Analysis of Charcoal Gasification on a Downdraft Fixed Bed Gasifier by CFD Modeling

Abstract

Gasification is a remarkable alternative to produce either thermal or electrical energy using renewable resources like biomass. Several developments have been performed in order to increase the energy value of syngas resulting from biomass gasification; nevertheless, the complexity of the reactions occurring inside the reactors and the differences of the raw materials employed have created the necessity of analyzing the internal performance of the equipment. This work aims to simulate the thermal performance of a lab-scale downdraft fixed bed gasifier by means of a Euler-Euler multiphase Computation Fluid Dynamics (CFD) model, and validate it in accordance with experimental data of charcoal gasification. The model, built on a 2D axis-symmetric domain, comprises heat, mass transfer equations, and homogeneous and solid-gas heterogeneous reactions. Mesh quality was evaluated in accordance with squish, skewness and aspect ratio criteria. The composition of the syngas varied with the airflow rate; however, CO content was between 24 and 27%, whereas CO2 was in the range of 4.4 and 2.8%. The higher the CO content, the greater the energy value of the syngas. Small prediction errors, obtained after comparing predicted CO and CO2 content in syngas with experimental data indicated good accuracy of the CFD model; nonetheless, further research is needed in order to decrease differences on the temperature distribution along the reactor.