Calorific value measurement
Calorific value is the amount of energy released in the form of heat during the combustion of a unit mass of fuel. It is also called heat of combustion. It is often expressed in J/kg, but also in kWh/kg.
Lower calorific value and superior calorific value
Two distinct calorific values are defined:
- Lower calorific value
- Superior calorific value
The difference between these two quantities lies in whether or not the heat recoverable by condensation of the water vapor emitted during combustion is taken into account. The lower calorific value considers that the water produced during combustion is not condensed and remains in vapor form. The higher calorific value conversely considers that the latent heat of water condensation is recovered, and is added to the total heat emitted during combustion. The difference between these two values thus depends on the fuels, because they do not all emit the same amount of water during their combustion. It is, for example, greater for natural gas than for coal.
Condensation boilers are boilers that incorporate a system for recovering latent heat from condensation of the water vapor emitted by internal combustion.
Examples of calorific value values
Here are some examples of the calorific values of usual fuels:
|Coal||15 to 27 MJ/kg|
The superior calorific value is measured by carrying out the combustion of the sample in a bomb calorimeter, in the presence of oxygen under pressure. The inferior calorific valud is calculated from the superior one and the elemental composition of the sample (mainly carbon and hydrogen content). Elemental analysis is in fact a prerequisite for estimating the quantity of condensable water during the combustion of the sample.