Catalytic effect of biomass ash on CO, CH4 and HCN oxidation under fluidised bed combustor conditions

Abstract

In fluidised bed combustion heterogeneous reactions catalysed by the bed material, CaO, and char are significant for the emission levels for instance of NO, N2O, and CO. The catalysts present in the bed affect significantly the selectivity of HCN and NH3 oxidation, which are known as precursors of NOx (i.e. NO and NO2) and N2O emissions from solid fuel combustion. Thus the catalytic activity of biomass ashes may also be responsible for the negligible N2O emissions from biomass combustion due to the presence of a large amount of solids in fluidised bed combustion, homogeneous oxidation may be suppressed within the bed by the quenching of the radicals. For this reason the catalytic oxidation of hydrocarbons and CO on the bed material may be of significance for the total burnout within the fluidised bed combustor.Within this study the effect of different ashes from spruce wood, peat, and for comparison bituminous coal on the oxidation of CH4, CO, and HCN was studied. The different ashes were shown to have a strong catalytic activity for the oxidation of CH4, CO, and HCN. In HCN oxidation the selectivity towards NO is high, whereas very little N2O is formed. The activity of the ashes is strongly dependent on the fuel, which may be explained by their composition.The kinetics of the oxidation of CO and HCN in the temperature range relevant for fluidised bed combustion, i.e. 800–900°C, has been evaluated for spruce wood ash.