Computational fluid dynamics modeling of thermo-chemical processes in an updraft biomass gasifier

Abstract

Biomass is recently gaining popularity in industry as a promising source of renewable energy. Gasification of biomass is a major thermal conversion method to improve the efficiency of raw biomass fuel. It is a process by which biomass is partially oxidized to produce a combustible gas named Syngas; a mixture of carbon monoxide, hydrogen and methane. Although the gasification technology is used throughout the history and there are a large number of gasification plants worldwide, their smooth operation remains questionable. This is due to a lack of understanding of proper design criteria. In order to gain insights to optimal design parameters, mathematical models and computer simulations based performance analysis can be used. Recently Computational Fluid Dynamics (CFD) analysis has been applied by many researchers as a tool for optimizing packed bed processes including gasification process. In this research study, a two dimensional CFD model has been developed for an updraft biomass gasifier. The model uses air as the gasifying medium and a fixed batch of biomass. The model is capable of tracking the movement of interface between solid packed bed and gas free board due to bed shrinkage. The two phase model is developed using the Euler-Euler approach. The model consists of several sub models, including reaction models, turbulence model for packed bed gas phase and free board, a radiation model for solid phase, a bed shrinkage model, and interphase heat transfer models. The final mathematical model is converted into a numerical model using open source CFD tool OpenFOAM. Required code was developed by using C++ language in OpenFOAM package, including all the relevant differential equations and procedures in the CFD model. To validate the CFD model, simulation results for gas temperature and gas compositions are compared against experimental gas temperatures and compositions measured from an operational laboratory gasifier. The validated model is used to perform air flow rate optimization. A series of CFD simulations were performed for air flow rates ranging from 3 m3/hr to 10 m3/hr for a computational geometry corresponding to the experimental gasifier and cumulative CO was calculated. It is found that cumulative CO production maximized at 7 m3/hr airflow rate. The maximum cumulative CO volume was 6.4 m3.