A first at Västhamn

When ABB Stal first announced an industrial gas turbine with the performance of an aeroderivative, the market eagerly awaited news of the first installation. PEI looks at Västhamn combined heat and power station, and why this first unit was ideal for this site.

Christer Olsson

Helsingborg Energi AB

Helsingborg, Sweden

Bertil Nilsson

ABB Stal AB

Finspong, Sweden

In mid-1997 ABB Stal AB introduced the GTX100 industrial gas turbine. With an output of 43 MW, the unit was claimed to be an industrial turbine which offered reliable, low cost power at efficiencies rivalling those of existing aeroderivative units.

It was recently announced that the first ever installation of the new machine will be for an extension of the Västhamn power station. The contract for the project was awarded to ABB Stal AB of Sweden by Helsingborg Energi AB (HEAB), owners of the plant. ABB Stal will have total responsibility for the project.

Securing low cost supply

HEAB is the municipally-owned energy utility in Helsingborg, a city of approximately 115 000 people in the south of Sweden. Since 1964, the city has operated a district heating network, which is now connected to more than 1700 commercial buildings and 5500 homes.

In 1997, HEAB had sales of more than SKr1 billion ($125 million) in the form of 1400 GWh of electricity, 900 GWh of district heat and 750 GWh of natural gas. The council has declared that Helsingborg should be an `eco-municipality`, which means that HEAB is striving to find more environmentally acceptable methods of energy production, at the same time as ensuring the security of its supply and low production costs.

Helsingborg Energi Produktion AB (HEPAB) is responsible for producing some of the power and all of the heat for HEAB, its parent company. The Västhamn combined heat and power (chp) plant is its principal facility, accounting for more than half its output of heat. HEPAB produces the rest of it by recovering waste heat from local industrial plants and utilizing heat pumps and boilers fired by oil and biofuels. In order to secure its heat supply, several reserve plants have been located around the district heating network.

HEPAB generates around 300 GWh of electricity a year, mainly at Västhamn, but also at relatively small-scale biogas, wind power and hydropower plants. The balance of HEAB`s electricity requirement is currently imported. The future of electricity supply in the region is somewhat uncertain, however, following the decision to close the nearby Barsebäck nuclear power plant.

The Västhamn extension

The chp plant at Västhamn (`west port`) is located in the harbour area of the city, just to the southwest of the city centre. The plant consists of a 220 MW boiler supplying 78.5 kg/s of steam at 110 bar/538°C to an ABB Stal single-casing, axial-flow steam turbine, type ATM, rated 64 MWe. This drives an ABB GTL 1350 ET generator, rated at 75 MVA/11.5 kV, at 3000 r/min. The steam turbine exhausts into two-stage district heating condensers supplying 129 MWth to the district heating network. The plant also includes a heat pump, rated 29 MWth, and a hot water accumulator of 36 000 m3 capacity, which can store up to 1500 MWh.

The boiler is furnished with ABB three-stage flue gas cleaning equipment, including an electrostatic precipitator, SO2 scrubber and fabric filter. When the plant opened in 1983, it was fired by coal, with oil used as a back-up and to stabilize combustion at low loads. However, wood pellets – a lower-cost fuel – have been increasingly used in recent years, and will account for half the fuel used in 1998. The burning of wood pellets has reduced ash production, as well as sulphur and NOx emissions. A drawback of pure wood pellet firing is that the maximum capacity of the boiler cannot be achieved.

The key objective of the extension project was to increase the electrical output of the plant, in both MW and MWh terms. This was to be done in a way that did not increase heat production greatly and had a positive impact on the plant`s environmental performance.

When the project was first envisaged, two options were considered. The first was an extension of the plant based on a condensing steam turbine. This would make electricity generation possible at times when the heat produced was in excess of the district-heating requirement. The other option was an extension based on a gas turbine, to be integrated into the existing steam turbine cycle, thus converting Västhamn into a combined cycle plant.

The gas turbine option was preferred because the electricity generated would be of higher value, since periods of heavy demand for electricity and heat tend to coincide in Helsingborg. Another important issue concerned the fuels that would be used in each option. Coal would have been used in the condensing steam turbine application, whereas natural gas will be the main fuel for the gas turbine. HEAB has an ongoing project to reduce its consumption of coal.

The GTX100

The gas turbine selected for the extension was the GTX100, manufactured by ABB Stal. Launched in 1997, it has a number of features that make it highly suitable for this project. It has been optimized for cogeneration and combined cycle applications, with a pressure ratio of 20:1. Its electricity-to-heat production ratio is also high. Operating on natural gas, simple cycle output is 43 MW (ISO), at 37 per cent thermal efficiency, a level of efficiency maintained down to 70 per cent load. It has an exhaust mass flow of 122 kg/s, at 546°C.

Another important factor is its compact footprint of just 27 x 7 m. As can be seen in Figure 2, the plant occupies a rather small site bounded by harbour, road and rail facilities. The space available for new plant was further limited by the condition that any extension should not rule out the possibility of locating a new boiler on the site within the next ten years.

The gas turbine train and heat recovery steam generator (HRSG) will be housed in a separate building immediately adjacent to the existing plant. It will be 60 m long but only 20 m wide, which allows room for future expansion.

The GTX100 has been designed with the goals of high reliability and availability, and low life-cycle costs and maintenance requirements. Its development has therefore focused on using proven components and technologies. Where new components have been used, thorough testing has been carried out in order to validate them.

The GTX100 features a simple shaft arrangement. The compressor rotor and the three-stage bolted turbine module form a single shaft, which rests on two standard hydrodynamic bearings of the tilting pad type.

The compressor has 15 stages and uses controlled diffusion airfoils for high efficiency. The first three stages have variable geometry. To minimize leakage over the blade tips, abradable liners are applied to stages 4 to 15. The vane carrier of the high-pressure section, stages 11 to 15, where the blades are shortest, is made from IN 909, a low-expansion material that helps keep clearances to a minimum. The compressor rotor is built up from discs that are welded together into a robust unit using electronic beam welding, a technology used for many years and proven to be very reliable.

The combustor is of the annular type and is made from welded sheet metal. Its inner surface has a thermal barrier coating to reduce the level of heat transfer and extend its life. The combustor has 30 burners of ABB`s new Advanced EV (AEV) design. This dry, low-emission (DLE) technology will offer NOx and CO emissions below 15 ppmv (at 15 per cent O2) on natural gas and below 25 ppmv on liquid fuel over a 50 to 100 per cent load range. Extensive tests have been conducted for verification of these lower emission levels, covering the full load range of the machine. Dual-fuel DLE capability is a built-in feature. Natural gas will be the main fuel used at Västhamn, with oil being a standby. Switchover between fuels at full load will be possible.

The three-stage turbine is built as one module for ease of maintenance and bolted to the stub shaft of the compressor. The first blade is made of a single-crystal material. The turbine stator flanges are cooled by compressor air to reduce clearances and improve efficiency.

Balance of plant

The GTX100 features a cold-end drive arrangement, allowing an optimized axial diffuser section to be fitted for better performance. Particular care has been taken in the design of the diffuser connection to the HRSG to minimize losses.

The gas turbine will be connected to the generator via a gearbox of the double helical, parallel type, reducing the 6600 r/min turbine shaft speed down to a generator speed of 1500 r/min. The electric starter motor is also connected to the main gearbox, via a self synchronizing and switching (SSS) clutch and a separate starting gear. Since the two gas turbine bearings are lubricated with mineral oil, a common lube oil system is used for the gas turbine, gearbox and generator.

At Västhamn, a 4-pole, 66 MVA/11.5 kV generator will be fitted. This is larger than the standard generator for the GTX100, which is rated at 51 MVA. The larger generator is required since ABB has designed the plant extension in a way that avoids replacing the existing steam turbine generator – even though its active power output is due to rise from 64 MW to 82 MW. The steam turbine generator was originally rated at 75 MVA, but its capacity will be increased, through additional cooling, to some 83-85 MVA, allowing it to meet the new requirement by running at a power factor close to 1.0. In compensation, the GTX100 generator will have to be run at a much lower power factor (0.65) than normal (0.85), which means that an over-sized machine must be installed to handle the GTX100`s active power output of 43 MW.

Installation

The GTX100 is shipped in modules that are assembled and tested in the workshop, which will reduce the erection time in Helsingborg. Most of the piping and cabling work will also be carried out on the shop floor to minimize the time taken for this at site.

The building housing the GTX100 train and HRSG will lie behind the existing plant – i.e., in the foreground of Figure 2 – and, from the town, the bypass exhaust stack (60 m high) will be the only visible component. The normal exhaust flow line will be through the existing 120 m stack, which has room for extra piping. Noise from the plant will be low, and its impact on sensitive areas of the city will be further reduced due to the shield effect of the existing plant.

The gas turbine plant`s electrical and control equipment will be installed in the main Västhamn plant control room.

Process integration

The steam and condensate systems of the two plants will be fully integrated and the economizer of the HRSG will be connected to the district heating system. The bypass stack will allow independent operation of the gas turbine.

A total of 15.5 kg/s of steam, at 113 bar/520 degreesC, will be produced in the HRSG behind the GTX100 and fed to the ATM steam turbine in the existing plant. In addition, the steam flow from the main plant`s boiler will be increased, following an uprating, from an output of 78.5 to 82 kg/s of steam. This means that the steam turbine will be operating with its maximum design flow – 97.5 kg/s at the inlet. An ATM steam turbine, installed at a plant in Luleå, northern Sweden, currently operates at this load, but the unit at Västhamn will have to be retrofitted with upgraded blading to allow for the larger steam flow. The GTX100 part of the steam will pass through the whole steam turbine, resulting in a high level of electrical efficiency. The output will be increased to 82 MWe. (As explained earlier, the existing steam turbine generator will be used in spite of the higher output.)

Another component that will operate at a higher load than presently is the district-heating condenser. Thorough investigations have concluded that the greater load can be handled.

A feature of the configuration, when compared with a standard combined cycle, is the larger (district heating) economizer that is placed downstream of the HRSG. It delivers 16 to 18 MWth, depending on the operating mode and the chosen exhaust flow temperature.

Operation

The new set-up will have a wide operating range, since the GTX100 combined cycle will be covering the heat demand up to the point where the existing boiler has its minimum load. The latter is the least flexible component, as it should not be put through more than a handful of stop/start operating sequences each district heating season. For this reason, the gas turbine cycle must be flexible and able to operate intermittently, both in periods of low demand, when the plant`s main boiler is not operating, and in periods of intermediate demand, when the boiler is in operation.

The gas turbine plant can be fully integrated with the existing chp plant and its steam turbine, but the gas turbine may also be run when disconnected from the HRSG. It will also be possible to run the steam turbine purely on input from the HRSG.

In its standard combined cycle configuration, the GTX100 has 62 MW net output, at 54 per cent efficiency (based on a dual-pressure, non-reheat steam cycle). At Västhamn, however, the integration of the gas turbine cycle with the existing plant means that this level cannot be reached.

With the firing capacity at 121 MW, the GTX100 plant will produce 58 MWe and 51 MWth when operating in conjunction with the existing boiler at full load. The output will be 56 MWe and 53 MWth when operating in a pure combined cycle (i.e. with the main plant boiler shut down).

The combined cycle, chp plant will have an output of 126.7 MWe and 186.4 MWth at full load. The power-to-heat ratio will be 0.68, a substantial improvement on the present 0.48. The total plant efficiency will be 90.6 per cent, slightly higher than at present.

Conclusion

The GTX100 extension will meet HEAB`s need for a higher electrical output and a more flexible plant at Västhamn. The integration of the new gas turbine plant with the existing chp plant will reduce the overall investment cost substantially.

The extension will also cut the consumption of coal at the plant. This is mainly due to the fact that the full-load operating hours of the existing boiler will be reduced, making it possible to increase the usage of wood pellets in the boiler. Part-load operation with biofuels has proved to be both environmentally and economically advantageous for HEAB.

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Figure 1. Helsingborg, a city of around 115 000 people in the south of Sweden

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Figure 3. GTX100 performance vs. ambient temperature

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