Impact of particle-scale models on CFD–DEM simulations of biomass pyrolysis

Abstract

The performance of biomass pyrolysis reactors depends on the interplay between chemical reactions, heat and mass transfer, and multiphase flow. These processes occur over a wide range of scales ranging from molecular to reactor level. Accurate predictions of the reactor behavior necessitate integrating adequate kinetic and particle-scale biomass devolatilization models with reactor-level CFD simulations. Global kinetic schemes and homogeneous particle models neglecting spatial variations are commonly used in CFD simulations. Recent CFD investigations have focused on using a spatially resolved particle description modeled by the mass, species, and energy conservation equations. However, the impact of these particle-scale models on the CFD predictions is unclear. This work investigates the role of particle-scale models of biomass devolatilization in CFD–DEM simulations of biomass pyrolysis in fluidized beds. To this end, spatially resolved and homogeneous particle models using a multistep kinetic scheme (with 24 reactions, 19 solid species, and 20 gas species) are integrated with a CFD–DEM framework. The impact of particle-scale models on three-dimensional CFD–DEM simulations is assessed for low (Bi = 0.26) and high (Bi = 1.6) Biot numbers. The relevant time scales are computed to analyze the coupling among various processes. We show that the particle-scale models primarily affect the transient behavior of species composition and bed hydrodynamics within the fluidized bed and have negligible impact on the product composition and yield at the reactor outlet. The cost of CFD–DEM simulations remained unchanged while using the homogeneous model. In contrast, it increased by 20% using the spatially resolved intraparticle model. This increase in cost is attributed to solving the governing equations of the intraparticle model and storing data for a spatially resolved biomass particle.