Integrated process of lignocellulosic biomass torrefaction and pyrolysis for upgrading bio-oil production: A state-of-the-art review

Renewable fuels or chemicals from lignocellulosic biomass have the potential to be a substitute for fossil fuels, thereby reducing greenhouse gas emissions and diversifying energy supplies. Integrating torrefaction and pyrolysis is a feasible and promising technology that converts biomass into fuels or chemicals. Understanding of the relevant process designs and mechanisms is favorable to the cohesion and optimization of these two processes and the innovation of reactors for commercial-scale biorefineries. First, biomass properties and their corresponding pyrolysis behaviors have been discussed in consideration of the challenge presented by complex biomass structures that limit the in-depth research on torrefaction or pyrolysis. Second, torrefaction fundamentals are illustrated in detail, and many kinetic models with comprehensive mechanism schemes, such as pseudo-mechanistic model and one-, two-, or multi-step models, are summarized. The effect of torrefaction on biomass characteristics and subsequent pyrolysis is reviewed to further elucidate the integrated process. The novel integration of torrefaction and up-to-date pyrolysis techniques is also outlined to improve product quality. Finally, future directions and technological challenges associated with the integrated process are proposed and its economic potential is also evaluated.