The fluidized bed furnace comprises a combustion chamber containing a sand bed acting as the heat transfer medium. Commercial units are generally large-scale and capital intensive and as such are less suited for application in developing countries. These systems are also suitable for particulate biomass residues of relatively high moisture content. The biomass is transported by a screw-feeder through a specially constructed trough into the middle of the furnace. From the sides of the trough primary air is forced through the biomass mound.
Secondary air is introduced near the top of the mound allowing complete fuel combustion.
There has been relatively little application of under-fed stokers in developing countries. The biomass is pyrolysed to produce combustible gases and wood tars. These products of pyrolysis are then used as a fuel. Since good combustion control can be obtained with gasifiers the hot gases of combustion can be employed to direct-fired dryers. The experience of briquetting agricultural residues has been mixed.