Co-pyrolysis of coal and raw/torrefied biomass: A review on chemistry, kinetics and implementation

Thermochemical conversion via co-pyrolysis has the potential to be an efficient route for converting biomass to bio-energy and bio-refinery products. In this review, the implementation of co-pyrolysis of torrefied biomass and coal was critically assessed against co-pyrolysis of raw biomass and coal from both a fundamental and engineering perspective. This evaluation showed fundamental advantages for torrefaction of biomass prior to co-pyrolysis such as a decrease in mass and heat transfer limitations due to an increase in permeability and thermal conductivity of biomass. Co-pyrolysis volatiles may also be upgraded through the catalytic activity of the torrefied biomass surface, producing higher quality oil. Due to properties more similar to coal, torrefied biomass requires less energy for milling (lower operating costs) and can be more easily blended with coal in reactor feeding systems. A state-of-the-art research on co-pyrolysis kinetics revealed that reactivities of blends may be predicted from kinetic parameters of individual feedstocks using an additive approach. To conclude on the preferred reactor design for this process, different reactors were evaluated based on heat transfer mode, operation and product formation. Although both the fluidized bed and rotating cone reactor provide high oil yields, the rotating cone has been more successful commercially. This design shows great promise for specifically co-pyrolysis due to the intimate contact that may be achieved between fuels to maximize synergy. The co-pyrolysis of torrefied biomass and coal may be encouraged from a scientific point of view, however further research is recommended on the effective integration of torrefaction and co-pyrolysis technologies.