Heat of combustion
This article deals with the thermodynamic quantity; for the energy content of food, see Physiological calorific value; for its use in optics, see Refractive power.
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The calorific value Hs (obsolete calorific value or upper heating value Ho) is a measure of the specific chemically bound energy contained in a substance per unit of measurement. The calorific value is identical to the absolute amount of the standard enthalpy of combustion ΔVH° of general thermodynamics, which is given with a negative sign.
The calorific value of a fuel indicates the chemically bound energy (reaction enthalpy) released during combustion and subsequent cooling of the combustion gases to 25 °C and their condensation.
The calorific value takes into account both the energy required to heat the combustion air and the flue gases and the enthalpy of condensation of the liquids condensing during cooling, in particular the water produced during the combustion of hydrogen-containing fuels. In contrast, the calorific value (formerly "lower heating value") refers to the energy released during combustion and subsequent cooling to the initial temperature of the combustible mixture, with the combustion water still in gaseous form. The calorific value of hydrogen-rich fuels is therefore significantly lower than their calorific value by the amount of the enthalpy of condensation of the water formed during combustion.
The physical quantity
The calorific value is given as a mass-related calorific value in kilojoules per (kilo)gram in kJ/g or kJ/kg, as a volume-related calorific value per litre in kJ/l or per cubic metre in kJ/m3. For gaseous substances, the calorific value is related to the volume at 101.325 kPa and 25 °C (standard conditions). The specification is then made in kilojoules per standard cubic metre as kJ/m3 i.N., where "i.N." means means "in standard conditions".
The current formula symbol is Hs or B. The subscript s stands for Latin superior "higher". The formula symbol Ho (the o stands for upper calorific value) is obsolete; the information in the obsolete unit kcal or related to the indexed unit of measurement such as in kJ/mN3 does not correspond to the legal units and may therefore not be used in the movement of goods.
Energy and heating technology
In every combustion process of hydrocarbons (such as natural gas, crude oil, coal, wood), water vapour is produced by the combination of oxygen from the combustion air and hydrogen from the fuel. With conventional technology, the energy contained in the water vapour and thus in the flue gas is lost via the hot flue gases through the chimney. Modern condensing technology condenses the water vapour contained in the flue gas via a heat exchanger. In this way, the condensation heat contained in the flue gas is recovered as far as possible.
In the gas supply industry, the gas calorific value is usually expressed in kilowatt hours per standard cubic meter (kWh/m3).
For calculation and tables of calorific values of fuels see the article Calorific value
Questions and Answers
Q: What is meant by the term heat of combustion?
A: The heat of combustion refers to the amount of energy that is released when a substance is burned in standard conditions.
Q: What is the other name given for the heat of combustion?
A: The heat of combustion is also known as the calorific value or energy value of a substance.
Q: What happens to the energy released during combustion?
A: The energy released during combustion is released in the form of heat.
Q: Is the amount of energy released during combustion different for different substances?
A: Yes, the amount of energy released during combustion is different for different substances.
Q: What factors determine the amount of energy released during combustion?
A: The amount of energy released during combustion is determined by the nature of the substance being burned and the standard conditions under which the combustion takes place.
Q: How is the heat of combustion useful?
A: The heat of combustion is useful in determining the energy content of fuels and other combustible substances.
Q: Is the heat of combustion important for fuel efficiency?
A: Yes, the heat of combustion is an important factor in determining the fuel efficiency of a particular substance.
