Detailed kinetic mechanism of devolatilization stage and CFD modeling of downdraft gasifiers using pelletized palm oil empty fruit bunches

Abstract

Oil palm residues, such as palm oil empty fruit bunches (PEFBs), are typically abandoned due to high moisture and bulky. Pelletization and downdraft gasification constitute a promising solution for small-scale power and heat production. In this study, the two-dimensional, steady-state computational fluid dynamics (CFD) model of a modified Imbert downdraft gasifier consuming pelletized PEFBs is developed. The detailed kinetic mechanism of biomass devolatilization coupled with particle-scale transport equations is incorporated to enable accurate prediction of the pyrolysis zone in the gasifier. Pelletized-biomass-particles packed bed is simulated by using a porous media model with volumetric species-transport CFD model, which is validated by comparing the prediction of the species volume fractions at the gas outlet with published experimental data. Velocity and temperature profiles of the downdraft gasifier are revealed, along with tar evolution pathline. The effect of the equivalence ratio on the producer gas composition, tar concentration, and cold gas efficiency is investigated. The optimum equivalence ratio is found at 0.30 based on the tar concentration of 5 mg Nm−3 and the cold gas efficiency of 62%.