Combined Cycle Systems: Fast track to efficiency

Willibald Fischer, Lothar Balling, Power Generation Group (KWU), Siemens AG, Germany

The San Pedro de Macoràƒ­s power project will be one of the cheapest generators of electricity in the Dominican Republic. The 3×100 MW combined cycle plant is currently under construction on the south coast of the Dominican Republic, east of San Pedro. The plant will use technology designed to reduce life-cycle costs and reduce NOx and CO2 emissions.

The project was initiated by the Dominican utility Corporacàƒ­

A Siemens led consortium submitted its bid in November 1997 based on combined cycle technology. A letter of intent between the owners and Siemens was signed by November 1997. This consortium was selected as the winning bidder in December 1997 with financial close achieved in April 2000. Construction began immediately.

The project comes at a time when the power market in the Dominican Republic continues to undergo changes toward a competitive environment. The San Pedro de Macoràƒ­s plant is well placed to take advantage of these changes. The project is scheduled to begin delivering power to CDE in the first quarter of 2002.

There are several companies involved in the construction and development of the plant, including Cogentrix Energy Inc, the lead project developer.

The Commonwealth Development Corporation (CDC) entered the project when it purchased the interests of Scotia Energy of Scotland, the original development partner of Cogentrix. Siemens AG of Germany is providing the engineering, procurement and construction services for the combined cycle power plant.

Motherwell Bridge of Scotland is providing the engineering, procurement and construction services for the delivery, storage and piping of fuel to the project.

Exxon Trading InterAmerica will deliver the fuel, a light kerosene product, to the project under a long-term contractual arrangement. CDE will purchase the electricity under the conditions of a long-term Power Purchase Agreement.

Project financing

The V64.3A incorporates advanced technologies such as aerodynamics, film cooling and thermal barrier coating technology
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Several companies were involved in the project financing, including the Inter-American Development Bank and West LBl, export credit agencies, and numerous institutional lenders. Financing consists of five funding tranches, totalling over $200 million.

The involvement of these contributors demonstrates the importance that the project holds for the international lending community and how they view the Dominican Republic’s economic future.

The San Pedro de Macoràƒ­s plant is based on Siemens’ 100 MW singleshaft 1S.64.3A combined cycle plant which is the latest, and smallest of Siemens’ advanced combined cycle power plants. The design benchmarks were high efficiency, high reliability and availability, short project implementation, low capital investment and low life-cycle costs.

Plant configuration

The San Pedro de Macoràƒ­s plant will consist of three GUD1S.V64.3A modules. Each module will be equipped with a V64.3A gas turbine, generator, single-casing industrial-type steam turbine and gearbox aligned on a single shaft. Duct-fired, dual-pressure, non-reheat HRSGs, ancilliary systems and I&C equipment will also be included. The condenser is cooled by a forced-draft cell-type cooling tower.

The plant will be operated exclusively on No. 2 fuel oil which will be unloaded at a jetty and pumped to the plant site via intermediate storage tanks. Due to the choice of No. 2 fuel oil, water injection is necessary to reduce NOx emissions.

Using a configuration with three identical blocks allows a reduction of redundancies for the main pumps in the feed and condensate system. This reduces the capital costs and maintenance efforts. A tailor-made spare parts concept reduces potential unit outage time. Under the given ambient conditions with fuel oil and water injection, this plant will achieve an output of 3 x 99.2 MW and 48.2 per cent net efficiency.

The latest design in combined cycle power plants is the single shaft power train with the gas turbine and steam turbine driving one common generator positioned in the middle, with a clutch between the generator and steam turbine. This will maximize operating availability at the lowest operating cost.

Additional advantages include a reduced space requirement and fast start-up and shut-down.

The heart of the plant

The heavy-duty V64.3A is a downscaled version of the larger V94.3A and the V84.3A. It is a 5400 r/min (90 Hz) engine geared down to either 50 Hz or 60 Hz, depending on the grid. The 1190à‚°C-class engine is equipped with a hybrid burner ring (HBR) combustion chamber with low-NOx burners and state-of-the-art cooling techniques. The engine was full-load shop-tested in 1998 at the company’s manufacturing plant in Berlin, Germany.

The V64.3A incorporates the use of a cold-end generator drive with a straight axial exhaust to the stack eliminating the negative impact of thermal expansion and stresses. The stiff and lightweight two-bearing rotor consists of individual discs for each compressor and turbine stage. These discs are lined up on a central tie bolt with radial Hirth serrations transmitting the torque.

The high-temperature Model V64.3A gas turbine is a single-shaft machine of single-casing design incorporating aero-engine features such as advanced aerodynamics, materials, film cooling and thermal barrier coating technology.

The V64.3A is equipped with a 17-stage axial compressor designed for the same mass flow and pressure ratio as the V64.3. Stationary and moving blading and vanes have been redesigned to optimize compressor performance without sacrificing reliability or maintainability. It also uses aero-engine technology such as single-crystal blades, sophisticated coating technologies and film cooling extensively for efficient performance and reliable long-term operation. Free-standing blading is utilized in all four stages to minimize stresses and maximize efficiency.

State-of-the-art components

Cooling air is provided at several pressure and temperature levels providing optimal cooling and improved thermal performance. The use of an internal cooling-air supply system eliminates external components such as cooling-air coolers, boosters or filters with their negative effect on net heat rate and plant availability.

The generator is a two-pole type, which uses a TEWAC cooling system. The primary cooling circuit is in a closed-loop design. The cooling air at the generator outlet is cooled in a secondary cooling circuit. The coolers are mounted on one side of the stator frame.

The single-casing axial-exhaust steam turbine allows steam admissions from the front-end inlet casing. The turbine also consists of extension sections situated at the centre section, and the rear exhaust-end section. The main steam admission comprises a valve block of four control valves, two at the upper part and two at the lower part of the steam chest.

The turbine rotor, including the control wheel disk, is made out of a solid forging. Despite the large number of stages, the bearing spacing remains relatively short since each stage requires little space.

The condenser is a box-type, two-pass, undivided surface condenser. It is located behind the LP turbine and forms an integral part of it. The steam dome, shell and hotwell are all steel fabrications. The steam space is a rectangular cross section to achieve optimum use of the enclose volume for the necessary condensing surface.

Because the gas turbine runs at 5400 r/min, a reduction gearbox is installed between the gas turbine and generator which runs at 3600 r/min. This special HET gearbox reduces losses and thus contributes to increased plant efficiency.

The horizontal-flow, dual-pressure, non-reheat heat recovery steam generator is a natural-circulation drum type and generates steam in a high and low-pressure section. A No.2 fired duct burner system provides supplementary firing for additional steam generation.

Operating the plant

The V64.3A is at the heart of the power plant
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The whole plant is operated by the Teleperm XP, a state-of-the-art distributed digital control system (DCS). It ensures user-friendly, economic and reliable plant operation. It is open to coupling of automation systems supplied by other vendors and to extension of the plant with additional components or devices. The system is based on a hierarchical structure and programmable control system. This means that only compressed information has to be transferred to the next higher automation level. Intercommunication between the I&C components and external systems are realized by a bus system.

The single air-cooled generator feeds into the grid via bus ducts and the main transformer at 10.5 kV. The high-voltage side was selected at 110 kV for the facility, but would be designed to meet the requirements of a specific application.

Success in the market

Single shaft combined cycle power plants have quickly attracted the interest of the power plant market.

Six such plants from Siemens are now in commercial operation including Tapada do Outeiro, Portugal, 3×330 MW, which has clocked more than 70 000 EOH with an availability of more than 95 per cent; Otahuhu, New Zealand, 400 MW with an efficiency of 57 per cent; and St. Francis 1, USA, 250 MW.

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