Fuel

“Fuel” generally refers to a substance that will release heat that can be used as heat, light, and power. On our planet, that means three things: elemental (and highly flammable) hydrogen, organic hydrocarbons that can be burned, and radioactive elements that can be coaxed into a (controllable) nuclear reaction. There are many of the former, only two of the latter (uranium and plutonium). All hydrocarbons are of organic origin, meaning that they were manufactured by a living organism. The fossil fuels (coal, natural gas, and petroleum), are the result of organic materials being subjected to temperature, pressure, and time. As a result of this process, they are extremely concentrated and powerful fuel sources. Hydrocarbons extracted from living plants are not as concentrated — the advantage, of course, is that they are renewable. Hydrocarbons have three general sources:

• Plant material that can be directly burned as fuel: e.g., wood, charcoal, peat, manure, and bagasse (the plant material remaining after the process of extracting sugar from sugar cane).
• Gas (methane) arising from the decomposition of plant material, e.g. the use of manure from dairy cows in methane collection.
• Fats, oils, and sugars produced by plants and animals. For example, ethanol is a fuel manufactured from a variety of sources, including sugar cane, maize, sugar beets, and palm oil.