Performance of the CFD model of the pyrolysis of a single cylindrical and spherical maple wood particles in a thermally thick regime over a range of temperatures and sizes

Predicting pyrolytic conversion of larger particles of wood (thermally thick regime) is a complex task due to heat transfer limitations, which detailed influence is still not fully known. The CFD modelling allows for the real-time investigation of parameters that are impossible to be analysed experimentally. That can provide new insights into the conversion mechanism and help to fill the knowledge gaps. The aim of this study was to establish a comprehensive single particle model of wood pyrolysis in the thick regime, valid over a wide range of parameters, to serve as a reliable tool for process optimisation and product tailoring. The investigation was based on the 2D-axisymmetrical models of the particles (without surroundings), developed in the commercial software COMSOLTM v5.3. The particles were modelled as porous matter with solid and fluid phases. As the kinetic was used the RAC scheme. Models included heat transfer (conduction, convection, and radiation), the vapours flow (Darcy’s) and mass diffusion (Fick’s), but not shrinking (software’s limitations). The thermochemical properties and the boundary conditions were based on the experimental data of Attreya et al. [1], also used for the validation. The experimental data consisted results of the maple wood particles with cylindrical shape (D/H [mm/mm] = 10/20, 15/20 and 20/20) and spherical shape (D [mm] = 10, 15 and 20), pyrolysed in temperatures 500 °C, 610 °C, 720 °C and 840 °C (in total 24 scenarios). All models were checked for grid independence to avoid numerical errors in the results of the simulations. The fit of the predicted profiles of temperature at the centre and mass loss to experimental data had more than satisfactory considering a broad range of conditions. The absolute error between experimental and predicted char yield was 1.3±0.7 wt. % and 1.8±1.5 wt. % for the spheres and cylinders, respectively. The yields of products per initial mass did not change linearly with the particle’s size for the same shape, and such an outcome was assigned to the influence of the heat transfer limitations. The temperature distribution and the release profiles of vapours indicated the existence of the outer thermal layer (c.a. 2.5 mm thickness), which had a significantly higher heating rate than the inner areas of the particle. The developed model is suitable for the detailed investigation of the pyrolysis process. The particle shrinking and the adjustment of the heat transfer have to be implemented to increase the model’s accuracy. The existence of the layer has to be experimentally proven considering the numerical foundation of the observation. If confirmed, the relation between, e.g., the thermal conductivity of the wood and the outer layer thickness should be investigated. [1] Atreya A, Olszewski P, Chen Y, Baum HR. The effect of size, shape and pyrolysis conditions on the thermal decomposition of wood particles and firebrands. International Journal of Heat and Mass Transfer. 2017;107:319-28