Coal, oil, peat are some of the most common fuels and they are all based on debating material. What makes material like this such a great fuel when it’s aged over huge period of time.
In: Geology
For starters, the original organic material *before* it sat around is pretty decent fuel. Carbon and hydrogen both bond strongly and readily to oxygen (which is what burning is). The trick is that the vast majority of carbon and hydrogen on Earth are *already* bonded to oxygen (CO2 and H2O) or something else, and it takes energy to break those bonds. In fact, it takes *more* energy to break those bonds than you get later by burning them.
Sunlight is an abundant source of energy and while photosynthesis is a horribly inefficient way of capturing that energy, there is a lot of surface area for photosynthetic organisms to use and a lot of time to do it. That sunlight energy is captured by breaking strong carbon and hydrogen bonds and storing those atoms in hydrocarbons – which, as the name suggests, are made primarily of hydrogen and carbon (and oxygen). There are a lot of them in there, which means plenty of fuel to burn.
Coal, oil, and peat are great specifically because they have *not* decayed. Most of the world’s fossil fuels came from a time in Earth’s history when decomposers had not evolved yet, or did not yet evolve to break down things like the tough lignin in tree bark. Even if they had already evolved, the decomposers just didn’t get to the organic material before it settled under mud, silt, or some other environment where the decomposers could not live or didn’t have what they needed to actually do the job of decomposing.
Instead, those lovely energy-rich hydrocarbons stuck around. As they were buried by centuries of additional sediment, they were put under enormous pressure and heated up. That heat and pressure put more energy into the hydrocarbons, causing more chemical reactions that broke down big, more stable molecules like that lignin or big polysaccharide chains like cellulose into smaller molecules. It also packed the hydrogen and carbon atoms together into bigger, but also less stable molecules. Less stable means it falls apart easier, which means the hydrogen and carbon atoms are more able to react with oxygen once the process is started.
That means you have even *more* potential energy packed into the molecules. And while there are plenty of other molecules that are [even more energy-dense](https://blogs.sciencemag.org/pipeline/archives/2011/11/11/things_i_wont_work_with_hexanitrohexaazaisowurtzitane), it still takes energy to build those molecules, and that energy still has to come from somewhere. Plus, it takes time. Plants and algae had millions of years to do it, and then the Earth had millions more years to do it more, and there’s just *so damn much* of the stuff that it’s economical to use fossil fuels, albeit not environmentally friendly.
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