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 processes 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 was proposed and tested in a low-tar biomass (LTB) gasifier to evaluate the effect on tar reduction. The design process was based on a swirling flow created by an inclined nozzle that allows good mixing between pyrolysis gases and gasifying air. The swirl combustor created 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 induced 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 showed that the tar yields in pyrolysis and partial oxidation zones were on average 1693 mg/Nm3 and 255 mg/Nm3, respectively which accounted for a reduction of 86.6% and 12.7% tar compounds in these zones, respectively. Besides, the producer gas leaving the gasifier had a tar content below 1%, and measured tar content was 7.4-27.14 mg/Nm3 for the tested biomass with moisture contents of 9–34%. Tar removal efficiency was found on average 84.9% after passing the pyrolysis gas through the partial oxidation zone, whereas tar in the producer gas was removed with an efficiency of 99.1% at the gas outlet zone. 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%. The achieved producer gas could be directly fed to an internal combustion engine or a gas turbine at small to medium level power generation in remote rural off-grid areas.