Coffee Break Training: Fire Dynamics: Spontaneous Heating of Agricultural Products [open pdf - 503KB]
"In many parts of the Northern Hemisphere, farmers now are reaping their first cutting of hay to be formed into bales and stored for later use. Spontaneous heating to ignition of improperly dried hay in storage is a leading cause of agricultural fires each year. Many agricultural products are susceptible to spontaneous heating, including corn meal feed, linseed, rice, bran and pecan meal. Spontaneous heating in agricultural products is the result of plant and microbial respiration, where the microbes consuming sugars in the presence of oxygen results in the production of carbon dioxide, water vapor and heat. With hay, heat produced in the center of the stored mass -- such as when the hay is tightly baled -- cannot easily escape and may increase to the point where the hay is ignited. Ignition generally occurs at about 175 F (80 C) and can occur within days of the hay being baled."
Coffee Break Training - Fire Protection Series No. FP-2013-25
U.S. Fire Administration: http://www.usfa.dhs.gov/