Exergy In

The heating value of methane (see Chapter 6) is almost equal to its exergy value [11,14]. For the sake of simplicity in the ensuing analysis, we will set this exergy value of the gas to be equal to the energy value or, equivalently, the value of the heat of reaction. In Table 9.2, we have tabulated the exergy of methane at a number of different conditions. 

As the table shows, the exergy is not a strong function of temperature or pressure. For the illustrative purposes of our analysis, we use the value of 831.6 kj/mol. 

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The second law of thermodynamics is instrumental in providing insights into environmental impact. The most appropriate link between the second law and environmental impact has been suggested to be exergy, in part because the magnitude of the exergy of a system depends on the states of both the system and the environment and because exergy is a measure of the departure between these states. This departure is zero only when the system is in equilibrium with its environment. The authors have discussed this concept extensively previously [1, 2, 9, 14-16],... 

In the last chapter, the concepts of exergy and physical exergy, in particular, were introduced. This chapter deals with three other important concepts, namely, exergy of mixing, chemical exergy, and cumulative exergy consumption, and their numerical evaluation. 

The calculations for Table 10.1 have been scaled such that they relate to the input of one unit of exergy into the column. Figure 10.12 shows that the sum of the lost work (lost exergy) in the reboiler, column, and condenser... 

Overview of Exergy in Distillation Column Exergy In Exergy Out... 

Because chemical transformation does not occur, we do not need to include the chemical exergy in the exergy flows. We can now write expressions for the terms in the exergy balance based on 1 mol feed entering the system for both the reversible and real separation processes. 

It is interesting to realize that, eventually, all exergy in the sun s radiation to the earth is dissipated. Heat released to the environment, without exergy,... 

Nielsen SN. 1992. Application of maximum exergy in structural dynamic models [PhD thesis]. Roskilde (Denmark) Roskilde University. 

A chemical substance has its chemical energy in terms of the chemical potential and has its chemical exergy as well. Let us consider a chemical substance present at unit activity in the normal environment at temperature T0 and pressure p0 and examine its chemical exergy in relation with the exergy reference species in the atmospheric air, in seawater, and in lithospheric solids (Refs. 9 and 11). 

In calculating the numerical values of the standard molar exergy e°of chemical elements and compounds, we usually make clear the exergy reference species at zero level of exergy in our natural environment of the atmosphere, the hydrosphere and the lithosphere. 

From the foregoing discussion in this chapter we realize that all the terms in Eq. 10.42 can be expressed in the form (H- H0) - T0(s S0). We can therefore express the total exergy in a simple form as follows ... 

The exergy in the steam can be easily evaluated using the property relations represented by the steam tables ... 

At the location (G) the gases are at the same pressure as the environment but are not in thermal or chemical equilibrium with it. Even if cooled to T, the stack gases at a total pressure Pq would still not be in complete equilibrium with the environment, because the composition is different. Using thermostatic property relations (10), the total exergy in the stack gases may be calculated ... 

To calculate the consumptions of exergy in the combustion process and the heat transfer process, it is supposed that the boiler may be separated into two distinct hypothetical entities an "adiabatic combustor" and a "heat exchanger." After determining the state of the combustion products, using an energy balance, the transport of exergy from the combustion process with product gases can be determined with the same procedures as above ... 

The stack losses, while not insignificant, represent only 5.5% of the exergy in the coal in contrast, they represent nearly 15% of the coal s energy content. 

Figure 6. Flow of exergy in the U.S. (All values reported are 1015 Btu [1.055(1018)J] (20).) Footnote a all values in ( ) are energy, other values reported are exergy. Footnote b the energy value for hydropower is reported in the usual manner of coal equivalent actual energy is 1.12 x 1015 Btu.

There are large amounts of thermal exergy in the "waste heat" from cement kilns. A specific proposal would install a 4700 kW power cycle driven by the otherwise wasted exergy at a cost of 2.7 million. The resultant power would be used in-plant, replacing electricity that would need to be purchased at 2.6CkWhr. 

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