Oxidative torrefaction and torrefaction-based biorefining of biomass: A critical review

Document Type : Review Paper

Authors

1 Department of Chemical and Process Engineering, University of Moratuwa, Moratuwa, 10400, Sri Lanka.

2 Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan 701, Taiwan.

3 Research Center for Smart Sustainable Circular Economy, Tunghai University, Taichung 407, Taiwan.

4 Department of Mechanical Engineering, National Chin-Yi University of Technology, Taichung 411, Taiwan.

Abstract

Torrefaction is a vital pretreatment technology for thermochemical biorefinery applications like pyrolysis, gasification, and liquefaction. Oxidative torrefaction, an economical version of torrefaction, has recently gained much attention in the renewable energy field. To provide necessary guidance for future research and commercial implementations, recent literature on inert and oxidative torrefaction was critically reviewed. The critical performance parameters of torrefaction for thermochemical biorefinery applications, such as solid yield, energy yield, carbon enhancement, higher heating value (HHV) enhancement, and energy-mass co-benefit index (EMCI), were analyzed. Agricultural waste, woody biomass, and microalgae were considered. The analysis reveals that woody biomass could benefit from oxidative or inert torrefaction equally. In contrast, inert torrefaction was more suitable for agricultural wastes and microalgae. The use of flue gas as the oxidative torrefaction medium and waste biomass as the feedstock could achieve a circular economy, improving the sustainability of oxidative torrefaction for thermochemical biorefineries. The significant challenges in oxidative torrefaction include the increase in ash content in torrefied agricultural waste, the oxidative thermal runaway of fibrous biomass during torrefaction, temperature control, and scale-up in reactors. Some proposed remedies are combined washing and torrefaction pretreatment, balancing oxygen content, temperature, and residence time, depending on the biomass type, and recirculating torrefaction gases.

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