|The assembly of an Alstom high pressure turbine, which features a distinctive shrink ring design Source: Alstom|
Thomas Olböter, Alstom Power, China
In 2007, the European Union agreed to cut CO2 emissions by 20 per cent until 2020, compared to the emissions level in 1990. In Germany the commitment is even more ambitious, with a 40 per cent reduction in the same time frame.
Combined heat and power generation is one of the key features that will help to fulfil these commitments. European metropolitan areas have a huge demand for electricity and heat for which combined heat and power generation is particularly suited. In recent years, Alstom has received orders/awards for eight ultra-supercritical (USC) steam turbines for district heating applications.
|Alstom’s Gigatop 2-pole turbogenerator is a hydrogen- and water-cooled turbogenerator designed to deliver up to 1400 MVA and to reach up to 98.96 per cent efficiency Source: Alstom|
One of the largest German district heating systems currently serves major parts of the cities of Mannheim and Heidelberg. The system will be further extended with the addition of Grosskraftwerk Mannheim’s (GKM) Unit 9. Due to the capacity increase provided by Unit 9, which will provide 911 MW and up to 500 MW heating output, the district heating grid will also serve the city of Speyer, located 30 km south of the GKM site.
By 2013, GKM will supply 25 per cent of the electrical needs of the region, serving a population of 2.4 million. Unit 9 will save around 1 million tons of CO2 per year due to its higher efficiency than the installations it replaces. The steam turbines will work with state-of-the-art USC steam parameters, i.e. main steam pressure at boiler outlet of 28.5 MPa, main steam temperature of 597 °C, and reheat temperature of 609 °C at the turbine inlets.
Although the power plant is designed for combined heat and power generation, no compromise was made in efficiency. In pure condensing mode it will deliver a plant net efficiency of more than 46.4 per cent (based on LHV), hence, becoming one of the most efficient coal fired power plants worldwide. In district heating mode the heat utilization rate reaches up to 70 per cent.
The compact STF100 five-casing USC steam turbine, to be installed in the GKM power plant, is equipped with Alstom’s largest steel last stage blade, at 45 inches. The STF100, Alstom’s largest steam turbine frame, together with the Gigatop 2-pole generator can generate a power output up to 1000 MW for 60 Hz or up to 1200 MW for 50 Hz applications. This turbine generator set contributes to cost efficient electricity generation while saving resources and minimizing environmental impact.
Increasing steam cycle parameters
Improving the steam cycle by increasing steam parameters is one of the most effective ways to enhance the efficiency of modern coal fired power plants. By raising the steam temperature, the plant net efficiency can be increased by approximately 0.25 per cent per 10 K (relative) for the main steam and approximately 0.2 per cent per 10 K (relative) for the reheat steam.
Steam temperatures established towards the end of the 1990s are 565 °C for the main steam and 585 °C for the reheat steam. Since the beginning of the 21st century, these steam temperatures have been further significantly raised due to the strongly increased pressure to conserve both resources and the environment.
The state-of-the-art GKM Unit 9 steam turbine will operate with a main steam temperature of 597 °C and pressure of 285 bar, and a reheat steam temperature of 610 °C, i.e. in the upper range of the steam conditions commonly referred to as ultra-supercritical.
Advanced steam cycle design
In addition to the use of advanced steam parameters, enhancing the design of the water-steam cycle can further increase the plant net efficiency. In order to expand the steam as much as possible, the condenser pressure is minimized.
In the case of GKM Unit 9, water from the River Rhine will be used to directly cool the condenser, which will have a relatively low temperature difference of 2.5 K. The temperature of the cooling water will rise by around 7.8 K.
With an average cooling water inlet temperature of 14.1 °C, the condenser pressure is about 31 mbar. A ten-stage preheater line preheats the feedwater up to a final temperature of 308 °C. The preheater consists of five LP (low pressure) heaters, a de-aerator, and three HP (high pressure) heaters.
Steam from the IP (intermediate pressure) turbine feeds an additional separated de-superheater on top of the HP heaters. All of these features help to achieve a net efficiency in pure condensing mode of more than 46 per cent.
Another major contribution to reducing CO2 emissions is the high capacity of this power plant for district heating. Compared to a conventional decentralized domestic gas fired heating system, district heating helps to reduce the usage of conventional fuels like coal, gas or oil by around 50 per cent. Through use of heat from the steam cycle, preferably at low temperatures, the fuel utilization rate of the power plant can be increased significantly.
The Mannheim 9 power plant is able to deliver a huge amount of heat for district heating. To allow a very flexible and effective steam extraction, the power plant is equipped with a three-stage district heating line and a topping heater. Two heaters are connected to the LP turbines and the third heater is fed by steam from the IP exhaust.
The topping heater is connected to the cold reheat line. The focus is on the range of 100 MJ/s to 300 MJ/s heat with outlet temperatures of 100 °C and 120 °C, respectively. In the first case, only heater 1 and 2 are used.
Steam is extracted from the LP turbines at a pressure level close to the corresponding district heating water temperature. This leads to a highly efficient heat extraction. For part load operation, larger heat extractions or higher water outlet temperatures, the steam extractions are shifted to higher pressure levels. In the case of 300 MJ/s heat extraction, heater 3 is also in operation.
Compact layout and routing of the steam extraction piping, which contributes to reducing the specific investment cost of the overall power plant Source: Alstom
For 300 MJ/s heat extraction, the pressure at the IP exhaust is still high enough to ensure, also in part load operation, the outlet temperature of 120 °C. The maximum heat extraction of 500 MJ/s with 130 °C outlet temperature is done with the topping heater in operation.
To reduce the pressure losses from the steam path to the extraction nozzles, as well to increase the steam extraction flow to the preheating and district heater line, the LP extractions feature more nozzles than a standard turbine without district heating. Furthermore, the size of the slots, which transfer the steam out of the steam path, have been increased.
The design of the Alstom LP turbines, with their cast inner casing, provides the flexibility to adjust these slots and front stages according to the district heating demand. In this way, there is nearly no impact on the performance in a pure condensing mode, which enables the plant to be operated over a very wide range of conditions, according to the requirements of the district heating or power demand.
Raising Steam Turbine Performance
In parallel to efficiency improvements through advanced steam cycle design and optimization according to the requirements of district heating, Alstom has put significant efforts into increasing the efficiency of its steam turbines. New features have been developed from proven elements in a step-by-step evolutionary development process.
Two of the key features to emerge from recent development activities that have been applied to the Mannheim 9 steam turbine are an advanced 3D blade and optimized steam path design and an advanced low pressure turbine with a 45-inch steel last stage blade.
Alstom uses reaction type blading with 3D state-of-the-art profiles. The HP and IP turbine blading is specifically adapted to each application. The blades are milled in one piece from bar material with integrated shrouds and roots. They are assembled pre-twisted to form a clearance free ring with excellent vibration behaviour.
This blade design has been further improved in recent years, particularly by minimizing secondary losses, improving the aerodynamics at the trailing edge, and reducing the gap and leakage flow interaction with the main steam flow. The stage efficiency gain accumulates to approximately 0.5–0.9 per cent, mainly dependent on the aspect ratio. All these measures require high manufacturing and quality standards.
The last stage blade (LSB) is of essential importance for the overall turbine efficiency. In addition to the efficiency of the LSB itself, the exhaust area plays a central role because exhaust losses with a low pressure turbine are much higher than with high and intermediate pressure turbines, and cannot be recovered.
Therefore, the availability of a wide range of LSBs with different exhaust areas is highly beneficial. It allows the optimization of the cold end design with regard to both efficiency and economics. Using a larger exhaust area can reduce the exhaust losses (improved efficiency) or eliminate the need for an extra LP casing (optimized cost).
Cold end optimization
The cold end optimization for the power plant Mannheim 9, taking into account different load cases with and without extraction for district heating, led to the selection of a six-flow configuration using Alstom’s longest steel LSB. This 45 inch blade provides an exhaust area of 10.7 m² per flow.
The six-flow configuration together with the large LSB leads to an exceptionally large overall exhaust area, which allows maximum benefit from the available cooling conditions. The LSB’s stage efficiency is far higher than that of the former freestanding blades with a length up to 43 inches.
In addition, these LSBs are equipped with shrouds to reduce the leakage flow over the tip compared with freestanding blades. At the same time, the shrouds, together with snubbers, form a rigid and robust blade ring with excellent vibration behaviour.
Several measures protect the blade against water droplet erosion. The last stationary row is of a diaphragm design, built up by hollow vanes with slots. The vanes are welded into inner and outer half rings with integral channels to connect the slots with the condenser.
The pressure difference between stage and condenser induces a steam flow through the slots and the hollow system. The water film or droplets at the vane surface will be sucked out through the slots by means of this driving steam flow. This system is carefully designed to achieve the optimal water removal effect with minimal steam losses. In addition, the distance between the stationary vane outlet and LSB inlet is raised with increasing diameter.
The remaining water droplets can accelerate with and follow the steam flow. The energy when hitting the leading edge of the last runner blade is reduced to a minimum. Centrifugal forces cause large water droplets to travel towards the inner casing wall, where they are collected and then removed by means of a circumferential cavity in the casing wall.
The combination of these three measures results in a highly effective system that is able to almost completely negate the risk of water droplet erosion. Nevertheless, Alstom applies an additional measure to provide a further degree of safety. A kind of nose at the leading edge of the blades, close to the shrouds, on the one hand protects the contact surface of the shrouds against possible droplets and on the other hand provides a form of sacrificial material in order to avoid possible mechanical risks due to water droplet erosion.
Following the latest LSB design, the diffuser and the exhaust have also been redesigned, in order to achieve highest efficiencies over a wide operating range. Naturally, the new low pressure turbines, based on this LSB, are also equipped with the well known 360° inlet scroll with a radial axial first stage.
Mannheim 9 Steam Turbine Configuration
The Mannheim 9 STF100 steam turbine is configured from Alstom’s modular steam turbine portfolio. The steam path in the high pressure and intermediate pressure turbine module is specifically adapted and optimized to the Mannheim 9 application.
Thanks to the modular concept, the materials of the turbine parts such as the inner and outer casing can be easily adapted to match the steam cycle parameters. This specific material selection is also possible for the rotor sections because all large HP, IP and LP rotors are of a welded design.
The unique shrink ring design of the Alstom HP turbine was introduced in the 1960s. It eliminates bolt flanges on the inner casing and results in a radially symmetrical structure with superior thermo elasticity. Therefore, the shrink ring design almost completely eliminates casing distortions during operation, which is of essential importance for USC applications.
The benefits are long-term stable clearances and sustained efficiencies combined with long-term reliability and operational flexibility. The steam extraction for the top heater is maintained by slots in the inner casing and a simple and robust shrunk-on extraction chamber located between two shrink rings. The axial position of these slots is determined via the targeted final feed water temperature.
Due to the double shell design, only the outer casing is exposed to the exhaust steam, which allows relatively small flanges at the outer casing. A preheating of the casings prior to a startup is not required. Generally, Alstom’s reheat turbines are designed so the casings do not limit thermal transients.
Design features of the IP turbine
The lower pressure level of the intermediate pressure turbine allows the inner and outer casing to be horizontally split via conventional flanges. However, standard double shell designs would lead to large casing distortions as a result of the increased steam temperatures experienced in modern ultra-supercritical steam cycles.
Alstom has conducted intensive research and development to reduce the possible thermal casing distortion, in particular in the inlet sections where the inlet scrolls cause additional asymmetries. Alstom’s largest IP turbine module, as used in Mannheim 9 and all other large Alstom USC units, features a triple shell design.
The inlet section with the inlet scrolls and some first stages form the inner casing, which supports the blade carriers of the other steam path sections. The pressure drop over the blade carriers is comparably small and, therefore, their flanges can also be kept small. The result is an overall more rotationally symmetrical design, which almost completely eliminates thermal distortions. The benefits are stable clearances and sustained efficiencies.
Thanks to the inlet scroll design, with an integrated radial first stationary blade row, and the welded rotor design, a secondary cooling of the steam with its negative impact on the efficiency, is not required, not even for the highest reheat temperatures used today.
COMPACT TURBINE LAYOUT
A compact turbine generator layout contributes to reducing the specific investment cost of the overall power plant. The large exhaust areas provided by the three double flow LP turbines equipped with 45 inch last stage blades enable the compact five-casing layout of the STF100 steam turbines for the power plant GKM with 1 x 911 MW power output.
Alstom’s compact turbine casings and the single bearing concept lead to the very compact layout of the steam turbines, even though they deliver steam extractions to provide a maximum of 500 MJ/s of heat to the district heating system of the city of Mannheim.
This high amount of steam extraction can be realized through optimization of the extraction bleed path of the LP turbines, thus enabling the maximum district heating requirements to be met, even if one steam turbine is out of operation.
Gigatop 2-pole turbogenerator
Gigatop 2-pole is Alstom’s hydrogen- and water-cooled turbogenerator product line that can deliver up to 1400 MVA and reach up to 98.96 per cent efficiency. Its high reliability in operation has been proven – for example, a unit in the United States achieved 607 days of uninterrupted operation before a schedule shutdown.
To optimize the maintenance and operating costs and increase the product‘s availability and performances, it is equipped with several technologies and low maintenance features.
The water-cooling system in the stator winding is designed to provide optimum reliability. De-ionized water flows through the stainless-steel cooling tubes to remove the heat dissipated by the stator winding. The corrosion resistance of stainless steel ensures that the cooling tubes do not clog, the losses are minimized and efficiency enhanced.
The tubes themselves are welded to water clips located at both ends of each bar, next to the brazed lugs. The electrical and cooling circuits are therefore kept separate, further improving reliability. Over the past 30 years, Alstom’s stainless-steel tube technology has demonstrated excellent reliability in operation.
The Gigatop 2-pole turbogenerator is equipped with a stator end-winding support system. Its unique design reduces the maintenance effort and increases the Gigatop 2-pole’s availability. Axially flexible to allow thermal expansion, it is rigid in the radial and tangential directions to withstand high electromagnetic forces. To facilitate maintenance, the end-winding has been designed so that it can be easily re-tightened during regular maintenance, which accelerates maintenance operations.
Safety has been given top priority for the Gigatop 2-pole, with a triple-circuit hydrogen sealing system instead of a double-circuit system. The resulting very low hydrogen consumption also helps to reduce operational costs and keeps the hydrogen at very high purity levels. The Gigatop 2-pole’s efficiency is thereby sustained at a high level over the long term.
Performance with reliability and integrity
In addition to increasing the steam cycle parameters for such USC power plants, Alstom has improved the steam turbine performance without compromising reliability and mechanical integrity. Advanced 3D blade and steam path design and advanced top performing last stage blades with optimized diffuser and exhaust contribute to the increased efficiency and support the cost saving compact turbine layout at the same time.
To cover the higher steam cycle parameters Alstom adopted proven design principles. The adaptation has mainly been achieved through appropriate material selection. Alstom’s design features, like the shrink ring construction of the high pressure inner casing and the welded rotor design, ensure high operational flexibility also under USC conditions. The inner shell design of the intermediate pressure turbine has been optimized in order to minimize thermal distortions and to ensure long-term stable clearances and thus sustained efficiencies.
Combined heat and power generation can further increase the fuel utilization rate. As a result, the specific CO2 emissions of the power plant are reduced, because not only electricity but also useful heat is ‘produced’, whereas waste heat is minimized. With regard to extraction flexibility, the GKM Mannheim Unit 9 power plant is an excellent example of what can be achieved. Due to the flexible design of the steam path and the steam extractions, and the use of four district heaters, the steam turbine is able to provide up to 500 MJ/s of heat for district heating, at maximum electrical load.
The Mannheim 9 power plant will achieve a net efficiency of more than 46 per cent in condensing mode. Therefore, the specific CO2 emission of this combined heat and power plant will be up to 38 per cent lower than the worldwide average and more than 23 per cent lower than the average of existing fossil fired units in Germany.
This article was co-authored by Alexander Tremmel and Christoph Brandt of Alstom Power, Germany. Andy Birrell of Alstom Power, Switzerland.
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