Seminar Report

On

“Energy Conservation in a Steam combined cycle Power Plant”

Hoozefa J. Shaikh08BME040

GuideProf. R N Patel

Mechanical Engineering DepartmentInstitute of Technology

Nirma University of Science and TechnologyAhmedabad – 382 481

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Certificate

This is to certify that Mr. Shaikh Hoozefa J, Roll No 08BME040, of B. Tech. Sem. VI has successfully completed the report of seminar on “Energy Conservation in a Steam combined cycle power plant”.

Date: 23/4/2011

Guide: Prof R N Patel

Head of the Department: Prof V R Iyer

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Contents

Page No.Acknowledgements IAbstract IIList of Figures / Diagrams IIIList of Tables IVNomenclature VI

Chapter 1 Introduction to Combined Cycle power plant 8-91.1 Combine cycle power plant (CCPP) 1.2 Basic Layout of CCPP 1.3 Working Principle of a CCPP 1.4 Thermodynamic Cycles Involved in a CCPP

1.4.1 Toppling Cycle 1.4.2 Bottoming Cycle

Chapter 2 Methods of energy Conservation of Combined Cycle Power Plant 102.1 Various Methods for Energy Conservation

Chapter 3 Performance Improvement of Combined Cycle Power Plant Based 11-14on the Optimization of the Bottom Cycle and Heat Recuperation3.1 Influence of HRSG inlet temperature on the efficiency of steam bottoming cycle 3.2 Gas to Gas Recuperation 3.3 Conclusion

Chapter 4 Effect of Inlet Air Cooling in Gas turbine 15-18 4.1 Effect on inlet air temperature 4.1.1 Case study for determining effect of inlet air cooling in GT4.2 Result

4.2.1 Power Output and Efficiency 4.2.2 Specific Fuel Consumption 4.2.3 Energy of Exhaust Gas

4.3 ConclusionChapter 5 Comparison of various types of Cooling Method for inlet air Cooling 19-235.1 Case Study comparing various cooling technique for inlet air cooling 5.2 Various techniques considered

5.2.1 Evaporative coolers5.2.2 Heat absorption chillers5.2.3Mechanical chillers5.2.4 Heat absorption chillers with ice storage

5.3 Economic Result5.4 Conclusion

References 24

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Acknowledgement

I wish to express my sincere gratitude to Prof. V R Iyer, HOD of Mechanical engineering department of Institute of Technology, Nirma University for providing me an opportunity to do my seminar work on “Energy Conversation of a steam combined cycle power plant”. I sincerely thank to my seminar guide, Prof. R N Patel, Mechanical Engineering Department of Institute of Technology, Nirma University for guidance and encouragement in carrying out this seminar work. I also wish to express my gratitude to seminar instructor Prof B A Shah, who rendered his help during the period of my seminar work. Last but not least I wish to avail myself of this opportunity, express a sense of gratitude and love to my friends and my beloved parents for their manual support, strength, help and for everything.

Hoozefa J Shaikh

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Abstract

It is of great interest to investigate the efficiency improvement of CCPP plant. A combined cycle with three-pressure reheat heat recovery steam generator (HRSG) is selected for study in this paper.The optimization of the HRSG operating parameters is performed. The influence of HRSG inlet gas temperature on the steam bottoming cycle efficiency is discussed. The result shows that increasing the HRSG inlet temperature has less improvement to steam cycle efficiency when it is over 590ºC. Partial gas to gas recuperation in the topping cycle is studied. Joining HRSG optimization with the use of gas to gas heat recuperation, the combined plant efficiency can rise up to 59.05% at base load. In addition, the part load performance of the GTCC power plant gets much better. The efficiency is increased by 2.11% at 75% load and by 4.17% at 50% load.Also, Despite their high efficiency, GT performance strongly depends on ambient air temperature. It is well known the negative effect of this parameter on GT efficiency and power output. In a CC, the high temperature and relative humidity of the ambient air also influence the cooling tower behaviour, causing higher pressure in the condenser, and consequently reducing the steam turbine cycle efficiency.A proper solution to minimise this negative effect is to reduce GT inlet airtemperature by means of an air cooling system. Nevertheless, although the effect of differentcooling systems on GT operation has been widely analysed.

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List of Figures / Diagrams

Fig. No. Title Page No. 1.1 Basic Layout Of CCPP 8 1.2 P-V Diagram of Brayton Cycle 9 1.3 T-S Diagram of Rankine Cycle 9 3.1 Steam bottoming cycle efficiency as a function of HRSG Inlet Temp 11 3.2 Steam turbine exergy efficiency and HRSG Exergy loss v/s 12

HRSG inlet Temp 3.3 Gas Turbine with Partial Gas to Gas recuperation 13 3.4 HRSG inlet Temperature after recuperation v/s efficiency at 14

75% and 50% load 4.1 Power Output Temperature v/s Inlet air temperature 16 4.2 Efficiency variation v/s Inlet air Temperature 16 4.3 Specific fuel consumption v/s Inlet air temperature 17 4.4 Energy of exhaust gas v/s Inlet air temperature 18 5.1 CCPP power output performance over a year with and without 19 Evaporative cooler. 5.2 CCPP power output performance over a year with and without 20 Heat absorption Chiller 5.3 CCPP power output performance over a year with and without 21 Mechanical Chillers.

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List of Tables

Table No. Title Page No.5.1 Cooling systems investments depending on the selected technology 23

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Nomenclature CCPP = Combined Cycle Power PlantHRSG = Heat Recovery Steam Generator T = TemperatureS= Entropy

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Chapter 1 Introduction to Combined Cycle power plant

1.1 Combine cycle power plant (CCPP)

In electric power generation a combined cycle is an assembly of heat engines that work in tandem off the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators. The principle is that the exhaust of one heat engine is used as the heat source for another, thus extracting more useful energy from the heat, increasing the system's overall efficiency

1.2 Basic Layout of CCPP

Fig 1.1 Basic Layout of CCPP

1.3 Working Principle of a CCPP

In a combined cycle power plant (CCPP), or combined cycle gas turbine (CCGT) plant, a gas turbine generator generates electricity and heat in the exhaust is used to make steam, which in turn drives a steam turbine to generate additional electricity. This last step enhances the efficiency of electricity generation.

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1.4 Thermodynamic Cycles Involved in a CCPP

1.4.1 Toppling CycleThe thermodynamic cycle associated with Gas Turbine Generator is referred to as Toppling cycle. Brayton cycle is the most commonly used cycle in Gas turbine.

Fig 1.2 P-V Diagram of Brayton Cycle

1.4.2 Bottoming Cycle The thermodynamic cycle involved in Steam turbine generator is known as Bottoming cycle Rankine cycle is the most common cycle in Steam Turbine Generator.

Fig 1.3 T-S Diagram of Rankine Cycle

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Chapter 2Methods of energy Conservation of

Combined Cycle Power Plant

2.1 Various Methods for Energy Conservation are

1. Optimization of Bottoming Cycle

2. Partial Gas to Gas Heat Recuperation In Gas Turbine(GT)

3. Inlet Air Cooling Methods using

a. Evaporative Coolers

b. Mechanical Chillers

c. Heat Absorption Chillers

d. Heat Absorption Chillers using Ice Storage

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Chapter 3

Performance Improvement of Combined CyclePower Plant Based on the Optimization of the Bottom Cycle and Heat Recuperation

3.1 Influence of HRSG inlet temperature on the efficiency of steam bottoming cycle

For a given HRSG configuration, the efficiency of the steam bottoming cycle is the function of inlet gas temperature of HRSG. From Fig. 3.1 it appears that there is an upper limit value for the inlet temperature of the exhaust gas to HRSG.

The increase in HRSG inlet temperature over a value of 590 C will lead to a less increase in the efficiency of steam bottoming cycle.

Fig 3.1 Steam bottoming cycle efficiency as a function of HRSG Inlet Temp

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Fig 3.2 Steam turbine exergy efficiency and HRSG Exergy loss v/s HRSG inlet Temp

This can also be confirmed by the analysis of the exergy losses rate in HRSG and turbine exergy efficiency, as shown in Fig. 4. By the temperature of 620 , the exergy losses rate in HRSG will ℃be minimal.

Hence, we conclude that efficiency of HRSG increases with increase in inlet air temperature, but above 590 C due to increase in exergy loss rate it is not advisable to raise the temperature further.

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3.2 Gas to Gas Recuperation

IF TEMPERATURE IS ABOVE 590 C, efficiency improvement is done usingGas to Gas RecuperationThe gas turbine exhaust gas temperature reaches 615C at base load and even higher than 640C under 75% load. To increase the efficiency of the combined cycle plant, heating the compressed air through partial gas to gas recuperation heat exchanger is done.

Fig 3.3 Gas Turbine with Partial Gas to Gas recuperation

The compressed air from compressor is divided into two streams: one directly to combustion chamber and the other to the exchanger and then to combustion chamber,Partial gas to gas recuperation does not decrease the steam bottoming cycle efficiency, but can save the fuel consumption in Brayton cycle and increase the topping cycle efficiency.

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Fig 3.4 HRSG inlet Temperature after recuperation v/s efficiency at 75% and 50% load

The combined cycle will operate at the best point when the gas to gas recuperation temperature is regulated at the point of 620C at 75% load. The combined cycle plant efficiency is as high as 57.13%,At the load of 50%, the best efficiency of the combined cycle plant is 54.25%, 4.17% more than that of original value.

3.3 Conclusion

Hence, we conclude that increasing inlet HRSG temperature would lead to increase in efficiency of bottoming cycle, but if temperature of GT outlet temperature is above 590C,Partial gas to gas Recuperation in the toppling cycle will help in increasing the efficiency.

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Chapter 4Effect of Inlet Air Cooling in Gas turbine

4.1 Effect on inlet air temperature The power output and efficiency of a gas turbine plant depends among on the temperature of inlet air. The efficiency and power output of a gas turbine during hot condition is less than power output during cold condition.Cooling the inlet air of gas turbine, decreases the temperature which increases the air density, hence increasing the mass flow rate.

Ability to cool the inlet air will facilitate the production of consistent gas turbine power output throughout the year, irrespective of the changes in ambient temperature. Also cooling the inlet air increases the mass flow of air into the gas turbine and at the exhaust outlet. The increased exhaust mass flow increases steam production in the heat recovery steam generator downstream of the gas turbine due to higher energy availability in the exhaust gas. For the same power output, decreasing the inlet air has the effect of decreasing the fuel consumption.

4.1.1 Case study for determining effect of inlet air cooling in GTThe data used for the analysis is obtained from the manufacturer data sheet of TAURUS 60 gas turbine model. TAURUS 60 is a simple gas turbine, the nominal performance at ISO condition (15oC and 60% RH), power output is 5670 KW, heat rate 11425 KJ/KW.hr, exhaust temperature 783 K and natural gas fuel flow, no inlet and exhaust losses and no accessory losses.

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4.2 Result

4.2.1 Power Output and EfficiencyThe power output and efficiency of the cycle were calculated for various ambient air temperatures, for relative humidity of 60% and 100%, and the results of the analysis are presented in Fig.4.1 and Fig4.2 respectively. A rise in the ambient temperature by 1oC result 0.75% drop from gas turbine rated capacity.

Fig 4.1 Power Output Temperature v/s Inlet air temperature

Fig4.2 Efficiency variation v/s Inlet air Temperature

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4.2.2 Specific Fuel ConsumptionThe specific fuel consumption (SFC) of gas turbine decrease when the ambient temperature decreases, Fig.4.3 shows that at low ambient temperature of 24oC decreases to ISO standard condition, the specific fuel consumption drop by 1.8% and at high ambient temperature of 35oC decreases to ISO standard condition the specific fuel consumption drop by a 4.7%.

Fig 4.3 Specific fuel consumption v/s Inlet air temperature

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4.2.3 Energy of Exhaust GasBy reducing the intake air temperature, the flow rate of the exhaust gas will correspondingly increase. Fig.4.4 shows at the low ambient temperature of 24oC decreases to ISO standard condition, the energy of exhaust gases increases by about 3.55% and at the high ambient temperature of 35oC decreases to ISO standard condition, the energy of exhaust gases increases by about 8.3%.

Fig 4.4 Energy of exhaust gas v/s Inlet air temperature

4.3 Conclusion The performance of gas turbine can be successfully improved by decreasing the temperature of inlet air. Reducing the temperature from ambient condition to ISO standard condition could help to increase the power output between 6.3% to 13.65%.

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Chapter 5Comparison of various types of Cooling Method

For inlet air Cooling

5.1 Case Study comparing various cooling technique for inlet air coolingA three-level steam pressure combined cycle has been chosen for analysis. It is basedon a 260 MW Industrial GT and a 140 MW steam turbine.

5.2 Various techniques considered

5.2.1 Evaporative coolers:Evaporative systems cool the inlet air by pulverising water into the air stream. The water evaporation causes the air temperature to decrease.Locations of low humidity climate are suitable to use this cooling technology. Two considerations must be taken into account.a) The maximum relative humidity that it is possible to reach with an evaporative system is hardly over 90%.b) The difference between wet and dry bulb temperatures in the outer section of the evaporative system is recommended not to be under 1ºC.

Fig5.1 CCPP power output performance over a year with and without Evaporative cooler.

It can be inferred from figure 5.1, using an evaporative cooling system, a betterImprovement is reached in summer months than in winter ones. In addition, in the central hours of summer days the improvement is still higher than the average result

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5.2.2 Heat absorption chillers:These systems are one of the best options for CCPP, due to the possibility of working with a low-pressure steam extraction. That extraction is condensed and returns to the steam cycle via desgasificator. The required steam mass flow rate is taken to avoid unnecessary power loses in the bottoming cycle. The absorption chiller selected uses the CC cooling tower to refrigerate both absorber and evaporator. As a consequence, steam cycle performance is slightly deteriorated due to the losses in steam mass flow and the increase of the condenser pressure.

Fig.5.2 CCPP power output performance over a year with and without Heat absorption Chiller

Results using a heat absorption chiller are shown in figure 5.2. A 20 MW cooling power heat-absorption LiBr system has been selected for the study. It allows to reach the maximum recommended cooling of the inlet air stream (7ºC) during 88% of the period.

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5.2.3Mechanical chillers:Since the chiller compressor needs electricity to work, the power output improvement is partially reduced with mechanical cooling systems. However, the temperature reduction could be as high as desired. Mechanical chiller results are similar to those obtained with an absorption chiller.

Fig 5.3 CCPP power output performance over a year with and without Mechanical Chillers.

5.2.4 Heat absorption chiller with ice storage.: When the differences between peak and no-peak demand prices are high, the use of ice storage is advisable. This system allows to storage cool energy, i.e. when electricity prices are low, and to use the storage energy to reduce air temperature when electricity prices increase. In places with small variation of electricity prices, this option is not economically competitive.Although an important temperature reduction could be achieved during some period, themonth average power output improvements are worse as compared to other chillers. During part of the day, the cooling system is off but a power consumption exists to generate and storage ice. This system could work some hours a day in critical months (summer), but the same ice tank capacity allows to work more hours (sometimes all day) in winter months.Normally, the selected hours to work are those with higher electricity prices, and the energy storage is made along the whole day.

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5.3 Economic Result

90% CC availability per year is considered and values of equipment prices derived from the economic study are shown in table

Table 5.1 Cooling systems investments depending on the selected technology

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5.4 Conclusion

In evaporative cooling, the inlet air is cooled to near the ambient wet bulb temperature. InRefrigerated cooling, the inlet air is cooled to below the dew point temperature. Since larger temperature reductions are possible with refrigerated cooling, capacity enhancements are correspondingly higher compared to evaporative cooling.The main advantage of evaporative cooling is simple design and low initial cost. The payback is quicker for evaporative cooling even though the capacity increment is limited by ambient wet bulb temperature.Since electricity power generates higher incomes per kilowatt, the maximum temperature reduction is the recommended goal. A cooling system with the same technology but with higher chilling power always produces higher cash flows. Therefore, sizing must be made in order to reach the minimal air temperature that is technologically allowable in the GT inlet duct (5-7ºC). Combining technical and economic results, evaporative coolers generally have a good economic behaviour, due to their low cost. Their main disadvantage is that it is impossible to fix the inlet temperature below a point (dew temperature) which depends on the weather conditions (site climate). Their payback periods are less of a year when the climate is favourable and near a year with adverse humidity. Mechanical coolers have lower cash flow as compared to heat absorption onesWith the former, a slightly lower power output improvement and a slightly fuel decrement is obtained in comparison with the latter. Both effects make the mechanical chillers cash flow to decrease.

Hence, it is advisable to use evaporative coolers only if climate are favourable, else mechanical type or heat absorption type chillers are to be used depending on the requirement of the plant.

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References

1. www.wikipedia.org2. Wenguo XIANG, Yingying CHEN Journal of Thermal Science Vol.16, No.1 84―893. Waiel Kamal Elsaied, Zainal AmbriBin Abdul Karim, Effect of Inlet air cooling, University of Technology, Petronas, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia4. Raquel Gareta, Luis M. Romeo,Antonia Gil, The Effect of Inlet air Cooling System in CC performance, CIRCE (Centre of Research for Power Plants Efficiency). University of Zaragoza

 

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