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Evaluation of the energy efficiency of combined cycle gas turbine. Case study of Tashkent thermal power plant, Uzbekistan
Accepted Manuscript Evaluation of the energy efficiency of combined cycle gas turbine. Case study of tashkent thermal power plant, uzbekistan Zarif Aminov, Nobukazu Nakagoshi, TRAN Dang Xuan, Osamu Higashi, Khusniddin Alikulov PII: DOI: Reference:
S1359-4311(16)30467-7 http://dx.doi.org/10.1016/j.applthermaleng.2016.03.158 ATE 8026
To appear in:
Applied Thermal Engineering
Received Date: Accepted Date:
2 December 2015 29 March 2016
Please cite this article as: Z. Aminov, N. Nakagoshi, T.D. Xuan, O. Higashi, K. Alikulov, Evaluation of the energy efficiency of combined cycle gas turbine. Case study of tashkent thermal power plant, uzbekistan, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.03.158
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Submission to APPLIED THERMAL ENGINEERING Entitle: “EVALUATION OF THE ENERGY EFFICIENCY OF COMBINED CYCLE GAS TURBINE. CASE STUDY OF TASHKENT THERMAL POWER PLANT, UZBEKISTAN” Zarif Aminov a , Nobukazu Nakagoshi b , TRAN Dang Xuan b , * , Osamu Higashi c , Khusniddin
Alikulov b
a
Institute of Power Engineering and Automation, Academ y of Sci ences of Uz bekistan, 29
Durmon yuli, Akademgorodok, Ta shkent, 100125, Uz bekistan b
Department of Devel opment Technology, Graduate School for International Devel opment and
Cooperation , Hiroshima University, 1-5-1 Kagamiyama, Higashi -Hiroshima 739-8529, Japan c
MI Consulting Corporation, AIM Bldg., 3-8-1 Asano, Kokurakita-ku, Kitakyushu-shi, Fukuoka 802-0001, Japan
* Corresponding author Tran Dang Xuan (Ph.D) Hiroshima University Kagamiyama 1-5-1 Higashi Hiroshima, 739-8529 Japan E-mail: [email protected] Tel/Fax: +81-42-424-6927 1
Abstract
The power generation of Tashkent Thermal Power Plant (TPP) is based on conventional power units. Moreover, the facility suffers from limited efficiency in electricity generation. The plant was constructed during the Soviet era. Furthermore, the power plant is being used for inter-hour power generation regulation. As a result, the efficiency can be reduced by increasing specific fuel consumption.
This research focuses on the evaluation of
the energy efficiency of the combined cycle gas turbine (CCGT) for the Tashkent TPP. Specifically, the objective is an evaluation of fossil fuel savings and reduction of CO 2 and NOx emissions with the using CCGT technology at conventional power plant. The proposed combined cycle power plant (CCPP) includes an existing steam turbine (ST) with 160 MW capacity, heat recovery steam generator (HRSG), and gas turbine (GT) technology with 300 MW capacity. The performance of a three pressure CCGT is modelled under different modes. As a result, the efficiency of the combined cycle was evaluated at 58.28 percent, while the conventional cycle had an efficiency of 34.5 percent. We can achieve an annual reduction of 1760.18 tNOx/annum and 981.25 ktCO2/annum. Keywords: Energy efficiency; Gas turbine; Heat recovery steam generator; Combined cycle power plant; Cycle gas turbine
1. Introduction Conventional power plants suffer from limited efficiencies, on average 30percent[1]. Owing to many thermal power plants were constructed during the Soviet era, including Tashkent Thermal Power Plant (TPP). Specifically, 20 percentage of existing generation capacity is past the useful service life, and this will increase to 40 percent by 2017 [2].In the Uzbek energy sector, the growth in the electricity demand is satisfied mainly by replacement with efficient Combined Cycle Power Plant (CCPP) and natural gas has a major role as energy resources by 2040 [1]. The installed capacity of Tashkent TPP is 1860 MW. The basic equipment are 12 condensing power units 155-165 MW each. The main fossil fuel is natural gas with 85 %, and oil as reserve fuel, which is 15 %[3]. The power plant is being used for inter-hour power generation regulation. Moreover, a steady growth in the consumption of fuel for power generation is 2
observed at the power plant, due to the expiration of the normative life of the basic equipment, in connection with which impact on the environment influence raises into the environment. CCPP is a system in which two types of the turbines are used to generate electricity[4]. Typical heat rates of the CCPPs are lower compared with steam turbine(ST) or a simple cycle gas turbine(GT), therefore a performance of the CCPPs is effectively [5]. Efficiency of power plants may reach up to 65 % due to enhancement in the gas turbine technology and optimization in heat recovery steam generator (HRSG) of combined cycle gas turbine (CCGT)[6].Reduction of the emission of polluting gases can be implemented by the using the combined cycle[7]. The effects of turbine inlet temperature (TIT) and compressor pressure ratio (Cpr) on the parameters that measure cycles’ performance, environmental impact and costs of the power plant [8]. The reconstruction of the existing SG for the purpose of using exhaust gases from GT is feasible and the investment cost could be covered by fuel saving[9]. According to the power generation method and their controls, GTs can vary significantly from fossil-fuel conventional power plants [10]. The aim of the current study is to evaluate energy efficiency of CCGT in case of Tashkent TPP, which takes into account the frequent off-design operation of the power plant with the corresponding decrease in efficiency. Thus, this research was conducted to: 1) to evaluate energy efficiency of TPP, 2) to evaluate energy efficiency of CCGT, and 3) to evaluate fossil fuel savings and reduction of CO2 and NOx emission 2. Experimental There are several methods to simulate CCPPs. Salehi, Seifi & Safavi [11], designed a simulator based on object-oriented programming, C-programming, and SIMULINK® toolbox of MATLAB®. The dynamic modelling of CCPP was designed by El-Hefni, Bouskela, & Lebreton with an introduction ThermoSysPro library, and presents the power plant test-case [12]. Simulation of the heat transfer by fluid-water/steam – integrated - solar combined cycle system is implemented by using Thermoflex thermal engineering software [13]. In this paper, the thermodynamic cycle and energy balance of the thermal systems in the conventional and combined cycle power plants are simulated with Ebsilon Professional 11.01 software. This Ebsilon software is used for evaluation, design and optimization of various power plants [14].
3
A short description of two global calculation modes in the software, which is used for these simulations, is explained below: Design mode. In this mode, we define appropriate specification values for all components, e.g. according to specifications from manufacturer. Partload mode. This mode is used for a cycle already completed and calculated in design mode. It is calculated for different loads. 3. Theory/Calculation 3.1.
Simulation of Current TPP
In order to perform the objectives shown above, three steps are required. Energy efficiency is widely used in the industrial practice to measure power plants technical performance [15]. In this section, the first step is the simulation of the current TPP at the design and off-design conditions, in order to evaluate its energy efficiency. The selected modes of operation of the power plant fit a reasonable scenario of energy production. The chosen Rankine cycle for the design of conventional power unit has ST 160 MW capacity. The process simulation diagram of the steam/water cycle is shown in Figure 1. FF - fluegas fan AF - air fan AH - air heater CC - combus on chamber SG - steam generator
PH - feed wate preheater HP, LP G - generator D - deaerator C - condenser M - motor ST - steam turbine
Air line Fluegas line
ST HP
Steam line Water line
ST HP
ST HP
Electric line
G
160.000 MW
Gas line Mechanical sh
SG
C
CC
M
M
AH
FF
AF
M
D PH LP PH HP
M
M
M
Figure 1. Schematic diagram of TPP
Steam generator (SG) with a single reheat system is performed to verify the accuracy of the simulation. Moreover, based on the traditional seven-stage extraction steam regenerative system, the stages high (HP), intermediate(IP) and low (LP) pressures turbines are modelled 4
with different mean isentropic efficiencies (ETAI), which are defined during the simulation. The live steam parameter is improved to 135 bar and 545⁰C, and the reheat steam is heated to 545⁰C. The outlet steam of the HP turbine enters the reheater and then goes into the IP turbine. The outlet steam of the IP turbine enters the LP turbine and then goes into the steam condenser after expansion in the LP turbine. Steam with 95.37 kg/s and 27.62⁰C is converted to condensate by 4537.2 kg/s and 13⁰C cooling water in the steam condenser. Temperature of the feed water reach up 227.9⁰C at inlet of steam generator, which is delivered through the LP preheaters, deaerator and HP preheaters. In off-design mode, the power plant was simulated with the nominal load 160 MW and partloads, such as 140 and 130 MW. Usually, conventional power plants work with automatically regulation of steam generation. In that case, pressure and temperature are not fluctuated at different load of turbine. Because of breaking ranks the system regulation, inlet pressure same as mass flow of the steam is decreased at partload in this study, as practical operation of steam turbine. Temperature is stable as nominal value by using injection control system (Figure 1). In Table 1, variation of parameters in Off-design is shown, when the power is specified. The mass of fuel and air has to be changed for obtaining the required power, taking into account that, with that the smaller amount of fuel and air the less power is delivered. Table 1: Parameters of the power plant in Off-design modes Partload,
W,
Mfuel,
Mair,
Msteam,
Mgas,
TOT,
HP,
IP,
Mexh,
Texh,
Pexh,
%
MW
kg/s
kg/s
kg/s
kg/s
⁰C
bar
bar
kg/s
⁰C
bar
100
160
11.58
235.38
128.92
288.5
155.88
135
26.5
95.37
27.62
0.037
88
140
10.22
207.71
113.61
254.58
141.88
118.95
23.47
84.58
26.08
0.034
81
130
9.52
193.45
105.68
237.11
134.66
110.64
21.89
78.96
25.27
0.032
In this simulation, the following are the main set of equations for Rankine cycle based on general thermodynamic relationships [16]. The efficiency of SG is calculated as: (1) Where Qout: Output heat energy of SG (MW), H: Enthalpy of output power of SG (kJ/kg), M: Mass flow of fluid type logic line, Qfuel: Input energy of the fuel is determined by the following equation (2): H
NCV (2)
5
Where Mfuel: Fuel consumption (kg/s), H: Enthalpy of the fuel (kJ/kg), NCV: Specified net calorific value of natural gas at the existing power plant is 37889 (kJ/kg) [3]. The thermal efficiency of ST is defined as: (3) Where WST: Output power of ST generator (MW). The overall thermal efficiency of the cycle is determined by the equation (4): (4) Where Wnet: Net power of TPP (MW) is expressed in the following equation (5), Wown: Own use power for auxiliary equipment (MW): (5) The annual net power of TPP as: t Where to: The operation hours equal to
) (6) , t: Number of hours a year (h), F:
Load factor of ST generator Using the equation (6), the annual fuel consumption is calculated as: (7) A fossil fuel required for power generation of 1 kWh is defined [8] by the following equation (8): (8) Where HR: Heat rate (kJ/kWh) of TPP is presented in the following equation (9): HR
(9)
3.2.Simulation of CCGT The second objective concerns the definition of the energy efficiency of the different loads of energy production at CCPP. The load dependent efficiency of power units with high capacity is considered primarily. These large scale power units remain a decisive role in large systems [17]. Modelling a simple CCGT is carried with triple pressure HRSG and a large scale power unit which has 429.57 MW capacity. The Brayton cycle represents a thermodynamic process of GT technology with 300 MW nominal load. The current ST with 133.6 MW capacity is presented as a Rankine cycle in this simulation.
6
Figure 2 presents describes a schematic diagram of the three pressures CCPP. In the simulation, the vertical HRSG design is chosen based on some attractive advantages mainly connected with the possibility of inserting an HRSG in an existing utility boiler steel structure for re-powering [18]. In the construction of the cycle of the GT, the selected components are a compressor, a combustion chamber and a turbine.
7
Nominally, the turbine has a pressure ratio of 18. We set an “internal set value of power” at the gas pipeline before the combustion chamber for controlling a load of GT. In design mode, we set the nominal steam temperature as 545 ⁰C and the mass flow, while the pressure is set in the HRSG component. After simulating the performance of the CCPP, HRSG provides ST with HP, IP and LP steam of 135, 26.5 and 2.2 bar respectively. During the modelling of ST, the seven-stage extraction steam system was discontinued from using. Thus, heat expansion of steam occurs with 78.58, 90.08 and 97.21 kg/s respectively in HP, IP and LP turbine. Steam is delivered to the steam condenser for cooling after work execution in LP turbine with 0.037 bar and 27.6 ⁰C parameters. Using the deaerator, oxygen and other dissolved gases
G
M
M
M
M M
G
Net output
removed from condensate, steam is taken from LP evaporator of HRSG with 1.1 bar and 117.5 ⁰C. The feed water, after being preheated and leaving the deaerator, is divided in two pipes and pumped to the different pressure levels. Figure 2. Schematic diagram of combined cycle power plant with a three pressure HRSG
The partload differs from the nominal conditions of the CCPP performance based on partloads of ST, when the amount of power smaller than in nominal conditions is enough for satisfying the demand. We wanted to observe what happens with variation in power, for
8
instance if TOT is set as 613 ⁰C after GT. Table 2 presents the parameters of the CCGT in off-design mode. Table 2: Variation of parameters of CCGT in Off-design modes when the power is specified Partloads of the GT, % 100
80
69
300
280
250
Cpr, bar
18.00
17.41
16.40
λ
1.86
1.88
1.93
TIT, ⁰C
1320.1
1305.6
1280.8
Mfuel1, kg/s
19.41
18.59
17.24
Mfuel2, kg/s
0
0
0
610.39
593.16
563.57
MHP steam, kg/s
78.58
75.53
70.84
MIP steam, kg/s
11.46
11.27
10.89
MLP steam, kg/s
19.95
19.24
17.97
Tfluegas, ⁰C
110.8
110.7
110.4
Mfluegas, kg/s
629.8
611.75
580.81
WST, MW
133.6
129.18
121.85
Mexh2, kg/s
97.21
94.35
89.34
Texh2, ⁰C
27.6
27.2
26.4
Pexh2, bar
0.037
0.036
0.035
GT performance WGT, MW
Mair, kg/s HRSG performance
ST performance
The following are the main set of governing equations based on general thermodynamic principles of the combined-cycle plant [19]. These equations were used to predict the performance of CCGT after evaluation of the performance of GT and ST such as for Brayton and Rankine cycles. Thus, it is relationship between profits and expenditure. The efficiency of Brayton cycle is calculated with equation (10): (10) Where WGT: Output gross power of GT generator (MW), Qfuel: Input energy of the fuel (MW), which is determined by the equation (11): (11) Where Mfuel: Fuel consumption in CCPP (kg/s), H: Enthalpy of the fuel (kJ/kg), NCV: Specified net calorific value of the fuel is 37889 (kJ/kg) [3]. The thermal efficiency of HRSG is expressed in the following equation (12): 9
(12) Where M1: Flue gas mass flow (kg/s), H1: Enthalpy of flue gas (kJ/kg), Exhaust gas mass flow (kg/s), H2: Enthalpy of exhaust gas (kJ/kg). The thermal efficiency of ST is calculated as: (13) Where WST: Output gross power of ST generator (MW), Qin: input power of ST which is equal to the output energy (
) of SG (MW).
The combined cycle efficiency is the ratio of addition of GT and ST output to the input to the cycle i. e. heat input to GT. By inserting equation (11) into equation (14), the efficiency of combined cycle is presented as: (14) Where Wnet: Net power of CCPP (MW), which is equal to the difference between the gross power of CCPP and own use power, Wown: own use power in CCPP (MW), Qfuel: Input energy of the fuel (MW) is expressed in Equation (11). The annual net power output of CCPP is defined as: (15) Where to: The operation hours equal
, t: Number of hours a year (h), F: Load factor
of CCGT A fossil fuel required for power generation of 1 kWh at the CCPP is determined as the following equation (16) [8]: (16) Where HR: Heat rate (kJ/kWh) of CCPP is indicated in the following equation (17):
(17) 3.3.Comparative Study and Environmental Analysis The fuel saving is evaluated through the difference between the fossil fuel required of the CCPP and the conventional power plant per 1 kWh: (18)
10
Where Mr1, Mr2: Fossil fuel required per 1 kWh power generation at TPP and CCPP, respectively (kg/s). Annual saved fossil fuel is the amount of the annual net power output multiplied by the saved fossil fuel: (19)
Figure 3. Schematic diagram of reduction of CO2 and NOx emissions
As final step, CO2 and NOx emissions reduction are evaluated based on saved fossil fuel. The simulation is performed by using combustion chamber, a three selective splitter which is for each emission i.e. CO2, NO2 and NO. Figure 3 shows the model of reduction of CO2 and NOx emissions. The amount of the fossil fuel for burning in the combustion chamber is equivalent to the saved natural gas which is marked with a purple line. Input data is taken from the simulation of CCPP. Using the equations of selective splitter models [14], CO2, NO2 and NO are evaluated as below: (20) Where XCO2: CO2 fraction is equal to 0.12803 (21) Where XNO2: NO2 fraction is equal to 0.00021 11
(22) Where XNO: NO fraction is equal to 0.00001 Taking into account the annual power generation, the amount of CO 2, NO2 and NO reductions per year are calculated by these the equations (23, 24 and 25): (23) (24) (25) 4. Results and Discussion In off-design conditions, the temperature of the steam extractions is defined by the ETAI of the turbine implemented in Ebsilon. The heat transfer capacity of heat exchanger varieties with the varying in mass flow through the heat exchanger. We can determine the transfer surfaces in the heat exchanger in design mode and it will be constant in every mode. In Ebsilon, the turbine has an established default value of ETAI. The default ETAI for each part of the turbines is from a different index. For instance, ETAI of the HP turbine is 0.85, and this value is determined in partload by some correction curves. Figure 4 represents the temperature/heat diagram in the heat exchangers of Rankine cycle. The feed water is preheated with seven-stage extraction steam regenerative system at high and low pressures. Thus, temperature is preheated from 27.6 ⁰C up to 227.9 ⁰C by increasing the cycle efficiency. Moreover, deaerator is also preheat the condensate for 12 ⁰C during the deaerating process.
12
Figure 4. Heat transfer in the heat exchangers of Rankine cycle
Thermodynamic principles of Rankine cycle with vapor reheating occurs based on the temperature/specific entropy diagram, which represents input energy of fossil fuel, power generation and efficiency of steam turbine cycle (Figure 5).
Figure 5. T-S diagram of the Rankine water-steam cycle
Table 3 presents the main results of the first simulation which are calculated with the equations presented in at the previous section. At nominal mode, high temperature source is equal to 438.87 MW and power generation is 151.54 MW. Considering the load factor of 13
generator, annul net power output of 1.128*109 is produced. Heat rate of 10434.69 kJ/kWh at the cycle with specific fuel consumption of 275 g per 1 kWh. According to equations (2, 4 and 5), steam turbine cycle efficiency of 34.5 % is obtained at nominal mode (Table 3). Table 3: Performance of the conventional power plant Parameters
Loads (%) 100
88
81
438.87
387.28
360.7
SG (%)
89.98
90.82
91.26
ST (%)
40.49
39.77
39.46
151.54
133.16
123.85
Qin (MW)
Wnet (MW) cycle (%)
34.5
Wannual (kWh/annum)
1.128*10
HR (kJ/kWh)
10434.69
Mr1 (g/kWh)
275.4
34.35 9
0.992*10
34.31 9
0.922*109
10.478.87
10493.7
276.57 6
Mannual (kg/annum)
310.74*10
274.22*10
F (%)
85
85
276.96 6
255.39*106 85
Such as steam extractions are shut down at steam turbine, temperature of feed water is lower, which need more fuel consumption for power generation. For this reason, steam generator is designed as HRSG. Steam generation is developed with effective using the exhausted gas after gas turbine due to extra heat exchangers and evaporators. Since the enthalpy of the exhaust gases of the GT is enough to achieve nominal conditions with 27.6 ⁰C and 97.21 kg/s in steam condenser at Rankine cycle. As shown in Table 2, no necessity for fuel consumption (Mfuel2) in HRSG as supplementary. Using a three pressure stage, we can achieve a widely heat transfer in the HRSG (Figure 6). Exhaust gases with 613 ⁰C is capable to produce a steam with HP, IP and LP. Thereby, the feed water of 436 kJ/kg can be increased up to 3557 kJ/kg for delivering to HP ST.
14
Figure 6. Heat transfer in the HRSG in a three pressure CCPP
Steam turbine is constructed with certain amount for heat expansion and power generation. According to these facts, steam turbine is reconstructed as 133.287 MW capacity. In this case, steam is delivered to LP turbine from LP evaporator through the control valve with 2.2 bar. Depending on the required power, Ebsilon calculates by itself the amount of the necessary air and fuel. GT delivers exactly that amount. Figure 7 illustrates the combined cycle that consist the steam turbine cycle (Rankine cycle) and gas turbine cycle works through the Brayton cycle. Therefore, the performance is almost doubled. In particular, the area under the T-s curve of the process represents the heat to be transferred to the system, and the effectiveness of the power generation during the thermodynamic process. The growth of energy dissipation is observed at the off-design modes. This means that the measures of deviation of the actual from the ideal process were increased. The highest index of the efficiencies of GT, ST and CC conform at a nominal load, except of effectiveness of HRSG. Thereby the energy efficiencies decrease at the partload modes with the incrementation of the fuel consumption.
15
Figure 7. T-S diagram of the combined cycle
Working in partload conditions means changes in its power output and thereby in the pressure ratio of GT. In a real GT the mass of air entering the compressor is regularized by changing the inlet guides vanes. The given operating range of TIT and Cpr are significant in the analysis, as mentioned in the study of Mahto & Pal [20]. Furthermore the other components of the turbine also have to be considered in the overall efficiency of GT and the CCPP, which experience a variation in the performance working at the unload conditions. In Ebsilon, compressors and turbines have an established default value of isentropic efficiency. ETAI in a compressor or a turbine is a comparison between the real power obtained or consumed and the isentropic case. The default ETAI for turbines is 0.92 and for compressors is 0.88. In partload that value is defined by some correction curves. The variation of ETAI is directly proportional to the change of mass flow which is going through the compressor or turbine. According to the results from the previous conventional power plant model, the HP, IP and LP STs have an established default value of ETAI. In general, the GT provides two-thirds of the total capacity [21]. In this simulation, GT supplies 69.2 % of the power in nominal load, while ST delivers only 30.8 % of the energy of the CCPP power output of 433.292 MW.
16
Using the equations (10, 11, 12 and 13), CC efficiency of 58.28 % is obtained at nominal mode (Table 4). So, it is able to reduce SFC from 275.4 g/kWh until 163 g/kWh, heat rate of CC amount to 6176.81 kJ/kWh. In result, a fuel saving of 112.38 g per 1 kWh and annual amount is 359.45 million kg/annum. Table 4: Performance of the CCPP with a three pressure HRSG Loads (%)
Parameters 100
80
69
Wnet (MW)
429.57
405.27
368.12
Qin (MW)
735.37
704.46
653.14
GT (%)
40.7
39.66
38.19
HRSG (%)
82.86
82.87
82.92
ST (%)
37.19
37.04
36.84
58.28
57.4
56.23
Wannual (kWh)
3.199*109
3.018*109
2.741*109
HR (kJ/kWh)
6176.81
6272.04
Mr2 (g/kWh)
163
165.5
Ms (g/kWh)
112.38
CC
(%)
Annual fuel savings (kg/annum)
6401.88 169
111.03 6
107.99 6
296.02*106
359.45*10
335.05*10
85
85
85
MCO2 (tCO2)
981.25*103
914.64*103
808.09*103
MNO2 (tNO2)
1641.25
1529.1
1351.22
MNO (tNO)
118.93
110.81
97.3
Ϝ (%)
Figure 8 shows the fluctuation of the efficiencies of the steam turbine and combined cycles at the various partloads. In the both model, effectiveness of steam generation is increased. In case of ST, efficiency is decreased by 3.3, 2.73 and 2.62 % at partloads, respectively. But, cycle efficiency of power generation is increased by 23.78 % at nominal mode. Environmental analysis was implemented on the grounds of the burning a saved fuel by the CCPP. The economics of power generation are affected mainly by the amount of CO2 produced [22], for instance by a large amount of harmful emissions, created by the burning process, turning natural gas into carbon dioxide. In Table 4, the saved fuel with 0.1123 kg/kWh fuel consumption is delivered by the gas pipeline. According to the calculation using Equations (20,
17
90.00
Steam generator Steam turbine 1 Cycle
Efficiency, %
60.00
Gas turbine
HRSG
30.00
Steam turbine 2 0.00 1
2 Partload cases
3
Combined cycle
Figure 8. Effect of partloads on energy efficiency for steam turbine and combined cycles
21 and 22), CO2, NO2 and NO are equal to 306.781 g/kWh, 0.513 g/kWh and 0.037 g/kWh, respectively. Table 4 presents the amounts of CO2, NO2 and NO reductions per year when the power generation is equal to 3.199 billion kWh/annum. 5. Conclusion The present study deals with evaluation of the thermal efficiency of CCGT with a multipleshaft type. Comparison method is applied to estimate a fossil fuel saving. The first simulation showed a conventional power plant with ST 160 MW capacity, and the model of CCPP with net power 429.57 MW. CCGT includes GT 300 MW, ST 133 MW and HRSG. SG was updated to a three pressure HRSG in the vertical configuration. The HRSG is designed with a supplementary burner, but fuel is not burned in this furnace. This is because HRSG is capable of providing the ST with the necessary heat energy. In the CCGT model, efficiency of ST and HRSG are reduced compared with the previous model, but overall thermal efficiency of CCPP is 58.28 %, while TPP represents only 34.5 %. The fuel consumption of CCPP is decreased to 163 gram per 1kWh and the fuel saving is expressed as 112.38 g/kWh. As a result, we could achieve an annual reduction in harmful 1760.18 tNOx and 981.25 ktCO2 emissions by power generation in the CCGT power unit.
18
Nomenclature Abbreviations and Initializations ETAI
isentropic efficiency (%)
CAPS
united Central Asian power system
NCV
specified net calorific value (kJ/kg)
TPP
thermal power plant
CO2
carbon dioxide (kg/s)
CCPP
combined cycle power plant
NOx
nitrogen oxides (kg/s)
CCGT
combined cycle gas turbine
M
mass flow (kg/s)
HRSG
heat recovery steam generator
W
Power (kW, MW)
ST
steam turbine
P
pressure (bar)
SG
steam generator
T
temperature (⁰C)
HP
high pressure
H
enthalpy (kJ/kg)
IP
intermediate pressure
Q
heat energy (MW)
LP
low pressure
HR
heat rate (kJ/kWh)
FWPH
feed water preheater
Cpr
compressor pressure ratio
DH
duplex heat exchanger
t
time (h)
TIT
turbine inlet temperature
TOT
the outlet temperature
Subscripts net
net power output
Greek letters
r1
Fuel required per 1kWh in the TPP
SG
SG efficiency
r2
fuel required per 1kWh in the CCPP
GT
GT efficiency
out
output value
HRSG
HRSG efficiency
in
input value
cycle
TPP efficiency
own
own use electricity
CC
CCPP efficiency
0
operating hours
ST
ST efficiency
exh
exhaust steam
λ
air ratio
s
saved fuel consumption
Ϝ
load factor
a.s
annual saved fuel consumption
19
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M. P. Boyce, GasTurbine Engineering Handbook. Houston: Butterworth-Heinemann, 2002, p. 816.
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T. K. Ibrahim, M. M. Rahman, and A. N. Abdalla, “Optimum Gas Turbine Configuration for Improving the performance of Combined Cycle Power Plant,” Procedia Eng., vol. 15, pp. 4216–4223, 2011.
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K. Qiu and a. C. S. Hayden, “Performance analysis and modeling of energy from waste combined cycles,” Appl. Therm. Eng., vol. 29, no. 14–15, pp. 3049–3055, 2009.
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D. Tucakovic, G. Stupar, T. Zivanovic, M. Petrovic, and S. Belosevic, “Possibilities for reconstruction of existing steam boilers for the purpose of using exhaust gases from 14 MW or 17 MW gas turbine,” Appl. Therm. Eng., vol. 56, no. 1–2, pp. 83–90, 2013.
[10] P. Pourbeik, “Modeling of combined-cycle power plants for power system studies,” 2003 IEEE Power Eng. Soc. Gen. Meet. (IEEE Cat. No.03CH37491), vol. 3, pp. 1–6, 2003. [11] A. Salehi, A. R. Seifi, and A. A. Safavi, “Combined-Cycle Plant Simulation Toolbox for Power Plant Simulator.,” Pacific J. Sci. Technol., vol. 9, no. 1, pp. 97–109, 2008. [12] B. El-Hefni, D. Bouskela, and G. Lebreton, “Dynamic Modelling of a Combined Cycle Power Plant with ThermoSysPro,” Proc. 9th Model. Conf., pp. 365–375, 2011. [13] J. Rodriguez Martin, E. Bernardos Rodriguez, I. Lopez Paniagua, and C. Gonzalez Fernandez, “Thermoeconomic Evaluation of Integrated Solar Combined Cycle Systems,” Entropy, vol. 16, no. 8, pp. 4199–4245, 2014. [14] Steag Energy Services GmbH, “Ebsilon Professional,” 2014. [Online]. Available: http://www.steag.us/us-our-service.html. [Accessed: 28-Jun-2015]. 20
[15] P. Regulagadda, I. Dincer, and G. F. Naterer, “Exergy analysis of a thermal power plant with measured boiler and turbine losses,” Appl. Therm. Eng., vol. 30, no. 8–9, pp. 970–976, 2010. [16] J. R. Simões-moreira, “Fundamentals of Thermodynamics Applied to Thermal Power Plants,” Therm. Power Plant Perform. Anal., 2012. [17] P. Bihari, G. Gróf, and I. Gács, “Efficiency and cost modelling of thermal power plants,” Therm. Sci., vol. 14, no. 3, pp. 821–834, 2010. [18] G. Volpi, M. Penati, and G. Silva, “Heat Recovery Steam Generators for large combined cycle plants ( 250 MWe GT output ): experiences with different design options and promising improvements by once-through technology development,” 2005. [19] R. Kehlhofer, Combined-Cycle Gas and Steam Turbine Power Plants. Penn Well, 1997, p. 388. [20] D. Mahto and S. Pal, “Thermodynamics and thermo-economic analysis of simple combined cycle with inlet fogging,” Appl. Therm. Eng., vol. 51, no. 1–2, pp. 413–424, 2013. [21] K. Arshad, “Technical and financial analysis of combined cycle gas turbine,” Therm. Sci., vol. 17, pp. 931–942, 2013. [22] J. H. Horlock, Advanced Gas Turbine Cycles, no. 4. 2003, p. 203.
21
Highlights
The combined cycle power plant (CCPP) has a steam turbine and a gas turbine. Fossil fuel savings and reduction of the CCGT of was evaluated. The performance of a three pressure CCGT is modelled under different modes. Energy efficiency of the combined cycle was 58.28%. An annual reduction of 1760.18 tNOx/annum and 981.25 ktCO2/annum can be achieved.
22
S1359-4311(16)30467-7 http://dx.doi.org/10.1016/j.applthermaleng.2016.03.158 ATE 8026
To appear in:
Applied Thermal Engineering
Received Date: Accepted Date:
2 December 2015 29 March 2016
Please cite this article as: Z. Aminov, N. Nakagoshi, T.D. Xuan, O. Higashi, K. Alikulov, Evaluation of the energy efficiency of combined cycle gas turbine. Case study of tashkent thermal power plant, uzbekistan, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.03.158
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Submission to APPLIED THERMAL ENGINEERING Entitle: “EVALUATION OF THE ENERGY EFFICIENCY OF COMBINED CYCLE GAS TURBINE. CASE STUDY OF TASHKENT THERMAL POWER PLANT, UZBEKISTAN” Zarif Aminov a , Nobukazu Nakagoshi b , TRAN Dang Xuan b , * , Osamu Higashi c , Khusniddin
Alikulov b
a
Institute of Power Engineering and Automation, Academ y of Sci ences of Uz bekistan, 29
Durmon yuli, Akademgorodok, Ta shkent, 100125, Uz bekistan b
Department of Devel opment Technology, Graduate School for International Devel opment and
Cooperation , Hiroshima University, 1-5-1 Kagamiyama, Higashi -Hiroshima 739-8529, Japan c
MI Consulting Corporation, AIM Bldg., 3-8-1 Asano, Kokurakita-ku, Kitakyushu-shi, Fukuoka 802-0001, Japan
* Corresponding author Tran Dang Xuan (Ph.D) Hiroshima University Kagamiyama 1-5-1 Higashi Hiroshima, 739-8529 Japan E-mail: [email protected] Tel/Fax: +81-42-424-6927 1
Abstract
The power generation of Tashkent Thermal Power Plant (TPP) is based on conventional power units. Moreover, the facility suffers from limited efficiency in electricity generation. The plant was constructed during the Soviet era. Furthermore, the power plant is being used for inter-hour power generation regulation. As a result, the efficiency can be reduced by increasing specific fuel consumption.
This research focuses on the evaluation of
the energy efficiency of the combined cycle gas turbine (CCGT) for the Tashkent TPP. Specifically, the objective is an evaluation of fossil fuel savings and reduction of CO 2 and NOx emissions with the using CCGT technology at conventional power plant. The proposed combined cycle power plant (CCPP) includes an existing steam turbine (ST) with 160 MW capacity, heat recovery steam generator (HRSG), and gas turbine (GT) technology with 300 MW capacity. The performance of a three pressure CCGT is modelled under different modes. As a result, the efficiency of the combined cycle was evaluated at 58.28 percent, while the conventional cycle had an efficiency of 34.5 percent. We can achieve an annual reduction of 1760.18 tNOx/annum and 981.25 ktCO2/annum. Keywords: Energy efficiency; Gas turbine; Heat recovery steam generator; Combined cycle power plant; Cycle gas turbine
1. Introduction Conventional power plants suffer from limited efficiencies, on average 30percent[1]. Owing to many thermal power plants were constructed during the Soviet era, including Tashkent Thermal Power Plant (TPP). Specifically, 20 percentage of existing generation capacity is past the useful service life, and this will increase to 40 percent by 2017 [2].In the Uzbek energy sector, the growth in the electricity demand is satisfied mainly by replacement with efficient Combined Cycle Power Plant (CCPP) and natural gas has a major role as energy resources by 2040 [1]. The installed capacity of Tashkent TPP is 1860 MW. The basic equipment are 12 condensing power units 155-165 MW each. The main fossil fuel is natural gas with 85 %, and oil as reserve fuel, which is 15 %[3]. The power plant is being used for inter-hour power generation regulation. Moreover, a steady growth in the consumption of fuel for power generation is 2
observed at the power plant, due to the expiration of the normative life of the basic equipment, in connection with which impact on the environment influence raises into the environment. CCPP is a system in which two types of the turbines are used to generate electricity[4]. Typical heat rates of the CCPPs are lower compared with steam turbine(ST) or a simple cycle gas turbine(GT), therefore a performance of the CCPPs is effectively [5]. Efficiency of power plants may reach up to 65 % due to enhancement in the gas turbine technology and optimization in heat recovery steam generator (HRSG) of combined cycle gas turbine (CCGT)[6].Reduction of the emission of polluting gases can be implemented by the using the combined cycle[7]. The effects of turbine inlet temperature (TIT) and compressor pressure ratio (Cpr) on the parameters that measure cycles’ performance, environmental impact and costs of the power plant [8]. The reconstruction of the existing SG for the purpose of using exhaust gases from GT is feasible and the investment cost could be covered by fuel saving[9]. According to the power generation method and their controls, GTs can vary significantly from fossil-fuel conventional power plants [10]. The aim of the current study is to evaluate energy efficiency of CCGT in case of Tashkent TPP, which takes into account the frequent off-design operation of the power plant with the corresponding decrease in efficiency. Thus, this research was conducted to: 1) to evaluate energy efficiency of TPP, 2) to evaluate energy efficiency of CCGT, and 3) to evaluate fossil fuel savings and reduction of CO2 and NOx emission 2. Experimental There are several methods to simulate CCPPs. Salehi, Seifi & Safavi [11], designed a simulator based on object-oriented programming, C-programming, and SIMULINK® toolbox of MATLAB®. The dynamic modelling of CCPP was designed by El-Hefni, Bouskela, & Lebreton with an introduction ThermoSysPro library, and presents the power plant test-case [12]. Simulation of the heat transfer by fluid-water/steam – integrated - solar combined cycle system is implemented by using Thermoflex thermal engineering software [13]. In this paper, the thermodynamic cycle and energy balance of the thermal systems in the conventional and combined cycle power plants are simulated with Ebsilon Professional 11.01 software. This Ebsilon software is used for evaluation, design and optimization of various power plants [14].
3
A short description of two global calculation modes in the software, which is used for these simulations, is explained below: Design mode. In this mode, we define appropriate specification values for all components, e.g. according to specifications from manufacturer. Partload mode. This mode is used for a cycle already completed and calculated in design mode. It is calculated for different loads. 3. Theory/Calculation 3.1.
Simulation of Current TPP
In order to perform the objectives shown above, three steps are required. Energy efficiency is widely used in the industrial practice to measure power plants technical performance [15]. In this section, the first step is the simulation of the current TPP at the design and off-design conditions, in order to evaluate its energy efficiency. The selected modes of operation of the power plant fit a reasonable scenario of energy production. The chosen Rankine cycle for the design of conventional power unit has ST 160 MW capacity. The process simulation diagram of the steam/water cycle is shown in Figure 1. FF - fluegas fan AF - air fan AH - air heater CC - combus on chamber SG - steam generator
PH - feed wate preheater HP, LP G - generator D - deaerator C - condenser M - motor ST - steam turbine
Air line Fluegas line
ST HP
Steam line Water line
ST HP
ST HP
Electric line
G
160.000 MW
Gas line Mechanical sh
SG
C
CC
M
M
AH
FF
AF
M
D PH LP PH HP
M
M
M
Figure 1. Schematic diagram of TPP
Steam generator (SG) with a single reheat system is performed to verify the accuracy of the simulation. Moreover, based on the traditional seven-stage extraction steam regenerative system, the stages high (HP), intermediate(IP) and low (LP) pressures turbines are modelled 4
with different mean isentropic efficiencies (ETAI), which are defined during the simulation. The live steam parameter is improved to 135 bar and 545⁰C, and the reheat steam is heated to 545⁰C. The outlet steam of the HP turbine enters the reheater and then goes into the IP turbine. The outlet steam of the IP turbine enters the LP turbine and then goes into the steam condenser after expansion in the LP turbine. Steam with 95.37 kg/s and 27.62⁰C is converted to condensate by 4537.2 kg/s and 13⁰C cooling water in the steam condenser. Temperature of the feed water reach up 227.9⁰C at inlet of steam generator, which is delivered through the LP preheaters, deaerator and HP preheaters. In off-design mode, the power plant was simulated with the nominal load 160 MW and partloads, such as 140 and 130 MW. Usually, conventional power plants work with automatically regulation of steam generation. In that case, pressure and temperature are not fluctuated at different load of turbine. Because of breaking ranks the system regulation, inlet pressure same as mass flow of the steam is decreased at partload in this study, as practical operation of steam turbine. Temperature is stable as nominal value by using injection control system (Figure 1). In Table 1, variation of parameters in Off-design is shown, when the power is specified. The mass of fuel and air has to be changed for obtaining the required power, taking into account that, with that the smaller amount of fuel and air the less power is delivered. Table 1: Parameters of the power plant in Off-design modes Partload,
W,
Mfuel,
Mair,
Msteam,
Mgas,
TOT,
HP,
IP,
Mexh,
Texh,
Pexh,
%
MW
kg/s
kg/s
kg/s
kg/s
⁰C
bar
bar
kg/s
⁰C
bar
100
160
11.58
235.38
128.92
288.5
155.88
135
26.5
95.37
27.62
0.037
88
140
10.22
207.71
113.61
254.58
141.88
118.95
23.47
84.58
26.08
0.034
81
130
9.52
193.45
105.68
237.11
134.66
110.64
21.89
78.96
25.27
0.032
In this simulation, the following are the main set of equations for Rankine cycle based on general thermodynamic relationships [16]. The efficiency of SG is calculated as: (1) Where Qout: Output heat energy of SG (MW), H: Enthalpy of output power of SG (kJ/kg), M: Mass flow of fluid type logic line, Qfuel: Input energy of the fuel is determined by the following equation (2): H
NCV (2)
5
Where Mfuel: Fuel consumption (kg/s), H: Enthalpy of the fuel (kJ/kg), NCV: Specified net calorific value of natural gas at the existing power plant is 37889 (kJ/kg) [3]. The thermal efficiency of ST is defined as: (3) Where WST: Output power of ST generator (MW). The overall thermal efficiency of the cycle is determined by the equation (4): (4) Where Wnet: Net power of TPP (MW) is expressed in the following equation (5), Wown: Own use power for auxiliary equipment (MW): (5) The annual net power of TPP as: t Where to: The operation hours equal to
) (6) , t: Number of hours a year (h), F:
Load factor of ST generator Using the equation (6), the annual fuel consumption is calculated as: (7) A fossil fuel required for power generation of 1 kWh is defined [8] by the following equation (8): (8) Where HR: Heat rate (kJ/kWh) of TPP is presented in the following equation (9): HR
(9)
3.2.Simulation of CCGT The second objective concerns the definition of the energy efficiency of the different loads of energy production at CCPP. The load dependent efficiency of power units with high capacity is considered primarily. These large scale power units remain a decisive role in large systems [17]. Modelling a simple CCGT is carried with triple pressure HRSG and a large scale power unit which has 429.57 MW capacity. The Brayton cycle represents a thermodynamic process of GT technology with 300 MW nominal load. The current ST with 133.6 MW capacity is presented as a Rankine cycle in this simulation.
6
Figure 2 presents describes a schematic diagram of the three pressures CCPP. In the simulation, the vertical HRSG design is chosen based on some attractive advantages mainly connected with the possibility of inserting an HRSG in an existing utility boiler steel structure for re-powering [18]. In the construction of the cycle of the GT, the selected components are a compressor, a combustion chamber and a turbine.
7
Nominally, the turbine has a pressure ratio of 18. We set an “internal set value of power” at the gas pipeline before the combustion chamber for controlling a load of GT. In design mode, we set the nominal steam temperature as 545 ⁰C and the mass flow, while the pressure is set in the HRSG component. After simulating the performance of the CCPP, HRSG provides ST with HP, IP and LP steam of 135, 26.5 and 2.2 bar respectively. During the modelling of ST, the seven-stage extraction steam system was discontinued from using. Thus, heat expansion of steam occurs with 78.58, 90.08 and 97.21 kg/s respectively in HP, IP and LP turbine. Steam is delivered to the steam condenser for cooling after work execution in LP turbine with 0.037 bar and 27.6 ⁰C parameters. Using the deaerator, oxygen and other dissolved gases
G
M
M
M
M M
G
Net output
removed from condensate, steam is taken from LP evaporator of HRSG with 1.1 bar and 117.5 ⁰C. The feed water, after being preheated and leaving the deaerator, is divided in two pipes and pumped to the different pressure levels. Figure 2. Schematic diagram of combined cycle power plant with a three pressure HRSG
The partload differs from the nominal conditions of the CCPP performance based on partloads of ST, when the amount of power smaller than in nominal conditions is enough for satisfying the demand. We wanted to observe what happens with variation in power, for
8
instance if TOT is set as 613 ⁰C after GT. Table 2 presents the parameters of the CCGT in off-design mode. Table 2: Variation of parameters of CCGT in Off-design modes when the power is specified Partloads of the GT, % 100
80
69
300
280
250
Cpr, bar
18.00
17.41
16.40
λ
1.86
1.88
1.93
TIT, ⁰C
1320.1
1305.6
1280.8
Mfuel1, kg/s
19.41
18.59
17.24
Mfuel2, kg/s
0
0
0
610.39
593.16
563.57
MHP steam, kg/s
78.58
75.53
70.84
MIP steam, kg/s
11.46
11.27
10.89
MLP steam, kg/s
19.95
19.24
17.97
Tfluegas, ⁰C
110.8
110.7
110.4
Mfluegas, kg/s
629.8
611.75
580.81
WST, MW
133.6
129.18
121.85
Mexh2, kg/s
97.21
94.35
89.34
Texh2, ⁰C
27.6
27.2
26.4
Pexh2, bar
0.037
0.036
0.035
GT performance WGT, MW
Mair, kg/s HRSG performance
ST performance
The following are the main set of governing equations based on general thermodynamic principles of the combined-cycle plant [19]. These equations were used to predict the performance of CCGT after evaluation of the performance of GT and ST such as for Brayton and Rankine cycles. Thus, it is relationship between profits and expenditure. The efficiency of Brayton cycle is calculated with equation (10): (10) Where WGT: Output gross power of GT generator (MW), Qfuel: Input energy of the fuel (MW), which is determined by the equation (11): (11) Where Mfuel: Fuel consumption in CCPP (kg/s), H: Enthalpy of the fuel (kJ/kg), NCV: Specified net calorific value of the fuel is 37889 (kJ/kg) [3]. The thermal efficiency of HRSG is expressed in the following equation (12): 9
(12) Where M1: Flue gas mass flow (kg/s), H1: Enthalpy of flue gas (kJ/kg), Exhaust gas mass flow (kg/s), H2: Enthalpy of exhaust gas (kJ/kg). The thermal efficiency of ST is calculated as: (13) Where WST: Output gross power of ST generator (MW), Qin: input power of ST which is equal to the output energy (
) of SG (MW).
The combined cycle efficiency is the ratio of addition of GT and ST output to the input to the cycle i. e. heat input to GT. By inserting equation (11) into equation (14), the efficiency of combined cycle is presented as: (14) Where Wnet: Net power of CCPP (MW), which is equal to the difference between the gross power of CCPP and own use power, Wown: own use power in CCPP (MW), Qfuel: Input energy of the fuel (MW) is expressed in Equation (11). The annual net power output of CCPP is defined as: (15) Where to: The operation hours equal
, t: Number of hours a year (h), F: Load factor
of CCGT A fossil fuel required for power generation of 1 kWh at the CCPP is determined as the following equation (16) [8]: (16) Where HR: Heat rate (kJ/kWh) of CCPP is indicated in the following equation (17):
(17) 3.3.Comparative Study and Environmental Analysis The fuel saving is evaluated through the difference between the fossil fuel required of the CCPP and the conventional power plant per 1 kWh: (18)
10
Where Mr1, Mr2: Fossil fuel required per 1 kWh power generation at TPP and CCPP, respectively (kg/s). Annual saved fossil fuel is the amount of the annual net power output multiplied by the saved fossil fuel: (19)
Figure 3. Schematic diagram of reduction of CO2 and NOx emissions
As final step, CO2 and NOx emissions reduction are evaluated based on saved fossil fuel. The simulation is performed by using combustion chamber, a three selective splitter which is for each emission i.e. CO2, NO2 and NO. Figure 3 shows the model of reduction of CO2 and NOx emissions. The amount of the fossil fuel for burning in the combustion chamber is equivalent to the saved natural gas which is marked with a purple line. Input data is taken from the simulation of CCPP. Using the equations of selective splitter models [14], CO2, NO2 and NO are evaluated as below: (20) Where XCO2: CO2 fraction is equal to 0.12803 (21) Where XNO2: NO2 fraction is equal to 0.00021 11
(22) Where XNO: NO fraction is equal to 0.00001 Taking into account the annual power generation, the amount of CO 2, NO2 and NO reductions per year are calculated by these the equations (23, 24 and 25): (23) (24) (25) 4. Results and Discussion In off-design conditions, the temperature of the steam extractions is defined by the ETAI of the turbine implemented in Ebsilon. The heat transfer capacity of heat exchanger varieties with the varying in mass flow through the heat exchanger. We can determine the transfer surfaces in the heat exchanger in design mode and it will be constant in every mode. In Ebsilon, the turbine has an established default value of ETAI. The default ETAI for each part of the turbines is from a different index. For instance, ETAI of the HP turbine is 0.85, and this value is determined in partload by some correction curves. Figure 4 represents the temperature/heat diagram in the heat exchangers of Rankine cycle. The feed water is preheated with seven-stage extraction steam regenerative system at high and low pressures. Thus, temperature is preheated from 27.6 ⁰C up to 227.9 ⁰C by increasing the cycle efficiency. Moreover, deaerator is also preheat the condensate for 12 ⁰C during the deaerating process.
12
Figure 4. Heat transfer in the heat exchangers of Rankine cycle
Thermodynamic principles of Rankine cycle with vapor reheating occurs based on the temperature/specific entropy diagram, which represents input energy of fossil fuel, power generation and efficiency of steam turbine cycle (Figure 5).
Figure 5. T-S diagram of the Rankine water-steam cycle
Table 3 presents the main results of the first simulation which are calculated with the equations presented in at the previous section. At nominal mode, high temperature source is equal to 438.87 MW and power generation is 151.54 MW. Considering the load factor of 13
generator, annul net power output of 1.128*109 is produced. Heat rate of 10434.69 kJ/kWh at the cycle with specific fuel consumption of 275 g per 1 kWh. According to equations (2, 4 and 5), steam turbine cycle efficiency of 34.5 % is obtained at nominal mode (Table 3). Table 3: Performance of the conventional power plant Parameters
Loads (%) 100
88
81
438.87
387.28
360.7
SG (%)
89.98
90.82
91.26
ST (%)
40.49
39.77
39.46
151.54
133.16
123.85
Qin (MW)
Wnet (MW) cycle (%)
34.5
Wannual (kWh/annum)
1.128*10
HR (kJ/kWh)
10434.69
Mr1 (g/kWh)
275.4
34.35 9
0.992*10
34.31 9
0.922*109
10.478.87
10493.7
276.57 6
Mannual (kg/annum)
310.74*10
274.22*10
F (%)
85
85
276.96 6
255.39*106 85
Such as steam extractions are shut down at steam turbine, temperature of feed water is lower, which need more fuel consumption for power generation. For this reason, steam generator is designed as HRSG. Steam generation is developed with effective using the exhausted gas after gas turbine due to extra heat exchangers and evaporators. Since the enthalpy of the exhaust gases of the GT is enough to achieve nominal conditions with 27.6 ⁰C and 97.21 kg/s in steam condenser at Rankine cycle. As shown in Table 2, no necessity for fuel consumption (Mfuel2) in HRSG as supplementary. Using a three pressure stage, we can achieve a widely heat transfer in the HRSG (Figure 6). Exhaust gases with 613 ⁰C is capable to produce a steam with HP, IP and LP. Thereby, the feed water of 436 kJ/kg can be increased up to 3557 kJ/kg for delivering to HP ST.
14
Figure 6. Heat transfer in the HRSG in a three pressure CCPP
Steam turbine is constructed with certain amount for heat expansion and power generation. According to these facts, steam turbine is reconstructed as 133.287 MW capacity. In this case, steam is delivered to LP turbine from LP evaporator through the control valve with 2.2 bar. Depending on the required power, Ebsilon calculates by itself the amount of the necessary air and fuel. GT delivers exactly that amount. Figure 7 illustrates the combined cycle that consist the steam turbine cycle (Rankine cycle) and gas turbine cycle works through the Brayton cycle. Therefore, the performance is almost doubled. In particular, the area under the T-s curve of the process represents the heat to be transferred to the system, and the effectiveness of the power generation during the thermodynamic process. The growth of energy dissipation is observed at the off-design modes. This means that the measures of deviation of the actual from the ideal process were increased. The highest index of the efficiencies of GT, ST and CC conform at a nominal load, except of effectiveness of HRSG. Thereby the energy efficiencies decrease at the partload modes with the incrementation of the fuel consumption.
15
Figure 7. T-S diagram of the combined cycle
Working in partload conditions means changes in its power output and thereby in the pressure ratio of GT. In a real GT the mass of air entering the compressor is regularized by changing the inlet guides vanes. The given operating range of TIT and Cpr are significant in the analysis, as mentioned in the study of Mahto & Pal [20]. Furthermore the other components of the turbine also have to be considered in the overall efficiency of GT and the CCPP, which experience a variation in the performance working at the unload conditions. In Ebsilon, compressors and turbines have an established default value of isentropic efficiency. ETAI in a compressor or a turbine is a comparison between the real power obtained or consumed and the isentropic case. The default ETAI for turbines is 0.92 and for compressors is 0.88. In partload that value is defined by some correction curves. The variation of ETAI is directly proportional to the change of mass flow which is going through the compressor or turbine. According to the results from the previous conventional power plant model, the HP, IP and LP STs have an established default value of ETAI. In general, the GT provides two-thirds of the total capacity [21]. In this simulation, GT supplies 69.2 % of the power in nominal load, while ST delivers only 30.8 % of the energy of the CCPP power output of 433.292 MW.
16
Using the equations (10, 11, 12 and 13), CC efficiency of 58.28 % is obtained at nominal mode (Table 4). So, it is able to reduce SFC from 275.4 g/kWh until 163 g/kWh, heat rate of CC amount to 6176.81 kJ/kWh. In result, a fuel saving of 112.38 g per 1 kWh and annual amount is 359.45 million kg/annum. Table 4: Performance of the CCPP with a three pressure HRSG Loads (%)
Parameters 100
80
69
Wnet (MW)
429.57
405.27
368.12
Qin (MW)
735.37
704.46
653.14
GT (%)
40.7
39.66
38.19
HRSG (%)
82.86
82.87
82.92
ST (%)
37.19
37.04
36.84
58.28
57.4
56.23
Wannual (kWh)
3.199*109
3.018*109
2.741*109
HR (kJ/kWh)
6176.81
6272.04
Mr2 (g/kWh)
163
165.5
Ms (g/kWh)
112.38
CC
(%)
Annual fuel savings (kg/annum)
6401.88 169
111.03 6
107.99 6
296.02*106
359.45*10
335.05*10
85
85
85
MCO2 (tCO2)
981.25*103
914.64*103
808.09*103
MNO2 (tNO2)
1641.25
1529.1
1351.22
MNO (tNO)
118.93
110.81
97.3
Ϝ (%)
Figure 8 shows the fluctuation of the efficiencies of the steam turbine and combined cycles at the various partloads. In the both model, effectiveness of steam generation is increased. In case of ST, efficiency is decreased by 3.3, 2.73 and 2.62 % at partloads, respectively. But, cycle efficiency of power generation is increased by 23.78 % at nominal mode. Environmental analysis was implemented on the grounds of the burning a saved fuel by the CCPP. The economics of power generation are affected mainly by the amount of CO2 produced [22], for instance by a large amount of harmful emissions, created by the burning process, turning natural gas into carbon dioxide. In Table 4, the saved fuel with 0.1123 kg/kWh fuel consumption is delivered by the gas pipeline. According to the calculation using Equations (20,
17
90.00
Steam generator Steam turbine 1 Cycle
Efficiency, %
60.00
Gas turbine
HRSG
30.00
Steam turbine 2 0.00 1
2 Partload cases
3
Combined cycle
Figure 8. Effect of partloads on energy efficiency for steam turbine and combined cycles
21 and 22), CO2, NO2 and NO are equal to 306.781 g/kWh, 0.513 g/kWh and 0.037 g/kWh, respectively. Table 4 presents the amounts of CO2, NO2 and NO reductions per year when the power generation is equal to 3.199 billion kWh/annum. 5. Conclusion The present study deals with evaluation of the thermal efficiency of CCGT with a multipleshaft type. Comparison method is applied to estimate a fossil fuel saving. The first simulation showed a conventional power plant with ST 160 MW capacity, and the model of CCPP with net power 429.57 MW. CCGT includes GT 300 MW, ST 133 MW and HRSG. SG was updated to a three pressure HRSG in the vertical configuration. The HRSG is designed with a supplementary burner, but fuel is not burned in this furnace. This is because HRSG is capable of providing the ST with the necessary heat energy. In the CCGT model, efficiency of ST and HRSG are reduced compared with the previous model, but overall thermal efficiency of CCPP is 58.28 %, while TPP represents only 34.5 %. The fuel consumption of CCPP is decreased to 163 gram per 1kWh and the fuel saving is expressed as 112.38 g/kWh. As a result, we could achieve an annual reduction in harmful 1760.18 tNOx and 981.25 ktCO2 emissions by power generation in the CCGT power unit.
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Nomenclature Abbreviations and Initializations ETAI
isentropic efficiency (%)
CAPS
united Central Asian power system
NCV
specified net calorific value (kJ/kg)
TPP
thermal power plant
CO2
carbon dioxide (kg/s)
CCPP
combined cycle power plant
NOx
nitrogen oxides (kg/s)
CCGT
combined cycle gas turbine
M
mass flow (kg/s)
HRSG
heat recovery steam generator
W
Power (kW, MW)
ST
steam turbine
P
pressure (bar)
SG
steam generator
T
temperature (⁰C)
HP
high pressure
H
enthalpy (kJ/kg)
IP
intermediate pressure
Q
heat energy (MW)
LP
low pressure
HR
heat rate (kJ/kWh)
FWPH
feed water preheater
Cpr
compressor pressure ratio
DH
duplex heat exchanger
t
time (h)
TIT
turbine inlet temperature
TOT
the outlet temperature
Subscripts net
net power output
Greek letters
r1
Fuel required per 1kWh in the TPP
SG
SG efficiency
r2
fuel required per 1kWh in the CCPP
GT
GT efficiency
out
output value
HRSG
HRSG efficiency
in
input value
cycle
TPP efficiency
own
own use electricity
CC
CCPP efficiency
0
operating hours
ST
ST efficiency
exh
exhaust steam
λ
air ratio
s
saved fuel consumption
Ϝ
load factor
a.s
annual saved fuel consumption
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Highlights
The combined cycle power plant (CCPP) has a steam turbine and a gas turbine. Fossil fuel savings and reduction of the CCGT of was evaluated. The performance of a three pressure CCGT is modelled under different modes. Energy efficiency of the combined cycle was 58.28%. An annual reduction of 1760.18 tNOx/annum and 981.25 ktCO2/annum can be achieved.
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