Fractionation of biomass and plastic wastes to value-added products via stepwise pyrolysis: a state-of-art review

Pyrolysis has been considered as a promising thermochemical process that can convert biomass in nonoxidizing atmospheres to value-added liquid bio-oil, solid biochar, and noncondensable gas products. Fast pyrolysis has a better economic return because of the valuable biofuel production (e.g. bio-oil, syngas). Because of the complexity and heterogeneity of the feed-stocks, the one-step pyrolysis often leads to the mixed, acidic, and highly oxygenated liquid products. Moreover, the downstream processes (e.g. deoxygenation) for the desired fuels require high costs on energy and catalysts consumption. Stepwise pyrolysis is defined as a temperature-programmed pyrolysis that can separately obtain the products from each temperature step. It is a feasible approach to accomplish the fractionation by optimizing the pyrolysis process based on the decomposition temperature ranges and products among the biomass constituents. In recent years, the stepwise pyrolysis technology has gained attentions in thermochemical conversion of complex organic solid wastes. Through the stepwise pyrolysis of a real waste, oxygenated and acidic products were concentrated in the first-step liquid product, whereas the second-step product normally contained a high portion of hydrocarbon with low acidity. The stepwise pyrolysis of biomass, plastics, and their mixtures is comprehensively reviewed with the objective of fully understanding the related mechanisms, influence factors, and challenges.