One aspect of energy calculations that causes many problems in interpretation is the conversion factor used to convert fuel mass to energy. The conversion factor is called the calorific value of the fuel and represents the quantity of energy that can be derived from unit mass of fuel. There are, however, two types of calorific value—gross and net.
Gross calorific value is the energy obtained from a fuel when the combustion products are returned to the thermodynamic standard state and any water produced in the combustion reaction is liquid. This definition yields a value equal to the total energy resource available from the fuel. Gross calorific value is sometimes referred to as the high heat value.
Net calorific value is the energy obtained when the combustion products all remain as gases. This definition is widely used in engineering design when the aim is to ensure that any water produced in the combustion reaction leaves the combustion reactor as a gas, thus minimizing corrosion. Since the energy content of gaseous water is not recovered, the net calorific value is lower than the gross calorific value. For natural gas the difference is about 10%, and for oil products the difference is about 5%. Net calorific value is sometimes referred to as the low heat value.
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