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?