Gas

Building Better Gas Turbines

Issue 6 and Volume 116.

By Russell Ray, Managing Editor

7FA Gas Turbine
The rotor of a 7FA Gas Turbine manufactured by General Electric. Photo courtesy GE

Electricity generated from natural gas is projected to rise 17 percent this year, according to the Energy Information Administration, the statistical arm of the Department of Energy. Another study is projecting that natural gas will replace coal as the leading fuel for generating electricity in the U.S. by 2025.

The EIA estimates the U.S. will add 222 GW of generation capacity between 2010 and 2035. More than half of that new capacity will be fired by natural gas, the agency estimates. In addition, more than 250 gas-fired power plants are expected to come online between 2011 and 2015.

The transition to gas-fired generation and renewable power means gas-turbine efficiency and flexibility will be critical for power producers planning to build a new fleet of modern-day power plants fueled with natural gas.

To understand the options available to power producers, Power Engineering examined the latest gas-turbine innovations from Alstom, Mitsubishi, General Electric, Siemens and Hitachi. The following is a description of what each company has to offer.

ALSTOM

The ability to deliver more than 450 MW (in KA24-2 CCPP-configuration) to the grid within 10 minutes and achieving low level emissions even though the turbine is “parked” below 20 percent of the maximum load makes the GT24/KA24 configuration attractive for clients who want maximum flexibility and reduced cost for fuel and maintenance.

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Alstom’s GT24 gas turbine. Photo courtesy Alstom

During periods of high demand, operators want to maintain peak output reliably while during off-peak periods combined cycle power plants (CCPPs) are shut down or operated at minimum stable load.

To meet these market challenges, the Alstom GT24 gas turbine – or its sister product for the 50Hz market, the GT26 – has been designed to deliver more than 230 MW at 40 percent efficiency (respective values for GT26: 326 MW at 40.3 percent efficiency). The corresponding KA24-2 CCPP can achieve 700 MW output in a 2-on-1 configuration and operate at more than 60 percent efficiency. Together with Alstom’s Low Load Operation (LLO) technology, operators can park their plant below 20 percent CCPP load with both gas turbines and the steam turbine in operation. Compared to a 40 to 50 percent plant load, which is the current industry standard, the LLO feature helps operators enhance competitiveness, guarantee grid stability on demand and reduce the environmental footprint.

The GT24 supports the operator’s efforts to add renewable energy to the grid. In case the sun is not shining or the wind is not blowing, the operator can deliver GT24/KA24’s power reserves within minutes to the grid. Using the flexibility of this gas turbine technology as a backup simplifies the integration of renewable energy to the grid and supports its stability.

One of the main technology differentiators of the GT24 is the “sequential (2-stage) combustion” principle: The EnVironmental (EV) burner in a first annular combustor followed by the high pressure (HP) turbine, Sequential EnVironmental (SEV) burners in the second annular combustor, and the low pressure (LP) turbine. The improved LP turbine can now switch on-line between two operation modes thereby enabling extended operation intervals of up to 30 percent between inspections. To further enhance operational flexibility of the GT24/GT26, different types of fuel can be used to generate power without modifying setup or configuration of the turbine. Additionally, the plant efficiency remains nearly constant from 100 percent full load down to 80 percent combined cycle power output.

With the GT24/GT26, power producers can improve the competitiveness of the asset by enabling higher operational flexibility with improved performance and extended intervals between inspections. For example, the intervals for the inspection of the hot gas path were extended to up to 8,000 operation hours while increasing the output and decreasing the heat rate.

The latest GT24/26 configuration has been extensively tested at a test facility in Birr, Switzerland as well as in a commercial unit in Spain. Performance guarantees were exceeded and various validation tests have been successfully conducted on the LP turbine, the compressor and the SEV burner.

Mitsubishi

Mitsubishi’s current gas turbine line up for both 50 and 60 Hz includes large frame gas turbines featuring efficiencies among the highest in the industry. The efficiencies are achieved by increasing the turbine inlet temperature (TIT) and by applying other technology enhancements that include, among others, higher pressure ratio compressors. Depending on the TIT, Mitsubishi applies air or steam cooling technology.

The source of new technology allowing these gas turbines to achieve leading efficiencies is a Japanese government funded project lead by Mitsubishi called the “Japanese National Project.”

T-Point Combined Cycle
T-Point Combined Cycle demonstration plant. Photo courtesy Mitsubishi

State-of-the-art technology developed since 2004, when the national project started, includes Exhaust Gas Recirculation (EGR), higher cooling efficiency, lower thermal conductivity TBC, higher loading turbine aerodynamics, a higher pressure ratio compressor and advanced turbine materials. The main target of this project is the development of a 1,700°C (3,092°F) TIT class gas turbine to achieve 62 to 65 percent (LHV) combined cycle efficiency. This represents a 5 to 10 percent increase compared with the current available technology. Mitsubishi has retrofitted several of the developed technologies into existing gas turbine developments.

In the 60Hz arena, Mitsubishi offers two high efficiency gas turbines, the M501GAC and the M501J. The GAC is an air-cooled gas turbine featuring combined cycle efficiency in the 59 percent range. This frame is based on the steam cooled M501G, which has accumulated more than 1.5 million operating hours. The M501J is Mitsubishi’s largest and most advanced 60 Hz gas turbine. It operates at a record TIT of 1,600°C and takes advantage of advanced technology developed for the “Japanese National Project”.

Similar to the 60 Hz, air and steam cooled gas turbines are available for 50Hz. The M701F4 and the M701F5 are Mitsubishi’s latest air cooled F class gas turbines, while the M701J is the steam cooled version designed as a scale up of the 60 Hz counterpart.

Mitsubishi’s approach to testing new frames offers a key differentiator. Rather than testing new designs in shop test facilities for a few hours, Mitsubishi owns and operates a full scale 1:1 combined cycle on the premises of the Takasago Machinery Works, where newly designed or upgraded 60 Hz units are dispatched with the associated revenue. Compared with off-grid testing, this revenue stream allows sustained long term validation under real demand conditions.

General Electric

Close collaboration between its energy and aircraft businesses continues to drive new technology into GE’s heavy duty gas turbine portfolio. GE’s latest 7FA Gas Turbine utilizes aircraft engine compressor technology that has been proven in more than 1.1 billion reliable revenue hours.

General Electric's 7FA Gas Turbine
General Electric’s 7FA Gas Turbine. Photo courtesy GE

More than 750 GE 7FA Gas Turbines are installed today and produce about 15 percent of North America’s power. The latest 7FA offers improvements in output, thermal efficiency, operability and lower life cycle costs – all without compromising the reliability, availability, maintainability and operational flexibility. .

At ISO conditions, the latest 7FA Gas Turbine will produce about 216 MW in simple-cycle operation, an increase of nearly 31 MW over the previous 7FA Gas Turbine. In a 207FA combined-cycle configuration, the output will be 648 MW, an increase of more than 100 MW. The ISO efficiency has been increased to 38.5 percent in simple cycle and 58.5 percent in a 207FA combined-cycle configuration.

The advanced 7FA is part of GE’s overall strategy to develop gas turbine technology with the flexibility to take full advantage of natural gas. By providing the ability to start up quickly and to rapidly ramp electricity production up and down in response to fluctuations in wind and solar power, this technology offers efficient integration of renewable energy onto the grid.

The latest 7FA Gas Turbine also employs GE’s proven DLN2.6 combustor, which has more than 17 million hours and 400,000 starts of additional experience. Only minor modifications to the DLN2.6 combustion system are required for improved output and efficiency. With DLN2.6 technology, the latest 7FA Gas Turbine is capable of reducing NOx emissions to 9 ppm.

The turbine section of the newest 7FA Gas Turbine features advanced hot gas path (AGP) enhancements, including improved cooling, sealing and clearances for increased efficiency. This was developed with significant technology shared with GE aviation, including advanced 3D aerodynamics and the use of innovative materials capable of providing higher efficiency levels with increased part lives.

GE has made significant investments in a full-speed, full-load test facility in Greenville, S.C. that allows gas turbines to be tested under extremely demanding conditions. The new facility enables the testing of the latest 7FA Gas Turbine beyond the normal operating conditions that would be expected in the field. Because of this validation process, operators will know in advance how their gas turbines will respond to the most extreme conditions. In July 2011, GE validated the 7FA compressor and will complete full scale validation of the latest 7FA Gas Turbine in 2012.

Following an internal and third party environmental and operational evaluation, the latest 7FA Gas Turbine has become part of GE’s ecomagination portfolio.

Siemens

In the past, there was an expectation that combined cycle power plants would run at baseload. Today, these plants not only need to be powerful and efficient but also need to respond to increasingly strict environmental requirements, rapidly changing load demands, and increased non-dispatchable megawatts to compete.

Siemens' SGT6-5000F gas turbine
Siemens’ SGT6-5000F gas turbine. Photo courtesy Siemens

Siemens has been designing flexible engines and power plants for almost a decade. For the North American market, Siemens offers a range of gas turbines with high start up times in simple and combined cycle mode.

The 60 Hz version of the SGT-8000 gas turbine series has been validated in the Siemens manufacturing plant in Berlin and is already marketed all over the world. Six SGT6-8000H turbines have been sold to Florida Power & Light and are now being installed. The FPL contract is specifically for the renovation of its power plants in Riviera Beach and Cape Canaveral. Each plant will make use of three 274-MW-rated SGT6-8000H gas turbines, which are rated for 60 percent efficiency in combined cycle operation and promise a 50 percent reduction in carbon dioxide emissions. They are expected to return to service in 2013 and 2014, with enough combined output to power 500,000 area homes.

The combined cycle based on this machine, the SCC6-8000H, is the first power plant to break the 60 percent efficiency barrier, Siemens said. The plant also has a high reliability and provides half a GW in half hour. The SCC6-8000H is available in single shaft, and multishaft configurations. In 2010, Siemens introduced the SGT6-5000F with shaping power and has sold more than 25.

Siemens Flex-Plant 10 is a high efficiency peaker 1×1 F class combined cycle with low start up emissions. The SCC6-5000F 1×1 Flex-Plant™ 10 is designed around the SGT6-5000F, two of which have reached more than 8000 equivalent operating hours in 2011. This turbine is the only frame gas turbine-generator that can generate 150 MW in less than 10 minutes, reaching full load in 12 minutes without imposing additional maintenance impacts. The power plant uses a simplified single-pressure non-reheat bottoming cycle with an air-cooled heat exchanger for backpressure steam condensing to provide a net efficiency of more than 48 percent. Through the use of air cooling, the SCC6-5000F 1×1 Flex-Plant™ 10 uses minimal water to operate. It is the only combined cycle power plant to qualify for the non-spinning reserve market. A standard SCR is another available feature.

The El Segundo Flex-Plant™ 10 (FP10) is a 550 MW project, consisting of two 1×1 combined cycles and boasts being the first combined cycle plant offered with a capability of reaching 150 MW per block within 10 minutes of gas turbine ignition. The El Segundo Flex-Plant 10 consists of two power blocks each with a natural gas fired SGT6-5000F gas turbine, a single pressure drum-type boiler, a SST-800 steam turbine and a positive pressure air cooled heat exchanger. The Flex-Plant™ 10 technology allows for the fast start up time of a stand alone gas turbine but with better efficiency and lower emissions than a combined cycle provides.

The El Segundo Flex-Plant™ 10 project is on the scenic beach in El Segundo, Calif. The project is a redevelopment of two previous decommissioned once-through cooled units owned and operated by NRG. Unlike the old units, which heated seawater to cool the plant, the new Flex-Plant™ 10 plant will use atmospheric air as the plant’s cooling media, thus lowering the environmental impact. Commercial operation is scheduled for early 2013.

The Siemens Flex-Plant™ 30 is a fully integrated high efficiency 3 pressure reheat combined cycle which can ramp on and off and up and down like a simple cycle. This concept is unique because of its fast-start capability designed to deliver approximately 200 MW of power to the grid in only 30 minutes. These short start-up times reduce the emissions of the plant significantly. The SCC6-5000F Flex-Plant™ 30 is a highly efficient combined cycle plant designed for intermediate to continuous duty that is capable of daily cycling at efficiencies of more than 57 percent.

The Lodi power plant in California starting up in June 2012 is a 280-MW, natural gas-fired combined cycle power plant based on a Siemens Flex-Plant™ 30 power island. It generates efficient and environmentally friendly power. New features for the Flex-Plant™ 30 are emissions of only 2 ppm NOx and <5 ppm CO while load following a 2×1 CC at up to 75 MW/min.

Hitachi

Hitachi’s combustion turbine generator product line includes models H-15 and H-25. The H-15 is a scaled down design of the H-25.

The H-25 is a cost effective and schedule effective replacement option for the aging fleet of General Electric Frame 5 combustion turbine generators, respectively. It is designed to utilize the same foundation and auxiliary equipment as the respective GE unit and moreover, it results in a significant increase in output at reduced fuel consumption and higher efficiency. Over 145 H-25 units are in successful operation worldwide.

Hitachi’s H-25 and H-15 combustion turbine generators are heavy duty type frame machines with high efficiencies and high reliabilities, suitable for not only simple cycle but also combined cycle, cogeneration and mechanical drive applications.

The H-25 and H-15 combustion turbines are single shaft machines equipped with conventional or dry low NOx combustors and able to burn a variety of fuels, including natural gas and distillate oil. The generator is geared, which allows the same turbine design to be applied in markets with 50 Hz or 60 Hz grid frequencies.

The units are enclosed by an all-weather proof housing suitable for indoor or outdoor installation. They are shipped as a modular equipment package, which minimizes site installation time and labor cost.

The intervals between scheduled inspections are maximized, while outage times are minimized.

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