Targeting a niche

In response to changing customer needs in the deregulating power markets, ABB Stal is introducing its latest gas turbine – the GT10C. Aimed at the 30 MW market and based on the GT10B, this unit will offer high efficiencies and robustness in cogeneration and combined cycle applications.

The ability to produce power and heat efficiently is of growing importance to a power generation industry faced with deregulation, privatization and fast-changing customer needs. Utilities, industries and independent power producers (IPPs) are all searching for technologies that will give them high levels of performance without compromising reliability or safety.

As a result of deregulation, large industrial power users have been able to realise considerable cost savings and boost revenues by installing either combined cycle cogeneration or pure generation plants. Heat-intensive industries such as paper, oil and chemicals, can benefit from having their own cogeneration plants. Even where the need for heat is greater than that for electricity, a relatively larger power generation capability can be considered, since income from the sale of surplus electricity can shorten the payback time significantly. The last decade has seen a sharp rise in the number of combined cycle plants rated from 20-50 MW being installed across the world in a variety of applications.

Since heat cannot be transported over long distances, it is usually more economic to build a number of small-scale combined cycle cogeneration plants near the heat consumers rather than one very large, central plant, as might be the case when only electricity is to be generated.

The new regulatory environment in Europe as well as other regions has brought about major structural changes in the power supply industry. For example, companies which previously only distributed electricity – including the municipal utilities – are now able to generate it in relatively small-scale plants.

These changes in the market have been key factors in the transformation of a technology which was once considered to be mainly suitable for very large gas turbines and steam turbines.

ABB Stal is introducing the GT10C to meet the market`s demand for an industrial high efficiency gas turbine in the 30 MW slot. The GT10C is an up-rate of the ABB GT10B, and will produce 29 MW in a power generation application at ISO conditions (15 degrees C, sea level, and no losses in air intake and exhaust).

The company`s ambition is to position the new GT10C as a high efficiency unit yet robust and simple in terms of its design. This unit will meet a strong demand in the market place for low life cycle costs and will be introduced to the market in 2000.

A natural progression

The GT10B was the design platform for the GT10C. Many of the B-version`s solutions remain very much the same for the new machine. Similarly the length of the hot section, combustor and compressor turbine is the same for both versions. This provides the possibility of retrofitting this section of the GT10B in order to improve the environmental performance. Also, the GT10B packages for both power generation and mechanical drive will remain almost identical. The experience within ABB`s turbine world has been extensively utilised for this new gas turbine.

The GT10C packages with the auxiliary systems will remain almost identical for power generation and mechanical drives. In combined cycle applications the GT10C can be arranged in a single string configuration with the steam turbine driving a common generator. This well-proven configuration has been used in many GT10B combined cycle plants and offers a compact solution.

The compressor technology is common to the GT10B and also to the GT8C/C2. The compressor rotor, like the rotor of the B-version, is EB (electron beam) welded but the turbine discs will be bolted to the shaft and not welded, as is the case for the GT10B power turbine. This will give the client the option of removing the entire power turbine rotor at an inspection or repair, or of only removing the discs and leaving the shaft in its bearings.

The cooling and the flow path of the turbine section have been designed in much the same way as the GTX100, the GT8C/C2 , the GT24/26 and the GT11N2. Staying within the envelope of current ABB experience provides for reliability and has been important for satisfying the market`s requirements.

A comparison between the new GT10C and the GT10B shows the following:

In order to increase the unit power rating, the mass flow has been increased from 80.5 kg/s in the GT10B to 91.1 kg/s in the new unit. This has been achieved by adding an extra stage in the compressor. This has been introduced as the number 3- stage, thereby decreasing the distance between the stages of the new compressor. This is particularly true for the first part of the compressor, and the distance between the bearings, i.e. the length of the rotor, has been kept unchanged .

The reason for not zero-staging the compressor, as is common in the uprating of gas turbines, is that the power of the new gas turbine would then have been increased to above 30 MW, bringing it closer to the power range of the GTX100. The compressor has also been made wider at the intake end, resulting in a larger flow.

The inner compressor house, being the vane carrier, has also been made wider at the intake end in order to cope with the larger flow, but the external compressor casing is kept unchanged from the B version.

In order to improve the simple cycle efficiency, the compressor`s pressure ratio has been increased from 14:1 for the GT10B to 18:1 for the GT10C. This has taken the efficiency of the unit from 35.1 per cent to 37.3 per cent measured at the drive shaft.

The open cycle efficiency for power generation is sometimes of minor importance, while for cogeneration and combined cycle applications high efficiency is essential, mainly because the operating time per year is long for cogeneration and combined cycle applications. The efficiency in a simple cycle is, however, important for the drive of compressors in the oil and gas industry where also the speed of the drive shaft is crucial for such an application. The speed of the GT10C free power turbine is 6500 r/min. For the GT10B the speed is 7700 r/min. A gear is utilised in generation plants to reduce the speed down to 3000/3600 r/min for the 50 Hz and 60 Hz generators respectively.

The focus on improving the open cycle efficiency has resulted in a slight drop in the exhaust temperature from 545 degrees C for the B version to 518 degrees C for the new version. However, because of the better electrical efficiency, the efficiency of a combined cycle plant built around one GT10C (KA10C-1) in a cold-condensing application, i.e. one in which only electrical energy is produced, will be 51.1 per cent while for a two unit installation (KA10C-2), it will be 51.8 per cent. This will still make the GT10C very competitive in combined cycle and cogeneration applications and able to meet one of the key market requirements.

Experience shows that changing around the auxiliary systems is not without difficulties – the reliability may suffer in the short term. Because of this, ABB Stal has kept the package, auxiliaries and the control and electrical systems from the GT10B very much intact for this new unit. This is of course only natural since the dimension and weight of the new core engine is very much the same as that of its forerunner.

ABB Stal has also maintained the very good environmental performance of its GT10B industrial gas turbine. The GT10B has offered a dry low NOx emission of less than 25 ppm (15 per cent O2) since the introduction of the EV burners in the unit back in 1991. This is equivalent to 50 mg NOx/MJ fuel and has given ABB a leading position in the environmental race for a long period. The B-version has to date logged close to a million operating hours with the EV burners. The C-version offers a lower than 15 ppm dry emission of NOx. For liquid fuel the NOx emission is as low as 42 ppm. The CO emission is limited to 25 ppmv (15 per cent O2) for both fuels.

Like the other gas turbines marketed by ABB Stal, including the GT35, GT10B and the GTX100, the GT10C is able to automatically change between gas and liquid fuels. This dual fuel capability provides for fuel flexibility, which has proved to be important in markets where there is a premium put on gas contracts under which temporary cuts in the gas supply can be accepted. The dual fuel system will also allow for operating on the fuel which is, for a certain period of time, the most economical choice.

There is of course an extra bonus in being able to run dual fuel with dry low-emission burners for both fuels, which the GTX100 and the new GT10C are both capable of doing.

Design validation

A comprehensive test programme has been conducted to validate the design of the critical components of the GT10C. Tests in models of the compressor inlet have verified the flow characteristics and the pressure drop. Model tests have also been carried out on the diffuser section behind the compressor to validate the design.

The burners used for the GT10C are identical to those used in the GTX100. These burners have been tested extensively at the ABB works in Finspong, Sweden, at ABB in Switzerland, and at the CIAM test centre in Moscow. Two combustor cylinders have been built for design validation under hot conditions, at atmospheric as well as at full pressure.

A complete combustor has been built to verify design details such as surface design, pressure drop and vortex breakdown behaviour during operation in atmospheric conditions.

The turbine cooling technology has been verified in tests conducted at a newly-built hot test rig in Finspong, where cooling flows and heat transfer coefficients of blades and vanes of the turbine can be measured.

Closing the gap

The aeroderivative gas turbines have traditionally enjoyed a high level of efficiency – a feature of the jet engines from which they have been developed. The traditional industrial gas turbine, in open cycle, offers a lower efficiency but greater robustness. The GT10C, like the other modern industrial gas turbines, is closing this gap with the aero engines and with an expected low deterioration in the efficiency of this industrial design, the specific fuel consumption over a longer period will most probably match or even be superior to that of some of the aeroderivatives.

It is clear that the main elements which make up life cycle cost are fuel and maintenance. For example, in an environment of higher fuel prices, an increase in efficiency of two percentage points over a 15 year cycle means a saving of close to $5 million.

In contrast, in a market with lower fuel costs, the cost of maintenance becomes more significant. Add to this effect the down-time for maintenance on the availability of a plant, and maintenance becomes an even more important factor. The robust, less sophisticated design of the GT10C, in comparison with its aeroderivative competitors, provides for a lower maintenance cost and longer time between overhauls.

A $1/kWh saving in maintenance costs provides for a life cycle cost saving of around $4 million over the same 15-year life cycle mentioned. Even so, the efficiency of this industrial unit remains competitive – not least in the combined-cycle and cogeneration applications.

A demanding market

When the GT10B was first introduced by ABB Stal in the early 90s, it quickly became the leader on the European combined cycle scene. Combined cycle plants built around the GT10B were mushrooming in the Netherlands, Germany and in many other countries, not only in Europe. The leading position of the dry low NOx technology, below 25 ppm NOx, was one of the main factors behind its popularity. Another important factor was that the unit offered the best combined cycle efficiency of the units in the market in its power range.

In terms of performance, the competitors in the market place soon developed an improved efficiency and a dry emission performance which equalled the GT10B, but without the operating experience gained by ABB. The power of these new competing units also increased and ABB`s response was to introduce the GT10C.

Today, the marketing of the GT10C has begun and ABB Stal is certain that it will be received with the same appreciation by the market as the GTX100 has received since its introduction in 1997. The conclusion is that only by a steady pace in the development work to improve the performance of the turbine programme can a manufacturer stay in the race for competitive advantage in the market.

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