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

<p>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.</p>

<p>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 &amp; it oxidizes with atmospheric oxygen combustion can occur.</p>

CC BY-SA 3.0
2017-07-27T12:35:25.310Z
Thanks! Now that you mention sulphides, it calls to mind that pyrite mines can catch fire as well, e.g. http://www.bisbeeminingandminerals.com/mine-fires and https://link.springer.com/article/10.1007/s40789-015-0085-y and an aide, just for fun, http://gizmodo.com/the-9-deadliest-minerals-weve-ever-mined-1638317799
uhoh
CC BY-SA 3.0
2017-07-28T14:42:31.037Z
And precautions opening certain refinery units are necessary as otherwise the iron sulfide will ignite.
blacksmith37
CC BY-SA 3.0
2017-07-27T12:22:11.203Z
2017-07-27T12:22:11.203Z
Fred
<p>Reference [6] under that <a href="https://en.wikipedia.org/wiki/Spontaneous_combustion#Coal" rel="nofollow noreferrer">Wikipedia article on spontaneous combustion</a> gives you some answers:</p>

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

<blockquote>
  <p>How Coal Self-Ignites<br /><br />
  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.</p>
  
  <p>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.</p>
</blockquote>

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

<p>Don't forget to read the <em>A Lesson Learned from the Titanic</em> 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'.</p>

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

CC BY-SA 3.0
2017-07-27T13:04:21.613Z
"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!
uhoh
CC BY-SA 3.0
2017-07-27T13:12:28.973Z
[Start here](https://chemistry.stackexchange.com/questions/16027/when-is-coal-considered-to-be-oxidized)
Jan Doggen
CC BY-SA 3.0
2017-07-27T17:06:30.247Z
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.
uhoh
CC BY-SA 3.0
2017-07-31T11:49:49.227Z
The use of Fahrenheit hurts my brain
Gimelist
CC BY-SA 3.0
2017-07-27T12:57:03.117Z
2017-07-27T12:57:03.117Z
Jan Doggen
<p>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 <em>and</em> it provides more hydrogen to the decomposition chemistry while excluding some oxygen which creates more volatile compounds with increasingly low ignition points.</p>

CC BY-SA 3.0
2017-08-07T20:27:56.280Z
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.
uhoh
CC BY-SA 3.0
2017-08-08T08:31:11.203Z
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.
Andrew Jon Dodds
CC BY-SA 3.0
2017-08-08T11:24:34.383Z
@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.
Ash
CC BY-SA 3.0
2017-08-08T11:27:10.780Z
@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.
Ash
CC BY-SA 3.0
2017-08-07T18:12:00.723Z
2017-08-07T18:12:00.723Z
Ash
<p>Coal, which is essentially pure carbon, <strong>slowly oxidizes</strong>, it is the same reaction as it burning just much slower. 
<strong>This is the <a href="http://www.sciencedirect.com/science/article/pii/S036012850300042X" rel="nofollow noreferrer">major</a> source of heat for self-heating of coal</strong>. 
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 <a href="http://Oxocarbon" rel="nofollow noreferrer">oxocarbons</a> that can be produced from this, some of which can contribute to more rapid heating the right circumstances. </p>

CC BY-SA 3.0
2017-07-28T03:20:49.417Z
Coal is certainly *mostly carbon*, but there are plenty of other components. A major causal factor of [acid rain](https://en.wikipedia.org/wiki/Acid_rain) is the sulfur content of coal for example, and there are plenty of other compounds in coal as well. (See [this answer](https://earthscience.stackexchange.com/a/11939/6031).) 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.
uhoh
CC BY-SA 3.0
2017-07-28T03:23:31.193Z
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.
uhoh
CC BY-SA 3.0
2017-07-28T03:54:47.893Z
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.
John
CC BY-SA 3.0
2017-07-28T03:57:26.890Z
Wow, I had no idea such things existed. OK I will read further. Thank you!!
uhoh
CC BY-SA 3.0
2017-07-27T23:17:45.967Z
2017-07-28T03:56:57.390Z
2017-07-28T03:56:57.390Z
John
<p>I finally broke down and read the Wikipedia article on <a href="https://en.wikipedia.org/wiki/Spontaneous_combustion" rel="nofollow noreferrer">spontaneous combustion</a> 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.</p>
<p>Then I saw the following:</p>
<blockquote>
<p><strong>Coal</strong></p>
<p>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.</p>
</blockquote>
<p>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)?</p>

CC BY-SA 3.0
2017-07-26T21:21:34.160Z
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.
blacksmith37
CC BY-SA 3.0
2017-07-27T08:11:13.830Z
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](http://dx.doi.org/10.1021/acs.est.5b01978).
daniel.heydebreck
CC BY-SA 3.0
2017-07-27T09:07:08.077Z
@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.
uhoh
CC BY-SA 3.0
2017-07-27T12:44:00.147Z
@blacksmith37 Note that many coal deposit/mine fires start *at the surface* where the opportunities for fire are much larger. See [Coal seam fire](https://en.wikipedia.org/wiki/Coal_seam_fire) at Wikipedia. This question seems to be about ignition without those external factors.
Jan Doggen
CC BY-SA 3.0
2017-07-27T12:50:57.543Z
@JanDoggen I believe so, thanks. I'm asking about the "spontaneity" of combustion; how can a large amount of coal get so hot with only the presence of surface air (with its oxygen) and/or moisture that it catches fire, without any external heat or ignition source.
uhoh
CC BY-SA 3.0
2017-07-29T19:32:09.570Z
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.
blacksmith37
CC BY-SA 3.0
2017-07-30T02:49:22.873Z
@blacksmith37 Yep, I remember seeing remnants of coal bins in the basements of very old houses myself. There seems to be several factors involved in the spontaneous combustion of coal, including the total size or quantity stored, particular type of coal, and how recently it was mined. As you point out, by the time it might reach an individual consumer, a few meter pile was probably not at risk. But from what I've read an occasional train-car full leaving the mine might start smoldering.
uhoh
<fossil-fuel><coal>
Coal self-heating and spontaneous combustion, how does this happen?
uhoh
# Coal self-heating and spontaneous combustion, how does this happen?

(Asked by: username_0 on 2017-07-26)

I finally broke down and read the Wikipedia article on [spontaneous combustion](https://en.wikipedia.org/wiki/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)?

**Tags:** fossil-fuel, coal

> **Comments:**
> **username_1**: 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.  
> **username_2**: 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](http://dx.doi.org/10.1021/acs.est.5b01978).  
> **username_0**: @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.  
> **username_3**: @username_1 Note that many coal deposit/mine fires start *at the surface* where the opportunities for fire are much larger. See [Coal seam fire](https://en.wikipedia.org/wiki/Coal_seam_fire) at Wikipedia. This question seems to be about ignition without those external factors.  
> **username_0**: @username_3 I believe so, thanks. I'm asking about the "spontaneity" of combustion; how can a large amount of coal get so hot with only the presence of surface air (with its oxygen) and/or moisture that it catches fire, without any external heat or ignition source.  
> **username_1**: 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.  
> **username_0**: @username_1 Yep, I remember seeing remnants of coal bins in the basements of very old houses myself. There seems to be several factors involved in the spontaneous combustion of coal, including the total size or quantity stored, particular type of coal, and how recently it was mined. As you point out, by the time it might reach an individual consumer, a few meter pile was probably not at risk. But from what I've read an occasional train-car full leaving the mine might start smoldering.  

---

## 4 answers

### Answer by username_4 on 2017-07-27. Score: 4

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.

> **Comments:**
> **username_0**: Thanks! Now that you mention sulphides, it calls to mind that pyrite mines can catch fire as well, e.g. http://www.bisbeeminingandminerals.com/mine-fires and https://link.springer.com/article/10.1007/s40789-015-0085-y and an aide, just for fun, http://gizmodo.com/the-9-deadliest-minerals-weve-ever-mined-1638317799  
> **username_1**: And precautions opening certain refinery units are necessary as otherwise the iron sulfide will ignite.  

---

### Answer by username_3 on 2017-07-27. Score: 4

Reference [6] under that [Wikipedia article on spontaneous combustion](https://en.wikipedia.org/wiki/Spontaneous_combustion#Coal) 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.

> **Comments:**
> **username_0**: "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!  
> **username_3**: [Start here](https://chemistry.stackexchange.com/questions/16027/when-is-coal-considered-to-be-oxidized)  
> **username_0**: 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.  
> **username_5**: The use of Fahrenheit hurts my brain  

---

### Answer by username_6 on 2017-08-07. Score: 3

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.

> **Comments:**
> **username_0**: 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.  
> **username_7**: 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.  
> **username_6**: @username_7 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.  
> **username_6**: @username_0 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.  

---

### Answer by username_8 on 2017-07-27. Score: 2

Coal, which is essentially pure carbon, **slowly oxidizes**, it is the same reaction as it burning just much slower.
**This is the [major](http://www.sciencedirect.com/science/article/pii/S036012850300042X) 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](http://Oxocarbon) that can be produced from this, some of which can contribute to more rapid heating the right circumstances.

> **Comments:**
> **username_0**: Coal is certainly *mostly carbon*, but there are plenty of other components. A major causal factor of [acid rain](https://en.wikipedia.org/wiki/Acid_rain) is the sulfur content of coal for example, and there are plenty of other compounds in coal as well. (See [this answer](https://earthscience.stackexchange.com/a/11939/6031).) 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.  
> **username_0**: 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.  
> **username_8**: 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.  
> **username_0**: Wow, I had no idea such things existed. OK I will read further. Thank you!!  

---

