Modelling air flow and ambient temperature effects on the biological self-heating of compost piles
We formulate and investigate a one-dimensional model for self-heating in compost piles. The self-heating occurs through a combination of biological and chemical mechanisms. Biological heat generation is known to be present in most industrial processes handling large volumes of bulk organic materials. The heat release rate due to biological activity is modelled by a function which at sufficiently low temperatures is a monotonic increasing function of temperature. At higher temperatures, it is a monotonic decreasing function of temperature. This functionality represents the fact that micro-organisms die or become dormant at high temperature. The heat release due to oxidation reaction is modelled by Arrhenius kinetics. The model consists of mass balance equations for oxygen and energy. Steady-state temperature diagrams are determined as a function of the size of the compost pile and the flow rate of air through the pile. We show that there is a range of flow rates for which elevated temperatures, including the possibility of spontaneous ignition, occur within the pile. We also investigate the effects of variations in the ambient temperature.