2-D CFD modeling for steam gasification of a large biomass char particle

Abstract

A detailed nonisothermal, two-dimensional computational fluid dynamics (CFD) model was developed for steam gasification of a large biomass char particle and computed by COMSOL Multiphysics. It incorporated chemical kinetics and transport processes inside the particle and the gas boundary layer, pore evolution, and thermophysical property changes. The model was satisfactorily validated with the present and reported experimental results over a range of reactor temperature (923–1123 K) and particle size (8–32 mm) for casuarina and acacia chars. The nonisothermal model was far more accurate than the isothermal model with the RMS relative error being 0.04 and 0.71 for casuarina char. Similarly, the RMS relative error was found to be 0.017 (2-D) and 0.113 (1-D) for casuarina and 0.019 (2-D) and 0.117 (1-D) for acacia char. The use of nonisothermal and 2-D features assumes greater importance at higher reactor temperatures and smaller L/D ratio. Casuarina was found to be more reactive and a better-suited suitable biomass than acacia for gasification. A simulation study investigated the effect of the reactor temperature, particle size and char reactivity on the conversion, gasification rate of biomass char, and gas composition within the particle. The reaction started with shrinking core model, shifting subsequently to shrinking reactive core model. A higher temperature favoured the peak rate and total production of H₂. The sensitivity analysis indicated the highest sensitivity for the reactor temperature. The proposed and validated simple but robust model has provided crucial insight and guidelines on biomass gasification behaviour and the design of an industrial biomass gasifier.