Modeling Tar Recirculation in Biomass Fluidized Bed Gasification

A biomass gasification model is proposed and applied to investigate the benefits of tar recirculation within a gasification plant. In the model, tar is represented by the four species phenol, toluene, naphthalene, and benzene. The model is spatially one-dimensional, assuming plug flow for the gaseous compounds and perfect mixing of the fuel particles in the bed; it includes fuel particle heating and drying, bed fluidization, pyrolysis, a kinetic reaction mechanism, entrainment of char particles from the fluidized bed into the freeboard, the motion of char particles in the freeboard and in pipes, and the particle and gas flow within a cyclone. An uncertainty analysis identifies the most critical parameters of the model that severely affect the results of simulation. The model is validated against experimental data obtained at a pilot gasifier plant. A detailed parameter study suggests that an efficiency rise can be expected, the tar and soot content in the produced gas will be lowered, the heating value of the gas can be increased, and the bed temperature will decrease when recirculation is applied. Critical operational conditions leading to an unlimited tar accumulation in the gasifier are identified.