International Journal of Mechanical Engineering and Computer Applications, Vol 2, Issue 2, March- April 2014, ISSN 2320-6349
Exergy Analysis of Combined Cycle Power Plant using Steam Jet Inlet Cooling Kumar Sourav
Dhaneshwar Mahto
Department of Mechanical Engineering Birla Institute of Technology Ranchi,Jharkhand
[email protected]
Department of Mechanical Engineering Birla Institute of Technology Ranchi,Jharkhand
[email protected]
Abstract— Combined cycle power plant integrates two cycles – Brayton cycle (Gas Turbine) and Rankine cycle (Steam Turbine) with the objective of increasing overall efficiency. The combined cycle power plant employed in this project uses Steam jet cooling in order to cool the inlet-air entering the compressor. The cooling is done in order to increase the efficiency of the overall combined cycle power plant as well as to reduce the losses due to the high temperature occurring inside the compressor. Also, the power consumed by the compressor increases in proportion to the intake air temperature without there being a corresponding increase in the output from the turbine. The theory involved in the jet cooling is that an evaporation of 1 kg of water leads to cooling of about 5ºC. The exergetic analysis has been done using MATLAB®. Index Terms—HRSG, Steam Turbine, Refrigeration, Turbine Inlet Temperature
Steam
Jet
I. INTRODUCTION Electric power generation involves transformation of some kind of energy like chemical, mechanical or thermal into electrical energy. A combined cycle as the name states is a combination of two cycles operating at different temperatures, each of which could operate independently. The heat rejected by the higher temperature cycle is used by a low temperature cycle like Rankine Cycle to produce additional power for an improved overall efficiency. For the combination, the separate cycles must operate on separate fluids. Gas turbine works without any change in the phase of the working fluid whereas in Steam turbine the phase of fluid changes. The gas/steam combined cycle efficiency ranges from 47% to 60%. The heat rejected by gas turbine varies from 450ºC-650ºC depending upon pressure ratio and turbine inlet temperature. Turbine inlet temperature and pressure ratio are the two most important variables of a combined cycle. Heat leaving the gas turbine goes as a waste if it is rejected to the atmosphere and therefore can be utilized to drive a different system. This waste heat energy can be utilized to produce steam in a heat recovery steam generator (HRSG) and the superheated steam produced by may be expanded in steam turbine to develop additional power based on Rankine cycle. This combination is termed as gas/steam combined cycle power
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plant. Combined Cycle Power Plant stands out among other technologies because of existence of two thermodynamic power producing cycles in one system and a high calorific fuel such as natural gas for power generation. II. EXERGY ANALYSIS The exergy of a system is the maximum work obtainable as the system comes to equilibrium with the surrounding Bejan et al. The higher the value of exergy, more is the work obtainable from the system. The first law of thermodynamics makes only an energy balance of a system or a control volume. It does not make any distinction of different forms of energy, particularly between work and heat, or heat (internal energy) available at different temperatures. It is the second law which asserts that from engineering viewpoint, all forms of energy are not of the same quality. Energies of two systems may be quantitatively equal, but qualitatively they may be different. When steam is adiabatically throttled, its energy does not change, but its quality degrades. Exergy is a measure of energy quality and exergetic (or second law) efficiency is a measure of the perfectness of a thermal system. While energy of a system in any process remains constant, a part of its energy is always destroyed.
III. MODEL CONSTRUCTION The system comprises of a the system comprises of a Heat recovery steam generator (HRSG), steam turbine, boiler feed pump, a condensate pump and a deaerator. The HRSG used is a single pressure system without supplementary firing. The HRSG consists of an evaporator, an economizer and a superheater. By superimposing a high temperature power plant as a topping unit to the steam plant, higher energy conversion efficiency from fuel to electricity can be achieved, since the combined plant operates through a higher temperature range. In the basic form of combined cycle power plant, a gas turbine exhausting into a heat recovery steam generator that supplies steam to a steam turbine cycle is the most efficient system of generating electricity today.
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International Journal of Mechanical Engineering and Computer Applications, Vol 2, Issue 2, March- April 2014, ISSN 2320-6349 The HRSG is basically a heat exchanger, or rather a series of heat exchangers. The HRSG can rely on natural circulation or utilize forced circulation using pumps. As the hot exhaust gases flow past the heat exchanger tubes in which hot water circulates, heat is absorbed causing the creation of steam in the tubes. The tubes are arranged in sections, or modules, each serving a different function in the production of dry superheated steam. These modules are referred to as economizers, evaporators, superheaters/reheaters and preheaters. V. PROPOSED MODEL Fig.1. Basic Combined Cycle Plant
IV. WORKING Combined cycle power plant as in name suggests, it combines existing gas and steam technologies into one unit, yielding significant improvements in thermal efficiency over conventional steam plant. The air which is purified then compressed and mixed with natural gas and ignited, which causes it to expand. The pressure created from the expansion spins the turbine blades, which are attached to a shaft and a generator, creating electricity. In second step the heat of the gas turbine’s exhaust is used to generate steam by passing it through a heat recovery steam generator (HRSG) with a live steam temperature between 420 and 580 °C. In Heat Recovery Steam Generator highly purified water flows in tubes and the hot gases passes a around that and thus producing steam .The steam then rotates the steam turbine and coupled generator to produce Electricity. The hot gases leave the HRSG at around 140 degrees centigrade and are discharged into the atmosphere.
The model proposed here is consists of a steam jet refrigration system where steam bleeding takes place at a certain pressure. Using the ejector compression a low pressure system is created inside and entrainment takes place. Water gets cooled due to evaporation and is supplied to the heat exchanger where ambient air at higher temperature gets cooled and is finally transferred to the inlet of compressor. Since the mass flow rate of air increases as a result of increase in the density of air , more steam gets generated in the HRSG. Finally, the overall plant efficiency increases.
Fig.3. Temperature(T) vs. Entropy(S) diagram
VI. PARAMETRIC ANALYSIS TABLE I.
Table Head
Effect of TIT on other system variables
Tg4(K)
ṁs(kg /s)
ṁs1(k g/s)
ṁs2(k g/s)
Steam Turbine Work
% overall efficienc y
1300
519.01
0.129
0.10
0.005
102.07
47.4
1400
510.72
0.158
0.10
0.007
130.61
50.09
1500
502.44
0.188
0.10
0.008
159.41
52.18
1600
494.15
0.218
0.10
0.009
188.31
53.86
TIT
Fig.2. Combined Cycle Power Plant employing Steam Jet Refrigeration
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International Journal of Mechanical Engineering and Computer Applications, Vol 2, Issue 2, March- April 2014, ISSN 2320-6349 1700
Table Head
485.87
0.248
0.10
0.01
217.50
55.20
Fig.5. TIT vs Overall Efficiency
TABLE II. Variation of TIT and exergy analysis of components
TIT
Ωcomp (KJ/Kg )
Ωcombu .(KJ/ Kg)
ΩGT (KJ/K g)
ΩHRSG( KJ/Kg)
ΩST(KJ /Kg)
exergetic efficienc y(%)
1200
25.23
263.9
12.69
71.90
20.51
41.06
1300
25.23
297.8
12.72
81.72
26.58
43.83
1400
25.23
329.7
12.76
93.54
32.67
46.23
1500
25.23
359.8
12.79
107.39
38.82
48.14
1600
25.23
388.3
12.83
122.73
44.99
49.69
1700
25.23
415.5
12.87
139.37
51.18
50.99
VII. GRAPHS
Fig.4.TIT vs mass of steam generated in HRSG
Fig.6. TIT vs Exergetic Efficiency
VIII. CONCLUSION The overall plant efficiency approaches 55.2% when the turbine inlet temperature is 1700 K. The efficiency can be increased much more if the temperature difference between the ambient air-conditions and the actual inlet air temperature in compressor increases. It should be also noted that the evaporation of 1kg of water leads to a drop of 5 degree Celsius, therefore in order to have to difference of 20 degree Celsius, evaporation of 4kg of water should take place theoretically. Exergy analysis of the system shows that the maximum exergy loss takes places in the combustion chamber, therefore controlling various parameters like air-fuel ratio etc. can lead to much more efficiency. Although, the COP of the steam jet refrigeration is low, the system is still beneficial since it does not use ammonia which is environmentally detrimental and also the steam jet refrigeration is a very efficient system. REFERENCES [1] Irimescu Adrian, Lelea Dorin, Thermodynamic analysis of gas turbine powered cogeneration systems, Journal of Scientific and Industrial Research, Vol 69, July 2010. [2] Tiwari A.K., Islam Mohd, Khan M.N., Thermodynamic analysis of combined cycle power plant,International Journal of Engineering Science and Technology, Vol.2(4),2010. [3] Sharma Meeta, Singh Onkar, Thermodynamic evaluation of whrb for its optimum performance in Combined Cycle Power Plants, IOSR Journal of Engineering, Vol.2 Issue 1, Jan.2012. [4] US Department of Energy,Energy tips-compressed Air ,Compressed Air Tip-Sheet #14. [5] A.M. Kler, A.S. Maksimov, E.L. Stepanova, High-speed mathematical models of cogeneration steam turbines: optimization of operation at heat and power plants, Journal Thermo-Physics and Aeromechanics 13(2006)141–148.
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International Journal of Mechanical Engineering and Computer Applications, Vol 2, Issue 2, March- April 2014, ISSN 2320-6349 [6] IEEE Committee Report, Dynamic models for steam and
hydro turbines in power system studies, IEEE Power Engineering Society, Winter Meeting, NY,1973.
[7] Khurmi R.S., Gupta J.K., A textbook of Refrigeration and
Air-Conditioning, S.Chand & Company Ltd., ISBN: 81219-2781-1 [8] Nag P.K., Power Plant Engineering, Tata McGraw Hill Education Private Limited, Third Edition, ISBN-13: 978 [9] Yadav R., Steam & gas turbines and power plant engineering, Central Publishing House, Allahabad, 7th Edition/2003
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