Chemical thermodynamics is concerned with the description of the equilibrium states of reacting multicomponent systems. Compared to single-component systems in which only thermal equilibrium is required, we are now also interested in chemical equilibrium among all of the components. Since practical combustors are designed to ensure that fuel and air have sufficient residence time to mix, react, and attain thermodynamic equilibrium, global performance parameters such as the heat and power output can frequently be estimated by assuming thermodynamic equilibrium of the combustion products. Thus, the scientific elements of a large part of combustion engineering are covered by the subject of this chapter.

In Section 1.1, we introduce the concept of stoichiometry, which sensitively controls the temperature of a combustion process. In Section 1.2, the criterion for chemical equilibrium is derived and the methodology for calculating the equilibrium composition of a mixture, for given pressure and temperature, is discussed. We then apply this calculation procedure to hydrocarbon–air mixtures as an example in Section 1.3. In Section 1.4, energy conservation is considered, which enables the simultaneous determination of the final composition and temperature of a reactive mixture after equilibrium is established. This final temperature, called the adiabatic flame temperature, Tad, is perhaps the most important parameter of a reactive mixture, indicating not only its potential to deliver heat and power, but also the rates of progress of the various chemical reactions constituting the entire combustion process.