Since gasification involves only partial oxidation of the fuel, the heat released inside a gasifier is only a fraction of the fuel’s heating value, and part of it is absorbed by the gasifier’s endothermic reactions. Thus, it is undesirable to extract any further heat from the main gasifier column. For this reason, the height of a fluidized-bed gasifier is not determined by heat-transfer consider­ations as for fluidized-bed boilers. Instead, gas and solid residence times are major considerations.

The total height of the gasifier is made up of the height of the fluidized bed and that of the freeboard above it:

Total gasifier height = bubbling bed height (depth) + freeboard height (6.31)