11 - 11 Chemistry o f Combustion The thermodynamics of...

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11 Chemistry of Combustion The thermodynamics of combustion were considered in Chapter 10, and it was stated that adiabatic combustion could be achieved. The concept of adiabatic combustion runs counter to the experience of many engineers, who tend to relate combustion to heat addition or heat release processes. This approach is encouraged in mechanical engineers by the application of the air standard cycle to engines to enable them to be treated as heat engines. In reality combustion is not a process of energy transfer but one of energy transformation. The energy released by combustion in a spark ignition (petrol) engine is all contained in the mixture prior to combustion, and it is released by the spark. It will be shown that the energy which causes the temperature rise in a combustion process is obtained by breaking the bonds which hold the fuel atoms together. 11.1 Heats (enthalpies and internal energies) of formation can be evaluated empirically by ‘burning’ the fuel. They can also be evaluated by consideration of the chemical structure of the compound. Each compound consists of a number of elements held together by certain types of bond. The bond energy is the amount of energy required to separate a molecule into atoms; the energy of a particular type of bond is similar irrespective of the actual structure of the molecule. This concept was introduced in Chapter 10, and heats of formation were used to evaluate heats of reaction (Hess’ law). The process of breaking the chemical bonds during the combustion process can be depicted by a diagram such as Fig 1 1.1. It is assumed that element molecules can be atomised (in a constant pressure process) by the addition of energy equal to AHa. If these atoms are then brought together they would combine, releasing dissociation energy of c A H(X - nR, to form the reactants. The sum of the dissociation and atomisation energies (taking account of the signs) results in the enthalpy of formation of the reactants. In a similar way the enthalpy of formation of the products can be evaluated. Using Hess’ law, the enthalpy of reaction of the fuel can be evaluated the difference between the enthalpies of formation of the products and the reactants These energies are essentially the bond energies of the various molecules, and some of these energies are listed in Table 1 1.1. Figure 11.2 shows how the energy required to separate two atoms varies with distance: the bond energy is defined as the minimum potential energy relative to that at infinity. The point of minimum energy indicates that the molecule is in equilibrium. Bond energies and heats of formation
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Gaseous atoms Fig. 11.1 Relationship between enthalpies of formation and reaction n n 4 energy cm Elemental n molecules - Distance Fig. 11.2 Variation of bond energy with distance between atoms CAH(X-Y), CAH(X-Y), n n m3, Wfh Table 11.1 Some atomisation, dissociation and resonance energies (based on 25OC)* Reactant molecules Bond Bond Energy dissociation Energy Energy atomisation (AH,) (MJ/kmol) (AH(X- Y)) Resonance (MJ/kmol) . V V V H-H 435.4
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11 - 11 Chemistry o f Combustion The thermodynamics of...

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