Drying of Thermally Thick Wood Particles: A Study of the Numerical Efficiency, Accuracy, and Stability of Common Drying Models

Abstract

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.

Review on Modelling Approaches Based on Computational Fluid Dynamics for Biomass Pyrolysis Systems

“…The kinetic model assumes a first-order Arrhenius reaction of the liquid water phase turning into vapour. In work by Haberle et al [64] a summary of the commonly used parameters for this model can be found. The kinetic model is very convenient, but it treats a physical phenomenon via a chemical description, so it does not reflect the process well in real terms.…”

“…The heat sink model (thermal drying model, heat flux model) [57,64,65] assumes that water evaporation in a representative volume occurs only at the boiling temperature, and the temperature stays constant until all water is evaporated. To maintain a constant temperature, the evaporation reaction needs to consume all the energy transferred to the representative volume.…”

“…The heat sink model of Lu et al [65] assumes that the boiling temperature of water is fixed at 373 K. Nevertheless, strong local evaporation can cause noticeable changes in pressure which shifts the boiling temperature. The pressure effect on the boiling temperature can be modelled as [64]:…”

“…(15.23)), which is hard to handle by a numerical solver and results in numerical instability [57,66]. The step function was investigated by Haberle et al [64], who advised using an evaporation fraction factor (fevap) as the multiplier of the heat flux. The purpose of this limiting factor is to reduce the amount of the heat sunk by the evaporation reaction.…”

A Two-Dimensional Study on the Effect of Anisotropy on the Devolatilization of a Large Wood Log

“…The presented equations only consider the devolatilization stage, since in the current test run, drying and char conversion are not modeled. Further details on the model development can be found in earlier works by Haberle et al [11,12]. Table 1.…”