ARS | Publication request: CFD modeling of space-time evolution of fast pyrolysis products in a bench-scale fluidized-bed reactor

Submitted to: Applied Thermal Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 25, 2011
Publication Date: October 29, 2011
Citation: Boateng, A.A., Mtui, P.L. 2012. CFD modeling of space-time evolution of fast pyrolysis products in a bench-scale fluidized-bed reactor. Applied Thermal Engineering. 33-34:p.190-198.

Interpretive Summary: The design of reactor systems for the production of biofuels from biomass resources can be complex. This often requires lengthy experiments on prototype machines to understand and extract parameters needed for the design of larger systems. This undertaking could be expensive especially when smaller units that defy economies of scale are to be mass-produced. Computer models can take the place of such expensive experiments and might be used in the virtual design of systems large and small. We applied such computer models to simulate the fast pyrolysis process, the production of liquid fuels and associated coproducts (biochar and gas) in the absence of air to understand the mechanics underlying the evolution of the products so experiments would not be repeated before designing new systems. The model results confirmed our experimental observations that the space-time evolution of products occurred at a very fast rate. The quantities of liquid, biochar and gas predicted for the pyrolysis of switchgrass, corn stover and soybean straw compared favorably with experimental data. The model could be advantageous in the virtual design of fast pyrolysis reactors and their optimization to meet economic scales required for distributed or satellite units.

Technical Abstract: A model for the evolution of pyrolysis products in a fluidized bed has been developed. In this study the unsteady constitutive transport equations for inert gas flow and decomposition kinetics were modeled using the commercial computational fluid dynamics (CFD) software FLUENT-12. The model system described herein is a fluidized bed of sand externally heated to a predetermined temperature prior to introduction of agricultural biomass. We predict the spontaneous emergence of pyrolysis vapors, char and non-condensable (permanent) gases and confirm the observation that the kinetics are fast and that bio-oil vapor evolution is accomplished in a few seconds, and occupying two-thirds of the spatial volume of the reactor as widely reported in the open literature. The model could be advantageous in the virtual design of fast pyrolysis reactors and their optimization to meet economic scales required for distributed or satellite units.