Furnace fuel is a type of fuel used to heat buildings and other structures. Common furnace fuels include COKE (4 letters), COAL (4 letters), GAS (3 letters) and OIL (3 letters). COKE is a solid fuel made from COAL that is used in furnaces, while COAL is a combustible black or brownish-black sedimentary rock. GAS is a form of energy derived from natural GAS, petroleum or other sources, and OIL is a combustible liquid hydrocarbon mixture. All of these fuels can be used to power furnaces and provide heat.