February 12, 2008
Spontaneous Combustion Possible in Tree Mulch Piles
Dear Action Line: The towering tree-mulch piles at Johnson Park, 61st Street and Riverside Drive, are steaming. On the farm we learned that putting damp hay bales in the barn caused it to spontaneously combust and burn down the barn. What safeguards are being taken to keep these huge mounds of tree mulch from catching fire? -- W.L., Tulsa.
Dan Crossland, Tulsa Public Works Department's deputy director of public facilities, said the mulch mounds are emitting fermentation odor and steam because the ambient temperature is so much colder than the piles' interiors. "Working with these materials does present the potential for spontaneous combustion when certain conditions exist," he said. "The contractor is aware of the hazard and is managing the debris to ensure spontaneous combustion will not occur."
The National Ag Safety Database fact sheet "Hay Fires: Prevention and Control," found at www.tulsaworld.com/spontaneouscombustion, says fires in "freshly cut forage materials" (like pulverized tree heartwood and sapwood) can catch fire spontaneously under the right conditions of material moisture
content, types of bacteria present and atmospheric conditions."
Hay fires can occur in loose hay, small bales, large bales or in stacks -- stored inside or outside. Regardless of when or where the fires occur, the most common cause is excessive moisture. The tree debris contains green limbs and trunks with a moisture content of 60 percent. The bark and roots are covered with microorganisms that cause decay of cellulose and fermentation of plant sugars, once the trees' immune systems cease keeping them in check. Both processes generate heat, and the insulation provided by the piled material keeps the heat from escaping.
Heating continues to around 140 degrees, the temperature at which many microorganisms are killed (most are killed at 170 degrees). The temperature then can decrease slowly, but other microorganisms may survive and become reactivated by rainfall or ground moisture. The debris still contains leaves and branch sapwood that continue respiring (inhaling carbon dioxide, exhaling oxygen. "Thermophilic bacteria" and the heat they generate convert the material to "carbon sponge" with microscopic pores. In this form and at high temperature, it combines readily with oxygen and can self-ignite -- "with a tendency to burn that is almost unbelievable," the fact sheet says.
Assistant Fire Marshal Ron Fegaly, who handles Fire Code Enforcement for the city of Tulsa, said the Fire Department is concerned about this "but there is not a great deal that can be done about it. We could dump gallons of water into the pile, but its dampness is already the problem -- the moisture causes decomposition and this causes heat to build up. If it starts to smolder, all we could do is rake it out. I don't think it's going to be a big problem, but this is hard to predict.
"We're not proactively involved with the mulch piles and will respond if a problem does arise. It's steaming, and if it evolves into something we'll get over there, dig through it and put it out. They (Public Works) have done this for years and mulch piles have ignited in the past, but there's really not a great deal that can be done about it -- unless they decide to just keep turning that over every so often. But they don't have the equipment or the space."
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