Managing Shed Fires

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19 Nov 2004
By Jeff Harper and Colin Peace

Hay Shed Fires Expalined

Spontaneous combustion is a common cause of hay fires. In the rush of hay making, with impending rain, farmers and contractors are under great pressure to complete hay baling and have hay ‘safely’ in the shed.

Obviously in this haste, problems can occur and high moisture hay can lead to spontaneous combustion and loss of valuable hay and sheds.

A prime example of this problem was in southern New South Wales last spring. Due to the rebound in hay production following the drought of season 2002/03, the high volume of hay making lead to some short cuts with disasterous consequences.

Neil and Fiona Muller farmers and contractors at Henty were aware of up to nine fires in the district. Neils says that “farmers need to be aware of the risks and be more patient when baling hay. The moisture tolerance with the popular large square bales is much less than others and should be baled at around 13% moisture” said Neil. Attention should also be paid to the sap content of the plant’s nodes as this has the potential to lift the overall moisture and temperature of stacked hay.

Fiona who often rakes hay said that “they tend to bale the headlands first. These bales are considered high risk and are stored either separately or on the outside of the hay stack.”

According to Neil, “with grain harvest following hay making, most farmers don’t spend the time monitoring of hay sheds for heat.”

The Australian Fodder Industry Association (AFIA) took these issues on board with a presentation at the recent National Conference held in Launceston Tasmania. Information on spontaneous combustion in hay sheds is not easy to find but Jeff Harper, Field Officer with the Tasmania Fire Service provided a great presentation.

According to Jeff, the real danger with shed fires is its unpredictability. After stacking, hay fires can occur any time from a few days to several months. Time is not a particularly important aspect of self ignition of hay.

When hay is stacked we already have two of the three elements required for fire, fuel and oxygen and all we need is the third component, heat.

For a hay stack to combust it must undergo a process of self heating that even now is not fully understood. What we do know is that there are several sources of heat that can contribute to the process.

Primary heating Respiration- Respiration of freshly cut plants and subsequent bacterial action causes a temperature increase to a maximum of between 48-70deg C. If the heating hay is located on the outside of the stack where the heat can dissipate the process may very well stop here.

If however it is located at the bottom or inside the stack then the heat may continue to build. This would appear to be the time that the moisture content of the hay may be most critical.

Biological- For vegetable material to continue to heat it is often a biological process reliant on oxygen and living microorganisms. If the hay is well cured (dried), it will not allow the destructive fermentation to occur; if too wet, not enough oxygen can diffuse into the mass and no fire will occur.

It would appear that partially cured hay (moisture content between 12-21%) is the most prone to fire Biological heating will heat the hay to the thermal death point of the organisms involved ie in the vicinity of 70deg C.

Regardless of how well the heated hay is insulated, temperatures of 70 deg C fall far short of the ignition point of hay, which is in the vicinity of 280degC.

Secondary heating Exothermic- Once the hay reaches the limits of the primary heating (70 deg C), the exothermic process can be initiated to raise the temperature much higher.

Many theories can explain this process including the production of “pyrophoric carbon,” pyrophoric iron, heat from enzyme action, and even the auto-oxydation of the oils contained in seeds. What we do know is that this process is generally accompanied by the production of much acid in the early stages and is accompanied by a marked browning of the hay.

If insulated, this process can progressively raise the temperature inside the stack to 240-280degC at which point the slightest introduction of oxygen will result in the ignition of the stack.

Jeff said if anyone finds a hay bale with dark brown parts smelling like tobacco its time to buy a Tatts ticket as this hay has been ready to ignite.

Some research has shown that hay does not need to reach such high temperatures to ignite spontaneously. If hay is subjected to long periods of heating at temperatures as low as 88degC and remains in the presence of volatile gases produced by oxidation, it may ignite when introduced to air. In other words a relatively low temperature for long periods may have the same effect on hay as high temperatures for short periods.

Combustion can therefore occur in two different ways; in a hot pocket of carbonised hay or in a larger volume surrounding the hot pocket.

Time line Spontaneous fires in stacked hay do not usually occur in less than 10 to 14 days after stacking, and generally require 5 to 10 weeks. Under ideal conditions of moisture and high ambient temperatures ignition has been observed in 6-7 days.

Monitoring (a simple test) The temperature of a stack may be checked by the use of a “temperature probe”, a crow bar or other piece of steel. The bars should be left in place and checked regularly. A pipe or tubing should not be used as this may entrain air into the stack and cause ignition to occur.

The feel of the bar and corresponding temperatures are:

• Comfortable 27 - 38 degC
• Tolerable 38 – 55 degC
• Tolerable for a short time 55 – 60 degC
• Tolerable to touch only 60 – 66 degC
• Too hot to touch and above the danger point 71 + deg C

Any stack that is known to be heating should be checked even more regularly to determine if temps are rising or falling. If the stack continues to rise the only solution is to pull it apart. Water and fire fighting equipment should always be on hand to extinguish a possible fire. Bales that have reached a high enough temperature may ignite as they are introduced to a more available oxygen supply.

Shed layouts Jeff concluded his presentation by saying that shed lay outs should provide adequate air flow throughout the stack to provide for the dissipation of any heated hay. The dissipation of this heat can help to stop the stack from reaching those critical temps at which the next phase of heating can begin.

Segmenting the shed to allow for this can also enable regular monitoring of the temperatures enabling earlier warning of any bales that are continuing to heat and allow for easier removal. Also a leaky shed roof could result in an insulating blanket of moisture across the top of a stack.

Despite these precautions its not a perfect science. According to Fiona Muller, “things can still go wrong even if you do everything right. Its important to monitor stacks and take precautions to minimise the chances of spontaneous combustion of hay.”

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