Flexible future for a tested turbine

Power plant owners needing to maximise their returns on investment and operation are calling for flexibility. Alstom’s GT24/GT26 gas turbines aim to provide just that.

The development of gas turbine technology is moving steadily towards higher operational flexibility and reduced cost of electricity. In parallel, reliability, availability and maintainability are strong requirements from power generators. Thus, depending on market conditions at any particular time and the commitments of each individual plant, it is sometimes more economical to run the plant at part-load, or at other times, to switch it off, or restart it and run it at full load. The plant needs to be flexible, able to operate in different conditions efficiently, without incurring extra costs and without producing unacceptable levels of NOx emissions. Alstom’s GT24/GT26 gas turbine is designed to respond to these operational requirements and sustain performance.

Behaviour and efficiency

The start-up behaviour of a power plant is determined by both time and reliability.

The GT24/GT26 has been shown to reach combined cycle full load within 50 minutes, while the simple cycle plant can start up in around half an hour. This short start-up time allows revenues to be realized almost immediately after the decision to start up the power plant. It allows the operator to optimally adapt to market changes and to take opportunities with minimal time delay.

The second important attribute is gas turbine start-up reliability, and units consistently register 90 per cent or higher. This again is of prime importance to merchant plants as a missed start is extremely expensive in terms of buying in previously committed power. There may also be further benefits, such as revenues from non-spinning reserve payments.

Figure 1. Operation of the EV Burner is based on the vortex breakdown principle
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The GT24/GT26 in combined cycle can achieve excellent part-load efficiency. In low-price periods, the part-load efficiency is the crucial factor in respect of flexibility and profitability. Only high part-load efficiency allows the operator to run the machine during low-price periods, by reducing the power output to a minimum without losing too much efficiency. This operation mode reduces the number of starts and therefore decreases the associated starting costs.

Additionally the lifetime consumption that is related to the number of starts of the gas turbine can be controlled by freely choosing whether the gas turbine is shut down during low-price periods or not, significantly increasing the flexibility in outage planning. Furthermore, the high part-load efficiency indirectly allows control of the cumulative emissions of the power plant, since a decrease in the number of starts reduces the absolute emissions produced during start-up.

Sequential combustion

One of the main technological differentiators of the GT24/GT26 engines is the combustion system, which is based on two concepts – the environmental (EV) burner in annular combustion, and sequential combustion. The end result is a low-footprint, high-power density engine of the same basic size as the Alstom GT13E2 but producing some 100 MW more power and giving very low NOx levels.

The EV burner is a dual-fuel premix burner for gaseous and liquid fuels and is shaped like two half cones slightly offset sideways to form two slots of constant length along the sides of the burner. Combustion air enters the cone through these slots and gaseous fuel is injected through a series of holes that runs along its edge. This arrangement ensures that the fuel and the air are completely mixed shortly after entering the burner cone. Alternatively, liquid fuel (premixed with water) can be directed to the centre of the burner through the EV lance.

Operation of the EV burner is based on the vortex breakdown principle. Due to the high velocity and swirl in the cone itself, ignition can only take place near the flow stagnation point of the recirculation zone. This means that the lean fuel/air mixture leaves the cone without being ignited. At the exit of the cone the vortex breaks down, forming a recirculation zone, and the fuel/air mixture is ignited. The flame is stabilized outside the burner in free space within the combustion chamber.

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The EV combustor has an annular burner arrangement fitted with individual retractable EV burners (24 in the GT26 and 20 in the GT24). All burners are in operation across the full load range and radial temperature uniformity is accomplished by mixing virtually all incoming compressor air with the fuel in the EV burner and by the absence of film cooling in the convection-cooled combustor walls. This produces a single uniform flame ring in the free space of the EV combustor.

In the annular Sequential Environmental (SEV) combustor, the combustion is repeated in a similar fashion as in the EV: vortex generation, fuel injection, premixing and vortex breakdown. The SEV combustor consists of annually distributed burners followed by a single annular combustion zone. Exhaust gases from the high-pressure turbine enter the SEV combustor through the diffusor area. Combustion temperature uniformity in the SEV is determined by the spatial homogeneity of the air/fuel mixture, again accomplished by the use of vortices. Each SEV burner contains delta-shaped wings that swirl combustion air into vortices.

Fuel is injected into the SEV burner through air-cooled fuel nozzles, distributing it in a manner that forms a perfect fuel/air mixture prior to combustion. The fuel jet is surrounded by cool carrier air that postpones spontaneous ignition until the mixture reaches the combustion chamber. There the vortexes break down and, as in the EV, combustion occurs in a single stable flame ring, operating across the entire load range.

Where the incoming hot gas has considerably lower oxygen content than normal air, less oxygen is available for NOx formation. Furthermore, because the SEV air is at considerably higher temperature than conventional combustion air, it requires less heating to reach flame temperature.

These two combustion systems result in NOx emissions on fuel gas regularly measured at much lower than the current statutory limits of 25ppm both at full and part loads without the need for injection water or selective catalytic reactors.

Fuel flexibility

More and more gas turbines are using fuels that are outside the previous normal range. A high hydrocarbon (C2+, containing two or more carbon atoms) content will have a limited effect on turbulent flame propagation and heating values, in comparison with pure methane gas, but a significant effect on the spontaneous or auto-ignition properties of the fuel-air mixture. Ignition in low-NOx premix combustor starts earlier and at lower temperatures compared with methane. Consequently C2+ affects the flame position and stability. A shift in flame position bears the potential risk of flashback, i.e. overheating the combustor hardware.

Alstom investigated the basic premix combustion effects of high hydrocarbon gases in test rigs and on the GT26 gas turbine at Alstom’s test centre in Switzerland. Extensive testing with more than 16 per cent high hydrocarbon gases found that no design modifications to the EV burner were necessary, as the flashback resistance is unaffected by the C2+ content. For the SEV combustor, testing showed that the burner inlet temperature alone can control the flame position, thus giving a very easy operation concept when burning fuels with high hydrocarbon content.

Alstom uses a fast-acting measurement system for gas concentration changes, which allows GT24/GT26 engines to automatically adjust their operating parameters as required and so remain in operation during any sudden fuel quality changes. This is particularly useful because the composition of high carbon fuels can vary significantly during operation.

Fuel gases with a high inert nitrogen and carbon dioxide content require higher fuel flow for a given power output. Consequently, the fuel distribution systems must be able to cope with the bigger mass flows and pressures. The momentum of the fuel entering the combustor will have an impact on the mixing field. As a result, the flame position and stability as well as the low NOx characteristics of the engine can be affected.

To ensure the full burner and gas turbine performance for fuel gases outside the Lower Heating Value (LHV) range of 35-50 MJ/kg with a Wobbe index below 36 000 kJ/m3, the EV burner needs no change in the burner design itself, but a simple adjustment in the EV burner hole pattern, i.e. a few additional fuel holes, compensates for the higher mass flow. Therefore no changes to the standard SEV burner are necessary.

Combined cycle benefits

There are factors other than the gas turbine itself that go toward providing operational flexibility, an example of which is the 100 per cent steam bypass that Alstom uses on its combined-cycle plant arrangements. A steam bypass is needed on all gas turbine combined-cycle plants in order to divert the steam from the boiler directly into the condenser during steam turbine trips or during the start-up procedure. Alstom’s use of a 100 per cent bypass is different to the usual arrangement – other manufacturers typically bypass only 30 per cent of the steam to the condenser and the remaining 70 per cent is blown into the atmosphere.

Figure 2. The sequential combustion systems result in NOx emissions well below 25ppm
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While this may be slightly more expensive, Alstom believes that the 100 per cent bypass is a solution that optimises plant cost against additional operating flexibility. This is particularly true in cases where the cost of demineralised water is high, or the plant is in intermediate or daily start and stop operation where a 30 per cent bypass would lead to high water loss.

In the case of a steam turbine trip, the availability of a 100 per cent steam bypass enables the operator to maintain the full power of the gas turbines for a longer period and allows a normal gas turbine shutdown to be made.

Advanced testing

Alstom has been greatly aided in the introduction of improved gas turbine design features by having a dedicated test centre where it can verify, in full field conditions, every new design development. The Gas Turbine Test Center is home to a GT26 gas turbine and a GT8C2 gas turbine. Both units run in simple-cycle operation and the power produced is fed into the local grid. Since 1996, numerous test campaigns have been carried out to validate product enhancements and gas turbine technology, critical to continual improvement of the gas turbine portfolio.

The analysis of measurement points from the test engine is an important element of the validation of the GT24/GT26 engine improvements. Validation relies on various levels of testing at component and sub-system level, depending on the design change. It is recognized that various levels of testing are required to isolate the influence of individual features, parts and components on the overall engine performance and operation. The Alstom design and validation routine is guided and maintained by routine design reviews.

Figure 3. Validation concept for the GT24/GT26 product line
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A scaling philosophy between the 50 Hz GT26 and the 60 Hz GT24 is the basis for the similarity in the aerodynamic and mechanical design, allowing the translation of the validation for the GT26 unit to the GT24. In the first fleet implementation in a GT24 unit, measurements are reconfirmed with a reduced instrumentation scope.

The GT24/GT26’s 22-stage compressor was redesigned for an increased mass flow of 5 per cent, which was aimed to give a similar increase in the power output. This was achieved through optimized airfoil design and – principally – a re-staggering of the blades. This means that the changes required no modifications to the rotor and only minor machining to the compressor vane carriers to optimize the blow-off throat area. Blade and channel height and length, rotor and vane fixation grooves and blade and vane materials all remain unchanged.

Although the turbine required no changes due to the increased flow, the exhaust housing on the engine was optimized for the higher mass flow. This was again a relatively simple modification consisting of aerodynamically improved strut covers that result in a reduced pressure loss. When this is installed together with the compressor, there is an additional power output benefit available for the operator.

Figure 4: Gas turbine under installation at the Cartagena combined cycle power plant in Spain
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To test these design changes, 400 probes were installed along the full length of the compressor. The four-month test schedule was comprehensive and covered start-up and variable guide vane optimisation, combustion mapping, part-load and base-load measurements and transient and protection tests. As a final test and as part of the compressor mapping runs, the engine was run into actual surge conditions to confirm the safety margins. In addition, compressor operation was also tested in all required modes including dual-fuel operation with fuel switchover, and with inlet cooling. Data from the tests confirmed the expected improved results: there was an increase of mass flow by 5-6 per cent in all operating parameters in the expected design ranges, and also confirmed were the cold and hot restart capabilities.

Future developments

With the fleet now exceeding one million hours of operating experience, the performance and robustness of Alstom’s GT24/GT26 have been thoroughly proven. The GT24/GT26 is distinctive because of its flexibility and unique technical features.

However, this is not the end of the development trail. Future developments of the engine are already ongoing. These developments are not just focused on improved hardware designs in order to increase output and efficiency but also to provide wider operating ranges.

Stephen Philipson, Alstom, Switzerland

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