Straw combustion in a fixed bed combustor

Straw and herbaceous energy crops are key biomass materials for greenhouse gas neutral energy production. Combustion of straw and two herbaceous crops was investigated in a fixed-bed reactor for a range of air flow-rates (234-1170 kg/m² h). The fixed bed tests simulate the moving bed combustion where the distance along a grate corresponds to the time on the fixed bed. Measured temperatures, gas composition and mass loss were used to evaluate the combustion characteristics in terms of ignition front speed, burning rate, percentage of mass loss and the equivalence ratio. The average burning rates of herbaceous fuels reached a peak of 220-250 kg/m² h at air flow rates of 700-900 kg/m² h. An overlap of the two distinct periods of combustion was observed at high air flow rates, as the air supply was sufficient to simultaneously burn the char above the ignition front. When uncut straw was burned, less uniform packing of the long fibres increased the effect of air channelling and led to irregular propagation of the ignition front.