3D-CFD simulation of catalytic filter candles for particulate abatement and tar and methane steam reforming inside the freeboard of a gasifier

Biomass gasification is a very promising process to produce energy from agricultural wastes; however, tar and particulate have to be removed in order to make the product gas exploitable. Catalytic filter candles inserted in the freeboard of a fluidized bed gasifier, perform hot syngas cleaning from particulate and promote the steam reforming reactions of tar and methane thanks to the addition of Ni-catalyst in their inner space. In this work, a 3D-CFD model is implemented in the FLUENT software, to simulate 6 catalytic candles housing in the freeboard of a pilot scale dual bubbling fluidized bed steam gasifier (100 kWth as biomass input). Main model parameters were derived from experimental results obtained with a bench scale gasification reactor, equipped with a ceramic filtering candle containing a commercial catalyst, to allow conditioning the raw syngas produced on site. Simulations were carried out to study the conversion of tar and methane in a real case application. The temperature drop along the gasifier freeboard causes very low conversion (≈15%) of tar produced inside the gasifier fluidized bed. It was found that small injections of O2 in the freeboard can rise the temperature and increase tar conversion up to 77%. Finally, simulations with raw syngas input subjected to primary tar removal by means of Fe-olivine as fluidized bed material resulted in high conversion rates and residual tar content lower than 1 g/Nm3.