Plant efficiency exergy

On the other hand, second law analysis of a Brayton cycle (Example 5 Section 23.6.5) reveals a high exergy content of the turbine exhaust. For this reason, great improvements in gas turbine plant efficiency can be achieved by recovering exergy in the turbine outlet with either recuperation (Example 7 Section 23.6.7) or through the use of a heat recovery boiler used to supply a secondary steam cycle in a cogeneration plant (Example 6 Sechon 23.6.6). 

As in fhe case of a turbine, rjj is generally higher than t] because the additional work supplied raises the exergy of the exit stream. It is possible to recover this exergy in later processes. For example in a Brayton cycle plant, efficiency less than unity results in a higher compressor exit temperature and enthalpy, which reduces the heat requirements of the combustion chamber. 

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. 2.9 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... 

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. 

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. 

We wish to alert the reader that in the analyses presented above, the results were essentially independent of the type of fuel used. From an efficiency point of view, this may be true, but from a sustainability point of view, it is not. In general, gas is a much cleaner burning fuel than coal and requires less pre- and posttreatment. Even though the standard power generation plants can be made more efficient using thermodynamic analysis (lost work, availability, or exergy analysis), we note that power generation based on fossil fuels is not sustainable since the combustion of these fuels leads to increased... 

A noteworthy point is, however, that the heat for the reboiler is supplied at 377 K, which is often available as waste heat in a chemical plant. Waste heat is usually rejected (hence the term "waste"), so heat integration could improve the process efficiency and reduce total exergy losses for the entire chemical plant (see Figure 10.14). Now, what Figure 10.14 shows is that heat integration can sometimes reduce the exergy losses of a chemical plant as well as reduce... 

Figure 10 shows the flows of exergy between the major groups of equipment in the Synthane process. Also shown is the consumption in each group and its efficiency. Furthermore, unit costs of exergy are shown in parentheses. A notable feature of the SYNTHANE process is that more electric power is produced from the gasifier by-product char than is consumed by the process itself hence the excess is available for "export" from the plant. 

From the foregoing analysis 1t is clear that a process modification that helps eliminate or minimize the irreversibility associated with the reactor feed preparation will lead to a major reduction in the thermal mismatch, reduce the exergy dependence on the power plant, and increase the overall energy efficiency. In the author s opinion, this conclusion would not be evident as readily without the thermodynamic analysis of process irreversibilities, which attests to the value of such exergy analyses. 

Table 4.17 shows that column 2 operates with efficiency as low as 4.1%, while the efficiency of column 1 is 50.6%. The exergy values for the whole separation system of two columns are also low and need to be improved. The individual values of PI show that it is possible to reduce by 13.1% the total loss of 0.834 MW in column 1 and by 19.1% the total loss of 27.813 MW in column 2. Despite the heat integration, the separation section of the methanol plant performs poorly in utilizing the exergy in the distillation columns. 

The author asserts that an isothermal plant arrangement (process integration therefore not applicable) is essential for greater thermodynamic efficiency, and hopefully for cheapness. An equilibrium diagram for such a plant is shown in Figure A.4. The plant is arranged so that exergy is supplied as electrical power and not by combustion heat, an entirely theoretical concept, and therefore a difficult development problem, for Methanex, Air Products and Ballard. 

An efficient plant must be based on making an approach, as near as is practical and economic, to the perfect processes of an equilibrium diagram, such as Figure A.4, which was initially composed as a calculation route for methane chemical exergy, and then realised to have larger implications. Methane had to be consumed in an isothermal equilibrium reversible process. 

The energy consumption figures discussed so far represent a thermodynamic analysis based on the first law of thermodynamics. The combination of the first and second laws of thermodynamics leads to the concept of ideal work, also called exergy. This concept can also be used to evaluate the efficiency of ammonia plants. Excellent studies using this approach are presented in [1061], [1062], Table 39 [1061] compares the two methods. The analysis in Table 39 was based on pure methane, cooling water at 30 °C (both with required pressure at battery limits), steam/carbon ratio 2.5, synthesis at 140 bar in an indirectly cooled radial converter. 

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