Cfd simulations of fluidized bed Biomass gasification

Abstract

CFD simulation of fluidized bed biomass gasification process has been carried out in the present work. The gas-solid interaction, thermal-flow behavior and gasification process inside a fluidized-bed biomass gasifier are studied using the commercial CFD solver ANSYS/FLUENT13.0. Velocity profile, bed expansion, solid movement, temperature profile, species mass fractions have been focused in the present work. Three phases are used to model the reactor (sand, solid phase for the fuel, and gas phase). All phases are described using an Eulerian approach to model the exchange of mass, energy and momentum. In the present work rice husk is considered as feed material and sand is taken as the inert bed material. The influences of particle properties viz. particle size (530μm, 856μm) and other operating parameters namely, gas velocity (0.05-2 m/s) and temperature (600-1000K) of the gasifier have been investigated comprehensively. It is found that superficial gas velocity has a strong influence on the axial solids velocity and subsequently on the down flow of solids. Gas temperature and species distributions indicate that reactions in the instantaneous gasification model occur very fast and finish very quickly. Temperature of 1000K, superficial velocity of air of 0.7m/s is found to be most favourable for gasification of rice husk with an indication of 100% carbon conversion. On the other hand the reactions in the finite-rate model involve gas-solid reactions which occur slowly with unburnt chars at the exit. The mass fractions of product gas are also validated with the experimental data. Thus the developed simulation model will be a powerful theoretical basis for accurate design of FBG.