A comprehensive pyrolysis model for lignocellulosic biomass particles with a special emphasis on the anisotropic characteristics

Abstract

The porous and fibrous structural tissue of lignocellulosic biomass serves as a transport channel for water and nutrients along the growth direction, indicating a highly anisotropic nature of biomass. The anisotropies of the thermal and structural properties of biomass have substantial influences on particle-scale reactions, transport processes, and morphological evolution during thermal conversions. An in-depth investigation of the anisotropic characteristics of these physicochemical phenomena in pyrolysis benefits in the understanding of biomass gasification and combustion processes. In this study, a comprehensive 2D axisymmetric mathematical model is developed to quantitatively analyze the pyrolysis behaviors of a cylindrical wood particle with or without anisotropy. Correlative unsteady energy and mass conservation equations are combined with a multistep kinetic model of biomass devolatilization. To improve the numerical descriptions of volatiles flowing in a porous biomass matrix, the constitutive equations of the Brinkman equation and dusty gas model are adopted. Moreover, a binary differential equation is used to calculate the simultaneous changes in the pore structure and shrinkage of the wood particles. When considering anisotropy, validations of the model against literature data demonstrate that it can reliably predict the intraparticle temperature profiles, weight loss, and particle shrinking degree of lignocellulosic biomass. Using this model, the coupling effects of multi-physical fields and reactions inside pyrolyzing wood particles are explored. It is revealed that the anisotropic characteristics of biomass influence not only the orientations of intraparticle heat and mass transport processes but also their mechanisms, resulting in varying degrees of secondary reactions and particle shrinkage.