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PEDCo- ENVIRONMENTAL
SUITE 13 • ATKINSON SQUARE
CINCINNATI. OHIO 4524-6
513 / 771-4330
950R75006
ANALYSIS OF THE EFFECT OF
COAL PROPERTIES ON FURNACE/
BOILER COMBUSTION CHARACTERISTICS
Prepared by
PEDCo-Environmental Specialists, Inc.
Suite 13, Atkinson Square
Cincinnati, Ohio 45246
Contract No. 68-02-1076
Task Order No. 11
EPA Project Officer: Rayburn Morrison
Prepared for
Foster Associates, Inc.
1101 Seventeenth Street, N.W.
Washington, D.C. 20036
April 1975
BRANCH OFFICES
Suite 110, Crown Center
Kansat City, Mo. 64108
Suite 104-A, Professional Village
Chapel Hill, N.C. 27514
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This report was furnished to the Environmental Protection
Agency by Foster Associates, Inc., 1101 Seventeenth Street,
N.W., Washington, D.C., in fulfillment of Contract No. 68-
02-1076. The contents of this report are reproduced herein
as received from the contractor. The opinions, findings,
and conclusions expressed are those of the author and not
necessarily those of the Environmental Protection Agency.
ii
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TABLE OF CONTENTS
Page
LIST OF FIGURES V
LIST OF TABLES vi
1.0 INTRODUCTION 1-1
1.1 Purpose of Study 1-2
1.2 Natural Gas-Fired and Oil-Fired Units 1-3
1.3 General Considerations When Switching 1-3
Coal Sources
1.4 General Effects of Coal Characteristics on 1-5
Boiler Performance
2.0 DISTRIBUTION OF U.S. COALS 2-1
3.0 COMBUSTION SYSTEM AND COAL COMBUSTION 3-1
CHARACTERISTICS
4.0 DETAILED DISCUSSION OF THE EFFECT OF COAL SWITCHING 4-1
ON THE PERFORMANCE OF COMBUSTION SYSTEMS
4.1 General Description of Combustion Systems 4-1
4.2 Genetal Description of Heat Transfer 4-5
4.3 Effects of Fuel Switching on Combustion 4-5
Performance
4.3.1 Coal Handling and Storage 4-8
4.3.2 Coal Crushing or Pulverizing Equipment 4-8
4.3.3 Coal Burners, Combustion Chamber, and 4-18
Grate Area
4.3.4 Combustion Furnace and Heat Transfer 4-19
Area
4.4 Other Factors 4-26
4.5 Clean Coal 4-31
iii
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TABLE OF CONTENTS (continued)
Page
APPENDIX A DETAILED DESCRIPTION OF FURNACE/BOILER A-l
SYSTEMS
APPENDIX B TEST METHODS FOR PHYSICAL AND CHEMICAL B-l
PROPERTIES OF COAL
REFERENCES
iv
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LIST OF FIGURES
Figure Page
3-1 Coal and Ash Characteristics which, if 3-4
Changed, May Adversely Affect Boiler
Performance
4-1 Traveling-Grate Spreader Stoker with Front 4-2
Ash Discharge
4-2 Pulverized Coal Unit 4-3
4-3 Cyclone Furnace Operation 4-4
4-4 Typical Pulverized-Coal-Fired Boiler Heat 4-6
Transfer Surface Arrangement
4-5 Relative Amount of Heat Transfer Surface and 4-7
Relative Amount of Heat Absorbed
4-6 Pulverized Capacity as a Function of Fineness 4-12
4-7 Relative Pulverizer Capacity as a Function of 4-13
Hardgrove Grindability
4-8 Effect of Coal Heating Value on Pulverized 4-14
Capacity
4-9 Effect of Moisture on the Temperature of 4-16
Pulverizer Air
4-10 Direct-Firing with Predrying Bypass and 4-17
Tempering Air
4-11 Influence of Ash Characteristics on Furnace 4-20
Size
4-12 Slagging and Fouling Zones 4-23
4-13 Effect of Sulfur Content and Temperature on 4-27
Fly Ash Resistivity
A-l Shell-Type Boiler A-2
A-2 Fire-tube Boiler A-3
A-3 Water-Tube Boiler A-4
A-4 Pulverized-Coal-Fired Unit A-5
v
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LIST OF FIGURES (continued)
Figure Page
A-5 Heat Transfer Zones in a Pulverized-Coal- A-6
Fired Boiler
A-6 Single-Retort, Horizontal Underfeed Stoker A-9
with Side-Ash-Discharge
A-7 Double-Retort, Horizontal-Feed Type of Side- A-10
Ash-Discharge Underfeed Stoker
A-8 Multiple-Retort, Gravity Feed Underfeed Stoker A-ll
with Rear Ash Discharge
A-9 Chain-Grate Stoker with Rear Ash Discharge A-13
A-10 Traveling-Grate Spreader Stoker with Front A-15
Ash Discharge
A-ll Spreader Stoker with Gravity Flow Fly Ash A-16
Return
A-12
Circular Burners for Firing Pulverized
Coal
A-18
A-13
Cyclone Furnace Operation
A-19
A-14
Schematic of Two-Drum Bent Tube Boiler
A-21
B-l
Burning Profiles of Coals of Different
Rank
B-5
B-2
Grindability Index of Coal
B-6
LIST OF TABLES
Table Page
2-1 Approximate Distribution of U.S. Coal Reserves 2-2
by Location and Type and Percent of Coal
Reserves Which Meet a Regulation of 1.2 Pounds
of so2/mm BTU
3-1 Physical and Chemical Properties of Coal 3-3
Affecting Coal Combustion Subsystem Performance
3-2 Principal Test Methods Used in Measuring Coal 3-5
Combustion Characteristics
4.1 Ash Characteristics Indices 4-24
A-l Modern Furnace/Boiler Combinations and Size A-7
Ranges
vi
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LIST OF TABLES (continued)
Table Page
B-l Classification of Coals by Rank, ASTM D 388-38 B-3
B-2 Ranges Found in Constituents and Properties B-4
of Selected Coals
B-3 Comparison of Ash Analyses of Selected Coals B-7
B-4 Average Ash Constituents of Three Ranks of Coal B-8
vii
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1.0 INTRODUCTION
Realization that foreign oil producers can seriously
affect the country's economy has caused the United States to
embark on a national policy of energy independence. Major
efforts are being made to reduce the importation of oil by
recovering untapped reserves of off-shore and Alaskan oil,
by chemical conversion of coal to oil-type liquids and by
instigating oil conservation practices (e.g. 55 mph speed
limit). The impact of this policy on controlling SOx
emissions from industrial and utility combustion sources, is
significant. Many combustion sources have switched from
coal to low-sulfur oil and natural gas as an economical
means of controlling atmospheric emissions of S0x and par-
ticulate. Oil and gas shortages and higher energy costs are
causing a redirection of fuel supplies from oil and gas back
to coal. Further, coal supplies are being examined to
establish whether or not low-sulfur coal supplies can be
used in place of high-sulfur fuel supplies.
The conversion of a coal-burning unit to gas or oil
firing is relatively straight forward with no major un-
solvable fuel handling, storage or combustion problems
occurring. The switch from an oil- or gas-fired combustion
1-1
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unit (originally designed for oil or gas) to coal firing is
a formidable problem and essentially requires a complete
combustion system redesign. The switch from one type of
coal to another type of coal can usually be achieved.
However significant changes in combustion practice, fuel
storage and handling, and in plant capacity may be encoun-
tered.
1.1 PURPOSE OF THE STUDY
This study addressed one facet of the pre-combustion
control technique for S0x limitation the use of low-
sulfur coal. There is no scarcity of low-sulfur coal in the
U.S. It is estimated that of the 45 billion tons of strip-
pable reserves of coal and lignite in the U.S., approxi-
mately 31 billion tons contain less than one percent sulfur.
In addition, some of the coal with higher sulfur contents
can be beneficiated via washing techniques. Aside from the
problems faced in the extraction of these low-sulfur coals
or the beneficiation of coals having higher sulfur contents,
the use of these coals in existing furnace/boiler facilities
can result in operational problems. The purpose of this
study is to describe these problems and their solutions.
This is achieved by acquainting the reader with:
1) the various furnace/boiler facilities in use
2) the properties of coal which affect combustion
characteristics
3) specific operation problems and possible solutions.
1-2
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Unfortunately, specific equipment recommendations
and/or operational changes cannot be delineated since many
interrelated variables of coal composition and boiler
operation effect required changes. Currently coal switching
remains more of an art than a science.
1.2 NATURAL GAS-FIRED AND OIL-FIRED UNITS
Notwithstanding the move toward energy independence, it
is not likely that furnaces originally designed to burn
natural gas or oil will switch to burning coal. Such a
switch cannot be made without a major derating of the unit.
Oil and gas-fired furnaces have a much smaller volume per
unit of steam production than comparable coal-fired units.
Subsequently, size is the limiting factor and no modifica-
tions can ameliorate the situation. Deratings on the order
of 50 to 60 percent may be expected. For this reason, this
report does not consider the effects of coal-firing on units
originally designed for burning oil or natural gas.
1.3 GENERAL CONSIDERATIONS WHEN SWITCHING COAL SOURCES
The coal user has two principal methods to use in
evaluating the effects of switching coal sources.
First the user may obtain detailed laboratory analysis
on coal chemical and physical properties and with the
assistance of competent combustion experts, evaluate the
results of such tests as ash viscosity, ash composition,
ignitability, grindability, etc. The evaluations are gen-
erally made in light of previous combustion experience with
1-3
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the new fuel in a combustion unit of similar design and the
new coals' similarity with the coal presently being burned.
Lack of this type of information or obtaining lab test
results which indicate marginal performance, lead to the
second method of evaluating coals; namely full scale testing
by burning a test load of the fuel.
Full scale combustion testing is the most reliable
method of evaluating a new coal source. Predicting combus-
tion performance and associated ash fouling and slagging
problems by lab methods, is costly, requires expert data
evaluation and since most of the tests are empirical and
rely on statistical correlations, the lab tests at best must
be considered indicative rather than conclusive. Full scale
testing over a minimum one-week period, together with the
lab data, is the best procedure available to evaluate new
coal sources for a particular combustion unit. Even the
short-term, full scale test may not disclose all combustion
problems since some ash deposition problems occur only after
a significant (up to a few months) "conditioning period".
In practice, when a new coal source is considered,
utility and industrial coal consumers rely heavily on their
own boiler operating experience, combustion equipment vendor
recommendations and other utility experience with similar
coals. Few eastern utilities have any experience with
western coals. Few utilities have any long-term experience
in switching from a high-sulfur eastern coal to a low-sulfur
1-4
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cleaned eastern coal- There has been some short-term ex-
perience in coal-switching but the effects of such switching
has not been reported. Much work in these areas remains to
be done.
1.4 GENERAL EFFECTS OF COAL CHARACTERISTICS ON BOILER
PERFORMANCE
When converting a steam generating unit from one source
of coal to another, the physical and chemical properties of
the coal are of particular interest in determining effects
on operation characteristics and related furnace/boiler
problems. The relationship of coal fuels and furnace/boiler
design shows that knowledge of the analysis of the specific
coal to be used after the conversion, is needed to properly
assess the changes in operating characteristics that will
occur. Complete coal and ash information of a specific fuel
are items that are measurable from coal samples, and form
the basis for judgment and engineering calculations.
The power generation industry utilizes coal from fields
throughout the country. In general the eastern fields
contain anthracite and medium and high volatile bituminous
coals while subbituminous and lignite coals are found in the
western fields. When converting from one coal type to
another, significant changes in operating characteristics
occur only with marked changes in the physical and chemical
properties of the new fuel. For example, in the case of
converting an existing furnace/boiler from bituminous coals
having medium-to-high sulfur content to those with low
1-5
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sulfur content, some of the possible characteristics of a
low-sulfur fuel which must be considered are:
1. Anthracite is commonly low in sulfur, but this low
volatile fuel is difficult to pulverize and burn,
and is not a suitable conversion fuel for a steam
generator arranged to burn high-sulfur, medium-to-
high volatile bituminous coal.
2. Almost all western subbituminous and lignitic
coals have sulfur contents of less than two per-
cent, and in most the sulfur under one percent.
These western coals are medium-to-high volatile
fuels, easy to burn; in this respect they are
similar to the eastern and midwestern bituminous
coals. However, many western coals have other
characteristics that are markedly different from
eastern fuels, and cause significant changes in
operating characteristics. While there is no
typical western coal analysis, some of the major
features include:
a. The total moisture content is frequently from
25 percent to over 30 percent.
b. Heating value is typically low, varying from
6000-10,000 BTU/lb as-received.
c.. The ash content seldom exceeds 12 percent,
and usually ranges from 5 to 8 percent.
Sodium oxide in the ash ranges from 0.5 to 8
percent or more. Potassium and iron in the
ash are frequently low, but calcium oxide can
exceed 2 5 percent.
d. Grindability is moderate, ranging from 40 to
7 0 Hardgrove Grindability Units.
Therefore, because of the possible difference in coal
characteristics when converting from one type of coal to
another, several of the major affects on performance which
can occur are:
1. The quantity of fuel required to sustain a given
output will change in relation to the changes in
heating value. Converting to western fuels will
generally require more coal, and may require
additional pulverizer and primary air system
capability.
1-6
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2. The efficiency of the unit will change in relation
to the moisture in the fuel, heating value of the
fuel, and changes in the gas weight produced by
the combustion of coal. Converting to western
coal will generally reduce the furnace/boiler
efficiency.
3. Coals with higher moisture content and lower
heating value will produce higher flue gas weights,
resulting in increased gas velocity and draft loss
in the convection passes. Fan power and capacity
requirements will also be higher.
4. Superheater steam temperatures will increase in
relation to increased flue gas weight and higher
flue gas temperature. Boiler types with radiant
superheaters in the furnace and convection super-
heaters in the gas pass are affected by conversion
to coals that produce more furnace slagging by
reduction of heat absorption in the lower furnace,
and increased heat absorption in the upper furnace.
Higher steam temperature, and increased metal
temperature occur in both radiant and convection
superheaters, with a corresponding increase in
steam temperature control duty. Adjustment to
heating surface may be required.
5. Adjustment to air heater performance may be re-
quired in relation to amount of moisture in the
fuel. Primary air temperatures may be too low for
coals with higher moisture content. In addition,
adjustment to the tempering air system may be
required for proper control of fuel systems with
variable moisture content.
6. Additional fuel and ash handling and storage
facilities may be required if these tonnages are
significantly different than that now being
produced.
These effects together with information on coal combustion
characteristics are discussed in detail in this report.
1-7
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2.0 DISTRIBUTION OF U.S. COALS
Current coal-fired power generating capacity in the
U.S. is approximately 180,000 MW. In 1973 the utilities
consumed approximately 374,000,000 tons of coal of which
26.7 percent was 1 percent sulfur or less. Most of this
low-sulfur tonnage was eastern bituminous coal. Conse-
quently SO2 emissions from burning this low-sulfur coal
easily met regulations allowing as low as 1.2 pounds of
SO2/MM BTU. Low-sulfur, low-BTU coal such as some western
subbituminous coal and all lignites would produce an SO2
emission higher than 1.2 pounds of SO2/MM BTU. Table 2-1
shows the distribution of U.S. coal reserves and the quan-
tity of each rank of coal which will meet an emission reg-
ulation similar to the New Source Performance Standard of
1.2 pounds of S02/MM BTU of input. However it should be
pointed out that existing sources are governed by the
various state implementation plans not the new source
performance standards. Allowable emission rates for ex-
isting utility sources range from 0.55 to 5.0 lb S02/
million BTU.
2-1
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Table 2.1 APPROXIMATE DISTRIBUTION OF U.S. COAL RESERVES
BY LOCATION AND TYPE AND % OF COAL RESERVES WHICH
MEET A REGULATION OF 1.2 POUNDS OF S02/MM BTU1
Western
Pa., W.Va.
E. Ky.
111., Ind.
W.Ky.
All
Other
Lignite
Subbitununous
Bituminous
Bituminous
Bituminous
% of all Coal Reserves
7. 63
13.47
4. 39
26.43
30.82
17.21
% of Reserves which
meet 1.2 pound
S02/MM Btu
0
a
1.44
5.22
0.24
1.84
\ of Reserves which
can be cleaned to
meet 1.2 pound SO-/
MM Btu
0
a
0
3.10
0.26
9.09
Note: There are conflicting data with respect to the quality of Western Bituminous and Subbitummous
coal reserves. Above values represent a conservative estimate of the availability of low-sulfur
reserves.
a Unable to quantify at this time.
Lignite comprises almost 11 percent of U.S. coal reserves.
Although the sulfur level of lignite is low, the heating
value of lignite is also low. Consequently, lignite having
a sulfur content of 1.4 percent, can produce as high as four
pounds of SO2/MM BTU when burned. Thus it is unlikely that
eastern coal consumers will switch to lignite as a source of
low-sulfur fuel. It should be noted that future uses of
lignite include conversion to pipeline gas which may ulti-
mately be used as a fuel in
-------
more detailed discussion of western subbituminous coal
combustion properties follows later.
Other sources of naturally occurring low-sulfur coal
are anthracite, low-sulfur western bituminous and eastern
bituminous coal. With the exception of anthracite coal,
which because of its low volatile matter is difficult to
burn in coal-fired systems designed for bituminous coal,
eastern utilities and industrial coal consumers encounter no
major furnace/boiler problems when switching to a low-sulfur
eastern coal.
Another source of low-sulfur coal is a coal which can
be cleaned by conventional coal cleaning methods; in par-
ticular heavy media cleaning, to yield a low ash, low-
sulfur, high-BTU product. Utilities have been utilizing
coal cleaning methods to yield a low ash coal, and therefore
increase boiler performance. However, combustion experience
with coal which has been specifically cleaned to yield a
low-sulfur product is very limited. In using a washed low-
sulfur coal, the general guidelines identified above apply.
These guidelines are discussed in detail in Section 4.0 of
this report.
2-3
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3.0 COMBUSTION SYSTEMS AND COAL COMBUSTION CHARACTERISTICS
Although there are many different types of coal combus-
tion systems, each system will include certain general
subsystems necessary to burn the design coal at full design
load. These subsystems include coal handling and storage,
coal pretreatment such as crushing and drying, coal feed
systems, coal pulverizers, burner or grate, combustion
chamber, heat transfer equipment, dust collection equipment,
fans and ash handling and storage facilities. Water treatment
and cooling stacks, turbine generator, and steam-side
systems may also be included in coal combustion systems but
are not part of this discussion. Details of specific coal
combustion systems are presented in Appendix A.
Each combustion system is designed to accommodate a
specific coal or coals as the case may be. Switching to a
nondesign fuel may have a significant affect on all or
portions of the combustion system and its operation. The
degree of combustion system problems encountered by switch-
ing coal is directly related to:
a. the difference in chemical and physical properties
between the new and existing coals, and
3-1
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b. the range of operation of the components of the
combustion system.
Table 3-1 identifies the physical and chemical prop-
erties of coal which affect each of the significant coal
combustion subsystems. The range of possible effects and
potential solutions for each problem are discussed in the
sections which follow. These discussions are based on
published information, such as that presented by coal com-
bustion equipment vendors.
Figure 3-1 indicates the coal properties which may
adversely affect the performance of the five types of coal
combustion systems. The principal test methods for mea-
suring these properties are identified in Table 3-2 and
discussed in detail in the Appendix B. Some of the tests
listed in Table 3-2 are standard methods, others are special
tests, such as viscosity, which only a few laboratories are
capable of conducting. Other tests are also used, such as
sintering tests, forms of sulfur (pyrite, organic, sulfate),
bed moisture, inherent mineral matter, coal plasticity, etc.
The tests identified in Table 3-2 are considered to be most
pertinent to this discussion.
3-2
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Table 3-1 PHYSICAL AND CHEMICAL PROPERTIES OF COAL
AFFECTING COAL COMBUSTION SUBSYSTEM PERFORMANCE
Coal combustion subsystem
Coal properties
Potential effects
Coal Handling and Storage
Bulk Density
Size Consistency
Heating Value
Pyritic Sulfur
Significant changes effect quantity of coal
which can be handled. Fine coal increases
fugitive dust problems. High pyritic sulfur
can cause spontaneous combustion in coal
storage piles.
Coal Crushing and Pulverizing
System
Grindability
Moisture Content
Heating Value
Size Consistency
Volatile Matter
Ash Content
Problems encountered relate to reducing
pulverizer capacity, pulverize-drying air
(quantity and temperature), size consistency of
product, preignition and explosion conditions
Coal Burners or Grate Area
Heat Content
Size Consistencv
Ignitability
Caking & Swelling
Problems relate to capacity, firing rate,
and complete combustion. Stokers may be
plugged by caking and swelling coals
Combustion Furnace and Heat
Transfer Surfaces
Heat Content
Ash Composition
Ash Viscosity
Ash Content
Moisture
Sulfur Content
Complex relationships of ash components
may cause slagging, fouling, deposition
problems. Boiler tube erosion and corrosion,
cold end air preheater problems. Loss of
rated capacity.
Ash Collection and Handling
Ash Content
Sulfur Content
ESP performance may be adversely effected
by low sulfur coals. Increased ash content
will effect ash handling and removal system.
-------
COAL OR ASH
CHARACTERISTICS
FURNACE
TYPE
BOILER TYPE
Natural
circulation
Forced
circulation
Stoker
Pulverized
Coal
Cyclone
Pulverized
Coal
Cyclone
Heating value
•
•
•
9
9
Grindability
•
•
9
9
Moisture content
•
9
•
9
9
Caking tendency
•
Slagging tendency
•
9
e
9
9
Fouling tendency
G
9
®
9
9
Ash erosion tendency
e
©
9
9
9
Ash viscosity
9
e
9
9
9
Ash content
©
9
•
9
9
Figure 3-1 Coal and ash characteristics which, if changed,
may adversely affect boiler performance.
3-4
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Table 3-2 PRINCIPAL TEST METHODS USED IN MEASURING
COAL COMBUSTION CHARACTERISTICS
Method
Brief description
Results achieved
1.
Heating Value
Measure heat content of
coal in adiabatic bomb
calorimeter
Theoretical heat evolved
when fuel is burned
2.
Hardgrove Index
Coal is ground in a
special grinding
machine under standard
test conditions
Relative hardness of
coal is determined
3.
Ash Content
ASTM ashing procedure
Heat 1 gram of coal in
muffle furnace at standard
conditions
Determines the total
amount of noncombustible
residue
4.
Ash Composition
Spectrographic determina-
tion of elemental composi-
tion of ash
Results are reported as
oxides. Used to calculate
various empirical ash
slagging and fouling
factors
5.
Ash Viscosity
High temperature ash
viscosimeter. Specialized
test
Measures the plastic
zones of ash
6.
Ash Fusion
Temperature
Ash melting characteristics
in oxidizing and reducing
atmospheres are measured
Recently has been used
to develop a slagging
index for lignitic type
ash
7.
Sulfur Content
Total elemental sulfur
determined by ASTM method
Determines environmental
acceptance. Used to assess
ash collectability and
part of ash slagging index
8.
Burning Profile
Measures the rate of weight
change when coal is oxidized
at constant rates of temper-
ature increase.
Data are used to assess
ignitability, false
ignition, speed of com-
bustion and burnout
9.
Proximate Analysis
Measures ash and volatile
matter
Gives general information
on coal
10.
Ultimate Analysis
Measures elemental C, H, N,
0, S.
Useful in making com-
bustion calculations
11.
Free Swelling
Index
Measures the relative
swelling tendency of coal
under ASTM procedures
Identifies caking and
swelling coals for use
in evaluating stoker
coals
3-5
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4.0 DETAILED DISCUSSION OF THE EFFECTS OF COAL SWITCHING
ON THE PERFORMANCE OF COMBUSTION SYSTEMS
4.1 GENERAL DESCRIPTION OF COMBUSTION SYSTEMS
Three general types of coal combustion systems cur-
rently in industrial and utility use in this country are
stokers, pulverized-coal-fired units, and cyclone-fired
units. These are shown in Figures 4-1, 4-2 and 4-3.
Stokers are the general class of combustion units in
which sized coal with a minimum of fines is burned on or
above a grate. Stoker designs include hand-fired units,
stationary grates, vibrating grates, spreader stokers,
underfeed stokers, traveling grates. The large industrial
coal stokers are primarily traveling-grate and spreader stokers.
Pulverized-coal-(p.c.) fired combustion units are those
in which an air suspension of pulverized coal (approximately
200 mesh or finer) is burned in a combustion chamber. The
p.c. unit may be wet wherein the ash is removed as a molten
slag or dry wherein the ash is removed as a fine powder.
P.C. firing is by far the most common method of burning coal
in utility and large industrial coal combustion systems.
Cyclone-fired coal combustors are those which burn
coarse coal, approximately -4 mesh, in a horizontal cylinder
4-1
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in which part of the combustion air is introduced tangen-
tially imparting a whirling or centrifugal motion to the
coal. The combustion temperatures are high causing the ash
to melt and adhere to the walls of the cyclone furnace. The
ash is removed as slag. Cyclone-firing represents less than
two percent of total utility coal combustion systems.
Various types of furnace/boiler configurations are
supplied by vendors. Details of these commercial systems
are found in Appendix A. A general description of the heat
transfer (boiler) section of the combustion system follows.
COAL
HOPPER
OVERTHROW
^ ROTOR
FEEDER
TRAVEL DIRECTION
SIDE-WALL HEADER
STOKER CHAIN
ASH! 3
HOPPER
AIR PLENUM
Figure 4-1 Traveling-grate spreader stoker
with front ash discharge.2
4-2
-------
Figure 4-2 Pulverized coal unit.
(Courtesy of Babcock & Wilcox)
4-3
-------
HOT GASES
MOLTEN
SLAG
Figure 4-3 Cyclone furnace operation.
4-4
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4.2 GENERAL DESCRIPTION OF HEAT TRANSFER
Heat transfer in a combustion system is arranged to
economically and efficiently transfer heat by radiation and
convection from the burning fuel and hot gases to water,
steam and combustion air. In power generating equipment the
end use of the steam is to drive a turbine. Steam generation
in industrial use may be principally for heating and process
uses. Several basic types of boiler systems have been
developed, and are discussed in detail in Appendix A. This
discussion will focus on the general class of utility
boilers which produce high temperature and pressure steam
for turbine operation.
A typical distribution of the heat transfer surfaces in
a boiler is shown in Figure 4-4. Water walls and pendant
superheaters in the furnace extract heat by radiation from the
burning fuel. Secondary pendant superheater extracts heat
from high temperature flue gas by convection. Primary
superheater, reheater and economizer sections remove heat by
convection. The bulk of the heat is absorbed in the water
walls although most of the heat transfer surface is in the
convection passes. Figure 4-5 illustrates the relative
amount of heat transfer surface and relative amount of heat
absorbed in a typical combustion system. The air heater
preheats the combustion air. Hot flue gas exhausts at
temperatures between 200-350°F.
4.3 EFFECTS OF SWITCHING FUEL ON COMBUSTION PERFORMANCE
The effect of changes in the physical and chemical
4-5
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Attemperator
Windbox
Air Heater
!=Primary
LAir Duct
Tempering
Air Duct
nAshrt f
Hopper
Primary Air Fan
Stack
Penthouse
Pendant
Reheater
Steam Drum
Secondary
Superheater
Primary
Superheater
Economizer
l_l L
Feeders
Pulver zers
Steam Coil E1
Air Heater
Forced Draft Fan
Figure 4-4 Typical pulverized-coal-fired boiler heat
transfer surface arrangement.
(Courtesy of Babcock & Wilcox)
Electrostatic
Precipitator
-------
RELATIVE AMOUNT OF RELATIVE AMOUNT OF
HEAT TRANSFER SURFACE HEAT ABSORBED
Figure 4-5 Relative amount of heat transfer surface and
relative amount of heat absorbed.3
4-7
-------
properties of a new coal on the performance of the combus-
tion subsystems can be significant. Each subsystem is
discussed separately.
4.3.1 Coal Handling and Storage
Bulk coal storage is normally designed to accommodate a
30 day minimum coal supply. Larger quantities of coal may
be stored from time to time in special instances such as
preparation for a strike of coal miners or utility workers.
Site requirements may control the quantity of fuel which can
be stored. Coal handling equipment such as belts, conveyors,
and breakers are specifically designed to handle coal ton-
nages and sizes with only slight additional capacity.
Working bins or silos are designed to accommodate a fuel
requirement of between 8 and 24 hours. The coal parameters
associated with coal storage and handling facilities are
coal size distribution, bulk density, tonnage, sulfur con-
tent (high pyrite sulfur coal is amenable to coal pile
fires). When switching to a low-sulfur coal, the principal
changes which may occur are (1) the quantity of fuel in-
creases, and (2) reduced sulfur content may improve coal
storage by reducing risk of fire.
4.3.2 Coal Crushing or Pulverizing Equipment
Each of the different combustion systems have different
crusher or pulverizer requirements. Stokers usually burn a
coarse-sized coal over a range of 2 inches to 1/4-inch,
stoker-size coal is a commercial product and usually crushers
4-8
-------
are not required. Cyclone-firing requires coal with a top
size of -4 mesh; the size distribution below 4 mesh is not
critical. Crushers designed to produce -4 mesh coal usually
receive a feed of 2 inch x 0 coal. The grindability char-
acteristics of the coal are not critical in the design of
crushers for cyclone F/B systems. Coal tonnage, as related
to the unit's fuel requirements (BTU input), is the con-
trolling factor.
Design of pulverizers for pulverized coal (p.c.)
firing is affected by a significant number of coal char-
acteristics in addition to fuel requirements (tonnage/BTU
input). Grindability, surface moisture, ignitability,
volatile matter all affect the size and number of pulver-
izers. In addition, all pulverizers for p.c. B/F units are
swept with a fixed quantity of hot air. This air is called
primary combustion air and serves several purposes (a) to
convey the pulverized coal to the burners, (b) to dry the
coal, and (c) to provide a significant portion of the com-
bustion air. The principal coal factors associated with the
quantity of pulverizer air are volatile matter and surface
moisture. The temperature of the pulverizer air is related
to the potential for explosion or premature ignition of the
coal air mixture.
4.3.2.1 Grindability, Heating Value, and Fineness - A
principal pulverizer design element is the grindability of
the coal. The accepted method for determining the grind-
4-9
-------
ability of coal is that specified by ASTM as Standard D409
(see Appendix B). This method determines the relative
hardness of a coal compared with a standard coal. The
standard coal has an assigned index of 100. Relatively
harder coals have a grindability less than 100. In general,
p.c.-fired units require a minimum fineness of coal to
ensure complete combustion of the coal in the p.c. furnace.
The degree of fineness is expressed as the amount of coal
passing through a -200 mesh sieve. As a rule of thumb p.c.-
firing requires a coal with a fineness of 70 percent -200
mesh material. For optimum firing, published data on the
fineness of various types of coal are presented below:
ASTM CLASSIFICATION OF COALS BY RANK4
Fixed carbon, %
Fixed carbon below 69%
85.9-78
77.9-69
BTU above
BTU 12,900
BTU below
% -200 Mesh
13,000
11,000
11,000
Material
75
70
70
65a
60a
8 Extremely high ash content coals will require higher fineness
than indicated.
The required fineness, together with grindability index
and required coal tonnage (BTU release), determines the
number of pulverizers and their size. Higher grindability
index coals require less pulverizer horsepower. In general
lower grindability index coal requires more pulverizer air
(primary air). The relative effects of changes in coal
grindability, coal fineness and heat content of the fuel are
4-10
-------
shown in Figures 4-6, 4-7 and 4-8. When switching to
Western subbituminous coal for example, with 10,000 BTU/lb
heating value and Hardgrove Grindability Index (HGI) of 50,
from an eastern bituminous coal with 12,000 BTU/lb heating
value and HGI of 70, the required fineness may increase from
70 percent -200 mesh to 75 percent -200 mesh. This may
significantly reduce overall pulverizer capacity. The
required increased fineness reduces pulverizer capacity by
about 10 percent, the lower HGI reduces pulverizer capacity
by 20 percent while the lower heating value, at full load
requires handling about 20 percent more coal leaving a
deficit in pulverizer capacity of about 50 percent. Since
design pulverizer capacity may be higher than full load
requirements, the overall net effect may be somewhat less.
Increasing pulverizer capacity is a major change in the
combustion system since the capacities of all associated
equipment must be increased, this includes bins, belts,
feeders, pulverizer air, coal feed distribution systems to
burners, and possibly additional burners may be required.
The alternative to these changes includes derating the
combustion unit.
4.3.2.2 Moisture - In general higher moisture content coals
reduce pulverizer capacity. An increase in moisture content
also requires significantly higher temperature air and
generally more air to dry and carry the fuel to the burners.
Two types of moisture are present in coal, surface and
4-11
-------
1.2
1.0
0.8
0.6
0.4
60
J L
J L
70 80
COAL FINENESS, I THROUGH 200 MESH
90
Figure 4-6 Pulverizer capacity as a function of fineness.
4-12
-------
HARDGROVE GRINDABILITY
Figure 4-7 Relative pulverizer capacity as a
function of Hardgrove grindability.
4-13
-------
BOILER LOAD, %
Figure 4-8 Effect of coal heating value
on pulverizer capacity.
4-14
-------
inherent. Surface moisture is effected by wet weather
conditions, method of mining and preparation, and method of
transportation (slurry pipelines), however, typical surface
moisture is 8-10 percent with intermittent surges. Usually
surface moisture is independent of the type of coal used.
Inherent moisture is that which is intimately associated
with the coal in the pore structure. Western coals, espe-
cially lignites and subbituminous coals have high inherent
moistures. Figure 4-9 shows the effect of total moisture on
the temperature of pulverizer air. Usually most utility and
industrial combustion systems are capable of handling wet
coal (up to 20% moisture content) on an intermittent basis.
Combustion units designed for lignite and subbituminous
coals must handle total moisture contents as high as 40
percent. Significant changes in moisture content will
require modifications in the air preheater system to obtain
higher pulverizer air temperatures. One air drying system
used in a combustion system designed to burn lignite is
shown in Figure 4-10. The system utilizes 250°F drying air
in 2 stages of drying. The drying air represents about 30
percent of the total combustion air. Modifications in fuel
drying systems are possible but generally costly.
4.3.2.3 Other Factors - Other coal factors which influence
pulverizers include character and quantity of ash. These
factors generally affect pulverizer repair and maintenance
costs. Lower ash and sulfur content, especially pyrite
4-15
-------
Figure 4-9 Effect of moisture on the
temperature of pulverized air.
4-16
-------
J
PRIMARY AIR DUCT
TEMPERING AIR DUCT
/ MECHANICAL
ROTARY FEEDER COLLECTOR
Figure 4-10 Direct-firing with predrying bypass and tempering air.
-------
sulfur content, will reduce pulverizer repair and main-
tenance costs.
4.3.3 Coal Burners, Combustion Chamber, and Grate Area
The optimum combustion of the fuel is affected by coal
properties. In stoker firing, caking, swelling, volatile
matter, heating value and size consistency are significant.
In p.c. or cyclone firing coal size consistency, heating
value, moisture content, ash fusion temperature, and burning
characteristics influence the coal burner system. In gen-
eral, switching to a low-sulfur western coal has little
effect on the combustion performance of a stoker since most
western coals burn readily on a grate with little or no
trouble. The principal negative effect is the lower heating
value of the fuel which may cause derating of the combustion
unit due to the unit's ash-handling capability.
Burner requirements and combustion chamber size are
influenced by many factors. The burning profile is used by
one combustion equipment vendor to assess coal combustion
properties for design of the burner/combustion chamber
section. The burning profile is a lab test which measures
the rate of oxidation of coal at various rates of tempera-
ture rise. Figure B-l in Appendix B shows typical burning
profiles. Low rank western coals oxidize at lower temper-
atures than bituminous coals. These data indicate that the
subbituminous coals can be burned as easily as eastern
coals, however their high moisture content requires a larger
4-18
-------
combustion area to complete combustion. Slagging and foul-
ing characteristics also determine combustion chamber size
and soot blower location. Figure 4-11 shows the relative
size of combustion chambers designed for various coal types.
Compatability of combustion chamber volume with the fuel
burned is a necessity. Otherwise severe derating and
combustion problems can result.
In other stoker-firing, grate area is fixed by heat
release rate requirements. Principal combustion considera-
tions are the swelling and caking tendencies of the fuel
which if severe causes blockage of combustion air through
the grate tuyeres. Lignites and subbituminous coals are
generally noncaking and nonswelling.
4.3.4 Combustion Furnace and Heat Transfer Area
The principal coal properties which affect combustion
system operation in the combustion furnace and heat transfer
zones are slagging, fouling, corrosion and erosion. Basi-
cally these are all properties of the ash constituents of
coal. Combustion system operation can be significantly
reduced by (a) slagging - the tendency of a coal ash to form
deposits on the high temperature heat transfer surface, and
(b) fouling - the tendency of ash to form bonded deposits on
boiler tubes in the convection passes.
These problems are controllable only to a limited
extent. Many F/B systems encounter major ash problems when
burning eastern coals. Operating practice for many systems
4-19
-------
BOILER C
SUBBITUMINOUS
WYO. COAL
Figure 4-11 Influence of ash characteristics
on furnace size.5
4-20
-------
is to derate the unit in order to operate as a base load
unit even though the operation is at a lower than design
output. This section summarizes the present status of
knowledge on ash related problems. The state of the art of
controlling ash problems is not sufficiently advanced to
offer universal solutions or even to accurately predict the
extent of ash problems prior to fuel scale combustion tests.
4.3.4.1 Coal Ash - Behavior When Heated - What is commonly
termed coal ash has its origin in coal as specific complex
mineral entities. This mineral matter includes complex
metal and organic silicates, chlorides, carbonates, sul-
g
fides, sulfates, phosphates, etc. The principal elemental
constituents are calcium, aluminum, iron, silica, magnesium,
sulfur, sodium and potassium and manganese. Minor and trace
elements include most of the naturally occurring elements of
the periodic table.
When coal is burned, the flame temperature generally
exceeds 3000°F, however the expanding hot gas is rapidly
cooled by heat absorption in the water walls and convection
passes of the F/B system. During combustion and subsequent
cooling, the mineral content of the coal is thermally de-
composed and generally melted and fused in particles which
become fly ash, vapor or slag. Fly ash is that material
which has passed through the F/B convection passes as
discrete particles. Vaporized mineral matter includes
sodium and potassium oxides which can deposit on boiler
4-21
-------
tubes in the lower temperature zones of the F/B system.
Slag is molten ash which is removed in wet bottom or cyclone
furnaces in a molten state but which can form hard deposits
in the high temperature zone of the boiler. Figure 4-12
shows the slagging and fouling zones of a furnace.
The study of boiler ash problems has resulted in the
development of empirical relationships which are used to
evaluate the slagging and fouling characteristics of a given
coal. In general these indices rely on the relationships
between the elemental components of coal ash as determined
by chemical means and as expressed as oxides. Thus a slag-
ging index for eastern bituminous coals has been established
which relates the ratio of basic ash constituents to acidic
ash constituents. This is the B/A index.
In reality the composition of material called ash or
slag in the boiler is complex usually to a great extent
amorphous. The B/A index has been derived from a study of
slag deposits, elemental ash composition, and boiler slag-
ging problems. Table 4-1 identifies the several indices
used in assessing ash problems.
4.3.4.2 Burning Western Subbituminous Coal - Operators
experience ash deposit problems when burning subbituminous
coals in boilers designed to burn that type of coal. The
problems are aggravated in boilers designed to burn coals
with bituminous ash. As mentioned previously subbituminous
coals differ from bituminous coal. Ash characteristics
4-22
-------
Figure 4-12 Slagging and fouling zones.
(Courtesy of Babcock & Wilcox)
4-23
-------
Table 4-1 ASH CHARACTERISTICS INDICES
Index and Formula
Banae of Values and/or Significance
App1icabi11ty
Fc-O, > C.-lO ~ MqO
FejOj < CaO + MgO
Height percent
n i tumi nous Anh
Lignitic Ash
Clasnifleation of ash
FejOj + CaO +¦ MgO 4
NajO 4 KjO
Weight
Base-to-Acid Ratio (B/A)
Information required to
SiOj + A1203 4
Ti02
percent
-
calculate indices
Slagging Index, R„
E
B/A x S (weight percent)
Slagging Type
Slagging Index
Low
Medium
High
Severe
less than 0.6
0.6 - 2.0
2.0 - 2.6
2.6 or greater
Measures
tendency
coal ash
slagging
of bituminous
Fouling Index, R_
Fouling Type
Fouling Index
B/A x Na^O (weight percent)
Low
Medium
High
Severe
Less than 0.2
0.2 - 0.5
0.5 - 1.0
1.0 or greater
Measures
tendency
coal ash
the fouling
of bituminous
Slagging Index, Rc
W
(Max. HT) + 4(Min. IT)
Where:
Slagging Class
Slaqqing Index
Medium
High
Severe
2450 - 2250
2250 - 2100
less than 2100
Measure slagging
tendency of lignite ash
Max. HT =• Maximum hemispherical
softening temperature (oxidizing
or reducing conditions).
Min. IT = Minimum initial deformation
temperature (oxidizing or reducing
conditions)
S • Sulfur content, percent
Fouling Index, R_
W
NajO (weight percent)
Fouling class
Low to Medium
High
Severe
ft Na2o
less than 3
3-6
greater than 6
Measure the fouling
tendency of lignite
coal ash
Slagging Index, R^
S
T250 P0186 (ox-) " T10 000 P°iae
97. B F, '
Wherei
Slagging Claaa Slagging Index
Medium
High
Severe
0.5 - 0.99
1.0 - 1.99
greater than 2.0
Used to measure
slagging tendency of
bituminous and lignite
coal ash
Tjsq poise (ox.) - Temperature corresponding
to <(sh viscosity of 250 poise under oxidizing
conditions.
T10 goo poise (red.) - Temperature corresponding
to 10,000 poise ash viscosity under reducing
conditions
F» ¦ Slagging Factor
-------
differ also. Most subbituminous coals contain lignitic
ash, and have high to severe slagging and fouling poten-
tials.
Lignitic ash contains a higher percentage of CaO + MgO
than Fe^^. Severe slagging potential occurs with coals
having low ash furnace temperatures and fouling occurs with
coal having a high ^£0 content (see Table 4-1) .
Slag deposits form in the radiant heat receiving
section. These deposits may form only after an extended
conditioning period. The mechanism of slag formation is
complex. One possible explanation is that molten ash par-
ticles strike a boiler tube, then freeze, fracture and break
up. A small portion of the deposit remains on the tube.
After some time and many ash particles striking the residual
bonded deposit, a large deposit forms. As the deposit
thickens, the outside temperature of the deposit becomes
very hot relative to the tube side. As the temperature
grows higher the deposit grows faster. The deposit inhibits
heat transfer causing the possible formation of deposits
higher in the furnace. To minimize the affects of slagging,
furnaces should be designed so that the size of the burners
and the combustion chamber are sufficient to prevent the
molten ash particles from impinging on boiler walls. This
is done by keeping the molten ash in suspension until it is
cooled. Remedial measures include the use of wall soot
blowers to remove deposits on a routine basis. A change in
4-25
-------
slagging characteristics may require relocation of the soot
blowers. However this has a limited remedial effect.
Fouling problems occur in the convection passes. These
deposits contain a high percentage of calcium sulfate and
sodium sulfate and are formed when volatile constituents
condense on the boiler tubes and subsequently react with SC^
or SO^ in the gas stream or form bonded deposits. To
control fouling, combustion gases must be kept at design
temperature, and soot blowers must be placed in proper areas
and allowed to operate at maximum effectiveness.
Burning western coals with the above described char-
acteristics in boilers designed to accommodate eastern coals
can probably be achieved without extensive boiler modifica-
tion by derating the unit.
4.4 OTHER FACTORS
Switching to coals with low sulfur generally affects
electrostatic precipitator (ESP) performance. Numerous
studies in this area have indicated the adverse effect of
low sulfur coals on ESP performance. High fly ash resis-
tivity limits the power input to the electrostatic pre-
cipitator and hence the driving force for particle capture.
Fly ash resistivity is a function of the sulfur content
of the coal, the gas temperature and fly ash composition.
The effect of sulfur content and temperature on resistivity
are illustrated in Figure 4-13. The effect of ash com-
position is more subtle. Several indices, such as the
4-26
-------
10'
101
iz
u
J
£
-C
o
^ -
on
LH
LiJ
n:
1U'
10'
100
0.5-1 .0% S
IN 5 1 IIJ III! A SHU I flEIII Al"
TYPICAL H ? 0 5-8% VOL .
POORER
PPTN
r R I riCAL
RANGE
RESISTIVITY
GOOD
PPTN
200 300 400 500 600
GAS TEMPERATURE, T
700
800
Figure 4-13 Effect of sulfur content and temperature
on fly ash resistivity.
4-27
-------
alkalai index, have been established by precipitator manu-
facturers and others to aid in the design of precipitators
for low-sulfur coal application.
To illustrate the impact of converting from a high to
low-sulfur coal, a precipitator operating at about 98 per-
cent efficiency on a 2.5 percent sulfur coal can easily drop
to below 90 percent collection efficiency on a 1% sulfur
coal. The extent of efficiency degradation is highly
variable, depending in large part upon the plate area of the
installed precipitator and the plate rapping efficiency.
Methods for overcoming this performance degredation are:
° Flue gas and fly ash conditioning.
° Operating at higher or lower temperatures.
0 Derating the boiler.
Flue gas conditioning affects the surface conductivity
of the fly ash whereas fly ash conditioning affects the
particle's bulk resistivity. Both conditioning methods have
merit, their applicability dependent upon coal chemistry and
the precipitators operating temperatures. Flue gas con-
ditioning, however, has been more widely used. Usually
sulfuric acid, sulfamic acid, or SO^ is injected in the flue
gas in quantities sufficient to reproduce the concentrations
of SO^ that would result from high-sulfur coal combustion
(e.g., 30 to 40 ppm). Sodium salts, and to a somewhat
lesser extent iron, have been used to condition fly ash.
These are added with the coal to yield a fly ash with
4-28
-------
sodium, iron, etc. concentrations comparable to easily
collectable fly ash.
Although there are "off-the-shelf" conditioning systems,
each application is unique. The performance of the existing
precipitator on both the high- and low-sulfur coal applica-
tions must be carefully analyzed to specify the necessary
conditioning system and precipitator modifications (e.g.,
increased power supply).
The capital cost for flue gas conditioning systems is
generally in the range of $1.5 to $4.0/KW of installed plant
capacity. Operating costs (including capital charges) are
typically between 0.05 and 0.1 mills/KWH.
The effect on resistivity operating at either higher or
lower temperatures can be seen from Figure 4-13. It is
apparent that precipitator operation above about 600°F is
relatively insensitive to coal sulfur content, hence the
popularity of "hot" precipitators for low-sulfur coal
application. However, this entails the installation of a
completely new precipitator. Only a limited number of
existing plants have opted for this approach; these plants
initially had only marginal collectors (e.g., multiple
cyclones) even for their high-sulfur coal application.
A few plants have improved precipitator efficiency by
reducing operating temperatures. However, this option also
finds limited application; the plants must either be able to
reduce temperatures or make the necessary modifications to
4-29
-------
the air preheater. Many plants are also reluctant to
operate at lower temperatures because of concerns about
"cold-end" corrosion of the air preheater. Thus, this
approach must be carefully evaluated to determine whether it
will have the desired effect on precipitator performance and
its overall viability for the plant in question.
Precipitator performance can also be increased by
reducing the gas flow through the unit. This can be accom-
plished by derating the boiler. However deratings of at
least 20 to 30 percent would be required to compensate for
most low-sulfur coal conversions. Thus this approach would
find only very limited use (e.g., boilers being phased out
in a few years).
The most straight forward method for upgrading pre-
cipitator performance is to install additional plate area.
This yields a precipitator properly sized for the low-sulfur
coal application. Most plants have sufficient space for
installing the required additional plate area, but capital
costs can be high (25 to $70/KW). If the plant has limited
remaining life or low capacity factor, then operating costs
can also be high (e.g., 6 mills/KWH for a plant with a
remaining life of 10 years and a 30 percent capacity factor
vs. 2.5 mills/KWH for a plant with over 20 years remaining
life and a 50% average capacity factor).
Thus the preferred options for upgrading precipitator
efficiency are usually to install a carefully designed flue
4-30
-------
gas conditioning system and for newer plants, install the
collection plate area if required.
4.5 CLEAN COAL
Ash impurities and pyritic sulfur can be removed from
coal by conventional coal cleaning. Some eastern bituminous
coals can be sufficiently cleaned to meet a 1.2 pounds of
SC^/MM BTU regulation. This deep coal cleaning produces a
low ash (2-5 percent), low sulfur (1 percent or less), and
high BTU content (at least 14,000 BTU/lb). To date there
are no reports of utilities using such low-sulfur cleaned
coal, therefore actual operating experience is unknown. The
extent to which clean coal ash will exhibit slagging and
fouling tendencies remains to be determined. Some unpub-
lished laboratory tests on a Pennsylvania bituminous coal
indicate that even though the quantity of ash is low, the
relative percentage of ash elements tends to remain the
same. If this is true, then a slagging dirty coal will
produce a slagging clean coal but the degree of slagging
might be minimized because of the lower quantity of ash in
the clean coal. However the reduced ash content will,
itself, reduce operating and maintenance costs.
Blended coals present no combustion problems unique
from those discussed earlier. Many plants receive coals
from multiple sources and blend these coals to various
extents: from essentially nil to the extensive blending
facilities proposed by some plants to provide a uniform
4-31
-------
supply of low-sulfur coal to the boilers. The properties of
the individual coals should be compatible with the boiler
requirements because of the possibilities of acute blending
inadequacies or coal segregation subsequent to blending.
4-32
-------
APPENDIX A
DETAILED DESCRIPTION OF
FURNACE/BOILER SYSTEMS
A-l
-------
APPENDIX A DETAILED DESCRIPTION OF
FURNACE/BOILER SYSTEMS
It is generally considered that a "boiler" is a device
which burns fuel to produce steam. However, technically
this is not true. A boiler is in reality a facility which
includes, (1) a furnace in which the fuel is burned and (2)
a boiler in which steam is produced. Since there is a
distinct difference between the two entities, the term
"furnace/boiler" is used throughout this report to identify
the entire facility.
In order to appreciate the workings of steam generation
facilities, it is helpful to look at the basic designs.
Figure A-l illustrates the simple shell-type boiler = this
is very much akin to a tea kettle arrangement.
STEAM OUT
TO
ATMOSPHERE
WATER IN
STACK
HEAT
TRANSFER
-SURFACE
Figure A-l Shell-type boiler.
A-2
-------
As can be seen from the Figure, the heat transfer surface is
small relative to the water volume. A refinement on this
method is the firetube boiler shown in Figure A-2. This
unit resembles the type of boiler often found in residences.
Note that the heat transfer surface, relative to the water
volume has been increased over the shell-type boiler by
ducting the hot combustion gases thru multiple tubes.
TO
ATMOSPHERE
STEAM OUT
2-
Figure A-2 Fire-tube boiler.
In order to increase heat transfer efficiency even
further, the water-tube boiler was developed. This is shown
in Figure A-3. This principle is used in all large fossil-
fuel-burning steam producing facilities today. The logical
extension of the water-tube concept is the modern pulverized-
coal-burning unit shown in Figure A-4. In this type of
facility, the entire boiler is encompassed in the furnace in
A-3
-------
an effort to wring out every bit of available heat from the
combustion gases. Thus the gases: (1) preheat the water
prior to entering the steam drum; (2) convert the water to
steam thru heat exchange with the furnace wall tubes; (3)
superheat the steam to achieve an even higher temperature,
and finally; (4) preheat the incoming combustion air.
TO
ATMOSPHERE
Figure A-3 Water-tube boiler.
Part of this preheated combustion air is ducted to the coal
pulverizer to preheat the coal and to control the coal's
moisture content to facilitate pulverizing. Most furnace/
boiler facilities have radiant and convection surfaces. In
other words, there are zones which "see" the brilliant
combustion process (radiant zone) and zones which are shielded
from the glare (convection zone). Heat transfer in the
latter zone is solely due to convection. These zones are
shown in Figure A-5'.
A-4
-------
Figure A-4 Pulverized coal-fired unit.
(Courtesy of Babcock & Wilcox)
A-5
-------
Figure A-5 Heat transfer zones in a pulverized-
coal-fired boiler.
(Courtesy of Babcock & Wilcox)
A-6
-------
There are generally three types of furnaces which have
been designed for use with modern boilers. Table A-l shows
the various modern furnace/boiler combinations and size
ranges. Each of these is discussed in detail in subsequent
sections.
Table A-l MODERN FURNACE/BOILER COMBINATIONS
AND SIZE RANGES
Furnace type
Boiler type
/V // /
/ /^/ /
Natural circulation
e
0
©
Once-thru forced circulation
6
%
Approximate size range, MW
up to
35
up to
1300
50 to
1050
A.1 FURNACE TYPES
A.1.1 Stoker Units
A stoker is generally thought of as a conveying system
used to feed coal into a furnace. It also provides a moving
grate upon which the coal is burned. There are three gen-
eral types of stoker furnaces: 1) underfeed retort; 2)
chain grate, and 3) spreader. Stoker furnaces are limited
in feed rates and are generally used on units rated at less
than 600 million BTUH heat input.
A-7
-------
A.1.1.1 Underfeed Retort Stokers - There are many varia-
tions of the underfeed retort stoker depending on:
a. whether the coal is horizontally fed or gravity fed
b. whether the ash is discharged from the end or the
sides
c. number of retorts.
Single or double retort units generate up to 30,000 pounds
of steam per hour (40 million BTUH heat input). Multiple
retort gravity fed stokers generate up to 400,000 pounds of
steam per hour.
In the side discharge, horizontal underfeed stoker,
shown in Figure A-6, coal is intermittently force-fed to the
fuel bed by a ram or, in very small units, is continuously
fed by a screw. The coal moves in a longitudinal channel,
called a retort, usually assisted by an auxiliary push rod
with small pusher blocks at the bottom of the retort. After
the retort is full, the fuel is forced upward by the ram and
spills over the top on each side to form, and to feed, the
fuel bed. Air is supplied through tuyeres on each side of
the retort and through openings in the side grates. Combus-
tion air is force-fed from a plenum chamber or windbox
through the tuyere openings and perforated grates. Manually-
operated dampers admit air from the central plenum to the
ash pits for final burnout before the ash is dumped.
Some designs of side-dump underfeed stokers rely on
pressure from the incoming raw fuel to achieve distribution
A-8
-------
STROKE
ADJUSTER
PUSHER ROD
PUSHER BLOCKS
AIR CHAMBER
OVERFIRE
AIR
COAL
HOPPER
COAL RAM
TUYERE
BLOCKS
COMBUSTION
, GRATE
COMBUSTION
GRATE v
RETORT
PUSHER
BLOCK
DUMP
PLATE
DUMP
PLATE
AIR
ZONING
DAMPER
AIR
ZONING
DAMPER
AIR CHAMBER
ASH PIT
ASHPIT
Figure A-6 Single-retort, horizontal underfeed stoker
2
with side ash discharge.
A-9
-------
over the side grates. For those wider than 8 feet, however,
distribution is obtained from reciprocating tuyere-block
action between the retort section, as shown in Figure A-7.
Other designs, with a single center retort, depend upon
agitating grates for fuel distribution.
Figure A-7 Double-retort, horizontal-feed type of
2
side-ash-discharge underfeed stoker.
The rear-ash-discharge, gravity-feed type of underfeed
stoker is usually longer than the side-ash-discharge,
horizontal-feed type is always fitted with multiple re-
torts. The multiple retort stoker, Figure A-8, consists of
a series of inclined single retorts placed side by side with
tuyeres, and sloped from front to rear for constant ash
discharge. Each retort is equipped with a primary ram,
which feeds the coal into the ram at the head of the retort.
From this point, the fuel bed is moved slowly toward the
A-10
-------
rear and at the same time is forced upward over the bank of
tuyeres by secondary pushers or by the moving bottom of the
retort. Most of the combustion air enters through the boxes
supporting the tuyeres, which are located between the re-
torts. An overfeed or clean-up section is provided at the
rear end of the bank of tuyeres to complete combustion
before the ash is discharged to the pit for disposal.
COAL HOPPER
Figure A-8 Multiple-retort, gravity feed underfeed
stoker with rear ash discharge.
Overfire-air is commonly used with underfeed stokers to
provide some combustion air and turbulence in the flame zone
directly above the active fuel bed. This air is provided by
a separate overfire-air fan and is injected through small
nozzles in the furnace walls. Overfire-air is effective in
preventing smoke, especially at low loads with a "lazy"
fire, or when sudden increases in coal feed occur.
A-11
-------
Underfeed stokers are started by placing kindling or
oiled waste on each side of the retort. Light air pressure
is applied to hasten burning after ignition is well estab-
lished.
An underfeed stoker has the capability of burning a
wide range of coals at the time of design including coking
coals and anthracite. Once designed to burn a particular
coal, a switch will effect the unit's efficiency. Effi-
ciency may increase or decrease depending on the coal ash
characteristics. For example, a unit designed to burn
anthracite may experience a higher efficiency upon switching
to an easier-to-burn low sulfur bituminous. On the other
hand, if the new fuel has a higher ash content, the stoker
may have to be slowed down to accommodate the ash handling
system, thereby derating the unit. The underfeed stoker is
best suited for free burning bituminous coals. The size
factor of the coal represents a direct capacity and effi-
ciency factor on the underfeed stoker. The most desirable
size consists of 1-1/4 inch and under with not more than 50
percent fines that will pass through a 1/4 inch screen.
A.1.1.2 Chain-Grate or Moving-Grate Stokers - Moving-grate
stokers are classified as overfeed stokers. They are
equipped with chain or traveling grates and with refractory
arches or overfire-air jets to improve combustion. This
type of stoker is usually designed for forced draft; natural
draft designs are gradually becoming obsolete.
A-12
-------
Chain- and traveling-grate stokers, can produce up to
300,000 pounds of steam per hour. A continuous fuel burning
rate of 500,000 BTU per square foot of grate per hour can be
achieved.
In chain-grate and traveling-grate stokers, assembled
links of grates are joined in endless belt arrangements that
pass over sprockets or return bends located at the front and
rear of the furnaces. As shown in Figure A-9, coal is fed
from the hopper onto the moving assembly and enters the
furnace after passing under an adjustable gate that regulates
the thickness of the fuel bed. At the far end of the travel,
combustion is completed and ash is discharged over the end
of the grate into the ashpit.
Figure A-9 Chain-grate stoker with rear ash discharge.2
A-13
-------
The links of the chain-grate are so assembled that they
break at the drum end of the furnace in a scissors-like
manner so as not to retain siftings and clinkers adhering to
the grate surface. Conversely, the traveling-grate does not
have any relative scissor-type movement to free clinkers.
Most stoker-fired furnaces are provided with water
cooling. Completely water-cooled furnaces require less
maintenance and form less slag than refractory or air-cooled
constructions.
The furnace is ignited by placing kindling over the
coal and lighting it. Sufficient air is admitted to the
compartments to cause the kindling to burn briskly. After
the kindling has ignited the coal, the stoker is started and
maintained at a speed adequate to light the entire bed.
The chain-grate and traveling-grate stoker can initially
be designed to burn the entire spectrum of solid fuels.
A. 1.1.3 Spreader Stokers - The spreader stoker combines
suspension burning and a thin, fast-burning fuel bed on a
grate. A rotating paddle system throws coal into the grate.
Spreader stoker capacities range from 5,000 to 400,000
pounds of steam per hour.
The modern spreader stoker, as shown in Figure A-10
consists of feeder units (arranged to distribute fuel over
the grate area), a grate, forced-draft systems for both
undergrate and overgrate air, and combustion controls to
coordinate air and fuel supply.
A-14
-------
COAL
HOPPER
OVERTHROW
• ?°T9* -
FEEDER
.J -¦¦ ¦ ' TRAVE-DIRECTION
SIDE-WALL HEADER ' ~ " "*>
STOKER CHAIN J.
AIR PLENUM
Figure A-10 Traveling-grate spreader stoker with
2
front ash discharge.
An integral part of many spreader-stoker firing systems
is the provision for fly ash recirculation wherein the fly
ash that is removed from the flue gas stream is reinjected
into the furnace. A gravity flow fly ash return is shown in
Figure A.11. Pneumatic conveying systems are used for
reinjection in the high temperature zone above the grate for
burning.
Traveling-grate spreader stokers are generally in-
stalled with one large plenum or air chamber under the
entire grate surface. Overfire-air systems are useful in
promoting good combustion and reducing smoke formation,
especially at low load.l
A-15
-------
Figure A-ll Spreader stoker with gravity
flow fly ash return.
(Courtesy of Babcock & Wilcox)
Before light-off, the spreader distributes coal evenly
over the grate to a depth of approximately one inch.
Kindling is then placed over a large portion of the coal
surface and ignited. As the fuel ignites, the fan speeds or
dampers are adjusted to supply sufficient combustion air.
Spreader stokers are versatile and can be designed to
burn almost any type of solid fuel. Free burning bituminous
and lignite coals are commonly used and other fuels such as
bagasse (refuse and sugar cane), and wood waste are also
satisfactory. Anthracite is generally not satisfactory
because it is a low volatile fuel and does not suspension
burn adequately.
A-16
-------
A.1.2 Pulverized-Coal-Fired Units
Pulverized-coal-fired units operate on the principle of
suspension burning. Coal is pulverized to the consistency
of talcum powder and pneumatically injected into the furnace.
These furnaces are classified as dry-bottom or wet-bottom
depending on whether the ash is removed in the solid or
molten state. Figure A-4 illustrates a pulverized-coal-
fired unit. As shown in Figure A-4 the coal is pulverized
at the boiler. This is called a direct firing system. In
the direct-firing system, hot primary air is ducted to the
pulverizer where the raw coal is dried, pulverized, and
pneumatically conveyed to the burners in a continuous
pattern. Coal and primary air are mixed before entering the
burner.
Another pulverized coal firing system is the now out-
dated bin system. The coal is processed at a location apart
from the furnace. It is dried, pulverized, classified
within the pulverizer, and then stored. From storage, the
pulverized coal is conveyed pneumatically to utilization
bins. This system was used extensively before reliable
pulverizers were developed. However, the direct-firing
system, described above, is now used predominantly.
There are several types of burners. The circular type,
shown in Figure A-12, is most frequently used. Either gas
or oil can be burned effectively as alternate fuels. The
maximum capacity of these individual burners is 165 million
A-17
-------
BTU per hour. Up to as many as 70 burners may be used
although 16 to 30 burners is more commonly found.
Figure A-12 Circular burners for firing pulverized coal.2
A.1.3 Cyclone-Fired Units
As with pulverized coal-fired units, cyclone-fired
units require that the coal be processed prior to combus-
tion. However, the coal is merely crushed as opposed to
pulverized. These furnaces are best suited for steam rates
of 200,000 pounds per hour or higher. Principal use is for
steam and electric generation in public utilities.
Figure A-13 illustrates operation of the cyclone
furnace. The furnace operates at temperatures sufficiently
high to melt the ash into a liquid slag which coats the
walls of the cyclone. Incoming coal, after being crushed,
is thrown to the walls of the cyclone barrel, held in a slag
layer and contacted by high or low velocity tangential air.
The gaseous products of combustion are discharged through
A-18
-------
2
Figure A-13 Cyclone furnace operation.
the center portion of the barrel into the boiler. Molten
slag in excess of the thin layer retained on the walls
continually drains toward the rear and discharges through
the tap hole to a slag tank. The furnace features a high
fuel heat-release and gas temperatures exceed 3000°F. The
cyclone furnace has a wide range of usable fuels. Generally
speaking, the poorer and cheaper fuels are better suited for
the cyclone furnace. In general, it is required that the
slag formed from the ash have a viscosity of not more than
250 poises at 2600°F.
A-19
-------
Babcock and Wilcox, the major manufacturer of the
cyclone unit has discontinued production of this type of
unit because of the high bulk furnace gas and peak flame
temperatures necessary for combustion resulting in higher
than allowable NO emissions.
x
A. 2 BOILER TYPES
A.2.1 Natural Circulation Boiler
As its name implies, the natural circulation boiler
provides water circulation by the difference in density
between steam and water. Various fuels and a broad range of
steam conditions are pre-engineered into the natural cir-
culation boiler. The furnaces utilized in conjunction with
this boiler type are the cyclone and pulverized-coal-fired
units burning bituminous, subbituminous and lignite type
coals. The furnace walls are generally water cooled and the
furnace utilizes a balance draft or pressure system.
Modernly these boilers consist of two or more drums
connected by tubes which enter the drums radically as shown
in Figure A-14. Since the tubes must be bent, the boiler is
generally known as a bent tube boiler. These boilers are
also referred to as two, three, or four drum Stirling
boilers, depending on the number of drums utilized. Most
boilers used today are of the two-drum type. The bottom
drum is always filled with water while the top drum, known
as the steam drum, is filled with both water and steam.
A-20
-------
Figure A-14 Schematic of the two-drum bent tube boiler.
The capacity range for a natural circulation boiler is
approximately 300,000 pounds per hour minimum, to an un-
determined maximum, which exceeds 7,000,000 pounds per hour
(700-800 megawatts). The operating pressure for a unit of
this type ranges from 1800 to 2400 psi and the steam and
reheat temperatures usually run approximately 1000°F.
A.2.2 Forced Circulation Boiler
The forced circulation boiler is a drumless boiler.
The principle of operation is that of the once-through or
"Benson" Cycle. The working liquid is pumped into the unit,
it then passes sequentially through all the pressure-part
heating surfaces, where it is converted to steam as it
absorbs heat, and leaves as steam at the desired temperature.
There is no recirculation of water within the unit and, for
this reason, a drum is not required to separate water from
A-21
-------
steam. On the surface, this arrangement would appear to be
technically simpler than the drum-type boiler. However, it
is a child of modern boiler technology providing a means by
which highly efficient plants can operate in the super-
critical range that is the water becomes a working fluid
above the vapor dbme of the p-V and T-s diagrams. Forced
circulation is needed because, as the fluid approaches and
passes the critical limits (3206.2 psi and 705.4°F), natural
circulation (due to the difference in densities between the
hot and cold fluids) cannot take place. In this state,
density differentials are too small to adequately overcome
the friction between the fluid and the tube walls.
The capacity range on this boiler type ranges from a
minimum of 300,000 pounds per hour to an undetermined
maximum exceeding 10,000,000 pounds per hour (1000 mega-
watts) . Operating pressure is approximately 24 00 psi at
subcritical condition and 3500 psi at the supercritical
condition. The steam and reheat temperatures range approxi-
mately 1000°F. Constant steam temperature can be maintained
to one-fourth load and the reheat temperature can be main-
tained, to 60 percent load with its respective fuel. To
maintain these temperatures, the fuel characteristics cannot
vary to any large extent.
The furnaces that are normally utilized with this
boiler type are the cyclone and the pulverized-coal-fired
units utilizing water or steam-cooled radiant surfaces.
A-2 2
-------
APPENDIX B
TEST METHODS FOR
PHYSICAL AND CHEMICAL
PROPERTIES OF COAL
B-l
-------
APPENDIX B TEST METHODS FOR PHYSICAL AND
CHEMICAL PROPERTIES OF COAL
B.l CLASSIFICATION
ASTM defines coal rank as the degree of metamorphism,
or progressive alternation, in the natural series from
lignite to anthracite. Coals are classified by rank according
to their volatile matter, fixed carbon, bed moisture and
calorific value as shown in Table B-l.
The four basic ranks of coals; namely anthracite,
bituminous, subbituminous and lignite can be found predom-
inantly in various parts of the United States. Anthracite
is found in the northeast; bituminous coal in the Eastern
Appalachian region and midwest; subbituminous in the midwest
and farwestern states; and lignite in the Dakotas and
Montana.
B.2 CHEMICAL AND PHYSICAL PROPERTIES OF COAL
B.2.1 Analysis of Selected U.S. Coals
Table B-2 lists the constituents and properties of
selected coals. This table gives some idea of the ranges
and differences encountered between both coals of different
type, or rank, and within a rank itself.
B-2
-------
Table
B-l CLASSIFICATION OF
COAL
BY RANK
d.
f
ASTM D 388
-386
Class
Cioup
l'i\ed CaiUm
Limits, %
(Diy, Mineial-
Matter-Fire
Basis)
Volatile Matter
Limits, %
( Diy, Muicial-
Mattci-l'iee
basis)
Calorific Value
Limits, lltu/lb
(Moist,1'
Mineial-Matter-
Free Basis)
Agglomerating
Character
'jqual oi
Cieater Less
Than Than
Greater
Than
Equal
oi Less
Than
Equal or
(I renter
Than
Less
Than
i. Anthracitic
1. Meta-authiacite
2. Anthracite
3 Si'inianthiacite'
98 —
92 98
86 92
2
8
2
8
1-1
—
z •
Nonagglomeiating
II Bituminous
1. Low volatile bituminous coal 78 86
2. Medium volatile bituminous coal (>9 78
3. High volatile A hituniinoiis coal — 09
4. High volatile H bituminous coal — —
5 lligli volatile C bituminous coal — —
14
22
.31
22
¦31
14,000'!
.13,000'!
111,500
\ 10,500"
14,000
13,000 j
11,500
Commonly
~aggloinciatingc
Agglomerating
III Siibhituminoiis
1 Siibbituminous A
2 Siibbitunimoiis 11
3. Si ibl ii t mi n no us ('
coal
coal
coal
—
10,500
9,500
8,300
1 1,500
10,500
9,500
^Nonagglomeiating
IV Ligiutu
1 1.ignite A
2 1,ignite li
—
0,300
8,300
6,300
"This classification dots nol include a few coals, pnncipally non-
handed vaneties, winch have unusual physical and chemical piop-
cili's and wliuh conic willnn the hunts of fixed carhnn 01 calonfic
\;ilue of the hn^h-volaljlc bituminous and siibhitunnnnus ranks. All
ill these co.lis eithei (ontain less than 48% di), imnei al-iuattei -
lnr li\cd c.tilnin <>! have moie than 15,500 moist, nnneial-inatter-
11ci linlish tl in mal Hints pn |><>uml.
'*Mi>isi iclns to coal tout.lining its natuial inhcicnt moisture but
not including visible watei on the surface of the coal.
' If agglomeiating, classify in low-volatile gioup of the bituminous
class.
''Coals having 69% or moic fned carbon on the div, mmcral-
niattcr-fice basis shall be classified aecouling to fixed caibon.
legaidlcss of calorific, \alur.
''It is lecogni/ed that tlicie may be nonagglomeiating varieties in
these groups of the bituminous class, and there aie notable excep-
tions in high volatile C bituminous group
B-3
-------
Table B-2 RANGES FOUND IN CONSTITUENTS AND
PROPERTIES OF SELECTED COALS
State
Analysis of Coals, As Received
Coal
Type
h2o
VM
FC
Ash
Sulfur
BTU
Pa.
2.0
1.8
86.2
10
0.79
13070
Anthracite
Pa.
4.0
17
69
10
1.63
13430
Bituminous
Pa.
3.0
23.1
63. 9
10
2.17
13600
Bituminous
Ky.
3.0
34.4
56.6
6.0
0.72
13800
Bituminous
Ohio
6.0
34.8
49.2
10
2.44
12450
Bituminous
111.
14
34.3
39.7
12
4.07
10470
Bituminous
Iowa
13.9
36.9
35.2
14
6.15
10244
Subbituminous
Col.
24
30.2
40.8
5
0.36
9200
Subbituminous
Wyoming
24
30
36
10
0.33
8450
Subbituminous
N. Dak.
40
27.6
23.4
9.0
1.42
6330
Lignite
B-4
-------
B.2.2 Burning Profiles of Selected Coals
Figure B-l compares burning profiles of coals of different
rank. Low rank coals exhibit a weight loss at low temperatures
due to the evaporation of its moisture. These coals also
start oxidation at a lower temperature and burn out sooner
than coals of high rank. Because of the high moisture
content found in most western coals, they must remain in a
hot zone of the furnace long enough to complete combustion.
300 500 700
FURNACE TEMPERATURE, °C
1100
Figure B-l Burning profiles of coals of different rank.
B-5
-------
B.2.3 Grindability
Grindability is a measure of the ease with which a coal
may be pulverized or ground. This property is described in
terms of a Hardgrove Grindability Number, which is an inverse
function of the energy required to pulverize a given quantity
of coal. The standard is 100 and is indicative of excellent
grindability. Figure B-2 indicates the variability of
grindability by rank of coal.
120
30 —— — —'
LIGNITE SUBBITUMINOUS BITUMINOUS ANTHRACITE
Figure B-2 Grindability index of coal.
B.2.4 Ash Analysis
Table B-3 compares the ash analyses of selected coals
of different rank. These values will vary from coal-to-
coal. However, the ash analysis can give some indication of
the fouling and slagging tendencies of the coal.
B-6
-------
Table B-3 COMPARISON OF ASH ANALYSES OF
SELECTED COALS
Type of Coal
Coal Analysis
Lignite
Subbituminous
Bituminous
BTU/lb
6500
9086
10,290
% Ash
10
10.6
18
% Moisture
40
27. 2
10.4
% Sulfur
0.8
1.0
5.1
Ash Analysis, %
Si02
28. 4
34.2
44.6
Al2°3
11
15
18
Ti02
0.4
0.8
0.6
Fe2°3
14
12
18
CaO
18
18
15
MgO
5
4.5
1.2
Na20
7.0
0.3
1.35
k2o
0.7
0.3
1.9
S03
19. 8
17
5.0
B/A
1. 04
0.70
0. 59
"^sion tempera-
ture
(Reducing)
IT
2050
2180
1900
HT
2110
2270
2300
FT
2310
2500
2450
Fouling
potential
High
Medium
High
Slagging
potential
Severe
High
Severe
B-7
-------
Table B-4 AVERAGE ASH CONSTITUENTS OF THREE RANKS OF COAL
Bituminous States (averages percent of ash constituents)
State
Ash
S
Si02
Al2°3
Fe2°3
Ti02
P2°5
CaO
Mgo
Na20
KJ
o
S°3
Def.
temp.,
"F
Soften
temp.,
•F
Fluid
temp.,
°F
Alabama
9.0
1.6
43.7
26.4
19.9
1.18
0.23
2.98
1.29
0.27
2.36
2.13
2385
2318
2490
Arkansas
8.25
2.5
24.8
19.75
23.4
0.95
1.06
13.1
4.9
1.45
1.25
10.15
2175
2270
2400
Illinois
10.05
3.35
45.5
19.1
23.33
0.95
0.157
5.16
0.89
0.373
1.62
1.73
2016
2085
2290
Indiana
8.62
2.88
46.9
22.78
20.7
1.08
0.145
3.39
0.88
0.45
2.43
1.07
2214
2325
2512
Iowa
13.4
5.15
34.3
13.95
33.4
0.85
0.29
9.65
1.25
0.5
1.2
3.05
1975
2025
2165
Kansas
10.45
4.0
38.2
16.35
37.75
0.65
0.16
6.75
0.55
0.3
1.0
2.7
1970
2025
2200
E. Kentucky
6.32
1.07
46. 2
27.5
10.5
1.43
0.13
2.16
1.04
0.45
1.86
2.17
2463
2615
2710
W. Kentucky
10.14
3.03
47.86
23.05
21.76
1.20
0.16
2.19
0.92
0.25
2.37
1.00
2034
2352
2501
Maryland
9.5
1.3
51.65
30.35
10.05
1.4
0.21
1.85
0.65
0.6
2.55
0.85
2705
2790
2740
Missouri
11.73
4.6
42. 2
15.8
31.05
0.7
0.1
4.9
0.65
0.15
2.1
2.45
1978
2028
2295
Ohio
11.6
3.6
31.6
22.9
28.0
1.0
0.21
2.0
0.69
0.24
1.5
1.14
2092
2206
2411
Pennsylvania
10.23
1.95
45.43
27. 55
21.15
1.05
0.27
1.85
0.55
0.21
1.95
1.26
2377
2456
2579
Tennessee
10.4
2.0
47.7
36.32
15.9
1.19
1.86
1.91
1.25
0.31
2. 68
1.6
2411
2456
2610
Utah
7.7
0.76
51.4
15.1
7.4
0.96
0.58
11.8
3.3
1.7
0.6
6.0
2166
2250
2409
Virginia
7.8
1.09
45.6
27.8
14.6
1.34
0.24
4.5
1.5
0.88
2.1
2.5
2377
2485
2623
N.W. Virginia
10.21
2.56
41.20
26.11
23.38
1.16
0.40
3.39
0.85
0.40
1.62
2.36
2331
2376
2529
S.W. Virginia
7.73
1.00
50.86
30.89
10.50
1.52
0.27
2.07
0.81
0.56
1.74
1.67
2682
2638
2737
Subbituminous States (averages percent of ash constituents)
State
Ash
S
Si02
A12°3
Fe2°3
Ti02
P2°5
CaO
MgO
Na20
k2o
S03
Def.
temp.,
°F
Soften
temp.,
°F
Fluid
temp.,
° F
Montana
12.6
0.59
35.4
21.5
5.31
0.83
0.41
13.46
4.63
2.8
0.67
13.33
2355
2435
2505
Hew Mexico
10.53
1.13
49.2
21.82
13.76
1.0S
0.06
6.38
2.0
0.67
0.58
4.68
2318
2372
2474
Wyoming
10.4
1.2
31.6
16.9
9.7
1.4
0.36
20.1
4.6
0.15
0.55
15.3
2450
2510
2630
Lignite State (average percent of ash constituents)
State
Ash
S
Si02
A12°3
Fe2°3
Ti02
P2°5
CaO
MgO
Na20
k2o
so3
Def.
temp.,
°F
Soften
temp.,
°F
Fluid
temp.,
•F
North
Dakota
11.8
0.98
2,6.3
12.1
6.85
0.73
0.21
21.1
6.4
4.42
0.33
20.6
2180
2237
2303
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B.3 COAL TEST METHODS
Coal test methods used in assessing coal combustion
properties are identified below. Where standard methods
exist, only the ASTM designation is given. Where special
methods are used, a brief description or reference is pro-
vided.
B.3.1 Proximate Analysis
Measure of ash, volatile matter, fixed carbon and
moisture - ASTM Method D-271-70.
B.3.2 Ultimate Analysis
Measure of elemental carbon, hydrogen, nitrogen, sulfur
and oxygen - ASTM Method D-271-0.
B.3.3 Heating Value
BTU or calorific content of coal - ASTM Method D-
20155-66.
B.3.4 Total Sulfur and Forms of Sulfur
Total sulfur, sulfate sulfur and pyrite sulfur are
determined analytically. Organically bound sulfur is
obtained by subtracting pyrite and sulfate sulfur from total
sulfur content. ASTM Methods D-271-70 and D-2942-68.
B.3.5 Ash Fusion Temperature
Measure of ash melting characteristics under oxidizing
and reducing conditions - ASTM Method D-1857-68.
B.3.6 Ash Composition
Elemental composition of ash is determined by spectro-
graph^ methods, ASTM Method D-2795-69. Results of elemental
composition are reported as oxides.
B-9
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B.3.7 Ash Viscosity
Viscosity of ash at various temperatures and atmospheres
is determined in a special viscosimeter. "Relationship of
Coal-Ash Biscosity to Chemical Composition", W„ L. Sage and
J. B. Mcllnoy, Journal of Eng. for Power, April I960, pp.
145-155.
B.3.8 Burning Profile
The rate of oxidation of coal is measured at various
rates of heating. "Further Development of the Burning
Profile", C. L. Wagoner and E. C. Winegartner, Journal of
Eng. for Power, April 1973, pp. 119-123.
B.3.9 Grindability
Measures relative hardness of coals - ASTM Method D-
409-71.
B.3.10 Swelling Index
Measures relative swelling characteristics of coal -
ASTM Method D-720-67.
B-10
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REFERENCES
1. Assessment of Alternative Strategies for the Attainment
and Maintenance of National Ambient Air Quality Stand-
ards for Sulfur Oxides. PEDCo-Environmental Special-
ists, Inc., Cincinnati, Ohio. Prepared for the U.S.
Environmental Protection Agency, Research Triangle
Park, North Carolina, Contract No. 68-02-1375, Task
Order No. 17. December 6, 1974. P. 42.
2. Devitt, T. W., R. W. Gerstle, N. J. Kulujian. Field
Surveillance and Enforcement Guide: Combustion and
Incineration Sources. U.S. Environmental Protection
Agency, Research Triangle Park, North Carolina. Pub-
lication Number APTD-1449. June, 1973.
3. Bender, R. J. Steam Generation. Power. June, 1964.
Special Report. P. 48.
4. The Babcock and Wilcox Co. Steam - Its Generation and
Use. New York, Babcock and Wilcox, 1955. P. 17-2.
5. Blue, J. D., J. L. Clement, V. L. Smith. Effect of
Coal and Multi-Fuel Firing on Industrial Boiler Design.
Babcock and Wilcox. (TAPPI Engineering Meeting.
Seattle, October 21-24, 1974.) P. 9.
6. The Babcock and Wilcox Co. Steam - Its Generation and
Use. New York, Babcock and Wilcox, 1972. 607 p.
7. Moore, G. F., R. F. Ehrler. Western Coals - Laboratory
Characterization and Field Evaluations of Cleaning
Requirements. Babcock and Wilcox; Diamond Power Specialty
Corp. (ASME Winter Annual Meeting. Detroit. November
11-15, 1973.) 6 p.
8. Wilson, E.B., J. W. Leonard, R. W. Borio. A Mining
Operations Systems Approach for Elimination of Corrosion
at Power Plants. Coal Research Bureau, West Virginia
University, Report No. 37.
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
subject: Rep0rt on the Effect of Switching Coals on date:
Furnace/Boiler Operations
October 15, 1975
FROM:
Joseph Padgett, Director
Strategies and Air Stan
TO:
See Below
Enclosed for your information and use is a copy of a final
report entitled "Analysis of the Effect of Coal Properties on Furnace/
Boiler Combustion Characteristics." The report was prepared to pro-
vide guidance on the review and analysis of proposals for switching
boiler fuels. The report focuses on the effects on boiler operations
of switching from a high sulfur to a low sulfur coal. Coal properties
are discussed and various furnace/boiler operations are described. Be-
cause of the complexity of furnace/boiler operations and equipment and
the numerous variables in coal composition, the report does not pro-
vide specific fuel and equipment recommendations. However, it does
provide general information on the problems caused by changes in fuel
on furnace/boiler operations and possible solutions to these problems.
As stated in the report, "coal switching remains more of an art than
a science."
This report highlights those problems which could occur during
the switching from one coal to another. For example, included are
discussions of the effect of ash characteristics on boiler slagging
and fouling, the impact of changes in ash fusion temperatures on wet
bottom and cyclone furnace operations, and the effect of differences
in coal sulfur levels on ESP efficiencies. Suggestions are provided
for-estimating the seriousness of these changes on the equipment and
operations mentioned. Suggested solutions to some of these problems
are also provided.
We hope this report will assist you in your work with power
plants and industrial plants involved in switching fuels. If you
have any questions or comments, contact Rayburn Morrison, Mail Drop
12, U. S. Environmental Protection Agency, Research Triangle Park,
North Carolina 27711 or call (919) 688-8201.
Enclosure
Addressees:
Air and Hazardous Material Division
Directors, Regions I, III-X, w/o enclosure
Air Programs Branch, Regions I-X
Library (APTIC), Regions I-X
John Butler, DSSE
Robert Hangebrauck, IERL
EPA Form 1320=6 (Rev. 6-72)
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