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My question is about the formation of coal seeming to break the laws of the conservation of energy (that energy can neither be created nor destroyed but instead transferred from one thing to another). So I am taking a chemistry class and we were learning about the heat of combustion, and suddenly I found myself wondering how the heck wood can be turned into coal.

I did some research and made myself more confused when I found that 0.59 kg of coal releases as much energy as 1 kg of wood (https://www.euronuclear.org/info/encyclopedia/coalequivalent.htm). Since I knew that coal was made from trees (wood) from millions of years ago that died and were compressed, I was wondering how it could be possible to seemingly double your energy output without any energy input. Is this because of potential energy is already high in wood or something? Is this just because coal is more efficient and it didn't actually take any energy to change the wood to coal and the extra energy output is because of the increase in efficiency (I am not really sure about this though because wood already burns at 70 % efficiency – http://www.daviddarling.info/encyclopedia/W/AE_wood_heat_value_BTU.html – and the difference between the energy released between coal and wood is 1.97 times larger). So where is this extra energy coming from?

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    $\begingroup$ Well, living wood has a lot of water in it. Coal doesn't. So, per pound, coal has more energy per kg. $\endgroup$ – Jon Custer Jun 10 '19 at 21:41
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    $\begingroup$ I think this question is off-topic and perhaps more about chemistry than physics. The question is really "what happens to wood to increase its energy density when it turns into coal?". $\endgroup$ – rghome Jun 10 '19 at 21:58
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the quick answer is that it took a lot of plant matter to make a smaller amount of peat, and it took a lot of peat, compressed and heated for millions of years, to make a lesser amount of coal. So it is not a one to one relationship.

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  • $\begingroup$ but wouldn't that small amount of coal have the same weight as the larger amount of wood? $\endgroup$ – Yu. Sep 13 '19 at 16:18
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    $\begingroup$ @Yu: No, it would take many kilos of plants to make a kilo of coal so an equal weight of coal has much more carbon than living wood. $\endgroup$ – Adrian Howard Sep 14 '19 at 8:10

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