How does this happen? We had a truck that was hauling bales of hay catch fire yesterday on the freeway. I suspect spontaneous combustion. But wet hay? Come on!
It's most likely bacteria converting the hay's glucose to co2, h20 and heat. Under the right moisture conditions, the bacteria can double in 10 minutes.
Moisture is essential for many types of spontaneous combustion. Dry compost remains pretty cool; wet compost can reach 170F even during cold weather. With hay, 25% moisture content is the ideal setting for spontaneous combustion. Any less than that and the odds of combustion go way down.
Where does the process get enough oxygen to combust? From my understanding, the process occurs deep inside the bale of hay.
I think you may be right. Does the bacteria produce alcohol that gets ignited? And how does the bacteria produce so much heat?
Moist decaying organic matter often emits more CH4 than CO2 - I.e. makes fuel, but I do not have any reason to think it would spontaneously begin to burn.
From the air. A bale of hay is mostly air. Once combustion begins, rising smoke draws new air from below the bale.
It's a very real danger if you bale and store hay before it has properly dried. When I was a kid, I remember coming across bales with warm ashes inside of them due to the hay being put up too soon.
From Washington State University: The process of spontaneous combustion involves both microbial growth and chemical changes and may be slow to develop. The wet hay will first stimulate microbial growth and as these organisms grow they produce heat while drying out the surrounding surfaces of the hay for energy. More drying surfaces produces more microbial growth and different types of microbes live and die as the internal bale temperature climbs. When the bale temperature reaches about 150 Fº the hay is on a one-way street and going the wrong direction! The larger the haystack and the more densely packed the hay is the longer it may take to show signs of internal bale burning. Internal bale temperature may take several weeks before reaching 150 Fº, but from this point on more heat resistant bacteria, called exothermic bacteria, start a process of chemical change that rapidly increases temperatures to the point of spontaneous combustion.
Hay is a very good insulator - heat produced in the middle of a haystack is largely trapped there. The air flow is restricted, so the combustion is slow - a smoldering, like a coal bed under ash - until something shifts or is removed, or the burn tunnels close enough to the outside, and a chimney opens up. Then look out. http://www.extension.org/pages/66577/preventing-fires-in-baled-hay-and-straw#.VdQ2fYtRc4M
