Gas exergy

Subsequently, we refer briefly to other comparable studies, including the calculations of exergy losses and rational efficiency. Finally, we show the real gas exergy calculations for two practical plants—[CBT]i and [CBTX]i. 

Using the molar fraction of water x h20. the thermomechanical exergy is split into the dry gas exergy ei, ,tm,dry and moisture exergy e , ,tm,H20 both in kj/kmol, the latter referring to the condensed state of water. 

The objective of the gas turbine designer is to make all the proces.ses in the plant as near to reversible as possible, i.e. to reduce the irreversibilities, both internal and external, and hence to obtain higher thermal efficiency (in a closed cycle gas turbine plant) or higher overall efficiency (in an open gas turbine plant). The concepts of availability and exergy may be used to determine the location and magnitudes of the irreversibilities. 

Fig. 2.8. Exergy fluxes in actual CBT ga.s turbine plant with combustion.

Fig. 2.9 illustrates this approach of tracing exergy through a plant. The various terms in Eq. (2.49) are shown for an irreversible open gas turbine plant based on the JB cycle. The compressor pressure ratio is 12 1, the ratio of maximum to inlet temperature is 5 1 (T,nax = 1450 K with To = 290 K), the compressor and turbine polytropic efficiencies are... 

Fig. 2.9. Work output and exergy losses in CBT ga.s turbine plant (all. as I ractions of fuel exergy).

Therefore, plots of exergy loss or irreversibility like Fig. 2.9, for a particular plant operating condition, do not always provide the complete picture of gas turbine performance. 

Horlock, J.H., Manfrida, G. and Young, J.B. (2000), Exergy analysis of modem fossil-fuel power plants, ASME J. Engng Gas Turbines Power 122, 1-17. 

El-Masri, M.A. (1987), Exergy analysis of combined cycles. Part 1. Air-cooled Bray ton-cycle gas turbines, ASME J. Engng Power Gas Turbines 109, 228-235. 

In both cases heat is taken from the exhaust gases to feed the reaction process, enhancing the heating value of the resulting modified fuel, which is then fed to the combustion chamber. But the main thermodynamic feature is that the exergy loss in the final exhaust gas is thus reduced and the efficiency increased. 

The definition of the exergy of the fuel with the thermodynamic state of the fuel cell is now shown more detailed. Similar methods are used for the reversible heating of the air and the reversible cooling of the flue gas. The reversible air heating requires... 

This work is called the chemical exergy of methane and is the maximum amount of work that this compound has available for performing work in the environment (Figure 6.3). Indeed, natural gas is an important fuel for a power station. Table 6.1 gives the chemical exergy or available work of a number of compounds energy carriers, raw materials, and pure products. 

From these data, we can calculate the chemical, exergy values of these components in the pure state at P0 and T0. Air at these conditions can, to a good approximation, be considered as an ideal gas therefore, separation... 

The first term on the right-hand side of this equation is the standard Gibbs energy of formation of methane, which is listed [2] as -50.460 kj/mol and thus EXch,cH4(g) can be calculated to be 831.6 kj/mol. Chapter 9 illustrates the use of this exergy value in the analysis of a natural gas-driven power station. 

The concept of cumulative chemical exergy consumption is very useful and accounts for the fact that when a compound (e.g., ammonia) is introduced into a process, its chemical exergy has to be corrected for the exergy consumption accumulated since this compound was manufactured from its natural constituents (air and natural gas in the case of ammonia). 

The conditions of a natural gas reservoir are 30 MPa and 100°C. The gas, assumed to be pure methane, is spontaneously expanded to a pressure of 7MPa (Figure 8.1). Assuming that this expansion is adiabatic, calculate the amount of work that is lost in the process, and express it as a fraction of the originally available amount of work per mole of gas in the reservoir. Carry out this calculation while making a distinction between the physical and chemical exergy of the gas. 

Next, we wish to calculate which fraction this lost work is of the work originally available in the gas. The chemical exergy of the gas, assumed to be methane, is significant, 831.65 kj/mol, but it should be excluded from the calculation because no chemistry is involved in the expansion step. The work available in the gas at initial and final conditions can be calculated from Equation 6.11 ... 

The fraction of nonchemical work available in the gas that has been lost in the expansion process can now be calculated from W /Exj = 3.325/13.501 = 0.246. If we had included the chemical exergy of the gas, this number would have been reduced to 0.00393, but as the expansion step is strictly nonchemical, this result is meaningless. Of course, the calculation of Wlost itself would not be affected as the chemical exergy would have to be included in both Exj and Ex2 and would drop out. 

In this chapter, we explore how the exergy concept can be used in the analysis of energy conversion processes. We provide a brief overview of commonly used technologies and analyze the thermodynamic efficiency of (1) coal and gas combustion, (2) a simple steam power plant, (3) gas turbine, and (4) combined cycle and cogeneration. At the end of this chapter, we summarize our findings with some concluding remarks. 

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. 

In adiabatic combustion, the chemical energy of the natural gas is converted into the same amount of thermal energy stored in the effluent stream. For the effluent stream, however, the exergy will no longer be equal to the energy content, since some work would have been lost. (See Figure 9.17. The numbers are calculated in Section 9.7.3.)... 

---