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program and recommended that advances in gasification technologies be
pursued aggressively in the Vision 21 program and that fluidized-bed
combustion research be continued as part of DOE's main program to improve
power-generating technologies (NRC, 2000b).
Fluidized-Bed Coal Combustion
Fluidized beds suspend solid fuels on upward-blowing jets of air during
the combustion process (DOE, 2001). This turbulent mixing of gas and solids
makes chemical reactions and heat transfer more effective. While NO
x
forms at
2,500°F, fluidized-bed combustion burns fuel well below that at temperatures of
1,400 to 1,700°F. In addition, a sorbent inside the boiler can capture more than
95 percent of the sulfur pollutants. The fuel flexibility makes this technology
popular—almost any combustible material, from coal to municipal waste, can
be burned. Also, fluidized-bed combustion can meet SO
2
and NO
x
emission
standards without expensive external controls.
Approximately 12 years ago, atmospheric fluidized-bed combustion
crossed the commercial threshold, and most boiler manufacturers currently offer
fluidized-bed boilers as a standard package (DOE, 2001). Fluidizedbed coal
combustors have been called “the commercial success story of the last decade in
the power generation business.” More than 6 gigawatts of electricity are
produced by fluidized-bed power plants operate in the United States (Arey,
1997).
A newer technology that promises greater efficiency is the pressurized
fluidized-bed combustor (
Figure 7.4
). The first-generation pressurized fluidized-
bed combustor, which has been demonstrated by a joint DOE-American
Electric plant in Ohio, the Tidd Plant, uses a bubbling-bed technology. A
relatively stationary fluidized bed is established in the boiler using low air
velocity to fluidize the material, and a heat exchanger (boiler tube bundle)
immersed in the bed to generate steam.
Currently, investigators are developing a second-generation pressurized
fluidized-bed combustor to enhance efficiency. Circulating fluidized-bed
technology can improve operational characteristics by using higher air flows to
entrain and move the bed material, and by recirculating nearly all the bed
material with adjacent high-volume, hot cyclone separators. The relatively clean
flue gas goes on to the heat exchanger. This approach theoretically simplifies
feed design, extends the contact between sorbent and flue gas, reduces
likelihood of heat exchanger tube erosion, and improves SO
2
capture and
combustion efficiency (DOE, 2001). With all these features, second-generation
pressurized fluidized-bed combustion is expected to achieve a 52
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