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The ENVIRONMENTAL, HEALTH SERIES of reports was
established to report the results of scientific and engineering
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Public Health Service Publication No. 999-AP-24
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CONTENTS
ABSTRACT v
I. INTRODUCTION 1
II. SUMMARY OF EMISSION DATA 3
III. PROPERTIES AND DISTRIBUTION OF COAL .... 7
Introduction 7
Coal Production and Distribution 7
Chemical Properties of Coal 9
Classification of Coal 9
Typical Properties of Coal by Producing
Districts 11
Coal Ash 12
Sulfur in Coal 14
Chlorine in Coal 14
Physical Properties of Coal 19
Coal Sizing 19
Fusibility of Coal Ash 19
Coking and Caking Properties of Coal .... 21
IV. COAL COMBUSTION THEORY 23
Combustion of Coal 23
Combustion in Fuel Beds 25
Combustion of Coals in Suspension 28
V. HOW COAL IS UTILIZED 31
Brief History of Development of Mechanical
Firing Methods 31
Description and Size Ranges of Mechanical
Firing Equipment 32
Underfeed Stokers, Single-Retort,
Residential 32
Underfeed Stokers, Commercial, Institu-
tional, and Small Industrial 32
Multiple-Retort Underfeed Stokers 33
Traveling-Grate and Chain-Grate Stokers . . 33
Vibrating-Grate Stoker 35
BCR Automatic "Packaged" Boiler 35
Spreader Stoker 36
Pulverized-Fuel Firing Units 37
Cyclone Furnace 38
Summary of Related Coal-Firing Equipment
and Use 39
VI. SMOKE EMISSIONS AND COMBUSTION PLUME ... 41
Theoretical Considerations 41
111
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Plume Emission Measurement Methods 42
Ringelmann Chart 42
Equivalent Opacity 42
Soiling Potential 42
Smoke Spot Tester 43
Plume Emission Data 43
Smoke in Average Percent Density 43
Plume Equivalent Opacity 44
Soiling Potential 45
Smoke Spot Data 45
Mass Emission and Smoke Plume 46
Reducing Smoke Emissions 48
Hand Firing 49
Small Underfeed Stokers 49
Large Boiler-Firing Equipment 49
VII. PARTICULATE EMISSIONS 51
Theoretical Considerations 51
Emission Units 53
Physical Properties of Particulates 54
Particle Size Distribution 54
Particle Description 58
Particle Density 59
Chemical Composition of Particulates 60
Combustible Content of Particulates 63
Mass Emission Factors 65
Effect of Firing Rates on Emissions 73
Hand-Fired Units 74
Control of Particulate Emissions 76
Variables Affecting Efficiency of Control
Equipment 79
VIII. GASEOUS EMISSIONS FROM COAL COMBUSTION . . 83
Sulfur Oxides 83
Theoretical Considerations 83
Emission Data 83
Oxides of Nitrogen 87
Theoretical Considerations 87
Emission Data 89
Other Gaseous Emissions 91
IX. FUTURE NEEDS FOR DATA AND RESEARCH .... 93
Emission Data Needs 93
Research Needs 93
Suggested Research Directions 95
REFERENCES . 97
ACKNOWLEDGMENT !. . Ill
IV
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ABSTRACT
Information concerning atmospheric emissions arising
from the combustion of coal was collected from the published
literature and other sources. The data were abstracted,
assembled, and converted to common units of expression to
facilitate comparison and understanding. From these data,
emission factors were established that can be applied to coal
combustion processes to determine the magnitude of air pollu-
tant emissions. Also discussed are the composition of coal,
theory of coal combustion, emission rates, gaps in emission
data, and future research needs.
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ATMOSPHERIC EMISSIONS
FROM COAL COMBUSTION -
AN INVENTORY GUIDE
CHAPTER I.
INTRODUCTION
Although smoke, liquid and solid particles, and gases
from the combustion of coal have long been an almost universal
contributor to air pollution, information published on the subject
has been largely fragmentary. For this reason the Technical
Assistance Branch of the Division of Air Pollution undertook a
project to draw together existing knowledge concerning emis-
sions resulting from the combustion of coal. In this effort, a
literature search was performed and over 300 separate refer-
ences were studied. Information from other reliable sources
and tests' was also utilized.
As the gathering of information progressed, the most
appropriate nomenclature and units were selected; thereafter,
information covered was converted to the selected terms and
standard units.
Information required to support data was often missing,
and no data were used unless adequate supplementary informa-
tion was available to justify whatever assumption had to be made
in order for the data to merit inclusion.
In the process of organizing information, each possible
contaminant •was evaluated and the significance and interrela-
tionships of the quantities of materials present were carefully
studied. The principal product of this effort was the establish-
ment of "emission factors. " An emission factor is the typical
value for the amount of a specific pollutant emitted. Emission
factors were determined for pollutants from different types of
firing equipment and from different types of coal.
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The information in this report is presented in the hope
that it will be useful in accomplishing the following purposes:
1. Development of community or area-wide inventories
of emissions from coal combustion.
2. Evaluation of emissions from specific existing or
proposed coal-burning installations where detailed
data are not available.
3. Projection of the effects of coal combustion on the
future air quality of communities.
4. Development and expansion of a central depository for
emission data within the Technical Assistance Branch
of the Division of Air Pollution.
5. Indication of the gaps in the knowledge and understand-
ing of the variables that influence emissions.
6. Dissemination of information on the effectiveness of
various types of control equipment and processes.
EMISSIONS FROM COAL COMBUSTION
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CHAPTER II.
SUMMARY OF EMISSION DATA
The mass emission rates of participates and gases arising
from the combustion of coal before stack gas collection is applied
are summarized in Tables 2-1 and 2-2, respectively. The
Table 2-1. PARTICULATE EMISSION FACTORS
FOR COAL COMBUSTION WI THOUT
CONTROL EQUIPMENT
Type of unit
Particulate per ton
of coal burned, a Ib
Pulverized
General
Dry bottom
Wet bottom without
fly-ash reinjection
Wet bottom
with fly-ash reinjection
Cyclone
Spreader stoker
without fly-ash reinjection
with fly-ash reinjectionb
All other stokers
Hand-fired equipment
16A
17A
ISA
24A
2A
13A
20A
5A
20
The letter A on all units other than hand-fired
equipment indicates that the percent ash in the
coal should be multiplied by the value given.
Example: If the factor is 17 and the ash content
is 10 percent, the particulate emission before
the control equipment would be 10 times 17 or
170 pounds of particulate per ton of coal.
Values should not be used as emission factors.
Values represent the loading reaching the control
equipment always used on this type of furnace.
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factors are expressed as pounds of emission per ton of coal with
a heat content of 13,000 Btu per pound. The data are divided
into three categories: (1) power plants (1 x 10^ Btu/hr input
or more), (2) industrial plants (107 to 108 Btu/hr input), and
(3) domes tic-commercial plants (107 Btu/hr input or less). The
factors in Tables 2-1 and 2-2 should not be used if the heating
value of the coal used in an area varies significantly from
13,000 Btu per pound. Nomographs have been constructed to
convert the emission values (or estimate emissions from a given
unit) to those appropriate for the coal used in a particular area
(Figures 2-1 and 2-2).
The quality of the emission control effort within the area
under study must not be neglected. The estimate of particulate
emissions for various degrees of control are generalized in
Table 2-3. If the emission without control is less than the value
found in Table 2-3, the smaller number should be used.
Table 2-2. GASEOUS POLLUTANT EMISSION FACTORS
FOR COAL COMBUSTION
Pollutant
Nitrogen oxides
as NO2
Sulfur oxides
as SO
Carbon monoxide
Hydrocarbons
as methane
Aldehydes as
formaldehyde
Pollutant per ton of coal burned, Ib
Electric generating
plants
20
38 Sa
0.5
0.2
0.005
Industrial
plants
20
38 Sa
3
1
0.005
Domestic and
commercial plants
8
38 Sa
50
10
0.005
S indicates that the percent sulfur in the coal should be multiplied
by 38. Example: If the sulfur content is 2 percent, the sulfur
emission would be 2 times 38, or 76 pounds of SO_ per ton of coal.
EMISSIONS FROM COAL COMBUSTION
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PARTICULATE EMISSION,
REFERENCE lb/!06Btu 1 b/ 1031 b f 1 ue gas at
L| NE 5055 excess ai r
ASH
HEATING VALUE
I.OOOBtu/lb C0™,
20-t
18
1 O —
16 -
15 -
14-
13 -
12-
11 -
10-
9-
8-
7 -
6 -
5-
""" -- .
-30
-20
-15
-IP.
9^
-8
-7
-6
-5
-4
-3
-2
-1
( Bituminous coal )
0.2-1
0.3-
0.4-
0.5-
0.6-
0.8-
A-i i-
*
'.
B 2 =
3-
C- 4-
E- 5n
M 6-
8-
10-
15-
20-
-0.2
-0.3
-0.4
-0.5
-0.6
-0.8
-1
-2
-3
-4
-5
-6
-8
-15
A. CYCLONE UNITS
B. ALL STOKERS OTHER THAN SPREADER STOKERS
C. WET BOTTOM, PULVERIZED, OR SPREADER STOKERS
WITHOUT FLY-ASH REINJECTION
D. DRY BOTTOM PULVERIZED
E. SPREADER STOKERS WITH FLY-ASH REINJECTION
F. WET BOTTOM PULVERIZED WITH FLY-ASH REINJECTION
Figure 2-1. Nomograph for estimating participate emissions from coal
combustion (without air pollution control equipment).
Table Z-3. ESTIMATES OF CONTROLLED PARTICULATE
EMISSIONS FROM COAL COMBUSTION
Particulate per ton of coal burned, Ib
Degree of
control
Average
Good
Electric generating
plants
25
10
Industrial
plants
25
15
Domestic and
commercial plants
25
20
Summary of Emission Data
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SOX as S02,
% SULFUR
IN COAL
10.0 -
9.0 -
8.0
7.0 -
6.0 ~
5.0 ~
4.0 -
3.0 -
2.0 --
1.5 -
1 .0
0.9
0.8
0.7
0.6
0.5
HEATING VALUE,
Btu/lb
ppm at
5055 EXCESS AIR
(Bitumi nous coal )
400'
500-
600-
700.
800-
900-
1.000-
Ib/IO6 Btu
-20,000
-15_,OPO -•
-5.000
1 ,500-E
r 4
2 ,000 -|- 5
6
7
8
3,000 - -
4..000--
5.000'
6,000•
7,000-
7,500•
-1 .5
'- 2
r 3
9
10
--15
- 20
Figure 2-2. Nomograph for calculating SOX emissions.
EMISSIONS FROM COAL COMBUSTION
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CHAPTER III.
PROPERTIES AND DISTRIBUTION OF COAL
INTRODUCTION
During the geological ages vast deposits of vegetable ma-
terial accumulated to form the parent material of coal. Through
many thousands of years this material underwent a process
involving changes in temperature, pressure, submersion in water,
and biochemical action to form coal. Although predominantly
carbon, coal contains varying amounts of about half of the known
elements. Coal is broadly classified as (1) anthracite (hard
coal), (Z) bituminous (soft coal), or (3) lignite (brown coal).
COAL PRODUCTION AND DISTRIBUTION
The U. S. Geological Survey estimates recoverable coal
reserves to be 830 billion short tons, the equivalent of 17. 3
quadrillion Btu of untapped energy. The Department of Interior
reports coal underlying 350,000 square miles, or approximately
one-ninth of the total area of the United States. Bituminous coal
is mined in 26 states, with West Virginia, Kentucky, Pennsylvania,
Illinois, Ohio, Virginia, Indiana, and Alabama, in that order,
leading the tonnage output in 1963. 1 The United Spates produced
452 million tons of bituminous coal in 1963; 409 million tons of
it was consumed in this country. 1 Of the total energy from
fossil fuels and water power, coal supplies about 23 percent;
liquid petroleum, 41 percent; natural gas, 32 percent; and water
power, 4 percent.
The bituminous and lignite fields iare organized into pro-
ducing districts as defined in the Bituminous Coal Act of 1937. 3
These districts are shown in Figure 3-1. The anthracite fields
not included in the numbered producing districts are in Penn-
sylvania, Rhode Island, and Arkansas.
Since the type of coal used in any area being studied is
important, it is necessary to have information on coal distribu-
tion "and utilization. The Bureau of Mines^ publishes data on
the distribution of bituminous coal and lignite in the various
states and geographic areas. These data include the producing
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00
1
i
O
I
O
O
I
Figure 3-1. Map of the coal-produci nq districts of the United States. 3
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districts of origin, method of transportation, and types of con-
sumer use. Table 3-1 shows the distribution of bituminous coals
and lignite to the various states in 1962 from all districts of
origin, and the percentage of coal supplied by each district.
Tabulation of the amount of coal produced in each district is
shown in Table 3-2.
Table 3-3 shows the distribution of coal among the various
major users for the year 1963 and the predicted usage for 1975. *
"Keystone Buyers Guide"^ contains a directory of fuel usage
(including coal) for all major utilities in the United States; for
all cement plants, including capacities; and a directory of bee-
hive and by-product coke-oven plants with their capacities.
Also helpful to the air pollution survey is the directory of the
Retail Coal Merchants Association. ^ More complete data on
the amount of fuel used by the electric utilities can be found in
the National Coal Association publication "Steam-Electric Plant
Factors. "6
Analyses of coal used in producing districts can be found
in "Keystone Buyers Guide, "' as can typical analyses from
seams within the various states. The Bureau of Mines also
publishes coal analyses. ' • °» °» *"
CHEMICAL PROPERTIES OF COAL
Classification of Coal
The most common method of classifying coal is shown in
Table 3-4. The criteria for the various classes of coals are
determined by "proximate analysis. " This analysis determines
the weight percent of moisture, volatile matter, fixed carbon,
and ash in a given coal, usually on an "as received" basis. The
amount of moisture is determined by heating a coal sample to
about 110°C for 1 hour; the loss in weight is then termed
"moisture. " This same sample is then heated to 950°C for
7 minutes, and the further loss in weight is called volatile
matter; it represents the hydrocarbons and other organics
driven off by the heat. The remainder is fixed carbon and ash,
which are separated by combustion. H> 12
Although the amount of sulfur, the heating value, and the
ash-softening temperature are not part of the analysis, they are
usually reported with it. *•* Table 3^5 lists typical ranges of
data from analyses of coals used in the United States. 13
Properties and Distribution of Coal 9
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Table 3-1. DISTRIBUTION OF BITUMINOUS AND LIGNITE COAL IN 1962
BY STATES FROM PRODUCING DISTRICTS1' 3> 4
S
55
en
M
§
g
O
O
>
f
O
g
w
C!
CO
Region
New England
Mid-Atlantic
East N. Central
We. IN. Central
S. Atlantic
East S. Central
Mountain
Pacific
Canada
Mexico
Destination not
revaalable
State
Massachusetts
Connecticut
Maine. Rhode Uland, 1
New Hampshire, Vermont)
New York
lew Jersey
ennsy van a
Indiana
Illinois
Michigan
Wisconsin
Minnesota
tow a
Missouri
North Dakota)
South Dakota /
Nebraska, Kansas
Delaware , Maryland
District of Columbia
Virginia
West Virginia
North Carolina
South Carolina
Georgia, Florida
Kentucky
Tennessee
Alabama, Mississippi
Oklahoma, Texas I
Colorado
Utah
Montana, Idaho
Wyoming
New Mexico
Arlaona, Nevada
Washington, Oregon
California
Alatka
Total
market
in
thousand
ten*
4.342
4,047
1,608
21,737
6.901
'
31,624
39.259
27.255
5.766
5.047
7,685
2, 390
1,630
9.884
81
12,82
15.27
9,98
3,92
11.873
14, 120
16.716
3, 40
2, 17
1. 08
1. 38
07
488
964
1,426
893
i. 105
DUtrlct of origin in percent of total 1962 market*' b
1
24.4
31.0
9.9
30.3
29. 1
< 0.1
< 0. 1
1.0
38.6
30.6
0.3
0.1
0. 3
4.2
2
3.B
0.3
0. 1
16.2
4.9
< 0.1
< 0. 1
1.0
1.0
21.6
2.2
J
and
17.1
36.1
33.7
43.2
59.2
0.1
0.2
2.7
0, 4
39.2
1.9
0.1
25.3
1.3
4
1.6
< 0. 1
26.4
9.6
3. 1
7
5.9
4.4
2.2
0.9
2.6
12.4
2.5
7.2
7.6
27.4
11.4
7.4
6.2
2.8
0. 1
3. 3
2.7
1.2
25.6
9
48.8
28.2
54. 1
7.8
4.2
25.2
8. 1
53.6
3.6
0. 1
8.2
6.
3,
7.
4.
B.
0.2
0.4
31.8
9
16 1
11 2
2 6
0.1
26.2
49.4
2. 6
4.6
10
6 5
72 5
3 1
0. 3
0.8
28.8
1. 6
7.9
11
39.7
5.5
0.4
2.0
19.1
3.9
81. 5
1. 2
12.7
0. 1
54. 6
7.0
0.5
100
with-
out
0.7
5. 0
35.0
0,2
81.4
37.2
4.6
24. 4
0. 2
0. 4
6. 5
65.3
11. 1
1. 3
15.0
29.9
85.0
69.7
0. 3
12.0
10. 1
0 1
10.3
5.6
14.4
97.8
3.8
O.I
1.0
< 0. 1
76. 3
52.8
0.9
30. 3
69.6
69.3
21
12.6
81.7
2.5
22
and
23
0.4
32.8
24.7
100
"Figure 3-1 ahows the location ol producing dlatrlcU in the continental United Statea. ^Production from District S is negligible.
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Table 3-2. BITUMINOUS COAL PRODUCTION IN 1962
BY DISTRICT1'3'4
District number
and name
1 . E a s te rn P enns ylvania
2. Western Pennsylvania
3. Northern West Virginia
4. Ohio
5. Michigan
6 . Panhandle
7. Southern numbered 1
8. Southern numbered 2
9. West Kentucky
10. Illinois
1 1 . Indiana
1 2 . Iowa
13. Southeastern
14. Arkansas -Oklahoma
15. Southwestern
16. Northern Colorado
17. Southern Colorado
18. New Mexico
19. Wyoming
20. Utah
21. North-South Dakota
22. Montana
23. Washington
United States total
Production, thousand
net tons
30. 649
36,080
36, 516
34, 500
4, 475
33, 720
113,851
31, 300
, 48,400
15, 780
1, 150
15,934
924
4,406
790
3, 103
367
2, 570
4, 270
2,780
370
1,065
423, 000
Percent of total
production
7.2
8.5
8.6
8. 1
-
1. 1
8.0
27.0
7.4
11.4
3.7
0.3
3.8
0.2
1.0
0.2
0.7
0. 1
0.6
1.0
0.7
0.1
0.3
100
From the air pollution viewpoint, the amounts of volatile
matter, ash, and sulfur and the heating value are the most
important part of the fuel analysis. Volatile matter is related
to the emission of smoke, ^ the ash, to particulate emission;
and the sulfur content, to sulfur oxide emissions, whereas the
heating value is related to the total amount of pollutant produc-
tion. Another coal variable connected with smoke and flue dust
emission is the size of coal. The optimum size for coal is
determined by the method of firing and will be discussed in a
later section.
Typical Properties of Coal by Producing Districts
The average sulfur contents of coals mined in this country have
been estimated at 2 percent for bituminous, and 0. 6 percent^, •'•"for
Properties and Distribution of Coal
11
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Table 3-3. BITUMINOUS COAL CONSUMPTION
IN UNITED STATES FOR 1963 AND PREDICTED FOR 1975
Major user
Electric power utilities
Coking coals
Steel and rolling mills
Cement mills
Other manufacturing and
mining
Retail deliveries
Export
Motive power
Totals
Consumption in
1963,
millions of short tons
209.0
77.7
7.4
8. 1
83.5
23.5
47.1
-
456.3
Predicted consumption
in 1975,
millions of short tons
440
90
I 89
20
30
2
671
anthracite. Several authors estimate that 10 percent ash and
2.5 percent sulfur are reasonable average figures for coal used
to produce electrical energy. 1°> *7 Of equal importance is the
range of volatiles, ash, and sulfur found in coal. Such values
are presented in Table 3-6. These values were calculated from
reference 8 and probably are representative of the retail coal
sold from these districts, which are shown in Figure 3-1.
Coal Ash
The ash-forming mineral matter in coal consists principally
of slate, clay, sandstone, shale, carbonates, pyrite, and gypsum.
Many other constituents occur in trace amounts. Table 3-7
shows the relative frequency of occurrence of the ash-forming
mineral matter in coal. Typical ranges of coal-ash constituents
found in United States coal are presented in Table 3-8.
Some mineral matter is derived from the soil above and
below the seam of coal being mined. With the advent of mechan-
ical mining processes, the amount of mineral matter has in-
creased. This and some of the pyrites in the coal mav be
removed by washing or other mechanical processes. '
Generally, coal shipped long distances is of low-ash content
for economic reasons. Also power plants usually burn higher-
ash coals, whereas lower-ash coals go to the retail market.
12
EMISSIONS FROM COAL COMBUSTION
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Table 3-4. CLASSIFICATION OF COALS BY RANK11
Class a
I. Anthracite
II. Bituminous e
III. Subbituminous
IV. Lignitic
Group
I. Meta-anthracite
2. Anthracite
3. Semianthracite
1. Low-volatile
2. Medium -volatile
3. High -volatile A
4. High-volatile B
5. High-volatile C
1 . Subbituminous A
2. Subbituminous B
3. Subbituminous C
1 . Lignite
2. Brown coal
Limits of fixed carbon or Btu
mineral-matter-free basis
Dry FCb 90% or more (dry VMC
2% or less)
Dry FC 92% or more and less than
98% (dry VM 8% or less and
more than 2%)
92% (dry VM 14% or less and
more than 8%)
Dry FC 78% or more and less than
86% (dry VM 22% or less and
more than 14%)
Dry FC 69% or more and less than
78% (dry VM 31% or less and
more than 22%)
Dry FC less than 69% {dry VM
more than 31%). Moist Btu^
14, OOflS or more
Moist1 Btu 13, 000 or more and
less than 14,0008
Moist Btu 11,000 or more and
less than 13,000^
Moist Btu 11,000 or more and
less than 13,0003
Moist Btu 9, 500 or more and less
than 11,0008
Moist Btu 8,300 or more and less
than 9, 5008
Moist Btu less than 8, 300
Moist Btu less than 8,300
Requisite physical
properties
Nonagglomerating
Either agglomerating
or nonweathering
Both weathering and
nonagglom crating
Consolidated
Unconsolidnted
aStandard Specifications for Classification of Coals by Rank (ASTM D388-38, ASA M20.1-1938).
This classification does not include a few coals that have unusual physical and chemical properties
and that come within the limits of fixed carbon or Btu of the high-volatile bituminous and sub-
bituminous ranks. AH these coals either contain less than 48 percent dry mineral-matter-free
fixed carbon or have more than 15,500 moist mineral-matter-free Btu.
bFC = fixed carbon.
CVM = volatile matter.
**!£ agglomerating, classify in the low-volatile group of the bituminous class.
elt is recognized that there may be noncaking varieties in each group of the bituminous class.
fMoist Btu refers to coal containing its natural moisture, but not including visible water on the
surface of the coal.
SCoals having 69 percent or more fixed carbon on the dry mineral-matter-free basis shall be
classified according to the fixed carbon, regardless of Btu.1
"There are three varieties of coal in the high-volatile C bituminous coal group, viz., variety 1,
agglomerating and nonweathering; variety 2, agglomerating and weathering; variety 3, non-
agglomerating and nonweathering.
An apparent linear relationship exists between the heat
content and the ash content (both on a dry basis). This relation-
ship is shown in Figure 3-2. For clarity, the individual points
have been deleted. The accuracy of each line is about plus or
minus 10 percent.
Properties and Distribution of Coal
13
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Table 3-5. RANGES OF VALUES FROM ANALYSES
OF COALS USED IN UNITED STATES13
Moisture, weight %
Volatile matter, weight %
Ash, weight %
Sulfur, weight %
Heating value, Btu/lb
Bituminous
2-15
14-40
4-15
0.5-4.5
11,000-14,000
Anthracite
4-10
4-8.5
7-20
0.4-0.8
11,000-13,500
One would expect a direct relationship between ash content
and particulate emission; but as shown by the data in Figure 3-2,
this is not the case. A 100 percent increase in the ash content
decreases the heating content 5 to 15 percent; the resulting
increase in particulate emissions is 110 to 130 percent.
Sulfur in Coal
Sulfur occurs in coal in three forms: pyritic, organic,
and sulfate sulfur. The proportions of each sulfur compound
vary widely. The amount of sulfur as sulfate is usually small
in freshly mined coal. The pyritic sulfur is found in small,
discrete particles within the coal, and a percentage of this
sulfur may be removed by washing or other mechanical means.
The organic sulfur is usually evenly distributed'throughout the
coal and cannot be removed without changing the chemical
nature of the coal. 18
Although there is no definite relationship, sulfur has been
found to be a contributing factor in the formation of clinkers and
slag in stokers. A study conducted by the Bureau of Mines
showed that Pennsylvania coals with high ash-softening tem-
peratures usually have a low sulfur content. This, however,
does not mean that low ash-softening-temperature coals have
high sulfur content, as shown in Figure 3-3. HI 20
Chlorine in Coal
As noted in Table 3-7, various salts are found in coal
mineral matter. Some of these salts are chlorides, such as
potassium and sodium chlorides. Until the last decade, this
14
EMISSIONS FROM COAL COMBUSTION
-------
TJ
i-t
o
V
a>
(D
in
B
o
H"'
0)
I-t-
H
H-
CT1
H-
§
O
o
Table 3-6. SELECTED PROPERTIES OF UNITED STATES COALS
BY PRODUCING DISTRICTS, 19618
(Analysis on a dry basis)
Producing
district
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Pennsylvania
(anthracite)
Volatile matter, %
Low
15.5
32.3
27.7
32.6
36.3
12.5
21.8
39.8
37.8
40.7
38.8
27. 1
21.8
31. 1
37.7
29.5
44.4
41.6
40.0
38.8
34.7
38, 1
3.5
Average
25.9
35.8
37.3
41.3
38.8
21.0
34.0
42.6
42.4
43.6
42.7
31.4
24. 1
38.1
38.1
37.6
44. 5a
42.8
43.2
42.8
38.2
39. 9a
5.9
High
39.5
42.6
42.7
46.2
43.3
25.9
42.6
47.3
48.3
45.9
45.0
39.5
30.4
44.6
38.5
42.6
44.5
44.4
48.7
49.7
41.6
41.6
9.4
Ash, %
Low
7.4
6.9
2.3
3.8
4. 1
2.5
1.5
5.5
6.0
6.9
10.8
1.7
5. 1
4.3
5.2
6.3
7.5
3.5
4.3
9.8
7.1
9.1
6.4
Average
8.8
9.7
7.3
10.4
6.4
5.7
6.7
7.9
9.6
9.0
16.1
8.3
8.9
8.9
6.4
8.7
7.8a
5.9
6.3
13.3
9.5
12.2
11. 1
High
20.2
19.1
12.0
26.1
8.7
31.7
18.4
12.6
13.5
11.2
22. 1
17.0
12.0
13.2
7.9
14. 1
8. I
7.6
8. 1
22.7
12.6
15.8
15.8
Sulfur, %
Low
1.0
0.9
0.6
1.9
0.6
0.6
0. 1
2.8
2. 1
1.0
5.0
0.6
1.2
0.6
0.3
0.4
0.8
0.5
0.3
0.9
0.5
4.9
0.4
Average
1.8
2.1
1.8
3.9
1.7
0.8
1.2
3.5
3.3
3.3
6.4
0.9
2.7
2.3
0.4
0.7
0. 9a
0.6
0.5
1.5
0.9
4. 9b
0.6
High
4.5
4.2
3.7
9.4
4.0
1.6
4. 5
4.8-
4. 4
4.2
8.0
2.2
3.2
5.6
0.4
1.2
0.9
0.8
1.0
1.8
1.4
4.9
1.4
Heat content, Btu
Low
11,770
13,220
13,000
10,330
13,730
10,240
12,230
12,490
12,700
12,870
10,690
12,340
13,550
12,450
12,380
12, 150
13,010
12,120
13,290
9,490
10,580
10,890
12,360
Average
14,100
13,650
13,850
13,000
14,200
14,800
14, 100
13,400
13, 100
13,190
11,700
12,800
14, 100
13,600
12,460
13,280
13,050a
12,500
13,600
10,600
12,200
12,000
13,300
High
14,580
14,290
15, 180
14,800
14,690
15,390
15,380
13,940
13,760
13,580
12,490
14,940
14,650
14, 380
12,620
14,230
13,080
13,220
13,900
11,000
12,870
12, 920
14,250
Two samples.
One sample.
-------
Table 3-7. RELATIVE FREQUENCY OF OCCURRENCE
OF MINERALS IN COAL19
Mineral
Clay and shale
11 lite
Sericite
Montmorillonite
Kaolinite
Halloysite
Sulfides, Sul fates
Pyrite
Marcasite
Sphalerite
Chalcopyrite
Galena
Pyrrhotite
Barite
Gypsum
Carbonates
Siderite
Ankerite
Calcite
Dolomite
Oxides, hydroxides
Hematite
Quartz
Magnetite
Rutile
Lirnonite
Goethite
Diaspore
Phosphate
Apatite
Silicates
Zircon
Biotite
Staurolite
Tourmaline
Granite
Epidote
Orthoclas e
Augite
Hornblende
Cyanite
Chlorite
Salto
Halite
Sylvite
Melanterite
Alunogen
Kieserite
Bischofite
Glaubers salt
Formula
KNaO 3AI203- 6S»O2- 2H2O
KNaO- 3MeO- A12O3 - 24SiO2 • 12H2O (Me = Fe, Ca, Mg>
Al2O3-4SiO2-nH2O
Al2O3'2SiO2'2H2O
Al2O3'2SiO2'4H2O
FeS2
FeS2
ZnS
CuFeS2
PbS
FeS
BaSO.
CaSO4- 2H2O
FeCO,
CaFe(C03)2
CaC03
CaMg
-------
14
12
0
NOTE: NUMBERS SIGNIFY
PRODUCING DISTRICTS,
- DISTRICTS 5 AND 23 NOT
SHOWN BECAUSE OF IN-
SUFFICIENT DATA.
10,000 11,000 12,000 13,000 14,000 15,000 16,000
COAL HEATING VALUE (DRY BASIS), Btu/lb
Figure 3-2. Relationship between ash content and heatinq value of coal
from various producing districts.
Properties and Distribution of Coal
17
-------
Table 3-8. TYPICAL LIMITS OF COAL
ASH IN UNITED STATES19
Constituent
Silica, SiO
Lj
Alumina, Al O
Ferric oxide, Fe O
b J
Calcium oxide, CaO
Magnesium oxide, MgO
Titanium oxide, TiO
Li
Alkalies, Na_O + K O
Sulfur, as SO
Weight percent
20-60
10-35
5-35
1-20
0.3-4
0.5-2.5
1-4
0.1-12
Table 3-9. CHLORINE CONTENT OF SELECTED
AMERICAN COALS19
Source of coal
State
Ohio
Illinois
Indiana
West Virginia
Pennsylvania
Illinois
Oklahoma
Bed
Sharon
No. 6
No. 4
Pittsburgh
Lower Freeport
Central Illinois
Henryetta
Chlorine content, %
0.01
0.01
0.06
0.07
0. 14
0.35
0.46
18
EMISSIONS FROM COAL COMBUSTION
-------
O
o
UPPER LIMIT OF
RELATIONSHIP
J| RANGE'OF VALUES!!!;
vm;.;i;:;;::m m^m^ms
LOWER LIMIT
Q|OF RELATIONSHIP
2,000 2,200 2, TOO 2,600 2,800 3,000
SOFTENING TEMPERATURE OF ASH,°F
Figure 3-3. Relation between percentaqe of sulfur in
Pennsylvania coals and ash-softeninq temperature.''1^
PHYSICAL PROPERTIES OF COAL
Coal Sizing
Commercially, coals are referred to by such terms as
"run of mine, " which is unscreened broken coal from the mine-
"slack coal, " which is all the coal passing through a screen of'
a given size, such as 1- or 2-inch slack; and double-screened
sizes, such as "egg," "stove," "nut," "pea," and "stoker."12
For anthracite, the double-screened sizes are standardized,
and the names, such as "egg," refer to a definite size
(Table 3-10). For other coals, however, these terms are just
trade names, having no fixed meaning unless they are accom-
panied by the numerical sizes. 10 Table 3_n Ustg several
sizes of bituminous coal and their most common use.
Fusibility of Coal Ash
One important property of coal ash is the "ash-softening"
or "fusion" temperature. This is the temperature at which the
coal ash softens and fuses. The composition of the ash deter-
mines the ash fusion temperature (Table 3-12). In general,
mixtures high in silica or alumina, or low in pyrites usually
have a high fusion temperature. A coal high in pyritic sulfur
is necessarily high in iron; the possible resultant lower silica-
iron ratio lowers the fusion temperature. H.21 (See Figure
Properties and Distribution of Coal
19
-------
N>
O
Table 3-10. STANDARD SIZING OF PENNSYLVANIA ANTHRACITE APPROVED AND
ADOPTED BY ANTHRACITE INSTITUTE, APRIL 1, 193121
(Round-mesh screen)
Name
Stove
Pea
No. 1 Buckwheat
No. 2 Buckwheat (rice)
No. 3 Buckwheat (barley)
Breaker rnesh, in.
4.1/4 . 4-1/2
3-1/8 - 3-3/8
2-3/8 - 2-4/8
1.-9/16 - 1-11/16
24/32 - 27/32
17/32 - 19/32
9/32 - 11/32
6/32 - 7/32
3/32 - 4/32
Test mesh, in.
Through
4-3/8
3-1/4
2-7/16
1-5/8
1-3/16
9/16
5/16
3/16
M
V
>
O
3-1/4
2-7/16
1-5/8
1-3/16
9/16
5/16
3/16
3/32
Oversize maximum, %
5
5
5
5
10
10
10
10
Undersize, %
Maximum
15
15
15
15
15
15
15
20
Minimum
7-1/2
7-1/2
7-1/2
7-1/2
7-1/2
7-1/2
7-1/2
10
Maximum
impurities , %
o>
•M
rt
a
1-1/2
1-1/2
2
3
5
-------
Table 3-11. GENERAL USES OF SEVERAL BITUMINOUS
COAL SIZES
11
Type
Most common use
5 lump
5x2 egg
2 x 1-1/4 nut
1-1/4 x 3/4 stoker
1-1/4 x 5/16 stoker
3/4 x 3/8 stoker
3/4 x 0 slack
5/8 x 0 slack
1/2x0 slack
1/4x0 slack
1-1/4 x 0 nut and slack
2x0 nut and slack
Hand-firing, domestic and industrial
Domestic hand-firing and gas producers
Domestic hand-firing, industrial stokers,
and gas producers
Domestic and small industrial stokers
Domestic and small industrial stokers
Domestic and small industrial stokers
Industrial stokers and pulverizers
Particularly suited to pulverizers
Particularly suited to pulverizers
Particularly suited to pulverizers
Industrial stokers
Industrial stokers
Coking and Caking Properties of Coal
Coke is the fixed carbon and ash, which are left after the
coal has been heated and the volatile matter has been driven
from it. In this sense, all coals coke; however, the term
"coking coal, " which is used synonymously14with "caking coal, "
refers to a coal that melts and fuses to form, larger lumps, even
though the coal may have been in small pieces. Thus, the caking
process takes place to varying degrees and is described by
various adjectives, such as "strongly caking," "weakly caking,"
or "non caking" coals. A free-burning coal is the same as a
noncaking coal. ^ Bituminous is usually a caking coal, whereas
anthracites and most subbituminous coals are free-burning coals.
Properties and Distribution of Coal
21
-------
Table 3-12. ASH-SOFTENING TEMPERATURES AND ASH COMPOSITION OF SELECTED COALS20
Sample
Montana subbituminous
Illinois bituminous
Pennsylvania bituminous
West Virginia semibituminous
Kentucky bituminous
Softening
temperature,
°F
2,060
2,320
2,500
2,730
> 2,900
Analysis of ash, %
Si02
30.7
46.2
49.7
51.0
58.5
A12°2
19.6
22.9
26.8
30.9
30.6
Fe2°3
18. 9
7. 7
11.4
10.7
4.2
TiO
LJ
1. 1
1.0
1.2
1.9
1.8
CaO
11.3
10. 1
4.2
2.1
2.0
MgO
3.7
1.6
0.8
0.9
0.4
Na O + K O
LJ £*
2.4
1.5
2.9
1.4
1.6
so3
12.2
8.9
2.5
0.6
0.9
w
§
CO
g
n
o
n
i
td
c!
CO
§
-------
CHAPTER IV.
COAL COMBUSTION THEORY
COMBUSTION OF COAL
The heating value of coal is principally a function of its
carbon and hydrogen content. In order for heat to be released,
the coal must be oxidized, or burned. The process is a chemi-
cal reaction of carbon and hydrogen with oxygen (from the air)
that forms carbon dioxide and water, and releases heat; however,
several necessary conditions must exist before this reaction can
take place. For the reaction to go to completion, there must be
an excess of oxygen in the presence of proper temperature and
turbulence of the combustion gases for a necessary period of
time.
Coal will not burn as a solid; no fuel will. The combustion
process must vaporize, gasify, or break down a solid into indi-
vidual molecules by the addition of heat. When coal burns in a
bed on a grate, the incoming or primary air through the grate
is heated by the ash or burning fuel. As the air temperature
rises, the heat begins to vaporize and scrub off volatile and
carbonaceous material from the coal particles. In this vaporous
state, the combustible material is oxidized. 23, 24 jn suspension
firing, a similar process takes place, with the energy of the
flowing gases replacing the function of the grate.-
Air is approximately 21 percent oxygen and 79 percent
nitrogen by volume. As air travels through a bed of fuel, oxygen
is consumed by combustion, the oxygen concentration is reduced,
and the possibility of oxygen contacting the fuel decreases.
Because of the lack of oxygen,gases leaving the bed carry with
them a high concentration of carbon monoxide and other com-
bustible matter. Above the bed, more air (secondary air) must
be introduced to oxidize all of the combustible material. Nitrogen
from the air tends to dilute and prevent contact between oxygen
and combustibles. To overcome this effect, in a reasonable
period of time, there must be an overabundance or excess of air;
in other words, an increase of air over and above the chemically
required (stoichiometric) amount. The amount of excess air
needed varies for each type of furnace (see chapter 5).
23
-------
To increase the amount of contact of oxygen with the
combustible material, a high degree of turbulence must be main-
tained. Turbulence reduces the amount of excess air necessary
for complete combustion. Figure 4-1 compares the flue gas
analysis for poor mixing to that of ideal mixing. With ideal
mixing, the theoretical air-to-fuel ratio is all that is necessary
to achieve complete combustion. As the mixing becomes less
ideal, excess air is needed to completely burn the combustible
matter.
POOR MIXING
IDEAL MIXING
DEFICIENCY ^ EXCESS AIR
CHEMICALLY CORRECT
AIR-FUEL RATIO
Figure M~i. Effect of ai r-fuel ratio on flue aas analysis.25
It might seem logical to assume that the primary function
of a furnace is to attain 100 percent combustion. This, however,
is not true. The primary purpose is to help attain the highest
overall efficiency for the energy system. Usually this means
the point at which the most steam is raised for the least amount
of coal. Starting from the low side, an increase in excess air
will usually increase the combustion efficiency while at the same
time diluting and cooling the combustion gases. After a given
point, more heat is lost in the stack gases by the increase in
excess air than is gained by releasing the remaining heat-of
combustion. This point would be that of maximum overall
thermal efficiency (Figure 4-2). Usually, from 0.5 to 5 percent
of the thermal energy of the fuel is sacrificed for optimum
operation. ^°
24
EMISSIONS FROM COAL COMBUSTION
-------
iS
O (/)
(J <
UJ
Q a
LJ U
z z
ce. —
OQ
INCREASING EXCESS AIR-
(J ID
U- O)
LL) <
LJ
-I K
-i a
< z
CC —
MAXIMUM OVERALL
EFFICIENCY
INCREASING EXCESS AIR
Figure 4-2. Effect of excess air on combustion efficiency.
COMBUSTION IN FUEL BEDS
When coal is burned 3n grates, one of two types of feeding
mechanisms is generally used, overfeed or underfeed. The
overfeed operation introduces coal to the grate from the top and
the primary air under the grate, and burning occurs from the
bottom to the top of the fuel bed. The underfeed operation intro-
duces the primary air and the fuel from below the grate, and
the fuel burns from the top to the bottom of the bed. There is
also a third operation called cross-feeding, which is a com-
bination of the two types. H»2?
Coal Combustion Theory
25
-------
The idealized overfeed fuel bed is a series of layers,
which merge into each other as shown in Figure 4-3. At the
bottom of the bed and above the grate, a layer of ash serves
to protect the grate and to preheat the primary air. The ash
layer merges into the actively burning oxidation zone. Here,
the distilled coal undergoes the exothermic reaction,
C + O2 = CC>2, which consumes almost all of the oxygen
from the primary air. This is the hottest part of the fuel bed
with temperatures above 3,000°F. Above this is a reduction
zone where, in the presence of high temperatures and a high
concentration of CC>2, an endothermic reaction, CC>2 + C = 2CO,
reduces the temperature of the gases and the fuel bed. The top
layer is the distillation zone where volatile matter is distilled
off the fresh or green coal. 12,24,27,28
Figure 4-3 shows the relative concentrations of the
various combustion gases and the temperature as the gases
travel through the bed.
That part of the bed termed "ignited fuel" contains both
the oxidation and reduction zone. As shown by the relative
concentration curves next to the diagram, the two zones blend
together with no definite division.
RAW
COMBUSTION FUEL
GASES A JlL
TEMPERATURE
IGNITION_
PLANE
GRATE
0 10 20 30
COMPOSITION . % by volume
PRIMARY
AIR
Figure t-3. Idealized overfeed fuel bed and relative distribution of
temperature and products of combustion. 28
26
EMISSIONS FROM COAL COMBUSTION
GPO 825-629—3
-------
The underfeed fuel bed is the reverse of the overfeed
operation (Figure 4-4). Raw fuel is fed in from the bottom
above the grate and under the actively burning coal. Incoming
air below the grate enters the bed, is heated, and distills
volatiles from the coal. This mixture of volatiles and oxygen
rises to the ignited zone, where it first oxidizes the carbon
and hydrogen in the volatile matter, and then reduces the CO2
to CO as the gases travel upward. On top of the bed is the
ash.24'27'28^
TEMPERATURE-
I GN I T I ON-
PLANE
GRATE
CO
CO
0 10 20 30
COMPOSITION, % by volume
i TED' FUEL"
V#xRAW' FUEL •#:
RAW A I R
FUEL
Figure 4-M-. Idealized underfeed fuel bed and relative distribution of
temperature and products of combust ion .28
After the primary air has passed through either the over-
feed or underfeed fuel bed, virtually all of the oxygen has
combined with the carbon to produce CO and CO£. The gases
leaving the fuel bed are rich in volatile hydrocarbons and tars,
carbon monoxide, and nitrogen. Unless secondary air is intro-
duced, these hydrocarbons and tars crack, decompose, or
condense, and are emitted to the atmosphere as a white,
yellow, or black smoke. 12,24,27,28 Black carbon is not
produced by gases coming in contact with cool heating surfaces,
but is formed at or near the surface of the fuel bed.
The velocity of the combustion reaction is faster than the
velocity of decomposition. If oxygen is present in sufficient
quantity at the time of distillation, hydrocarbons oxidize com-
pletely without forming soot and smoke through thermal crack-
ing and condensation reactions. For this reason, secondary
air should be admitted as near the surface of the fuel bed as
Coal Combustion Theory
27
-------
possible and should have sufficient velocity to penetrate to the
combustion zone so that oxygen is available for completing the
combustion reaction. ^9
For the combustion process to take place, sufficient heat
must be provided for each fuel component to reach the "self-
ignition" temperature and sufficient air must be available to
supply the necessary oxygen. If a given combustion temper-
ature is maintained and primary air is decreased, the burn-
ing rate in the fuel bed is decreased. In practice, the main
method of controlling the burning rate is by the regulation of
the primary air. Secondary air controls the efficiency of the
combustion over the fuel bed. The depth or thickness of the
fuel bed does not control the burning rate to any great extent;
however, it does control the amount of carbon monoxide leav-
ing the top of the bed. A thick bed produces higher concen-
trations of carbon monoxide because of the larger reduction
zone.12'24'27
COMBUSTION OF COALS IN SUSPENSION
Combustion of coal in suspension is similar in principle
to combustion in an overfeed fuel bed. The volatile matter
is first distilled off and burned; the fuel particle is thus sur-
rounded by a highly reducing atmosphere. Secondary air and
sometimes highly turbulent gases move the reducing atmos-
phere away so that more oxygen comes in contact with the
particle for complete combustion. For some suspension-fired
units, such as the spreader stoker, final oxidation takes
place on grates, whereas in pulverized-coal-fired and cyclone
units, complete combustion takes place in the suspended fuel
bed.12
Various arrangements for suspension-fired units are shown
in Figure 4-5.
28 EMISSIONS FROM COAL, COMBUSTION
-------
FANIAIL MULIIPLE INTERIUBE
(a) VERTICAL FIRING
PRIMARY AIR
AND COAL
5
SECONDARY
PLAN VIEW 01 (URNACE
f IANGEN1I AL FIRING
PRIMARY AlR
AND COAL
Vcl
SECONDARY AIR
MULTIPLE INTERIUBE
i'rt ;M/. ;• Y ;. j i
..« CO/.L
SECONDARY Al R _J
CIRCULAR
(c) HORIZONIAL FIRING
SECONDARY AlR
PRIMARY A IR
AND COAL
CYCLONE•
(d) CYCLONE FIRING
AND COAL
(c) OPPOSEO-INCLINEO F IRING
Figure 4-5. Various methods of firing coal in suspension.
12
Coal Combustion Theory
-------
CHAPTER V.
HOW COAL IS UTILIZED
BRIEF HISTORY OF DEVELOPMENT OF MECHANICAL
FIRING METHODS
The widespread use of mechanical firing has been a major
factor in reducing the visible smoke plume from coal-fired
boilers and furnaces.
Underfeed stokers of various designs were built before
1900, with major improvements being developed as early as
1906. Both single- and multiple-retort units were being
installed at that time. Chain-grate and traveling-grate stokers
were introduced between 1900 and 1920; the first forced-draft
units were made in 1922. Although spreader stokers of crude
design were manufactured in the early 1900's, they did not
become a successful firing unit until about 1925. Their popu-
larity increased rapidly in the 1930's. 2^, 31
The development of the small underfeed stoker for home
boilers and furnaces in the early 1930's made automatic coal
firing available to every coal user, regardless of size of
equipment.
Pulverized-fuel firing was first applied to boilers for
steam generation in 1920 and has progressed in development.
Cyclone furnaces appeared about 1947. Today pulverized-coal
burners and cyclone furnaces are the universal methods of
firing coal in the new large electric-generating stations.
The newest entry into the firing equipment field is the
vibrating-grate stoker, which has been applied to large industrial
boilers since about 1954. This type of firing unit, utilizing a
water-cooled inclined grate, has been the focal point in the
development by Bituminous Coal Research of a small-to-medium-
sized, completely packaged boiler.
31
-------
DESCRIPTION AND SIZE RANGES OF MECHANICAL
FIRING EQUIPMENT24- 31, 33
Underfeed Stokers, Single-Retort, Residential
In the residential underfeed stoker, the coal is fed from
a hopper or directly from the coal storage bin to the retort by
a continuous, rotating screw (see Figure 5-1). Coal rises
into the firing zone from underneath, thus the term "underfeed
firing. " Air is delivered to the firing zone through tuyeres
(grate openings), also from underneath the actively burning bed.
The coal and primary air control is "all on" or "all off. " Ash
is removed as a clinker from a refractory hearth through the
furnace firing door. Burning rates range from 1 to 60 pounds
of coal per hour.
FIRE BOX
BURNER HEAT
Figure 5-1. Residential underfeed stoker
Underfeed Stokers, Commercial, Institutional, and Small
Industrial
The general arrangement is as described in the previous
paragraph, with "dead" plates replacing the refractory hearth
(Figure 5-2). As sizes become larger, screw feeders are re-
placed by a mechanical ram, which feeds coal to pusher blocks
that distribute the coal in the fire box. Ash is discharged by
side-dump grates. Modulating combustion controls, i.e., vari-
able control of both fuel and air rates, are often used. Forced
draft is automatically regulated, and separate over fire-air sys-
tems are used, particularly when on-off controls are used. A
32
EMISSIONS FROM COAL COMBUSTION
-------
bridge wall retains the coal over the stoker grates. The size
ranges for screw-feed stokers are 60 to 1, 200 pounds of coal per
hour and for ram-feed stokers , from 300 to 3 , 500 pounds per hour.
••^'TRANSVERSE SECTION
w
LONGITUDINAL SECTION
Figure 5-2. Single-retort underfeed stoker.
Multiple-Retort Underfeed Stokers
As the name implies, these units usually consist of
several inclined retorts side by side, with rows of tuyeres in
between each retort (Figure 5-3). Coal is worked from the
front hopper to the rear ash-discharge mechanism by pushers.
The forced-air system is zoned beneath the grates by means
of air dampers, and the combustion control is a fully modu-
lating system. In the larger furnaces the walls are water-
cooled, as are the grate surfaces in some units. Multiple-
retort underfeed stokers are losing their popularity, giving
way to spreaders and traveling-grate units. Sizes range
from 20, 000 to 500, 000 pounds of steam per hour with burning
rates up to 600, 000 Btu per square foot of grate per hour.
Traveling-Grate and Chain-Grate Stokers
Traveling-grate and chain-grate units (Figure 5-4) are
essentially moving grate sections, moving from the front to the
rear and carrying coal from the hopper in front through a gate
How Coal is Utilized
33
-------
COAL HOPPER
f COAL RAMS
ASH-
DISCHARGE PLATE
FUEL
DISTRIBUTORS
Figure 5-3. Mul t i pi e-retort underfeed stoker.
OVERFI RE-AIR
NOZZLES
AIR-CONTROL
DAMPERS
, L/AHrcno *
-RETURN
BEND
m BTB STQ flTG HVQ fl^CT ^STtt 6jTfl flTfl ATfl tflTn mfl AfiSl WH »!U
1
DRAG
PLATE
STOKER
CHAIN
DRIVE
SPROCKET
HYDRAULIC
DRIVE
Figure 5-4. B & W jet-ignition chain-grate stoker.
into the combustion zone. The fuel bed burns progressively to
the rear, where the ash is continuously discharged. Older
units with natural draft are fast disappearing; modern units have
zone-controlled forced draft. Complete combustion-control
systems are utilized, and overfire air, especially in the front
wall, is an aid to burning the volatiles in the fuel. Units range
in size from 20 to300xl(PBtu per hour input.
34
EMISSIONS FROM COAL COMBUSTION
-------
Vibrating -Grate Stoker
This unit consists of a water-cooled grate structure on
which the coal moves from the hopper at the front of the boiler
through the burning zone by means of a high-speed vibrating
mechanism automatically operated on a time-cycling control
(Figure 5-5). As in the traveling grate, the fuel bed progresses
to the rear, where the ash is continuously discharged. Forced
air is zone-controlled and regulated, along with the complete
coal and air system, through an automatic combustion-control
regulator. Grate heat release may range from 350, 000 to
500, 000 Btu per square foot per hour. The size range for this
unit is from 5,000 to 100,000 pounds of steam per hour.
COAL HOPPER-
COAL GATE-
OVERFI RE-AIR NOZZLES
Figure 5-5. Vi brat i nq-qrate stoker furnace.
BCR* Automatic "Packaged" Boiler
This unit is a complete steam or hot water generating
system, incorporating a water-cooled vibrating grate as the
firing mechanism (Figure 5-6). Coal is delivered from the
storage bin to a hopper from which it travels on the vibrating
grate to the fuel bed. Ash is discharged automatically with
a screw conveyor. The unit has completely automatic com-
bustion controls so that coal feed to the hopper from the bin
and ash discharge is coordinated with load conditions. Forced
and induced draft fans are used. The size range is from 3 to
20 million Btu per hour input.
^Bituminous Coal Research, Inc.
How Coal is Utilized
35
-------
FLUE GAS
EXHAUST
-STEAM
Figure 5-6. Bituminous Coal Research, Inc., packaged boiler
Spreader Stoker
The spreader stoker combines suspension and fuel bed
firing by the stoker mechanism feeding from the hopper onto
a rotating flipper mechanism, which throws the fuel into the
furnace (Figure 5-7). Because fuel is burned partly in sus-
pension and partly on the grate, the fuel bed is thin, and
response to fluctuations in load is rapid. The grates are
either stationary or continuously moving from the rear to the
front. Vibrating, oscillating, traveling, and chain grates are
designed for moving the fuel toward the ash receiving pit.
GRATE
Figure 5-7. Spreader stoker-fired furnace.
36
EMISSIONS FROM COAL COMBUSTION
-------
Zoned undergrate air is important, as is the careful
application of a responsive combustion control system. Over-
fire air is necessary. Fly-ash carryover is strongly influenced
by high burning rates, whereas smoke emission is increased
at low burning rates. In large units, cinders are often returned to the
grate from the fly-ash collector to reduce unburned carbon losses.
Spreader stokers range in size from 6 to 500 x 1CP Btu per hour
input or from 5, 000 to 400, 000 pounds of steam per hour output.
Pulverized-Fuel Firing Units
In this system, coal is pulverized to particles, at least
70 percent of which pass through a 200-mesh sieve, and is
fired in burners similar to those used for liquid fuel (Figure 5-8).
In direct-firing systems, raw coal is dried and pulverized simul-
taneously in a mill and is fed to the burners as required by the
furnace load. The control system regulating the flow of both
coal and primary air is so designed that a predetermined air-
coal ratio is maintained for any given load. The indirectly fed
unit utilizes storage bins and feeders between the pulverizers
and the burners. Some bin-and-feeder systems are in use, but
the majority of plants use direct-firing units.
Radiant superheater
Convect ion
superheater
Economi zer
Ai r heater
Figure 5-8.
Pulveri zed-coal-fi red
uni t.
Burners are characterized by their firing position,
i. e. , horizontal, vertical, or tangential. Arrangements for
the introduction of primary, secondary, and, in some cases,
tertiary air vary with burner manufacturers. One manufacturer
How Coal is Utilized
37
-------
uses an adjustable burner, which is tilted upward or downward
to control the furnace outlet temperature, so that steam temper-
ature can be regulated over a wide range of capacities.
Pulverized-coal-fired units are usually one of two basic
types, wet bottom or dry bottom. The temperature in a wet-
bottom furnace is maintained above the ash fusion temperature,
thus the slag is melted so that it can be removed from the
bottom as a liquid. The dry-bottom furnace maintains a temper-
ature below this point so that the ash will not fuse.
Pulverized-fuel-fired boilers range in capacity from
200, 000 to several million pounds of steam per hour.
Cyclone Furnace
The cyclone furnace is a water-cooled horizontal cylinder,
in which the fuel is fired and heat is released at an extremely
high rate for the given volume (Figure 5-9). Coal is crushed
so that approximately 95 percent passes through a 4-mesh
screen. Coal is introduced into the burner end of the cyclone,
and air for combustion is admitted tangentially. Combustion
occurs at heat-release rates of 500, 000 to 900, 000 Btu per
cubic foot per hour at gas temperatures sufficiently high to
melt a high percentage of the ash into a liquid slag, which is
discharged from the bottom of the furnace through a slag tap
opening. The size range of boilers fired are comparable to
those with pulverized-fuel units.
SCREENED-FURNACE OPEN-FURNACE
ARRANGEMENT ARRANGEMENT
OPEN-FURNACE
ARRANGEMENT
Figure 5-9. Types of cyclone furnaces.
38
EMISSIONS FROM COAL, COMBUSTION
-------
SUMMARY OF RELATED COAL-FIRING EQUIPMENT AND USE
Since coal firing is utilized in such a wide range of equip-
ment, a reference chart relating the various kinds of coal-firing
equipment to several size-range scales and then to the types of
buildings in which the equipment is utilized has been prepared to
aid in emission inventory studies (see Figure 5-10).
The classification of building occupancy and plant grouping
is that shown in Table 5-1.
Table 5-1. BUILDING AND PLANT HEATING REQUIREMENTS
Group
Building or plant category
Range of heat
input, 10*" Btu/hr
II
III
IV
V
VI
VII
Residential (primarily space heating).
Residential, 1-4 family.
Residential, multiple dwelling, large apartment.
Institutional and commercial (primarily space heating).
Schools, churches, small colleges, small hospitals, librar-
ies, other public buildings.
Office buildings, hotels, theaters, stores (core area and
business district).
Business and manufacturing without high process steam re-
quirements (primarily space heating). Manufacturing,
warehousing, wholesaling.
Large institutional and manufacturing (primarily space heating
Large colleges, hospitals, large housing projects, or other
institutional complex with large central boiler plant.
Community central heating plants (utility).
Small industrial (with high process steam requirement).
Dairies, laundries, dry cleaners, food process, etc.
Large industrial (with high process steam required or electric
steam generating facilities). Large industrial plants.
Public utility steam electric generation station.
0-1.0
0.5-5.0
1-50
1-50
1-50
10-200
100-500
1-100
10-600
100 up
"Groups have been arbitrarily numbered for purposes of this report.
Size ranges of boilers are also commonly stated in pounds
of coal per hour input and boiler output in thousands of pounds
of steam per hour. In order to relate the boiler input in pounds
of coal per hour to 10& Btu per hour, the average heating value
of 13,100 Btu per pound for United States coal was used. * Boiler
output was determined by applying the coal-to-steam efficiencies
shown on Figure 5-10. These are the typical efficiencies found
for the size and type of equipment indicated.
The general relationship between combustion gas condi-
tions of temperature and excess air for the various sizes of
equipment is included only as an indication of what might be
expected. These relationships are important in standardizing
stack gases.
How Coal is Utilized
39
-------
40 EMISSIONS FROM COAL COMBUSTION
CLASSIFICATION
OF
BUILDI»C MO |
PUHT ]
FIRING METHODS
EFFLUENT
TEMPER* TU RE, °F
COAL-TO-STEAN
EFFICIENCY. <
EXCESS AIR, J
STACf
EFFLUENT? cf»
ELECTRICAL
GENERATION MX
STEW OUTPUT,
1,000 Ib/hr
COAL IKPUT. Ib/hr
HEAT INPUT
106 it u /hr,
0
|
j 1 GROUP VI i - PUBLIC UTILITY STEAM ELECTRIC GENERATION STATION
(GROUP VI - LARGE INDUSTRIAL WITH PROCESS STEAM [ ! 1
IGROUP V SHALL INDUSTRIAL WITH PROCESS STEAM DAIRIES, Etc.| | J
1 IGROUP IV8ICENTRAL PLANTS) 1 |
GROUP IV A - LAHGt INST. HOSPS, fete. | | j
GROUP MB- OFFICE BLOS. HOTELS, THEATERS. Etc, ! j
GROUP II A - SCHOOLS, CHURCHES. SH1COUEGES. Etc ; 1 [
| GROUP 1 6 - APARTMENT BUILDUPS | 1 i ! ',
GROUP IA l-to 4 FAMlDWELLINGSI 'III
! ( CHAIN or TRAVELING GRATE | | 1
| WATER COOLED VIBRATING GKATT | | [
J SPRa- TRAVELING GRATE 1 1 1
[SPP^-STATIONARY or DUMPING GRATE | GRATE 1 |
! ISPR"IVIB. GRATE| ' 1
(6CR AUTO|PACK»GED BOILER] | MULTIPLE RETORT UNDERFEED STOKER ] j 1
SINGLE RETORT UNDERFEED STOKER j _J j i J
HAND-FIRED EQUIPMENT 1 1 " j
! '" " ! ! ' 1
i i ! !
1 |
1 1 Illl II
SCO 750 700 650 ! 600 550 i 500 150 400 j 350 300
: 1 * ' i
! ! : : i
j i i i i ii i
55 60 65 ' 70 i 75 80 i 85 ',
! 1 i 1 I
! i ! !
L 1 1 III
1 1 1 1 1 1 1 1 1
100 90 85 75 70 65 60 56 50 j 15 40 35 30 ' 25
i ! : j :
,1111111 1 1 1 1 1 Illl 1 1 1 1 1 1 III 1 1 1 1 1 1 III 1 1 1 1 1 1 1 II 1 1 1 II III
50 100
500 1.000 j 5.000 10,000 j 50.000 100.000 I 500,000 1.000,000 1 5.000.000 10.000.000
1 1 1 1 1 Mill 1 I 1 1 I Mil 1 1 1 1 1 Illl 1 1 1 Mill
i JIO 50 100 500 1.000 5.000
! i i i
Mill 1 1 1 Illlll 1 1 1 1 1 Illl 1 1 Mllll 1 1 1 1 Mill 1 1 1 1 1 Illl 1 1 1 1 III
D.I 0.5
1.0 5 1 10 50 ] 100 500 jl.OOO 5.000 .10.000
1 ,' « 1
Illl
1 1 1 1 1 Mill 1 1 1 II 1 III 1 1 1 1 Mill 1 1 1 1 II III 1 1 1 1 II III 1 1 1 1 II
10 50
! 1 1 1 1 1 III
100 500 ' 1.000 5.000 ! 10. 000 50000 i 100,000 500.000 I 1.000. 000 5.000.000
i ! '
I
1 1 1 1 1 III 1 1 1 1 Mill 1 1 1 1 1 Illl 1 1 1 1 1 Illl 1 1 1 1 1 1 II
.1 0.5 1.0 5 lo 50 500 1.000 5.000 10.000 50.000
-------
CHAPTER VI.
SMOKE EMISSIONS AND COMBUSTION PLUME
THEORETICAL CONSIDERATIONS
The combustion plume is a visual manifestation flowing
from a stack or chimney, which reduces visibility by the
scattering or absorption of light. The plume may result from
the presence of submicron-size solids, liquid particles ranging
in size from 0.01 to 2 microns with the greatest number of
particles being approximately 0. 3 to 0. 6 micron, 34> 35
gases that manifest visible color.
The visible plume from the combustion of coal may be
caused by one or all of the following: condensed water vapor,
sulfur trioxide, sulfuric acid, organic liquids and gases,
particulates, and smoke. Water vapor condenses and produces
a white plume, which dissipates rapidly. Sulfur trioxide and
sulfuric acid cause a detached bluish-white plume that does not
dissipate readily. Organic liquids and solids may cause a white,
yellow, or brown plume, whereas the particulates (including fly
ash) cause the plume to be white, brown, or black in color.
Although much has been written on the subject, the theory
of smoke formation is not well understood. As far back as 1913,
Porter and Ovitz3" explained that visible smoke consists of solid
carbon particles and solid or liquid hydrocarbon particles, or
"tar vapors, " resulting from the incomplete combustion of the
volatile products of the fuel. The carbon of the smoke is not
derived from the free carbon in the fuel, but result from, the
cooling of hot, dissociated hydrocarbon gases. Thus, the smoke
as referred to in this report, is defined as the black portion of
the combustion plume.
Once formed, carbon soot is difficult to burn. For this
reason, air supplied over the fuel bed should be admitted at or
as near the surface as possible and mixed with the hydrocarbons
so that they will burn before they are decomposed by heat into soot
and smoke. 29,37
41
-------
PLUME EMISSION MEASUREMENT METHODS
Ringelmann Chart
The standard method of evaluating the severity of smoke
plume is a visual comparison of the color shade of the plume
with shades of gray of the Ringelmann Chart. Other devices
have been used, but, in general, they are standardized against
or related to the Ringelmann numbers.
The Ringelmann Chart, as described by a Bureau of Mines
Publication, 38 establishes shades known as Ringelmann No. 1,
2, 3>, and 4, respectively, with No. 0 being clear and No. 5
being 100 percent black. Thus, No. 1 is related to 20 percent
density; No. 2, to 40 percent density; and so on.
To evaluate smoke emission over a period of time, the
average percentage density of the smoke for the entire period
of observation is obtained by the formula:
Equivalent units of No. 1 Ringelmann x 20 Average percentage
Number of observations smoke density.
By the same methodology, the "average smoke density" of a
large number of combustion sources over a time period can be
determined.
Equivalent Opacity
The evaluation of a plume of any color may be accomplished
by comparing the opacity of the plume to an equivalent shade of
gray on the Ringelmann Chart. ' ' This method evaluates not only
smoke but also non-settling particulates, sulfur trioxide, etc.
The evaluation is reported in terms of percentage of plume
opacity and can be calculated in a manner similar to that of the
smoke calculations for average density.
Soiling Potential
A procedure of drawing a measured volume of air through
a white filter paper tape and evaluating the resultant stain;by
optical means has been used for many years as an index for
atmospheric pollution buildup. It was first applied by Hemeon
in 1953 to evaluate the severity of smoke emission from a
42 EMISSIONS FROM COAL COMBUSTION
GPO 825-629-^*
-------
power plant chimney. 4" Since that time, the continuing measure-
ment of soiling index has been used by many communities as one
of the basic, outdoor air quality appraisals. 41, 42 This method,
however, has not been used extensively as a means of quanti-
tating smoke emissions from the combustion of fuels until
recently. 42 The procedure is similar to that recommended by
the Air Pollution Control Association for ambient air measure-
ments. 43 The quantity of emissions are reported as Coh-ft^
per pound of coal when evaluation is by light transmittance and
as Rud-ft per pound of coal, when evaluation is by light reflect-
ance. The advantage of this method over those mentioned earlier,
is that it provides data that can be inventoried from all sources
and compared with conventional atmospheric measurement
(soiling index).
Smoke Spot Tester
For a number of years , the smoke spot method of testing
smoke density in the flue gases from distillate fuel oils has been
used with success to evaluate oil burner performance, particu-
larly of smaller units. This procedure is described as a pro-
posed method, published by Committee D-2 of the American
Society for Testing Materials. 44 Although the method produces
a relative value of the soiling potential, it has not been extended
to quantitating emissions.
The Air Pollution Control Division of the Department of
Works, Metropolitan Toronto, evaluates combustion equipment
fired by all fuels, oil, gas, and coal, with the Bachrach Smoke
Tester, which conforms with the American Society for Testing
Materials method.
PLUME EMISSION DATA
Smoke in Average Percent Density
Values of average percent smoke density for a large number
of units .operating in a given community are difficult to find. One
such project was conducted in the City of Cincinnati in 1939-
1940. 45 Smoke emission readings in Ringelmann numbers were
taken from vantage points throughout the entire city. The number
of operating chimneys, mainly residential units, were known,
and the smoke readings in Ringelmann numbers were compiled
into average percentage density values as shown in Table 6-1.
In 1939, Cross, et al. , conducted a field survey of 22
small stoker-fired boiler plants and found the average Ringel-
mann Number to be 0. 5 with the stokers on and 1. 0 with the
Smoke Emissions and Combustion Plume 43
-------
Table 6-1. AVERAGE PERCENTAGE SMOKE
DENSITY FROM OPERATING CHIMNEYS,
CITY OF CINCINNATI, 1939-1940
All chimneys (except basin area)
Basin area only
Coal-fired railroads
River boats only
Smoke density, %
7.8
21.0
28.0
23.7
stokers off. Corresponding percentage of smoke density are
10 and 20 percent, which were explained earlier.
It would be expected that improvement in stoker-firing
equipment has reduced the average percentage smoke density
for a given population of small stoker-fired plants to approx-
imately 10 percent average smoke density or one-half Ringel-
mann average.
Estimated average percentage smoke densities for 24-
hour operation, based on the above information, are shown in
Table 6-2.
Table 6-2. ESTIMATED AVERAGE SMOKE EMISSION FROM
SMALL STOKER-FIRED PLANTS
Where good air pollution controls are exercised
Where average operation is experienced
Where poor operation is experienced
Average
smoke density, %
10
20
40
Plume Equivalent Opacity
There is very little published work evaluating equivalent
opacity of the combustion plume, although most smoke recorders,
44
EMISSIONS FROM COAL COMBUSTION
-------
mounted in the boiler stacks, record the light transmission or
opacity of the whole plume, not just black smoke. In 1963,
Haugebrauck, et al. , 47 measured total particulate ( after the con-
trol equipment) and, incidentally, noted the equivalent opacity of
the plume ( Table 6-3). As shown by the data of Table 6-3, no
direct relationship seems to exist between the total particulate
loading and the opacity of the smoke plume.
Table 6-3. PLUME OPACITIES FROM VARIOUS
TYPES OF EQUIPMENT47
Firing method
1. Pulverized
2. Pulverized
3. Chain- grate stoker
4. Spreader stoker
5. Underfeed stoker
6. Underfeed stoker
7. Underfeed stoker
8. Hand -fired
Total particulate,
pounds per 10 Btu
'0. 59
2.23
1.31
0.82
0.62
0.25
0.44
1.29
Plume opacity,
percent
30-40
60
20-40
0-20
20-40
0-20
0-20
40-80
Soiling Potential
Data from 17 tests by the Division of Air Pollution Control,
City of Cincinnati,42 showed an average value of 134 Rud-ft2 per
pound of coal burned; the measured values ranged from 9 to 1,250
Rud-ft2 per pound of coal burned. Results from these tests indi-
cated that good combustion should yield values of less than lOORud-flr
per pound of coal, whereas poor operation would be well above
1, 000 Rud-ft2 per pound of coal.
Smoke Spot Data
In 1939, the Bureau of Air Pollution Control, City of
Cincinnati, applied the smoke spot method to smoke performance
tests of various coals fired in a small space heater (not published).
Bachrach smoke spots were taken every 4 minutes for 1 hour after
a uniform charge of coal was fired by hand upon an established
Smoke Emissions and Combustion Plume
45
-------
fire bed. Figure 6-1 shows the 1-hour average values of smoke
spot numbers versus percent volatile matter in the coal.
Mass Emission and Smoke Plume
In this country, little interest has been shown in relating
the severity of the plume to mass emission units. Many authors
have pointed out quite explicitly that most smoke plumes contain
only infinitesimal weights of particulate matter, even though at
times black smoke produced .by the incomplete burning of hydro-
carbons may seem so dense as to appear to be solid black. The
opacity is due to the presence in the plume of a tremendous num-
ber of small particles in the size range of 0. 3 to 0. 6 microns,
which have a highly effective light absorbing or scattering effect,
0 1
Figure 6-1.
23456 789
AVERAGE BACHRACH SMOKE NUMBER
(ONE STROKE OF PUMP)
Relative soilinq potential of various coals as related
to their volatile content. ^
46
EMISSIONS FROM COAL, COMBUSTION
-------
but contribute little to the mass of the emission in relation to the
larger particulates in the plume. The mass of the emission is
contributed by the larger particles, which may have little light
absorbing or scattering effects.
40
Some of the work done is of interest, however. One author
related total loading to percent of light absorption for a stoker-
fired, warm-air furnace, burning approximately 20 pounds of coal
per hour and determined particulate sizes to be mostly 1 micron
or less (Figure 6-2). 48
2.4
o > 900
A 700-900
0500-700
<500
0 10 20 30 40 50 60 70 80 90
"TOTAL LOADING" AT 60 °F. grains/ft3
Figure 6-2. Relationship between "total" particulate emission
and light absorption.^8
In England, Hurley and co-workers4*?, 52 investigated the
relationship between mass emission rates and opacity on hand-
fired and small stoker equipment (Figure 6-3). Of greater in-
terest than total emission is the composition of the particulate
( Figure 6-4), which shows a marked rise in both carbon (soot)
and tar (benzene soluble) as smoke density increases. This
rapid rise in the tar content as smoke increases is a most im-
portant consideration in assessing the overall effect of the
"visible smoke" plume upon the community.
Smoke Emissions and Combustion Plume
47
-------
0
6 8 10 12 14
SMOKE NUMBER
6 18 20 22
Figure 6-3. Relationship between solids emission and opacity.^
s? -
60.
50
40
30
20
10
ASH
s^hK
7
HYDROGEN
I
12
10 ^
8 £
Q
6 i
4|
o:
o o
8 10 12 14 16 18 20 22
SMOKE NUMBER
0 2 4 6
Figure 6-M-. Relationship between pollutant, emissions and smoke opacity.1*9
REDUCING SMOKE EMISSIONS
Techniques for reducing smoke formation from the burning
of coal are very well understood and are generally applied, par-
ticularly in areas having air pollution control programs. The un-
bridled ^mission-of black smoke from home and industry chimneys
motivated smoke control programs in many communities at the
turn of the 19th century.
48
EMISSIONS FROM COAL COMBUSTION
-------
Hand Firing
The only practical method of controlling excessive smoke
from hand-fired furnaces is to use a coal of relatively low vola-
tile content, varying from 26 percent down to 20 percent or less
on a moisture- and ash-free basis, depending upon the degree of
control desired. Good firing practices, assisted by well-designed,
over-fire air jets, are partially effective in some larger furnaces
when trained firemen are used, but such installations are fast
disappearing, being replaced with automatic firing.
Small Underfeed Stokers
The construction of a smokeless installation requires attention
to numerous details, which can be grouped into five general guides.
1. The firebox dimensions, .including combustion volume,
flame clearance, and burning rates, should meet the
standards contained in the "Technical Manual on Single-
Retort Underfeed Stokers" published by the Air Pollution
Control Association. 3
2. Stoker controls should match the load requirements; and
for units consuming more than 800 pounds per hour, step
control for the coal feed rate and combustion air should
be provided. Automatic furnace draft control is also
essential.
3. Over-fire air systems are beneficial on all stokers and,
in particular, on those with on-off control. Design should
comply with the recommendations developed by Bituminous
Coal Research. 54
4. An electric smoke-indicating and/or alarm system can be
of assistance to the boiler operator.
5. Adequately sized chimneys for draft are necessary, as well
as adequate air openings, to supply combustion air to the
boiler room.
Large Boiler-Firing Equipment
As the size of boiler and firing equipment increases, the
inherent premium for complete combustion and smokeless
operation is greater. As a result, less control need be exer-
cised by the control official over the dimensional specifications
Smoke Emissions and Combustion Plume 49
-------
of the combustion unit. Larger units are generally well de-
signed by experienced engineers striving for the maximum Btu
recovery, the end result being a relatively smoke-free plant.
This same motivation does not usually transfer to the selection
of fly-ash-prevention equipment. In this regard, much influ-
ence is exercised by the local air pollution control regulation.
Heretofore, the degree of control over the smoke fraction
of the particulate emission was judged solely by a reduction in
the visible emission. Utilizing soiling potential (expressed as
either Rud-ft2 or Coh-ft2 per 10^ Btu input), the factors con-
tributing to soiling or haze-producing effects in the atmosphere
can be determined more precisely, resulting in improvement
in the effectiveness of control methods.
50 EMISSIONS FROM COAL COMBUSTION
-------
CHAPTER VII.
PARTICULATE EMISSIONS
THEORETICAL CONSIDERATIONS
The emission of solid matter from a given furnace is
related to many factors, mainly gas velocity, particle size,
particle density, fuel-burning rate, combustion efficiency, flue
gas temperature, furnace configuration, coal composition and
size, and the initial state of the raw coal. An indication of how
these variables affect the emission rate is shown in Table 7-1.
For any specific furnace, the composition of the fuel is the
largest variable. The primary consideration in burning a fuel
is to maximize heat release while minimizing costs. This does
not always mean 100 percent combustion. As noted in chapter IV,
Table 7-1. SOME VARIABLES AFFECTING
PARTICULATE EMISSION RATES
Variable increasing
Gas velocity
Particle size
Particle density
Coal ash
Coal size
Coal fired in suspension
Coal -burning rate
Coal heat value
Combustion efficiency
Boiler efficiency
Mass particulate rate
Increasing
X
X
X
X
Decreasing
X
X
X
X
X
X
51
-------
the optimum efficiency is usually about 95 to 99. 5 percent of
complete combustion. 31 Ideally, the only particulate emission
would be the mineral ash contained in the coal; however, 0. 5 to
5 percent of the combustible content of the coal can also be
emitted as par ticulate matter. (There cannot be more than
100 percent of the ash in the coal emitted as noncombustible
matter.) Thus, more particulate matter can be emitted than
there is ash in the coal because of the combustible fraction in
the emissions. If reinjection of fly ash is practiced, there can
be an accumulation in the furnace of suspended solids repre-
senting more than 100 percent of the ash in the fuel and, thus, a
factor representing the solids leaving the furnace (before the
fly-ash collector) can be greater than the total ash entering in
the fuel; however, when the collector is included in the emission
calculations, this is not true.
As the velocity of the gases passing through the furnace
increases, larger particles of coal and ash are carried out of
the furnace. The velocity of the gases is directly proportional
to the firing rate of a given furnace; thus the size of the particle
and rate of emission should be a function of the firing rate. In
a similar manner, the excess air, pressure, and temperature
are related to the particulate emissions in that they control the
gas velocity.
The method of burning the coal also influences particulate
emission rates. When coal is thrown or blown into a furnace,
combustion takes place in suspension. As the pieces of coal
burn, they get smaller, and thus their chance of being exhausted
•with stack gases is increased. When coal is pushed or pulled
into a furnace, to form a bed, the coal or ash has less chance of
being entiained by the flue gases because of impingement onto
larger particles. When coal is introduced tangentially into a
cylinder, such as in the cyclone furnace, the burner acts as a
cyclone separator and thus reduces emission of larger particles.
If all of the variables were known, the amount of particu-
lates emitted from a given unit could be predicted. The problem
is that none of the above variables are completely known. The
following variables are felt to be the most important in relation
to particulate emissions:
1. Amount of ash in the coal.
2. Heat content or heating value of the coal.
3. Method of burning the coal.
4. Rate at which the coal is burned.
52 EMISSIONS FROM COAL, COMBUSTION
-------
Hand-fired equipment is treated separately from mechan-
ically fired furnaces because of the difficulties in obtaining,
representing, and interpreting the data.
EMISSION UNITS
A wide variety of units have been used by various authors
for reporting emission rates, such as a percentage of the ash
in the coal, a percentage of the coal burned, pounds per 10° Btu
input, grains per cubic foot of stack gas, and pounds per thousand
pounds of flue gas. Some authors have reported the conditions
at which their units are standardized, such as the temperature,
percent carbon dioxide or excess air, or type of coal, whereas
others have assumed that conditions considered "standard" are
understood.
In the selection of emission units for this report, primary
consideration was given to the effect that variation in the com-
position of coal has on emission rates. Consideration was also
given to the principal usage of the coal, namely to produce heat.
In an attempt to combine these two facets into one factor, several
correlations were developed. The heat content (on a dry basis)
was plotted against the ash content (on a dry basis) for coals
from the individual producing districts of the country (Figure 3-2,
chapter III), and the nomograph in Figure 7-1 was developed to
show this general relationship.
Because of the many different units used in reporting emis-
sion data, utilizing conversion factors from standard handbooks
was convenient to produce a series of nomographs to assist in
converting units and making elementary combustion calcula-
tions. ' 12,31,55 pertinent relationships developed are given
in Figures 7-2 and 7-3.
With these relationships, one can see that the composition
of the fuel is related to the stack gas concentrations only through
the heat content of the coal. Thus, since the composition of the
fuel is so highly variable, the emissions should be stated in
terms related to composition, i. e. , pounds of pollutant per
10° Btu input. An estimate of particulate emissions, therefore,
requires knowledge of the ash content and heating value of the
coal, type and size of the combustion unit, and control equip-
ment efficiency. With this knowledge, an estimate may be made
of the mass rate of emission of particulate pollutants per unit
time or stack concentrations of particulate from various units
with and without various types of control equipment.
Particulate Emissions 53
-------
ASH CONTENT,
20-i
18-
16-
14-
12->v
\
10-
8-
6-
4-
2-
0-
HEATING VALUE,
Btu/lb
r 16.000
H»
G»
'S.
F«
E»
O
B»
A«
- 15,000
- 14.000
-13.000
- 12,000
- 11 ,000
L 1 0.000
POINT
A
B
C
D
E
F
G
H
PRODUCING
DISTRICTS
21
19
16
17,22
12
4,9,10.11
3,6,15
2,8
1,13,14
7
18,20
ANTHRACITE
Figure 7-1. Relationship between ash content and heatinq value for coals
from various producing districts.
PHYSICAL PROPERTIES OF PARTICULATES
Particle Size Distribution
Many authors have reported particle size distributions for
various types of equipment. Most of these distributions were
termed "typical, " although a few were based on specific stack
test data. Some authors reported other data with the size
analysis, such as combustible content or firing rate. Some data
represented the size analysis of dust taken from a collector or
precipitator, whereas other data represented size distributions
54
EMISSIONS FROM COAL COMBUSTION
-------
of dust passing uncollected through control equipment. Figures
7-4 through 7-7 present only those data believed to represent
the size of the particles leaving the boiler or furnace before
any control equipment. Attempts were made to separate the
data, according to broad types of combustion equipment. The
data were equally scattered for all types of stokers other than
spreader stokers and were therefore combined into one grouping.
100-
90-
80
70-
*«• 60-
oT
-------
100 T
I
K
Cfl
cn
i
n
o
o
cn
8
o:
<
80-
70-
60-
50-
to-
30-
20-
- METHANE
- AVERAGE NATURAL GAS
-ETHANE
-PROPANE
- BUTANE
-PENTANE
60.. 'GASOLINE
'*5- -
> MEDIUM VOLATILE
' BITUMINOUS
.SUBBITUMINOUS
AND LIGNITE
Figure 7-3.
HIGH VOLATILE
BITUMINOUS
LOW VOLATILE
BITUMINOUS '
SEMIANTHRACIT
ANTHRACITE-
COKE-
Relationship between type of fuel burned and excess air, and
resulting percent oxygen and carbon dioxide in flue gases
(adapted from reference 55).
KEROSENE
35- - No. 2 FUEL OIL
25-- No. 4 FUEL OIL
15- - No. 5 FUEL OIL
10-- No . 6 FUEL OIL
5~ > BUNKER
2-J
'C" OIL
v
I \
-------
No difference was found between wet- or dry-bottom pulverized-
fuel-fired furnaces; therefore, in Figures 7-4 through 7-7, the
size analysis ranges (dashed lines) and a typical analysis were
chosen by the authors to represent the very scattered data.
10
O.Ot 0.050.10.2 05 I 2 5 10 20 30 40 50 60 70 00 90 95 96 99 99.090.9 99.99
PERCENT BY 1EIGHT LESS THAN STATED PARTICLE SIZE
Figure 7-M-. Estimated size distribution for particles emitted from pulverized-
fuel-fired furnaces (before collectors).
One important variable was found with respect to the
pulverized-fuel-fired units. Some of the data°2,85 revealed
that one could expect larger particles when the combustible
content was high and smaller particles when the combustible
content was low. This is only a generalization, and numbers
cannot be assigned to various size analyses because this relation-
ship varied so much between units. This relationship may be
true of other types of units also, but because of a lack of data
with supporting operating information, no definite conclusions
can be drawn. It might be expected that the particle size -would
increase with an increase in firing rate or exhibit differences
with the use or nonuse of fly-ash reinjection; however, no such
correlations were found.
Participate Emissions 57
-------
20
10
0.01
I I I 1—I T
CURVES 3ASEO ON DATA IN
REFERENCES 96 through 52
~\ 1—I 1 T
-T—r
RANGE REPORTED
I I I I_J L
J L
I I I L
i L
I I
0.090.1 0.2 0.5
5 10 20 30 « 50 60 70 80 90
PERCENT BY HEIGHT LESS THAN STATED PARTICLE SIZE
96 99
98.B 99.0 96.99
Figure 7-5. Estimated size distributions for particles emitted from cyclone
furnaces (before collectors).
Particle Description
Microscopic analysis of fly ash, using reflected light, will
indicate the type of firing unit that was the source as well as the
combustion efficiency (Table 7-2). Additional information can
be found in reference 86.
Table 7-2. CHANGING VARIABLES WITH
MICROSCOPIC ANALYSIS
62
Type of unit
Pulverized units
Spreader stoker
Other stokers
Domestic units
Small particles
i
Large particles
Glassy and
spherical
Flaky and
agglomerated
Low carbon
i
I
High carbon
58
EMISSIONS FROM COAL COMBUSTION
GPO 625-629-5
-------
100
~r~i i I i I i 1—i—i—i—i—i—i 1 1 1—i r~r
CURVES BASED ON DATA IN
REFERENCES 58. 59. 66.
and 71-77.
RANGE REPORTED
/ // I I | I I | | L. I I I
10
0.01 0.050.1 0.2 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.B 99.9 99.99
PERCENT BY iEIGHT LESS THAN STATED PARTICLE SIZE
Figure 7-6. Estimated size distribution for particles emitted from spreader-
stoker-fired furnaces (before collectors).
Particle Density
The density of fly ash depends primarily on its particle
size, particle structure, and carbon content. 56, 87 ln general,
the large, coarse particles, containing a high percentage of
carbon, have a low density. It appears that the volatile portion
burns out, leaving black, coke-like particles, having low densi-
ties and a specific gravity on the order of 0. 6 to 1. 0. ", 87 One
investigator^ reports a specific gravity of 0. 7 as compared to
the average value for fly ash of 2. 0 to Z. 7. 57, 89 in evaluating
the importance of the physical and chemical properties of fly ash
for commercial use, values of 2. 28°° and 2. 0°"> '3 for specific
gravity have been reported.
Finer particles of ash, which tend to be low in carbon
content have a much higher specific gravity, usually in the range
of 1. 5 to 3. 56, 59, 88 -phe very small particles may run well
over 4. 0^9. 88 an(j ,jo not exhibit the porous structure of the
larger particles, although many of them may be hollow spheres
or cenospheres.
Particulate Emissions 59
-------
CURVES BASED OH DATA IN
REFERENCES II. 19 57-59
62-66. and 71-84.
M.8 H.t H.99
PERCENT BY IEIQHT LESS THAN STATED PARTICLE SIZE
Figure 7-7. Estimated size distributions for particles emitted from stoker-
fired (other than spreaders), furnaces (before collectors).
The variation in density with particle size for typical fly
ash from a modern pulverized-fuel-fired boiler is shown in
Table 7-3. Also given are the corresponding bulk densities of
the size fractions. The bulk density of fly ash usually ranges
from 30 to 50 pounds per cubic foot^k, 88 but may be as high as
90 pounds per cubic foot. ^° Freshly collected, hot fly ash is
normally very fluid and has a somewhat lower density than cold
fly ash. The fresh fly ash is probably aerated by the exposure
of the individual particles to the carrier gas, which results in
adsorption of gas layers on the particle surface. De-aeration
of the ash tends to occur after standing and cooling, which cause
the ash to compact and become less fluid. ^6 One author reports
that the bulk density of freshly precipitated fly ash may be as
low as 15 pounds per cubic foot, but upon standing and complete
removal of occluded gases, the ash may have an increased
density of 40 to 60 pounds per cubic foot. 91
CHEMICAL COMPOSITION OF PARTICULATES
Chemical compositions of particulate emissions are as
variable as emission rates. The inorganic portions vary with
60 EMISSIONS FROM COAL COMBUSTION
-------
the ash analysis of the coal (see chapter III). Tables 7-4 and
7-5 show representative data found in the literature. These
analyses show that the major constituents of most fly ashes
Table 7-3. DENSITIES OF PARTICLE-SIZE FRACTIONS
FOR A TYPICAL PULVERIZED-COAL FLY ASH
56
Particle-size
fraction, microns
Total sample
< 44
44 to 74
74 to 149
149 to 297
>297
Percent present
100
78
10
8.3
3.6
0. 1
Density,
g/cm3
1.75
1.78
1.70
1.60
1.57
1.02
Bulk density,
g/cm3
0.58
0.60
0.44
0.38
0.25
0.21
lb/ft3
36
37
27
24
16
13
Table 7-4. RANGES IN ANALYSIS OF FLY ASH
Compound
Carbon, C
Iron, Fe,O, or Fe,C3
23 34
M-agnesium, MgO
Calcium, CaO
Aluminum, Al O,
2 3
Sulfur. SO
Titanium, TiO,
2
Carbonate, CO
Silicon, SiO,
2
Phosphorus, P2°c
Potassium, K O
Sodium, Na O
Undetermined
Reference
Percentage of fly ash
0.37-36.2
2.0 -26. 8
0. 06- 4. 77
0. 12-14.73
9.81-58.4
0. 12-24. 33
0. 50- 2.8
0.05- 2.6
17.3 -63.6
0.07-47.2
0.08-18.9
56
0. 56-31. 56a
3.86-26.43
0.55- 1.91b
1.00-10. 59
15. 12-34. 04
0.23- 3. 59°
28. 1 -51.26
1
92
1.4-13.5a
6. 1- 9.0
1. 3- 2. 0
2.6- 4. 3
26. 7-28.5
45.2-46. 9
2.8- 3.0
0.2- 0. 9
93
1.49-19. 51a
6. 62-26. 43
0.55- 1.63
0. 99- 9. 68
17. 50-30. 39
0.23- 3.59
34.01-47.54
94
"Ignition loss.
Usual range, extreme range: trace - 3.0%.
cUsual range, extreme: as high as 12%.
Particulate Emissions
61
-------
are silica, alumina, and iron oxide. The first two are present
primarily as silicates, which give fly-ash particles their typical,
glassy appearance. Iron oxide may be present as Fe2O3, which
in appreciable amounts imparts a tan or reddish color to fly ash.
The presence of iron as magnetite, Fe3
-------
COMBUSTIBLE CONTENT OF PARTICULATES
The combustible content has a direct relationship to the
mass emission rates and, therefore, is treated separately
from other chemical properties of fly ash. The combustible
contents of fly ash from various types of units were compiled
and separated in an attempt to determine what might be con-
sidered average or typical values (see Figures 7-8 through
7-10). Only three values were found for the cyclone unit
(14.2 and 11. 1 percent, 95 an
-------
reason, the value of 10 percent combustible is believed to better
represent the values for pulverized and cyclone units. Figure 7-9
shows the values found for spreader stokers. Here the most
common value, about 50 percent, appears to be representative
of spreader stokers (with or without fly-ash reinjection).
1C
o
o
NUMBERS IN BLOCKS ARE
REFERENCES CITED
RVALUE CHOSEN
N
71
71
71
66
101
oq
82
91
1
"•
0 10 20 30 40 50 60 70 80 90 100
COMBUSTIBLES IN PARTI CULATES, %
Figure 7-9. Combustible content of particulates from spreader-
stoker-fired furnaces.
Figure 7-10 shows the values found for other types of
stokers. The data for each stoker category were so meager
and scattered that all stoker data were combined. In this case,
the authors chose 40 percent combustible matter as a repre-
sentative value for stokers other than spreader stokers.
a
-i
Q
1s-
U-
o
o
NUMBERS IN BLOCKS ARE
REFERENCES CITED
* VALUE CHOSEN
81 81 OC
6S| 95 80 80 95J67 67 1 4
66J , , 102, 17
67
67 95
103 67|67| |67|67| ,
10 20 30 40 50 60 70 80 90 100
COMBUSTIBLES IN PARTICULATES, %
Figure 7-10. Combustible content of particulates from stoker-fired
furnaces (except spreader stokers).
64
EMISSIONS FROM COAL, COMBUSTION
-------
The values for the combustible content of particulate
matter are extremely scattered because of many variables,
the most important of which is probably the firing rate.
Figures 7-11 and 7-12 show correlations between firing rate
and load, with combustible content of the fly ash reported by two
authors. The actual values may not apply to the average unit
operating today, but the relative increase could be representative.
One author attempted to correlate the carbon content of ash to
particle size (Figure 7-13). ^6
100
z 60
8
UJ
f 40
S20
I I I I I
0.2 0.4 0.6 0.8 1-0
LOAD, I06 Btu/ft2 hr
Figure 7-11. Combustible content of particulates versus load fora
multiple-retort underfeed stoker.67
Associated with the combustible content are the poly-
nuclear hydrocarbons. There is much interest in these sub-
stances because of their carcinogenic properties. Concentra-
tions of polynuclear hydrocarbons in particulate emis sions
found in the literature are shown in Tables 7-6 and 7-7. There
was little, if any, reduction in the polynuclear hydrocarbons
after the effluent passed through control equipment. This seems
to indicate that polynuclear hydrocarbons are found in particles
of less than 1 micron and are not easily collected. 47> 104> 105, 106
MASS EMISSION FACTORS
The literature contains vast amounts of data for stack gas
particulate concentrations. The majority of these data have
Particulate Emissions 65
-------
50
^ 40
I—~
UJ
£ 30
S
LU
= 20
GO
z>
ca
S
o
0 10
25
I
I
50 75
PERCENT LOAD
100'
125
Figure 7-12. Combustible content of particulates versus load for a
slag-tap pul veri zed-coal-fi red unit.96
50
•HO
30
Al
20
10
i i i i i
0 5 10 15 20 25 30 35
CARBON IN ASH, %
Figure 7-13. Relationship between
ash particle size and carbon content
of fly ash from pulverized units.56
66
EMISSIONS FROM COAL COMBUSTION
-------
Table 7-6.
FROM STOKER AND HAND-FIRED UNITS
SOME POLYNUCLEAR HYDROCARBONS EMITTED
47
(Values expressed in lb/10 Btu input)
Compound
Benzo(a)pyrene
Pyrene
Benzo(e)pyrene
Perylene
Benzo(ghi)perylene
Anthanthrene
Coronene
Anthracene
Phenanthrene
Fluor anthene
Benz(a)anthracene
Type of unit
Chain-grate
stoker
0.082
0.860
0.290
1. 50
Spreader
stoker
0.057
1. 30
0.770
0. 057
0.790
Underfeed
stoker
22
35
17
3.5
9.9
0. 64
0.73
1.9
22
83. 9
8. 6
0.26
3.70
0. 510
2.2
7. 1
8. 4
17
11.9
1.28
2.64
64
103
1.23
Hand-
fired
880
1,320
220
132
660
198
66
880
2,200
2,200
aA blank indicates that the compound was not detected.
little value for the purpose of establishing emission inventory
factors. Particulate emissions are mainly a function of
(1) the ash content of the coal, (2) the heating value of the fuel,
(3) the method by which the coal is burned, and (4) the rate at
which the coal is burned. If an author who reports the particu-
late emissions in the form of a concentration does not report
the ash and heating content, and the method and rate of com-
bustion, the values are not useful in estimating emissions from
similar coal-burning units.
Authors also have neglected to include information about
control equipment through which the flue gas has passed before
the sample was taken. Such an omission, along with the others
previously mentioned, has caused much concern. An attempt
was made to use dust concentrations reported in the literature;
but since companion data were often lacking, the dust concentra-
tion values were of little value. The data used were principally
those expressing the emission as a fraction of the ash introduced
to the unit. The amount of combustible material was added before
a representative value could be ascertained. In a previous
Particulate Emissions
67
-------
Table 7-7. SOME POLYiSTUCLEAR HYDROCARBONS EMITTED
104
FROM VARIOUS SUSPENSION-FIRED UNITS
(Values expressed in Ib/10 Btu input)
Compound
Fluoranthcne
Pyrene
nenzo(a)pyrene
Bcnzo(e)pyrene
Benzo(j(hi)perylene
Corone.no
Pcrylem;
Type of Firing
Pulverized fuel
Vertical
0. 44
o. •»•;
0.04
Corner
0. 8S
0. 31
0. 11
0. 19
o. •»»
o. 02
0. IS
Front-wall
0. 18
0. 40
0.04
0. OS
0.02
Horizontally
opposed
0.41
0.20
0. 18
0. 18
1.42
0. 12
Spreader
stoker
0. 11
0.25
0.04
0. 11
0.02
Cyclone
0. 17
2.2S
0.49
(I. 87
0.44
0.01
0.04
section of this report, the percentage of combustibles for dif-
ferent units was estimated. These values were used in this
section when reported emissions indicate that the reported
number refers only to the noncombustible portion of the particu-
late emission. (Example: in a hypothetical case, 40 percent
of the ash is slagged in a wet-bottom pulverized-fuel-fired unit
and, therefore, 60 percent of the ash is emitted from the stack.
From a previous section of this report, a value of 10 percent
combustible was estimated for particulate emissions from
pulverized firing. This would mean that the 60 percent ash
value represents only 90 percent of the total emission, and the
total emission expressed as a percentage of the ash would be
60/0. 9 or 66. 7 percent.)
In this report, all ash fractions represent the total particu-
late emission (ash and combustible content) expressed as a
percentage of the ash in the as-fired coal. The values are
assumed to represent the emissions leaving the boiler before
any control equipment but include emissions from soot blowing.
(Cinder catchers in the boiler are assumed to be part of the
combustion unit and not control equipment.) If fly-ash reinjec-
tion is practiced, the emission value may exceed 100 percent
because of recirculation and accumulation of the fly ash within
the boiler passages. It must be understood, however, that in
order to recirculate the fly ash, some of it must be collected.
This means that any unit utilizing fly-ash reinjection must have
a fly-ash collector.
68
EMISSIONS FROM COAL COMBUSTION
-------
Figures 7-14 through 7-19 show the total particulate
values found for various units expressed as a percentage of the
ash in the as-fired coal. Several values were given for pul-
verized-fuel-fired units in general (Figure 7-14). The most
common value centered around 80 percent. Figure 7-15 illus-
trates values found for dry-bottom pulverized-fuel-fired units,
with 85 percent selected as the most representative value. Fly
ash is often reinjected into wet-bottom pulverized-fuel-fired
units and, therefore, it must be represented by two values
(Figure 7-16). Values chosen are 65 percent for units without
reinjection and 120 percent with reinjection. For the cyclone
unit (Figure 7-17), 10 percent was chosen as a representative
number. Operation of spreader stokers, like wet-bottom
pulverized-fuel-fired units, often utilizes fly-ash reinjection.
Useful data found for spreader stokers are shown in Figure 7-18.
Values chosen to represent these data are 65 percent for spreader
stokers without fly-ash reinjection and 100 percent for those with
reinjection. Values for other stokers, such as underfeed, chain -
or vibrating-grate stokers, of all sizes are shown in Figure 7-19.
15-
A VALUE OF 10% COMBUSTIBLE MATTER
WAS ADDED WHEN AN AUTHOR INDICATED
HIS VALUES REPRESENTED ONLY ASH EMISSION.
NUMBERS IN BLOCKS ARE
REFERENCES CITED
* VALUE CHOSEN
108
95|l09|62|6l]
62
62
62
62
62
07
69
60
107
80 |l07|
20 30 40 50 60 70 80 90 100 110 120
TOTAL PARTICULATES AS PERCENT OF ASH IN COAL
Figure 7—14. Particulate emissions from pulverized-coal-f i red
units (qeneral ).
Particulate Emissions
69
-------
CO
S'O-
«*
>•
s
en
o
0-
LU
:»-
o
0
A VALUE OF 10% COMBUSTIBLE MATTER
WAS ADDED WHEN AN AUTHOR INDICATED
HIS VALUES REPRESENTED ONLY ASH EMISSION.
NUMBERS IN BLOCKS ARE
REFERENCES CITED
•¥• VALUE CHOSEN
104
1(M 66
105 106 62
104
62
113
58
95 95 62
111
112110
70 1 14 97
58 95
23 63 62 62 |62|62] |62| 62
50 60 70 80 90 100 110 120 130 140 150
TOTAL PARTICULATES AS PERCENT OF ASH IN COAL BURNED
Figure 7-15. Total particulate emissions from dry-bottom pulverized-
coal—fi red units.
A VALUE OF 10% COMBUSTIBLE MATTER WAS
ADDED WHEN AN AUTHOR INDICATED HIS
VALUES REPRESENTED ONLY ASH EMISSION.
30 40 50 60 70 60 90 100 110 120 130 140 150 160 170
TOTAL PARTICULATES AS PERCENT OF ASH IN COAL BURNED
Figure 7-16. Particulate emissions from wet-bottom, pul veri zed-coal-
fi red units.
70
EMISSIONS FROM COAL COMBUSTION
-------
o
i
en
i
A VALUE
OF \0% COMBUSTIBLE
MATTER WAS ADDED WHEN AN
AUTHOR INDICATED HIS VALUES
REPRESENTED ONLY ASH
EMISSION.
«• VALUES CHOSEN
112
112
112
12
112
112
112
112
112
112
60
112
112
112
112
112
112
12
112
112
95
1
ME
112
112
1C
62
112
112
[95
66
56
112
58
116
61
62
56
58
117
61
NUMBERS IN BLOCKS ARE
REFERENCES CITED
i
5FJ| (il2] fr \\C>4 ,
0
TOTAL PARTICIPATES AS PERCENT OF ASH IN COAL BURNED
Figure 7-I7. Particulate emissions from cyclone units.
20'
15-
5-
I 1
A VALUE OF 50% COMBUSTIBLE MATTER WAS
ADDED WHEN AN AUTHOR INDICATED HIS VALUES
REPRESENTED ONLY ASH EMISSION.
VALUE FOR UNIT WITHOUT REINJECT10N
r—\
L.j VALUE FOR UNIT WITH REINJECTION
NUMBERS IN BLOCKS ARE
REFERENCES CITED
VALUE CHOSEN
WITHOUT ASH REINJECTION
58
RVALUE CHOSEN
fnsfee]
118|l 19J71
58
23
58
80
71 !
MOI^Tl
58 !
23
._Si33
58 !
TO 261
99
20
30
40 50 60 70 80 90 100 110 120 130
TOTAL PARTI CULATES AS PERCENT ASH IN COAL
HO
Figure 7-18. Participate emissions from spreader-stoker-fired units.
Particulate Emissions
71
-------
Scattering of these data probably results from varying firing
rates. A value of 25 percent was chosen to represent any type
of stoker other than spreader stokers.
A summary of the particulate emission factors, expressed
in terms of the ash content of coal, is shown in Table 7-8.
5-
im
67
81
96
67
66
47
95
58
80
•»
67
120
47
47
66
67
120
120
1 2O
47
tw
67
120
120
95
81
62
58
kLUE CHOSEN
A VALUE OF 40% COMBUSTIBLE MATTER WAS
ADDED WHEN AN AUTHOR INDICATED HIS VALUES
REPRESENTED ONLY ASH EMISSION.
120
63
64 I2o|67
"80 I67I I67I
23 1 |67|67l
o
o
30
Figure 7-19.
40 50 60 70 80 90 100 110 120
TOTAL PARTICULATES AS PERCENT OF ASH
130 140 150
Particulate emissions from stoker-fired units
(except spreader stokers).
Table 7-8. PARTICULATE EMISSION FACTORS FOR COAL
COMBUSTION WITHOUT CONTROL EQUIPMENT
Type of unit
Pulverized
General
Dry bottom
Wet bottom without
fly-ash reinjection
Wet bottom with
fly-ash reinjection
Cyclone
Spreader stoker
without fly-ash reinjection
with fly-ash reinjectionb
All other stokers
Pounds of particulate
per ton of coal burned3
(Values represent emissions
before control equipment)
16A
17A
13A
24A
2A
13A
20A
5A
Percent of ash in
coal as particulate
emission
80
85
65
120
10
65
100
25
ttThe letter A to be used for all units other than hand-fired equipment, indicates
that the percent ash in the coal should be multiplied by the .value given. Example:
If the factor is 17 and the ash content is 10 percent, the particulate emission
before the control equipment would be 10 times 17, or 170 Ib participate/ton
of coal.
bValues should not be used as emission factors. Values represent the loading
reaching the control equipment always used on this type of furnace.
72
EMISSIONS FROM COAL COMBUSTION
-------
Effect of Firing Rates on Emissions
Emissions from stokers are greatly dependent on the
firing rate, as shown in Figures 7-20 and 7-21. Figure 7-20
• FROM REFERENCE NO. 67
D FROM REFERENCE NO. 8!
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
GRATE LOADING, ,I06 Btu/ft2 - hr
Figure 7-20. Effect of firing rate on parti cul ate emissions from large
underfeed-stoker-fi red units.
Particulate Emissions
1.1
73
-------
shows the total particulate emission, expressed as a percentage
of the ash in the coal, as a function of the grate heat release.
The data were taken from two references, both representing
large underfeed stokers. Many authors have reported stack
concentrations as a function of the grate loading, but these data,
as explained earlier, were too diverse to permit definite con-
clusions and did not include information on ash in the coal fired.
Figure 7-21 shows trends in emission rates for different types
of stokers. This figure might be used to indicate the relative
discharge as burning rates increase, although it is not based
on fuel-ash content.
I- o
z o
3. 5
3.0
2. 5
2. 0
O
O CO
£ ^ 1.5
1=3
O LL. 1.0
a: —
-------
80
VS.
^ 60
_J
•a.
o
o
>- 40
a:
o
z
_ 20
•SLAGGED IN FURNACE
EMI TIED
AS FLY ASH
EMITTED DURING SOOT BLOWING
25
50 75 100
PERCENT LOAD
1 25
Figure 7-22. Participate emission
versus load for a slaq-tap furnace. *
25
50
75
-100
125
PERCENT LOAD
Figure 7-23. Particulate concentration
in stack gas versus load for a slaq-tap
furnace96 (at stack C02 concentrations).
i
Although the number of hand-fired units in urban areas
is rapidly diminishing and this mode of combustion is usually a
minor contributor, data to determine a representative emission
factor are given in Table 7-9 so that the presentation of emis-
sion factors is complete.
From these data, it is estimated that approximately 1 per-
cent of the coal is emitted as particulate matter from hand-fired
furnaces and stoves. This estimate is equal to about 0. 8 pound
per 10^ Btu, or 20 pounds per ton of coal burned.
Particulate Emissions
75
-------
Table 7-9. PARTICULATE EMISSIONS FROM HAND-FIR ED
COAL-BURNING EQUIPMENT
Reference
64
122
138
136
137
Particulate emission
As percent
of coal
1.85
0.7-1.7
0.8-2.5
1 -2
3
0.3
0.5
1.3
0. 1
As percent
of ash
69
17
22
3
Percent
combustible
in particulate
46
90
0
60
45
20
Remarks
Lighting fire
Refueling
Usual range
Burning
bituminous
Burning
s emibituminous
or anthracite
Burning
anthracite
Burning coke
Burning
subbituminous
CONTROL OF PARTICULATE EMISSIONS
The influence of control equipment is often neglected by
persons making emission inventories. The general level of
control of any community is determined by the quality of air
pollution control programs, the length of time they have been
in existence, the attitude of the citizeny toward the programs,
the prevailing methods of coal utilization, and the characteristics
of the coal used throughout the area. All of these factors, applied
with judgment and skill as the emission inventory is developed,
will enhance the accuracy of a survey.
The efficiency of particulate control equipment for the area
as a whole can be judged by looking at a number of typical units,
applying the factors for emission without control, "plugging in1'
the regulatory limit of emission and the ash content of the coal
(see Table 3-6, chapter III), and calculating the efficiency of
flue-gas-cleaning equipment to meet the air pollution regulations.
76
EMISSIONS FROM COAL COMBUSTION
-------
Example: The local ordinance in effect at the time the
plant was built placed the emission limit for particulates at
1 pound per million Btu input. The plant under consideration is
a spreader stoker with fly-ash reinjection, burning Illinois coal
with a heat content of 13, 000 Btu per pound (dry basis). From
Table 3-6, chapter III, select 10 percent (dry basis) as the ash
content. According to Table 7-8, the emissions would be equal
to 100 percent of the ash in the coal. Since both heating content
and ash content are on the same basis, the moisture content
would affect both to the same degree and, therefore, it can be
considered as if it were an as-fired basis. The emission from
this unit without any control equipment would be
1. 00 (ash out)(0. 10 Ib ash/lb coal)(lQ6 Btu) _ . 6
7 or 7.7 lb/10 Btu
(13,000 Btu/lb coal)( 10 Btu)
The collection efficiency of (1 1/7.7) x. 100 or, 87 percent,
would be necessary to comply with the ordinance.
Most coal-burning plants have some type of control equip-
ment, ranging from the settling-chamber effect of large breeches
and chimney bases to a combination mechanical-electrical pre-
cipitator for large central stations. The efficiency of each type
of collector depends primarily upon the size, specific gravity,
and resistivity of the particles acted upon. In general, the
smaller the unit is, the less the total emission and the larger
the particle size. As unit size increases, the total quantity of
particulate carried to the collector increases and particle size
decreases; therefore, the need for more efficient gas-cleaning
equipment is compounded. Table 7-10 delineates operating
conditions and use limitations for major categories of particulate
collectors. Efficiency ranges generally achieved by commonly
used collectors on various units are given in Table 7-11.
No generalization can be made for collection efficiency
values to be expected for any specific unit. In making an emis-
sion inventory, one looks at local codes and ordinances to estab-
lish maximum allowable emissions for that community. Then,
using emission factors for uncontrolled equipment established
in this report, the emissions from each type of unit are calculated.
If the calculated values are greater than the prevailing codes, it
can be assumed that control equipment is being used. It can
usually be assumed that the emissions are equal to or less than
the prevailing codes, and in some specific cases, much less.
Particulate Emissions 77
-------
~J
CO
Table 7-10. DUST COLLECTORS FOR COAL-FIRED HEATING AND POWER PLANTS32
H
to
S
to
g
O
O
i
tu
cj
to
H
i—i
§
Collector
Type
Cinder trap
Medium draft
loss
Single cyclone
(large diameter)
Multicyclone
(small diameter
tubes)
Wet scrubber
Electrostatic
precipitator
Siliconized glass
filter
Collecting
action
Gravity
Inertia
Centrifugal force
and inertia
Centrifugal force
and inertia
Gravity
Electrical
attraction
Filtering
Recommended
application
Smaller plants with under-
feed, vibrating, chain, and
traveling-grate stokers
Smaller plants with very
critical on-grate firing
On-grate firing at high
rates and some spreader
stokers
Spreader stoker
Spreader stoker and pul-
verized-coal-firing units
Pulverized-coal-firing unit
Pulverized-coal-firing
units
Efficiency relative
to particle size
30 to 40% for 45 \j.
and smaller; 75%
or more for
particles over >
45 PL
Overall - to 65%,
100% over 2 5 -pi
size
50 to 90% for
particles over
ZO (i
75 to 90% for
particles over
10 pL
70 to 90%, depend-
ing on particle
size; 75% over
2n
85 to 99% - < 1
to 10 IL (high effi-
ciencies call for
series installation
with multicyclone
collector)
98 to 99% for < 1
to 44 (i
Draft loss,
inches of water
0. 1 to 0. 5
(natural draft
usually
sufficient)
0. 4 to 1.5
0. 5 to Z. 0
2. 0 to 6. 0
13 to 20
0. 1 to 0. 5
1 to 6
Other considerations
Used mainly to elimi-
nate cinder nuisance
in immediate plant
area.
Abrasion may occur:
made in variety of
designs to fit job.
Made in variety of
designs. Care re-
quired to fit design
to job.
Abrasion may be a
problem.
Caking and corrosion
may be a problem,
also water recovery.
Continuous cleaning
necessary.
Exit temperature
limited to 600° F
-------
Table 7-11. USUAL EXPECTED EFFICIENCY RANGES FOR
COMMONLY USED CONTROL EQUIPMENT (percent)
Type of firing
or furnace
Cyclone
Pulverized
Spreader stoker
Other stokers
Type of control equipment
Electrostatic
precipitator
65-99a
80-99. 9*
High-
efficiency
cyclone
30-40
65-75
85-90
90-95
Low-
resistance
cyclone
20-30
40-60
70-80
75-85
Settling chamber,
expanded
chimney bases
20-30
25-50
The higher efficiencies can only be attained with high-efficiency cyclones in
series with electrostatic precipitatoi's.
For those areas where specific emission limitations are not
known or cannot be determined, average control practice based
upon the present American Society of Mechanical Engineers Example
Ordinance, ^^ i. e. , 0. 85 pound of particulates per 1, 000 pounds of
flue gases at 50 percent excess air, can be assumed to be applicable.
For areas of better-than-average control practice, consideration
might be given to applying one of the emission limitations considered
by the Subcommittee of the American Society of Mechanical Engineers
Committee on Air Pollution Control. Figure 7-24 contains one of
1 ^ c
those considered. 1^-J
VARIABLES AFFECTING EFFICIENCY OF CONTROL
EQUIPMENT
Many variables other than particle size and density affect
the collection efficiencies. For centrifugal collectors, the
efficiency of collection increases as load increases (Figure 7-25),
whereas the reverse is true for electrostatic precipitators.
Thus, the centrifugal collector tends to improve its efficiency
with increasing exit gas loadings, which are associated with
increased boiler load, thereby tending to maintain a constant
emission concentration at the outlet of the collector. Conversely,
as the load increases, the efficiency of an electrostatic precipitator
decreases,thus total emissions are increased. For example,
assume that the efficiency curves in Figure 7-25 represent a unit
that generates 7 pounds of flue dust per 10° Btu at 50 percent load
and 10 pounds of flue dust per 10° Btu at 100 percent load at the
collector inlet. Emissions from the centrifugal collector will be
1.75 pounds per 10° Btu for either load, whereas the emissions
from the electrostatic precipitator will be 0. 21 pound per 10° Btu
for a 50 percent load and 0. 5 pound per 10° Btu for a 100 percent
load, a 240 percent increase in particulate emission.
Particulate Emissions
79
-------
i2.0
o
a.
1-^
ll.O
= 0.8
-H
m 0.6
0.2
PRESENT ASME MODEL
10'
IO6 IO7 IO8 IO9 lo'°
INPUT, Btu/hr
Figure 7-24. Proposed revision to ASME model ordinance for
permissable fly-ash emissions '" (At the present
time the ASME Air Pollution Standards Committee
is considering proposals for the control of dust
emission from combustion equipment that differ
from those shown above.)
100
95
s.
\ 90
U
I**
c
3
3 80
j
5
i
75
-CENTRIFUGAL, ELECTRICAL SERIES UNIT
_ ELECTRICAL PRECIPITATOR
CENTRIFUGAL COLLECTOR
j I
I
I
50 60 70 80 90
PERCENT LOAD
100 110
Figure 7-25. Typical performance curves for
fly-ash collectors serving large pulveri zed-
coal-fired furnace.''1*
The carbon content in the fly ash affects the collection
efficiency of both centrifugal and electrostatic precipitators.
An increase in carbon content is usually associated with an
80
EMISSIONS FROM COAL, COMBUSTION
-------
increase in size distribution and electrical resistivity, and a
decrease in specific gravity. In general, the centrifugal col-
lector becomes more efficient because of particle size increase
as the carbon content increases, and the electrostatic precipita-
tor becomes less efficient because of the increase in electrical
resistivity. Electrostatic precipitators are not generally used
for high-carbon ash, such as that derived from stokers, because
the particles lose their charge too rapidly.
Particulate Emissions 81
-------
CHAPTER VIII.
GASEOUS EMISSIONS
FROM COAL COMBUSTION
SULFUR OXIDES
Theoretical Considerations
The sulfur content of coal ranges from less than 1 percent
to greater than 10 percent (by weight). During combustion, a
high percentage of the sulfur in coal is oxidized to sulfur dioxide
(SO2) or sulfur trioxide (803). Some of the sulfur oxide (SOX)
complexes •with fly ash and ash residue or slag, but most is
emitted as a part of the stack gases. If combustion is very
inefficient, hydrogen sulfide (t^S) may be evolved. The oxida-
tion of sulfur to the sulfur oxides is similar to the oxidation of
carbon. If large amounts of carbon monoxide are detected, one
might suspect the presence of I^S. The majority of the sulfur
should, however, be oxidized to SC>2 in modern furnaces.
The amount of sulfur emitted as SC>2 may be inferred from
a material balance. The total sulfur effluent is emitted from the
chimney as a gas or in the particulate matter, or is removed
after combination with the slag or ash residue. Data compiled
in reference 126 show that about 2 percent of the sulfur goes to
the fly ash and soot (Figures 8-1 and 8-2). Figure 8-3 shows
that less than 1 percent of the sulfur usually goes into the slag
or residue, whereas data in Figure 8-4 indicate that 1 to 2 per-
cent of the sulfur usually goes to SOj. Thus, if no appreciable
amount of I^S is formed, about 95 percent of the sulfur is
emitted to the atmosphere as SC^.
Emission Data
Attempts were made to separate the data for various
classes of equipment and to find other relationships that might
account for large differences in the amount of sulfur going to
products other than SO2. One author reports that stoker-fired
units emit from 65 to 75 percent of the sulfur as SO2, whereas
pulverized-fuel-fired units emit as much as 95 percent of the
sulfur. 13 Such values cannot be confirmed by other information
83
-------
20-
i f\m
1 Ctm
D
i i ^
•'•'•'•
! i i
***!
)
/
m
ALL VALUES TAKEN FROM REFERENCE 126
EXCEPT WHERE INDICATED.
[§|] VALUES FROM LOCOMOTIVES126
| J OTHER SOURCES
;;S; p gs |:;j;SJ
H;: :;j;g m 83 |'2v| [127 12?
§11 Hll 3* 11 > HI • iii
6 8 10 12 14 16 33 40 42 4
SULFUR IN COAL FOUND IN PARTI CULATE EMISSIONS,
Figure 8-1. Percentage of sulfur in coal found in
parti cul ate emissions.
reported in the literature. It is of interest to note that Figures
8-1 through 8-3 show values in excess of 10 percent of the sulfur
in the fly ash and slag. No reason for these high values could
be established except that data from references 127 and 129 were
for the combustion of coke in hand-fired stoves. Many of the high
values for sulfur in the slag were from coal combustion in loco-
motives, and the low values for gaseous sulfur products were
also from locomotives (Figure 8-5). This seems to indicate that
inefficient combustion might direct more sulfur into the slag than
would efficient combustion.
All of the values found in the literature for the proportion
of the sulfur in the coal emitted as SC>2 are shown in Figure 8-5.
These data are for equipment ranging in size from domestic
stoves to large steam-electric power plants. Only the values for
the locomotives were consistently lower than those previously
84
EMISSIONS FROM COAL, COMBUSTION
-------
50-
45- ,
25.
15.
S.
10'
ALL VALUES TAKEN FROM REFERENCE 126
6 8 10 12 14 16 18
S03 IN PARTICIPATE EMISSIONS, %
20 22 24 26
Figure 8-2. 803 content of the parti cul ate emissions.
determined by a material balance. Previous experience with the
material balance for sulfur oxides emissions135 indicated that the
measurement of SOX by itself is not always a true representation
of the SOX emission. The measurement of SOX must be accom-
panied by a complete material balance to confirm the measured
gaseous value. For the above reasons, a value of 95 percent of
the sulfur in the coal is chosen for the emission of SO2 from the
stack, and a value of 1 percent of the sulfur in the coal is selected
for the emission of SOj.
One of the reaction products of sulfur, hydrogen sulfide, has
been given little consideration in the study of coal combustion.
One author reported he found an average of 0. 4 percent of the
sulfur in the coal converted to H2S in a hand-fired stove, whereas
only a trace of H2S was found from the burning of coke. 129
Gaseous Emissions
85
-------
128
128
128
128
j | "T-inV VY RO •
m
24 26 28 30 32
SULFUR IN COAL FOUND IN SLAG,
Figure 8-3. Percentage of sulfur in coal found in slaq.
10
NUMBERS -IN BLOCKS ARE
REFERENCES CITED
104
62
62
62
128
128
128
128
126
126
126
126
104
131
132
129
129
133
89
134
130
126
104
126
126
126
126
126
126
Il04
I04|126
13
i 69 I . i
" 0 2 4 6 8 10
SULFUR IN COAL
EMITTED AS S03, %
Figure 8-4. Percentage of sulfur in coal
emitted as SO^.
86
EMISSIONS FROM COAL COMBUSTION
-------
Zb"
20-
co
LU
1^
— J
^0.4, and N2O5 but calculated as NO2- During combustion, oxygen
and nitrogen gas combine to form nitric oxide (NO) as follows:
Gaseous Emissions
S7
-------
0
2
2 NO
(1)
If time permits , reaction (1) continues to equilibrium, butitdoesnot
go to completion as does the carbon to carbon dioxide reaction. The
NO will, however, react with more oxygen and form nitrogen dioxide
(NO^) and other nitrogen oxides. The N£ to NO equilibrium may shift in
either direction, depending upon many variables. If the concentration
of one of the gases is increased, the equilibrium shifts to the opposite
side. There is anabundance of nitrogen butvery little oxygenpresent
for this reaction. If the amount of oxygen (excess air) is increas ed with-
out reducing the flame temperature, the NO concentration will also in-
crease, and the reverse is true. As the NO reacts with oxygen to produce
NO2, there is a reduction in the concentration of NO, which removes it
from the equilibrium in reaction (1) above. The NO is replaced by re-
action (1) returning to equilibrium.
Other variables that affect this equilibrium are the different
temperature, pressure, and concentration zones through which
the gases pass. Most of the NO is formed in the flame, where
very high temperatures are present. The residence time of the
gases at this temperature, however, is relatively short, and the
NO reaction is prevented from reaching equilibrium. Figure 8-6
shows the theoretical concentration of NO, assuming typical fuel
analysis, typical excess air, and a residence time of 0. 5 second
at various flame temperatures. 136
1,000
800
600
200
2,800
3,000 3,200
FLAME TEMPERATURE, °F
3,400
Figure 8-6. Theoretical formation of nitric oxide versus flame
temperatu re. 136
88
EMISSIONS FROM COAL COMBUSTION
-------
The main factors in NOX production are: the flame and
furnace temperature, the length of time that combustion gases
are maintained at this flame temperature, the rate of cooling of
the gases, and the amount of excess air present in the flame. 104,
105,106, 135
Emission Data
Very little stack-sampling data on oxides of nitrogen in
coal burning plant emissions have been reported in the literature.
From the theoretical considerations, one might expect lower
flame temperature to be found in domestic units and higher flame
temperature to be found in pulverized fuel units. Woolrich^^
proposed a method for estimating NOX emissions from coal com-
bustion based on an empirical approach using data from the com-
bustion of oil and gas. His resulting equation is:
1 18
T-, j -*T^ /T Btu/hr input
Pounds NO /hr c—
x
3.8 x 10
6
(2)
When NOX emission data for oil and gas 138 are plotted on six-
cycle log-log graph paper, the data tend to follow a straight line,
as represented by equation (2), but with a different denominator.
If, however, these emission data were presented as pounds of
NOX per 10° Btu input (instead of pounds NOX per hour) versus
10° Btu per hour, the data stay in a consistent order of magnitude
(approximately 0.1 to 1.0 pound NOX per 10° Btu), but do not
follow any real relationship. This lack of correlation results
from the many factors involved in the production and decomposi-
tion of the oxides of nitrogen. Equation (2), however, does
permit the selection of an emission range.
Three articles report ranges of concentrations representa-
tive of large power plants, 100 to 1,400 ppm69' 132 and 650 to
1,460 ppm. ' When these concentrations were standardized to
a stack gas containing 12 percent CO2 from a bituminous coal,
they represented emission ranges of 0. 17 to 2. 5 and 1. 1 to 2. 6
pounds per 10& Btu, respectively. Two authors, referring to
data similar to the above along with oil and gas data, derived
NCx emission factors of 0. 01 pound of NOX per pound of c
and 0. 004 ton NOX per ton of coal, 1^1 values that are equivalent
to about 0. 8 and 0. 3 pound of NOX per 106 Btu, respectively.
There is an indication that small units (commercial and domestic)
may emit less NOX than large units (see Tables 8-1 and 8-2).
One author confirmed the above supposition by measuring NOX
emissions of from 0. 0014 to 0. 047 pound per 106 Btu from a.
i OQ
domestic stove in England. l '
Gaseous Emissions 89
-------
Table 8-1. EMISSION OF NITROGEN OXIDES FROM UNITS
i
FIRING COAL IN SUSPENSION
Test
Full
loada
Partial
loadb
Burner configuration
or type
Vertical
Corner
Front wall
Spreader stoker
Cyclone
Horizontally opposed
Vertical
Corner
Front wall
Spreader stoker
Cyclone
Horizontally opposed
NO , lb/10 Btu
X
Before fly-ash
collector
0. 38
0.95
0. 68
0. 65
2.5
0. 65
0.28
0.73
0. 82
0.73
1.9
0.66
After fly-ash
collector
0. 55
0.71
0. 95
0.76
2.2
0. 59
0. 31
0. 57
0. 74
0. 68
1.8
0. 56
Average values for three or four tests at each unit.
Average values for two tests at each unit.
Table 8-2. EMISSIONS OF NITROGEN OXIDES
FROM SMALL UNITS47
(3x10 Btu/hr input or smaller)
Type unit
Underfeed stoker
Underfeed stoker
Hand-fired stoker
NOX,
lb/106 Btu
0.30
0.36
0. 11
Size of unit,
106 Btu/hr
3
0.066
0. 115
In view of the limited data available, arriving at a suggested
emission factor for oxides of nitrogen is difficult; however, the
90
EMISSIONS FROM COAL COMBUSTION
GPO 825-629-7
-------
following factors are suggested, pending the development of a
more reliable body of data:
0. 8 pound NO /10^ Btu for large units (10^ or more Btu
per hour input)
0. 2 pound NO /106 Ecu for small units (less than 106 Btu
per hour input)
OTHER GASEOUS EMISSIONS
Some work has been reported for gaseous emissions other
than SOX and NOX. These values are shown in Tables 8-3 and
8-4. Data used to determine a heat balance can also be found in
the literature. These data are old and/or refer to hand-fired
units (see references 49, 50, 67,' 129, 142, 143). The values
given by these data are not thought to be representative of those
found today. If values for carbon monoxide (CO), hydrocarbons,
or formaldehyde are needed, one can judge from the above data
Table 8-3. COMBUSTIBLE GASEOUS EMISSIONS
FROM SUSPENSION-FIRED UNITS1 4
Type of boiler firing
Vertical
Corner
Front-wall
Spreader -stoker
Cyclone
Horizontally opposed
Emissions, lb/10 Btu
CO
0.017
0.011
0.005
0.029
-
0.044
Hydrocarbons
0.010
0.004
0.010
0.009
-
0.001
Formaldehyde
2.5 x 10"4
_4
1.7 x 10
_4
1. 4 x 10
.4
0.6 x 10
_4
1.7 x 10
1.0 x 10"4
Gaseous organic gases at room temperature expressed as a
single carbon atom hydrocarbon, measured using infrared
and flame ionization techniques.
Gaseous Emissions
91
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what might be expected. Formaldehyde seems to be consistently
about 0. 0002 pound per 106 Btu, whereas both CO and hydro-
carbons vary 3 to 4 orders of magnitude. Suggested estimating
factors are shown in Table 8-5.
Table 8-4. COMBUSTIBLE GASEOUS EMISSIONS FROM
GRATE-FIR ED UNITS47
Type unit
Chain grate
Spreader stoker
Underfeed stoker
Underfeed stoker
Underfeed stoker
Hand-fired stoker
Size (infant),
106 Btu/hr
147
59.2
4.4
3.0
0.066
0. 115
Emissions, lb/106 Btu
CO
0.51
<0. 1
0. 16
0. 14
1. 1
3.5
CH4
0.005
0.006
0. 116
0.036
0. 12
0.73
Formaldehyde
1.4 x 10-*
2.Z x lO-4
2. 1 x lO-4
3.8 x lO-4
-
Table 8-5. SUMMARY OF COMBUSTIBLE
GASEOUS EMISSION FACTORS
Source
Power plants
Industrial stokers
Domestic units
Emissions, lb/10 Btu
CO
0.02
0. 1
2
Hydrocarbons
0. 007
0.05
0.5
Formaldehyde
2 x 10"4
2 x 10"4
2 x 10"4
Another pollutant of possible importance is hydrogen
chloride (HC1). As shown in chapter III, chlorine occurs in coal
in concentrations of about 0. 1 percent. Calcium chloride may
also be added in concentrations of 0. 1 to 0. 5 percent as an anti-
freeze or dust-proofing agent. ** If all of this were emitted as
HC1, then from 0. 08 to 0. 3 pound of HC1 per 106 Btu might be
emitted. One author recorded a concentration of 49 ppm HC1 at
stack conditions when burning a coal containing 0. 066 percent
chlorine. This value corresponds to about 60 percent of the
chlorine being emitted as HC1.
EMISSIONS FROM COAL COMBUSTION
-------
CHAPTER IX.
FUTURE NEEDS FOR DATA AND RESEARCH
EMISSION DATA NEEDS
This report presents emission factors based on existing
data, which are, in many instances, meager. Much of the data
in the literature could not be used because the information neces-
sary to calculate a useful emission factor was not reported. Re-
finement of the emission factors presented in this report could
be expedited if future reported stack sampling is accompanied by
a complete material balance and a good description of both the
sampling equipment used and plant operating conditions that
existed at the time of sampling. If sampling data were presented
in this manner, the following needs for more emission data could
be satisfied:
1. The establishment, by types of equipment, of emission
values for nitrogen oxides, carbon monoxide, hydro-
carbons, and soiling potential.
2. The effect of design variables on emissions of nitrogen
and sulfur oxides, particulates, hydrocarbons, and
soiling potential.
3. The effect of various types of control equipment on emis-
sion of particulates, sulfur oxides, nitrogen oxides, and
hydrocarbons.
4. The actual operating characteristics of emission control
equipment compared to its design criteria.
RESEARCH NEEDS
During the past several decades, coal-burning equipment
has been markedly improved, and many substandard plants have
been replaced by plants fired with other fuels. A coal-fired
plant with maximum controls can compete favorably in many
respects with one fired with fuel oil, but it cannot match the
performance of a gas-fired plant as judged by the air pollution
potential of the combustion products.
93
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Intensive research effort is needed to create the technical
capability of matching the air pollution potential of coal combus-
tion to that of any fuel. It can be done; and unless it is done,
there will always be the temptation to require by ordinance
(directly or indirectly) the least offensive fuel in the interest of
community welfare.
In the immediate future, the areas in which the overall
emission potential of coal could be reduced include:
1. Improvement of coal quality by lowering the ash and
sulfur content; producing sizes more acceptable to the
firing equipment; and expanding the availability of low-ash,
low-sulfur coals at attractive prices (see reference 144).
2. Improvement of fuel-burning equipment as follows:
a. Over-fire air systems should be made more effective,
should have better controls, and should provide for
better combustion at low loads.
b. Equipment should be improved to reduce or prevent
formation of nitrogen oxides.
c. Boilers should be so designed so that soot blowing is
either not necessary or may be accomplished without
overloading particulate collectors, and overall effi-
ciency should be improved to reduce fuel requirements.
3. Development of better air cleaning equipment. Reliabil-
ity and efficiency of existing particulate removal equip-
ment should be improved. Uses and markets for con-
taminants collected should be developed to ease the
economic burden of collection. New, more practical
systems for reducing sulfur- and nitrogen-oxide emis-
sions, or methods for preventing their formation during
the combustion process should be devised.
The development of a long-range effort should include
consideration of new concepts of burning coal, such as gasifica-
tion or liquifaction. In another direction, continued improve-
ment in the heat rate of central steam-electric generation and
reduction of electricity transmission costs could result in re-
/ '
placement of thousands of small, poorly controlled sources with
a single coal-burning plant with highly efficient emission control.
94 EMISSIONS FROM COAL COMBUSTION
-------
SUGGESTED RESEARCH DIRECTIONS
The literature reviewed in preparing this report gave
some insight into the direction future research might proceed.
Much of the current research on control of sulfur oxides is
directed toward either collecting sulfur oxides in the stack gases
or removing sulfur from the coal. Some of the reports studied
indicate a possibility of tying up the sulfur in the slag. This
might be done by a two-stage combustion operation in which the
first stage maintains a highly reducing atmosphere and the
second stage completes combustion. Examples of similar oper-
ations are the blast furnace and the kraft paper mill recovery
furnace.
Oxygen could replace combustion air and be used in con-
junction with the above method or be used only as a means of
reducing the volume of stack'gases to make treatment of such
gases more economical.
The nitrogen oxides data indicate that emissions could
possibly be reduced by changing burner positions. Staged com-
bustion and very low excess air might yield better results than
those from changing burner configuration. The replacement of
combustion air by pure oxygen would, of course, essentially
eliminate emissions of nitrogen oxides.
More effective particulate control might be accomplished
by a change in furnace or burner design. Data examined in pre-
paring this report indicate that actual operating efficiencies of
control equipment are not close enough to design efficiencies.
With expectations of more stringent air pollution ordinances,
application of fabric filtration to particulate emission control
may become desirable. Such possible use should be studied.
Since the day coal was first fired, it has created significant
air pollution. Although much progress has been made toward
control, it is unlikely that tomorrow's cities will tolerate emis-
sions experienced today from coal combustion.
Future Needs for Data and Research 95
-------
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ACKNOWLEDGMENT
Grateful acknowledgment is extended to William Bye, who
started the work on this report, and to Richard Wromble, who
helped in its early phases. Thanks are also extended to
Donald F. Walters of the Technical Assistance Branch, Division
of Air Pollution, without whose guidance in the preparation,
engineering review, and editing of the final text this work would
not have been completed.
Acknowledgment is also extended to Jean J. Schueneman
and others of the Technical Assistance Branch for their efforts
in preparing this report.
Some of the data used in this report were supplied by a
number of contributors. The assistance of the following organ-
izations is gratefully acknowledged:
Department of Safety, Bureau of Air Pollution Control and Heat-
ing Inspection, Cincinnati, Ohio
Department of Buildings and Safety Engineering, Bureau of Air
Pollution Control, Detroit, Michigan
Department of Public Safety, Division of Air Pollution Control,
St. Louis, Missouri
Department of Service and Buildings, Division of Building Inspec-
tion, Bureau of Combustion Control, Dayton, Ohio
Bureau of Air Pollution Control, Indianapolis, Indiana
Tennessee Valley Authority, Knoxville, Tenn.
Bureau of Mines, U. S. Department of the Interior, Washington,
D. C.
The Air Pollution Control Association, Pittsburgh, Pa.
Pennsylvania Electric Co. , Johnstown, Pa.
Public Service Electric and Gas Company, Maplewood, N. J.
Ill
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Consolidated Edison Co. of New York, N. Y. , N. Y.
Baltimore Gas and Electric Co. , Baltimore, Md.
Combustion Engineering, Co. , N. Y. , N. Y.
The Babcock and Wilcox Co. , N. Y. , N. Y.
Koppers Co. , Inc. , Baltimore, Md.
Western Precipitation Div. , Joy Mfg. Co. , Los Angeles, Calif.
Research-Cottrell, Inc., Bound Brook, N.J.
APRA Precipitator Corp. , N. Y. , Y. Y.
American Air Filter Co. , Inc. , Louisville, Ky.
Aerotec Corp. , Greenwich, Conn.
112 EMISSIONS FROM COAL COMBUSTION
GPO 825-629-9
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BIBLIOGRAPHIC: Smith, W. S. , and C. W. Gruber.
Atmospheric emissions from coal combustion -
an inventory guide. PHS PUBL. No. 999-AP-24.
1966. 112pp.
ABSTRACT: Information concerning atmospheric
emissions arising from the combustion of coal
was collected from the published literature and
other sources. The data were abstracted, as-
sembled, and converted to common units of ex-
pression to facilitate comparison and understand-
ing. From these data, emission factors were
established that can be applied to coal combustion
processes to determine the magnitude of air pol-
lutant emissions. Also discussed are the com-
position of coal, theory of coal combustion, emis-
sion rates, gaps in emission data, and future
research needs.
ACCESSION NO.
KEY WORDS:
Coal
Burning
Emission
Particles
Gases
Smoke
Air pollution
Combustion
Devices
Research
BIBLIOGRAPHIC: Smith, W. S. , and C. W. Gruber.
Atmospheric emissions from coal combustion -
an inventory guide. PHS PUBL. No. 999-AP-24.
1966. 112 pp.
ABSTRACT: Information concerning atmospheric
emissions arising from the combustion of coal
was collected from the published literature and
other sources. The data were abstracted, as-
sembled, and converted to common units of ex-
pression to facilitate comparison and understand-
ing. From these data, emission factors were
established that can be applied to coal combustion
processes to determine the magnitude of air pol-
lutant emissions. Also discussed are the com-
position of coal, theory of coal combustion, emis-
sion rates, gaps in emission data, and future
research needs.
ACCESSION NO.
KEY WORDS:
Coal
Burning
Emission
Particles
Gases
Smoke
Air pollution
Combustion
Devices
Research
BIBLIOGRAPHIC: Smith, W. S., and C. W. Gruber.
Atmospheric emissions from coal combustion
an inventory guide. PHS PUBL. No. 999-AP-24.
1966. 112 pp.
ABSTRACT: Information concerning atmospheric
emissions arising from the combustion of coal
was collected from the published literature and
other sources. The data were abstracted, as-
sembled, and converted to common units of ex
pression to facilitate comparison and understand-
ing. From these data, emission factors were
established that can be applied to coal combustion
processes to determine the magnitude of air pol-
lutant emissions. Also discussed are the com-
position of coal, theory of coal combustion, emis-
sion rates, gaps in emission data, and future
research needs.
ACCESSION NO.
KEY WORDS:
Coal
Burning
Emission
Particles
Gases
Smoke
Air pollution
Combustion
Devices
Research
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