Preparing for the Millennium

Merchant power plants operate without conventional power purchase agreements. Therefore the most efficient power plants, operating with the latest technology, stand to generate the most profit. It is against this background that US Generating Company chose to build an advanced combined cycle plant in Massachusetts known as the Millennium Project.

The advanced turbine combined cycle plant is uniquely capable of responding to a market driven by a strong economy, increasing demands for power, high electricity prices in certain markets, falling reserve margins and the divestiture of generating assets. This combination of forces is particularly evident in the USA, where the need for low-cost generation in some parts of the country is extremely high, with demand propelling sales of power to levels as high as $7000/MWh. With the availability of merchant plants in the Northeast comes the ability to sell generated electricity at whatever price the market will bear due to this overwhelming demand.

Merchant+Massachusetts = Millennium

Because merchant power plants operate with some or all of their output at risk, uncommitted through conventional power purchase agreements, independent power producers are filling the need created by the deregulation of the power industry. Therefore, the most efficient power plants – operating with the latest technology – stand to generate the most profits. For example, Bethesda, Maryland-based US Generating Company (USGen) purchased 4000 MW of divested generating capacity from the New England Electricity System, and proceeded to augment this purchase by expanding its merchant construction programme. (The company currently has plants under development in various areas throughout the US, including California.)

The Charlton, Massachusetts plant, called the “Millennium” Project, is a 360 MW combined cycle facility using a Siemens-Westinghouse 501G gas turbine. The plant, scheduled to begin commercial operation in June 2000, will enable USGen to become a strong market power in the region with the ability to provide power at prevailing market prices into the grid.

“There were two key factors in identifying the site at Charlton,” explains Gary Lambert, Vice President of Development for the Northeast Region for USGen. “First was its proximity to electric transmission. With the Millennium plant in the heart of the New England Electric Transmission System, it sits at the very hub of the power system for all of New England. Second was its proximity to a main gas pipeline. In this case, it`s the Tennessee Pipeline – the major pipeline for Central Massachusetts.”

New England presents its own set of challenges because its permitting process is intricate and complex. “Perhaps the most important of our concerns was acceptance at both the local and state level,” Lambert continues. “Because of New England`s political climate and concentrated population, a long-standing process of determining what, if any, impact each new generating facility will have on the electricity transmission grid is in place with a variety of interrelated pieces which must mesh together to ensure approval. Here, the Millennium plant successfully negotiated the process of conducting a system impact study and by agreeing to upgrade the transmission system as necessary to fully integrate the plant in accordance with its New England Power Pool approval,” he concludes.

Environmental concerns produced other obstacles in the development of the plant. Nearly one-half of the site`s 120 acres was set aside as permanent conservation land to be used as a habitat for the endangered spotted blue salamander. In addition, special permitting was necessary for the plant to utilize waste water effluent from the Southbridge Water Treatment Plant as wet cooling.

A growing portfolio

USGen, a wholly-owned subsidiary of PG&E Corporation, is one of North America`s premier power generation companies, serving both wholesale and large industrial customers. USGen, through affiliates, has ownership and management interests in a $6 billion portfolio of reliable and efficient generating assets representing more than 7000 MW of capacity. Currently, it has more than 4500 MW under development for merchant sales into the competitive power market.

The Millennium Project is one of the first highly efficient combined cycle merchant plants in New England which the company has developed to displace older, less efficient and less environmentally-friendly plants to capture anticipated load growth. The $200 million, 360 MW, natural gas-fuelled power plant is part of a new wave of merchant power plants in New England and the country being built to sell power as a commodity in the competitive marketplace without relying on long-term contracts. Millennium will compete to sell the plant`s electricity on a wholesale basis to local utilities and power marketers who, in turn, will sell it to industrial, commercial and residential electricity customers.

Making the pieces fit

The majority of merchant capacity is gas-fired, often in a combined cycle arrangement, because of a low installed cost, short construction time and higher efficiency than coal-fired and oil-fired units. Gas turbine units are an attractive choice for power producers, offering a high degree of flexibility in providing cost-effective power in all capacities.

The main plant components are the 501G combustion turbine; a three-pressure natural circulation heat recovery steam generator; and a 120 MW axial flow two-case condensing steam turbine.

For the Millennium Project, USGen tapped Siemens-Westinghouse for its time-proven combustion turbine. Siemens-Westinghouse`s 230 MW, 60 Hz 501G was particularly attractive to USGen because of its low capital cost and low fuel consumption. An evolutionary design in the company`s combustion turbine product line, the 501G features a two-bearing rotor, axial exhaust, cold-end drive, individual combustors and other time-proven features that provide high reliability. Enhanced by the latest technology airflow design and a reputation for world-class efficiency, the combustion turbine was a perfect fit for USGen`s needs.

“USGen came to us with the knowledge that we had an advanced, high-efficiency gas turbine that met their needs and they asked us to provide them with preliminary information. Immediately afterward, we entered into negotiations on a term sheet,” explains marketing manager John Wilson. “They were already aware of the 501G`s capabilities, having done a due diligence on the technology for a prior project that never materialized. They still had the comfort level and intimate knowledge of our product, and decided it was a good fit.”

One of the driving issues here is the efficiency of this plant. Its level of efficiency is 50 per cent greater than the existing generating assets in New England. Emissions are 3.5 ppm on NOx and 4 ppm on CO. With numbers such as these, Millennium is the most efficient and most environmentally benign plant in New England.

This high level of efficiency is a direct result of gradual improvements to Siemens-Westinghouse`s 501 series. The 501G combined cycle plant employs a highly integrated cycle which is designed to achieve the highest practical plant output and efficiency while simultaneously meeting the requirements of several cooling systems.

The 501G combustion turbine features steam-cooled combustor transitions which require a reliable source of intermediate pressure steam. The cooling steam is supplied from the intermediate pressure section of the Nooter HRSG – one of the largest three-wide (three modules wide), modular units in the world – and returned to the intermediate pressure steam turbine as hot reheat. The balance of the intermediate pressure steam developed in the HRSG (not used for cooling) is mixed with the cold reheat prior to entering the reheater section of the HRSG.

The system also includes two separate streams of air extracted from the combustion turbine compressor which require cooling within the cycle. The cooled air streams have the important function of cooling several stages of rotating blades, vanes and the turbine rotor. The 501G combustion turbine has a higher pressure ratio and firing temperature than the 501F in order to achieve enhanced efficiency.

Due to these characteristics, and the desire to minimize the cooling air flow extracted from the turbine to maximize the compressor flow to the combustor for stable combustion with the lean pre-mixed system, the temperature differential of the cooling air is high. The most efficient way of capturing this cooling air energy within the cycle was to do so in multiple stages in a manner which paralleled the HRSG steam systems. Two kettle-type reboilers (a flooded vessel with heating on the tube side) are included within the cycle for this function. The kettle boilers utilize the cooling air energy to produce steam in parallel with the HRSG intermediate pressure and low pressure steam systems.

An additional feature which is included in the cycle to enhance plant efficiency is the fuel gas heater, which uses water from the intermediate pressure system of the HRSG to heat the fuel gas in excess of 204°C (400°F). Subsequent to heating the fuel, this water is recirculated to the condensate system. The CT cooling steam system, the cooling air system and the fuel gas system are all linked within the cycle. The HRSG is the cycle component which links these systems together.

“It is how the 501G fits into the cycle and is meshed with the HRSG to achieve the overall plant efficiency that makes the cycle unique,” explains Michael McManus, strategic and product development engineer. “It is the next level of integration between combustion turbines and the combined cycles they fit into.” For example, traditional single cycle steam plants approach 38 per cent efficiency, while the 501G is capable of 58 per cent efficiency. “This advanced combustion turbine of ours is more efficient than any other in the world and it produces exhaust energy that creates a thermal cycle that`s more efficient than any other cycle,” Wilson adds. “Not only is the engine more efficient, but it provides more waste energy – higher grade waste energy – that makes the whole plant more efficient,” he concludes.

“In the merchant market, the least cost units are turned on – dispatched – first, and the highest cost units are dispatched last,” Lambert explains. “To put a new unit in a system, depending on the efficiency of the plant and its operating cost, which is directly proportional, it tells you where it goes in the dispatch “curve”. So this unit will go to the bottom of the dispatch curve…the first turned on and the last turned off against the need in the area.”

Built for the future

The Millennium plant is completely new from the ground up. This provides maximum flexibility in tailoring the pieces to fit together as a cohesive unit for peak efficiency, since it ensures more compatibility and smoother operation than a retrofit or renovation of an existing system. Siemens-Westinghouse provided USGen with a “thermal island” which integrates the combustion turbine, the steam turbine and the heat recovery equipment so that what USGen gets is a guarantee for the performance of the system rather than individual components.

Siemens-Westinghouse`s thermal cycle worked with USGen specifications to conform to manufactured equipment. Bechtel, the engineer constructor, is basically building the plant around the thermal island. With a long-term parts and maintenance programme signed along with the power island contract, Siemens-Westinghouse takes responsibility for repairing and replacing parts as necessary on USGen`s schedules, thereby guaranteeing operating costs for the company to help protect their investment. “It`s our guarantees that the thermal cycle works that make the project financeable,” explains Wilson. “This is the engine in the race car. It still comes down to the fact that USGen picked the 501G because it was the most advanced gas turbine on the market and could be delivered on their schedule.”

Critical timing

Contracts for the Millennium Project were signed in February and development began shortly thereafter in preparation to meet the project`s mandated completion date. June 2000 is a critical date for the plant to go on-line, due to the power demands of the region it serves. In the merchant market, capacity is more valuable than energy, and given that the New England market is a summer peaking market, capacity tends to be more valuable during the summer than the rest of the year. Therefore, being on-line June 1 is tremendously more important than being on-line September 1.

And that dictates that the Millennium Project lives up to its name, ready to meet the challenges of the 21st Century.

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Figure 1. The 501G compressor

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Figure 2. Location of the Millennium project

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Figure 4. Artist`s impression of the completed Millennium Project