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

Nils Erland L Haugen,Yvind Skreiberg

doi:10.3390/en12234430

Abstract

In this work, a two-dimensional (2D) model for wood particle drying, devolatilization, and char conversion is presented and validated against experimental studies of devolatilization of a large, dry, cylindrical birch wood log. The wood log’s internal distributions of temperature, solid species densities, gas density, and gas species mass fractions, together with the internal pressure and the gas flow velocities in radial and longitudinal directions, were studied. It was found that the internal pressure peak significantly depended on the permeability of char. The velocity profiles changed as the pressure wave traveled radially inward in the same manner as the devolatilization zone. The wood log internal pressure was not only dependent on wood and char permeabilities but also depended on the devolatilization rate, which itself is a heat-controlled conversion stage. In addition to the study on wood anisotropy, which can be accounted for in detail by the presented 2D model, the authors compared the 2D results to results obtained from a corresponding 1D version of the model, where only radial heat and mass transfer properties were used. It was found that even though the predictions of the 2D model were in better agreement with experimental observations than the 1D modeling results, for these parameters, 1D models can still be used without too much loss in accuracy. Finally, the paper concludes with a clear recommendation of when higher dimensional models should be used and when they should not.

Highlights

Since 1980, the world’s primary energy consumption has dramatically increased
In the experiments, which were conducted by Larfeldt [10], the thermocouples were fixed at different radial positions of a wood log with a radius of 25 mm and a length of 300 mm
Besides the experimental work by Larfeldt [10], experimental research has rarely been focused on thermochemical degradation and char conversion of comparably large wood logs

Summary

Introduction

Since 1980, the world’s primary energy consumption has dramatically increased. This increase is mainly due to an intense rise in the use of oil, coal, and natural gas [1]. Biomass can be used to generate heat via combustion, while at the same time being accompanied by limited greenhouse effects [2]. Thermochemical degradation of biomass, from which energy carriers can be generated, and combustion, used for heat generation, are currently both intensively studied [3]. One-dimensional (1D) models were, and still are, one of the main tools [4] This is because they allow for fundamental studies on conversion-related physical processes, while at the same time remaining computationally efficient

Objectives

Results

Conclusion