaCOa), is used with water to
neutralize the absorbed acid gases in a packed-bed
scrubber. These two soluble alkali materials are
preferred because they minimize the possibility of
scale formation in the nozzles, pump, and piping. For
the typical case of using NaOH as the neutralizing
agent, the HC1 and SOz collected in the scrubber
react with NaOH to produce sodium chloride (NaCl)
and sodium sulfite (Na2SO3) in an aqueous solution.
One of the major problems with these scrubbers is the
accumulation of solids at the entry to the bed and
within the bed. The dissolved and suspended solids
levels in the liquor must be monitored carefully to
maintain performance.
Spray Towers.2 Spray towers are relatively simple
scrubbers. Most units consist of an empty cylindrical
steel vessel containing nozzles that spray the liquid
scrubbing media into the vessel. Most units use
countercurrent flow with the exhaust gas stream
entering the bottom of the vessel and moving upward,
while the liquid is sprayed downward. Figure 3-5
shows a typical countercurrent flow spray tower.
Countercurrent flow exposes the exhaust gas with
the lowest pollutant concentration to the freshest
scrubbing liquid.
Many nozzles are placed across the tower at different
heights to spray all of the exhaust gas as it moves up
through the tower. The major purpose of using many
nozzles is to form a tremendous amount of fine
droplets for impacting particles and to provide a large
surface area for absorbing gas. Theoretically, the
smaller the droplets formed, the higher the collection
efficiency achieved for both gaseous and particulate
pollutants. However, the liquid droplets must be
large enough to not be carried out of the scrubber by
the exhaust stream. Therefore, spray towers use
nozzles to produce droplets that are usually 500 to
1000 um in diameter. The exhaust gas velocity is
kept low, from 0.3 to 1.2 m/s (1 to 4 ft/s) to prevent
excess droplets from being carried out of the tower.
Because of this low exhaust velocity, spray towers
must be larger than other scrubbers that handle
similar exhaust stream flow rates. Another problem
occurring in spray towers is that after the droplets
fall short distances, they tend to agglomerate or hit
the walls of the tower. Consequently, the total liquid
surface area for contact is reduced, thus reducing the
collection efficiency of the scrubber.
Liquid
Sprays
Figure 3-5. Counterourrent-flow spray towerJ"
Spray towers are low-energy scrubbers. Contacting
power is much lower than in venturi scrubbers, and
the pressure drops across such systems are generally
less than 2.5 cm (1 in.) of water. The collection
efficiency for small particles is correspondingly lower
than in more energy-intensive devices. They are
adequate for the collection of coarse particles larger
than 10 to 25 um in diameter, although with
increased liquid inlet nozzle pressures, particles with
diameters of 2.0 um can be collected. Smaller droplets
can be formed by higher liquid pressures at the
nozzle. The highest collection efficiencies are
achieved when small droplets are produced and the
difference between the velocity of the droplet and the
velocity of the upward-moving particles is high.
Small droplets, however, have small settling
velocities, so there is an optimum range of droplet
sizes for scrubbers that work by this mechanism.
Because of their inherent design, particulate matter
emissions from controlled-air incinerators are
usually composed of relatively fine particulate
matter. Therefore, spray towers are not commonly
applied to controlled-air units. Multiple-chamber
incinerators, however, can emit large quantities of
relatively large particle particulate matter and are
often controlled by spray towers.
28
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3.3.3.2 Fabric Filters
Fabric filters (i.e., baghouses) are used on a limited
number of hospital incinerators for control of
particulate matter emissions. They have some
advantages over wet scrubbers in that they are
highly efficient at removing fine particles if they are
properly operated and maintained. However, poor
operation and maintenance (O&M) can result in bag
blinding, bag corrosion, or bag erosion.
Filtration principles. U Fabric filtration is one of the
most common techniques used to collect particulate
matter. A fabric filter is a collection of bags
constructed of a fabric material (nylon, wool, or
other) hung inside a housing. The combustion gases
are drawn into the housing, pass through the bags,
and are exhausted from the housing through a stack
to the atmosphere. When the exhaust stream from
the incinerator is drawn through the fabric, the
particles are retained on the fabric material, while
the cleaned gas passes through the material. The
collected particles are then removed from the filter by
a cleaning mechanism typically by using blasts of air.
The removed particles are stored in a collection
hopper until they are disposed.
With a new filter, the open areas in the fabric are of
sufficient size that particles easily penetrate the bag.
Over time, a cake builds on the bag surface, and this
cake acts as the primary collection medium. Particles
are collected on a filter and cake by a combination of
several mechanisms. The most important are
impaction and direct interception.
In collection by impaction, the particles in the gas
stream have too much inertia to follow the gas
streamlines around the fiber and are impacted on the
fiber surface. In the case of direct interception, the
particles have less inertia and barely follow the gas
streamlines around the fiber. If the distance between
the center of the particle and the outside of the fiber
is less than the particle radius, the particle will graze
or hit the fiber and be "intercepted". Impaction and
direct interception mechanisms account for
99 percent collection of particles greater than 1 um
aerodynamic diameter in fabric filter systems.
Fabric filter performance. Generally, fabric filters are
classified by the type of cleaning mechanism that is
used to remove the dust from the bags. The three
types of units are mechanical shakers, reverse air,
and pulse jet. To date, the only hospital incinerators
that have been identified as having fabric filters use
pulse jet units. The paragraphs below briefly describe
the design and operating characteristics of pulse jet
filters and identify key design parameters.
A schematic of a pulse jet baghouse is shown in
Figure 3-6. Bags are supported internally by rings or
cages. Bags are held firmly in place at the top by
clasps and have an enclosed bottom (usually a metal
cap). Dust-laden gas is filtered through the bag,
depositing dust on the outside surface of the bag.
Pulse jet cleaning is used for cleaning bags in an
exterior filtration system.
Clean Air
Plenum •
Plenum Access H
Blow Pipe •
Induced -
Flow
Bag Cup &
Venturi
Tube Sheet
Bag Retainer -
To Clean Air
Outlet and Exhauster
/*•
1
Housing
Filter
Tube
Dirty Air Inlet & Diffuser
. -
Rotary
Air Lock
Figure 3-6. Pulse jet baghouse.12
The dust cake is removed from the bag by a blast of
compressed air injected into the top of the bag tube.
The blast of high pressure air stops the normal flow of
air through the filter. The air blast develops into a
standing or shock wave that causes the bag to flex or
expand as the shock wave travels down the bag tube.
As the bag flexes, the cake fractures and deposited
particles are discharged from the bag. The shock
wave travels down and back up the tube
approximately 0.5 seconds.11
in
The blast of compressed air must be strong enough for
the shock wave to travel the length of the bag and
shatter or crack the dust cake. Pulse jet units use air
supplies from a common header which feeds into a
nozzle located above each bag. In most baghouse
29
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designs, a venturi sealed at the top of each bag is used
to create a large enough pulse to travel down and up
the bag. The pressures involved are commonly
between 414 and 689 kPa (60 and 100 psig). The
importance of the venturi is being questioned by
some pulse jet baghouse vendors. Some baghouses
operate with only the compressed air manifold above
each bag. n
Most pulse jet filters use bag tubes that are 10 to
15 cm (4 to 6 in.) in diameter. Typically the bags are
3.0 to 3.7 m (10 to 12 ft) long, but they can be as long
as 7.6 m (25 ft).H Generally, these bags are arranged
in rows, and the bags are cleaned one row at a time in
sequence. Cleaning can be initiated by a pressure
drop switch, or it may occur on a timed sequence.
The key design and operating parameters for a pulse
jet filter are the air-to-cloth ratio (or the filtration
velocity), the bag material, operating temperature,
and operating pressure drop.
The air-to-cloth ratio is actually a measure of the
superficial gas velocity through the filter medium. It
is a ratio of the flow rate of gas through the fabric
filter (at actual conditions) to the area of the bags and
is usually measured in units of acfm/ft2. No operating
data were obtained for hospital incinerators, but
generally, the air-to-cloth ratio on waste combustion
units is in the range of 0.025 to 0.05 m3/s/m2 (5 to
10 acfm/ft2) of bag area. 13
Generally, bag material is specified based on prior
experience of the vendor. Key factors that generally
are considered are: cleaning method, abrasiveness of
the particulate matter and abrasion resistance of the
material, expected operating temperature, potential
chemical degradation problems, and cost. To date, no
information has been obtained on types of material
typically used for hospital incinerator applications.
The operating temperature of the fabric filter is of
critical importance. Since the exhaust gas from a
hospital incinerator can contain HC1, the unit should
be operated at sufficiently high temperatures to
assure that no surfaces drop below the acid dewpoint.
Otherwise, condensation of HC1 will result in
corrosion of the housing or bags. The boiling point of
HCl (aqueous hydrochloric acid) is 110°C (230°F); gas
temperatures should be maintained at 150°C (300°F)
to ensure that no surfaces are cooled below the
dewpoint. Above a maximum temperature that is
dependent on filter type, bags will degrade or in some
cases fail completely. Gas temperatures should be
kept safely below the allowed maximum.
Pressure drop in fabric filters generally is
maintained within a narrow range. Pressure drops
below the minimum indicate that either (1) leaks
have developed, or (2) excessive cleaning is removing
the base cake from the bags. Either condition results
in reduced performance. Pressure drops greater than
the maximum indicate that either (a) bags are
"blinding", or (b) excessive cake is building on the
bags because of insufficient cleaning. The primary
problem that results from excessive pressure drop is
reduced flow through the system and positive
pressure at the combustor. Over time, high pressure
drops also lead to bag erosion and degradation.
3.3.3.3 Dry Scrubbers
Dry scrubbers use absorption for the removal of
sulfur dioxide, hydrogen chloride, hydrogen fluoride,
and other acid gases. Some adsorption of vapor state
organic compounds and metallic compounds also
occurs in some dry scrubber applications. Basically,
dry scrubbers use an alkaline sorbent to react with
and neutralize the acid gas. The reaction product is a
dry solid which can be collected by a particulate
control device. Dry scrubbers usually are followed by
either fabric filters or electrostatic precipitators
(ESP's) for collection of the reaction products and the
unreacted sorbent. Currently, there are not any
hospital incinerators known to be using spray dryer
absorption systems. There are at least three hospital
waste incinerators which are known to be using dry
injection control systems.*
Components and operating principles of dry scrubber
systems.1-3,14: There is considerable diversity in the
variety of processes which are collectively termed dry
scrubbing. Dry scrubbing techniques that could be
applied to hospital incinerators can be grouped into
two major categories: (1) spray dryer absorbers, and
(2) dry injection absorption systems. Specific types of
dry scrubbing processes within each group are listed
below. Alternative terms for these categories used in
some publications are shown in parentheses.
1. Spray dryer absorption (semiwet)
• Rotary atomizer spray dryer systems
• Air atomizing nozzle spray dryer systems
2. Dry injection absorption (dry)
• Dry injection without recycle
• Dry injection with recycle (sometimes termed
circulating fluid bed absorption)
Simplified block diagrams of the two major types of
dry scrubbing systems are presented in Figures 3-7
and 3-8. The main differences between the two
systems are the physical form of the alkaline reagent
and the design of the vessel used for contacting the
acid gas-laden stream. The alkaline feed
1 Southland Exchange Joint Ventures, Hampton, South Carolina
(dry injection/ESP); Fairfax County Hospital, Falls Church,
Virginia (dry injection/baghouse); Borgess Medical Center,
Kalamazoo, Michigan (dry injection/baghouse).
30
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Lime
Storage
Lime
Slaker
1
Slurry
Mixing
Tank
Slurry
Feed
Tank
,
Combustion
Gases
Figure 3-7. Components of a spray dryer absorber system (semiwet process).
Feeder
Combustion
Gases from
Incinerator
Figure 3-8. Components of a dry injection absorption system (dry process).
31
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requirements are much higher for the dry injection
system. Conversely, the spray dryer system is much
more complicated.
Spray dryer absorbers. In this type of dry scrubbing
system, the alkaline reagent, usually pebble lime, is
prepared as a slurry containing 5 to 20 percent by
weight solids. This reagent must be slaked in order to
prepare the reactive slurry for absorption of acid
gases. Slaking is the addition of water to convert
calcium oxide to calcium hydroxide. Proper slaking
conditions are important to ensure that the resulting
calcium hydroxide slurry has the proper particle size
distribution and that no coating of the particles has
occurred due to the precipitation of contaminants in
the slaking water. The prepared slurry is atomized in
a large absorber vessel having a residence time of 6 to
20 seconds. Atomization of the slurry is achieved
through the use of: (1) rotary atomizers or (2) air
atomizing nozzles. Generally, only one rotary
atomizer is included in a spray dryer absorber.
However, a few applications have as many as three
rotary atomizers.
In rotary atomizers, a thin film of slurry is fed to the
top of the atomizer disk as it rotates at speeds of
10,000 to 17,000 revolutions per minute. These
atomizers generate very small slurry droplets having
diameters in the range of 100 microns. The spray
pattern is inherently broad due to the geometry of the
disk.
High pressure air is used to provide the physical
energy required for droplet formation in nozzle type
atomizers. The typical air pressures are 70 to
90 pounds per square inch, gage (psig). Slurry drop-
lets in the range of 70 to 200 microns are generated.
This type of atomizer generally can operate over
wider variations of the gas flow rate than can be used
in a rotary atomizer. However, the nozzle atomizer
does not have the slurry feed turndown capability of
the rotary atomizer. For these reasons, different
approaches must be taken when operating at varying
system loads.
The shape of the scrubber vessel must be designed to
take into account the differences in the slurry spray
pattern and the time required for droplet evaporation
for the two types of slurry atomizers. The length-to-
diameter ratio for rotary atomizers is much smaller
than that for absorber vessels using air atomizing
nozzles.
All of the slurry droplets must evaporate to dryness
prior to approaching the absorber vessel side walls
and prior to exiting the absorber with the gas stream.
Accumulations of material on the side walls or at the
bottom of the absorber would necessitate an outage
since these deposits would further impede drying.
Proper drying of the slurry requires generation of
small slurry droplets and adequate mixing with the
hot flue gases.
Drying that is too rapid can reduce pollutant
collection efficiency since the primary removal
mechanism is absorption into the droplets. There
must be sufficient contact time for the absorption. For
this reason, spray dryer absorbers are operated with
exit gas temperatures 90° to 180°F above the
saturation temperature.
Dry injection absorption systems. This type of dry
scrubber uses injection of a finely divided alkaline
sorbent such as calcium hydroxide (hydrated lime) or
sodium bicarbonate for the absorption of acid gases.
The reagent feed has particle sizes which are
90 percent by weight through 325 mesh screens. This
size is approximately the consistency of talcum
powder. This size is important to ensure an adequate
alkaline sorbent surface area for high efficiency
pollutant removal.
Proper particle sizes are maintained by transporting
the sorbent to the dry scrubber system by means of a
positive pressure pneumatic conveyor. This provides
the initial fluidization necessary to break up any
clumps of reagent which have formed during storage.
The air flow rate in the pneumatic conveyor is kept at
a constant level regardless of system load to ensure
proper particle sizes.
Fluidization is completed when the alkaline sorbent
is injected countercurrently into the gas stream. The
alkaline sorbent may be injected directly into a
reaction vessel or may be injected directly into the
ducting with a reaction vessel (expansion chamber)
located downstream to increase residence time. The
gas stream containing the entrained sorbent
particles and fly ash is then ducted to a fabric filter or
ESP. When a fabric filter is used for particulate
control, acid gas removal may be further enhanced by
the reaction with the sorbent in the filter cake.
In one version of the dry injection system, solids are
recycled from the particulate matter control device
back into the flue gas contactor. The primary purpose
of the recycle stream is to increase reagent utilization
and thereby reduce overall calcium hydroxide costs.
3.4 References for Chapter 3
1. U. S. Environmental Protection Agency. Hospital
Waste Combustion Study: Data Gathering Phase,
EPA-450/3-88-017. December 1988.
2. Joseph, J. and D. Beachler. APTI Course SI:412C,
Wet Scrubber Plan Review - Self Instructional
Guidebook. EPA 450/2-82-020. U. S. Environ-
mental Protection Agency. March 1984.
32
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3. Schifftner, K. and Patterson, R. Engineering
Efficient Pathological Waste Incinerator
Scrubbers. Paper presented at First National
Symposium on Incineration of Infectious Waste.
Calvert, Inc. May 1988.
4. Holland, O. L., and Means, J. D. Utilization of
Hydro-Sonic® Scrubbers for the Abatement of
Emissions from Hazardous, Industrial, Municipal
and Biomedieal Wastes; Technical Paper
No. 7802, John Zink and Company, 1988.
5. Reference 2. p. 1-4.
6. Reference 2. p. 3-2.
7. Reference 2. p. 3-4.
8. U. S. Environmental Protection Agency. Wet
Scrubber System Study: Volume I-Scrubber
Handbook. EPA-R2-72-118a, PB 213016.
August 1972. p. 5-122.
9. Reference 2. p. 5-3.
10. Reference 2. p. 3-4.
11. Beachler, D. and M. Peterson. APTI Course
SI:412A, Baghouse Plan Review - Student
Guidebook. EPA 450/2-82-005. U. S. Environ-
mental Protection Agency. April 1982.
12. U. S. Environmental Protection Agency. Control
Techniques for Particulate Emissions from
Stationary Sources, Volumes 1 and 2. EPA 450/3-
81-005a,b. (PB83-127498). U. S. Environmental
Protection Agency. September 1982.
13. Sedman, C. and T. Brna. Municipal Waste
Combustion Study: Flue Gas Cleaning
Technology. EPA/530-SW-87-021d (NTISPB87-
206108) U. S. Environmental Protection Agency.
June 1987.
14. Richards Engineering. Air Pollution Source Field
Inspection Notebook; Revision 2. Prepared for the
U. S. Environmental Protection Agency, Air
Pollution Training Institute. June 1988.
33
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Chapter 4
Operation
The success of incineration as a technique for
treating hospital waste depends on the proper
operation of the incinerator and its air pollution
control devices. Proper operating techniques can
affect equipment reliability, on-line availability,
combustion efficiency, and compliance with air
pollution regulations. The operator is in control of
many of the factors that affect the performance of a
hospital incinerator including: (1) waste charging
procedures, (2) incinerator startup and shutdown,
(3) air pollution control device startup and shutdown,
(4) monitoring and adjusting operating parameters
for the incinerator and air pollution control system,
and (5) ash handling. This section identifies key
operating parameters, identifies operating ranges for
key parameters, and discusses general operating
procedures that can minimize unexpected malfunc-
tions and improve the performance of the incinerator
and air pollution control devices. Appropriate
monitoring procedures also are discussed in this
section.
4.1 General Objectives
The primary objectives associated with the proper
operation of a hospital waste incineration system are
to operate the system in a manner so that infectious
materials in the waste are rendered harmless, waste
volume is reduced, good ash quality (from an
aesthetic standpoint) is ensured, and air pollution
emissions of particulate matter, organic compounds,
carbon monoxide, and acid gas are minimized. The
operator is responsible for knowing key operating
parameters and the proper operating ranges for these
parameters to assure that the system is operated at
its design efficiency. The operator also should know
the operating procedures and monitoring techniques
which will assist in maintaining those parameters
within the acceptable operating range.
Because each incinerator model is designed
differently, design criteria, operating parameters,
and operating procedures will vary. Nonetheless,
recommended operating ranges can be established for
some general key parameters to assist in meeting the
objectives of the incineration process - destruction of
pathogens and reduction of waste volume - while
minimizing air pollution and maintaining good ash
quality. To identify these key parameters and discuss
proper operating procedures, incineration systems
have been categorized as follows:
l.Bateh/controlled-air incinerators;
2.1ntermittent-duty, controlled-air incinerators;
3.Continuous-duty, controlled-air incinerators;
4. Multiple-chamber incinerators; and
S.Rotary kilns.
The operation of air pollution control systems is
addressed later in this chapter.
Currently, most hospital waste incineration systems
do not include an air pollution control device mainly
because with proper combustion control through
proper operation, the systems can meet current
emission regulations. However, a growing number of
States have promulgated or will soon promulgate
more stringent regulations governing hospital waste
incinerators that will increase the numbers of
incinerators requiring add-on air pollution control
devices to meet the regulations. When an air
pollution control system is included as part of the
incineration system, the operation of the incinerator
and the control device are interrelated. Therefore, the
optimum approach to describing the proper operation
of incineration systems (including air pollution
control devices) would be to discuss the operation of
each of the possible combinations of incinerators and
control devices. However, because of the large
number of combinations of equipment types that may
constitute an incineration system, the proper
operation of incinerators and air pollution devices
will be discussed separately.
This document provides a discussion of operating
ranges for the key operating parameters and presents
an overview of proper operating procedures and
monitoring techniques. This information will provide
the reader with a basic understanding of what
constitutes good operating practice. However, it does
not substitute for manufacturers' operating
procedures. Specific steps and operating procedures
for equipment will be specified by each
manufacturer; furthermore, the control and
monitoring systems used by each manufacturer may
be different. Consequently, a specific manufacturer's
35
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recommended and detailed operating procedures
should be followed.
4.2 Incinerator Key Operating
Parameters
4.2.1 Introduction
The interrelationship among an incinerator's
thermal capacity, waste feed characteristics, heat
input rate from the waste feed, and combustion air
was discussed in Chapter 2. This section identifies
the operating parameters and presents recommended
operating ranges for these parameters. The majority
of the key parameters and the recommended
operating ranges remain the same for the different
incinerator systems, but because they do differ
somewhat for different types of incinerators, the
parameters are presented for each type system that
was described in Chapter 2. Controlled-air
incinerators, multiple-chamber incinerators, and
rotary kilns are discussed in order.
4.2.2 Controlled-Air Incinerators
Table 4-1 summarizes the key incinerator operating
parameters and the operating range for each
parameter. These parameters assume the waste is
primarily a heterogeneous mixture of hospital
infectious waste with a Btu content that will sustain
combustion (i.e., greater than 6,000 Btu/lb). For
"pathological" incinerators, burning pathological
wastes, the recommended operating ranges are
slightly different because waste characteristics differ
significantly. An explanation of why each key
operating parameter in Table 4-1 is important and a
discussion of each operating range is presented
below.
4.2.2.1 Primary and Secondary Combustion
Chamber Temperatures.
Maintaining the desired operating temperatures
within each combustion chamber is critical to proper
operation of a controlled-air incinerator. Both upper
and lower limits on the temperature range for each
chamber are of interest. The desired range of
operation is different for the ignition (primary) and
combustion (secondary) chambers because the
functions of these chambers differ.
The literature indicates that typical operating ranges
for the ignition chamber of controlled-air
incinerators range from 400° to 980°C (750° to
1800°F).l.2 The ignition chamber must be maintained
at a minimum temperature sufficient to sustain
combustion, combust the fixed carbon in the ash bed,
and kill any microorganisms in the waste bed so that
the remaining ash is sterile. The temperature also
must be maintained below a level that will damage
the refractory and result in slagging of the waste.
Few studies have been conducted to determine the
conditions necessary to achieve complete pathogen
kill in incinerators. Barbeito et al., conducted a study
on a multiple-chamber, industrial-refuse incinerator
to determine the minimum operating temperatures
required to prevent release of viable microorganisms
into the atmosphere. His research indicated that the
destruction of the microorganisms within the
incinerator depends on the temperature and time of
exposure. These parameters are affected by many
factors, including charging beyond incinerator
capacity and exceeding design linear velocities,
which reduces retention time. Barbeito recommended
a minimum primary chamber temperature of 760°C
(1400°F).1,3
From an operational (operating efficiency)
standpoint, it is desirable to operate the primary or
ignition chamber at a temperature high enough to
sustain combustion in the chamber and to generate
sufficient volatile combustion gases and heat to
maintain the desired secondary combustion
temperature without the use of auxiliary fuel.
Consequently, the desired primary chamber
temperature will depend somewhat on the waste
composition. Furthermore, a sufficiently high
temperature to effectively combust the fixed carbon
in the waste bed is desired. One manufacturer's
experience indicates that this temperature is in the
760° to 870°C (1400° to 1600°F) range for continuous-
duty incinerators.2 This temperature may be as low
as 540°C (1000°F) for batch feed and intermittent-
duty units because the burnout time can be extended
in these units.2 Operating batch feed incinerators at
a low primary chamber temperature (relative to
continuous-duty incinerators) helps assure that
volatiles are not generated at a rate which cannot be
handled by the secondary chamber.
When the waste stream contains a significant
amount of plastics, "cracking" of light hydrocarbons
can be significant and can affect the combustion gas
volume in the secondary chamber. Operating the
primary chamber at lower temperatures (540° to
650°C [1000° to 1200°F]) may help to minimize rapid
increases in flue gas volume.4 Some incinerator
manufacturers use water quenching in the primary
chamber to maintain gas temperatures below the
930° to 980°C (1700° to 1800°F) range and minimize
cracking. 5
At the same time, the primary chamber temperature
must be maintained below the point that will result
in refractory damage. Modern incinerators commonly
use nominal 1540° to 1650°C (2800° to 3000°F)
quality refractory. Although this refractory is rated
at 1540° to 1650°C (2800° to 3000°F), reactions can
occur with contaminants in the combustion gas,
making it desirable to expose this refractory to no
higher than 1200°C (2200°F) on a continual basis.2
Another, perhaps more important, limiting factor for
36
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Table 4-1. Key Incinerator Operating Parameters and Recommended Operating Range: Controlled-Air Incinerator
Parameter
Ignition chamber temperature, °F
Combustion (secondary) chamber temperature, °F
Charging rate, Ib/hr
Ignition chamber combustion air (percent of stoichiometric)
Total combustion air (percent excess air)
Combustion gas oxygen concentration, percent
Ignition chamber draft, in w.c.
Burndown period, h
Batch feed
1000° to 1800°
1800° to 2200°
Fill chamber once at
beginning of cycle
30 to 80
140 to 200
12 to 14
-0.05 to -0.1
2 to 5
Incinerator type
Intermittent feed
1000° to 1800°
1800° to 2200°'
10 to 25 percent of
rated capacity at 5 to
15 min intervals
30 to 80
140 to 200
12 to 14
-0.05 to -0.1
2 to 5
Continuous duty
1400° to 1800°
1800° to 2200°
1 0 to 25 percent of
rated capacity at 5 to
1 5 min intervals
30 to 80
140 to 200
12 to 14
-0.05 to -0.1
Not applicable
the upper operating temperature of the primary
chamber is slagging of the waste. Most ash residues
begin to become soft at temperatures in the range of
1200° to 1370°C (2200° to 2500°F).2 While
thermocouples in the primary chamber indicate the
temperature of the combustion gas exiting the
primary chamber, the temperature in the ash bed at
the hearth adjacent to the underfire air ports can be
consistently higher. Consequently, although the
combustion gas temperature can indicate that
clinker (fused slag) formation should not be a
problem, the ash bed can be hot enough to form
clinkers. Experience has shown that a control
temperature of 980°C (1800°F) can be used in most
cases with acceptable performance with regard to
clinker formation and carbon burnout, but the
performance problems due to slagging may begin to
occur at primary chamber temperatures as low as
760°C (1400°F).2,6
The secondary chamber serves to complete the
combustion process initiated in the primary chamber.
As with the primary chamber, it is desirable to
operate within a lower and upper range. At
temperatures that are too low, complete combustion
may not occur. At temperatures that are too high,
refractory damage may occur, residence time may be
decreased, and auxiliary fuel may be unnecessarily
wasted.
A minimum temperature is needed to prevent the
discharge of potentially toxic products of incomplete
combustion. Experimental work conducted at the
University of Dayton Research Institute (UDRI)
indicates that temperature is the primary factor that
affects these emissions.7 The experimental data for
thermal decomposition indicate that the destruction
of a species depends predominantly on the
temperature, and effectively a threshold temperature
exists above which the compound will rapidly
combust. The threshold temperature found for
polychlorinated dibenzo-p-dioxin (PCDD) and
polychlorinated dibenzofurans (PCDF) and potential
precursors (e.g., hexachlorobenzene) is near 930°C
(1700°F)J
Sufficient temperature in the secondary chamber is
necessary to kill microorganisms entrained in the gas
stream from the primary chamber. Again, little
information on the required temperature is
available, and data from studies conducted on specific
incinerators are specific to those incinerators and the
experimental operating conditions, e.g., residence
time and turbulence. Based upon his incineration
study, Barbeito recommended a minimum secondary
combustion chamber temperature of 980°C (1800°F).l
The limiting factor for the secondary combustion
chamber temperature upper operating range is
refractory damage. The upper limit will depend on
the refractory used, but a typical limit is 1200°C
(2200°F) on a continual basis.2
In summary, temperatures in both chambers should
be maintained at high enough levels to ensure
complete combustion of the waste but not so high that
refractory damage or slagging of the ash occurs. A
minimum temperature for the primary chamber in
the range of 540° to 760°C (1000° to 1400°F) should be
maintained to ensure pathogen kill, sterile ash, and
good ash quality. The minimum temperature
required for proper operation will depend on waste
characteristics as well as the retention time of the
ash in the primary chamber and consequently the
incinerator design. To prevent clinker formation, the
recommended upper bound primary chamber
temperature is 980°C (1800°F). To assure complete
combustion, yet conserve auxiliary fuel and prevent
refractory damage, an operating range of 980° to
1200°C (1800° to 2200°F) is recommended for the
secondary combustion chamber.
4.2.2.2 Charging Rate
An incinerator system is designed for a particular
thermal input rate. The thermal input comes from
the waste and, as necessary, the auxiliary fuel.
Under ideal conditions, the incinerator operates
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under conditions of a constant thermal input. For
controlled-air incinerators, the thermal release from
the "fuel" (the waste in the chamber) is controlled, to
the extent possible, by controlling the available
combustion air (the typical ranges for combustion air
will be discussed in the next section). Additional
control of the thermal input is obtained by
controlling the quantity and frequency of waste
charges to the incinerator. The charging process,
thermal input, and combustion approaches a steady
state condition as the waste homogeneity increases,
the charge loads decrease in size, and the frequency of
charges increase. Therefore, a charging scenario of
more frequent charges of smaller volume is more
desirable than one of a single large charge.8
4.2.2.2.1 Batch feed incinerators. By nature of
their design, batch feed incinerators are intended to
accept a single load of waste at the beginning of their
incineration cycle; the heat release rate is controlled
solely by controlling the size of the initial charge and
the available air for combustion. The primary
chamber acts as a fuel storage area. Manufacturers of
batch operated incinerators recommend that the
primary chamber be filled to capacity, but not
overfilled or stuffed so full that the flame port to the
secondary chamber or the ignition burner port
assembly are blocked.9 If wastes containing an
unusually high volatile content are charged to the
unit, a full load may contain enough volatiles that,
even if the primary air is controlled, the capacity of
the secondary combustion chamber will be exceeded.
Therefore, it may be necessary to decrease the size of
the charge to the incinerator. That is, volume of
waste is an imperfect method for determining the
thermal input to the incinerator. The incinerator
chamber size is designed for a particular volume of
waste with a particular Btu content. If the waste
being charged is significantly higher in Btu content,
even though the volume capacity of the chamber is
not exceeded, the thermal capacity of the incinerator
may be exceeded. Consequently, it will be necessary
to decrease the thermal input of the batch charge by
decreasing the volume input.
Regardless of the type, incinerators should never be
overcharged above the manufacturer's specifications.
In the case of batch feed incinerators where all
charging occurs at the beginning of the incineration
cycle, there may be a tendency to stuff that last bag of
waste into the incinerator so that no waste is left over
for the next burn. This practice can lead to excessive
emissions, incomplete combustion, and damage to the
incinerator. Overcharging with waste adds more fuel
to the incinerator than it can handle, may block the
incinerator air ports, and damage the primary
burner. Excessive fuel combined with an inadequate
air supply causes excessive amounts of volatile
material to be passed to the secondary chamber
which cannot be handled effectively in the secondary
chamber and results in high particulate matter
emissions. Additionally if a large quantity of plastic
material is charged, the excessive temperatures
produced can cause refractory damage and clinker
formation. Higher primary chamber temperatures
also cause higher gas volumes and velocities which
effectively reduces the secondary chamber retention
time.
4.2.2.2.2 Intermittent-feed and continuous-duty
incinerators. Intermittent- and continuous-duty,
controlled-air incinerators are designed to
accommodate semicontinuous charging. The primary
design change for this type unit is that the charging
system is set up with some type of a mechanical
device (either manually or automatically operated)
which allows the operator to safely add charges while
the system is operating. The mechanical device is
designed to protect the operator from exposure to the
flame and heat of the primary combustion chamber.
Additionally, the mechanical charging device may be
designed to limit air in-leakage to the incinerator
during charging to maintain control of the
combustion air level. The primary difference between
the "intermittent" and "continuous" duty units is not
in the charging rate or operation, per se, but that the
continuous-duty system has a means for continuously
removing the ash generated by the waste.
Consequently, this type unit can maintain
continuous "steady state" operation. The
intermittent unit can maintain continuous "steady-
state" operation only for a limited time; once the ash
in the incinerator builds up to an unacceptable level,
the unit must be shutdown and the ash removed.
To approach a steady thermal input, manufacturers
recommend that multiple charges be made at equally
timed intervals. The recommended charge size is 10
to 25 percent of the rated capacity charged at 5 to
15 minute intervals. 10.u The charging frequency
may need to be adjusted based upon variations in
moisture content, volatile content, and overall Btu
value. The procedures that the operator can use to
monitor the charging rate and to make appropriate
decisions is discussed in Section 4.4.
4.2.2.3 Primary and Secondary Combustion Air
Rate and Combustion Gas Oxygen
Concentration
For controlled-air units, the volatilization/combus-
tion rate in the primary chamber is controlled by
using combustion airflow rate to control the chamber
temperature. The typical combustion air operating
range for the ignition chamber is 30 to 80 percent of
stoichiometric conditions. 1.2 Typically, about
20 percent of the total air requirement to the
incinerator is supplied as underfire air to the ignition
chamber. 2 The remainder of the air is supplied to the
secondary mixing chamber.
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Approximately 80 percent of the total combustion air
required during incineration is supplied to the
secondary combustion chamber. Typically, the total
combustion air level is 140 to 200 percent excess
air. 12.13 Measurement of the oxygen concentration
provides a convenient way of determining and
monitoring the excess-air level. Operating the
incinerator at a total excess-air level in the range of
140 to 200 percent will result in stack gas oxygen
concentrations in the range of 12 to 14 percent.
is somewhat experimental and will vary depending
upon the waste composition and combustion air rate.
According to one manufacturer, a rule of thumb for a
trial burndown setting for an intermittent-duty,
small capacity incinerator is 1 hour plus an
additional 20 minutes for each hour of operation. 10
Therefore, a trial burndown setting for a unit that
operated 6 hours would be (1 hour) + (6 x 20 minutes)
= 3 hours. A typical burndown period for batch and
intermittent units is in the range of 2 to 4 hours.11.14
4.2.2.4 Combustion Chamber Pressure (Draft)
A typical draft for controlled-air incinerators is in the
range of-0.05 to -0.1 in. water column (w.c.). Exces-
sive draft is not desirable because increased carry-
over of particulate matter to the secondary chamber
can occur.
4.2.2.5 Burndown Period
After volatilization diminishes, sufficient time must
be provided for the fixed carbon in the waste bed to
combust.
For continuous-duty incinerators, there is no distinct
burndown period since the burndown occurs
continuously as the waste moves through the system
during the "steady state" operation. However, on
some continuous-duty incinerators with internal
transfer rams, once the last load of waste is charged,
each ram may go into a burndown mode sequentially
where the rams' strokes are increased to push the
waste from one hearth to the next. For example, the
ram on the first or drying hearth will stroke at the
same time intervals but will stroke several inches
more on each stroke. During this period, the ash
underfire air to this part of the hearth is shut off. On
the last stroke, the ram is at the edge of the hearth,
and all of the waste falls to the next hearth which
then goes into its burndown mode, and so on. This
burndown sequence is usually preset at the factory
but can be changed by the manufacturer if ash
quality is poor. On these units, the burndown
sequence is normally in the range of 4 to 6 hours.14
For batch and intermittent-duty operations, there is
a distinct burndown period associated with the
operating cycle. During this period, the volatiles
have been combusted, and the fuel remaining in the
waste bed is slowly diminishing. In order to combust
the fixed carbon remaining in the waste, a higher
temperature is required and, therefore, more
underfire air is introduced into the primary
chamber, io As the burndown proceeds and the fuel is
further diminished, the temperature slowly
decreases. During burndown, a minimum
temperature level (e.g., 1400°F) is maintained for a
predetermined time period; auxiliary fuel may be
required to maintain this temperature. It is difficult
to recommend a required burndown period because it
4.2.3 Multiple-Chamber Incinerators
Traditionally, multiple-chamber units were designed
specifically to burn pathological waste and contained
a fixed (solid) hearth. Other multiple-chamber
incinerators use a grate type of hearth. Combustion
of medical waste containing significant quantities of
fluids and/or infectious material is not recommended
in multiple-chamber units with grates. Both type
units employ large quantities of excess air for
combustion and will operate at similar temperature
ranges. The operating parameters and ranges for a
multiple-chamber incinerator are summarized in
Table 4-2 and are briefly discussed below.
4.2.3.1 Primary and Secondary Chamber
Temperatures
As discussed for controlled-air incinerators, the
temperature in the primary chamber must be
maintained at such a level as to assure pathogen kill.
Most multiple-chamber incinerators operate on a
batch or intermittent-duty basis. An extended
burndown period is available to assure adequate ash
burnout and sterilization of the waste. A minimum
primary chamber temperature in the range of 540° to
760°C (1000° to 1400°F) is recommended. The upper
operating range for the primary chamber should be
established to prevent refractory damage. As
previously mentioned for controlled-air units,
sustained operation above 2200°F is generally not
desirable.
Pathological wastes have a high moisture content
and low volatile and fixed carbon content. For
pathological wastes, continuous operation of an
auxiliary burner is required to maintain the primary
chamber temperature. To facilitate burndown of the
pathological wastes, a minimum primary combustion
chamber temperature of 1600°F is recommended. 15
A minimum secondary chamber temperature of
1600°F for pathological incinerators is recommended
in the Air Pollution Engineering Manual to assure
complete combustion. 15 As discussed in Sec-
tion 4.2.2.1 for controlled-air incinerators, a min-
imum temperature to prevent the discharge of
potentially toxic products of incomplete combustion
is required; a minimum secondary combustion
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Table 4-2. Key Incinerator Operating Parameters and Recommended Operating Range: Multiple-Chamber Incinerator
Parameter Pathological waste General refuse
Ignition chamber temperature, °F
Combustion (secondary) chamber temperature,
Charging rate
1600 to 1800
1800 to 2200
Single layer on hearth
1000to 1400
1800 to 2200
10 to 25% of rated capacity at
5- to 15-min intervals
Ignitkxi chamber combustion air (percent excess air)
Total combustion air (percent excess air)
Combustion gas oxygen concentration, percent
Ignitkxi chamber draft, in. w.c.
80
120 to 200
10 to 14
-0.05 to -0.1
150
250 to 300
15 to 16
-0.05 to -0.1
chamber temperature of 980°C (1800°F) is recom-
mended for multiple-chamber incinerators.
4.2.3.2 Charging Rate
Whether a multiple-chamber unit is incinerating red
bag waste or pathological waste affects the charging
rate and procedures because the volatile content,
moisture content, and Btu value of these wastes are
so different. An incinerator system is designed and
sized for a particular thermal input rate. The thermal
input comes from the waste and, as necessary, the
auxiliary fuel. Unlike a controlled-air unit, the
combustion rate in a multiple-chamber unit cannot
be controlled by controlling the combustion air to the
primary chamber. Accordingly, the controlling factor
for the combustion rate is essentially the availability
of fuel; i.e., the amount of waste charged when
general refuse/red bag waste is being incinerated.
The thermal input to the incinerator and the
combustion approaches a steady-state condition as
the waste homogeneity increases, the charge loads
decrease in size, and the frequency of charges
increase. Therefore, charging procedures using more
frequent charges of smaller volume are more
desirable than one single large charge. A charge rate
of 10 to 15 percent of the rated capacity at regular
intervals is recommended. Note that if a mechanical
feeder is not used, the more frequently the charging
door is opened, the greater the risk to the operator.
Consequently, safety considerations make less
frequent charging desirable, which contradicts the
more frequent charging procedures desired from a
thermal input standpoint. Thus, the objective should
be to assure that the individual charges to the
incinerator are not so large and infrequent that the
capacity of the incinerator is exceeded by rapid
volatilization of combustibles resulting in excess
emissions. Therefore, charging frequency and load
size will depend on waste characteristics and
incinerator design.
Pathological wastes have a low Btu content and a low
volatiles content. Consequently, rapid volatilization
and rapid changes in heat release to the primary
chamber are not a concern when charging
pathological wastes. The primary chamber burner(s)
provides a steady heat input to the incinerator.
Because of the high moisture and low volatile carbon
content of pathological waste, the burner flame must
impinge on or near the waste for efficient combustion
to occur. Accordingly, fresh pathological waste
should not be charged into the incinerator until the
previous waste charge has burned down almost
completely. Completeness of burndown can be
determined by inspection through a view port and
examining the waste bed.
4.2.3.3 Primary and Secondary Chamber
Combustion Air Levels
For multiple-chamber incinerators the primary
chamber excess-air level is typically 150 percent or
greater. Overall excess-air levels for multiple-
chamber incinerators are typically 250 to
300 percent.15 This roughly corresponds to a
combustion gas oxygen concentration of 15 to
16 percent. When incinerating pathological wastes,
since the primary heat release is coming from the
auxiliary burner, the combustion air can be
controlled at a lower level than for a typical multiple-
chamber incinerator.
4.2.3.4 Chamber Pressures
A negative draft is maintained in the combustion
chambers. A typical range for the ignition chamber
draft is -0.05 to -0.1 in. w.c. 15
4.2.4 Rotary Kiln Incinerators
Currently, there are relatively few rotary kiln
incinerator installations at hospitals for incinerating
hospital medical waste. Consequently, little
information is available on the application of rotary
kilns to this waste type. However, rotary kilns are a
proven technology and have been used for a wide
variety of industrial applications including
hazardous waste incineration. The operating
parameters identified below are based mainly on the
application of rotary kilns to the incineration of
hazardous waste because of the availability of
information in this area.
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4.2.4.1 Ignition and Combustion Chamber
Temperatures
As discussed in Chapter 2, a rotary kiln incineration
system consists of an inclined rotary kiln which
serves to volatilize the waste and a secondary
combustion chamber which completes the
combustion process by burning the volatiles in the
kiln off-gas. Temperatures in rotary kiln incinerators
usually range from about 760° to 1670°C (1400° to
3000°F) depending on the types of waste being burned
and on the location in the kiln.
In evaluating the application of rotary kiln
incineration to hospital medical waste, it is likely
that the kiln (primary chamber) and secondary
chamber operating temperatures will be similar to
those identified for other hospital medical waste
incineration systems, i.e., a primary chamber
temperature of 760°C (1400°F) and a secondary
chamber temperature of 980°C(1800°F).
4.2.4.2 Charging Rate
Rotary kiln incinerators may be batch or
continuously fed and have a feed capacity that ranges
from 600 to 2,000 kg/h (1,300 to 4,400 Ib/h).
The waste throughput of a rotary kiln is determined
both by the speed of rotation and by the incline angle
(rake). Typically, rotary kilns rotate at a speed that
ranges from 0.25 to 1.5 revolutions per minute and
are oriented at a rake of less than 3 percent (i.e.,
around 11° from horizontal). Both of these
parameters may be adjusted depending on the
throughput desired. However, once the rake is set, it
is usually not altered. Assuming that all other
operating parameters are in the normal operating
ranges, incompletely burned material in the ash is an
indication that waste is traveling through the kiln
too quickly. The waste throughput may be reduced by
reducing the speed of rotation of the kiln which in
turn must include a corresponding decrease in the
charging rate. Some experimentation with charge
rates and speed of rotation may be required to obtain
optimum ash quality and throughput.
4.2.4.3 Ignition and Combustion Chamber Air
Levels
Based on information available on hazardous waste
rotary kiln incinerators, the rotary kiln excess-air
level ranges from 140 to 210 percent or greater,
depending on the desired operating temperature and
the heating value of the waste. When high aqueous
wastes are being burned, lower excess-air rates may
be needed to maintain adequate temperature.
Secondary chamber excess-air levels are
approximately 80 percent of the rotary kiln excess-air
levels. For example, in a typical system operating at
820°C (1500°F) in the kiln and 980°C (1800°F) in the
secondary chamber, approximately 160 to
170 percent excess air would be maintained in the
secondary chamber compared to about 210 percent in
the kiln. Manufacturers are now designing rotary
kilns utilizing special kiln seals and air injection that
operate with substoichiometric air levels in the kiln.
Operating the kiln at substoichiometric air levels
decreases the kiln size and reduces auxiliary fuel
usage.4
4.3 Waste Feed Handling
The physical and chemical properties of wastes and
the effects of these properties on the incineration
process were presented in Chapter 2. Furthermore,
typical characteristics of hospital infectious wastes
were presented and discussed. In most cases, the
operator of an incinerator does not have control over
the quantity or types of wastes which are transported
to the incinerator for disposal. However, the operator
does have control over two of the most important
parameters - how the waste is charged and at what
rate. Waste charging procedures are specific to the
type incinerator and the incinerator's mode of
operation; proper charging procedures will be
discussed in Section 4.4. In this section, some general
concepts with regard to types of waste and handling
of the wastes are discussed. Although the operator
does not have control (or has very little control) over
the waste which is to be incinerated, the operator
must understand how waste composition affects
operation of the incinerator and must learn to
recognize problems related to waste composition or to
identify significant changes in waste composition.
The operator then can modify charging procedures,
modify incinerator combustion parameters (if
properly trained to make such adjustments), and/or
notify the appropriate hospital administrators of
continued waste related problems which are severely
affecting incinerator operation. The remainder of this
section addresses the following concerns:
1. Safe waste handling procedures; and
2. Wastes that should not be incinerated.
4.3.1 Proper Waste Handling
Infectious waste will be delivered to the incinerator
area in "red bags." The primary concern is to avoid
exposure to pathogens; consequently red bag wastes
should be handled in a manner which maintains the
integrity of the container. Proper procedures dictate
that:
1. Sturdy containers are used;
2. Waste handling is minimized;
3. The waste storage area is secure if wastes are to
be stored; and
4. Mechanical handling/loading systems are
properly operated and maintained.
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Plastic bags are the typical containers used for
infectious waste. Tear-resistant bags that will
maintain their integrity during the
handling/transporting process should be used. The
two main criteria used to evaluate bag durability are
bag thickness and the ASTM dart test.3,16
Some State regulations specify minimum bag
requirements based on one of these two criteria.3 For
example, Massachusetts specifies a minimum bag
thickness while California requires use of the ASTM
dart test.3 Even if appropriate bag materials are
used, overloading of bags, placement of sharp objects
within the bags, or mishandling can result in tearing.
"Sharps" (e.g., needles, broken glass) should be
placed in special rigid sealed containers; these
containers can then be placed in the red bag. The
options that can be used if bag tearing is a problem
include double bagging and the use of rigid
containers to contain the bags. Rigid containers such
as reusable metal/plastic drums or disposable
cardboard drums or boxes can be used to contain the
individual plastic bags. If reusable containers are
used, the plastic bag would be placed as a liner in the
container (much like one might do at home for
household garbage). When filled, the bag is sealed,
and a lid is placed on the container. The container is
then transported to the incineration area. Reusable
containers have some disadvantages in that they
require additional handling and will need to be
disinfected after each use.1 Because the waste is to be
incinerated, disposable cardboard cartons that can be
loaded directly into the incinerator and disposed with
the waste is a reasonable alternative. Use of
disposable containers with mechanical charging
systems also minimizes the potential for spillage in
and contamination of the charging equipment during
operation of the incinerator.
Another approach to enhancing integrity of the red
bags is to use rigid carts, usually made of rigid plastic
or of metal, to collect and transport the red bags.
These rigid carts protect the waste bags from
bumping, tears, etc.; the integrity of the bags is more
easily maintained. Also, should a bag tear or break,
the waste is contained, even if liquids are involved.
Finally, handling of the waste can be minimized by
the use of rigid carts. Once the red bags are placed in
the cart, they need not be handled again until they
are loaded directly into the incinerator or into the
mechanical charging system for the incinerator.
Since waste handling is minimized, the approach of
keeping the wastes in the transport cart up until the
time it is charged to the incinerator is preferred over
the approach of unloading the waste to a storage area
or charging pile on the floor. When rigid transport
carts are used for large automated incineration
systems, the waste may never need to be handled
manually after collection. Charging systems are
available which will automatically lift and dump the
contents of the collection transport carts into the
charging hopper without handling of the waste by the
operator.8,17 Consequently, use of rigid type
transport carts can be effective in minimizing waste
handling.
To assure the integrity of the red bags, mechanical
means of transporting the waste should not be used,
except for final charging into the incinerator. For
example, dumb-waiters, chutes or conveyors should
not be used to transport the waste.3 The mechanical
charging systems for the incinerator should be
designed and operated to minimize bag-breakage,
spillage, and possible contamination. For example,
trash compactors should not be used since when the
bags are compacted, they will likely break open.
Furthermore, compaction will affect the wastes'
bulk/density (lb/ft3) which will consequently affect
the combustion process.3
The treatment of infectious waste as soon as possible
after generation is preferable. However, because
same-day treatment is not always possible, the
incinerator operator may be responsible for waste
storage. If the waste must be stored prior to
incineration, four factors should be considered:
1. Maintaining container integrity and minimizing
handling;
2. Storage temperature;
3. Storage duration; and
4. Location of the storage area.
The waste storage area should be a "secure" area, out
of the way from normal hospital traffic and should
have restricted access. Certainly, the area should be
secure from public access. The storage area and/or the
containers should be secure from rodents and vermin
which can contract and transmit disease.
As temperature and storage time increases, decay
occurs and unpleasant odors result. There is no
unanimous opinion on acceptable storage
temperature or times. The EPA Office of Solid Waste
simply recommends that storage times be kept as
short as possible.3 Some States do regulate storage
times. For example, Massachusetts allows infectious
waste to be stored for 24 hours (1 day) at room
temperature or for 72 hours (3 days) at refrigerated
temperatures (34° to 45°F).3
Obviously, the level of control that the incinerator
operator has over the items just discussed will vary
with the situation and with the particular waste
handling issue. That is, the operator may have little
control over what strength red bag is used, but may
have a great deal of control over whether red bags are
removed from transport carts to a "charging floor" or
whether the bags are simply loaded into the
incinerator directly from the cart - thus eliminating
handling each bag two more times.
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The operator does, however, have control of personal
protection items used. It is prudent that when
handling infectious waste, an operator should always
wear hard-soled shoes to avoid the potential for
punctures and thick rubber gloves to resist cuts and
punctures and to prevent direct contact with fluids.
Safety precautions are discussed further in
Chapter 9. The operator must remember that his/her
safety is at stake and should bring deficiencies in
waste handling practices (e.g., consistent tearing of
bags due to poor bag quality) to the hospital
administrator's attention.
4.4.1 Batch Feed Controlled-Air Incinerator
This type incinerator typically is a small unit, up to
500 Ib/h, but more typically less than 200 Ib/h
capacity. The incinerator is operated in a "batch
mode" over a 12- to 24-h period which entails a single
charge at the beginning of the cycle, followed by
combustion, ash burnout, cooldown, and ash removal.
The operating cycle from startup to shutdown is
discussed in the following sections. Parameters
which can be automatically controlled and
monitoring techniques also are discussed.
4.3.2 Restricted Wastes
It is inappropriate to dispose of some wastes in a
hospital waste incinerator unless special permits are
obtained. Wastes which should not be incinerated,
unless special permits are obtained include:
(1) radioactive wastes, and (2) hazardous wastes
regulated under the Resource Conservation and
Recovery Act (RCRA).
Some State regulations or specific operating permits
may dictate charging limits for certain types of
wastes. For example, an operating permit may allow
only up to 5 percent of the hourly charging rate to be
human or animal parts (i.e., "pathological waste").
Obviously, the operator cannot tell whether the
waste contains 4 percent or 5 percent of pathological
wastes; nor should the operator ever open red bag
wastes to examine the contents. Nonetheless, the
operator must be aware of the regulations and must
be attuned to spotting problem wastes, either by the
obvious (e.g., gallon jugs of liquids, containers
marked "radioactive") or by more subtle means (i.e.,
identifying large quantities of human/animal
anatomical parts in the incinerator waste bed, while
viewing the bed through a glass covered viewport).
4.4 Incinerator Operation, Control, and
Monitoring
The key operating parameters for incinerators were
presented in Section 4.2. This section provides a
summary of the operating procedures, parameters
which can be automatically controlled, and
monitoring techniques for incineration systems. The
operation, monitoring, and control of these four
"typical" systems are discussed:
1. Batch feed controlled air;
2. Intermittent-duty controlled air;
3. Continuous-duty controlled air; and
4. Multiple chamber.
The operation, control, and monitoring of rotary kilns
is not presented here because very few units are used
to incinerate hospital waste and detailed information
regarding their operation was not available.
4.4.1.1 Incinerator Operating Procedures
4.4.1.1.1 Ash removal. Startup of the incinerator
actually begins with removal of the ash generated
from the previous operating cycle. The following are
guidelines for good operating practice:10
1. In general, allowing the incinerator to cool
overnight is sufficient for the operator to remove
the ash safely. This cooling can take as long as
8h.
2. The operator should open the ash cleanout door
slowly both to minimize the possibility of damage
to the door stop and seal gasket and to prevent
ash from becoming entrained.
3. The operator should exercise caution since the
refractory may still be hot and the ash may
contain local hot spots, as well as sharp objects.
4. The ash and combustion chamber should not be
sprayed with water to cool the chamber because
rapid cooling from water sprays can adversely
affect the refractory.
5. A flat blunt shovel, not sharp objects that can
damage the refractory material, should be used
for cleanup.
6. Avoid pushing ash into the underfire air ports.
7. Place the ash into a noncombustible heat
resistant container, i.e., metal. Dampen the ash
with water to cool and minimize fugitive
emissions.
8. Once the ash has been removed and prior to
closing the ash cleanout door, the operator should
inspect the door seal gasket for frayed or worn
sections. Worn seal gaskets should be replaced.
9. To prevent damage to the door seal gasket, the
operator should close the ash cleanout door
slowly and should not overtighten the door
clamps. Overtightened door clamps may cause
the seal gasket to permanently set and allow
infiltration of outside air around the door face.
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4.4.1.1.2 Waste charging. The operator may have
the option of selecting which items are included in a
particular charge. Waste properties which should be
considered when the waste is segregated into charges
include: (1) the heating value, (2) the moisture
content, (3) the plastics content, and (4) the amount of
pathological wastes. The heating value and moisture
content of waste affects the performance of an
incinerator. A charge of waste with a very high
heating value may exceed the thermal capacity of the
incinerator. The result is high combustion
temperature, which can damage the refractory of the
incinerator and can result in excessive emissions.
Similarly, a charge of waste with a very high
moisture content will not provide sufficient thermal
input, and the charge will require the use of more
auxiliary fuel than usual.
Plastic items are an example of materials with high
heating values. Large quantities of plastic, which
may contain poly vinyl chloride, should be distributed
through many waste charges, not concentrated in one
charge, if possible. When sorting loads of waste to be
incinerated, the operator should try to create a
mixture of low, medium, and high heating value
wastes in each charge, if possible, to match the design
heat release rate of the incinerator. In general,
lighter bags and boxes will contain high levels of low
density plastics which burn very fast and very hot.
Heavier containers may contain liquids (e.g., blood,
urine, dialysis fluids) and surgical and operating
room materials which will burn slowly. As a general
rule for segregating waste into charges, the operator
may mix light bags and heavy bags to balance the
heating value of each charge. If several different
types of waste, (i.e., red-bag, garbage (cafeteria
wastes), and trash) are being charged to the
incinerator, charging the incinerator with some of
each waste type is better than charging it with all of
one waste type. Special care should be taken to avoid
overcharging the incinerator (beyond its intended
use) with anatomical wastes.
Prior to initiating charging, operation of the
combustion air blowers and ignition and secondary
burners should be checked. Follow the manufac-
turers' recommendations. The proper operation of the
primary and secondary burners is best achieved by
observing the burner flame pattern through the
viewports in the incinerator wall or in the burner
itself. Some burners are equipped with one
observation point to view the main flame and another
to view the pilot flame. The flame pattern will likely
vary with the type of burner. However, the length of
the flame should be such that the flame touches the
waste but does not impinge directly on the refractory
floor or wall. Obviously, the absence of a flame
indicates a problem with the burner or the system
that controls the burner.
Most burners are equipped with a flame safeguard
system that includes a flame detector that effectively
cuts off the fuel (gas or oil) supply to the burner if a
flame is not detected. When the burner is first
started, the burner blower starts and when it reaches
full speed, a purge timer starts. When the purge
timer times out, the flame safeguard energizes the
pilot relay that opens the pilot fuel supply and
igniter. When the pilot lights, a flame detector (either
an ultraviolet scanner [gas or oil] or flame rod circuit
[gas only]) detects the flame and causes the main
flame relay to activate the fuel supply to the main
burner. The pilot then ignites the main burner. The
flame detector continues to operate and shuts the
burner down if the main burner fails. Additionally, if
the air supply is lost both pilot and flame relays shut
off the fuel supply. The pilot usually is ignited for no
more than 15 seconds (interrupted pilot). If the main
burner does not ignite during the period, the flame
safeguard system shuts the entire system down. 18
The incinerator is charged cold. Because these units
generally are small, they are usually loaded
manually. The waste is loaded into the ignition
chamber, which is filled to the capacity recommended
by the manufacturer. Typically, the manufacturer
will recommend filling the incinerator completely,
but not overstuffing the chamber. Overstuffing can
result in blockage of the air port to the combustion
chamber and in premature ignition of the waste and
poor performance (i.e., excess emissions) during
startup. Overstuffing also can result in blockage of
the ignition burner port and damage to the burner.
After charging is completed, the charge door seal
gasket is visually checked for irregularities. The door
is then slowly closed and locked. The charge door seal
gasket should then be inspected for any gaps that
would allow air infiltration into the primary
chamber. Once operation is initiated, no further
charges will be made until the next operating cycle is
initiated, i.e., after cooldown and ash removal.
4.4.1.1.3 Waste ignition. Prior to ignition of the
waste, the secondary combustion chamber is
preheated to a predetermined temperature by
igniting the secondary burner. A minimum
secondary chamber temperature of 980°C (1800°F) is
recommended prior to ignition of the waste. The
manufacturer should be consulted regarding proper
preheat procedures; improper preheat can result in
refractory damage.
After the secondary chamber is preheated, the
secondary combustion air blower is turned on to
provide excess air for mixing with the combustion
gases from the primary chamber.
The primary chamber combustion air blower is
activated and the primary burner is ignited to
initiate waste combustion. When the primary
chamber reaches a preset temperature (i.e., the
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minimum operating temperature for the primary
chamber, see Table 4-1) and the waste combustion is
self-sustaining, the primary burner is shutdown.
The primary combustion air and secondary
combustion air are adjusted to maintain the desired
primary and secondary chamber temperatures.
(Typically this adjustment is automatic and can
encompass switching from high to low settings or
complete modulation over an operating range.)
During operation, the primary burner is reignited if
the ignition chamber temperature falls below a
preset temperature. Similarly, the secondary burner
is reduced to its lowest firing level if the secondary
chamber rises above a preset high temperature
setting. Again, control of the burners, like the
combustion air, is typically automated. A barometric
damper on the stack is used to maintain draft. The
incinerator chambers should both be maintained
under negative draft.
4.4.1.1.4 Burndown. After the waste burns down
and all volatiles have been released, the primary
chamber combustion air level is increased to
facilitate complete combustion of the fixed carbon
remaining in the ash. The temperature in the
primary chamber will continue to decrease indicating
combustion is complete. During the burndown period,
the primary burner is used to maintain the primary
chamber temperature at the predetermined
minimum level of the operating range. The length of
time required for the burndown period depends on the
incinerator design, waste characteristics, and degree
of burnout desired. A typical burndown period is 2 to
4 h.14 When combustion is complete, the primary and
secondary burners are shutdown.
Shutdown of the secondary burner which initiates
the cooldown period usually is automatically
determined by a preset length of time into the
cycle.9.11 The combustion air blowers are left
operating to cool the chambers prior to subsequent
ash removal. The blowers are shutdown when the
chambers are completely cooled or prior to opening
the ash door for ash removal. Cooldown typically
lasts 5 to 8 h. 14
The final step in the cycle is examination of ash
burnout quality. Inspection of the ash is one tool the
operator has for evaluating incinerator performance.
The operator should look for fine gray ash with the
consistency of ash found in the fireplace at home or in
the barbeque grill. Ash containing large pieces of
unburned material (other than materials which are
not combustible, such as cans) shows that incinerator
performance is poor. It may be necessary to return
these large pieces of material to the incinerator to be
reburned because poor quality ash may be refused at
landfills for disposal. Ash color also is an indicator of
ash quality. White or gray ash indicates that a low
percentage of carbon remains in the ash. Black ash
indicates higher carbon percentages remaining.
Although carbon remaining in the ash indicates that
available fuel has not been used and combustion has
not been complete, the fact that carbon remains in
the ash is not in itself an environmental concern or
an indicator that the ash is not sterile. Nonetheless,
ash color can be used to assist the operator in
evaluating burnout and incinerator performance.
4.4.1.1.5 Special considerations. If pathological
waste is being burned, the ignition burner should be
set to remain on until the waste is completely burned.
Further, the volume of waste charged likely will need
to be significantly reduced. The time required to burn
an equivalent volume of Type 4 waste will be
extended, since the waste contains high moisture and
low volatile content. To destroy pathological waste
efficiently, the waste must be directly exposed to the
burner flame; consequently piling pathological waste
in a deep pile (e.g., filling the entire chamber) will
result in inefficient combustion. 15 If large volumes of
pathological wastes are to be incinerated, an
incinerator which is especially designed for
pathological waste should be used.
4.4.1.2 Automatic Controls
Various levels of automatic controls are available for
hospital incinerators, even for the smallest units
sold. The smallest batch type units can be designed to
use the same automatic control concepts and
hardware for the key combustion control parameters
(e.g., temperature and combustion air) as the larger
continuous-duty incinerators.19 The use of control
systems allows key combustion parameters to be
adjusted automatically based on data (e.g.,
temperature) from the incineration system. These
automatic controls permit the combustion process to
be more closely controlled.
For batch feed systems, parameters that can be
automatically controlled include:
1. Charging frequency;
2. Combustion air rate;
3. Primary and secondary burner operation;
4. Temperature; and
5. Combustion, burndown, and cooldown cycle
times.
4.4.1.2.1 Charging frequency. The charging
frequency can be automatically controlled by
providing an interlock system on the charging door.
The interlock will prevent the charging door from
being opened after the primary burner is ignited and
will prevent additional charges to the batch system
until after burndown and cooldown have been
completed. The operation of the interlock can be set
up in one of two ways. The interlock can be activated
by a timer so that the door cannot be opened until
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after a preset time has elapsed (e.g., 24 hours).
Alternatively, operation of the interlock can be based
on temperature; the interlock is set to deactivate only
after the proper temperature in the cool down cycle is
attained. The latter approach truly controls operation
of the unit in a manner that assures the incinerator
will run through the complete charge, combustion,
burnout, and cooldown cycle.
4.4.1,2.2 Combustion air rate. Combustion air rate
can be automatically controlled by on/off settings,
low/high settings, and full modulation of the flow
over an entire operating range. The control settings
can be activated based upon the value of a monitored
parameter (e.g., temperature) or by a specified time
in a cycle. Some manufacturers are now using
different types of more advanced combustion air
controls that sense parameters such as gas flow,
opacity, oxygen concentration, and loading cycles to
assure that adequate combustion air is available in
the secondary chamber.*
4.4.1.2.3 Primary and secondary burner operation.
Like the combustion air, the ignition and main
combustion (secondary chamber) burners can be
controlled over an entire operating range (i.e.,
modulated), at low/high levels, or in the on/off
position. The settings can be based upon a monitored
value (e.g., temperature) or as part of a timed
sequence. The use of such settings in a control system
is discussed in the following sections.
4.4.1.2.4 Temperature. Both the primary ignition
and secondary combustion chamber temperatures
can be automatically controlled. At least two levels of
control are available. The first control approach is
designed to control temperature in each chamber by
modulating the available combustion air to each
chamber. In this type system, the temperature in
each chamber is monitored, and the combustion air to
each chamber is separately controlled based on
feedback from the thermocouples. The combustion air
is modulated continuously over a wide range in order
to maintain the set point temperatures. Auxiliary
burners are used as necessary to maintain minimum
temperatures. Since instantaneous peaks in the
combustion gases generated in the primary chamber
can occur, some manufacturers recommend operating
the secondary burner at all times (in a low fire
position if heat is not required) to assure complete
combustion of primary chamber gases.4
This type control system effectively controls the
waste combustion rate in the primary chamber by
controlling the combustion air. Controlling the
combustion rate in the primary chamber
subsequently limits the combustion rate in the
secondary chamber. Controlling the secondary
chamber combustion air controls that chamber's
temperature by increasing or decreasing the excess-
air level. The firing rate of the main combustion
burner (secondary chamber) may be modulated
upward or downward if low and high temperature set
points are reached in the secondary chamber.
Similarly, the ignition burner can be activated
automatically if the primary chamber temperature
falls below the minimum set point. This type system
provides a true control of the key parameter,
combustion temperature, by modulating the
combustion air.
One manufacturer controls the combustion rate in
the primary chamber and the temperature in the
secondary chamber by monitoring the oxygen
concentrations in each combustion chamber and
using these data to control/adjust the combustion air
levels.20
Review of manufacturers' information indicate that a
second approach to automatically controlling the
combustion process is used for batch type units.9.11 In
this approach, a series of timers are used to establish
a timed sequence of events and the key process
equipment (i.e., burners and combustion air) have
high/low or on/off settings. The switching of the
burners and combustion air from one setting to
another is controlled by the timed sequence.
Overrides for the timed sequence are typically
provided to assure that minimum or maximum set
point temperatures are not exceeded.
This type control system does not offer the same level
of combustion control as the modulated system
previously described. Instead of continuously
modulating to achieve a specific set point (i.e.,
temperature), this type system utilizes on/off or
low/high settings to maintain control between two set
points.
4.4.1.2.5 Combustion, burndown, and cooldown
cycle times. An automatic control sequence operating
on a timer cycle typically is used to initiate and
control the length of burndown and cooldown cycles.
An example timed control cycle for a batch operated
incinerator is presented in Table 4-3.
4.4.1.3 Monitoring Operations
The monitoring of key operating parameters provides
several benefits. First, monitoring provides the
operator with information needed to make decisions
on necessary combustion control adjustments. For
example, continuous monitoring of the temperature
and carbon monoxide level of the secondary
combustion chamber effluent gas stream allows the
operator to determine whether optimum combustion
conditions are being maintained. Indications of an
abnormally high CO level and low temperature can
immediately be used to adjust the secondary burner
rate to raise the combustion chamber temperature.
Second, properly maintained monitoring records can
provide useful information for identifying operating
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Table 4-3. Example Timed Control Cycle for Batch Mode Incinerator
Cycle
Controlling Parameter/Level
Resulting Automatic Control Action
1. Secondary combustion
chamber preheat
2. Ignition
3. Combustion
4. Burndown
5. Cooldown
Manual start
Combustion chamber temperature reaches 1100°F
Primary chamber temperature reaches 1000°F
Secondary temperature reaches high set point, 2000 °F
Secondary temperature reaches low set point, 1800°F
(Burner will cycle as high/low set-points are reached)
5 hours from ignition
High primary temperature override
1 hour after override activated
8 hours from ignition
16 hours from ignition; cycle completed
Secondary burner-High fire
Secondary combustion air-high level
Primary combustion air-high level
Primary burner on
Primary burner off
Secondary burner switches to low fire
Secondary burner switches back to high fire
Primary air switched to high
Primary air switched back to low
Primary air switched to high level
Secondary burner shuts off
Combustion air blowers shut off
trends and potential maintenance problems. This
historical information can be used to make decisions
with respect to modifying standard operating
procedures or control set points. For example, careful
visual inspection of ash quality each day
complemented by comments in a daily operator's log
book can be used to track ash burnout patterns. If a
gradual trend towards poor ash burnout becomes
evident, identification of possible reasons and
corrective actions can be initiated. Finally,
monitoring generally is needed to satisfy regulatory
requirements.
Monitoring can be divided into three broad
categories:
1. Continuous monitoring - involves continuous
instrumental measurement with continuous data
recording; e.g., use of a temperature sensor with a
strip chart recorder.
2. Continuous measurement - involves continuous
instrumental measurement but requires the
operator to monitor the data output manually;
e.g., use of a temperature sensor with a digital
meter display.
3. Manual monitoring - involves inspection by the
operator on a noncontinuous basis, e.g., visual
stack gas opacity readings.
All of these techniques can be used to monitor the
operation of batch type incinerators. Continuous
monitoring of operating parameters will, in most
cases, provide more information than manual
monitoring. For example, continuous opacity
monitoring of stack gas opacity will provide opacity
data for the entire operating period and will provide a
permanent record of changes in opacity over the
operating cycle and trends over time. On the other
hand, visual inspection of the stack gas opacity will
provide limited data, and a record will be available
only if the observer records the result.
The following operating parameters can be
monitored:
1. Charge rate;
2. Combustion gas temperature;
3. Condition of the waste bed and burner flame;
4. Combustion gas oxygen level;
5. Combustion gas carbon monoxide level;
6. Combustion gas opacity;
7. Auxiliary fuel usage; and
8. Ash quality.
The techniques for monitoring each of these
parameters arp briefly described in the following
paragraphs.
Note that this section is not intended to recommend
specific monitoring requirements, but is intended to
present the key parameters which can be monitored
and provide a brief description of how the parameters
can be monitored and what useful information
monitoring can provide. Chapter 6 further discusses
actual instrumental monitoring methods.
4.4,1.3.1 Charge rate. The weight of each charge
can be recorded in a log book. This procedure will
help to indicate whether the rated capacity of the
incinerator is being exceeded. Note that incinerators
are designed for a specific thermal input. Since the
heat value (Btu/lb) of wastes will vary, the weight of
a charge is not a true indicator of whether the rated
capacity is being exceeded. Variations in heating
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value in the waste will need to be considered in
determining charge size.
Nonetheless, monitoring the weight of the charge
will provide historical data which can be used to
make systematic modifications to operating
procedures, i.e., reduction of the charge rate by
20 percent to evaluate the effect on resolving poor
burnout problems. Monitoring this parameter will
require a scale or weigh bin which can be used to
weigh individual waste bags or transport carts.
4.4.1.3,2 Combustion gas temperature. The
combustion gas temperatures (primary and
secondary) are critical operating parameters. These
parameters typically are monitored continuously
using thermocouples. Permanent strip chart (or data
logger records) may be kept; for small units only
meters without permanent recordkeeping typically
are provided.
4.4.1.3.3 Condition of waste bed and burner flame.
If viewports are provided in each chamber, the
operator can view the flame pattern and the waste
bed. Visual observation of the combustion process
provides useful information on the burner operation
(i.e., potential problems such as flame impingement
on the refractory, or smoking of the secondary burner
flame can be identified) and on the combustion
process in general (i.e., quantity and condition of
remaining waste charge can be identified).
Viewports should be sealed glass covered by
blastgates. "Inspection doors" which open the
chamber to atmosphere should not be used for
viewing the combustion process because these pose
safety problems and affect the combustion air control.
4.4.1.3.4 Combustion gas oxygen level. The
combustion gas oxygen concentration is a direct
measure of the excess-air level. Although this
measurement is not essential, it provides real-time
information about changing conditions in the
combustion chamber. Typically, the oxygen level of
the combustion gas exiting the secondary chamber is
used to assure that excess air is always available for
complete combustion. A continuous oxygen
monitoring system typically is used (see Chapter 6).
However, a portable instrumental measurement
system also can be used occasionally for monitoring
oxygen levels to confirm proper operation or to
provide information necessary for adjusting
combustion air dampers.
4.4.1.3.5 Combustion gas carbon monoxide level.
Carbon monoxide is a product of incomplete
combustion; excessive levels indicate that a poor
combustion condition exists. The CO concentration of
the combustion chamber effluent can be monitored
continuously to alert the operator to poor combustion
conditions. An instrumental CO monitoring system
generally is used for this procedure (see Chapter 6).
Alternatively, a portable instrumental measurement
system can be used to make occasional measurements
of CO to monitor performance. As with the oxygen
measurements, this approach often is used to check
performance in conjunction with maintenance or
adjustment of combustion control level settings.
4.4.1.3.6 Combustion gas opacity. Combustion gas
opacity provides an indirect measurement of
particulate matter concentration in the stack gas;
hence opacity is an indicator of incinerator per-
formance. As particulate concentration increases, so
does the opacity. Continuous emission monitoring
systems (CEMS) for measuring opacity (referred to as
"transmissometers") are available (see Chapter 6).
An alternative, and simpler monitoring technique, is
visual observation of the stack emissions. The opacity
of the stack gas can be determined accurately by a
trained observer. Even an untrained observer can
detect gross changes in stack gas opacity and can use
visual observations to note combustion problems.
4.4.1.3.7 Fuel usage. Monitoring fuel usage
provides historical data for identifying maintenance
problems and for identifying the need for
adjustments to control system settings to increase
efficiency. Fuel usage can be monitored with a simple
metering system. Fuel usage can than be determined
by logging meter readings or continuously recording
the metering system's output.
4.4.1.3.8 Ash quality. Ash quality is one indication
of combustion performance. Visual inspection is the
simplest means of determining ash quality and
incinerator performance. For example, large pieces of
uncombusted materials (e.g., paper) indicate very
poor ash quality. The operator should inspect the ash
visually and record comments on ash quality in a log
book routinely.
For a more technical determination of ash quality, a
chemical analysis of a sample of the ash can be
conducted. The amount of combustible materials
remaining in the ash can be determined by a
laboratory test which subjects a sample of the ash to a
combustion atmosphere and then determines the
weight loss. This procedure measures the ash's
"burnout" quality. A 100 percent "burnout" means no
volatile or combustible materials remain in the ash;
some incinerators are capable of 90 percent
burnout.21 Occasional checks of ash burnout are
useful for monitoring performance of the incinerator.
4.4.2 Intermittent-Duty, Controlled-Air
Incinerators
Intermittent-duty, controlled-air incinerators
typically are used for "shift" type operation. The
incinerator must be routinely shutdown for ash
removal. Hence, there is a distinct operating cycle.
48
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The main feature which distinguishes this type
incinerator from the batch incinerator is the
charging procedures which are used. The charging
system is designed to accommodate multiple charges
safely throughout the operating cycle rather than
rely on a single batch charge at the beginning of the
operating cycle. Either manual or automated
charging systems can be used.
4.4.2.1 Operation.
4.4.2.1.1 Ash removal. The residual ash from the
previous operating cycle must be removed before a
cycle can be initiated. Ash removal procedures are
essentially the same as those described in Section 4.3
for batch mode incinerators.
4.4.2.1.2 Startup. Before the operator initiates
startup, proper operation of the primary and
secondary burners and combustion air blowers
should be checked according to manufacturer's
instructions. The following steps are conducted
during startup:
1. The primary and secondary burner(s) are ignited,
and preheat of the combustion chambers is
initiated. The manufacturer should be consulted
regarding proper preheat procedures, since
improper preheat can damage the refractory.
2. The secondary chamber must reach a
predetermined temperature (e.g., 1800°F) before
the incinerator is ready for charging; and
3. After the predetermined secondary chamber
temperature is attained, the primary and
secondary combustion air blowers are activated.
The incinerator is ready to be charged.
4.4.2.1.3 Waste charging. Stable combustion can
be maintained most readily with a constant thermal
input to the incinerator. Feeding too much waste in a
charge causes the incinerator to overload. These
overloads can result in poor burndown (because of
waste pile buildup on the hearth) or can cause
excessive emissions because the rapid generation of
volatiles overloads the capacity of the secondary
chamber. Feeding too little waste results in
inadequate thermal input and consequent excessive
auxiliary fuel use.10 A charge frequency and
quantity recommended by two manufacturers is 15 to
25 percent of the rated capacity (Ib/h) at 10 to
15 minute intervals.10'1! Another rule of thumb is to
recharge the incinerator after the previous charge
has been reduced by 50 to 75 percent in volume,
determined by observation of the waste through the
view ports or operating experience.11 Charging
Volume and frequency will vary with waste
composition, and the operator must use some
judgment to determine appropriate rates.
The temperature profile of a combustion chamber is a
picture of how the temperature in the chamber
fluctuates during the course of the incineration
process. The variations in temperature are shown on
the strip or circular chart recorder that records the
temperatures measured by the thermocouples in the
combustion chambers. The temperature fluctuations
are affected by the frequency of waste charging and
the size of the charge. Insufficient or infrequent
charging may cause the temperature to become too
low and necessitates the use of auxiliary fuel to help
maintain the desired set point temperature.
Charging too much waste may cause a rapid increase
in secondary combustion chamber temperature if the
waste has a high volatile content. On the other hand,
if the waste has a low Btu content or contains a lot of
moisture, overcharging may have the effect of first
decreasing the primary combustion chamber
temperature (while the moisture is being volatilized)
before a temperature increase is noted. Charging on a
regular basis with the same volume of waste in each
charge helps to "flatten" the temperature variation
and allow proper combustion while preventing
refractory damage and excessive auxiliary fuel
consumption. Monitoring the temperature profile of
the combustion chambers will assist in determining
the proper charging rates.
After the last charge of the day is completed, the
incinerator is set to initiate the burndown cycle. The
limiting factor on how long the charging period can
be sustained without initiating the burndown cycle is
the degree of ash buildup on the hearth. Typically the
charging period is limited to 12 to 14 hours.14
4.4.2.1.4 Burndown. The burndown cycle is
essentially the same as described for batch
incinerators and is initiated after the last charge of
the day is made. For intermittent-duty incinerators
the burndown sequence can be initiated manually or
may be initiated automatically. One manufacturer's
control system is designed to include a control timer
which is activated by the charging door.11 Whenever
the charging door is opened and then closed, the
burndown timing cycle is initiated by resetting the
burndown timer to a preset time period, e.g., 5 hours
(the actual length of the burndown time is
determined by experience with waste stream).
4.4.2.1.5 Cooldown. The cooldown period is the
same as for batch incinerators.
4.4.2.2 Automatic Controls
The parameters which can be controlled
automatically for intermittent-duty controlled-air
incinerators are essentially the same as those already
discussed for the batch-operated incinerators. The
automatic control techniques used also are
essentially the same. Either timed sequences with
high/low burner and air settings or control systems
49
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which have fully modulated burner and air controls
that are continually adjusted based on control
feedback (e.g., temperature) can be used. For
intermittent-duty units, the charging rate can be
controlled automatically if an automatic mechanical
charging system is provided with the unit. Automatic
control for a ram feed charger consists of a timing
sequence which will initiate the charging sequence at
regularly timed intervals. The size of the charge is
controlled by the volume of the feed hopper. An
override on the automatic feeder can be provided so
that the control system will not allow a charge to be
fed at the regular interval if certain conditions are
not met. For example, if the primary chamber
temperature exceeds a high level set point, the
incinerator is not yet ready for another "fuel" charge
and the charging cycle will not initiate.
4.4.2.3 Monitoring
The operating parameters that can be monitored for
intermittent-duty incinerators are the same as those
parameters discussed in Section 4.4.1.3 for batch
incinerators. The same monitoring techniques
generally apply. However, continuous monitoring
and recording of the ignition and combustion
chamber temperatures have added importance for
intermittent-duty incinerators. The trends in these
measured values are useful to the operator in
determining the appropriate charging frequency and
in adjusting the charging frequency as waste
characteristics change. Similarly, the use of
appropriate viewports has added significance in
making decisions on charging frequency based upon
appearance of the waste bed.
4.4.3 Continuous-Duty, Controlled-Air
Incinerators
Continuous-duty incinerators have the capability of
continuously removing the ash from the incinerator
hearth. Consequently, the incinerator can be
operated at a near-steady-state condition by
continuously charging the unit at regularly timed
intervals and similarly by removing the ash at
regularly timed intervals.
4.4.3.1 Operation
4.4.3.1.1 Startup. Startup procedures for
continuous-duty incinerators are essentially the
same as for the intermittent-duty incinerators. The
chambers are first preheated before the initial charge
is loaded to the incinerator. The manufacturer should
be consulted regarding proper preheat procedures,
since improper preheat can damage the refractory.
4.4.3.1.2 Charging. To approach steady state
operation, consistently sized charges should be fed at
regularly timed intervals. Practically speaking,
continuous-duty incinerators are likely to be more
automated than the types of units previously
discussed. Continuous-duty incinerators will include
a mechanical feed system, which may be fully
automated so that charges are automatically fed at
regularly timed intervals.
During operation, the waste bed/ash bed is typically
moved through the incinerator by one of two
methods. In the first method, the waste bed is
continually pushed towards the back end of the
chamber when each new waste charge is pushed into
the incinerator by the feed ram. In the second
method, where the incinerator hearth is
exceptionally long, the hearth is built in a stepped
fashion, and special ash rams are provided for
pushing the ash from one step to the next step.
Ultimately, in both systems, the ash is pushed off the
hearth into an ash discharge chute.
During operation, the operator must assure that the
ash is being removed from the system. For manual
ash removal systems, an ash bin must be emptied
routinely. The ash bin is located inside a sealed
chamber at the end of the ash discharge chute or is
directly sealed to the combustion chamber ash
discharge chute. For fully automated ash systems,
the ash is mechanically removed by a rake or
conveyor from a water quench pit located at the end
of the ash discharge chute. For these systems, the
operator need only monitor the ash discharge system
to assure that no mechanical problems have
developed and to assure that the quench pit water
level is maintained.
4.4.3.1.3 Shutdown. Shutdown of the incinerator
involves stopping the charging process and
maintaining temperatures in the combustion
chamber until the remaining waste burns down to
ash and is finally discharged from the system in the
normal manner.
4.4.3.2 Automatic Controls
The parameters which can be controlled
automatically for this type unit are essentially the
same as those for the intermittent duty unit.
However, when separate ash transfer rams are a part
of the system, ash removal may also be automatically
controlled. If the waste feed system is on an
automatically timed sequence, the ash removal
system likely will be integrated into the timed
sequence.
The operating controls for the parameters discussed
in Section 4.2.2 are more likely to be fully automated
for continuous-duty incinerators than for batch units,
simply because of the size and frequency of use of the
units. Automated controls relieve the operator from
the burden of continuously making adjustments.
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4.4.3.3 Monitoring
The operating parameters that can be monitored for
continuous-duty incinerators are the same as those
parameters discussed in Section 4.4.2.3 for
intermittent-duty incinerators. The use of auto-
matic/mechanical feed systems simplifies the mon-
itoring of waste feed charge frequency (and
consequently volume). A recording indicator can be
installed that indicates how frequently the waste
feed charge system is activated. This information can
be used to provide a rough estimate of the charging
rate (Ib/h) for the unit and also may be helpful in
diagnosing other operational problems, such as
reasons for temperature excursions.
4.4.4 Multiple-Chamber Incinerators
Typically, multiple-chamber incinerators are used to
burn pathological waste. However, other medical
waste including red bag waste are sometimes burned
in these units; when burning red bag waste special
precautions should be taken during charging to
assure the incinerator's capacity is not exceeded.
Typical applications of multiple-chamber
incinerators include batch or intermittent operation;
continuous-duty operation automatic ash removal is
atypical. Operation of these units is discussed briefly
below.
4.4.4.1 Operation
4.4.4.1.1 Ash removal. The residual ash from the
previous operating cycle must be removed before a
cycle can be initiated. Ash removal procedures are
essentially the same as those described in Sec-
tion 4.4.1.1.1 for batch mode controlled-air incin-
erators.
4.4.4.1.2 Startup. Startup of the excess-air
incinerator is similar to startup for the batch-mode,
controlled-air incinerators. The secondary chamber is
first preheated to a predetermined chamber tem-
perature. The incinerator is then charged with the
waste.
4.4.4.1.3 Charging. Multiple-chamber incinerators
may be either batch or intermittent duty and may be
charged manually or with a mechanical loading
device. Because of the significant difference in heat
contents between pathological waste and red bag
waste and because incinerators are designed to burn
a waste with a specific heat input, the proper
charging (addition of fuel) procedures are different
for different wastes.
Pathological waste has a low heat content, a high
moisture content, and a low percentage of volatiles.
Consequently, it must be constantly exposed to the
auxiliary burner(s') flame(s) to be combusted. The
following proper pathological waste charging
procedures are recommended:
1. The waste should be placed on the hearth in an
even layer that provides maximum exposure to
the burner(s) flame(s). The waste should not be
deeply piled.
2. Recharging the incinerator should not be done
until considerable reduction in volume (greater
than 75 percent) of the previous charge has
occurred.
3. When recharging the incinerator:
a. Turn off the primary burner (some units may
have an interlock system that automatically
turns the burner off when the charge door is
opened);
b. Place the fresh charge in a single layer on the
hearth so that the burner(s) impinge on the
waste; and
c. Close the charge door before restarting the
primary burner.
The heat content of red bag waste will be variable
depending on the contents of the bag. Proper
operation of the incinerator dictates that:
(1) sufficient waste should be charged to the unit to
sustain the desired temperature without excessive
use of the primary burner; and (2) to maintain the
primary chamber temperature below the upper limit
and to prevent emissions, the charge rate should not
exceed the capacity of the incinerator at any time.
Obviously, if the incinerator was designed for
pathological waste, then a significantly smaller red
bag waste charge should be made than is typically
made of pathological waste. The following guidelines
are recommended for charging red bag waste into a
multiple-chamber incinerator:
1. Use of frequent, small batches rather than one
large batch. The objective is to avoid causing a
rapid release of volatile compounds that exceeds
the combustion capacity of the incinerator. The
frequency and size of each charge will be
determined by the incinerator being operated and
the type of waste. A recommended procedure is to
charge about 1/10 the rated capacity (Ib/h) every
6 minutes.
2. Keep a fairly consistent waste bed in the
incinerator. The incinerator should not be
jammed full, nor should it be empty.
3. Avoid "stuffing and burning" in the incinerator;
that is, do not fill the incinerator chamber to full
capacity, floor to ceiling, ignite the waste, and
allow the incinerator to operate unattended.
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4. When recharging the incinerator:
a. Turn the primary burner off (some units may
have an interlock system that automatically
turns the burner off when the charge door is
opened);
b. The partially burned waste from the previous
charges should be pushed towards the back of
the hearth with a rake; and
c. The new waste charge should be fed to the
front end of the hearth (near the charge door).
This procedure allows good exposure of the
partially combusted waste to the overfire air
and allows a good flame from the waste bed to
be maintained. On the other hand, if cold,
newly charged waste is thrown on top of the
existing waste bed it partially smothers the
burning bed which can result in increased
emissions.
5. If the incinerator has a grate, it is important that
the entire grate be covered with a waste bed.
4.4.4.1.4 Operation. When burning pathological
waste, the primary burner is operated throughout the
combustion cycle and thermal output should be
relatively constant without rapid fluctuations. The
primary chamber combustion air is preset to
maintain a constant excess-air level, and primary
chamber temperature is controlled by modulating the
primary burner. Since the pathological waste does
not contain large amounts of combustible volatiles,
secondary chamber combustion settings also will
remain relatively constant. The combustion air
damper settings and burner settings necessary to
attain the desired secondary chamber temperature
and excess-air levels are set and normally do not need
to be adjusted. Temperature control normally is
achieved only by modulating the primary burner
settings.15
Modulating the primary burner does not offer the
same degree of control over temperature as the
adjustment of air supply. If a multiple-chamber
incinerator was overcharged with red bag waste,
modulation of the primary burner (or even shutdown
of the burner) would not reduce the temperature
because once ignited, the red bag waste will continue
to burn out of control with the large excess-air level
carrying excessive unburned material to the
secondary chamber. In such a scenario, the secondary
chamber would be unable to respond to such a high
carryover of volatiles. Therefore, while red bag waste
may be burned in a multiple-chamber incinerator,
the potential exists for high temperatures that may
damage the unit and for high emissions if the unit is
overcharged. Some multiple-chamber units may have
an automatically modulated air supply system that
adds air if the temperature rises (cools the
combustion process) or restricts air if the
temperature falls. However, the large excess-air level
in the primary chamber will still produce carryover
of particulate matter to the secondary chamber.
Therefore, multiple-chamber incinerators are better
suited to burn pathological waste than they are to
burn red bag waste because of the relatively stable
and constant heat output of pathological waste and
the fact that pathological waste combustion is
controlled entirely by the firing of the primary
burner (i.e., pathological waste cannot sustain
combustion without the primary burner and will not
burn out of control as the red bag waste can).
4.4.4.1.5 Burndown. There is no burndown period
in the operation of multiple-chamber incinerators.
The degree of burnout achieved is dictated by the
length of time that the primary burner is left in
operation after the last charge. After complete
destruction of the waste has been achieved (as noted
by visual observation through a viewport), the
primary burner is shut down. The secondary burner
is not shut off until all smoldering from residual
material on the hearth in the primary chamber has
ceased. *5
4.4.4.1.6 Cooldown. After all smoldering in the
ignition chamber has ceased, the secondary burner is
shut down, and the incinerator allowed to cool.
4.4.4.2 Automatic Controls
The primary operating parameters which are
automatically controlled are the primary and
secondary chamber temperatures. Control of the
temperatures is achieved by modulating the burner
rates. The primary and secondary combustion air
levels typically are not automatically controlled, but
are preset. Incinerator draft typically is controlled by
a barometric damper. Charging rate also is not likely
to be controlled automatically in this application. 15
4.4.4.3 Monitoring
The same operating parameters that are monitored
on controlled-air incinerators can be manually or
continuously monitored during operation of the
multiple-chamber unit.
4.5 Add-On Air Pollution Control Systems
Key operating parameters and procedures for wet
scrubbers, fabric filters, and dry scrubbers are
discussed in this section. Addition of an air pollution
control system (APCS) to a hospital waste incinerator
increases the number of operating parameters the
operator must control, monitor, and adjust.
Furthermore, addition of an APCS to an incinerator
significantly modifies how at least one important
incinerator operating parameter is controlled; this
parameter is the incinerator draft. Operation of the
APCS will require use of an induced draft (ID) fan to
provide the necessary airflow through the system.
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Natural draft control will no longer be applicable; gas
flow through the system will be controlled by the ID
fan.
4.5.1 Wet Scrubbers22-24
4.5.1.1 Key Operating Parameters
All wet scrubbers utilize a liquid scrubbing media to
remove pollutants from the incinerator exhaust gas
stream. Actual collection of the pollutant in the
liquid is primarily through impaction for particulates
and absorption for acid gases. The liquid/gas (L/G)
ratio is an important parameter for all wet scrubbers.
The appropriate L/G ratio varies with scrubber type.
Venturi scrubbers require a relatively low L/G ratio
with 7 to 10 gallons per thousand actual cubic feet
(gal/Macf) recommended.22 This low L/G ratio is
acceptable because venturi scrubbers rely on high
energy pressure drops to create high gas velocities,
turbulence, and lots of small water droplets for the
collection of fine particulate matter by impaction.
Therefore, pressure drop (AP) and L/G ratio are the
two key performance parameters for venturi
scrubbers. Packed-bed scrubbers, however, rely on
creating large surface areas of gas to liquid interface
for collection of gaseous pollutants through
absorption. Large pressure drops are not required by
packed-bed scrubbers. However, relatively large L/G
ratios are required to adequately cover the packing
media and create a large wetted surface. The L/G
ratio is the key performance parameter for packed-
bed scrubbers used for acid gas control with 10 to
15 gal/Macf recommended.23 Spray towers also
require a L/G ratio of 5 to 20 gal/Macf.23 While these
are the most important key performance parameters,
there are several key parameters for each scrubber
type that can be monitored and adjusted by the
operator to achieve the desired performance of the
unit. Table 4-4 lists the key parameters and
identifies typical operating ranges for venturi
scrubbers, packed-bed scrubbers, spray towers, and
mist eliminators for application to hospital waste
incineration systems.
Control of scrubber liquor pH is important to
minimize corrosion from acid or alkaline conditions.
A scrubber liquor pH range of 5.5 to 7.0 is
recommended.22 A pH in this range represents
neutral water. A lower pH indicates acidic water
which can cause corrosion. Higher pH values may
result in scaling.
Turbidity of the scrubber liquor feed also is an
important operating parameter with regard to
maintaining particulate matter control efficiencies.
High turbidity (a measure of the dissolved and
suspended solids in the liquid) can result in increased
particulate matter emissions and plugging of nozzles.
Proper design and operation of the mist eliminator
system is necessary for maintaining particulate
matter control efficiencies. Scrubber system design
sometimes includes a clean water spray for rinsing
mesh pad eliminators and maintaining performance.
4.5.1.2 Scrubber Operation
Proper operation of a scrubber requires that the
operator (1) establish a fixed liquid flow rate to the
scrubbing section, (2) initiate gas flow through the
system by starting a fan, and (3) set up the liquid
recirculation system so that suspended and dissolved
solids buildup does not create operating problems.
Once the system has been started and operation has
stabilized, little additional operator attention will be
needed. Operators should refer to the instruction
manual provided by the scrubber manufacturer for
adjustment of site-specific operating conditions.
4.5.1.2.1 Scrubber Startup. The scrubber should be
started prior to charging waste feed to the incinerator
but not necessarily prior to preheat. The specific
manufacturer's startup instructions should be
followed. The following procedures in sequence are
typical during startup of a scrubbing system to insure
proper operation:
1. Turn on the liquid recirculation system or liquid
supply(s) to the scrubber(s) and mist eliminator.
2. Adjust the liquid flow rates to those specified in
the instructions supplied by the scrubber
manufacturer.
3. If the induced draft or forced draft fan feeding the
scrubbing system has a damper installed at its
inlet or outlet, close the damper.
4. Start the induced draft or forced draft fan.
5. If the system is equipped with a damper,
gradually open the damper until the proper gas
flow rate is established.
6. Again recheck the liquid flow rate(s) and adjust
as necessary.
7. Check the differential pressure across the
scrubber and compare with the design pressure
drop specified in the operating manual. If the
differential pressure is too low across the
scrubber, either the liquid rate is too low or the
gas flow rate is too low. To correct this condition,
either increase the gas flow rate by opening a
damper, or increase the liquid flow rate to the
scrubber. If the scrubber is a venturi unit with an
adjustable throat, the pressure can be increased
by decreasing the throat area.
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Table 4-4. Wet Scrubber Performance Parameters for Hospital Waste Incinerators2*-**
Parameter Typical range* units of measure Operating range*"
Ventur! scrubbers
Pressure drop
Liquid feed rate0
Liquid-to-gas ratio
Liquid feed pressure
Liquid feed turbidity
Gas flow ratec
Liquid feed pH
Packed-bed scrubbers
Pressure drop
Liquid feed rate0
Liquid feed pH
Liquid-to-gas ratio
Liquid feed pressure
Gas flow rate3
Liquid feed turbidity
Spray towers
Pressure drop
Liquid-to-gas rate
Mist eliminator
Pressure drop
Liquid feed rate0
Liquid feed pressure
Liquid-to-gas ratio
Liquid turbidity
Gas flow rate0
Liquid feed pH
15-60
>35
4-10
20-60
1-10
> 5,000
5-10
1-5
>5
5.5-7.0
10-30
20-60
> 5,000
1-10
0.5-3.0
5-20
1-3
>5
20-60
1-6
0-3
> 5,000
5-10
in. w.c.
gal/min
gal/Macf
psi
Percent suspended solids
acfm
PH
in. w.c.
gal/min
PH
gal/Macf
psi
acfm
Percent suspended solids
in. w.c.
gal/Macf
in. w.c.
gal/min
psi
gal/Macf
Percent suspended solids
acfm
pH
20-50
7-10
20-60
0-3
5.5-7.0
5.5-7.0
15-25
30-60
1-3
1-3
5-20
1-3
30-60
2-3
0-0.5
7
• The typical range is the range of operating parameters that can exist on a broad range of source
categories,
bThe operating range is the range of operating parameters specified by manufacturers for combustion
sources similar to hospital waste incinerators.
c Values, or range of values, are dependent on the size of the scrubber system.
8. Initiate the liquid bleed to treatment or disposal
as specified in the manufacturer's manual. If the
bleed is taken by an overflow from the
recirculation tank, the flow rate at this point is
established by the rate at which makeup water is
introduced to the recirculation tank. The manu-
facturer's manual should show the anticipated
water evaporation rate in the scrubbing system.
If, as an example, the evaporation rate is 1 gallon
per minute, and if you wish to establish a bleed
rate of 1 gallon per minute, it will be necessary to
feed 2 gallons per minute of total water to the
recirculation tank. The bleed rate is determined
by the rate at which the solids build up in the
scrubbing system. These solids can be either
suspended or dissolved solids or both. A scrubber
is capable of handling a maximum of 3 percent
(weight) suspended solids, and it is suggested
that the dissolved solids not exceed 10 percent
(weight).22 Based on design data, a recommended
bleed rate from the system should be provided by
the manufacturer. The operator should combine
this figure with the evaporation figures to give a
total recommended makeup water rate to the
recirculation tank if an overflow type bleed
system is used. If a bleed system is provided from
a slip stream off the pump feeding the venturi
scrubber, liquid makeup is normally provided by
a level control device in the recirculation tank.
The flow rate required will be the same as the
flow rate required for the overflow bleed system.
However, it is only necessary to insure that an
adequate water supply is available to the level
control device on a continuous basis.
4.5.1.2.2 Scrubber shutdown. To shut the system
down without overloading the fan or causing any
54
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damage to the scrubbing equipment, the following
procedures in sequence are typical:
1. Shut off the induced draft or forced draft fan
feeding the scrubbing system.
2. Wait until the fan impeller has stopped rotation
and then shut off the scrubbing water
recirculation pump.
3. Shut off the makeup water supply system.
4.5.1.3 Automatic Control
Key operating parameters which may be
automatically controlled for a scrubber system
include:
1. Venturi pressure drop;
2. Scrubber liquid flow rate;
3. Scrubber liquid pH; and
4. Gas flow rate.
The venturi pressure drop is the key performance
parameter for venturi scrubbers and the scrubber
system can be designed to monitor this parameter
and provide automatic control based on adjustment of
the venturi throat.
For Venturis, packed beds, and spray towers, the
scrubber liquid flow rate is a key parameter. This
parameter usually remains constant during
operation but, nonetheless, can be designed for
automatic flow rate control.
When caustic materials are added to the scrubber
liquid, control of scrubber liquid pH is necessary to
prevent damage to the scrubber equipment. A pH
meter is used to monitor the pH of the liquid. The
output of the meter is used to control the amount of
caustic material added to the scrubber sump.
Gas flow rate through the system can be controlled
automatically by monitoring and adjusting fan rpm
for a variable speed fan, or by monitoring and
controlling fan static pressure using a damper for a
constant rpm fan. Gas flow rate is controlled to
maintain the desired draft in the incinerator
combustion chambers, as well as the desired pressure
drop across the scrubber.
4.5.1.4 Monitoring
Scrubber parameters which can be monitored by the
operator include:
1. Venturi pressure drop;
2. Scrubber liquid flow rate;
3. Scrubber liquid pH;
4. Fan static pressure, rpm, or amperage; and
5. Gas inlet temperature.
The first four items listed were discussed in the
section above with regard to using these parameters
for automatic system control. Gas inlet temperature
is of interest to the operator because excessive
temperatures can damage the pollution control
equipment, especially in systems where fiber
reinforced plastic materials are used. A thermocouple
is used to monitor the gas inlet temperature.
4.5.2 Fabric Filters25-27
4.5.2.1 Key Operating Parameters
The key operating parameters for fabric filter control
systems are summarized in Table 4-5. To prevent
damage to the system the gas temperature must be
maintained in a range between the dewpoint of the
gas and the upper temperature limit of the fabric.
The upper operating temperature of the fabric will
depend on the fabric type; the manufacturer should
be consulted regarding the upper temperature limit.
Table 4-5. Key Operating Parameters for Fabric Filter
Control Systems
Parameter
Operating range26
Upper gas temperature, °F
Lower gas temperature, °F
Pressure drop, in. w.c.
Cleaning air pressure, psig
Below upper limit for fabric3
Above dewpointb
5-9
60-100
a The upper temperature limit will be dependent on fabric type.
Consult manufacturer.
b The gas temperature usually is maintained above 300 °F.
The pressure drop across the system for pulse jet
baghouses typically is maintained within a range of 5
to 9 in. w.c.25 Pressure drop gives an indication of
filter cake formation. Filter cake formation is
dependent on the effectiveness of the bag cleaning
cycle. The frequency and dwell of bag cleaning must
be set to maintain the desired pressure drop. The
cleaning pressure must be maintained at sufficient
pressure to properly clean the bags; the desired range
is 60 to 100 psig. 25
4.5.2.2 Operation of Fabric Filters
While the performance of a fabric filter is dependent
on proper design and the timely detection of upset
conditions, proper operation and preventive
maintenance procedures dictate satisfactory long-
term performance. This section discusses general
operating procedures that can minimize unexpected
malfunctions and improve the 'performance of the
fabric filter. Preventive maintenance practices are
discussed in Chapter 5. Proper operating procedures
are important during startup, normal operation,
shutdown, and emergency conditions.
55
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4.5.2.2.1 Startup Procedures. Prior to operation of
new fabric filters, a complete check of all components
is recommended including the cleaning system, the
dust-discharge system, and the isolation dampers
and fans. Clean, ambient air should be passed
through the system to confirm that all bags are
properly installed. New bags are prone to abrasion if
subjected to high dust loadings and full-load gas
flows, particularly during the initial startup before
new bags have the benefit of a dust buildup cake to
protect the fibers from abrasion or to increase their
resistance to gas flow. Full gas flow at high dust
loadings can allow the particulate matter to impinge
on the fabric at high velocity and result in abrasion
that may shorten bag life. In addition, the dust may
penetrate so deeply into the fabric that the cleaning
system cannot remove it, and a "permanent" pressure
drop results. Bag abrasion may be prevented by
either (1) initially operating the incinerator at a low
throughput and reduced gas volume to allow the dust
cake to build gradually or (2) precoating the bags to
provide a protective cake before the incinerator
exhaust is introduced. The baghouse manufacturer
should be consulted regarding proper startup
procedures and acceptable precoat materials.
The fabric filter should not be operated at
temperatures approaching the dewpoint of water
and/or the hydrochloric acid formed by the
combustion of chlorinated plastics because if the
dewpoint is reached, serious operating problems may
arise. Warm moist gas that is introduced into a cool
or cold fabric filter will cause condensation on the
bags or on the fabric filter shell. Condensation can
cause a condition known as "mudded" bags where the
bags are blinded by dust and moisture. The acid
dewpoint depends on the amount of moisture and
acidic material in the gas stream. Condensation of
acid can cause corrosion of the fabric filter
components, sticky particulate and cake-release
problems, and acid attack on some fabrics.
Preheating the fabric filter to a temperature above
the acid dewpoint will prevent condensation and
enhance fabric filter performance. Since the
incinerator goes through a warmup period using
natural gas or fuel oil burners prior to waste combus-
tion, the problems associated with condensation of
water or hydrochloric acid are unlikely to occur.
Unstable combustion during startup can cause some
carbon carryover, which may result in a sticky
parfciculate. This situation creates the potential for
fires in the fabric filter when a combustion source
and an adequate oxygen supply are available.
Therefore, during startup, the fabric filter hoppers
that collect the particulate should be emptied
continually. More importantly, unstable combustion
conditions during startup should be minimized by
going through proper incinerator startup procedures.
4.5.2.2.2 Normal operating procedures. Under
normal conditions, operation of the fabric filter is
straightforward. The operator has to do very little
other than monitor the key parameters, as discussed
later in Section4.5.2.4. Combustion gas flow rate
through the system must be maintained at the level
necessary to maintain negative draft in the
combustion chambers.
4.5.2.2.3 Shutdown procedures. The top priority
during shutdown of a fabric filter is avoiding
dewpoint conditions. Bag cleaning and hopper
emptying are lower priority items.
When processes operate on a daily cycle, the last
operation of the day should be to purge moisture and
acidic materials from the fabric filter without passing
through the dewpoint. In the case of a hospital waste
incinerator, the operator should leave the secondary
chamber burner on for a few minutes after
combustion is completed to remove moisture from the
fabric filter. Ambient air could then be drawn
through the system to purge the remaining
combustion products.
After shutdown, the fabric filter should be allowed to
go through a complete bag cleaning cycle. This
procedure will help prevent blinding of the bags.
Additionally, continuing to operate the hopper
discharge system while the cleaning system is in
operation will minimize the potential of hopper
plugging.
When emergency conditions are encountered such as
high temperatures, spark detection, or other process
upsets, the fabric filter is usually bypassed to prevent
damage to the system. If a fire occurs in the hopper,
the addition of water under oxygen-starved
conditions is not advisable because the water will
hydrolyze forming hydrogen and causing the
potential for an explosion inside the fabric filter.
Both the fabric filter manufacturer and insurance
carrier should be contacted whenever a known
potential for fires/explosions exists.
Other process failures may necessitate only tem-
porary bypassing of the fabric filter, and operation
can be restored in a matter of minutes. In these cases,
the fabric filter generally does not have to be shut
down completely and purged. If the upset cannot be
corrected within a reasonable amount of time,
however, shutdown and the subsequent startup of the
fabric filter may then be necessary to prevent
dewpoint problems.
It is important to note that bypassing the fabric filter
during startup, soot blowing, or an emergency may
not be acceptable to the applicable regulatory agency.
Such occurrences should be investigated and
addressed during the design stages of development.
56
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4.5.2.3 Automatic Controls
Operating parameters which are controlled
automatically for fabric filter control systems include
the bag cleaning cycle and the ash removal system.
The combustion gas flow rate through the system also
can be controlled automatically. Control is achieved
by adjusting fan rpm's (variable speed fans) or fan
static pressure by way of a damper. Proper gas flow
through the system must be maintained to assure
sufficient negative draft in the combustion chambers.
Reduction of the gas stream temperature to the fabric
filter typically is achieved by passing the combustion
gases through a waste heat boiler. Temperature
monitoring systems are available that will cause
automatic bypass of the fabric filter under high
temperature conditions.
4.5.2.4 Monitoring
A well-designed and maintained fabric filter should
provide adequate control of particulate matter
emissions. Parameters that can be monitored to
maintain optimum performance include opacity,
pressure drop, fan motor amperage, and temperature.
Opacity readings can help to determine the presence
of pinholes and tears in bags and, in some cases, the
general location of the bag failure. These visible
emissions are usually the first indicator of poor fabric
filter performance. The failed bags should be replaced
as necessary.
The pressure drop across the fabric filter is another
indicator of poor performance. A high pressure drop
outside the normal operating range may indicate
inadequate cleaning of the fabric, bag blinding, or
excessive gas volume through the system. In some
cases, the pressure drop before and after cleaning
increases steadily as the bags age. Excessively low
pressure drop may indicate inadequate filter cake
formation resulting from too frequent cleaning.
When used in conjunction with the pressure drop
across the fabric filter, measurement of fan motor
amperage also can provide an indication of the gas
flow rate. In general, an increase in current combined
with an increase in pressure drop indicates an
increase in gas volume, and a decrease in amperage
reflects a decrease in gas volume. These changes,
however, must be normalized for temperature
(density) changes because temperature influences
the energy required to move the gas through the
system.
High-temperature operations such as hospital waste
incinerators should be equipped with continuous
strip chart recorders and high-temperature alarms.
The high-temperature alarms should provide some
margin for corrective action, i.e., set points of 50° to
75°F below the high temperature limit of the fabric.
The temperature alarm/recorder also may be
connected to an automatic damper system to control
the temperature or to bypass the fabric filter.
Although some differential between the maximum
temperature and the alarm activation must be
provided, the temperature set point should not be so
low that the alarm is continually activated. The
temperature indicator also will monitor against
excessively low temperatures and dewpoint
problems.
4.5.3 Spray Dryers7,28
4.5.3.1 Key Operating Parameters
The key parameters that are necessary for effective
operation of a spray dryer are slurry feed rate, slurry
sorbent content, and the outlet gas wet and dry bulb
temperatures. Effective operation of a spray dryer
system requires that adequate sorbent is provided in
the slurry for reaction with the acid gases and that
all slurry moisture is evaporated in a time frame that
allows for reaction with the acid gases. Drying that is
too rapid can reduce acid gas collection efficiency.
Drying that is too slow will result in solids buildup on
the sides of the reaction vessel that will further
impede drying. The liquid slurry feed rate and
sorbent content should be balanced with the hot flue
gas volume and acid gas content to ensure the desired
removal of acid gases and the timely evaporation of
all water. The recommended slurry sorbent content is
5 to 20 percent by weight solids. The slurry moisture
added to the flue gas serves to cool and increase the
moisture content of the gas stream. The difference
between the wet bulb and dry bulb temperatures
gives an indication of the saturation of the gas stream
and the potential for evaporation of the slurry
moisture. As the slurry feed rate is increased, the wet
bulb and dry bulb temperature difference will
decrease. The recommended wet bulb/dry bulb outlet
gas temperature difference (for Municipal Solid
Waste Systems) is 30° to 80°C (90° to 180°F).28
4.5.3.2 Spray Dryer Operation
The design specifications for the dry scrubber should
identify the sorbent-to-water ratio for the mix tank.
The sorbent content should be set at a level that will
provide ample sorbent for the range of acid gas
concentration expected to occur. Proper operation of
the dry scrubber requires that the operator adjust the
slurry flow rate to the atomizer in the reaction vessel
to maintain an acceptable wet bulb/dry bulb
temperature difference. If the wet bulb/dry bulb
temperature difference is too low, the slurry feed
should be decreased.
4.5.3.2.1 Spray dryer startup. Startup of a spray
dryer should follow procedures that ensure
evaporation of all slurry moisture in the reaction
57
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vessel and that prevent condensation of moisture in
the fabric filter. One method of ensuring evaporation
and prevention of condensation in the fabric filter is
to use auxiliary fuel firing in the incinerator to bring
the exhaust gas temperature up to the normal
operating range before injecting the slurry. Slurry
injection should be initiated before charging the
incinerator with waste feed material. An alternative
method would be to gradually increase the slurry feed
rate during startup to maintain a 90° to 180°F wet
bulb/dry bulb temperature differential.
4.5.3.2.2 Spray dryer shutdown. Proper shutdown
procedures for spray dryers should ensure that no
liquid moisture remains in the spray dryer or that
condensation in the fabric filter does not cause bag
blinding or corrosion. Slurry remaining in the system
after shutdown can cause solids buildup. Also, the
reaction products resulting from the neutralization of
the acid gases include highly corrosive salts such as
NaCl and CaC^. After the waste material in the
incinerator has been combusted, auxiliary fuel firing
should be used to maintain temperatures above
saturation until all sorbent is purged from the
system. To prevent bag blinding and reaction product
salt corrosion that could occur in the presence of
condensation, the fabric filter should go through a
complete cleaning cycle to remove the filter cake
after shutdown of the incinerator and spray dryer.
4.5.3.3 Automatic Control
The most important key parameter from a control
standpoint is the slurry feed rate. The slurry feed rate
determines the amount of sorbent available for acid
gas collection (along with the sorbent content of the
slurry) and the outlet gas wet bulb/dry bulb
temperatures. Slurry feed rate can be automatically
controlled in response to monitoring of HC1 outlet gas
concentration or wet bulb/dry bulb temperature. In
less complex systems, the slurry sorbent content is
fixed and the slurry feed is automatically increased
or decreased in response to the outlet gas
temperatures. Under conditions of low inlet HC1
concentrations these systems will use excessive
amounts of sorbent. More complex systems are
available that automatically adjust the sorbent feed
to the slurry mix tank in response to HC1
concentration and slurry feed rate to the atomizer in
response to outlet gas temperatures.
4.5.3.4 Monitoring
Spray dryer parameters which can be monitored by
the operator include:
1. Sorbent feed to the mix tank;
2. Slurry feed rate to the atomizer;
3. Gas inlet temperature;
4. Gas outlet wet bulb and dry bulb temperature;
and
5. HC1 and SOz outlet gas concentrations.
The most important parameters are the outlet gas
HC1 concentration which indicates the performance
of the unit and the outlet gas wet bulb/dry bulb
temperatures which determine evaporation rate. As
explained earlier, the evaporation rate is important
to ensure that adequate absorption time is available
for acid gas removal and that all moisture is
evaporated prior to leaving the reaction vessel.
4.5.4 Dry Injection
4.5.4.1 Key Operating Parameters
The key operating parameters for a dry injection
system are the sorbent injection rate and the particle
size of the sorbent. The sorbent injection rate should
provide adequate sorbent for neutralization of the
acid gases and is dependent on the acid gas content of
the flue gas. The sorbent feed rate for dry injection
systems is usually three to four times the
stoichiometric requirements.2'''
The acid gas/sorbent reaction requires that the acid
gas come in physical contact with the surface of the
solid sorbent particles. To maximize the efficiency of
the collection process it is necessary to maximize the
surface area of the sorbent material. Because the
surface area-to-volume ratio increases with
decreasing particle size, acid gas removal efficiency
increases with the decreasing particle size of the
sorbent. Generally, the sorbent feed should have a
particle size where 90 percent by weight will pass
through a 325 mesh screen.28
4.5.4.2 Dry Injection Operation
Dry injection systems are relatively simple to operate
compared to spray drying systems. The particle size
of the sorbent should be specified to the supplier. To
ensure that the particles do not agglomerate prior to
injection, the airflow rate in the pneumatic transfer
line should be set and maintained at the manu-
facturer's specified rate. Proper airflow in the
pneumatic line will fluidize the sorbent particles and
prevent agglomeration. Sorbent injection rates for
dry injection systems, as stated above, are usually
three to four times stoichiometric requirements. The
large amount of extra sorbent can handle moderate
variations in inlet acid gas concentrations. For
systems that are not equipped with outlet acid gas
continuous monitors, the sorbent injection rate can be
set at a constant level or varied with flue gas flow
rate. If the system is equipped with an outlet monitor
for HC1, the sorbent injection can be varied as needed.
58
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4.5.4.2.1 Dry injection startup. There are no special
considerations for dry injection startup. At startup of
the incinerator, sorbent injection can be initiated.
4.5.4.2.2 Dry injection shutdown. The only special
concern for shutdown of a dry injection system is to
put the fabric filter through a cleaning cycle after
sorbent injection is stopped. This prevents possible
blinding from condensation and reaction product salt
damage to the fabric filter components.
4,5.4.3 Automatic control
Dry injection systems can be equipped with outlet gas
HC1 continuous monitors. The sorbent injection rate
can be controlled automatically based on the outlet
HC1 concentration.
4.5.4.4 Monitoring
Dry injection system parameters which can be
monitored by the operator include:
1. Sorbent injection rate;
2. Pneumatic transfer line airflow rate;
3. Flue gas flow rate; and
4. HC1 and SOz outlet gas concentrations.
4.6 References for Chapter 4
1. Ontario Ministry of the Environment. Incin-
erator Design and Operating Criteria, Volume II
- Biomedical Waste Incinerators. October 1986.
2. McRee, R. Operation and Maintenance of
Controlled-Air Incinerators. Ecolaire Combus-
tion Products. (Undated)
3. U. S. Environmental Protection Agency Office of
Solid Waste. EPA Guide for Infectious Waste
Management, EPA/530-SW-86-014 (NTIS PB 86-
199130). May 1986.
4. Letter from Ken Wright, John Zink Company, to
J. Eddinger, U. S. EPA. January 25,1989.
5. Personal conversation between R. Neulicht, Mid-
west Research Institute, and J. Kidd, Cleaver-
Brooks. February 22,1989.
6. Personal conversation with representatives of the
National Solid Waste Management Association.
December 15,1988.
7. U. S. Environmental Protection Agency.
Municipal Waste Combustion Study: Combustion
Control of Organic Emissions, EPA/530-SW-87-
021C (NTIS PB87-206090). June 1987.
8. Ecolaire Combustion Products, Inc. Technical
Paper: Controlled Air Incineration. (Undated)
9. Simonds Incinerators. Operation and
Maintenance Manual for Models 751B, 1121B,
and 2151B. January 1985.
10. Ecolaire Combustion Products, Inc. Equipment
Operating Manual for Model No. 480E.
11. John Zink Company. Standard Instruction
Manual: John Zink/Comtro A-22G General
Incinerator and One-Half Cubic Yard Loader.
12. Brunner, C. Incineration Systems Selection and
Design. Van Nostrand Reinhold. p. 22.1984.
13. Personal conversation between Roy Neulicht,
Midwest Research Institute and Larry Doucet,
Doucet and Mainka Consulting Engineers.
November 29,1989.
14. Doucet, L. C. Controlled-Air Incineration:
Design, Procurement, and Operational
Considerations. American Hospital Association
Technical Series, Document No. 055872. January
1986.
15. Air Pollution Control District of Los Angeles
County. Air Pollution Engineering Manual, AP-
40 (NTIS PB 225132). U.S. Environmental
Protection Agency. May 1973.
16. American Society for Testing and Materials.
ASTM Standard D1709-75. Philadelphia,
Pennsylvania. 1975.
17. Consumat Systems, Inc. Technical Data Sheet.
(Undated)
18. Ecolaire Combustion Products, Inc. Equipment
Operating Manual for Model No. 2000TES.
19. Personal conversation between Roy Neulicht,
Midwest Research Institute, and Steve Shuler,
Ecolaire Combustion Products.
20. Ashworth, R. Batch Incinerators - Count Them
In. Technical paper prepared for the National
Symposium of Infectious Wastes. Washington,
D.C. May 1988.
21. Cross, F. and J. Tessitoire. Incineration of
Hospital Infectious Waste. Pollution
Engineering. November 1988.
22. Engineering Manual With Operation and
Maintenance Instructions. Anderson 2000, Inc.
Peachtree City, Georgia. (Undated)
59
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23. Joseph, J. and D. Beachler. APTI Course SI:412C,
Wet Scrubber Plan Review - Self Instructional
Guidebook. EPA 450/2-82-020. U. S. Environ-
mental Protection Agency. March 1984.
24. U. S. Environmental Protection Agency. Wet
Scrubber Inspection and Evaluation Manual.
EPA 340/1-83-022 (NTIS PB85-149375).
September 1983.
25. U. S. Environmental Protection Agency. Fabric
Filter Inspection and Evaluation Manual.
EPA 340/1-84-002 (NTIS PB86-237716).
February 1984.
26. Beachler, D.S. APTI Course SI:412, Baghouse
Plan Review. U. S. Environmental Protection
Agency, EPA-450/2-82-005. April 1982.
27. U. S. Environmental Protection Agency,
Operation and Maintenance Manual for Fabric
Filters, EPA/625/1-86/020. June 1986.
28. Richards Engineering, Air Pollution Source Field
Inspection Notebook; Revision 2. Prepared for the
U. S. Environmental Protection Agency, Air
Pollution Training Institute. June 1988.
60
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Chapter 5
Maintenance
Efficient operation of a hospital waste incinerator
and any associated air pollution control device
requires an aggressive preventive maintenance (PM)
program. Effective maintenance will prolong the
service life of the equipment, reduce the frequency of
upset conditions and air pollution episodes, and save
the hospital money by avoiding costly repairs.
Information on the types and frequency of equipment
inspections and maintenance procedures for
incinerators, wet scrubbers, and fabric filters is
provided in this chapter. The information provided
represents only general guidance and should not be
substituted for the equipment manufacturer's
recommended maintenance schedule. Recordkeeping,
which is an important part of a PM program, is
mentioned briefly below and is discussed more fully
in Chapter 8. Records of maintenance activities can
help pinpoint problems and show trends in
maintenance activities that may increase the life of
the equipment and minimize emissions.
5.1 Hospital Waste Incinerators
Typically, the incinerator operator does not perform
PM but makes hourly and daily inspections to ensure
proper operation of the incinerator and its air
pollution control device and the identification of
potential problems. The hospital's maintenance
department performs PM on the incinerator on a set
schedule and corrects any potential problems
identified by the operator. At some hospitals, the
maintenance department performs PM on a work
order system. 1 In practice, minimizing the number of
PM items per work order ensures that PM is
performed properly and with less downtime.1
Because of the diversity in both size and design of
hospital waste incinerators, specific recommenda-
tions on PM practices and frequency that would be
applicable to all units are impossible to make. Also,
as noted above, the manufacturer's or vendor's
recommended PM schedule for a particular unit
should be followed, and the information presented
here should only be used to supplement, not replace,
that schedule. In general, however, PM activities for
any hospital waste incinerator involve inspection,
cleaning, and lubrication of incinerator components
on a regular basis. Table 5-1 presents a typical PM
program for a hospital waste incinerator. Note that
both the components listed on the table for
maintenance and the maintenance frequency will
vary depending on the type and amount of waste
being incinerated and the type of incinerator used.
The sections below discuss the items on Table 5-1 in
greater detail and are organized according to
hourly/daily maintenance, weekly/biweekly main-
tenance, and monthly/semiannual maintenance.
5.1.1 Hourly/Daily Maintenance
The hourly maintenance activities described in
Table 5-1 are simple inspections that are applicable
to large, continuous-feed incinerators. These large
units may be equipped with a water quench pit and
an ash removal conveyor system that continuously
removes ash from the incinerator.2 The operator
must routinely replenish the water in the quench pit
because water is constantly removed by evaporation
and by the ash removal system.2,3 The water quench
pit is used to quench or extinguish the hot ash as it is
removed from the incinerator and to provide an air
seal for the ash removal conveyor.3 Therefore, it is
essential that sufficient water be available. In
addition, the operator should check the ash removal
conveyor system frequently to remove any debris that
could cause it to jam.3 Small, batch-feed incinerators
will likely have neither a water quench pit nor an
automatic ash removal conveyor system.
Daily maintenance activities involve checking the
operation of any opacity, oxygen, and CO monitors.
These monitors indicate whether the
operating/combustion conditions of the incinerator
are within an acceptable range; for some units, the
oxygen monitor may also be used to control the
combustion air rate. Before the incinerator is started
up, the opacity monitor and the CO monitor should
read 0 percent opacity and 0 parts per million (ppm),
respectively. The oxygen monitor should read about
21 percent oxygen (ambient air). If these monitors are
equipped to conduct for daily calibration checks, the
calibration checks should be conducted and the
calibration values noted to assure that they are in the
proper range. If not, maintenance personnel should
be informed. An additional check for the opacity
monitor is to observe the stack emissions and
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Tabla 5-1. Typical Maintenance Inspection/Cleaning/Lubrication Schedule for a Hospital Waste Incinerator4
Activity frequency Incinerator component Procedure
Hourly
Daily
Weekly
Biweekly
Monthly
Semiannually
Ash removal conveyor
Water quench pit
Opacity monitor
Oxygen monitor
Thermocouples
Underfire air ports
Limit switches
Door seals
Ash pit/internal dropout sump
Heat recovery boiler tubes
Blower intakes
Burner flame rods (gas-fired units)
U.V. scanner flame sensors
Swing latches and hinges
Hopper door support pins
Ram feeder carriage wheels
Heat recovery induced-draft fans
Hydraulic systems
Ash removal conveyor bearings
Fuel trains and burners
Control panels
External surface of incinerator and stack
Refractory
Internal rarn faces
Upper/secondary combustion chamber
Large combustion air blowers and heat recovery
induced draft fans (those fans whose bearings are not
sealed)
Hydraulic cylinder clevis and trunnion attachments to Lubricate.
all moving components
Burner pilots
Hot external surfaces
Inspect and clean as required.
Inspect water level and fill as required.
Check operation of the opacity monitor and check
exhaust for visible emissions.
Check operation of the oxygen monitor.
Check operation of the thermocouples.
Inspect and clean as required.
Inspect for freedom of operation and potential
obstructing debris.
Inspect for wear, closeness of fit, and air leakage.
Clean after each shift on batch units that do not have
continuous ash conveyor cleaning system.
Inspect and clean as required. (Clean weekly for 6
weeks to determine optimum cleaning schedule.)
Inspect for accumulations of lint, debris; clean as
required.
Inspect and clean as required.
Inspect and clean as required.
Lubricate.
Lubricate.
Lubricate.
Inspect and clean fan housing as required. Check for
corrosion and V-belt drives and chains for tension
and wear.
Check hydraulic fluid level and add the proper
replacement fluid as required. Investigate sources of
fluid leakage.
Lubricate.
Inspect and clean as required. Investigate sources of
fuel leakage as required.
Inspect and clean as required. Keep panel securely
closed and free of dirt to prevent electrical
malfunction.
Inspect external "hot" surfaces. White spots or
discoloration may indicate loss of refractory.
Inspect and repair minor wear areas with plastic
refractory material.
Inspect for wear. These skinless steel faces may
wear out and may require replacement in 1 to 5
years depending on service.
Inspect and vacuum any particulate matter that has
accumulated on the chamber floor.
Lubricate.
Ambient external surfaces
Chains
Inspect and adjust as required.
Inspect and paint with high-temperature paint as
required.
Inspect and paint with equipment enamel as required.
Inspect and brush clean as required. Lubricate.
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compare the observation to the opacity monitor
reading. For example, if the opacity monitor, which
normally reads below 5 percent, reads 25 percent, and
a simple check of the exhaust stack shows an opacity
of 0 percent, then something is wrong with the
opacity monitor.
The thermocouples that indicate the temperature of
the primary and secondary chambers should also be
checked daily.4 It is essential that these
thermocouples be operating properly because the
temperature monitored by these thermocouples
typically is the parameter that controls the
combustion airflow into the incinerator (controlled-
air units) and burner operation.2 The temperature
readings should be ambient prior to incinerator
startup. A thermocouple problem may be indicated if
the operator notices a significant change in startup
conditions, i.e., if significantly more auxiliary fuel is
required than normal, or if it takes significantly
longer to heat the incinerator than normal.4 Also, a
noticeable change in response time of the
thermocouple is an indication of problems. Unfortu-
nately, there is no simple way to check the accuracy
of a thermocouple; either the thermocouple must be
removed and tested or a second thermocouple must be
inserted in a second thermocouple port, if available.
The remaining daily maintenance activities involve
inspection for slagging and plugging (underfire air
ports); inspection for obstructing debris (e.g., limit
switches on charging doors, charging rams, hopper
doors, etc.); inspection of door seals for wear,
closeness of fit, and air leakage; and cleaning the ash
from the hearth on batch and intermittent-duty
incinerators.4 The underfire air ports must be kept
clean so that the proper amount of underfire air can
reach the burning waste in a well distributed
manner. Limit switches must be kept clear of debris
because they control the proper adjustment of the
charging door and the ram feeder (if installed).2
Proper functioning of charging door and hopper/ram
limit switches is essential because these switches
ensure that the doors are fully opened/closed at the
proper times and help prevent excessive air in-
leakage to the incinerator during charging and
operation.2 Improperly sealed doors, which result in
excessive air leakage, will not only affect incinerator
operation, but also may result in more severe
maintenance requirements (damage to seals or
warpage due to excessive heat), as well as causing a
fire/safety hazard. Finally, the ash typically is
allowed to cool overnight and is removed from the
hearth on batch and intermittent-duty incinerators
the day after the burn.1
5.1.2 Weekly/Biweekly Maintenance4
Some of the large hospital waste incinerators (20 to
25 tons waste/d) may employ a waste heat recovery
boiler to produce steam for use in the hospital. When
first installed, the heat recovery boiler tubes should
be cleaned weekly. The first indication of fouled tubes
on the gas side will be the gradual increase of boiler
outlet gas temperature. All blower intakes and the
heat recovery induced-draft fan housing should be
inspected weekly and cleaned as required. The
burner flame rods (gas-fired units) or ultraviolet
scanner flame sensors (other units) should also be
inspected weekly and cleaned as required. The swing
latches and hinges, the hopper door support pins, and
the ram feed carriage wheels should all be lubricated
weekly.
The biweekly maintenance items include the
inspection of the fuel trains, burners, and control
panels. These pieces of equipment should also be
cleaned as required. Any sources of fuel leakage
should be investigated. The hydraulic fluid level
should be checked on all hydraulic systems, and the
appropriate replacement fluid should be added as
needed. Only the fluid recommended by the
manufacturer should be used because other fluids
may damage the hydraulic seals. The ash removal
conveyor bearings on large incinerators should be
lubricated biweekly.
5.1.3 Monthly/Semiannual Maintenance4
On a monthly basis, the incinerator operator should
inspect the external hot surfaces of both the
incinerator vessels and stack for white spots or other
discoloration that may indicate a loss of refractory
inside. Additionally, whether white spots are evident
or not, the refractory lining inside the incinerator
should be inspected for wear areas and any buildup of
ash or metal oxides such as calcium, potassium, and
sodium. Any minor wear areas in the refractory
should be repaired using plastic refractory material.
The metal oxides can cause a marked decrease in the
softening temperature of the refractory that effec-
tively reduces the operating temperature range. Any
ash and metal oxide buildup should be washed off
only after the incinerator has cooled to avoid
damaging the refractory if the temperature of the
wash water is significantly different from that of the
refractory.
Large incinerators may be equipped with internal
ash rams that push burning waste from one hearth to
the next. The rams typically are made of refractory
material with the exception of the front face of the
ram, which is made of stainless steel. These stainless
steel ram faces should be inspected for wear and
damage each month initially until an optimum
inspection schedule can be established. The ram faces
may last from 1 year to 5 years depending upon the
type of service in which they are used.
In controlled-air incinerators, the primary chamber
is operated at less than stoichiometric air (starved-
air) conditions, which minimizes the amount of fly
63
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ash carryover to the secondary (excess-air) chamber.
However, in multiple-chamber (excess-air) designs,
carryover of fly ash to the secondary chamber should
be expected; the carryout of carryover will depend on
the amount of excess air at which the primary
chamber is operating. Therefore, a monthly
inspection and cleaning schedule for the secondary
chamber is appropriate for multiple-chamber units
but may be unnecessary for controlled-air units.
Manufacturers of controlled-air units specify
vacuuming ash from the secondary chamber at least
every 6 months. Therefore, an effective cleaning
schedule for these units may be every 3 to 4 months
until an optimum cleaning schedule can be
established.
Large fans (those that do not have sealed bearings)
and the hydraulic cylinder clevis and trunnion
attachments should be lubricated monthly. Lastly,
the burner pilots should be inspected and adjusted as
required.
The semiannual maintenance activities include the
inspection of all hot external surfaces, all ambient
external surfaces, and any conveyor chains. All
external surfaces should be cleaned; high-
temperature paint should be applied to hot surfaces,
and equipment enamel should be applied to ambient
surfaces as required. The chains should be brushed
clean if necessary and lubricated.
5.2 Wet Scrubberss-7
A typical maintenance inspection, cleaning, and
lubrication schedule for a wet scrubber is presented
in Table 5-2. As with incinerators, the frequency with
which these activities take place will depend on a
number of variables including the size and
complexity of the scrubber, the number of hours per
day it operates, and the volume and pollutant
concentration of the exhaust gas it handles. In
addition, not all of the maintenance activities listed
in Table 5-2 will be required at each scrubber
installation. The type and frequency of maintenance
activities will depend in large part on the scrubber
vendor's recommendations and the experience of
personnel with the unit.
5.2.1 Daily/Weekly Maintenance
The daily maintenance activities described in
Table 5-2 are simple inspections that are applicable
to all scrubbers. Most of these inspections involve
checking various components for proper operation
and fluid leakage. These inspections should be
carried out after the scrubber has been started up and
prior to startup of batch-feed incinerators so that any
necessary repairs can be made before the scrubber
has to control emissions. Scrubbers controlling
continuous-feed incinerators should be inspected
while both units are in operation.
Failure of components such as the scrubber liquid
pump and variable throat activator (on venturi
scrubbers) will cause the scrubber to be ineffective in
controlling emissions from the incinerator. In such
cases, the incinerator may have to be shut down
depending on both the severity of the problem and
any State regulations concerning operation of the
incinerator without its attendant air pollution
control device. Liquid leakage from these components
also can have a detrimental effect on the performance
of wet scrubbers. The sources of such leaks should be
investigated and repaired as required.
Fans in wet scrubber service operate under
substantially more severe conditions than those
serving dry systems because humid conditions
enhance the possibility of condensation of acids on
the fan and fan housing. In the case of hospital waste
incinerators, scrubbers and fans are likely to
experience hydrochloric acid mist as a result of the
combustion of chlorinated plastic material. There-
fore, acid-resistant materials should be used in the
construction of the scrubber interior and more
frequent cleaning of the fan system serving the
scrubber should be practiced. Unusual noise or
vibration from any part of the fan system, including
the bearings or belt, may indicate that replacement is
necessary.
The weekly maintenance items include more
inspections of components. The fan and the scrubber
liquid pump oil levels should be checked and adjusted
as appropriate. Additionally, the fan oil temperature
and color should be checked. Temperature or color
readings that are outside of the manufacturer's
recommended ranges may indicate excessive wear
and/or contamination of the oil. Both the fan and
scrubber liquid pump bearings should be lubricated
weekly.
5.2.2 Monthly/Semiannual Maintenance
The monthly maintenance activities consist of
additional inspections and appropriate corrective
actions as required. Clogged pipes, manifolds, and
spray nozzles can hinder the proper operation of the
scrubber. The pressure drop of the scrubber will be
lower than normal if plugging is experienced. The
pressure drop is a critical indicator of scrubber
performance in removing particulate from the
incinerator exhaust, and the pressure gauges should
be checked and calibrated for accuracy. Some
regulations may require a minimum pressure drop to
be achieved across the venturi to assure a minimum
pollutant removal efficiency. Therefore, it is
important that the pressure gauges operate properly.
The main body of the scrubber, fan blades, and
internal fan housing are subject to buildup of
particulate matter that can adversely affect both
scrubber and fan performance. These components
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Table 5-2. Typical Maintenance Inspection/Cleaning/Lubrication Schedule for a Wet Scrubber
Component Procedure
Inspection
frequency
Daily Scrubber liquid pump
Variable throat activator
Scrubber liquid lines
Mist eliminator pressure lines
Reagent feed system
Fan
Fan bearings
Fan belt8
Weekly Fan
Scrubber liquid pump
Damper air purge system
Monthly Duct work
Fan and motor bearings
Fan blades and internal housing
Drain chain drive mechanism
Pipes and manifolds
Dampers
Spray bars
Pressure gauges
Main body of scrubber
Semiannually Fan, pump, motor, and drag chain
bearings and gear reducers
Flowmeters
Damper drive mechanism
Damper seals, bearings, blades, blowers
Check for proper operation and leakage. Investigate sources of fluid
leakage and repair as required.
Check for proper operation and leakage.
Check for leakage. Repair as required.
Check for leakage. Repair as required.
Check for leakage. Repair as required.
Check for vibration and proper operation.
Check for abnormal noise.
Check for abnormal noise.
Check oil level, oil color, oil temperature, and lubricate.
Check oil level and lubricate pump motor bearings.
Check for proper operation.
Inspect for leakage.
Inspect for leaks, cracks, and loose fittings.
Inspect for material buildup and clean as required. Inspect for abrasion
and corrosion and repair as required.
Check chain tension, sprocket wear and alignment, and oil level.
Inspect for plugging/leaking and clean/repair as required.
Check for leakage.
Inspect for nozzle wear and plugging and clean as required.
Check for accuracy.
Inspect for material buildup and clean as required. Inspect for abrasion
and corrosion and repair as required.
Inspect clearances and wear, pitting, and scoring. Inspect for leaks,
cracks, and loose fittings.
Check for accuracy.
Check for proper operation and alignment.
Check for wear and leakage.
a Check fan belt tension whenever fan is out of service.
should be inspected and cleaned monthly. In
addition, these components should be inspected for
any abrasion caused by particulate matter or
corrosion caused by hydrochloric acid mist that might
adversely affect performance.
Fan and motor bearings should be inspected for leaks,
cracks, and loose fittings. Dampers and ductwork
should be checked for any air leakage. Leakage of air
can cause condensation of moisture and acid gases
due to the introduction of cool ambient air. Also, a
large number of leaks in the ductwork will cause the
induced-draft fan to pull more air, and the
incinerator operator will have less control over the
air introduced into the incinerator. The drag chain
drive mechanism used on some automatic ash
removal systems should be inspected for wear, chain
tension, and sprocket alignment. The oil level also
should be checked on this component and adjusted as
required.
The semiannual maintenance activities include
inspections of all bearings on fans, pumps, motors,
and gear reducers for leaks, cracks, and loose fittings.
Additionally, the clearances and any wear, pitting, or
scoring should be checked on these components. The
flow meters on the scrubber should be checked for
accuracy. State regulations, in addition to requiring
a pressure drop, also may require a specific water
flow rate. Therefore, the flow meters should operate
properly. The damper drive mechanism should be
checked for proper operation and alignment; and the
damper seals, bearings, blades, and blowers should
be inspected for wear and leakage.
5.3 Maintenance of Fabric Filters8,9
Although the frequency and components of a PM
program for a fabric filter system will depend on the
type of system and the vendor's recommendations,
the major components should be inspected on a
routine basis, and any needed maintenance should be
performed. The following sections describe the daily,
weekly, monthly/quarterly, and semiannual/annual
inspections/maintenance that are recommended. A
specific PM program should be established based on
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the manufacturer's recommendations. A typical
maintenance inspection, cleaning, and lubrication
schedule for a fabric filter is presented in Table 5-3.
5.3.1 Daily Inspection/Maintenance
At least twice per shift (and perhaps as often as every
2 hours), plume opacity (visual inspection of stack
outlet) and pressure drop should be checked. Sudden
changes in these values along with those of
temperature and gas volume may indicate a problem.
For example, the failure or partial failure of the
cleaning system generally will cause a relatively
rapid increase in pressure drop in most systems.
Timely identification, location, and correction of this
problem can minimize operating problems and long-
term effects on bag life. Although identification and
subsequent correction of relatively minor problems
have little effect on fabric life, some minor problems
tend to turn into major failures. The ability to
perform on-line maintenance depends on the design
of the control equipment.
Routine checks of the fabric filter include pressure
drop (and patterns if a AP indicator and recorder are
used), plume opacity at the outlet, dust discharge
operation, and external checks (i.e., visual
inspection) of the cleaning system operation. Other
factors that can be checked include temperature
(range) and fan motor current. If a check of these
factors reveals a sudden change, maintenance should
be scheduled as soon as possible.
5.3.2 Weekly Inspection/Maintenance
The extent of the weekly maintenance program
depends greatly on access and design of the fabric
filter. Where possible, quick visual inspections
should be conducted; however, not all systems or
processes are amenable to this type of review. A
weekly lubrication schedule should be established for
most moving parts. Manometer lines should be blown
clear, and temperature monitors should be checked
for proper operation.
When a fabric filter is the air pollution control device
of choice for a hospital waste incinerator, it is almost
always a pulse-jet fabric filter. The following items
should be checked weekly. On the dirty side of the
tubesheet, bags should be checked for relatively thin
and uniform exterior deposits. Bags also should be
checked for bag-to-bag contact (points of potential
bag wear). On the clean side of the tubesheet, each
row of bags should be examined for leakage or holes.
Deposits on the underside of the blowpipes and on the
tubesheet may indicate a bag failure. The cleaning
system should be activated (the inspector should use
hearing protection), and each row of bags should fire
with a resounding "thud." The blowpipes should
remain secured, and there should be no evidence of oil
or water in the compressed air supply. The surge tank
or oil/water separator blowdown valve should be
opened to drain any accumulated water. Misaligned
blowpipes should be adjusted to prevent damage to
the upper portion of the bag. The compressed air
reservoir should be maintained at about 90 to 120 psi.
On a weekly basis, the dust removal system
including hopper and screw conveyor should be
inspected to make sure that dust is being removed
from the system by checking the conveyor for
jamming, plugging, wear and broken parts, etc.
Problems with the conveyor system are indicated
when the conveyor appears to be moving but no dust
is dropping into the dust storage container, when the
conveyor does not move, or when the conveyor makes
unusual sounds.
5.3.3 Monthly/Quarterly
Inspection/Maintenance
Requirements for monthly or quarterly maintenance
and inspection for fabric filters are very site specific.
Clear-cut schedules cannot be established for such
items as bag replacement and general maintenance
of the fabric filter. Some items, however, may
warrant quarterly or monthly inspections, depending
on site-specific factors. Items to be checked include
door gaskets and airlock integrity to prevent
excessive in-leakage (both air and water) into the
enclosure. Any defective seals should be replaced.
Baffles or blast plates should be checked for wear and
replaced as necessary, as abrasion can destroy the
baffles. Some facilities prefer to use fluorescent dye to
check the integrity of the bags and bag seals. Any
defective bags should be replaced, and leaking seals
should be corrected.
Bag failures tend to occur shortly after installation
and near the end of a bag's useful life. A record of bag
failures and replacements is invaluable for
identifying recurrent prob'^ms and indicating when
the end of bag life has been reached. Initial bag
failures usually occur because of installation errors
or bag manufacturing defects. When new bags are
installed, a period with few or no bag failures is
normally expected unless serious design or operation
problems exist. As the bags near the end of their
useful life, however, the number of bag failures may
increase dramatically. When weighed against factors
such as downtime for rebagging, the cost of new bags,
and the risk of limited incinerator operating time as
the result of keeping the old bags in service, the most
economical approach may be to replace all the bags at
one time to eliminate or minimize failure rate.
In some cases, bags can be washed or drycleaned and
reused, e.g., when dewpoint limits are approached or
the bags are blinded in some manner. Although
cleaning may shorten bag life somewhat, cleaning
may still be more economical than replacement if
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Table 5-3. Typical Maintenance Inspection/Cleaning/Lubrication Schedule for a Fabric Filter System*^
Component Procedure
Inspection
frequency
Daily
Weekly
Monthly
Quarterly
Semiannually
Annually
Stack
Manometer
Compressed air system
Collector
Damper valves
Rotating equipment and drives
Dust removal system
Filter bags
Cleaning system
Hoppers
Shaker mechanism
Fan(s)
Monitor(s)
Inlet plenum
Access doors
Shaker mechanisms
Motors, fans, etc.
Collector
Check exhaust for visible dust.
Check and record fabric pressure loss and fan static
pressure. Watch for trends.
Check for air leakage (low pressure). Check valves.
Observe all indicators on control panel and listen to
system for properly operating subsystems.
Check all isolation, bypass, and cleaning damper
valves for synchronization and proper operation
Check for signs of jamming, leakage, broken parts,
wear, etc.
Check to ensure that dust is being removed from the
system.
Check for tears, holes, abrasion, proper fastening,
bag tension, dust accumulation on surface or creases
and folds.
Check cleaning sequence and cycle times for proper
valve and timer operations. Check compressed air
lines including oilers and filters. Inspect shaker
mechanisms for proper operation.
Check for bridging or plugging. Inspect screw
conveyor for proper operation and lubrication.
Inspect for loose bolts.
Check for corrosion and material buildup and check
V-belt drives and chains for tension and wear.
Check accuracy of all indicating equipment.
Check baffle plate for wear; if appreciable wear is
evident, replace. Check for dust deposits.
Check ail gaskets.
Tube type - (tube hooks suspended from a tubular
assembly): inspect nylon bushings in shaker bars and
clevis (hanger) assembly for wear.
Channel shakers - (tube hooks suspended from a
channel bar assembly): inspect drill bushings in tile
bars and connecting rods for wear.
Lubricate all electric motors, speed reducers, exhaust
and reverse-air fans, and similar equipment.
Check all bolts and welds. Inspect entire collector
thoroughly, clean, and touch up paint where
necessary.
more than half a bag's "normal" life expectancy
remains.
filter. Additionally, the unit should be cleaned and
painted as appropriate.
5.3.4 Semiannual/Annual
Inspection/Maintenance
Some motors and packaged blowers are supplied with
sealed bearings and, therefore, require no
lubrication. Semiannual fabric filter system main-
tenance activities include the lubrication of the
following components having nonsealed bearings: all
electric motors, speed reducers, exhaust and reverse
air fans, and similar equipment.
Annual maintenance activities include checking the
tightness and fit of all bolts^nd welds on the fabric
5.4 References for Chapter 5
1. Personal Conversation between Mark Turner,
MRI, and William Tice, Rex Hospital, Raleigh,
North Carolina. August 16,1988.
2. Murphy, P., and Turner, M. Report of Site Visit to
Ecolaire Combustion Products, Charlotte, North
Carolina. July 20,1988.
3. Allen Consulting and Engineering, Municipal
Waste Combustion Systems Operation and
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Maintenance Study. EPA-340/1-87-002.
June 1987.
4. Ecolaire Combustion Products, Inc. Equipment
Operating Manual for Model No. 2000TES;
Equipment operating manual for Model
No. 480E.
5. U. S. Environmental Protection Agency. Wet
Scrubber Inspection and Evaluation Manual,
EPA 340/1-83-022 (NTIS PB 85-149375).
September 1983.
6. Joseph, J. and Beachler, D., APTI Course
SI:412C, Wet Scrubber Plan Review, Self-
Instructional Guidebook. U. S. Environmental
Protection Agency. EPA 450/2-82-020.
March 1984.
7. Engineering Manual with Operation and
Maintenance Instructions. Anderson 2000, Inc.
Peachtree City, Georgia. (Undated).
8. U. S. Environmental Protection Agency.
Operation and Maintenance Manual for Fabric
Filters. EPA/625/1-86/020. June 1986.
9. Beachler, D. APTI Course SI:412, Baghouse Plan
Review. U. S. Environmental Protection Agency.
EPA 450/2-82-005. April 1982.
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Chapter 6
Control and Monitoring Instrumentation
6.1 Operating Parameters that Should
be Monitored
Proper operation of incinerators depends to a large
extent on certain operating parameters that are
commonly monitored to provide necessary
information to properly control the process. These
usually consist of:
1. Temperature;
2. Pressure;
3. Oxygen;
4. CO;
5. Opacity; and
6. Charge rate.
Although all of the above parameters provide useful
information, many hospital incinerators monitor
only temperature and pressure. The other param-
eters listed are sometimes monitored on larger incin-
erators or if regulatory agencies require that
monitoring be conducted (e.g., opacity).
The temperature and oxygen level are key
parameters, and incinerators are normally designed
to monitor one or both of these parameters to provide
the necessary information for automatic control of
combustion air input and auxiliary fuel rates. To
maintain the desired draft in the incinerator, control
loops may be used to adjust incinerator draft by
means of barometric dampers or the induced draft
fan. The other parameters provide additional
information that operators can use to maintain
proper operation (e.g., charge rate, oxygen levels).
Proper operation of wet scrubber systems is
dependent on several operating parameters that are
commonly monitored, usually consisting of:
1. Pressure and pressure drop;
2. Scrubber liquid flow;
3. Scrubber liquid pH;
4. Temperature.
For venturi scrubbers, pressure drop is normally
monitored and automatically controlled to a fixed
level by adjusting the venturi throat. For Venturis
and packed beds, the scrubber liquid flow is usually
established at a constant rate and is not auto-
matically controlled during operation. For both types
of scrubbers, control of pH is necessary to prevent
damage to the scrubber equipment. Most scrubbers
are designed to monitor and control pH automat-
ically. Gas flow and temperature are often monitored
to provide operators with additional information
useful in maintaining proper operation by indicating
potential problems and the need to make manual
adjustments. Other instrumentation and control
often includes an automatic liquid level control for
the scrubber liquid sump, and a temperature switch/
solenoid valve to activate an emergency quench spray
and/or an emergency bypass stack.
For fabric filters, the key operating parameter which
typically is monitored is pressure drop across the
bags. Temperature also is monitored to assure that
the temperature does not decrease below the dew-
point or increase to a level which will cause bag
damage.
The next section describes the typical
instrumentation used to monitor/control these key
parameters and briefly discusses its proper use and
operation.
6.2 Typical Instrumentation
6.2.1 Temperature Sensors
Thermocouples are used to monitor temperatures in
the combustion chambers and the air pollution
control system. The thermocouples are always
enclosed in a thermowell to protect the small thermo-
couple wires and the critical thermocouple junction
from direct exposure to the combustion gases and
entrained dust particles, among other vagaries.
Thermocouples are usually located near the exit of
the combustion chamber to provide a representative
temperature reading away from the flame zone,
which can otherwise cause erratic temperature
readings as well as damage to the thermocouple.
Thermowells may extend several inches past the
inner wall of the refractory into the gas stream, or
may extend only to the depth of the refractory.
Thermowells that extend past the refractory provide
a more accurate measure of the gas temperature and
respond more quickly to temperature changes;
however, this type also may be subject to dust and
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slag buildup, which can slow response to temperature
changes. Generally, thermocouples also are located
upstream of the air pollution control system to
provide a warning or control mechanism for high
temperature excursions that could damage control
equipment.
The different types of thermocouples are identified by
the metal used for the thermocouple junction wires.
The most common types are listed in Table 6-1.
Replacement thermocouples must always be the
same type as the original because the receiver to
which a thermocouple is connected is designed to
receive the signal from a specific type of
thermocouple. Thermocouples generate a small
millivolt signal that increases with increasing
temperature, but the amount of voltage for a given
temperature is different for each type of thermo-
couple. It is important to realize that thermocouples
operate on the basis of a junction between two
different metals that generates only a small millivolt
signal. Consequently, any wiring connections from
the thermocouple to the receiver or any interfering
electrical signals can affect the resulting
temperature reading. This sensitivity necessitates
the special shielding of the wire in electrical conduit.
Table 6-1. Thermocouple Types
Type
J
E
K
S
R
B
Materials
Iron/Constantan
Chromel/Constantan
Chromel/Alumel
Pt 10% Rhodium/Pure Pt
Pt 13% Rhodium/Pure Pt
Pt 30% Rhodium/Pt 6% Rhodium
Upper temp., °F
1400
1650
2300
2650
2650
3100
Although thermocouples typically are very reliable,
they can fail or give erroneous readings. For example,
the thermocouple junction or wire may break after
long exposure to high temperatures. However, a
thermocouple can give erroneous readings for reasons
that are not as obvious as a broken junction or wire.
For example, if mechanical vibration abrades the
insulation and one of the thermocouple wires comes
into contact with the metal wall of the thermowell or
other grounded metal surface, an erroneous
temperature reading will likely result. As noted
earlier, faulty thermocouple readings may also be the
result of external conditions; for example, excessive
dust buildups around a thermowell can insulate it
from the gas stream and result in erroneously low
temperature readings. To have the ability to compare
readings to identify a faulty thermocouple, dual
thermocouples are often used at nearby locations in
the incinerator chamber. The second thermocouple
enables continued monitoring of temperatures while
the faulty thermocouple is being checked or replaced.
Periodic replacement of thermocouples, and checking
the physical integrity of the thermowell and any
outer dust buildup, is probably the best maintenance
procedure. Because it is not practical to perform a
high temperature calibration of the thermocouple,
only periodic replacement ensures that a properly
operating thermocouple is in place. The receiver
should be checked periodically using calibrated
equipment that produces a known millivolt signal
equivalent to a specific temperature reading for a
particular type of thermocouple. The generated
signal can be applied to the thermocouple leads to
check that the receiver's output produces the correct
"temperature" reading.
Close monitoring of temperatures is essential to good
incinerator operation. It is essential, therefore, to
identify possible thermocouple problems because the
temperature signal is usually the primary
measurement used for the automatic control of
auxiliary fuel burners and combustion air input. Any
problem with the thermocouple temperature reading
can cause the automatic control system to make
inappropriate changes in the controlled variable in
an attempt to maintain the desired (setpoint)
temperature.
6.2.2 Pressure
The incinerator draft is measured in units of gauge
static pressure, but it is actually a measure of the
differential pressure (AP) between the inside of the
chamber and the outside air. Monitoring of AP can be
done with a common U-tube manometer, but for
incineration systems, a differential pressure gauge
(e.g., Magnehelic®) or a differential pressure
transmitter typically is used. All of these
instruments use the same basic method to monitor
incinerator draft. One side (the high-pressure side) of
the instrument is always open to the ambient air; the
other (low-pressure) side is connected by tubing or
piping to the incinerator.
These types of pressure monitors also are used to
measure differential pressure across an air pollution
control system (e.g., venturi scrubber throat or fabric
filter baghouse). For this application, the low-
pressure side is connected to a pressure tap in the
ductwork downstream from the control device and
the high-pressure side is connected upstream from
the control device.
A differential pressure transmitter contains a dia-
phragm with two tubes connected on each side of the
diaphragm; the diaphragm moves or deflects as a
result of changes in pressure. The transmitter is
designed so that any change or deflection causes a
change in an electrical output signal from the
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transmitter. The electrical signal is sent to the
monitor in the control room that indicates the
incinerator draft or the pressure differential across
the air pollution control system.
Several problems can cause faulty incinerator draft
readings. Transmitters used to measure AP are
sensitive devices that can be damaged by excessive
vibration or sudden shocks. The severe hot and dirty
conditions in the incinerator may also cause
problems such as plugging in the tubing and its
connections. When faulty incinerator pressure
readings are suspected, several procedures for
correcting them can be used. Air can be blown
through the tubing to clear any plugging, and a
known AP can be applied to the transmitter to check
its response and span. Disconnecting any
instrumentation should be done by experienced
instrumentation personnel and must be coordinated
with the operator so that the appropriate instrument
can be put on manual control. For example, if the
incinerator's automatic control system maintains
incinerator draft via a damper on the ID fan inlet
duct, disconnecting the pressure tap would probably
cause the ID fan to increase flow in an effort to
maintain the draft setpoint, thereby upsetting the
temperature and other process operating parameters.
6.2.3 Oxygen Concentration^
Some incinerators may be equipped with oxygen
analyzers to monitor the oxygen concentration in the
combustion gases from the secondary combustion
chamber to help ensure that adequate oxygen is
available for proper combustion (i.e., that the excess-
air level is sufficient). In some incinerators, the
oxygen levels measured by the monitor are used to
control air feed rates for control of the combustion
process. In essence, lower waste feed rates require
lower air feed rates in order to maintain essentially
constant temperature and constant percent oxygen in
the combustion gases, so long as the characteristics of
the waste feed are the same. However, if the
composition of the feed changes (i.e., its heating value
changes), more or less air will be needed to maintain
the same temperature and oxygen content in the
combustion gases. Thus, the oxygen content is a
useful parameter to monitor and is an important
control parameter if the incinerator is set up to
control based on oxygen level rather than tem-
perature.
Oxygen monitors may be of two types: in situ or
extractive. In situ merely means that the analyzer's
sensor is mounted in direct contact with the gas
stream. In an extractive system, the gas sample is
continuously withdrawn (extracted) from the gas
stream and directed to the analyzer which may be
located several feet or several hundred feet away.
Extractive analyzers include a conditioning system
to remove dust and moisture from the gas sample;
thus, the oxygen concentration measurement is on a
dry basis. In situ analyzers, on the other hand, do not
include a conditioning system, and the oxygen
concentration measurement is on a "wet basis." For
the same gas stream, the oxygen measurement
obtained with an in situ analyzer will be slightly
lower than that obtained with an extractive analyzer.
For example, a typical combustion gas stream that
contains 10 percent water vapor will yield a reading
of 8 percent oxygen using an in situ analyzer and a
reading of 10 percent oxygen using an extractive
analyzer.
Regardless of the type of analyzer, the location of the
sampling point is very important to ensure that a
representative sample is obtained. The monitoring
point may be in the stack, the combustion chamber
exit, or other locations within the process (e.g., duct
between air pollution control system and the stack).
In most cases, the intent is to monitor the combustion
gases at the exit of the combustion chamber and that,
in fact, is where the sampling point is often located.
One of the main problems related to sampling
location that can occur is obtaining a nonrepresen-
tative sample due to in-leakage of air. For example,
substantial air in-leakage can occur through
emergency stack dampers. Any such in-leakage
affects the analyzer reading if the sample point is
downstream of the in-leakage. However, there are
practical limitations with regard to locating oxygen
monitors. The in situ sensor or probe for extracting
the sample must be located in an environment
consistent with the monitor's design. High tem-
peratures, particulate concentrations, and acid gas
concentrations can have adverse effects on the
monitoring system resulting in operational and/or
maintenance problems.
Oxygen analyzers are capable of good accuracy
(+1 percent of full scale) as long as the actual gas to
be sampled reaches the analyzer (no plugging or in-
leakage of air), the conditioning system (if one is
present) is operating properly, and the instrument is
calibrated. Eleetrocatalytic in situ monitors have
rapid response times (i.e., seconds). The response
times for polarographic and paramagnetic extractive
analyzers are slower (several seconds to a minute).
Extractive systems inherently involve longer
response times, usually on the order of 1 to 2 minutes,
depending on the sampling rate and the volume of the
sampling line and conditioning system.
Problems with oxygen analyzer systems may be
difficult to discern since they commonly are
associated with slowly developing plugging in the
system, or small air in-leaks, etc. The extractive
systems should be checked daily by the operators, and
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maintained and calibrated on a weekly basis by the
incinerator instrument personnel.
6,2.3.7 In-S'rtu Oxygen Analyzers
In situ analyzers provide rapid response to changes in
the oxygen content of the gas because the sensor is in
direct contact with the gas stream. In most cases, the
sensing element is enclosed in a sintered stainless
steel tube, which allows the gas to permeate through
the tube but prevents particles in the gas stream from
entering. Most in situ oxygen analyzers are equipped
with connections so that zero gas (nitrogen) or
calibration gas (air) can be flushed through the
permeable tube and in contact with the sensing
element. Flushing provides a means of zeroing and
spanning the analyzer, and also creates reverse flow
of gas through the permeable tube that helps to
remove dust particles that eventually will clog the
tube and slow the detector's response time. Even so,
the tube periodically must be removed for cleaning or
replaced if warranted.
Most in situ oxygen analyzers are of the
electrocatalytic type, sometimes referred to as fuel-
cell analyzers. Operation of these analyzers is based
upon an electron flow created by reaction of oxygen
with a solid zirconium oxide electrolyte.
Consequently, manufacturers recommend that the
sensing element be replaced after several months of
service.
6.2.3.2 Extractive Oxygen Analyzers
Extractive analyzers always involve a "conditioning
system" for removal of water, dust, and sometimes
other constituents that would interfere with
operation of the analyzer. An example extractive
system is illustrated in Figure 6-1. The moisture
knockout for removal of water vapor and the normal
connections for zeroing and calibrating the analyzer
are shown.
The integrity of the sample line and the conditioning
system is crucial to obtaining a representative
sample and accurate results. Any in-leakage of air
can drastically distort the reading. As shown in
Figure 6-1, the extractive system requires a pump to
draw the sample gas continuously through the
sample line, conditioning system, and analyzer. Most
systems include a small rotameter (flowmeter) which
shows that sample gas is flowing through the system.
This flowmeter is always one of the first items that
should be checked if any problem is suspected
because loss of flow will occur if the pump fails or the
system is plugged. However, even if the flow rate is
correct, the measured gas concentration will not be
correct if there is any problem with in-leakage of air.
Two types of extractive oxygen analyzers,
paramagnetic and polarographic analyzers, are
available in addition to the electrocatalytic type
described previously for in situ analyzers.
Paramagnetic analyzers measure the oxygen
concentration as the strength of a magnetic field in
which oxygen molecules are present. Oxygen
molecules are somewhat unique in displaying a
permanent magnetic moment (paramagnetism),
allowing oxygen concentration to be differentiated
from the stack gas sample. Calibration is performed
by monitoring an inert gas such as nitrogen (zero)
and a gas of known oxygen concentration (span). A
potential problem with this type of analyzer is its
susceptibility to paramagnetic molecules other than
oxygen. Nitrogen oxide and nitrogen dioxide in
particular display a high degree of paramagnetism
(about one-half that of oxygen), but their concen-
tration is usually low compared to that of oxygen.
Polarographic analyzers monitor oxygen
concentration by allowing oxygen to pass through a
selective, semipermeable membrane and react at an
electrode in an oxidation-reduction reaction.
Measuring the current produced by the reaction
indicates the oxygen concentration. Improper
conditioning of the sample gas is a potential problem
with these analyzers, since moisture and particles
will hinder performance of the semipermeable
membrane. Calibration is performed by zeroing with
an oxygen-free gas (nitrogen) and spanning with a
gas of known oxygen concentration (e.g., air).
Furthermore, these monitors contain a liquid
electrolyte that has a limited life span and must be
replaced at regular intervals.
6.2.4 Carbon Monoxide1
Carbon monoxide (CO) analyzers are used to measure
emissions of CO and indicate whether proper
combustion conditions are being maintained. Carbon
monoxide analyzers typically are not part of the
automatic process control system. In general, a
problem with CO levels (i.e., high level) indicates
some other problem in the process and its control
system (e.g., feed rate or temperature).
Location of the CO sampling point may vary,
although it is most commonly in the stack or at the
exit of the combustion chamber. As with oxygen
monitors, CO analyzers can be affected by upstream
in-leakage of air, but usually not as dramatically as
oxygen monitors.
Carbon monoxide analyzers also may be in situ or
extractive, but by far the most common type is
extractive. Both types are based on the principle that
CO will absorb specific wavelengths of light in the
infrared region. They are therefore referred to as
nondispersive infrared (NDIR) analyses.
In situ CO monitors involve an infrared signal that is
transmitted across the duct or stack to a receiver or
.
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J
Y 1
Secondary
Combustion
Chamber f—
\
r
Sample Probe
f
"T — Filter —
N
* <
r1!
Vent
Analyzer
Back Flush
Purge Air
Sample Pump
Drain
Zero
Figure 6-1. Schematic of an extractive monitoring system.
reflector on the opposite side. The change in the
signal, due to absorbence by CO, is processed by the
analyzer system and output as an equivalent CO
concentration. In situ CO monitors are difficult to
calibrate directly because the duct cannot be filled
with a gas of known concentration. Therefore, calib-
ration is performed using an optical filter that can be
moved into the signal path, or a calibration gas that
can be put through a separate cell through which the
infrared signal can be sent.
In most cases, in situ systems are installed at a
location in a stack after pollution control devices
have removed most of the particulate matter because
solid particles also absorb the IR signal. Water vapor
and carbon dioxide also may interfere with the
signal. Because all these interferences can be
removed by a sample conditioning system, the
extractive systems are more commonly used.
Much of the previous discussion on extractive oxygen
systems applies to extractive CO systems. In fact, the
same extraction/conditioning system often is used for
both monitors. Daily checks of the system should be
made by the operator, with weekly maintenance and
calibration by the instrument personnel.
Currently, there are no specific EPA performance
requirements for CO analyzers on hospital
incinerators. However, EPA is developing guidelines
for CO monitors that are required on hazardous
waste incinerators permitted under the Resource
Conservation and Recovery Act (RCRA).
6.2.5 Opacityi,2
Like CO monitors, opacity monitors (or transmis-
someters) are used as indicators of proper operation
rather than as a part of the automatic control system
and almost always, are located in the stack or in the
ducting to the stack downstream of air pollution
control devices. The operating principle for these
transmissometers involves measurement of the
absorbence of a light beam across the stack or duct.
Consequently, they are not applicable in saturated
wet streams downstream of wet scrubbers unless the
gas has been reheated to vaporize any water droplets.
Transmissometers involve a light source directed
across the stack and a detector, or reflector, on the
opposite side. The amount of light absorbed or
scattered is a function of the particles in the light
path, path length (duct diameter), and several other
variables that are considered in the design and
installation. A typical transmissometer is depicted in
Figure 6-2.
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Transceiver
Assembly
Retroreflector
Assembly
Preseparator Air Inlet
Ambient Air
Blower
Figure 6-2. Typical transmissometer installation for measuring opacity. 1
Blower
The lenses on the light source and the detector must
be kept clean of any buildup of dust. For this reason,
the systems often include air blowers and filters that
continuously blow clean air past the lenses to prevent
contact with the gas and dust in the stack (see
Figure 6-2). These blowers and filters should be
checked and cleaned regularly but often are
neglected because of their location on the stack.
The EPA has promulgatedS performance specifica-
tions for opacity monitoring systems that are
required at some types of plants as summarized in
Table 6-2. The calibration procedure for this
performance specification, or normal checking of the
instrument, is based on filters having known light
absorbence that can be placed in the light path. Some
systems automatically calibrate on a regular
schedule that is evident to the operator because it
usually activates a high opacity alarm or a spike on
the opacity recorder.
6.2.6 Charge Rate
The amount of waste charged into the incinerator and
the charging frequency are important operating
variables for an incinerator. Some incinerators may
be equipped with systems that weigh the amount of
each charge and automatically record this weight,
but, at most facilities, the amount charged is based on
operator experience and hopper charge size and is not
closely monitored. Thus, operator experience is relied
upon to avoid overcharging (stuffing), which may
Table 6-2.
Performance Specifications for Opacity
Monitors^
Parameter
Specifications
Calibration error3
Response time
Conditioning periodb
Operational test period13
Zero drift (24-hour)a
Calibration drift (24-hour)a
Data recorder resolution
s 3 percent opacity
<10 seconds
<168 hours
<168 hours
s 2 percent opacity
<2 percent opacity
50.5 percent opacity
a Expressed as the sum of the absolute value of the mean and
the absolute value of the confidence coefficient.
b During the conditioning and operational test periods, the CEMS
must not require any corrective maintenance, repair,
replacement, or adjustment other than that clearly specified as
routine and required in the operation and maintenance
manuals.
decrease temperature, increase emissions (CO), and
cause poor burnout. Undercharging will require
excessive auxiliary fuel usage.
For mechanically charged systems, the charging
frequency is often automatically set by the system at
preselected time intervals. As long as the loader fills
the charging hopper to a certain level within this
time interval, the charging rate will be maintained
constant by the control system, unless the charging
frequency is changed by the operator. Altering the
charging frequency is sometimes necessary when the
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characteristics or composition of the waste has
changed. For example, if a charge contains large
amounts of plastic or other high heating value
material, temperatures may rise rapidly. Thus, the
operator may decide to delay the next charge until
the system can again handle another charge. For
mechanical systems, a recorder can be set up to
indicate the frequency of changes automatically, i.e.,
when a charge is fed to the incinerator.
Having a charging record available is useful for
monitoring and troubleshooting problems and for
determining whether adjustments to the charging
procedures are necessary.
Operator judgment and experience are usually key to
proper control of charging rate; pressure on the
operator to increase the charging rate when the
amount of accumulated waste has increased cannot
be allowed to compromise proper incinerator
operating conditions.
6.2.7 Scrubber Liquor pH
There are two types of pH sensors: immersion (dip-
type) and flow-through. The immersion sensor is
merely inserted into a tank and can be removed for
maintenance and calibration. A flow-through sensor
depends upon a continuous flow in the sample line.
Both have advantages and disadvantages. The
immersion sensor is easier to operate and maintain.
Performance can also be improved by locating the
sensor in a special sampling tank, by using
redundant sensors, and by frequent cleaning and
calibration. The flow-through pH sensor is prone to
wear and abrasion. Redundant sensors are also
desirable for the flow-through type but are not easy to
provide.
The pH measurement probe consists of a pH
measuring electrode, a reference electrode, and a
high input impedance meter. The voltage of the pH
electrode varies with the pH of the solution it is in
contact with, while the reference electrode delivers a
constant voltage. The difference between the two
voltages is measured by the impedance meter,
allowing pH of the system to be monitored. Such a
system is susceptible to signal loss and noise, making
it desirable to locate the preamp, controller, or signal
conditioning unit as close to the electrodes as
possible.
Calibration of a pH monitoring system is performed
through the use of known pH buffer solutions.
Typically, pH 7 is used for calibration, although pH 4
and pH 10 may also be used, depending upon the
expected ranges of scrubber operation. For greatest
accuracy, buffer solutions should be selected that are
close to expected pH values.
The pH monitoring system electrodes may become
fouled over time by dirt, particulate matter, and
bacteria. This fouling can damage or inhibit correct
electrode operation, introducing drift and inaccuracy
in the unit's calibration. To maintain reliability,
electrodes can be boiled in a solvent or cleaned with a
built-in ultrasonic device. Calibration checks usually
are necessary every few weeks.
Control systems for pH meters range in complexity
from simple ON/OFF types to more sophisticated
proportional controllers. In an ON/OFF type, as pH
goes out of range, a valve opens and neutralizing
reagent is added until the pH is corrected.
Proportional controllers add an amount of
neutralizing reagent in proportion to deviation of pH
from the setpoint, which provides much smoother
control. The exact type of controller necessary for the
system is based upon individual needs, but typically
a proportional controller is used.
The pH measurement probe should be placed at a
representative location within the system. Turbulent
flow is needed to ensure a well-mixed solution and to
limit dead time (reaction time of the controller) to
30 seconds or less. Excessive dead time will cause the
system to overshoot the pH setpoint continually as
the controller is unable to react quickly enough and
continues to add neutralizing reagent beyond the
desired pH. Dead time can be reduced by locating the
pH probe a short distance downstream from where
the reagent is added.
6.3 References for Chapter 6
1. U. S. EPA. Continuous Air Pollution Source
Monitoring Systems Handbook, EPA 625/6-79-
005. June 1979.
2. Jahnke, J. APTI Course SI:476A,
Transmissometer Systems Operation and Main-
tenance, an Advanced Course. EPA 450/2-84-004.
U.S. Environmental Protection Agency, Research
Triangle Park, N.C. September 1984. p. 6-9.
3. Code of Federal Regulations, Title 40 Part 60
(40CFR60), Appendix B, Performance Specifica-
tion 1. Specifications and Test Procedures for
Opacity Continuous Emission Monitoring
Systems in Stationary Sources.
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Chapter 7
Operational Problems and Solutions
Potential operational problems associated with
hospital waste incinerators, wet scrubbers, and fabric
filters are described in this chapter. The cause of the
problems and possible solutions to them are discussed
also. Unfortunately, some operational problems are
the result of deficiencies in design, fabrication, and/or
installation of the equipment. Deficiencies in
incinerator design are usually the result of
insufficient information on the waste characteristics
and/or quantity. The assumption is made in the
following paragraphs that the incinerator and its air
pollution control system have been properly
designed, fabricated, and installed. Therefore, no
deficiencies in these areas are addressed. Purchasers
of hospital waste incinerators and air pollution
control systems should consult with reputable
manufacturers. In many cases, these companies can
provide complete turnkey service that includes
evaluation of the purchaser's needs, proper design of
the incinerator based on waste characteristics, proper
design of the air pollution control device based on
expected combustion exhaust gas characteristics,
fabrication of the incinerator with appropriate
quality control, installation and shakedown of the
entire system, and operator training.
7.1 Operational Problems and Solutions
Associated with Hospital Waste
Incinerators
Incinerator operational problems include excessive
stack emissions in the form of white or black smoke,
smoke leakage from the charging door or other
openings, excessive auxiliary fuel usage, and
incomplete burnout of the waste. These operational
problems can be minimized through proper operation
of the incinerator together with an effective
preventive maintenance program.
7.1.1 Excessive Stack Emissions - Controlled-
Air Units'! .2
The proper operation of controlled air incinerators
results in relatively low emission rates. Excessive
emission rates can usually be attributed to one of the
following causes:
1. High setpoint for the secondary burner
temperature is not high enough;
2. Excessive negative draft in the primary chamber;
3. Excessive infiltration air (from charging door);
4. Excessive underfire air in the primary chamber;
5. Operating at too high a primary chamber
temperature;
6. Overcharging;
7. Problem wastes; and
8. Inadequate secondary combustion air.
7.1.1.1 Black Smoke
The appearance of black smoke indicates the
presence of unburned carbonaceous material. Dense
black smoke is caused because incomplete combus-
tion is occurring. Incomplete combustion is due to
insufficient amounts of combustion air for the
quantity of volatiles/soot present and is usually the
result of overcharging the unit, charging of a highly
volatile material, or operating the primary chamber
at too high a temperature. The following steps may
assist in eliminating black smoke:
1. Check/increase secondary chamber combustion
2.
air;
Check/decrease underfire air (if necessary); an
air decrease should result in reducing the
primary chamber operating temperature;
3. Check/increase secondary chamber temperature.
Should these steps fail to eliminate the black smoke,
evaluate the composition of the material remaining
to be charged. Highly combustible materials (i.e.,
rubber, plastics, etc.) that are charged in too great a
proportion to the other refuse will result in a
combustion rate which is too rapid for the incinerator
to handle. These materials may be charged in very
small quantities and in relatively small pieces along
with general refuse. If such materials must be burned
frequently, experimentation as to the quantity that
may be charged along with other materials may be
necessary. Generally, highly combustible materials
must be charged at less than 10 percent by weight of
the total charge. If the waste contains a significant
amount of plastics, then reducing the primary
chamber temperature (to the 540° to 650°C [1000° to
1200°F] range) may assist in reducing emissions.
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Operating the primary chamber at too high a
temperature can cause the plastics to rapidly
volatilize.
5. Inadequate secondary combustion air; and
6. Operating at too high a primary chamber
temperature.
7.1.1.2 White Smoke
The appearance of a steady stream of white smoke
from the stack indicates the presence of small
aerosols in the effluent gas. There are several causes
for _this problem. Either excess air is entering the
incinerator causing entrainment of micron-sized
particles or the secondary chamber is operating at too
low a temperature (i.e., too much air) causing
premature cooling of the combustion gases. The
following steps may assist in eliminating white
smoke:
1. Check to see that the secondary burner is
operating and that secondary chamber
temperature is above 1200°F.
2. Check/decrease underfire air.
3. Decrease secondary air.
4. If all the secondary burner capacity is not being
used, gradually increase the operating rate of the
burner until full capacity is reached.
If adjustment of the combustion parameters fail to
stop the issuance of white smoke, examine the
material to be charged. Possibly the white smoke is
the result of finely divided noncombustible mineral
material present in the waste charge which is being
carried out the stack. Paper sacks that contain
pigments or other metallic oxides, and minerals such
as calcium chloride, generate fine inorganic
particulate which causes white smoke.
The appearance of a white plume (other than a
condensing water vapor plume) a short distance away
from the stack probably indicates that hydrogen
chloride is condensing. Unfortunately, there is no
incinerator adjustment that will solve this problem.
One solution is to reduce the amount of chlorinated
waste incinerated in each load. This option is often
impractical because the waste is mixed together in
packages and, therefore, cannot be separated.
Probably the best solution is to eliminate chlorinated
plastics from use in the hospital or to install an acid
gas scrubbing system.
7.1.2 Excessive Stack Emissions - Multiple-
Chamber Units
Excessive emission rates from multiple-chamber
units can usually be attributed to one of the following
causes:
1. High setpoint for the secondary burner
temperature is not high enough;
2. Excessive draft in the primary chamber;
3. Overcharging;
4. Problem wastes;
7.1.2.1 Black Smoke
The appearance of black smoke indicates the
presence of unburned carbonaceous material. Dense
black smoke is caused by incomplete combustion as a
result of insufficient amounts of combustion air for
the quantity of volatiles/soot present. Black smoke is
often the result of overcharging of the incinerator or
of too large an amount of highly volatile materials in
the waste charged.
Steps which can be taken to eliminate black smoke
include:
1. Decrease the charging rate;
2. Increase the secondary combustion air; and
3. Reduce the percentage of highly volatile
materials in the waste feed; and
4. Reduce the primary chamber operating
temperature.
7.7.2.2 White Smoke
The appearance of a steady stream of white smoke
from the stack indicates the presence of small
aerosols in the effluent gas. Steps which can be taken
to eliminate white smoke include:
1. Check to see that the secondary burner is
operating. If all the secondary burner capacity is
not being used, increase the operating rate of the
burner to full capacity; and
2. Decrease the secondary and/or primary air in
order to increase the secondary temperature.
The appearance of a white plume (other than a
condensing water vapor plume) a short distance away
from the stack probably indicates that hydrogen
chloride is condensing. No incinerator adjustment
will solve this problem.
7.1.3 Leakage of Smoke From Primary
Chamber
The leakage of smoke through charging doors or
other openings indicates that a positive pressure
differential exists in the primary chamber. Positive
pressure can be caused by excessive combustion air,
by excessive charging of a highly volatile material,
by too high a primary chamber operating
temperature, or by too much hot ash being discharged
to a wet sump all at one time. The following steps
may help eliminate leakage of smoke:
1. Check/decrease underfire air (controlled-air
units);
2. Decrease feed rate;
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3. Adjust ash discharge ram cycle; and
4. Adjust draft control (damper or ID fan) setpoints.
If a positive pressure persists, operation of the
incinerator draft control system (draft monitor,
barometric damper, induced draft fan) should be
checked.
7.1.4 Excessive Auxiliary Fuel Usage1
For controlled-air units, improper underfire air
distribution, excessive air infiltration, or improper
setting of the underfire and secondary combustion air
levels can result in excessive fuel usage. If the
underfire air distribution in the primary chamber is
incorrect or if there is excessive air inleakage, even a
substoichiometric air/fuel mixture, which generates
combustible gases for maintaining the secondary
combustion chamber temperature, will not exist in
the primary chamber. Instead, the waste bed will
completely burn in some areas and not burn at all in
other areas. The gases conveyed to the secondary
chamber will already be fully oxidized, and auxiliary
fuel will be necessary to maintain the secondary
chamber temperature.2
Another cause of excessive auxiliary fuel usage is
that the incinerator is not consistently charged. If the
incinerator is not receiving enough heat input in the
form of waste to maintain its temperature setpoints,
then the unit will supply its own heat in the form of
auxiliary fuel, i.e., natural gas or oil. Consistent
charging of waste at regular timed intervals at a rate
near 100 percent of the incinerator's design capacity
will reduce the amount of auxiliary fuel required and
will enhance the incinerator's performance. It is best
to charge waste in batches which are 10 to 15 percent
of rated capacity. Therefore, if a unit is rated at 500
pounds of waste per hour, then the unit should be
charged at 6-minute intervals with charges of
approximately 50 pounds each. The following actions
may assist in reducing auxiliary fuel usage:
1. Charge waste at regularly timed intervals at a
rate consistent with 100 percent of the design
(thermal input) capacity;
2. Check/reduce secondary combustion air levels;
3. Check primary combustion air levels and
distribution;
4. Check charging door seals and other seals for air
leakage (controlled-air units); and
5. Check the fuel trains and burners for fuel
leakage.
f
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7.1.5 Incomplete Burnout - Poor Ash Quality
The causes of incomplete burnout include primary
burner malfunction, insufficient primary chamber
combustion air or poor underfire air distribution,
overcharging the incinerator with waste, and
charging too much wet waste. As with other
operational problems, the causes of incomplete
burnout can be minimized by proper operation of the
incinerator and an effective preventive maintenance
program.
7.1.5.1 Primary Burner Malfunction.
Primary burner malfunction causes incomplete
burnout because the primary chamber temperature
will not be maintained and because the flame is
insufficient to ignite the waste and keep it burning.
Primary burner malfunction may arise due to burner
power loss, burner pluggage, failure of the flame
safeguard, or leaking fuel trains. Each of these
problems can be eliminated by preventive
maintenance performed on a regular basis as
outlined in Section 5.1. If the primary burner should
fail, the following steps are recommended:
1. Check power supply to the primary burner;
2. If power is available, check burners for pluggage
and clean as required;
3. Check the operation of the burner safeguard
system as outlined in the owner's manual; and
4. Check fuel trains and burner assembly for fuel
leakage and repair as required.
7.7.5.2 Insufficient Underfire Air (Controlled-Air
Units)
In a controlled-air incinerator, insufficient underfire
air can cause the combustion process to stop
completely. Primary causes of the lack of combustion
air are an improper underfire air setting, clinker
buildup around the underfire air ports, and air ports
clogged with ash or slag from previous charges.
Clinker buildup around primary chamber air ports is
usually the result of too much air, resulting in local
hot spots that cause the ash to soften, agglomerate,
and then harden as clinker during the cool down
cycle. Some manufacturers utilize steam injection
into the ash bed to facilitate burnout of fixed carbon
in the ash bed and at the same time help prevent hot
spots and clinker formation.3 Maintaining proper air
levels and air distribution through all underfire air
ports is important. Proper operation and preventive
maintenance can prevent these problems. If poor
burnout occurs, the following items should be
checked:
1. Check underfire air ports for ash pluggage and
rod out as required;
2. Check around the underfire air ports for clinker
buildup and clean as required; and
3. Check underfire air setting and adjust to the
proper setting (increase) as required.
7.1.5.3 Waste Charging
Charging of waste into the incinerator should be
performed as described in the vendor's literature.
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Two conditions to be avoided that can cause
incomplete burnout are overcharging the incinerator
and charging too much wet waste as part of a charge.
These problems may be encountered with the so
called "stuff-and-burn" type batch units operating on
a timed-cycle. When the incinerator is overcharged
and tightly packed with waste, the combustion air
cannot circulate freely through the compacted waste,
thereby inhibiting combustion. When incinerating
wet waste, the waste must first be dried by
evaporating the moisture in the waste; if the batch
contains a large fraction of wet waste, insufficient
time for complete combustion may be a problem. Both
of these conditions should be prevented by not
overcharging the incinerator or by increasing the
burnout cycle time.
For intermittent-duty, controlled-air, and multiple-
chamber incinerators, reduction of the charge rate
may improve the ash burnout quality. The burndown
period also can be increased in an attempt to improve
burnout.
For continuous-duty incinerators, reducing the waste
feed charge rate will increase the residence time of
the solids in the primary chamber and may help to
improve ash burnout quality.
7.2 Operational Problems and Solutions
Associated with Wet Scrubbers4,5
The performance of a wet scrubber system is
dependent on the key operating parameters discussed
in Chapter 4, on effective preventive maintenance,
and on the integrity of the scrubber components.
Premature failure of these components will lead to
increased costs, extended downtime of the system,
and/or operation in temporary noncompliance. The
main problems experienced by wet scrubber systems
include corrosion, scaling, and erosion. An effective
preventive maintenance program can minimize these
problems by correcting malfunctions, observing
trends in maintenance activities and making
modifications to prolong equipment life, and
correcting minor problems before they become costly,
time consuming repairs. The following sections
describe the problems, identify affected equipment,
and recommend solutions.
7.2.1 Corrosion
Corrosion of wet scrubber components is the result of
absorption of sulfur dioxide, sulfur trioxide, or
hydrochloric acid gas from the dirty gas to the
scrubbing liquid. The resulting acidic conditions
cause corrosion of the wet scrubber system if it is
made of carbon steel. Wet scrubbers controlling
hospital waste incinerators are likely to experience
corrosion from hydrochloric acid as a result of
combustion of chlorinated plastics. The types of
equipment that are likely to be subject to corrosion
include scrubbers, absorbers, fans, dampers,
ductwork, and exhaust stack. If the scrubbing liquid
is recirculated (i.e., the system does not use once-
through water), the importance of maintaining the
pH of the scrubbing liquid above the level at which
carbon steel is attacked cannot be stressed enough.
An appropriate pH of 6 or greater is usually
maintained through additions of alkaline reagents
such as soda ash, lime, or limestone. The operation of
the pH monitor used to control the rate of alkaline
addition should be checked on at least a daily basis to
minimize short-term low pH excursions.
Corrosion also can be a problem in the pumps, pipes,
valves, tanks, and feed preparation areas in slurry
service. Slurry tanks and associated feed preparation
equipment should be checked daily for leaks.
Equipment temporarily removed from slurry service
should be thoroughly flushed. Typically, slurry
pumps are disassembled at least annually to verify
lining integrity and to detect wear and corrosion or
other signs of potential failure. Bearings and seals
are checked but not necessarily replaced. Pipelines
also must be periodically disassembled or tested in
other ways (e.g., hand-held nuclear and ultrasonic
devices) both for solids deposition and for wear.
Valves must be serviced routinely, especially control
valves.
The solution to corrosion problems in wet scrubber
systems is to maintain the pH of the scrubbing liquid
by the following:
1. Check the proper operation of the pH monitor
that controls alkaline additions daily;
2. Check the alkaline addition system for leaks
daily and repair as required; and
3. Perform regular preventive maintenance on
pumps, pipes, valves, and tanks to minimize and
correct corrosion problems.
7.2.2 Scaling
Scaling is a common problem that arises in scrubber
components in wet service such as mist eliminators,
fans, dampers, and ductwork. Scale deposits in mist
eliminators can cause nonuniform flow and plugging.
Fans in wet service can develop vibrations as a result
of scale deposition on the fan blades. Dampers may
become stuck in place.
The best solution to scaling and/or plugging is an
effective preventive maintenance program.
Observation of the differential pressure across the
mist eliminator can alert personnel to scaling
problems. Periodic cleaning of equipment can
minimize scaling problems.
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7.2.3 Erosion
Erosion is a common problem associated with
scrubber components in dry service such as fans,
dampers, and ductwork. Erosion is also a problem for
scrubber spray nozzles due to the suspended solids
recovered in the scrubbing liquid from the dirty gas.
For fans in dry service, fan vibration may be caused
by erosion of fan blades. In such cases, the fan blades
may have to be replaced. Erosion may cause holes in
the ductwork. These holes must be repaired to
prevent air in-leakage.
Scrubber spray nozzles are extremely susceptible to
erosion and pluggage problems due to the high
velocities of the liquid stream and the suspended
solids within the stream. Pluggage and erosion in the
spray nozzles can be determined by observing the
spray angle. If the spray angle has become enlarged,
then the nozzle orifice probably has been enlarged by
erosion. Conversely, if the spray angle has decreased
or if a distinct spray pattern is no longer achieved,
then the nozzle orifice probably is partially or
completely plugged. The potential for erosion of the
scrubber components is directly related to the percent
suspended solids. The greater the recirculation flow
rate relative to the makeup and purge (blowdown)
flow rates, the greater the potential for buildup of
solids. Infrequent purging also can cause high solids
buildup.
Erosion in dry service components is to be expected.
Effective preventive maintenance and equipment
replacement when required are the best solutions.
Erosion in scrubber spray nozzles and other scrubber
components may be prevented by proper operation
and maintenance as follows:
1. Rod out spray nozzles on a regular basis to
prevent plugging;
2. Purge the system frequently to prevent solids
buildup; and
3. Adjust the recirculation rate as appropriate.
7.3 Operational Problems and Solutions
Associated with Fabric Filters^
The two main operational problems associated with
fabric filters are high opacity and high pressure drop.
Well designed, operated, and maintained fabric
filters will generally have a very low opacity
(between 0 to 5 percent), and the pressure drop will
fall within a general operating range for the
particular fabric filter type (5 to 9 in. w.c. for pulse-jet
fabric filters). Opacity and/or pressure drop
deviations from the baseline values are good
indicators of fabric filter performance deterioration.
Higher or lower than normal inlet temperatures can
cause opacity and pressure drop problems. The inlet
temperature should be monitored continuously. The
following sections describe each operational problem,
the cause, and possible solutions.
7.3.1 Opacity
Large fabric filter installations may have an opacity
monitor coupled with a strip-chart recorder.
Typically, smaller installations have no opacity
monitor and must rely on visible emission
observations. In any event, opacity measurements
are useful in determining trends in the performance
of the fabric filter. Typically, the opacity plume of a
properly operated and maintained filter will be very
low, except when a condensable plume is present
(even in this case the condensing plume should be
detached). In general, high opacity is a good indicator
of fabric failure. A consistently elevated opacity level
relative to the baseline level is an indication of major
leaks and tears in the filter bags. A puffing,
intermittent opacity observed after cleaning that is
higher than the baseline opacity level is a good
indication of pinhole leaks in the filter bags. The
factors that cause fabric failure include improper
filter bag installation, high temperature, chemical
degradation, and bag abrasion.
Improper installation of filter bags can result in leaks
around seals, improper bag tensioning, and damage
to the bags. Lack of training of maintenance
personnel in filter bag replacement and poor access to
the fabric filter housing are contributing factors to
improper installation.
High temperatures are the result of process
malfunction(s) upstream of the fabric filter.
Therefore, in the fabric filter design phase, a fabric
must be chosen on the basis of expected temperature
range with an adequate margin for error. High
temperature breaks the polymer chains in most
commercially available fabrics causing loss of
strength and reduced bag life. High temperature
attacks the finish on fiberglass bags causing
increased bag abrasions. In general, high
temperatures shorten bag life considerably. High
enough temperatures can cause filter bags to ignite.
Some installations may have an alarm system to
warn of high temperature excursions and a bypass
system to prevent damage to the filter bags. Such a
system may be appropriate should the cooling system
prior to the fabric filter fail. Sparks may also be a
problem with a combustion source such as a hospital
waste incinerator. A spark arrestor can help
eliminate the potential for sparks in the fabric filter.
Chemical degradation of the filter bags occurs as the
result of acid gas condensation. In addition to
specification of a temperature range for filter bags, a
chemical resistance rating must also be specified for
the fabric. In the case of hospital waste incinerators,
fabric should have good chemical resistance to
hydrochloric acid due to combustion of chlorinated
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plastics. Additionally, operation at a high enough
temperature to prevent acid gas condensation will
reduce chemical degradation.
Filter bag abrasion can be caused by contact between
a bag and another surface (e.g., another bag or the
walls of the fabric filter) or by the impact of higher-
than-average gas volumes and particulate loading on
the bags. Bag abrasion can also be a problem when
the fabric filter experiences a high pressure drop.
Usually, when bag abrasion is a problem, the
greatest abrasion occurs within 18 to 24 inches from
the bottom of the bags. A blast plate or diffuser will
redirect the larger particles away from the bags and
reduce bag abrasion.
While the only solution to the problem of high opacity
is replacement of the failed filter bags, it can be
prevented by the following:
1. Train maintenance personnel in the proper
installation of replacement filter bags;
2. Design fabric filters for ease in bag replacement;
3. Choose filter bag fabric with an appropriate
operating temperature range and chemical
resistance rating;
4. Blast plates or diffusers are recommended for
those facilities experiencing severe bag abrasion;
5. Monitor inlet temperature - an alarm and bypass
system may be installed to prevent filter bag
damage from high temperature excursions;
6. Install a spark arrestor to prevent sparks and fire
inside the fabric filter; and
7. Install a bypass/alarm system to prevent damage
to filter bags during high temperature
excursions.
7.3.2 Pressure Drop
The pressure drop across a fabric filter is an
important indicator of performance. An increase in
pressure drop indicates greater resistance to flow and
can be a symptom of a high air-to-cloth ratio or an
increase in dust cake thickness due to condensation
or cleaning system failure.
The air-to-cloth ratio is a design parameter that is
determined during the fabric filter design phase. In
the pulse-jet baghouses used to control emissions
from hospital waste incinerators, the air-to-cloth
ratio and therefore pressure drop are relatively high
(5 to 9 in. w.c.). Bag abrasion and/or fugitive
emissions may result if the pressure drop is too high
(10tol4in.w.c.).
Condensation of moisture on filter bags is caused by
temperatures in the fabric filter below the dewpoint.
"Mudding" or blinding of the bags increases the
resistance to flow and occurs because the cleaning
system cannot remove the dust. Condensation can be
prevented by preheating the fabric filter during the
startup operation and by purging moist gases from
the unit prior to shutdown. During operation it is
critical to maintain the operating temperature above
the dewpoint of the gas stream at all times.
Cleaning system failures in pulse-jet systems are
usually the result of worn or undersized compressors,
and failed solenoids and/or timers. Compressor
problems are indicated by a low compressed-air
pressure. Because of the low pressure the system
cannot clean the bags properly and an increased
pressure drop across the fabric filter results due to
dust cake buildup. Compressor capacity may be a
problem and should be checked against the needs of
other systems the compressor serves. Routine
preventive maintenance can prevent premature
failure of the compressor and can prevent worn
compressor seals from passing oil into the fabric filter
and blinding the filter bags. Both reduced
compressed-air pressure and bag blinding can cause
an increase in pressure drop.
Failure of solenoids and or timers can prevent the
filter bags from being cleaned at all. These systems
require clean, dry mountings to operate properly.
Solenoid failures affect the only row of filter bags
that the solenoid services, while timer failures tend
to affect most, if not all, of the fabric filter system.
An additional problem is that associated with the
pulse pipe that discharges the compressed air into the
bags. Pulse pipes may sometimes be damaged by the
force of the compressed air causing ineffective
cleaning and pressure drop increase, improper pipe
alignment that may blow holes in the filter bags, or a
loose pipe that may damage the interior of the fabric
filter. The sound of a loose pulse pipe is unmistakable
because it moves whenever compressed air is fired
into the pipe.
An increase in pressure drop may indicate operation
and maintenance problems that may be corrected.
However, blinded bags resulting from condensation
or the accidental discharge of compressor oil into the
fabric filter will likely have to be replaced. An
increase in pressure drop can be prevented by the
following:
1. Preheat the fabric filter prior to process
operation;
2. Purge the fabric filter of moist air prior to
shutdown;
3. Always maintain the temperature of the gas
entering the fabric filter above its dewpoint;
4. Perform preventive maintenance of the
compressor system and solenoid/timer system;
5. Make necessary repairs to pulse pipes as
required; and
6. Adjust the cleaning cycle to shorten the cycle
between cleanings.
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7.4 References for Chapter 7
1. McRee, R. Operation and Maintenance of Con-
trolled-Air Incinerators. Ecolaire Environmental
Control Products. Undated.
2. Letter from K. Wright, John Zink Company to J.
Eddinger, U. S. EPA. January 25,1989.
3. Personal conversation between R. Neulicht, MRI,
and G. Swan, Ecolaire Combustion Products and
J. Kidd, Cleaver Brooks. February 22,1989.
4. Joseph, J., and D. Beachler. APTI Course
SI:412C, Wet Scrubber Plan Review - Self In-
structional Guidebook. EPA-450/2-82-020. U. S.
Environmental Protection Agency. March 1984.
5. U. S. Environmental Protection Agency. Wet
Scrubber Inspection and Evaluation Manual.
EPA-340/1-83-022. (NTIS PB85-149375). Sep-
tember 1983.
6. U. S. Environmental Protection Agency.
Operation and Maintenance Manual for Fabric
Filters, EPA/625/1-86/020. June 1986.
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Chapter 8
Recordkeeping
Recordkeeping is an integral part of an equipment
operation and maintenance (O&M) program. The
purpose of recordkeeping is to document major O&M
events and to collect historical data on key operating
parameters. The objective of recordkeeping is to
prevent premature failure of equipment, increase the
life of the equipment, and to minimize emissions.
Recordkeeping allows facility and regulatory agency
personnel to track performance, to evaluate trends, to
identify potential problem areas, and to determine
appropriate solutions. The magnitude and scope of
recordkeeping activities will depend on a
combination of factors, including personnel
availability and training, size and sophistication of
the equipment, and the level of maintenance
required. Only records of key performance
parameters and activities should be maintained to
avoid accumulation of unnecessary information.
The following information should be readily
available to O&M personnel: (1) the manufacturer's
equipment specification and instruction manuals,
(2) compliance emission tests, (3) operating permits,
(4) operating logs, and (5) maintenance activities log.
The operating history provided by this information is
useful in evaluating current and future performance,
maintenance trends, and operating characteristics. A
spare parts inventory also should be maintained with
periodic updates so that parts can be obtained and
installed in a timely manner. A recommended spare
parts inventory typically is supplied with the
equipment manufacturers' O&M manuals. Whether
a facility maintains an extensive or minimal parts
inventory is dependent on the available space for
parts storage and whether the facility has a service
agreement with the equipment vendor.
8.1 Manufacturer's Specifications and
Literature
The manufacturer's information is the foundation of
a recordkeeping program. A copy of the information
and literature supplied by the incinerator and air
pollution control device manufacturers should be
easily accessible for use and review by personnel
responsible for O&M. This literature includes the
manufacturer's design specifications, performance
guarantee, piping and instrumentation diagram,
process flowsheet, material balance information for
normal and maximum design conditions, and an
instruction manual for O&M.
The O&M manuals discuss the theory and design of
the equipment; operating procedures for startup,
shutdown, normal operation, and troubleshooting;
maintenance procedures; and a recommended spare
parts list. The guarantee or warranty provisions in
the contract are conditioned on the proper care and
treatment of the equipment as specified in the O&M
instruction manual.
8.2 Compliance Emission Test Records*
Records of initial compliance test results and records
of incinerator process and air pollution control device
operating conditions during the compliance test form
the baseline information against which subsequent
operating data are compared. The baseline period
normally occurs soon after shakedown of new
equipment and represents conditions when the
control device is operating in compliance with
applicable regulations. Comprehensive baseline test
results and equipment operating data establish the
relationship between operating conditions and
emission levels for a specific process/air pollution
control device combination. Once this relationship
exists, operating personnel and regulatory groups
can use subsequent measurements of the same
parameters to compare against baseline conditions
and thereby identify excursions from acceptable
performance.
8.3 Operating Records*-3
There is a practical limit to the operating parameters
that should be routinely checked and to the frequency
with which the data are logged. Table 8-1 presents a
list of recommended operating parameters for
incinerators, wet scrubbers, fabric filters, and
continuous emission monitors that should be
monitored and logged on a regular basis. However,
the decision on which operating parameters will be
monitored and recorded and with what frequency is
largely site-specific. This decision depends on the size
and complexity of the equipment, the number of
hours per day that it operates, and the availability of
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personnel. The greater the frequency of data
gathering, the more sensitive the operators will be to
equipment operational problems. However, as the
amount of data increases, the effort required to
collect and manipulate the data also increases. The
optimal frequency of data gathering may be every
4 hours (twice per shift). If sudden and dramatic
changes in performance occur, if the source is highly
variable (such as an incinerator), or if the air
pollution control device operation is extremely
sensitive, shorter monitoring intervals, such as once
per hour, are required.
Table 8-1. Recommended Operating Parameters that
Should be Included in Operating Logs for
Incinerators, Wet Scrubbers, Fabric Filters, and
Continuous Emission Monitors
A. Incinerator Operating Parameters
1. Charging rate/frequency
2. Pnmary combustion chamber temperature
3. Secondary combustion chamber temperature
4. Incinerator draft
5. Exhaust gas O2 concentration
6. Auxiliary fuel feed rate
B. Wet Scrubber Operating Parameters
i. Gas temperature, inlet and outlet
2, Static pressure drop, total
3. Static pressure drop, mist eliminator
4. Liquor feed rate
5. Liquor pH
6, Water makeup rate
7. Fan(s) current, rpm
8. Nozzle pressure
9. Solids content of liquor
C. Fabric Fiitor Operating Parameters
1. Gas temperature, inlet and outlet
2. Static pressure drop, total
3. Cleaning cycle frequency
D. Continuous Emission Monitors
1. CO emission concentration, ppm
2. Opacity of emissions, percent
In addition to the numerical values of the operating
parameters, a checklist should be included to confirm
operation of fans, limit switches, burners, ram
feeders, pumps, nozzles, and the other general
physical considerations that can adversely influence
performance.
8.4 Maintenance Records*
Maintenance records provide an operating history of
equipment. They can indicate what equipment has
failed, where, when, and how often; what kinds of
problems are typical; what actions were taken; and,
over time, the efficacy of the remedial actions. These
records can be used in conjunction with a spare parts
inventory to maintain and update a current list of
available parts and the costs of these parts.
The work order system is one way to keep useful
maintenance records. This system would be
administered by the engineer in charge of the proper
O&M of the incinerator. Whenever maintenance is
required, the engineer will issue a work order to the
appropriate department (e.g., maintenance) detailing
the work to be performed. Additionally, the work
order form should include the following information
to assure both good communication between
departments and prompt completion of the work: a
work order number, the date of the work request, the
name of the person requesting the work and his/her
department, the department to which the work order
is sent, any special equipment required, any special
precautions that should be taken, a signature block
for the person performing the work, and the date of
completion of the work. The format of the work order
form is variable as long as it fits the needs of the
particular hospital. The engineer, upon return of the
completed work order, may check the satisfactory
performance of the work requested. When properly
designed and used, this system provides information
on the suspected problem, the problem actually
found, the corrective action taken, time and parts
required, and any additional pertinent information.
Additionally, such a system can also be used to
ensure that the monthly, quarterly, and semiannual
routine preventive maintenance is performed. In
developing a work order maintenance log system,
facility personnel should consult both the schedules
in Chapter 5.0 and the manufacturer's recommended
maintenance schedule. The work order system may
involve the use of triplicate carbon forms or it may be
computerized. Triplicate carbons provide a copy of the
work order to the engineer (requestor), the
department performing the work, and the hospital
administration. These copies would become part of
the O&M log. Daily and weekly preventive
maintenance activities should be included as part of
the routine operation of the incinerator and
associated air pollution control device. Preventive
maintenance activities and schedules for
incinerators, wet scrubbers, and fabric filters are
discussed more fully in Chapter 5.0.
Another approach is to use a log book in which a
summary of maintenance activities is recorded.
Although not as flexible as a work order system (e.g.,
copies of individual work orders can be sent to
appropriate departments), it does provide a
centralized record and is probably better suited for
the small facility.
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8.4.1 Retrieval of Records2
A computerized storage and retrieval system is ideal
for recordkeeping. A computer can manipulate and
retrieve data in a variety of forms and also may be
useful in identifying trends. A computerized system
is not for everyone, however. The larger the data set
to be handled, the more likely it is that a computer
can help to analyze and sort data. Sorts might include
a history of maintenance activities and identification
of recurring operating problems that might indicate a
need for more frequent maintenance or inspections.
For a small source that presents few problems and
that has a manageable set of operating parameters to
be monitored, a computer system may not be cost
effective.
Retention time is also a site-specific variable. If
records are maintained only to meet a regulatory
requirement and are not used or evaluated, they can
probably be disposed of at the end of the statutory
limitation (typically 2 years). It can be argued that
these records should not be destroyed because if the
equipment fails prematurely, the data preserved in
the records could be used to troubleshoot the problem.
In some cases, records going back 10 to 12 years have
been kept to track the performance, cost, and system
response to various situations and the most effective
ways to accomplish remedial actions. These records
serve as a learning tool to optimize performance and
minimize emissions, which is the underlying purpose
of recordkeeping. Some of these records may be kept
throughout the life of the equipment. After several
years, however, summaries of O&M activities are
more desirable than the actual records themselves.
These can be created concurrently with the daily
O&M records for future use. If needed, actual data
can then be retrieved for further evaluation.
8.5 References for Chapter 8
1. U. S. Environmental Protection Agency. Wet
Scrubber Inspection and Evaluation Manual.
EPA-340/1-83-022. (NTIS PB85-149375).
September 1983.
2. U. S. Environmental Protection Agency.
Operation and Maintenance Manual for
Electrostatic Precipitators. EPA 625/1-85/017.
September 1985.
3. U. S. Environmental Protection Agency.
Operation and Maintenance Manual for Fabric
Filters. EPA/625/1-86/020. June 1986.
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Chapter 9
Safety Guidelines
This section provides general safety guidelines in the
O&M of hospital incinerators and associated air
pollution control devices. The information presented
here is intended to supplement safety information
provided by manufacturers and/or safety standards
established by individual hospitals. There are two
primary concerns with respect to the operator's
safety. The first concern is the potential for exposure
to pathogens during handling of infectious waste.
The second concern is the prevention of injury due to
the general hazards normally associated with
industrial equipment such as incinerators and air
pollution control devices.
9.1 Prevention of Infection During Waste
Handlingi
The major risk of infection to an operator is from
puncture wounds caused by contaminated objects
such as surgical instruments, needles, and broken
glass. Most, if not all, hospitals are using special
rigid, secure containers to prevent injury from
infected sharp objects. Nonetheless, to prevent
possible injury which could result in infection, safety
rules which should be followed include:
1. Minimize the handling of red bag waste;
2. Keep red bag wastes in a secure location prior to
incineration;
3. Never open red bags to inspect the contents;
4. Follow procedures which maintain the integrity
of the red bag waste. If breakage/spillage of red
bag waste occurs, then handling procedures must
be changed. The use of double bags, stronger
bags, rigid carts, or cardboard containers to
contain the red bag waste should be instituted.
The hospital administration should be contacted
if continued problems with bag integrity occur;
5. Wear thick rubber gloves when handling red bag
wastes;
6. Wear hard-soled rubber shoes when handling
waste and working in the area around the
incinerator; and
7. Wear safety glasses.
9.2 Equipment Safety Procedures*^
General safety procedures to prevent injury when
working around the incinerator and air pollution
control equipment include:
1. Containers of flammable liquids or explosives
should never be fed into the incinerator;
2. The incinerator charging door should not be
opened if the incinerator is under positive
pressure, if the ignition burner is on, or if other
conditions recommended by the manufacturer
are not met (i.e., minimum time between
charges). Always exercise caution when opening
the charging door. Wearing safety glasses, the
operator should stand behind the door, open the
door several inches and pause, then open the door
fully;
3. Never open cleanout ports to view into the
incinerator during operation;
4. Never enter the chambers of mechanical ram
feeders, ash conveyors, or the incinerator without
first turning off all power sources and assuring
that the units are "locked out." If a chamber must
be entered, after locking the unit out, make sure
a second person is standing by;
5. Observe caution around all moving belts,
hydraulic cylinders, and doors;
6. For systems requiring manual ash removal,
exercise extreme caution when removing the ash.
Do not enter the incineration chamber to remove
the ash. Instead, use the mechanical ash ram or
conveyor (if available) or rakes or shovels with
handles of sufficient length to reach the back of
the incinerator ash compartment. When using
rakes or shovels, use caution so as not to damage
the incinerator refractory;
7. Wear proper personal safety equipment when
operating the incinerator and removing/handling
the ash to prevent burns, cuts, punctures, or eye
injury. Proper safety equipment includes gloves,
hard-soled rubber shoes, and safety glasses;
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8. Avoid direct contact with the hot surfaces of the
incinerator chamber, heat recovery equipment,
ductwork, and stack;
9. The scrubber liquor from wet scrubbers likely
will be caustic. Avoid contact with the liquid and
wear eye protection near and around the
scrubber;
10. Venturi scrubbers operate at high positive
pressures. Be cautious of leaks in the scrubber
vessel, ductwork, or piping;
11. Entry into fabric filters other than for
maintenance should be avoided. Only persons
who have been properly trained should enter
fabric filters, and they should be alert to the
following hazards and necessary precautions.
(a) Before personnel enter the unit, the filter
bags should be thoroughly cleaned and dust
dislodged and discharged from the hopper by
mechanical vibration to prevent fugitive
emissions and dust inhalation. Hopper doors
should only be opened when the unit
(including fan and hopper evacuation
systems such as screws and drag chains) has
been shut down.
(b) Oxygen deficiency as is common in
incinerator combustion gases makes fabric
filter entry especially dangerous. Purging of
the unit does not always replace the exhaust
gases with ambient air. Inspectors should
know the dangers associated with oxygen
deficiency.
(c) Explosion is possible in a confined space such
as a fabric filter. Ventilation/purging prior to
entry is recommended.
(d) Exposure to toxic chemicals in the collected
dust is another danger of fabric filter entry. A
quantitative assessment of the expected
compounds in the dust and threshold dose
levels should be made prior to entry. Proper
personnel protection equipment such as
respirators should be worn.
12. Eye protection, hearing protection, long sleeved
shirts, and gloves should be worn during fabric
filter inspection. Inspectors also should be
cognizant of heat/thermal stress associated with
the length of time required for inspection/repairs.
Because of the dusty, humid conditions and
limited access, thermal effects may be severe.
9.3 Fire Safety^
Fire safety is particularly important when working
around incinerators due to the nature of the high
temperature combustion process. The areas in which
the operator should exercise caution include: waste
storage, waste charging to the incinerator, and ash
removal.
The location of the waste storage area relative to the
incinerator is an important consideration regarding
fire safety. Waste should be stored away from the
incinerator and ash storage areas such that access to
the unit is not blocked and premature ignition of the
stored waste from stray sparks or burning flyash is
eliminated.
Charging of the waste into the incinerator has a
particularly high fire hazard associated with it
because of the potential exposure of high temperature
and flaming conditions to the operator and
surrounding building areas. When charging
manually charged incinerators, the operator should
exercise extreme caution. A fire safety feature that
should be included on manually loaded incinerators
is an interlock system that shuts off the primary
chamber burner(s) when the charging door is opened-
Some systems shut off the underfire air when the
charge door opens. The charge door should operate
freely and be free of any sharp edges or projections
that could catch or hang up waste materials arid
cause spillage. When charging automatically
charged incinerators, a potential fire hazard exists
when waste material (particularly plastics) adheres
to the hot charging ram. If these materials do not
drop from the ram during its loading cycle, they may
ignite and be carried back into the charging hopper
where the remaining waste will become ignited.
Automatic loading systems usually have automatic
safety systems activated by a flame scanner that
spray water to quench the hot ram face and any
flames and/or automatic systems that reinitiate a
loading sequence to charge the burning waste into
the incinerator.
Handling of ash is another area where the operator
should exercise extreme caution regarding fire
safety. Manual ash removal involves removing ash
with rakes and shovels followed by a water quench.
Because the ash may contain hot spots, the operator
should wear proper personal safety equipment
including gloves, hard-soled rubber shoes, and safety
glasses. Water quenching should occur only after
removing the ash from the incinerator to prevent
damage to the incinerator refractory. Automatic ash
removal systems quench the ash as it is removed from
the incinerator and dump the quenched ash into ash
carts. Quenched ash should be stored in a fire rated
area in a location isolated from stored waste
materials.
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9.4 References for Chapter 9
1. U. S. Environmental Protection Agency. EPA
Guide for Infectious Waste Management.
EPA/530-SW-86-014. (NTIS PB86-199130). U. S.
EPA Office of Solid Waste. May 1986.
2. U. S. Environmental Protection Agency.
Operation and Maintenance Manual for Fabric
Filters. EPA/625/1-86/020. June 1986.
3. U. S. Environmental Protection Agency. Wet
Scrubber Inspection and Evaluation Manual.
EPA 340/1-83-022. (NTIS PB85-149375).
September 1983.
4. Richards Engineering. Air Pollution Source Field
Inspection Notebook; Revision 2. Prepared for the
U. S. Environmental Protection Agency, Air
Pollution Training Institute. June 1988.
5. U. S. Environmental Protection Agency. Air
Pollution Source Inspection Safety Procedures:
Student Manual, EPA-340/l-85-002a.
September 1985.
6. Doucet, L. Fire Protection Handbook, National
Fire Protection Association Waste Handling
Systems and Equipment. Chapter 12. 1986
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Chapter 10
Glossary
ABSORPTIONS
ACID GASES*
ACTUAL CUBIC
FEET PER MINUTE
(acfm)3
AERODYNAMIC
DIAMETER*
The process by which gas molecules are transferred to (dissolved in) a liquid phase.
Corrosive gases formed during combustion of chlorinated or halogenated
compounds and sulfur-containing compounds, e.g., hydrogen chloride (HC1), or
sulfur oxides (SOX).
A gas flow rate expressed with respect to temperature and pressure conditions.
The diameter of a unit density sphere having the same aerodynamic properties as
an actual particle. It is related to the physical diameter according to the equation:
where:
ipa
p
C
= aerodynamic diameter
= physical diameter
= particle density
= Cunningham correction factor
Air containing no water vapor.
The noncombustible inorganic residue remaining after the ignition of combustible
substances.
The reduction of liquid to a fine spray.
The loading, or accumulation, of filter cake on the bag fabric to the point where
capacity air flow rate is diminished.
BAROMETRIC SEAL* A column of liquid used to hydraulically seal a scrubber, or any component thereof,
from the atmosphere or any other part of the system.
AIR, DRY*
ASH5
ATOMIZATION*
BAG BLINDING
BOTTOM ASH5
BURN RATE*
The solid material that remains on a hearth or falls through the grate after
incineration is completed.
The total quantity of waste that is burned per unit of time that is usually
expressed in pounds of waste per hour.
BURNDOWN PERIOD The period of time in an incinerator's operating cycle during which no additional
waste is charged to the incinerator and the primary combustion chamber
temperature is maintained above a minimum temperature (using auxiliary
burners as necessary) to facilitate the solid phase combustion of the waste bed.
GEMS
Continuous Emission Monitoring System (CEMS). The total equipment required
for the determination of opacity or an emission rate for stack gases.
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CHARGE RATE4
CLINKERS5
COCURRENTOR
CONCURRENT4
COLLECTION
EFFICIENCY!
COMBUSTION*
CONDENSATION*
CONTROLLED AIR
INCINERATIONS
COOLDOWN PERIOD
CROSSFLOW4
CYCLONE4
DAMPER2
DEHUMIDIFYl
DEMISTER4
DENSITY2
DIFFUSION
(AEROSOL)4
DRAFT!
DUST2
DUST LOADING2
Quantity of waste material loaded into an incinerator over a unit of time but which is
not necessarily burned. Usually expressed in pounds of waste per hour.
Hard, sintered, or fused pieces of residue formed in an incinerator by the
agglomeration of ash, metals, glass, and ceramics.
Flow of scrubbing liquid in the same direction as the gas stream.
The ratio of the weight of pollutant collected to the total weight of pollutant entering
the collector.
A thermal process in which organic compounds are broken down into carbon dioxide
(CO2) and water (H2O).
The physical process of converting a substance from the gaseous phase to the liquid
phase via the removal of heat and/or the application of pressure.
Incineration utilizing two or more combustion chambers in which the amounts and
distribution of air to each chamber are controlled. Partial combustion takes place in
the first zone (chamber) and subsequent zones are used to complete combustion of the
volatilization gases.
The period of time at the end of an incinerator's operating cycle during which the
incinerator is allowed to cool down. The cooldown period follows the burndown
period.
Flow of scrubbing liquid normal (perpendicular) to the gas stream.
A device in which the velocity of an inlet gas stream is transformed into a confined
vortex from which inertial forces tend to drive particles to the wall.
An adjustable plate installed in a duct to regulate gas flow.
To remove water vapor from a gas stream.
A mechanical device used to remove entrained water droplets from a scrubbed gas
stream.
The ratio of the mass of an object to the volume of the object.
Random motion of particles caused by repeated collisions of gas molecules.
A gas flow resulting from pressure difference; for example, the pressure difference
between an incinerator and the atmosphere, which moves the products of combustion
from the incinerator to the atmosphere. (1) Natural draft: the negative pressure
created by the difference in density between the hot flue gases and the atmosphere.
(2) Induced draft: the negative pressure created by the vacuum action of a fan or
blower between the incinerator and the stack. (3) Forced draft: the positive pressure
created by the fan or blower, which supplies the primary or secondary air.
Solid particles less than 100 micrometers created by the breakdown of larger
particles.
The weight of solid particulate suspended in an airstream (gas). Usually expressed in
terms of grains per cubic foot, grams per cubic meter, or pounds per thousand pounds
of gas.
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ENDOTHERMIC*
ENTRAINMENT3
EXOTHERMIC*
FEEDBACK
CONTROLS
FIXED CARBON*
GRID1
HEADERi
HEAT RELEASE
RATE*
HEATING VALUE*
HUMIDITY,
ABSOLUTE2
HUMIDITY,
RELATIVE2
INCINERATOR*
INDUCED DRAFT
FAN3
INERTIA*
LIQUID-TO-GAS
RATIO3
LIQUOR*
MAKEUP WATERS
MIST ELIMINATORS
OPACITY*
PACKED-BED
SCRUBBERS
PARTICLE*
PARTICLE SIZE*
A chemical reaction that absorbs heat from its surroundings. For example:
C + H2O + heat -» CO+H2.
The suspension of solids, liquid droplets, or mist in a gas stream.
A chemical reaction that liberates heat to its surroundings. Combustion is an
exothermic reaction. For example: C + O2 -» CO2 + heat
An automatic control system in which information about the controlled parameter is
fed back and used for control of another parameter.
The nonvolatile organic portion of waste.
A stationary support or retainer for a bed of packing in a packed bed scrubber.
A pipe used to supply and distribute liquid to downstream outlets.
The energy released over a unit of time during combustion. Calculated as the
heating value (Btu/pound) X burn rate (pound/hour). Usually expressed as Btu/hour
(Btu/h).
The amount of heat that is released when a material is combusted, usually expressed
as Btu/lb.
The weight of water vapor carried by a unit weight of dry air or gas.
The ratio of the absolute humidity in a gas to the absolute humidity of a saturated
gas at the same temperature.
A thermal device which combusts organic compounds using heat and oxygen.
A fan used to move a gas stream by creating a negative pressure.
Tendency of a particle to remain in a fixed direction, proportional to mass and
velocity.
The ratio of the liquid (in gallons per minute) to the inlet gas flow rate (in acfm).
A solution of dissolved substance in a liquid (as opposed to a slurry, in which the
materials are insoluble).
Water added to compensate for water losses resulting from evaporation and water
disposal.
Equipment that removes entrained water droplets downstream from a scrubber.
Measure of the fraction of light attenuated by suspended particulate.
Equipment using small plastic or ceramic pieces, with high surface area to volume
ratios for intimate gas/liquid contact for mass transfer.
Small discrete mass of solid or liquid matter.
An expression for the size of a liquid or solid particle, usually expressed in microns
(or micrometers).
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PARTICULATE
EMISSION*
PARTICULATE
MATTER*
PATHOGENIC
PATHOLOGICAL
WASTE
PENETRATION*
pHl
PRESSURE DROPS
PRESSURE, STATIC*
PROXIMATE
ANALYSIS*
PYROLYSIS
QUENCHi
REAGENT3
RED BAG WASTE
RESIDENCE TIME
SATURATED GAS*
SIZE DISTRIBUTION*
SLURRYl
SMOKE*
SPECIFIC GRAVITY!
SPRAY NOZZLEl
Fine solid matter suspended in combustion gases carried to the atmosphere. The
emission rate is usually expressed as a concentration such as grains per dry standard
cubic feet (gr/dscf) corrected to a common base, usually 12 percent CO2 or 7% oxygen.
As related to control technology, any material except uncombined water that exists
as a solid or liquid in the atmosphere or in a gas stream as measured by a standard
(reference) method at specified conditions. The standard method of measurement and
the specified conditions should be implied in or included with the particulate matter
definition.
Waste material capable of causing disease.
Waste material consisting of anatomical parts.
Fraction of suspended particulate that passes through a collection device.
A measure of acidity-alkalinity of a solution.
The difference in static pressure between two points due to energy losses in a gas
stream.
The pressure exerted in all directions by a fluid; measured in a direction normal
(perpendicular) to the direction of flow.
The determination of the amounts of volatile matter, fixed carbon, moisture, and
noncombustible (ash) matter in any given waste material, usually expressed in
percentages by weight.
The chemical destruction of organic materials in the presence of heat and the
absence ofoxygen.
Cooling of hot gases by rapid evaporation of water.
The material used in a scrubbing system to react with the gaseous pollutants.
As used in this document, red bag waste refers to infectious waste; the name comes
from the use of red plastic bags to contain the waste and to clearly identify that the
waste should be handled as infectious.
Amount of time the combustion gases are exposed to mixing, temperature, and
excess air for final combustion.
A mixture of gas and vapor to which no additional vapor can be added at specified
conditions.
Distribution of particles of different sizes within a matrix of aerosols; numbers of
particles of specified sizes or size ranges, usually in micrometers (um).
A mixture of liquid and finely divided insoluble solid materials.
Small gasborne particles resulting from incomplete combustion; particles consist
predominantly of carbon and other combustible material; present in sufficient
quantity to be observable independently of other solids.
The ratio between the density of a substance at a given temperature and the density
of water at 4°C.
A device used for the controlled introduction of scrubbing liquid at predetermined
rates, distribution patterns, pressures, and droplet sizes.
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STANDARD CUBIC
FEET PER MINUTE
(scfm)3
STARVED-AIR
INCINERATION
STOICHIOMETRIC
AIR
STUFF AND BURN
ULTIMATE
ANALYSIS
VAPOR*
VOLATILE MATTER
A gas flow rate expressed with respect to standard temperature and pressure
conditions (20°C [68°F] and 29.92 in. Hg [760 mm]).
Controlled air incineration in which the primary chamber is maintained at less than
stoichiometric air conditions.
The theoretical amount of air required for complete combustion of waste to CO2 and
H2O vapor.
A situation in which the charging rate is greater than the burning rate of the
incinerator.
A determination of the quantities of the various elements (i.e., carbon, hydrogen,
sulfur, nitrogen, and oxygen) and ash of which a substance is composed, usually
expressed in percentages by weight.
The gaseous form of substances that are normally in the solid or liquid state and
whose states can be changed either by increasing the pressure or by decreasing the
temperature.
That portion of waste material which can be liberated with the application of heat
only.
10.1 References for Chapter 10
1. Industrial Gas Cleaning Institute. Wet Scrubber
Technology. Publication WS-1, July 1985.
2. Industrial Gas Cleaning Institute. Fundamentals
of Fabric Collectors and Glossary of Terms.
Publication F-2, August 1972.
3 Flue Gas Desulfurization Inspection and
Performance Evaluation. EPA/625/1-85-019.
October 1985.
4. U. S. Environmental Protection Agency, Control
Techniques for Particulate Emissions from
Stationary Sources. Volume I. EPA-450/3-81-
005a. September 1982.
5. Brunner, C. R. Incineration Systems Selection
and Design. Van Nostrand Reinhold Company,
1984.
• U.S. GOVERNMENT PRUNING OFFICE:! 993 .750 .002/ 6016K
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