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GAS TURBINE OPERATION METHODS DURING WOBBIE INDEX OF FUEL GAS CHANGE WITHOUT FUEL GAS NOZZLES MODIFICATION

Charcrist Kuhakarn

Electricity Generating Authority of Thailand 53 Moo 2, Charansanitwong road, Bang kruai, Nonthaburi, Thailand, 565911@egat.co.th

Poramut Tattanon Electricity Generating Authority of Thailand

53 Moo 2, Charansanitwong road, Bang kruai, Nonthaburi, Thailand, 576220@egat.co.th Jirayu Janpool

Electricity Generating Authority of Thailand 53 Moo 2, Charansanitwong road, Bang kruai, Nonthaburi, Thailand, 593651@egat.co.th

ABSTRACT

When wobbie index of fuel gas is changed out of its designed criteria, fuel gas nozzles should be modified in accordance with the change of fuel gas’s wobbie index. The modified fuel gas nozzles and their spare parts should be supplied in time. Unfortunately, the period of the change of fuel gas’s wobbie index is not fixed, the supply of modified fuel gas nozzles and their spare parts are quite difficult. If these parts are supplied too early, expense from reservation will be occurred. Otherwise, supply of these parts can be late. If fuel gas nozzles cannot be modified, gas turbine especially with dry low emission (DLE) or dry low NOX (DLN) combustion chamber can trip from combustion unstable. Moreover, hardware of combustion part may be damaged as well. From the difficulty of spare part’s management, gas turbine should be operated appropriately without trip and less damage of combustion part if the fuel gas nozzle cannot be modified. The gas turbine can be operated appropriately by detecting the change of fuel gas’s wobbie index in advance. Then, the appropriate power demand was defined from the change of fuel gas’s wobbie index, such as gas turbine load was decreased to 50% rated load. After that, the control parameter, which is concerned with combustion, was adjusted in advance, including with applying the auto tuning system. The decrease of gas turbine load can expand the range of combustion stability, which decreases the opportunity of gas turbine trip and the damage of combustion part. This paper shows how to operate gas turbine appropriately during wobbie index of fuel gas changes without fuel nozzle modification, especially heavy industry gas turbine with Dry Low Emission (DLE) combustion chamber. This can reduce the possibility of the unit tripped from combustion trouble and make the power plant more stable and reduce the possibility of the combustion chamber damage when the wobbie index of fuel gas is changed nearly out of fuel gas designed criteria.

KEYWORDS: Gas turbine, Wobbie index, Combustion, Combustion stability, Combustion trouble 1. INTRODUCTION

From the discovery of new fuel gas source, the fuel gas from new source was used by mixing with the old source and supplied to the power plant. This mixture made the wobbie index of fuel gas change, which effects the power plant operation especially the gas turbine part with dry low emission (DLE) or dry low NOX (DLN) combustion chamber. When wobbie index of fuel gas is changed out of its designed criteria, gas turbine’s fuel gas nozzles should be modified in accordance with the change of fuel gas’s wobbie index. The modified fuel gas nozzles and their spare parts should be supplied in time. Unfortunately, the period of the change of fuel gas’s wobbie index is not fixed, the supply of modified fuel gas nozzles and their spare parts are quite difficult. If these parts are supplied too early, expense from reservation will be occurred. Otherwise, supply of these parts can be late. If fuel gas nozzles cannot be modified, gas turbine can trip from combustion unstable. Moreover, hardware of combustion part may be damaged as well. Fortunately, the wobbie index of this fuel gas mixture was nearly out of fuel gas designed criteria. If gas turbine operate appropriately, the possibility of the unit tripped from combustion trouble and the combustion chamber damage can be reduced

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2. MAIN CONTENTS 2.1 Location and structure of the power plant and fuel gas characteristic 2.1.1. Power plant location

This paper considered South Bangkok power plant block III located at Samutprakarn province, which

is at the south of Bangkok. The location of these power plants is shown in figure 1.

Figure 1: South Bangkok power plant location.

Figure 2: Structure of MHI gas turbine at South Bangkok power plant block III.

2.1.2. Power plant structure South Bangkok power plant block III or SBK-C3 is the combined cycle power plant, which consists of

two gas turbines, two HRSGs (Heat Recovery Steam Generator), one steam turbine and three generators. They were produced by Mitsubishi Heavy Industries Company or MHI. Gas turbine which is produced by MHI can be called MHI gas turbine. MHI gas turbine at South Bangkok power plant block III was M701F3 type. Its combustion part of MHI gas turbine are Dry Low Emission (DLE) or Dry Low NOX (DLN). Structure of MHI gas turbine at South Bangkok power plant block III was shown in figure 2.

South Bangkok Power plant,

Samut Prakan

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2.1.3 Fuel gas characteristic before and after fuel gas change Before fuel gas change, fuel gas source comes from west side of Thailand and its Wobbie index is between 950 Btu/scf and 1,051 Btu/scf, as shown in figure 3. After fuel gas change, fuel gas source was mixed between west side of Thailand and east side of Thailand and its Wobbie index is between 1,030 Btu/scf and 1,050 Btu/scf. Fuel gas supply’s route of each power plants was shown in figure 4.

Figure 3: Fuel gas’s Wobbie index before fuel gas changes

Figure 4: Route of Fuel gas supply for South Bangkok power plant block III.

WI of west side is 952 – 1,051 Btu/scf.

Mixing point

South Bangkok Power plant block III

Wobbie index (Btu/scf)

(Wobbie index of South Bangkok Power plant)

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2.2 How to operate MHI gas turbine when fuel gas change

From the delay of fuel gas pipe construction, period of fuel gas change was not fixed so the schedule of fuel gas change was not set. Finally, schedule of fuel gas change is set on November 27, 2015. Unfortunately, the acknowledgement of fuel gas change’s period is quite short, fuel gas nozzle of gas turbine at South Bangkok power plant block III cannot be supplied in time. Fortunately, the Wobbie index of fuel gas change was not out of fuel gas designed criteria. From combustion characteristic of MHI gas turbine (Dry Low NOX combustion), MHI gas turbine can be operated. To understand how to operate MHI gas turbine when fuel gas change, the principle of Dry Low NOX combustion for MHI gas turbine, as shown in 2.2.1 and how to tune or adjust Dry Low NOX combustion for MHI gas turbine, as shown in 2.2.2, should be known. Then, these will be applied for MHI gas turbine when fuel gas change in 2.2.3.

2.2.1 Principle of Dry Low NOX Combustion Dry low NOX combustion of MHI gas turbine integrates premixed combustion and diffusion combustion. Premixed combustion is occurred by injecting mixture of fuel gas and air into combustion zone while diffusion combustion is occurred by diffusing fuel gas and air into combustion zone. Combustion temperature of premixed combustion is lower than combustion temperature of diffusion combustion so premixed combustion produces NOX lower than diffusion combustion. Unfortunately, stable region of premixed combustion is small cause of its lower temperature. Comparison between premixed combustion and diffusion combustion are shown in table I. If dry low NOX combustion is prone to diffusion combustion, NOX value will be high. Otherwise, if dry low NOX combustion is prone to premixed combustion, combustion stability is low. Appropriate combustion is combustion that produces NOX value lower than the desired value and stays in stable region, all gas turbine’s operating period. Moreover, when MHI gas turbine’s load is low, combustion temperature is low and region of combustion stability is wide.

TABLE I: COMPARISON TABLE BETWEEN PREMIXED AND DIFFUSION COMBUSTION

Detail Diffusion Combustion Premixed combustion Combustion temperature High Low Stable region Large Small Oxides of Nitrogen (NOX) High Low

Figure 5: Dry low NOX combustion chamber of MHI gas turbine at South Bangkok power plant block III.

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2.2.2 Tuning Method for Dry Low NOX combustion at South Bangkok Power plant block III

When fuel gas change was received at South Bangkok power plant block III, combustion tuning should be done. Component of dry low NOX combustion chamber, shown in figure 5, consists of multi-main nozzle, multi-top hat nozzle, pilot nozzle, air-bypass valve, combustor basket, etc. Fuel flow of main nozzle is controlled by main flow control valve. Fuel flow of top hat nozzle is controlled by top hat flow control valve. Fuel gas flow of pilot nozzle is controlled by pilot flow control valve.

Control equipment to control dry low NOX combustion of MHI gas turbine divides in two groups. First group, concerned with combustion air, consists of Inlet Guide Vane or IGV, and air-bypass valve. Other group, concerned with fuel gas flow, consists of main flow control valve, top hat flow control valve and pilot flow control valve. IGV is not appropriate for combustion tuning because it controls air not only for combustion but for cooling gas turbine blade also. This means that equipment to control combustion air for dry low NOX combustion is air-bypass valve only. In case of fuel gas flow, fuel gas flow is concerned with not only combustion characteristic but electrical power of gas turbine also. To keep electrical power constant, fuel gas flow must be constant. Combustion tuning in case of fuel gas flow is to split (1) fuel gas ratio of fuel gas from top hat flow control valve and all fuel gas flow, called top hat ratio, and (2) fuel gas ratio of fuel gas from pilot flow control valve and all fuel gas flow, called pilot ratio, appropriately.

From all above control equipment, control parameters that use for combustion tuning are position of air-bypass valve, top hat ratio value and pilot ratio value. Combustion from main nozzle and top hat nozzle is premixed combustion while combustion from pilot nozzle is diffusion combustion. Objective for tuning dry low NOX combustion is to guarantee that NOX values is always kept in the desired value and combustion zone is always in stable region. Combustion stability of MHI gas turbine is known from combustion pressure fluctuation value. If combustion pressure fluctuation value is high, combustion stability is low. If NOX value exceeds its desired value, pilot ratio or position of air-bypass valve must be decreased to reduce diffusion combustion. Otherwise, if combustion pressure fluctuation value exceeds alarm criteria, pilot ratio or position of air-bypass valve must be increased to reduce combustion pressure fluctuation value. Moreover, if dry low NOX combustion cannot be tuned, gas turbine must be shut down to check its hardware.

2.2.3 Operation method when fuel gas changed

Low MHI gas turbine’s load, Low combustion temperature and region of combustion stability is wide. If MHI gas turbine’s load was decreased, the possibility of the unit tripped from combustion trouble and the combustion chamber damage can be reduced. So before the fuel gas change, MHI gas turbine’s load should be decreased to increase the boundary of combustion stability. Mostly, MHI gas turbine’s load was set at 50% rated load. Then combustion tuning should be done at each load to guarantee that MHI gas turbine can be operated safely. If combustion tuning at each load is completed, gas turbine’s load can be increased. Gas turbine’s load can be increased until combustion pressure fluctuation value is high or NOX is higher than the designed criteria. If dry low NOX combustion cannot be tuned, MHI gas turbine’s load should be hold.

Figure 6: MHI gas turbine’s load schedule

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Figure 7: Operation of MHI gas turbine unit 31 (SBK-C31) at South Bangkok power plant block III during fuel gas change period.

GT-31 Swing Load Test

Combustion Pressure fluctuation of SBK-C31

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Figure 8: Operation of MHI gas turbine unit 32 (SBK-C32) at South Bangkok power plant block III during fuel gas change period.

GT-32 Swing Load Test

Combustion Pressure fluctuation of SBK-C32

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2.3 Result

MHI gas turbine’s load schedule before fuel gas change was shown in figure 6. Before the fuel gas change on September 27, 2015, MHI gas turbine’s load was decreased to 50% rated load. Then combustion tuning should be done at each load to guarantee that MHI gas turbine can be operated safely. When combustion tuning at each load is completed, gas turbine’s load can be increased. MHI gas turbine’s load was increased to base. Combustion pressure fluctuation value and NOX are in the designed criteria as shown in figure 7 and 8. Moreover, fuel gas change can be detected for 4 hours after the Wobbie Index of fuel gas was change, as shown in figure 9. If operators of South Bangkok power plant check the Wobbie Index of fuel gas from mixing point every 2 – 3 hours, fuel gas change will be known in advance. If the fuel gas change was found, MHI gas turbine’s load can be decreased to 50% rated load (Appropriate load) in advance to decrease the possibility of the unit tripped from combustion trouble and reduce the combustion chamber damage.

Figure 9: The relation of Wobbie Index from mixing ping and South Bangkok power plant block III

3. OTHER POWER PLANT, CONCERNING WITH THIS FUEL GAS CHANGE

This fuel gas change effects two more power plants. These power plants consist of North Bangkok power plant block I and II. North Bangkok power plant is at Nonthaburi province, which is at the North of Bangkok. The location of these power plants is shown in figure 10.

North Bangkok power plant block I or NBK-C1 is the combined cycle power plant which consists of two gas turbines, two HRSGs, one steam turbine and three generators. Gas turbines at North Bangkok power plant block I were produced by General Electric Company or GE, called GE gas turbine, while steam turbine was produced by Hitachi Company. North Bangkok power plant block II or NBK-C2 consists of two single shaft combined cycle power plant. Each single shaft combined cycle power plant consists of one gas turbine, one HRSG, one steam turbine and one generator. Gas turbines and steam turbines at North Bangkok power plant block II were produced by ALSTOM Company. Gas turbine which was produced by ALSTOM Company can be called ALSTOM gas turbine. The combustion part of these gas turbine are dry low emission.

From this fuel gas change, combustion part of GE gas turbine at North Bangkok Power plant block I was changed from DLN2.0+, to DLN2.6+ (figure 11), to increase Wobbie index range while combustion part of ALSTOM gas turbine at North Bangkok Power plant block II, which was double stage combustion (figure 12), can receive this Wobbie index.

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Figure 10: North Bangkok power plant location.

DLN2.0+ Combustion chamber DLN2.6+ Combustion chamber (Before fuel gas change) (After fuel gas change)

Figure 11: Combustion chamber of GE gas turbine at North Bangkok power plant block I

Figure 12: Combustion chamber of ALSTOM gas turbine at North Bangkok power plant block II

North Bangkok Power plant, Nonthaburi

South Bangkok Power plant,

Samut Prakan

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4. CONCLUSION

When fuel gas was changed not more than its designed criteria (WI between 950 to 1,050 Btu/scf), MHI gas turbine can be operated. In case of fuel gas condition exceeds than its designed criteria, MHI gas turbine should be decreased to 50% rated load. After that, combustion pressure fluctuation and NOX value should be checked. If combustion pressure fluctuation and NOX value is not more than the designed criteria, MHI gas turbine’s load can be increased and combustion tuning should be done. If combustion pressure fluctuation or NOX value is almost near its designed criteria, MHI gas turbines’ load should be hold. This will be make MHI gas turbine operate without fuel gas nozzle modification and decrease the possibility of the unit tripped from combustion trouble and reduce the combustion chamber damage. Moreover, the fuel gas change can be known in advance for 4 hours before MHI gas turbine of South Bangkok power plant block III receive this change. So the Wobbie index of mixing point every 2 – 3 hours to know the fuel gas change.

REFERENCES [1] A. H. Lefebvre, Gas Turbine Combustion, USA: McGraw Hill, 1983. [2] Mitsubishi Heavy Industry, Combustion tuning Manual for South Bangkok power plant block III,

Presentation, 2008. [3] Hiddemann M., Morey M,. Zheng C., Swami S., “Latest Upgrade GT24/GT26 gas turbines for

meeting the requirements of the Asian markets”, Power Gen Asia 2013 Presentation, 2013. [4] Instruction manual of North Bangkok power plant block I. [5] Instruction manual of North Bangkok power plant block II. [6] Instruction manual of South Bangkok power plant block III. BIOGRAPHY

Charcrist Kuhakarn has 14 year experience concerning with the power plant control system of gas turbine, combined cycle and thermal power plant. Moreover, Charcrist Kuhakarn has experience in the engineering of control system and instrumentation at Electricity Generating Authority of Thailand, Nonthaburi, Thailand. Charcrist Kuhakarn receives B.Eng. in electrical engineering and M.Eng. in electrical engineering (control system) from Chulalongkorn university, Bangkok, Thailand. Moreover, Charcrist Kuhakarn receives Bachelor of Economics, Political Science and Public Health Science from Sukhothaithamathiratch university, Nonthaburi, Thailand.

Poramut Tattanon has 7 year experience concerning with the power plant control system of gas turbine and combined cycle power plant. Porrmut Tattanon receives B.Eng. in electrical engineering from Kasetsart university, Bangkok, Thailand.

Jirayu Janpool has 2 year experience concerning with the power plant control system of gas turbine and combined cycle power plant. Jirayu Janpool receives B.Eng. in electrical engineering from King mongkut's university of technology ladkrabang Bangkok, Thailand.