Performance of different dolomites on hot raw gas cleaning from biomass gasification with air

Calcined dolomites (CaO-MgO) from four different quarries have been tested for the upgrading of the hot raw gas from a fluidized bed gasifier of biomass with air. These calcined dolomites have big macropores (900-4000 Angstrom) and low (3.8-12 m(2)/g) BET surface areas. They have been tested in a fixed bed of 6 cm i.d. downstream from the air-blown biomass gasifier. The change in gas composition (contents in H-2, CO, CO2, CH4,...), tar content, gas heating value, etc., has been studied at different temperatures (780-920 degrees C) as well as space-times for the gas in the bed (0.03-0.10 kg.h/m(3)) and the type of dolomite. Increasing the equivalence ratio used in the gasifier and decreasing the WC ratio of the gas increases the refractoriness of the tars to be eliminated by the calcined dolomite. Activation energies (100 +/- 20 kJ/mol) and preexponential factors for the overall tar elimination reaction have been calculated for the different dolomites under realistic conditions. The activity of the dolomite for tar elimination can increase by 20% on increasing its pore diameter or its Fe2O3 content. Comparison of results with similar ones obtained in biomass gasification with steam is also presented.