Thermodynamics

756 | Thermodynamicsoxygen, giving a total of 95.2 moles of air. The air–fuel ratio (AF) is determinedfrom Eq. 15–3 by taking the ratio of the mass of the air and the massof the fuel,AF m airm fuel1NM2 air1NM2 C 1NM2 H2 24.2 kg air/kg fuel120 4.76 kmol2 129 kg>kmol218 kmol2 112 kg>kmol2 19 kmol2 12 kg>kmol2That is, 24.2 kg of air is used to burn each kilogram of fuel during thiscombustion process.Fuel n CO 2mC n H mH2 O2AIRCombustionchamberCH 4 + 2(O 2 + 3.76N 2 ) →CO 2 + 2H 2 O + 7.52N 2• no unburned fuel• no free oxygen in productsExcess O 2N 2FIGURE 15–8A combustion process is complete ifall the combustible components of thefuel are burned to completion.FIGURE 15–9The complete combustion processwith no free oxygen in the products iscalled theoretical combustion.15–2 THEORETICAL AND ACTUALCOMBUSTION PROCESSESIt is often instructive to study the combustion of a fuel by assuming that thecombustion is complete. A combustion process is complete if all the carbonin the fuel burns to CO 2 , all the hydrogen burns to H 2 O, and all the sulfur (ifany) burns to SO 2 . That is, all the combustible components of a fuel areburned to completion during a complete combustion process (Fig. 15–8).Conversely, the combustion process is incomplete if the combustion productscontain any unburned fuel or components such as C, H 2 , CO, or OH.Insufficient oxygen is an obvious reason for incomplete combustion, but itis not the only one. Incomplete combustion occurs even when more oxygenis present in the combustion chamber than is needed for complete combustion.This may be attributed to insufficient mixing in the combustion chamberduring the limited time that the fuel and the oxygen are in contact.Another cause of incomplete combustion is dissociation, which becomesimportant at high temperatures.Oxygen has a much greater tendency to combine with hydrogen than itdoes with carbon. Therefore, the hydrogen in the fuel normally burns tocompletion, forming H 2 O, even when there is less oxygen than needed forcomplete combustion. Some of the carbon, however, ends up as CO or justas plain C particles (soot) in the products.The minimum amount of air needed for the complete combustion of a fuelis called the stoichiometric or theoretical air. Thus, when a fuel is completelyburned with theoretical air, no uncombined oxygen is present in theproduct gases. The theoretical air is also referred to as the chemically correctamount of air, or 100 percent theoretical air. A combustion processwith less than the theoretical air is bound to be incomplete. The ideal combustionprocess during which a fuel is burned completely with theoreticalair is called the stoichiometric or theoretical combustion of that fuel (Fig.15–9). For example, the theoretical combustion of methane isCH 4 2 1O 2 3.76N 2 2 S CO 2 2H 2 O 7.52N 2Notice that the products of the theoretical combustion contain no unburnedmethane and no C, H 2 , CO, OH, or free O 2 .