Exergy Based Analysis of an Open Cycle Gas Turbine Power Plant

Transcription

1 Canadian Journal of Basic and Applied Sciences PARL publication, 2015 CJBAS Vol 03(10), , October 2015 ISSN xergy Based Analysis of an Open Cycle Gas urbine Power Plant Mukesh Gupta, Raj Kumar Department of Mechanical ngineering, YMCA University of Science & echnology Faridabad, Haryana, India Keywords: Abstract xergetic analysis, Open cycle gas turbine, xergy destruction Open cycle gas turbine power plants play a significant role in the power generation industry In India alone, 105% of the total thermal power capacity is generated by gas turbine power plants Hence, analysis of these power plants is of significant interest from thermodynamic point of view Over the years, many researchers have analysed the performance of open cycle gas turbine power plants using the approach based on first law of thermodynamics which uses energy as the criterion for defining the performance of the power plants However use of this approach has its limitations as it is unable to take into account the irreversibilities which are inherent part of the system o take into account these irreversibilities, a new approach was developed based on the second law of thermodynamics In this approach, exergy is the criterion for defining the performance of a thermal system his approach allows us to take into consideration the irreversibilities associated with the various components of the system In the current study, an exergy based approach has been illustrated for an open cycle gas turbine power plant For formulation, a 25 MW open cycle gas turbine power plant has been considered as an example Detailed exergy analysis has been done for the variou s plant components xergy destruction has been calculated for various components and the effect of thermodynamic variables on the exergy destruction in various components has been analyzed Finally, equations have been developed which provide a correlatio n between the exergy destruction in different components as a function of the thermodynamic variables under consideration he current study provides a robust method which can be used to analyze open cycle gas turbines of different capacities 1 Introduction Open cycle gas turbine power plants play a significant role in the power generation industry In India alone, 105% of the total thermal power capacity is generated by gas turbine power plants Corresponding Author : -mail, mukeshmg@gmailcom el, (+91)

2 Mukesh Gupta et al - Can J Basic Appl Sci Vol 03(10), , October 2015 Many researchers have put forward different approaches to analyze the performance of gas turbine power plants Most of these approaches are based on the first law of thermodynamics However, there is a major drawback with these approaches In the first law of thermodynamics, energy is the criterion based on which the performance of a thermal system is defined However, energy based approach fails to consider the effects of irreversibilities which are inherent with any thermal system Hence, for better understanding of the performance of a thermal system, a new approach based on the second law of thermodynamics was proposed In this approach, exergy is the criterion for analyzing the performance of a thermal system xergy analysis and its applications in calculating entropy generation has been described [1] xergy based methods have been used for optimization of a single and double effect vapor absorption refrigeration system [2, 3, 4] An elaborate method to analyze the operation of a plant using various exergetic variables such as exergetic efficiency, the rates of exergy destruction, exergy destruction ratio has been provided [5] Different researchers have used thermodynamic relations between the energy and exergy losses to analyze the performance of a modern coal fired electrical generation station [6, 7, 8, 9, 10] Various efficiencies of fossil-fuel power plants have been studied in detail using the exergy concepts [11] Comparison between conventional and fluidized bed power plant have been made and improving techniques have also been given using exergy based methods for the conventional plants [12] Graphical exergy analysis has been used to locate inefficient segments in the combined cycle plant [13] xergy based analysis has been used for performance analysis of different processes such as production of hydrogen and hydrogenderived fuels, electrical and thermal power generation, thermal energy storage [14, 15, 16] o evaluate the exergy losses in the individual components of a cogeneration system, exergy analysis for each component in the subsystems has been done [17] stimation of avoidable and unavoidable exergy destruction and investment costs associated with different thermal components has been done [18] he primary way of keeping the exergy destruction, in a combustion process, within a reasonable limit is to reduce the irreversibility in heat conduction [19] 2 Methodology A typical open cycle gas turbine power plant is shown in Figure 1 274

3 Mukesh Gupta et al - Can J Basic Appl Sci Vol 03(10), , October 2015 Figure 1 Schematic layout of an open cycle gas turbine plant Steady flow conditions can be closely approximated by devices that are considered for continuous operation such as compressor, combustor and gas turbine of the power plant he conservation of mass principle for a general steady flow system with multiple inlets and outlets is given in quation 1 mi me (1) Where m denotes the mass flow rate and subscripts i and e stand for inlet and exit respectively A general exergy balance equation, applicable to any kth component of a thermal system, has been formulated [1] and is given by quation 2 W (2) e e, k k q, k i, k i he thermo-mechanical exergy of any stream may be decomposed into its thermal and mechanical components and is represented by quation 3 and quation 4 mcp[ 0 0(ln )] (3) 0 Where is the thermal exergy of a component p m R0 ln (4) p M Where M 0 is the mechanical exergy of a component he exergy destruction for the kth component is calculated from the exergy balance as given in quation 5 (5) D, k i, k e, k i e 275

4 Mukesh Gupta et al - Can J Basic Appl Sci Vol 03(10), , October Illustrative xample For analysis purpose, an open cycle gas turbine power plant of 25 MW capacity has been considered he system comprises of an air compressor, a combustion chamber and a gas turbine he mass flow rate of air is kg/ s and air enters the compressor at a temperature of 200 C and a pressure of 0981 bars he pressure increases to 481 bars through the compressor whose isentropic efficiency has been taken as 80% he inlet temperature to the gas turbine is 11230C and a pressure of bars he isentropic efficiency of the turbine has been taken as 80% he exhaust gases from the turbine are at 8170C and 110 bars he fuel (natural gas) is injected at 200C and 22 bars 3 Results and Discussion 31 xergy calculations for plant he net flow rates for different streams entering and leaving the system are shown in able 1 Positive values indicate the exergy flow rates of the products and the negative values represent the exergy flow rates of resources or fuel for a particular component State able 1 Property values and thermal, mechanical, chemical and net exergy flow rates at various state points in the m (kg/s) P (bar) gas turbine power plant (K) (MW) M (MW) C (MW) (MW) stream Where C represents the chemical exergy [1] and xergy balance values for each component are given in able 2 Component represents the net exergy flow for a able 2 xergy balance for each component in the gas turbine power plant W (MW) C (MW) (MW) M (MW) Compressor Combustion chamber Gas urbine Overall Plant D 276

5 Mukesh Gupta et al - Can J Basic Appl Sci Vol 03(10), , October 2015 he values of exergy destruction calculated in able 2 are plotted in Fig 2 Figure 2 xergy Destruction in various components and plant From Figure 2, it can be seen that maximum exergy destruction takes place in the combustion chamber followed by the gas turbine and the air compressor Hence the combustion chamber is least efficient from the exergetic view point 32 ffect of thermodynamic variables on plant performance he next step in the analysis is to study the effect of thermodynamic variables on the performance of the gas turbine power plant For this the following two thermodynamic variables have been considered: 1 Compressor pressure ratio 2 Air inlet temperature he effects of these two thermodynamic variables have been analyzed with respect to the exergy destruction values for various components 321 ffect of compressor pressure ratio (r p ) he effect of variation of the compressor pressure ratio on exergy destruction in compressor, combustion chamber and gas turbine are shown in Fig 3, Fig 4 and Fig 5 respectively 277

6 Mukesh Gupta et al - Can J Basic Appl Sci Vol 03(10), , October 2015 Figure 3 xergy destruction in compressor Vs Compressor pressure ratio Figure 4 xergy destruction in combustion chamber Vs Compressor pressure ratio Figure 5 xergy destruction in Gas turbine Vs Compressor pressure ratio From Fig 3, 4 and 5 it is clear that the exergy destruction increases for all the three components of the plant with increase in compressor pressure ratio he effect of increase in compressor pressure ratio is felt most in the gas turbine For the given range of compressor pressure ratios, the increase in exergy destruction ratio is terms of percentage are given in able 3 able 3 Maximum exergy destruction variation in various components 278

7 Mukesh Gupta et al - Can J Basic Appl Sci Vol 03(10), , October 2015 Component Maximum exergy destruction variation (% ) Compressor Combustion chamber 9003 Gas turbine Based on the analysis done, following equations have been developed which express the exergy destruction in compressor, combustion chamber and gas turbine as a function of compressor pressure ratio hese equations have been checked for different ranges of compressor pressure ratio and the results have been fairly satisfactory hese are represented as quations (6), (7) and (8) D Comp p D CC = ln(r ) (7) D = ln(r ) p G = ln(r ) (8) p (6) Where D Comp, gas turbine D CC, D G represent the exergy destruction in compressor, combustion chamber and 322 ffect of air inlet temperature he effects of variation in inlet air temperature on the exergy destruction in compressor, combustion chamber and gas turbine are shown in Fig 6, Fig 7 and Fig 8 Figure 6 xergy destruction in compressor Vs inlet air temperature 279

8 Mukesh Gupta et al - Can J Basic Appl Sci Vol 03(10), , October 2015 Figure 7 xergy destruction in combustion chamber Vs inlet air temperature Figure 8 xergy destruction in gas turbine Vs inlet air temperature From Fig 6, 7 and 8 it is clear that the exergy destruction decreases for all the three components of the plant with increase in inlet air temperature he effect of increase in inlet air temperature is felt most in the combustion chamber For the given range of compressor pressure ratios, the increase in exergy destruction ratio is terms of percentage are given in able 4 able 4 Maximum exergy destruction variation in various components Component Maximum exergy destruction variation (%) Compressor 829 Combustion chamber 8607 Gas turbine 7927 Based on the analysis done, following equations have been developed which express the exergy destruction in compressor, combustion chamber and gas turbine as a function of inlet air temperature hese equations have been checked for different ranges of inlet air temperatures and the results have been fairly satisfactory hese are represented as quations (9), (10) and (11) D Comp = ln( ) (9) D a CC = ln( ) (10) D a G = ln( ) (11) a 280

9 Mukesh Gupta et al - Can J Basic Appl Sci Vol 03(10), , October Conclusions his study makes use of this concept of exergy to analyze the open cycle gas turbine power plant It shows in detail the performance of various components of the open cycle gas turbine power plant xergy destruction has been taken as an important parameter to understand the performance of different components of the power plant Combustion chamber has the maximum exergy destruction followed by the gas turbine and compressor It means that the combustion chamber is the least efficient among the three components from exergetic view point Further the effect of two thermodynamic variables; (1) Compressor pressure ratio and (2) Inlet air temperature, on the exergy destruction in different components have been studied in detail Maximum effect of variation in compressor pressure ratio is felt in the gas turbine and is felt least in the compressor Maximum effect of inlet air temperature is felt in the combustion chamber and least in the gas turbine Finally, equations have been developed to correlate the exergy destruction in different components as a function of the two thermodynamic variables hese equations provide a robust mathematical model which is valid for different ranges of the thermodynamic variables under consideration References [1] Bejan A: Fundamentals of xergy Analysis, ntropy Generation Minimization, and the Generation of Flow Architecture International Journal of nergy Research, 26(7), ,(2002) [2] Mishra RD, Sahoo PK and Gupta A: Application of exergetic cost theory to LiBr/H2O vapour absorption system nergy, , (2002) [3] Mishra RD, Sahoo PK and Gupta A: hermoeconomic optimization of single effect water/ LiBr vapour absorption refrigeration system International Journal of Refrigeration, 26, ,(2003) [4] Mishra RD, Sahoo PK and Gupta A: hermoeconomic optimization of double effect water/ LiBr vapour absorption refrigeration system International Journal of Refrigeration, 28, ,(2005) [5] Moran MJ and Sciubba : xergy analysis: Principles and practice Journal of ngineering for Gas urbines and Power, 116, ,(1994) [6] Rosen M: Clarifying thermodynamic efficiencies and losses via exergy xergy International Journal, 2, 3-5, (2002) [7] Rosen M: ditorial-xergy in industry: Accepted or not? xergy, An International Journal, 2, 67,(2001) 281

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