Computational Modeling of Biomass Thermochemical Conversion in Fluidized Beds: Particle Density Variation and Size Distribution

The design and scale-up of fluidized-bed reactors is an important step to commercialize viable conversion pathways (such as fast pyrolysis) for biomass into hydrocarbon intermediates and fuels that lead to “drop-in” replacements for jet fuel, diesel, gasoline, and other petroleum-based products. Detailed information about the particle size distribution (PSD) and particle density evolution throughout the fluidized-bed reactor can play a critical role in determining in situ catalyst selectivity, intermediate components, and reactor performance. This work presents an Euler–Euler computational fluid dynamics (CFD) model applied to biomass thermochemical conversion for use in fluidized-bed reactor simulations. The complex chemical and physical processes of particle devolatilization and their interaction with the reacting gas environment are described within a multifluid framework based on the kinetic theory of granular flows. The direct quadrature method of moments is used to describe the biomass PSD. Continuously varying particle density due to mass evolving to the gas flow was applied to describe the evolution of particles’ physical properties. The global kinetic model is based on superimposed hemicellulose, cellulose, and lignin reactants. The calculations of the stiff chemical source terms and convection are decoupled using a time-splitting method. The CFD model is applied to simulate the fast pyrolysis of red oak in a laboratory-scale fluidized-bed reactor and validated against experimental data. The simulated product yields at the reactor outlet are presented and compared with monodisperse results and the experimental measurements. It is demonstrated that our current CFD model is able to predict in detail the dynamic particle processes, mixing and segregation, char particle elutriation, and produced gas composition at the reactor outlet needed to optimize the reactor operating conditions