Analysis of the Imbert downdraft gasifier using a species-transport CFD model including tar-cracking reactions

Abstract

Primary tar reduction is an effective method to diminish tar concentration in producer gas obtained from a downdraft biomass gasification process. It tends to be more economical than secondary tar reduction in post-gas-cleaning processes. This study used a two-dimensional computational fluid dynamics (CFD) model to analyze the performance of an Imbert downdraft biomass gasifier. The spatial distribution of producer gas compositions in the gasifier was explored. The CFD model was validated, using the same conditions and design, by published experimental data. The effects of the key design parameter, i.e., throat diameter, represented by the ratio of throat to gasifier diameters and the height of air nozzle from the throat, on the gasifier performance were predicted. The tar concentration and cold gas efficiency (CGE) were computed. Although the modeling results indicated that decreasing the throat diameter can decrease the tar content up to 0.005 g Nm−3, the CGE decreased by approximately 7% compared with the scenario with no throat. Decreasing the nozzle-to-throat lengths abated tar and hydrogen composition, but the CGE exhibited fluctuation. Therefore, a compromise between tar reduction and the CGE is required to improve the design of the Imbert downdraft gasifier.