The GE H System 50 Hz steam-cooled gas turbine combined cycle power plant to be built in South Wales is the most advanced of next generation gas turbines. It incorporates all the US DOE advanced turbine system (ATS) programme elements on which the 60 Hz 7H machine will be based. It will have a nominal combined cycle output of 480 MWe and a thermal efficiency of over 60 per cent. For further details on this and future DOE programmes, see p45 of this issue.

The steam cooling permits a radical increase in firing temperature while reducing the operating temperature of turbine blading as well as eliminating loss of cooling air flow for traditional turbine blade cooling. The original site was the 1000 MWe 9H Fleetwood Power project in Lancashire, England, which was abandoned due to the UK government’s de facto moratorium on new gas-fired power plants, as also was the Partington project near Manchester which was to have the first 9FA++ gas turbines.

A single 500 MWe 9H combined cycle chp power plant unit is now planned for the old BP Chemicals site at Baglan Bay near Neath. The project will replace a more ambitious 1200 MWe project on the same site – with three 9FA gas turbines plus a single 550 MWe steam turbine – for which Section 36 application was made in December 1996.

Project development

Following the submission of a significantly revised application for the construction of the new 500 MWe power station at the Baglan Energy Park, South Wales, UK Secretary of State for Trade and Industry Stephen Byers confirmed his approval of the application under Section 14 of the Energy Act.

The proposal will now be submitted to the local planing process to secure final consent to trigger development of Baglan Energy Park – a joint initiative by the Welsh Development Agency, Neath Port Talbot County Borough Council and BP Chemicals. Although the proposal flies in the face of issues cited in the UK government’s White Paper supporting the ‘moratorium’ on new gas-fired power plants, it unlikely that further permit applications will be refused.

The notification to Baglan Cogeneration Company Project Manager Ken Allison pointed out that “… certain types of generating stations may, however, have benefits that outweigh the government’s concerns about new gas-fired power stations (paragraph 10.41 of the white paper).”

The government’s determination to promote chp technology is well known, but the notification stresses the desperate lack of employment in the area which started with the closure of coal mines in the area.

“The Secretary of State has noted that the Neath/Port Talbot area is in a proposed European Union Objective 1 area for the purpose of eligibility for EU Structural Funds grants. It suffers a relatively high unemployment rate and the area has been historically dependent on ageing industries which are fast disappearing.” Employment in manufacturing in the region has fallen by 59 per cent since 1980 compared with 27 per cent in Wales as a whole.

Some 2800 jobs will be lost this year. The Pembroke 2000 MWe oil-fired power station, which was closed down after it was denied a licence to burn orimulsion, was a major employer, and the loss of this facility has resulted in a power supply deficit in the area.

Also, the BP Chemicals plant producing styrene and isopropanol, a major employer in the area in the 1970’s, is now running down and moving production to its east coast and Grangemouth complexes using natural gas feedstock from the North Sea instead of Welsh coal. It is increasingly becoming a brown field site on which Baglan Energy Park is to be built.

“We believe that by locating this new turbine in South Wales, the Baglan Energy Park can become the core for a new centre of technological excellence in Wales which will set global standards for performance, efficiency and emissions control well into the 21st century” said Bob Nardelli, President and CEO of GE Power Systems, which will invest $450 million in constructing the new power plant.

BP Chemicals already has a 100 MWe oil-fired chp plant on the site. The new 500 MWe plant will supply bulk steam, water, nitrogen and oxygen to local industrial users. The station will also have a black-start facility to re-energise the grid in event of system failure, as well as providing security of supply to the chemicals plant.

After five years of the most intensive component, system and materials testing ever applied to a heavy industrial gas turbine, the first complete machine will be installed with its integrated steam bottoming cycle for commercial field operation in a merchant chp plant.

9H combined cycle

Recently demonstrated to invited potential utility customers, the GE 9H will only be introduced to the market as a single shaft combined cycle power unit with a purpose-built exhaust heat recovery boiler and the new specially designed GE Mark 6 control system.

This new control system is perhaps the most recently developed and advanced constituent of the technology. It not only incorporates the digital algorithms necessary to optimise supply of blade cooling steam to gas turbine, it must also integrate control algorithms to optimise chemical process plant dynamics for every operating scenario of the BP Chemicals complex it will supply, as well as the project district heating loads in local government buildings and new factories to be established in and around the Energy Park.

The major advances in turbine output and efficiency mainly derive from the use, for the first time, of closed-cycle steam cooling of turbine stationary and rotating blades and incorporation of GE’s advanced aircraft engine technology, including optimised compressor aerodynamics, single crystal turbine blades and advanced thermal barrier coating. The rotor for the first 9H.

The exhaust heat recovery steam generator will not be very different from a typical three pressure level combined-cycle boiler, except that a substantial proportion of the cold reheat steam from the HP exhaust system will be diverted into the gas turbine steam cooling system to be returned into the IP section of the condensing steam turbine. At an output level of 395 MWe, some 25 per cent of the cold reheat steam is used for gas turbine cooling.

The gas turbine

GE’s MS9001H and MS7001H turbines contain an 18-stage compressor, a can-annular dry low NOx (DLN) combustion system, and a four-stage turbine. A 2600°F/1427°C firing temperature and closed-circuit steam-cooling are used in the gas turbine. The rotor is similar to prior GE gas turbines, being supported by two bearings and the first rotor bending critical above the operating range. Through-bolt rotor construction is used in both compressor and turbine rotors. The rotor thrust bearing is at the inlet end of the gas turbine.

The MS9001H and MS7001H compressors provide a 23:1 pressure ratio with 685 kg/s and 558 kg/s airflow for the MS9001H and MS7001H turbines respectively. This is a higher pressure ratio and airflow than GE’s FA gas turbine compressors. The higher airflow provides increased output and economic scale efficiencies, while the higher pressure ratio is necessary to keep exhaust temperatures at an acceptable level. Without the higher pressure ratio, elevated exhaust temperatures driven by gas turbine firing temperature and closed steam cooling in the turbine would have an undesirable cost and life impact on the exhaust system and HRSG.

The H System compressors are derived from GE’s high-pressure compressor used in the CF6-80C2 aircraft engine and the LM6000 aeroderivative gas turbine. This compressor has recorded many million hours of experience providing reliable operation. For use in the H gas turbines, the CF6-80C2 compressor is scaled up (2.6: 1 for the MS7001H and 3.1:1 for the MS9001H) with four stages added to achieve the desired combination of airflow and pressure ratio. On the MS9001H, the four additional stages are on the back of the CF6-80C2 compressor. On the MS7001H, the last stage from the MS9001H compressor is eliminated and a zero stage added at the front.

The H compressors have four stages of variable stator vanes (VSV) at the front of the compressor. They are used, in conjunction with the IGV, to control compressor airflow during turndown as well as optimise operation for variations in ambient temperature.

The H can-annular combustor is a lean premix DLN system similar to current GE systems. Fourteen cans are used on the MS9001H and 12 cans on the MS7001H. The combustion system is a reverse-flow type, with double wall construction with impingement sleeves surrounding the transition ducts and combustion liners. These sleeves provide impingement and convective cooling of the liners and transition pieces, using compressor discharge air. The DLN technology was developed for and proven on the F class machines.

A four-stage turbine is used for compatibility with the compressor 23:1 pressure ratio. Previous GE gas turbines have operated successfully with three turbine stages. However, with the increase in pressure ratio, three turbine stages would have increased the loading on each stage causing reduced stage efficiency. By using four stages, the H turbine is able to achieve optimum work loading on each stage and high turbine efficiency.

The turbine uses closed-loop steam cooling of Stage 1 and 2 nozzles and buckets plus Stage 1 shroud. Steam from the combined-cycle steam system is introduced into the turbine components, provides cooling, and is returned to the steam bottoming cycle for work extraction in the steam turbine. Air cooling is used for the Stage 3 nozzle and bucket with the fourth stage being uncooled.

In operation, the turbine will be taken up to approximately 10 per cent load on air-cooled blades, and then switched over from air cooling to steam cooling.

A single crystal material with thermal barrier coating (TBC) is used for both the Stage 1 nozzle and bucket. The single crystal alloy is a nickel-based cast superalloy possessing excellent high temperature properties which was developed and patented by GE. It has been used by GE Aircraft in full scale production since 1988. Stages 2 to 4 rotating blades utilise a directionally solidified material used in GE’s F gas turbines today. Stage 2 is also thermal barrier coated. Stages 2 through 4 stationary blade materials are also used in GE’s gas turbines and aircraft engines. Stages 2 and 3 are also thermal barrier coated.

No steam or water injection is used for NOx reduction, since single figure NOx – 9 ppm, has already been demonstrated with the GE DLN combustors in the F and FA marque.

Nominal output for the 50 Hz 9H is 480 MWe, compared to 400 MWe of the 60 Hz 7H, which will be the first of the US Department of Energy’s ATS specification machines. All enabling technology for the ATS has been built into the Baglan Bay 9H, but the ATS designation applies exclusively to the 60 Hz version. Favourable site ambient conditions are cited as the reason for increased output of 50 MWe at Baglan Bay. Detailed characteristics of the H System machines were first published in the June 1995 issue of Modern Power Systems.

Test programme

It is, of course, not possible to test a new turbine of some 500 MWe output on a factory test-bed using a dynamometer, the first test operation of any GE H System turbine will be the Baglan Bay combined cycle chp unit.

The first running up and operation of the Baglan Bay machine took place during no-load testing at GE’s Greenville facility from April to June 1998. Currently the machine is being stripped down for extensive inspection and analysis before rebuilding for delivery to the site. The machine will be very highly instrumented and the first year of test operation on power will be critical to both H System development and the ATS programme.

Baseline compressor test results

A baseline compressor rig was used to validate the fundamental design approach of using the CF6-80C2 derived compressor in heavy duty gas turbine operation during 1995. Test objectives included validation of performance, power turndown operability, stall margin and aeromechanics. The rig was tested for over 200 hours. Nearly 600 data points were recorded verifying the design approach by meeting all test objectives.

The baseline compressor had excellent efficiency confirming the design value of product efficiency. Figure 6 shows a plot of compressor adiabatic efficiency as a function of airflow.

Power turndown performance and operability testing was successfully conducted to validate the baseline compressor at conditions which are not encountered in aircraft operation. During power turndown, airflow is reduced at constant speed which compares to the CF6-80C2 aircraft engine where airflow reduction is accompanied by a speed reduction. The baseline compressor, at constant speed, achieved 50 per cent airflow reduction, exceeding the design objective of 40 per cent. During power turndown testing, the added flexibility of having VSVs was explored.

A plot of compressor efficiency at ISO conditions as a function of flow turndown. Testing revealed that up to 0.8 points of compressor efficiency improvement are achieved by using the IGV plus VSVs for airflow reduction compared to the IGV only. This testing also validated that using the IGV plus VSVs at maximum flow turndown avoids pressure or temperature drop through the first compressor stage. This eliminated the need for inlet heating on the H gas turbines.

An important part of the baseline test was establishment of the high speed compressor map. Testing was used to validate the no load operating line, full load operating line and stall margin. There is an ample stall margin at ISO full load conditions. The minimum stall margin occurs at cold day, maximum flow turndown but still exceeds GE’s design practice value for minimum stall margin.

All the compressor blades were instrumented with strain gages to obtain aero-mechanical data over the entire compressor operating range to validate airfoil operation. Data from the tests have since been used to complete mechanical, aerodynamic and aero-mechanical design modification necessary to convert from the baseline compressor to the commercial MS9001H compressor design.

Turbine test programme

An extensive test programme has been conducted on the H turbine to confirm that successful operation has been fully defined and validated for operation with steam cooling.

The Stage 1 nozzle ring comprises single vane castings with 42 vanes in the MS9001H and 36 vanes in the MS7001H.

The airfoil and sidewall band areas are steam cooled with heat transfer enhancement by a similar impingement process to that used in the FA Stage 1 nozzle. The difference is that rather than the spent impingement air being exhausted to the flow path as film cooling in air-cooled designs, in the H System the spent impingement steam is collected and returned to the steam bottoming cycle.

Stage 1 nozzle material

The Stage 1 nozzle single crystal material is a high nickel and chromium containing alloy which possesses superior pitting resistance which is also very stable at the steam cooling conditions to which it will be exposed. It is not expected to exhibit a loss of chromium at the grain boundaries, loss of alloy to precipitates or other destabilising effects. The testing programme to validate its use in a steam environment included:

Oxidation testing – Isothermal oxidation testing was conducted to study the long term isothermal oxidation, corrosion and stress corrosion cracking (SCC) susceptibility. Specimens were loaded into steam autoclaves for aging. The samples included coated specimens and potential braze and weld materials. Samples were removed at set intervals for evaluation and quantification of oxidation, corrosion and SCC susceptibility, the majority of which showed no detrimental effects of steam on the properties of the single crystal alloy.

Cyclic oxidation tests – were also conducted to determine if thermal cycling conditions in steam changed the oxidation behaviour of the materials. At H conditions, test results reported no effects of thermal cycling on the oxidation behaviour as compared to the isothermal results.

Mechanical property testing – Mechanical property testing on both bulk properties and structure-sensitive properties was initiated at H temperature and stress conditions. Tests include creep testing, low cycle fatigue (LCF) testing, slow strain rate tensile testing and fatigue crack propagation.

TBC coating

Thermal barrier coating previously used on the flow path surfaces of the Stage 1 nozzle operate at different conditions than those in steam-cooled components. Multi-path development of TBCs to perform at H conditions have been tested under a validation programme including laboratory durability tests and field trials.

A high thermal gradient electron beam facility was constructed for testing of TBC specimens duplicating the thermal-mechanical conditions which will be experienced in the H Stage 1nozzle and other airfoils. The electron beam is the energy source providing flux to the hot side of the specimens, with the backside of the metal substrate cooled to maintain the thermal flux, thereby generating the desired temperature gradient through the TBC and/or metal.

Nozzle heat transfer

Internal and external heat transfer tests fully characterised the thermal environment of the Stage 1 nozzle. A test programme was performed to obtain design data on the effects of surface roughness levels, inlet free-stream turbulence intensity and vane Reynolds number on H Stage 1 nozzle external heat transfer. This facility used a half-scale, linear airfoil model representative of the pitch line section of the Stage 1 nozzle, operating at an overall pressure ratio of 1:86.

Airfoil heat transfer distributions were measured using a thin-walled stainless steel airfoil having imbedded thermocouples. A thin-foil surface heater was used to provide a known heat flux condition, with room temperature mainstream air at approximately 5 atm pressure.

Airfoil surface roughness was varied from smooth to very rough, with either uniform or distributed roughness features. Inlet turbulence intensity could also be varied. In addition to acquiring design data for the H Stage 1 nozzle, the results are used for the verification and improvement of the predictive methods used in designing turbine airfoils.

Nozzle cascade tests

Heat transfer, steam purity and steam compatibility test results have been incorporated into three dimensional aerodynamic, thermal and stress models to confirm that the Stage 1 nozzle will meet operating life requirements.

Further validation has been acquired from cascade tests on actual full size prototype, steam cooled Stage 1 nozzle segments under H System thermal conditions. This involves mounting two nozzle segments were mounted in a test stand behind a DLN combustion system and transition piece. The good correlation between aerodynamic test results and pretest analysis.

Table 1. H System combined cycle plant performance characteristics