Calorific value

The calorific value or heat of combustion (noted Δ_cH⁰, in English Heating Been worth) of a material Combustible is the Enthalpie of reaction of Combustion per unit of mass. It is the energy released in the form of heat by the reaction of combustion by oxygen (in other words quantity of heat). Generally, one considers a Hydrocarbure reacting with the Oxygène of the air to give Carbon dioxide, Eau and Chaleur.

It is expressed in general in kilo Joule by Kilogram (noted kJ/kg or kJ·kg^-1), but one also meets the molar calorific value (in kilo Joule by Mole, kJ/mol) or the voluminal calorific value (in kilo Joule by Liter, kJ/L). For the Natural gas, it is expressed in kilo Watt hour by normal Cubic meter (noted kw·h/m ³ (N)).

There exist 2 types of calorific value:

the higher calorific value (PCS) : it is energy resulting from the combustion to which one adds energy that the steam restores with its environment by condensing. (in other words quantity of heat produced by a combustion whose produced water is in liquid form).
the lower calorific value (PCI) : it is energy resulting from combustion without taking account of the energy released by the Condensation of water. (in other words quantity of heat produced by a combustion whose produced water is in the form of vapor).

These two types of calorific value explain why certain salesmen of boiler with condensation speak to you about output superiors at 100% (104%, 110% etc) In this case they refer to the NCV. The output calculated on GCV is always lower than 100%.

Calorific value of some fuels

Calorific value inferiors of pure organic compounds (with 25°C)

Reference

" Fuels and moteurs" , J-C Guibet, Publication of the French Petroleum Institute, ISBN 2-7108-0704-1

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