The effect of oxygen transport by catalytic bed materials in biomass gasification

Conference poster, 2013

Catalytic bed material is the most common primary measure to get a reasonable quality of the product gas in fluidized bed biomass gasifiers. The catalytic behavior is in most cases related to some sort of metal, often iron or nickel, which promote the cracking of the tars. However, the active material will also be oxidized and reduced and, hence, transport oxygen from the oxygen rich to the reducing areas in the gasifier. For an indirect gasifier this means that the bed material will not only crack some of the tars, it will also burn part of the product gas and, thereby, reduce the chemical efficiency, at the same time as the product gas will be diluted with carbon dioxide. In this work this effect has been investigated in the 2-4 MWfuel indirect gasifier at Chalmers, using silica sand as bed material blended with two different fractions(2 and 12 %) of Ilmenite (iron-titanium oxide) as active catalytic bed material, where the higher fraction used, correspond to the oxygen transport capacity. The effect of oxygen transport by catalytic bed materials in a biomass gasifier of the commonly used Olivine. The result shows an oxygen transport between the reactors in the same order as the theoretical maximal oxygen transport of Ilmenite, which means that this transport can significantly reduce the chemical efficiency of the process. The work also shows that the effect will be hidden; if not the mass balance of the process is properly closed. Considering the rapid oxidation and reduction of the active bed material a further conclusion from the work is that this will also have a significant effect in a direct gasifier. The provided oxygen in a direct gasifier will be consumed by the bed material and subsequently contribute to gas combustion instead of char conversion; as a matter of fact the conversion of the char is pushed towards the much slower gasification reactions