Catalytic decomposition of biomass tars: Use of dolomite and untreated olivine

Although biomass is getting increased attention as a renewable energy source, one of the remaining problems still to be solved is the reduction of the high level of tar present in the product gas from gasification of biomass. The purpose of the present work is to study the activity of olivine and dolomite for tar destruction. Some researchers investigated olivine as bed material for biomass gasification. But it is not yet known how tars behave in the presence of olivine and whether olivine has some activity towards tar destruction. A slipstream from a lab-scale atmospheric bubbling-fluidised-bed gasifier (located at ECN) is passed through a secondary fixed-bed reactor where the additives are placed. For easy understanding, the results are represented in terms of the following tar classes; GC-undetectable tars (class 1), heterocyclic compounds (class 2), aromatic compounds (class 3), light polyaromatic compounds (class 4), heavy polyaromatic compounds (class 5). The general observation is that the conversion of all tar classes increases as the temperature was raised from 800 to 900 °C for both additives. The water-soluble heterocyclic compounds can be easily converted by thermal treatment. At the temperature of 900 °C, the water-soluble heterocyclic compounds are completely converted. A 48% decrease in heavy PAHs is observed with pure sand. Addition of 17 wt% olivine to the sand leads to a 71% decrease of PAHs at 900 °C, whereas addition of 17 wt% (pre-calcined) dolomite converted 90%. Also improvement in conversion of other tar classes is observed when olivine and dolomite are added during hot gas cleaning. A total tar amount of 4.0 g m0-3 could be reduced to 1.5 and 2.2 g m0-3 using dolomite and olivine, respectively, at a temperature of 900 °C. Inspite of this reduction in total tar concentration, a limited impact on the tar dewpoint is observed.