Beauty and the beast – siting the 878 MW Langage power plant close to a national park required a little extra aesthetic sensitivity
Lee Dawes, Alstom Power, UK
Close proximity to a national park hardly seems the ideal spot for a power station, much less one large enough to deliver 878 MW of electricity. Yet the use of clever architectural design and strict adherence to environmental constraints has meant that the recent start up of the new Langage power station near Plymouth, Devon in the UK has plenty of heads turning.
Notably, Langage is the first large-scale gas fired power station, built by Alstom, to begin commercial operation in the UK for five years, and is one of a clutch of new gas fired combined-cycle power plants that will help Britain bridge the looming gap between electricity supply and demand.
UK’s move towards Gas fired Power Plants
The UK is estimated to need an additional 35 GW of capacity by 2020. Gas fired combined-cycle power plants are still seen by many as a good option for satisfying the UK’s short to mid-term power needs, as well as complementing the increasing emphasis on renewable and environmental goals.
The second half of last decade therefore saw a revival in the construction of gas fired combined-cycle plants. In the last few years, contracts have been awarded for the construction of five large combined-cycle power plants, four of which – Langage, Grain, Staythorpe C and Pembroke – were awarded to Alstom.
The Langage power station project was acquired by Centrica Langage Limited, a wholly owned subsidiary of Centrica Energy, in August 2004. It marks the first plant that Centrica has constructed in the UK and is in line with the company’s strategy of developing the largest fleet of gas fired power plants in Britain.
The power station project also underpins the development of the Langage Energy Park, a major strategic employment development site to be built in Plymouth. This separate development by Carlton Power, will take some of the power directly from the power plant to supply its mixed-use energy park.
Like its sister power stations at Grain, Staythorpe and Pembroke, the new Langage power station is based on the Alstom GT26 gas turbine. The plant is a nominal 878 MW, KA26-2 multi-shaft 2-on-1 combined-cycle power plant, which features two Alstom GT26 gas turbines, each driving its own Alstom TOPAIR (air-cooled) turbogenerator, and each with an associated heat recovery steam generator (HRSG) feeding into a single common type STF30C steam turbine, also driving an Alstom TOPAIR (air-cooled) turbogenerator.
|The GT26 gas turbine’s sequential combustion system provides high operational flexibility|
Langage has been built under a €400 million ($493 million) turnkey engineering, procurement and construction contract, where Alstom was responsible for the design, supply and installation of the gas turbines, HRSGs, steam turbine and a large air-cooled condenser, as well as commissioning the entire power plant, including the site preparation, civil works and final testing. Alstom will also provide maintenance and operation support for an initial 12-year period under a long-term service agreement with Centrica.
Best available technology
One of the main reasons for selecting Alstom for the project is the performance of the KA26 in terms of power output, operational flexibility and environmental capabilities.
The KA26 combined-cycle provides Centrica with a highly efficient and flexible plant, which can be run at part load in order to meet variable demand requirements while maintaining low levels of emissions.
The key feature of the KA26 is the GT26 gas turbine’s sequential combustion system or reheat cycle concept. Compressed air is heated in the first combustion chamber (the ‘EV’ combustor for EnVironmental) by adding about 50 per cent of the total fuel (at baseload). After this, the combustion gas expands through the single stage high pressure turbine,
which lowers the pressure by about a factor of two.
The remaining fuel is added in the second combustion chamber (the ‘SEV’ combustor for Sequential EV combustor), where the combustion gas is heated a second time to the maximum turbine inlet temperature and finally expanded in the 4-stage low-pressure turbine.
During operation, loading/de-loading of the gas turbine is carried out by varying the set points for the high-pressure turbine inlet and low-pressure turbine inlet temperature, as well as the variable inlet guide vanes. This is performed in a controlled manner at a loading gradient that is defined with respect to thermal stresses in blades, vanes, rotor and casings.
Loading is carried out in several phases: the initial ignition of the EV burners; the subsequent ignition of the SEV burners; and then loading of the gas turbine by opening the variable inlet guide vanes to allow a greater air mass flow through. With these open the turbine inlet temperature of the SEV combustor is increased further to achieve the baseload value.
The sequential combustion system allows operational flexibility ranging from 100 per cent power plant load down to 40 per cent or even lower. This operational flexibility comes with:
- High part load efficiency
- Low nitrogen oxides emissions
- Fuel flexibility regarding gas composition
Part of the air extracted from the compressor for cooling the gas turbine hot gas components is first cooled down in once-through coolers and the energy is passed into the HRSG to improve overall cycle output and efficiency.
Each gas turbine is connected to a highly efficient triple-pressure reheat HRSG, with additional duct firing, that are the largest heat recovery steam generators Alstom has installed to date. The HRSG is of a horizontal once-through design technology that is optimized for frequent cycling. The HRSG incorporates ‘FAST’ start-up features, where ‘FAST’ denotes
Flexibility And Stepped-component Thickness (i.e. components thicknesses are stepped in size to reduce thermal stresses and lower cycle fatigue).
The HRSG has an 18-module design and includes duct firing. The use of a once-through design permits higher live steam parameters to be used in the water-steam cycle. In the fired case, the HRSG is designed to operate at significantly higher pressures and temperatures than more conventional combined-cycle power plant units.
There is also a reheat cycle where the steam at the exhaust of the high-pressure steam turbine is returned to the HRSG mixed with intermediate-pressure steam and reheated leading to higher efficiency, improved flexibility for load gradients and shorter start-up and shutdown times.
The Langage power plant also utilizes Alstom’s ALSPA P320 state-of-the-art, plant distributed control system to ensure total plant integration and high all-round operational flexibility. The plant’s flexibility is demonstrated by the various available operating modes. It will predominantly be operated in baseload with some cycling, the ability to go from zero to full load in less than one hour is a key differentiator.
Blending Into the environment
The Langage power station is situated in the southwest of England, an area that has little existing generation capacity. It will therefore reduce the need to import power from the rest of the country and hence reduce transmission costs. This will also help the UK’s wider security of supply position.
The power plant however is located just outside Plympton, a small historical town a little over three km away from the Dartmoor National Park, which means that environmental compliance was of paramount importance. To blend in with its surroundings, Langage is of a smaller size, height and area than plants of a comparable power output and is architecturally designed to minimize its visual impact.
The plant building is covered with a special cladding, which is not only visually appealing but is designed to keep noise to a minimum. The noise guarantee, which is 35 dBA at specific locations near the plant, means that the noise produced from the plant is actually less than the ambient background noise from nearby highways, rivers and even local wildlife.
Clever and stylish architecture has also been used to enable it to blend in with the surrounding landscape. The plant has a curved green-coloured roof and has been set down into the ground to minimize its impact on the horizon. The stack has also been made into an architectural feature through the use of a special mesh covering, creating a visually stunning power plant in a similar design style to that of a modern airport.
Construction challenges overcome
The contract for the construction of the plant was placed in June 2006. Following civil works, the first major milestone was the installation of the HRSG modules in October 2007. This was shortly followed by the start of the steel structure erection of the turbine hall in November. The first gas turbine was placed on its foundation in January 2008, the steam turbine generator arrived on site in February 2008 and was on its foundation in April of that year.
There were numerous challenges that had to be overcome in constructing this plant while respecting the environment and the needs of the local community. These included significantly lowering the profile of the plant to reduce the visual impact.
This meant that the construction team had to deal with a high ground water table and onerous discharge limitations. Numerous filters and silt removal systems had to be used so that water was returned to the local stream clear of all pollutants.
In addition to the civil works challenges, there were also challenges on the logistics side. For example, although there was around 1500 staff on site at the peak of construction, disturbance to local traffic had to be minimized especially through the residential areas surrounding the site.
Langage’s successful Commissioning
The first hot commissioning milestone, i.e. first firing of the gas turbines, occurred in July 2009 and the plant achieved full load in December last year. The plant reached the important landmark of commencing the 30-day tests in January 2010. Both gas turbines were fired in sequence and were run-up in stages to full load. Dynamic tests of the gas turbines were then carried out. Once the correct performance of the gas turbines was verified, steam was then admitted to the steam turbine. The safety valves and duct firing were all tested, tuned and optimized, and finally the commissioning of the entire combined cycle plant.
These tests also included the 100-hour performance tests, where the plant output power and efficiency were proven. The tests represented the final stage of the project before handing over the plant to Centrica. They involved running the GT26 gas turbine generator units, the STF30C steam turbine generator unit and all pumping and auxiliary equipment at maximum operational loads including daily cycling to ensure everything was running smoothly and without any problems before the plant was finally handed over in March 2010. The plant produces a nominal 847 MW without supplementary firing and 878 MW with supplementary firing.
The plant can be described as both powerful and stylish, and will have a major impact on the security of electricity supply in both the southwest region as well as the wider UK. Langage also furthers Centrica’s strategy of being the UK’s largest electricity producer from gas fired plants.