A computational framework for the pyrolysis of anisotropic biomass particles

This work aims presenting a new computational framework (bioSMOKE) for modeling the thermochemical conversion of anisotropic biomass particles. It allows for the solution of the Navier-Stokes equations, applied to porous media, providing a better insight into the relative role of transport phenomena and chemical kinetics. The code is embedded within the OpenFOAM® framework and allows managing the spatial discretization of the governing equations on arbitrary geometries. One of the original aspects of this work is represented by the development of a comprehensive approach to model the anisotropic particle shrinking during the thermal decomposition. It is based on the evaluation of a shrinking wave front which progressively evolves during the transient and allows for handling arbitrary multi-dimensional particle deformations due to anisotropy and/or non-uniform boundary conditions. Despite the outstanding importance of this topic, only few models are available in the literature, usually based on empirical correlations or fitting parameters. The code has been validated against comparisons with experimental data of particle pyrolysis in different ranges of operative conditions. Three different geometries were analyzed (sphere, cylinder and slab), to show the reliability and the capability of bioSMOKE in handling particle shrinking. The model turns out to be in reasonable agreement with the experimental data and it is able to properly predict the temperature history of the particle, the mass/volume loss, as well as the evolution of the major species released during the pyrolysis. The deep understanding of the main chemical/physical phenomena occurring at the particle scale is an essential step toward the multiscale-based analysis of thermochemical processes involving solid particles. In view of this, the proposed approach results being extremely useful for a rational modeling at the reactor scale.