Energetics of combustion - Fundamental aspects

Combustion is an important and widespread technical operation. It has two main purposes: to produce heat or, ultimately, to supply mechanical energy. In the first case, energy systems are based on boilers or furnaces. The aim is to obtain flue gases with the lowest possible outlet temperature, in order to recover as much heat as possible. The second case corresponds to engines of all kinds, where combustion can be adiabatic. The enthalpy transferred to the flue gases is then transformed into mechanical energy.

To master these combustions, the energy engineer needs to know certain basic elements, based on physical and chemical knowledge. This is the aim of this article, which will be followed by a more technical one outlining the elements used in practice in industry.

After giving the essential definitions, reactions are presented in their global aspect, i.e. without any consideration of chemical kinetics. However, a few notions relating to chain reactions are provided to help understand the conditions required to initiate and maintain combustion. Emphasis is placed on the energy changes that take place during combustion. A link is also made between the heats of combustion and the heats of formation of pure bodies.

The boundary conditions of isothermal combustion on the one hand, and adiabatic combustion on the other, are considered in order to define the maximum heat released by combustion or the maximum temperature reached by the flue gases. The intermediate case is of course also considered, even with partial condensation of water in the flue gas, but without taking dissociation reactions into account, for which the reader is referred to specialized works on combustion chemistry.

The exergy aspects of combustion are discussed at the end of this article. Their interest, as is always the case with exergy, is to highlight the irreversibilities associated with this process. These are of the order of a quarter of the energy contained in the fuel mixture.