Fluidized bed gasifiers have a lower gas flowrate, higher residence time, uniform temperature, and good mixing which avoids clinker formation and possible defluidization. They are well suited for active reactants such as bio­mass and low-rank coal. If char particles are entrained, they can be recycled back by a cyclone. Ash particles that are removed at the bottom can exchange heat with incoming recycled gas and steam. Fluidized bed reactors require moderate oxygen and steam requirements and extensive char recycling. They can accept a wide variety of feedstock (e. g., solid waste, wood, and high ash coals) with a larger particle size then what is normally required for the entrained flow gasifiers. Commercial applications of fluidized bed reactors are high-temperature Winkler (HTW) and KRW designs, the latter gasifier is a part of the Pinon Pine Coal gasification plant [6].

Different fluidized bed gasifiers can vary in ash conditions (dry ash or agglomerated) and design configurations to improve char use. Both dry ash and agglomerating fluidized beds are operated with crushed feed (about M in. size). The acceptability of fines is good for the dry ash bed and better for the agglomerating bed. The reactor is generally operated at 925-1025°C. The caking coal may be processed in a dry ash fluidized bed, but it can cer­tainly be processed in an agglomerating bed. The dry ash bed is preferred for low-rank coal and agglomerating bed can process any rank of coal. The main technical issue of the fluidized bed is the carbon conversion per pass. For this reason, for a less reactive coal mixture, a circulating fluidized bed is preferred.

Mixed feedstock has been tested in fluidized bed reactors at the Royal Institute of Technology at Stockholm, Sweden. They found that for a coal- biomass mixture, the char from woody biomass is very sensitive to the ther­mal annealing effect which occurred at low (650°C) temperature and short soak time (less than 8 min). A mixture of birch and coal gasification [106] showed synergies by an enhanced gasification rate in the presence of oxy­gen and reduction of char formation. Also both tar and ammonia formations were lowered in the mixture gasification. Currently, there are very few large fluidized bed gasifiers in operation.