A substance that can be economically burned to produce heat energy for domestic or industrial purposes. Fuels include compounds of carbon and hydrogen and exclude other 1296 substances that can be burned. Fuels can be subdivided into recent plant fuels, fossil fuels, such as peat and coal, and products of distillation of plant or fossil fuels. According to their state of aggregation, fuels can be subdivided into solid, liquid, and gaseous fuels.