How can a compost heap combust?

  • Thread starter Guineafowl
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In summary, the conversation discusses the concept of spontaneous combustion, specifically in relation to piles of vegetation or hay. The cause of this phenomenon is believed to be microbial action, which leads to a rise in temperature that can start a self-sustaining chemical reaction. However, specific conditions, such as the presence of polyunsaturated fatty acids and sufficient oxygen, are necessary for the fire to occur. The conversation also mentions the importance of the fire triangle, which includes fuel, heat, and oxygen, and how turning over or exposing the biomass to oxygen can result in a flaming fire.
  • #1
... spontaneously, that is.

I’ve heard many a story of a heap of vegetation, or even hay that’s baled too wet, bursting into flames.

Where does the heat come from? “Microbial action” is the sage answer given. But how? The ignition temperature of wood, which I assume applies at least roughly to dried vegetation, is 180 degC.

But microbial action will surely cease through denaturation well before this temp is reached. I assume some chemical process takes over - is this correct?
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  • #3
Per wiki:

Microbial action first. Then enough composition change with a reasonable rise in temperature to start a self-sustaining chemical reaction, seems to be the general rule. Specific conditions have to be present, for the reaction to proceed beyond biological.

I find the Wiki link and its references mostly unfulfilling.
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  • #4
Polyunsaturated fatty acids (PUFAs) found in drying oils oxidize exothermically, e.g., linseed oil (57% alpha-linolenic acid) residues on cotton cloth rags. And rags are in a pile that insulates the interior. Ask any Fire Marshal about the safety of rag piles.

No bacterial action required.

Seeds and vesicles in plant materials contain PUFAs and cellulose. Haystacks are very like rag piles in that regard.
  • #5
See section 5.1 for biological action.

Note that you need three things for a fire, the fire triangle, and that is apt for these types of fires also.

Too dry and the microbes cannot survive.
Too wet and, well it is too wet to burn.
But with the right conditions biomass,
Microbiological action would raise the temperature somewhat.
As is said in the article, above 70-80 C, exothermic chemical reactions take over.
If insufficient oxygen is available, the temperature can rise, but no fire.
If one turns over the biomass allowing access to oxygen, or if leaky to oxygen penetration, the flaming fire can be a result.
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1. How does a compost heap combust?

A compost heap combusts when the organic materials inside it break down and release heat. This heat, combined with the lack of oxygen in the center of the heap, can cause spontaneous combustion.

2. What factors contribute to a compost heap combusting?

There are several factors that can contribute to a compost heap combusting. These include the size and density of the heap, the moisture content, and the types of materials being composted. If these factors are not properly managed, the heap is more likely to combust.

3. Is a combustible compost heap dangerous?

In most cases, a combustible compost heap is not dangerous. However, if the heap is near flammable materials or structures, it can pose a fire hazard. It is important to monitor a compost heap for signs of combustion and take precautions to prevent fires.

4. How can I prevent my compost heap from combusting?

To prevent a compost heap from combusting, it is important to properly manage the size and density of the heap, maintain proper moisture levels, and regularly turn the heap to allow for proper air flow. It is also recommended to keep the heap away from flammable materials and structures.

5. What should I do if my compost heap combusts?

If your compost heap combusts, the first step is to safely extinguish the fire. This can be done with water or a fire extinguisher. Next, remove any remaining combustible materials from the heap and allow it to cool down. It is important to properly manage your compost heap to prevent future combustions.