The Cottam plant, which was first fired on March 1, 1999, will allow advanced equipment to be put through prolonged trials under commercial power plant operating conditions. The V94.3A gas turbine based combined cycle plant has been connected to the grid since April 1999.

The plant is operated by Cottam Development Centre Ltd, a joint venture between PowerGen and Siemens Project Ventures, in which both companies own equal shares.

High R&D costs for complex power plant components and shorter product cycles with decreasing market prices are placing new demands on development of plant technology. To ensure that risks to manufacturers and plant operators are reduced, advanced power plant equipment has to be one-hundred per cent market-ready when orders are placed.

Siemens operates a gas turbine test facility at its development and manufacturing centre in Berlin, but turbines can only be tested in such facilities for a limited number of hours. Possible creep problems, which only become evident after several thousand operating hours, therefore mostly remain undetected.

Through evolutionary advances in the design of the major components – gas turbine, steam turbine and heat-recovery steam generator – and further development of the combined cycle process, the objective is to use the experience gained at Cottam to raise plant efficiency to over 60 per cent, while at the same time reducing the plant’s power generating costs.

Gas turbine development

In order to rise above the 60 per cent efficiency mark, it is necessary to achieve higher exhaust gas temperatures and exhaust gas flows. To do this, the entire combined-cycle process as well as the key components – the gas and steam turbines and the heat-recovery steam generator – must be further improved. By the end of 2000, sufficient meaningful data will have been collected to enable all of these components to be further enhanced.

The most effective starting point for raising the efficiency of a combined-cycle power plant is the gas turbine, since it accounts for around two-thirds of the entire plant output. The first step is to install an advanced version of the Siemens Model V94.3A gas turbine. This gas turbine has a 15 stage compressor with an approximately 5 per cent higher intake air flow and a slightly higher pressure ratio than the 16.4:1 of the previous V94.3A design.

Based on the .3 Series designed experience the .3A Series gas turbine technology was introduced in several steps.

The compressor of the introductory V84.3A gas turbine featured the advanced flow path design with 15 compressor stages but it still had a welded outer casing design with four adjustable guide vane rows. The introductory V94.3A gas turbine featured the original 17-stage compressor but a cast casing design with one adjustable guide vane row.

The final design of units for both 60 and 50 Hz applications will feature the most advanced 15 stage compressor design with the cast casing with only the first compressor guide vane row being adjustable. The compressor casing has been designed as a three dimensional system which provides not only stiffness to carry static and dynamic loadings, but also expands in a fully concentric manner without adversely affecting the operating clearance in axial and radial directions. Blade carriers with external clearance adjustment are utilized for the latter section of compressor blading and for the turbine section to handle high temperatures and still keep minimum clearances.

The turbine section of the introductory V84.3A gas turbine featured the most advanced blade design in all four turbine stages as will the advanced V94.3A design. The introductory V94.3A units did, however, have the most advanced blade design in the first two stages.

Real-life testing

The early testing of the V84.3A gas turbine on Siemens’ full-load test bed in Berlin with two horizontally arranged cylindrical combustors was mainly done to compare the original design with the improved compressor and turbine flow path design under exactly the same boundary conditions. The later test phases of the V84.3A turbine were performed with the advanced hybrid burner ring combustor.

All the tests met or exceeded expectations and only minor modifications of the final design of the .3A Series gas turbines were required. The final design of Siemens V84.3A and V94.3A gas turbines will feature equal technology with scaled components and dimensions for the compressor and turbine flow path design as well as the hybrid burner ring combustor.

In the annular combustion chamber with its 24 hybrid burners for diffusion and premix combustion of natural gas and distillate, uncooled, all-ceramic combustion-chamber tiles are used instead of the ceramic-coated metallic heat shields. These tiles need less air for cooling, which in turn makes more combustion air available for the hybrid burners, which produce less thermal NOx as a result. Potential thus exists for increasing the turbine inlet temperature.

The first and second stage turbine blades have a ceramic thermal barrier coating which lowers the temperature of the underlying metal blade surface, thus also providing potential for increasing the turbine inlet temperature.

In order to exceed the 60 per cent efficiency mark, it is necessary to optimally utilize the higher exhaust-gas temperatures and exhaust-gas mass flow rates of advanced gas turbines. To achieve this, the cycle as well as the key components – the heat-recovery steam generator and the steam turbine – have to be further improved.

Single-shaft combined-cycle

The Cottam combined cycle plant is based on the single-shaft, combined-cycle concept. The power output will be substantially higher than the 350 MWe rating for the introductory 1S.94.3A units of the kind used in the Tapada do Otera three unit, 1000 MWe, plant now operating in Portugal.

The gas turbine, generator and steam turbine are coupled together on a single shaft and form a modular, standardized system. The single-shaft concept is characterized by the simplification and reduction in the number of components of auxiliary plant systems, low plant complexity and resulting simplification of open and closed loop control functions.

Plant complexity was further reduced by eliminating redundancies of pumps and auxiliary and ancillary systems, and this is not expected to adversely affect plant availability, but installation space, auxiliary power requirements and investment costs are substantially reduced. The steam cycle can also be simplified and improved using this layout.

By using improved materials the main HRSG steam temperature can be further increased. With future exhaust-gas temperatures expected to exceed 600°C, compared with the present 587°C, an increase in the main steam pressure is also reasonable.

The high-pressure section of the water/steam cycle was designed for an operating pressure of up to 160 bar so that better use is made of the high temperature heat due to the lower evaporation enthalpy resulting from the higher boiling temperature. If volumetric flow rates remain sufficiently high, no decrease in expansion efficiency is expected.

A new HRSG concept

A triple-pressure reheat steam cycle with fuel preheating was chosen as optimum solution. The increase in efficiency gained by further increasing the number of evaporator stages in the heat-recovery steam generator drops off as the gas-turbine exhaust-gas temperature increases, making it no longer economical.

The horizontal triple-pressure heat-recovery steam generator supplied by Babcock Borsig uses a new concept combining a natural-circulation system for the low-pressure stage with a forced-circulation system for the intermediate and high pressure stages.

The design of the forced-circulation system is similar to that of the natural-circulation system, but does not require a drum. In terms of flow dynamics, the evaporators are designed for low pressure losses so that the mass flow rate in the series-connected tube bundles adapts itself to the amount of heat provided by the exhaust-gas mass flow.

The advantage of this arrangement is that despite the decreasing amount of heat in the latter rows of tubes, the differences in steam content and temperature at the outlets of the tube bundles remain very small. The new boiler exhibits improved dynamic behaviour, short start-up times and great potential for future technical development.

Steam turbine improvements

The currently installed steam turbine is designed for main steam conditions of up to 160 bar/600°C. The turbine is able to withstand the stresses produced by the higher pressures and temperatures primarily because 10 per cent chromium steel is used, but also through the incorporation of targeted design features in the inlet areas of the turbine cylinders.

The single-flow high-pressure turbine cylinder has a service-proven barrel-type casing. The rotationally symmetrical construction without interfering mass concentrations caused by axial flanges results in a highly compact design. The advantage of this concept over jointed casings becomes more significant as steam conditions increase.

The combined intermediate-/low-pressure turbine cylinder is designed as a reverse-flow turbine with a split casing. The use of a new IP/LP turbine cylinder with welded rotor, straight-through flow path and longer titanium blades for the last stage is planned.

Operating regime

In the first phase beginning in mid-1999, the power plant is expected to begin producing electrical power commercially under grid guidance. Efforts to increase the gas turbine inlet temperature and to optimize the combustion process will continue in accordance with a well-defined schedule. New maintenance concepts will be tested, and the performance of the new components will be observed and further improved during the operation of the plant.

By the end of the year 2000, enough meaningful data should have been gathered to allow further increases in efficiency and power output to be achieved. This will be accomplished by replacing the gas turbine, the generator and the combined intermediate-/low-pressure steam turbine cylinder during an upgrade phase lasting several months. The details of these upgrades are not yet known but it is certain that Siemens will be making use of know-how gained through its recent acquisition of Westinghouse.