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Energy, exergy and parametric analysis of a combined cycle power plant
Journal Pre-proofs Energy, Exergy and Parametric Analysis of a Combined Cycle Power Plant Mansur Aliyu, Ahmad B. AlQudaihi, Syed A.M. Said, Mohamed A. Habib PII: DOI: Reference:
S2451-9049(19)30255-0 https://doi.org/10.1016/j.tsep.2019.100450 TSEP 100450
To appear in:
Thermal Science and Engineering Progress
Received Date: Revised Date: Accepted Date:
24 June 2019 6 November 2019 8 November 2019
Please cite this article as: M. Aliyu, A.B. AlQudaihi, S.A.M. Said, M.A. Habib, Energy, Exergy and Parametric Analysis of a Combined Cycle Power Plant, Thermal Science and Engineering Progress (2019), doi: https://doi.org/ 10.1016/j.tsep.2019.100450
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Energy, Exergy and Parametric Analysis of a Combined Cycle Power Plant Mansur Aliyu, Ahmad B. AlQudaihi, Syed A. M. Said, Mohamed A. Habib Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, 31261 Dhahran, Saudi Arabia.
Abstract This paper presents the thermodynamics (energy and exergy) analysis of a power plant using the design data. The plant is a triple pressure combined cycle power plant (CCPP) equipped with reheat facilities. The temperature gradient as well as the exergy destruction were determined across each components of the heat recovery steam generator (HRSG). Among the components of the HRSG high-pressure evaporator experienced a large temperature gradient which accounted for high irreversibility while intermediate-pressure superheater experienced lowtemperature change and therefore low irreversibility. Exergy analysis showed that the major source of irreversibility (exergy destruction) in the steam turbine cycle (STC) of the CCPP is the stack followed by the HRSG, turbine, and condenser. The exergetic efficiency of the turbine is the highest in the STC with more than 92% while the exergetic efficiency of the condenser was the lowest one with less than 63%. Parametric analyses were conducted where the effects of some operating parameters on the turbine output, efficiencies, and exergy destruction were investigated. The results indicated that superheat pressure, reheat pressure, and steam quality at the exit of the low-pressure steam turbine significantly affect the output of the turbine and efficiencies. Keywords: Combined cycle power plant, Triple pressure, Energy, Exergy, Efficiency, Parametric analyses Corresponding author: Syed A. Said, King Fahd University of Petroleum and Minerals, e-mail: [email protected], Tel: 0096638601559
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Nomenclature AMSL CCPP h HRSG ๐ธ ๐ธ๐ฅ ex LP ๐ MSF s STC STG ๐บ๐ธ GT GTG ๐ T ๐ x
Above mean sea level Combined cycle power plant Specific fluid enthalpy Heat recovery steam generator Rate of energy Rate of exergy Specific exergy Low pressure Mass flow rate Multi-stage flashing Specific entropy Steam turbine cycle Steam turbine generator Excess free Gibbs energy Gas turbine Gas turbine generators Rate of heat input to the system Temperature Rate of work produced by the system Mole fraction
Subscript chem D exit i II in mix o phy Q
Chemical Destroyed Exit Component in the mixture Second law Inlet Mixture Reference (environment) condition Physical Heat
Symbol ๐
Efficiency
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Introduction Attention has been given to combined cycle power plants (CCPP) recently because of their improved thermal efficiency, power output and less emission as compared to the individual gas turbine (Brayton) or steam turbine (Rankine) cycles. A combined cycle (CC) is a wellestablished technology which has been commonly adopted in power plants where fossil fuels are used [1]. Thermodynamically, a combined cycle power plant (CCPP) is a combination of hightemperature Brayton cycle and moderate to low temperature Rankine cycle. Such a combination becomes necessary when there is high demand for power generation as compared to the heat demand [2]. The CC technology basically consists of gas and steam turbines with heat recovery steam generators (HRSGs) lie within them [1,2]. Heat recovery steam generator (HRSG) is a device that utilizes gas turbine exhaust gases to produce steam [3]. More than a single pressure level need to be considered in order to enhance the HRSG heat recovery process and increase the CCPP efficiency[4,5]. The net power is obtained from both; the gas turbine ( topping cycle) and the steam turbine ( bottoming cycle) with the former supplying 65-70% of the network [6]. Combined cycles are flexible and stable for range of loads. Their benefits include low capital costs, low environmental impact, high efficiency (50-60%) [1,2,7] and short construction period compared to other fossil fuel power plants. The only shortcoming of this technology is the complexity in combining the Brayton and Rankine technologies in a single plant [2]. Combined cycle power plants with high capacity exist in several regions of Saudi Arabia to meet electrical energy demand. These plants include Rabigh-2 with an installed capacity of 2100 MW located around the southern coast of the Arabian Gulf, Qurayyah with an installed capacity of 4000 MW and Riyadh with an installed capacity of around 2000 MW. As a result of CC benefits, accurate design and analysis of such plants are very essential due to the increase in population which leads to more energy (power) demand. One of the major challenges facing the designers and engineers is how to efficiently utilize the temperature of the exhaust gases of the gas topping cycle in the steam bottoming cycle to optimize the output of the steam turbine. A quite number of researchers and texts [8] had proved it beyond doubt that the key tool used in designing, analyzing and improving the performance of power plants is exergy analysis. Exergy which is also referred to as availability is the highest amount of useful work that could be acquired from a given system in a specified environment at a given state [9]. The major objective of the exergy analysis is to significantly discover and assess the thermodynamic inefficiencies of the process in Page 3 of 27
question. Some of the notable work include Pattanayak et al. [10] that analyzed the effects of some design parameters on the performance of the combined cycle power plant (CCPP) equipped with reheat using energy and exergy analysis. They reported that as the compressor air inlet temperature, inlet and exhaust pressure loss increase there is a decrease in the CCPP efficiency and that the total efficiency of the CCPP can be enhanced by decreasing the bottoming cycle losses. Abuelnuor et al. [11] evaluated the exergy destruction and exergetic efficiency of Garri โ2โ combined cycle power plant located in Sudan. They reported that 63% of the total exergy destruction is from the combustion chamber followed by gas turbines, steam turbines, HRSGs, stacks, compressors, and cooling systems at 13.6%, 6.4%, 6.3%, 4.7%, 3.8%, and 2.3% respectively. They also reported that the exergetic and thermal efficiencies of the system are 49% and 38% respectively. Tiwari et al. [12] presented exergy analysis of a dual pressure combined cycle power plant (CCPP) located in the Dadri area of India. The components of the CCPP considered for the analysis include; GT unit, HRSG without supplementary firing, and a steam turbine unit. They reported that about 35% of the total exergy destruction occurred in the combustion chamber of the gas turbine. Adibhatla and Kaushik [13] performed exergy, energy, and economic analyses of a solar energy integrated (at intermediate temperature level) natural gas-fired dual pressure combined cycle power plant (CCPP). They reported that the solar field exergy and energy efficiencies are 27.39% and 53.79% respectively with an overall increase of 7.84% in the plant power output. Kotowicz and Brzeczek [14] presented the cooling effect of coolant (air) in the Rankine cycle of a triple pressure combined cycle power plant (CCPP). They reported that the net efficiency of a CCPP can reach 65% by using known industrial methods of cooling such as sequential combustion and steam cooling. Kilani et al. [15] studied the effect of methods used in steam generation for steam injection in the combustion chamber and the ambient temperature on the overall efficiency of the combined power plant. They reported that generating the steam outside the HRSG increase the overall efficiency of the combined circle up to about 6% and that the ambient temperature has a significant effect on the overall efficiency of the system. Kaviri et al. [16] studied the effect of HRSG inlet gas temperature on the HRSG exergy destruction and combined cycle efficiency. They reported that as the temperature of the inlet gas to the HRSG is increased the first and the second law efficiencies increase until the temperature reaches 650 oC after which the performance reduces. Also, the exergy analysis revealed that the high-pressure evaporator and high-pressure superheater 2 have the highest Page 4 of 27
amount of exergy destruction. Sharma and Singh [17] revealed that the main causes of irreversibility in a dual pressure HRSG are high and low-pressure superheaters and high-pressure evaporator. Ahmadi and Dincer [18] optimized the design parameters (isentropic efficiencies of compressor and gas turbine, inlet temperatures of combustion chamber and turbine, and compressor pressure ratio) of a combined power and heat plant with the intentions of producing 50 MW and 33.3 kg/s worth of electricity and saturated steam at 13 bar respectively through genetic algorithm. They reported that as the net power output is increasing the design parameters values also increases for a precise unit cost of fuel. Khana and Tlilib [19] conducted a parametric analysis to enhance the performance of combined cycle power plant which includes the bypass valve. They reported that when the turbine inlet temperature of the topping cycle rises from 1000 K to 1400 K there is a gain in the output network of about 45% and the gain in the net cycle efficiency from 15% to 31%. They also recommended that for the high and small value of compression ratio the bypass valve should remain closed and opened respectively. Ahmadi and Dincer [20] analyzed a combined cycle power plant equipped with an auxiliary firing system thermodynamically using exergy and energy analysis after which optimization was carried out using a generic algorithm by identifying an objective function. They reported that the deviation of the results of the model developed from the actual data is around 1.41%. Ganjehkaviri et al. [21] developed inclusive thermodynamic modeling and then performed an extensive optimization to come up with the optimum design parameters of a CCPP. They reported that with the optimum variable, the second-law efficiency is increased by about 6% while 5.63% reduction in CO2 emission was recorded. Regarding the retrofitting of CCPP with fuel cells, Thattai et al. [22] reported that an existing integrated gasification combined cycle power plants can be successfully operated with comparatively high electrical efficiencies of more than 40% (lower heating value) and without major modifications when retrofitted with solid oxide fuel cells and partial CO2 capture (using oxy-combustion technique). To achieve full-scale CO2 capture, they stated that the main process will require major modification which might call for the redesign of some components such as HRSG and gas turbine unit. Promes et al. [23] developed a model to investigate the performance of the steady-state operation of the integrated gasification combined cycle power plant using the Page 5 of 27
parameters of an existing plant. They reported that the temperature, flow rates, efficiency (42%), and pressure were accurately predicted by the model and that the rate of exergy destruction is more in the gas turbine combustion chamber [7] and the gasifier. In an effort to study the effect of vapor quality at the turbine exit, Ganjehkaviri et al. [24] reported from their modeling result that with respect to the economic, efficiency, and environmental, a system with the vapor quality of 88% at the turbine exit is more realistic. The literature on combined cycles indicates that one of the major challenges is the losses encountered in the water-steam bottoming cycle. Hence, it is important to identify the components of the cycle that are contributing to these losses as well as the magnitude of such losses in order to improve overall efficiency of the combined cycle. Exergy analysis is an effective way in identifying the location and magnitude of irreversibility and potential for improvement in a thermal system. Therefore, utilizing such a tool with plant actual field data; do help in addressing the major challenge facing the CCPP. The objective of this study is to perform detailed energy, exergy and parametric analyses of a triple pressure combined cycle power plant by given consideration to the exhaust gasses of the gas turbine used in the Rankine cycle. The results of such study will provide useful information and guidelines to power plant designers, engineers and operators.
Description of the Power Plant The power plant is one of the largest combined cycle power plants comprising of triple pressure steam turbine cycle (STC) and heat recovery steam generators (HRSG) with facilities for reheat located in the eastern province of Saudi Arabia. The plant consists of 5 units and each unit comprises of 3 gas turbine generators (GTG), 3 heat recovery steam generators (HRSG), and 1 steam turbine generator (STG) as shown in Fig. 1. The exhaust gas of the gas turbines (GT) is utilized in HRSG to generate steam used in driving STG. The layout of the plant showing 1 of the 3 GTG and HRSG is shown in Fig. 2. In addition to power generation, the plant also supplies steam to 2 multi-stage flashing (MSF) water desalination plants which is an integral part of the plant base on the requirement. The plant runs on natural gas with 43,196 kJ/kg heating value and the fuel flow rate per GT is 10.05 kg/sec. The exhaust of the gas turbine of the mass flow rate of Page 6 of 27
418.0 kg/sec and temperature of 617.0 oC is fed into the HRSG to generate the steam that runs the STG. The gross electrical output of each of the gas and steam turbines are 126 and 263.6 MW respectively while the net electrical output of the 5 STG is approximately 1.24 GW. The plant is 2.00 m above mean sea level (AMSL) while the average ambient dry bulb temperature and relative humidity are 33.0 oC and 70% respectively. A continuous power generation is achieved by each unit through a 19.6 m3/sec (19462 kg/sec) of cooling water at an average temperature of 38.0 oC from a nearby sea which is ejected back to the sea at a temperature of 43.6 oC.
Figure 1: Schematic of a single unit.
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Figure 2: Layout showing a typical part of a unit.
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Energy Analysis Basically, the conservation of mass and energy (i.e. first law of thermodynamics) is used in analyzing the energy of a system [25]. To perform the energy analysis of a given system through which energy, mass, and work cross the boundaries of the systemโs components such conservation laws need to be applied. Under a steady-state condition and considerable losses, the system is considered as a control volume and the conservation equations 1 and 2 (i.e. mass and energy balance respectively) were applied to the components of the system. โ๐๐๐ = โ๐๐๐๐๐ -
-
-
-
-
-
-
-
-
- (1)
๐ธ+๐ฌโ๐พ=๐-
-
-
-
-
-
-
-
-
- (2)
Where ๐ธ is the rate of heat input to the system, ๐ฌ is the rate of flow energy which is given by equation 3, and ๐พ is the rate of work produced by the system. ๐ฌ = โ๐๐๐๐๐๐ โ โ๐๐๐๐๐๐๐๐๐๐ -
-
-
-
-
-
-
- (3)
Where h is the fluid enthalpy and subscript in and exit represent inlet and exit respectively. Exergy Analysis Exergy analysis is a technique that employs the first and second laws of thermodynamics in addition to the conservation of mass principle to design and analyze the energy systems [10,20,25]. Exergy analysis is essential in evaluating the thermodynamic losses of a given system [26]. Recognizing the components with major exergy destruction is the key step to exergy performance optimization [26,27]. Basically, exergy is divided into four components out of which two are given much attention and the other two are less important because in most cases they got neglected. The two important components of the exergy are chemical and physical exergies while the negligible components are kinetic and potential aspects of exergy. The later components are negligible because the changes in the altitude and speed of an energy system during a process are negligible compared to the changes in the chemical and physical states of the system. The chemical exergy is linked with the parting of the systemโs chemical composition from its chemical equilibrium, while the physical exergy is the maximum theoretical useful work obtained by the system as it interacts with an equilibrium state [10]. Using the thermodynamicsโ first and second law, the steady-state exergy balance equation in a rate form is; Page 9 of 27
๐ฌ๐๐ซ = ๐ฌ๐๐ธ + โ๐ฌ๐๐๐ โ โ๐ฌ๐๐๐๐ โ ๐พ -
-
-
-
-
-
-
- (4)
Where ๐ฌ๐๐๐ and ๐ฌ๐๐๐๐ denote the rate of flow exergy into and out of the control volume
[ (
respectively, ๐ฌ๐๐ซ is the rate of exergy destroyed, ๐ฌ๐๐ธ ๐.๐. ๐ โ
)๐ธ] is the rate of exergy
๐ป๐ ๐ป
transfer by heat (๐ธ) at a temperature ๐ป, and ๐พ is a useful work. The flow exergy consists of two integral parts as express below; ๐ฌ๐ = ๐ฌ๐๐๐๐ + ๐ฌ๐๐๐๐๐, ๐๐๐ - -
-
-
-
-
-
-
-
- (5)
Where ๐ฌ๐๐๐๐ and ๐ฌ๐๐๐๐๐ represent the physical and chemical exergy respectively. For a negligible change in altitude and speed, the two terms are further expressed as follows; ๐ฌ๐๐๐๐ = ๐โ(๐ โ ๐๐) โ ๐ป๐(๐ โ ๐๐)โ -
-
-
-
-
-
-
- (6)
๐ฌ๐๐๐๐๐, ๐๐๐ = ๐๐๐๐๐๐๐, ๐๐๐ Where ๐ is the mass flow rate, ๐ is specific enthalpy, s is specific entropy, and ๐๐๐๐๐๐, ๐๐๐ is specific a chemical exergy of the mixture on a molar basis which can be obtained from the expression given in equation (7) [28][29]. ๐
๐
๐๐๐๐๐๐, ๐๐๐ = โ๐ = ๐๐๐๐๐๐๐๐๐,๐ + ๐น๐ป๐โ๐ = ๐๐๐๐ฅ๐ง ๐๐ + ๐ฎ๐ฌ - -
-
-
-
- (7)
Where ๐๐, ๐๐๐๐๐๐,๐, ๐ป๐, and ๐ฎ๐ฌ are the mole fraction of the component i-th in the mixture, molar chemical exergy, reference (environment) temperature, and excess free Gibbs energy respectively. When the pressure of the gas mixture is low ๐บ๐ธ is negligible. The second law efficiency of the system is obtained as follows; ๐๐ผ๐ผ =
๐ธ๐ฅ๐๐๐๐ฆ ๐๐๐๐๐ฃ๐๐๐๐ ๐ธ๐ฅ๐๐๐๐ฆ ๐ ๐ข๐๐๐๐๐๐
๐ธ๐ฅ๐ท
= 1 โ ๐ธ๐ฅ ๐๐
-
-
-
-
-
-
-
- (8)
Results and Discussion The energy and exergy analysis of a CCPP equipped with triple pressure STC and HRSG with reheat were conducted using engineering equation solver (EES). The results presented in this section are typically for a single unit of the plant which comprises of 3 GT, 3 HRSG, 1 STG, and a condenser. The flue gas temperature at every stage of the HRSG was calculated using the Page 10 of 27
conservation of energy principle. Figure 3 shows the temperature variation along with the components of the HRSG which was validated with the stack designed temperature. The horizontal axis represents the HRSG components. The components located between the numbers are shown in Table 1.
Figure 3: Flue gas temperature variation along with the components of the HRSG.
Table 1: Components of the HRSG Numbering
Components of the HRSG
1-2
High-pressure superheater 3 (HP 8-9
High-pressure economizer 2 (HP
SPHT 3)
ECON 2)
Reheater superheater 2 (RHT SPHT 9-10
Intermediate-pressure
2)
(IP EVAP)
High-pressure superheater 2 (HP 10-11
Low-pressure
SPHT 2)
SPHT)
Reheater superheater 1 (RHT SPHT 11-12
Intermediate-pressure economizer
1)
(IP ECON)
High-pressure superheater 1 (HP 12-13
High-pressure economizer 1 (HP
2-3 3-4 4-5 5-6
Numbering
Components of the HRSG
evaporator
superheater
(LP
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SPHT 1) 6-7
High-pressure
ECON 1) evaporator
(HP 13-14
EVAP) 7-8
Intermediate-pressure
Low-pressure
evaporator
(LP
EVAP) superheater 14-15
Preheater
(IP SPHT)
It could be observed from Fig. 3 that the evaporator pinch which is the difference in temperature between the saturation temperature of the water flowing through the evaporator and the flue gas leaving the evaporator of high (6-7), intermediate (9-10), and low-pressure (13-14) evaporators are in the range of 8 to 14 K which are in agreement with the design limitations [2]. The highpressure evaporator experienced a large temperature gradient while a very low-temperature change occurs across the intermediate-pressure superheater (7-8), low-pressure superheater (1011), and intermediate-pressure economizer (11-12). In the case of high-pressure evaporator large amount of heat (energy) is required to transform the high flow rate feed water from saturation liquid to saturation vapor which resulted in a large temperature drop of the flue gas. In the cases of intermediate and low-pressure components, the feedwater flow rate is lower in addition to the fact that less amount of energy is required to superheat a vapor and economizer basically preheat the feed water. The ratios of the mass flow rate of the steam generated through the high, intermediate, and low-pressure equipment of the HRSG are 8.03:1.14:1.00 at a pressure ratio of 20.85:5.94:1 respectively. Figures 4 and 5 show the exergy destruction rate of the components of the HRSG and STC respectively. It could be observed from Fig. 4 that HP evaporator has the largest exergy destruction rate of about 27% of the total value follow by preheater then HP superheaters. This is contrary to what was reported for the case of single-pressure CCPP [11]. The variation in the exergy destruction rate of the HRSG components may not necessarily be attributed to inefficient of the facilities but rather to the amount of heat transfer processes taking place in these facilities. The high exergy destruction rate associated with the HP evaporator was due to the high level of energy exchange taking place in this equipment as explained with the high-temperature gradient across the component in Fig. 3. Therefore, looking at Fig. 3 closely, a clear understanding of the relationship between the exergy destruction rate and the temperature gradient (mainly energy transfer) could be deduced. Page 12 of 27
Figure 4: Exergy destruction rate of the HRSG components.
From Fig. 5, stack appeared to be the major source of exergy destruction with almost 60% of the total value in the steam turbine cycle (STC) follow by the HRSG, turbine, and condenser. The large value of exergy destruction rate in the stack is as a result of the high level of irreversibility due to the flue gas leaving the stack with a substantial amount of energy. In addition to the irreversibility, the exergy destruction rate in the turbine is also due to the mechanical losses associated with the energy transformation.
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Figure 5: Exergy destruction rate of the STC components.
Figure 6 shows the second law efficiency of the turbine and HRSG of the STC. It could be seen from the figure that the turbine is having the highest second law efficiency; this is expected according to equation (8) because the exergy destruction rate of the turbine is lower than that of the HRSG. 95
90.86
92.05
Efficiency (%)
90 85 80 75 70 62.98
65 60 HRSG
Turbine
Condenser
Components of the steam turbine cycle
Figure 6: Second law efficiency of the components of the STC.
The results obtained from this study are compared with what is available in the literature. Figure 7 shows the comparison between the results obtained from this study with that of the Pattanayak Page 14 of 27
et al. [10] who presented the energy and exergy analysis of triple pressure CCPP. The rate of the exergy destruction per unit output of the steam turbine is presented in Fig. 7 for the condenser and steam turbine. In either case, the results obtained from the current study are a bit lower than what was reported by Pattanayak et al. [10]. There are 7.7% and 1.7% differences in the condenser and steam turbine exergy destruction rate per ST output respectively between the current study and that of the Pattanayak et al. [2]. The differences in the results obtained may be attributed to several factors. There are significant differences in operating parameters of the two plants since the power produced by the two plants are not the same. For instance, the pressure of the steam exiting the low-pressure turbine is 0.09 bars in the case of Pattanayak et al. [2] while it is 0.105 bar in the present study. The condensing pressure and the inlet temperature of the cooling water in the case of Pattanayak et al. [2] are 0.089 bars and 33 oC while 0.11 bar and 38 oC
respectively were considered in the current study, therefore, there will be a distinction in the
heat transfer rate of the two studies. Also, the ratio of steam to cooling water flow rates is 0.010 and 0.013 for the current study and Pattanayak et al. [2] respectively with the current study being 13% lesser. This may likely explain the 7.7% reduction in the exergy destruction rate per ST output in the condenser of the current study as shown in Fig. 7.
Figure 7: Comparison of the exergy destruction rate per ST output obtained from this study with Pattanayak et al. [2].
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Parametric studies were conducted in which the effects of variations in some operating parameters on the efficiencies and output of the steam turbine were considered. Figures 8 โ 10 show the effect of variation in the quality of steam leaving the low-pressure (LP) turbine on the turbine gross power, exergy destruction, efficiencies, exergy destruction of the condenser, and the cooling water exit temperature. Figure 8 shows how steam turbine gross power and exergy destruction varies with steam quality. As the quality of the steam leaving LP turbine increases the expansion of the steam is being restricted thereby leading to a decrease in the output of the steam turbine.
Figure 8: Effects of variation in steam quality on the turbine gross power and exergy destruction.
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Figure 9: Effects of variation in steam quality on the turbine first and second law efficiencies.
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Figure 10: Effects of variation in steam quality on the exergy destruction rate of the condenser and the cooling water exit temperature.
The gross power of the steam turbine can be increased by lowering the steam quality below the saturated value, however, lower the steam quality increases the moisture content of the steam which may lead to the corrosion of the turbine blade and eventually threatening the turbineโs life. Hence, a good balance needs to be maintained between the gross power and steam quality. On the other hand, the loss of potential to do work increases with an increase in the steam quality, this is expected since the gross power of the turbine decreases with an increase in the steam quality. Restricting the expansion in the LP turbine by exiting the steam at a saturated state (i.e. x = 1) increases the irreversibility of the processes thereby, given rise to the exergy destruction rate. Since the turbine energetic (first law) and exergetic (second law) efficiencies are directly related to the turbine gross power and the exergy destruction rate of the turbine the variation in Fig. 9 can be clearly understood. Therefore, the lower the restriction in the steam expansion at the LP turbine end the higher the efficiencies. Figure 10 shows the variations in the exergy destruction rate of the condenser and the cooling water exit temperature as the steam quality changes. As the quality of the steam exiting the LP turbine increases the rate of heat transfer process taking place in the condenser also increase which is evidenced by the increase in the
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cooling water outlet temperature. The increase in the condenser exergy destruction rate is as a result of an increase in heat transfer rate associated with an increase in steam quality. Tripping of the cooling water pump(s) is a phenomenon normally experience in some power plants. This phenomenon leads to the shortage or reduction in the availability of water needed for the steam condensation. Figure 11 shows how the cooling water flow rate varies with the exergy destruction rate of the condenser and cooling water exit temperature. When the cooling water flow rate is not enough the exergy destruction rate is low meaning less heat transfer and the exit cooling water temperature is high. An increase in exit cooling water temperature beyond the required limit is a strong indication of improper condensation and violation of environmental regulations by threatening the life of aquatic animals. It is important to note that more than 15% decrease in the required cooling water flow rate leads to about 1 oC increase above the required/designed temperature of the cooling water ejected back to the sea.
Figure 11: Effects of variation in cooling water flow rate on the exergy destruction rate of the condenser and the cooling water exit temperature.
Since the inlet turbine pressure is one of the sensitive parameters in Rankine cycle [30], Figs. 12 and 13 show the effect of superheat pressure while Figs. 14 and 15 show the effect of reheat pressure on the turbine output, exergy destruction, and turbine efficiencies. From Figs. 12 and Page 19 of 27
14, it could be observed that increasing the superheat and reheat pressures decrease the power produced by the turbine by increasing the lost work potential. At lower pressures, the turbine output power is high due to a high level of expansion that will be taking place in the first (high pressure) and second (intermediate pressure) stages turbines. The life of the turbine blade may be the likely price to be paid when the superheat and reheat pressures were lowered beyond a reasonable value. Figures 13 and 15 show that the efficiencies of the turbine decrease by increasing either the superheat or reheat pressure. The first and second law efficiencies are the function of energy and exergy respectively which implies that a decrease in gross power and an increase in the irreversibility of the process lead to a decrease in the efficiencies of the system.
Figure 12: Effects of variation in superheat pressure on the turbine gross power and exergy destruction.
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Figure 13: Effects of variation in superheat pressure on the turbine first and second law efficiencies.
Figure 14: Effects of variation in reheat pressure on the turbine gross power and exergy destruction.
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Figure 15: Effects of variation in reheat pressure on the turbine first and second law efficiencies.
Conclusion The current study presents detailed energy and exergy analysis of a Rankine cycle of a triple pressure combined cycle power plant (CCPP) using the design data. The effects of several operating parameters on the turbine output, efficiencies, and exergy destruction were investigated. The results obtained were validated against published data. The study results indicated that: a.
The components that have a great potential for improvement include; the stack, the high-
pressure evaporator, preheater and the high-pressure superheater of the HRSG. Therefore, more attention needs to be given to these components for improving their efficiencies and hence the overall efficiency of the combined cycle. b.
The exergetic efficiency of the turbine is the highest in the steam turbine cycle (STC)
c.
More than 15% decrease in the required cooling water flow rate leads to about 1 oC rise in
the temperature of the ejected cooling water Page 22 of 27
d.
The superheat pressure, reheat pressure and steam quality at the exit of the low-pressure
steam turbine significantly affect the output of the turbine and efficiencies. e.
The study results will provide useful information and guidelines to power plant engineers
and operators; such as choosing possible performance enhancement modifications to gas-fired combined cycle power plants. f.
It is also recommended to investigate the potential of utilization of exhaust flue gas to run
more than one energy device.
Acknowledgment The authors would like to acknowledge the support of the King Fahd University of Petroleum and Minerals (KFUPM) through Project No DUP18101 in carrying out this study.
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Date: June 24/2019 To: The Editor, Thermal Science and Engineering Progress Subject: Conflicts of interest statement
Page 26 of 27
Manuscript title: Energy, Exergy and Parametric Analysis of a Combined Cycle Power Plant Authors: Mansur Aliyu, Ahmad B. AlQudaihi, Syed A. M. Said, Mohamed A. Habib.
This is to inform you that, all authors whose names are listed above and in the manuscript certify that have NO affiliations or involvement in any organization or entity with any financial interest in the subject matter or materials discussed in the manuscript.
Best Regards Syed A.M. Said Corresponding Author
Energy, Exergy and Parametric Analysis of a Combined Cycle Power Plant
Highlights 1. 2. 3. 4. 5.
Energy and exergy analyses of the components of combined cycle power plant. Exergetic efficiency of the components of combined cycle power plant. Computation of temperature gradient across the heat recovery steam generator (HRSG). Exergy analysis of every component of HRSG. Parametric analyses of some operating parameters were investigated.
Page 27 of 27
S2451-9049(19)30255-0 https://doi.org/10.1016/j.tsep.2019.100450 TSEP 100450
To appear in:
Thermal Science and Engineering Progress
Received Date: Revised Date: Accepted Date:
24 June 2019 6 November 2019 8 November 2019
Please cite this article as: M. Aliyu, A.B. AlQudaihi, S.A.M. Said, M.A. Habib, Energy, Exergy and Parametric Analysis of a Combined Cycle Power Plant, Thermal Science and Engineering Progress (2019), doi: https://doi.org/ 10.1016/j.tsep.2019.100450
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ยฉ 2019 Elsevier Ltd. All rights reserved.
Energy, Exergy and Parametric Analysis of a Combined Cycle Power Plant Mansur Aliyu, Ahmad B. AlQudaihi, Syed A. M. Said, Mohamed A. Habib Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, 31261 Dhahran, Saudi Arabia.
Abstract This paper presents the thermodynamics (energy and exergy) analysis of a power plant using the design data. The plant is a triple pressure combined cycle power plant (CCPP) equipped with reheat facilities. The temperature gradient as well as the exergy destruction were determined across each components of the heat recovery steam generator (HRSG). Among the components of the HRSG high-pressure evaporator experienced a large temperature gradient which accounted for high irreversibility while intermediate-pressure superheater experienced lowtemperature change and therefore low irreversibility. Exergy analysis showed that the major source of irreversibility (exergy destruction) in the steam turbine cycle (STC) of the CCPP is the stack followed by the HRSG, turbine, and condenser. The exergetic efficiency of the turbine is the highest in the STC with more than 92% while the exergetic efficiency of the condenser was the lowest one with less than 63%. Parametric analyses were conducted where the effects of some operating parameters on the turbine output, efficiencies, and exergy destruction were investigated. The results indicated that superheat pressure, reheat pressure, and steam quality at the exit of the low-pressure steam turbine significantly affect the output of the turbine and efficiencies. Keywords: Combined cycle power plant, Triple pressure, Energy, Exergy, Efficiency, Parametric analyses Corresponding author: Syed A. Said, King Fahd University of Petroleum and Minerals, e-mail: [email protected], Tel: 0096638601559
Page 1 of 27
Nomenclature AMSL CCPP h HRSG ๐ธ ๐ธ๐ฅ ex LP ๐ MSF s STC STG ๐บ๐ธ GT GTG ๐ T ๐ x
Above mean sea level Combined cycle power plant Specific fluid enthalpy Heat recovery steam generator Rate of energy Rate of exergy Specific exergy Low pressure Mass flow rate Multi-stage flashing Specific entropy Steam turbine cycle Steam turbine generator Excess free Gibbs energy Gas turbine Gas turbine generators Rate of heat input to the system Temperature Rate of work produced by the system Mole fraction
Subscript chem D exit i II in mix o phy Q
Chemical Destroyed Exit Component in the mixture Second law Inlet Mixture Reference (environment) condition Physical Heat
Symbol ๐
Efficiency
Page 2 of 27
Introduction Attention has been given to combined cycle power plants (CCPP) recently because of their improved thermal efficiency, power output and less emission as compared to the individual gas turbine (Brayton) or steam turbine (Rankine) cycles. A combined cycle (CC) is a wellestablished technology which has been commonly adopted in power plants where fossil fuels are used [1]. Thermodynamically, a combined cycle power plant (CCPP) is a combination of hightemperature Brayton cycle and moderate to low temperature Rankine cycle. Such a combination becomes necessary when there is high demand for power generation as compared to the heat demand [2]. The CC technology basically consists of gas and steam turbines with heat recovery steam generators (HRSGs) lie within them [1,2]. Heat recovery steam generator (HRSG) is a device that utilizes gas turbine exhaust gases to produce steam [3]. More than a single pressure level need to be considered in order to enhance the HRSG heat recovery process and increase the CCPP efficiency[4,5]. The net power is obtained from both; the gas turbine ( topping cycle) and the steam turbine ( bottoming cycle) with the former supplying 65-70% of the network [6]. Combined cycles are flexible and stable for range of loads. Their benefits include low capital costs, low environmental impact, high efficiency (50-60%) [1,2,7] and short construction period compared to other fossil fuel power plants. The only shortcoming of this technology is the complexity in combining the Brayton and Rankine technologies in a single plant [2]. Combined cycle power plants with high capacity exist in several regions of Saudi Arabia to meet electrical energy demand. These plants include Rabigh-2 with an installed capacity of 2100 MW located around the southern coast of the Arabian Gulf, Qurayyah with an installed capacity of 4000 MW and Riyadh with an installed capacity of around 2000 MW. As a result of CC benefits, accurate design and analysis of such plants are very essential due to the increase in population which leads to more energy (power) demand. One of the major challenges facing the designers and engineers is how to efficiently utilize the temperature of the exhaust gases of the gas topping cycle in the steam bottoming cycle to optimize the output of the steam turbine. A quite number of researchers and texts [8] had proved it beyond doubt that the key tool used in designing, analyzing and improving the performance of power plants is exergy analysis. Exergy which is also referred to as availability is the highest amount of useful work that could be acquired from a given system in a specified environment at a given state [9]. The major objective of the exergy analysis is to significantly discover and assess the thermodynamic inefficiencies of the process in Page 3 of 27
question. Some of the notable work include Pattanayak et al. [10] that analyzed the effects of some design parameters on the performance of the combined cycle power plant (CCPP) equipped with reheat using energy and exergy analysis. They reported that as the compressor air inlet temperature, inlet and exhaust pressure loss increase there is a decrease in the CCPP efficiency and that the total efficiency of the CCPP can be enhanced by decreasing the bottoming cycle losses. Abuelnuor et al. [11] evaluated the exergy destruction and exergetic efficiency of Garri โ2โ combined cycle power plant located in Sudan. They reported that 63% of the total exergy destruction is from the combustion chamber followed by gas turbines, steam turbines, HRSGs, stacks, compressors, and cooling systems at 13.6%, 6.4%, 6.3%, 4.7%, 3.8%, and 2.3% respectively. They also reported that the exergetic and thermal efficiencies of the system are 49% and 38% respectively. Tiwari et al. [12] presented exergy analysis of a dual pressure combined cycle power plant (CCPP) located in the Dadri area of India. The components of the CCPP considered for the analysis include; GT unit, HRSG without supplementary firing, and a steam turbine unit. They reported that about 35% of the total exergy destruction occurred in the combustion chamber of the gas turbine. Adibhatla and Kaushik [13] performed exergy, energy, and economic analyses of a solar energy integrated (at intermediate temperature level) natural gas-fired dual pressure combined cycle power plant (CCPP). They reported that the solar field exergy and energy efficiencies are 27.39% and 53.79% respectively with an overall increase of 7.84% in the plant power output. Kotowicz and Brzeczek [14] presented the cooling effect of coolant (air) in the Rankine cycle of a triple pressure combined cycle power plant (CCPP). They reported that the net efficiency of a CCPP can reach 65% by using known industrial methods of cooling such as sequential combustion and steam cooling. Kilani et al. [15] studied the effect of methods used in steam generation for steam injection in the combustion chamber and the ambient temperature on the overall efficiency of the combined power plant. They reported that generating the steam outside the HRSG increase the overall efficiency of the combined circle up to about 6% and that the ambient temperature has a significant effect on the overall efficiency of the system. Kaviri et al. [16] studied the effect of HRSG inlet gas temperature on the HRSG exergy destruction and combined cycle efficiency. They reported that as the temperature of the inlet gas to the HRSG is increased the first and the second law efficiencies increase until the temperature reaches 650 oC after which the performance reduces. Also, the exergy analysis revealed that the high-pressure evaporator and high-pressure superheater 2 have the highest Page 4 of 27
amount of exergy destruction. Sharma and Singh [17] revealed that the main causes of irreversibility in a dual pressure HRSG are high and low-pressure superheaters and high-pressure evaporator. Ahmadi and Dincer [18] optimized the design parameters (isentropic efficiencies of compressor and gas turbine, inlet temperatures of combustion chamber and turbine, and compressor pressure ratio) of a combined power and heat plant with the intentions of producing 50 MW and 33.3 kg/s worth of electricity and saturated steam at 13 bar respectively through genetic algorithm. They reported that as the net power output is increasing the design parameters values also increases for a precise unit cost of fuel. Khana and Tlilib [19] conducted a parametric analysis to enhance the performance of combined cycle power plant which includes the bypass valve. They reported that when the turbine inlet temperature of the topping cycle rises from 1000 K to 1400 K there is a gain in the output network of about 45% and the gain in the net cycle efficiency from 15% to 31%. They also recommended that for the high and small value of compression ratio the bypass valve should remain closed and opened respectively. Ahmadi and Dincer [20] analyzed a combined cycle power plant equipped with an auxiliary firing system thermodynamically using exergy and energy analysis after which optimization was carried out using a generic algorithm by identifying an objective function. They reported that the deviation of the results of the model developed from the actual data is around 1.41%. Ganjehkaviri et al. [21] developed inclusive thermodynamic modeling and then performed an extensive optimization to come up with the optimum design parameters of a CCPP. They reported that with the optimum variable, the second-law efficiency is increased by about 6% while 5.63% reduction in CO2 emission was recorded. Regarding the retrofitting of CCPP with fuel cells, Thattai et al. [22] reported that an existing integrated gasification combined cycle power plants can be successfully operated with comparatively high electrical efficiencies of more than 40% (lower heating value) and without major modifications when retrofitted with solid oxide fuel cells and partial CO2 capture (using oxy-combustion technique). To achieve full-scale CO2 capture, they stated that the main process will require major modification which might call for the redesign of some components such as HRSG and gas turbine unit. Promes et al. [23] developed a model to investigate the performance of the steady-state operation of the integrated gasification combined cycle power plant using the Page 5 of 27
parameters of an existing plant. They reported that the temperature, flow rates, efficiency (42%), and pressure were accurately predicted by the model and that the rate of exergy destruction is more in the gas turbine combustion chamber [7] and the gasifier. In an effort to study the effect of vapor quality at the turbine exit, Ganjehkaviri et al. [24] reported from their modeling result that with respect to the economic, efficiency, and environmental, a system with the vapor quality of 88% at the turbine exit is more realistic. The literature on combined cycles indicates that one of the major challenges is the losses encountered in the water-steam bottoming cycle. Hence, it is important to identify the components of the cycle that are contributing to these losses as well as the magnitude of such losses in order to improve overall efficiency of the combined cycle. Exergy analysis is an effective way in identifying the location and magnitude of irreversibility and potential for improvement in a thermal system. Therefore, utilizing such a tool with plant actual field data; do help in addressing the major challenge facing the CCPP. The objective of this study is to perform detailed energy, exergy and parametric analyses of a triple pressure combined cycle power plant by given consideration to the exhaust gasses of the gas turbine used in the Rankine cycle. The results of such study will provide useful information and guidelines to power plant designers, engineers and operators.
Description of the Power Plant The power plant is one of the largest combined cycle power plants comprising of triple pressure steam turbine cycle (STC) and heat recovery steam generators (HRSG) with facilities for reheat located in the eastern province of Saudi Arabia. The plant consists of 5 units and each unit comprises of 3 gas turbine generators (GTG), 3 heat recovery steam generators (HRSG), and 1 steam turbine generator (STG) as shown in Fig. 1. The exhaust gas of the gas turbines (GT) is utilized in HRSG to generate steam used in driving STG. The layout of the plant showing 1 of the 3 GTG and HRSG is shown in Fig. 2. In addition to power generation, the plant also supplies steam to 2 multi-stage flashing (MSF) water desalination plants which is an integral part of the plant base on the requirement. The plant runs on natural gas with 43,196 kJ/kg heating value and the fuel flow rate per GT is 10.05 kg/sec. The exhaust of the gas turbine of the mass flow rate of Page 6 of 27
418.0 kg/sec and temperature of 617.0 oC is fed into the HRSG to generate the steam that runs the STG. The gross electrical output of each of the gas and steam turbines are 126 and 263.6 MW respectively while the net electrical output of the 5 STG is approximately 1.24 GW. The plant is 2.00 m above mean sea level (AMSL) while the average ambient dry bulb temperature and relative humidity are 33.0 oC and 70% respectively. A continuous power generation is achieved by each unit through a 19.6 m3/sec (19462 kg/sec) of cooling water at an average temperature of 38.0 oC from a nearby sea which is ejected back to the sea at a temperature of 43.6 oC.
Figure 1: Schematic of a single unit.
Page 7 of 27
Figure 2: Layout showing a typical part of a unit.
Page 8 of 27
Energy Analysis Basically, the conservation of mass and energy (i.e. first law of thermodynamics) is used in analyzing the energy of a system [25]. To perform the energy analysis of a given system through which energy, mass, and work cross the boundaries of the systemโs components such conservation laws need to be applied. Under a steady-state condition and considerable losses, the system is considered as a control volume and the conservation equations 1 and 2 (i.e. mass and energy balance respectively) were applied to the components of the system. โ๐๐๐ = โ๐๐๐๐๐ -
-
-
-
-
-
-
-
-
- (1)
๐ธ+๐ฌโ๐พ=๐-
-
-
-
-
-
-
-
-
- (2)
Where ๐ธ is the rate of heat input to the system, ๐ฌ is the rate of flow energy which is given by equation 3, and ๐พ is the rate of work produced by the system. ๐ฌ = โ๐๐๐๐๐๐ โ โ๐๐๐๐๐๐๐๐๐๐ -
-
-
-
-
-
-
- (3)
Where h is the fluid enthalpy and subscript in and exit represent inlet and exit respectively. Exergy Analysis Exergy analysis is a technique that employs the first and second laws of thermodynamics in addition to the conservation of mass principle to design and analyze the energy systems [10,20,25]. Exergy analysis is essential in evaluating the thermodynamic losses of a given system [26]. Recognizing the components with major exergy destruction is the key step to exergy performance optimization [26,27]. Basically, exergy is divided into four components out of which two are given much attention and the other two are less important because in most cases they got neglected. The two important components of the exergy are chemical and physical exergies while the negligible components are kinetic and potential aspects of exergy. The later components are negligible because the changes in the altitude and speed of an energy system during a process are negligible compared to the changes in the chemical and physical states of the system. The chemical exergy is linked with the parting of the systemโs chemical composition from its chemical equilibrium, while the physical exergy is the maximum theoretical useful work obtained by the system as it interacts with an equilibrium state [10]. Using the thermodynamicsโ first and second law, the steady-state exergy balance equation in a rate form is; Page 9 of 27
๐ฌ๐๐ซ = ๐ฌ๐๐ธ + โ๐ฌ๐๐๐ โ โ๐ฌ๐๐๐๐ โ ๐พ -
-
-
-
-
-
-
- (4)
Where ๐ฌ๐๐๐ and ๐ฌ๐๐๐๐ denote the rate of flow exergy into and out of the control volume
[ (
respectively, ๐ฌ๐๐ซ is the rate of exergy destroyed, ๐ฌ๐๐ธ ๐.๐. ๐ โ
)๐ธ] is the rate of exergy
๐ป๐ ๐ป
transfer by heat (๐ธ) at a temperature ๐ป, and ๐พ is a useful work. The flow exergy consists of two integral parts as express below; ๐ฌ๐ = ๐ฌ๐๐๐๐ + ๐ฌ๐๐๐๐๐, ๐๐๐ - -
-
-
-
-
-
-
-
- (5)
Where ๐ฌ๐๐๐๐ and ๐ฌ๐๐๐๐๐ represent the physical and chemical exergy respectively. For a negligible change in altitude and speed, the two terms are further expressed as follows; ๐ฌ๐๐๐๐ = ๐โ(๐ โ ๐๐) โ ๐ป๐(๐ โ ๐๐)โ -
-
-
-
-
-
-
- (6)
๐ฌ๐๐๐๐๐, ๐๐๐ = ๐๐๐๐๐๐๐, ๐๐๐ Where ๐ is the mass flow rate, ๐ is specific enthalpy, s is specific entropy, and ๐๐๐๐๐๐, ๐๐๐ is specific a chemical exergy of the mixture on a molar basis which can be obtained from the expression given in equation (7) [28][29]. ๐
๐
๐๐๐๐๐๐, ๐๐๐ = โ๐ = ๐๐๐๐๐๐๐๐๐,๐ + ๐น๐ป๐โ๐ = ๐๐๐๐ฅ๐ง ๐๐ + ๐ฎ๐ฌ - -
-
-
-
- (7)
Where ๐๐, ๐๐๐๐๐๐,๐, ๐ป๐, and ๐ฎ๐ฌ are the mole fraction of the component i-th in the mixture, molar chemical exergy, reference (environment) temperature, and excess free Gibbs energy respectively. When the pressure of the gas mixture is low ๐บ๐ธ is negligible. The second law efficiency of the system is obtained as follows; ๐๐ผ๐ผ =
๐ธ๐ฅ๐๐๐๐ฆ ๐๐๐๐๐ฃ๐๐๐๐ ๐ธ๐ฅ๐๐๐๐ฆ ๐ ๐ข๐๐๐๐๐๐
๐ธ๐ฅ๐ท
= 1 โ ๐ธ๐ฅ ๐๐
-
-
-
-
-
-
-
- (8)
Results and Discussion The energy and exergy analysis of a CCPP equipped with triple pressure STC and HRSG with reheat were conducted using engineering equation solver (EES). The results presented in this section are typically for a single unit of the plant which comprises of 3 GT, 3 HRSG, 1 STG, and a condenser. The flue gas temperature at every stage of the HRSG was calculated using the Page 10 of 27
conservation of energy principle. Figure 3 shows the temperature variation along with the components of the HRSG which was validated with the stack designed temperature. The horizontal axis represents the HRSG components. The components located between the numbers are shown in Table 1.
Figure 3: Flue gas temperature variation along with the components of the HRSG.
Table 1: Components of the HRSG Numbering
Components of the HRSG
1-2
High-pressure superheater 3 (HP 8-9
High-pressure economizer 2 (HP
SPHT 3)
ECON 2)
Reheater superheater 2 (RHT SPHT 9-10
Intermediate-pressure
2)
(IP EVAP)
High-pressure superheater 2 (HP 10-11
Low-pressure
SPHT 2)
SPHT)
Reheater superheater 1 (RHT SPHT 11-12
Intermediate-pressure economizer
1)
(IP ECON)
High-pressure superheater 1 (HP 12-13
High-pressure economizer 1 (HP
2-3 3-4 4-5 5-6
Numbering
Components of the HRSG
evaporator
superheater
(LP
Page 11 of 27
SPHT 1) 6-7
High-pressure
ECON 1) evaporator
(HP 13-14
EVAP) 7-8
Intermediate-pressure
Low-pressure
evaporator
(LP
EVAP) superheater 14-15
Preheater
(IP SPHT)
It could be observed from Fig. 3 that the evaporator pinch which is the difference in temperature between the saturation temperature of the water flowing through the evaporator and the flue gas leaving the evaporator of high (6-7), intermediate (9-10), and low-pressure (13-14) evaporators are in the range of 8 to 14 K which are in agreement with the design limitations [2]. The highpressure evaporator experienced a large temperature gradient while a very low-temperature change occurs across the intermediate-pressure superheater (7-8), low-pressure superheater (1011), and intermediate-pressure economizer (11-12). In the case of high-pressure evaporator large amount of heat (energy) is required to transform the high flow rate feed water from saturation liquid to saturation vapor which resulted in a large temperature drop of the flue gas. In the cases of intermediate and low-pressure components, the feedwater flow rate is lower in addition to the fact that less amount of energy is required to superheat a vapor and economizer basically preheat the feed water. The ratios of the mass flow rate of the steam generated through the high, intermediate, and low-pressure equipment of the HRSG are 8.03:1.14:1.00 at a pressure ratio of 20.85:5.94:1 respectively. Figures 4 and 5 show the exergy destruction rate of the components of the HRSG and STC respectively. It could be observed from Fig. 4 that HP evaporator has the largest exergy destruction rate of about 27% of the total value follow by preheater then HP superheaters. This is contrary to what was reported for the case of single-pressure CCPP [11]. The variation in the exergy destruction rate of the HRSG components may not necessarily be attributed to inefficient of the facilities but rather to the amount of heat transfer processes taking place in these facilities. The high exergy destruction rate associated with the HP evaporator was due to the high level of energy exchange taking place in this equipment as explained with the high-temperature gradient across the component in Fig. 3. Therefore, looking at Fig. 3 closely, a clear understanding of the relationship between the exergy destruction rate and the temperature gradient (mainly energy transfer) could be deduced. Page 12 of 27
Figure 4: Exergy destruction rate of the HRSG components.
From Fig. 5, stack appeared to be the major source of exergy destruction with almost 60% of the total value in the steam turbine cycle (STC) follow by the HRSG, turbine, and condenser. The large value of exergy destruction rate in the stack is as a result of the high level of irreversibility due to the flue gas leaving the stack with a substantial amount of energy. In addition to the irreversibility, the exergy destruction rate in the turbine is also due to the mechanical losses associated with the energy transformation.
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Figure 5: Exergy destruction rate of the STC components.
Figure 6 shows the second law efficiency of the turbine and HRSG of the STC. It could be seen from the figure that the turbine is having the highest second law efficiency; this is expected according to equation (8) because the exergy destruction rate of the turbine is lower than that of the HRSG. 95
90.86
92.05
Efficiency (%)
90 85 80 75 70 62.98
65 60 HRSG
Turbine
Condenser
Components of the steam turbine cycle
Figure 6: Second law efficiency of the components of the STC.
The results obtained from this study are compared with what is available in the literature. Figure 7 shows the comparison between the results obtained from this study with that of the Pattanayak Page 14 of 27
et al. [10] who presented the energy and exergy analysis of triple pressure CCPP. The rate of the exergy destruction per unit output of the steam turbine is presented in Fig. 7 for the condenser and steam turbine. In either case, the results obtained from the current study are a bit lower than what was reported by Pattanayak et al. [10]. There are 7.7% and 1.7% differences in the condenser and steam turbine exergy destruction rate per ST output respectively between the current study and that of the Pattanayak et al. [2]. The differences in the results obtained may be attributed to several factors. There are significant differences in operating parameters of the two plants since the power produced by the two plants are not the same. For instance, the pressure of the steam exiting the low-pressure turbine is 0.09 bars in the case of Pattanayak et al. [2] while it is 0.105 bar in the present study. The condensing pressure and the inlet temperature of the cooling water in the case of Pattanayak et al. [2] are 0.089 bars and 33 oC while 0.11 bar and 38 oC
respectively were considered in the current study, therefore, there will be a distinction in the
heat transfer rate of the two studies. Also, the ratio of steam to cooling water flow rates is 0.010 and 0.013 for the current study and Pattanayak et al. [2] respectively with the current study being 13% lesser. This may likely explain the 7.7% reduction in the exergy destruction rate per ST output in the condenser of the current study as shown in Fig. 7.
Figure 7: Comparison of the exergy destruction rate per ST output obtained from this study with Pattanayak et al. [2].
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Parametric studies were conducted in which the effects of variations in some operating parameters on the efficiencies and output of the steam turbine were considered. Figures 8 โ 10 show the effect of variation in the quality of steam leaving the low-pressure (LP) turbine on the turbine gross power, exergy destruction, efficiencies, exergy destruction of the condenser, and the cooling water exit temperature. Figure 8 shows how steam turbine gross power and exergy destruction varies with steam quality. As the quality of the steam leaving LP turbine increases the expansion of the steam is being restricted thereby leading to a decrease in the output of the steam turbine.
Figure 8: Effects of variation in steam quality on the turbine gross power and exergy destruction.
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Figure 9: Effects of variation in steam quality on the turbine first and second law efficiencies.
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Figure 10: Effects of variation in steam quality on the exergy destruction rate of the condenser and the cooling water exit temperature.
The gross power of the steam turbine can be increased by lowering the steam quality below the saturated value, however, lower the steam quality increases the moisture content of the steam which may lead to the corrosion of the turbine blade and eventually threatening the turbineโs life. Hence, a good balance needs to be maintained between the gross power and steam quality. On the other hand, the loss of potential to do work increases with an increase in the steam quality, this is expected since the gross power of the turbine decreases with an increase in the steam quality. Restricting the expansion in the LP turbine by exiting the steam at a saturated state (i.e. x = 1) increases the irreversibility of the processes thereby, given rise to the exergy destruction rate. Since the turbine energetic (first law) and exergetic (second law) efficiencies are directly related to the turbine gross power and the exergy destruction rate of the turbine the variation in Fig. 9 can be clearly understood. Therefore, the lower the restriction in the steam expansion at the LP turbine end the higher the efficiencies. Figure 10 shows the variations in the exergy destruction rate of the condenser and the cooling water exit temperature as the steam quality changes. As the quality of the steam exiting the LP turbine increases the rate of heat transfer process taking place in the condenser also increase which is evidenced by the increase in the
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cooling water outlet temperature. The increase in the condenser exergy destruction rate is as a result of an increase in heat transfer rate associated with an increase in steam quality. Tripping of the cooling water pump(s) is a phenomenon normally experience in some power plants. This phenomenon leads to the shortage or reduction in the availability of water needed for the steam condensation. Figure 11 shows how the cooling water flow rate varies with the exergy destruction rate of the condenser and cooling water exit temperature. When the cooling water flow rate is not enough the exergy destruction rate is low meaning less heat transfer and the exit cooling water temperature is high. An increase in exit cooling water temperature beyond the required limit is a strong indication of improper condensation and violation of environmental regulations by threatening the life of aquatic animals. It is important to note that more than 15% decrease in the required cooling water flow rate leads to about 1 oC increase above the required/designed temperature of the cooling water ejected back to the sea.
Figure 11: Effects of variation in cooling water flow rate on the exergy destruction rate of the condenser and the cooling water exit temperature.
Since the inlet turbine pressure is one of the sensitive parameters in Rankine cycle [30], Figs. 12 and 13 show the effect of superheat pressure while Figs. 14 and 15 show the effect of reheat pressure on the turbine output, exergy destruction, and turbine efficiencies. From Figs. 12 and Page 19 of 27
14, it could be observed that increasing the superheat and reheat pressures decrease the power produced by the turbine by increasing the lost work potential. At lower pressures, the turbine output power is high due to a high level of expansion that will be taking place in the first (high pressure) and second (intermediate pressure) stages turbines. The life of the turbine blade may be the likely price to be paid when the superheat and reheat pressures were lowered beyond a reasonable value. Figures 13 and 15 show that the efficiencies of the turbine decrease by increasing either the superheat or reheat pressure. The first and second law efficiencies are the function of energy and exergy respectively which implies that a decrease in gross power and an increase in the irreversibility of the process lead to a decrease in the efficiencies of the system.
Figure 12: Effects of variation in superheat pressure on the turbine gross power and exergy destruction.
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Figure 13: Effects of variation in superheat pressure on the turbine first and second law efficiencies.
Figure 14: Effects of variation in reheat pressure on the turbine gross power and exergy destruction.
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Figure 15: Effects of variation in reheat pressure on the turbine first and second law efficiencies.
Conclusion The current study presents detailed energy and exergy analysis of a Rankine cycle of a triple pressure combined cycle power plant (CCPP) using the design data. The effects of several operating parameters on the turbine output, efficiencies, and exergy destruction were investigated. The results obtained were validated against published data. The study results indicated that: a.
The components that have a great potential for improvement include; the stack, the high-
pressure evaporator, preheater and the high-pressure superheater of the HRSG. Therefore, more attention needs to be given to these components for improving their efficiencies and hence the overall efficiency of the combined cycle. b.
The exergetic efficiency of the turbine is the highest in the steam turbine cycle (STC)
c.
More than 15% decrease in the required cooling water flow rate leads to about 1 oC rise in
the temperature of the ejected cooling water Page 22 of 27
d.
The superheat pressure, reheat pressure and steam quality at the exit of the low-pressure
steam turbine significantly affect the output of the turbine and efficiencies. e.
The study results will provide useful information and guidelines to power plant engineers
and operators; such as choosing possible performance enhancement modifications to gas-fired combined cycle power plants. f.
It is also recommended to investigate the potential of utilization of exhaust flue gas to run
more than one energy device.
Acknowledgment The authors would like to acknowledge the support of the King Fahd University of Petroleum and Minerals (KFUPM) through Project No DUP18101 in carrying out this study.
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Date: June 24/2019 To: The Editor, Thermal Science and Engineering Progress Subject: Conflicts of interest statement
Page 26 of 27
Manuscript title: Energy, Exergy and Parametric Analysis of a Combined Cycle Power Plant Authors: Mansur Aliyu, Ahmad B. AlQudaihi, Syed A. M. Said, Mohamed A. Habib.
This is to inform you that, all authors whose names are listed above and in the manuscript certify that have NO affiliations or involvement in any organization or entity with any financial interest in the subject matter or materials discussed in the manuscript.
Best Regards Syed A.M. Said Corresponding Author
Energy, Exergy and Parametric Analysis of a Combined Cycle Power Plant
Highlights 1. 2. 3. 4. 5.
Energy and exergy analyses of the components of combined cycle power plant. Exergetic efficiency of the components of combined cycle power plant. Computation of temperature gradient across the heat recovery steam generator (HRSG). Exergy analysis of every component of HRSG. Parametric analyses of some operating parameters were investigated.
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