Journal of Scientific & Industrial Research SRINIVAS et al: OPTIMUM PRESSURE HEAT RECOVERY STEAM GENERATOR IN COMBINED CYCLE Vol. 67, October 2008, pp.827-834 827
Thermodynamic modeling and optimization of multi-pressure heat recovery steam generator in combined power cycle
T Srinivas1*, A V S S K S Gupta2 and B V Reddy3 1School of Mechanical and Building Sciences, VIT University, Vellore 632 014 2Department of Mechanical Engineering, J N T U College of Engineering, Kukatpally, Hyderabad 500 080 3Faculty of Engineering and Applied Sciences, University of Ontario Institute of Technology, Oshawa, ON, L1H 7K4, Canada
Received 27 September 2007; revised 17 July 2008; accepted 21 July 2008
Optimum configuration for single pressure (SP), dual pressure (DP) and triple pressure (TP) heat recovery steam generator (HRSG) is presented to improve heat recovery and thereby exergy efficiency of combined cycle. Deaerator was added to enhance efficiency and remove dissolved gases in feed water. A new method was introduced to evaluate low pressure (LP) and intermediate pressure (IP) in HRSG from local flue gas temperature to get minimum possible temperature difference in heaters instead of a usual fixation of pressures. Optimum location for deaerator was found at 1, 3 and 5 bar respectively for SP, DP and TP in heat recovery at a high pressure (HP) of 200 bar. Results also showed optimum pressure for air compression and steam reheater by means of three categories of heat recovery.
Key words: Combined cycle, Deaerator, Exergy analysis, Heat recovery, Multi pressure, Steam generator
Introduction Ragland & Stenzel6 compared four plant designs using Combined cycle (CC) power plants are gaining wider natural gas with a view of cost benefits achieved through acceptance due to more and more availability of natural HRSG optimization. Casarosa et al7 determined operating gas, higher overall thermal efficiencies, peaking and parameters means both of a thermodynamic and of a two-shifting capabilities, fast start-up capabilities and thermoeconomic analysis, minimizing a suitable lesser cooling water requirements. Optimization of heat objective function by analytical or numerical recovery steam generator (HRSG) is particularly mathematical methods. interesting for combined plants design in order to Optimal design and operation of a HRSG is possible maximize work obtained in vapor cycle. Pasha & with minimization of entropy generation8. Reddy et al9 Sanjeev1 presented a discussion about parameters that applied second law analysis for a waste HRSG, which influence type of circulation and selection for HRSG. consists of an economizer, an evaporator and a super Ongiro et al2 developed a numerical method to predict heater. Multi pressure steam generation in HRSG of a performance of HRSG for design and operation combined power plant improves performance of plant10. constraints. Ganapathy et al3 described features of Pelster et al11 compared results of a reference CC with HRSG used in Cheng cycle system, where a large dual and triple pressure HRSGs and also with and without quantity of steam is injected into a gas turbine to steam reheating models. Bassily12,13 modeled a dual and increase electrical power output. Subrahmanyam4 triple pressure reheat CC with a preset in constraints on discussed factors affecting HRSG design for achieving minimum temperature difference for pinch points, highest CC efficiency with cheaper, economical and temperature difference for superheat approach, steam competitive designs. Noelle & Heyen5 designed once- turbine inlet temperature and pressure, stack temperature, through HRSG, which is ideally matched to very high and dryness fraction at steam turbine outlet without a temperature and pressure, well into supercritical range. deaerator in steam bottoming cycle. *Author for correspondence This study presents optimization of single pressure E-mail: [email protected] (SP), dual pressure (DP) and triple pressure (TP) of 828 J SCI IND RES VOL 67 OCTOBER 2008
HRSG in CC with a deaerator. Variations in CC exergy P31 P31 efficiency are plotted with compressor pressure ratio, sO ,31′ − sO ,30 + 3.76(s ' − sN ,30 ) − Rlog + 3.76log = 0 2 2 N2 ,31 2 P P gas turbine inlet temperature, high pressure (HP) in 30 30 HRSG, steam reheat pressure, deaerator pressure and …(1) pinch point (PP). Optimized pressures for combustion, Temperature of air after isentropic compression is HP steam, steam reheater and deaerator for each of estimated from iteration of Eq. (1). Actual temperature HRSG configuration are presented. of compressed air is determined from compressor isentropic efficiency. Combustion equation in gas turbine combustion chamber is Materials and Methods Thermodynamic Model of Combined Cycle with SP, DP and CH + x (O + 3.76 N ) CO + 2 H O TP HRSG 4 2 2 → 2 2 Optimized configuration in HRSG improves steam + (x - 2)O2 + 3.76x N2 ...(2) generation rate and hence steam turbine output. Simple In Eq. (2), x is amount of air to be supplied. Air supply gas cycle gives higher efficiency at low compressor is determined by energy balance of Eq. (2) to get required ratio compared to intercooled-reheat gas cycle, thereby combustion temperature. ∆s for isentropic expansion in in this work simple gas cycle is selected as a topping gas turbine is cycle for CC 14. Steam cycle with a reheater is adopted as a bottoming cycle in CC, which is modeled separately ∆s = 0 = s – s (3) with SP, DP and TP level configurations in HRSG 32 33' … (Fig. 1). where, s32 = [sCO + 2 sH O + (x - 2) sO Temperature-heat transferred diagram for each 2 2 2
pressure level is depicted in Fig. 2. In each configuration + 3.76 x sN2]32 - R [log (P32/ mpcc) of HRSG, a deaerator is located to gain higher efficiency as well as to remove dissolved gases in feed water. + 2 log(2 P )/ m )+ 3.76x log (3.76x P / m ) Condensate preheater (CPH) is located in HRSG as the 32 pcc 32 pcc last heat transfer surface to improve heat recovery. In + (x - 2) log ((x - 2) P32)/ mpcc)] ...(4) this work, steam turbine inlet temperatures, low pressure (LP) and intermediate pressure (IP) are not
get fixed as in a regular manner. Steam temperature and s332 = [sCO + 2 sH O + (x - 2) sO + 3.76.xsN2]33' 2 2 2 and pressures are determined with local flue gas - R [log (P33/ mpcc) temperature in heaters. Pinch point (PP)-minimum temperature difference between gas turbine exhaust
leaving evaporator and saturation temperature of steam + 2 log (2 P33 / mpcc) + 3.76x log (3.76x P33/ mpcc) in evaporators and terminal temperature difference + (x - 2) log ((x - 2) P )/ m )] ...(5) (TTD)-temperature difference between exhaust entry 33 pcc and super heated steam in super heaters are maintained where mpcc (kg mol) is total mass of chemical elements in constant. Zero approach point (temperature difference products of combustion. between saturation temperature of steam and incoming water temperature) is assumed in economizer. Gas temperature after expansion in gas turbine is determined with iteration of entropy in Eq. (3). Actual temperature of expanded gas is obtained from gas turbine Thermodynamic Analysis of Combined Cycle isentropic efficiency. IP and LP are evaluated from Assumptions for analysis of CC are tabulated saturation temperatures. Steam flow rates and local (Table 1). Energy efficiency of CC is determined based exhaust temperature in heating devices are determined on lower heating value (50, 145 kJ/kg) of fuel. Net work from heat balance equations. Saturation temperature of output (%) of CC of standard chemical exergy of fuel IP evaporator is (52, 275 kJ/kg) is expressed as exergy efficiency of CC 15. For an isentropic compression process, entropy T = T – TTD – DSH …(6) change for air is zero, ∆s = 0. IP sat IP ex out IP IP SRINIVAS et al: OPTIMUM PRESSURE HEAT RECOVERY STEAM GENERATOR IN COMBINED CYCLE 829
Fuel Atm. Fuel 31 Atm. 31 air air 32 30 32 30 CC CC G G 33 GT 42 38 LP HRSG 33 HP SH HP LP EVAP LP HP HT LT ECO SH HPSH EVAP RH EVAP ECO CPH ECO ECO RH CPH 34 35 36 37 34 35 36 37 38 39 40 41
9 16 13 12 22 21 17 20 3 18 19 1 4 HP FP 11 5 LPST ST G 1 2 6 3 HPST G 2 Condenser 8 6 Condenser 12 4 Deaerator 7 FP Deaerator 11 8 14 10 FP CEP
LP 15 FP 13
(a) (b) Fuel Atm. air 31 32 CC 30 G
33 GT IP IP ECO LP Exhaust HP HT EVAP EVAP ECO HP LP HP IP HP LT LP SH SH RH EVAP SH ECO ECO 34 35 36 37 38 39 40 41 42 43 44 16 10 21 20 15 26 25 19 22 24
1 4 5 8 9 3 G 2 6 11 Condenser Deaerator 17 13 LP FP 18 14
23 19 HP FP IP FP (c)
Fig. 1—Schematic representation of combined cycle with: a) Single pressure HRSG; b) Dual pressure HRSG; and c) Triple pressure HRSG (HP, high pressure; IP, intermediate pressure; LP, low pressure; HT, high temperature; LT, low temperature; GT, gas turbine; ST, steam turbine; RH, reheater; SH, superheater; EVAP, evaporator; ECO, economizer; CPH, condensate preheater; FP, feed pump; HRSG, heat recovery steam generator; CEP, condensate extraction pump) 830 J SCI IND RES VOL 67 OCTOBER 2008
Fig. 2—Temperature-transferred heat diagram for: a) Single pressure; b) Dual pressure; and c) Triple pressure HRSG in combined cycle
Saturation temperature of LP evaporator is Total net work output by CC, wnet, cc = wnet, gc + wnet, sc …(11)
TLP sat = TLP ex out – TTDLP – DSHLP …(7) To determine exergetic losses, irreversibilities At HP evaporator, outlet temperature of flue gas associated in all components have to be estimated for exergy analysis. In this analysis, efficiency and losses = T + PP (8) HP sat HP … are determined for 1 kg mol of fuel. Chemical exergy At IP evaporator, outlet temperature of flue gas and physical exergy can be determined at each state as
= TIP sat + PPIP …(9) Chemical exergy, e At LP evaporator, outlet temperature of flue gas ch 0 = n ε k + RT Σ n ln .[ P.x ] …(12) = TLP sat + PPLP …(10) ∑k k 0 k k k
Work outputs and inputs in gas and steam cycles are where x is mole fraction of kth component related to unitary mass flow of fuel. k SRINIVAS et al: OPTIMUM PRESSURE HEAT RECOVERY STEAM GENERATOR IN COMBINED CYCLE 831
Table 1—Assumptions made for thermodynamic evaluation of HRSG in combined cycles
Atmospheric condition 25°C and 1.01325 bar
Gas cycle pressure ratio and maximum temperature 12 and 1200°C
Inlet pressure for HP steam turbine 200 bar
Terminal temperature difference (TTD) -temperature difference between flue gas and 25 reheated/superheated steam
Condenser pressure 0.05 bar
Steam reheat pressure 50 % of HP pressure
Pinch Point (PP) - minimum temperature differences between flue gas and steam in 25 evaporators of HRSG
Degree of superheat (DSH) in superheater 50
Isentropic efficiency of gas turbine 90 %.
Isentropic efficiencies of compressor and steam turbine 85 %
Pressure drop in combustion chamber 5 % of combustion chamber pressure
Pressure drop in HRSG, deaerator and condenser is neglected; Heat loss in HRSG, turbines, condenser, and deaerator is neglected
Exergetic loss in condenser,
Physical exergy = eph = h - Σk T0 sk …(13) Tw , out i = T m ( s − s ) + m × 4.18 × log Exergy, e = e + e …(14) co 0 co , st out in w T ch ph w , in Exergetic loss (irreversibility) in compressor, …(21) Exergetic loss of hot water from condenser, ic = e30 + wc - e31 …(15) T Exergetic loss in gas turbine combustion chamber, w, out iw,ic =mw ×4.18×(Tw, out − Tw, in )− T0mw × 4.18× log T w, in igtcc = eCH4 + e31 - e32 …(16) …(22) Exergetic loss in gas turbine, igt = e32 - e33 - wgt…(17) Total (internal + external) exergetic loss in condenser, Exergetic loss in HRSG, iHRSG icot = ico+ iw …(23) = (e - e ) + (e - e ) …(18) HRSG in HRSG out Σ water in steam out Exergy loss in deaerator, Exergetic loss in exhaust from ide = T0 [mde steam (safter mix – sbefore mix) HRSG, iex = eex …(19) + mfeed water (safter mix - sbefore mix)] …(24) Exergetic loss in steam turbines,
ist = T0 Σ m(sout - sin) …(20) Total exergetic loss of combined cycle, 832 J SCI IND RES VOL 67 OCTOBER 2008
Fig. 3-Exergy efficiency of combined cycle with different configurations (SP, DP and TP) of HRSGs versus: (a) Compressor pressure ratio; (b) Gas turbine inlet temperature; (c) HP pressure; (d) Steam reheater pressure; (e) Deaerator pressure; and (f) Pinch point
i = i + i + i + i + i + i + i + i …(25) Results and Discussion tot c gtcc gt HRSG ex st cot de CC with SP, DP and TP HRSG has been studied Exergy efficiency of combined cycle, parametrically to find maximum obtainable exergy efficiency from CC. Deaerator pressure, at which steam
wnet cc is separated from steam turbine to heat feed water, is a η2, cc = ×100 0 …(26) key parameter to get optimum heat recovery from ε CH 4 SRINIVAS et al: OPTIMUM PRESSURE HEAT RECOVERY STEAM GENERATOR IN COMBINED CYCLE 833
0 Table 2—Comparison steam properties in three HRSGs and performance (rc = 12, Tg = 613 C, mg = 650 kg/s
and mfuel = 13.1 kg/s)
SP HRSG DP HRSG TP HRSG HP Steam 83.5 kg/s HP Steam 83.5 kg/s HP Steam 83.5 kg/s Pressure 200 bar Pressure 200 bar Pressure 200 bar Temperature 525°C Temperature 525°C Temperature 525°C
LP Steam 16.7 kg/s IP Steam 17 kg/s Pressure 8.5 bar Pressure 12 bar Temperature 230°C Temperature 239°C LP Steam 15 kg/s Pressure 2 bar Temperature 165°C Net output: Net output: Net output: Gas cycle 236 MW Gas cycle 236 MW Gas cycle 236 MW Steam cycle 125 MW Steam cycle 140 MW Steam cycle 147 MW Combined cycle 361 MW Combined cycle 376 MW Combined cycle 383 MW Energy efficiency 55 % Energy efficiency 57.2 % Energy efficiency 58.3 %
recovery improves from SP to TP configuration and hence efficiency of CC also rises with single to multi pressure effect. Optimum pressure ratio with SP, DP and TP HRSGs is 8, 10 and 12 respectively at gas turbine inlet temperature of 1200°C. At a fixed compressor ratio of 12, for SP, DP and TP configurations, heat recovery and exergy efficiency of CC increases with increase in temperature (Fig. 3b). But, at high temperature, there is a less increment in efficiency with multi pressure HRSG compared to at low temperature. Efficiency of cycle increases with increase in steam turbine inlet pressure for all configurations in HRSG (Fig. 3c). There is a significant increase in efficiency from DP to TP effect at high pressure (HP). At a fixed steam turbine inlet pressure, 200 bar, SP HRSG exergy efficiency of CC increases all time with increase in steam Fig. 4—Comparison of exergetic losses in combined cycle reheat pressure (Fig. 3d). A steam reheater is added to components with SP, DP and TP HRSG increase dryness fraction of steam at turbine exit and to avoid erosion of blades during wet expansion. For DP exhaust. Effects of topping cycle parameters (compressor and TP HRSGs, exergy efficiency maximizes at optimum pressure ratio and gas turbine inlet temperature) and steam reheater pressure of 100 bar with HP pressure at bottoming cycle parameters (HP pressure, steam reheater 200 bar. Optimum deaerator pressure with SP, DP and pressure, deaerator pressure and PP in HRSG) on exergy TP levels in HRSG is around 1, 3 and 5 bar respectively efficiency of CC are plotted to identify maximum with HP pressure at 200 bar and steam reheating at 100 efficiency. bar (Fig. 3e). Optimized exergy efficiency at these Optimum pressure ratio increases with increase in conditions is 52.5, 54.5 and 55.5 % with SP, DP and TP pressure level in HRSG (Fig. 3a). Gas turbine outlet HRSGs respectively. Exergy efficiency decreases with temperature decreases with increase in pressure ratio. increase in PP (Fig. 3f). Irreversibility losses in heat Therefore, efficiency of cycle decreases with further transfer between exhaust and steam increases with increase in pressure ratio from optimum value. Heat increase in temperature difference. Heat recovery also 834 J SCI IND RES VOL 67 OCTOBER 2008 drops with increase in temperature difference (PP). References Therefore, at low PP, higher efficiency for CC can be 1 Pasha A & Sanjeev J, Combined cycle heat recovery steam achieved. But, close temperature difference increases generators optimum capabilities and selection criteria, Heat Recovery systems & CHP, 15 (1995) 147-154. size of heating device and hence the cost. Their choice 2 Ongiro A, Ugursal VI, Taweel A M & Walker J D, Modeling is the consequence of a compromise between of heat recovery steam generator performance, Appl Thermal thermodynamic efficiency and investment costs16. Thus, Engg, 17 (1997) 427-446. gain in efficiency from SP to DP is more compared to 3 Ganapathy V, Heil B & Rentz J, Heat recovery steam generator the gain from DP to TP level in HRSG. for Cheng cycle application, in ASME Ind Power Conf – PWR, San Diego, 4 (1988) 61-65. 4 Subrahmanyam N V R S S, Rajaram S & Kamalanathan N, For SP, DP and TP heat recovery, compressor pressure HRSGs for combined cycle power plants, Heat Recovery ratio and gas turbine inlet temperature are fixed. Systems & CHP, 15 (1995) 155-161. Therefore, topping cycle components exhibit no change 5 Noelle M & Heyen G, Mathematical modeling and design of on exergetic losses, which are compressor, gas an advanced once-through heat recovery steam generator, combustion chamber and gas turbine. Steam generation Compu Chem Engg, 28 (2004) 651-660. 6 Ragland A & Stenzel W, Combined cycle heat recovery rate increases from SP to TP in HRSG, thereby exergetic optimization, in ASME Proc 2000 Int Joint Power Generation loss associated with heat transfer increases (Fig. 4). Conf, IJPGC2000-15031 (Miami Beach, Florida) 23-26 July Exhaust temperature and hence exergetic loss in exhaust 2000, 1-6. decreases from SP to TP levels in HRSG. Exergetic loss 7 Casarosa F, Donatini & Franco A, Thermoeconomic optimization of heat recovery steam generators operating in expansion of steam in turbine rises with pressure levels parameters for combined plants, Energy, 29 (2004) 389-414. due to excessive steam flow rate. This also increases 8 Nag P K & De S, Design and operation of a heat recovery condenser exergetic loss with increase in pressure levels. steam generator with minimum irreversibility, Appl Thermal More amount of steam is required for feed water heater Engg, 17 (1997) 385-391. in case of SP HRSG whereas in DP and TP effect, amount 9 Reddy B V, Ramkiran G, Kumar A K & Nag P K, Second law analysis of a waste heat recovery steam generator, Int J Heat of bled steam and hence exergetic loss in deaerator Mass Transfer, 45 (2002) 1807-1814. decreases. On overall basis, total exergetic loss decreases 10 De S & Biswal S K, Performance improvement of a coal from SP to TP HRSG. Keeping exhaust gas flow rate gasification and combined cogeneration plant by multi-pressure fixed (650 kg/s), results (Table 2) for SP, DP and TP steam generation, Appl Thermal Engg, 24 (2004) 449-456. 7 11 Pelster S, Favrat D & Spakovsky M R, Thermoeconomic and heat recovery agreed with literature values . environomic modeling and optimization of the synthesis, design, and operation of combined cycles with advanced Conclusions options, ASME J Gas Turbines Power, 123 (2001) 717-726. 12 Bassily A M, Modeling, numerical optimization, and Optimum pressure ratio for compressor with SP, DP irreversibility reduction of a dual-pressure reheat combined and TP effects in heat recovery are 8, 10 and 12 cycle, Appl Energy, 81 (2005) 127-151. respectively at 1200°C of gas turbine inlet temperature. 13 Bassily A M, Modeling, numerical optimization, and Optimum deaerator pressure is obtained at 1, 3 and 5 irreversibility reduction of a triple-pressure reheat combined cycle, Energy, 32 (2007) 778-794. bar for SP, DP and TP levels respectively at steam turbine 14 Srinivas T, Gupta A V S S K S & Reddy B V, Performance inlet pressure of 200 bar. Similarly, at 200 bar of HP simulation of 210 MW natural gas fired combined cycle power pressure for DP and TP, steam reheater demands 100 plant, J Energy Heat Mass Transfer, 29 (2007) 61-82. bar to maximize exergy efficiency for CC. Parametric 15 Kotas T J, The Exergy Method of Thermal Plant Analysis analysis exhibits that gain in efficiency form single (Krieger publishing company, Malabar, F L) 1995, 236-269. 16 Franco A. & Russo A, Combined cycle plant efficiency increase pressure heat recovery to DP and TP recovery increasing based on the optimization of the heat recovery steam generator with diminishing rate. operating parameters, Int J Thermal Sci, 41 (2002) 843-859.