Microwave torrefaction for viable fuel production: A review on theory, affecting factors, potential and challenges

•Power level was a major parameter in affecting performance of torrefied products.•Dielectric materials and particle size affect significantly heating rate.•150 to 250 W was sufficient to produce a good quality torrefied products.•Clean-products derived from microwave heating technique can be obtained.•Dedicated operating conditions in MW heating resulted in high EROI (6.1 to 34). Microwave (MW) heating offers numerous advantages over conventional heating. Its characteristics of fast internal heating, volumetric heating, and low energy consumption enable microwave torrefaction (MWT) of biomass to be performed. The purpose of this article is to review the current state of MWT of biomass. A brief explanation of MWT, including experimental setups and the effects of several important parameters, are reviewed and discussed. Potential future directions and challenges regarding MWT are also discussed. It is known that the MW power level, types of MW absorbers (catalyst), and particle size can significantly influence the performance of torrefied biomass in terms of energy and mass yields, mass loss, H/C and O/C ratios, fuel ratio, and high heating value. The effect of operating condition on products derived from MWT needs to be further investigated in order to make the technology feasible and reliable. Overall, MWT has the potential to serve as energy-efficient method to improve the performance of biomass.