Simulation of biomass particle evolution under pyrolysis conditions using lattice Boltzmann method

Abstract

To accurately characterize biomass fast pyrolysis at the particle scale, intra-particle transport phenomena needs to be considered together with the gas flow surrounding the particle. In this study, a detailed numerical method was used to simulate the evolution of biomass particles under fast pyrolysis conditions. To conduct particle-scale simulations, the lattice Boltzmann method (LBM) was employed to solve the conservation equations, and pyrolysis kinetics were implemented to describe the chemical reactions. The present model was validated by comparing the numerical results with experimental data for a single biomass particle under pyrolysis conditions. The predicted temperature and conversion history agree with the experimental data. The temperature and density fields in the particle were found to be anisotropic due to the effect of the gas flow surrounding the particle, which was ignored by the 1D model. The non-uniform distributions of surface temperature indicate that using a constant temperature or heat flux as boundary conditions may cause numerical errors. Sensitivity analysis shows that density is the most influential parameter while porosity is the least. The heat of reaction converting the intermediate solid to char is more dominant than those of the three primary reactions. A parametric study was conducted to investigate the effect of particle shape on conversion time and final product yields. The simulation results show that the conversion time decreased when using the elliptic particle instead of the regular (circular) particle. The model

Overview of Computational Fluid Dynamics Simulation of Reactor-Scale Biomass Pyrolysis

“…It is worth noting that with the increased computational power in recent years, a few attempts have been oriented to simulate biomass pyrolyzers by the so-called particle-resolved direct numerical simulation approach. − In such studies, the grid size to simulate gas flow is below the particle scale. Intraparticle transport phenomena are directly coupled with surrounding gas flow to substitute artificial boundary conditions at particle surface.…”

Heat-Transfer-Corrected Isothermal Model for Devolatilization of Thermally Thick Biomass Particles

“…Such a number of meshes can be used for a 2D coarse grid simulation of a pilot- or large-scale fluidized bed. As compared to the nonisothermal models of Papadikis et al , and Ström and Thunman and the lattice Boltzmann method, which requires additional computational cost for internal grid points, the biomass particles are not required to be discretized radially by using the heat-transfer-corrected isothermal model. Therefore, we expect that the corrected isothermal model has a better performance on computational efficiency, compared to the above-mentioned models.…”

“…Nonisothermal models have been implemented in CFD modeling of biomass devolatilization in fluidized bed reactors. − The individual biomass particles are discretized with a large number of mesh grid points (e.g., 50) to calculate the intraparticle heat transfer and devolatilization rate, resulting in a significant increase of computational demand in CFD. ,, To reduce the computational cost, Thunman et al proposed a four-layer model, in which a biomass particle is discretized into four layers (moist wood, dry wood, char, and core layers). This method needs only additional equations to describe the heat transfer between the different layers, thereby improving the computational efficiency in CFD modeling, as compared to other nonisothermal models. , However, it is still complicated to apply the four-layer model to simulate biomass devolatilization in fluidized beds because of highly diverse particle boundary conditions, and the computational cost is still considerably higher than that of the conventional isothermal model.…”

In situ observation of the evolution of polyaromatic tar precursors in packed-bed biomass pyrolysis

“…During pyrolysis, biomass is thermally decomposed in the absence of external oxidizing agents, to form solid (char), liquids (bio-oils, tars) and gaseous products. 1 The yield of these three products varies significantly depending on biomass precursors, reactor, sample configuration and process parameters. 2 During slow pyrolysis (∼50 K min −1 heating rate) in a fixed-bed reactor, carbon content originally present in the untreated feedstock progressively concentrates in the solid phase to form a porous carbonaceous material, known as biochar.…”