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Thermodynamic Modeling and Optimization of Multi-Pressure Heat Recovery Steam Generator in Combined Power Cycle

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Thermodynamic Modeling and Optimization of Multi-Pressure Heat Recovery Steam Generator in Combined Power Cycle 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.
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