The intensive mixing in a fluid bed has its advantages in promoting excellent mass and heat transfer. In fact, a fluid bed approaches the ideal of a continuously stirred tank reactor. This does, however, have its disadvantages. There is wide range of residence times for the individual particles, which are distributed evenly over the whole volume of the bed. Thus removal of fully reacted particles, which consist only of ash, will inevitably be associated with removal of unreacted carbon. The best of existing fluid-bed processes only have a carbon conversion of 97%. This is in contrast to moving-bed and entrained-flow processes, where carbon conversions of 99% can be obtained. Only in pressurized biomass gasification have fluid-bed processes efficiencies of 99% been reported (Kersten 2002).
Many attempts (e. g., Synthane or Ну-Gas) have been and are being made to introduce some staging effect by which the last carbon in the ash to be discharged is converted with (part of) the incoming blast, but this remains difficult as fluid beds lend themselves poorly to staging because of the “no-go” temperature zones mentioned earlier in this chapter.
The ash of fluid-bed gasifiers comes available in a highly leachable form. This problem is exacerbated when limestone is added in order to bind the sulfur present in the gas and to avoid the need for wet sulfur removal processes. The limestone is never completely converted into gypsum, and hence the ash will always contain unconverted lime. Similar problems are associated with fluid-bed combustion processes. Adding limestone to the fluid bed is an example of doing two things at once. It looks elegant, but you are never free to optimize both desulfurization and gasification or combustion.
Ash particles are removed from below the bed and/or from the cyclones in the top of the bed. In the former case they are sometimes used for preheating the blast. Fluid-bed gasifiers may differ in the manner in which ash is discharged (dry or agglomerated) and in design aspects that improve the carbon conversion. Some “carbon stripping” of the char containing ash to be discharged with incoming oxygen is possible.