TABLE OF CONTENTS The flexibility in fuel utilization, mainly the ability to use low-quality fuels, makes FBC boilers an attractive technology. These fuels can be fired solely, in combination with other low-grade fuels, or cofired with coal. The lower combustion temperatures permit burning high-fouling and -slagging fuels at temperatures below their ash fusion temperature. This greatly reduces operating problems associated with these fuels; however, care is still required because these fuels are those containing significant concentrations of alkali and alkaline earth elements, as discussed in more detail in Section 8.7. In addition, fuels with low heating values, due to high moisture and/or ash contents, or low volatile matter contents can be successfully burned using an FBC boiler because of the large mass of hot bed material and long residence time that the fuel spends in the bed. Examples of these include fuels such as brown coal, peat, and sludge with moisture contents up to 60%; waste coals with ash contents up to 76% and higher heating values as low as 2,600 Btu/lb; and petroleum coke with volatile matter content less than 10%. The fact that many of these low-grade fuels are difficult to reduce to fine size, due to high ash contents or fibrous structures (as in the case of biomass), makes them candidates for FBC technology, since the fuel does not need to be pulverized but can be processed to sizes of 0.25 inches 0.
Although a main advantage of FBC boilers is...
