Calorific value

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The gross calorific value H s (outdated calorific value or upper calorific value H o ) is a measure of the specific chemically bound energy contained in a substance per measurement unit . The calorific value is identical to the absolute amount of the standard enthalpy of combustion Δ V H ° of general thermodynamics, given with a negative sign .

The calorific value of a fuel indicates the chemically bound energy ( enthalpy of reaction ) that is 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 for heating the combustion air and the exhaust gases and the condensation enthalpy of the condensing on cooling liquids, in particular of the produced during combustion of hydrogen-containing fuels water . In contrast, the calorific value (formerly "lower calorific value") describes the energy that is released during combustion and subsequent cooling to the starting temperature of the combustible mixture, whereby the combustion water is still in gaseous form. The calorific value of hydrogen-rich fuels is therefore significantly lower than their calorific value, namely 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 liter in kJ / l or per cubic meter in kJ / m 3 . In the case of 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 meter as kJ / m 3 iN, where "iN" means "in standard condition".

The current symbols is H s or B . The subscript s stands for Latin superior "higher". The symbol H o (the o stands for upper calorific value) is out of date; the information in the outdated unit kcal or in relation to the indicated unit of measurement such as B. in kJ / m N 3 do not correspond to the legal units and must therefore not be used in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), the combination of oxygen from the combustion air and hydrogen from the fuel produces water vapor. With conventional technology, the energy contained in the steam and thus in the exhaust gas is lost through the chimney through the hot exhaust gases . Modern condensing technology condenses the water vapor contained in the exhaust gas via a heat exchanger . This means that the heat of condensation contained in the exhaust gas is largely recovered.

In the gas supply industry, the gas calorific value is usually given in kilowatt hours per standard cubic meter (kWh / m 3 ).

For the calculation and tables of calorific values of fuels, see the article Calorific value

Norms

  • EN 437: 2003 Test gases - Test pressures - Appliances categories ; German: DIN EN 437: 2003-09 test gases - test pressures - device categories and ÖNORM EN 437: 1994-05-01 devices for operation with fuel gases - test gases - test pressures and device categories
    This Euro norm performs well in the sense of the international harmonization of the symbols H s one
  • DIN 5499 calorific value and calorific value, terms (January 1972)
  • DIN 1343 reference condition, standard condition, standard volume; Concepts and values (January 1990)
  • DIN 51900 Determination of the calorific value with the bomb calorimeter and calculation of the calorific value
    • Part 1 General information, basic devices, basic procedures (April 2000)
    • Part 2 method with isoperibolic or static-jacket calorimeter (May 2003)
    • Part 3 procedure with adiabatic jacket (July 2004)
For relevant standards of heating and energy technology, see calorific value #Norms

See also

Web links