Heat Value

(or calorific value, heat of combustion), the quantity of heat liberated upon the complete combustion of a unit weight or unit volume of fuel. It is expressed in, for example, kilojoules (kJ) or kilocalories (kcal) per kg or m³. In Great Britain and the USA, heat values are often measured in Btu/lb; 1 Btu/lb = 2.326 kJ/kg. The heat value is an important indicator of the usefulness of a fuel. Heat values are determined by calorimetry.

If, after combustion, the water originally contained in the fuel and the water formed from the burning of the hydrogen in the fuel are present in liquid form, the quantity of heat liberated is characterized by the high, or gross, heat value Q_h. If the water is in the form of vapor, the heat liberated is characterized by the low, or net, heat value Q_l. The relation between the high and low heat values is given by the equation

Q_I = Q_h – K(W + 9H)

Here, W is the amount of water in the fuel, in percent of the total weight; H is the amount of hydrogen in the fuel, in percent of the total weight; and k is a constant equal to 25 kJ/kg, or 6 kcal/kg.

In some countries, such as the USSR and the Federal Republic of Germany, the low heat value is generally used in thermal calculations. In the USA, Great Britain, and France, the high heat value is used.

Heat values may be defined in special ways. For example, Q^w is the heat value with respect to the working weight of the fuel, that is, the weight of the fuel in the form in which it is supplied to the user; Q^d is the heat value with respect to the dry weight of the fuel; and Q^c is the heat value with respect to the combustible weight of the fuel, that is, the weight of the fuel excluding moisture and ash.

In approximate calculations, heat values are found from empirical formulas. For example, the heat values of solid and liquid fuels can be calculated from Mendeleev’s formula

where C^w, H^w, O^w, , and W^w are the amounts, in percent, of carbon, hydrogen, oxygen, volatile sulfur, and water, respectively, in the total working weight of the fuel.

Comparative calculations make use of a standard fuel having the heat value 29,308 kJ/kg, or 7,000 kcal/kg.