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Improvement of overall performance of Benghazi North combined power plant by retrofitting the Inlet
Cooling with a single effect absorption chiller Salah Masheiti*, Jamal Abdusamad**,Awad Shamekh*, and Awad Bodalal*
*Faculty of engineering University of Benghazi, Libya
[email protected] [email protected] [email protected]
**School of Mechanical and systems Engineering, University of Newcastle upon Tyne, UK
Abstract---The work performed and reported in this paper examined the overall performance of the Benghazi plant throughout the year at different load and different ambient conditions. This was done by simulating the power plant using the IPSEpro commercial software. An absorption refrigeration unit was then added to the model and the simulation repeated with the absorption unit stabilising the gas turbine inlet conditions. The performance of the gas turbine and steam turbine units is presented and discussed. The results of the simulation indicate that retrofitting an absorption refrigerator to the existing power plant is thermodynamically possible and has the potential for improving the overall efficiency and reducing the greenhouse gases produced by the plant.
Keywords--- Ccombined cycle, thermal efficiency, Lithium Bromide, absorption chiller, greenhouse gases, IPSEpro.
I. INTRODUCTION
Gas turbine power plants operating in hot climate condition suffer a decrease in output power and efficiency during the hot summer months. The typical combustion turbine on a hot summer day, for instance, produces up to 20% less power than on a cold winter day. As a result, a number of cooling techniques and technologies have evolved over the years to maximize turbine output. In the combined cycle mode the steam plant takes the energy in the gas turbine exhaust. Because the mass flow drops with increased ambient temperature, it is clear that the combined cycle overall power output will significantly drop with temperature, given that both the gas turbine and the steam turbine output are reduced [1]. Inlet air-cooling system is a popular choice worldwide to reduce the power loss especially at times of peak demand in the summer. Cooling the turbine inlet air back to the ISO condition can restore the design point performance. M. Ameri, and S. Hejazi [2] stated in their study paper that for each 1ºC increase in ambient temperature, the power output will decrease by 0.74%, and also that the variation in the ambient temperature in summer months produces a typical loss of 20%
of the rated capacity. Performance and economic enhancement of cogeneration gas turbines through compressor inlet air cooling has been described by M. De Lucia, R. Bronconi, and E. Carnevale [3] who showed that gas turbine air cooling systems serve to raise performance to peak power levels during the hot months when high atmospheric temperatures
cause reductions in net power output. This work describes the
technical and economic advantages of providing a compressor inlet air cooling system to increase the gas turbine's power rating and reduce its heat rate. Power output decreases during the hot summer months, just when electricity demand is at its highest and capacity is most valuable. Reason for the performance drop is that hot air entering the turbine is less dense than cool air, so mass flow through the machine is reduced. Lower mass flow through the gas turbine also means there’s less exhaust energy supplied to the heat-recovery steam generator (HRSG), so steam production is reduced as well. One way to avoid the performance degradation is by installing an inlet air-cooling system which is old technology that’s becoming increasingly popular in light of electric industry restructuring. Nasser, A.E.M. and M.A. El-Kalay [4] in 1991 proposed the use of absorption chillers powered by the waste heat of the exhaust gases to cool the compressor inlet air. This study suggested that using a stream of exhaust gases with a flow rate of 300 kg/s at 450ºC, to cool the ambient air from 40ºC. He showed that, cooling the ambient air entering the compressor, from 40ºC to 30ºC increased the power output by 10%, (1% per °C ΔT). E. Kakaras, A. Doukelis, and Karellas [5] pointed out that the variation in the air ambient temperature could lead to increase the specific fuel consumption. He presented a computer simulation of the integration of an evaporative cooler and an absorption chiller to reduce the compressor air intake temperature. The absorption chillers was powered by waste heat through a heat exchanger, and found to be able to cool the air intake to 5ºC, for an ambient temperature less than 18.5ºC, and to 15ºC for an ambient temperature of 40ºC. It can be concluded that for
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the type of industrial gas turbine that is being considered in this study as opposed to an aero derived unit the absorption chiller energised by the exhaust gas waste heat is the preferred form of inlet air-cooling.
The plant model in this study is an existing power plant situated in the city of Benghazi Libya and it has been connected to the Libyan grid since 1979. The power plant site is located North of Benghazi city, on the cost of the Mediterranean Sea 13 km from Benghazi harbour. Figure 1 shows the flow diagram for Benghazi North combined cycle power plant (CCPP) and the parameters used for the simulation project are also shown in table 1. The power plant consists of 2 gas turbine (GT) GT13E1 & GT13E2 units and 1 steam turbine (ST) unit. The main operating parameter of gas turbine units as shown in fig.1are as follows:
• Gas turbine 1- (GT13 E1) consumes 9.263 kg/s of natural gas (fuel), 450.8 kg/s of air at ambient temperature of 37 °C to produce 149.5 MW of electricity and emits 460.5 kg/s of exhaust gases at a temperature of 568 °C.
• Gas turbine 2- (GT13 E2) consume s 9.263 kg/s of natural gas (fuel) and 468 kg/s of air at ambient temperature of 37 °C to produce 150 MW of electricity and emits 460.8 kg/s of exhaust gases at a temperature of 580.5 °C.
The ISO performance of the ABB GT13E1 and GT13E2 units at Benghazi North power plant are shown in Table I.
TABLE I BASE PLANT PARAMETERS SIMULATED AT ISO DAY
Parameters at
ISO Day At Design point
At Ambient
temperature
of 37 °C
Manufacturer ABB
Model (power) GT13E1 (156.64 MW)
GT13E2 (172.2 MW)
GT13E1 (149.5 MW)
GT13E2 (150.0 MW)
Frequency 50 MHz 50 MHz
Overall thermal
efficiency at
generator terminal
36.4%
34.4%
Compressor
pressure ratio
GT13E1 (15.4:1)
GT13E2 (16.5:1)
GT13E1(15.4:1)
GT13E2(16.5:1)
Number of
compressor stages
21
21
Number of turbine
stages 5 5
Shaft rotor type Hollow concentric Hollow concentric
Shaft speed 3000(rpm) 3000(rpm)
Type of the shaft Single one spool Single one spool
Number of
compressors 1 Axial flow 1 Axial flow
Fuel type Natural gas Natural gas
Figure 1. Benghazi North combined cycle Power Plant Diagram [6]
II. MODELLING OF THE COMBINED CYCLE PLANT
The heat exhaust of gas turbine can be utilized for useful work; as the case of this study, waste heat is utilized to operate absorption chillers to cool the ambient temperature for the gas turbine air compressor inlet. In this stage the two plants will be attached together in a way that the gas turbine exhaust heat energy leaving the heat recovery steam generator (HRSG) units, will be linked through a counter current heat exchanger to power the absorption chillers as shown in fig. 2. The simulated model indicated that the exhaust stream exits the HRSG unit for (GT1) at a temperature of 95.28°C, enters the counter current heat exchanger with a heat transfer area of 735.28kW/K, where 3.1% of the remaining exhaust’s heat energy is utilized to power a single effect absorption chillers to produces 2328RT of chilled water at temperature of 6°C. For (GT2) HRSG unit the exhaust stream temperature is 95.9°C, enters the counter current heat exchanger with heat transfer area of 722.92kW/K, where 3.28% of the remaining exhaust heat energy is utilized to power the second absorption chillers and have refrigeration capacity of 2328RT of chilled water temperature of 6°C. Water circulation stream has mass flow rate of 330kg/s and transfer heat energy from absorption chillers counter current heat exchanger at a temperature of 95.28 & 95.9°C to the absorption chiller generator. The water then pumped back to the heat exchanger at a temperature of 77°C. The chilled water stream is circulated between the evaporator in the absorption chiller at temperature of 6°C and the gas turbine inlet heat exchanger, where it cools the inlet air of the gas turbine to the desired temperatures. It is then pumped back to the evaporator at a temperature of 12°C, with mass flow rate of 330kg/s. Furthermore, a stream of seawater at a temperature of 25°C is used to cool the absorption
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chillers. Firstly, it cools the condenser and leaves with a temperature of 33°C and then proceeds to cool the condenser, and its outlet temperature is 40.7°C. The first simulation was performed for ISO conditions i.e. ambient temperature 15°C, humidity 60%, atmospheric pressure 1.013 bar and seawater temperature at 25°C at full load. The part load condition was examined by varying the output between 25% and 100% with equal load distribution between the gas turbines.
Figure 2. Benghazi North Power Plant retrofitted with Absorption Chillers (IPSEpro Model)
III. RESULTS
Basic CCPP model parametric studies sensitivity to changes in ambient conditions This study is set to investigate the plant performance under the fluctuating weather conditions i.e. ambient temperature. The maximum and the minimum ambient temperature values considered were determined from the weather data that obtained from the planning department of the power plant. The sensitivity studies were performed using the PSExcel module, which has been used to create the required variation and to exchange the input values and results between the IPSEpro process simulator and MS Excel spreadsheet. Figures 3 through to 7 illustrate the outcome of this simulation. As expected the power output and efficiency both decrease with increased ambient temperature with the sensitivity being largest at the high load condition for the power output but the low load case for efficiency. The fuel flow rate decreases significantly at high load with increased ambient temperature compared to the low load case. This is reflected in the emission rate of CO2, which is highest for the low load high ambient temperature condition. Figure 3 shows the effect of ambient temperature on the performance of the plant output
power at different load before the improvement, the CCPP at full load and operating at ambient temperature of 37˚C above the ISO condition, power output decreased by 12.17%, CO2 emission increased by 1.84% and the plant efficiency dropped by 1.97%. The CCPP operating at 50% part load and under ambient temperature of 37˚C, the power output decreased by 10.24%, CO2 increased by 6.50% and the plant efficiency also dropped by 2.80%.
Figure 3. Power Output variations with Ambient Temperature without AC
The CCPP with absorption chillers performed much better than the plant without absorption chiller as shown in fig. 4. When operating with ambient temperature between 15°C and 37°C at full load and with the absorption chiller, the power output drooped only by 1.10%, CO2 decreased by 0.24% and the plant efficiency increased by 0.16%. When the plant operated at 50% part load the CO2 emissions increased slightly by 0.30% and efficiency decreased by 0.18%.
0
100
200
300
400
500
600
700
0 10 20 30 40 50 60
Pow
er o
utpu
t (
MW
)
Ambient Temp. (°C)
100% without chiller
75% without chiller
50% without chiller
25% without chiller
0100200300400500600700
0 20 40 60
Pow
er o
utpu
t (
MW
)
Ambient Temp. (°C)
100%with chiller75% with chiller50% with chiller25% with chiller
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Figure 4. Ambient Temperature vs. Power output with AC and at different load
Figures 5 and 6 show the efficiency of the power plant at all loads, with and without AC and under different ambient temperature. The plant operating at 100% load and without absorption chillers and at ISO condition, for every 5ºC increase in ambient temperature the efficiency drop by 0.27% although with the absorption chiller the efficiency improved by 0.04%.
Figure 5. Plant thermal efficiency vs. Ambient Temperature and at different loads without AC
Figure 6. Ambient Temperature vs. Plant thermal efficiency at different loads with AC
Table II shows the effect of ambient temperature fluctuation on the efficiency of the combined cycle power plant. The efficiency for the plant operating at part load of 25% without absorption chiller decreased by 0.7% for every 5˚C increase in ambient temperature but with the absorption chiller the
efficiency increased by 0.27% at ISO condition. At 75% load the efficiency increased by 0.31% and at 50 % load the efficiency increased by 1.57%.
TABLE II AMBIENT TEMPERATURE VS. CCPP AT DIFFERENT LOADS WITH AND WITHOUT AC
Temp
(ºC)
CCPP Efficiency (%)
without Absorption
Chiller
Different loads
CCPP Efficiency (%)
with Absorption Chiller
Different loads
Loads
%
25
%
50
%
75
%
100
%
25
%
50
%
75
%
100
%
ISO 30.3 42.3 50 53.1 31.6 43.9 50.2 53.8
25˚C 28.9 41.0 49 52.6 31.5 43.8 50.2 53.4
35˚C 27.5 39.8 48.1 52.2 31.4 43.7 50.2 53.5
45˚C 26.6 38.7 47.3 51.9 31.4 43.7 50.3 53.7
50˚C 26.2 38.2 47 51.8 31.4 43.7 50.4 53.8
One effect of ambient temperature variation is a change in the fuel consumption per MW electricity produced. The fuel consumption was calculated and results are given in Table III. Examination of this table shows that fuel consumption decreased with increase of ambient temperature. However, the rate of fuel consumption decrease is lower than the rate of decrease of electricity production which reduces the overall efficiency. This is in agreement with the work of Hasan Hüseyin, and Süleyman Hakan [8] who examined two gas turbine models and seven-climatic regions of Turkey. They analyzed electricity production and fuel consumption for the cold and hot region and found that electricity output in hot regions was reduced by 2.87% whereas in cold regions the output increased by 7.85%. Not only the electricity production decreases in hot region, but it also increases the fuel consumption per unit of electricity production.
TABLE III AMBIENT TEMPERATURE VS. CCPP FUEL CONSUMPTION WITH AND WITHOUT AC
Temp
(˚C)
CCPP Fuel consumption
(kg/s) without AC and
different loads
CCPP Fuel consumption
(kg/s) with AC and
different loads
25
%
50
%
75
%
100
%
25
%
50
%
75
%
100
%
ISO 9.6 13.5 17.6 21.8 9.7 13.7 18.2 22.7
25˚C 9.6 13.2 17 20.8 9.7 13.7 18.1 22.6
35˚C 9.6 13 16.3 19.7 9.7 13.6 18 22.4
15202530354045505560
0 10 20 30 40 50 60
Pla
nts
The
rmal
Eff
icie
ncy
(%)
Ambient Temp. (°C)
100% with outchiller75% without chiller50% without chiller25% without chiller
15202530354045505560
0 10 20 30 40 50 60
Pla
nt th
erm
al
effi
cien
cy
(%)
Ambient Temp. (°C)
100% with chiller75% with chiller50% with chiller25% with chiller
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45˚C 9.5 12.7 15.7 18.7 9.6 13.6 17.9 22.3
50˚C 9.5 12.6 15.3 18.1 9.6 13.6 17.8 22.2
Figure 7 shows the behaviour of the HRSG in recovering the energy available in the gas turbine exhaust. It is the most efficient at the design case full load and is not sensitive to changes in ambient temperature. At low load however the performance is very largly effected by the ambient Temperature.
Figure 7. Ambient Temperatures vs. Heat Energy Utilization by ST
Combined gas and steam Turbines with Inlet Cooling and at different load
When the combined plant is integrated with an absorption refrigeration unit for inlet air cooling the energy in the gas turbine exhaust is distributed in the following manner. At ISO condition the first HRSG utilises 83.5% of the available energy and a further 3.26% is used by the absorption chiller. The remaining 13.24% is rejected to the atmosphere as waste at a temperature of 77°C and very low enthalpy 80.75 kJ/kg. The second HRSG also utilises 83.97% of the exhaust gas energy, and the AC unit utilises 3.08% and the remaining 12.95% is rejected to the atmosphere (T = 77°C and h = 80.65 kJ/kg). The absorption refrigerator produces a refrigeration effect of 4656 RT at a COP of 0.79. (Note at this condition the absorption refrigerator is producing chilled water, as inlet air cooling is not required). The emission of CO2 from the exhaust system is 424.57 kg/kWh for the CCPP which is less than the base power plant produced before assembly. The result indicate that the absorption chiller performance has not been negatively affected by being attached to the power plant, neither has the power plant itself. The absorption chillers stably generates in total 4657 RT of cooling effect regardless of the ambient conditions or plant configurations. As the cycle is now producing a usable commodity in terms of the refrigeration effect it is acceptable to include the refrigeration effect in the plant energy audit. The traditional form of
efficiency does not reflect the additional energy use as it takes into account only the work output. It is necessary now to replace the overall plant efficiency with the energy utilisation factor (EUF) which is defined as the work and useful energy output divided by the energy input. This is shown in figure 8. The EUF is larger than the efficiency due to recovering an additional 3.17% of the exhaust gas heat energy in the absorption chillers.
Figure 8. Ambient Temperatures vs. EUF with AC
The analysis has been taken a step further by cooling the inlet air below the ISO condition to 10ºC. This produces an increase in electricity production in the region of 2.88% and the specific fuel consumption per unit electricity is 0.13 kg/kWh. Figures 9 and 10 illustrate the effect on fuel consumption of the plant before and after the improvement. Figure 9 shows the fuel consumption at different loads without the improvement. The plant at full load and 75% partial load, showed that with increase of ambient temperature, the fuel flow rate decreases. The reason for that as discussed above is the ambient temperature effect on air density. The plant at 25% and 50% partial load the fuel consumption has faintly decreased.
40
50
60
70
80
90
100
0 10 20 30 40 50 60
Heat
ene
rgy
utili
zatio
n b
y S
T (%
)
0.25% without AC0.50% without AC0.75% without AC100% without AC
Ambient Temperature (ᵒ C)
40
50
60
70
80
90
100
0 10 20 30 40 50 60
Hea
t ene
rgy
util
izat
ion
by S
T (
%)
0.25% with AC0.50% with AC0.75% with AC100% with AC
Ambient Temperature (ᵒ C)
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Figure 9. Fuel Consumption at all Loads and without AC
Figure 10 also shows the plant fuel consumption after improvement. As seen from the chart below the plant fuel consumption increased slightly with ambient temperature increase, and that’s due to the absorption chiller cooling and stabilizing the inlet air temperature intering the compressor.
Figure 10. Ambient Temperature vs. Fuel Consumption at all loads and with AC
Figure 11 and 12 shows the plant CO2 emission rate versus ambient temperature fluctuation before and after improvement. The plant’s emission rate improved with inlet-air cooling to 10ºC, for ISO condition the plant at full load improved CO2 emission by 0.47%, plant at 75% part load the emission of CO2 improved by 0.44%, plant at part load of 50% the emission rate improved by 3.55% and the plant operating at 25% part load the improvement was 4.15%. This indicate that using absorption refrigeration units for the compressor inlet air cooling of the gas turbine will increase production
capacity of the plant and reduction in the CO2 emission released to the environment.
Figure 11. Ambient Temperatures vs. CCPP CO2 Emission Rate
Figure 12. Ambient Temperatures vs. CCPP CO2 Emission Rate
Table IV show results for the combined cycle power plant, CO2 emission rate achieved by IPSEpro simulation. The table also shows the plant result with and without AC at different ambient temperature.
0369
1215182124
0 20 40 60
Fue
l co
nsum
ptio
n (
Kg/
s)
Ambient Temp. (°C)
100% without chiller75% without chiller50% without chiller25% without chiller
0369
1215182124
0 10 20 30 40 50 60
Fue
l c
onsu
mpt
ion
( k
g/s)
Ambient Temp. (°C)
100% with chiller75% with chiller50% with chiller25% with chiller
150250350450550650750850950
0 10 20 30 40 50 60
Co2
Em
issi
on (
Gra
ms/
kW/h
r)
Ambient Temp. (°C)
100% without chiller75% without chiller50% without chiller25% witout chiller
150250350450550650750
0 20 40 60
Co2
Em
issi
on (
Gra
ms/
kW/h
r)
Ambient Temp. (°C)
100% with chiller75% with chiller50% with chiller25% with chiller
Temp
(˚C)
CCPP CO2 Emission
(kg/kWh) without
AC & at different loads
CCPP CO2 Emission
(kg/kWh)
with AC & at different loads
25
%
50
%
75
%
100
%
25
%
50
%
75
%
100
%
ISO
15˚C 746 535 453 426 715 516 451 424
25˚C 782 552 462 430 718 517 451 424
35˚C 821 569 470 433 719 518 451 423
45˚C 851 584 478 436 720 518 450 421
50˚C 864 592 482 437 720 517 449 420
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TABLE IV AMBIENT TEMPERATURE VS. CCPP CO2
EMISSION RATE
IV. CONCLUSION
All the results obtained indicate that modifying an existing combined gas steam turbine power plant, currently operating in Benghazi Libya, to include inlet air cooling will have beneficial effects. The simulation predicted the proposed plant would work in a stable and efficient condition at different loads. Installing absorption chillers greatly benefits the plant in more than one way. Firstly, it utilizes the waste heat energy that usually will be rejected into the environment. Secondly, it stabilizes the effect of the ambient temperature fluctuation. Thirdly, it increases the power output and improves the plant overall efficiency. Finally, it increases the gas turbine lifetime and reduces maintenance costs.
ACKNOWLEDGMENT
The authors would like to thank Engineer Salah El-Safarani form Benghazi North combined power plant for his great help.
REFERENCES
[1] P.J. Dechamps, P.H. Mathieu, D. Magain, “Advance Combined Cycle Alternative with Advance Gas Turbine”, in IGT1 Vol. 8, pp. 387, 1993.
[2] M. Ameri, and S. Hejazi, “The study of capacity enhancement of the Chabahar gas turbine installation using an absorption chiller”, Applied Thermal Engineering, Vol. 24 (1), pp.5968, 2004.
[3] M. De Lucia, R. Bronconi, and E. Carnevale, “Performance and Enhancement of Economic Enhancement of Cogeneration Gas Turbines Through Compressor Inlet Air Cooling”, Journal of Engineering of Gas Turbine Power, Vol. 116 (2,360), (6) doi:10.1115/1.2906828, April 1994.
[4] A.E.M. Nasser, and M.A. El-Kalay, “A heat-recovery cooling system to conserve energy in Gas-Turbine power station in the Arabian Gulf”, Applied Energy, Vol. 38(2), pp.133-142, 1991.
[5] E. Kakaras, A. Doukelis, and Karellas, “Compressor intake-air cooling in Gas Turbine plants”, Energy, Vol. 29 (12-15), pp. 2347-2358, 2004.
[6] GECOL 2006, GECOL Annual Report, General Electricity Company of Libya, Tripoli-Libya, 2006.
[7] M. Pogoreutz, I. Giglmayr, M. Nixdorf, “Comparison of software for calculation of thermodynamic processes”, Report of VGB research project Nr.177 VGB, (German), 2000.
[8] Hasan Hüseyin, and Süleyman Hakan, “Effect of ambient temperature on the electricity production and fuel consumption of a simple cycle gas turbine in Turkey”, Applied Thermal Engineering. Vol. 26,(2-3), pp.320-326, February2006.
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