The 9F platform: A history of refinement

9FB gas turbine installation in Spain

With options to suit a range of applications and decades invested in their development, General Electric’s range of 9F turbines have evolved to become an effective, adaptable heavy-duty unit for gas fired applications.

Randy Morrell, Laurent Cornu, Jeffrey Goldmeer, GE, USA

General Electric (GE) has been producing the 9F family of heavy-duty gas turbines for more than 20 years. Launched in 1991, the first had a simple cycle output of 212 MW and efficiency of around 35 per cent. Refinements to this design led to the 9FA.01 and then 9FA.02 turbines, which each improved upon the capacity and efficiency of their predecessors. The current version, the 9FA.03, offers many improvements that directly affect performance, operational flexibility and availability. Some of these new systems include an enhanced compressor, a combustion system that reduces NOx emissions to between 9 ppm and 15 ppm, hot gas path cooling enhancement, and blade health monitoring.

GE expanded the 9F family with its most advanced 50 Hz air-cooled gas turbine, the 9FB (see Figure 1). Using the same compressor aerodynamics as the 9FA but operating at increased pressure ratio, and with new components capable of operating at higher firing temperatures, the 9FB further increased output and efficiency. Meanwhile, its modular accessory systems reduced installation times.

Figure 1: Evolution of the 9F heavy duty gas turbine platform

The 9FB gas turbine tends to be used where fuel is a critical component in operating cost, while the 9FA has been designed for F-class calibre simple-cycle peaking power plants and combined-cycle power plants. Both the 9FA and 9FB offer the flexibility to meet changing demand by rapidly adjusting the output through flexible start-ups and the ability to turn down to low power levels. Both designs can handle frequency changes in regions where transmission grid fluctuations are a concern, or the grids operate under-frequency.

Of the 230 currently installed 9F turbines, which have accumulated more than 8.2 million fired hours and 84 000 fired starts, most have historically operated in a baseload operation (see Figure 2). However, recent shifts in electricity generation and demand have produced a strong trend toward more cyclic operation and are likely to lead to the 9F fleet being more heavily represented in the cyclic and peaking duty operating modes, given its notable reliability and availability. As well as GE’s traditional markets in Western Europe, including the UK, Italy and Spain, it has also found customers for the 9FA and 9FB gas turbines in Eastern European countries such as Latvia and Lithuania, as well as Algeria, Egypt, the Middle East, Chile, Argentina and China.

Figure 2: Fleet statistics for GE’s F-class heavy duty gas turbine fleet

Turbine features

Many of the components in the 9F turbine platform have been developed to provide extra power, higher efficiency, reduced emissions and increased turndown, as well as increased availability and reliability (see Figure 3). Enhancements to the compressor system are aimed at increasing durability and availability, while maintaining or increasing reliability, a vital consideration for plant operators. GE claims that enhancements to the inlet guide vanes, the forward and mid-stage rotating blades, the forward stage rotor vanes, and the aft stage vanes dramatically reduce the degradation of and stresses placed on wear-related components. Meanwhile, advanced material treatments combine to lengthen the compressor’s effective lifecycle.

Figure 3: Common 9F gas turbine technology developments and enhancements

These reduced wear characteristics and greater tolerance to normal operating stresses translate into a robust compressor that is much more tolerant to a wider range of real world operating environments, reducing the likelihood of service interruptions due to unplanned maintenance outages. The overall result is a more reliable gas turbine that is available more often. This also translates into longer intervals between inspections and lower maintenance costs. Specifically, GE claims the enhanced compressor will increase the average lifetime availability of its 9F gas turbine by almost 1.9 per cent.

This improved compressor is fitted to all of GE’s new 9F gas turbines. It is also available as an upgrade for the currently installed fleet, with pre-scoped packages on offer to complement standard planned maintenance outages. These packages range from a control curve change, requiring only a single day to implement, to full replacement requiring a major inspection. Each field package removes applicable technical information letters (TILs) and periodic maintenance requirements, thus increasing the availability entitlement of the gas turbine. Field retrofit outages have already been completed on some 9FA and 9FB gas turbines, with many more under way and scheduled. GE has also developed special tools for these procedures to further minimize outage durations and the impact on operational schedules.

The blade health monitoring system was developed to monitor the R0, R1 and R2 compressor blades by providing real-time measurements of key static and dynamic deflection features. Specifically, this system uses a set of non-contacting probes that are placed on the compressor casing, allowing for data to be collected on the blades’ passing frequency and amplitude signals. These probes are connected to an on-site data acquisition system, which in turn connects to an off-site analysis system. Once collected, this data is compared with GE’s proprietary models. This allows the data to be used, for example, to examine blade trends over multiple starts, and to analyze static deflection at full-speed and full-load to measure deterioration in the blades, time-related trends and significant shifts. Since February 2010, this system has been installed on more than 18 units.

The dry low NOx (DLN) 2.6+ combustor combines the technological advantages of the 9FA gas turbine DLN2+ combustion system with the 7FA gas turbine DLN2.6 combustion system, as well as the 9H gas turbine and 6C gas turbine DLN2.5H systems. The DLN2.6+ has the same configuration as the DLN2.6: five outer fuel nozzles and a single central nozzle. The centre fuel nozzle ensures a stable combustion across a wide range of operating conditions. To the DLN2.6 system architecture, the DLN 2.6+ adds an advanced fuel nozzle, called the ‘swozzle’, from the DLN2+ and DLN2.5H combustion systems. The swozzle combines the fuel injection gas ports into the swirler vanes, all within the fuel nozzle body, to provide a better mixed, more stable combustion zone. A patented asymmetric fuel strategy allows the DLN2.6+ to maintain low emissions levels while extending the available operational load range. Further advanced materials, coatings and cooling technology extend the combustion interval beyond the standard 8000 hours of operation.

Specifically, on the 9FA gas turbine, the DLN2.6+ combustion system offers NOx and CO emissions of 20 mg/Nm3 (9.7/16.2 ppm) to 50 per cent load. With the use of OpFlex Extended Turndown software, NOx and CO emissions are rated at 30 mg/Nm3 (15/24.3 ppm) down to 35 per cent load with a 24 000 hour combustion interval. On the 9FB gas turbine, NOx and CO emissions of 30/12.5 mg/Nm3 (9.7/10.1 ppm) are possible down to 38 per cent load with a 12 000 hour combustion interval. This combustor was launched on the 9FA gas turbine in 2005, on the 9FB gas turbine in 2007, and as of July 2010 is in operation in more than 45 gas turbines with a combined service of more than 340 000 fired hours, and more than 5000 starts. These units are shown operating in peaking duty, cycling duty and base load as shown in Figure 4.

Figure 4: Fleet statistics for DLN2 combustion systems

The DLN2.6+ combustion system also improves on the fuel flexibility of prior DLN systems. Based on the advanced fuel nozzle and control technology, the DLN2.6+ can handle up to à‚±5 per cent variation in natural gas energy content, as measured by modified Wobbe index (MWI), with low combustion dynamics. With the optional model-based control OpFlex Autotune software, the DLN2.6+ can handle up to à‚±20 per cent MWI in primary combustion mode, with up to à‚±10 per cent MWI across all combustion modes. The DLN2.6+ system maintains the ability to burn liquid distillate fuel in a dual-fuel configuration, using water injection to control NOx.

GE has also developed aviation-proven technology for controlling gas turbines using detailed, thermodynamic models of the unit and its sub-systems. A real-time physics-based model increases the gas turbines’ overall performance, operability, and reliability. This model-based control (MBC) has been used for several years on GE’s 7FA gas turbines, but was introduced to the 9FA gas turbine in 2009.

The MBC all load cycle control (ALCC) replaces the current schedule-based control that has been in use since the inception of digital control systems for GE gas turbines. ALCC controls the overall operation of the gas turbine by changing the angle of the inlet guide vanes to adjust the airflow, and fuel flow via the valve position to maintain the set power output within the physical limits of the gas turbine, such as firing temperature, compressor surge margins and combustor operability limits.

Rapid response

The ALCC constantly analyzes the turbine during operation and compares it to a theoretical model, then adjusts its operating parameters over time based on changes in ambient conditions and degradation of the turbine hardware. Since the system can compile an accurate picture of the gas turbine’s physical state, it can increase its flexibility and performance by enabling operations closer to its current physical boundaries, while still increasing the overall reliability of the system.

Running in conjunction with ALCC system, OpFlex Autotune software uses the same model-based control approach for the combustion system. This software replaces a fixed split schedule between the various combustion fuel circuits with a DLN2.6+ combustion model and control logic to tune the splits in real time to lower emissions and dynamics. This means the combustion system can adapt to external changes, such as ambient temperature, fuel temperature and composition, without the need to retune it, eliminating the need for all seasonal system adjustments, and enables the DLN2.6+ combustion system to handle up to à‚±20 per cent variation in natural gas energy content as measured by the MWI. This process of continual tuning helps prevent periods of high dynamics, which can rapidly damage hardware, leading to higher repair costs and reduced combustion intervals.

GE offers a fast start package for the 9F family that reduces the time to fire up the turbine across the entire start sequence for both simple- and combined-cycle applications. For simple-cycle gas turbines, start-up to full load takes only 15 minutes, enabling the gas turbine to be used as a non-spinning reserve. In combined-cycle configurations, the power plant start-up time can be reduced by up to 20 minutes. Combining the turbine fast start with combined-cycle rapid response products enables the turbines to be brought on-line at similar speeds to simple-cycle units and provides opportunities as a non-spinning reserve, while also maintaining the higher efficiency of a combined-cycle power plant during normal operation.

The package utilizes six techniques to achieve this faster start time. The purge cycle has been moved to shortly after the gas turbine shuts down, eliminating the need to enter the purge cycle during the start sequence. A load-commutated inverter enables the rotor to engage more quickly with the LCI system, allowing for faster starts of the gas turbine. The LCI turns the generator into a motor, providing the initial torque to spin the turbine until the combustion system is firing and the turbine is producing enough power to accelerate on its own accord. When a turbine must be restarted, fire-on-the-fly technology eliminates the hold on rotor speed during the combustion ignition process, enabling the gas turbine to continue to accelerate.

Fast grid synchronization uses closed-loop acceleration control and incorporates a feedback to the control system of the actual acceleration rate versus target rate to ensure a more consistent start time to full speed and no load across the ambient temperature range. Meanwhile, advanced control logic allows faster load acceleration to reduce the time needed to synchronize the gas turbine generator with the grid frequency.

The turbines are also able to load from full speed and no load to full speed with full loading roughly twice as fast in simple-cycle configurations, or with a rapid response combined-cycle plant.

Flexibility and performance

The 9FA gas turbine can be integrated into a variety of power generation applications, and can operate on a range of fuels. GE continues to improve the 9FA turbine’s thermal efficiency, power output, operating flexibility, availability, reliability and emissions. These product enhancements to materials, aerodynamics, combustion and controls can improve baseload efficiency by up to 5 per cent and raise power by up to 15 per cent in simple-cycle applications. GE claims these enhancements also increase reliability and availability up to 2 per cent while reducing NOx and CO emissions. Given the need for increased fuel and operational flexibility, this unit can now provide turndown to 35 per cent gas turbine load as well as an increased range on MWI.

When combined with a multi-nozzle quiet combustor (MNQC), a diffusion combustion system with more than 1 million fired hours in the field, the 9FA can operate on many low heating value fuels. The 9F Syngas turbine, which is a derivative of the 9FA gas turbine, is capable of operating on syngas or high hydrogen (H2) fuels, and can be used in integrated gasification combined-cycle (IGCC) plants, or in other applications that generate syngas or high H2 fuels. Using the 9F Syngas turbine as part of an IGCC plant with pre-combustion carbon capture allows for reduced emissions. A cooling optimization package can provide up to a 1.5 per cent increase in power and a 1 per cent improvement in heat rate, by reducing clearances on the first stage bucket using abradable first stage shrouds and case temperature management to control thermal expansion, thereby maintaining closer clearances.

The high firing temperatures found in advanced gas turbines such as the 9FA call for the hot gas path components to be cooled below their required material property limits. GE uses internal air-cooling and external film cooling on the 9FA components. The second and third stage nozzles are cooled by air extracted from the 13th and 9th compressor stages, respectively. Due to the pressure ratios over the majority of the ambient and operating ranges, the second stage nozzle cooling air can be mixed in an ejector to use less 13th stage and more 9th stage air. As less work has been expended to compress air at this stage, the trade-off results in an overall efficiency improvement. The extraction control system manages how much air from each stage is mixed in the ejector, based on load and ambient conditions. It can also reduce the overall cooling air for the third stage nozzle, leading to further gains in efficiency.

With a focus on efficiency, the 9FB gas turbine is designed for applications in regions where fuel costs are a critical consideration. The turbine is able to operate on natural gas or distillate fuel, allowing the operator to take full advantage of the best fuel prices or supplies available at any given time. There are currently 28 of these gas turbines operating in the field, which have accumulated more than 230 000 fired hours and 3800 fired starts. The fleet leader has more than 22 500 fired hours and more than 320 fired starts.

GE is also offering a performance enhancement package for the 9FB model that delivers incremental gas turbine performance through reducing secondary flow and drops in combustion pressure, as well as by modulating extraction flow and reducing the pressure loss across the inlet duct. With these improvements, the 9FB gas turbine is capable of generating 291 MW of electricity with a heat rate of 8880 BTU/kWh of thermal energy at 38.4 per cent efficiency in simple-cycle configuration (at ISO conditions). In a 109FB combined cycle configuration, the unit delivers 444 MW with a heat rate of 5778 BTU/kWh and 59.1 per cent thermal baseload efficiency (reference plant at Gas Turbine World conditions). Another benefit from the design enhancements in this package is improved off-frequency operation.

Modular construction

The time required to construct and start up a new power plant is always going to be a critical portion of any infrastructure schedule. The modular 9FB system requires suppliers to assemble as much of the piping and supports as possible before reaching the field, to reduce installation costs. Five modules are pre-assembled and assembled at the final site. Although this decreases installation time, it also offers the opportunity to improve quality and reduce waste, since pre-assembly ensures the pipes are the right size. It also creates a safer working environment for the labourers, with more ground level and shop work compared to work on-site. There are also logistical benefits since there are fewer crates and loose parts to store, and a smaller lay-down area. Lastly, the modular process allows faster combustion inspection and increased gas turbine availability.

Efficency without compromising flexibility

The 9FA and 9FB gas turbines are another step in an evolving portfolio of heavy-duty gas turbines, tackling the changing demands of global power generation. These enhancements have combined to create turbines that offer improved power output, thermal efficiency, emissions and field installation benefits, without compromising the operational flexibility traditionally delivered by GE’s F-class fleet.

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