(390f) CFD Investigation of Autothermal Biomass Pyrolizers

Autothermal biomass pyrolysis processes [1] are being developed by reaction engineers to address one of the main bottlenecks to the scale-up of biomass pyrolysis, which is the heat transfer limitation [1,2] due to the small heat transfer flux that can be sustained in the fluidized bed pyrolyzers used to conduct the process. Autothermal pyrolysis aims at addressing this limitation by allowing partial oxidation [1,3] of pyrolysis products, which allows the in-situ generation of heat needed to sustain the pyrolysis process.

The performance of autothermal pyrolizers may be affected by several design choices and by the composition of the biomass feed stream. The geometric configuration of the pyrolyzer and the location of the biomass feed may affect mixing of the biomass in the pyrolizer, impacting the residence time of the biomass and the product yield.

In order to investigate these effects, we have formulated a computational model to describe biomass fast pyrolysis. The model is based on the Euler-Euler multi-fluid model [4,5] with kinetic theory closures for polydisperse granular phases [6,7], supplemented by a comprehensive chemical kinetic mechanism [8–18] accounting for devolatilization, char combustion and gas-phase reactions. Results demonstrating the predictive capabilities of the computational models in comparison to experimental data of pyrolysis product yield will be presented.

The model was then used to study mixing of biomass and sand in a fluidized bed pyrolizer to understand the level of mixing obtained in the fluidized bed with different injection location

of the biomass. To this purpose, a mixing index insensitive to the spatial discretization of the computational domain was formulated, in order to systematically quantify the quality of the mixing of the granular mixture. Numerical experiments were first performed considering a cold flow in a laboratory scale fluidized bed with sand and biomass without accounting for the effect of eventual effects of the temperature distribution and of the chemical reactions.

The effects of heat transfer and chemical reaction are then introduced to investigate the impact of the location of the injection point of the biomass on the products in realistic operating conditions.

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