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I finally broke down and read the Wikipedia article on spontaneous combustion and I was surprised to see how serious of a problem this can be. Wet hay, oily rags, there really are exothermic reactions in these combinations that speed up as the temperature increases.

Then I saw the following:

Coal

Self-heating in coal has been extensively studied. The tendency to self-heat decreases with increasing rank of the coal. Lignite coals are more active than bituminous coals, which are more active than anthracite coals. Freshly mined coal consumes oxygen more rapidly than weathered coal, and freshly mined coal self-heats to a greater extent than weathered coal. The presence of water vapor may also be important, as the rate of heat generation accompanying the absorption of water in dry coal from saturated air can be an order of magnitude or more than the same amount of dry air.

I understand that coal is a complex material, it's more than a lump of carbon. But what are the types of chemical reactions within coal that will cause a pile of it to self-ignite? Is moisture always required? Is there moisture in freshly mined coal naturally, or is it absorbed hygroscopically from the atmosphere or is a liquid source necessary (getting wet)?

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  • $\begingroup$ This can partly explain why there are so many ongoing fires in coal deposits/ mines. I have seen an estimate that these fires in China produce more CO2 that all the vehicles in the US. Fortunately , it has been decreed that this CO2 does not count in China's' total CO2 production. $\endgroup$
    – blacksmith37
    Jul 26, 2017 at 21:21
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    $\begingroup$ Is it maybe related to radioactive material in the coal? Radioactive decay could explain the heating ... . It is just a guess. Coal (at least some types of coal) contains radioactive atoms - see e.g. Lauer et al., 2015. $\endgroup$
    – daniel.heydebreck
    Jul 27, 2017 at 8:11
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    $\begingroup$ @daniel.neumann that might work for a lump of coal 1,000 km in diameter, but for a railroad car full to start on fire, that would have to be incredibly radioactive. I think endothermic chemical reactions with oxygen are much more likely. $\endgroup$
    – uhoh
    Jul 27, 2017 at 9:07
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    $\begingroup$ @blacksmith37 Note that many coal deposit/mine fires start at the surface where the opportunities for fire are much larger. See Coal seam fire at Wikipedia. This question seems to be about ignition without those external factors. $\endgroup$
    – Jan Doggen
    Jul 27, 2017 at 12:44
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    $\begingroup$ This is apparently not a problem with stored coal. Up through the 50's most buildings and homes in Chicago were heated with coal. So most homes had 1 to 4 tons of coal stored in the basement and more in larger buildings. I expect most of the northern part of the country was the same. This was true for many grades of coal; Sears stores had booths with 10 different coals displayed for customers. And today , power plants have mountains of stored coal. $\endgroup$
    – blacksmith37
    Jul 29, 2017 at 19:32

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Sulphur/sulphides in coal may be one of the causes of self combustion.

A slight diversion - in some metal sulphide mines, such as copper, at high level of sulphides in the ore, the sulphides can oxidize thus creating sulphide fires. If a lot of dry dust is produced during stope blasts, during the mining process, sometimes the sulphide dust can cause a secondary, unwanted, explosion.

Most coal deposits contain sulphides/sulphur. Most lignite (brown coal, also low quality or low grade coal) deposits are nearer the surface, thus if lignite has lots of sulphur & it oxidizes with atmospheric oxygen combustion can occur.

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Reference [6] under that Wikipedia article on spontaneous combustion gives you some answers:

"The Fire Below: Spontaneous Combustion In Coal". DOE/EH-0320, Issue No. 93-4. US Department of Energy. May 1993

How Coal Self-Ignites

The coal's temperature begins to climb above ambient. At about 150-300 degrees F, it begins to give off minute, but measurable, quantities of gas--aerosols, hydrogen, and CO(2)--precursors of combustion. As the temperature increases further--at about 600-700 degrees F--relatively, large, visible particulates are emitted. Soon, as the heating rate increases in intensity to about 750-800 degrees F, incipient combustion, and ultimately self-ignition and flame, will occur.

Spontaneous combustion has long been recognized as a fire hazard in stored coal. Spontaneous combustion fires usually begin as "hot spots" deep within the reserve of coal. The hot spots appear when coal absorbs oxygen from the air. Heat generated by the oxidation then initiated the fire.

The list of recommendations towards the end of that article specifically mentions as contributing factors: sulphur content, moisture, age of the mined coal and the obvious availability of oxygen and ambient temperature.

Don't forget to read the A Lesson Learned from the Titanic at the end, describing how in April 1912, their bunker coal was on fire 'from the day we put out of Southhampton until we hit the iceberg'.

I think it is crucial to note that the tiniest amount of oxidation will lead to a rise in temperature, accelerating the process exponentially.

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  • $\begingroup$ "But what are the types of chemical reactions within coal that will cause a pile of it to self-ignite?" I'm really trying to understand why it gets hot. The Wikipedia does describe various scenarios for the rates of increase of temperatures in different cases (hay, oily rags, etc.), but the why is what's bugging me. So anything you can add on the actual exothermic chemistry that is causing the sizable and sometimes catastrophic temperature increases would be greatly appreciated! $\endgroup$
    – uhoh
    Jul 27, 2017 at 13:04
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    $\begingroup$ Start here $\endgroup$
    – Jan Doggen
    Jul 27, 2017 at 13:12
  • $\begingroup$ Well, I've started here, but I'll try to get a copy of that 1940 paper next week and see if it has the answer. It's possible some Earth scientist here knows what's in coal that reacts with air and/or moisture. $\endgroup$
    – uhoh
    Jul 27, 2017 at 17:06
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    $\begingroup$ The use of Fahrenheit hurts my brain $\endgroup$
    – Gimelist
    Jul 31, 2017 at 11:49
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Coal and other organics like wood and straw decompose when exposed to oxygen, that decomposition is an exothermic oxidation reaction so it heats the material around it which accelerates the decomposition. This can be reasonably harmless if there's enough airflow to cool the pile and carry away the small volumes of flammable volatile gases formed by what is essentially decaying plant matter. The presence of water makes the process more dangerous in two ways, it fills pore spaces that would otherwise provide air cooling to the pile as a whole and it provides more hydrogen to the decomposition chemistry while excluding some oxygen which creates more volatile compounds with increasingly low ignition points.

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  • $\begingroup$ Thank you for your post. This adds some important factors that haven't been addressed yet. It's hard to imagine how getting something like coal or hay wet can increase it's likelihood of catching fire, but it seems that this is in fact true. $\endgroup$
    – uhoh
    Aug 7, 2017 at 20:27
  • $\begingroup$ You can also get (in hay) faster bacterial growth, which heat the pile by their metabolism. It's possible that a similar effect can happen with coal. $\endgroup$
    – Andrew Jon Dodds
    Aug 8, 2017 at 8:31
  • $\begingroup$ @AndrewJonDodds Any carbon source can potentially support bacterial growth, if the water is urban rainfall with dissolved nitrates from exhaust fumes it's even worse because the nitrates help to balance bacterial biochemistry allowing more rapid growth. $\endgroup$
    – Ash
    Aug 8, 2017 at 11:24
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    $\begingroup$ @uhoh It's a balance thing, no water isn't much of a problem and lot of water isn't a problem at all, but a little water can be a big problem. $\endgroup$
    – Ash
    Aug 8, 2017 at 11:27
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Coal, which is essentially pure carbon, slowly oxidizes, it is the same reaction as it burning just much slower. This is the major source of heat for self-heating of coal. Many things that will burn will also slowly oxidize under normal temperatures, it is just not a sustained chain reaction that a fire is. Since this is weak slow process there is a large variety of oxocarbons that can be produced from this, some of which can contribute to more rapid heating the right circumstances.

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    $\begingroup$ Coal is certainly mostly carbon, but there are plenty of other components. A major causal factor of acid rain is the sulfur content of coal for example, and there are plenty of other compounds in coal as well. (See this answer.) It's a complicated material. I am sure there is some room-temperature oxidation of the carbon, it may be oxidation of other components that's responsible for making some kinds of coal more likely to spontaneously catch fire than others. $\endgroup$
    – uhoh
    Jul 28, 2017 at 3:20
  • $\begingroup$ I'll try to access the paper next week, it looks very promising! But the phrase "formation of solid oxygenated complexes" in the abstract suggests its more than just C + O2 -> CO2 for example. $\endgroup$
    – uhoh
    Jul 28, 2017 at 3:23
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    $\begingroup$ true, Co2 is only the final produce of complete combustion, when your oxygen to carbon ratio is high enough and you have enough energy available. I suggest a look at Oxocarbons which is all the weird things you can get with just carbon and oxygen. Mellitic anhydride (C12O9) is an example of a solid at room temprature oxocarbon. $\endgroup$
    – John
    Jul 28, 2017 at 3:54
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    $\begingroup$ Wow, I had no idea such things existed. OK I will read further. Thank you!! $\endgroup$
    – uhoh
    Jul 28, 2017 at 3:57

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