How can a compost heap combust?

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SUMMARY

The discussion centers on the phenomenon of spontaneous combustion in compost heaps, specifically how microbial action and subsequent chemical reactions contribute to this process. Initially, microbial activity raises the temperature of the biomass, but once it exceeds 70-80°C, exothermic chemical reactions, particularly involving polyunsaturated fatty acids (PUFAs), take over. The presence of sufficient oxygen is crucial for ignition, as it allows for the transition from microbial heating to a self-sustaining fire. The fire triangle—fuel, heat, and oxygen—must be balanced for combustion to occur.

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Guineafowl
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... 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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Per wiki:
https://en.wikipedia.org/wiki/Spontaneous_combustion

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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Example:
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.

https://www.nps.gov/articles/fire-prevention-52-spontaneous-combustion.htm

No bacterial action required.

Seeds and vesicles in plant materials contain PUFAs and cellulose. Haystacks are very like rag piles in that regard.
 
https://www.apsei.org.pt/media/recu...es-incendio/CFPA_E_Guideline_No_31_2013_F.pdf
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.
Fuel
Heat
Oxygen

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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