Computational Fluid Dynamics Modeling of Biomass Gasification in a Fluidized Bed Reactor

Computational Fluid Dynamics (CFD) modeling of gasification is proven to be an effective strategy to examine the multi-physics of a gasification system and also as a means to estimate or measure internal parameters of a gasifier which otherwise will be indeterminate through costly and time-consuming experimentation. An Eulerian--Eulerian CFD model was developed to simulate the biomass gasification process in a bubbling fluidized bed gasifier (BFBG). The gasification model considered three individual phases of biomass, bed material of sand and gas in a BFBG. This model considered the complicated gasification reaction kinetics including moisture evaporation, devolatilization or primary pyrolysis, gas-solid heterogeneous reactions and gas-gas homogeneous reactions. The biomass particles were modeled as a composite based on their proximate compositions. The solid-solid contact heat transfer due to the stochastic interactions of biomass particles and the heating sand bed material were also modeled. As a direct result of the implementation of particle-particle heat transfer model due to the collision of the biomass particles and the bed material of the gasifier, CH4 content in the syngas increased by 32% (from 3.6 to 4.7 vol. %) at the gasifier outlet whilst CO2 content increased by 52% (from 5.5 vol. % to 8.3 vol. %) for gasification conducted at an equivalence ratio (ER) of 0.3. A similar trend was seen at 0.4 ER with CH4 and CO2 mole fractions increasing by 18% (from 3.4 to 4.0 vol. %) and 7% (from 7.6 to 8.1 vol. %) at the gasifier outlet, respectively. It was also clear from the wide variations in percentage increments of CO 2 and CH4 produced for gasification at 0.3 ER and 0.4 ER that, while particle-particle heat transfer may have some impact on the production of CH4 and CO2, the influence of ER on the synthesis gas composition and quality was significant.