Fluidized Bed Pyrolysis of Biomass: A Model-Based Assessment of the Relevance of Heterogeneous Secondary Reactions and Char Loading

A model of non-catalytic fast fluidized bed pyrolysis of biomass has been developed with the aim of investigating the relevance of secondary heterogeneous reactions between primary products of biomass decomposition and the char accumulated in the bed. The fate of biomass and the extent of biomass and char holdup in the pyrolyzer have been modeled by considering entrainment and elutriation of biomass and char particles, char attrition, as well as bed drain/regeneration. The kinetics of primary and secondary pyrolitic reactions is modeled according to a semi-lumped reaction network using kinetic parameters selected among published correlations. The rate of heterogeneous volatile-char secondary reactions has been modeled borrowing a kinetic expression from the neighboring area of tar adsorption/decomposition over char during biomass gasification. Model computations are helpful to assess the role of heterogeneous vapor-char secondary reactions. The sensitivity of the pyrolyzer performance on char loading in the bed as a result of combined char entrainment, elutriation, attrition, and bed drain/regeneration, the extent of gas-phase backmixing, the process temperature, the gas superficial velocity and residence time, and the particle size is demonstrated and discussed. Model results provide useful guidelines and pinpoint future research priorities for optimal design and control of fluidized bed pyrolyzers.