Geology Reference
In-Depth Information
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 350-400 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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