Numerical modeling of a deep, fixed bed combustor

A computational model to evaluate the anticipated performance characteristics of a deep, fixed bed combustor/gasifier utilizing whole trees as the source of fuel is presented. This combustor/ gasifier is the heat source for a proposed steam-driven electric power plant utilizing whole trees as the source of fuel. In the simulation model presented, hardwood logs 20 cm in diameter are burned in a 3.7 m deep fuel bed. Solid and gas velocity and CO, CO2, H2O, hydrocarbon, and O-2 profiles are calculated. This deep bed combustor obtains high energy release rates per unit area due to the high inlet air velocity and extended reaction zone. The lowest portion of the overall bed is an oxidizing region and the remainder of the bed acts as a gasification and drying region. The overfire air region completes the combustion. Approximately 40% of the energy is released in the lower oxidizing region. The wood consumption rate obtained from the computational model is compared with test results obtained from full scale testing. The wood consumption rate predicted by the model is 2630 kg/(m(2) h) which matches well the consumption rate of 2670 kg/(m(2) h) observed during the 2 h test period of the field test. This corresponds to a heat release rate of 9.6 MW/m(2). The model is used to investigate the performance of the combustor under a variety of load conditions, fuel sizes, and moisture conditions.