Simulating Thermal Wood Particle Conversion: Ash-Layer Modeling and Parametric Studies

摘要

In this work, we study the thermochemical degradation and char conversion of wet wood particles. The work is split in two main parts: (1) the effect of the ash layer handling approach and (2) a parametric study over different relevant parameters. In the study of the ash layer handling, we investigate the effect of allowing the ash to remain on the surface of the particle when the char is converted (Model A), in contrast to removing the ash such that the reacting char layer is always exposed (Model B). It was found that the two modeling concepts yield significantly different mass losses and surface and center temperature predictions. Model B presents a faster thermal conversion, while the results predicted by Model A are in better agreement with what has been observed experimentally. A parametric study was also done, where the sensitivity to variations in thermal conductivity, specific surface area, and gas permeability was studied. It was found that thermal conductivity influences the time when drying and devolatilization are accomplished. This is because these conversion stages are heat-transfer-controlled. Char conversion is primarily affected by a shift to earlier times for the initialization of the final char conversion when higher thermal conductivities are used. It is found that the specific surface area smaller than a critical value can significantly affect the final char conversion time. Since char conversion is a key stage of wood combustion, the full conversion time is also affected. The gas permeability primarily affects mass diffusion into the particle. It was found that, up until a critical effective gas permeability, the modeling results are sensitive to assigned permeabilities.