Fossil Fuels — Combustion, Energy Density, and Environmental Impact

Fossil fuels — coal, oil, and natural gas — are the decayed and compressed remains of organisms that lived hundreds of millions of years ago. Over geological time, heat and pressure transformed buried organic matter into hydrocarbons and carbon-rich solids, locking away chemical potential energy that originated as sunlight captured by ancient photosynthesis. When we burn these fuels today, we release that stored energy in a single rapid step, which is why fossil fuels are classified as non-renewable: they form far too slowly to be replenished on any human timescale.

The energy is released by combustion, an exothermic chemical reaction in which the fuel combines with oxygen. For a simple hydrocarbon the reaction produces carbon dioxide and water and liberates heat. Methane, the main compone