CFD Modeling of a Downdraft Gasifier with Woodchips used as Feedstock

Abstract

CFD analysis is a useful tool that can be used to optimize the design of gasifiers. Although plenty of CFD analysis has been performed on fluidized bed gasifiers, little analysis has been performed on the biomass gasification of solid fuels due to the difficulty in modeling the chemical phenomena in the combustion of solids. In order to simplify calculations, previous CFD simulations have modeled fuel beds by creating homogenous beds with uniform spacing or beds with simplified shapes like spherical particles. However, in real experimental conditions, fuel beds are non-homogenous and particle position is randomized. This study aimed to fill the gap of research simulations on realistically modeled fuel beds. In this study, a downdraft gasifier was modeled using cubic woodchips as fuel and air as a gasifying agent. Woodchips were individually placed in the gasifier in random orientation to imitate the positioning of when biomass is fed from the top of a gasifier. CFD analysis was performed on the effect of air flow rate and feedstock properties on air velocity, air trajectory, and average pressure throughout the gasifier. The applications of these analyses were demonstrated in this paper; they include blower size optimization, pressure drop calculation for non-uniform beds, combustion zone range determination, air inlet design, and velocity and pressure profile generation and analysis. An optimal flow rate was chosen based on previous experimentation and results of the optimized flow rate for this gasifier were given in this paper.