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Role of coal cleaning in control of air toxics
FUEL PROCESSING TECHNOLOGY ELSEVIER
Fuel Processing Technology 39 (1994) 73-86
Role of coal cleaning in control of air toxics D a v i d Akers*, R o b e r t D o s p o y CQ Inc., One Quality Center, P.O. Box 280, Homer City, PA 15748-0280, USA
(Received 19 April 1993; accepted in revised form 16 January 1994)
Abstract Twelve of the potentially hazardous air pollutants listed in the 1990 Clean Air Act are elements commonly found in trace amounts in coal. An option for controlling the release of these elements into the atmosphere is to remove them before combustion. Based on commercial-scale tests, conventional physical coal cleaning processes are effective in reducing the concentration of many of these trace elements in coal. In addition, advanced cleaning processes directed toward reduction of various elements may perform better than conventional processes. The degree to which a specific trace element can be reduced by coal cleaning depends on the mode of occurrence of the trace element, the method of cleaning employed, and the way in which the cleaning process is operated. For example, higher trace element reductions can often be achieved if the cleaning plant is operated to remove as much ash as possible. In addition to reducing the concentration of many trace elements, coal cleaning can improve overall boiler performance by increasing thermal efficiency. Also, cleaning changes ash loading and ash chemistry, impacting the performance of particulate collection equipment. For maximum effectiveness, coal cleaning should be considered as part of a system that includes the boiler as well as other emissions control systems.
1. Introduction The Clean Air Act Amendments of 1990 have identified 189 potentially hazardous air pollutants (PHAPs), of which 12 are elements and their compounds commonly found in coal. These elements are antimony, arsenic, beryllium, cadmium, chlorine, chromium, cobalt, lead, manganese, mercury, nickel, and selenium. In addition to these specific elements, radionuclides are also listed as PHAPs; these also occur naturally in some coals. The geological processes that form coal can also concentrate trace elements. For example, the average concentration of arsenic in bituminous coal (20 ppm) is ten times
* Corresponding author. Tel.: (1-412)479-3503. Fax: (1-412)479-4181. 0378-3820/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 3 7 8 - 3 8 2 0 ( 9 4 ) 0 0 0 2 8 - R
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D. Akers, R. Dospoy/Fuel Processing Technology 39 (1994) 73-86
the average concentration found in all the other rocks that compose the earth's crust (2 ppm). Similarly, antimony, cadmium, mercury, and selenium are generally more concentrated in coal than in the earth's crust. When coal is burned, trace elements can be further concentrated. Volatile elements such as arsenic, mercury, and selenium are a special concern because they tend to vaporize during combustion. Then, as the flue gas cools, these elements have a tendency to condense on the surface of flyash particles. Because the finest-size flyash particles provide the greatest surface area, volatilized elements tend to become concentrated on ultra-fine flyash. These flyash particles are the most difficult to collect and can escape up the stack. Although no new restrictions on trace element emissions were placed on the power generation industry under the new Clean Air Act amendments, the act mandates a study of utility toxic air emissions by the United States Environmental Protection Agency (EPA). In addition, EPA is required by law to recommend new air toxic regulations found necessary to protect human health and the environment. Furthermore, the legislation requires three separate studies of mercury emissions, deposition, and health effects. New regulations aimed at coal-fired electric utilities may be forthcoming when these federally mandated studies are complete. While a significant research effort aimed toward the development of new methods of air toxics control is likely to develop, three basic options are currently available to utilities: (i) Installation of improved particulate collection devices to increase capture of the finest-size flyash particles that usually contain the highest concentration of volatile trace elements. (ii) Installation of wet scrubbers that serve to cool boiler flue gas, thereby promoting condensation of volatile trace elements. Wet scrubbers provide a high fine-particle capture efficiency and are, therefore, likely to be highly effective in capturing most P H A P s (with the possible exception of highly volatile species such as mercury). (iii) Use of coal cleaning or coal switching to reduce the concentration of trace elements entering a boiler. As a P H A P control technology, coal cleaning offers the advantages of relatively low cost, improved boiler thermal efficiency, and reduction of other regulated emissions such as sulfur dioxide. Also, since that proportion of trace elements removed by coal cleaning never enters the boiler, a smaller amount can be expected to be released into the atmosphere. It is unlikely, however, that any physical coal cleaning technology will provide complete removal of any trace element. Therefore, for some coals additional post-combustion control systems may be required.
2. Factors affecting trace element removal Many trace elements in coal are associated with the coal's mineral matter. For example, arsenic is commonly associated with pyrite; cadmium with sphalerite; chromium with clay minerals and chromite; mercury with pyrite and cinnabar; nickel With millerite, pyrite, and other sulfides; and selenium with lead selenide, pyrite, and other sulfides. There are also cases in which some of these elements are organically bound [1, 2]. Just as both organic and pyritic sulfur can occur in the same coal, the
D. Akers, R. Dospoy/ Fuel Processing Technology 39 (1994) 73-86
75
Table 1 Trace element removal by conventional cleaning (% reduction)
Ash Arsenic Cadmium Chromium Lead Mercury Nickel Selenium
AL
KY
OK
PA
MT
75 43 XX 53 62 39 63 46
85 43 XX 79 89 48 82 62
57 53 91 76 25 55 25 - 42
72 83 63 69 44 78 48 - 4
31 35 3 - 62 3 17 31 59
Notes: XX Data not available. Minus sign indicates an increase in concentration with cleaning.
same trace element may be both organically bound and also be present as part of the coal's mineral matter. Physical coal cleaning techniques are effective in removing mineral matter from coal and can, therefore, potentially remove some of the trace elements associated with specific minerals. Work by the Electric Power Research Institute, the United States Department of Energy, Bituminous Coal Research Inc., and others has demonstrated that large reductions in the concentration of many trace elements are possible if conventional coal cleaning techniques are properly applied. Five examples are given in Table 1. In each example, the results shown were generated by cleaning the coal at commercial scale at CQ Inc.'s Coal Quality Development Center (CQDC) located in Homer City, PA [3]. In general, these data demonstrate that physical coal cleaning is effective in reducing the concentration of many trace elements. In some cases, trace element reductions correlate with ash reduction. For example, Fig. 1 is a plot of ash reduction verses lead reduction for the five coals in Table 1. Notice that the correlation between ash and lead reduction is very strong even though coals from all over the United States were cleaned by a variety of technologies. In other cases the situation is more complex as shown by the plot of mercury reduction verses ash reduction in Fig. 2. Here, a general relationship between ash and mercury reduction exists; however, the degree of reduction achieved appears to be both coal-specific and a function of the method of cleaning employed. Finally, for elements such as chromium and selenium, coal cleaning may or may not reduce concentration. An important coal characteristic is the degree of liberation of the trace elementbearing mineral. Fig. 3 is a plot of ash reduction versus arsenic reduction for Kentucky No. 11 seam coal. In this ease additional arsenic liberation occurs when the raw coal is crushed to a topsize of 3/8 inch. In addition, the type of equipment used in a cleaning plant affects trace element reduction. Figs. 4 and 5 are plots of ash reduction versus arsenic reduction for two different cleaning methods: froth flotation and heavy-media cycloning. Notice that
D. Akers, R. Dospoy/ Fuel Processing Technology 39 (1994) 73-86
76 1 00 gO
1:3
80 70 A
O
60 5,0 40 30 20 10 r't
0
I
I 30
I
I
50
I
I
70
I
90
ASH REDUCTION ( ~ )
Fig. I. Lead reduction trend.
90
80
70
60 O
50
t,J "i
4O
30
2O
10
Q I 3O
I
I
I
50
I
70
I
I
90
ASH REDUCTION ( Z )
Fig. 2. Mercury reduction trend.
froth flotation (Fig. 4) did not produce as great a reduction in arsenic as projected by the theoretically perfect separation derived from laboratory float-sink data (washability) of the feed to the froth cell. At an 80% ash reduction, the actual arsenic reduction was about 65%, while the washability data indicates that an arsenic reduction of over
D. ,4kers, R. Dospoy / Fuel Processing Technology 39 (1994) 73-86
77
100 x
9O
x
x
x
++ +
X
.-~ 80
X
.t. +
X
70
60 50
60
i
i
65
70
i
75 Arsenic
x Uncrushed
i
80 85 Reduction (%)
90
+ Crushed to
-3/8
95
lOO
in.
Fig. 3. Liberation by grinding. Kentucky No. 11 seam coal.
j
100 -
8O
g "o
60
4o
20
i
20
40
60
i
w
80
100
Arsenic Reduction (%) -'~Washability
Data
+
Flotation
Test
Fig. 4. Flotation performance.
80% is possible. However, the heavy-media cyclone (HMC) test shown in Fig. 5 produced results similar to the washability data. This figure shows arsenic reductions around 95% for both the theoretical washability data and the HMC data at a 90% ash reduction. The comparison of froth flotation to heavy-media cycloning illustrates the concept that physical cleaning processes do not remove trace elements as such, but rather remove trace element-bearing minerals. Arsenic commonly occurs in coal within the pyrite structure. As pyrite is a very dense mineral, it is easily removed by a density based process such as a heavy-media cyclone. However, cleaning processes such as froth flotation remove minerals based on surface characteristics. Because coal and pyrite have similar surface characteristics, conventional froth flotation may not provide high reductions of either pyrite or pyrite associated trace elements. Table 2 contains a second comparison of a heavy-media cyclone and froth flotation for trace element reduction. In this case, Pratt seam coal from Alabama was cleaned
78
D. Akers, R. Dospoy/Fuel Processing Technology 39 (1994) 73-86 1oo X
X
X +X X
80
60
40
2o 40
2o
60 Arsenic
x
Washability
80
Reduction
(%)
Data
HMC
+
100
Test
Fig. 5. Heavy-media cyclone performance.
Table 2 Equipment performance comparison (% reductions)
Chromium Mercury Pyritic sulfur Ash
Heavy-media cyclone
Froth flotation
63 26 35 70
56 - 20 17 62
Note: Minus sign indicates an increase in concentration with cleaning.
by both technologies. Notice that chromium removal is roughly proportional to ash removal for both cleaning devices; however, while mercury is reduced by the heavymedia cyclone, it is increased by froth flotation. As before, the difference in performance seems to relate to the superior ability of the heavy-media cyclone to reduce pyrite.
3. Impact of flowsheet design In addition to the impacts of specific coal cleaning devices, the manner and order in which coal cleaning and sizing devices are arranged to produce a flowsheet can increase or decrease the removal of trace elements. The flowsheets shown in Figs. 6 and 7 illustrate this point. Flowsheet No. 1 (Fig. 6) is a state-of-the-art conventional flowsheet which involves cleaning all size fractions of the coal with the best available conventional technology while the flowsheet presented in Fig. 7 (Flowsheet No. 2) involves the use of less efficient conventional technologies especially in the fine coal circuit. Both of these flowsheets were configured at commercial scale at CQ Inc's Coal Quality Development Center and used to clean Pratt seam coal from Alabama.
D. Akers, R. Dospoy / Fuel Processing Technology 39 (1994) 73-86
79
Raw Coal Conveyor
Heavy Media Cyclone
Desl[me Screen
3/8"x 28m
28m xr0 O4
Primary W.O. Cyclone
x ~o t~
Two Stage •
i Secondary W.O. Cyclone
j c~
\/
--~ 325m x 0
Thickening
cl Conditioning I Tank 1
Flotation Cells
/ o
Clean Coal Conveyor Clean Coal Storage Pile Fig. 6. Flowsheet No. 1--State-of-the-art conventional flowsheet.
J
j o
8O
D. Akers, R. Dospoy / Fuel Processing Technology 39 (1994) 73-86
eei:x28: D ester Concentrat ng Table
Derrick Screen
3/8"x 28m
3/8" x 28m
Thickening Cyclones
128m x 0
28m x 325m Spiral Concentrator
325m x 0
c~:,one
• Refuse Storage Pile
-••I
A / T.a S,age
jt/~Z8m x O
28m x C
Conveyor
00m x 0
[
28rex lO0~,t Clean Cool Conveyor Cleon Coal Storage Pile
Fig. 7. Flowsheet No. 2--All-water cleaning without a fines circuit.
J D
81
D. Akers, R. Dospoy/Fuel Processing Technology 39 (1994) 73-86
Table 3 Pratt seam coal trace element reductions(%) Element
Flowsheet No. 1
Flowsheet No. 2
Ash Sulfur Arsenic Barium Chromium Fluorine Lead Mercury Nickel Selenium
75 26 43 89 53 72 62 39 63 46
75 25 28 89 64 64 41 45 39 28
Zinc
76
66
The results of these commercial scale cleaning tests are presented in Table 3. Notice that both flowsheets produced essentially identical results relative to ash and sulfur reduction; however, Flowsheet No. 1 provided higher reductions of arsenic, fluorine, lead, nickel, selenium, and zinc. The main advantage of Flowsheet No. 1 for trace element removal is the efficient cleaning of fine size coal which often contains the highest concentration of liberated trace elements. In the case of chromium and mercury, Flowsheet No. 2 produced higher reductions than Flowsheet No. 1. The higher performance of Flowsheet No. 2 for mercury reduction was caused in part by poor rejection of the mercury-bearing mineral pyrite in the froth flotation cells of Flowsheet No. 1. Flowsheet No. 2 did not use froth flotation. There is insufficient data to explain the higher chromium reductions of Flowsheet No. 2.
4. Removal by advanced coal cleaning technology Advanced coal cleaning techniques may offer even further advantages in reducing trace elements. First, advanced processes typically crush or grind coal to very small particles to increase the chance of liberating sulfur-bearing and ash-forming mineral matter, possibly also liberating trace element-bearing mineral matter. Second, advanced processes are specifically designed to clean fine-sized coal, making them more efficient than conventional processes in removing mineral matter from this material. As many trace elements are concentrated in fine-sized coal, efficient methods of removing them from the finer sizes are especially important [3]. Table 4 provides a comparison between an advanced physical coal cleaning process based on heavy-media cycloning (the custom coals process) and conventional coal cleaning techniques. This evaluation is based on extensive washability and liberation tests and the use of sophisticated models such as the Department of Energy's coal cleaning model.
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D. Akers, R. Dospoy/Fuel Processing Technology 39 (1994) 73-86
Table 4 Conventional and advanced physical cleaning (ppm except where noted)
Ash Content (Wt %) Antimony Arsenic Cadmium Chromium Cobalt Lead Mercury Nickel Selenium
Raw
Conventional cleaning
Custom coal advanced process
29 0.8 14.0 0.2 16.1 0.3 14.7 0.16 13.4 1.1
15 0.5 7.2 0.6 8.4 0.2 7.0 0.14 9.1 1.5
14.0 0.3 3.5 0.3 8.2 0.2 6.2 0.14 8.2 1.2
Table 5 Illinois No. 5 trace element reductions {%) Element
Feed/100 mesh product
Feed/200 mesh product
Arsenic Barium Beryllium Chlorine Cobalt Chromium Fluorine Lead Mercury Manganese Nickel Zinc Ash
64 79 48 16 - 11 52 50 47 68 77 38 77 77
61 79 49 9 15 43 58 48 68 70 42 74 79
Note: Minus sign indicates an increase in concentration with cleaning.
As shown, c o n v e n t i o n a l cleaning techniques reduced the c o n c e n t r a t i o n of a n t i m o n y , arsenic, c h r o m i u m , cobalt, lead, mercury, a n d nickel while the c u s t o m coals cleaning technology provided a further r e d u c t i o n in all cases except mercury. F o r example, c o n v e n t i o n a l cleaning reduced the arsenic c o n c e n t r a t i o n of the coal from 14 to 7 ppm, while the c u s t o m coals t e c h n o l o g y provided a further r e d u c t i o n to 4 p p m [3]. T a b l e 5 illustrates the performance of a second a d v a n c e d physical cleaning process, selective agglomeration. In these tests, Illinois No. 5 coal was cleaned in a two t o n - p e r - h o u r proof-of-concept facility at Wilsonville, AL, by S o u t h e r n C o m p a n y Services. D u r i n g testing, the p l a n t was operated c o n t i n u o u s l y for a p p r o x i m a t e l y 80
D. Akers, R. Dospoy/Fuel Processing Technology 39 (1994) 73-86
83
Table 6 Conventional and advanced chemical/physical cleaning (ppm) Raw coal Antimony Arsenic Beryllium Cadmium Chromium Lead Mercury Nickel Selenium
< 0.5 7.0 1.1 < 0.1 37.8 13.1 0.31 32.2 1.8
Conventional cleaning < 0.5 4.4 0.8 < 0.1 12.5 4.4 0.19 12.7 0.4
Midwest ore process < 0.5 2.0 0.7 < 0.1 11.7 1.8 0.11 9.3 0.5
hours at optimized operating conditions for each coal tested. Diesel oil in concentrations less than 1.5% (by weight on a dry coal feed basis) was added as an agglomerant before high-shear vessels. The microagglomerates produced from the high-shear mixer were pumped to flotation cells where the clean coal was recovered. The results of the cleaning tests with Illinois No. 5 seam coal are presented in Table 5. This coal was crushed to two different topsizes (100 mesh and 200 mesh) to investigate the effects of crushing on the selective oil agglomeration process. For the Illinois No. 5 seam coal, selective oil agglomeration provided significant reductions in all elements studied except cobalt, which increased 11%o in the 100 mesh product. The reductions ranged from 9% for chlorine to 79% for barium. Crushing to 200 mesh produced either no change or a very small change in the reductions of arsenic, barium, beryllium, lead, mercury, nickel, and zinc. The data appear to indicate that the additional grinding aided the removal of cobalt and fluorine, while reducing the ability of the process to remove chlorine, chromium, and manganese. Cases in which additional grinding reduces the ability of a cleaning process to remove a trace element could occur if the characteristics of the trace element-bearing mineral prevent it from being rejected by the cleaning process employed as illustrated earlier in the comparison between a heavy-media cyclone and froth flotation for mercury reduction. In such a case, while the specific mineral is part of a larger particle containing a variety of coal and mineral types, the cleaning process may be able to remove it based on the average characteristics of the particle. If liberated by additional grinding, however, a new particle composed of a single mineral may be created. If this occurs, the particle will behave according to the characteristics of that specific mineral alone. If these characteristics cause it to report to the clean coal, the concentration of the associated trace elements will increase with additional grinding. Conversely, if the newly liberated mineral can be removed by the specific cleaning process, the concentration of the associated trace element in the clean coal is reduced by additional grinding. An evaluation of a combined chemical and physical cleaning process (the Midwest Ore process) is presented in Table 6. While details of this process are confidential,
84
D. Akers, R. Dospoy / Fuel Processing Technology 39 (1994) 73-86
analysis of feed and clean coal samples indicates high removals of many trace elements. This process also proved superior to conventional coal cleaning in lowering the concentrations of measured trace elements, especially arsenic, lead, mercury, and nickel [3].
5. Boiler impacts Trace elements that volatilize during combustion represent a greater health and environmental concern than those that do not because volatile elements are the most likely to be discharged into the atmosphere. A volatile trace element may leave the boiler as a vapor within the flue gas or, as the gas cools, condense preferentially on fine-sized flyash. As this ash is difficult to collect, it is more likely to escape out the stack than coarser fly ash. Combustion parameters such as the temperature profile and residence time in the boiler impact trace element volatility. If coal cleaning removes a trace element that might normally exit as a vapor, the benefits of cleaning are directly proportional to the reduction in concentration of that element on a heat unit basis. In other words, if a highly volatile element such as mercury would exit the boiler entirely as a vapor, reducing the mercury concentration in the boiler's feed coal from six grams per billion Btu to three grams per billion Btu would reduce mercury emissions by 50%. However, if the trace element first volatilizes and then condenses on the flyash, the situation is more complex. Coal cleaning always reduces ash. If in addition to ash reduction, a volatile trace element is partially removed by coal cleaning, three things can happen if the relative size distribution of the flyash is unchanged by cleaning Case 1. Ash is removed at a greater rate than the trace element. In this case, the proportionally greater ash removal will increase the concentration of the trace element on the flyash because there is a proportionally smaller amount of ash available to receive the element as it condenses. In this case, the concentration of the element in the flyash will increase. Case 2. The ash and trace element are removed at the same rate. Here, the concentration of the trace element in the flyash is unchanged by cleaning. Case 3. The trace element is removed at a greater rate than the ash, therefore, the concentration of the trace element in the flyash is decreased. In addition, coal cleaning reduces the total amount of ash that must be collected and can also change the overall characteristics of the ash. Reducing the total amount of ash entering the boiler tends to improve the performance of a flyash collection system by reducing the quantity of ash that must be collected. Coal cleaning, however, can change the chemistry of the ash, and possibly increase ash resistivity, thereby lowering ESP collection efficiency. Coal cleaning also tends to increase the moisture content of the coal, which is often beneficial to ESP performance, and reduce the sulfur content, which negatively impacts ESP performance. If coal cleaning increases the particle size of the flyash, ESP performance tends to improve. If particle size is reduced, ESP performance tends to degrade.
D. Akers, R. Dospoy/Fuel Processing Technology 39 (1994) 73-86
85
The net effect of these and other variables is both coal- and boiler-specific. For example, ESP collection efficiency will be reduced by increasing flyash resistivity. If coal cleaning increases flyash resistivity and if this increase is large enough to overcome the reduced ash loading on the ESP, more particles will be emitted, thereby increasing trace element emissions in Cases 1 and 2 and, if the increase in particulates is sufficiently large, even in Case 3. Conversely, if a flue gas conditioning system is added to the boiler to restore ESP performance, trace element emissions will decrease in Cases 2 and 3 and possibly in Case 1 if the relative increase in the trace element on the flyash is more than offset by the decrease in total particulate emissions created by reduced ash loading. Finally, the mode of occurrence of trace elements may play a significant role in their volatilization during combustion. If a trace element exists in a coal in two forms, one highly volatile and one less volatile, the amount of the more volatile form removed by coal cleaning will have a greater impact on emissions than the total quantity of the trace element removed.
6. Waste disposal concerns
The process of removing trace elements by coal cleaning naturally results in a waste that is enriched in these same trace elements. This waste appears to be stable if proper disposal techniques are practiced; if acid conditions form however, trace metals may be leached from the waste [4].
7. Conclusions
Physical coal cleaning techniques are effective in removing ash-forming mineral matter along with many mineral-associated trace elements from coal. Data gathered from commercial scale cleaning tests indicate that reductions of certain trace elements are directly related to ash reductions. In other cases, the situation is more complex and factors such as the mode of occurrence of the trace element-bearing mineral and the type of cleaning equipment employed affect trace element reduction. Knowledge of the interplay among the characteristics of the trace element-bearing mineral and various types of coal Cleaning equipment can be used to enhance trace element removal during coal cleaning. If coal cleaning removes a trace element that might normally exit a power plant stack as a vapor, the benefits of cleaning are directly proportional to the reduction in the concentration of that element. However, the situation is more complex for trace elements that volatilize in the boiler and recondense as flue gas temperatures drop. As these elements tend to become surface enriched .on fine-sized flyash particles, the interplay between ESP performance and the effects of coal cleaning on ash loading and ash chemistry are very important. Coal cleaning can best be used to control air toxics if it is viewed as part of a strategy for power station emissions controls rather than a standalone technology.
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Because of the probability of future legislation directed at air toxics emissions from fossil-fired utilities, it is important that utilities and their coal suppliers understand available control options. If legislation is passed, power plant operators will reevaluate their fuel supplies to determine what, if any, control options exist through fuel switching. Moreover, suppliers of coal with high concentrations of potentially toxic elements may be forced to remove these elements in order to market their fuel. Removing volatile trace elements prior to combustion is an attractive toxics control option as coal cleaning is likely to be less expensive than post-combustion control. In addition, coal cleaning reduces the total amount of ash that must be collected and usually improves the overall performance of flyash collection systems unless the coal's chemistry is negatively changed. Coal cleaning also offers improved thermal efficiency and reduction of other regulated elements such as sulfur dioxide. However, conventional coal cleaning processes are unlikely to provide complete removal of any trace element and the overall impact of cleaning on the specific boiler and ash collection system must be considered.
Acknowledgments Most of the work reported in this paper was performed as part of a project funded by the Electric Power Research Institute, David O ' C o n n o r project manager.
References [1] Electric Power Research Institute, 1989. Trace Elements in Coal and Coal Wastes, Interim Report Project 1400-6, -11, EPRI GS-6575. [2] Finkelman, R.B., 1980. Modes of Occurrence of Trace Elements in Coal, Ph.D. Dissertation, University of Maryland, College Park, MD. [3] Akers, D. and Dospoy, R., 1993. An overviewof the use of coal cleaning to reduce air toxics. Miner. Metall. Process., 10: 124-127. [4] Dospoy, R. and Akers, D., 1993. Predicting acid and toxic element production of coal refuse sites. Environ. Mining, 1: 8-15.
Fuel Processing Technology 39 (1994) 73-86
Role of coal cleaning in control of air toxics D a v i d Akers*, R o b e r t D o s p o y CQ Inc., One Quality Center, P.O. Box 280, Homer City, PA 15748-0280, USA
(Received 19 April 1993; accepted in revised form 16 January 1994)
Abstract Twelve of the potentially hazardous air pollutants listed in the 1990 Clean Air Act are elements commonly found in trace amounts in coal. An option for controlling the release of these elements into the atmosphere is to remove them before combustion. Based on commercial-scale tests, conventional physical coal cleaning processes are effective in reducing the concentration of many of these trace elements in coal. In addition, advanced cleaning processes directed toward reduction of various elements may perform better than conventional processes. The degree to which a specific trace element can be reduced by coal cleaning depends on the mode of occurrence of the trace element, the method of cleaning employed, and the way in which the cleaning process is operated. For example, higher trace element reductions can often be achieved if the cleaning plant is operated to remove as much ash as possible. In addition to reducing the concentration of many trace elements, coal cleaning can improve overall boiler performance by increasing thermal efficiency. Also, cleaning changes ash loading and ash chemistry, impacting the performance of particulate collection equipment. For maximum effectiveness, coal cleaning should be considered as part of a system that includes the boiler as well as other emissions control systems.
1. Introduction The Clean Air Act Amendments of 1990 have identified 189 potentially hazardous air pollutants (PHAPs), of which 12 are elements and their compounds commonly found in coal. These elements are antimony, arsenic, beryllium, cadmium, chlorine, chromium, cobalt, lead, manganese, mercury, nickel, and selenium. In addition to these specific elements, radionuclides are also listed as PHAPs; these also occur naturally in some coals. The geological processes that form coal can also concentrate trace elements. For example, the average concentration of arsenic in bituminous coal (20 ppm) is ten times
* Corresponding author. Tel.: (1-412)479-3503. Fax: (1-412)479-4181. 0378-3820/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 3 7 8 - 3 8 2 0 ( 9 4 ) 0 0 0 2 8 - R
74
D. Akers, R. Dospoy/Fuel Processing Technology 39 (1994) 73-86
the average concentration found in all the other rocks that compose the earth's crust (2 ppm). Similarly, antimony, cadmium, mercury, and selenium are generally more concentrated in coal than in the earth's crust. When coal is burned, trace elements can be further concentrated. Volatile elements such as arsenic, mercury, and selenium are a special concern because they tend to vaporize during combustion. Then, as the flue gas cools, these elements have a tendency to condense on the surface of flyash particles. Because the finest-size flyash particles provide the greatest surface area, volatilized elements tend to become concentrated on ultra-fine flyash. These flyash particles are the most difficult to collect and can escape up the stack. Although no new restrictions on trace element emissions were placed on the power generation industry under the new Clean Air Act amendments, the act mandates a study of utility toxic air emissions by the United States Environmental Protection Agency (EPA). In addition, EPA is required by law to recommend new air toxic regulations found necessary to protect human health and the environment. Furthermore, the legislation requires three separate studies of mercury emissions, deposition, and health effects. New regulations aimed at coal-fired electric utilities may be forthcoming when these federally mandated studies are complete. While a significant research effort aimed toward the development of new methods of air toxics control is likely to develop, three basic options are currently available to utilities: (i) Installation of improved particulate collection devices to increase capture of the finest-size flyash particles that usually contain the highest concentration of volatile trace elements. (ii) Installation of wet scrubbers that serve to cool boiler flue gas, thereby promoting condensation of volatile trace elements. Wet scrubbers provide a high fine-particle capture efficiency and are, therefore, likely to be highly effective in capturing most P H A P s (with the possible exception of highly volatile species such as mercury). (iii) Use of coal cleaning or coal switching to reduce the concentration of trace elements entering a boiler. As a P H A P control technology, coal cleaning offers the advantages of relatively low cost, improved boiler thermal efficiency, and reduction of other regulated emissions such as sulfur dioxide. Also, since that proportion of trace elements removed by coal cleaning never enters the boiler, a smaller amount can be expected to be released into the atmosphere. It is unlikely, however, that any physical coal cleaning technology will provide complete removal of any trace element. Therefore, for some coals additional post-combustion control systems may be required.
2. Factors affecting trace element removal Many trace elements in coal are associated with the coal's mineral matter. For example, arsenic is commonly associated with pyrite; cadmium with sphalerite; chromium with clay minerals and chromite; mercury with pyrite and cinnabar; nickel With millerite, pyrite, and other sulfides; and selenium with lead selenide, pyrite, and other sulfides. There are also cases in which some of these elements are organically bound [1, 2]. Just as both organic and pyritic sulfur can occur in the same coal, the
D. Akers, R. Dospoy/ Fuel Processing Technology 39 (1994) 73-86
75
Table 1 Trace element removal by conventional cleaning (% reduction)
Ash Arsenic Cadmium Chromium Lead Mercury Nickel Selenium
AL
KY
OK
PA
MT
75 43 XX 53 62 39 63 46
85 43 XX 79 89 48 82 62
57 53 91 76 25 55 25 - 42
72 83 63 69 44 78 48 - 4
31 35 3 - 62 3 17 31 59
Notes: XX Data not available. Minus sign indicates an increase in concentration with cleaning.
same trace element may be both organically bound and also be present as part of the coal's mineral matter. Physical coal cleaning techniques are effective in removing mineral matter from coal and can, therefore, potentially remove some of the trace elements associated with specific minerals. Work by the Electric Power Research Institute, the United States Department of Energy, Bituminous Coal Research Inc., and others has demonstrated that large reductions in the concentration of many trace elements are possible if conventional coal cleaning techniques are properly applied. Five examples are given in Table 1. In each example, the results shown were generated by cleaning the coal at commercial scale at CQ Inc.'s Coal Quality Development Center (CQDC) located in Homer City, PA [3]. In general, these data demonstrate that physical coal cleaning is effective in reducing the concentration of many trace elements. In some cases, trace element reductions correlate with ash reduction. For example, Fig. 1 is a plot of ash reduction verses lead reduction for the five coals in Table 1. Notice that the correlation between ash and lead reduction is very strong even though coals from all over the United States were cleaned by a variety of technologies. In other cases the situation is more complex as shown by the plot of mercury reduction verses ash reduction in Fig. 2. Here, a general relationship between ash and mercury reduction exists; however, the degree of reduction achieved appears to be both coal-specific and a function of the method of cleaning employed. Finally, for elements such as chromium and selenium, coal cleaning may or may not reduce concentration. An important coal characteristic is the degree of liberation of the trace elementbearing mineral. Fig. 3 is a plot of ash reduction versus arsenic reduction for Kentucky No. 11 seam coal. In this ease additional arsenic liberation occurs when the raw coal is crushed to a topsize of 3/8 inch. In addition, the type of equipment used in a cleaning plant affects trace element reduction. Figs. 4 and 5 are plots of ash reduction versus arsenic reduction for two different cleaning methods: froth flotation and heavy-media cycloning. Notice that
D. Akers, R. Dospoy/ Fuel Processing Technology 39 (1994) 73-86
76 1 00 gO
1:3
80 70 A
O
60 5,0 40 30 20 10 r't
0
I
I 30
I
I
50
I
I
70
I
90
ASH REDUCTION ( ~ )
Fig. I. Lead reduction trend.
90
80
70
60 O
50
t,J "i
4O
30
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10
Q I 3O
I
I
I
50
I
70
I
I
90
ASH REDUCTION ( Z )
Fig. 2. Mercury reduction trend.
froth flotation (Fig. 4) did not produce as great a reduction in arsenic as projected by the theoretically perfect separation derived from laboratory float-sink data (washability) of the feed to the froth cell. At an 80% ash reduction, the actual arsenic reduction was about 65%, while the washability data indicates that an arsenic reduction of over
D. ,4kers, R. Dospoy / Fuel Processing Technology 39 (1994) 73-86
77
100 x
9O
x
x
x
++ +
X
.-~ 80
X
.t. +
X
70
60 50
60
i
i
65
70
i
75 Arsenic
x Uncrushed
i
80 85 Reduction (%)
90
+ Crushed to
-3/8
95
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Fig. 3. Liberation by grinding. Kentucky No. 11 seam coal.
j
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8O
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60
4o
20
i
20
40
60
i
w
80
100
Arsenic Reduction (%) -'~Washability
Data
+
Flotation
Test
Fig. 4. Flotation performance.
80% is possible. However, the heavy-media cyclone (HMC) test shown in Fig. 5 produced results similar to the washability data. This figure shows arsenic reductions around 95% for both the theoretical washability data and the HMC data at a 90% ash reduction. The comparison of froth flotation to heavy-media cycloning illustrates the concept that physical cleaning processes do not remove trace elements as such, but rather remove trace element-bearing minerals. Arsenic commonly occurs in coal within the pyrite structure. As pyrite is a very dense mineral, it is easily removed by a density based process such as a heavy-media cyclone. However, cleaning processes such as froth flotation remove minerals based on surface characteristics. Because coal and pyrite have similar surface characteristics, conventional froth flotation may not provide high reductions of either pyrite or pyrite associated trace elements. Table 2 contains a second comparison of a heavy-media cyclone and froth flotation for trace element reduction. In this case, Pratt seam coal from Alabama was cleaned
78
D. Akers, R. Dospoy/Fuel Processing Technology 39 (1994) 73-86 1oo X
X
X +X X
80
60
40
2o 40
2o
60 Arsenic
x
Washability
80
Reduction
(%)
Data
HMC
+
100
Test
Fig. 5. Heavy-media cyclone performance.
Table 2 Equipment performance comparison (% reductions)
Chromium Mercury Pyritic sulfur Ash
Heavy-media cyclone
Froth flotation
63 26 35 70
56 - 20 17 62
Note: Minus sign indicates an increase in concentration with cleaning.
by both technologies. Notice that chromium removal is roughly proportional to ash removal for both cleaning devices; however, while mercury is reduced by the heavymedia cyclone, it is increased by froth flotation. As before, the difference in performance seems to relate to the superior ability of the heavy-media cyclone to reduce pyrite.
3. Impact of flowsheet design In addition to the impacts of specific coal cleaning devices, the manner and order in which coal cleaning and sizing devices are arranged to produce a flowsheet can increase or decrease the removal of trace elements. The flowsheets shown in Figs. 6 and 7 illustrate this point. Flowsheet No. 1 (Fig. 6) is a state-of-the-art conventional flowsheet which involves cleaning all size fractions of the coal with the best available conventional technology while the flowsheet presented in Fig. 7 (Flowsheet No. 2) involves the use of less efficient conventional technologies especially in the fine coal circuit. Both of these flowsheets were configured at commercial scale at CQ Inc's Coal Quality Development Center and used to clean Pratt seam coal from Alabama.
D. Akers, R. Dospoy / Fuel Processing Technology 39 (1994) 73-86
79
Raw Coal Conveyor
Heavy Media Cyclone
Desl[me Screen
3/8"x 28m
28m xr0 O4
Primary W.O. Cyclone
x ~o t~
Two Stage •
i Secondary W.O. Cyclone
j c~
\/
--~ 325m x 0
Thickening
cl Conditioning I Tank 1
Flotation Cells
/ o
Clean Coal Conveyor Clean Coal Storage Pile Fig. 6. Flowsheet No. 1--State-of-the-art conventional flowsheet.
J
j o
8O
D. Akers, R. Dospoy / Fuel Processing Technology 39 (1994) 73-86
eei:x28: D ester Concentrat ng Table
Derrick Screen
3/8"x 28m
3/8" x 28m
Thickening Cyclones
128m x 0
28m x 325m Spiral Concentrator
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• Refuse Storage Pile
-••I
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jt/~Z8m x O
28m x C
Conveyor
00m x 0
[
28rex lO0~,t Clean Cool Conveyor Cleon Coal Storage Pile
Fig. 7. Flowsheet No. 2--All-water cleaning without a fines circuit.
J D
81
D. Akers, R. Dospoy/Fuel Processing Technology 39 (1994) 73-86
Table 3 Pratt seam coal trace element reductions(%) Element
Flowsheet No. 1
Flowsheet No. 2
Ash Sulfur Arsenic Barium Chromium Fluorine Lead Mercury Nickel Selenium
75 26 43 89 53 72 62 39 63 46
75 25 28 89 64 64 41 45 39 28
Zinc
76
66
The results of these commercial scale cleaning tests are presented in Table 3. Notice that both flowsheets produced essentially identical results relative to ash and sulfur reduction; however, Flowsheet No. 1 provided higher reductions of arsenic, fluorine, lead, nickel, selenium, and zinc. The main advantage of Flowsheet No. 1 for trace element removal is the efficient cleaning of fine size coal which often contains the highest concentration of liberated trace elements. In the case of chromium and mercury, Flowsheet No. 2 produced higher reductions than Flowsheet No. 1. The higher performance of Flowsheet No. 2 for mercury reduction was caused in part by poor rejection of the mercury-bearing mineral pyrite in the froth flotation cells of Flowsheet No. 1. Flowsheet No. 2 did not use froth flotation. There is insufficient data to explain the higher chromium reductions of Flowsheet No. 2.
4. Removal by advanced coal cleaning technology Advanced coal cleaning techniques may offer even further advantages in reducing trace elements. First, advanced processes typically crush or grind coal to very small particles to increase the chance of liberating sulfur-bearing and ash-forming mineral matter, possibly also liberating trace element-bearing mineral matter. Second, advanced processes are specifically designed to clean fine-sized coal, making them more efficient than conventional processes in removing mineral matter from this material. As many trace elements are concentrated in fine-sized coal, efficient methods of removing them from the finer sizes are especially important [3]. Table 4 provides a comparison between an advanced physical coal cleaning process based on heavy-media cycloning (the custom coals process) and conventional coal cleaning techniques. This evaluation is based on extensive washability and liberation tests and the use of sophisticated models such as the Department of Energy's coal cleaning model.
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D. Akers, R. Dospoy/Fuel Processing Technology 39 (1994) 73-86
Table 4 Conventional and advanced physical cleaning (ppm except where noted)
Ash Content (Wt %) Antimony Arsenic Cadmium Chromium Cobalt Lead Mercury Nickel Selenium
Raw
Conventional cleaning
Custom coal advanced process
29 0.8 14.0 0.2 16.1 0.3 14.7 0.16 13.4 1.1
15 0.5 7.2 0.6 8.4 0.2 7.0 0.14 9.1 1.5
14.0 0.3 3.5 0.3 8.2 0.2 6.2 0.14 8.2 1.2
Table 5 Illinois No. 5 trace element reductions {%) Element
Feed/100 mesh product
Feed/200 mesh product
Arsenic Barium Beryllium Chlorine Cobalt Chromium Fluorine Lead Mercury Manganese Nickel Zinc Ash
64 79 48 16 - 11 52 50 47 68 77 38 77 77
61 79 49 9 15 43 58 48 68 70 42 74 79
Note: Minus sign indicates an increase in concentration with cleaning.
As shown, c o n v e n t i o n a l cleaning techniques reduced the c o n c e n t r a t i o n of a n t i m o n y , arsenic, c h r o m i u m , cobalt, lead, mercury, a n d nickel while the c u s t o m coals cleaning technology provided a further r e d u c t i o n in all cases except mercury. F o r example, c o n v e n t i o n a l cleaning reduced the arsenic c o n c e n t r a t i o n of the coal from 14 to 7 ppm, while the c u s t o m coals t e c h n o l o g y provided a further r e d u c t i o n to 4 p p m [3]. T a b l e 5 illustrates the performance of a second a d v a n c e d physical cleaning process, selective agglomeration. In these tests, Illinois No. 5 coal was cleaned in a two t o n - p e r - h o u r proof-of-concept facility at Wilsonville, AL, by S o u t h e r n C o m p a n y Services. D u r i n g testing, the p l a n t was operated c o n t i n u o u s l y for a p p r o x i m a t e l y 80
D. Akers, R. Dospoy/Fuel Processing Technology 39 (1994) 73-86
83
Table 6 Conventional and advanced chemical/physical cleaning (ppm) Raw coal Antimony Arsenic Beryllium Cadmium Chromium Lead Mercury Nickel Selenium
< 0.5 7.0 1.1 < 0.1 37.8 13.1 0.31 32.2 1.8
Conventional cleaning < 0.5 4.4 0.8 < 0.1 12.5 4.4 0.19 12.7 0.4
Midwest ore process < 0.5 2.0 0.7 < 0.1 11.7 1.8 0.11 9.3 0.5
hours at optimized operating conditions for each coal tested. Diesel oil in concentrations less than 1.5% (by weight on a dry coal feed basis) was added as an agglomerant before high-shear vessels. The microagglomerates produced from the high-shear mixer were pumped to flotation cells where the clean coal was recovered. The results of the cleaning tests with Illinois No. 5 seam coal are presented in Table 5. This coal was crushed to two different topsizes (100 mesh and 200 mesh) to investigate the effects of crushing on the selective oil agglomeration process. For the Illinois No. 5 seam coal, selective oil agglomeration provided significant reductions in all elements studied except cobalt, which increased 11%o in the 100 mesh product. The reductions ranged from 9% for chlorine to 79% for barium. Crushing to 200 mesh produced either no change or a very small change in the reductions of arsenic, barium, beryllium, lead, mercury, nickel, and zinc. The data appear to indicate that the additional grinding aided the removal of cobalt and fluorine, while reducing the ability of the process to remove chlorine, chromium, and manganese. Cases in which additional grinding reduces the ability of a cleaning process to remove a trace element could occur if the characteristics of the trace element-bearing mineral prevent it from being rejected by the cleaning process employed as illustrated earlier in the comparison between a heavy-media cyclone and froth flotation for mercury reduction. In such a case, while the specific mineral is part of a larger particle containing a variety of coal and mineral types, the cleaning process may be able to remove it based on the average characteristics of the particle. If liberated by additional grinding, however, a new particle composed of a single mineral may be created. If this occurs, the particle will behave according to the characteristics of that specific mineral alone. If these characteristics cause it to report to the clean coal, the concentration of the associated trace elements will increase with additional grinding. Conversely, if the newly liberated mineral can be removed by the specific cleaning process, the concentration of the associated trace element in the clean coal is reduced by additional grinding. An evaluation of a combined chemical and physical cleaning process (the Midwest Ore process) is presented in Table 6. While details of this process are confidential,
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D. Akers, R. Dospoy / Fuel Processing Technology 39 (1994) 73-86
analysis of feed and clean coal samples indicates high removals of many trace elements. This process also proved superior to conventional coal cleaning in lowering the concentrations of measured trace elements, especially arsenic, lead, mercury, and nickel [3].
5. Boiler impacts Trace elements that volatilize during combustion represent a greater health and environmental concern than those that do not because volatile elements are the most likely to be discharged into the atmosphere. A volatile trace element may leave the boiler as a vapor within the flue gas or, as the gas cools, condense preferentially on fine-sized flyash. As this ash is difficult to collect, it is more likely to escape out the stack than coarser fly ash. Combustion parameters such as the temperature profile and residence time in the boiler impact trace element volatility. If coal cleaning removes a trace element that might normally exit as a vapor, the benefits of cleaning are directly proportional to the reduction in concentration of that element on a heat unit basis. In other words, if a highly volatile element such as mercury would exit the boiler entirely as a vapor, reducing the mercury concentration in the boiler's feed coal from six grams per billion Btu to three grams per billion Btu would reduce mercury emissions by 50%. However, if the trace element first volatilizes and then condenses on the flyash, the situation is more complex. Coal cleaning always reduces ash. If in addition to ash reduction, a volatile trace element is partially removed by coal cleaning, three things can happen if the relative size distribution of the flyash is unchanged by cleaning Case 1. Ash is removed at a greater rate than the trace element. In this case, the proportionally greater ash removal will increase the concentration of the trace element on the flyash because there is a proportionally smaller amount of ash available to receive the element as it condenses. In this case, the concentration of the element in the flyash will increase. Case 2. The ash and trace element are removed at the same rate. Here, the concentration of the trace element in the flyash is unchanged by cleaning. Case 3. The trace element is removed at a greater rate than the ash, therefore, the concentration of the trace element in the flyash is decreased. In addition, coal cleaning reduces the total amount of ash that must be collected and can also change the overall characteristics of the ash. Reducing the total amount of ash entering the boiler tends to improve the performance of a flyash collection system by reducing the quantity of ash that must be collected. Coal cleaning, however, can change the chemistry of the ash, and possibly increase ash resistivity, thereby lowering ESP collection efficiency. Coal cleaning also tends to increase the moisture content of the coal, which is often beneficial to ESP performance, and reduce the sulfur content, which negatively impacts ESP performance. If coal cleaning increases the particle size of the flyash, ESP performance tends to improve. If particle size is reduced, ESP performance tends to degrade.
D. Akers, R. Dospoy/Fuel Processing Technology 39 (1994) 73-86
85
The net effect of these and other variables is both coal- and boiler-specific. For example, ESP collection efficiency will be reduced by increasing flyash resistivity. If coal cleaning increases flyash resistivity and if this increase is large enough to overcome the reduced ash loading on the ESP, more particles will be emitted, thereby increasing trace element emissions in Cases 1 and 2 and, if the increase in particulates is sufficiently large, even in Case 3. Conversely, if a flue gas conditioning system is added to the boiler to restore ESP performance, trace element emissions will decrease in Cases 2 and 3 and possibly in Case 1 if the relative increase in the trace element on the flyash is more than offset by the decrease in total particulate emissions created by reduced ash loading. Finally, the mode of occurrence of trace elements may play a significant role in their volatilization during combustion. If a trace element exists in a coal in two forms, one highly volatile and one less volatile, the amount of the more volatile form removed by coal cleaning will have a greater impact on emissions than the total quantity of the trace element removed.
6. Waste disposal concerns
The process of removing trace elements by coal cleaning naturally results in a waste that is enriched in these same trace elements. This waste appears to be stable if proper disposal techniques are practiced; if acid conditions form however, trace metals may be leached from the waste [4].
7. Conclusions
Physical coal cleaning techniques are effective in removing ash-forming mineral matter along with many mineral-associated trace elements from coal. Data gathered from commercial scale cleaning tests indicate that reductions of certain trace elements are directly related to ash reductions. In other cases, the situation is more complex and factors such as the mode of occurrence of the trace element-bearing mineral and the type of cleaning equipment employed affect trace element reduction. Knowledge of the interplay among the characteristics of the trace element-bearing mineral and various types of coal Cleaning equipment can be used to enhance trace element removal during coal cleaning. If coal cleaning removes a trace element that might normally exit a power plant stack as a vapor, the benefits of cleaning are directly proportional to the reduction in the concentration of that element. However, the situation is more complex for trace elements that volatilize in the boiler and recondense as flue gas temperatures drop. As these elements tend to become surface enriched .on fine-sized flyash particles, the interplay between ESP performance and the effects of coal cleaning on ash loading and ash chemistry are very important. Coal cleaning can best be used to control air toxics if it is viewed as part of a strategy for power station emissions controls rather than a standalone technology.
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Because of the probability of future legislation directed at air toxics emissions from fossil-fired utilities, it is important that utilities and their coal suppliers understand available control options. If legislation is passed, power plant operators will reevaluate their fuel supplies to determine what, if any, control options exist through fuel switching. Moreover, suppliers of coal with high concentrations of potentially toxic elements may be forced to remove these elements in order to market their fuel. Removing volatile trace elements prior to combustion is an attractive toxics control option as coal cleaning is likely to be less expensive than post-combustion control. In addition, coal cleaning reduces the total amount of ash that must be collected and usually improves the overall performance of flyash collection systems unless the coal's chemistry is negatively changed. Coal cleaning also offers improved thermal efficiency and reduction of other regulated elements such as sulfur dioxide. However, conventional coal cleaning processes are unlikely to provide complete removal of any trace element and the overall impact of cleaning on the specific boiler and ash collection system must be considered.
Acknowledgments Most of the work reported in this paper was performed as part of a project funded by the Electric Power Research Institute, David O ' C o n n o r project manager.
References [1] Electric Power Research Institute, 1989. Trace Elements in Coal and Coal Wastes, Interim Report Project 1400-6, -11, EPRI GS-6575. [2] Finkelman, R.B., 1980. Modes of Occurrence of Trace Elements in Coal, Ph.D. Dissertation, University of Maryland, College Park, MD. [3] Akers, D. and Dospoy, R., 1993. An overviewof the use of coal cleaning to reduce air toxics. Miner. Metall. Process., 10: 124-127. [4] Dospoy, R. and Akers, D., 1993. Predicting acid and toxic element production of coal refuse sites. Environ. Mining, 1: 8-15.