To gasify or not to gasify torrefied wood? Investigating the effect of torrefaction on oxygen steam blown circulating fluidized bed gasification of wood, focusing on permanent gas and tar composition, and environmental performance

Biomass is a sustainable biofuel as long as it does not compete with food and feed production. Gasification is a versatile technology that produces a gas which can be converted into various high value products. Torrefaction is a technology that converts biomass to a more coal alike product with upgraded properties. As it was confirmed that torrefaction offers environmental benefits in co-combustion for electricity generation, it was decided to assess gasification of torrefaction wood from technical and environmental perspectives. Gasification of commercial torrefied mixed wood pellets resulted in in an increased gas quality and decreased tars quantity, leading in a positive affecting the gasification performance positively. On the other hand, the gasification of torrefied pure woods affected the gasification performance negatively; the effects on the gas quality were limited and the effect on the tar quantity was not the same for both pure woods. Furthermore, for a better understanding of the tar formation in the gasifier additional experiments were performed. Torrefaction resulted in increasing the phenol and decreasing the naphthalene mass fractions at the high temperature range. However, it did not show a significant effect on the polyaromatics species heavier than fluorene. Lastly, the environmental performance of a biorefinery based on gasification of torrefied wood for car fuels production showed significant environmental benefits in CO2 emissions reduction and air quality improvement of the broader area as well. To conclude, torrefaction adds benefits in the technical performance of the gasifier especially regarding the problematic tar content of the gasification gas, but it may decrease the gasification performance, and wood torrefaction integrated in a biorefinery system shows significant benefits to important environmental impacts.