Thermodynamic Analysis of Combined Cycle Power Plant

Transcription

1 Thermodynami Analysis of Combined Cyle Power Plant A.K.Tiwari 1, Mohd Islam 2, M.N.Khan 3 1 Greater.Noida.Institute of Tehnology, Greater Noida, India. 2 Jamia Millia Islamia,New Delhi, India. 3 Krishna Institute of Engineering & Tehnology,Ghaziabad, India. 3 Corresponding Author: khanrkgit@rediffmail.om, Abstrat Air Bottoming Cyle (ABC) an replae the heat reovery steam generator and the steam turbine of the onventional ombined yle plant. The exhaust energy of the topping gas turbine of existing ombine yle is sent to gas-air heat exhange, whih heats the air in the seondary gas turbine yle. In 1980 s the ABC was proposed as an alternative for the onventional steam bottoming yle. In spite of the ost of reduing hardware installations it ould ahieve a thermal effiieny of 80%. The omplete thermodynami analysis of the system has been performed by using speially designed programme, enabling the variation of main independent variables. The result shows the gain in net work output as well as effiieny of ombined yle is 35% to 68%. Keywords: Topping Cyle, Bottoming yle, Compression Ratio, Power Output 1. Introdution The ombined yle has gained wide spread aeptane in the land based power generating industry and is definitely a proven tehniogy. But due to HRSG, the ost has been found to be too high. As a onsquene of high inlet temperature, a higher flue gas temperature results, whih allows for improved ombined yle or ogeneration effiieny. A ombined system whih has not been widely investigated is the air bottoming yle, where air, instead of steam, is used in a bottoming yle to reover partially the energy from the turbine exhaust & onvert it into useful power (Bolland O. et al, 1996). Wiks (1991) derived the onept of Air Bottoming Cyle (ABC) from the theory of ideal fuel burning engines by omparing the engines with the Carnot yle. The air bottoming yle may be employed to utilize the heat rejeted from the gas turbine. The ABC together with topping Gas Turbine Cyle is another type of ombined yle. It offers effiieny lose to that of ombined Gas Turbine/Steam Turbine Cyle and offers the potential of low weight as ompared to ombined Gas Turbine/Steam Turbine Cyle. The ombination of a gas turbine and an ABC represents a high-effiieny plant that provides lean, hot air for proess needs. The tehnial-eonomi analysis showed that an implementation of this sheme at the industries that require hot air will result in signifiant fuel savings ampared to the urrent tehnology and will have a paybak time of 3 year. The air turbine yle applied as a heat reovery yle behind an industrial furnae will onvert waste heat to eletriity with an effiieny of upto 26% (31% on exergy basis). This result in onsiderable avoided osts and a paybak time of 3-4 year additional fuel savings are obtained by passing the air turbine exhaust to the furnae as pre-heated ombustion air. Two projets have been proposed to a governmental energy ageny to demonstrate the viability of the ABC in these appliations. The projets will be implemented at milk power prodution and industrial bakeries, and in the glass industry.(korobitsyn, 2002) In this system an air turbine is used to onvert the exhaust energy into mehanial power, dispensing with all the hardware relvant to steam power plants (Boiler, Steam Turbine, Condenser, Pumps, Water Treatment plant, Cooling Towers et). The literature survey relvant to this paper are: Korobitsyn (2002) onsidered and used the yle as a ompat and simple bottoming yle in various appliations suh as upgrading option for simple gas turbines in the off shore industries. The tehnial and eonomial feasibility of ABC has also been evaluated; where hot air from the air turbine is supplied to food proessing industries. ISSN:

2 Kaikko (2001) presented the air bottoming yle as an eonomial onept to inrease the power generation effiieny of small and medium size gas turbines. A thermodynami analysis has been presented for o-generative system where a fration of ompressed air in an inter-ooled ABC was taken to a Reversed Brayton Cyle to provide old airflow. Korobitsyn (1998) analyzed the performane of duel gas turbine yle. Analysis of the duel gas turbine yle with various topping gas turbines were performed and implementation of ABC at the gas turbine to resulted in inrease in power output of 20 to 35 % when ompared to steam bottoming yle. The distint features of this set up were its simpler and robust design. Bolland et al (1996) performed the thermodynami analysis of ABC and the result revealed that the ABC alternative is eonomial as ompared with the other alternatives. Najjar et al (1996) onluded that ABC ould ahieve the thermal effiieny of about 60%, whih is not possible in simple gas turbine yles. Figure 1. Shemati diagram of Air Bottoming Cyle of ombined yle power plant Going through the literature, it is revealed that the use of Air Bottoming yle in onjuntion with the gas turbine topping yle inreases the overall effiieny of the yle. However, the use of interoolers and reuperators in the bottoming yle has not been studied. So, in the present work, power optimization analysis of air bottoming yle has been attempted. 2. Analysis of Topping Cyle In topping yle the air is ompressed in ompressor from 1 to 2 where its temperature raises from T 1t to T 2t.The ompressed air then enter the ombustion hamber where the ombustion of fuel takes plae. This results in rise of temperature of ombustion produt from T 2t to T 3t. The high temperature gases enter the turbine where it expands to the final temperature T 4t. Therefore expression for turbine work in topping yle (W t ) is given by W m T T ) (1) t g ( pg 3t 4t ISSN:

3 Where T T 1 (1 r )} (2) 4t 3t{ t p1 And expression for ompressor work in topping yle (W ) is given by W p1 m T T ) Where {1 } a ( pa 2t 1t T 2t ( r T 1) 1t (3) Network of topping yle W nett W W (4) t The expression for heat supplied in the ombustion hamber (q) is ( mg pgt3 t ma pat2t ) q (5) omb. W nett topping Therefore effiieny of the topping yle is (6) q Figure 2. Shemati diagram of Topping Cyle of ombined yle power plant 3. Analysis of Bottoming Cyle In Air Bottoming yle air at a temperature of T 1b (298K) enter the Low Pressure Compressor (LPC) where it ompresses and leave it at temperature higher than T 1b.There is Low Pressure Interooler (LPI) between Low Pressure Compressor (LPC) and Intermediate Pressure Compressor(IPC) and High Pressure Interooler (HPI) between Intermediate Pressure Compressor(IPC) and High Pressure Compressor (HPC). The temperature of air after leaving High Pressure Interooler (HPI) is same as the temperature of air at the entry of Low Pressure Compressor (LPC) and after ompression in High Pressure Compressor (HPC) its temperature raises to T 2b.This high temperature air enter the reuperator where it absorb heat from the exhaust gas of topping yle and in this way the temperature of air raises to T 3b. The high pressure and high temperature air after leaving the reuperator enters the turbine where it expands to the final temperature of T 4b. Therefore the expression for temperature of air leaving the High Pressure Interooler (HPI) in bottoming yle is given by /( n1) ( rp2 1) T T {1 } (7) 2b 1b The expression for temperature of air leaving the turbine in bottoming yle is given by T 4b T3 b{ 1t (1 rp2 )} (8) And the expression of effetiveness for the heat exhanger (ε) is ISSN:

4 T3b T2b (9) T4t T2b Therefore the expression for turbine inlet temperature of air in bottoming yle is given by ( T T T (10) T3 b 4t 2b) 2b And the expression for turbine work in bottoming yle (W t ) is given by W m T T ) (11) t a pa ( 3b 4b Expression for ompressor work in bottoming yle (W ) is given by W m T T ) (12) a pa ( 2b 1b So net work of bottoming yle W netb W W (13) t Figure 3. Shemati diagram of Bottoming Cyle of ombined yle power plant The net work of ombine yle is the net work of topping yle plus net work of bottoming yle is W W W (14) net nett netb And the expression for net effiieny of the ombine plant is W net ombine (15) q 4. Results and Disussion From the above analysis it is lear that the values of work output, effiieny of topping yle as well as bottoming yle effeted by number of independent variables like turbine inlet temperature of topping yle (T 3t ), pressure ratio of topping yle (r p1 ), pressure ratio of bottoming yle (r p2 ), mass flow rate in bottoming yle (m ab ) and number of interoolers (n) in the bottoming yle. This study analyzes those fators that affet the output of topping yle, bottoming yle as well as ombined yle. 4.1 Topping yle From equations (1), (2) and (5) it is lear that the work output and effiieny of topping yle depends on pressure ratio of topping yle (rp1) and turbine inlet temperature (T 3t ). From equation (1) and (2) it is easy to ISSN:

5 predit that as the turbine inlet temperature as well as pressure ratio of topping yle (r p1 ) inreases, turbine work inreases but with more inrease in pressure ratio of topping yle ompressor work also inreases that will results in slight derease in work output of topping yle. In this study, pressure ratio of topping yle (r p1 ) vary from 4 to 12 and turbine inlet temperature of topping yle (T 3t ) vary from 1000k to 1400k, on this data results shows that on partiular pressure ratio as the turbine temperature inreases net work of the yle inreases as well as on the partiular turbine inlet temperature as the pressure ratio inreases net work of the yle also inreases. In fig.4.1 (a) the network of topping yle are plotted against turbine inlet temperature as well as pressure ratio of topping yle simultaneously. Here it is lear that as the turbine inlet temperature inreases the rate of inrease of network of topping inreases with inrease in pressure ratio. Figure 4.1 (b) shows the variation of effiieny of topping yle with respet to pressure ratio of topping yle as well as turbine inlet temperature of topping yle. It is indiated from the graphs that the effiieny of topping yle inreases with inrease in pressure ratio as well as turbine inlet temperature of topping yle and attain its maximum value (32%) at 1400k of turbine inlet temperature and at the pressure ratio of 12 of topping yle. Figure 4.1a Surfae of power output of topping Cyle w.r.t and T3t Figure 4.1b Surfae of effiieny of topping yle w.r.t rp1 and T3t 4.2 Bottoming yle From equations (11), (12) and (13) it is lear that the network of bottoming yle depends on pressure ratio and turbine inlet temperature of bottoming yle. From equation (10) the turbine inlet temperature of bottoming yle depends on pressure ratio of topping yle, turbine inlet temperature of topping yle and number of interoolers in bottoming yle. ISSN:

6 Figure 4.2a Surfaes of power output of bottoming Figure 4.2b Surfaes of power output of bottoming yle w.r.t rp1 and rp2 at mab=52, n=1 yle w.r.t rp1 and rp2 at mab=57, n=1 ISSN:

7 Figure 4.2 Surfaes of power output of bottoming Figure 4.2d. Surfaes of power output of bottoming Cyle w.r.t rp1 and rp2 at mab=62, n=1 Cyle w.r.t rp1 and rp2 at mab=67, n=1 Figure 4.2e Surfaes of power output of bottoming Cyle w.r.t rp1 and rp2 at mab=72, n=1 Figure 4.2f Surfaes of power output of bottoming yle w.r.t rp1 and rp2 at T3t=1400K, mab=72 The graphs 4.2a to 4.2e shows variation of net work of bottoming yle with respet to pressure ratio of topping yle (r p1 ) and pressure ratio of bottoming yle (r p2 ) for different turbine inlet temperature varying from 1000K 1400K and different mass flow rate in bottoming yle (m ab ) varying from 52kg/s to 72 kg/s for one interooler. It is lear from the graph that as the pressure ratio of topping yle inreases the net work of bottoming yle dereases this is beause as the pressure ratio of topping yle inreasing the exhaust temperature of topping yle dereases whih results in derease in turbine inlet temperature of bottoming yle. In the opposite way as the pressure ratio of bottoming yle inreases the net work of bottoming yle inreases this is beause as the pressure ratio of bottoming yle inreases the turbine inlet temperature of bottoming yle inreases and exhaust temperature of bottoming yle dereases whih results in inrease in net power output, further more inrease in pressure ratio of bottoming will slightly derease in net power output beause of inrease in ompressor work. Here the maximum work output is for r p1 =4 and r p2 =12 ISSN:

8 It is shown in graph as the turbine inlet temperature of topping yle inreases the surfae of net work output shifted towards above beause it inreases the turbine inlet temperature of bottoming yle and attains its maximum value when turbine inlet temperature is 1400K Similarly as the mass flow rate in bottoming yle inreases the net work output also inreases beause work output is diretly proportional to the mass flow rate and attains its maximum value when mass flow rate is 72 kg/s. Also by inreasing the number of interooler in bottoming yle from one to two it saves in ompressor work and net work output also inreases. Here the maximum value of net work output is 25635KW for r p1 =4, r p2 =12, T 3t =1400K, m ab =72kg/s and n=2 shown by green surfae in figure 4.2f. 4.3 Combined yle Equation (14) gives the expression for network of ombine yle, and from the above analysis the results for network of ombined yle are shown in figures 4.3(a) to 4.3(e). Figure 4.3a Surfaes of power output of ombined Figure 4.3b. Surfaes of power output of ombined Cyle w.r.t rp1 and rp2 at mab=52, n=1 Cyle w.r.t rp1 and rp2 at mab=57, n=1 Figure 4.3 Surfaes of power output of ombined Figure 4.3d. Surfaes of power output of ombined Cyle w.r.t rp1 and rp2 at mab=62, n=1 Cyle w.r.t rp1 and rp2 at mab=67, n=1 ISSN:

9 Figure 4.3 e Surfaes of power output of ombined Cyle w.r.t rp1 and rp2 at mab=72, n=1 Figure 4.3f. Surfaes of power output of ombined yle w.r.t rp1 and rp2 at T3t=1400K, mab=72 Figure 4.3g. Surfaes of net effiieny of ombined yle w.r.t rp1 and rp2 at T3t=1400K, mab=72 The above graphs (4.3a to 4.3e) indiate variation of net work of ombined yle with respet to pressure ratio of topping yle and pressure ratio of bottoming yle for different turbine inlet temperature of topping yle and mass flow rate in bottoming yle for one interooler. It is lear from the graph as the pressure ratio of topping yle inreases the net work output of ombined yle inreases and attains its maximum value at r p1 =6 then dereases slightly. This position shifted towards above when inreases in turbine inlet temperature; mass flow rate in bottoming yle and number of interooler in bottoming yle. Similarly as the pressure ratio of bottoming yle inreases the net work output of ombined yle inreases and attains its maximum value at r p2 =12. This position shifted towards above as shown in graphs by inrease in turbine inlet temperature of topping yle; mass flow rate in bottoming yle and number of interooler in bottoming yle. Figure 4.3f shows the optimum position of network output of ombined yle i.e kw for r p1 =6, r p2 =12, T 3t =1400K, m ab =72 and n=2 by green surfae. The graph 4.3g shows the variation of net effiieny of ombined yle w.r.t rp1 and rp2 orresponding to maximum power output ondition ie at T3t =1400K, mab = 72 kg/s for one and two interooler in ABC. It is lear from graph that as pressure ratio of topping yle and bottoming yle inreases the net effiieny inreases and attains its maximum value at rp1=12, rp2=12. Also by adding two interooler in ABC the net inrease in effiieny is 8%. ISSN:

10 5.0 Conlusions On the basis of above analysis on topping, bottoming as well as ombined yle outputs the following onlusion are made by varying the pressure ratio of topping yle (r p1 ), pressure ratio of bottoming yle (r p2 ), turbine inlet temperature of topping yle (T 3t ), mass flow rate in bottoming yle (m ab ) and number of interoolers in the bottoming yle (n): a) The network of ombined yle inreases as inreases in turbine inlet temperature of topping yle for every partiular value of mass flow rate of ABC and pressure ratio of topping yle of ABC. b) The network of bottoming yle inreases as inrease in pressure ratio of bottoming yle and dereases as inrease in pressure ratio of topping yle for every partiular value of turbine inlet temperature of topping yle as well as mass flow rate in bottoming yle and attains its maximum value for r p1 =4, r p2 =12, T 3t =1400K, m ab =72kg/s for two interooler. ) The use of two interooler onsiderable improve the performane of ABC and inreasing the bottoming yle power output by 16% in the relation to the one interooler ase d) For any partiular value of pressure ratio of topping yle any partiular value of pressure ratio of bottoming yle, turbine inlet temperature of topping yle, and any mass floe rate in bottoming yle of ABC, the network of ombined yle attain its maximum value when two interoolers are used in bottoming yle. e) A ombined system with air bottoming yle an improve the net power output as well as effiieny by about 35% to 68%. f) The optimum point for this system is found to be, r p1 = 6, r p2 =12, m ab =72 of air bottoming yle with two interoolers at turbine inlet temperature of 1400k. g) The net power output and effiieny of ombined yle inreases up to 8% and 18% respetively, by adding two interooler in ABC in relation to the one interooler ase. The performane of this system an also be analysed by onsidering the pressure drop in ombustion hamber, introduing interooler in topping yle and with reheat in topping and bottoming yle both in future work. Nomenlature T 1t Compressor inlet temperature in topping yle T 2t Compressor exit temperature in topping yle T 3t Turbine inlet temperature in topping yle T 4t Turbine exit temperature from topping yle T 1b Compressor inlet temperature in bottoming yle T 2b Compressor exit temperature in bottoming yle T 3b Turbine inlet temperature in bottoming yle T 4b Exhaust Temperature from air turbine W Work output W nett Net work of topping yle W netb Net work of bottoming yle W net Net work of Combine Cyle Є Effetiveness of Heat Exhanger n Number of interooler r p1 Pressure ratio in topping yle r p2 Pressure ratio in bottoming yle η Compressor effiieny η t Turbine effiieny η omb Combustion Effiieny η topping Effiieny of Topping Cyle η ombine Effiieny of Combined yle q Heat Supplied in ombustion hamber Speifi Heat C p Variables r pt (Pressure Ratio Of Topping Cyle) : 4 to 12 r pb (Pressure Ratio Of Bottoming Cyle) : 4 to 12 ISSN:

11 T 3t (Turbine Inlet Temperature of Topping Cyle) : 1000 to 1400K m ab (Mass flow rate of air in bottoming Cyle) : 52 to 72 Kg/s n (Number of Inter-Cooler) : 1to 2 Constants η omb = 0.96, η t = 0.90, η = 0.90 T 1t (Compressor inlet temperature of topping yle) = 298K T 1b (Compressor inlet temperature of Bottoming Cyle) = 298K m a (Mass flow rate of air in Topping Cyle) = 69Kg/s m f (Mass flow rate of fuel) = 1.32Kg/s m g (Mass flow rate of gas in topping yle) = m a +m f =70.32Kg/s Є (Effetiveness of heat exhanger) = 0.90 C pg (Speifi Heat Of Gas) = 1.14KJ/KgK C pa (Speifi Heat of air) = 1.005KJ/KgK γ (For air) = 1.4 α= (γ-1)/γ = γ (For Gas) = 1.33 β= (γ -1)/γ = Referenes [1] Anonymous, 1991 Low Cost Air Bottoming Cyle for Turbines, Gas Turbine World, May-June, [2] Bathie W.W., 2000 Fundamentals of Gas Turbines, John Wiley & Sons, 2000 [3] Boland O., Forde M., and Hande B., Air Bottoming Cyle: Use of Gas turbine Waste Heat for Power Generation, ASME Paper 95- CTP-50, [4] Cohen H., Rogers G.F.C. and Satavanmutto H.H., 1996 Gas Turbine Theory. 4 th Edn., Longmans, [5] Happenstall T., 1998 Advane Gas Turbine Cyle for Power Generation: A ritial Review ASME-98-GT-846. [6] Horlok J.H; 1995 Combined power plant-past, present and future Trans of the ASME, J. of Engg. For Gas Turbine and Power, Vol. 117, [7] Kaikko J., Air Bottoming yle for Cogeneration Power, Heat and ooling Department of Energy Tehnology, Stokholm, Sweden, [8] Kakaras E., Doukelis A., Saharfe J.,2001 Appliation of gas turbine plant with ooled ompressor intake air, ASME 2001-GT-110. [9] Korobitsyn M.A., New and Advane Energy Conversion Tehnologies. Analysis of Cogeneration, Combined and Integrated Cyles, CHP 7 ( ), [10] Korobitsyn Mikhail, Industrial appliation of Air Bottoming Cyle pentagon, Vol.43, pp [11] Najjar Y.S.H. and Zaamout M.S., Performane Analysis of Gas Turbine Air Bottoming Combined System, Energy Conversion and Management, vol. 37, pp , [12] Wiks F., 1991 The Thermodynami Theory and Design of an Ideal Fuel Burning Engine, Proeedings of the 25 th Intersoiety [13] Energy Conversion Engineering Conferene IECEC 90, Boston, Vol.2, pp , [14] Yahaya S.M., Turbine Compressor and Fans 2000 Biographial notes Mr. A.K. Tiwari is Senior Leturer in Department of Mehanial Engineering, Greater Noida Institute of Tehnology, Gr.Noida, India. He is presently pursuing M.Teh from Jamia Millia Islamia, New Delhi. He has engaged in teahing and researh ativities sine last 7 years. His field of speialization is Thermal Engineering. Mr. M.N.Khan is Assoiate Professor, Department of Mehanial Engineering, Krishna Institute of Engg. & Teh. Ghaziabad, India. He has done his M.Teh. from Delhi College of Engineering and presently pursuing Ph.D. from Jamia Millia Islamia, New Delhi. He has engaged in teahing and researh ativities sine last 11 years. His field of speialization is Thermal. Mr. Mohd Nadeem Khan has published several papers in various national, international onferenes and journals. He has guided students for their M. Teh. work. Prof. Mohd. Islam is working as a professor in Department of Mehanial Engineering, Jamia Millia Islamia, New Delhi, India. He has engaged in teahing and researh ativities sine last 25 years. Prof. M. Islam has published several papers on various National and Internal Journals and guided several M.Teh and Ph.D. thesis in Thermal areas. ISSN: