A volume-of-fluid model for biomass particle pyrolysis

TL;DR

The paper addresses the gap in biomass pyrolysis modeling where solid-phase evolution and surrounding gas-phase dynamics were treated separately. It introduces a single-grid Eulerian Volume-Of-Fluid (VOF) framework that resolves both phases, incorporating porosity evolution, particle shrinkage, and anisotropic transport within a porous biomass pseudo-phase, with interface conditions computed directly from the surrounding flow. The numerical method couples mass, momentum, energy, and species transport with a stiff chemical-kinetics solver, includes Darcy–Forchheimer drag, and ensures mass conservation while allowing complex shapes and shrinking to be captured. Validation against isotropic, wooden, and anisotropic particles shows good agreement with temperature profiles, mass loss, and volatile species trends, while highlighting the need for deeper understanding of internal biomass structure evolution. The open-source Basilisk implementation enables reproducibility and further development toward predicting yields, degradation times, and pollutant formation in sustainable pyrolysis processes.

Abstract

Numerical models of biomass particle pyrolysis focus on either the solid particle evolution or on the surrounding gas-phase dynamics, neglecting the coupled interactions between the two. This work addresses this limitation by proposing a single-grid model that fully resolves both phases without relying on sub-grid-scale correlations. The model adopts an Eulerian representation of the two-phase system, using a Volume-Of-Fluid (VOF) method to track the interface between the biomass and the surrounding gas phase. Solid-phase pyrolysis reactions are included, and a novel approach is proposed to capture the coupling between the evolution of biomass porosity and the particle shrinkage. The anisotropic nature of the biomass particle is accounted for in this multidimensional framework. The resulting model demonstrates mass conservation and numerical convergence. Extensive validation with experimental data shows excellent agreement in terms of mass and temperature profiles and correct volatiles trends. Shrinking profiles reveal correct trends, but they also highlight the need for a better fundamental understanding of the evolution of the biomass structure. Overall, the model takes a step forward in aiding the development of sustainable pyrolysis processes. The code and simulation setups, developed within the open-source Basilisk framework, are made publicly available.

Figures (13)

• Figure 1: Control volume encapsulates both the external environment and the pseudo-phase with interface $\Gamma$ and interface normal $\mathbf{n}_\Gamma$
• Figure 2: Results for the simulation of the idealised pyrolysis case for the different $Z$ functions. The reported data refer to the simulation performed at the maximum grid refinement case. (a) Radial porosity distribution: the black line represents the initial conditions, while the vertical lines highlight the position of the interfaces. (b) Map of the porosity field at end time for the smooth function case, the dashed line represents the initial position of the interface
• Figure 3: Relative error on the mass conservation at increasing grid refinement. (a) Conservation error in solid mass. (b) Conservation error in produced gas mass. The abscissa represents the maximum number of cells for each dimension of the domain.
• Figure 4: Computational domain for isotropic sphere with imposed surface temperature profile (case \ref{['sec:isotropic-sphere']})
• Figure 5: Temperature profiles inside the particle, experimental (points)corbetta2014pyrolysis versus numerical (lines) results