Clean coal technologies reduce emissions either in the combustion cycle or through gasification of the coal. Circulating fluidized-bed combustion (CFB) and supercritical pulverized-coal systems use temperature and pressure in the combustion cycle. Integrated gasification combined-cycle (IGCC) systems convert coal to a gas before combustion. Until recently, all of them have been less reliable than conventional subcritical pulverized-coal (PC) powerplants, the backbone of the U.S. generation fleet. Some also cost more to construct, operate and maintain.
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In CFB, a bed of coal particles is "fluidized" by suspension in flowing combustion air and entrained with a sorbent such as limestone to remove SO2. Fluidizing promotes complete combustion at relatively low temperatures of 1,400 to 1,700°F, inhibiting NOx production. Efficiencies of fluidized-bed combustion can be similar to those of pulverized-coal. CFBs environmental performance with low-rank coals and its fuel flexibility make it attractive.
In a PC plant, finely ground coal is burned to make steam and flue gases are cleaned up. Most U.S. coal-fired plants are "subcritical" units in the 2,400-2,600-psi range. Supercritical steam is above the supercritical point of water (3,208 psi). Temperature of main steam and reheat steam is 1,050° to 1,100°F. Higher-efficiency steam may be designated ultrasupercritical. With "back-end" emission controls, modern plants can have very low emissions of pollutants. Supercritical units can be 45% efficient, compared with subcritical efficiencies of 35%. Supercriticals low emission profile results from its greater efficiency, because less coal is burned per kW-hour of power generated.
Gasification of coal is a chemical process, involving reaction with oxygen and heat or steam under pressure to produce a "synthesis gas" containing carbon monoxide, hydrogen and methane. For IGCC, sulfur and particulates are removed from the gas before it is burned in a gas turbine. The exhaust heat is used to make steam to drive a steam turbine. NOx is controlled in the turbine by combustion controls. Postcombustion, selective catalytic reduction can further reduce NOx. Sulfur emissions can be lowered by more than 99.5% and NOx can be comparable to gas combined-cycle emissions. Impurities removed can be sold as commercial chemicals or byproducts. Fuel is high-rank bituminous coal or low-rank coal with petroleum coke. New IGCC designs may be better for low-rank coal. The largest constructed to date is 300 MW.
