Thermodynamics

Chapter 15 | 773which is lower than 5,646,081 kJ. Therefore, the actual temperature of theproducts is between 2350 and 2400 K. By interpolation, it is found to beT prod 2395 K.(b) The balanced equation for the complete combustion process with 400percent theoretical air isC 8 H 18 12 50 1O 2 3.76N 2 2 S 8CO 2 9H 2 O 37.5O 2 188N 2By following the procedure used in (a), the adiabatic flame temperature inthis case is determined to be T prod 962 K.Notice that the temperature of the products decreases significantly as aresult of using excess air.(c) The balanced equation for the incomplete combustion process with90 percent theoretical air isC 8 H 18 12 11.25 1O 2 3.76N 2 2 S 5.5CO 2 2.5CO 9H 2 O 42.3N 2Following the procedure used in (a), we find the adiabatic flame temperaturein this case to be T prod 2236 K.Discussion Notice that the adiabatic flame temperature decreases as aresult of incomplete combustion or using excess air. Also, the maximum adiabaticflame temperature is achieved when complete combustion occurs withthe theoretical amount of air.15–6 ENTROPY CHANGE OF REACTING SYSTEMSSo far we have analyzed combustion processes from the conservation ofmass and the conservation of energy points of view. The thermodynamicanalysis of a process is not complete, however, without the examination ofthe second-law aspects. Of particular interest are the exergy and exergydestruction, both of which are related to entropy.The entropy balance relations developed in Chap. 7 are equally applicableto both reacting and nonreacting systems provided that the entropies of individualconstituents are evaluated properly using a common basis. Theentropy balance for any system (including reacting systems) undergoingany process can be expressed asS in S out S gen ¢S system 1kJ>K215253 123 123Net entropy transfer Entropy Changeby heat and mass generation in entropy(15–18)Using quantities per unit mole of fuel and taking the positive direction ofheat transfer to be to the system, the entropy balance relation can beexpressed more explicitly for a closed or steady-flow reacting system as(Fig. 15–28)aQ kT k S gen S prod S react 1kJ>K2(15–19)where T k is temperature at the boundary where Q k crosses it. For an adiabaticprocess (Q 0), the entropy transfer term drops out and Eq. 15–19reduces toS gen,adiabatic S prod S react 0(15–20)ReactantsS reactSurroundingsReactionchamber∆S sysProductsS prodFIGURE 15–28The entropy change associated with achemical relation.