FBC in boilers at atmospheric pressure can be particularly useful for high ash coals, and/or those with variable characteristics. Relatively coarse particles at around 3 mm size are fed into the combustion chamber. Two formats are used, bubbling beds (BFBC) and circulating beds (CFBC).
There was rapid growth in the coal-fired power generation capacity using FBC between 1985 and 1995, but it still represents less than 2% of the world total.
Combustion takes place at temperatures from 800-900°C.
Bubbling beds use a low fluidizing velocity, so that the particles are held mainly in a bed which will have a depth of about 1 m, and has a definable surface. Sand is often used to improve bed stability, together with limestone for SO2 absorption. As the coal particles are burned away and become smaller, they are elutriated with the gases, and subsequently removed as fly ash. In-bed tubes are used to control the bed temperature and generate steam. The flue gases are normally cleaned using a cyclone, and then pass through further heat exchangers, raising steam.
Atmospheric BFBC is mainly used for boilers up to about 25 MWe, although there are a few larger plants where it has been used to retrofit an existing unit. There are hundreds of small BFBC units in China.
Overall thermal efficiency is around 30%.
Flue gas cleaning/emissions
Combustion takes place at temperatures from 800-900°C resulting in reduced NOx formation compared with PCC. Air staging can further reduce NOx formation. N2O formation is, however, increased. SO2 emissions can be reduced by the injection of sorbent into the bed, and the subsequent removal of ash together with reacted sorbent. Limestone or dolomite are commonly used for this purpose. A disadvantage of BFBC is that in order to remove SO2, a much higher Ca/S ratio is needed than in atmospheric CFBC. This increases costs, and in particular the cost of residues disposal.
The residues consist of the original mineral matter, most of which does not melt at the combustion temperatures used. Where sorbent is added for SO2 removal, there will be additional CaO/MgO, CaSO4 and CaCO3 present. There may be a high free lime content and leachates will be strongly alkaline. Carbon-in-ash levels are higher in FBC residues that in those from PCC.