There was a photograph with a paragraph attached to it this past week in the paper about a hay fire in the area. The information indicated the cause of the fire appeared to be spontaneous combustion. Several students in the college’s agriculture program were curious exactly what the term spontaneous combustion meant and what caused it to happen. So what is spontaneous combustion and why does it happen to baled hay?
Spontaneous combustion can happen with a variety substances including hay bales, cotton, linseed oils, coal, and even under the right circumstance cow manure piles. The simple short definition is combustion occurring without an outside ignition source. So how does it happen?
• You need a substance with a relatively low ignition point (temperature) beginning to release heat. Normally this involves an oxidative process where you have moisture, oxygen, and often bacteria involved in a fermentation process and this generates heat.
• As the heat is generated, it is unable to dissipate (escape), especially if the material and/or storage method provide good thermal insulation. This results in rising temperatures in the material.
• As the process continues, the temperature rises above the ignition point.
• If enough oxygen, or other oxidative agent, is present along with enough fuel for the fire, combustion will take place and you end up with a fire.
It helps to remember that processes like fermentation and respiration occur so organisms can break down “food” for among other things energy. As sugars and more complex compounds are broken down several things happen including the release of energy in the form of heat as the molecules are broken down. Another product from these processes is water vapor. Think of what happens when you are in a close area, say a car and no fresh air is being circulated in. It doesn’t take long for the windows to fog up since we release water vapor every time we breathe. Now we have heat and we have rising moisture levels. So how do hay bales catch on fire spontaneously?
• Excessive heat is generated by bacteria fermenting hay and by the hay itself since plant cells continue to respire until the hay moisture levels fall below 20 percent as do bacteria and yeast.
• If this takes place in the field with unbaled hay, there should be no problem. The problem happens when baling, combined with the method of stacking/storing prevents the dissipation of the heat generated and allows moisture to be trapped.
• For most hay, once the internal bale temperature reaches on 180 – 190 degrees F a producer should be really concerned. It is advisable to move the bales before they get this hot and really suggested to at this temperature.
• Once that internal temperature reaches >210 degrees F, you should call the Fire Department before moving bales since moving them can provide more oxygen at this stage and bad things can happen quickly.
In a nutshell, how/why does this happen? Hay is baled when it is too wet or marginally wet. Then it is typically stacked and that further reduces heat dissipation. Usually this is more of a problem in the higher humidity regions as you move east from here and where hay is stored in an enclosed structure.
Ideally, you want to bale at an ideal moisture but sometimes that isn’t possible. One aid is the use of a preservative (there are several) that will inhibit mold and yeast growth. Another is to not tightly stack the bales or if you must, purchase something like a compost thermometer and monitor interior temperature and be ready to move bales. The most critical period is about two weeks after baling but combustion can occur for up to 60 days later.
With the scarcity and price of hay now, a small hay fire can mean pretty significant dollars.
Dr. Victor L. Martin is the agriculture instructor/coordinator for Barton Community College. He can be reached at 620-792-9207, ext. 207.