Parametric Analysis of Triple Pressure HRSG in Combined Cycle Power Plant (September 2006)

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International Energy Journal: Vol. 7, No., September 6 www.serd.ait.ac.th/reric arametric Analysis o riple ressure HRSG in ombined ycle ower lant (September 6) N. Ravi Kumar, Sk. Jaheeruddin, Dr. K. Rama Krishna, and Dr.

1 International Energy Journal: Vol. 7, No., September 6 arametric Analysis o riple ressure HRSG in ombined ycle ower lant (September 6) N. Ravi Kumar, Sk. Jaheeruddin, Dr. K. Rama Krishna, and Dr. A. V. Sita Rama Raju Abstract - ombined cycle power plants play an important role in the present energy sector. ombined ycle plants couple a Brayton cycle with a Rankine cycle. For the combined power plants, the optimization o the heat recovery steam generator (HRSG) is o particular interest in order to improve the eiciency o the heat recovery rom gas turbine exhaust to maximize the power production in the steam cycle. In the present study eect o dierent operating variables such as exhaust gas low rate, exhaust gas temperature, pinch point, approach point, steam temperature, steam pressure on the perormance o HRSG has been investigated. riple pressure steam cycle is considered or the bottoming cycle to reduce irreversibilities during heat transer rom gas to water/steam. he cycle is analysed by using energy and exergy analysis. It is observed that in triple pressure cycle selection o I and L pressures, and L pinch are identiied as dominant parameters having impact on heat recovery steam generator perormance. Key words - ombined cycle, exergy analysis, heat recovery steam generator, optimization. INRODUION ombined cycle power plants pose a very good alternative to conventional thermal power plants due to better thermodynamic perormance and reduced environment pollution. In a typical combined cycle plant gas turbine is the topping cycle and the steam turbine is the bottoming cycle. he major components that make up a combined cycle are the gas turbine, the HRSG, the steam turbine. here are many concepts o the combined cycle, these cycles range rom the single pressure cycle, in which the steam or the turbine is generated at only one pressure, to the triple pressure cycles where steam generated or the steam turbine is at three dierent levels. he schematic o Simple combined cycle plant is shown in Fig..he practical design o the HRSG is usually based on the concepts o pinch point and approach point that govern the gas and steam temperature proiles given by Linho and Hindmarsh []. he pinch point represents the dierence between the gas temperature leaving the evaporator and the saturation temperature, while the approach-point temperature is the dierence between the water temperature leaving the economizer and the saturation temperature. inch and approach points take into account both thermodynamic and economical points o view. Subrahmanyam et al. [] discussed various actors aecting the HRSG design or achieving highest combined cycle eiciency with cheaper, economical and competitive designs and with highest requirements to meet the shorter N. Ravi Kumar is working as Assistant roessor in Gudlavalleru Engineering ollege, Gudlavalleru, A, India-556 (orresponding author, hone: , Fax: , ravi_naradasu@yahoo.com). Sk. Jaheeruddin is working as Assistant roessor in Gudlavalleru Engineering ollege, Gudlavalleru, A, India-556. Dr. K. Ramakrishna is working as proessor in Narasaraopet Engineering ollege ( nec_mech@redimail.com). Dr. A. V. Sita rama raju is working as roessor in JN University, Kakinada, A, India ( avsr_raju@redimail.com). deliveries..k. Nag and D.Raha [] made thermodynamic analysis o a coal based combined cycle power plant rom the view points o both irst law and second law. Horlock [4] presented detailed thermodynamic analyses o various schemes o combined power plants. A comprehensive computer simulation o an HRSG was developed by A.Ongiro et al. [5] to predict the perormance o heat recovery steam generator (HRSG).Nag and Mazumder, [6] reported the thermodynamic optimization o a waste heat recovery boiler with an economizer and evaporator and producing saturated Steam. hey have reported the eect o dierent operating parameters on entropy generation or the waste heat recovery boiler. Fig.. Schematic o combined cycle power plant. Dallenback [7] proposed an alternative regenerator coniguration to improve the eiciency o gas turbine cycle. Ravi kumar et al. [8] has done exergy analysis or the alternative coniguration to know exergy losses in dierent components. It is observed that the irreversibility in exhaust gases is low which indicates eective utilization o heat energy, but the speciic work output o the turbine is less. Gas turbine is seen to oer high speciic work output i the turbine inlet temperature (I) could be increased. Increase in I has strict metallurgical limitations in terms o maximum temperature that the turbine stage could withstand. Ravi kumar and Sita rama raju [9] analyzed the eect o inlet

2 International Energy Journal: Vol. 7, No., September 6 cooling on HRSG perormance. It is ound that the inlet cooling reduces the work input o the compressor and increases the mass low rate o air. he eiciency o steam cycle can be improved by increasing the temperature o steam entered into the steam turbine. he maximum temperature o steam that can be used is ixed rom metallurgical considerations.. K. Nag and S. De [] applied thermodynamic analysis to the optimal design and operation o a heat recovery steam generator (HRSG) generating saturated steam or a combined gas and steam power cycle. he aim o the present paper is to study the eect o dierent operating parameters on triple pressure HRSG coniguration in combined cycle power plant by using energy and exergy analysis.. ANALYSIS In a combined cycle power plant the HRSG represents the interace element between the gas turbine and the steam cycle. Here, the gas turbine exhaust gas is cooled down and the recuperated heat is used to generate steam. In order to provide better heat recovery in the HRSG more than one pressure level is used. HRSGs or gas turbine exhaust are usually designed in unired conditions and the perormance evaluated at other unired or ired conditions. he reason or this is that two o the important variables which aect the gas and steam temperature proiles, namely the pinch and approach points, cannot be arbitrarily selected in the ired mode. he pinch point is the dierence between the gas temperature leaving the evaporator and the temperature o saturated steam. Approach point is the dierence between the temperature o saturated steam and the temperature o water entering the evaporator. he schematic diagram o the combined cycle power plant with triple pressure HRSG, or which the analysis has been conducted, is shown in Fig..he air is taken into the compressor at atmospheric pressure and compressed to the desired pressure. he compressed air is sent into the combustion chamber and raised its temperature by burning the uel. he steam turbine utilizes the energy in the exhaust gas o the gas turbine as its input energy. he ollowing assumptions are taken or the present analysis () he system is in steady state () here is no pressure drop in water and steam side () here is no external heat and mass transer (4) he speciic heats o exhaust gas and water are constant (5) he pressure drop on the gas side does not have a signiicant eect on its temperature, Maximum H steam superheat temperature is 5, Minimum pinch point temperature dierence 5, Minimum stack temperature, Maximum working pressure bar, pa.5 kj/kgk, pg.47kj/kgk, R g.87, γ a.4, γ g., c 89%, t 87%, hl., pl., ψ.4. s ( r p ) ( γ a ) ( s ) + pa ( ) pl * γ 4 γ g 4 s ( ) net t t a pg ( 4 s For the gas turbine process net m * V ( γ g ) he combined eiciency o the cycle is + st m * V ΔH m * V ΔG ϕ * ΔH * Δs ΔG ΔH sec + ΔG st ) () () (4) (5) (6) (7) (8) (9) () () () () (4) p r p * p () Fig.. Schematic o triple pressure HRSG.

3 International Energy Journal: Vol. 7, No., September 6 Fig.. emperature heat diagram or triple pressure HRSG. low pressure steam temperature, the dierence in temperature between the high pressure steam ater expansion and the low pressure steam at the mixing point in the turbine must be taken into account. I the dierence is too high, it causes unnecessary thermal stresses with in the turbine. But a high low pressure steam temperature presents the advantage o a kind o a low reheating, reducing the risk o erosion due to wetness in the turbine. Figure 5 represents the change in mass low rate o steam with gas turbine exhaust (HRSG inlet) temperature. ith increase in temperature the H mass o steam increases and L mass o steam is decreased. 7 A computer program is developed to calculate the mass o steam generated in dierent sections, dryness raction o steam, stack temperature, work output, irreversibilities in turbine and HRSG are calculated. he results obtained rom the analysis are represented in orm o graphs and discussed in the next section.. RESULS AND DISUSSION Mass o steam (kg/s) m hps m lps Figure 4 discussed the eect o maximum steam temperature on mass low rate o steam. ith increase in steam temperature the H mass is decreased because more heat is required and the I, L mass o steam is increased. Mass low (ons/hr) HMASS I MASS LM ASS M aximum steam temperature ( ) Fig. 4. Eect o steam temperature on mass low o steam. Figure 5 discussed the eect o steam temperature on eiciency o the bottoming cycle and stack temperature. ith increase in steam temperature the stack temperature and eiciency also increased. For steam turbine increasing the live steam temperature means less erosion in the inal stages but too high a live steam temperature can also cause a disproportionate increase in plant costs since a great amount o expensive material is required or the piping, the superheater and the steam turbine. In most cases the exhaust gas temperature sets the limit or the live steam temperature level because a suicient dierence in temperature is necessary between the exhaust gas and the live steam temperature in order to limit the size o the superheater. In triple cycle while selecting the intermediate pressure and HRSG inlet temperature (K) Fig. 5. Eect o Gas turbine exhaust on mass o steam. Figs. 6 and 7 shows the eect o steam pressure on optimum I and L pressures and mass o steam. ith increase in steam pressure the optimum I and L pressures are increased. In a combined cycle plant, a high live steam pressure brings an increased eiciency o the water/steam cycle due to the greater enthalpy gradient in the turbine. he rate o waste heat energy utilization in the exhaust gases however drops o sharply. From economical point o view it is advisable raising the steam pressure above the thermodynamic equilibrium, because it causes a reduction in exhaust steam low which requires a smaller condensor and less cooling water requirement. ressure (Ma) I RESSURE L RESSURE Max. Steam pressure (Ma) Fig. 6. Eect o maximum steam pressure on optimum I and L steam pressures.

4 4 International Energy Journal: Vol. 7, No., September 6 Mass low rates(ons/hr) 9 8 HMASS 7 IMASS 6 L MASS Max. Steam pressure (Ma) Fig. 7. Eect o maximum steam pressure on mass o steam. able. Eect o inch oints on Stack emperature, Eiciency and Stack Irreversibility H pinch I pinch L pinch Stack Eiciency I stack Figure 8 discussed the eect o steam temperature on dryness raction o exhaust steam. ith increase in steam temperature the dryness raction o steam also increases, which is helpul in avoiding the erosion in inal stages o turbine. Figure 9 shows the change in irreversibilities o steam turbine and exhaust o HRSG. he irreversibility o steam turbine decreases with increase in temperature, but the stack irreversibility increases with increase in steam temperature. o reduce the stack temperature the L pressure must be low. Dryness raction Steam temperature( ) Irreversibility(k) I st I exh Steam emperature( ) Fig. 9. Eect o steam temperature on Stack and steam turbine irreversibility. able shows the eect o steam pressure on eiciency o the cycle, stack temperature and stack irreversibility. At higher pressures the change in stack temperature and stack irreversibility is small. For better exergetic utilization the high pressure must be as high as possible. able shows the change in work output and mass o lue gases with plant capacity. From the table it is clear that or any capacity o the plant one third o total power is produced by the steam turbine and the remaining two third power is produced by the gas turbine. able. Eect o Maximum Steam ressure on Stack emperature, Eiciency and Stack Irreversibility ressure (bar) Eiciency Stack I stack able. hange in Steam urbine Output with lant apacity or Fixed Steam Inlet onditions at Optimum onditions lant apacity (M) Mass o lue gas (kg/s) ork Output (k) Stack (K) Eiciency Fig. 8. Eect o steam temperature on dryness raction.

5 International Energy Journal: Vol. 7, No., September 6 4. ONLUSIONS hermodynamic analysis o triple pressure HRSG is done using energy and exergy analysis. Eect o various operating parameters on perormance o HRSG is studied. It is observed that by increasing steam temperature the irreversibility o steam turbine decreases and dryness raction o steam is increased. For maximum pressure o 7 bar the optimum I and L steam pressures are ound as 47.6 bar and 9.8 bar. For given capacity o the combined cycle power plant / power is produced rom gas turbine and / power is produced rom the steam turbine. NOMENLAURE p speciic heat, kj/kg K V caloriic value, kj/kg pl ractional pressure loss G chemical exergy input H net caloriic value hl heat loss hp high pressure, bar lp low pressure, bar p pressure, bar ower output, M R gas constant r p pressure ratio emperature, K work output, M Suix 5 [] Subrahmanyam.N.V.R.SS, Rajaram.S, Kamalnathan.N HRSGs or combined cycle power plants, Heat Recovery Systems & H 5():55-6. [].K.Nag and D.Raha.995. hermodynamic analysis o a coal-based combined cycle power plant, Heat Recovery Systems &H, 5(): 5-9. [4] Horlock, J.H. 99. ombined ower lants Oxord, ergamon ress. [5] A. Ong iro, V. I. Ugursal, A. M. A aweel, and J. D. alker.997. Modeling o heat recovery steam generator perormance, Applied hermal Engineering, 7(5): [6].K.Nag, S. Mazumder,99. hermodynamic optimization o a waste heat boiler in: roceedings o the th National Heat and Mass ranser onerence, Madras, India. [7] Dellenback,.A.. Improved gas turbine eiciency through alternative regenerator coniguration. rans ASME, Journal o Engineering or Gas turbines and power [8] Ravi kumar.n. Rama krishna.k. Sita rama raju.a.v. 5. Exergy Analysis o Gas urbine ower lant with Alternative oniguration o Regenerator. roceedings, nd International Exerg, Energy Environment Symposium(IEEES). Kos-Greece. [9] Ravi kumar.n., Sita rama raju.a.v. 5. he Study o the Eects o Gas urbine Inlet ooling on lant and HRSG erormance, roceedings o National onerence on Advances in Mechanical Engineering (AIM-5).Hyderabad []. K. Nag and S. De Design and operation o a heat recovery steam generator with minimum Irreversibility, Applied hermal Engineering, 7(4)85-9. a air ap approach point c compressor g gas gas turbine pp pinch point s steam st steam turbine Greek Symbols ç eiciency γ speciic heat ratio ø ratio o chemical exergy to net caloriic value Δ dierential REFERENES [] Linho B., Hindmarsh E. 98. he pinch design method or heat exchanger networks. hemical Engineering Science (8):

EFFECT OF AMBIENT TEMPERATURE, GAS TURBINE INLET TEMPERATURE AND COMPRESSOR PRESSURE RATIO ON PERFORMANCE OF COMBINED CYCLE POWER PLANT

EFFECT OF AMBIENT TEMPERATURE, GAS TURBINE INLET TEMPERATURE AND COMPRESSOR PRESSURE RATIO ON PERFORMANCE OF COMBINED CYCLE POWER PLANT Harendra Singh 1, Prashant Kumar Tayal 2 NeeruGoyal 3, Pankaj Mohan