Thermodynamic performance analysis of a combined cycle system with wet compression

Transcription

1 1 JEST-M, Vol.1, Issue 2, 2012 Thermodynamic performance analysis of a combined cycle system with wet compression Shweta Agrawal 1, S K Maharana 2, K S Badarinarayan 3 1 PG Student, Thermal Power Engineering, Dept. of Mechanical Engineering 2 Professor and Head, Dept. of Aeronautical Engineering 3 Professor, Dept. of Mechanical Engineering and Principal MVJ College of Engineering, Bangalore, Karnataka, India Abstract-- Thermodynamic model of wet compression in a combined cycle system is established in this paper. The objective of this paper is to improve the performance of combined cycle system by injecting suitable quantity of water into the compressor. A Microsoft excel based tool is developed to do the thermodynamic sensitivity analysis for the system. The results show that the pressure ratio, water injection quantity, ambient temperature etc strongly influence the overall performance. Keywords: combined cycle system, wet compression, gas turbine I. INTRODUCTION Combined cycle power plants have both gas and steam turbines based on more than one thermodynamic cycle Brayton (gas) and Rankine (steam). The gas turbine cycle in a combined cycle power plant, first produces power and the waste heat from the exhaust gas is used to make steam to generate additional power through a steam turbine, which enhances the efficiency of electricity generation. Compare to other power plants the advantages of combined cycle plant are high flexibility, quick part-load starting, suitability for both base-load and cyclic operation, and a high efficiency over a wide range of loads. In this paper the objective is to improve the net work done and efficiency of the combined cycle system by wet compression. Wet compression is a process in which water droplets are injected into the compressor inlet air and allowed to be carried into the compressor. As the water droplets evaporate in the front stages of the compressor, it reduces the air temperature and therefore reduces the amount of compressor work [1]. A. Overview II. SYSTEM ANALYSIS The schematic diagram of combined cycle system with wet compression is shown in Figure 1. limited by the ability of air to absorb water. After that air water mixture is compressed into the compressor. Now the compressed air is sent to the combustor where there is addition of fuel takes place. Due to combustion, hot gases generated into the combustor runs the gas turbine. In HRSG the incoming feed water is converted into steam by absorbing heat from the exhaust gas that steam runs the steam turbine. B. Assumptions The following assumptions are made to simplify the calculations however they can be refined to reach more real solutions. The main assumptions are Natural gas is used as a fuel (CV=43000 KJ/Kg K) Working fluid is assumed to be air as an ideal gas Air water mixture flow rate at inlet is taken 1Kg/s Water amount is between 0-27% of air mass flow Water and air inlet temperature and pressure is atmospheric (298 K, 1atm) Compression and expansion processes in compressor and turbine are assumed to be adiabatic considering compressor and turbine efficiency varying with pressure ratio [4]. The efficiency of steam turbine assumed as 82% Combustion chamber pressure loss is negligible Steam injection temperature 700 K and pressure 40 bar Pressure ratio 10 bar where ever applicable Fuel injection rate 3% of air mass flow rate Feed water temperature in HRSG is taken as 300 K at condenser pressure of 0.04 bar Pump work is neglected during performance calculations First, in the wet compression system air is cooled by introducing water into the air in the form of fine water droplets (spray form). The reduction in the air temperature is C. Thermodynamic calculations The performance analysis of combined cycle system is based on Steady Flow Energy Equation (SFEE).

2 2 JEST-M, Vol.1, Issue 2, 2012 Figure 1 Combine cycle system thermodynamic behavior among its subsystem and establishes the mathematical relationship to arrive on various performance or cycle parameters like max cycle temperature, net work, efficiency etc. The different combinations of water injection, pressure ratio, compressor inlet temperature, combustor exit temperature and several such parameters are considered as variable. Some of the important observations are highlighted in this paper and detailed as below. Figure 2 shows effect of water injection on compressor work at different pressure ratio. By increasing the pressure ratio compressor work is also increasing. Also by increasing the water injection into the compressor at constant pressure ratio, the compressor work is reducing. In this paper our aim is to reduce compressor work so that net work output will increase. With the help of graph it can be seen that compressor work is maximum at zero water injection, if the water injection exceeds 0.24 Kg/s we will get negative compressor work. From the references [1] [6] [10] and [18] the recommended water injection quantity is 1-3%. Process 1-2 (Adiabatic compression) Compressor work W C = m*c pc * (T 2 -T 1 )/ƞ c Process 2-3 (Constant pressure heat addition) Energy equation (m+f)*c pp *T 3 = f*cv + mc pc * T 2 Process 3-4and a-b (Adiabatic expansion) Work done by the Turbine, W GT = (m+f)*c pp * (T 3 -T 4 )*ƞ GT W ST = y*(ha-hb)* ƞ ST Process 4-5- (Constant pressure heat rejection in HRSG) Energy equation- (m+f)*c pp *(T 4 -T 5 ) =y*(h a -h d ) Overall Performance Net work, W net = W GT - W c + W ST Thermal Efficiency: ƞ th = W net / Q 1, Heat addition, Q 1 = f* CV Specific fuel consumption (sfc) = 3600*f/W net Heat rate = 3600/ƞ th III. RESULTS & DISCUSSIONS Performance analysis of a combined cycle system links the Figure 3 shows effect of water injection on combustor exit temperature at different fuel injection flow rate. Turbine inlet temperature increases when fuel injection increases but it decreases with the increase in water injection. When fuel addition is more, more heat is generated into the combustion chamber; this heat raises the temperature of gas which comes out of the combustor. Figure 4 shows effect of pressure ratio on gas turbine work at different fuel injection. With the increase in pressure ratio turbine work is also increasing. It has been also seen that with the increase in fuel injection into the combustor at constant pressure ratio the work done by the turbine has improved. From the figure, the work done by the turbine is highest at maximum pressure ratio and maximum fuel injection Figure 5 shows effect of steam turbine efficiency on steam turbine work at different steam injection. With the increase in steam turbine efficiency turbine work is also increasing. It has been also seen that with the increase in steam injection into the turbine, work done by the turbine has improved. From the figure, the work done by the turbine is highest at maximum turbine efficiency and maximum steam injection. Figure 6 shows effect of steam injection on net work output at different water injection. Net work done is continuously increasing with the increase in water and steam injection, the relationship is linear. Work done is highest at highest steam injection and highest water injection. Figure 7 shows effect of pressure ratio on thermal efficiency with different water injection and constant steam injection of 20% of air mass flow rate. Thermal efficiency increases with

3 Turbine inlet temperature, T 3 (K) (KW) 3 JEST-M, Vol.1, Issue 2, 2012 the increase in pressure ratio and by increasing the water injection into the compressor at constant steam injection. Thermal efficiency is highest at maximum pressure ratio and maximum water injection. Figure 8 shows the effect of compressor inlet temperature on specific fuel consumption at different fuel injection. In case of combined cycle it can be seen that specific fuel consumption increase is very less with the increase in inlet temperature it means that the effect of inlet temperature increase is very less but sfc improves with the increase in fuel consumption. Figure 9 shows the effect of compressor inlet temperature on heat rate at different fuel injection. In case of combined cycle it can be seen from the figure that when the value of fuel injection is minimum i.e Kg/s the heat rate is minimum and suddenly there is much improvement in heat rate value when there is slight increase in fuel injection. Also when fuel injection value crosses Kg/s there is not much improvement in heat rate value. IV. CONCLUSION The system formulation and computer implementation described in the preceding section repertoire the performance calculations of the combined cycle system. This section summarizes the sensitivity of performance parameters of the major modules of combined cycle system. A design methodology has been developed for parametric study and performance evaluation of a combined cycle system which shows that pressure ratio, inlet air temperature, water injection etc played a very vital role on overall performance of gas turbine system. MS excel based tool is developed to carry out the sensitivity studies of performance parameters. V. APPENDIX The appendix provides the performance charts of cogeneration system with wet compression and steam injection. Compressor work, W C Pressure ratio, r p x = 0 x = 0.03 x = 0.06 x = 0.09 Figure 2 Effect of water injection on compressor work at different pressure ratio Fuel injection, f (Kg/s) x = 0 x = 0.03 x = 0.06 x = 0.09 x = 0.12 x = 0.15 x = 0.18 x = 0.21 x = 0.24

4 Net Work Wnet (KW) (KW) (KW) 4 JEST-M, Vol.1, Issue 2, 2012 Figure 3 Effect of fuel injection on turbine inlet temperature at different water injection Gas turbine work, W GT Pressure ratio, r p f = 0.01 f = f = 0.02 f = f = 0.03 f = f = 0.04 f = f = 0.05 Figure 4 Effect of pressure ratio on turbine work at different fuel injection Steam Turbine Work, W ST Steam Turbine efficiency, ƞ ST y = 0 y = 0.05 y = 0.1 y = 0.15 y = 0.2 y = 0.25 y = 0.3 y = 0.35 y = 0.4 Figure 5 Effect of steam turbine efficiency on steam turbine work at different steam injection Steam injection, y (Kg/s) x = 0 x = 0.03 x = 0.06 x = 0.09 x = 0.12 x = 0.15 x = 0.18 x = 0.21 x = 0.24 x = 0.27 Figure 6 Effect of steam injection on net work output at different water injection

5 Heat rate (KJ/kW-h) Specific fuel consumption, sfc (Kg/KWh) 5 JEST-M, Vol.1, Issue 2, 2012 Overall efficiency, ƞ th Pressure ratio, r p x = 0 x = 0.03 x = 0.06 x = 0.09 x = 0.12 x = 0.15 Figure 7 Effect of pressure ratio on thermal efficiency with different water injection 0.22 f = f = f = 0.02 f = f = f = f = f = f = f = Compressure inlet temperature, T 1 (K) Figure 8 Effect of compressor inlet temperature on sfc at different fuel injection Compressure inlet temperature, T 1 (K) Figure 9 Effect of compressor inlet temperature on heat rate at different fuel injection f = 0.01 f = f = 0.02 f = f = 0.03 f = f = 0.04

6 6 JEST-M, Vol.1, Issue 2, 2012 Acknowledgment Authors would like to thank MVJ College of Engineering for providing the opportunity to carry out the paper work and assisting to avail required support. NOMENCLATURE ƞ c = Thermal efficiency of compressor ƞ GT = Thermal efficiency of turbine ƞ ST = Thermal efficiency of turbine ƞ th = Overall thermal efficiency of cycle C pc = Specific heat of mixture (KJ/Kg K) C ps = Specific heat of steam (KJ/Kg K) C pp = Specific heat of products of combustion (KJ/Kg K) C Vf = Calorific value of fuel (KJ/Kg) f = Mass flow rate of fuel supplied () h s = Enthalpy of steam (KJ/Kg) h w = Enthalpy of water (KJ/Kg) m = Mass flow rate of mixture (air + water) (Kg/s) Q 1 = Heat addition (KW) T 1 =Compressor inlet temperature (K) T 2 = Compressor exit temperature (K) T 3 = Combustor exit temperature (K) T 4 = Turbine exit temperature (K) T 6 = Feed water inlet temperature in (K) T 5 = HRSG exit temperature (K) T 7 = Steam injection temperature (K) W C = Compressor work (KW) W GT = Gas turbine work (KW) W net = Net work done (KW) W ST = Gas turbine work (KW) x = Mass flow rate of water injection (Kg/s) y = Mass flow rate of steam injection () REFERENCES [1] Sanjeev Jolly, Wet Compression A powerful means of enhancing combustion turbine capacity, Power-Gen International, Orlando, Florida, pp 10-12, December [2] Yousef S.H. Najjar, Gas turbine cogeneration systems: a review of some novel cycles, Applied Thermal Engineering, no. 20, pp , [3] A. Bouam, S. Aïssani and R. Kadi, Donald W. Shepherd, Donald Fraser, Impact of heat rate, emissions and reliability from the application of wet compression combustion turbines, Seimens Florida, [4] Kyoung Hoon Kim a, Horacio Perez-Blanco, Potential of regenerative gas turbine systems with high fogging compression, Applied Energy, no. 84, pp.16-28, [5] Maria Jonssona, Jinyue Yana, Humidified gas turbines- a review of proposed and implemented cycles, Energy, no. 30, pp , [6] Marcus Thern, Humidification processes in gas turbine cycles, Lund University Sweden, December [7] F.J. Wang, J.S. Chiou, Performance improvement for a simple gas turbine GENSET- a retrofitting example, Applied Thermal Engineering, no. 22, pp , [8] Sanjeev Jolly, Performance enhancement of GT24 with wet compression, presented at the Power-Gen International, pp.9-11, Dec [9] A. Ragland, W Stenzel, Combined Cycle Heat Recovery Optimization, International joint power generation, July [10] Mehmet Kanoglu a, Ibrahim Dincer, Performance assessment of cogeneration plants, Energy Conversion and Management, no.50, pp , [11] Anthony Giampaloa, Gas turbine handbook- Principle and practices, 3rd edition. [12] Ashok Kumar, S SKachhwaha and R S Mishra, Thermodynamic analysis of regenerative gas turbine cogeneration plant, Journal of scientific & Industrial research,vol 69, pp , March [13] Devki Energy Consultancy Pvt. Ltd. India, Best practice manual on cogeneration, [14] OzerArnas, Daisie D boettner, On the teaching of performance evaluation and assessment of a combined cycle cogeneration system, Journal of energy resources technology, [15] A Khalid and K Choudhary, Combined first and second law analysis of gas turbine cogeneration system with inlet air cooling and evaporative aftercooling of the compressor discharge, ASME journal of engineering of gas turbines and power, October [16] QunZheng, Yufeng Sun, Shuying Li, Yunhui Wang, Thermodynamic analysis of wet compression process in the compressor of gas turbine, ASME Journal of turbo machinery, July [17] Mustapha Chaker, Cyrus B. Meher-Homji, Gas turbine power augumentaion: Parametric study relating to fog droplet size and its influence on evaporative efficiency, Journal of engineering for gas turbine and power, vol 133, September [18] I. Roumeliotis, K. Mathioudakis, Evaluation of interstage water injection effect on compressor and engine performance, Journal of engineering for gas turbine and power, vol. 128, October [19] A. J. White, A. J. Meacock, An evaluation of the effects of water injection on compressor performance, ASME Journal of engineering for gas turbine and power,vol. 128/749, October [20] Mohsen Ghazikhani, NimaManshoori, DavoodTafazoli, Influence of steam injection on thermal efficiency and operating temperature of GE- F5 gas turbine applying Vodoley system, ASME international engineering congress and exposition, November 2005.