United States Industrial Environmental Research EP<'« ' ' '
Environmental Protection Laborato^ t?9
Agency Research Tnangle Park NC 2771 1
Environmental
Assessment of Coal
Cleaning Processes:
Technology Overview
Interagency
Energy/Environment
R&D Program Report
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RESEARCH REPORTING SERIES
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EPA-600/7-79-073e
September 1979
Environmental Assessment of Coal Cleaning
Processes: Technology Overview
by
P.W. Spaite (Consultant), G.L Robinson, A.W. Lemmon, Jr
S. Min, and J.H. McCreery
Battelle Columbus Laboratories
505 King Ave.
Columbus, Ohio 43201
Contract No. 68-02-2163
Task No. 212
Program Element No. EHE624A
EPA Project Officer: James 0. Kilgroe
Industrial Environmental Research Laboratory
Office of Environmental Engineering and Technology
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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FOREWORD
Many elements and chemical compounds are known to be toxic to man and
other biological species. Our knowledge concerning the levels and condi-
tions under which these substances are toxic is extremely limited, however.
Little is known concerning the emission of these pollutants from industrial
processes and the mechanism by which they are transported, transformed,
dispersed, or accumulated in our environment.
Portions of the Federal Clean Air Act, the Resource Conservation
Recovery Act, and the Federal Water Pollution Control Act require the
U.S. Environmental Protection Agency (EPA) to identify and regulate
hazardous or toxic substances which result from man's industrial activities.
Industrial pollutants are often identified only after harmful health or
ecological effects are noted. Remedial actions are costly, the damage to
human and other biological populations is often irreversible, and the
persistence of some environmental contaminants may endanger future populations.
EPA's Office of Research and Development (ORD) is responsible for
health and ecological research, studies concerning the transportation and
fate of pollutants, and the development of technologies for controlling
industrial pollutants. The Industrial Environmental Research Laboratory,
an ORD organization, is responsible for development of pollution control
technology and conducts a large environmental assessment program. The
primary objectives of this program are:
• The development of information on the quantities of
toxic pollutants emitted from various industrial
processes—information needed to prioritize health
and ecological research efforts.
• The identification of industrial pollutant emissions
which pose a clearly evident health or ecological
risk and which should be regulated.
• The evaluation and development of technologies for
controlling pollution from these toxic substances.
The coal cleaning environmental assessment program has as its specific
objectives the evaluation of pollution control problems which are unique to
coal preparation, storage, and transportation. The coal preparation industry
is a mature yet changing industry and in recent years significant achievements
have been made in pollution abatement. Specifically, this report provides
an overview of physical coal cleaning technology and its environmental impacts.
ii
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ABSTRACT
This report reviews U.S. coal cleaning process technologies and related
technologies for environmental control. It provides a background against
which requirements for further developments of coal cleaning technology and
control techniques for the associated pollutants can be established.
The state of the art of physical coal cleaning is summarized. The status
of coal cleaning technology is summarized with respect to cost, energy effi-
ciency, applicability, extent of development, and commercialization prospects.
Current technologies are described. The various physical coal cleaning
operations necessary to produce systems capable of producing minimum, inter-
mediate, and maximum effectiveness of coal cleaning are discussed.
The coverage of the subject is felt to be complete in the sense that
all applications of the technology and all potentially polluting discharges
have been considered. The report does not, however, present detailed infor-
mation on composition of discharges, control technologies, economics, and
the like. It is designed for use in development of programs and studies
needed to quantify potential pollution control problems, prioritize environ-
mental protection needs, and related activities. It was felt that inclusion
of all background data would detract from, rather than enhance, its useful-
ness for broad analysis. For those needing more detailed information,
references to background documents have been supplied.
iii
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TABLE OF CONTENTS
Page
Foreword ii
Abstract ill
List of Figures v
List of Tables vi
Acknowledgments vii
1.0 Introduction 1
2.0 Status of Technology ..... 3
2.1 Energy Efficiency 4
2.2 Costs 6
2.2.1 Introduction 6
2.2.2 Physical Coal Cleaning Costs 7
2.2.3 Case Studies of PCC Costs 11
2.2.4 Benefits of Physical Coal Cleaning 11
2.3 Applicability 19
2.4 Extent of Development 21
2.5 Prospects for Commercialization 24
3.0 Description of Technology 27
3.1 Physical Coal Cleaning Systems 27
3.1.1 Coal Pretreatment Processes 29
3.1.2 Coal Cleaning Processes 31
3.1.3 Product Conditioning Processes 32
iv
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TABLE OF CONTENTS
(Continued)
Page
3.1.4 Auxiliary Processes 32
3.2 Raw Materials 34
3.2.1 Washability 35
3.2.2 Trace Element Content 36
3.2.3 Leachability 36
3.3 Products 38
4.0 Environmental Impacts 40
4.1 Coal Pretreatment Operations 42
4.2 Coal Cleaning Operations 42
4.3 Product Conditioning Operation 43
4.4 Auxiliary and Ultimate Disposal Operations 44
4.4.1 Coal Handling and Storage 44
4.4.2 Water Treatment 45
4.4.3 Solid Waste Disposal 47
5.0 References 49
Appendix A. Environmental Assessment/Control Technology
Development Diagram 51
Appendix B. Nomenclature Definitions for Energy Technologies .... 52
Appendix C. Glossary of Terms Associated with
Physical Coal Cleaning (PCC) 56
Appendix D. Description of Processes for Physical Coal Cleaning ... 61
LIST OF FIGURES
Figure 1. Process for Physical Coal Cleaning Systems 28
Figure A-l. Environmental Assessment/Control Technology
Development Diagram 51
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LIST OF TABLES
Page
Table 1. Variation in Cleaning Performance by Region 5
Table 2. Cost of Btu Loss for R = 0.95 9
Table 3. Approximate Ranges of PCC Costs 10
Table 4. Description of Coal Preparation Plants 12
Table 5. Total Annualized Costs for Eight
Coal Preparation Plants 13
Table 6. Postulated Conditions of Availability 16
Table 7. Summary of Costs for a 500-MW Coal-Burning Power Plant . . 17
Table 8. Summary of Costs for Power Generation Using
Various Control Modes 18
Table 9. Estimated Fuel Consumption for U.S. Boilers (1975) .... 20
Table 10. Homer City Plant Performance 22
Table 11. Physical Coal Cleaning Plant Types 30
Table 12. Processes Employed for Physical Cleaning of Coal 32
Table 13. Trace Elements in 101 Coals 37
Table 14. Distribution of Elements in Float-Sink Separation
of Illinois Coals at Varying Specific Gravity to
Achieve 75 Percent Weight Recovery 39
Table 15. Typical Coal Trace Element Fractions
Released by Combustion Process 41
Table 16. Composition of Drainage from Coal Piles at Eleven
Steam Electric Power Generating Plants 46
Table 17. Classification of Water Treatment Technologies
Used in Coal Cleaning 47
vi
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ACKNOWLEDGMENTS
This study was conducted as a task in Battelle's Columbus Laboratories'
program, "Environmental Assessment of Coal Cleaning Processes", which has
been supported by the U.S. Environmental Protection Agency, Industrial
Environmental Research Laboratory, Research Triangle Park (IERL/RTP),
North Carolina. The contributions of G. Ray Smithson, Jr., Program Manager;
Linda M. Curran; and Steven A. Barker are gratefully acknowledged. The
Deputy Program Manager, Mr. Alexis W. Lemmon, Jr., and the Task Leader,
Dr. Gerald L. Robinson, were coauthors of this report.
The assistance of Dr. Harold L. Lovell, consultant to Battelle, in
providing a technical review of the report is gratefully acknowledged.
The advice and council of the EPA Project Officer, Mr. James D. Kilgroe,
and others at the IERL/RTP facility were invaluable in performance of
this work.
vii
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1.0 INTRODUCTION
The Environmental Protection Agency's Industrial Environmental
Research Laboratory (IERL), Research Triangle Park, North Carolina, is
conducting a series of environmental assessments. These activities involve
continuing iterative studies to (1) identify and characterize industrial
process discharges, (2) evaluate pollution control and waste disposal options,
(3) compare estimates for environmental loadings with applicable standards
and projected environmental goals, and (4) prioritize potential pollution
problems and control technology needs. This overview report deals with
physical coal cleaning (PCC)* and is one of a series of reports being developed
for coal processing technologies. It was developed in connection with
activities to evaluate current process technology for the overall assessment
program, which is described in Figure A-l of Appendix A.
The objective of this report is to describe the systems** or combi-
nations of processes that are likely to be used for physical cleaning of
coal for boiler firing. This involves making judgments as to types of coal
that will be processed, types of equipment (and auxiliary processes) that
will be employed, and markets that will develop for physically cleaned coal.
Data supporting statements in this overview report generally are
from reports prepared by Battelle's Columbus Laboratories under Contract
No. 68-02-2163 with the U.S. Environmental Protection Agency (EPA). Where
other sources of information were used, they are cited in the text.
* Physical coal cleaning processes are those that remove ash*** and
pyritic sulfur from coal without chemical modification or destruction
of the coal or other mineral matter.
** Certain terms, such as "systems'1, which have a number of commonly
accepted meanings, have been defined specifically for use in environ-
mental assessment activities of the U.S. EPA-IERL/RTP. A glossary
of these terms is Included in Appendix B.
*** PCC does not remove ash; it removes ash-forming minerals. However,
common usage of the word "ash" in the coal cleaning industry to denote
ash-forming minerals will be followed subsequently in this report.
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The balance of this overview report is divided into three sections.
Section 2, "Status of Technology", presents information intended to define
the future prospects for physical coal cleaning in relatively broad terms.
Section 3, "Description of Technology", presents more information on
individual processes that are considered likely to be employed commercially.
Section 4, "Environmental Impacts", discusses the kinds of pollutant discharges
that must be anticipated.
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2.0 STATUS OF TECHNOLOGY
Commercially available physical coal cleaning (PCC) processes have been
used worldwide for many years to upgrade coal quality. Relatively simple
systems were used to remove ash-forming constituents from coals supplied
for boiler fuels. More elaborate systems were used to remove pyritic
sulfur from metallurgical coals. In the mid-1960's the U.S. Environmental
Protection Agency (EPA) initiated programs to develop information on the
usefulness of physical coal cleaning for reducing sulfur oxide emissions
from utility boilers. It was soon determined that such information was
not available. As a result, the U.S. EPA, working with the U.S. Bureau of
Mines and others, conducted numerous projects aimed at (1) evaluating the
degree to which steam coals of the U.S. could be desulfurized using physical
coal cleaning techniques, (2) determining the effectiveness of commercially
available coal preparation equipment for sulfur removal, and (3) evaluating
processes that could utilize the coal associated with the rejected mineral
matter and thereby increase the degree to which coal could be economically
cleaned with physical methods.
These activities, along with the work of others, has led to a growing
appreciation of the possibilities for using physical coal cleaning, alone or
in combination with other methods, to minimize sulfur oxides pollution. In
addition, there is interest in using PCC for minimizing other potential
pollutants (e.g., fly ash and trace metals) and for realizing other economic
benefits such as reduced transportation costs and reduced boiler maintenance.
While physical coal cleaning systems are commercially available and have
been in use for many years for traditional applications, only one system
(discussed in Section 2.4) has been designed and operated specifically to
reduce the sulfur content in coal burned by a utility.
There has been considerable testing of U.S. coals to estimate what sulfur
and ash reductions are economically achievable. The U.S. Bureau of Mines
conducted washability studies of 455 raw coal channel samples from six major
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coal producing regions. These samples were obtained from mines which
provide more than 70 percent of the annual U.S. utility coal production. The
results of these studies indicate that substantial reductions in sulfur and
ash content can be attained for numerous coals. They also indicate, however,
that maximizing the effectiveness of PCC will require modified approaches to
accommodate variations in the properties of U.S. coals as they occur in different
regions, different coal beds within regions, and within individual coal beds.
Table 1 shows the differences in results obtained when different coals
are given similar processing. These results were obtained from Battelle's
RPAM (Reserve Processing Assessment Model) computer program for a process
equivalent to a physical separation at a specific gravity of 1.6 after crushing
to a maximum size of 3/8 inch. As will be noted, sulfur removal levels are
high except for Southern Appalachia and Alabama regions. The variations shown
are attributable to the differences in coal characteristics and indicate the
need to optimize the cleaning process used in terms of the coal properties.
Estimating the degree to which PCC is applicable to boiler fuel production
is further complicated by uncertainties regarding the levels of future pollu-
tion control standards. Physical coal cleaning is by far the cheapest method
per unit of sulfur removal for "cleanable" coals, and for some small indus-
trial and commercial boilers it may be the only economically feasible route
to control. However, precise control of product sulfur content at a parti-
cular level, on a continuous basis, is not presently possible due to sulfur
variability within a coal seam.
In summary, PCC technology is an established technology which is now
being adopted and applied as a pollution control method. While a considerable
amount of information relative to this usage has been accumulated, there is
still much to learn.
2.1 Energy Efficiency
The overall energy efficiency of PCC is influenced by the energy required
to operate the process and by the energy value of the coal lost with the
refuse. Power consumption by a PCC facility would be the equivalent to
something less than 1 percent of the Btu input to the process. Btu's lost
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TABLE 1. VARIATION IN CLEANING PERFORMANCE BY REGION
Ul
Btu
Recovery.
Region percent
1.
2.
3.
4.
5.
6.
7.
(N. Appalacbta)
(S. Appalachia)
(Alabau)
(1. Midwest)
(V. Midwest)
(western)
(Entire USA)
96.0
97.0
96.5
96.4
9S.2
97.9
97.1
Haas
Recovery,
purcent
93.4
95.0
94.2
93.5
90.3
97.3
95.1
Percent
Sulfur in
Raw Coel
2.79
1.04
1.28
3. 89
4.42
0.65
1.90
Percent
Sulfur in
Cleaned Coal
1.93
0.90
1.13
2.80
3.23
0,45
1.35
Pi>rrent
Reduction
in Sulfur
30.8
13.5
11.7
28.0
26.9
30.6
28.9
Ib S02/106 Btu
in Raw Coal
4.25
1.51
1.90
6.32
7.23
1.09
3.07
Ib SOj/106 Btu
in Cleaned Coal
2.86
1.28
1.64
4.41
5.01
0.75
2.14
Percent Percent
Reduction in Ash In
Ib S02/106 Btu Raw Coal
32.7
15.2
13.7
30.2
30.7
31.2
30.3
11.87
8.78
10.82
13.51
15.54
8.07
10.27
Percent
Ash in
Cleaned Coal
7.32
5.07
6.61
8.04
7.96
5.84
6.57
Percent
Ash
Reduction
38.3
42.3
38.9
40.5
48.8
27.6
36.0
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with the refuse are by far the more important of the two and might amount
to as much as 5 to 15 percent in practice. It should be noted, however,
that while there has been a tendency to consider Btu's in the waste material
as equal in value to Btu's in the coal, this is misleading. Much of the
carbonaceous matter which is rejected is low-grade material. Further, being
in association with other unwanted minerals, it is of limited value for high
efficiency combustion processes. Also, the removal of such constituents,
which is the primary purpose of physical coal cleaning, provides monetary
benefits to operators of coal-fired boilers in terms of lower costs for
coal transportation and handling, ash disposal, coal pulverizing, and
boiler maintenance. Other cost benefits result from increases in boiler
capacity factor and availability. Normally, the monetary vlaue of the Btu's
associated with the unwanted coal constituents is charged as a cost of coal
cleaning, but, unless credits for the monetary benefits of using cleaned
coal are also considered, the true cost (or value) of coal cleaning would
not be properly assessed. The importance of these credits are discussed
in Section 2.2.
It is also important to appreciate that while the loss of coal in PCC
refuse is a significant economic factor, the losses are not large when compared
to those associated with other possible techniques for sulfur control. For
example, flue gas desulfurization (FGD) systems consume energy in amounts
(2)
equal to 3 to 10 percent of the fuel input to the boiler. Technologies
being developed to produce low-sulfur gaseous or liquid fuels from coal may
attain efficiencies of 75 to 80 percent, but gasification or liquefaction
technologies that can be used today are more likely to have efficiencies of
60 to 70 percent.
2.2 Costs
2.2.1 Introduction
Evaluation of the economics of PCC requires consideration of both costs
and benefits which are highly specific to individual coal supply/coal user
combinations. Further, some of the most important factors are difficult to
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quantify, e.g., the value of improved boiler operability and efficiency
which may result from lower throughput of inert mineral matter and sulfur.
Also where PCC is being assessed as an approach to control of sulfur oxide
pollution, it must be compared with available alternatives including flue
gas desulfurization (FGD), the use of naturally occurring low-sulfur coals,
and the use of PCC in combination with FGD. Finally, the economic factors
can be assessed from several perspectives, e.g., that of a coal producer
evaluating the return on investment that might be realized by selling
cleaned coal or that of a coal user evaluating what might be saved using
physically cleaned coal. All of these factors combine to make generalization
relative to costs difficult. However, one recent study demonstrates a
definite economic advantage for PCC combined with FGD for S02 emission
control, as compared to the exclusive use of FGD. Other data which have
been accumulated have made it increasingly apparent that PCC can be very
attractive economically in many situations, and it can ease the problems
(4)
in meeting environmental objectives. Some of the findings considered
most significant are presented in this section.
2.2.2 Physical Coal Cleaning Costs
For simplicity, only those costs which would be incurred by the operator
of a PCC plant are considered here. No attempt is made to estimate the
incentives for selling ROM coal, as opposed to physically cleaned coal,
or the probable selling price of physically cleaned coal. Also, it should
be noted that the overall cost of coal preparation may be expressed in units
such as $/ton of clean coal, $/ton of raw coal, $/10 Btu, or $/lb of sulfur
removed. In this report, costs are frequently stated in several ways in an
attempt to prevent confusion associated with the use of different terms.
The total cost of PCC includes three main elements, i.e., capital
charges, operating and maintenance (O&M) costs, and the costs of the Btu's
lost when combustible matter is rejected in the refuse stream from PCC
processes.
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Capital Charges. Annualized capital charges depend on three factors,
i.e., capital investment, estimated useful life of the plant, and the
interest or discount rate.
The capital investment required for a PCC plant is heavily dependent
upon its size and complexity. With new plants being designed to achieve
economy of scale, the complexity is probably the dominant cost-related
factor for modern plants. Complexity is in turn dependent upon the cleana-
bility of the coal and product requirements. The complexity-cost relationship
can be illustrated by considering a range of costs* which have been reported.
For a simple jig plant processing, a 6-inch x 0 coal with the 48 mesh x 0
fraction processed in hydrocyclones to produce a 48 x 120 mesh clean coal
fraction, the cost was $6,600^ ' per ton/hour of feed capacity. For the
multistream coal cleaning plant which has been built to help control
emissions from a power plant (see Section 2.4), the reported cost was
$42,000 per ton/hour of capacity.
Capital investment costs are converted to fixed annual charges on
capital by using appropriate estimates for the useful life, L, of the
facility (in years) and the interest or discount rate, i (annual percentage).
The values of L and i determine the capital recovery factor (CRT) which is
multiplied times the capital investment cost to obtain the fixed annual
charges on capital. These charges allow for amortization of the capital
investment and the time value of the moeny invested over the useful life
of the physical asset. The approximate PCC capital investment cost range
of $6,600 to $42,000 per ton/hour of capacity can be converted to a range
for charges on capital as follows. With a plant useful life of 15 years
and a discount rate of 18 percent**, the capital recovery factor (CRF) is
0.19640, which yields a range for fixed annual charges on capital of
approximately $1,296 to $8,249 per ton/hour of capacity. With a plant
capacity of 1000 tons/hour and a plant utilization of 42 percent, or 3690
hours/year, the range for charges on capital is approximately $0.35 to
* 1977 dollars.
** Based on a combination of interest on borrowed funds and return on
equity.
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$2.24/ton of feed coal or $0.44 to $2.79/ton of clean coal if the weight
yield is O.8.*
Operating and MaintenanceCost. O&M costs for PCC typically include
raw materials and supplies, utilities, labor, and overhead. Other costs
which may be included are payments to union welfare fund, insurance,
payroll taxes, and property taxes. O&M costs** for PCC may range approxi-
mately from $0.6(T to $4.0CP ' per ton of feed coal ($0.75 to $5.00 per
ton of clean coal if the weight yield is 0.8). The lower figure is for
coarse beneficiation, i.e., washing of +3/8-inch material only with the
3/8-inch x 0 fraction being shipped dry as is. The higher figure applies
to full beneficiation of a coal with large amounts of -28-mesh material
(above 20 percent) using heavy-medium circuits and full treatment of the
fines.
Cost of Btu Loss. The cost of Btu loss is a function of the cost
and heating value of the feed coal and the Btu recovery. Formulas for
(9)
calculating and reporting these costs have been shown by Battelle
The costs for Btu loss as a function of these two variables is shown in
Table 2.
TABLE 2. COST OF BTU LOSS FOR R = 0.95
$/ton of Clean Coal
F, Cost of Feed Coal, 100
$/ton
10
15
20
25
60
0.83
1.25
1.67
2.08
Y, Weight Yield, Percent
70
0.71
1.07
1.43
1.79
80
0.63
0.94
1.25
1.56
90
0.56
0.83
1.11
1.39
95
0.53
0.79
1.05
1.32
* Cost per ton of cleaned coal is obtained by dividing the cost per
ton of feed coal by the weight yield.
** 1977 dollars.
9
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The values in Table 2 reflect a cost which assumes that the value of
Btu's associated with coal in refuse is equal to that of Btu's in clean
coal. It should be recognized, however, that, as indicated earlier, this
is not a true relationship because constituents removed by PCC have a
relatively low heating value and, being associated with sulfur compounds
and inert mineral matter, they are difficult to utilize effectively.
However, calculation of the cost for Btu loss without differentiation
with respect to the actual value of Btu in the coal product versus those
in the refuse gives a good approximation. It should be noted also that
the cost of coal after washing is not simply a function of the cost of
input coal and the amount of coal lost. For example, with a $20/ton
cost of feed coal and an 80 percent weight yield, the cost of "raw materials"
per ton of clean coal is $25.00. With a Btu recovery of 95 percent and a
weight yield of 80 percent, the cost of Btu loss is $1.257ton of clean
coal as shown in Table 2. Part of the increase in raw material of $5.00
is offset by a $3.75/ton increase in the value of the coal product
(attributable to its higher heating value) giving a net cost of $1.25 per
ton of cleaned coal.
Total PCC Costs. The range in costs previously cited are summarized
in Table 3.
TABLE 3. APPROXIMATE RANGES OF PCC COSTS
Approximate Cost Range,
Cost Category $/ton of clean coal
Capital Charges 0.44 - 2.79
O&M 0.75 - 5.00
Btu Loss 0.53 - 2.08
Total 1.72 - 9.87
10
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The calculated value of fixed annual charges on capital is very sensitive
to the estimated useful plant life and discount rate. Both the capital and
O&M costs per ton of feed or clean coal are very sensitive to the estimated
plant utilization. Thus, while most plants would fall in the cost range
shown, the costs in specific situations are subject to further variation
as estimates for these parameters differ from the representative values
used here.
2.2.3 Case Studies of PCC Costs
A recent report , based on a study by The Hoffman-Muntner Corporation,
presents the results of detailed case studies of the costs of eight PCC
plants. These plants represent a sepctrum of PCC technology from a relatively
simple jig process to relatively complex circuits involving heavy medium
cyclones, froth flotation cells, and thermal dryers. These configurations
were considered to be typical of those currently in use. The estimated
capital investment costs are given in Table 4, and the annualized costs,
which include operating and miantenance costs, capital charges, and cost
of Btu loss, are shown in Table 5. The data are all within the ranges
shown in Table 3 except for the cost of Btu's lost in Plant No. 3. For this
plant, coal losses combined with high rejection of mineral matter, give a
low yield. As a result, the high cost per ton of raw coal, when converted
to cost per ton of cleaned coal comes to $4.49/ton, a value more than twice
that for any other plant in the study. This is a good illustration of the
degree to which site-specific factors can cause wide variations from any
rule of thumb even though it may be generally applicable. The assumptions
made in the Hoffman-Muntner study to determine a capital recovery factor
(9)
varied widely from those used for the Battelle study , but the differences
were in large part offsetting and, when adjusted to a common basis, the
agreement was good.
2.2.4 Benefits of Physical Coal Cleaning
The benefits associated with the use of physically cleaned coal include
the following.
11
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TABLE 4. DESCRIPTION OF COAL PREPARATION PLANTS
(5)
N>
Plant
No.
1
2
3
4
5
6
7
8
Type of Procoas
Jig
Jig
Jig
Jig
Heavy Medium
Heavy Medium
Heavy Medium
Heavy Medium
Raw Coal
Capacity (TPH)
600
1.000
1,000
1,600
1,400 (720) removed/ton of raw coal processed.
(b) 680 tph of raw coal Is not processed by the cleaning plant.
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TABLE 5. TOTAL ANNUALIZED COSTS FOR EIGHT COAL PREPARATION PLANTS
(Mid-1977 Dollars)
u>
Operating and
Maintenance Cost*3'
Plant
No.
1
2
3
4
5(c>
6
7
8
$/Ton of
Raw Coal
1.59
1.82
1.51
1.76
2.37
2.23
1.27
2.10
$/Ton of
Clean Coal
2.70
2.55
2.67
2.96
3.20
3.04
2.J2
2.44
Capital Charges
S/Ton of
Raw Coal
0.38
0.80
0.71
0.84
0.42
1.31
0.82
O'.81
$/Ton of
Clean Coal
0.65
1.12
1.25
1.40
0.56
1.79
1.36
0.94(d>
Cost of Btu Loss
$/Ton of
Raw Coal
1.26
0.54
2.54
0.95
0.81
1.62
1.06
0.88
$/Ton of
Clean Coal
2.14
0.75
4.49
1.60
1.10
2.21
1.76
1.02
$/Ton of
Raw Coal
3.23
3.16
4.76
3.55
3.60
5.16
3.15
3.79
Tota| Annualized Cost
$/Ton of
Clean Coal
5.49
4.42
8.41
5.96
4.86
7.04
5.24
4.40
S/10" Btu
Recovered
0.227
0.183
0.338
0.222
0.239
0.258
0.206
0.176
$/Ton of
Ash Removed
9.92
13.39
15.71
10.47
27.69
30.53
9.55
52.28
S/Ton of
Sulfur Removed
1,746
271
344
789
244
187
1.000
421
(a) Includes labor, supervision, overhead, supplies, fuel, electricity, and subcontract services.
(b) Based on a 10-year amortization period, 9 percent discount rate, and 30 percent utilization factor, except as noted.
(c) Costs .shown for Plant No. 5 are based on 1400 TPH.
(d) Fifty percent utilization factor.
-------
(1) Reduced coal transportation cost
(2) Reduced coal handling and coal pulverization cost
(3) Reduced ash collection and disposal cost
(4) Increased boiler efficiency, capacity, and availability
(5) Reduced O&M costs
(6) Savings in cost for control of sulfur oxide pollution.
Items (1) through (5) above are savings in operating cost which result from
general upgrading of coal quality. Item (6) is a savings which is realized
where PCC is used to minimize sulfur pollution control costs. To date there
has been no comprehensive assessment of the value of all these benefits. A
number of relevant studies have been conducted, however, and some under-
standing of the potential importance of some of the factors above is being
(4)
developed.
Penalties Associated With Poor Quality Coal. According to a recent
study, power plants can experience incremental costs from poor coal quality
starting at about $1.00 per ton for coal containing 13 percent minerals (ash
+ sulfur) and ranging to about $8.00/ton for coal containing 25 percent
minerals. These costs, which could be minimized by reducing the mineral
content of the coal, are approximately in the range of costs shown in
Table 3 for PCC. Based upon the study of cost penalties for poor quality
coal , power plants using clean coal produced by the PCC plants listed
in Tables 4 and 5 could expect to realize substantial cost reductions
because of the amounts of ash and sulfur removed from the raw coal.
However, accurate estimates of these cost reductions cannot be made
because of insufficient data. Some but not all of the six benefits
listed above were considered in the referenced study , the most notable
exception being savings in cost for control of sulfur oxide emissions.
This factor is discussed in the section following.
Savings in Sulfur Pollution Control. The control of sulfur oxide
emissions from fossil-fuel-fired combustors currently is achievable by the
use of one of four methods: (1) naturally-occurring low-sulfur coal;
(2) flue gas desulfurization (FGD); (3) physical coal cleaning (PCC);
and (4) combinations of these three approaches. Evaluation of the relative
merits of the different approaches requires that they be assessed from the
14
-------
standpoint of their impact on the cost of boiler output. One such comparison
which considers the impact of sulfur control on power generation was reported
(9)
recently by Battelle.v Data derived from that report are presented below
to illustrate the possible magnitude of savings which could be realized
using PCC alone, or with FGD, for sulfur oxide control.
The cost of power generation is determined by the capital charges, fuel
cost, and O&M costs for the power plant. All three components are influenced
in different ways by the method selected for sulfur oxide pollution control.
Application of FGD increases both fixed charges and O&M costs but makes it
possible to use readily available fuels. The use of PCC likewise increases
both fixed and O&M costs for the total system but, when used in combination
with FGD, reduces costs for the gas cleaning system. Also, it upgrades the
quality of the fuel. The use of low-sulfur western fuels has no impact as
far as increased fixed costs are concerned. Its use in boilers designed for
eastern bituminous coal is judged, because of lower heating value and other
properties, to reduce boiler availability from 0.8 to 0.7 for purposes of
the Battelle comparison. Some increased boiler maintenance might also be
anticipated, but none is assumed for purposes of this comparison.
Because costs for generation of electricity are greatly dependent upon
the hours the plant is operated, any comparison of sulfur oxide control
methods must consider their effect on plant availability. The differences
in availability reflect differences in coal quality on boiler and scrubber
operation and the effect of scrubber operability on system availability.
The effect of scrubbers was estimated for different degrees of redundancy
as far as spare scrubber modules were concerned. The availabilities
estimated for the earlier study are shown in Table 6.
Assumptions made for cost factors for the system configurations shown
in Table 6 are shown in Table 7. The costs and benefits for PCC are
generally consistent with data presented earlier. The relationship for
incremental maintenance and mineral content is based on recent work reported
for TVA boilers. Other costs are considered reasonable in light of the
latest estimates. The direct benefits shown for PCC include those discussed
earlier such as reduced transportation costs, etc. The indirect benefits
are associated with FGD, e.g., reduced energy requirements for reheat of
stack gases.
15
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TABLE 6. POSTULATED CONDITIONS OF AVAILABILITY
Case
No.
1
2
3
4
Case Description*
Raw high-sulfur eastern coal, no FGD (baseline)
Raw low-sulfur western coal, no FGD
Cleaned high-sulfur eastern coal, no FGD
Raw high-sulfur eastern coal, with FGD
System
Availability
0.8
0.7
0.9
0.627
(4 modules + 1 spare)(Boiler - 0.8,
FGD = 0.65/module)
5 Cleaned high-sulfur eastern coal, with FGD 0.806
(3 modules + 1 spare)(Boiler » 0.9,
FGD = 0.75/module)
6 Cleaned high-sulfur eastern coal, with FGD 0.864
(3 modules + 2 spares)(Boiler = 0.9,
FGD - 0.75/module)
* Individual availabilities for boilers and FGD modules are given in
parentheses where applicable.
The overall comparison for the six system configurations is shown in
Table 8. From results for the two cases comparing the use of ROM eastern
coal and cleaned eastern coal (1 and 3), it is apparent that burning cleaned
coal is the cheapest way to produce power. For those cases which achieve
desired levels of sulfur oxide control, the advantage is even greater,
ranging from 0.241 c/kWh to 0.563 c/kWh, even in situations where supple-
mental control with FGD is needed. These results are confirmed by a paper^
discussing a partially completed study being conducted by Bechtel National,
Inc., for the Electric Power Research Institute. The paper concludes that
"from the results obtained so far, it is judged that the cost of coal
cleaning can be offset by savings in transportation costs, power plant
capital costs, and operating and maintenance costs."
In the development of those cost comparisons, a number of simplifying
assumptions were made which require that any conclusions reached be substan-
tially qualified. First of all, the analysis applies only to utility boilers
which are required to meet the former NSPS of 1.2 Ib S02/10 Btu or SIP
16
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TABLE 7. SUMMARY OF COSTS FOR A 500-MW COAL-BURNING
POWER PLANT
Power Plant
Annual Fixed Charges - (0.235)($215,375,000) = $50,663,000
Fuel
Eastern high-sulfur coal - $0.84/10^ Btu
Western low-sulfur coal - $1.41/10^ Btu
Production - (0.176)(Fuel Costs)/106 Btu
Incremental Maintenance - $0.15 (% ash + % sulfur - 12.5)/ton of coal
Flue Gas Desulfurization System
Annual Fixed Charges
Five modules - (0.235)($45,435,000) = $10,688,000
Four modules - (0.235)($40,485,000) = $9,523,000
Operating and Maintenance - $0.23/10 Btu
Physical Coal Cleaning
Capital Cost - $15,870 per ton/hr
Annual Fixed Charges - (0.235)($6,852,500) » $1,612,000
Operating and Maintenance - $0.089/10 Btu
Direct Benefits - $0.041/10 Btu
Indirect Benefits (when used with FGD) - $0.031/10 Btu
17
-------
TABLE 8. SUMMARY OF COSTS FOR POWER GENERATION USING VARIOUS CONTROL MODES
Operating
Number
! Fixed
7008
6132
7884
5493
1.446
1.652
1.285
1.845
Coal Cleaning Costs and
Power Plant Costs, C/kWh FGD Costs, Savings, C/kWh
Fuel
0.840
1.410
0.898
0.840
Incremental c/kWh PCC PCC/FGD 1
Production Maintenance Fixed O&M Fixed O&M Savings Savings
0.148 0.093
0.248
0.158 0.015 — — 0.041 0.089 -0.041
0.148 0.093 0.389 0.230
total Costs,
C/kWh
2.527
3.310
2.445
3.545
5 Cleaned high-sulfur eastern 7061 1.435 0.898 0.158
coal, with FGD (3 modules +
1 spare)(Boiler • 0.9,
FGD - 0.75/module)
6 Cleaned high-sulfur eastern 7569 1.339 0.898 0.158
coal, with FGD (3 modules +
2 spares)(Boiler - 0.9,
FGD - 0.75/module)
0.015 0.270 0.230 0.046 0.089 -0.041 -0.031
3.069
0.015 0.282 0.230 0.043 0.089 -0.041 -0.031 2.982
(a) Not in compliance with NSPS promulgated December 23, 1971 (36FR24876).
(b) Based on Tables 6 and 7.
-------
regulations in this same range. Additional analysis will be needed to deter-
mine definitely that similar conclusions will apply to operation of commercial/
industrial boilers and those utility boilers which will be subject to the
recent NSSPS (June 11, 1979).
Also, at least two areas of uncertainty are evident in the estimates
of costs and benefits. First, the savings estimated for reduced boiler O&M
costs (and associated increases in boiler availability) assume that these
costs are a function of only the amount of ash and sulfur present in the
coal. They are not based on results of operation with run-of-mine coal
versus cleaned coal from the same source. Second, the estimates for the
fixed, operating and maintenance cost for the FGD systems were based on
average conditions and not related specifically to flue gas volumes to be
treated, amount of sulfur oxide to be removed, etc. In any future analyses,
(12)
a more rigorous approach based on recent work by Kilgroe would be possible.
It does not appear, however, that the elimination of uncertainties
would substantially change the results. And the cost advantage for PCC
indicated in Table 8 represents a potential annual savings of $9 million
to $21 million for a 500 MW plant. The magnitude of national savings which
appear to be possible is such that activity to promote the use of PCC would
be in the national interest.
2.3 Applicability
Potential markets for low-sulfur, low-ash coal appear to be large, but
the availability of cleanable coals in proximity to potential users is ill-
defined. Further, the extreme sensitivity of coal cleaning economics to
highly variable site-specific requirements is such that generalities can be
misleading. It appears, however, that some comment in this connection is
appropriate.
Potential market areas for physically cleaned coals include (1) utility
boilers, (2) industrial and commercial boilers producing process steam or
providing space heating, (3) metallurgical applications, and (4) systems
converting coal into gaseous or liquid fuels. In 1975, U.S. boiler fuel
consumption was approximately 26 x 10 Btu (about 1/3 of the total for
19
-------
fossil fuels), exceeding even transportation (19 x 10 Btu) which is
(14)
by far the next largest fuel use. Coal-fired utility boilers burned
about 430 x 10 tons of coal. Coal-fired boilers for industrial or commer-
cial applications burned about 44 x 10 tons. Table 9 breaks down the
estimated fuel consumption, by usage sector and fuel type for the U.S. in
1975. From these data it is apparent that, despite the large amounts of
coal burned, gas is used in about equal quantities as a boiler fuel, and
gas and oil combined provided about 60 percent of the total. The ways in
which patterns will change as gas and oil become unavailable or prohibitively
expensive are very uncertain. It seems certain, however, that the use of
coal will increase substantially, and it seems reasonable to assume that
considerably more will be burned in small boilers which make up the industrial/
commercial sectors where gas and oil consumption is now the dominant fuel.
TABLE 9. ESTIMATED FUEL CONSUMPTION FOR U.S. BOILERS (1975)
Use Sector Fuel Consumption,
Fuel Type 1012 Btu
Utility
Coal 9,310.0
Residual Oil 2,590.8
Distillate Oil 129.7
Natural Gas 3,016.7
15,047.2
Indust rial/Commerc ial
Coal 1,101.6
Residual Oil 1,762.3
Distillate Oil 1,129.6
Natural Gas 6,381.2
10,374.7
20
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PCC is a potentially useful S02 emission control method for existing
utility and large industrial boilers that are subject to state and local
regulations but not subject to New Source Performance Standards (NSPS).
For boilers where present NSPS are applicable, PCC is useful as a comple-
mentary control technique (e.g., along with FGD) in a number of situations.
For many of the smaller coal-burning boilers not subject to NSPS
(those less than 250 x 10 Btu/hr heat input), PCC may be the only feasible
near-term method for minimizing SO- emissions and, therefore, may be used,
or at some point even required, for control of such boilers. Boilers of
this class represent an estimated 40 percent of the existing boiler fuel-
(13)
burning capacity and, with pressure to convert the 85 percent that are
oil- or gas-fired to coal, the number of small coal-burning boilers can be
expected to increase. Burning physically cleaned coal could be helpful
in minimizing the pollution from such boilers by removing potential pollutants
prior to combustion.
As conversion processes for generating gas or oil from coal begin to
find application, it seems reasonable to suppose that they will create
additional demand for physically cleaned coals. Removal of substantial
amounts of sulfur, trace elements, and ash-forming minerals from the coal
prior to conversion is likely to be less expensive than putting such
impurities through the system and having to remove them from discharge
streams. In addition, there may be process-reaction reasons for using
PCC. While such markets are likely to develop some years in the future,
it seems reasonable to consider such applications as a part of the contribu-
tion that PCC can make to future environmental control.
2.4 Extent of Development
Physical coal cleaning systems can be designed with reasonable confi-
dence for well-characterized coals. Systems designed for removal of ash
and pyritic sulfur may have circuits for treatment of three size fractions
of coal: (1) coarse (e.g., 3 x 3/8 inch)*, (2) fine (e.g., 3/8 inch x 28
* In practice, the size of coal fractions processed may vary widely.
For purposes of this report, size ranges shown as examples are considered
to be representative of coarse, fine, and ultrafine coal.
21
-------
mesh [M]), and (3) ultrafine (e.g., 28 M x 0). Such arrangements are
characteristic of systems which have been used for preparation of metallur-
gical coals. A more advanced system, alluded to earlier, which has been
installed near the Homer City power generating plant and is jointly owned
by Pennsylvania Electric Company and the New York State Electric and Gas
Corporation is the first multistream coal cleaning system (MCCS) designed
to provide an optimized strategy for meeting emission stnadards and utilizinh
coal resources. In this facility, two clean coal products, with low and
medium sulfur contents, are developed for consumption in separate boilers
which are regulated under different sulfur limitations. This system meets
emission levels and maximizes recovery of Btu's from the raw coal.
This facility, now in the start-up phase, has a design capability of
1,200 tons per hour and will process 5.2 million tons of ROM coal per year.
It has a number of unique design features and incorporates four processing
circuits which utilize heavy medium cyclones for processing of coarse and
fine coal and hydrocyclones combined with wet concentrating tables for very
fine coal. The plant is expected to produce a medium sulfur coal and a
low sulfur coal. The characteristics of the output streams are shown in
Table 10. The medium sulfur coal will be used in two existing 600 MW
generating units to meet the Federal and state emission regulation of
TABLE 10. HOMER CITY PLANT PERFORMANCE
(6)
Medium-Sulfur
Coal
Yield, weight percent
Recovery, Btu percent*
Product Btu/lb (dry basis)
Ash, in Product, weight percent
Sulfur, in Product, weight percent
Pounds SO./IO6 Btu
Total Sulfur Reduction, weight percent
56.2
61.6
12,549**
17.8
2.2
3.6
52.6
Low-Sulfur
Coal
24.7
32.9
15,200
2.8
0.9
1.2
91.8
Refuse
19.1
5.5
3,367
69.7
6.2
36.5
* Figures account for 94.5 percent of Btu recovery. Overall plant Btu
recovery is 93.5 percent which includes 1 percent loss for thermal drying.
** Mixture of middling and deep-cleaned coal.
22
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4.0 Ib S02/10 Btu for existing boilers. The low sulfur coal will be used
for a new 650 MW unit to meet the New Source Performance Standards (NSPS)
of 1.2 Ib S02/10 Btu without the use of a flue gas desulfurization (FGD)
system.
The original plan for the new Homer City unit called for installation
of an FGD system for S02 emission control. However, following comparative
studies of the effectiveness and costs of FGD and MCCS for controlling
S02 emissions, the plan to install FGD was dropped.
As a sequel to the design and construction of this facility, a program
to evaluate the performance of this advanced design will be undertaken. A
cooperative program for this evaluation will involve the owners, the U.S. Environ-
mental Protection Agency (IERL-RTP), the U.S. Department of Energy, and the Electric
Power Research Institute. A 3-year period of performance for the evaluation
is anticipated. It is expected that the results of the evaluation program
will confirm the soundness of the design principles on which the plant is
based.
The use of PCC systems for ash and pyritic sulfur removal is not, however,
without technical limitations. Research and development aimed at improving
these systems is being conducted by the U.S. EPA, the U.S. Department of
Energy, and others. This work is intended to:
1. Define and evaluate the potential sources of pollution that
are associated with PCC systems and assess the environmental
impacts that might be associated with widespread application
of the technology.
2. Collect additional information needed for full characterization
of the washability of both existing coal supplies and reserves.
While a substantial amount of testing has been completed for
some operating mines, all deposits now being mined have not
been intensively analyzed. Very little work has been done to
evaluate the washability of reserves not being mined.
3. Develop information needed to better understand the relationship
between variability in the amount and type of ash and sulfur
in the input coal and to develop improved systems for
controlling the quality of the cleaned coal.
23
-------
4. Generate background information needed to define the potential
of new systems which employ presently available technology
in configurations that involve crushing of the entire coal
feed to a relatively fine size such as 3/8 inch x 0.
5. Develop information needed to define all the benefits
associated with PCC including savings in pollution control
costs, transportation costs, and boiler maintenance costs
as well as savings associated with boiler availability.
6. Reduce the coal losses associated with coal cleaning and
investigate ways to recover, or otherwise utilize, coal
rejected by the cleaning process.
7. Determine the possible role of PCC in combination with,
or in competition with, other emission control techniques
such as flue gas desulfurization (FGD) systems or
using clean fuels produced by advanced coal conversion
systems.
2.5 Prospects for Commercialization
Physical coal cleaning is a long-established but still developing techno-
logy whose commercial application is being expanded at a low rate in light of
the magnitude of previously discussed economic and environmental benefits
that are attainable where coals with good washability characteristics are
available. Some factors cited as retarding the rate of application of PCC
are:
1. Coal companies that could operate PCC orocesses at the
mine site may orefer to use available capital to open
new mines unless there is a clear financial incentive
for their use of PCC.
2. Utility and other industrial groups, because coal cleaning
is most practical near the mine, may have to go into the
mining business if they want to produce their own physically
cleaned coal. This is considered unlikely to happen in
many situations.
24
-------
3. The costs and benefits of PCC, being highly specific to
each coal producer and coal user combination, require careful
evaluation. Many users, being unfamiliar with PCC technology,
may find evaluation difficult. Unless the benefits can be
demonstrated, it is unlikely that users will offer a premium
price for cleaned coal.
4. While washability data are available for many of the oresent
coal supplies, it is difficult to draw conclusions with
respect to the overall potential impact of PCC. Additional
information on the washability of coal reserves is needed
so that future applicability can be predicted with more
confidence.
5. Uncertainties with respect to sulfur oxide emission standards
make PCC difficult to evaluate as a pollution control measure.
Further, it is uncertain what pollution control measures will
be required for the PCC plants themselves. Since some PCC
plants will be large (1500 T/hr capacity or more) and expensive
(perhaps $40,000,000 and up), uncertainties of any kind are a
great deterrent to building a plant.
6. There is a considerable investment in existing mines, manv of
which are producing coal whose sulfur is not significantly
lowered by PCC. Further, many utilities have long-term
purchase contracts for coals that are not amenable to substantial
sulfur reduction by physical cleaning. Whether other benefits
associated with PCC could be realized in such situations has not
been established. Hence, there aopears to be little documented
incentive to change the status quo in such situations.
7. A significant amount of our total coal production comes from
small companies which could not afford to invest in PCC systems.
This list of barriers to commercialization suggests that positive
coordinated actions on the part of authorities on the Federal and state levels
will be necessary if the apparent benefits from use of PCC are to be realized.
This situation has been analyzed by Battelle^ ' and the following recommen-
dations were presented.
25
-------
1. Federal and state Environmental Protection Agencies should take
actions needed to define SO. emission standards so that the long-
term usefulness of PCC for control of SC< can be assessed. In
addition, environmental regulations applicable to operation of
PCC systems must be defined. Removal of these environmental
uncertainties will make it possible to make meaningful investment
decisions.
2. The authority conferred by the "Energy Policy and Conservation
Act" should be used to provide loan guarantees for central
coal cleaning facilities to orocess the output of small producers.
3. Congress should appropriate funds to construct PCC plants
which would be paid back out of profits when the olant is in
operation. (A program similar to the "Rubber Reserve" program
during World War II is envisaged.)
4. The Internal Revenue Service should rule that PCC plants qualify
as pollution control investments for tax purposes. This would
qualify the investment for accelerated depreciation and tax credit,
and it would permit financing with the proceeds from tax exempt
pollution control bonds.
5. The Interstate Commerce Commission should change its regulations
to allow shipment of cleaned coal at unit train rates.
6. EPA should take action to develop a public information program
to educate coal producers and users to the advantages of using PCC.
7. Research and development needed to further improve PCC and
generate firm data for economic evaluations should be pursued
vigorously so that the advantages offered by PCC can be maximized.
In summary, it appears that PCC has considerable potential for producing
overall benefits nationally. The national benefits would be in the form of
reduced pollutant emissions and also reduced costs for (1) transportation of
coal, (2) pollution control, and (3) generation of electricity or production
of steam for process or soace heating. Realization of these benefits would
require development of PCC systems tailored to meet the requirements of many
unique coal production-usage combinations and would require actions of the
type discussed above by responsible government agencies.
26
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3.0 DESCRIPTION OF TECHNOLOGY
Physical coal cleaning (PCC) systems that might be employed today range
in complexity from simple systems for removing coarse refuse to highly sophis-
ticated systems designed for maximum removal of sulfur and ash from a specific
coal supply. The simple systems have been employed widely in the past and may
need to be considered from the standpoint of their environmental impacts. They
are of little interest, however, for their potential for minimizing the environ-
mental impacts associated with coal combustion. The more sophisticated systems
that are designed for maximum removal of sulfur and ash are identified in this
section, and feedstock and product characteristics are discussed.
3.1 Physical Coal Cleaning Systems
The separation processes that divide the product stream from the refuse
fraction are the central components of PCC systems, which involve three
primary operations—coal pretreatment, coal cleaning, and product conditioning.
Other auxiliary processes are employed for coal handling, water treatment,
and solid waste disposal. All operations employ a variety of processes with
different potentials for discharging pollutants. These processes and asso-
ciated waste streams are discussed individually in Appendix D. Figure 1
schematically displays the processes and operations which are incorporated
in PCC systems. The indicated sizes may vary in practice and are shown
only as typical size ranges for coarse, fine, and ultrafine coal cleaning
circuits.
Cleaning effectiveness is a function of the effectiveness of a PCC
system in removing impurities (inorganic sulfur compounds and other mineral
matter) and the effectiveness in recovering high percentages of the Btu's
available in the raw coal. Unfortunately, these two objectives are often
difficult to achieve simultaneously and can be mutually exclusive. Three
levels of cleaning effectiveness which involve a compromise between ash
and sulfur removal and Btu recovery are currently practiced:
27
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OO
FMXMCf COMUHONING OflMIIOH
) • 0
LIQUID Mill STRUM
W.IB NUK SIMMS
- HIMKftUMS
10
FROM
" notation
II
f.
>>
FIGURE 1. PROCESS FOR PHYSICAL COAL CLEANING SYSTEMS
-------
1. Minimum cleaning effectiveness involves crushing and sizing of
raw coal, cleaning the coarse fraction (3 x 3/8 inch) in jigs
or dense-medium vessels, and then combining or not combining
the uncleaned fine and ultrafine fractions (3/8 inch x 0) with the
cleaned coarse fraction. With this level, it is possible to
obtain either a high quality coal product or a high Btu recovery,
but not both. A high quality coal product may be obtained by
use of a low specific gravity of separation (near 1.3) for the
coarse fraction and by not combining it with the uncleaned
fine and ultrafine fractions. A high Btu recovery may be obtained
by use of a high specific gravity of separation (say 1.8) for
the coarse fraction and by combining it with the uncleaned
fine and ultrafine fractions.
2. For the intermediate level of cleaning effectiveness, the coarse
fraction is cleaned as in Level 1, and the combined fine and
ultrafine fraction is cleaned with air tables, wet concentrating
tables, or dense medium cyclones. Where dense medium cyclones
are used, the 3/8 inch x 0 feed may be passed over desliming
screens which yield a 3/8 inch x 200 M material, which is fed
to the dense medium cyclones.
3. Maximum effectiveness in cleaning is obtained by cleaning the
coarse fraction as in Level 1, splitting the 3/8 inch x 0
fraction into two fractions (e.g., 3/8 x 28 M and 28 M x 0),
and cleaning the two fractions separately with equipment
designed for maximum removal of sulfur and ash from fine and
ultrafine size coals. The 3/8 inch x 28 M fraction is cleaned with
wet concentrating tables or dense-medium cyclones. The 28 M x
0 fraction is cleaned using either hydrocyclones or froth
flotation. PCC plants which incorporate processing of the
28 M x 0 fraction tend to employ thermal drying.
Table 11 illustrates eight basic systems that may be employed.
3.1.1 Coal Pretreatment Processes
Coal pretreatment processes are employed to generate feed streams with
liberated impurities and particle size distributions that have been shown
29
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TABLE 11. PHYSICAL COAL CLEANING PLANT TYPES*
System
1
7
/.
5
6
7
8
Level of Cleaning
Effectiveness
In ten media te
Maximum
Maximum
Maximum
Maximum
Coal
Coarse,
3" x 3/8"
rcj-j or DMV
CS+J or DMV
CS+J or DMV
CS+J or DMV
CS+J or DMV
CS+J or DMV
CS+J or DMV
Size and
Fine
3/8" x
WT
WT
DMC
DMC
Process
, Ultrafine,
28M 28 M x 0
- —-.AT— —
HC+TD
F+TD
HC+TD
F+TD
Legend:
CS Crushing and Sizing
J Jigs
DMV Dense Medium Vessels
AT Air Tables
WT Wet Concentrating Tables
DMC Dense Medium Cyclones
HC Hydrocyclones
TD Thermal Dryers
F Froth Flotation Units
Note: All wet separation processes will have associated dewatering processes
as shown in Figure 1.
* The eight types of coal cleaning plants listed are representative, but many
other types exist or may be constructed. For example, in some cases jigs
have been used for 3" x 0 coal instead of 3" x 3/8" coal at the minimum
cleaning effectiveness levels. The raw coal may be crushed to a finer top
size than 3" to maximize impurity liberation. At the maximum cleaning
effectiveness level, hydrocyclones and froth flotation units may be used
in combination for treatment of fine coal.
30
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to be most effective for processing in the available coal cleaning processes.
Raw coal is crushed to a top size of about 3 inches and screened to provide
a coarse stream and a fine to ultrafine stream. The latter is either sold,
processed directly, or further divided into fine and ultrafine size fractions
for wet cleaning. The initial screening of crushed raw coal may be either
wet or dry. Dry screening is used if air tables are to be used for coal
cleaning. Wet screening is used where fine and ultrafine size streams are
processed by wet cleaning processes.
3.1.2 Coal Cleaning Processes
A number of approaches have been studied for non-destructive separations
of coal from unwanted, associated mineral matter. These include methods based
on specific gravity differences, surface properties, and responses to elec-
trical fields or magnetic forces. Almost all methods with demonstrated
commercial potential are based on specific gravity differences between the
coal and other mineral matter. The only exception is froth flotation where
differences in surface properties of coal allow it to be removed from aqueous
suspensions of coal and inert minerals with air and organic flotation reagents.
The specific gravities of the principal coal constituents are: organic
constituents—1.2 to 1.7, non-combustible ash (clay, shale, sandstone, etc.)—
2.0 to 2.6, and pyrite—4.9 to 5.0. With differences of this magnitude,
methods that separate fractions by specific gravity can be very effective
where the components can be isolated from each other in reasonably pure
form. Unfortunately, some impurities, pyritic sulfur in particular, are
often present in very small, highly disseminated forms that require extensive
size reduction to separate them from the coal. For very fine materials, the
effectiveness of separation based on specific gravity decreases because of
immobility of particles, particle-to-particle attractions, etc. Hence,
froth flotation is finding increased application and other methods for fine
coal cleaning are being studied.
Some historical trends, including the growth of froth flotation usage,
are illustrated in Table 12. Pneumatic methods, because of ineffectiveness
and problems associated with dust, moisture content, and size of equipment,
have declined in use from 14 percent in 1942 to 4 percent in 1972.
31
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TABLE 12. PROCESSES EMPLOYED FOR PHYSICAL CLEANING OF COAL
(Percentage of Clean Coal Produced, Year)
Process
Jigs
Dense Medium Processes (Dense Medium
Vessels and Cyclones)
Wet Concentrating Tables
Froth Flotation
Pneumatic (Air Tables, etc.)
Other (Classifiers and Launders)
1942
47.0
8.8
2.2
-
14.2
20.5
1952
42.8
13.8
1.6
-
8.2
13.7
1962
50.2
25.3
11.7
1.5
6.9
4.3
1972
43.6
31.4
13.7
4.4
4.0
2.9
3.1.3 Product Conditioning Processes
The product conditioning processes are primarily for removal of excess
water. The processes employed vary with the size of coal being processed.
For coarse coal, simple mechanical dewatering which involves the use of
natural drainage from perforated bucket elevators or dewatering screens is
employed. For finer coal, the surface area is much larger and the amount of
moisture retained per unit of surface area is also larger. Tight packing and
capillary action tends to hold water in void spaces. Hence, finer coals
(less than 1/4 inch) require something more effective than natural drainage.
For coals which contain both fine and ultrafine fractions, e.g., 3/8 inch
x 0 or 3/8 inch x 200 M, a sieve bend combined with a centrifuge for addi-
tional water removal may be used. For fractions made up exclusively of
ultrafine materials (e.g., 28 M x 0) vacuum filtration is required. Fine
and ultrafine materials are thermally dried.
3.1.4 Auxiliary Processes
Auxiliary processes, i.e., processes that are in some way incidental to
the main functions involved in PCC but are still of great importance to plant
32
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operation, are coal storage and handling, water handling, and solid waste
disposal.
A typical coal cleaning plant with a cpaacity of 1000 tons/hr will operate
16 hr/day and can process over 5,000,000 tons of raw coal annually. Storage
of raw coal and cleaned coal often is necessary to assure continuity of
operation and continuous availability of coal for shipment to market. As
production and transportation capacity increase, coal cleaning plants
require larger storage facilities to achieve the maximum utilization of
coal cleaning and transportation equipment. Coal is stored in open piles
or enclosed bins and silos.
Transportation of coal from mines or preparation plants to the point
of consumption is one of the most important factors affecting coal utiliza-
tion. Transportation modes are rail, waterway, truck, pipeline, and belt
conveyor. Often, more than one mode of transportation is used to convey
coal from the mine to the consumer.
In conjunction with transportation and storage of coal, a wide variety
of material handling operations is needed. This includes loading and unloading,
stacking and reclaiming, and transferring coal in a plant. As the amounts
of coal to be handled have grown larger, the material handling systems have
become more mechanized and tend to be equipped with more automatic and
integrated control systems. Movement of coal in and out of storage and
in-process transfer of coal on such a scale makes coal handling one of the
most important of the activities associated with PCC.
The high process-water requirement (up to about 10,000 gal/min for a
1000 ton/hr plant) makes overall water management another important consider-
ation. Recycling of water is necessary to minimize consumption and the
release of contaminated water to the environment. Water treatment is required
to meet process requirements for recycling or environmental standards for
discharge. The total requirement for PCC plants in the U.S. amounts to
hundreds of billions of gallons per year. About 80 percent is recycled
water. Most of the remainder is retained on the clean coal or refuse surfaces.
Some is also discharged to the atmosphere from the thermal dryer. This loss
must be replaced by fresh or make-up water.
PCC plants represent one of the largest sources of solid waste in the
U.S. It has been estimated that 25 percent of raw coal is disposed as
33
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waste. In 1974, U.S. coal production was 590 million tons. Of this, 289
million tons were washed, producing 96 million tons of refuse for disposal.
A single 1000-ton/hr plant would produce in the neighborhood of 1,000,000
tons/year of refuse. The disposal of ultrafine coal refuse is especially
costly and difficult, and it presents complex environmental control problems.
3.2 Raw Materials
The characteristics of coal that are most relevant to the potential
suefulness of physical coal cleaning in minimizing the pollution from coal
combustion are as follows.
1. Specific Gravity. The specific gravity of coal varies from
1.2 to 1.7 as compared to 2.0 to 2.6 for non-combustible ash
and 4.9 to 5.0 for pyritic sulfur. Differences in specific
gravity provide the basis for most separation processes.
2. Washability. Coal washability is a measure of the degree to
which pyritic sulfur and other non-coal minerals can be
liberated, isolated from the coal, and removed using methods
which discriminate between materials with different specific
gravity. Only the sulfur contained in pyrite may be rejected
from the ROM coal by PCC processes. The relative amount rejected
from any coal seam varies widely with the nature of the coal and
the processing technology employed.
3. Surface Properties. Differences in the hydrophobic or
hydrophilic character of coal and ash particles influence
the effectiveness of froth flotation for separation of fine
coal from fine ash.
4. Grindability. This characteristic is a measure of the ease
with which size reduction can be accomplished; it varies widely
among coals and among components in a given coal.
5. Friability. The tendency of coal to degrade in size upon
handling is termed friability.
6. Weatherability. The resistance of coal to disintegration or
"slacking" on exposure to weather is called weatherability.
Weatherability is an important factor in coal storage.
34
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7. Trace Metal Content. Many potentially hazardous trace metals
are present in mineral forms which tend to be removed by PCC.
Properties of great interest from the standpoint of environmental protection
are trace element content, leachability, and washability. These properties
have all received a substantial amount of study but greatest emphasis has
been centered on washability.
3.2.1 Washability
The previously-mentioned studies of washability by the U.S. EPA and the
U.S. Bureau of Mines have been combined with data from the U.S. Bureau of
Mines on the coal reserves of the U.S., using Battelle's Reserve Processing
Assessment Model (RPAM).O) This model computes, by region, the cleanability
of the coal reserves for different coal cleaning processes. Major results
from RPAM are as follows.
1. Thirty-seven percent, by weight, of the U.S. coal reserves
(excluding Pennsylvania anthracite and Alaskan coals,
for lack of appropriate data) meet an SO- emission standard
of 1.2 lb/10 Btu without any cleaning prior to combustion.
2. If the reserves that do not meet an S02 emission standard
g *
of 1.2 lb/10 Btu are cleaned by physical separation at
1.6 specific gravity after crushing to 1-1/2-inch top size,
then an additional 9 percent of the reserves will neet the
standard, giving 46 percent in total.
3. Coal from the western region is of low sulfur content, and
71 percent of the raw coal meets an SC^ emission standard
of 1.2 lb/10 Btu. If the remaining coal is separated at
1.6 specific gravity after crushing to 1-1/2-inch top size,
then, in total, 86 percent of the coal meets the standard.
4. Northern Appalachian, Eastern Midwest, and Western Midwest
coals are high in sulfur, and only 6, 1, and 6 percent,
respectively, of the raw reserves in the regions would
meet an SO, emission standard of 1.2 lb/10 Btu. Pyritic
sulfur has limited liberation potential in the coal, and,
even after crushing to 1-1/2-inch top size and separating
at 1.6 specific gravity, the percentages of the reserves
35
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that will meet the standard overall are only 12, 3, and 7
percent, respectively.
5. Southern Appalachian coal is low in sulfur, and 53 percent
of the raw coal meets an SC^ emission standard of 1.2 lb/10
Btu. After cleaning at 1.6 specific gravity and 1-1/2-inch
top size, 63 percent of the reserves meet the standard.
6. Twenty-seven percent of the reserves in Alabama meet an
S02 emission standard of 1.2 lb/10 Btu without cleaning
while 36 percent meet the standard after cleaning at 1.6
specific gravity and 1-1/2-inch top size.
From these results, it is seen that for many coal reserves, physical coal
cleaning at 1.6 specific gravity will produce low sulfur levels.
However, in meeting the New Stationary Sources Performance Standards
(NSSPS) (promulgated June 11, 1972) which require a 90 percent sulfur
removal between 1.2 and 0.6 Ib SO-/10 Btu and a 70 percent removal below
6
0.6 Ib SO /10 Btu, physical coal cleaning essentially will be eliminated
as a single control technology for compliance. For meeting this regulation,
physical coal cleaning must be supplemented with flue gas desulfurization.
Also, it should be noted that some of the reserves of low-sulfur coal in the
eastern U.S. are dedicated to use as metallurgical coal, and they should
not be assumed to be available for combustion.
3.2.2 Trace Element Content
The trace element content for 101 coals is shown in Table 13. While it
is not known if utilization of coal results in serious environmental impacts
when these materials are liberated to the environment, concentrations are
such that potentially harmful effects must be considered. Removal of signif-
icant quantities of trace elements using FCC prior to combustion may prove
to be a substantial benefit. This thought is duscussed further in Section 3.3.
3.2.3 teachability
The teachability of coal is a poorly understood function of its
chemical composition. The presence of oxidizable sulfur compounds which
36
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TABLE 13. TRACE ELEMENTS IN 101 COALS
(16)
Constituent
As
B
Be
Br
Cd
Co
Cr
Cu
F
Ga
Ge
Hg
Mn
Mo
Ni
P
Pb
Sb
Se
Sn
V
Zn
Zr
Al
Ca
Cl
Fe
K
Mg
Na
SI
Ti
Mean
Value
14.02
102.21
1.61
15.42
2.52
9.57
13.75
15.16
60.94
3.12
6.59
0.20
49.40
7.54
21.07
71.10
34.78
1.26
2.08
4.79
32.71
272.29
72.46
1.29
0.77
0.14
1.92
0.16
0.05
0.05
2.49
0.07
rt.,.1*. Standard
Unit Deviation
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
PPM
Percent
Percent
Percent
Percent
Percent
Percent
Percent
Percent
Percent
17.70
54.65
0.82
5.92
7.60
7.26
7.26
8.12
20.99
1.06
6.71
0.20
40.15
5.96
12.35
72.81
43.69
1.32
1.10
6.15
12.03
694.23
57.76
0.45
0.55
0.14
0.79
0.06
0.04
0.04
0.80
0.02
Minimum
Value
0.50
5.00
0.20
4.00
0.10
1.00
4.00
5.00
25.00
1.10
1.00
0.02
6.00
1.00
3.00
5.00
4.00
0.20
0.45
1.00
11.00
6.00
8.00
0.43
0.05
0.01
0.34
0.02
0.01
0.00
0.58
0.02
Maximum
Value
93.00
224.00
4.00
52.00
65.00
43.00
54.00
61.00
L43.00
7.50
43.00
1.60
181.00
30.00
80.00
400.00
218.00
8.90
7.70
51.00
78.00
5350.00
133.00
3.04
2.67
0.54
4.32
0.43
0.25
0.20
6.09
0.15
37
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can form acidic runoff when exposed to the elements is a factor of known
importance. The presence of compounds containing potentially hazardous
metals such as arsenic, beryllium, and selenium, which can react with
acidic runoff and pollute surface or underground water, is also a matter
of concern as far as adverse effects of leaching are concerned. Other
soluble materials can contribute to runoff high in dissolved solids, and
finely divided insoluble materials can be leached out producing water
polluted with suspended solids.
3.3 Products
As indicated earlier, coals are physically cleaned at any of three levels
of intensity. Each type of cleaning can be considered to yield a different
product which has been given minimum cleaning, intermediate cleaning, or
maximum cleaning. The primary uses of cleaned coals are for boiler firing and
as metallurgical coals. In the past, only metallurgical coals were given
maximum cleaning. Boiler fuels (steam coals) were given minimum or inter-
mediate level cleaning. At present, two factors have changed, thereby providing
new incentives for the use of FCC for steam coals. Coal prices increased in
the 1969 to 1974 period with steam coal cost tripling and that for metallur-
gical coals doubling. This increase in value of the coal provides additional
justification for coal cleaning facilities which will upgrade coal quality and
increase product yield. In addition, the growing pressure to minimize sulfur
oxide pollution gives impetus to the application of PCC to production of steam
coals. As a result, most new PCC plants probably will be designed for maximum
reduction of ash and sulfur consistent with cost constraints and high Btu recovery.
For coals that are "washable", significant quantities of the potential
pollutants can be removed in more concentrated form. As much as 25 to 30 percent
of the raw coal feed will be rejected as refuse by PCC svstems. The sulfur
content of the refuse might be 10 to 15 percent and trace elements generally
will have larger concentrations in the rejected material than in the clean
coal. The effect of float-sink separation on trace elements for selected
Illinois coals is illustrated in Table 14, which shows "concentration factors"
for clean coal and refuse where concentration factor for an element is defined
as:
38
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Concentration Factor for _ Concentration in Clean Coal
Element in Clean Coal Concentration in Raw Coal
Concentration Factor for _ Concentrationin Refuse
Element in Refuse ~ Concentration in Raw Coal
From these data, it is apparent that, on the whole, trace elements generally
will tend to concentrate heavily in the refuse.
TABLE 14. DISTRIBUTION OF ELEMENTS IN FLOAT-SINK SEPARATION
OF ILLINOIS COALS AT VARYING SPECIFIC GRAVITY TO
ACHIEVE 75 PERCENT WEIGHT RECOVERY(16>
Element
Al
Ca
Fe
K
Mg
Na
P
S
Si
Ti
As
B
Be
Cd
Co
Cr
Cu
Ga
Ge
Hg
Mn
Mo
Ni
Pb
Sb
Se
V
Zn
Zr
ppm
Raw Coal
- 2.3 %
0.38%
2.9 %
0.28%
81.2
400.
20.9
4.4 %
4.2 %
1300.
11.5
28.7
3.0
2.0
5.8
14.0
29.1
3.0
6.7
0.28
69.6
11.5
30.5
110.
0.57
2.8
15.2
510.
3.6
Concentration
unless otherwise
Clean Coal
0.5 %
0.07%
0.80%
0.09%
34.5
200.
19.2
1.6 %
0.9 %
450.
1.5
31.7
2.9
0.2
3.0
8.7
16.2
2.7
8.1
0.07
7.4
3.8
19.6
22.
0.34
1.3
9.1
12.0
1.9
»
stated
Refuse
7.8 %
1.32%
9.3 %
0.87%
215.1
1000.
26.2
12.9 %
14.2 %
3900.
41.0
19.8
3.3
7.2
14.2
29.9
68.1
4.1
2.3
0.92
258.
34.8
63.5
.377.
0.87
7.3
33.4
2019.
8.8
Concentration
Clean Coal
0.22
0.19
0.28
0.32
0.42
0.50
0.92
0.36
0.21
0.35
0.13
1.10
0.97
0.10
0.52
0.62
0.56
0.90
1.21
0.25
0.11
0.33
0.64
0.20
0.60
0.46
0.60
0.02
0.53
Factor
Refuse
3.39
3.47
3.21
3.11
2.65
2.50
1.25
2.93
3.38
3.00
3.57
0.69
1.10
3.60
2.45
2.14
2.34
1.37
0.34
3.29
3.71
3.03
2.08
3.43
1.53
2.61
3.67
3.96
2.44
39
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4.0 ENVIRONMENTAL IMPACTS
Coal is inherently a "dirty" source of energy. Raw coal contains non-
coal minerals that will be released as polluting discharges at some point
in the extraction and use cycle unless precautions are taken to collect and
dispose of them in a controlled way. Pyrite is probably the most pervasive
source of pollution. It can contribute to acid mine drainage in extraction
operations, sulfur oxide air emissions from ignited coal refuse piles, acid
drainage with leaching of potentially toxic materials from coal refuse or
coal storage piles, and emissions of sulfur compounds from utilization
operations. Also, trace elements are, as indicated earlier, present in
amounts that might pollute if they are discharged from coal processing systems
under the wrong conditions. The trace constituents of major concern include
arsenic, beryllium, cadmium, lead, manganese, mercury, and selenium, as well
as radioactive materials. The extent to which humans and the environment
could be exposed to potentially hazardous concentrations of such materials
where coal is being consumed has not been established. But with coal likely
to be consumed in steadily increasing quantities, all possibilities of harmful
environmental impacts must be considered.
while removing pollutants before the coal is used is an effective way
to contain them, the greater concentrations in the refuse require that it
be disposed of in ways that will not allow leaching by rainwater or surface
flows, lead to spontaneous combustion, or lead to future collapse of unstable
waste piles.
Potential pollutants which are not removed prior to combustion will exit
from boilers in the slag or ash or as particulates and/or gaseous discharges
into the atmosphere. Air discharges can be minimized using control equipment
in some situations. For large boilers, scrubbers can be employed for sulfur
oxide and particulate control. For control of particulates only, electro-
static precipitators are generally used. Where particulates are controlled,
most of the trace metals which would otherwise be discharged to the atmosphere
are collected and can be disposed of as solid waste.
40
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Table 15 gives an indication of the typical fractions of elements in
feed coal which are released to the environment by combustion in boilers
equipped with electrostatic precipitators. If cleaned coal is burned, the
fractions of elements in the raw coal released to the atmosphere generally
are further reduced, as indicated by Table 14. Efficient wet scrubbers
may reduce these levels even further.
TABLE 15. TYPICAL COAL TRACE ELEMENT FRACTIONS
RELEASED BY COMBUSTION PROCESSC17)
Element
Al
As
Be
Ca
Cd
Co
Cr
Cu
Fe
Hg
Mg
Mn
Ni
Pb
Sb
Ti
V
Zn
Fraction
in
Slag
Ash
0.293
0.109
0.000
0.322
0.067
0.216
0.256
0.072
0.311
0.007
0.310
0.263
0.159
0.041
0.040
0.243
0.275
0.040
Fraction in
Gaseous and
Particulate
Combustion
Products
0.707
0.891
1.000
0.678
0.933
0.784
0.744
0.928
0.689
0.993
0.690
0.737
0.841
0.959
0.960
0.757
0.725
0.960
Electrostatic
Precipitator
Removal Effi-
ciency, Fraction
0.996
0.981
0.000
0.994
0.970
0.992
0.986
1.000
0.994
0.116
0.997
0.993
1.000
0.964
0.979
0.992
0.988
0.981
Fraction
Emitted
to Air
0.003
0.017
1.000
0.004
0.028
0.006
0.010
0.000
0.004
0.878
0.002
0.005
0.000
0.035
0.020
0.006
0.009
0.018
While collection of pollutants after combustion is possible, but costly,
control equipment which is economically feasible for large boilers is not
applicable to small boilers, as discussed in Section 2.3. For this portion of
the total boiler population, PCC may be the only practical near-term method
for pollution reduction.
41
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While PCC is effective in reduction of pollution, all operations asso-
ciated with PCC systems produce some form of potential pollution. All are,
however, amenable to effective control.
Unfortunately, undesirable practices in the past have resulted in serious
environmental damage. Examples include the Buffalo Creek disaster, which
resulted from a coal-waste embankment failure in 1972; many burning refuse
piles, which are continuing sources of air pollution; and many streams which
are polluted with runoff from abandoned refuse piles. At present, about 100
(18
million tons per year of refuse are being generated by coal preparation plants.
This figure will continue to grow as more coal is used and the percent which
is cleaned increases. It is, therefore, important to employ good refuse
management and pollution control technology.
The specific discharges and methods for control are as follows.
4.1 Coal Pretreatment Operations
Fugitive dust emissions from size reduction, dry screening, and coal
handling are potential pollutants from coal pretreatment operations. Dust
control measures that can be employed are wet scrubbers, and enclosure and
hooding of equipment with exhaust to fabric (bag) filters. The use of
chemical surfactants (polymers and hydrocarbons) are being used with increasing
frequency to minimize dust release. Contaminated water is generated by wet
separation processes and is passed on to coal cleaning processes where it
is discharged. Hence, the separation processes usually are not a source of
waste water per se. The exception to this is the desliming process, which
generates a wastewater stream containing dissolved and suspended solids.
This material is sent to water treatment facilities along with other aqueous
discharges.
4.2 Coal Cleaning Operations
The coal cleaning operations generate coarse solid waste when coarse coal
is cleaned in jigs or dense medium vessels. Finer waste, which is rejected
by air tables, wet concentration tables, dense medium cyclones, hydrocyclones,
or froth flotation units, may be combined with coarse waste for disposal or
disposed of separately. These materials, containing substantial amounts of
42
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finely divided mineral matter, are a potential source of contaminated leachate
or fugitive dust emissions unless disposal areas are well designed. Also, the
disposal sites must be designed to provide structural integrity and prevent
spontaneous combustion of coal residues.
Where air tables are used for coal cleaning, off-gases amounting to an
estimated 240,000 cfm would be generated in a 1000 ton/hr plant. For control
of dust emissions, a dry bag collector or a high-efficiency wet scrubber
preceded by cyclones for primary dust collection would be employed.
4.3 Product Conditioning Operation
Dewatering and drying of coal produces large amounts of water containing
dissolved solids and suspended particulate matter, and, where thermal drying
is used, potential air pollutants are generated.
The composition of the dissolved solids and suspended matter will reflect
the character of the non-coal minerals present in the raw coal feed but they
often will tend to be acidic and will contain trace metals.
Because large amounts of water are required, any modern coal cleaning
plant recycles process water. A 1962 estimate placed the total annual water
requirement for coal preparation at 170 billion gallons which included 81
(19)
percent recirculated water and 19 percent fresh water. When the increases
in total water requirements for coal preparation since 1962 and the projected
increases in the years ahead are considered, it becomes apparent that water
management for coal cleaning plants is a very important economic factor.
Water for recycling should contain less than 5 percent solid matter and
have a neutral pH. Removal of suspended solids by agglomeration and sedimenta-
tion is necessary to separate slimes from the water circuit, as slimes can
accumulate when water is recycled. The preferred practices for solids removal
involve the use of cyclones and/or thickeners for partial dewatering. The
underflow from these devices, which may contain 40 percent or more water,
is usually sent to settling ponds. Where space is unavailable for settling
ponds, dewatering equipment (centrifuges or vacuum filters) may be used to
process the slime. The dewatered fine refuse is then disposed of with the
coarse solid waste.
Thermal dryers are the greatest source of potential air pollution in
a coal cleaning plant. The drying involves contacting combustion gases,
43
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usually from the burning of coal, with fine wet coal to evaporate excess
water. Particulates entrained in the exit gases may amount to 15 to 25
pounds per ton of dried coal. The combustion gases also contain the usual
pollutants associated with combustion. Wet scrubbers are used for particulate
removal. Where high sulfur fuels are used as a heat source, scrubbing of
sulfur oxides may be needed. For a 1000 ton/hr plant, the thermal dryer
off-gases amount to an estimated 300,000 cfm. High efficiency wet
scrubbers, preceded by cyclones, are used for control of particulate emissions
from thermal dryers.
4.4 Auxiliary and Ultimate Disposal Operations
The auxiliary and ultimate disposal processes for PCC plants are (1)
coal handling and storage, (2) water treatment, and (3) solid waste disposal.
These are of great importance because of the very large amounts of materials
which are involved. At present, almost 300 million tons per year of coal are
processed by PCC plants. This is a materials handling problem of major propor-
tions. Water use runs to billions of gallons per year, an amount comparable
to that used by the utilities and other major industries consuming water.
Solid waste is produced in amounts approaching 100 million tons per year at
present levels of application of PCC. This is an amount comparable to that
from utilities and other major sources of solid waste. All of these activities
involve demonstrated potential for environmental impact if precautions are
not taken. The control options are discussed below.
4.4.1 Coal Handling and Storage
The principal waste stream from coal handling is fugitive coal dust.
Waste streams from the storage of coal are fugitive coal dust and precipitation
runoff and leachate.
Long, open conveyor belts carrying dry coal can be a significant source
of dust and all drop points onto or off conveyors are sources of dust emissions.
Fugitive dust can be suppressed by water spraying and dust proofing.
Outdoor coal piles have very large surface areas, and coal residence times
in them are relatively long so that rainwater has a chance to react and form
acids or extract sulfur compounds and soluble metal ions. Coal pile leachate
44
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is generally similar to acid mine drainage. The quantity of coal pile leachate
is highly variable, both in an absolute sense and with time, depending upon the
topography and drainage area of the coal pile site, the configuration and the
volume of the stock pile, and the type and intensity of precipitation.
The quality of leachate also varies with time and is dependent upon the
factors listed above plus the coal type and composition, the particle size,
and the reaction time which varies according to precipitation conditions. The
pyrite content of the coal is particularly important in determining the amount
of metal sulfates and sulfuric acid produced. The sulfuric acid dissolves
many other complex sulfides and metal salts, releasing many metals as ions,
including aluminum, zinc, copper, cadmium, beryllium, nickel, chromium,
vanadium, silver, and lead.
Because of the site dependence of coal pile drainage, it is extremely
difficult to generalize the emission characteristics from coal storage piles.
Table 16 tabulates the composition of drainage from coal piles at 11 steam
electric power generating plants. The national coal pile drainage volume is
reported to be approximately 7.9 x 10 gallons (30 x 10 m ) per year, based
on average rainfall rates and total coal storage of 93 million tons.
Dust control methods for coal handling or open storage include spraying
with water or surfactants; dust-proofing by treating the surface with oil
or calcium chloride; and providing wind screens. Float dust from loading
operations can be minimized by use of a telescoping chute.
The area around preparation plants should be properly designed with
diversion and drainage ways through proper slopes and collection sumps. These
waters must be collected for treatment and not be permitted to escape to the
environment.
Runoff from storage areas should be collected and sent to a retention
pond to settle out solids or to a water treatment facility. Lime can be
used in either case to neutralize acidity.
4.4.2 Water Treatment
Process and scrubbing water effluents from coal cleaning operations
contain two types of pollutants: suspended materials (solid or liquid) and
dissolved substances. The technology available for removing suspended
materials from the water includes mechanical dewatering, sedimentation, and
45
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TABLE 16. COMPOSITION OF DRAINAGE FROM COAL PILES AT . .
ELEVEN STEAM ELECTRIC POWER GENERATING PLANTS ( '
Parameter
Alkalinity (as CaC03)
BOD
COD
Total solids
Total suspended solids
Total dissolved solids
Ammonia
Nitrate
Phosphorus
Turbidity
Acidity (as CaCO_)
Total hardness (as CaCO )
Sulfate
Chloride
Aluminum
Chromium
Copper
Iron
Magnesium
Zinc
Sodium
PH
Concentration range.
0 -
3 -
85 -
1,330 -
22 -
247 -
0.4 -
0.3 -
0.2 -
2.8 -
0 -
130 -
133 -
3.6 -
825 -
0.1 -
1.6 -
0.1 -
89 -
0 -
160 -
2.1 -
, mg/l(a
82
10
1,099
45,000
3,302
44,050
1.8
2.3
1.2
505
27,810
1,850
21,920
481
1,200
15.7
3.4
93,000
174
23
1,260
7.8
(a) Appropriate for all values exceot pH.
46
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flotation. Dissolved substances can be removed from water or converted to
less objectionable forms by neutralization, adsorption, ion exchange, reverse
osmosis, freezing, or biological treatment. Table 17 lists the methodology
currently in use or contemplated for use in treating coal cleaning wastewater.
TABLE 17. CLASSIFICATION OF WATER TREATMENT
TECHNOLOGIES USED IN COAL CLEANING
Control of Control of
Suspended Materials Dissolved Materials
Mechanical Dewatering Neutralization
Centrifuges
Filters
Sedimentation
Settling Ponds
Sedimentation Tanks (Thickeners)
Flocculation
4.4.3 Solid Waste Disposal
The disposal of coal cleaning plant waste is a worldwide problem of
increasing magnitude. Over 3 billion tons of solid waste have accumulated
in the United States, and the total number of active and abandoned coal waste
(22)
dumps is estimated to be between 3000 and 5000.v About one-half of these
dumps pose some type of health, environmental, or safety problem.
In 1974, some 290 million tons of coal were cleaned by mechanical means,
(18)
resulting in an estimated 96 million tons of coal preparation refuse.
Although the refuse varies physically and chemically, depending upon coal
source, preparation process, and other factors, the refuse generally contains
waste coal, slate, carbonaceous and pyritic shales, clay, and other impurities
associated with a coal seam.
Depending upon the degree of size reduction employed in the coal cleaning
process, refuse may be coarse (+28 mesh) or fine (-28 mesh). Coarse refuse
may be generated dry or, if generated in a wet process, it drains fairly
47
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readily. Coarse refuse is usually disposed of in an embankment or landfill.
Fine refuse is generated by wet processes and is generally removed from the
preparation plant as a thickener underflow and impounded into nearby settling
ponds. In many cases, the settling pond embankment is constructed with
coarse refuse.
Since the 1972 Buffalo Creek disaster at Saunders, West Virginia, in
which a coal waste embankment failure caused heavy losses in lives and property,
greater emphasis has been placed on stability of coal waste embankments. This
consideration, as well as Federal water pollution and solid waste regulations,
now are causing most preparation plants to employ dewatering methods for the
fine refuse and to dispose of the dewatered fine refuse along with the coarse
refuse in landfills or non-impounding embankments.
In addition to acids in eastern and interior regions, the drainage from
coal refuse dumps contains high concentrations of Fe, Al, Ca, Mg, and SO,
ions. The concentrations of total dissolved species are up to 5 percent
(weight) in the highly acidic solutions. Another potential class of water
pollutants is trace or minor elements in coal refuse. Nearly every naturally
occurring element is likely to be present in coal refuse, and concentrations
of most elements of potential environmental concern are higher in the refuse
than in the raw coal, as shown in Table 9. Some of those elements are carried
into the environment by the aqueous leaching of refuse.
The chemical characteristics of refuse pile drainage are highly variable
depending upon local and regional geology of the coal and associated mineral
matter. Depending on meteorological and hydrologic conditions, the volume
of drainage water can vary from zero to millions of cubic meters per day.
Therefore, the emission characteristics of waste disposal areas must be defined
for each specific site.
In contrast to the highly acidic nature of drainage from coal fields
in eastern and interior regions, the runoff from western coal refuse may be
slightly alkaline and typically is rather high in dissolved solids. This
aspect may create problems in semi-arid or arid regions by affecting surface
and/or groundwaters. However, the annual precipitation in western coal
fields is generally so low that the chances of significant drainage of water
through those waste materials are remote.
Current research in waste disposal has been aimed at finding means of
utilizing the coal, pyrite, and other mineral values in the wastes in addition
to the improvement of disposal practices.
48
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5.0 REFERENCES
(1) Cavallaro, J. A., Johnston, M. T., and Deurbrouck, A. W., "Sulfur
Reduction Potential of the Coals of the United States", U.S. Bureau
of Mines, RI 8118 (1976).
(2) Rosenberg, H. S., and Choi, P.S.K., "Energy Aspects of FGD and Stack
Gas Reheat", Battelle's Columbus Laboratories, Paper No. 30d, 84th
National AIChE Meeting, Atlanta, Georgia (February 26-March 1, 1978),
p 31.
(3) Hoffman-Muntner Corporation, "Engineering/Economic Analysis of Coal
Preparation with SC>2 Cleanup Processes", EPA-600/7-78-002, report
prepared for Coal Preparation and Analysis Laboratory, U.S. DoE,
Pittsburgh, Pennsylvania (January, 1978).
(4) Isaacs, G., et al., "Cost Benefits Associated with the Use of Physi-
cally Cleaned Coal", prepared for U.S. EPA, by PEDCo Environmental
(October, 1979).
(5) Holt, E. C., Jr., "An Engineering/Economic Analysis of Coal Preparation
Plant Operation and Cost", final report to U.S. DoE, Solid Fuels Mining
and Preparation, by the The Hoffman-Muntner Corporation, Contract No.
ET-75-C-01-0925, Silver Spring, Maryland (February, 1978).
(6) McGraw, R. W., and Janik, G., "MCCS—Implementation at Homer City",
Proceedings of the Third Symposium on Coal Preparation, NCA/BCR Coal
Conference, Louisville, Kentucky (October, 1977), pp 107-122.
(7) Phillips, P. J., and DeRienzo, P. P., "Steam Coal Preparation Economics",
NCA/BCR Second Symposium on Coal Preparation, Louisville, Kentucky
(October, 1976), pp 50-63.
(8) Matoney, J. P., Moreland, C., Sehgal, R. S., and Thurman, A. L.,
"Physical Coal Preparation", prepared for Electric Power Research
Institute by Kaiser Engineers, Division of Kaiser Industries Corporation,
Oakland, California (May, 1977), pp 3-6.
(9) Hall, E. H., Lemmon, A. W., Jr., Robinson, G. L., Goodman, F. K.,
McCreery, J. H., Thomas, R. E., and Smith, P., "The Use of Coal
Cleaning for Compliance with S02 Emission Regulations", draft report
from Battelle's Columbus Laboratories to U.S. EPA (August, 1979).
49
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(10) Phillips, P. J., and Cole, R. M., "Economic Penalties Attributable to
Ash Content of Steam Coals", Coal Utilization Symposium, AIME Annual
Meeting, New Orleans, Louisiana (February, 1979).
(11) Buder, M. K., Clifford, K. L., Huettenhain, H., and McGowin, C. R.,
"The Effects of Coal Cleaning on Power Generation Economics", paper
presented at the American Power Conference, Chicago, Illinois (April
23-25, 1979).
(12) Kilgroe, J. D., "Combined Coal Cleaning and FGD", unpublished manuscript,
U.S. Environmental Protection Agency, Industrial Environmental Research
Laboratory, Research Triangle Park, N.C. (1979).
(13) Spaite, P. W., and Devitt, T. W., "Overview of Pollution from Combustion
of Fossil Fuels in Boilers of the United States", U.S. EPA, Contract
No. 68-02-2603, PEDCo Environmental, Cincinnati, Ohio (October, 1979).
(14) U.S. Department of Commerce, Bureau of the Census, Statistical Abstract
of the United States. 99th Edition, Washington, D.C. (1978), p 607.
(15) Deurbrouck, A. W., and Jacobson, P. S., "Coal Cleaning, State of the Art",
Coal Utilization Symposium - S02 Emission Control, National Coal
Conference, Louisville, Kentucky (October, 1974).
(16) Ruch, R. R., Gluskoter, H. J., and Shipp, N. F., "Occurrence and
Distribution of Potentially Volatile Trace Elements in Coal", Illinois
State Geological Survey, Urbana, Illinois, Environ. Geol. Notes.
No. 61 (April, 1971), No. 72 (August, 1974).
(17) Klein, D. H., Andren, A. W., and Bolton, N. E., "Trace Element Discharges
from Coal Combustion for Power Production", Water, Air, and^ Soil Pollution.
1, 71-77 (1975).
(18) Anderson, J. C., "Coal Waste Disposal to Eliminate Tailings Ponds",
Min. Cong. J.. £1, 42-45 (1975).
(19) Coal Preparation, edited by J. W. Leonard and D. R. Mitchell, 3rd Edition,
AIME, New York (1968), "Plant Waste Contaminants" (J. R. Lucas, D. Maneval,
and W. E. Foreman), Chapter 17.
(20) Walling, J. C., "Air Pollution Control Systems for Thermal Dryers",
Coal Age. 74 (9), 74-79 (1969).
(21) U.S. Environmental Protection Agency, "Development Document for Proposed
Effluent Limitations Guidelines and New Source Performance Standards
for the Steam Electric Power Generating Point Source Category", EPA
440/1-73-029, Washington, D.C. (March, 1974), p 132.
(22) National Academy of Sciences, "Underground Disposal of Coal Mine
Wastes", report to the National Science Foundation, Washington, D.C.
(1975).
50
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-------
APPENDIX B
NOMENCLATURE DEFINITIONS FOR ENERGY TECHNOLOGIES
Accidental Discharge - An accidental discharge is an abnormal discharge
(solid, liquid, gaseous, or combinations) which occur as a result of
upset process conditions.
Auxiliary Process - An auxiliary process is a process associated with a
technology, incidental to the main functions transforming raw materials
into end-products. Auxiliary processes are used to recover by-products
from waste streams, to furnish necessary utilities, and to furnish feed
materials for producing the end-product. Examples: some auxiliary
processes for physical coal cleaning include (a) water neutralization
for water treatment and (b) solid waste disposal.
By-product - A by-product is an output stream produced from waste streams
and marketed or consumed in the form in which it exits the process.
Control Equipment - Control equipment has the primary function of minimizing
the pollution to air, water, or land, from process discharges. While the
collected materials may be sold, recycled, or sent to final disposal, control
equipment is not essential to the economic viability of the process. Where
such equipment is designed to be an integral part of the process, e.g.,
scrubbers which recycle process streams, they are considered a part of the
basic process. Examples: electrostatic precipitators, wet scrubbers, and
adsorption systems.
Effluent Stream - An effluent stream is a confined liquid waste stream
(discharged from a source) which is potentially polluting.
Emission Stream - An emission stream is a confined gaseous waste stream
(discharged from a source) which is potentially polluting.
Energy Technology - An energy technology is made up of systems which are
applicable to the production of fuel, electricity, or chemical feedstocks
from fossil fuels, radioactive materials, or natural energy sources
(geothermal or solar). A technology may be applicable to extraction of
fuel, e.g., underground gasification; or processing of fuel, e.g., low-Btu
gasification, light water reactor, and conventional boilers with flue gas
desulfurization.
Final Disposal Process - A final disposal process is used to ultimately
dispose of liquid and solid wastes from processes, auxiliary processes,
and control equipment employed in a technology. Examples: landfills
and evaporation ponds.
Fugitive Effluent - A fugitive effluent is an unconfined discharge (including
accidental discharges) of potential water pollutants which are released as
leaks, spills, washing waste, etc., or as effluents in abnormal amounts
when accidents occur. These may be associated with storage, processing
or transport of materials as well as unit operations associated with
industrial processes. They may be disposed of in municipal sewers and can
lead to generation of contaminated runoff waters. They will escape from
a source.
52
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Fugitive Emission - A fugitive emission is an unconfined discharge (including
accidental discharges) of potential air pollutants. These may escape from
pump seals, vents, flanges, etc., or as emissions in abnormal amounts when
accidents occur. These may be associated with storage, processing, or
transport of materials as well as unit operations associated with a process.
They will escape from a source.
Input Stream - An input stream is a material that must be supplied to a
process for performance of its intended function. Input streams include
primary and secondary raw materials, streams from other processes, chemical
additives, etc. For auxiliary processes, a waste stream from which a by-
product is recovered is an input stream. Example: the input streams to
a flotation cell are the sized coal, water, air, and chemical flotation
agent.
Operation - An operation is a specific function consisting of a set of processes
that are used to produce specific products from specific raw materials.
Example: the operations for physical coal cleaning are coal pretreatment,
coal cleaning, and product conditioning. The processes used in each of
these operations are:
Coal pretreatment operation - size reduction, screening, desliming,
and fine coal separation.
Coal cleaning - separation of coal from waste by use of jigs, dense
medium vessels, air tables, wet concentrating tables, dense medium
cyclones, hydrocyclones, and froth flotation units.
Product conditioning operation - mechanical dewatering, medium plus
fine coal dewatering, fine coal dewatering, and thermal drying.
Output Stream - An output stream is a confined discharge from a process.
Output streams can be products, waste streams, streams to other processes,
or by-products. Examples: output streams from a physical coal cleaning
plant include waste water, solid wastes, and cleaned coal.
Plant - A plant is an existing system used to produce a specific product of
the technology from specific raw materials. A plant is comprised of some
combinations of processes which make up the technology. Example: the
Homer City coal cleaning plant, jointly owned by Pennsylvania Electric
Company and New York State Electric and Gas Corporation, is designed to
reduce the sulfur content in coal burned by the associated electric power
generating station.
Process - Processes are the basic units that make up a technology. A
process is used to transform, chemically or physically, input materials
into specific output streams. Every process has a definable set of
unique waste streams. The term "process" used without modifiers is used
to describe the generic processes. Where the term "process" is modified
(e.g., the Lurgi process), reference is made to a specific process which
falls in some generic class consisting of a set of similar processes.
Examples: a generic process in low/medium-Btu gasification technology
53
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is the fixed-bed/atmospheric/dry ash gasification process. Specific
processes which are included in this generic class are Wellman-Galusha,
Woodall-Duckham/Gas Integrale, Chapman (Wilputte), Riley-Morgan,
Foster Wheeler/Stoic and Wellman-Incandescent.
Process Module - A process module is a method used to display input and output
stream characteristics of a process. A combination of process modules can
be used to describe a technology, a system, or a plant. One example of
the "process module" approach to environmental studies of energy technologies
involved a study of emissions for petroleum refineries. A description of
each basic process of a petroleum refinery was developed, e.g., atmospheric
distillation, catalytic cracking, etc. Data on air emission as a function
of throughput were collected for each process, as part of the description
of each process module. Individual process modules were assembled to describe
plants with process configurations typical of specific areas of the U.S.,
e.g., a refinery in the Southwest which maximized gasoline output and another
in the Northeast which produced more distillate fuel.
Process Streams - Process streams are output streams from one process which
are input streams to another process in the technology. Example: the fine
coal from a fine coal dewatering process is an input stream to the thermal
dryer.
Product - A product is an output stream marketed for use or consumed in the
form in which it exited the process. Example: the cleaned coal is the
product from the physical coal cleaning plant.
Raw Materials - Raw materials are feed materials for processes. They are of
two types: (1) primary raw materials that are used in the chemical form in
which they were taken from the land, water, or air, and (2) secondary raw
materials that are produced by other industries or technologies. Examples:
primary raw materials for PCC technology include coal and water. Secondary
raw materials include neutralizing chemicals, flocculation reagents, flota-
tion reagents, and magnetite.
Residual - A residual is a gaseous, liquid, or solid discharge from control
equipment and final disposal processes. Examples: gaseous emissions from
control equipment, such as scrubbers, and vapors from an evaporation pond.
System - A system is a specified set of processes that can be used to produce
a particular end-product of a technology. A technology is comprised of
several systems. Examples: in the PCC technology, the simplest system
produces cleaned coal using crushing and sizing coupled with jigs or dense-
medium vessels. More complex systems will further clean medium and fine
coal using dense medium cyclones and froth flotation with thermal drying
in addition to the coarse coal cleaning.
Source - A source is a specific piece of equipment which discharges either
confined gaseous, liquid, or solid waste streams or significant quantities
of unconfined, potentially polluting substances in the form of leaks, spills,
and the like. Examples: coal storage and refuse piles are sources for
discharges of windblown dusts and of acidic runoff.
54
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Unit Operation - Unit operations, like processes described above, are
employed to perform a specific physical or chemical transformation on
input materials. The equipment making up a unit operation may have one
or more unit operations which have at least one source of waste stream(s).
Examples: distillation, evaporation, crushing, screening, etc.
Waste Stream - A waste stream is a confined gaseous, liquid, or solid stream
sent to auxiliary processes for recovering by-products, to pollution control
equipment, or to final disposal processes. Unconfined "fugitive" discharges
of gaseous or aqueous waste and accidental process discharges are also
considered waste streams. Example: the water drawn off from the fine
coal dewatering process is a waste stream in PCC technology.
55
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APPENDIX C
GLOSSARY OF TERMS ASSOCIATED WITH
PHYSICAL COAL CLEANING (PCC)
air tables: A particle sorting device utilizing air currents to separate
coal and mineral particles based upon mass differences.
biological treatment: Certain autotrophic bacteria under limited conditions
will enhance the oxidation of metal sulfides - as the dissolution of
pyrite to form coal mine drainage. Processes applying these principles
have been applied to coal mine drainage treatment and have been research
topics for pyrite removal from ROM coals.
centrifuge: A device designed to remove water in a continuous process from
the surface of coal and refuse particles by application of centrifugal
force.
cleanable coal: A coal type which is suitable for sulfur and ash removal by
physical techniques.
coarse coal: A term applied to coal particles whose size is greater than
3/8 inch.
concentration factors for element in clean coal: The ratio of the concentration
of the element in the clean coal to its concentration in the raw coal.
concentration factor for element in refuse: The ratio of the concentration of
the element in the refuse to its concentration in the raw coal.
crushing: A size reduction (comminution) process, applied to coal particles,
whose product top size particles are less than 14 mesh.
cyclones: A type of particle separating device utilizing centrifugal force
in which particles, entrained in fluid (liquid or gases), are separated
on a basis of size and density.
deep cleaned coal: A cleaned coal product that has been subjected to extensive
liberation and separation under conditions designed to provide maximum
economic rejection of minerals and sulfur-bearing components. A very
high quality (grade) of coal.
dense medium vessel: A coarse particle separating system utilizing a
suspension of water and a finely-divided, insoluble dense mineral (usually
magnetite) as the separating medium.
56
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desliming screens: A wet screen sizing system designed to remove particles
smaller than 28 M (0.6 mm) from a coal stream. They usually utilize
parallel rod, profile wire screen media arranged perpendicular to the
stream flow.
dewatering cyclones: A cyclone system designed to direct the bulk of solid
particles in the feed stream to the underflow orifice where pulp
densities may exceed 50 percent by weight.
fabric filter: A medium (synthetic polymers or fiber glass) designed to
remove particles with dimensions as small as 0.1 micron from a gaseous
stream.
fine coal: Coal particles whose dimensions are less than 3/8 inch. Such a
stream may or may not include particles smaller than 28 M (0.6 mm).
flocculation: A process in which existing charges on the surface of particles
are modified (usually with chemical reagents) to allow particles to form
larger-sized agglomerates.
freezing: The change in state from the liquid to the solid phase resulting
from the removal of energy. In stored coal products, coal will solidify
to larger particles when its moisture content exceeds about 5 percent
and temperatures are sustained below 0°C.
friability: Tendency of coal to degrade in sizes is a function of its fri-
ability. The tendency for coal to break during handling increases with
greater friability.
grindability: Characteristic which measures the ease with which size reduction
can be accomplished.
hydrocyclones: A cyclone system applied to fine coal suspended in water
designed to separate particles based upon particle density rather than
particle size.
hydrophilic: A solid, such as certain clays, whose surface properties have
a strong affinity.for water.
hydrophobic: A solid, such as certain coal components, whose surface properties
repel water.
intermediate cleaning effectiveness: A level of cleaning in which the raw
coal is crushed and sized, and the coarse coal is cleaned in jigs or
dense medium vessels while the combined fine and ultrafine coal fractions
are cleaned together using air tables, wet concentrating tables, or dense
medium cyclones.
ion exchange: A separating system designed to either remove or exchange
dissociated ionic species from solution (usually aqueous) by the use of
an insoluble solid medium (as natural zeolites, soils, or synthetic resin
polymers) whose structure has a reversible lattice holding ionic species
by relative weak forces. Used in water purification and demineralization.
57
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jig: A device used to continuously beneficiate coal (and other minerals) by
developing a particle bed (supported by a screen) segregated horizontally
from top to bottom with increasing particle density. The segregation is
accomplished by repeated expansion and contraction of the particle bed by
water velocities moving perpendicular to the bed flow. More coal is
processed in a wider particle size range by this technology than any other.
maximum cleaning effectiveness: A level of cleaning in which the raw coal is
crushed and sized, and the coarse coal is cleaned in jigs or dense medium
vessels. The 3/8-inch x 0 fraction is split into fine and ultrafine
fractions. The medium fraction is cleaned with wet concentrating tables
or dense medium cyclones, and the fine fraction is cleaned with hydro-
cyclones or froth flotation.
mechanical dewatering: The removal of surface water from coal or refuse
particles by any process other than increasing vapor pressure. Typical
systems utilize vibrating screens, centrifugal force, cyclones, thickeners,
filters, etc.
mesh size: An expression of the number of openings per linear inch used in
woven wire screen cloth. See ASTM Designation E-ll-70.
middling: A coal product of intermediate quality between the clean coal and
the refuse products. It may be saleable, rejected, or further processed.
minimum cleaning effectiveness: A level of cleaning in which the raw coal is
crushed and sized, and only the coarse coal is cleaned, using jigs or
dense-medium vessels. The cleaned coarse fraction may or may not be
combined with the uncleaned fine and ultrafine fractions.
neutralization: A chemical process utilized in water quality control in which
excess hydronium, hydroxyl, carbonate, or bicarbonate ions are reacted
with appropriate reagents to result in a water whose pH is near 7.0.
physical coal cleaning: The technologies which remove sulfur and ash from coal
using nonchemical techniques, such as screening, concentrating, and froth
flotation.
recovery, Btu %: The percentage of the heat content present in the ROM feed
to a PCC process that is recovered in the clean coal based on British
thermal units.
reverse osmosis: A water purification process utilizing specially prepared
semi-permeable membranes which constrain the passage of ionic species
across the membrane based upon differences in ionic concentrations and
osmotic pressures of the feed water. The process results in a highly
concentrated brine containing the ionic species (impurities) and a
purified water stream of low conductivity.
sedimentation: A process designed to remove suspended solids from a water
stream by settling under the influence of gravity; as a thickener, clar-
ifier, classifier, or settling pond.
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settling pond: An earthen enclosure developed by forming a flow constriction
(dam) or by excavation to create a static water storage volume having
minimal horizontal velocity, to allow the settling of suspended solids
and create a clarified effluent.
sieve bend: A specific type of stationary dewatering screen containing a
60° arc used to dewater and deslime particulate suspensions of coal,
refuse, and dense media suspensions. It was developed by the Dutch
State Mines in 1950.
sizing: A separation process creating product particles of specified physical
dimensions. May utilize screening or sedimentation principles.
slimes: Particles of coal and coal-minerals in aqueous suspension having
particle sizes less than about 28 mesh (0.6 mm).
sulfur forms: Sulfur in coal is reported in detailed chemical analyses as
organic, pyritic, and sulfate sulfur. Sulfate sulfur usually is of only
minor importance. The ratio of pyritic sulfur to organic sulfur is
important in determining the amount of sulfur removable with physical
coal cleaning.
surface properties: Difference in the hydrophobic or hydrophilic character
of coal and mineral particles which influence the effectiveness of froth
flotation for the separation of fine coal from ash.
thermal dryer: A device used to increase the vapor pressure of surface water
(thus enhance water removal) on coal particles by transferring heat prefer-
entially to the water than to the coal from hot combustion gases.
trace metal content: A metal occurring in coal (associated with either the
organic or mineral components) at concentrations expressed in yg/g (ppm).
Many such metals may be considered toxic to some degree. Such
components associated with the minerals may be rejected by PCC.
ultrafine coal: Coal particles whose dimensions are less than 28 mesh (0.6 mm).
vacuum filtration: A process used to remove surface water from fine coal or
refuse particles by continuously drawing water, from an aqueous coal
suspension, through a fabric (which traps the solid particles) to a
receiving chamber maintained at low pressure by'a vacuum pump. The
coal filter cake typically contains about 25 percent water.
washability: A measure of the degree to which pyritic sulfur and other inor-
ganic minerals can be removed from coal using methods which discriminate
between materials with different densities.
weathering: Tendency of coal to disintegrate or "slack" on exposure to weather.
Weathering is an important factor considered in coal storage.
wet concentrating table: A device used to separate fine coal and mineral
particles based on hindered settling and flowing film concentration. A
rhomboid-shaped, near horizontal deck fitted with surface riffles is fed
with an aqueous suspension of particles at the rate of 12 to 15 tons/hr
while the table is subject to 300 horizontal (3/4-inch) strokes per
minute. The low density particles are discharged from the table side into
a launderer.
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wet screening: A screening system employing spray and feed body water to
separate small particles through apertures.
yield, weight %: The percent weight (mass) of saleable, clean coal product
developed from a process ROM feed.
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APPENDIX D
DESCRIPTION OF PROCESSES FOR
PHYSICAL COAL CLEANING
Coal Pretreatment:
No. 1 - Size Reduction
No. 2 - Sizing
No. 3 - Desliming Screen
No. 4 - Fine Coal Separation
Coal Cleaning:
No. 5 - Jigs
No. 6 - Dense Medium Vessels
No. 7 - Air Tables
No. 8 - Wet Concentrating Tables
No. 9 - Dense Medium Cyclones
No. 10 - Hydrocyclones
No. 11 - Froth Flotation
Product Conditioning:
No. 12 - Mechanical Dewatering
No. 13 - Medium Plus Fine Coal Dewatering
No. 14 - Fine Coal Dewatering
No. 15 - Thermal Dryers
Auxiliary and Ultimate
Disposal Operations:
No. 16 - Coal Handling and Storage
No. 17 - Water Handling
No. 18 - Solid Waste Disposal
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COAL PRETREATMENT
Process No. 1 - Size Reduction
General Information
Raw coal is crushed to sizes suitable for cleaning and shipment to market.
The main purpose is to liberate impurities so that they can be removed in the
coal cleaning operation.
Process Information
Size reduction usually involves stage processing in a series of crushers.
The extent of the size reduction depends on the type of coal being processed
and the desired product characteristics. Raw coal with fines removed is
generally fed first to a roll-type crusher (rotary screen-type breakers may
be employed for rough cleaning) which reduces it to a top size of 3 to 8 inches.
When secondary crushing is used, the coal is reduced to a top size of
1-1/2 to 1-3/4 inches. A third stage, in which crushers further reduce
the top size to 1 to 3/8 inch, may be employed. The three-stage layout
represents the most economical arrangement for a majority of preparation
plants. Between crushing stages, scalping screens are often employed to
remove coarse rocks and other foreign materials.
Waste Streams
Size reduction of dry coal can be a major source of dust generation.
The quantity of dust generated during crushing operations varies depending
upon the friability, size, and surface moisture of coal and type of crusher.
The more friable coal produces larger percentages of fines. As the coal is
crushed to smaller sizes and as moisture decreases, more dust is generated.
Crushing operations are usually carried out in partially enclosed soaces
provided with sub-ambient pressure usin? exhaust hoods and suitable dust
collectors such as cyclones, bag filters, and wet scrubbers.
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COAL PRETREATMENT (continued)
Process No. 2 - Sizing
General Information
Sizing is the process of separating particles of mixed sizes into groups
in which, for each size fraction, all particles range between certain definite
maximum and minimum sizes. There are three general reasons for sizing prior
to coal cleaning operations:
1. To separate raw coal into various sizes for marketing,
2. To feed various cypes of washing units, and
3. To recover fines in the original feed and fines oroduced
by the size reduction process.
Process Information
Sizing of coal particles may be carried out by either wet or dry screens.
Screens may be stationary or moving. The screening surface may be a perforated
plate, a woven wire cloth, formed bars, or nonstationary parallel bars. By
far the most common screens are vibrating perforated plate and square-opening
woven wire screens.
Waste Streams
The major waste stream from sizing operations is fugitive dust generated
from dry screening, but this usually is minimal since ROM coals have increasing
levels of moisture. Dust can be controlled by the same techniques as described
for Process 1. Contaminated water is produced from wet screening, but the
contaminated water streams are generally directed to coal cleaning units and
are not considered waste streams.
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COAL PRETREATMENT (continued)
Process No.3 - Desliming Screen
General Information
The objective in "desliming" is to remove ultrafine coal (less than
28 M) which may interfere with fine coal processing and to collect the slimes
for direct recovery or separate processing. The size of separation may vary
between 28 and 100 M.
Process Information
Vibrating screens, stationary cross flow screens, sieve bends, Vor-Sivs,
etc., are used for slimes removal. Although tonnages vary over a wide range,
as an example for a 1000-ton/hr plant, about 300 tons/hr of 3/8-inch x 0
material may exit from wet screening and be input to desliming screens.
Streams from these screens in this case could be about 270 tons/hr of
cleaned 3/8-inch x 200 M coal and 30 tons/hr of reject material consisting
of wet coal and ash less than 200 M.
Waste Streams
Wastewater containing the rejected slimes and dissolved impurities asso-
ciated with the raw coal is generated. This stream is sent to a thickener
along with other process waters. The overflow water is recycled or treated
and discharged. The underflow slimes generally are reclaimed and processed
by froth flotation or hydrocyclones.
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COAL PRETREATMENT (continued)
Process No. 4 - Ultrafine Coal Separation
General Information
Separation of fine raw coal from ultrafine raw coal permits more efficient
isolation of the coal in both streams. The fine fraction is fed to wet
concentrating tables, dense medium cyclones, or hydrocyclones, and the
ultrafine fraction is fed to hydrocyclones or froth flotation cells.
»
Process Information
Sieve bends followed by wet vibrating screens are used to separate the
3/8-inch x 0 material into 3/8-inch x 28 M and 28 M x 0 fractions.
Waste Streams
The screening process generates contaminated water, but all process
streams are directed to coal cleaning units, and no waste is discharged from
the screening step.
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COAL CLEANING
Process No. 5- Jigs
General Information
Jigs are mechanical devices used for separating materials of different
specific gravities by the pulsation of a stream of liquid flowing through a
bed of the materials. The liquid pulsates up and down, causing the heavy
material to settle to the bottom of the bed and the lighter material to rise
to the top. Following the particle stratification, the particle is physically
"cut" at any desired particle density plane, thus producing the desired quality
products. Jigs are most widely used in coal cleaning. Over 125 million tons
of coal are cleaned by jigs annually in the United States.
Process Information
In coal preparation, jigs are applied to a wide size range of particles
with top sizes up to 8 inches and down to zero. With a very wide size range,
however, the efficiency of separation decreases with a decrease in size.
Effective separation can be obtained in the size range from 4 to 1/4 inch.
Other jig designs utilizing feldspar ragging beds will efficiently clean
coal to much smaller sizes (about 48 mesh). Jigs are simple in operation
and require low capital cost; however, power and water consumption are
high. For a 1000-ton/hr plant, the feed stream to jigs could amount to
about 690 tons/hr. Outputs then would be about 520 tons/hr of clean coal
and 170 tons/hr of refuse.
Waste Streams
Waste streams from the jigging operation are contaminated process
water and solid waste. The solid waste is primarily coarse refuse containing
waste coal and minerals such as silicates, sulfides, and carbonates associated
with the coal seam. The coarse refuse is normally disposed of in waste
heaps or embankments by dumping.
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COAL CLEANING (continued)
The principal pollutant present in process water is suspended solids.
A major portion of process water is recirculated through a suitable water
clarification circuit. Vibrating screens are usually employed for draining
the water from coal larger than 3/8 inch. The drained water mav be directly
recirculated without further treatment. If the solids concentration is -.00
high, thickeners or settling ponds are used to remove fines. In some instances,
treatment with lime is needed to maintain a neutral pH of process water.
Process No. 6 - Dense Medium Vessels
General Information
The application of dense medium separation is a practical extension of the
laboratory float-sink test. The dense medium used is usuallv a water suspen-
sion containing finely ground magnetite (-325 M) to provide the desired suspension
density. Magnetite is used because it can be recovered with magnetic separators.
Besides the magnetite, water solutions of calcium chloride and suspensions of
sand have been used as dense media. The use of magnetite (5.0 specific gravity)
permits practical suspension densities raneine from 1.3 to 2.0 specific gravity.
Dense medium vessels for washing coals are three basic types: cones, drums,
and troughs.
Process Information
Theoretically, any size particle can be treated by dense medium vessels;
practically, however, they are used for cleaning sizes from 6 to 1/6 inch.
The benefits of washing material finer than 1/6 inch are usually offset by the
increased medium loss and reduced cleaning capacity. Laminar-flow, dense
medium separators are efficient on coal coarser than 1/4 inch and coal recovery
is between 95 and 99 percent of the values expected from laboratory float-sink
tests. The magnetite used in the dense medium is recycled through a recovery
circuit which consists of a sump for collection of the dilute medium, classifying
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COAL CLEANING (continued)
cyclones, a magnetite thickener, and a magnetic separator. About 0.5 pound
of magnetite per ton of coal washed is usually lost in the recovery circuit.
For a 1000 ton/hr plant, the feed stream and output tonnages for dense medium
vessels would be approximately the same as those for jigs (Process No. 5).
Waste Streams
The waste streams generated from the dense medium vessels are the same
as those from the jigs (see Process No. 5), except that some of the unrecovered
magnetite ends up as solid waste. (The remainder of the unrecovered magnetite
is part of the clean coal product.)
Process No. 7 - Air Tables
General Information
Air tables are somewhat similar to jigs (Process No. 5), except that,
instead of water acting as the separating medium, a blast of air is driven
through a perforated deck.
Process Information
Air tables have been used for cleaning 3/8-inch x 0 and 1/4-inch x 0
coal. However, their use has declined because of problems with processing
wet coals and the associated dust problems. Close sizing is necessary to
obtain good results. Dry cleaning is difficult when surface moisture reaches
over 5 percent. For a 1000 ton/hr plant, air tables could process 300 tons/hr
of 3/8-inch x 0 material to produce 225 tons/hr of cleaned coal and 75 tons/hr
of refuse.
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COAL CLEANING (continued)
Waste Streams
Waste streams from the air tables are dust-laden air and dry solid waste.
Dust created by the pulsating air is sucked into an overhead hood and is
recovered in a cyclone collector followed by a cloth filter. This dust usually
is recovered, as typically it is a rather high-quality product. The dry solid
waste can be disposed of by dumping in refuse piles.
Process No. 8 - Wet ConcentratingTables
General Information
Tabling is a concentration process whereby a separation between two or
more minerals is effected by vibrating a ribbed, tilted surface. The separation
depends mainly on the difference in specific gravity and to a lesser degree
on the shape and size of the particles.
Process Information
A normal feed size consistency is about 3/8-inch x 0; but with widespread
acceptance of flotation, it is becoming quite common to remove the minus 48-
or 100-mesh material prior to tabling. Essential factors for good table
operation are feed rate, as to volume of both coal and water; slope of the
table; and the frequency and amplitude of the stroke. The water-to-solids
weight ratio normally used on a table is two to one. About 90 percent of the
water reports to the clean coal side of the table. For a 1000-ton/hr plant,
the concentrating tables could process 300 tons/hr of wet 3/8-inch x 0 material
producing 225 tons/hr of product and 75 tons/hr of wet refuse containing 10
tons/hr of drainable water.
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COAL CLEANING (continued)
Waste Streams
Waste streams generated from wet tabling are contaminated process water
and solid waste. The characteristics of the process water are similar to
those of the water from jigs (Process Ko. 3); however, the volume of water
from tables is larger, and it contains a higher concentration of suspended
solids. The solid waste consists of fine refuse which is normally impounded
into a nearby settling pond or disposed of with coarse refuse after dewatering.
Process No. 9 - Dense Medium Cyclones
General Information
Dense medium cyclones employ centrifugal forces to accelerate the separ-
ation of particles of different specific gravities. In the dense medium
cyclone, a mixture of the medium and the raw coal enters taneentially near the
top of the cylindrical section thus forming a vortex. The heavy refuse moves
along the wall of the cyclone and is discharged through the underflow orifice.
The clean coal moves toward the longitudinal axis of the cyclone and passes
through the vortex finder to the overflow chamber.
Process Information
Dense medium cyclones are generally operated at 12 to 14 psi. A medium-
to-coal weight ratio of about 5 to 1 is recommended. The dense medium cyclones
often have been used to treat a comparatively narrow size range, typically
3/8 or 1/4 inch to 1/2 mm. However, current practice is deviating from this
concept. Some units process coals from 1-1/2" to 100 M or even a larger
size range. Average loss of magnetite amounts to one pound/ton of feed coal.
For a 1000-ton/hr plant, a 270-ton/hr feed stream of wet 3/8-inch x 0 material
could be processed by dense medium cyclones to produce 203 tons/hr of clean
coal and 67 tons/hr of refuse containing 5 tons/hr of drainable water.
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COAL CLEANING (continued)
Waste Streams
Waste streams from dense medium cyclones are contaminated process water
and solid waste. The characteristics of waste streams are similar to those
of the wet tables (Process No. 8), except that some lost magnetite ends up
in the waste streams. The contaminated water is sent to a thickener. The
refuse is dewatered and sent to solid waste disposal.
Process No. 10 -Hydrocyclones
General Information
The hydrocyclone does not employ an artificial gravity suspension, but
it utilizes an autogenous dense medium developed from the raw coal being
cleaned. The specific gravity of separation of a hydrocyclone is regulated
by varying the dimensions of the discharge orifices or changing pressure.
Process Information
Hydrocyclones are used to clean flotation-size coal (-48 M) but can be
used for coal as coarse as 1/4 inch to 0. The size classification effect of
hydrocyclones is pronounced; therefore, best results can be obtained with
narrow-sized feed coal. The hydrocyclone can process large tonnages at
relatively low capital investment; however, power and water consumption is
high. For a 1000-ton/hr plant, hydrocyclones could process 100 tons/hr of
28 M x 0 material to produce 75 tons/hr of product and 25 tons/hr of refuse.
Waste Streams
Waste streams from the hydrocyclones are contaminated process water and
solid waste. The characteristics of the waste streams are similar to those
from the dense medium cyclones (Process No. 9), except that no magnetite is
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COAL CLEANING (continued)
present. Contaminated water is sent to a thickener. The overflow water is
recycled or treated and discharged.
Process No. 11 - Froth Flotation
General Information
In froth flotation, separation is effected by the difference in surface
characteristics between particles. This process consists of mixing the
finely divided coal and mineral suspensions with small amounts of reagents
(typically from 0.1 to 1.0 Ib of reagent per ton of coal feed) in the presence
of water and air. The reagents respond with the particle surfaces and with
the water to create conditions conducive to selective attachment of the small
air bubbles to the hydrophobic coal particles and carry them to the surface,
while the hydrophilic mineral matter is wetted by water and drawn off as
tailings. Certain agents in the water modify surface tension to assist in
forming air bubbles of proper stability.
Process Information
Froth flotation is used for both increased recovery and beneficiation
of ultrafine coals usually defined as 28 M x 0. The major factors affecting
coal flotation are particle size, solids concentration, pH value, and the
flotation agents. The optimum size for froth flotation is between 28 and
200 mesh. For coarser sizes, conventional gravity separation methods are
easier and less expensive. Below 200 mesh, separation efficiency decreases
because, for very fine particles, froth flotation becomes increasingly less
selective and consumes excessive amounts of reagent due to large total surface
areas. In current coal flotation practice, the solids concentration varies
from 3 to 10 percent by weight. As a general rule, the coarser the coal
particles, the higher the solids concentration. For a 1000 ton/hr plant
using froth flotation, the input and output tonnages would be similar to those
for hydrocyclones (Process No. 10).
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COAL CLEANING (continued)
Waste Streams
Waste streams from froth flotation are contaminated process water and
solid waste. The characteristics of waste streams are similar to those
from the hydrocyclones, except that these contain reagents used in the froth
flotation process. However, most reagent remains with coal and refuse.
Wastewater is sent to a thickener, and typically the overflow is recycled,
but it may be discharged after treatment. Concentrations of reagents released
to streams are likely to be very small.
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PRODUCT CONDITIONING
Process No. 12 - Mechanical Dewatering
General Information
Following the coarse coal wet cleaning process (jigs or dense medium
vessels), the product coal requires dewatering. Moisture left in the coal
decreases the combustion heat available and also causes shipping and handling
problems. For coarse coal with particle sizes greater than 1/4 inch, the
coal can be dewatered readily by natural drainage using perforated bucket
elevators or dewatering screens. For fine coal, the dewatering is considerably
more difficult and costly. .
Process Information
Dewatering methods commonly used for coarse coal are:
1. Drainage Methods. Natural drainage is rapid for coals
coarser than 1/2 inch when little or no fine coal is
contained therein. Under such conditions, drainage
conveyors or perforated bucket elevators can be used
to combine dewatering and transferring.
2. Dewatering Screens. Vibrating screens can dewater coal
larger than 1/4 inch to the extent required to meet
market requirements. Coarse coal may be sized and
dewatered on the same screen.
Waste Streams
The principal waste stream from mechanical dewatering operations is
contaminated water with suspended and dissolved solids containing ultrafine coal
and other minerals. This stream typically would be discharged to a thickener,
along with other process waters, with the overflow being recycled.
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PRODUCT CONDITIONING (continued)
Process No. 13 - Fine Plus Ulcrafine
Coal Dewatering
General Information
Dewatering of coal which contains slimes (less than 28 mesh) requires a
greater driving force for water removal than is obtained with simple mechan-
ical dewatering even if a substantial amount of coarse material is present.
Product streams requiring such dewatering are generated by wet cleaning of
3/8-inch x 0 or 3/8-inch x 200 M material with wet concentrating tables or
dense medium cyclones.
Process Information
Dewatering of products containing both fine and ultrafine coal is
accomplished with a variety of profile wire dewatering screens in series
with centrifuges. A 1000-ton/hr plant would produce around 225 tons/hr
of cleaned 3/8-inch x 0 material if concentrating tables are used or 200 tons/
hr of 3/8-inch x 200 M material when deslimed material is processed in dense
medium cyclones.
Waste Streams
The main waste stream is contaminated water which is similar to that
produced by mechanical dewatering. Because the coal is much finer, the loadings
of both dissolved and suspended solids may be higher. This stream is sent
to a thickener and the overflow water is recycled or treated and recycled.
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PRODUCT CONDITIONING (continued)
Process No. 14 - Ultrafine Coal Dewatering
General Information
Wet ultrafine coal (28 M x 0) is beneficiated by hydrocyclones and
froth flotation. This very fine material has high surface area and has
a tendency to pack tightly so that dewatering is not sufficiently effective.
Process Information
Ultrafine coal streams are dewatered using solid bowl centrifuges and
vacuum filters. For a 1000-ton/hr plant, about 75 tons/hr of fine material
could be processed with about 5 tons/hr of contaminated water removed. The
dewatered product may be sent to thermal dryers or blended with larger-size
clean coal products with lower moisture content.
Waste Streams
The vacuum filter produces contaminated water that would be the same
as that produced by mechanical methods except that it will contain flocculation
agents. However, most agent remains with the coal and refuse. This stream
is sent to a thickener along with other waste waters; typically, the overflow
is recycled, but it may be discharged after treatment. Concentration of
agents released to streams are likely to be very small.
Process No.15 - Thermal Dryers
General Information
Thermal drying is a process of accelerated evaporation where wet coal
and hot gases are brought into intimate contact with each other. The hot
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PRODUCT CONDITIONING (continued)
gases are usually the gaseous combustion effluent from a coal burner but
excess "tempering" air is added to the off-gas to control inlet dryer gas
temperature to achieve the optimum range for coal drying. After loading,
the thermally dried coal must be kept below its critical ignition temperature
of 130 to 150 F to prevent spontaneous combustion. Other important aspects
in product coal temperature control are oxidation and devolatilization which
will adversely affect coal quality if not controlled.
Process Information
Thermal dryers used in coal preparation can be grouped into six basic
types:
1. Rotary Dryers. The rotary-type dryer consists of cylindrical
drums in which the wet coal travels slowly from the feed to
the discharge end. The hot combustion gases usually travel
in the reverse direction in intimate contact with the coal.
2. Screen-Type Dryers. Screen-type dryers carry the coal on
reciprocating screens which promote evaporation by passing
hot gases through the bed. The gas flow is usually
alternated so as to transfer the suction from one screen
to the other section.
3. Multi-Louver Dryers. In the multi-louver dryer, the coal
falls in a thin stream over the face of the ascending
louver, and hot gases are passed between the filled louvers
and through the layers of solids descending in free fall.
4. Cascade Dryers. Cascade dryers accomplish their heat
transfer by introducing hot gases through and between the
wedge-wire shelves which are arranged to cause the coal
to cascade. The coal forms a curtain of flowing coal, and
hot gases are passed through it to impart heat for evaporation.
5. Suspension-Type Dryers. Suspension-type dryers introduce
the wet coal into a moving stream of hot gases which
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PRODUCT CONDITIONING (continued)
pneumatically convey and dry the material in transit. Cyclone
collectors separate the coal from the moisture-laden spent
gases.
6. Fluidized-Bed Dryers. In fluidized-bed dryers, the wet coal
is fed into a perforated or bar-type retention plate where
hot gases are blown or drawn through the bed. Fluidized beds
are characterized by a loose pulsating mass which is made
to act as a liquid by the gas stream. Fluidized bed dryers
are the most commonly used in coal cleaning plants. For a
1000-ton/hr plant, thermal dryers could process 220 tons/hr
of dewatered coal from the medium and fine coal cleaning
circuits.
In addition to these six types of thermal dryers, there is a new set
of dryer designs which carry the heat by means other than hot gases to minimize
dust control problems.
Waste Streams
Thermal dryers are the largest single source of air pollution in coal
cleaning plants. In thermal drying, uncontrolled emissions of particulate
matter may be in the range of 15 to 25 pounds per ton of thermally dried
coal. Thermal dryers also generate gaseous pollutants including sulfur
dioxide, nitrogen oxides, carbon monoxide, and hydrocarbons. These emissions
are variable depending on the coal used for combustion and the type of dryer.
Emissions from thermal dryers are universally controlled by cyclones
and wet scrubbers. Cyclones are integral parts of thermal dryers and are
used for recovery of fine coal particles. Venturi scrubbers are commonly
employed as secondary collectors. A consequence of wet scrubbing is the
generation of contaminated scrubber water, which is usually sent to thickeners
or settling ponds for clarification, after which chemical treatment commonly
is required prior to plant recirculation.
Coal-fired thermal dryers produce ash from combustion which must be
disposed of as solid waste.
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AUXILIARY AND ULTIMATE DISPOSAL OPERATIONS
Process No. 16 - Coal Handling and Storage
General Information
In conjunction with transportation and storage of coal, a wide variety
of material handling operations is needed. These include unloading, stacking
and reclaiming, and transferring coal in a plant. As the amounts of coal
to be handled have grown, the material handling systems have become more
mechanized and equipped with more automatic and integrated control devices.
Storage of coal is an economic necessity in coal preparation to provide
a reserve against production interruptions and also to facilitate intermittent
shipment. Coal is stored in huge open piles or enclosed bins and silos.
Process Information
Coal handling systems which relate to coal transportation are loading
and unloading facilities. The facilities required for coal loading and
unloading are different depending on the modes of transportation.
Coal storage can be divided into two categories according to purpose:
active storage, which supplies processing directly, and reserve storage
to guard against delays in shipments.
Active coal storage is generally in a covered structure such as a bin,
silo, or bunker, depending on the storage capacity required. Reserve coal
storage is usually in outdoor piles. Storage areas should be well drained
and raised to be protected from flooding. Drainage ditches should be
installed alongside the pile. Coal piles are constructed with the steepest
slopes possible to prevent rain or melting snow from penetrating into the
pile.
Stacking, reclaiming stored coal, and movement of coal between
processing units involves the use of various types of equipment ranging
from belt and bucket conveyors to bulldozers. The type of equipment used
for coal handling will depend on plant specific variables such as plant
capacity, storage capacity requirements, and configuration of available
land.
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AUXILIARY AND ULTIMATE DISPOSAL OPERATIONS (continued)
Waste Streams
Waste streams from the storage of coal are fugitive coal dust and
precipitation runoff or leachate. The amount of dust generated from the
open storage piles varies widely depending on climate, topography, and
characteristics of the stored coal. The generation of dust may occur at
any transfer point; control is critical at such locations. Outdoor piles
have large surface areas and rainwater has a chance to react with pyrite
and other mineral matter to form acids and to extract soluble metal ions,
resulting in acid water drainage. The area around preparation plants should
be properly designed with diversion and drainage ways through proper slopes
and collection sumps. These waters must be collected for treatment and
not be permitted to escape to the environment.
Dust control methods for open storage include spraying with water or
surfactants; dust-proofing by treating the surface with oil or calcium
chloride; and providing wind screens. Runoff from storage areas should be
collected and sent to a retention pond to settle out solids or to a water
treatment facility. Lime can be used in either case to neutralize acidity.
The principal waste stream from coal handling is fugitive coal dust.
Long, open conveyor belts carrying dry coal can be a significant source of
dust and all drop points onto or off conveyors are sources of dust emissions.
Fugitive dust can be suppressed by water spraying and dust proofing. Float
dust from loading operations can be minimized by use of a telescoping
chute.
Process No. 17 - Water Handling
General Information
Coal washing operations require large amounts of water. In 1962, coal
cleaning plants in the U.S. used over 170 billion gallons of water which
included 81 percent recirculated water and 19 percent fresh water. Water
SO
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AUXILIARY AND ULTIMATE DISPOSAL OPERATIONS (continued)
handling in coal cleaning plants can be divided into two main areas: (1)
obtaining a reliable source of fresh water for utilization in the coal
cleaning operations, and (2) treatment of waste water from the plant so
that it can be recirculated or discharged without presenting water pollution
problems.
Process Information
Possible sources of fresh water are municipal water, deep wells, man-
made lakes, nearby streams, and mine water. It is desirable for plant usage
that the process water contain less than 5 percent suspended solids for most
purposes. However, for rinse water these levels are generally unacceptably
high. Pump gland water should be essentially free of suspended solids and
of high quality. Process water should have a near neutral pH and low
conductivity, i.e., low dissolved solids. Although low conductivity water
is desirable, it is seldom attained due to the presence of varying levels
of soluble components in the ROM coal. Technology to remove dissolved
solids from process water are available, but unacceptably costly for coal
preparation systems.
As a result of stream pollution regulations and the coal industry's
desire to improve fine coal recovery, recirculation and treatment of wash
water are integral parts of the operation of a modern coal cleaning plant.
In particular, closed water circuits have grown in popularity because they
eliminate discharge to streams, reduce makeup water, and allow for recovery
of coal. Since closing the circuit results in the buildup of slimes, it is
necessary to remove a certain portion of these fine solids. Standard
equipment generally applied in a closed water circuit consists of thickeners,
cyclones, filters, and/or solid bowl centrifuge.
Waste Streams
The major waste stream from water handling operation is the solid waste
generated from water clarification. Since the solid waste consists of
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AUXILIARY AND ULTIMATE DISPOSAL OPERATIONS (continued)
extremely fine size materials and considerable amounts of water, the
disposal of it requires special care. Water pollution problems may be
encountered from acid water drainage and dust may be generated if the
waste is completely dried. In addition, accidental overflow of settling
ponds may create serious water pollution.
Waste streams from plant cleanup (wash down) are also significant.
Coal preparation plants should be frequently washed down with fresh water
to remove solids from accidental spills and settled dust, etc. These
waters and solids are collected in plant sumps and combined with slime
streams for solids removal and water recycle.
ProcessNo. 18 - Solid Waste Disposal
General Information
The disposal of coal cleaning plant waste is a worldwide problem of
increasing magnitude. Coal refuse consists of waste coal, slate, carbona-
ceous and pyritic shales, and clay and rock associated with a coal seam.
It is estimated that about 25 percent of the raw coal mined is disposed of
as waste. This enormous quantity of refuse varies considerably in physical
and chemical characteristics depending on the coal source and the nature of
the preparation process.
Process Information
The disposal of coal refuse involves two quite separate and distinct
materials—a coarse to fine refuse (+28 mesh) and an ultrafine refuse (-28
mesh). Coarse to fine refuse is transported to the disposal area by a variety
of material handling systems, which include aerial trams, conveyors, trucks,
side-dump mine cars, scrapers, and bulldozers. In mountain regions, the
types of disposal used include cross-valley fill, valley-fill dump, and
side-hill dump.
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AUXILIARY AND ULTIMATE DISPOSAL OPERATIONS (continued)
In flat land, the refuse is simply deposited in waste heaps, usually
ending up in the general shape of a truncated cone. Active and abandoned
strip mines can be used for the disposal of both slurry refuse and coarse
refuse from preparation plants. Layering, compaction, and revegetation may
be used to reclaim the disposal area.
Disposal of ultrafine refuse may be by slurry impoundment, underground
disposal, and disposal after dewatering. Transportation by pipeline may be
employed before disposal.
Waste Streams
Solid waste is the largest single source of potential pollution from
coal preparation. Adverse effects associated with coal refuse disposal include
air pollution, water pollution, land pollution, safety hazards, and ecological
and psychological impacts. Fugitive dust can pollute the air and leachate
can pollute surface or underground water. Methods for fugitive dust control
and leachate collection and treatment discussed for coal storage, as part of
the description of Process No. 16, are applicable to solid waste also.
Burning refuse piles can be especially troublesome sources of air
pollution. In 1968, the U.S. Bureau of Mines conducted a survey to locate
and examine coal refuse piles in 26 coal-producing states. At that time,
they located 292 burning piles in 13 states. Of these, at least 66 were
believed to be spontaneously ignited by air flowing through the pile.
(a) Magnuson, M. 0., and Baker, E. C., Stateof the Art in Extinguishing
Refuse Pile Fires, presented at First Symposium on Mine and Preparation
Plant Refuse Disposal, Louisville, Kentucky (October, 1974).
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AUXILIARY AND ULTIMATE DISPOSAL OPERATIONS (continued)
There are several ways of controlling coal refuse pile fires such as
digging out and cooling the affected material, covering up and sealing the
pile against air circulation, and grouting to solidify the affected material;
however, the most effective way to control coal refuse fires is to prevent
them. Important measures stressed for coal refuse pile fires include locating
refuse piles a safe distance from active mining operations and facilities
and from abandoned mine openings, cleaning vegetation from the disposal site,
compacting every 2-foot layer of refuse, and sealing the open slopes with
clay or other inert materials to prevent air circulation. Current Federal
regulations, if followed, should end fires in new coal refuse piles.
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
. REPORT NO.
EPA-600/7-79-073e
3. RECIPIENT'S ^CCESSIOW NO.
I. TITLE AND SUBTITLE
Environmental Assessment of Coal Cleaning Processes
Technology Overview
5. REPORT DATE
September 1979
6. PERFORMING ORGANIZATION CODE
7. AUTHORISt
P. W. Spaite (Consultant), G. L. Robins on, A. W. Lernmor
Jr. , S. Min, and J. H. McCreery
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Battelle Columbus Laboratories
505 King Ave.
Columbus, OH 43201
10. PROGRAM ELEMENT NO.
EHE 624A
11. CONTRACT/GRANT NO.
68-02-2163, Task 212
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final; July 1976 - May 1979
14. SPONSORING AGENCY CODE
EPA/600/13
. SUPPLEMENTARY NOTES
541-2851.
IERL-RTP project officer is James D. Kilgroe, Mail Drop 61, 919/
16. ABSTRACT
The report gives a background against which requirements for further develop-
ments of coal cleaning technology and control techniques for the associated pollutants
can be established, as part of a review of U.S. coal cleaning process technologies
and related technologies for environmental control. It summarizes the state of
the art of physical coal cleaning. It summarizes the status of coal cleaning
technology with respect to cost, energy efficiency, applicability, extent of
development, and commercialization prospects. It describes the manner in which
various physical coal cleaning operations (e.g., coal pretreatment, coal separation,
product conditioning, and auxiliary processes) are combined to produce systems
capable of producing minimum, intermediate, and maximum levels of coal
cleaning. It describes the physical and chemical properties of coal, and cites the
pertinent literature on washability of many U.S. coals. It gives technological
descriptions of coal cleaning processes (e.g., size reduction, sizing, desliming
screens, fine coal separation, jigs, dense-medium vessels, air tables, and wet
concentrating tables). It identifies potential pollutants evolved from these
processes and methods used for their control.
17.
KEY WORDS AND DOCUMENT ANALYSIS
a.
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pollution
Coal
Coal Preparation
Assessments
Comminution
Size Separation
Wet Classifiers
Jigs
Pollution Control
Stationary Sources
Coal Cleaning
Environmental Assessment
Air Tables
Wet Concentrating Tables
13B
08G
081
14B
13H, 07A
131
21. NO. OF PAGES
13. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report)
Unclassified
92
20. SECURITY CLASS (Thilpagtl
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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