Biomass oxy-CO2 gasification process for bio-methane production: an experimental and numerical activity

Biomass gasification is a thermochemical process in which the biomass is converted into a mixture of gases, called syngas, commonly utilised in thermal machines to produce electricity and heat. In the present research activity, the conventional air-gasification in downdraft gasifier is replaced by oxy-CO2 gasification technology. This strategy allows to obtain a nitrogen-free syngas, mainly composed by CO, H2, CO2 and CH4, which can be used into the synthesis of various bio-fuels like methanol or synthetic natural gas (SNG). Carbon dioxide is utilized as gasifying agent together with oxygen to mitigate the reactivity of the latter, which can lead to excessive reaction temperatures. In particular, the present work shows the preliminary results of an experimental campaign carried on utilizing a small scale downdraft gasifier (max thermal power of roughly 100 kW) fed with wood pellet and using a mixture of oxygen and CO2 as gasification agent. The experimental results have been utilized to calibrate a pseudo-kinetic model of the oxy-CO2 gasification process, implemented in Aspen Plus environment. To analyse the possibility to transform the produced syngas into methane, an additional numerical model of a methanation plant was then utilised. As a whole, the numerical analysis confirms its usefulness in predicting the performance of the gasifier, which can reach a cold gas efficiency of around 70%, while the methanation plant can achieve a production of roughly 20 kg of methane per 100 kg of gasified wood.