Hay Fire Burning In Mesa: How Spontaneous Hay Fires Start


Get used to the cloud of haze over Southeast Mesa… it’ll be there at least through the┬ánext few days – or even weeks.

A massive hay fire is burning at Superstition Farm on Hawes north of Elliot. The cause, given by Mesa Fire Captain Bob Tribbensee, is spontaneous combustion. Mesa Captain Forrest Smith announced later on Tuesday that the fire department had decided to let the blaze smolder and burn out on its own. Due to the smoke and haze that will blanket the East Valley while the fire burns out, they needed approval from the Maricopa county environmental office.

How does spontaneous combustion happen?

A few factors come in to play. First, the rains that we had over the past couple months have wet the hay. Spontaneous hay bale combustion tends not to happen with dry hay stored at 15% moisture or less. However, when it gets wet, there is potential for spontaneous combustion.

Chemical reactions from the moisture, microbial growth in the stack, and a change in the availability of oxygen can raise pressures and cause heat to the point of ignition and fire.

The wet hay stimulates microbial growth, and those microbes will produce heat. This heat further dries out the hay as the microbes seek energy. As the microbial growth intensifies, the heat intensifies, and the internal temperature of the hay bale climbs. Tightly packed bales may not allow enough space for the heat to dissipate, and the hay, in effect, cooks itself. The heat actually becomes so high to kill off the initial microbial growth, but then the conditions are perfect for a more heat resistant bacteria called exothermic bacteria, which then moves in. The exothermic bacteria does the same thing that the earlier microbial growth did – creates heat – but does so at a higher level.

Once the temperatures inside the bales reaches 150-160 degrees Fahrenheit,  there is a potential for fire. At temperatures above 160, gases have the potential to ignite once the hay is exposed to air.

The smoke from the fire will continue to cover the East Valley throughout the day and possibly beyond. Residents are being told to keep windows shut, especially for those with respiratory problems.