My favorite kind!
Coal is the most concentrated form of combustible fuel heat energy that is available by chemical means alone. Yet coal is not simply a conversion of carbon + oxygen to carbon dioxide, but burns WAY more efficiently when used in conjunction with - water. Coal, heated to near incandescence (about 1,000 degrees F.), when injected with water, forms carbon monoxide and hydrogen gas, both highly combustible fuels, and also important feedstock chemicals for a number of other industrial processes. The end result, when additional oxygen is introduced, is then carbon dioxide and water vapor (and a LOT of heat energy), both of which recycle in the ecology of green growing plants.
Blacksmiths using their forges knew the practical applications of this simple chemistry, as the hottest fire is from the combustion of hydrogen plus forced oxygen induction, via a bellows, used to forge iron by raising it to cherry or straw heat, for the purposes of forming by hammer and anvil. When iron approaches straw heat, is is almost as malleable as clay, though one would never attempt to handle it other than with tongs and while wearing protective gear.
The rise of industrialization and civilization itself depended on using this very concentrated form of carbon.