Performance of Derna Steam Power Plant under Varying Super-heater Operating Conditions Based on Exergy

Transcription

1 erorance o Derna Stea ower lant under Varyg Super-heater Operatg Condition Baed on Exergy Idri Eleituri a a Departent o Mechanical Engeerg, Univerity o Benghazi, Benghazi City, Libya Correpondg author. Tel.: ;.O.Box: 32834; e-ail: idri@yahoo.co ABST RACT In the current tudy, energy exergy analyi o a 65 MW tea power plant wa carried. Thi tudy vetigated the eect o variation o overall conductance o the uper heater on the perorance o an exitg tea power plant located Derna-Libya. The perorance o the power plant wa etiated by a atheatical odellg which conider the o-deign operatg condition o each coponent. A ully teractive coputer progra baed on the a, energy exergy balance equation ha been developed. The axiu exergy detruction ha been ound the tea generation unit. A 50% reduction the deign value o overall conductance o the uper heater ha been achieved, which accordgly decreae the aount o the electrical power that would be generated by at leat 13 MW, a well a the overall plant exergy eiciency by at leat 6.4%, at the ae tie that would caue an creae o the total exergy detruction by at leat 14 MW. The achieved reult howed that the uper heater deign operatg condition play an iportant role on the therodynaic perorance the uel utilization o the power plant. Moreover, thee conideration are very ueul the proce o the deciion that hould be taken at the occaion o decidg whether to replace or renovate the uper heater o the power plant. Keyword: Exergy, Super-heater, Foulg; Stea power plant; O-deign. 1. Introduction In the conventional theral cycle analyi, the perorance o a power plant are uually detered by it theral eiciency, where higher eiciency iplie better therodynaic perorance. ower plant cycle are oten aced with the proble o aeg the change operatg cot due to deterioration o perorance o dividual piece o equipent, the predictg o the eect on perorance due to change equipent or operatg procedure. The oil-ueled Derna tea power plant (D-S) i deigned with an overall theral eiciency o 32% [1]. Currently, the plant operate with the lowet eiciency. It i expected that the a reaon or the eiciency lo i due to the reduction the top teperature due to the deterioration o the uper heater perorance, hence, the reduction the overall teperature dierence between the hot gae the workg tea. The a role o the uper heater i to play the perorance o tea power plant it creae plant eiciency by creag the average teperature (T recipient) at which heat i upplied to the cycle ro product o cobution [2]. Thereore, the ueul energy (exergy) conuption or drivg heat traner procee creae with (T ource-t recipient), uper heater creae the power plant eiciency by decreag thi dierence [2]. The uper heater perorance i decreaed by decreag it overall theral conductance (UA). Thi variation the overall theral conductance ay reult ro: i- Reduction uper heater urace area (A) due to exoliation pierce o the uper heater tube which lead to leakage. In order to retore the uper heater to unctional operation, leakg tube are plugged to keep the o ervice thereby reducg the uper heater urace area. ii- Reduction overall heat traner coeicient (U), reulted ro oulg on the tube urace due to il build up ide ide the tube urace. Thee change caue decreae the overall conductance o the uper heater to the pot where the uper heater can no longer operate near the deign condition. A the uper heater eiciency drop o it aect the overall cycle eiciency, which caue an creae operatg cot due to the additional uel to be upplied to the boiler to brg the turbe-drivg tea teperature up to the deign value. I the additional uel cannot be upplied to the boiler, the tea teperature will be below the deign value, o to avoid the urnace tube overheatg due to exceive lue gae teperature. At certa tage, the deterioration o uper heater perorance becoe o great that the uper heater hould be taken o ervice or cleang or replaceent. Moreover, a the overall conductance (UA) decreae, the aount o heat traner decreae, which caue a reduction the tea let teperature. The rate o heat traner between the lue gae tea decreae, the exergy detruction creae due to the irreveribility o the heat traner proce. Thereore, the electrical power the overall perorance o the plant decreae The exegetic approach to reducg energy conuption theral power plant wa ued by everal author. Gaggioli Fehrg [3] carried detailed exergy analyi or a 350 MW tea power plant vetigated the eect o the boiler let eedwater teperature on the unit operatg cot. Lior [4] conducted a thero-econoic exergy analyi o the eect o oil-uel uperheatg a nuclear power plant. Benyo et al [5] troduced a iulation odel to decribe the theral proce o a gle tea uper heater tage. An energy-exergy analyi o a coal-bae theral power plant wa reported by Sureh et al [6], ug the deign data ro a 63 MWe plant under operation. Sriva at al [7] exaed the iproveent eiciency with creae boiler preure, turbe let teperature urnace teperature on the bai o exergy analyi. Dierent way o enhancg the perorance o tea power plant were preented by Bhatt Rajkuar [8]. Moreover, Roen Rayond [9] carried energy exergy analyi or a tea power plant evaluated poible odiication to iprove the eiciency o the plant, a well a,, Szargut [10] Eleituri [11] perored the irt econd law analyi o the luence o regenerative eedwater heater on the boiler let teperature, operation cot o tea power plant.

2 In current tudy, 65 MW tea power plant i odeled, analyzed the eect o the varyg uper heater operatg condition are vetigated. 2. ower lant Decription Derna tea power plant (D-S) wa deigned built 1980 by BBC at Derna-Libya [1]. A detailed proce lowdiagra o the plant i hown Figure 1, with three tage o extraction to the regenerative eedwater heatg yte. Feedwater heatg i carried one tage o low preure heater (LH) one tage o high preure heater (HH) along with one open heater, Deaerator (DA). A aturated tea ro the boiler dru i ed to the uper heater (SH) that elevate the heat to 520 ºC at a preure o 87 bar. The counter low, three tage uper heater ha a total heat traner rate urace area o 58.2 MW repectively, i copoed o a radiant a convection ection. The lue gae with high teperature pa through the, uper heater, evaporator, econoizer air heater ection ally exited to the urroundg through the tuck. The extraction preure at the deign condition o tea ( bar) ro the turbe are a ollow: High preure heater (HH): 21.8 Low preure heater (LH): 1.15 De-aerator (DA) : 6.03 Condener (CND) : 0.07 Thi tudy i baed on the o-deign condition with the ollowg auption: i-in order to calculate exergy low rate, the reerence abient condition were taken a bar 25 ºC, repectively. ii- The axiu lue gae teperature (T 3): 1300 ºC. iii- The axiu tea teperature at the uper heater let (T 2): 520 ºC. iv- The aount o energy upplied by the uel to the boiler (,o): MW. v- The heavy uel oil i ued with LHV o 44MJ/kg. vi- The tea generated the boiler ( 1): kg/. vii- The deign value o overall conductance o the uper heater (UA): 48. kw/k [1]. viii- The axiu reduction the overall conductance (UA): 50% o the deign value. ix- The eedwater teperature, preure low rate at the boiler let are contant. x- Dry aturated tea with contant enthalpy i upplied to the uper heater. xi- The paraitic power conuption by the plant equal the pupg power requireent only. (condener extraction pup, coolg water pup boiler eedwater pup). Figure 1: Scheatic diagra o Derna tea power plant. The power plant low diagra i hown Figure 1. Baed on the atheatical odel that conider the o-deign operatg condition or the whole theral yte, which reult ro the change o the tea teperature (T 2), at the uper heater let. The o-deign condition i accoplihed by reducg the uper heater overall conductance (UA). The energy upplied by uel ( ), boiler let eedwater teperature (T 6), tea put preure (p 2) low rate ( 2) are kept contant. The detailed equation are not preented here view o brevity. The proble i attepted by ean o a general ethod o deg the tate o theral yte at o-deign operatg condition. With regard o the Figure 1, each coponent the power plant wa conidered a a control volue analyzed eparately. Three balance equation were written or each coponent cludg a, energy exergy. The baic balance equation are; or a; 0. 0 (1) energy; h h 0. 0 (2) exergy: D 0. 0 (3) 2.1. Stea Boiler Model The irt law eiciency o the boiler i obtaed ro perorance tet data o the conidered plant [1]. The curve ittg wa ued to obta an equation or calculatg the boiler eiciency at varyg boiler heat capacity. The equation i a ollow:

3 2 3 B B B B B, o a b c d (4) B, o B, o B, o where B B, o are the boiler heat capacity at deign o deign condition repectively, coeicient: a=0.263, b=2.5, c=2.5 d= Super heater Model The haanic diagra o the uper heater it teperature ditribution are hown Fig. 2 3, which i a counter-low type o uper heater. For the given relative change (k) the uper heater overall conductance (UA), the tak o the uper heater odel i to calculate the let teperature or tea (T 2) lue gae (T 4). The odel equation are given a ollowg: T 3 T2 T 4 T1 T 3 T2 ln T T LMTD (8) 4 1 The relative change (k) i deed a the ratio o the actual value (UA) to the deign value (UA 0) o overall conductance the uper heater i given by: UA k UA 0 The ethod o nuber o traner unit ha been troduced or uper heater, to olve or the two unknown let teperature T 2 T 4 (9). The lue gae ( g) tea ide ( ) heat traner rate ay now be calculated through the ollowg equation: g p, g 3 T4 C T (10) g p, 2 T1 C T (11) Figure 2: Scheatic diagra o the uper heater. 1-aturated tea let, 2-uperheat tea let, 3-hot gae let, 4- hot gae let 2.3. ower lant erorance Model The overall energy eiciency o the power plant i deed a I *100% (12) * (13) gro Gen gro (14) par Figure 3: Teperature ditribution the uper heater. The uper heater eectivene, i deed a the ratio o the actual rate o heat traner a given uper heater to the axiu poible rate o exchange, i given by [12]: 1 exp NTU 1 R R T T 2 1 (5) 1 R exp NTU 1 T3 T1 where, gro are the gro electrical power produced repectively, par i the paraitic electrical power required to drive the water pup, i the total aount o energy upplied by the uel to the boiler at given uper heater condition, i equal to:, o, add (15) where The overall energy eiciency o the power plant i deed a *100% I (16) * gro (17) Gen where R i the ratio o the heat capacity rate o the hot cold trea, i expreed a (18) gro par R C T T p, 3 4 (6) gcp, g T2 T1 NTU i the nuber o traner unit i given a UA T2 T1 NTU (7) C LMTD p, Where, LMTD i the log-ean teperature dierence i calculated ro where, gro are the gro electrical power produced repectively, par i the paraitic electrical power required to drive the water pup, i the total aount o energy upplied by the uel to the boiler at given uper heater condition, i equal to:, o, add (19)

4 where, o i the aount o heat upplied by the uel to the boiler at given deign condition (equal MW) conidered a a contant through the calculation o the preent tudy, add i the additional aount o energy upplied by the uel to the boiler when the uper heater i deterioration condition, i aued equal to zero or thi tudy. For analyi, conidered additional aount o heat upplied by the uel to the boiler i, add * C T T p, B 4, o 4 (20) the a progra conitg o ubre, which conider the o-deign operatg condition or the power plant coponent, a boiler, turbe tage, condener, eedwater heater pup. Ug the ull load data, the relative change o the uper heater overall conductance (k), the atheatical odel or the coponent o the power plant a dicued the previou ection, one can get the therodynaic propertie a energy low rate at each tate pot Fig.1. Thee tep are repeated until the olution reache the required accuracy. The exergy low rate at each tate pot the exergy detruction each coponent, the overall energy exergy eiciencie, heat rate peciic uel conuption o the power plant are then calculated by ug thee propertie value. where T 4,o T 4 are the live tea teperature at deign o-deign condition repectively. Thu, the additional a o uel required, ay be calculated a: add, add, (21) LHV The exergy balance applied to the conidered power plant a decribed [2]. The yte turbe, pup hat work electrical energy, are ull traner o exergy. The energy low carried away ro the yte a uele or, are taken a zero exergy. The exergy detruction rate ith coponent i baed on the exergy balance or cog gog exergy low rate a: D (22) i Where i i the exergy low rate i calculated ro [2]: h h T i i i o o i o (23) Thereore, the total exergy detruction rate o the whole power plant i given by: DT D (24) i The overall exergy eiciency (econd-law eiciency) o the power plant can be ound by the ollowg orula: re covered detroyed II 1 100% (25) up plied up plied The exergy low rate o uel ( ) i calculated ro LHV (26) Where, i taken a1.064 [1,2] Analyi procedure The appropriate odel analyi i ued to carry the luence o change the uper heater overall conductance (UA) on the electrical power put o the tea power plant the change o exergy detruction rate the particular coponent o the power plant. The odel conit o the aenergy balance equation, equation volvg therodynaic paraeter plant perorance. A coputer progra or the energy exergy analyi ha been written to iulate the therodynaic perorance o the conidered power plant. The coputational procedure i led the low chart o the progra hown Fig.2. To peror it iulation, Figure 4: Flow chart o coputg tep. 3. Reult Dicuion The contructed coputer progra or thi analyi ha been ued to tudy the eect o the uper heater operatg condition (reduction overall conductance) on tea power plant perorance. The eect o the uper heater operatg condition i tudied on the bai o relative change o uper heater overall conductance (k) ro 1. to 0.5. In thi analyi, the value o energy upplied by uel ( ) i taken a contant equal 195.4

5 MW. The obtaed reult are repreented graphical or through Figure 5, 6,7,8,9 10. Figure 5 how the tea teperature at variou relative change o the overall conductance (k). The creae o the relative change (k) creae the tea teperature (T 1). It how that, when the relative change (k) creae ro 0.5 to 1.0 the tea teperature creae ro 390 º C to 520 º C. Thereore, the obtaed teperature dierence i ab 130ºC or the overall conductance reduction o a 50% ro the deign value. teperature, the creae exergy detruction the tea boiler unit. Figure 7: Relative Change o Overall Conductance V. Heat Rate. Figure 5: Relative Change o Overall Conductance v Stea Teperature. The eect o overall conductance variation on total exergy detruction ajor coponent o the plant i hown Figure.6. The analyi how an creae exergy detruction with decreae overall conductance. The doant exergy detruction take place tea boiler unit on account o carryg cobution heat traner procee with a large teperature dierence. Moreover, the decreag trend o the exergy detruction ay be noticed the turbe which ay be due to the decreae turbe capacity. Wherea, the eed water heater condener conta no variation exergy detruction. Thi ay be attributed to the contant heat traner rate through thi equipent due to the change the uper heater overall conductance. Figure 8: Relative Change o Overall Conductance V. SFC. Figure 6: Relative Change o Overall Conductance V Exergy Detruction lant Coponent. The variation o heat rate, peciic uel conuption electrical power put with overall conductance are illutrated Figure 7, 8 9 repectively. The decreae overall conductance creae heat rate peciic uel conuption decreae the electrical power put value. The change thee value i attributed to the decreae the enthalpy o tea at the turbe let, which decreae with a decreae the tea Figure 9: Relative Change o Overall Conductance V. Net Electrical ower. Fally, the variation o overall energy exergy eiciencie with the change overall conductance i hown Figure 10. It how, the overall energy exergy eiciencie decreae by 6.8% 6.4% repectively, with the reduction the uper heater overall conductance by 50% o the deign value. The decreae the overall energy eiciency can be accounted to the decreae the turbe put power, wherea the decreae overall exergy eiciency i attributed to the creae exergy detruction the tea boiler unit.

6 5. Reerence Figure 10: Relative Change o Overall Conductance V Overall Energy Exergy Eiciencie. 4. Concluion recoendation In the preent work, an energy-exergy baed therodynaic analyi o Derna tea power plant ha been carried. The analyi ha cluded the exergy detruction the coponent o the power plant aeent o the eect o uper heater operatg condition, under deign a well a deteriorated condition on the plant perorance. The reult lead to the ollowg concluion: 1. The tea teperature, total exergy detruction rate, power put, energy exergy eiciencie decreae while heat rate peciic uel conuption creae with decreae o the overall conductance o the uper heater. 2. The axiu exergy detruction rate wa ound the tea boiler unite. 3. The total exergy detruction o the power plant with deteriorated uper heater operatg condition reach the value o around 14 MW. 4. The o-deign calculation reulted an overall exergy eiciency power put o 24.3% MWe repectively, at 50% reduction uper heater overall conductance copared to 30.7% MWe at deign condition. 5. The analyi reult are expected to be beneicial to the reearcher engeer workg the area o theral power plant. They how that uper heater deign operatg condition ha an iportant luence on the perorance the uel utilization o the power plant, a well a, it ueulne on the deciion akg proce durg planng or renovation period, or replaceent o uper heater. In uture, the current reult will be ollowed by the econoic jutiication o uper heater replaceent. The operational cot will be jutiied, a well a, the additional uel conuption, while the additional uel be upplied to the boiler. Moreover, the ipact o exhaut gae that are aectg the environent urroundg. Acknowledgent The author i grateul to Mr. Ali Aer or upplyg the operation data hi technical advice. 1. Derna ower lant Workg Docuent, 1980, Turbe Boiler Unit, Brown Boveri, Vol Szargut, J., Morri, R., Steward, F., 1988, Exergy Analyi o Theral, Cheical Metallurgical rocee, Firt Edition, Heiphere ublihg, New York. 3. Gaggioli, R., Fehrg T., 1977,"Econoic o eedwater Heater replaceent," ASMA Journal o Engeerg or ower, Vol. 112, pp Lior, N., 1996," Energy, Exergy Thero-econoic Analyi o the Eect o Foil-Fuel Superheatg Nuclear ower plant," ECOS, Stockhol, pp Benyo, I., et al., 2004,"Modellg o Stea Teperature Dynaic o a Superheater," roceedg 16th European Siulation Sypoiu, Fl, pp Sureh, M., et al.,2004,"energy Exergy Baed Therodynaic Analyi o a 62.5 MWe Coal-Baed Theral ower lant- A Cae Study," Int. Conerence on Energy Environent, Malayia, pp Sriva, T.,, et al.., 2007,"Generalized Therodynaic analyi o Sea ower Cycle With 'n' Nuber o Feedwater Heater," Int. J. o Therodynaic, Vol. 10, pp Bhatt, M. Rajkuar, N., 1999," erorance Enhanceent Coal Fired Theral ower lant, art II: Stea Turbe:, Int. J. o Energy Reearch, Vol. 23, pp Ronen, M. Rayond, T., 2006.,"Aeg Iprovg the Eiciencie o a Stea ower lant ug Exergy Analyi part I: aeeent", Int. J. o Exergy, vol. 3(4), pp Szargut, J., 2005,"Inluence o Regenerative Feedwater Heater on the Operational Cot o Stea ower lant H lant", Int. J. o Therodynaic, Vol. 8(3), pp Eleituri A. I.," The Inluence o Feed water Heater Operation Condition on the Exergy Lo Econoic Eect o a Stea ower lant" hd Thei, Waraw Technical Univerity, ol, (1997). 12. Cengel, Y. A., 1997, Introduction to Therodynaic Heat Traner, McGraw-Hill, New York. NOMENCLATURE 2 A - urace area, UA - overall conductance, (W/K) C p - peciic heat, kj/kg. C - exergy low rate, kw D - exergy detruction rate, (kw) DT- total exergy detruction rate, (kw) h - peciic enthalpy, (kj/kg) k - relative change overall conductance, (-) LHV - lower heatg value, kj/kg LMTD- log-ean teperature dierence, K - a low rate, (kg/) NTU - nuber o traner unit, (-) p - preure, (bar) - power, (Kw) - heat rate, ( kw) R - ratio o heat capacity rate o hot cold trea (-) - peciic entropy, (kj/kg.k) T - teperature,(c or K) 2 U - overall heat traner coeicient, (W/.K) Greek Letter - eectivene, (-) I - overall energy eiciency, (%) II - overall exergy eiciency, (%) Subcript a - air add - added B - boiler - uel g - lue gae Gen - generator

7 - echanical - tea o - abient condition 0 -deign condition par - paraitic 1,2,3 - therodynaic tate Fig. 1. Abbreviation S -tea power plant DA -deaerator HH -high preure heater LH -low preure heater SH -uperheater