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OCR for page 131
7
Alternatives for Future Coal Waste Disposal
Coal system activities range from mining, to processing, to utilization,
to disposal, and involve resources, transportation, and the environment
(Figure 1.2~. Redesigning systems like this to eliminate or reduce
substantially waste streams is the first priority of industrial ecology (Sidebar
7.1~. In the coal system, this can be accomplished by identifying the
appropriate point in the process for reducing or removing noncombustible
material. Currently, waste is created during the mining and processing stages
and disposed of either in coarse refuse piles or fine waste slurry
impoundments. Consideration of the entire coal system concept leads to the
identification of alternatives to impoundment disposal of fine coal waste by
reducing the amount of waste generated, utilizing waste, or disposing of
waste elsewhere. These options attempt to minimize the waste stream,
transfer it to another part of the coal system, or redirect it, respectively.
Fine coal waste from the preparation plant can be reduced or eliminated
during both the mining and the preparation phases. For example, selective
mining reduces the amount of noncombustible material. However, many of
the coal seams currently mined are of sufficiently low quality that they
would not be mined if this were required. Options for preparation plant
waste elimination are more numerous during coal preparation. Cleaning
requirements are usually imposed on the mine operator, whose coal cleaning
and waste disposal may be constrained by site and environmental
considerations. One option is to transfer cleaning responsibilities to the
power plants (the majority of end-users). Another is to add fine coal waste to
the cleaned coarse coal product. Dewatering the slurry product solidifies (to
a degree) and reduces the volume of the waste.
Since 90 percent of the coal mined in the United States is used in power
plants (Freme, 2000), significant benefits can be achieved if advances in
power plant technologies are integrated into the coal system components.
Emerging power plant technologies, developed around fluidized-bed
combustion with appropriate flue gas desulfurization technology, allow
131
OCR for page 132
132
COAL WASTEIMPOUNDMENTS
burning of low value coals, i.e. coals with a large amount of noncombustible
material (ash). Another technology is the use of coal water slurry as Mel for
traditional combustors or gasifiers.
There are several alternatives to disposing of fine coal waste in
impoundments, such as disposing of it in surface fills and underground
workings. However, these options are often limited by factors such as
topography, cost, and safety.
The ramifications of all potential options may be explored to create a
Type III industrial ecology model for the coal/electric power industry
(Sidebar 7.1~. Figure 7.1 shows that through selective mining and other
means, coal preparation needs can be restricted to Level 3 (Sidebar 7.2) and
below, thus producing only limited fine coal waste slurry. If coal is cleaned
to Level 4, the options of waste utilization and alternative disposal locations
remain. Finally, existing ponds can be reclaimed, and the fine waste can be
used in a similar manner.
SIDEBAR 7.1 Industrial Ecology
In the past, many industries operated as individual entities; however, the
individual operations can have widespread impacts. The philosophies and
approaches of industrial ecology may be used to integrate an individual
industry, such as coal mining, with natural ecosystems and other industries.
Industrial ecology is defined by Graedel and Allenby (1995) as a rational way
for humans to maintain their existence with changing economies, cultures,
and technological capabilities. Individual systems must work with each other
to optimize the total materials cycle, including resources, energy, and capital.
In the biological world, metabolism is the key process for life, for
ecological balance, and for providing an increased capacity for living things.
When the ecological balance is disturbed, species perish or mutate until a
new balance is established. Industrial metabolism adapts the concept to the
industrial world. Related industries in the industrial system are designed to
work together to imitate or mimic the metabolic process. This process does
not exist in nature. Jelinski et al. (1992) present three models of industrial
ecosystems based on this analogy (see figure next page). In Type 1, the flow
process is unidirectional from resources through consumption to waste. With
time, the system's resources will be depleted, and its wastes will overwhelm
it. In Type 11, internal cycling loops are developed, leading to limited input of
resources and limited waste. This system is also not sustainable because
input flows to waste in only one direction. In the ideal Type 111 system,
industrial processes are similar to the biological ecosystems model, and full
cyclicity is achieved.
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ALTERNATIVESFOR FUTURE COAL WASTE DISPOSAL
Unlimited ~ Ecosystem
resources ~ ~ component
Ecosystem
/ / component`\ \
Energy and / ~ \> \
limited ~ Ecosystem Ecosystem
resources \;ompor ent componenj/
Energy
Limited
resources
—
·(
/
\
cosystem
component
/
Ecosystem
/ component `\
Unlimited
) waste
\
Ecosystem
component /
M—1~ ~
or grower j manufacturers
processor ~ · Consumer
- ~
Limited
waste
\
Limited
· waste
Reprinted with permission from Jelinski et al., 1992. Copyright 1992 by J.L. Jelinski.
133
OCR for page 134
734
Seie~i~
. .
mI 1ng
Level O,1
Comae
resect
Non-sele~i~
~ mining
al 2~ ~
~ Coal
Surface disposal
Advanced and
con~n1ional
pow> rants
Coal combustion /
by-p~ducts /
utilizing /
Haste
Disposal
Impoundment
remining
Underground
disposal
Fine coal cleaning
o!Iy
Fine
waste
/
/
FIGS 7.1 Coal system: Dining, prep ion, utHiz~ion, residues, Ed dispossL
OCR for page 135
ALTERNATIVES FOR FUTURE COAL WASTE DISPOSAL
135
However, these options for impoundment alternatives raise other issues.
For example, although the utilization of waste as a fuel eliminates the need
for disposal of fine coal waste from the preparation plant, the burden of
generating and disposing of coal combustion by-products is transferred to
the power plant. Although this material has been utilized in a number of
innovative ways (e.g., construction aggregates, synthetic soils, neutralizing
agent), whether it is truly benign has been questioned. Another question is
the safety and engineering aspects of alternative disposal locations, such as
underground injection of slurry.
Proven technologies can abet some of the issues with slurry disposal. To
make this a reality, major institutional, organizational, environmental, and
business issues must be addressed to encourage a shift from the traditional
mining, processing, and utilization practices to an approach based on
industrial ecology. Several alternatives are outlined in the sections that
follow.
REDUCING OR ELIMINATING SLURRY GENERATION
Of the more than 1 billion tons of coal mined in the United States, only
about 350400 million tons are cleaned in wet coal-processing circuits (C.
Raleigh, CQ Inc., personal communication, 2001~. The opportunities for
reducing slurry volume include mining and coal processing alternatives.
Modern methods of surface and underground coal mining offer only a
limited possibility for quality control during mining. Mining operations can
be planned to extract coal from the best quality seams and minimize dilution
with noncombustible material. This approach is commonly used in both
surface and underground mining, especially for coal in the western United
States. However, it is more difficult to apply in the eastern United States,
where the highest quality seams have already been mined. Run-of-mine coal
from both high and low quality sources can be blended to make a product of
direct marketable quality.
When coal is cleaned in wet-processing circuits, a fine waste stream
containing water, fine coal, and noncombustible particles (ash) is produced
in which the percentage of each depends upon the level and efficiency of the
fine coal cleaning methods employed (Sidebar 7.2~. Slurry volume can be
reduced by improving fine coal recovery and minimizing the mass of solids
for disposal. The slurry volume can be further reduced by dewatering, which
increases the proportion of solids to water. The ability to do either or both of
these depends on the method of extraction, the amount of slurry dilution, the
characteristics of the coal (e.g., the hardness of the coal, which affects the
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136
COAL WASTEIMPOUNDMENTS
SIDEBAR 7.2 Levels of Coal Preparation
There are five levels of coal preparation.
Level The coal is not cleaned, and run-of-mine coal is shipped directly to
the customer.
Level 1 The run-of-mine coal is crushed to below a maximum size, and
undesirable constituents are removed. The product of Level 1
preparation is commonly termed "raw coal."
Level 2 The product from Level 1 is sized as coarse and fine coal. The
coarse coal is cleaned to remove impurities; the fine material is added to
the cleaned coarse coal or marketed as a separate product.
Level 3- The fine product from Level 2 is sized into two products:
intermediate and fine. The intermediate product is cleaned to remove
impurities. The fine material is added to the cleaned product or marketed
separately.
Level ~CIeaning is extended to the fine material from Level 3.
SOURCE: BTU Magazine, 1982.
size and amount of fine particles produced), and the local geology (e.g.,
abundant clay in adjacent strata can produce a refuse stream that is more
difficult to dewater).
It has been nearly 20 years since dry coal preparation methods were
used in the U.S. coal industry. According to Couch (1991), in the early
1960s, dry coal cleaning accounted for about 10 percent of all coal that was
cleaned, but since then it has dropped to less than 1 percent. Dry cleaning is
usually accomplished with "air jigs" or "air tables" using oscillating and
fluidized bed principles. Most of the dry methods require closely sized and
moisture-free feed. A combination of factors associated with the dry
methods particle size, dust, transport, health and safety, noise, and the better
performance of wet processes have contributed to the near abandonment of
dry coal cleaning processes in recent years. The increased use of water for
dust control in underground mines, and the increased efficiency of wet
cleaning methods have continued the sharp decline in the use of dry cleaning
methods at the mines. However, dry coal preparation methods, which do not
create the same disposal challenges as slurry waste, can be effective in areas
where the water supply is restricted.
Currently, most coal preparation plants recover fine coal only from the
size fraction greater than 100 mesh (150 micrometer). Typically, this is done
with water-only cyclones, spirals, or both for 16 x 100 mesh (1.0 x 150
micrometer) material. Particles smaller than 100 mesh (150 micrometer)
=
OCR for page 137
ALTERNATIVESFOR FUTURE COAL WASTE DISPOSAL
137
account for 3 to 7 percent of the total plant feed, of which coal may comprise
as much as 50 percent (R. Honaker, University of Kentucky, personal com-
munication, 2001~. These fine coal particles may be untreated, partially
treated, or fully treated.
With no treatment, the fine material, usually smaller than 100 mesh (150
micrometer), is sent to a thickener and then pumped to a slurry impound-
ment. Alternatively, the fine coal and refuse could be blended with the
coarse coal product and sent to the power plant. This feed can be either
burned directly or further cleaned in the power plant after pulverization;
advances in magnetic and electrostatic separation hold promise for dry
cleaning the coal at this stage. In fact, pulverization liberates more of the
mineral matter in coal. Dry cleaning with magnetic and electrostatic separators
has shown encouraging results (Oder et al., forthcoming). However, this
process still generates a waste stream (albeit not a slurry) that requires
disposal.
Partial treatment utilizes classifying cyclones to remove coal particles to
approximately 325 mesh (45-micrometer) size. In full treatment, which
theoretically captures all of the fine coal particles, the fine waste from the
cyclone is subjected to flotation. As air bubbles rise through the flotation
tank, the coal particles attach themselves to the bubbles and are carried to
the top of a column of water.
Although the recovery of all or most of the coal fines will not eliminate
the need for slurry impoundments, it will reduce the required disposal
volume. At the same time, by increasing the clay content, a slurry that is
more difficult to stabilize may be produced.
In contrast, dewatering the refuse stream could eliminate the need for
slurry impoundments by changing the strength properties of the waste
material, although disposal of the resulting dewatered waste raises other
issues. Dewatering employs either sedimentation or filtration or both. In
sedimentation, the liquid is constrained, and the solid particles move freely.
This results in clarification of the liquid and thickening of the remaining
slurry. In filtration, a medium constrains the particles while the liquid flows
through. This is accomplished by screening and centrifugation (Osborne,
1988).
In coal preparation plants, wet refuse is usually sent to a thickener
(Figure 7.2~. The underflow Mom the thickener (30 to 35 percent solids by
weight), the fine waste stream, is sent to a slurry impoundment. Deep cone
or other paste thickeners produce an underflow with a higher solids content
than a conventional thickener. Their steep-sided deep cone construction
takes advantage of the high differential pressure applied by the depth of
solids to produce a paste (Steve Slottee, Eimco Process Equipment Company,
OCR for page 138
138
Walkwav
-
COAL WASTEIMPOUNDMENTS
Steel tank flat bottom
in\ Water level
· Feedwel/
7~
Cone scraper
~ Discharge cone
.
· Feedwell
_ _
,Arm
Bedded-in
~ Steel tank bottom and side
Concrete tank sloping bottom
T
1'
_
Walkway ~
as\ Water level
,Arm
Cone scraper
~ Discharge cone
FIGURE 7.2 Thickener tank design. Reprinted with permission of the Society for Mining,
Metallurgy, and Exploration, Inc., www.smenet.org.
-
OCR for page 139
ALTERNATIVESFORFUTURECOALWASTEDISPOSAL
139
personal communication, 2001~. Deep cone thickeners produce dewatered
paste in the range of 65 to 75 percent solids by weight (Osborne, 1988~.
While this material still requires disposal, its volume is less than that of
unthickened slurry.
Four types of filtration devices are used in coal preparation: gravity,
vacuum, pressure, and centrifugal. Although gravity and centrifugal methods
are used extensively, vacuum and pressure filtration methods offer the
greatest potential for dewatering material smaller than 100 mesh (150
micrometer) that typically leaves the conventional thickener as underflow.
Three basic types of vacuum filtration devices are rotary drum, rotary
disc, and horizontal belt or disc, depending on their filter configuration type.
With all of these devices, the feed is applied to the filter medium under
vacuum. This draws the water through the filter medium while retaining the
solids on the surface. The filter cake is then removed either by a burst of air
pressure, mechanical scraping, or both. Vacuum filters produce a filter cake
with a moisture content of 20 to 30 percent (Leonard, 1991~.
Pressure filtration devices are classified as either batch or continuous
press. Batch devices, such as the plate and frame filter press, have not been
widely accepted in the U.S. coal industry because they operate discon-
tinuously (Osborne, 1988~. However, solids recovery is high and effluent
water is clear.
Belt filter presses operate continuously and are more widely used than
batch methods (Figure 7.3~. Belt presses produce a cake with a moisture
content reportedly in the range of 20 to 30 percent (Osborne, 1988~;
however, in practice, moisture content ranging from 35 to 40 percent is more
common. Belt filter presses currently produce a dewatered product that still
must be disposed of behind a retaining structure.
Hyperbaric pressure filters, a fairly recent development, combine the
filtration technology of disc or drum filters and the low moisture levels of
discontinuous plate and frame presses. The specific solids throughput of a
hyperbaric filter is several times that of a vacuum filter or other batch-
operation pressure filters (B. K. Parekh, University of Kentucky, personal
communication, 2001~.
Chemical additives are almost always used in conjunction with any
dewatering mechanism. Most of the mechanical methods (thickeners, filters,
and presses) rely on chemical additives (usually flocculants) to expedite and
enhance the separation process. The additives represent a significant
operating cost, but dewatering would be largely ineffective without them.
Thermal drying is an effective way of removing moisture, but it is very
expensive and energy intensive and is currently used only for drying fine
coal. In addition' it entails the environmental cost of air pollutants produced
=
OCR for page 140
140
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OCR for page 141
ALTERNATIVES FOR FUTURE COAL WASTE DISPOSAL
141
during the drying processes. Other dewatering methods being developed
include ceramic capillary filters, electro-acoustic dewatering, microwave
dewatering, vacuum pressure hybrid filters, and pulsating vacuum filters (B.
K. Parekh, University of Kentucky, personal communication, 2001~.
Dewatering technologies represent one alternative to reduce the volume
of waste deposited in coal slurry impoundments, but they do not eliminate
the need for an impoundment. Many dewatering technologies are currently
available for specific applications, though none is likely to be universally
applicable. The committee believes that equipment vendors' current research
and development will lead to improvements in these technologies and that
operators of coal waste impoundments should monitor them carefully.
DIRECT UTILIZATION OF SLURRY
Slurry refuse can be utilized directly for power generation, either in
conventional boilers or with advanced combustion and gasification
technologies. These technologies have the advantage of turning a waste into
a resource. Some of them can reduce cleaning requirements for coal in the
preparation plant, but the use of low quality coal feed will increase the
amount of waste generated at the power plant.
Conventional Pulverized Coal-Fired Boilers
Direct utilization of fine coal waste in conventional pulverized coal-
fired boilers is an important alternative to its disposal in an impoundment.
This does not require a significant change to the system of mining, cleaning,
and burning coal. However, use of this material presents significant
challenges to existing boilers, because the moisture content is high and the
heating value and trace element quantities are inconsistent (Harrison and
Akers, 1997~. If the fine waste material alone does not meet the required
specifications for end-users, it can either be combined with a variety of other
feeds, such as cleaned coal or biomass, to achieve the desired filet
consistency or be agglomerated to improve handling.
Fine coal is difficult to handle, even when dewatered, because it clogs
equipment and is dusty and explosive. Agglomeration technologies can
reconstitute the fine coal by briquetting, pelletizing, or extrusion and can
solve the handling and transportation problems. The advantages and
disadvantages of the various forms of agglomeration are summarized in
Table 7.1. Which agglomeration technology is appropriate depends on the
OCR for page 154
154
COAL WASTEIMPOUNDMENTS
Even at this rate several cells must be permitted and in varying stages of
operation at any one time.
A variation of slurry cells is dewatered fine refuse cells. Mechanically
dewatered fine refuse can be placed in bermed cells. However, the combined
costs of dewatering and cell construction prevent this method from frequent
use in the coal industry.
Combined refuse disposal is another option. Combined refuse refers to
fine refuse from the static thickener that has been mechanically dewatered
and combined with coarse refuse for disposal on the surface. Depending on
the percentage of coarse and fine refuse and the moisture content of each,
the moisture content of combined refuse will range from 15 to 20 percent.
This material must be transported mechanically, since it is too dry to be
pumped. It is deposited in lifts approximately 2 feet thick, then graded with a
bulldozer, and compacted to create a stable surface. Once the fill reaches the
designed level, it is covered with 4 feet of soil and revegetated. Combined
refuse fills require underdrains to capture any water that leaches from the
fill, and perimeter drains to prevent runoff from entering it.
Problems associated with this method of disposal include the frequent
inability of mechanical dewatering to produce the moisture level required for
disposal of a combined refuse material (David Carris, J. T. Boyd Company,
personal communication, 2001~. Because the moisture and clay content are
still fairly high, trucks or conveyors cannot handle the material easily, and
compaction is difficult. Davies and others (1998) report using small amounts
of cement to create a more stable refuse material. They found that the
addition of 2 to 4 percent cement, by weight, results in a material that can be
compacted using standard procedures. Small amounts of fly ash and lime
have also been used to stabilize combined refuse. In addition, this method is
fairly expensive because of the high cost of mechanical dewatering and the
potential need for stabilizing chemical additives. The need for additives
may be seasonal and depends on climatic conditions. Finally, the method is
best suited to flat land.
Co-disposal, developed and practiced primarily in Australia, involves
the combination of fine refuse from the static thickener with coarse refuse
(Williams et al., 1995~. Since the fine refuse is not mechanically dewatered,
the combination has a fairly high moisture content that allows the combined
material to be pumped to the disposal area. The main difference between this
material and conventional slurry is the presence of coarse refuse in the mix.
The advantages are that it requires less total storage volume than separate
fine and coarse disposal methods, and the refuse stabilizes more quickly than
typical slurry.
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ALTERNATIVES FOR FUTURE COAL WASTE DISPOSAL
155
However, co-disposal does not eliminate the need for a starter
impoundment structure. After an initial dam is built, the mixed beach
sediments, settling out of the co-disposal material can be used to construct
the embankment for above-ground storage. Ideally, the fine particles fill the
voids between the coarse refuse fragments. As the mixture is discharged, it
forms a steep beach sloped at approximately 10 percent. This slope decreases
to approximately 1 percent at the end of the mixed beach. At this point, the
sediment consists mainly of fine coal suspended in ponded water in a small
impoundment. The success of this method is closely linked to the ratio of
coarse to fine particles. It also depends on gradation of the refuse (large gaps
in particle size are not acceptable) and proper particle shape (angular or
"platy" particles cause problems). Since the impounding structure is raised by
deposition of mostly coarse material, it does not compact as the structure
increases in elevation.
This method has been used primarily in sparsely populated areas with
low annual rainfall. Questions remain about its suitability for steep hills with
high annual rainfall. Unlike a conventional slurry impoundment, which
contains only fine refuse, the co-disposal system places all refuse (both
coarse and fine) in a slurry and deposits it behind an impounding structure.
Therefore, even though the refuse dewaters more quickly and forms a stable
bench, it requires more impoundment storage volume than an impoundment
designed only for fine refuse. So, for steep terrains, this factor negates the
advantage of less total storage area by actually requiring more material (both
coarse and fine) to be placed in an impoundment. Its use would hinge on
whether increased stability of the refuse outweighs the additional volume of
the impoundment.
If an effective dewatering approach, such as paste thickening, is used,
the resulting waste can be disposed of by thickened high-density residue
stacking (tech Brzezinski, LSB Consulting Services, personal communi-
cation, 2001~. Deep cone paste thickeners produce a homogeneous, non-
segregating paste with a solids content of approximately 60 percent. The
degree of dewatering is determined by the pumping capabilities. Under
controlled conditions, the paste can be deposited in thin layers over the
disposal site at uniform slopes of 2 to 5 percent arid does not require an
impoundment structure. This method is most suitable for homogeneous
residues of fine gradation, where the thickening process prevents segregation
of the coarse and fine particles during transportation.
Thickened high-density residue stacking was developed more than 20
years ago to handle red mud tailings generated by alumina plants. It has been
used for approximately 10 years for disposal of gold and base metal tailings;
=
OCR for page 156
156
COAL WASTEIMPOUNDMENTS
has recently found application at power plants for disposal of ash; and has
seen limited use in the coal industry for the disposal of fine coal refuse.
Although rainfall does not reconstitute a properly formed paste, and
erosion is not excessive on typical stacks with 2 to 5 percent slopes, this
method is best suited to areas of low rainfall and high evaporation. Under
these conditions, the surface of the stack can be traversed by bulldozers
within days of final placement. Regardless of the amount of precipitation,
perimeter drains are necessary to catch runoff and divert it around the stacks.
Underdrains are also needed for stability to prevent an increase in the
phreatic surface.
Three considerations land availability, steep terrain, and cost hamper
applying unsupported thickened high-density residue stacking to fine coal
refuse disposal. This method is best suited to areas where the slope of the
land is less than 5 percent. Secondly, although the technology is based on a
significant modification of the standard thickener, these thickeners are much
deeper and require a longer residence time. Therefore' the lower throughput
rate of deep cone thickeners compared with that of standard thickeners may
significantly affect the economic feasibility of the method.
Underground Disposal
In 1975, the National Research Council conducted a study entitled
Underground Disposal of Coal Mine Wastes, which evaluated the technical
and economic feasibility of several underground disposal methods; namely,
pneumatic backfilling, hydraulic backfilling, mechanical backfilling, hand
packing, controlled flushing, blind flushing, and pneumatic flushing. Much
of the study drew heavily on European experience, where backfilling has
long been used to control subsidence. The backfilling methods emphasized
the use of coarse refuse and did not specifically address the issue of fine
refuse or slurry; only the hydraulic methods dealt specifically with fine
refuse. The report did not discuss pneumatic flushing of fine coal waste, but
it did indicate that this method had been used to inject fly ash into
underground mines. The two primary methods for injecting fine coal refuse
into underground mines are controlled flushing, where the underground
workings are accessible, and blind or uncontrolled flushing, where the
underground workings are abandoned or have caved in.
The 1975 NRC report found that a number of underground disposal
methods were technologically feasible at that time; however, none was
universally economically feasible. The optimal solution varies from site to
site. In addition, the question of workers' health and safety was raised when
OCR for page 157
ALTERNATIVESFOR FUTURE: COAL WASTE DISPOSAL
157
considering injection into active mines. The report recognized that safe and
nonpolluting disposal is the operator's responsibility. If regulations are
imposed that exceed this obligation, society must recognize its responsibility
to avoid inequitable distribution of added costs to some operators. Under the
right set of conditions, underground disposal of slurry is an attractive
alternative to impoundments and is currently being used where suitable
opportunities exist (Table 7.2~.
Slurry injection creates additional factors that must be considered. Many
issues related to underground injection of slurry are independent of the
method of slurry injection. For example, it is essential to have an adequate
supply of water. This is especially true when water is not being recaptured
from the underground workings. Also, it is important to keep the solids
content below 20 percent, and preferably in the range of 10 to 12 percent.
Additional common issues include surface ownership, permits, surface
layout, and surface drainage.
In most cases, the right to mine coal is obtained after leasing the mineral
rights from the owner, but often, the rights to the surface have been severed.
Therefore, a lease to mine the coal does not automatically entitle the mine
operator to inject coal waste back into the voids created by the mining
process. Unless the mining company owns both the surface and the mineral
rights, the operator must obtain the landowner's permission to inject coal
waste back into the mine. In Kentucky, the state regulatory authority has
determined that a company cannot inject coal refuse into an underground
mine for which it has the mineral rights unless all of the surface owners
above the underground mine approve of the plan. In areas where multiple
ownerships are common, this policy seriously limits the areas available for
underground injection.
Under SMCRA, each state's regulatory authority has responsibility for
issuing permits to inject coal waste into underground mines. The permit
application must address such issues as:
Source and quantity of waste,
Area to be backfilled and the method to be used,
Approximate percentage of the voids that will be filled,
Design of underground bulkheads,
Potential influence on any active underground mines,
Source of the water to be used.
Methods of dewatering the backfill,
Amount of water that will remain underground, and
Water treatment system that will be used.
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158
COAL WASTEIMPOUNDMENTS
Underground injection also requires MSHA's approval. Depending on
the state, this can be handled independently or through a joint approval
process. Finally, underground injection of slurry requires approval by the
regional EPA office, whose primary concern is groundwater quality. States
may interpret the requirements of the Safe Drinking Water Act differently
for example, West Virginia limits the injection of any slurry from circuits
that employ petroleum products, such as those used in froth flotation.
In addition to obtaining permission from the surface owners to install
injection wells, the operator must obtain the right-of-way for the pipeline to
deliver the slurry to the boreholes (Marshall Hunt, Consol, personal
communication, 2001~. Variable terrain can impose severe pumping
problems if pipelines must be laid over hills and down into valleys to access
injection sites. The accuracy of mine maps, with regard to underground
workings and surface surveys, is important if the slurry boreholes are to
intercept the underground workings in the desired location.
Filling above-drainage mine workings with slurry may increase
hydraulic head on the coal barriers and result in a blow-out, making
evaluation of mine workings above a surrounding stream valley critical. The
mine maps must be evaluated for accuracy, and the underground barriers for
adequacy to contain the slurry. Mines below the surrounding natural
drainage level offer more secure underground disposal sites.
Blind flushing is used in mines where access has been obstructed, such
as by roof falls. ~ this method of slurry injection, the underground cavity
may be dry, partially filled with water, or completely filled with water. The
volume of the voids is estimated from old mine maps and any other
available, relevant data. Since it is nearly impossible to determine how much
slurry a borehole will accept, a series of holes is drilled; when one borehole
becomes clogged, injection moves to the next. The sluny is pumped to the
borehole and injected into the mine at a relatively high velocity. Once the
slurry leaves the turbulent area at the bottom of the borehole, the coarser
material will settle.
In flooded mines, the injected slurry will displace the water, which often
results in a new or increased discharge elsewhere (Marshall Hunt, Consol,
personal communication, 2001~. Depending on the quality of the displaced
water, treatment facilities may have to be upgraded to handle the additional
volume, or the mine pool may have to be pumped to avoid discharge at an
undesired location.
Paste backfilling has been demonstrated in other types of mines.
Although it has not been used to dispose of coal waste, it may be possible to
extend the technology to this application.
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Controlled placement is used where underground workings are
accessible. This method has a number of inherent advantages over blind
flushing. First, the engineer can verify the accuracy of the maps, inspect the
condition of the openings, and better estimate the available space. Second,
and possibly more important, bulkheads can be constructed to control the
direction and extent of flow. Finally, collection and reuse of the water is
more practicable.
Once areas have been identified and properly confined, controlled
flushing can proceed similarly to blind flushing. However, in controlled
flushing, the slurry can be delivered to the underground opening by a
pipeline that enters the mine through a borehole or other opening and then
extends laterally through the mine to the desired location. This allows the
operator to begin filling the most remote part of the mine and then to
withdraw the pipeline in stages as the voids are filled and sealed. The
advantages of this method are greater certainty of the location of the slurry,
better utilization of available space, and better reuse of the water needed to
transport the refuse. This method also protects against subsidence. One of
the main concerns is the safety of workers in the mine receiving the slurry
and in any adjacent, down-dip mines.
The committee concludes that although there are alternatives to
disposing of coal waste in impoundments, no specific alternative can be
recommended in all cases. Acceptable alternatives are highly dependent
upon regional and site-specific conditions. Also, the alternatives that have
been identified are in varying stages of technological development and
implementation. One of the factors limiting implementation to this point has
been the cost associated with the various alternatives. Additional research is
needed to develop these alternatives further and to evaluate the economics of
these processes. The committee recommends that a screening study be
conducted that (1) establishes ranges of costs applicable to alternative
disposal options, (2) identifies best candidates for demonstration of
alternative technologies for coal waste impoundments, and (3) identifies
specific technologies for which research is warranted. Input from MSHA
and OSM regarding regulatory issues will be valuable to such a study.
REMINING SLURRY IMPOUNDMENTS
While coal waste impoundments have generally been viewed as
permanent disposal sites, there may be situations in which impoundments
can be a resource (Sidebar 7.6~. In the past, recovery of fine coal was not as
efficient, resulting in many older slurry impoundments containing significant
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COAL WASTEIMPOUNDMENTS
SIDEBAR 7.6 Pond Recovery Facility
Ginger Hill is a coal slurry recovery facility in western Pennsylvania that
produces approximately 300,000 tons per year of synthetic fuel (Akers et al.,
2001~. A dredge is used to extract the fines from the impoundment. The slurry
is pumped to a surge tank and then into the cleaning plant. The coal is
classified at 150 mesh (106 micrometer) using 15-inch classifying cyclones.
Particles larger than 150 mesh (106 micrometer) are cleaned using water-
only cyclones and spirals, with an additional classification of the fine coal
product by a two-stage Van sieve. Material smaller than 150 mesh (106
micrometer) material is classified using a 6-inch classifying cyclone at 270
mesh (53 micrometers). The 150 x 270 mesh (106 x 53 micrometer) fraction
is cleaned by column flotation. After cleaning, all clean coal size fractions are
mixed and dewatered by two 40-inch decanter screen-bowl centrifuges. The
clean fines are pelletized using COVOL binder.
amounts of coal refuse larger than 28 mesh (600 micrometer) with recoverable
energy value. As processing technologies and the capacity of dewatering
equipment have improved, the proportion of particles smaller than 100 mesh
(150 micrometer) in slurry refuse in impoundments has increased. In many
instances, the finer slurry materials being disposed of today with less
recoverable and marketable coal can close off access to the more amenable,
profitable, and recoverable slurry (Sidebar 7.1~.
If an impoundment contains at least 1 million tons of in-situ slurry, a
recovery rate of at least 30 percent of a marketable fine coal product
(300,000 tons) from the slurry could prove to be a profitable venture. Fine
coal from is recovered from an impoundment through several stages:
investigation (preliminary site investigation, sampling and analysis of the
slurry and embankment materials, and engineering design and economic
evaluation), excavation and transport, and fine coal recovery.
The preliminary site investigation involves inspecting the impoundment
and reviewing maps and any other available information. The sampling and
analysis of slurry and embankment materials are the most critical and most
expensive phase of investigation. The preliminary sampling program requires
approximately three samples from each embankment and a detailed core
sampling plan throughout the basin. Samples are analyzed for size distribu-
tion, float and sink characteristics, froth flotation, and percentage of moisture,
ash, sulfur, and energy value per pound. In addition to these standard analyses,
it may be advantageous to determine whether other materials, such as
magnetite, may be recovered from the slurry. During engineering design and
economic evaluation the final phase of investigation- the economic
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161
feasibility is assessed for the project, and the marketability of the fine coal
product (i.e., potential buyers, final destination for the fine coal product,
means of transport) is investigated.
The excavation of the slurry material from the impoundment and
delivery to the fine coal recovery plant is the most critical consideration in
the development of a fine coal recovery project. Regardless of the simplicity
or sophistication in the design of the fine coal recovery plant, the slurry
delivered to the system requires a consistent feed rate (tons per hour) and
percentage of solids concentration (weight of slurry and water) to maintain
efficiency.
Three methods for coal slurry removal and delivery systems are used:
conventional excavation methods; stationary pump and wash-down systems;
and dredge operations. Conventional excavation and earth-moving equip-
ment includes earth-mover pans, excavators, backhoes, drag-lines, front-end
loaders, dump trucks, and farm tractors equipped with discs or harrows. The
stationary pump and wash-down system agitates, washes, and keeps in
suspension the slurry material within the area around and directed to the
suction of the floating agitator-feed pump. Dredging uses a floating pump
with a movable suction device. It excavates along a side-to-side arcing or
stewing motion with a positive forward movement, cutting materials from
beneath the surface of the water and creating a slurry to be delivered to the
pump suction. During removal of the fine coal, the site is vulnerable to storm
water runoff and to slumping of surrounding slopes, and adequate diversion
structures must be used to route excess water from the facility.
The design of a fine coal recovery plant is the same as that in
conventional coal preparation plants and typically processes the slurry
material that is smaller than 3/S inch. The recovery plant includes sumps,
pumps, vibrating screens, sieve bends, cyclones, spirals, flotation cells or
columns, centrifuges, conveyors, and thickeners each unit system being of
sufficient size and capacity to handle a specific feed rate and the size
characteristics of the slurry material. The refuse from a remining operation
can be handled with a variety of methods that are site specific because each
impoundment has its own design, site conditions, and slurry materials.
Methods include those outlined in this chapter such as underground
injection, thickening of slurry, multiple cell construction deposition within
the same impounding structure, a combination of these listed, or simply
disposing of the refuse in a separate part of the impoundment being remined.
Recovery of fine coal from slurry impoundments is an established
practice. The committee concludes that as advances are made in the use of
low value coal or coal water slurry, remining of slurry impoundments can
be an attractive source for fuel supply.
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COAL WASTEIMPOUNDMENTS
IMPLEMENTATION
Coal waste impoundments are part of the system for mining, preparing,
and combusting coal for energy production (Figure 1.2~. In order to assess
alternatives thoroughly, the whole system of mining, preparation, refuse
disposal, transportation, and power generation should be explored through
an in-depth life-cycle assessment, including cost assessment, with the goal
of optimizing the system to generate less fine coal waste while maintaining
the performance and economics of the system. Only through such modeling
can the benefits of system integration be fully explored. Of course, even if
there are such benefits, they may be difficult to realize because of
differences in interests and perceptions between the mining industry and the
utility industry, and the resistance to change embedded in these mature
industries.
Several types of policies can effectively promote the use of alternative
technologies. One approach is to phase out the dominant technology,
providing sufficient time for commercialization of alternatives. Such an
approach can have a significant economic impact on the industry affected. A
"safety valve" can be provided by extensions to the phase-out dates where
alternative technologies are not available and by site-specific variances. This
approach was used effectively in the Hazardous and Solid Waste
Amendments of 1986 (42 U.S.C. § 6924 ~ to phase out the landf~lling of
untreated hazardous waste and to give a clear signal to the market for
alternative treatment technologies. This approach has the benefit of
stimulating the market for alternatives without necessarily promoting
specific technologies. To bring on the alternatives, this approach can be
combined with other policies, such as financial incentives and research,
development, and demonstration.
A second type of policy for promoting alternative technologies is the use
of financial incentives. These can take the form of tax credits or direct
government subsidies. Such subsidies can be paid for by a fee on the activity
being discouraged, such as a few cents per ton of fine coal waste disposed of
in slurry impoundments, or through a general fee on coal mining, like the
Abandoned Mine Land program under SMRCA. Both the alternatives for
refuse disposal and increased use of finer coal products will aid in the
reduction of slurry impoundment quantity and additional and future areas
required for refuse disposal. These incentives may also be extended to
companies that reprocess, reduce, or relocate the slurry materials within an
existing impounding structure through the use of alternative technologies,
techniques, or methods. If this type of operation is utilized in abandoned or
"grandfathered" slurry impoundments, an additional incentive could be
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ALTERNATIVESFORFUTURECOALWASTEDISPOSAL
163
granting reduced bonding or regulatory obligations and moneys from
Abandoned Mine Land funds for environmental cleanup.
A third type of policy is research, development, and demonstration
funded wholly by government or jointly with the industry. The federal
government, and particularly the DOE, has long had research and
development programs in this area, and many of the alternatives discussed
above were first proven in these programs. There is always a gap between
research and development and widespread implementation of technologies,
but greater emphasis on joint large-scale demonstrations with industry can
help bridge this gap.
A fourth approach, particularly useful where alternative technologies are
already commercially available, is to assist the industry in evaluating these
alternatives in an integrated, objective, and thorough manner. Typically,
information about alternatives comes to the industry piecemeal through
vendors of new technologies. Rarely is it possible to perform an objective
evaluation of the full range of alternatives on the same basis. If such an
evaluation is performed by one company, it is rarely shared with others in
the industry. The EPA's Design for the Environment Program has developed
an approach for evaluating and comparing the relative costs, performance,
and health and environmental risks of alternative technologies that provide
the same Unctions. In this program multiple companies and vendors
participate and supply information to an independent academic institution
that evaluates the technologies. Costs are assessed using activity-based cost
accounting methods that allocate the All costs of technologies, bringing
costs out of overhead accounts that may have been overlooked in a
traditional cost estimate. Other stakeholders, such as government agencies,
labor organizations, and environmental groups, are involved in setting up the
evaluation and advising the team throughout. The academic institution can
keep proprietary data confidential and out of the hands of regulatory
agencies. The results of the evaluation are then widely disseminated to
members of the industry for their use in choosing technologies. EPA neither
makes these decisions nor recommends technologies. The committee
recommends that the total system of mining, preparation, transportation,
and utilization of coal and the associated environmental and economic
issues be studied in a comprehensive manner to identify the appropriate
technologies for each component that will eliminate or reduce the need
for slurry impoundments while optimizing the performance objectives
of the system. The committee concludes that a similar analysis of the waste
use and disposal technologies that make up the coal system would have
value. The committee recommends incorporating life-cycle assessment
of the costs and environmental impacts of the alternatives to evaluate
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COAL WASTEIMPOUNDMENTS
them on a more objective, comprehensive basis. In addition, a detailed
analysis of the economic and environmental impact of the various policy
alternatives should be performed.
A combination of policies can encourage the use of alternatives to coal
slurry impoundments. If there is no consensus on the need to phase out
slurry impoundments, financial incentives for alternatives and research and
development programs can expedite the switch to alternatives. In any event,
a thorough, systematic understanding of the whole system through life-cycle
assessment is needed for evaluating alternatives, as is the comparison of
alternatives. One of the factors limiting implementation to this point has
been the cost associated with the various alternatives. Additional research
and development is needed to refine these alternatives and to demonstrate
their economic implementation. The committee recommends that the use
of economic incentives be explored as a way of encouraging the develop-
ment and implementation of alternatives to slurry impoundments. The
development of incentives should be based on the full range of the portfolio
of technologies as well as the economics of the technologies. The incentives
should be linked directly to the reduction in slurry production of the
utilization of slurry.
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Representative terms from entire chapter:
coal waste
