While in fixed bed combustion the solid fuel is either burnt on a pile or on a grate and there is minimal relative motion between the fuel bed and the surface on which it is laying, in moving bed combustors fuel particles are suspended in a turbulent flow of air therefore even the presence of a fuel bed may be questioned. In moving bed combustors small particle size becomes essential to guarantee the suspension, and therefore the functioning of the burner, while in fixed beds small particle size may lead to unburned particles in the flue gas and consequent fouling and particulate emissions. Moving bed combustors are divided in two categories: suspension burners, where only fuel particles are present in the air stream, and fluidized bed burners, where hot inert sand is also suspended in the air stream, to increase the heat transfer to the fuel particles.

5.3.3.1 Suspension burners

In suspension burners (Fig. 5.19) biomass fine and dry particles are blown inside the combustion chamber in a high turbulent motion by a stream of preheated primary air and are burnt before
the air stream leaves the furnace. To guarantee complete combustion a massive fuel pretreatment is necessary and particle size and humidity usually do not exceed respectively 15% and 0.6 cm, therefore for economic reasons usually they are limited to ready-to-use biomass by-products such as wood dust, rice husk etc. Some burners would require an auxiliary oil or gas pilot burner to maintain combustion, especially if the radiation from the refractory lining is not adequate to maintain the necessary temperature.