Hydrothermal Response of Wood Pellets to Internal Heat Generation Due to Hygroscopic Wetting

Abstract

Highlights A unique model representing transient heat of wetting along with the dimensional expansion of wood pellets exposed to humid environment was developed. The model was validated by placing a pellet inside a controlled humidity chamber and continuously recorded the pellet’s temperature and moisture. The performance of the model for predicting the pellet’s cross-over temperature above the surrounding temperature was tested for a range of humid conditions from 0% to 95% RH. Abstract. Wood pellets are exposed to incidental humid conditions during postproduction and handling. It is well known that the heat of moisture adsorption contributes to the self-heating of the pellets. The aim of this research is to quantify the temperature rise of the pellets due to the heat of wetting. The modeling was accomplished by developing and validating a set of mass and energy conservation equations for solid, liquid, and vapor fractions. An exponential function represents the heat of wetting as a function of adsorbed moisture. A nonlinear model describes the increase in the pellet’s diameter. For model validation, the center temperature of a pellet was recorded in a humidity chamber set at 33°C. The model successfully predicted the pellet’s temperature increase from the initial 25°C to a maximum temperature of 36°C, surpassing 33°C after about 5 minutes. Keywords: Heat of moisture adsorption, Heat transfer, Moisture transfer, Self-heating, Wood pellet.