Burning coal and biomass to generate power while reducing emissions at the same time, Circulating Fluidized Bed (CFB) technology uses fluidization to mix and circulate fuel particles with limestone as they burn in a low-temperature combustion process. Unlike conventional steam generators that burn the fuel in a massive high-temperature flame, CFB technology does not have burners or a flame within its furnace.
As described in ASM International's Advanced Materials & Processes magazine, the limestone captures the sulfur oxides as they are formed, while the low burning temperature minimizes the formation of nitrogen oxides. The fuel and limestone particles are recycled over and over back to the process, which results in high efficiency for burning the fuel, capturing pollutants, and for transferring the fuel's heat energy into high quality steam to produce power.
Due to the vigorous mixing, long burning time, and low temperature of the combustion process, CFBs are fuel flexible, which means they can cleanly burn traditional coal fuels, as well as "carbon neutral" biomass and waste fuels. This ability to cleanly burn virtually any combustible material greatly surpasses the fuel limitation of conventional combustion processes.
Unlike conventional steam generators, CFBs capture and control harmful pollutants during the burning process and do not need to rely on add-on pollution control equipment. In addition, Foster Wheeler Power Group, Clinton, N.J., has applied highly efficient vertical-tube, supercritical steam technology to their utility-scale CFB designs. Supercritical steam enables more of the fuel's energy to be transferred to the steam. This improves power plant efficiency, reducing the amount of fuel needed for electricity production and further reducing emissions.
Due to its ability to burn carbon-neutral fuels such as biomass along with coal, CFB technology offers a unique solution to the CO2 issue. Biomass is considered carbon neutral since it absorbs and stores carbon from the atmosphere during its growth cycle through the natural photosynthesis process. When biomass is burned, it releases the same carbon back to the atmosphere, resulting in nearly zero net carbon emitted to the atmosphere. However, due to the world's limited and undeveloped biomass supply chain, existing biomass power plants are limited to about 25 to 50 MWe in size. The plant's small scale, coupled with its fuel supply limitation, translates into electricity costing about 20 to 30% more than that from conventional fossil power plants.
Again, the CFB offers a solution. Due to its fuel flexibility, a large scale (300 MWe or larger) CFB power plant can be built to burn a combination of coal and several types of biomass. This solution captures both the environmental benefit of reducing CO2 emissions, and the economic benefit of providing affordable electricity. It is also flexible enough to utilize more biomass when it is available, or fall back on coal when it is not. This concept can produce a substantial reduction in CO2 emissions.
The entire article, written by Robert Giglio and Justin Wehrenberg of the Foster Wheeler Power Group, Clinton, N.J., was published originally in the May 2009 issue of Advanced Materials & Processes magazine and may be accessed free of charge at www.asminternational.org/amp.
