A chemical process in which oxygen combines with carbon from the fuel at high temperatures, releasing heat, carbon dioxide and waste products. Combustion is usually a sequence of chemical reactions. A solid fuel with a complex molecular structure is first broken down by heat into simpler components, usually gaseous, which then react with oxygen. The heat generated serves to decompose other parts of the fuel and the reaction is thus self-sustaining as long as there is sufficient fuel and oxygen. For human civilization, combustion is the oldest, most familiar and most widely used method of heat generation. Both fossil fuels and combustible biomass, or biomass processed into combustible components (e.g. biogas, bioethanol), can be used as fuel.