Agawam merchant GT24 combined cycle plant uses once-through HRSG
20 September 1999Berkshire Power's 270 MWe combined cycle power plant in Agawam Massachusetts, USA, is one of the world's first true merchant power plants. It also embodies a number of firsts for combined cycle power plant technology.
The Agawam plant is the first installation to use ABB Alstom Power's KA24-1 ICS™ single-shaft combined cycle technology, the first to use ABB Alstom Power (Combustion Engineering) once-through HRSG (heat recovery steam generator) and the first to use imported LNG fuel. Owned by El Paso Energy of Houston and Power Development Co of Boston, the Agawam facility will supply electricity to the deregulated New England market on a purely merchant basis.
Commercial operation of the project, which represents many firsts in both technology and application, is planned for late 1999. Engineering for the 270 MWe combined-cycle power plant is completed, all equipment and purchases have been awarded, and construction (pictured above), which began in April 1998, is almost complete.
The GT24 gas turbine, which is reckoned to retain 90 per cent of its efficiency when it is brought down to 60 per cent load, is a key constituent in attaining the high level of efficiency and extraordinary flexibility needed for merchant power plant operation.
The KA24-1 ICS™ combined cycle plant is as near to an optimized standard power plant design as is achievable in practice. The first of these reference power plants are Agawam, described here, for operation in 1999 and two units in Monterrey, Mexico, for operation in early 2000. Plants have also been ordered for Midlothian (USA) and Hermosillo (Mexico), while ABB Alstom Power cites further commitments to purchase another ten plants comprising more than 25 power trains in all.
The project
The main focus of the new Power Merchant Division of El Paso Energy Corporation, headquartered in Houston, Texas, centres around two activities. The first involves restructuring contracts associated with generation assets to provide more competitive electric power to large users of electricity. The second is merchant generation plant development.
In August 1997, El Paso Energy Marketing announced plans to construct, own and operate three power plants in New England. The first is Agawam. El Paso Energy Marketing will provide fuel supplies for the facility and market the power generated by the plant.
The 270 MWe natural gas-fired, green field Agawam facility has been developed by Berkshire Power and was approved by the Massachusetts Energy Facilities Siting Board in April 1996. The Berkshire Power Company is a joint-venture company owned by El Paso Energy Corporation and Boston-based Power Development Corporation. Energy Ventures of Boston also holds a minority interest in the project.
ABB Alstom Power, in a consortium with Black & Veatch of Kansas City, Missouri, is supplying a complete combined-cycle power plant based on the KA24-1 ICS™ power plant concept, consisting of a GT24 advanced gas turbine, steam turbine, generator, heat recovery steam generator, and the overall power plant control systems as well as the engineering for the power island.
ABB Alstom Power is also responsible for the commissioning of the plant and in addition will operate and maintain the plant for the first ten years. Black & Veatch is providing the balance of plant equipment and is responsible for the engineering of their scope and the erection of the entire plant.
Fuel supply amounting to 45 000 MMBtu/d and power offtake marketing are being handled by El Paso Energy Marketing Co. The Massachusetts DPU has designated Berkshire Power as a public utility, exempting the Agawam facility from local zoning requirements.
The project was financed off the balance sheets of the owners, but ability to compete proved to be key to gaining the critical financial backing. Financing proposals were received from 12 banks, of which financing was eventually arranged by Kredietbank of New York. Construction began in December 1997.
The project is an important step in El Paso Energy Marketing's strategy to develop and build gas-fired generating assets in regions that are moving toward full electric competition and have a clear need for new, cost-effective generating capacity.
El Paso Energy owns the nation's only integrated coast-to-coast natural gas pipeline system. With over $9.5 billion in assets, El Paso Energy Corporation has operations in interstate natural gas transmission, gas gathering and processing, international infrastructure development, and energy marketing, through its five business units: Tennessee Gas Pipeline Company; El Paso Natural Gas Company; El Paso Field Services Company; El Paso Energy International Company; and El Paso Energy Marketing Company. The company owns interests in new projects under construction in Mexico, Peru, Indonesia and Hungary and deals with project financing, project development, construction and operation services for energy infrastructure projects.
Fuel supply
Distrigas of Massachusetts Corporation (DOMAC), the sole liquefied natural gas (LNG) importer on the United States' east coast, announced the signing of a long-term contract with El Paso Energy Marketing Company to provide natural gas to the Agawam plant.
The announcement of this contract followed receipt of the first LNG cargo delivered to the DOMAC LNG facility from the newly operational LNG export facility in Trinidad, owned and operated by the Atlantic LNG Company of Trinidad and Tobago. Cabot LNG Corporation, the parent company of DOMAC, is a 10 percent shareholder in Atlantic LNG and holds a 20-year purchase contract for 60 per cent of the plant's design capacity of about 400 million cubic feet of LNG per day.
DOMAC owns and operates an LNG import terminal in Everett, Massachusetts, where the company receives LNG shipments primarily from Trinidad and Algeria. LNG is delivered to customers throughout the Northeast via LNG tanker truck or regasified and sent out along one of three pipelines connected to the Everett facility.
Benefits
The Agawam plant will be 40 per cent more efficient than existing gas-fired generation facilities in the region. This equates to less expensive electricity and significantly lower CO2 emissions. Post-combustion treatment of the exhaust gases with SCR and CO catalysts reduce air emissions to 3.5 vppm NOx, making Agawam one of the lowest emissions plants ever to be built in the region. In addition, the plant has a low profile and adheres to the community's requirement for aesthetics and low noise.
New facilities, such as Agawam, consume two-thirds less water than conventional power plants and will provide efficient, clean and economic power, eventually replacing aging coal and oil fired and nuclear plants.
Being a merchant plant, there are no pre-arranged power sales agreements and, significantly, no capacity payments. A merchant plant must be not only efficient and environmentally friendly at its base load rating, but competitive and highly flexible throughout the entire operating load range. The flexibility of the KA24-1 ICS™ enables Agawam to reduce variable costs when electricity rates are low. It can also respond rapidly to profit opportunities when rates are high by working efficiently at peak-load.
The plant
The Agawam plant is based on the latest KA24-1 ICS™ reference power island design, which is optimized to maximize customer benefits by providing:
The turbine/generator train of the KA24-1 ICS™ plant is designed as a floor mounted single shaft power train. The automatic SSS clutch allows stand alone operation of the gas turbine during start-up or steam turbine bypass operation. In case of a steam turbine trip, the gas turbine and generator can remain on grid. If the steam turbine trips, the clutch automatically disengages and disconnects the steam turbine mechanically from the gas turbine generator unit.
The gas turbine generator set converts chemical energy contained in the gas turbine fuel into electrical energy at a gross efficiency of about 38.5 per cent in an ambient temperature of 15°C (59°F), while generating hot flue gas which then is directed via the exhaust duct into the heat recovery steam generator (HRSG).
Steam is generated in the HRSG by heat transfer from the GT exhaust gases to the feedwater. The HRSG is designed as a dual pressure reheat boiler providing:
The steam turbine is of the floor mounted two-casing reheat type with a gearbox between the HP and IP/LP part. The lateral steam turbine exhaust is connected to a two-pass horizontal condenser.
The net efficiency of the overall plant in combined cycle mode is 57.5 per cent.
Gas turbine
The GT24 gas turbine represents ABB Alstom Power's state-of-the-art in advanced generation heavy duty 3600 r/min, 60 Hz gas turbines. The rotor, which is rigidly coupled to the generator shaft, carries five turbine stages and 22 compressor stages. The turbine combines ABB's unique sequential combustion system with the proven annular combustion chamber, featuring dry low NOx technology for both HP and LP combustors.
The GT24 can be operated with either gaseous fuel or liquid fuel. In the case of liquid fuel operation, demineralized water has to be injected for NOx emissions control.
Three rows of variable inlet guide vanes (VIGV) are employed to control air flow through the turbine, securing the superior part-load characteristics of the gas turbine. In combination with the sequential combustion concept, the VIGVs maintain constant nominal exhaust gas temperatures down to 40 per cent load, thus securing the excellent combined cycle part-load performance of less than 115 per cent design full load heat rate at 50 per cent load.
The compressor is equipped with three stages of blow-off valves to prevent rotating stall, during start up and to reduce start-up power requirements. Blow-off air is directed to the exhaust system, thus minimizing noise emissions during start-up and shut-down.
For cooling and sealing purposes, air drawn off from the GT compressor at several stages is cooled in two steam generator heat exchangers which are tied into the HP section of the HRSG system. Combustion turbine inlet air chilling is provided by natural gas fired mechanical chillers.
Advanced HRSG
The ABB Alstom Power heat recovery steam generator (HRSG) unites proven equipment and systems to form an innovative HRSG concept with superior steam production capability and maximal simplicity. Its basic configuration is a dual pressure reheat design.
The HRSG is configured as a horizontal flow system combining a low pressure drum type steam generator and a high pressure once through monotube steam generator without any supplementary firing. A key design feature of the once-through boiler is that no thick walled, thermally sluggish high pressure drum is required.
The HP section of the boiler is designed as a once through system, whereas the LP part is a natural circulation drum-type boiler system. The HP once through system is fed with saturated LP water from the drum. The LP drum, therefore, takes on the function of a feedwater storage tank for the HP system. For reasons of steam quality and cleaning of the water/steam cycle, a separator is installed in the once through HP system. The reheat system operates dry during steam turbine bypass operation.
Due to the high gas turbine exhaust temperature, around 640°C, a compact once through boiler design is used.
The HRSG design represents an optimum in efficiency and cost. Because of the high exhaust-gas temperature, the efficiency of the HRSG is close to that of a standard industrial HRSG with three pressure levels.
Steam turbine
The KA24-1 ICS™ plant features a dual casing steam turbine design with a single flow low-pressure exhaust. One casing contains the IP/LP section, which runs at the normal 3600 r/min synchronous speed, and the other contains the HP section which runs at some 9000 r/min via a low loss gearbox. Integration of the high-speed, barrel-type, HP steam turbine is an advantage due to the low steam volumetric flow. Also, it features a more efficient blade design than a 3600 r/min machine, which more than compensates for the losses caused by the gearbox.
The superheated high pressure live steam is fed into the HP steam turbine. At the HP turbine exhaust the steam is reheated and fed back into the IP/LP turbine, where the reheat steam is expanded together with the LP steam flow.
The steam, partially expanded in the high-pressure turbine, passes back to the HRSG for reheating, and then finally expands together with steam from the low-pressure drum in the LP turbine to the condenser pressure. A 100 per cent steam bypass in combination with a condenser designed for full steam dump capacity ensures that the power plant does not have to be shut down in the unlikely event of a steam turbine outage, thereby improving the overall availability.
For numerical analysis of the transient processes involved, the powerful simulation package "Analyzer" was used to numerically model the start-up and shut-down processes, control sequences and disturbance responses as well as contributing to the increasingly important RAM analyses. Simulation results illustrated below show hot start-up parameters for a typical standard KA24-1 ICS™ combined cycle unit.
Air-cooled generator
The power plant uses a TEWAC type generator for simplicity and ease of maintenance. The type WY23 generator is a 2 pole 3 phase generator with a nominal rating of 280/345 MVA (ANSI class B/F) at 100 m.a.s.l. and 40°C (104°F ) cold air temperature.
Compared with the usual hydrogen-cooled machines, the TEWAC generator offers advantages in availability, operating costs, space requirements and handling – all results of the simpler technology.
The closed cooling air cycle used in TEWAC technology makes the generator cooling relatively insensitive to fluctuating ambient conditions. The higher noise emissions, compared with hydrogen-cooled generators, and the marginally lower efficiency due to a slight increase in ventilation losses, are negligible.
The generator breaker, which is part of the installed generator equipment, enables the gas turbine to continue operating in idling mode after being disconnected from the network, eg in the event of a grid outage. This increases the operational flexibility as the power plant can be quickly re-synchronized and connected to the grid. In this context it is also worth noting that the KA24-1 ICS™ is equipped with all the devices needed for black starts (optional) and emergency operation.
Steady-state as well as transient processes such as start-up, shut-down and load changes, are controlled automatically by ABB Alstom Power's Advant® Power distributed control system (DCS). The DCS comprises, as its main components, the Egatrol and Turbotrol control systems for gas and steam turbines, and also acts as the control interface for other plant equipment.
Deregulated power
The privatization of the power generation sector in the United Kingdom and deregulation of the US and European power markets started a trend in the industry that continues to fuel competition among the power producers. As a result, simply ensuring an uninterrupted supply of power has given way to market forces, with supply and demand controlling the trading of electricity as a commodity.
Increasingly, the electricity supply sector is being broken up into separate segments dedicated to power generation, transmission and distribution, and sales. Since the existing power networks will continue to be used, the T&D industry will mostly, for the time being, stay with the regulated structures.
The power utilities are having to compete today with new profit-oriented companies in the form of independent power producers (IPPs) and merchant power producers (MPPs). Both the IPPs and MPPs trade in the electricity market on the basis of supply and demand. Unlike the IPPs, who are active to a lesser degree in the spot market, the MPPs operate without fixed supply contracts and have a strategy which emphasises quick and flexible spot market trading.
For new plants to be competitive, they have to fulfil certain requirements: they must be up and running as soon as possible; specific capital costs must be kept low; they have to be environmentally compatible and offer no major obstacles to site approval; and they have to generate electricity at the lowest possible cost.
The competitive business development created by the liberalization of the electricity supply sector has made the combined cycle power plant, with its cost benefits and low environmental impact, the generating plant of choice for power producers.
ABB Alstom Power has developed standardized combined cycle reference plants for this new market based on GT24, GT26, GT13E2, GT11N2 and GT8C2 advanced gas turbines. The KA24-1 ICS™ reference plant, of the kind being constructed at Agawam, currently represents more than 75 per cent of all contracts for plants based on the GT24. Although conceived mainly for the deregulated power industry, it is also an interesting option for the traditional power utilities.
TablesProject profile Orders for KA24-1 ICS™ combined cycle power plants Agawam basic data
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