A Review of Torrefaction Technology for Upgrading Lignocellulosic Biomass to Solid Biofuels

The utilization of lignocellulosic biomass is closely related to one of the renewable sources of energy. However, a few inherent properties of lignocellulosic biomass such as high moisture content, low density, high volume, and heterogeneous composition make it unfavorable for bulk transportation, storage, handling, and conversion. The pretreatment of biomass is found to resolve a few of the aforementioned limitations while increasing the conversion efficiency of lignocellulosic biomass into biofuels. This article reviews the opportunities, challenges, and state-of-art research on torrefaction as a widely used biomass upgrading technique for solid fuel production and thermochemical biomass conversion. Some notable applications of such pretreatments in high-value solid biofuel production, densification, combustion, co-firing, gasification, and metallurgy have been reviewed. The broad changes in physicochemical characteristics and structural chemistry of lignocellulosic biomass because of torrefaction have been thoroughly described. This review also comprehends the effects of different process parameters and operating conditions of torrefaction of lignocellulosic biomass to produce high-value solid fuel. This article attempts to highlight some recent advancements in biomass torrefaction technology concerning the fundamental characteristics of biomass and process operation and optimization as well as the evolution of physicochemical features of torrefied biomass. Lastly, the value-added industrial applications of torrefaction technology and torrefied biomass are also elucidated.