The amount of heat released by a specific quantity of fuel. The gross heating value of a fuel is defined as the amount of heat released by a specified quantity (initially at 25°C) once it is combusted and the products have returned to a temperature of 25°C. The difference between the higher heating value and the lower heating value depends on the chemical composition of the fuel. In the case of pure carbon or carbon dioxide, both heating values are almost identical. See also higher heating value and lower heating value of natural gas.