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Fast starts and flexibility: Let the gas turbine battle commence

Issue 6 and Volume 19.

The new FlexEfficiency 50 combined-cycle plant was engineered from the ground up says GE Source: GE
The ‘big four’ combined-cycle gas turbine manufacturers – Alstom, GE Energy, MHI and Siemens – have launched their latest packages. While all promise greater thermal efficiency, the current buzzword is ‘flexibility’.

Tim Probert, Deputy Editor

Flexibility is the new black. De rigueur. The must-have accessory of the season. Or so the major gas turbine manufacturers claim.

In May, both GE and Siemens launched their next-generation gas turbines, both promising greater efficiency. GE rather rained on Siemens’ parade. The US firm claimed its new machine would achieve a thermal efficiency of 61 per cent, albeit on paper, thus topping its German counterpart’s verified operational efficiency of 60.75 per cent at E.ON’s Irsching 4 plant near Munich with its H Class unit.

In the same week Mitsubishi announced that its latest gas turbine, the J Series machine, had achieved a world record turbine inlet temperature of 1600 °C – 100 °C higher than its G Series turbines – while claiming gross thermal efficiency greater than 60 per cent at a similar NOx burn to current models.

In June, Alstom launched its latest GT26 combined-cycle gas turbine (CCGT) at Europe’s biggest power industry event, POWER-GEN Europe in Milan. The French company claimed that the latest version of the GT26 plant features an efficiency of over 61 per cent in combined-cycle operation.

EnergyAlstom’s next-generation GT26 gas turbine at its test power plant in Switzerland Source: Alstom

Yet while these latest turbines may have initially been conceived to push the efficiency envelope, the marketing and PR machines have been working on overdrive to hammer home that the new offerings are not just efficient, but crucially, flexible too.

According to Credit Suisse, gas fired power plants will make up about 25 per cent of global generation capacity additions in the next five years, boosting orders for gas turbines by 50 per cent to 63 GW, after a 34 per cent drop between 2008 and 2010. The spike in demand is largely due to rapid growth in developing nations and concerns about nuclear energy in the wake of the tsunami in Japan.

Furthermore, new sources of natural gas, as well as improvements to extraction techniques, have reduced prices. According to Navigant, a new gas fired plant can be built to generate electricity for 6 US cents/ kWh, compared with 7.5 US cents/kWh for a new coal fired plant.

But for developed nations with a significant installed base of renewable electricity gas turbines have a new, increasingly vital role. As more wind and solar plants feed a steadily increasing share of intermittent power into the grid, fast-reacting large-scale power plants are key to maintaining grid stability.

GE launches FlexEfficiency 50

To great fanfare, with grandiose claims of a breakthrough product set to revolutionize the power sector, GE launched the FlexEfficiency 50 on 25 May. The clue is in the name.

The machine is rated at 510 MW and GE claims a thermal efficiency in combined-cycle mode greater than 61 per cent. While this is undoubtedly impressive, GE’s figures for the air-cooled FlexEfficiency 50’s operational flexibility may most catch the eye.

GE says the plant reflects an investment of more than $500 million in research and development, drawing from the company’s jet engine expertise to engineer a plant that will ramp up at a rate of 51 MW per minute, twice the norm, and 15 MW per minute more than even Siemens’ new H Class machine. Furthermore, the new plant can achieve this while maintaining emissions limits of 50 ppm NOx and can go from hot start to full rated power in 28 minutes.

Paul Browning, GE Power and Water’s vice president of Thermal Products, says that his engineers avoided the typical trade-offs between flexibility and efficiency by approaching the plant design from a total equipment and control systems perspective. The FlexEfficiency 50 plant is engineered for flexible operation by integrating a next-generation 9FB gas turbine that operates at 50 Hz; a 109D-14 steam turbine, which runs on the waste heat produced by the gas turbine; a W28 generator; a heat recovery steam generator (HRSG); and a Mark VIe integrated control system that links all of the technologies.

“In support of fluctuations in renewables, fossil fuel prices, and energy demand, fewer plants will be operating in baseload mode,” said Browning. “That’s why GE technologies are engineered to deliver enhanced cyclic capabilities that allow utilities to ramp faster and more often, cycle on/off faster and more often, and provide more short-term reserves.”

Accounting for both the profitability of power production and the annual fuel consumption for cyclic operation, GE has defined the term FlexEfficiency as “profitable annual MWh (excluding during minimum turndown) over annual fuel consumption (including fuel consumption during start-up)”.

At an operating profile that includes 250 starts per year and a mix of baseload, part load and minimum turndown hours, typical advanced combined-cycle power plant ‘FlexEfficiency’ is 54 per cent. The ‘FlexEfficiency’ rating for the FlexEfficiency 50 plant is greater than 58 per cent on the same basis, including plant part load efficiency greater than 60 per cent down to 87 per cent of the plant’s baseload power output. The US firm also claims that its latest Frame 9FB machine allow a CCGT plant to turn down to 40 per cent of its load while maintaining emissions guarantees. Browning says a typical FlexEfficiency 50 CCGT single-shaft platform will offer annual fuel savings of 6.4 million m3 of natural gas, or about $2.6 million per year under a typical operating profile of 4500 hours per year at a natural gas price of $10 per million BTU.

Frame 9FB advances

In a break with the past, GE designed the first version of this technology to work on European and Asian power grids which use 50 Hz. The United States, of course, uses 60 Hz.

The gas turbine to be employed by FlexEfficiency 50 is not a new machine but an updated 9FB, of which GE can boast 28 units operating globally with than 230 000 fired hours and 3800 fired starts in 50 Hz configuration. GE claims that in FlexEfficiency 50 plant operation at ISO baseload conditions for 4500 hours per year, the 9FB will achieve 40 per cent efficiency in simple-cycle mode and an overall 1 per cent increase in combined-cycle efficiency compared to the prior model.

GE describes the 9FB as a 1500 °F (816 °C) class turbine, but for the latest version GE will increase the firing temperature by 50 °F. The improvements in the 9FB are built on advances ranging from its inlet system to its exhaust to the heat recovery steam generator, for which condensate polishing is not required.

The low-loss filtration system feeds a 14-stage three-dimensional, aerodynamically designed compressor utilizing greater use of 3D blade modelling. The hybrid radial diffuser recovers static pressure for the evolved DLN 2.6+ combustion system with advanced fuel staging for enhanced steady state and transient performance, and GE claims an extended turndown of 30 per cent gas turbine load while maintaining emissions guarantees.

Torque from the new four-stage hot gas path, with an inner shell for better managed clearances, is transmitted through a simplified rotor arrangement. Technologies from GE’s aviation and power generation experience include aerodynamics, heat transfer, cooling and sealing and materials technologies that are fully integrated with an advanced, model-based control system.

New 109D-14 steam turbine

Uniquely engineered for the FlexEfficiency 50, the new 109D-14 steam turbine, rated at 180 MW, is a three casing design featuring a high-pressure (HP) section, intermediate-pressure (IP) section and double flow, low-pressure (LP) section. This design configuration, says GE, enables effective management of clearances to deliver a high power density solution and overall steam turbine efficiency greater than 40 per cent.

The integrated clutch provides further enhanced operating flexibility in the FlexEfficiency 50 plant power train enabling ~85 per cent load attainment in less than 20 minutes under hot start conditions.

Advances in turbine maintainability and detectability, such as bearings external to the LP hood, adjustable stationary nozzles with the rotor in place and improved turbine monitoring sensors, enable improved turbine maintenance and shorter outage durations. On run-down, this integrated clutch enables earlier access to the gas turbine, with the steam turbine still on, turning gear for cool-down, thereby reducing the gas turbine maintenance cycle by about two days.

Also new is the W28 generator, a 550 MW unit incorporating hydrogen cooling of the field and stator core and direct water-cooling of the stator windings to provide improved cooling capability, as well as increased reliability and efficiency. GE says the overall efficiency of the 650 MVA generator exceeds 99 per cent at 0.9 power factor.

Prior to first fire, the latest 9FB gas turbine will be tested to full load in GE’s new, $170 million validation facility in Greenville, South Carolina. This dual fuel, non-grid connected facility will also provide part load, variable frequency, and transient capability validation.

On 3 June, GE announced that it had signed an agreement with China’s Harbin Electric to supply two Frame 9FB’s with FlexEfficiency technology, while on 8 June, the company named its first customer. MetCap Energy Investments, a Turkish project developer, is to integrate GE’s latest combined-cycle technology with solar thermal and wind power to create a innovative power plant that is scheduled to enter commercial operation in 2015

H Class: Siemens’ green banana now fully ripe

Siemens’ H Class programme was initially conceived as a gas turbine that would achieve 60 per cent efficiency at a capacity of 600 MW in combined-cycle operation. Unfortunately, Siemens has fallen just shy of the capacity target, but it still achieved a world record efficiency of greater than 60 per cent.

During a customer acceptance test run on 11 May the Siemens designed and built Irsching 4 CCGT, owned by E.ON, achieved an efficiency of 60.4 per cent (net), at a net power output of 570 MW. A few days earlier, on 6 May, during a TUV certified test run, the plant recorded an efficiency of 60.75 per cent (net) at a net power output of 578 MWe. The Irsching 4 CCGT plant in Bavaria, Germany, utilizes a SST5-5000 steam turbine, a SGen5-3000W generator and the SGT5-8000H gas turbine in a single-shaft configuration, but Siemens’ HRSG design unveiled at Irsching 4 is perhaps key to the H Class’s marked improvements in efficiency.

The steam parameters are indeed most impressive, particularly when compared to the F Class, Siemens’ erstwhile flagship gas turbine. The Benson HRSG, designed and built by Siemens following its 2007 acquisition of the Balcke Dürr HRSG business, is a step-change compared with the F Class. The steam temperature is 600 °C, an increase of 35 °C. Steam pressure is 180 bar, an increase of 35 per cent over the F class’s 130 bar, while steam mass flow is 100 kg/s, an increase of 30 per cent over 77 kg/s achieved by the F Class. The heating surface is increased by 45 per cent.

The HRSG at Irsching 4 eliminates the high-pressure (HP) drum to become a once-through system. This allows E.ON to increase the ramping rate of the plant, as well as increase the cycle frequency, crucial in achieving 35 MW/minute, or 500 MW in just under 30 minutes. Compared to the F class, this is an improvement of more than 5 MW/minute. Additional features of the Irsching 4 set-up include a condensate polishing plant that allows for easier start-up; stress controllers in the boilers and steam turbine; and high capacity steam de-superheaters which increase flexibility.

The need for speed

Lothar Balling, Siemens’ general manager for gas turbines who instigated the H Class programme in 2001, says that by 2020 it will be possible in Germany to meet the entire power demand in some periods on a sunny and windy day entirely from wind, solar, hydroelectric and biomass fuelled power plants.

“However,” Balling said, “if the weather situation changes at short notice, we will need about 20–50 GW of power from other sources to come on line within just a few minutes or hours. Up to 100 per cent of the non-renewable fleet will require daily start-stop operation, and load ramps of about 200 MW/minute will need to be covered.”

The H Class has been designed with this need in mind to be fast starting and flexible. Despite its size, the plant can run stably at around 100 MW – less than 20 per cent of its total rated output – in combined-cycle mode with an efficiency still typical of peak load power plants. This shows, says Balling, that while it was not designed to be a peaking plant, the H Class can be used efficiently throughout the base, intermediate and peak load ranges.

The operators of small or quasi-islanding grids such as in the UK or Singapore make very special demands in terms of grid stability, says Balling. “Testing according to the UK grid code frequency response requirements showed that we can use the features already operated in F Class plants in this plant, too, and can likewise significantly over-fulfil the requirements at values of 64 MW in 10 seconds.”

Another test prescribed by the UK grid code calls for a 45 per cent load rejection with stable continued operation to intercept rapid frequency rises that can occur due to sudden high power inputs from other sources, but also due to disconnection of large loads. Here, too, the H Class plant can shed 250 MW within less than six seconds and continue running stably, as demanded by the grid code.

Design features of the SGT5-8000H gas turbine

The engine concept was selected from a number of air-cooled engine design options and several gas turbine cycle variants after completion of a comprehensive feasibility analysis during the conceptual design phase. Siemens says the air-cooled concept selected offers maximum added value by virtue of its higher operational flexibility – an essential prerequisite in the deregulated power generation market environment.

Major gas turbine design features include a single tie-bolt rotor comprising individual compressor and turbine disks with Hirth serrations; hydraulic clearance optimization; axial 13-stage compressor with high mass flow, high component efficiency, controlled diffusion airfoils in the front stages and high performance airfoils in the rear stages, variable guide vanes and cantilevered vanes; high temperature, air-cooled, can annular combustion system; a four-stage, exclusively air-cooled turbine section; and an on-board variable dilution air system, with no external cooling system.

TESTING TESTING

Siemens was very keen to avoid the very costly mistake made by one manufacturer of launching a pioneering gas turbine without a thorough testing process.

The Irsching power plant, home to five units in total, three of which are operational, is 50 metres from the River Danube in Bavaria at Vohberg, Germany’s smallest city. The proximity of an abundant supply of cooling water, as well an E.ON Ruhrgas pipeline, means Irsching 4 was an ideal location to develop Siemens’ first new frame since its acquisition of Westinghouse Electric’s non-nuclear division in 1997.

The Irsching 4 H Class was first fired in combined-cycle mode in late December 2010 and has since clocked up 2500 operating hours, which will rise to 4000 hours once the reliability testing phase is complete. As of 19 May, the H Class had achieved 100 starts in combined-cycle mode, with a starting reliability that exceeded 90 per cent.

Market Launch, and First Commercial Projects

The SGT6-8000H, rated at 274 MW, is a direct scale of the 375 MW rated 50 Hz SGT5-8000H. The design of the SGT6-8000H is strictly based on Siemens’ proven aerodynamic scaling rules. A scaling factor of 1:1.2 is being applied consistently over the entire cross section of the turbine. The only exception is the combustion system, where exactly the same components, such as burners and baskets, are used as in the 50 Hz model.

To reflect the reduced mass flow of about 1:1.44, 12 rather than 16 can combustors are used on the 60 Hz model. Validation efforts for the SGT6-8000H can be based on the comprehensive information gained during the SGT5-8000H test programme and will require only limited additional efforts to fully prove the integrity of the 60 Hz model. Therefore, the first unit will be installed in Siemens’ Berlin test centre and be subject of a six-month test programme.

The first commercial H Class projects include the 1200 MW Cape Canaveral and Riviera Clean Energy Centers being built by US utility Florida Power & Light and the 410 MW unit Bugok III of GS Electric Power & Services in Seoul, South Korea. In all, eight H Class units have entered the market phase. In the long-term, Siemens targets 2015 for operating a 600 MW 50 Hz H Class combined-cycle gas turbine at an efficiency of 61.5 per cent.

Alstom going big on operational flexibility

French gas turbine manufacturer Alstom has viewed flexibility as the raison d’etre of its GT26 turbine for some time. Due to the increasing installation of intermittent renewables, Alstom sees efficiency under part load operation as even more important than efficiency in baseload.

Siemen’s H Class CCGT at Irsching 4 in Germany Source: Siemens

While Alstom is claiming baseload efficiency of 61 per cent, it is keener to stress that the latest GT26 has the best all-round efficiency over the entire load range. Alstom says its low load operation capability allows a 500 MW-plus GT26 CCGT power plant to be ‘parked’ at a much reduced minimum load point – about 100 MW – to provide fast responding stand-by and significantly reduced fuel consumption during such low load periods. It also claims a ramp up rate of 350 MW in 15 minutes from low load.

The latest upgrade of the sequential (two-stage) GT26 gas turbine is the fourth evolutionary development stage since the product’s initial introduction in the mid 1990s. From a technological standpoint, the upgrade is based on the development of the compressor, the second combustor and the low-pressure (LP) section of the turbine.

Compressor, combustor LP upgrades

Alstom claims the compressor upgrade results in an increased compressor inlet mass-flow at high compressor efficiency over a wide range of ambient and load ranges. The compressor architecture is based on the GT26’s usual 22-stage design. The outer annulus is increased to match the mass-flow increase.

Compressor blading is designed using tools developed by its technology partner Rolls-Royce, and the state-of-the-art blade design features controlled diffusion airfoils. To improve the part load performance, the variable vane row count has been increased from three to four.

The architecture and structural parts of the GT26’s second-stage combustion system remain unchanged, but there are several modifications to the burner, the lance and fuel injection, as well as improved seals to reduce leakages into the combustion chamber. The burner modifications ensure a better mixing of the fuel with the airflow, resulting in lower emissions over a wide operation range and fuel gas composition.

The combustion system is designed to operate over a wider wobbe index (WI) range and fuels with higher hydrocarbon content – such as ethane, propane and butane – than the current GT26. Alstom claims that NOx emissions will be below 25 vppm at 15 per cent O2 dry over a load range from 100 per cent down to 40 per cent, as well as the low load parking point.

The upgrade also includes an improved LP turbine. The high pressure turbine is unchanged. All four LP turbine stages contain airfoils with optimized profiles and cooling schemes. The shroud design was improved to reduce the over-tip leakages. In addition, the vane-part count per row is reduced from the current GT26 to minimize the hot gas surface, which requires cooling. The turbine annulus has also been increased to accommodate the higher hot gas mass flow delivered by the upgraded compressor. Alstom says these modifications enable higher component efficiency and the ability to switch on line between two operation modes, thereby offering an increase in scheduled maintenance schedules, i.e. hot gas path inspections, up to 30 per cent, resulting in higher availability and maintenance costs.

Testing of the full, upgraded GT26 package started in March 2011 at Alstom’s test plant in Birr, Switzerland. The LP turbine hardware has been released for a commercial KA26 unit in Spain owned by HC Energia. As PEi went to press, the turbine had clocked up 8000 operating hours.

Mitsubishi J Series – efficiency over flexibility?

Last but by no means least, in May Mitsubishi Heavy Industries (MHI) achieved the world’s highest gas turbine inlet temperature of 1600 °C, with its J Series gas turbine. The new turbine can withstand a temperature 100 degrees higher than the 1500° C G Series gas turbines, which operated at the highest temperature until now.

The J Series gas turbine has achieved a rated power output of about 320 MW (ISO basis) and 460 MW in combined-cycle power generation. MHI also confirmed gross thermal efficiency exceeding 60 per cent, but is aiming to achieve 61 per cent later this year. MHI says the J Series features improved 3D engineering of compressor blades, steam-cooling of stationary combustor components and advanced thermal barrier coatings. MHI has yet to highlight any particular efficiency features, but claims the 60 Hz M501J will be able to hold 55 per cent efficiency at 50 per cent load. MHI says high efficiency is particularly valuably in Japan, which imports all its fossil fuel; this has even greater significance after the Fukushima Daiichi nuclear accident.

The J Series poses a major challenge in combustor development. A hike in temperature raises emissions, creating a need for effective cooling. MHI tends to favour steam cooling for very high temperature applications because steam is a more effective medium for cooling. MHI is leveraging its long steam cooled combustor experience in the G Series, where durability has been achieved for higher turbine inlet temperatures, while keeping emissions at a level equivalent to current MHI models. The steam flow rate can be adjusted to maintain the metal temperature of combustor hardware. Improvements can be realized by MHI’s extensive steam-cooled combustion experience with the G-class.

A J Series combined-cycle power plant is being verification tested at MHI’s Takasago Machinery Works. Following the 60 Hz M501J, the Takasago Machinery Works is currently developing the 50 Hz M701J gas turbine, targeting first shipments in 2014. Six units of the J Series turbine are slated for delivery to Kansai Electric Power’s Himeji No.2 power station in Japan.


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