Experimental Validation of a Coupled Solid- and Gas-Phase Model for Combustion and Gasification of Wood Logs

Abstract

A solid-gas-phase model for thick wood gasification/combustion is extensively studied, after a re-examination of the kinetic constants for the char gasification reactions. The solid-phase model, which includes the description of all the relevant heat and mass transfer phenomena and chemical reactions, is coupled with a CFD code for the gas-phase processes. Both the gasification and combustion of single wood logs are simulated (log radius in the range of 0.06-0.1 m, initial moisture content, on a dry basis, 1-81%, inlet gas temperature 1253-1613 K, inlet gas velocity 0.5-1.0 m/s, and various compositions of the gaseous mixture). For comparison purposes, a solid-phase model, with global heat and mass transfer coefficients and a constant-property gas phase, is also considered. Although both models predict the mass loss dynamics to be qualitatively similar, the solid-phase model overestimates the total heat flux and underestimates the char combustion rate. Extensive experimental validation of both models is carried out in terms of conversion time and average mass-loss rates. Acceptable agreement is obtained for the comprehensive model, whereas in the other case, the conversion times are generally underestimated and the average mass loss rates are overestimated. However, improvements in the predictive capabilities of the solid-phase model could be achieved through the introduction of corrective factors for the external heat and mass transfer coefficients.