CFD Study on Fluidized Bed Pyrolyzers

Abstract

Fluidized bed pyrolyzers are commonly used for biomass fast pyrolysis to maximize the pyrolysis oil yield. A CFD study was performed on homogeneous secondary pyrolysis reactions that cause a reduction of liquid fraction yield and to propose certain design and operation considerations to reduce the extent of secondary reactions. Two models are utilized for the study. A particle pyrolysis model uses a kinetic model from literature to describe the pyrolysis progress of wood particle. The second model involves the use of commercial CFD code ANSYS FLUENT to model a fluidized bed pyrolysis scenario and secondary reactions based on the modelled pyrolysis progress from the first model. The effects and inter-relationship between feedstock size, fluidized gas temperature, vapour residence time and liquid fraction yield are discussed. It is found that the fluidized gas temperature is a more critical factor than vapour residence time in causing secondary reactions, while feedstock size only has a little effect. In addition, the downward flow of the pyrolysis volatiles observed along the reactor wall has to be considered in the design of the feeding and fluidized bed system.