Recent developments in investigating reaction chemistry and transport effects in biomass fast pyrolysis: A review

Fast pyrolysis of biomass converts it mainly into bio-oil, which is incapable of being utilized directly as drop-in fuel because of high oxygen content, unstable nature, and lower heating value. The composition of bio-oil decides its quality, fitness for upgrading, and environmental influence. However, it is controlled by numerous essential pyrolysis reactions, which are difficult to characterize because of the multiphase thermal degradation of biomass happening in short time scales with inter-related reaction chemistry and transport effects. This review paper critically analyses the current progress on essential pyrolysis reactions, from reaction-controlled pyrolysis experiments and molecular simulations. In experiments, recently employed Frontier Micropyrolyzer, PHASR reactor, Wire mesh reactor, and Pyroprobe with the allied analytical system revealed essential pyrolysis reactions (i.e., glycosidic bond cleavage, dehydration, and successive fragmentation of C-6 or C-5 compounds, etc.). The effect of transport on individual pyrolysis products, especially forming bio-oil, is described using transport controlled experiments. Besides, the role of catalysts in altering biomass pyrolysis reactions, and hence bio-oil composition, is highlighted through experimental and theoretical findings. The mechanistic insight of biomass compounds breaking (validated with experiments), with and without catalysts, is presented. Eventually, the particle level reaction-transport models capturing the inter-related effects of pyrolysis reactions (as reaction kinetics) and transport processes, under different pyrolysis conditions, are discussed. The collective information provided in this review would be beneficial for biomass pyrolysis investigators in designing operating conditions for the conversion of several biomass feedstocks into bio-oil, similar to drop-in fuel.