Design and Performance Evaluation of an Inclined Nozzle and Combustor of a Downdraft Moving Bed Gasifier for Tar Reduction

Abstract Tar formation is one of the major problems during biomass gasification due to blocking and fouling process in engines and turbines. Downstream gas cleanup process and conditioning are required to eliminate tar issues. For this reason, methods to reduce tar formation inside the gasifier are gaining more attention. In this work, a novel design for an inclined nozzle and a combustor is proposed and tested in a low-tar biomass (LTB) gasifier to evaluate the effect on tar reduction. The aim was to obtain a tar-free producer gas, which could be directly fed to an internal combustion engine or a gas turbine for power generation. The design process is based on a swirling flow created by an inclined nozzle that allows good mixing between pyrolysis gases and gasifying air. The swirl combustor creates large internal annular and reverse flow zones with the help of swirl flow. Thus, the combustor had a positive effect on the combustion of the mixture of air and pyrolysis gases and induces thermal cracking in the partial oxidation zone. Recycling ratio (recycle gas/air, v/v) and combustion degree of volatiles were used to evaluate the performance of the inclined nozzle and combustor. The result shows that the tar yield in pyrolysis and partial oxidation zone was on average 1693 mg/Nm3 and 255 mg/Nm3, which accounts for a reduction of 86.6% and 12.7% tar compounds in these zones, respectively. At an equivalence ratio of 0.35, the new design had the optimal performance with a recycling ratio of 0.35 and combustion degree of volatiles of 98.6%. Besides, the producer gas leaving the gasifier has a tar content below 1%, and measured tar content was between 7.4 and 27.14 mg/Nm3 for tested biomass with moisture contents of 9-34%.