How can a compost heap combust?

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Discussion Overview

The discussion centers on the phenomenon of spontaneous combustion in compost heaps, particularly focusing on the mechanisms behind the heat generation that can lead to combustion. Participants explore various factors including microbial action, chemical reactions, and environmental conditions that contribute to this process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how microbial action can generate enough heat for combustion, suggesting that chemical processes may take over before microbial denaturation occurs.
  • Another participant references a Wikipedia article that outlines the role of microbial action followed by compositional changes leading to self-sustaining chemical reactions, but expresses dissatisfaction with the article's depth.
  • A participant provides an example of polyunsaturated fatty acids oxidizing exothermically, arguing that this process can occur without bacterial action, drawing parallels between haystacks and rag piles.
  • Discussion includes the fire triangle concept, emphasizing the need for fuel, heat, and oxygen, and how varying moisture levels affect microbial survival and combustion potential.
  • One participant notes that temperatures above 70-80°C can trigger exothermic reactions, leading to combustion if oxygen is available.

Areas of Agreement / Disagreement

Participants express differing views on the primary mechanisms leading to combustion, with some emphasizing microbial action and others highlighting chemical reactions involving fatty acids. The discussion remains unresolved regarding the exact processes and conditions necessary for spontaneous combustion.

Contextual Notes

Participants acknowledge that specific conditions must be met for combustion to occur, including moisture levels and oxygen availability, but do not reach a consensus on the interplay of these factors.

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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