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UBC Theses and Dissertations

Moisture-related physical properties and self-heating of wood pellets Lee, Jun Sian


Wood pellets may unintentionally be exposed to liquid water (rain) and water vapor (humid air) during their storage, handling, and transportation to markets. This dissertation aims to analyze the impact of incidental rain on the reduction of durability and the potential for self-heating of wood pellets. Initially, wood pellets spread in a pan were exposed to sprayed water to simulate rainfall. The durability values for wetted pellets were assessed using the standard tumbler test. The constant durability 96.5% as a dependent value vs. varying rain intensity (mm/h) and rain duration (h) were graphically presented on a two-dimensional (x-y) coordinate. Subsequent tests were conducted to compare the durability of pellets exposed to liquid water and to water vapor. For liquid water, the dried pellets at 0% m.c. were immersed in water at 30 °C. For water vapor, the dried pellets were placed in a humidity chamber at 90% RH and temperature of 30 °C. When exposed to liquid water, the durability of wood pellets decreased from over 99% to below 80% when their moisture content increased to 20% (wb) within six minutes. When exposed to water vapor, the durability of wood pellets first decreased from over 99% to 95% when their moisture content increased from zero to 10.7% (wb) within four hours. The temperature rise due to self heating of wood pellets was measured using a thermocouple placed in 200 ml liquid water and 100 g pellets. The temperature rise in humid air was measured by inserting a thermocouple inside a pellet placed in the humidity chamber at 95% RH and 33 °C. The maximum heat of wetting of wood pellets immersed in water was determined to be 66 kJ/kg dry mass. An average differential heat of water vapor adsorption of 403 kJ/kg water was calculated as a derivative of the heat of wetting data. By including the equation for heat of wetting and differential heat of adsorption, the mathematical heat and moisture transfer model quantified the contribution of moisture adsorption to the self-heating phenomenon in wood pellets.

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Attribution-NonCommercial-NoDerivatives 4.0 International