Clean and Efficient Coal-Fired Power Plants: Development Toward Advanced Technologies
7 Alternative Coal-Fired Power Plant Concepts (FBC and IGCC)
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The challenge to burn coal most efficiently and with a minimum on emissions has led to a number of different power plant concepts besides pulverized-coal—fired plants. However, the competition of various technologies spurred also the development of advanced pulverized-coal—fired power plants. Fluidized bed combustion (FBC) had been introduced to the power-generation industry in the 1980s as a power plant concept that could especially be used to burn low-grade coal, even mixed with biomass or residues like wood, petroleum, coke, etc. The combustion in FBC takes place at a relatively low temperature of about 1560°F (850°C) to avoid ash softening, otherwise particles would stick together and the fluidization principle would no longer function. Along with the low combustion temperature, the formation of thermal NOX is suppressed, which accounts for low NOX emissions. This can be further improved by staging the combustion air in the FBC combustor. Feeding limestone into the combustor results in a sulfur retention up to about 90% (Ca/S ratio of 1 to 1.5). These measures eliminate the need for selective catalytic reduction (SCR) reactors and scrubbers and only a dust-removal system needs to be installed. Two FBC power plant concepts have been developed and built: atmospheric and pressurized FBC systems. The first featured an atmospheric fluidized bed steam generator providing steam to a steam turbine and the second was a pressurized fluidized bed steam generator integrated into a combined cycle with a gas turbine and a steam turbine.
A typical example of an atmospheric FBC steam-generation system is illustrated in Figure 7-1, which provides steam to a steam turbine.Worldwide, about 250 FBC power plant units have been built. Most of the atmospheric FBC steam generators generate steam for power plant units with a rating of 100 MW or less, but also larger unit sizes have been developed with the present maximum unit size of roughly 250 MW. The net power plant efficiency of atmospheric FBCs depends mainly on the fuel quality and the power plant cycle parameters and ranges up to 40% with the currently available technology.
The pressurized fluidized bed combustion (PFBC) power plant concept provided an improved plant performance, but requires the integration of a combined cycle power plant with the PFBC unit receiving pressurized air from the gas turbine compressor and supplying the hot combustion gas for the gas turbine.