Fluidized bed combustion systems have only recently been applied in hazardous waste incineration. Fluidized bed incinerators may be either circulating or bubbling bed designs (Chang et al. 1987). Both types consist of single refractory-lined vessels partially filled with particles of sand, aluminum, calcium carbonate or other such materials. Combustion air is supplied through a distributor plate at the bottom of the combustor (Figure 11.14.5) at a rate sufficient to fluidize (bubbling) or entrain part of the bed material (recirculating bed). In the recirculating bed design, air velocities are higher and the solids are blown overhead, separated in a cyclone, then returned to the combustion chamber (Figure 11.14.6). Operating temperatures are normally in the 1400-1600°F range. Excess air requirements range from 25-150%.
Fluidized bed incinerators are used primarily for liquids, sludges, or shredded solid materials, including soil. To allow good circulation of waste materials and removal of solid residues within the bed, all solids require pre-screening or crushing to a size less than 2 in in dia.
Fluidized bed incinerators offer: high gas-to-solids ratios, high heat transfer efficiencies, high turbulence in both gas and solid phases, uniform temperatures throughout the bed, and the potential for in-situ gas neutralization by lime, limestone, or carbonate addition. Fluidized beds also have the potential for solid agglomeration in the bed, especially if salts are present in waste feeds.
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