Energy optimization in gas turbine cycles using thermodynamic with environmental considerations

Transcription

1 Energy optimization in gas turbine cycles using thermodynamic with environmental considerations Jorge Amaya Pinos Professor of Salesiana Politecnic University Cuenca - Ecuador Abstract The use of gas turbines for different production processes has increased in the course of the last decades, so it has been proposed different strategies to improve the efficiency or performance of the same cycles The present work describes the behavior of the turbines after the implementation of a reheating system and a reheating system with recuperator, considering variables such as temperature, pressure, and thermodynamics The results demonstrate the benefit in terms of turbine work efficiency as well as the reduction of NOx emissions Keywords Gas Turbines, Optimization, Enthalpy Balance, Combined Cycles, Thermodynamic applications Objective The aim of the study is to know a problem in the environment applied to the thermodynamics This is necessary to know the advantages that it can get with the application of the new study 1 / 25

2 methods Introduction The gas turbine is a versatile mechanical element used by several productive activities such as power generation, oil and gas industries, processing plants, and others The work of the gas turbines begins by collecting air to compress it with a compressor module, then this air passes to a combustion chamber where it is mixed with fuel and the mixture is ignited, product of this process produces gases that are expanded by a turbine (Meherwan, 2002) After this process the axis of the turbine turns and drives the compressor that sits on the same axis, giving continuity to the cycle (See figure 1) The first movement of the rotor is given by an external starting unit, this starting must be given until the turbine achieves its design speed and can operate normally (Soares, 2008) Figure 1: Open Cycle Gas Turbine Source: IPIECA, 2016 To study the behavior or cycle of the turbine, we use the Brayton or Joule Cycle, it consists of diagrams representing the behavior Temperature-Entropy in an ideal Cycle (See Figure 2) 2 / 25

3 Figure 2: Temperature-Enthalpy diagram for a gas turbine cycle Source: Pathirathna, 2013 Figure 2 shows the T-S diagram where from point 1 to point 2 represents the compression phase, from point 2 to 3 the supply of heat at constant pressure and from 3 to 4 the expansion of air (Pathirahna, 2013) When analyzing the performance of gas turbines, it is necessary to consider the thermodynamic properties that govern the process since they are in the processes from points 1 to 4, in addition must consider other variables such as temperature, pressure, fuel-air ratio and relative humidity (GE-Power, 2016) The cycle of the gas turbine could be Simple Cycle (SC), Reheat Cycle (RC) and Cycle with a reheat and a recuperator (RHC); presents several flexibilities in its performance, that is to say, within the cycle can improve the efficiency and specific network work by incorporating extra components into the cycle (Malaver, 2014) An example of the above is to add a superheat combustion chamber, since it improves the performance of the turbine, when adding this chamber, the process of expansion in the turbine is divided in two, while the combustion chamber is placed between the Turbines of high and low pressure (Langston & Opdyle, 1997) By adding the superheat combustion chamber, the expansion process in the turbine is segmented into two parts and an extra combustion chamber is added between the high and low pressure turbines The exhaust gas from the high pressure turbine (HPT) enters a combustion chamber of reheating and the temperature of the gases increases by the action of the supplementary combustion In addition, the cooling air for the low pressure turbine (LPT) can be extracted from the intermediate stages of the compressor where the pressure is higher compared to the inlet pressure in the low pressure turbine (Sheikhbeigi & Ghofrani, 2007) If these assumptions are met, the performance parameters in the turbines could be improved by reducing compressor input work, and decreasing the cooling air temperature for LPT (Dahlq uist, 2016) This study describes the simple cycles, with overheating and with reheating and a recuperator, where all the variables that take place in cycles of real and practical gas turbines are involved, and also, after the modeling, a comparative analysis is carried out between them 3 / 25

4 Development A simple cycle is characterized by the direct extraction of cooling air from the compressor, in addition there is no recirculation of currents in any of the stages (See Figure 3) Figure 3: Flow diagram of the simple Cycle Source: Sheikhbeigi & Ghofrani, 2007 In the reheating cycle the gas coming from the first combustion chamber enters the high pressure turbine and expands partially; the gas enters the reheating combustion chamber where fuel is injected producing additional combustion, raising the temperature of the gas to the inlet of the second low pressure turbine where it expands to the atmospheric pressure value In this cycle the cooling air for high pressure turbines is extracted from the compressor output and for low pressure turbines are taken from the intermediate stage of the compressor (See Figure 4) Figure 4: Flow diagram of the reheat gas turbine cycle Source: Sheikhbeigi & Ghofrani, 2007 On the other hand, a heat exchanger is added in the Cycle with reheat and recuperator in 4 / 25

5 order to increase the temperature of the air prior to its entry into the first combustion chamber, where heat is recovered The extraction of air for the cooling process is similar to that previously described (See figure 5) Figure 5: Flow diagram of Reheat, and recuperator cycle Source: Sheikhbeigi & Ghofrani, 2007 Compressor analysis With calculations referring to the first law of thermodynamics, knowledge of the air temperature at the compressor outlet and the location of the extraction for cooling with air, it is possible to quantify the efficiency and the work consumed (See equation 1) The entropy change for a kilogram of air entering the compressor is described below (El-Masri, 1986) Normal 0 21 false false false ES X-NONE X-NONE Normal 0 21 false false false ES X-NONE X-NONE Normal 0 21 false false false ES X-NONE X-NONE Normal 0 21 false false false ES X-NONE X-NONE 5 / 25

6 Normal 0 21 false false false ES X-NONE X-NONE (1) Applying the Polytrophic efficiency concept, the final outlet temperature will be given by (See equation 2): Normal 0 21 false false false ES X-NONE X-NONE (2) Combustion chamber analysis The combustion reaction could be described by the general formula of C x H y (See equation 3) for hydrocarbon fuel and a theoretical air coefficient A (Sheikhbeigi & Ghofrani, 2007) (3) It is an adiabatic process, it is possible to use the first law of thermodynamics (see equation 4): 6 / 25

7 (4) The equation (5) could calculate the theoretical air coefficient and the fuel-air ratio (5) Turbine analysis First, the not cooled turbine is analyzed, where the refrigerant is not extracted from the compressor; the efficiency of the component is given by the outlet temperature, and the inlet pressure can be calculated according to equation (6), which is evaluated from the pressure drop in the combustion chamber to the resulting pressure of the compressor (Ranjan & Mohammad, 2014) (6) The produced work in the turbine comes from (see equation 7): (7) The behavior of the cooled turbine is described in Bolland's (2004) research The study describes the expansion process as a continuous event divided into several subscales with poor pressure relationship (Santos & Andrade, 2012) For the present study it is irrelevant the trajectory that will follow the expansion, the focus is on the mass of refrigerant that is given by (see equation 8): 7 / 25

8 (8) (9) In equation 8, mc represents the mass flow of the refrigerant, Mg is the gas mass flow, C pc and C pg are the specific heat of the refrigerant and the gas respectively dt g is the variation of inlet and outlet temperatures, T b is the blade temperature, and T c is the blade temperature upon contact with the refrigerant (Consonni, 1998) In Equation 9, St is the Staton number whose equivalence is equal to 0005 The ratio () has the value of 10 and the value of C is between 03 and 05 In the present review the values of σ for high technology turbines are equal to and for others , while the value of ε is equal to 03 for convection cooling and 05 for film cooling (Katsavou, 1995) Reheat combustion chamber analysis The equation describing the combustion in the reheating chamber is: (10) 8 / 25

9 Where x represents the fuel burned for each mole of fuel consumed in the primary combustion chamber, this parameter in conjunction with the air-fuel ratio can be found by performing the energy balance in the reheating combustion chamber (Sheikhbeigi & Ghofrani, 2007) Recuperator analysis In the heat exchanger, the gas flowing from the low pressure turbine is used to heat the air entering the first combustion chamber, reducing the fuel consumption by the combustion chamber The efficiency of the recuperator, considering air as a minimum fluid is given by (see figure 11): (11) Also, the energy balance for the recuperator analysis is: (12) Where T2, and T6 can be obtained from low pressure turbine models or from the models of compressors, while Ta and Tb can be quantified from the predecessor equations Results The results presented below are taken from the research carried out by Sheikhbeigi & Ghofrani, (2007), where the efficiency increase is analyzed according to variables such as: temperature, pressure ratio, and work produced mainly 9 / 25

10 Point of implementation of overheating Adequate values were sought with the priority of improving efficiency within the gas turbine cycles The results were extracted from graphs according to the efficiency, the relationship between the low and high pressure turbine, the turbine inlet temperature (TIT), the compressor pressure ratio (c, rc), and the ratio of Pressure between high and low pressure turbines e: (rc = rhpt rlpt) For the analysis, diagrams were used to identify the key parameters for cycle efficiency In Figure 6 the thermal performance of the Reheat cycle for various TITs and C, rc is evaluated The figure identifies that the efficiency peaks are in function of the increase of the pressure in the high pressure turbines, the recommended values are In addition it is noted that maximum efficiencies are reached as the turbine inlet temperatures (TIT) increase, having an efficiency of 50% when TIT = 1400 C, and the compressor pressure ratio is equal to 50 Figure 6: Efficiency versus e for different TIT and pressure ratios values Source Sheikhbeigi & Ghofrani, 2014, Design of cycles with overheating and with Reheating and Overheating with recuperator for maximum efficiency If it is considered a priority to have high efficiency values, it is advisable to design a cycle with overheating with recovery at low pressures (RHC), and with TIT = 1000 C and r = 5, with these parameters the efficiency achieved will be around 43%, with a Work of 252kJ for each kilogram of fuel However, it is important to emphasize that the cycles cannot be chosen only to present advantages in efficiency, an economic evaluation must be made and the production system for which the selected turbine will work (See figure 7) 10 / 25

11 Figure 7: Efficiency for the RHC Source: Sheikhbeigi & Ghofrani, 2014, Environmental considerations regarding the design of SC, RC, RHC To know the environmental benefits it is necessary to identify that the reduction of fuel, which in this case is the raw material represents a decrease in the burning of fuels, therefore the pollutant emissions will be reduced In the present study the behavior of the NOx pollutant against the adaptations to improve the efficiency was evaluated, the results were that the NOx will have between 15 to 25 parts per million less in the emissions, representing a rate of 15 to 20% in The production of this contaminating element Conclusions It can be concluded that the work compiles information on the cycles pertinent to the gas turbines, where it explores adaptations that improve the performance of the same, where they were related several factors that give greater confidence to the study Among the most important results achieved is that the correct design of a reheat; Or a reheat with a collector for a gas turbine would increase the efficiency by 43%, this increase of efficiency allows to obtain better economic benefits since the investment in fuel for the operation of the turbine will be smaller, in addition there are economic benefits since there are reductions In the emission of nitrogen compounds between 15% and 20% The application of these cycles turns out to be one of the most viable alternatives because it produces a less environmental impact Also, it use a cleaner fuel preserving so the nonrenewable resources and the reduction of the gas emission References 11 / 25

12 Consonni, S (1998) Gas turbine cycles performance evaluation In Proceings of ASME Cogen Turbo Power ASME Dahlquist, A (2016) Conceptual Thermodynamic Cycle and Aerodynamic Gas Turbine Desing Sweden: Lund UNIVERSITY El-Masri, M A (1986) On thermodynamics of gas turbine cycles Part 2: a model for expansion in cooled turbines J Eng Gas Turb, 108 Langston, L S, & Opdyle, G J (1997) Introduction to Gas Turbines for Non-Engineers Glo bal Gas Turbine News Malaver, M (2014) Optimización del trabajo en un ciclo Brayton con Irreversibilidades Ingeni ería, Meherwan, B P (2002) Gas Turbine Engineering Hnadbook Woburn: Butterworth - Heinemann Pathirahna, K (2013) GAS TURBINE THERMODYNAMIC AND PERFORMANCE ANALYSIS METHODS USING AVAILABLE CATALOG DATA Gavle: University of Galve Santos, P, & Andrade, C (2012) Analysis of Gas Turbine Performance with Inlet Air Cooling Techniques Applied to Brazilian Sites JATM, / 25

13 Sheikhbeigi, B, & Ghofrani, M (2007) Thermodynamic and environmental consideration of advanced gas turbine cycles with reheat and recuperator Int J Environ Sci Tech, Soares, C M (2008) Gas turbines in simple Cycle & Combined Cycle applications United States: US DEPARTMENT OF ENERGY Abstract The use of gas turbines for different production processes has increased in the course of the last decades, so it has been proposed different strategies to improve the efficiency or performance of the same cycles The present work describes the behavior of the turbines after the implementation of a reheating system and a reheating system with recuperator, considering variables such as temperature, pressure, and thermodynamics The results demonstrate the benefit in terms of turbine work efficiency as well as the reduction of NOx emissions Keywords 13 / 25

14 Gas Turbines, Optimization, Enthalpy Balance, Combined Cycles, Thermodynamic applications Objective The aim of the study is to know a problem in the environment applied to the thermodynamics This is necessary to know the advantages that it can get with the application of the new study m ethods Introduction The gas turbine is a versatile mechanical element used by several productive activities such as power generation, oil and gas industries, processing plants, and others The work of the gas turbines begins by collecting air to compress it with a compressor module, then this air passes to a combustion chamber where it is mixed with fuel and the mixture is ignited, product of this process produces gases that are expanded by a turbine (Meherwan, 2002) After this process the axis of the turbine turns and drives the compressor that sits on the same axis, giving continuity to the cycle (See figure 1) The first movement of the rotor is given by an external starting unit, this starting must be given until the turbine achieves its design speed and can operate normally (Soares, 2008) Figure 1: Open Cycle Gas Turbine Source: IPIECA, / 25

15 To study the behavior or cycle of the turbine, we use the Brayton or Joule Cycle, it consists of diagrams representing the behavior Temperature-Entropy in an ideal Cycle (See Figure 2) Figure 2: Temperature-Enthalpy diagram for a gas turbine cycle Source: Pathirathna, 2013 Figure 2 shows the T-S diagram where from point 1 to point 2 represents the compression phase, from point 2 to 3 the supply of heat at constant pressure and from 3 to 4 the expansion of air (Pathirahna, 2013) When analyzing the performance of gas turbines, it is necessary to consider the thermodynamic properties that govern the process since they are in the processes from points 1 to 4, in addition must consider other variables such as temperature, pressure, fuel-air ratio and relative humidity (GE-Power, 2016) The cycle of the gas turbine could be Simple Cycle (SC), Reheat Cycle (RC) and Cycle with a reheat and a recuperator (RHC); presents several flexibilities in its performance, that is to say, within the cycle can improve the efficiency and specific network work by incorporating extra components into the cycle (Malaver, 2014) An example of the above is to add a superheat combustion chamber, since it improves the performance of the turbine, when adding this chamber, the process of expansion in the turbine is divided in two, while the combustion chamber is placed between the Turbines of high and low pressure (Langston & Opdyle, 1997) By adding the superheat combustion chamber, the expansion process in the turbine is segmented into two parts and an extra combustion chamber is added between the high and low pressure turbines The exhaust gas from the high pressure turbine (HPT) enters a combustion chamber of reheating and the temperature of the gases increases by the action of the supplementary combustion In addition, the cooling air for the low pressure turbine (LPT) can be extracted from the intermediate stages of the compressor where the pressure is higher 15 / 25

16 compared to the inlet pressure in the low pressure turbine (Sheikhbeigi & Ghofrani, 2007) If these assumptions are met, the performance parameters in the turbines could be improved by reducing compressor input work, and decreasing the cooling air temperature for LPT (Dahlquist, 2016) This study describes the simple cycles, with overheating and with reheating and a recuperator, where all the variables that take place in cycles of real and practical gas turbines are involved, and also, after the modeling, a comparative analysis is carried out between them Development A simple cycle is characterized by the direct extraction of cooling air from the compressor, in addition there is no recirculation of currents in any of the stages (See Figure 3) Figure 3: Flow diagram of the simple Cycle Source: Sheikhbeigi & Ghofrani, 2007 In the reheating cycle the gas coming from the first combustion chamber enters the high pressure turbine and expands partially; the gas enters the reheating combustion chamber where fuel is injected producing additional combustion, raising the temperature of the gas to the inlet of the second low pressure turbine where it expands to the atmospheric pressure value In this cycle the cooling air for high pressure turbines is extracted from the compressor output and for low pressure turbines are taken from the intermediate stage of the compressor (See Figure 4) 16 / 25

17 Figure 4: Flow diagram of the reheat gas turbine cycle Source: Sheikhbeigi & Ghofrani, 2007 On the other hand, a heat exchanger is added in the Cycle with reheat and recuperator in order to increase the temperature of the air prior to its entry into the first combustion chamber, where heat is recovered The extraction of air for the cooling process is similar to that previously described (See figure 5) Figure 5: Flow diagram of Reheat, and recuperator cycle Source: Sheikhbeigi & Ghofrani, 2007 Compressor analysis With calculations referring to the first law of thermodynamics, knowledge of the air temperature at the compressor outlet and the location of the extraction for cooling with air, it is possible to quantify the efficiency and the work consumed (See equation 1) The entropy change for a kilogram of air entering the compressor is described below (El-Masri, 1986) Normal 0 21 false false false ES X-NONE X-NONE Normal 0 21 false false false ES X-NONE X-NONE 17 / 25

18 Normal 0 21 false false false ES X-NONE X-NONE Normal 0 21 false false false ES X-NONE X-NONE Normal 0 21 false false false ES X-NONE X-NONE (1) Applying the Polytrophic efficiency concept, the final outlet temperature will be given by (See equation 2): Normal 0 21 false false false ES X-NONE X-NONE (2) Combustion chamber analysis The combustion reaction could be described by the general formula of C x H y (See equation 3) for hydrocarbon fuel and a theoretical air coefficient A (Sheikhbeigi & Ghofrani, 2007) 18 / 25

19 (3) It is an adiabatic process, it is possible to use the first law of thermodynamics (see equation 4): (4) The equation (5) could calculate the theoretical air coefficient and the fuel-air ratio (5) Turbine analysis First, the not cooled turbine is analyzed, where the refrigerant is not extracted from the compressor; the efficiency of the component is given by the outlet temperature, and the inlet pressure can be calculated according to equation (6), which is evaluated from the pressure drop in the combustion chamber to the resulting pressure of the compressor (Ranjan & Mohammad, 2014) (6) The produced work in the turbine comes from (see equation 7): (7) 19 / 25

20 The behavior of the cooled turbine is described in Bolland's (2004) research The study describes the expansion process as a continuous event divided into several subscales with poor pressure relationship (Santos & Andrade, 2012) For the present study it is irrelevant the trajectory that will follow the expansion, the focus is on the mass of refrigerant that is given by (see equation 8): (8) (9) In equation 8, mc represents the mass flow of the refrigerant, Mg is the gas mass flow, C pc and C pg are the specific heat of the refrigerant and the gas respectively dt g is the variation of inlet and outlet temperatures, T b is the blade temperature, and T c is the blade temperature upon contact with the refrigerant (Consonni, 1998) In Equation 9, St is the Staton number whose equivalence is equal to 0005 The ratio () has the value of 10 and the value of C is between 03 and 05 In the present review the values of σ for high technology turbines are equal to and for others , while the value of ε is equal to 03 for convection cooling and 05 for film cooling (Katsavou, 1995) Reheat combustion chamber analysis The equation describing the combustion in the reheating chamber is: 20 / 25

21 (10) Where x represents the fuel burned for each mole of fuel consumed in the primary combustion chamber, this parameter in conjunction with the air-fuel ratio can be found by performing the energy balance in the reheating combustion chamber (Sheikhbeigi & Ghofrani, 2007) Recuperator analysis In the heat exchanger, the gas flowing from the low pressure turbine is used to heat the air entering the first combustion chamber, reducing the fuel consumption by the combustion chamber The efficiency of the recuperator, considering air as a minimum fluid is given by (see figure 11): (11) Also, the energy balance for the recuperator analysis is: (12) Where T2, and T6 can be obtained from low pressure turbine models or from the models of compressors, while Ta and Tb can be quantified from the predecessor equations 21 / 25

22 Results The results presented below are taken from the research carried out by Sheikhbeigi & Ghofrani, (2007), where the efficiency increase is analyzed according to variables such as: temperature, pressure ratio, and work produced mainly Point of implementation of overheating Adequate values were sought with the priority of improving efficiency within the gas turbine cycles The results were extracted from graphs according to the efficiency, the relationship between the low and high pressure turbine, the turbine inlet temperature (TIT), the compressor pressure ratio (c, rc), and the ratio of Pressure between high and low pressure turbines e: (rc = rhpt rlpt) For the analysis, diagrams were used to identify the key parameters for cycle efficiency In Figure 6 the thermal performance of the Reheat cycle for various TITs and C, rc is evaluated The figure identifies that the efficiency peaks are in function of the increase of the pressure in the high pressure turbines, the recommended values are In addition it is noted that maximum efficiencies are reached as the turbine inlet temperatures (TIT) increase, having an efficiency of 50% when TIT = 1400 C, and the compressor pressure ratio is equal to 50 Figure 6: Efficiency versus e for different TIT and pressure ratios values Source Sheikhbeigi & Ghofrani, 2014, Design of cycles with overheating and with Reheating and Overheating with recuperator for maximum efficiency 22 / 25

23 If it is considered a priority to have high efficiency values, it is advisable to design a cycle with overheating with recovery at low pressures (RHC), and with TIT = 1000 C and r = 5, with these parameters the efficiency achieved will be around 43%, with a Work of 252kJ for each kilogram of fuel However, it is important to emphasize that the cycles cannot be chosen only to present advantages in efficiency, an economic evaluation must be made and the production system for which the selected turbine will work (See figure 7) Figure 7: Efficiency for the RHC Source: Sheikhbeigi & Ghofrani, 2014, Environmental considerations regarding the design of SC, RC, RHC To know the environmental benefits it is necessary to identify that the reduction of fuel, which in this case is the raw material represents a decrease in the burning of fuels, therefore the pollutant emissions will be reduced In the present study the behavior of the NOx pollutant against the adaptations to improve the efficiency was evaluated, the results were that the NOx will have between 15 to 25 parts per million less in the emissions, representing a rate of 15 to 20% in The production of this contaminating element Conclusions It can be concluded that the work compiles information on the cycles pertinent to the gas turbines, where it explores adaptations that improve the performance of the same, where they were related several factors that give greater confidence to the study Among the most important results achieved is that the correct design of a reheat; Or a reheat with a collector for a gas turbine would increase the efficiency by 43%, this increase of efficiency allows to obtain better economic benefits since the investment in fuel for the operation of the turbine will be smaller, in addition there are economic benefits since there are reductions In the emission of nitrogen compounds between 15% and 20% The application of these cycles turns out to be one of the most viable alternatives because it 23 / 25

24 produces a less environmental impact Also, it use a cleaner fuel preserving so the nonrenewable resources and the reduction of the gas emission References Consonni, S (1998) Gas turbine cycles performance evaluation In Proceings of ASME Cogen Turbo Power ASME Dahlquist, A (2016) Conceptual Thermodynamic Cycle and Aerodynamic Gas Turbine Desing Sweden: Lund UNIVERSITY El-Masri, M A (1986) On thermodynamics of gas turbine cycles Part 2: a model for expansion in cooled turbines J Eng Gas Turb, 108 Langston, L S, & Opdyle, G J (1997) Introduction to Gas Turbines for Non-Engineers Glo bal Gas Turbine News Malaver, M (2014) Optimización del trabajo en un ciclo Brayton con Irreversibilidades Ingeni ería, Meherwan, B P (2002) Gas Turbine Engineering Hnadbook Woburn: Butterworth - Heinemann Pathirahna, K (2013) GAS TURBINE THERMODYNAMIC AND PERFORMANCE ANALYSIS METHODS USING AVAILABLE CATALOG DATA Gavle: University of Galve 24 / 25

25 Santos, P, & Andrade, C (2012) Analysis of Gas Turbine Performance with Inlet Air Cooling Techniques Applied to Brazilian Sites JATM, Sheikhbeigi, B, & Ghofrani, M (2007) Thermodynamic and environmental consideration of advanced gas turbine cycles with reheat and recuperator Int J Environ Sci Tech, Soares, C M (2008) Gas turbines in simple Cycle & Combined Cycle applications United States: US DEPARTMENT OF ENERGY 25 / 25