SELF HEATING OF AGRICULTURAL PRODUCTS
Charles C. Roberts, Jr. PhD. PE
Self heating (often called spontaneous combustion) is a condition where internal heat generation from a material exceeds its capacity to dissipate the heat, resulting in increased temperature and ignition of the material. A common type of self heating involves agricultural products such as hay or silage. Under certain storage conditions, hay will self heat, ignite and cause damage to buildings and property. During the fermenting process in a silo, heating of silage can get out of hand and a fire will ensue. Figure 1 is a case study of such an event. The photo at the left in Figure 1 shows a silo with smoke emanating from an auger vent at the roof (red arrow). The right photo is a thermogram showing various temperatures on the concrete block silo surface. The upper arrow in the thermogram points to hot gasses exiting from the auger opening at the roof. The lower right arrow points to a hot spot in the silage whose heat signature is showing on the wall. The lower left arrow shows the interface between the smoldering silage and the unheated silage. It should be noted that when the fire department arrived, no action was taken. Since other buildings were not in jeopardy, the fire department indicated that the fire would eventually burn itself out. Adding water to the contents in the silo would put the fire out temporarily but would eventually rekindle if the contents were not removed.
What has happened here is that internal heat generation from the fermentation of the biomass in the silo has gone undetected, causing ignition of the contents of the silo. Thermocouple temperature monitors which are available to monitor heating in silos and grain bins, were not present in this case. The fire in this silo caused severe damage to the electrical auger system and other hardware in the silo. Thermal cracking of some of the reinforced blocks also occurred. The electrical power to the silo had been disconnected at an outside electrical panel prior to the fire, eliminating an electrical malfunction as a cause. There was no lightning in the area at the time the fire started.
Figure 2 shows the remains of a barn used for storing hay. There was no electrical service to the barn and no lightning in the area at the time of the fire. The hay had been bailed and stored a year before; a year with relatively high rainfall. According to the owner, the metal building was totally packed with hay bales, suggesting that there was insufficient means to dissipate the heating that probably resulted from moist hay. Moisture content in hay at 20% or less usually does not result in self-heating since microbial activity is sufficiently reduced.
A fire related claim may result from a fire caused by self-heating of agricultural products, which may or may not be a covered peril. Identifying the existence of self-heating depends on the remaining evidence after a fire. In some instances, very little evidence remains. The following is a check list for evaluating the existence of self heating:
1. Check for electrical or mechanical malfunctions.
2. Check for environmental effects such as lightning.
3. Check thermocouple data for increased temperature trends.
4. Use an infrared camera to visualize thermal patterns if the smoldering is still occurring.
5. Check moisture level readings prior to storage.
6. Is the storage area confined, thereby limiting heat dissipation?