Fluidized bed combustion

Fluidized bed combustion (FBC) is a combustion technology used in power plants. FBC plants are more flexible than conventional plants in that they can be fired on coal, biomass, among other fuels. Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process. The result is a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides more effective chemical reactions and heat transfer.

FBC reduces the amount of sulfur emitted in the form of SOx emissions. Limestone is used to precipitate out sulfate during combustion, which also allows more efficient heat transfer from the boiler to the apparatus used to capture the heat energy (usually water pipes). The heated precipitate coming in direct contact with the pipes (heating by conduction) increases the efficiency. Since this allows coal plants to burn at cooler temperatures, less NOx is also emitted. However, burning at low temperatures also causes increased carbon dioxide, nitrous oxide, and polycyclic aromatic hydrocarbon emissions. FBC boilers can burn fuels other than coal, and the lower temperatures of combustion (800 °C) have other added benefits as well.

Fluidized-bed combustion evolved from efforts to find a combustion process able to control pollutant emissions without external emission controls (such as scrubbers). The technology burns fuel at temperatures of 1,400 to 1,700 °F, well below the threshold where nitrogen oxides form (at approximately 2,500 °F, the nitrogen and oxygen atoms in the combustion air combine to form nitrogen oxide pollutants). The mixing action of the fluidized bed results brings the flue gases into contact with a sulfur-absorbing chemical, such as limestone or dolomite. More than 95% of the sulfur pollutants in coal can be captured inside the boiler by the sorbent. The reductions may be less substantial than they seem, however, as they coincide with increases in carbon dioxide, nitrous oxide, and polycyclic aromatic hydrocarbons emissions.

Commercial FBC units operate at competitive efficiencies, cost less than today's units, and have NOx and SO2 emissions below levels mandated by Federal standards. FBC systems fit into essentially two major groups, atmospheric systems (FBC) and pressurized systems (PFBC), and two minor subgroups, bubbling or circulating fluidized bed.