GAS TURBINE TECHNOLOGY

First 60Hz ‘H’ takes shape at Riverside

1 September 2007



The term 7H identifies the 60 Hz version of GE's H System, the world's first combined-cycle technology platform designed with the capability to reach 60 % thermal efficiency. But because of its steam cooling requirement the claim cannot be proven until a suitable CC system is running. That didn’t happen at Baglan Bay. Now, in the race to achieve 60%, the baton passes to Riverside, Ca.


When in 1995 GE Energy set out the R & D goals for its collaboration with the DoE’s Advanced Turbine Systems Programme it had four specific aims for the chosen combined cycle operation – it had to run at the highest possible firing temperature, at a combustion temperature consistent with 10ppm NOx into the exhaust stack while achieving 60% CC efficiency and lower electricity production costs. By 2000 it could report that three of those goals had been achieved, that indications were that achievement of the 60% landmark was on target and by 2003 that the first commercial unit was producing electricity at Baglan Bay, Wales. This was, of course, a 50 Hz unit, designated 9H in GE’s parlance, and the world's first combined-cycle technology designed with the capability to reach 60 % thermal (ie fuel) efficiency.

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Now a 60 Hz version, designated 7H, or more formally a model MS7001H GT, has completed testing and has been installed at the Inland Empire Energy Center near Riverside, California, where engineering construction is expected to finish in time for first firing at the end of the year.

It is the first to be designed specifically for service in the USA. Four 50 Hz units have been sold in the UK and Japan but this will be the first installation of the 60 Hz technology, an important milestone for GE’s US customers.

The new unit, the first of two identical 107 H 405 MW combined-cycle units planned for the 775 MW project, is expected to enter commercial service by the summer of 2008, in time to help offset state-forecast energy shortfalls in Southern California. Operating on natural gas, they are also expected to reduce carbon dioxide emissions by more than 146 000 tons a year, compared to a typical gas-fired power plant of a similar size. GE confidently expects it to serve as a showcase for advanced combined cycle technology worldwide.

GE is keeping close financial and engineering control over this project. They will finance and own the Inland Empire Energy Center. Calpine Power Services will manage plant construction, and Calpine Energy Services will market the plant's output and manage fuel requirements under a long-term marketing arrangement with GE. Following an extended period of GE ownership, Calpine will purchase the plant and become its sole owner and operator.

Outline design

The main plant consists of the two ‘H’ combustion turbine-generators with heat recovery steam generators feeding a single steam turbine generator and associated pollution-control equipment. The four-stage gas turbine is equipped with an 18 stage compressor, and a can-annular dry low-NOx combustion system.

This arrangement is claimed, mainly because of its high efficiency combined with the single shaft design, to offer a 40% improvement in power density compared to other combined cycle systems.

Although GE has designed and built two models, the 60 Hz 7H at Riverside, and the 50 Hz 9H installed at Baglan Bay, Wales, the two version share similar design and capabilities. Both derive their performance primarily from their advanced materials and a new steam cooling system that enables the gas turbines to operate at 2600°F, or about 1425°C, firing temperature, more than 200°F above the previous generation ‘F’ GTs. This innovation, together with GE’s active clearance control system, enables the CC unit to achieve the planned increase in thermal efficiency to 60 %, an ambition that has been described as the ‘four minute mile’ of gas turbine technology.

First firing, second try

GE’s original plan was to demonstrate the 7H at the Sithe Energies Heritage plant in New York state, but after a series of delays the 2004-slated project, which had all major regulatory approvals, was put on hold in December 2001 and finally, in 2002, cancelled altogether. Sithe and GE cited ‘tough economic conditions’ and ‘changing energy markets’.

The deal with Calpine has been altogether more productive. Factory testing was completed in September 2006 and the first 7H, a 311 tonne machine, sent on its way to the Empire Energy Centre. After a seven week journey by land and sea it arrived, in November 2006, and was placed on its foundation. It has now been fully installed. A second 7H has now been delivered and is expected to be ready for first firing in early 2008.

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The second ‘H’ turbine arriving at Riverside

Closed-loop steam cooling

The biggest contributor to the improved thermal performance of the H resides in an innovative steam cooling system that allows the higher firing temperatures required. In a major departure from gas turbine practice, the H system uses steam to cool some nozzles and blades, as opposed to the air system conventionally employed for combustion and cooling.

Open-loop air-cooled gas turbines have a significant temperature drop across the first stage nozzles, which, for a given combustion temperature, reduces firing temperature. Also, steam cooling makes more air available to expand and produce work through the turbine stages, reducing what is usually referred to as ‘chargeable air.’ Closed-loop steam cooling therefore allows the turbine to fire at a higher temperature, 1430°C, with no increase in combustion temperature and with control of the resulting emissions (the firing temperature must not exceed the NOx formation combustion temperature of 1540ºC).

The use of steam as a coolant also improves cooling (and therefore efficiency) because steam has better heat transfer characteristics than air, and retains heat in the closed loop. The coolant reduces the temperature decrease to below 44°C which in turn makes it possible to achieve the aimed at 60% fuel efficiency capability while maintaining adherence to environmental standards.

Closed-loop steam cooling is used on the nozzle and buckets of the turbine's first two stages, with air cooling on the third stage, while the fourth (last) stage is uncooled. Closed-loop cooling also eliminates the film cooling on the gas path side of the blade, and increases the temperature gradients through its walls. This method however causes higher thermal stresses on the airfoil materials, and has led GE to use single-crystal nickel super-alloys for the first stage, with suitable protective ceramic thermal barrier coatings.

The H turbine uses tubular seals dubbed ‘spoolies’ to deliver and return the steam to the rotating buckets of the first and second stages, a technique that has used for many years on GE aircraft engines. According to GE, its engineers have conducted more than 50 component tests on H turbine spoolies to evaluate coating, lateral loads, fits, axial motion, angular motion, temperature, and surface finish.

After passing through the turbine's nozzles and buckets, the steam, with the thermal energy it picks up from the GT, is recycled to the HRSG. In this way the H turbine serves as a reheater for the bottoming cycle..

Compressor design

GE engineers based much of the H design on existing and proven turbine technology, starting with the high-pressure compressors. Those for the H were based on the compressor that GE designed for the CF6-80C2 aircraft engine and its industrial aeroderivative LM6000 gas turbine. The 9H compressor operates at a 23:1 pressure ratio and a 1510 lb/s (685 kg/s) airflow. The 7H turbine has a 23:1 pressure ratio with a 1230 pps (558 kg/s) airflow.

In addition to the variable inlet guide vane used on previous GE gas turbines to modulate airflow, H series compressors have variable stator vanes (VSV) in front of the compressor. They are used, together with the IGV, to control compressor airflow during turndown, as well as to optimise operation for variations in ambient temperature. These compressors also circulate cooled discharge air in the rotor shaft to regulate temperature, allowing the use of steel rather than Inconel.

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Basic flow chart of the standard H System combined cycle.

Single crystal materials

Single-crystal fabrication eliminates grain boundaries in the alloy, and improves thermal fatigue and creep characteristics. GE used a proprietary, single-crystal material the H turbine's first-stage buckets and nozzles. A dense, thermal barrier coating (ie applying several layers, including sacrificial layers, of a vertically cracked ceramic thermal barrier) was applied to first and second stage nozzles and buckets to increase their heat resistance, ensuring that these components would stand up to high firing temperatures over a long service life while meeting maintenance interval specifications.

Dry low NOx combustors

The H System employs a can-annular lean pre-mix DLN-2.5 dry low NOx combustor system. Fourteen combustion chambers are used on the 9H, and 12 on the 7H. The system premixes fuel and air prior to ignition to generate no more than 9 ppm of NOx for the 7H, and 25 ppm for the 9H. This emissions control system has logged millions of hours of service on other GE turbines around the world.

Controls

To manage all the various sub-systems, the H-Series incorporates an integrated control system that manages steam flows among the HRSG, steam turbine and gas turbine. It also schedules the distribution of cooling steam to the gas turbine. The H-Series uses digital control – the proven Mark VI system. A pyrometer system is installed to rapidly detect rises in temperature, enabling automatic turbine shutdown before damage occurs.

50Hz installations

The first two sales of GE's H System have both been 50 Hz projects: the Baglan Bay power station in Port Talbot, south Wales, where the first 9H began operation in 2002 and has since surpassed 18000 fired hours; and Tokyo Electric Power Company's 1520 MW Futtsu Thermal power station Group 4, which will feature three 9H gas turbines.

TEPCO is one of the largest utilities in the world, producing 60 GW, and is GE's largest single customer. Earlier-generation GE gas turbines are already in service at Futtsu No. 1 and 2 (9E) and Futtsu No. 3 (9FA).

GE will supply and build three 9H combined cycle systems. Toshiba will build the gas turbine compressors, steam turbines, and generators under the terms of a separate agreement with GE. The heat recovery steam generators, selective catalytic reduction systems, and accessory equipment will be built by Toshiba and Hitachi.

GE begin shipping equipment in April last year and expects to have the first unit in operation in mid-2008 and other two by mid-2010.

Futtsu No. 4 will be the fifth combined-cycle system GE has built for TEPCO.

Citing ‘tough economic conditions’ and ‘changing energy markets,’ Sithe and GE announced that the Heritage Station power plant in Scriba has been cancelled. The project, a joint venture of the two entities, which had all major regulatory approvals, was put on hold in December 2001 with the announced goal of resuming in the spring of 2002.

In an issued statement, Mark Little, vice president of energy products for GE Power Systems, said that ‘dramatic changes in the energy markets, linked to the national economic conditions, make the project not economically viable at this time.’ Dennis Murphy, manager of global communications for GE Power Systems, added that ‘the combination of gas prices and the economy made the project not as attractive.’

REPETITION The first 7H left GE's Greenville, South Carolina, manufacturing facility in September last year and was moved to the port of Charleston, S.C. where it began a voyage through the Panama Canal to Long Beach, California. The last section of the journey was a one-week road trip to the project site where after several days of site preparations, the GT was placed on its foundation on November 21. A second 7H has now been delivered and . GE has been commissioned to design and build the combined cycle system.

Baglan Bay 50 Hz ‘H’ will not achieve 60%

The first single-shaft, 50 Hz MS9001H gas turbine was built by GE in Greenville, S.C., and is 12 m long, 5 m in diameter, and weighs 370 U.S. tons. It may be the largest gas turbine ever made (although there is another contender – see p41). It began operating in summer 2002 at the south Wales plant Baglan Bay, GE’s flagship in the battle for 60% efficiency.

The main advantage provided by increased efficiency is economic. A single percentage point of efficiency gained can reduce operating costs by $15-20 million over the life of a typical, gas-fired combined-cycle plant. GE's previously most advanced gas turbines, the F series, top out at 57 to 58% combined-cycle thermal efficiency. But the Baglan CC system is unlikely now to be the unit that reaches the coveted figure. Because of the type of cooling water system that had to be installed to obtain the environmental permit, coupled with what GE calls normal plant performance degradation, Baglan Bay site performance will remain below 60% efficiency.

The Baglan turbine runs in tandem with a GE D10 steam turbine. This three-admission reheat turbine with a 33.3 inch last-stage bucket is used in many GE combined-cycle systems. The steam and gas cycles co-operate in the same way as other GE advanced single-shaft CC systems - that is, by solid coupling the gas turbine to the steam turbine and steam turbine to the generator.

NEM of Leiden, in The Netherlands, built the HRSG for the 9H at Baglan Bay. This three-pressure-level unit can reach 12.4 MPa, with main steam and reheat steam both at 565°C. It is similar to the NEM units used on GE 9FA and 7FA gas turbines serving in other combined-cycle applications, but is larger, to accommodate the greater exhaust flow of the H turbine. But because of the heat it picks up from the gas turbine during cooling, its reheater is smaller.

The Baglan plant is scheduled for its hot gas path inspection in early 2008. At that time improved parts will be installed and tested.


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