Calorific value The quantity of heat (Kcal) liberated by the combustion of unit quantity of fuel is called its calorific value. Unit of calorific value is Kcal/kg for solid and liquid fuels and Kcal/Nm3 for gaseous fuels. Nm3 means volume of gas in M3 at Normal Temperature and Pressure (NTP) which is zero deg0 C and 760 mmHg. Since the volume of gases varies sensitively with pressure (Boyle’s Law) and temperature (Charle’s Law) hence their volume is expressed at NTP in Nm3 to have a standard measurement.
Gross calorific value or higher heating value at constant volume is the quantity of heat liberated by combusting the fuel at constant volume in oxygen saturated with water vapour, the original material and final products of combustion being at a reference temperature (25degC) and the water obtained from the fuel being in the liquid state.
Gross calorific value at constant pressure implies that the combustion takes place at constant pressure and not at constant volume. In the laboratory determinations, solid and liquid fuels are burnt at constant volume and gaseous fuels are burnt at constant pressure. In the ovens and furnaces, however, the combustion takes place at constant pressure. The difference in the two corresponding values is small. For coal, the calorific value at constant pressure exceeds the calorific value at constant volume by about 5.5 Kcal/kg
Net calorific value or lower heating value at constant volume is the quantity of heat evolved when unit quantity of fuel is burnt at constant volume in oxygen saturated with water vapour, the originals material and final products of combustion being at a reference temperature (25degC) and the water obtained from the fuel being in the vapour state. The net calorific value is therefore less than the gross calorific value by the amount of the heat of condensation of water vapours, which at 25deg C is 583.5 kcal/kg of water. On the basis of hydrogen of water, this is equal to 5,252 kcal/kg 468.9 kcal/Nm3 of hydrogen. The following formula is used in calculating the net calorific value from gross calorific value of solid and liquid fuels approximately.
CN=CG-53 H
where CN and CG = net and gross calorific value in kcal/kg, respectively, and H= percentage of hydrogen of coal, including hydrogen of moisture and of water of hydration of minerals.
For a gaseous fuel, the formula is:
CN=CG-4.7V
where CN and CG = net and gross calorific values in kcal/Nm3, respectively, V= volume percentage (as H2) of total hydrogen of the gaseous fuel, including the hydrogen obtainable from other combustible components.
Net calorific value at constant pressure implies that the combustion takes place at constant pressure and not at constant volume.
Gross calorific value or higher heating value at constant volume is the quantity of heat liberated by combusting the fuel at constant volume in oxygen saturated with water vapour, the original material and final products of combustion being at a reference temperature (25degC) and the water obtained from the fuel being in the liquid state.
Gross calorific value at constant pressure implies that the combustion takes place at constant pressure and not at constant volume. In the laboratory determinations, solid and liquid fuels are burnt at constant volume and gaseous fuels are burnt at constant pressure. In the ovens and furnaces, however, the combustion takes place at constant pressure. The difference in the two corresponding values is small. For coal, the calorific value at constant pressure exceeds the calorific value at constant volume by about 5.5 Kcal/kg
Net calorific value or lower heating value at constant volume is the quantity of heat evolved when unit quantity of fuel is burnt at constant volume in oxygen saturated with water vapour, the originals material and final products of combustion being at a reference temperature (25degC) and the water obtained from the fuel being in the vapour state. The net calorific value is therefore less than the gross calorific value by the amount of the heat of condensation of water vapours, which at 25deg C is 583.5 kcal/kg of water. On the basis of hydrogen of water, this is equal to 5,252 kcal/kg 468.9 kcal/Nm3 of hydrogen. The following formula is used in calculating the net calorific value from gross calorific value of solid and liquid fuels approximately.
CN=CG-53 H
where CN and CG = net and gross calorific value in kcal/kg, respectively, and H= percentage of hydrogen of coal, including hydrogen of moisture and of water of hydration of minerals.
For a gaseous fuel, the formula is:
CN=CG-4.7V
where CN and CG = net and gross calorific values in kcal/Nm3, respectively, V= volume percentage (as H2) of total hydrogen of the gaseous fuel, including the hydrogen obtainable from other combustible components.
Net calorific value at constant pressure implies that the combustion takes place at constant pressure and not at constant volume.
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