Circulating fluidized beds burn various types of fuels without violating emission-control norms. This makes CFBs suitable for burning fuels - high-sulphur coal, lignite, peat, oil, sludge, petroleum coke, gas and wastes - cleanly and economically in CFB boilers. This technology does away with the need for complex scrubbers, catalytic systems or other costly chemical clean-up systems.
According to the US Department of Energy, 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 degrees F, well below the threshold where nitrogen oxides form (at approximately 2,500
degrees F, the nitrogen and oxygen atoms in the combustion air combine to form nitrogen oxide pollutants).
The mixing action of the fluidized bed brings the flue gases into contact with a sulphur-absorbing chemical, such as limestone or dolomite. More than 95 per cent of the sulphur pollutants in coal can be captured inside the boiler by the sorbent.
The popularity of fluidized bed combustion is due largely to the clean coal technology's fuel flexibility - almost any combustible material, from coal to municipal waste, can be burned - and the capability of meeting sulphur dioxide and nitrogen oxide emission standards without the need for expensive add-on controls.
The clean coal technology programme led to the initial market entry of 1st generation pressurized fluidized bed technology, with an estimated 1,000 mw of capacity installed worldwide. These systems pressurise the fluidized bed to generate sufficient flue gas energy to drive a gas turbine and operate it in a combined-cycle, the DoE report adds.