International Journal of Theoretical and Applied Mechanics. ISSN 0973-6085 Volume 8, Number 1 (2013) pp. 15-23 © Research India Publications http://www.ripublication.com/ijtam.htm
Study the Effect of Temperature Control on the Performance of the Output of Combined Cycle Gas Turbine J.N. Rai1, Naimul Hasan2, B.B.Arora3, Rajesh Garai4, Rishabh K. Gupta5, Rahul Kapoor6 1,4,5,6
Department of Electrical Engineering, Delhi Technological University, India. E-mail:
[email protected] 2 Department of Electrical Engineering, Jamia Millia Islamia, New Delhi, India. 3 Department of Mechanical Engineering, Delhi Technological University, India.
Abstract A combined cycle plant generates power by combining both gas turbine and steam turbine. For power generation purposes the Brayton Cycle in open or in combined cycle configuration is used for the gas turbine in which the air is directly taken from the atmosphere and mixed with the fuel to produce electrical energy, hence the ambient temperature of the air (atmosphere) is an important factor on which the power output of a gas turbine depends. The temperature control of the gas turbine is also important for preventing the excess rise in the temperature of the gas turbine. In this paper the effect of ambient temperature and temperature control on the performance of the output response of the gas turbine has been studied using the practical data from a combined cycle plant when the turbine was being operated at base load. Graphs have been plotted and analyzed on the basis of data obtained. Keywords: Gas turbine, ambient temperature, exhaust temperature, power.
1. Introduction Power generation is an important factor in the development of any economy. More efficient plants are required for producing more energy by utilizing less amount of fuel and controlled harmful emissions. Gas-fired combined cycle plants are the technologically advanced plants which can achieve efficiency as high as 60% and lower atmospheric emissions[6-8]. The Gas based power plants allow the plant to be compact using lesser amount of area. Because of these characteristics these plants have
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become a subject of research and work is required to understand their characteristics under various operating conditions in order to have a better control over their operation. Figure 1 shows the typical gas turbine configuration. In a combined cycle plant the load is controlled by the operation of the gas turbine and the output of the steam turbine depends on the exhaust of the gas turbine. Combined cycle plants because of their quick startup time provide high operational flexibility for adjusting the load output fast and load predictability. Changes in demand/load cause deviation in the frequency of the generated power. The frequency can be restored to its nominal value by various governing systems. This can be accomplished by Automatic Generation Control[9]. Because of the flexibility in operation of Combined Cycle Power Plants the units are generally AGC controlled. Figure 2 shows the start-up characteristics of a gas turbine [1].
Figure 1: Gas Turbine.
Figure 2: Starting Characteristics of Gas Turbine.
2. Gas Turbine Thermodynamics According to the ideal gas equation the variation of density with temperature is given as P (1) RspecificT
Study the Effect of Temperature Control on the Performance of the Output The efficiency of the combustor is given as [2] Wa C p (To Ti ) B W f Qr
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(2)
Where Wa is the rate of air flow in kg/sec, Wf is the rate of fuel flow in kg/sec, Cp is the specific heat at constant pressure, Ti is the inlet temperature in K, To is the discharge temperature in K and Qr is the heat flow rate. Once the fuel flow Wf, the airflow Wa, and the inlet temperature Ti are given, the temperatures Td, Tf, Tc, and the airflow Wa are determined. With these values, the net energy supplied to the gas turbine is given by
Eg K 0 {(T f Te ) (Td Ti )}Wa
(3)
Where Ko is a constant. This energy is converted to the power after a transport delay. The total horsepower produced by the turbine is given as [9]
HPt
T J (Wa W f )C p Ti [1 o ] 550 Ti
(4)
Where J is a constant. It also includes horsepower to drive the compressor. Gas turbine inlet temperature Tf (K) is given by [3]
T f Td T fo Tdo
Wf W
(5)
Figure 3: Exhaust Temperature control from Rowen Model [4]. Subscript “o” denotes the rated value and Td is the compressor discharge temperature.
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3. Temperature Control Simulation Figure 3 shows the simulation model of the exhaust temperature measurement and control of the gas turbine. It consists of blocks representing various transducer block for indicating electrical signals according to the changes in temperature, thermocouple block for measurement of the temperature, and a reference block for specifying exhaust temperature i.e. if the exhaust temperature is above which the turbine can handle then necessary control actions will take place which would reduce the exhaust temperature of the turbine. The temperature of the exhaust of the gas turbine is restricted to a constant value due to which fuel flow and air are varied which affect the output of the gas turbine.
Figure 4: Flow Diagram for calculation of variation of Gas Turbine parameters.
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4. Case Study In the following case study we have considered a multi-shaft combined cycle plant having two gas turbines and a steam turbine. This system is a part of a large interconnected system. The variation in the output of gas turbine with atmospheric temperature keeping exhaust temperature controlled has been plotted. The ratings of Gas turbines are 104 MW and that of steam turbine is 122 MW. [1]
Figure 5: Power Output versus Ambient Temperature. Figure 5 as per Table [I] and [II] shows the plot of power output versus temperature and a comparison is done with the plot given in [5]. The exhaust temperature of the turbine being adjusted to 5610 C for steam turbine operation. The graph shows the decrease in power output when the ambient temperature of the compressor inlet rises. This is because the gas turbine is a constant volume machine, when there is a rise in temperature the density of the air at the inlet decreases according to equation (1) so less air flows in the combustion chamber and more fuel remains unburnt due to which the power output of the turbine drops as can be seen by the equation (2). The graph deviates from the plot given in [4] because the demand of power (load) keeps changing due to which other factors like fuel flow and air flow are varied. The power output for a given ambient temperature can be increased by cooling the inlet air to the compressor. Figure 6 as per Table III [2] shows the graph between exhaust air flow and the power output near rated value of the output. Since the fuel flow is negligible compared to the air flow the exhaust flow can be assumed to be equal to the airflow. It can be seen from the equation (4) the power output of the gas turbine is proportional to the air flow. Also by varying the air flow in the turbine the temperature of the turbine is also controlled as shown in the next graph.
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Figure 6
Figure 7 Figure 7 as per the data in Table II [2] shows the air flow versus fuel flow characteristics keeping the turbine inlet temperature constant near rated power output. To increase the output of the gas turbine the fuel input has to be increased as can be seen by the equation (4). It can be seen that with the rise in fuel flow, air flow also increases so as to maintain the inlet temperature of the turbine at a specified value according to the equation (5) which shows that the air flow should be increased in direct proportion to the fuel flow.
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Appendix GT Model – Frame 6, MS9000 series units, 50Hz application (rotational speed 3000 rpm). Table I: Gas Turbine Ambient Temperature Data Gas Turbine #1. Temperature ( o C) 9 11 12 13 16 19 26 28 30 32 34 35
Load (MW) 110.6 109.9 108.6 107.9 105.7 104.1 103.6 102.7 99.9 99.4 96.9 96.1
Gas Turbine #2 Temperature ( o C) 10 11 12 13 15 20 27 28 30 33 34 35
Load (MW) 112 110.5 109.5 108.4 106.7 107.5 104.4 103 101.5 100.5 98.6 98.2
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J.N. Rai et.al. Table II: Air Flow in Gas Turbine Gas Turbine #1. Air Flow Exhaust (kg/s) 377.1 377.4 377.5 381.1 381.2 381.7 381.9 383 384.7
Load (MW) 101.3 100 100.8 99.7 102.7 102.9 103 103.2 103.4
Exhaust ( o C) 557 556 556 556 554 555 554 554 553
Table III: Air flow and fuel flow data in per unit. Air flow 0.943 0.945 0.955 0.957 0.962
Fuel flow 0.486 0.495 0.506 0.511 0.515
Power 0.981 1.000 1.019 1.038 1.038
Turbine Inlet Temperature ( o C) 1133 1132 1132 1133 1132
Conclusion It can be concluded from the above study that 1. With the rise in the ambient temperature of the atmosphere the output of the gas turbine falls and the output of the gas turbine can be increased by reducing the inlet temperature of the compressor by cooling of the air that is being fed to the compressor. 2. When the fuel input to the turbine is increased, for increasing the output, the air flow has also to be adjusted accordingly to prevent the turbine temperature to go above a reference temperature. There is a linear rise in air flow with the fuel flow when the turbine is being operated near its rated value. 3. The exhaust temperature is also controlled by increasing the air flow.
Acknowledgment The authors are grateful to the staff of Pragati Power Corporation Limited of Indraprastha Power Generation Corporation Limited for their in valuable advices and providing necessary data. Special thanks to Prof. Ibraheem of Jamia Millia Islamia for providing necessary guidance for the purpose.
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References [1] [2]
Pragati Power Corporation Limited, IPGCL, India. Tony Giampaolo “Gas Turbine Handbook: Principles and Practices”The Fiarmont Press 2006. [3] ‘Performance of Gas Turbine-Based Plants During Frequency Drops’, Naoto Kakimoto, , and Kazuhiro Baba, IEEE Transactions on Power Systems, Vol 18, No. 3 August 2003. [4] W.I. Rowen, “Simplified Mathematical Representation of Heavy-Duty Gas Turbines”, ASME Journal of Engineering for Power, Vol. 105, pp.865-869, Oct. 1983. [5] Gas Turbine Emissions and Control – Roointon Pavri, Gerald D. Moore, G.E. Energy Services Atlanta, G.A.2008 [6] Rolf Kehlhofer “Combined-Cycle Gas & Steam Turbine Power Plants” PennWell Publishing Company Copyright 2011 [7] Black & Veatch “Power Plant Engineering” Chapman &Hall [8] Operational Flexibility Enhancements of Combined Cycle Power Plants – Dr. Nobert Henkel, Erich Schmid and Edwin Gobrecht, Siemens AG, Energy Sector Germany Power – GEN Asia 2008 – Kuala Lumpur, Malaysia October 21-23, 2008 [9] C.N. Ning, and C.N. Lu., ‘Effect of Temperature Control on Combined Cycle Unit Output Response’ - 2006 [10] John H Horlock, "Advanced gas turbine cycles," Pergamon Press, 2003.
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