Great article on oil by the Torygraph

[biffvernon]

The heat from decaying organic matter is generated by the metabolism of bacteria. One needs to consider the maximum temperature at which, A, the bacteria can continue to live and, B, the ignition temperature of the vegetable matter.

Spontaneous combustion could occur if A>B.

There's a handy list of numbers here

Now if the compost heap was made of phosphorus...

For an explanation see http://www.psla.umd.edu/extension/publi ... ionp1c.pdf

[Little John]

Compost heaps can spontaneously combust.

What if I were to say, "No they can't!"?

I don't think that's right B. Though such spontaneous combustion requires a very special set of circumstances.

Firstly, we we both know that the temperature at which most organic matter will combust is well over the temperature at which the kind of bacteria that are generating the heat in the first place will have died. In which case, this makes such spontaneous combustion impossible, right? Well, not quite.

If the compost heap is big enough, there will be aerobic bacterial activity going on throughout it. The bacteria will be generating heat. Some of that heat will be radiating out of the pile and some of it will be radiating into the the pile. Ultimately, though, the heat will all eventfully radiate out of the pile. However, in the short term if some of the heat being radiated inwards also cannot escape easily or quickly, this can mean that the centre of the pile can reach very high temperatures indeed. Not because there is bacteria left alive in the centre (if the temperature reaches combustion levels they will have all died), but because of a build up of heat being radiated there from regions outside of the centre. This temperature can indeed, in theory, reach combustion levels of ordinary organic matter.

Then you need to considerer less ordinary organic matter.c Rapeseed or linseed plant matter, for example, would combust at much lower temperatures than is the case for other organic matter I would have thought.

[vtsnowedin]

Pretty common set of circumstances actually. All it takes is some hay that is baled up when not dry enough to keep without molding and then placing that in the hay mow with other hay pressing down on it. A constant concern for farmers harried by wet weather during haying season. A few heavy bales from a shaded edge of the field can be sorted out and placed on the edges of the mow or fed out promptly. But if the whole isn't dry enough it has to wait, storm coming or not. A wise farmer that wants to keep his barns standing takes great care and will monitor the interior temp. of the mow with a temperature probe that will reach ten feet into the stack and remove hay if it is getting too hot.
A compost heap is a bit of a stretch though as there is little probability of the moisture content in the center being low enough to allow combustion. The fag end on the sun dried surface plant material is much more likely.

[Little John]

Pretty common set of circumstances actually. All it takes is some hay that is baled up when not dry enough to keep without molding and then placing that in the hay mow with other hay pressing down on it. A constant concern for farmers harried by wet weather during haying season. A few heavy bales from a shaded edge of the field can be sorted out and placed on the edges of the mow or fed out promptly. But if the whole isn't dry enough it has to wait, storm coming or not. A wise farmer that wants to keep his barns standing takes great care and will monitor the interior temp. of the mow with a temperature probe that will reach ten feet into the stack and remove hay if it is getting too hot.
A compost heap is a bit of a stretch though as there is little probability of the moisture content in the centre being low enough to allow combustion. The fag end on the sun dried surface plant material is much more likely.

Ah well, I was not actually drawing a distinction between any particular size of pile of rotting organic material. That is to say, they all have their own context specific names. However, if they start to rot under the action of aerobic bacteria, then they can be said to be composting and so can be properly calledl compost heaps. Though, I agree, a compost heap of the size one might find in a garden is unlikely to self combust due to the large surfacer area by volume thus allowing the heat to dissipate too quickly for such self combustion to occur.

[biffvernon]

in the short term if some of the heat being radiated inwards also cannot escape easily or quickly, this can mean that the centre of the pile can reach very high temperatures indeed.

Nope, the laws of thermodynamics say that is impossible. There is NO way that bacteria can raise the temperature above their living temperature. If the temperature goes higher it is because a non-biological exothermic chemical reaction has been triggered.

The explanation lies in the article I linked to earlier. Here it is again for those who didn't bother

http://www.psla.umd.edu/extension/publi ... ionp1c.pdf
_________________

[Little John]

in the short term if some of the heat being radiated inwards also cannot escape easily or quickly, this can mean that the centre of the pile can reach very high temperatures indeed.

Nope, the laws of thermodynamics say that is impossible. There is NO way that bacteria can raise the temperature above their living temperature. If the temperature goes higher it is because a non-biological exothermic chemical reaction has been triggered.

The explanation lies in the article I linked to earlier. Here it is again for those who didn't bother

http://www.psla.umd.edu/extension/publi ... ionp1c.pdf
_________________

I'm not suggesting the process is in totality due to the aerobic bacteria. They may simply start a process of heating that then runs away chemically.

The process has been well studied:

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

In the end, the outcome is the same. That is to say, piles of composting organic material can spontaneously combust.

[JavaScriptDonkey]

Tut and Fi Steve! Biff has linked to one set of data and made his mind up already - no need for anyone else to go any further.

All compost heap and hay stack fires throughout history must conform to Biff's understanding of physics whether they like it or not.

[biffvernon]

That's a pretty silly post JSD. I suppose you didn't read the document I linked to explaining just which exothermic reaction allows hay to catch fire without violating the Laws of Thermodynamics.

[Little John]

That's a pretty silly post JSD. I suppose you didn't read the document I linked to explaining just which exothermic reaction allows hay to catch fire without violating the Laws of Thermodynamics.

Can that exothermic reaction occur independently of prior heating due to aerobic bacterial activity?

[biffvernon]

Arrrggghhhhh, read the paper - it explains about as much as I know about it.

Yes, the bacteria etc have to warm the stuff up and then, once warm enough, the chemical reactions take over sending the temperature high enough to kill the biological activity and then reach the ignition point of some of the material.

The initial biological heating will only happen if the hay is damp. Low moisture content prevents the bugs growing.

[Little John]

Arrrggghhhhh, read the paper - it explains about as much as I know about it.

Yes, the bacteria etc have to warm the stuff up and then, once warm enough, the chemical reactions take over sending the temperature high enough to kill the biological activity and then reach the ignition point of some of the material.

The initial biological heating will only happen if the hay is damp. Low moisture content prevents the bugs growing.

So, given that a runaway exothermic reaction can occasionally occur as a consequence of heating being initiated by bacterial actions inside a compost heap, we can indeed state that a compost heap can spontaneously combust can we not?

[biffvernon]

Yes. Did anybody dispute this?

[Little John]

Yes. Did anybody dispute this?

Was this not a refutation of the concept?

Compost heaps can spontaneously combust.

What if I were to say, "No they can't!"?

Also, where you state that:

The heat from decaying organic matter is generated by the metabolism of bacteria. One needs to consider the maximum temperature at which, A, the bacteria can continue to live and, B, the ignition temperature of the vegetable matter.

Spontaneous combustion could occur if A>B....

there is the implicit suggestion that spontaneous combustion cannot occur if A<B

However, as we know and as you have just clearly stated you agree with, spontaneous combustion can indeed occur even when A<B because of runaway exothermic reactions that have been triggered as a consequence of bacterial actions.

So, yes, I would say your initial responses on this specific topic did indicate you disputed it. not that it matters much. I occasionally make statements myself that turn out to be not right due to ignorance of all of the facts or due to error in reasoning.

Just admit that you have done so on this occasion, you'll feel better......

[vtsnowedin]

Arrrggghhhhh, read the paper - it explains about as much as I know about it.

Yes, the bacteria etc have to warm the stuff up and then, once warm enough, the chemical reactions take over sending the temperature high enough to kill the biological activity and then reach the ignition point of some of the material.

The initial biological heating will only happen if the hay is damp. Low moisture content prevents the bugs growing.

No bacteria need be present to achieve spontaneous combustion. Oxidation which is a chemical not biological process is all that is needed so having temperatures rise above the kill point for bacteria will not stop the process.

The Encyclopedia Britannica defines spontaneous combustion as the outbreak of fire without application of heat from an external source. This combustion can occur when flammable matter like oily rags, damp hay, leaves, or coal is stored in bulk. Spontaneous combustion, sometimes referred to as spontaneous ignition, begins when a combustible object is heated to its ignition temperature by a slow oxidation process. Oxidation is a chemical reaction involving the oxygen in the air around us gradually raising the inside temperature of something (like a pile of rags) to the point at which a fire starts

[JavaScriptDonkey]

Just admit that you have done so on this occasion, you'll feel better......