Top Plant: Langage Combined Cycle Power Plant, Plymouth, Devon, UK

The UK grid, focused on adding valuable renewable generation, will rely on natural gas–fired generation for many years to come. One of the most recent additions is the Langage Power Plant, designed for quick response and low load “parking” at night while remaining below air emissions limits. With an extraordinary architectural design that blends into the natural surroundings, Langage is now a local landmark.

The UK’s newfound interest in natural gas–fired electricity generation follows a decade of renewable energy development, principally wind farms, which followed an earlier “dash for gas” in the early 1990s. This build-pause cycle begins anew with the recent construction of several large gas-fired combined-cycle plants. The difference this time is their added ability to provide grid stabilization and spinning reserve services when integrated on the grid with wind energy.

The current gas-fired and renewable plants must also balance out the large number of plants scheduled for retirement over the next decade. The European Union (EU) has launched its Large Combustion Plant Directive (LCPD), which, in addition to setting emissions standards for power stations larger than 50 MW, requires power stations that don’t meet specified emissions standards to either add the necessary air quality equipment (“opt-in”) or close down (“opt-out”) by a date certain. Facilities that opt out of the standards can operate for a maximum of 20,000 hours after January 2008 and must shut down by 2015. Currently, about 11 GW of opt-out coal- and oil-fired stations are scheduled to close by 2015 and about 20 GW of opt-in coal-fired stations will have restricted operation post-2016. In addition, around 7 GW of nuclear power generation capacity is scheduled to close between now and 2020.

To fill the coming gap, over the past five years the UK has constructed six new gas-fired combined-cycle power plants, four of which were installed by Alstom under engineer-procurement-construction (EPC) contracts: Langage (878 MW), Grain (1,275 MW), Staythorpe (1,650 MW), and Pembroke (2,160 MW). Each of these KA26 plants includes unique features, such as high operational flexibility or district heating, and all used state-of-the-art, high-efficiency GT26 combustion turbines. Since 1990, Alstom has built and commissioned 16 combined-cycle power plants in the UK, including these four, totaling over 12 GW and accounting for more than 40% market share.

Building Langage

The 847-MW Langage Power Plant (LPP), owned by Centrica Langage Ltd. (formerly British Gas), is located near Plymouth, Devon along the southwestern shore of the UK, where the Pilgrims set sail for the New World. The plant is a KA26-2 multi-shaft combined-cycle plant configured with two GT26 gas turbines, two triple-pressure with reheat once-through heat-recovery steam generators (HRSGs) with duct firing, and one floor-mounted, three-casing, triple-pressure STF30C steam turbine. Each gas turbine and the steam turbine drive an identical air-cooled TOPAIR turbogenerator. Duct firing both HRSGs increases the power output more than 30 MW when peaking power is required, with little impact on plant efficiency.

The approach used in the EU for constructing EPC combined-cycle plants is significantly different from that used in the U.S., where a contractor typically assumes the overall project lead. For LPP, Alstom managed the entire EPC project, supplying all of the major equipment, including the plant control system; performed the engineering, integration, and design work internally; and performed the construction management.

Alstom uses a combination of contracting options, including subcontractors or direct hire of labor using its own construction company. Alstom also directly subcontracts the erection of purchased major equipment from suppliers, such as SPX for the supply and mechanical erection of the air-cooled condenser. Alstom’s staff of highly experienced commissioning experts then take charge of the start-up phase of the project and handover to operations.

Construction of LPP began in June 2006 when Alstom received its notice to proceed. The HRSGs were delivered in October 2007 and were followed by the combustion turbines in January 2008 and steam turbine the next month.

The Alstom lead center for HRSGs is located in Windsor, Conn., where the engineering design and project management is carried out for all Alstom HRSGs worldwide. The 18 HRSG modules for LLP were fabricated and shipped from Alstom’s facility in Portugal. Transformers were purchased from AREVA T&D, the same division that Alstom sold to Areva some years ago and just recently repurchased to form the newly created sector “Alstom Grid.”

By July 2009, the plant achieved first fire, and full load was reached in December 2009. The plant was formally turned over to the owner in March 2010. Alstom is also supplying operations and maintenance support for 12 years under a long-term service agreement.

Enviable Features

LPP exhibits many interesting design features that merited its inclusion as a POWER Top Plant. Several are unique to this project yet have much broader application for those seriously considering construction of a combined-cycle plant that has superior baseload efficiency plus the ability to “chase wind” when necessary.

High Reliability and Efficiency. According to Alstom, the thermal efficiency of the €400 million LPP is approximately 58% (LHV), making it one of the highest efficiency air-cooled combined-cycle plants in the world. The NOx guarantee on natural gas is 40 mg/Nm3 at 15% O2 dry without supplementary firing. Given Alstom’s long history with designing and building plants of similar design, its reliability guarantee was equally robust: >95% during the first 150 days of operation and 97.5% going forward.

Pleasing Design. LPP is constructed close to a national park, hardly an ideal spot for a power station. Yet its thoughtful architectural design (similar to that of a modern airport—see the cover photo) and strict adherence to environmental constraints has ensured that the plant blends in with the natural surroundings. For example, a green roof (Figure 1) minimizes the plant’s visual impact. Hiding a plant stack is always problematic, so LPP’s was sunk 10 meters into the ground to reduce its height, and the architects made it a key design feature. Today, the stack is a local landmark.

1. Site sensitive. A green roof and a stack sunk 10 meters into the ground minimize the visual impact of Langage Combined Cycle Power Plant. Nearby land is reserved for development of an energy park, a mixed-use industrial area that includes the plant’s administrative building, shown in the upper left corner. The plant design included provisions to add a “steam tail” to supply steam for the park’s district heating system. Courtesy: Alstom

VIP Parking. In the UK’s deregulated electricity markets, the plant offers extremely flexible operation, including the capability to turn down to 40% plant load and below. This turn-down capability is particularly useful for plants that must chase wind or provide spinning reserve as part of their dispatch duties. (See “Flexible Turbine Operation Is Vital for a Robust Grid” in this issue for a complete discussion of how Alstom’s GT24/GT26 sequential combustion design provides rapid response to meet the stringent UK Grid Code while maintaining low-load emissions.) LPP now supplies lucrative frequency support and spinning reserve to the UK grid with a responsive 8 MW/sec ramp rate based on the combined output of the plant.

Quiet Neighbor. Controlling the noise produced by the plant was a key design issue given its location near a residential area. Materials for the colored cladding on the facility’s walls were also selected for their special acoustic properties. The far-field noise limitations at an adjacent neighborhood were kept at a whisper-quiet 35 dBA—the local stream makes more noise than the power station.

Future Growth Option. Land is reserved nearby for development of an energy park, a mixed-use industrial area that includes the plant’s administrative building. The plant design also included provisions to add a “steam tail” to supply steam for district heating the park.

Dr. Robert Peltier, PE is POWER’s editor-in-chief.

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