Any fluid bed depends on having the solid particles of a size that can be lifted by the upward flowing gas. A large portion (over 95%) of the solids content of the bed of a gasifier is ash, which remains in the bed while the carbon leaves the reactor as syngas. If the ash content of the fuel, be it coal, biomass, or other, should start to soften sufficiently that the individual particles begin to agglomerate, these newly formed larger particles will fall to the bottom of the bed, and their removal poses a considerable problem. For this reason fluid-bed gasifiers all operate at temperatures below the softening point of the ash, which is typically in the range of 950°C- 1100°C for coal and 800-950°C for biomass.
On the other hand, the lower the temperature of the gasifier operation the more tar will be produced in the product synthesis gas. This is partly due to the fact that when the coal particles are heated slowly, more volatiles are produced, as discussed in Chapter 3. It is also due to the fact that at lower temperatures there is less thermal cracking of the tars produced.
The best known fluid-bed gasifiers that have no tar problems are regenerators of catalytic cracking units which operate under reducing—that is, gasification—conditions and can be found in some refineries. In these units the carbon residue that remains after gases and volatiles have been extracted from the feed, is gasified in a bubbling fluid-bed.
These operate at a few bar overpressure and temperatures of below 800°C. Maximum throughputs are 500 t/d carbon.