Avoiding Fires Caused By Spontaneous Combustion Recommendations

June 5, 2008

By: Richard Buggeln, University of Tennessee Center for Industrial Services

Let’s cut to the chase: how do you prevent fires from spontaneously erupting at mulching and composting facilities? There are some easy things to do…and some things not to do. We don’t want to go into a lot of complicated…but interesting…details* in this short space. However you should know how and why heating occurs in piles of mulch or compost. For a clearer understanding, we can make some good comparisons with the way things heat up—and sometimes catch on fire—in our daily life.

How and why… Everything that rots produces heat. Rotting happens as a result of microscopic bacteria and fungi feeding on moist wood and leaves…or on the cellulose sponge on your kitchen sink or on the fruit and vegetables in your refrigerator. Freshly cut grass spread over a lawn starts to rot but you can’t feel the heat that it generates until you rake the grass into a pile. Come back the next morning and you can feel the heat trapped inside the pile of grass! The bigger the pile, the more heat is trapped and the hotter the pile temperature. Our bodies give off heat. Pack a lot of people in a room and soon you have to open a window to let the heat out!

Goal: do not create conditions that trap heat inside piles of mulch or compost.

Avoid: making piles of ground material higher that 10-15’. NEVER run heavy equipment on top of piles. Heavy equipment compacts the rotting material and promotes the chance for “hot spots” to be created inside a pile. High piles should only be temporary. High piles should be taken apart and watered, then reformed. Let the heat out…just like opening a window in a crowded room!

A thoroughly and uniformly moistened pile is the best management practice. A potato on the stove in boiling water will not start to burn…not until all the water boils off!! Moisture inside a mulch pile acts as a “governor” that can keep a pile from getting excessively hot.

Avoid: alternating layers of wet or moist ground material with dry material, say ground pallet wood. Rotting and heat generation will occur in the wet layers and the heat will be transferred, via the “chimney effect” to the dry layers, possibly leading to spontaneous combustion in the dry layers.

Spontaneous combustion frequently occurs on very cold days in winter. Cold air is dry. When cold air enters a pile and warms up, it will hold more moisture than warm air entering a pile. Cold air can transfer a lot of heat-laden water vapor inside mulch and compost piles.

Remember: to minimize the risk of fires from spontaneous combustion, keep piles small and keep them moist.

*Self-Heating In Yard Trimmings: Conditions Leading To Spontaneous Combustion. Authors: Richard Buggeln (University of Tennessee Center for Industrial Services) and Robert Rynk (SUNY Cobleskill, NY). IN: Compost Science&Utilization, (2002), Vol.10,No. 2, 162-182:

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