MODELING THE PROPAGATION OF DRYING AND DECOMPOSITION FRONTS IN WOOD

Decomposition of moist wood is modeled using the shrinking unreacted-core approximation for a finite rate of reaction and the assumption of a thermally controlled evaporation of moisture across an infinitely thin front at constant temperature. The one-dimensional, quasi-steady (along the char layer) equations also take into account convective, conductive, and radiative heat transfer and different physical properties for char, dry wood, and moist wood. The use of realistic values for all the input parameters results in acceptable agreement between predicted and measured weight losses for 0.04-m-thick beech wood particles (external radiative heat fluxes between 40 and 80 kW/m2 and initial moisture contents between 0 and 47 wt% on a dry basis). A parametric analysis indicates that, apart from a linear dependence on the moisture content, the characteristic times are especially affected by water evaporation in the case of thick samples. It has also been found that assuming a coincident front for both wood decomposition and moisture evaporation, often done in gasification/combustion models, could be applied with sufficient accuracy only for thick samples with high moisture contents and subjected to severe thermal heating.