Olivine or dolomite as in-bed additive in biomass gasification with air in a fluidized bed: Which is better?

The raw gas from a fluidized-bed biomass gasifier should have very low tar, ammonia, and particulates content, to make it easy to clean for its eventual use in gas engines or gas turbines. Besides an optimized design and operation of the gasifier, it requires the use of in-bed catalytic additives. Four available and competitive additives have been compared in this work: a calcined dolomite (OCa.OMg), natural and sintered olivines ((Mg,Fe)(2)SiO4), and a Ni-olivine catalyst that was developed at the University of Strasbourg. They were tested under very similar experimental conditions in two small-scale pilot plants: the first pilot plant was based on a circulating fluidized-bed (CFB) gasifier, and the second pilot plant was based on a bubbling fluidized-bed (BFB) gasifier. The tar content at the gasifier exit when using dolomite was, on average, only 60% (+/-10%) of the tar content when natural or raw olivine was used. This showed that dolomite was 1.40 times more active than olivine in biomass gasification with air. Nevertheless, dolomite generates 4-6 times more particulates or dust and also some extra NH3 in the gasification gas than olivine. Under the conditions used in this work (gasification with air), the Ni-olivine catalyst was not very active for tar elimination and it deactivated very quickly. Much of the data on gasification gas provided in this paper was obtained under operations similar to those found in commercial biomass gasifiers.