IMPACT OF CONTROL STRATEGIES ON THE OFF- DESIGN OPERATION OF THE GAS TURBINE IN A COMBINED CYCLE GAS TURBINE (CCGT) POWER PLANT Zumg Liu and I A Karimi * Department of Chemical & Biomolecular Engeerg National Univerity of Sgapore 4 Engeerg Drive 4, Sgapore 117585 Abtract The off-ign performance of a combed cycle ga turbe (CCGT) plant for power generation i of great importance. In thi work, a detailed mathematical model i developed for a triple-preure reheat combed cycle power plant. Ug the model, CCGT off-ign operation and efficiency are tudied under four ga turbe (GT) trategie. Controllg the turbe let temperature (TIT) give better GT performance, but reduce the turbe exhaut temperature (TET) and the combed cycle efficiency. On the other hand, controllg the variable guide vane (VGV) along with TIT give higher combed cycle efficiency for GT load between 60% and 100%. Below 60% load, controllg the VGV and TET outperform the other operatg trategie combed cycle efficiency. Hence, a mixed operatg trategy eem the bet for improvg the CCGT off-ign performance. Keyword Modelg, Simulation, Ga turbe, Combed cycle power plant, Off-ign performance. Introduction Combed cycle ga turbe (CCGT) power plant are widely ued due to their high thermal efficiency and low emiion. However, due to the frequent peak regulation the power grid, they are often run at part-load condition. The part-load operation decreae thermal efficiency, and the vetigation of off-ign performance of CCGT i an important topic. CCGT uually comprie a ga turbe (GT) (toppg cycle) and a heat recovery team generator (HRSG) (bottomg cycle). Predictg their off-ign performance require an accurate imulation of the two cycle. Stone (1958) and Doyle and Dixon (1962) applied a tagetackg method to imulate the off-ign performance of multi-tage axial compreor with variable geometry angle. Thi method could obta ter-tage parameter (preure and temperature), and the compreor overall performance. By adoptg the tage-tackg method for the compreor and a tage-by-tage model for the turbe, Lee et al. (2011) developed a general off-ign performance prediction program to imulate imple, recuperative, and reheat cycle GT, which i ueful when component map are not available. Zhu and Sarvanamutto (1992) developed a mathematical model to predict the offign performance of a three-haft GT ug component matchg. They aumed chokg the GT, and began from the hot end to determe compreor operation. Al- Hamdan and Ebaid (2006) tudied GT imulation for power generation by uperimpog the turbe performance map on the compreor map. Hagld et al. (2009) developed one complex model (ug component map) and one imple model (ug turbe contant) for predictg the part-load performance of aero-derivative * To whom all correpondence hould be addreed
GT, and found both model to offer good agreement term of flow and preure characteritic. Jimenez- Epadafor Aguilar et al (2014) applied an off-le component baed program (GSP) to imulate the offign performance of a two-haft GT and tudied the regulation method of combed heat and power plant. Toutani et al (2015) propoed a novel method for modelg compreor and turbe map and tung their parameter to improve performance prediction and diagnotic under off-ign, teady tate, tranient, and degraded condition. HRSG off-ign modelg i maly devoted to the correction of overall heat tranfer coefficient. Kim and Ro (1997) and Kang et al (2012) correlated the overall heat tranfer coefficient of different HRSG ection under offign condition with ga flow propertie. Ganapathy (1990) developed a HRSG off-ign performance calculation procedure, and propoed a detailed method for etimatg heat tranfer coefficient by coniderg HRSG ign parameter, and turbe exhaut ga parameter. Zhang et al (2015) adopted Ganapathy procedure to model the off-ign performance of the bottomg cycle, and propoed ome emi-empirical and emi-theoretical formula. After overall heat tranfer coefficient under offign condition are calculated, HRSG imulation tart by applyg energy balance and heat tranfer equation. To achieve efficient off-ign operation, GT operatg trategie have been of great concern over the year. Kim et al (2003) vetigated the effect of variable let guide vane (VGV) on gle-preure combed cycle performance of gle-haft and two-haft GT configuration, and found that VGV modulation creae gle-haft combed cycle efficiency, epecially high load range, but doe not improve the efficiency of twohaft enge, due to the GT performance degradation. Kim (2004) analyzed the part-load performance of GT and combed cycle with different ign parameter, and tudied everal load control trategie, and oberved that a GT with higher ign performance exhibit uperior part load performance. Hagld (2010a, 2010b) analyzed the effect of variable geometry GT on part-load efficiency of combed cycle for hip propulion, and found that combed cycle part-load performance can be improved by variable area nozzle (VAN) and VGV control. From our literature earch, we concluded that no work ha addreed the full complexity of a CCGT cycle. While many have modeled dividual part, a complete, detailed, modular, and high-fidelity imulation model for the entire CCGT plant doe not exit the open literature. Thi i particularly true a far a the ue of actual compreor/ turbe performance map and the tudy of real triplepreure reheat combed cycle are concerned. Moreover, a the bottomg cycle i a paive ytem that utilize GT exhaut heat to generate team and power, uch a model would provide the bai for tudyg variou GT operatg trategie and gag ight to efficient operation. Latly, no work ha o far tudied combation of GT load control trategie to mata efficient CCGT operation over a wide load range. Thi work aim to addre all of thee gap the literature. Sytem cription Figure 2 how the chematic of a triple-preure reheat combed cycle power plant. An air compreor (AC) compree ambient air and ject it to a combutor. The fuel burn the combutor, and the flue ga expand a four-tage ga turbe (GT), of which the firt three tage are cooled by bleedg air from the AC to prevent blade overheatg. After expanion the GT, the exhaut ga enter an HRSG that generate team at three preure, namely high (HP), termediate (IP), and low (LP). The HP team expand the HP team turbe (HP- ST), and mixe with the IP team. The mixed team i reheated and expand the IP ST (IP-ST). The IP-ST exhaut mixe with the LP team, and expand the LP- ST. The exhaut from the LP-ST i condened a condener and pumped to an LP economizer to fih the bottomg cycle. Figure 2. Schematic of a triple-preure reheat combed cycle power plant. Mathematical model In what follow, VEC denote vane angle correction factor; η denote efficiency; α denote relative change VGV angle; m denote ma flow; h denote pecific enthalpy; P denote preure; T denote temperature; and LHV denote lower heatg value; κ i a contant; A denote area; g denote gravitational acceleration; R denote ga contant; and γ denote pecific heat ratio; r i outlet preure ratio; and ϕ denote flow coefficient. Furthermore, ubcript a i for air, c for AC, cc for combution chamber, for ign condition, cl for coolg air, f for fuel, w for water, p for BFW pump, g for ga, t for turbe, for let, for team, and upercript * for critical condition. We now write down the equation for modelg each component of the CCGT.
Air Compreor (AC) We aume an n-tage, adiabatic, axial flow compreor with an identical preure ratio for each tage. We ue a generic performance map (Fig. 1, Palmer et al, 1993) for predictg it off-ign operation. Three row of VGV at it let control the airflow. The VGV angle affect the AC efficiency a follow (Hagld, 2010a): 2 c, map 1 VEC c (1), P T cl cl, m cl mcl (4) Pcl, Tcl HRSG The HRSG i jut a erie of heat exchanger. At ign condition, the energy balance determe the team flow, temperature, and heat exchanger area. Under offign condition, we ue the effectivene-ntu method to model HRSG operation. The overall heat tranfer coefficient (Erbe and Gay, 1989) and effectivene (Kay and London, 1984) at off-ign condition are corrected a follow. 0.5 U U m m g g, 0.8 (5) C C m f NTU,, NTU (6) max UA C m Combutor Figure 1. Relativized compreor map For the combutor, we write the followg energy balance. m h a a m LHV m h m h (2) cc Ga turbe (GT) f f The off-ign operation of a GT can be modeled by a contant wallowg capacity (chockg condition) cribed by a turbe let ma flow, temperature and preure a follow (Streeter and Wylie, 1979; Palmer et al., 1993): m A t, T t, P nozzle t, C, f g g g 2 Rga 1 1 1 Durg off-ign operation, the ditribution of coolg air flow to nozzle vane and rotor bla i aumed to be unchanged, while the coolg air flow to each turbe tage i calculated from the preure and temperature of the bleedg tage a follow (Erbe and Gay, 1989): (3) Then, the heat tranferred i given by, Q Q max (7) The remag tate variable of the ga and water/team flow are obtaed from energy balance. Steam turbe (ST) An ST normally ha three ection (HP, IP, and LP). We model each ection eparately ug a modified form of Stodola equation (Erbe, 1986). m P BFW Pump r r 1 1 r (8) The ientropic efficiency of a water pump under offign condition i given by (Frank, 1995): pm pm, Deaerator m 2 m, w w m m, w w The operation of a deaerator i modeled ug the followg ma and energy balance. 2 (9)
m m m m m (10) fw fw fw lpw lpw hpw lpw vent m h m h m h m h m h (11) hpw hpw vent vent Thi complete our full model for the CCGT power plant. Operatg trategie The primary aim of a CCGT i to meet the power demand dynamically. Sce GT i the primary ource of power, the power i uually controlled by the GT. Two trategie are widely ued practice for tung the GT load under off-ign condition. TIT Control: Adjut fuel flow to change TIT and thu GT output. VGV-TET Control: Manipulate the VGV angle at the AC let and the fuel flow imultaneouly to change GT output, while matag contant the turbe exhaut temperature (TET). However, everal alternate trategie are poible. For tance, VGV-TIT control: Kim (2004) tudied a glepreure CCGT. They ued VGV to reduce air flow to 85% while matag TIT at it ign value. Then, they adjuted fuel flow alone to reduce GT load further to 30%, while keepg VGV angle contant. However, modern GT allow lower VGV angle (Hagld, 2010a), hence a traightforward modification i to contue the VGV-TIT trategy further lower than 85% air flow, leadg to an creae TET. Thi provi a better heat recovery performance HRSG and creae ST power. Sce TET uually ha an upper limit, a the lat tage of the GT ha no coolg; we top VGV-TIT trategy, when TET hit 650 C. For lower GT load, we imply witch to TIT control, while keepg the VGV angle contant. An alternative i to jut replace the VGV control by Inlet Air Throttle (IAT) control the above. In other word, adjut the IAT valve before the AC and fuel flow to reduce GT load until TET hit 650 C, while keepg TIT contant. We will call thi a IAT-TIT control. Thu, we tudy four operatg trategie for GT control. In thi work, we aume that the AC and GT hare a common haft, which ha the ame contant peed a the ST haft over the entire load range. Reult and dicuion Figure 3-9 how the off-ign performance of GT and combed cycle for the four GT operatg trategie. Firt, conider the TIT control. To reduce the GT load to 30%, the fuel flow reduce by 51.6%, and TIT decreae from 1353.1 C to 892.7 C. A we ee from Figure 4, the turbe exhaut flow (TEF) rema teady. Thi happen a the airflow creae by 0.9% to compenate for the decreag fuel flow. However, TET reduce harply from 605.2 C to 393.8 C, which lower the bottomg cycle efficiency from 31.1% to 21.2%. Thi clearly lower the total power output and overall efficiency of the plant. In VGV-TET control, TET rema at it ign value (605.2 C), a VGV cloe and the fuel flow reduce. In contrat to TIT control, both airflow and TEF decreae, but TET rema teady. The bottomg cycle efficiency rema nearly contant around 30.5%, uggetg the more critical role of TET veru TEF. In VGV-TIT control, TIT can be mataed at it ign value up to 73% GT load, when TET reache 650 C. TEF decreae by 20.1%, but the bottomg cycle efficiency creae. For GT load below 73%, TEF reduce by 0.6% only, but TET decreae by 150 C. Hence, the bottomg cycle efficiency decreae from 32% to 26.8%. The behavior of IAT-TIT i imilar to that of VGV- TIT. TET reache 650 C at about 70% GT load. TEF reduce by 18.9%, and the bottomg cycle efficiency creae by about 1% due to higher TET. For GT load lower than 70%, the IAT angle i kept unchanged. The TIT control reduce the fuel flow and TIT from 1353.1 C to 1025.3 C. TEF rema nearly unchanged, but TET reduce from 650 C to 495.5 C. Hence, the bottomg cycle efficiency reduce from 32.0% to 26.6% a with VGV-TIT control. A comparion of the four trategie ugget that the GT efficiencie are imilar, but TIT eem more efficient. However, TIT eem the wort for the combed cycle efficiency. IAT-TIT and VGV-TIT control are very imilar acro the load range term of both efficiencie. VGV-TET clearly outperform other under 60% GT load, and ha the bet overall performance. VGV-TIT ha only a margal edge for load exceedg 60%. Thi clearly ugget that a mixed GT operatg trategy may be the bet for a CCGT plant: VGV-TIT for GT load above 60% and VGV-TET for load below 60%. Concluion In thi work, a detailed and modular mathematical model i developed for imulatg the part-load performance a CCGT power plant. The model wa ued to tudy the effect of four operatg trategie on ga turbe and combed cycle performance. While TIT control may eem the bet for GT efficiency, it i the wort for the combed cycle efficiency. IAT-TIT and VGV-TIT control are very imilar. Fally, VGV-TET control eem the bet overall for the part-load control of a CCGT power plant. Our work highlight the diadvantage of ug GT control a the preferred control mode for CCGT power plant operation. Clearly, the GT and ST cycle are two tegral, ignificant, and teractg part of a CCGT power plant, and focug one at the detriment of the other a done mot literature i not a wie approach. Thi i
the focu of our ongog work, for which thi work ha laid the foundation by developg a rigorou high-fidelity model for CCGT plant imulation. Our work alo ugget the poibility of mix of operatg trategie that may prove the bet a the power plant load varie over a wide range. Figure 5. TIT veru GT load. Figure 3. Fuel flow veru GT load. Figure 6. TET veru GT load. Figure 4. TEF veru GT load. Acknowledgment Zumg Liu acknowledge ACTSYS Proce Management Conultancy Company for hotg hi dutrial ternhip under a rg-fenced Graduate Reearch Scholarhip from the National Univerity of Sgapore. The author thank Mr. Norman Lee, Dr. Yu Liu and Mr. Weipg Zhang for everal enlighteng dicuion on the GT operation. The author alo acknowledge the ue of GateCycle under academic licene. Figure 7. GT efficiency veru GT load.
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