Co-pyrolysis and co-gasification of coal and biomass in bench-scale fixed-bed and fluidised bed reactors

Tar and total volatile yields from the co-pyrolysis and co-gasification of coal and biomass samples were described (850 degrees C and 1000 degrees C; up to 25 bar). Two reactors were used: a fixed-bed reactor providing intimate contact between neighbouring fuel particles and a fluidised-bed reactor designed to provide near total segregation of sample particles. In pyrolysis experiments, neither intimate contact between fuel particles, nor their relative segregation appears to lead to synergistic effects. Small differences in tar yields were observed in the fixed-bed reactor when using a combination of Daw Mill coal and silver birch wood. Similar behaviour was observed under CO2-gasification. However this weak trend appears to be sample dependent, for a second sample set, total volatile yields were about 5% higher than would be expected in the absence of synergistic effects. The differences were systematic and larger than levels of experimental repeatability, but too small to provide information that might explain them. Isothermal combustion reactivities of Daw Mill coal and silver birch wood (mixture) chars prepared at 1000 degrees C, in the fixed-bed reactor, were almost twice as high as those of coal Char alone, despite the total gasification of the wood at this temperature. Mineral matter residue from the wood appears to have played a catalytic effect during combustion. When chars were separated with solvent, the effect could not be observed. The analogous effect observed with a second pair of samples was smaller, suggesting that the composition of ashes from different samples play a role. Apart from the catalytic effect of biomass ashes formed at high temperatures, observed synergistic effects were not of a magnitude that might influence process design. No evidence of synergy was found with the fluidised-bed reactor, thereby demonstrating the lack of contact between coal and biomass particles in the system. (C) 1999 Elsevier Science Ltd. All rights reserved.