Self-Heating In Yard Trimmings: Conditions Leading To Spontaneous Combustion

Spontaneous combustion (SC) is defined as combustion of material in the absence of “forced ignition”, that is, an externally applied spark or flame. This paper describes how SC occurs in piles of yard trimmings, starting with chemical and physical events that initiate heat-producing reactions via biotic and abiotic processes involving oxygen and a little water. The accumulation of heat generated within a pile depends on the balance between rate of internal heat production and rate of heat loss to the external environment. When the former rate is higher than the latter, a “critical” internal temperature may be reached at which time run away ignition, i.e., SC, is unavoidable. Water plays the interesting role of “governor” of temperature change and heat exchange within a pile of yard trimmings as well as heat loss from a pile. While heat, in the form of water vapor, is being lost from a self-heating pile, the pile temperature does not rise above 70 ?90°C until all free water has been removed. An analogy is potatoes in a pot of boiling water that only start to burn after the water has boiled off. Important experiments with Eucalyptus leaves, sawdust, and other types of plant material demonstrate the inverse relationship between mass of material and ambient temperatures than can lead to SC. That is, the bigger the pile, the lower the ambient temperature at which SC may occur. SC has been studied most intensively in coal. The roles of particle size, pile compaction, and other potentially catalytic phenomena observed in coal may have application to the propensity of SC to occur in piles of yard trimmings. Finally, several incendiary experiences at composting and mulching operations will be described together with guidelines on the prevention of costly fires that are very hard to extinguish.