Alternatives for Future Coal Waste Disposal - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 130

Syngle and Sinn, 1991). Fluidized-bed coal combustion technologies are also
being used in other countries (Sidebars 7.3 , 7.4 , and 7.5 ).
Fluidized-bed power plants can eliminate waste generation at the
preparation plant, but they increase the waste produced during the utilization
phase ( Figure 1.2 ). In general, fluidized-bed power plants produce more coal
combustion waste than conventional coal-fired boilers. However, because of the
alkalinity (high pH) created by combustion of limestone, the waste can be used
to reclaim acidic mining lands (Couch, 1998). Roughly 75 percent of the wastes
from fluidized-bed combustion are used beneficially, primarily in mine-fill (61
percent), followed by waste stabilization (6 percent), construction fill (5
percent), and agriculture (1 percent) (EPA, 1999). In addition, coal combustion
waste requires less volume for disposal and can be backfilled into mine
workings (Couch, 1998).
Gasification
Gasification is the process of converting various feedstocks to fuel gas or
syngas under reducing conditions, at high temperatures, with the addition of
oxygen and steam (DOE, 1999; Hebden and Stroud, 1981; J.L. Johnson, 1981;
Rousaki and Couch, 2000). The feedstock reacts, and the product is then cooled
and purified. This technology has been in use since the 1930s (Rousaki and
Couch, 2000). Feedstocks used in gasification include coal, coal slurry,
petroleum, gas, petroleum coke, and biomass (DOE, 1999; Rousaki and Couch,
2000). Coal has been used as an experimental gasification feedstock since the
1970s. The recent Vision 21 review concluded that coal gasification should be a
major focus for DOE's enabling technologies program (NRC, 2000b).
Incorporating gasification with other technologies has resulted in the
integrated gasification combined cycle, which improves energy conversion
processes by combining gasification and gas cleaning, synthesis gas conversion
and turbines ( Figure 7.5 ) (DOE, 1999). This process can theoretically reach 60
percent efficiency and has been reported at 42 percent efficiency (Arey, 1997;
DOE, 1999), whereas typical coal-powered plants achieve a maximum of 34
percent efficiency (Arey, 1997).
Emerging integrated gasification combined cycle power plant technologies
use many different types of coal gasification reactors based on entrained beds,
fluidized beds, and fixed- or moving-bed technologies. A feature to note is that
all of these coal gasification reactors use steam as a reactant. These
technologies could utilize coal-water slurry as a feed with