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cen84959_ch15 - cen84959_ch15.qxd 6:06 AM Page 773 Chapter...

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Chapter 15 CHEMICAL REACTIONS | 773 I n the preceding chapters we limited our consideration to nonreacting systems—systems whose chemical composi- tion remains unchanged during a process. This was the case even with mixing processes during which a homoge- neous mixture is formed from two or more fluids without the occurrence of any chemical reactions. In this chapter, we specifically deal with systems whose chemical composition changes during a process, that is, systems that involve chem- ical reactions. When dealing with nonreacting systems, we need to con- sider only the sensible internal energy (associated with tem- perature and pressure changes) and the latent internal energy (associated with phase changes). When dealing with reacting systems, however, we also need to consider the chemical internal energy, which is the energy associated with the destruction and formation of chemical bonds between the atoms. The energy balance relations developed for nonreact- ing systems are equally applicable to reacting systems, but the energy terms in the latter case should include the chemi- cal energy of the system. In this chapter we focus on a particular type of chemical reaction, known as combustion, because of its importance in engineering. But the reader should keep in mind, however, that the principles developed are equally applicable to other chemical reactions. We start this chapter with a general discussion of fuels and combustion. Then we apply the mass and energy balances to reacting systems. In this regard we discuss the adiabatic flame temperature, which is the highest temperature a react- ing mixture can attain. Finally, we examine the second-law aspects of chemical reactions. Objectives The objectives of Chapter 15 are to: Give an overview of fuels and combustion. Apply the conservation of mass to reacting systems to determine balanced reaction equations. Define the parameters used in combustion analysis, such as air–fuel ratio, percent theoretical air, and dew-point temperature. Apply energy balances to reacting systems for both steady- flow control volumes and fixed mass systems. Calculate the enthalpy of reaction, enthalpy of combustion, and the heating values of fuels. Determine the adiabatic flame temperature for reacting mixtures. Evaluate the entropy change of reacting systems. Analyze reacting systems from the second-law perspective.
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15–1 FUELS AND COMBUSTION Any material that can be burned to release thermal energy is called a fuel . Most familiar fuels consist primarily of hydrogen and carbon. They are called hydrocarbon fuels and are denoted by the general formula C n H m . Hydrocarbon fuels exist in all phases, some examples being coal, gasoline, and natural gas.
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