Integrated high temperature gas cleaning: Tar removal in biomass gasification with a catalytic filter

A nickel-based catalytic filter material for the use in integrated high temperature removal of tars and particles from biomass gasification gas was tested in a broad range of parameters allowing the identification of the operational region of such a filter. Small-scale porous alumina filter discs, loaded with approximately 2.5 wt% Al2O3, 1.0 wt% Ni and 0.5 wt% MgO were tested with a particle free synthetic gasification gas with 50 vol% N-2,, 12 vol% CO, 10 vol% H-2, I I Vol% CO2, 12 vol% H2O, 5 vol% CH4 and 0-200 ppm H2S, and the selected model tar compounds: naphthalene and benzene. At a typical face velocity of 2.5 cm/s, in the presence of H2S and at 900 degrees C, the conversion of naphthalene is almost complete and a 1000-fold reduction in tar content is obtained. Technically, it would be better to run the filter close to the exit temperature of the gasifier around 800-850 degrees C. At 850 degrees C, conversions of 99.0% could be achieved in typical conditions, but as expected, only 77% reduction in tars was achieved at 800 degrees C. Conversion data can be reasonably well described with first order kinetics and a dominant adsorption inhibition of the Ni sites by H2S. The apparent activation energies obtained are similar to those reported by other investigators: 177 kJ/mol for benzene and 92 kJ/mol for naphthalene. The estimated heat of adsorption of H2S is 71 kJ/mol in the benzene experiments and 182 kJ/mol in the naphthalene experiments, which points at very strong adsorption of H2S. Good operation of the present material can hence only be guaranteed at temperatures above 830 degrees C mainly due to the strong deactivation by H2S at lower temperatures. (c) 2005 Elsevier B.V. All rights reserved.