A Thermal Analysis and Optimization of a Combined Cycle by Several Technologies

Transcription

1 American Journal of Energy Researc, 2014, Vol. 2, No. 2, 5-41 Available online at ttp://pubs.sciepub.com/ajer/2/2/ Science and Education Publising DOI: /ajer-2-2- A ermal Analysis and Optimization of a Combined Cycle by Several ecnologies KILANI Nied *, KHIR aar, BEN BRAHIM Ammar National Scool of Engineers of Gabes, University of Gabes Omar Ibn El Kattab St, 6029 Gabes, unisia, *Corresponding autor: niedkilani@yaoo.com Received Marc 26, 2014; Revised April 17, 2014; Accepted April 21, 2014 Abstract ermodynamic optimization of four power plant installations wit different tecnology is presented and discussed in tis paper. Numerical optimization of te different cycles is performed in aim to obtain iger efficiency. ermal analysis performance for te fourt cycles is performed for a define range of operating parameters using a calculation code establised according to EES software. e first studied cycle is a simple combined cycle containing gas turbine cycle wit a injection system, one pressure eat recovery generator HRSG and a turbine cycle. e overall efficiency of tis cycle in inlet ambient conditions is about 46%. e second cycle is a combined one wit injection system for wic te injected is generated outside te HRSG using eat recovery system at te air compressor outlet. e performance of tis cycle in te same initial conditions is iger of about 1%. e tird plant is a combined cycle wit injection and two extractions from turbine and two open feedwater eaters. e performance of tis cycle is iger of about 1% compared to te first one. e last considered power plant tecnology is a combined cycle wit eat recovery at air compressor outlet and extraction. Obtained results sow tat te optimum operating parameters leading to te best performances are not te same for different cycles. Keywords: combined cycle, extraction, eat recovery, optimization, termal efficiency Cite is Article: KILANI Nied, KHIR aar, and BEN BRAHIM Ammar, A ermal Analysis and Optimization of a Combined Cycle by Several ecnologies. American Journal of Energy Researc, vol. 2, no. 2 (2014: doi: /ajer Introduction Due to excessive demand for electricity new tecnologies are used for power plant cycle in aim to improve its performances. One of te major used resources is te fossil fuels. Electricity production from fossil fuels as been known since 196. Simple turbine cycles are firstly used wit low termal efficiency. en te simple cycle is improved and combined wit te gas turbine plant. Gas turbine cycle is very popular due to its relatively low capital cost, its significant reliability and flexibility, Basrawi et Al [1]. In te oter and considering evolutionary need for electricity many optimization and new power plants are carried out driving an evolution of te efficiency and produced net power. In addition te requirements of safety, environmental conditions and te excess of fuels price made te combined cycle more required because of its advantages in front of te oter cycles Silveira et al. [2]. Several optimization studies, and researc works were carried out [,4]. Nord and Bolland [5] were developed new tecnologies to eliminate CO 2 and improve te termal efficiency for te same fuel flow rate used in traditional cycles. Sayyaadi and Merabipour [6] are used eat recovery system wit tubular eat excanger to improve te gas turbine cycle efficiency. Ibraim et al [7] obtained an efficiency of 64.5% for a combined cycle wit regeneration. Carcasci and Faccini [8] performed a comparison between two power plant installations wit multi pressure HRSG and obtained a termal efficiency of 60%. Najjar et al [9] acieved a comparison between tree processes for gas turbine blade cooling; te termal efficiency of about 5 % is obtained. o minimize te emission of polluted gas and increase te gas turbine mass flow rate Bouam et al [10] used a injection system. at permitted an increase of te gas turbine cycle efficiency of about 0.11% for eac 1% of injected. is study is focused on te comparison between four combined power plants aving te following design caracteristics: 1. Simple combined cycle wit injection. 2. Combined cycle wit injection and compressed air cooling.. Combined cycle wit injection into combustion camber and extraction from te turbine. 4. e fourt considered plant is a combination between te second and tird cycles Simple combined cycle wit injection

2 American Journal of Energy Researc 6 e first cycle is a combination between a Brayton cycle wit injection system in te combustion camber and using natural gas as combustible and a turbine cycle. A mono pressure HRSG is installed at gas turbine outlet in order to generate te required flow rates for te turbine. e cycle is sown in Figure Combined Cycle wit Air Combustion Cooling For te second cycle, te injected in te combustion camber is not produced in te HRSG as for te first installation but it is generated by a eat recovery system installed between te compressor and te combustion camber. A eat transfer is made between compressed ot air and water. e compressed air is cooled and ten entered into te combustion camber. e supplied water is eated and evaporated in order to be Figure 1. Simple combined cycle wit injection injected into te combustion camber as sown in Figure 2. is tecnique permits to increase te air density flowed troug te combustion camber. us te air mass flow rate increases, and consequently te gas turbine efficiency can be improved. In addition, generation is occurred outlet of te HRSG tis can preserve more eat to te turbine cycle. Noting tat a study is conducted by B. Glover study [11] on te metallurgy selection criterion of te eat excanger used for tis purpose. Figure 2. Combined cycle wit air combustion cooling

3 7 American Journal of Energy Researc Figure. Combined cycle wit extraction 1.. Combined Cycle wit Steam Extraction In te tird power plant is extracted from te turbine at two different pressure levels wit two opened feed water eater as sown in Figure. Steam extraction permits to reduce te flow rate troug te condenser and consequently minimize its sizes. at can decrease te investment cost of te installation Combined Cycle Power Plant wit Steam Extraction and Compressed Air Cooling e fourt installation is a combination between te oter studied power plants. It is a combined cycle wit injection in te combustion camber, compressed air cooling and two extractions from turbine. Despite of its important termal efficiency, tis installation is more complicated and need more control and regulation. Figure 4. Combined cycle wit extraction and compressed air cooling

4 American Journal of Energy Researc 8 2. Governing Equations and Heat Balance Governing equations and eat balance are widely expressed in several previous works [12,1,14]. In te following we present te main equations leading to define te termal efficiency of te different considered cycles Basic Installation Air Compressor Equation For an isentropic compression isentropic outlet temperature is calculated according te following equation: γa 1 P2 γa 2is 1 P 1 e compressor isentropic efficiency is given by: iscomp is e real outlet air temperature can be defined as: ( 2is 1 iscomp e need power compressor is calculated as: W. Cp.( comp air air Combustion Camber Equation Energy balance in te combustion camber can be establised as follows: fuel. LHV. combustion ( air fuel... Cp. air air 2 fuel fuel fuel Cp Cp.. Cp For unisian natural gas used as gas turbine fuel, te Lower Heating Value is LHV KJ/Nm. e fuel flow used for cycle is calculated according to te following equation: gaz fuel f air Cp. gaz + LHV. + Cp.. Cp.. LHV. + Cp.. Cp Cp combustion fuel fuel gaz air combustion fuel fuel gaz Gas urbine Equation For gas turbine and using te pressure ratio, te isentropic exit temperature is given by: γ g P2 γ g 1 4is. P1 e real outlet temperature 4 is calculated by: [ ] 4 is 4is Gas turbine produced power is determine by: W. Cp.( G g gaz 4 e termal efficiency of gas turbine cycle can be experimented as: GC WG W Q Steam turbine equation comb comp Steam cycle efficiency in te combined cycle is te ratio of net eat produced by te cycle and te eat of combustion produced in te combustion camber. Were: SC W pump S W Q comb pumpe WS W HRSG Equation e eat balance establised for HRSG is ca be expressed by: gaz. Cpgaz.( 4 5 water. Cpwater.( sat Cp.( 17 sat +. Cp.( + ( + L inj inj stea min j liqinj water inj v e teoretical HRSG effectiveness is calculated by: εt 4 4 lim it embiant For te natural gas limit is taken between 70 and 100 C [11]. And te real HRSG effectiveness is given by: εr 4 5 ε t 4 lim it Condenser Equation e condenser te eat balance is expressed as:. Cp.(.( sea sea out in Installation wit Compressed Air Cooling Water Evaporator In te eat excanger installed between te air compressor and combustion camber, water is eated and evaporated at ig pressure and ten injected into te combustion camber. e eat transfer equation is expressed as:

5 9 American Journal of Energy Researc HRSG. Cp.( air air in out. Cp.( +. L water water sat in water v Since tere is only one stream in te HRSG te eat balance becomes [15]:. Cp.(. Cp.( gaz gaz 4 5 water water sat Cp.( + L 17 sat water v e compression ratio effect on te overall cycle efficiency is mentioned in Figure 5. Numerical simulation for te different cycles is performed for te same operating conditions. e sapes of te overall cycle efficiency according to compression ratio agree wit Raman investigation [16]. 2.. Installation wit Steam Extraction Steam urbines e powers produced in te different turbine are defined as follows: HP turbine: MP turbine LP turbine Pumps W S1.( W ( f.( S W ( f f.( S e energy required for te different pumps can be expressed by: HP pump: MP pump: ( P16 P15 Wpump1 17. v15. ispump1 ( P14 P1 Wpump2 ( 17 f1. v1. ispump2 Wit υ is te water average specific volume. LP pump: ( P12 P11 Wpump ( 17 f1 f2. v11. ispump Figure 5. Cycle efficiency variations according compression ratio Compressed air cooling process increases te cycle efficiency of about 1% wile te optimal compression ratio is same compared wit te simple combined cycle. e extraction tecnology leads to an increase of te cycle efficiency of about 2% and a decrease te optimal compression ratio from 21 to 20. Combining compressed air cooling and extraction processes improve sensibly te cycle efficiency. e best cycle efficiency is about 5% obtained for a compression ratio of Air Excess Influence e excess of air can assure a complete reaction in te combustion camber, and reducing consequently te CO production. Also air excess contributes to te turbine blade cooling, avoids turbine blade corrosion and increases te gas flow rate. However increasing te air excess may decrease te gas turbine inlet temperature; wic decreases te gas turbine cycle efficiency. e gas feeding te HRSG will be also cooled and te flux using for generation will be decreased. at affects te overall cycle efficiency Feedwater Heater e operating conditions te FWH are defined suc as te mixture leaving te FWH is at saturated liquid. For tis reason extracted flow rates from bot HP and MP turbines are calculated as follows: And f f2 ( water f Simulation and Interpretation of Cycles.1. Compression Ratio Influence Figure 6. Air excess influence on te overall cycle efficiency of studied cycles e variation of cycle efficiency according to air excess is sown in Figure 6 for te different considered cycles. Cycle efficiency decreases significantly wit te increase

6 American Journal of Energy Researc 40 of air excess. For air excess of 5% all cycles ave best values of efficiency wit 50% for te simple cycle and 56% for te fourt cycle. e coice of te optimal value of air excess is related to te gas turbine inlet temperature. Actually maximum temperature accepted by turbine blade is about 187 K. e influence of air excess on te gas turbine inlet temperature I is presented in Figure 7. is temperature decreases significantly wit te increase of air excess. For te simple installation, turbine cycle efficiency is too low. Consequently te decrease of te eat flux transferred to te turbine cycle reduces te overall cycle efficiency. But for combined cycle wit extraction, turbine cycle is more efficient. us te overall cycle efficiency is better tan te simple cycle efficiency and it decreases sligtly wit injection ratio. Figure 7. Gas urbine inlet temperature wit air excess.. Steam Injection Influence Steam injection in te combustion camber is used to decrease NO, NO 2, CO emission. In oter and it s not recommended to attain more tan 10 % of injection rate due turbine blade protection. e effect of injection on te cycle efficiency is sown in Figure 8. For simple cycle and cycle wit extraction, injection as a negative effect on te overall cycle efficiency. is is can be explained by te fact tat increasing injection flow rate leads to te increase of te eat flux provided by HRSG to transform water into. us te remained eat flux transferred to te turbine cycle is reduced. Figure 8. Steam injection influence on te cycle efficiency For bot te cycle wit compressed air cooling and te fourt cycle, water eater-evaporator is installed at te compressor exit. A quantity of termal energy is extracted from te compressed air and used to product for injection into te combustion camber. e termal energy transferred from gas turbine to te HRSG is integrally used for te turbine cycle. On te oter and te decrease of te compressed air temperature in te eat recovery system is neglected in te combustion camber. us te overall cycle efficiency increases wit injection ratio increase..4. Cycles Comparison Operating parameters and obtained results of te four considered power plants are arranged in able 1. Simple combined cycle able 1. Operational parameters of studied cycle Combined cycle wit Combined cycle wit compressed air cooling extraction Combined cycle wit compressed air cooling and extraction Fuel: Natural gas (kg/s Inlet temperature ( C Relative umidity (% Ambient pressure (bar Compression ratio Steam injection (% Air excess (% Exaust mass flow (kg/s Exaust temperature ( C Gas cycle output (MW Steam cycle output (MW Hig pressure (bar Medium pressure (bar Low pressure (bar Gross output (MW Overall cycle efficiency (%

7 41 American Journal of Energy Researc 4. Conclusion ermal optimization study is performed on four different tecnologies of power plant combined cycles. Several processes are considered suc as compressed air cooling, injection into combustion camber and extraction. A comparison between te four combined cycles is developed for te same operating parameters. For eac installation te influences of important parameters are analyzed. Steam injection decreases te overall cycle efficiency if it generated in te HRSG. In te oter and, if a eat recovery system is externally added to generate for injection into te combustion camber, te overall cycle efficiency increase wit te increase of injection ratio. Compression ratio presents an optimum of about 20 for te different plants. e tecnical modification added to te simple cycle leads to a sligtly decrease of te optimal compression ratio. Air excess decreases te overall cycle efficiency of te different installations due to its influence on te gas turbine inlet temperature. e eat fluxes extracted from turbine and transferred to cycle are reduced. In te oter and, air excess ratio is very important to protect turbine blade from corrosion and excessive temperatures. e optimum value of air excess ratio is mainly determined according to te desired gas turbine inlet temperature. Steam ratio injected to te combustion camber decreases te overall efficiency of te simple combined cycle and te combined cycle wit extraction since te generation is realized in te HRSG. us increasing injection flow rate leads to te decrease of eat flux used for turbine cycle. If te is generated outside te HRSG, te injection into combustion camber improves overall cycle efficiency. Reference [1] F. Basrawi,. Yamada, K. Nakanisi, F. Naing, Effect of ambient temperature on te performance of micro gas turbine wit cogeneration system in cold region, Applied ermal Engineering 1, (2011, [2] J.L. Silveira, J.A. Carvalo, I.A. Villela, Combined cycle versus on tousand diesel power plants: pollutant emission, ecological efficiency and economic analysis, Renewable and Sustainable Energy Reviews 11, (2007, [] Y.S.H. Najjar, Efficien use of energy by utilizing gas turbine combined systems, Applied ermal Engineering 21, (2001, [4] R. Felipe, P. Arrieta, E. Electo, S. Lora, Influence of ambient emperature on combined-cycle power-plant performance, Applied Energy 80, (2005, [5] L.O. Nord, O. Bolland, design and off-design simulation of combined cycles for offsore oil and gas installation, Applied ermal Engineering 54, (201, [6] H. Sayyaadi, R. Merabipour, Efficiency enancement of a gas turbine cycle using an optimized tubular recupertive eat excanger, Energy 8, (2012, [7].K. Ibraim, M.M. Raman, A.N. Abdalla, Optimum gas turbine configuration for improving te performance of combined cycle power plant, Procedia Engineering 15, (2011, [8] C. Carcasci, B. Faccini, Comparison between two gas turbine solutions to increase combined power plant efficiency, Energy Conversion & Management 41, ( [9] Y.S.H. Najjar, A.S. Algamdi, M.H. Al-Beirutty, Comparative performance of combined gas turbine systems under tree different blade cooling scemes, Applied ermal Engineering 24, (2004, [10] A. Bouam, S. Aissani, R. Kadi, Améliorationdes performancesdes turbines à gaz par injection de la vapeur d eau en amontde la cambre de combustion, Revuedes Energies Renouvelables 11(2, (2008, [11] W.B. Glover, Selecting evaporator for process application, CEPmagazine, (2004, 26-. [12] M.P. Boyce, Gas turbine engineering andbook, Gulf Professional Publising, (2002. [1] J.H. Horlok, Advanced gas turbine cycles, Elsevier science, (200. [14] R. Kelofer, Combined cycle gas and turbine power plants, PennWell Company, (1997. [15] V. Ganapaty, Heat Recovery Steam Generator: Understand te basics, Cemical Engineering (1996, [16].K. Ibraim, M.M. Raman, Effect of compression ratio on performance of combined cycle gas turbine 2(1, (2012, 9-14.