Design Considerations for Heated Gas Fuel
By: D.M. Erickson
GE Power Systems Greenville, SC
Disclaimer: This document is owned by GE, being printed here for the reference only for readers. This is being printed in parts for easy reading on screen
Gas Fuel Performance Heating As the need for higher efficiency power plants increases, a growing number of combined-cycle power plants are incorporating performance gas fuel heating as a means of improving overall plant efficiency. This heating, typically increasing fuel temperatures in the range of 365°F/185°C, improves gas turbine efficiency by reducing the amount of fuel needed to achieve desired firing temperatures. For fuel heating to be a viable method of performance enhancement, feedwater has to be extracted from the heat recovery steam generator (HRSG) at an optimum location. Boiler feedwater leaving the intermediate pressure economizer is commonly used. Using gas-fired, oilfired or electric heaters for performance gas fuel heating will not result in a power plant thermal efficiency improvement.
Proper design and operation of the Gas Fuel Heating System is critical in insuring reliable operation of the gas turbine. Improper selection of components, controls configuration and/or overall system layout could result in hardware damage, impact plant availability and create hazardous conditions for plant personnel.
This paper addresses the critical design criteria that should be considered during the design and construction of these systems. Also included in this paper is the design of a “typical” GE Gas Fuel Heating System. This system has been developed taking into consideration the system requirements defined within.
Gas Compressor Heating
Gas compressors may be needed to meet specified minimum gas supply pressures levels. The use of a compressor adds heat to the gas and raises its operational temperature. The temperature level of the gas at the exit of the gas compressor is a function of its inlet conditions. This temperature may vary from site to site and should be evaluated against any combustion specific requirements defined in this document.
General System Requirements The following section identifies general system requirements that apply to all gas fuel heating systems. These requirements, in addition to those described in the Combustion Specific System Requirements section shall be followed during the design and development of the system.
Gas Fuel Cleanliness
Gas fuel supplied to the gas turbine shall meet the particulate requirements as specified in the latest revision of GEI-41040, “Process Specification –– Gas Fuels for Combustion in Heavy Duty Gas Turbines,” (Reference 1). If the components in the Gas Fuel Heating System are constructed of materials susceptible to corrosion, a method of final filtration upstream of the gas turbine interface is required. Particulate carryover greater than that identified in GEI-41040 can plug fuel nozzle passages, erode combustion hardware and gas valve internals and cause damage to first stage turbine nozzles. The new gas piping system must be properly cleaned prior to initial gas turbine operation. Additional design considerations related to gas fuel cleanliness may be found in GER-3942, Gas Fuel Clean-Up System Design Considerations for GE Heavy Duty Gas Turbines,” (Reference 2).
Gas Fuel Quality
As defined in GEI-41040, the fuel delivered to the gas turbine must be liquid free and contain a specified level of superheat above the higher of the hydrocarbon or moisture dewpoints. Saturated fuels, or fuels containing superheat levels less than specified, can result in the formation of liquids as the gas expands and cools across the gas turbine control valves. The amount of superheat provides margin to compensate for temperature decrease due to pressure reduction, and is directly related to incoming gas supply pressure. (Note: Within this document, gas fuel heating strictly for dewpoint considerations is still considered to be in a “cold” state. Heating for performance purposes is considered “heated” fuel.)
The design of the Gas Fuel Heating System shall prevent carryover of moisture or water to the gas turbine in the event of a heat exchanger tube failure. Water entrained in the gas can combine with hydrocarbons causing the formation of solid hydrocarbons or hydrates. These hydrates, when injected into the combustion system, can lead to operability problems, including increased exhaust emissions and mechanical hardware damage. Proper means of turbine protection, including heat exchanger leak detection, shall be provided.
Gas Fuel Supply Pressure
Gas being supplied to the gas turbine interface point (customer connection FG1) shall meet the minimum gas fuel supply pressure requirements as defined in the proposal documentation. These minimum pressure requirements are established to insure proper gas fuel flow controllability and to maintain required pressure ratios across the combustion fuel nozzles. The Gas Fuel Heating System shall be designed to insure that these requirements are met during all modes of operation over the entire ambient temperature range. The design of the Gas Fuel Heating System shall insure that the design pressure of the gas turbine gas fuel system is not exceeded.
Overpressure protection, as required by applicable codes and standards, shall be furnished. In addition to minimum and maximum pressures, the gas turbine is also sensitive to gas fuel pressure variations. Sudden drops in supply pressure may destabilize gas pressure and flow control. Sudden increases in supply pressure may potentially trip the turbine due to a high temperature condition. Limitations on pressure fluctuations are defined in the gas turbine proposal documentation.
Gas Fuel Supply Temperature
The Gas Fuel Heating System shall be designed to produce the desired gas fuel temperature at the interface with the gas turbine equipment. Guaranteed performance is based on the design fuel temperature at the inlet to the gas turbine gas fuel module (FG1). The gas fuel heating and supply systems shall compensate for heat losses through the system. Compensation shall include but not be limited to elevated heater outlet temperatures, use of piping and equipment insulation, and minimization of piping length from heater outlet to turbine inlet.
The Gas Fuel Heating System shall be designed to support specified gas fuel temperature setpoints required by the gas turbine. These setpoints include high and low temperature alarms, gas turbine controls permissives, and gas turbine controls functions. These setpoints are derived by GE Gas Turbine Engineering and are based on operability requirements and/or design limitations of components within the gas turbine gas fuel system.
During specified cold and hot gas fuel turbine operating modes, the Gas Fuel Heating System shall attain and maintain the fuel at a temperature that corresponds to a Modified Wobbe Index (MWI) within ±5% of the target value. The Modified Wobbe Index is a calculated measurement of volumetric energy content of fuel and is directly related to the fuel temperature and lower heating value (LHV). The Modified Wobbe Index is derived as follows:
The ±5% Modified Wobbe Index range insures that the fuel nozzle pressure ratios are maintained within their required limits. If gas fuel constituents and heating value are consistent, the 5% tolerance can be based strictly on temperature variation. If the heating value of the fuel varies, as is the case when multiple gas suppliers are used, heating value and specific gravity must be considered when evaluating the allowable temperature variation to support the 5% Modified Wobbe Index limit.
For the use of gas fuels having a significant variation in composition or heating value, a permanent gas chromatograph shall be furnished in the plant’s main gas supply line. LHV and specific gravity readings from the gas chromatograph are used to regulate the amount of fuel heating so that the ±5% Modified Wobbe Index requirement is satisfied. This control function shall be performed automatically by the plant control system.
Consideration shall be made to the location of the gas chromatograph relative to the inlet of the gas fuel module and the time delay from instrument reading to fuel gas control.
Combustion Specific System Requirements
The GE Gas Turbine product line incorporates the use of both Dry Low NOx (DLN) and Non-Dry Low NOx (conventional) combustion designs. Currently, there are five different DLN configurations offered by GE: DLN-1, DLN-2.0, DLN-2+, DLN-2.6 and DLN-2.5H. Each combustion design is applied to one or more gas turbine models. These designs have different hardware configurations and operability schemes and in turn have certain contrasting gas fuel heating requirements. Performance type gas fuel heating is not normally applied to conventional combustion systems, and thus will not be addressed in this document. Table 1 identifies the combustion designs that are applied to the various turbine models. This section will detail the system design and operability requirements that apply to the specific DLN combustion design.
On gas turbines that utilize DLN-1 combustion designs, the Gas Fuel Heating System and supporting control system shall be designed to provide either cold or heated fuel as based on the gas turbine’s requirements. The gas turbine control system will provide a permissive signal indicating when heated or unheated fuel is required. The plant control system shall use this signal to initiate gas fuel heating on start-up and to cease gas fuel heating on shutdown. For DLN-1 combustion designs, the fuel shall be in a cold state from ignition (Primary combustion mode) through Lean-Lean and into Secondary Premix combustion mode. The fuel can be heated only after Premix steady state is achieved. The gas can be hot or cold in Premix Mode, but must be cold in Primary, Lean-Lean or Extended Lean-Lean Mode. The gas must be cold prior to transferring out of Premix Mode.
During a “hot gas restart,” the DLN-1 combustion system has the ability to be fired on the hot fuel contained in the fuel supply system. Active heating of the fuel shall not be re-established until the combustion system reaches Premix Steady State Mode.
On gas turbines that utilize DLN-2.0 combustion designs, the Gas Fuel Heating System and supporting control system shall be designed to provide either cold fuel or heated fuel as based on the gas turbine’s requirements. DLN-2.0 combustion systems are designed to operate on both unheated and heated fuels at ignition as well as Primary and Lean-Lean Modes. While in Premix Transfer, Piloted Premix, and Premix Modes, the system is designed to operate on heated fuels only.
Permissives configured within the gas turbine controls permit or prevent changes in combustion mode until the gas is heated sufficiently in order to satisfy the Modified Wobbe Index requirements. Thermocouples located directly upstream of the gas turbine’s Stop Speed Ratio Valve initiate this permissive. During turbine shutdown, gas fuel heating shall be disabled only after transferring out of Premix Mode.
DLN-2+ Requirements (PG9351FA)
On gas turbines that utilize DLN-2+ combustion designs, the Gas Fuel Heating System and plant controls shall be designed to provide either cold or heated fuel as based on the gas turbine’s requirements. DLN-2+ combustion systems are designed to operate on heated or unheated fuel in Diffusion and Sub-piloted Premix Mode. Diffusion and Sub-piloted Premix Mode operation consists of ignition, acceleration to Full Speed No Load, and up to approximately 10% load. (See Figure 1.) During Piloted Premix Mode operation, from approximately 10% load to 25% load, the gas fuel temperature can be hot or cold. However, the gas must satisfy the Modified Wobbe Index hot temperature limits, in Piloted Premix Mode, from approximately 25% to 50% load. During Premix Mode operation, the gas temperature must be sufficient to satisfy the Modified Wobbe Index limit. In addition, Extended Piloted Premix Mode, from 50% load to Baseload, requires the gas to meet the Modified Wobbe Index hot limit. Permissives set within the gas turbine controls, prevent a transfer into the appropriate Piloted Premix load or Premix Mode, during loading, until the required temperature is attained.
Thermocouples located directly upstream of the gas turbine’s Stop Speed Ratio Valve initiate this permissive. During turbine shutdown, gas fuel heating shall be ceased only after transferring out of Piloted Premix Mode at approximately 25% load.
DLN-2+ FB Requirements (PG7251FB & PG9371FB)
On gas turbines that utilize the DLN-2+ FB combustion system design, the Gas Fuel Heating System and plant controls shall be designed to provide either cold or heated fuel as based on the gas turbine’s requirements. (See Figure 2.) DLN-2+ FB combustion systems are designed to operate on heated or unheated fuel in Diffusion and Sub-piloted Premix Mode.
Diffusion and Sub-piloted Premix Mode operation consists of ignition, acceleration to Full speed No Load, and up to approximately 10% load. (See Figure 2.) During Piloted Premix Mode operation, from approximately 15% load to 20% load, the gas fuel temperature can be hot or cold. However, the gas must satisfy the Modified Wobbe Index hot temperature limits, in Piloted Premix Mode, from approximately 20% to 40% load. During Premix Mode operation, the gas temperature must be sufficient to satisfy the Modified Wobbe Index limit. In addition, Extended Piloted Premix Mode, from 40% load to Baseload, requires the gas to meet the Modified Wobbe Index hot limit.
Permissives set within the gas turbine controls prevent operation in Premix Mode or in Piloted Premix Mode until the required fuel temperature is attained. Thermocouples located directly upstream of the gas turbine’s Stop/Speed Ratio Valve initiate this permissive. During turbine shutdown, gas fuel heating shall be ceased only after reducing load below 20%.
On gas turbines that use DLN-2.6 combustion designs, the Gas Fuel Heating System and plant controls shall provide either cold or heated fuel as based on the gas turbine’s requirements.(See Figure 3.) DLN-2.6 combustion systems are designed to operate on heated or unheated fuel in Modes 1, 2 and 3. Heated fuel operation in Modes 1, 2 and 3 is permitted, but not recommended, for normal operation. The gas must be heated to satisfy the Modified Wobbe Index hot temperature limits prior to transferring to combustion Mode 4, at approximately 25% load.
Thermocouples located directly upstream of the gas turbine’s Stop Speed Ratio Valve initiate a permissive to transfer into Mode 4. Fuel temperature must be maintained within the hot gas temperature limits at all modes above Mode 3 (approximately 25% load) during both unit operation and shutdown. During turbine shutdown, gas fuel heating shall be ceased only after transferring out of Mode 4 and into Mode 3. The gas fuel temperature is recommended, but not required, to be less than 120°F/49°C before transferring from Mode 3 to Mode 1.
On gas turbines that operate with DLN-2.5H combustion designs, the Gas Fuel Heating System and plant controls shall provide either cold or heated fuel based on the gas turbine’s requirements. The DLN-2.5H combustion systems are designed to operate on both unheated and heated fuels at ignition through Diffusion Mode and into Piloted Premixed Mode. The gas must be heated in order to satisfy the Modified Wobbe Index hot gas temperature limits prior to transferring to Premixed Mode.
Typical GE Gas Fuel Heating System The following section details the mechanical design and operational features of the typical GE Gas Fuel Heating System. The design intent of this system is to produce gas fuel that meets all requirements previously specified in this document. In addition to supporting heated fuel to the gas turbine, the typical system pro-vides safeguards that prevent gas fuel from entering the HRSG system. This commonly ignored condition can occur when a tube leak is present during gas turbine operation or unit shutdown.
This typical design is provided as a reference to the customer. Deviations from this design may be acceptable, providing that the requirements of the gas turbine are met.
Figure 4 identifies the equipment, instrumentation instrumentation and piping configuration of the typical GasFuel Heating System. This system, as described, was initially applied to the MS9001H combined cycle power plant, which used intermediate pressure feedwater as the medium for fuel heating.
The design criteria utilized during the development of this system shall be followed during the detailed design of all gas turbine gas fuel heating systems that utilize feedwater or steam as the heating medium. Job specific gas heating systems may deviate from this design based on gas conditions and interfacing balance of plant systems.
The standard Gas Fuel Heating System design meets the following design criteria:
Provide heated fuel that meets the Modified Wobbe Index requirement of the gas turbine’s combustion system.
Prevent water from being admitted to the gas turbine combustion system following a heat exchanger tube leak or rupture.
Provide early indication of heat exchanger tube failure.
Prevent gas fuel from entering the feedwater system following a heat exchanger tube failure.
Remove gas entrained particulate as specified in the latest revision of GEI-41040, Process Specification Gas Fuels for Combustion in Heavy Duty Gas Turbines (Reference 1).
Provide overpressure protection to the gas turbine Gas Fuel Heating System piping and components.
Ensure water pressure is higher than gas pressure during gas turbine operation and shutdown.
Heater Leak Detection Protection Philosophy
The heat exchanger leak detection scheme shall incorporate three levels of alarms or automated control. These three levels have been established to prevent the admission of water into the gas turbine while preventing inadvertent trips or load decreases due to failure of a single sensing instrument. (See Figure 5.) The heater leak detection controls have been established to provide early detection of a heat exchanger leak and to mitigate the effects of both the leak on the gas turbine and the balanceof plant systems.
Each gas fuel heater shell is furnished with a low point drain pot. The two drain pots house a series of level switches used in the tube leak detection controls. The lower heat exchanger drain pot is furnished with a single high level switch and three triple-redundant high-high level switches. A drain pot will open upon activation of the corresponding high level switch.
When two out of the three high-high level switches are activated, the feedwater to and from the heat exchanger will isolate. This action will quickly reduce the temperature of the gas fuel and initiate a transfer of the gas turbine to a cold mode of operation. Specifically for the typical Gas Fuel Heating System, the details of the three levels are as follows:
Level 1. At a minimum, a single sensing instrument (i.e., level switch) is implemented to alarm and evacuate the heating medium from the gas stream/liquid collection sump following a tube leak/rupture. This provides initial indication that the heat exchanger tube leak/rupture is present.
Level 2. At a minimum, triple redundant sensing instrumentation is implemented and set at a level higher than that of Level 1. Output from these signals shall alarm and automatically isolate the heating medium from the gas stream, (i.e., isolating the feedwater from the heat exchanger). This provides secondary indication that the heat exchanger leak/tube rupture is present and that action taken based on Level 1 has failed. Automatic isolation of the heating medium from the gas stream will initiate a transfer of the gas turbine to a cold mode of combustion operation and/or lower turbine load.
Level 3. At a minimum, triple redundant sensing instrumentation is implemented and set at a level higher than that of Level 2. Output from these signals shall be integrated into the customer’s master trip signal. This provides a final level of indication/mitigation following a rupture/leak event. Activation of these level switches switches prevents water from being admitted to the combustion system by either isolating the gas supply or tripping the gas turbine.