Biomass gasification with air in a bubbling fluidized bed is studied in a small pilot plant. Variables analyzed are equivalence ratio (from 0.20 to 0.45), temperatures of the gasifier bed (750−850 °C) and of its freeboard (500−600 °C), H/C ratio in the feed, use of secondary air (10% of the overall) in the freeboard, and addition (2−5 wt %) of a calcined dolomite mixed with the biomass used as the feedstock. Using advanced tar and gas sampling and analysis methods, the gas composition and tar content in the gas are determined and their variation with the operation parameters is given. A statistical analysis of the effects of the gasification variables is also here presented.

The upgrading of the raw gas from a biomass gasifier is studied with the commercial steamreforming BASF G1-25 S nickel-based catalyst. It is located downstream of the gasifier, a bubbling fluidized bed type in which air is used as gasifying agent. To increase the catalyst lifetime, a guard bed of a calcined dolomite at 800-850 °C is used. It decreases the throughput of tar entering the catalytic bed to amounts below 2 g tar/m 3 (NC). This work is focused only on the catalytic bed which easily decreases the tar content in the gas to only 1-2 mg/m 3 (NC). Variables studied include the particle diameter of the catalyst, time-on-stream, temperature of the catalytic bed, and gas and tar compositions. Both tar and gas compositions in the catalytic (Ni) reactor depend on the equivalence and H/C ratios existing in the gasifier and on the operating conditions of the guard bed of dolomite. A simple kinetic model is used to describe the overall tar elimination network. Its overall kinetic constant is used as index of the catalyst activity for tar elimination. Values of this overall kinetic constant are given for very different operating conditions.

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 Å) and low (3.8−12 m2/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 H2, CO, CO2, CH4, ...), tar content, gas heating value, etc., has been studied at different temperatures (780−920 °C) as well as space-times for the gas in the bed (0.03−0.10 kg·h/m3) and the type of dolomite. Increasing the equivalence ratio used in the gasifier and decreasing the H/C 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.

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