Another more efficient, lower-emissions technique is fluidized bed combustion (FBC). FBC evolved from efforts to find a combustion process able to control pollutant emissions without external emissions controls. It is a flexible method of electricity production using combustible material including coal, biomass, and general waste. FBC systems improve the environmental impact of coal-based electricity, reducing SOx and NOx emissions by 90%.
Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process. This leads to more effective chemical reactions and heat transfer. Combustion takes place at temperatures of 1,400 to 1,700 degrees F—below the threshold at which NOx gases form. Sulfur-absorbing chemicals, such as limestone or dolomite, are used to capture pollutants inside the boiler.
There are two main FBC systems: atmospheric systems (FBC) and pressurized systems (PFBC). FBC in boilers can be particularly useful for high-ash coals. In PFBC systems, steam generated from the heat in the fluidized bed is sent to a steam turbine, while the flue gasses are used in a gas turbine. Second-generation FBCs can include an integrated coal gasifier to produce syngas, which, when added to the energy entering the gas turbine, increases efficiency.
Coal is set to remain a major player in the global power market, and increasing the deployment of higher-efficiency, lower-emissions technologies and CCS will be essential to achieving global carbon emissions targets.