Energetics of combustion - Technical characteristics

The article [BE 8 311] presents only the basic aspects of combustion, and all specific quantities are relative to the mole of fuel. However, for industrial applications, this reference is not very practical. We prefer to refer to the mass of the fuel when it's solid or liquid, and to the volume when it's gaseous. This is how, for example, molar enthalpy of combustion is transformed into net calorific value (NCV) or gross calorific value (GCV), at constant volume or pressure. In addition, the differences between these various quantities must be expressed as a function of the fuel's content of its various elements and conventional thermodynamic parameters.

In industrial applications, it is also important to be able to determine the air flow rates required for combustion, as well as the flue gas flow rates according to the desired thermal or mechanical output, when sizing power or heating installations. The concepts of combustive and smoking powers enable these calculations to be made both for theoretical (stoichiometric) combustions and for combustions closer to reality, i.e. those that take place either with an excess of air, complete or incomplete, or with a lack of air.

In power plants (spark-ignition reciprocating engines or diesels, turbojet or turboprop engines, etc.), combustion is adiabatic or with a relatively low proportion of heat loss. For these applications, as for adiabatic furnaces, it is useful to know the temperature reached by the combustion products, or adiabatic combustion temperature. If heat exchange is required between a flame or fumes and walls or a heat exchanger, we need to know the thermal power exchanged, which depends on the temperature of the flame or fumes. This is particularly true of furnaces and boilers. In some cases, such as condensing boilers, we need to know the dew point temperature and the final temperature reached by the flue gases to determine the amount of additional energy that can be recovered thanks to the latent heat of condensation of the water.

Finally, to limit losses in complex installations such as refineries, we need to know how to calculate the irreversibility of each energy item. These irreversibilities are quantified through exergy losses. For combustions, this involves calculating exergy losses in adiabatic combustion. In fact, when heat is released, exergy losses mainly concern the transfer at the heat exchanger.

The aim of this article is to present these issues in their entirety....