Role of Oxygen Diffusion in Self-Heating of Forest Products
Keywords:
Oxygen diffusion, temperatures, heat conductionAbstract
This study deals with self-heating of forest products from oxygen diffusing into the material ("diffusion oxygen") to be consumed and generate heat. Some or all of this heat is lost through conduction and—in moist material—through diffusing vapor. When self-heating commences, the heating potential of "diffusion oxygen" exceeds heat loss, but since losses increase with temperature more rapidly than does diffusion of oxygen, the materials reach a stable temperature at which all heat that can be generated by oxygen is lost. At 20 C ambient temperature, this theoretical stable temperature is below 25 C in moist solid wood under side grain surfaces, below 40 C under end grain surfaces, but is on the order of 80 to 100 C in piles of moist chips and sawdust, as well as in stacks of fiber insulating board.
At low temperatures, heat lost by conduction exceeds heat transferred by diffusing vapor, even in fiber insulating board. Therefore the stable temperatures for dry solid wood are not much higher than for moist solid wood. Above 50 C, however, heat lost through vapor approaches heat lost through conduction. Therefore, the stable temperatures for permeable materials such as sawdust are much higher in the dry state than in the moist state.
Diffusion of oxygen, heat conduction, and vapor diffusion play a role in the self-heating of all forest products and are decisive for the course of self-heating, not only in the relatively impermeable products such as solid wood, but also in chips, sawdust, bark, and insulating board that are confined by impermeable walls. In open piles of these permeable materials, however, convection of hot air as in a chimney transfers additional heat out of the material. Therefore temperatures in piles of moist chips and sawdust rise to less than between 80 and 100 C, another reason being that not all oxygen entering the piles is consumed.
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