The calorific value of a fuel is the quantity of heat produced by its combustion- at constant pressure and under a conditions known as "normal" of temperature and pressure. Calorific value of different substances are: Coal 25.33 8.1 Petrol 50 Diesel 45 15.5 Hydrogen 150 48.5 Hydrogen as fuel : Hydrogen gas has the highest calorific value in the table given above. Thus hydrogen is the best fuel but since its transport and handling is difficult, hydrogen is used as fuel where it is absolutely necessary. Otherwise hydrogen gas as a source ofcommon fuel is not used in domestic or in industrial situations. Hydrogen gas jet along with oxygen gas is used for producing a very hot flame, which is used for cutting metals in industries.