Fuel Fuel Fuels are chemical substances which may be burned in oxygen to generate heat . They mainly consist of carbon and hydrogen and sometimes a small amount of sulfur or minerals. There are solid, liquid and gaseous fuels. Coal and Coke are examples of solid fuels.
Petroleum oils are usually a mixture of several liquid fuels.
The components before the combustion process are called reactants. The combustion process produces new components which are called products. Here, C and O2 are reactants and CO2 is the product.