Biomass Gasification with Steam in Fluidized Bed: Effectiveness of CaO, MgO, and CaO−MgO for Hot Raw Gas Cleaning

The upgrading of the raw hot gas from a bubbling fluidized bed biomass gasifier is studied using cheap calcined minerals or rocks downstream from the gasifier. Biomass gasification is made with steam (not air) at 750−780 °C and about 0.5−1.0 kg of biomass/h. Calcined solids used are dolomite (MgO−CaO), pure calcite (CaO), and pure magnesite (MgO). Variables studied have been temperature of the secondary bed (780−910 °C), time of contact or space-time of the gas (0.08−0.32 kg·h/m3n), and particle diameter (1−4 mm) and type of mineral. Their effects on tar conversion, tar amount in the exit gas, product distribution, and gas composition are presented. Using a macrokinetic model for the tar disappearance network, the activities of the stones are expressed by their apparent kinetic constant. Apparent energies of activation for tar elimination (42−47 kJ/mol) and preexponential and effectiveness factors are given for all tested solids of which the most active is the calcined dolomite.