Modeling Biomass Particle Evolution at Pyrolysis Conditions Considering Shrinkage and Chemical Kinetics with Conjugate Heat Transfer

Abstract

Biomass pyrolysis is an attractive method to produce renewable biofuel in a short time scale. This paper discusses a computational study considering particle shrinkage and reaction kinetics to characterize biomass particle evolution under pyrolysis conditions. A representative elementary volume (REV) scale simulation was conducted in three dimensions to resolve the gas and solid domains. The interface was modeled using the conjugated heat transfer (CHT) and the adapted partially saturated method (PSM) in the lattice Boltzmann (LB) framework. A sufficient gas thickness was considered to simulate the interface physics, such as the thermal boundary layer and product gas outflux. The current simulation was validated using three different experiments. The particle conversion time, temperature, and shrinkage were well predicted. A parametric study investigated the effect of different modeling approaches and operating conditions. Results show that the internal heat transfer is affected by the particle size, inlet gas velocity, reactor temperature, and geometry. The Fourier number decreases to a nearly constant value as the particle size increases. The Péclet number increases linearly with the reactor temperature. Sub-particle scale simulation results reveal that the CHT model and permeability need to be considered when simulating gaseous product efflux out of the biomass particle. Three regimes are identified based on the particle permeability: diffusion-limited, mixed, and advection-limited. The gaseous product efflux delays the overall conversion in the mixed regime. A modified heat transfer coefficient is formulated based on the current numerical study.