This paper reviews the current state of the art of numerical models used for thermochemical degradation and combustion of thermally thick woody biomass particles. The focus is on the theory of drying, devolatilization and char conversion with respect to their implementation in numerical simulation tools. An introduction to wood chemistry, as well as the physical characteristics of wood, is also given in order to facilitate the discussion of simplifying assumptions in current models. Current research on single, densified or non-compressed, wood particle modeling is presented, and modeling approaches are compared. The different modeling approaches are categorized by the dimensionality of the model (1D, 2D or 3D), and the one-dimensional models are separated into mesh-based and interface-based models. Additionally, the applicability of the models for wood stoves is discussed, and an overview of the existing literature on numerical simulations of small-scale wood stoves and domestic boilers is given. Furthermore, current bed modeling approaches in large-scale grate furnaces are presented and compared against single particle models.

The primary focus of this paper is on studying dierent numerical models for drying of wet wood. More specically, the advantages and disadvantages of the models with respect to numerical eciency, stability and accuracy are investigated. The two basic models that are studied in detail are the thermal drying model and the kinetic rate drying model. The drying models have been implemented in an in-house simulation tool that solves for drying and devolatilization of a one-dimensional cylindrical wood log.It is found that the choice of drying model can signicantly inuence the computational time associated with the thermal conversion. Furthermore, the occurrence of numerical pressure oscillations in the thermal drying model has been found and investigated. The numerical oscillations are reduced by introducing an evaporation fraction, f evap . When the thermal drying model is applied, the drying zone is very thin, commonly only including one grid point, which can result in numerical instabilities. The evaporation fraction allows the smearing of the drying zone by reducing the heat ux used for evaporation of liquid water and using the residual heat ux for heating up the grid points. Reducing the evaporation fraction also resulted in reduced CPU times. It was found that model accuracy was not signicantly inuenced by the choice of drying model.

A one-dimensional (1D) comprehensive combustion model for thermally thick wet wood particles, which is also applicable for studying large wood logs, is developed. The model describes drying, devolatilization and char gasification as well as char oxidation.Furthermore, CO oxidation is modeled, in order to account for the fact that exiting gas products can be oxidized and therefore limit the oxygen transportation to the active sites. The challenges for model validation are outlined. Model validation was done against experimental data for combustion of near-spherical wood particles. Furthermore, the validated model was up-scaled and the effect of wood log diameter on the thermal conversion time, the extent as well as the position of drying, devolatilization and char conversion zones were studied. The upscaling was done for cylindrical wood logs with an aspect ratio of 4. The thermal conversion time significantly increased with the size. It was also found that the relative extent of the drying, devolatilization and char conversion zones decreased as wood log size increased. The paper concludes with recommendations for future works.

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.

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