Numerical simulation of biomass fast pyrolysis in an auger reactor

• Integrated CFD and chemical kinetic simulations on biomass fast pyrolysis were conducted.
• Both fluidized-bed and auger reactors were studied.
• Simulation results agree well with experimental data and effects of operating conditions were characterized.

Fast pyrolysis is an effective means of converting solid low-energy density biomass to intermediate energy products, such as bio-oil. In this study, computational fluid dynamics (CFD) simulations of the biomass fast pyrolysis processes in both the fluidized-bed and the auger reactors were conducted. The numerical results were validated by experimental data. Materials of all phases were modeled as interpenetrating continua by use of a multi-fluid model. Chemical reactions were simulated using a multi-step reaction mechanism that considers multiple compounds in biomass and various gaseous species in the reactor. In the study of biomass fast pyrolysis in the fluidized-bed reactor, CFD simulation results were analyzed to obtain the average residence time of materials in the reactor. This residence time was used as the time for chemical reactions in estimating the final product yields. The estimated product yields agreed well with the experimental data. To simulate the complex geometry and fluid dynamics in the auger reactor, a numerical approach based on a rotating reference frame was incorporated into the multi-fluid model to account for the auger rotation. Good levels of agreement between the predicted and measured product yields were obtained. A parametric study was conducted to characterize the effects of operating conditions on product yields. The study shows that the optimal wall temperature for maximum tar production was approximately 823 K. The tar yield decreased with increased pre-treatment temperature of the biomass feedstock. It was also found that increased inlet nitrogen flow rate was beneficial to tar yield, while increased biomass feed rate had a negative effect on tar production.

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