Performance Evaluation of Selected Combustion Gas Turbine Cogeneration Systems Based on First and Second-Law Analysis

Abstract

The thermodynamic performance of selected combustion gas turbine cogeneration systems has been studied based on first-law as well as second-law analysis. The effects of the pinch point used in the design of the heat recovery steam generator, and pressure of process steam on fuel-utilization efficiency (first-law efficiency), powerto-heat ratio, and second-law efficiency, are examined. Results for three systems using state-of-the-art industrial gas turbines show clearly that performance evaluation based on first-law efficiency alone is inadequate. Decision makers should find the methodology contained in this paper useful in the comparison and selection of cogeneration systems. IntroductionCogeneration is an engineering concept involving the production of both electricity and useful thermal energy (steam or process heat) in one operation, thereby utilizing fuel more effectively than if the desired products were produced separately. Since the heart of a cogeneration system is a prime mover with waste heat at a useful temperature, it is not surprising that the requirements of cogeneration may be met in many ways ranging from steam and gas turbines to fuel cells and Stirling engines. An excellent summary of eight kinds of cogeneration systems with unique features of each kind has been given by Bazques and Strom (1983).Most cogeneration requirements are best met by back pressure steam turbines and combustion gas turbines. The latter are capable of producing significantly more electric power for a given amount of process heat. Since the cost effectiveness of a cogeneration system is directly related to the amount of power it can produce, gas turbines often possess significant advantages over steam tubines. Consequently combustion gas turbines have been widely used in cogeneration applications for many years. This paper is confined to cogeneration systems using combustion gas turbines as the prime movers, and producing thermal energy in the form of saturated steam.There are many combustion gas turbines on the market today. They differ in power output, cycle efficiency, cycle pressure ratio, firing temperature, exhaust temperature, and exhaust flow rate. An excellent treatment of the thermodynamic performance of combustion gas turbine cogeneration systems has been presented by Rice (1987) in which he has, based on the first law of thermodynamics, developed a unique graphic solution showing the interrelationship of the

GTA modeling of combined cycle power plant efficiency analysis

“…8. The amount of steam generated in the HRSG depends on the pinch point of the boiler [43][44][45]. Where pinch point is the difference between gas temperature leaving an evaporating section and the temperature at which boiling occurs.…”

“…7. The HRSG steam production for a given gas turbine goes down as the steam pressure and temperature goes up [42,43]. 8.…”

Engineering design and exergy analyses for combustion gas turbine based power generation system

“…Huang [5] shows that the performance evaluation of a CCPP based only on the first law of thermodynamics is not adequate, but the second law of thermodynamics must be taken into consideration to get a better evaluation. Verkhivker and Kosoy [6] pointed out the principal processes which cause the destruction of exergy in a power generation cycle are the combustion process, the subsequent heating of the working fluid and the heat transfer in the heat exchangers.…”

“…For this reason, the exergy analysis has drawn much attention by scientists and system designers in recent years. Some devoted their studies [1,2] to component exergy analyses [3] and efficiency improvement; others concentrate on systems design and analyses [4][5][6][7][8]. …”

Energy-Saving and Economical Evaluations of a Ceramic Gas Turbine Cogeneration Plant

“…A process is better thermodynamically if less exergy is consumed. Consequently, the ratio of the amount of exergy in the products to that supplied is a more accurate measure of the thermodynamic performance of a system [4]. By definition,…”