A method to estimate wood surface moisture content during drying

Abstract

A method to estimate the surface moisture content below the fibre saturation point that is a function of the surface temperature, wet-and dry bulb temperatures, equilibrium moisture content, and fibre saturation point was evaluated. The method is based on the premise that the surface temperature is solely influenced by the surface moisture content and the climate that the surface is exposed to. The prediction model contends that the surface moisture content is equal to the fibre saturation point when the surface temperature is equal to the wet bulb temperature, and equal to the equilibrium moisture content when the surface temperature is equal to the dry bulb temperature, with a linear interpolation between those two points. The model thus predicts that the average moisture content of a thin piece of veneer can be predicted with fairly good accuracy. Also, when drying boards in a fast changing climate, e.g. fan reversals in industrial kilns, the surface temperature and surface moisture content should change as abruptly as the climate does. Additionally, the surface moisture content should correlate to the known drying phases, with a consistently high surface moisture content during the capillary phase when the wet line is close to the surface, and a quickly decreasing surface moisture content when the wet line moves into the wood during the transition to the diffusion phase. The prediction model was tested in these three scenarios and the results suggest that the basic premise is reasonable, and that the method is useful for surface moisture content estimation.

Techno-Economics of Biocarbon Production Processes under Norwegian Conditions

“…Heat is supplied by flue gas and LP steam from the CHP unit. Drying rate is calculated based on a drying curve for woodchips, experimental data is gathered from Johansson et al, and the normalized drying rate v (α) according to eq is implemented in Aspen Plus and are shown in Figure and also included as supplementary data in Appendix D. where α is the normalized moisture content, Z is the current moisture content on dry basis [kg/kg], Z cr is the critical moisture content on dry basis (0.831 kg/kg), Z eq is the equilibrium moisture content on dry basis (0.01 kg/kg) and depends on the relative humidity and temperature of the drying medium and air. The drying rate is expressed in kg/(kg·s).…”

“…where α is the normalized moisture content, Z is the current moisture content on dry basis [kg/kg], Z cr is the critical moisture content on dry basis (0.831 kg/kg), Z eq is the equilibrium moisture content on dry basis (0.01 kg/kg) 24 gas, and the last heat exchanger is used when flue gas is not sufficient to provide all of the heat needed, and then lowpressure steam is used. Hot air is split into two streams that are directed to the second and third stages.…”