Why hay bales spontaneously combust

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It defies what we know about fire… something is wet enough to be flammable? It’s hard to believe. But that’s just the case for a hay bale.

Three things are needed for any fire to start…heat, fuel, and oxygen. Of course, the sun heats the hay bale but there is another process that helps it get even hotter, and that’s moisture.

For a bit after it’s cut down, hay is still going through its respiration process. This is when a plant is producing its own energy by intaking carbon dioxide and releasing oxygen. The byproduct is heat and moisture.

Water has a high heat capacity, meaning it can aid the hay bale into heating up easily – to well over 140 degrees inside. This is the beginning of the danger zone for a fire to occur and why a moist hay bale will be hotter than a dry hay bale.

Of course, the fuel is the actual hay itself and the fire typically starts on the outside of the hay bale because that’s where the oxygen is. Heat, fuel, and oxygen are all a recipe for a combustible hay bale.

Hay bales caught fire in McClusky over the weekend. The Goodrich Fire Chief – who was on the scene – told KX News about a thermal imaging tool they use to read what temperature an object is at.
He said once he came on the scene of the fire, another hay bale that hadn’t caught fire was up to 300 degrees. They were able to cool it before it caught fire.

Fire Chief Brian Galvin says, “It’s just another tool in the toolbox to make informed decisions… and especially with the drought this year, with water being a commodity that you don’t want to waste, you can pinpoint the water to where the hot bale is and you don’t have to waste people, resources or fuel or anything else.”

According to the National Ag Safety Database, hay fires usually occur within six weeks of baling but fires have been known to start in several-year-old bales.

If you need help monitoring the temperature of your hay bales, check with your local fire department and law enforcement offices. Many have these thermal imaging devices and can help.

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