Ultra-supercritical steam power plants meet the requirements for high efficiency that reduces both fuel costs and emissions, as well as providing a reliable supply of electric energy at low cost. This article examines how Siemens is applying its ultra-supercritical steam power plant technology to the 800 MW Trianel coal fired power project in Lünen, Germany.

Dr. Frank Cziesla & Andreas Senzel, Siemens,

Dr. Jürgen Bewerunge, Trianel Kohlekraftwerk Lünen GmbH & Co.KG, Germany

In 1999, several German municipal utilities joined forces to set up Trianel, a company that would supply electricity to municipal energy providers at low cost. Today, Trianel is the largest and most powerful platform for independent European municipal utilities.

Siemens’ SST 6000 series steam turbine, to be installed at Lünen has a power range of between 600-1200 MW
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In the autumn of 2005, Trianel announced its commitment to build an advanced coal fired steam power plant in Lünen, 15 km north of Dortmund in North-Rhine Westphalia, Germany. In September 2007, an engineering, procurement and construction (EPC) contract was signed with a consortium of Siemens Energy Sector and Trianel Boiler Consortium Lünen (TBCL). Construction of the power station is currently under way and progress can be followed via a webcam linked to the webpage of Trianel. Commissioning of the power plant is scheduled for November 2012.

The power plant. which will operate in baseload mode, is located on a greenfield site just outside Lünen. It will use international low sulphur bituminous coal delivered by river barges on the ‘Datteln-Hamm-Kanal’. An estimated €1.4 billion ($2 billion) is needed to cover the total capital requirements of the power plant and its associated infrastructure.

Siemens, as the EPC consortium leader, is responsible for overall planning, supply of the steam turbine generator set, the mechanical and electrical equipment, including the entire instrumentation and control system, the transformers and switchgear, as well as various auxiliary and supporting systems. Siemens is also responsible for construction, installation and commissioning.

Trianel Boiler Consortium Lünen, consisting of IHI Corporation and AE&E, supplies the once-through type steam generator, the air quality control equipment (electrostatic precipitators, SCR reactor including the ammonia supply system, and flue gas desulphurization unit), the coal feed and ash removal system, and the auxiliary boiler. Applying proven state-of-the-art technology while striving for cost-optimal efficiencies are key customer requirements in any new power plant project.

With this in mind, Siemens steam plant SPP5-6000 (1 x 800) is designed to meet these challenges with today’s technology. Trianel’s hard coal fired steam power plant in Lünen is the first application of this advanced 800 MW reference power plant design with ultra-supercriticalsteam parameters.

Protection of the Environment

The Lünen power plant will meet the most stringent environmental protection requirements of the German authorities and will have among the lowest environmental emissions of any coal fired power plant in the country. The flue gas cleaning system includes equipment for removing nitrogen oxides (SCR reactor), particulates (electrostatic precipitators) and sulphurous components (flue gas desulphurization unit).

Typical emissions limits are as follows: sulphure dioxide (SOx) 200 mg/Nm3; NOx 200 mg/Nm3; crbon monoxide (CO) <200 mg/Nm3 and particulates 20 mg/Nm3. Due to the high efficiency of the overall plant, specific carbon dioxide (CO2) emissions are well below 800g/kWh. Completely enclosed conveyor belts will supply the fuel from the ship unloading station to the closed coal silos and next to the coal bunkers of the steam generator. Such a complex coal handling system avoids emissions of respirable dust to a large extent.

Clean flue gas is supplied to the natural-draft wet cooling tower. Large amounts of moisturized air in the cooling tower ensure that the emissions are highly diluted before they are rejected to the environment. Noise emissions measured above background levels at a distance of 0.5 km from the power plant site will be no greater than 60 dB(A) during the day and 45 db(A) during the night, whereby the proportionate noise rating level generated exclusively by the power plant has to be 6 dB (A) lower at these immission points.

Pioneering steam turbine plant

Lünen is the first project based almost exclusively on Siemens SSP5-6000 (1×800) reference power plant for advanced steam power plants in the 800 MW class. Since the early 1990s Siemens has been working on reference power plant concepts both for steam power plants and combined-cycle power plants. Reducing investment costs by making use of modular pre-engineered reference power plant designs and at the same time providing sufficient flexibility to accommodate specific needs arising from customer requirements are major driving forces for all these development efforts.

The main focus of the SPP5-6000 (1×800) is the turbine building where all mechanical components of the water steam cycle, as well as all electrical equipments are optimized around the steam turbine generator set. The design is based on materials and technology that are available today and have proven reliability in use. Only a few modifications were required to adapt the SPP5-6000 (1x800TI) reference power plant design to the specific customer needs.

Lünen is a good example of the SPP5-6000 (1×800) concept in practice. Its turbine building with the turbine generator at a floor level of 17 m with no floor below the basement and with overall dimensions of 91x38x41 m shows a clear affinity with the reference power plant design. Lünen is also adopting the heater bay concept with the main components of the high and low pressure feedwater preheating line arranged within an annexe of the turbine hall. The annexe is located between the turbine building and the boiler island.

General layout planning attached particular importance to a compact and economic design. The arrangement of the steam turbine and the boiler results in short steam lines and a short electrical run to the switchyard. The side arrangement of the cooling tower in relation to the electrostatic precipitator allows efficient routing of the flue-gas exhaust system through the cooling tower, while at the same time optimizing the circulating water system.

The compact cost-effective plant design in the turbine building also allows for good accessibility during maintenance. Header-type high-pressure feedwater heaters and the separate desuperheater are located in front of the high-pressure steam turbine. The main components of the feedwater system including the feedwater tank, boiler feed pumps and low-pressure feedwater heaters are placed in the heater bay, which forms an integral part of the main structure. The central switchgear building is nearby the turbine building and accommodates the central control room.

Turbine generator

For the 50 Hz market, Siemens offers full speed tandem compound turbo-sets for steam power plants with ultra-supercritical steam parameters in the gross power output range of 600 MW to 1200 MW per unit. The steam turbine set SST5-6000 used at Lünen is a four-casing design with separate HP, IP, and two LP turbines. It is installed on the turbine floor on a spring-mounted foundation decoupled from the overall structure.

A push rod concept permits parallel axial thermal expansion of the LP rotor and inner casing. This reduces clearances between rotor and casing and improves the efficiency.

A conservative approach was chosen for the maximum steam temperatures to ensure a high availability and to improve the economic lifetime of the power plant. Raising the reheat temperature from 610 °C to 620 °C slightly improves the heat rate. However, the additional costs due to the increased material thickness outweigh this effect. High parameter values for the main steam (270 bar, ~600 °C) and reheat steam (60 bar, ~610 °C) at the turbine inlet pose special requirements on both the design and the materials.

For example, the HP cylinder is designed as a barrel-type turbine and has an inner casing. Ultra-supercritical steam conditions usually require the use of thick-walled components. The rate of heat transfer into these components is often the limiting factor for the duration of the start-up process.

In order to remove this restriction, a special feature has been developed for HP turbine modules: an internal bypass cooling system that allows for a more flexible operation (start-up/load changes). In a nutshell, a small amount of cooling steam passes through radial bores into the small annulus between the inner and outer HP casing.

This approach effectively protects the inner surface of the outer casing, which would be exposed to main steam temperature without the internal bypass cooling. As a consequence it was possible to reduce the wall-thickness of the outer casing and thus enable faster heat-up of the casing. An improved starting performance is the main customer benefit of this innovative concept.

Components exposed to high temperatures such as the HP inlet barrel as well as HP/IP rotors and inner casings are made of 9-12 per cent CrMoV steel. Siemens advanced 3DV technology (3-dimensional design with variable reaction levels) for HP/IP blades is used in Lünen’s steam turbine generator set.

With 3DV blades the stage reaction and stage loading for each row is optimized to gain highest HP and IP efficiencies. Stage reaction describes the split of pressure drop and velocity increase between stationary and moving blades, and is defined by the ratio of the enthalpy drop through the moving blade row to the enthalpy drop through the whole stage.

Both low-pressure turbines are double-flow designs. Free-standing 1150 mm steel last stage blades provide an annular area of 12.5 m2 per flow. Steam parameters have increased only slightly over recent years but gross power output capacity has increased considerably compared to the first large-scale ultra-supercritical application in Isogo, Japan. Chinese power suppliers favour higher electrical outputs, European customers very often consider 800 MW an optimum unit size. The generator in Lünen is an SGen5-3000W series (two-pole), directly coupled to the turbine. It has direct water-cooled stator windings, a hydrogen-cooled rotor, static excitation, a two-channel digital voltage regulator and the necessary auxiliary systems (i.e. seal oil, hydrogen- and water units).

High-energy piping costs have a significant share in the total capital expenditures. Main steam and hot reheat piping is made of P92 (X10CrWMoVNb9-2) each with four lines at the steam generator outlet that are already merged to two lines in the boiler island. The cold reheat piping (16MO3) consists of a single line at the turbine outlet that is split to two lines at the boiler inlet. Feedwater piping is a single line made of WB36.

Steam generator

The once-through steam generator manufactured by IHI has a tower design. Key features include: low NOx dual flow wide range burner, control of the reheat steam temperature by a parallel pass damper, a Ljungström-type air preheater, and dry bottom ash removal. About 600 kg/s ultra-supercritical main steam (280 bar/600 °C at boiler outlet) are generated. At design conditions, more than 94 per cent of the coal energy (LHV basis) is transferred to the water/steam cycle.

With regard to the steam generator, the overall plant efficiency is improved by optimizing the heating surface arrangement, raising the final feedwater temperature to 308 °C, keeping the excess air coefficient in the firing system less than 1.2, controlling the reheater outlet temperature without water injection, reducing the exhaust-gas temperature downstream of the air preheater to 120 °C, and minimizing pressure drops.

How the Trianel 800 MW coal fired plant at Lünen has been structured
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The boiler can be operated in once-through mode in the load range between approximately 35 per cent to 100 per cent. Under design conditions, heat transfer to the water/steam cycle reduces the temperature of the flue gases to approximately 350 °C at the outlet of the economizer. Preheating the combustion air decreases the temperature of the exhaust gases even further before they are supplied to the electrostatic precipitators. It should be noted that the values of the process parameters and the boiler efficiency depend on the coal being burnt.

The evaporator in the furnace consists of a spiral pass with smooth tubes and vertical water walls in the upper furnace section. Opposed firing is arranged on four burner levels with low NOx wide range burners. Flame characteristics can be adjusted with respect to the boiler design, load and coal quality. This improves flexibility and enables the operation from 60 per cent to 100 per cent load (at design conditions) without mill start/stop.

A special feature of the boiler design is the control of the reheater outlet temperature without spray water injection in normal operation.

This is achieved by a parallel design of reheater 1 and superheater 1/economizer 2 and the use of gas dampers in the upper convective part of the steam generator.

Water/steam cycle

Important features of the water/steam cycle include frequency control through condensate throttling, condensate polishing in bypass loop with a separate 1 x 100 per cent condensate polishing pump, and a steam bypass system including a 4 x 25 per cent HP bypass station with safety function and a 2 x 30 per cent LP bypass station.

For the given ambient conditions at the power plant site (+9 °C, 80 per cent relative humidity, 18 °C cooling water temperature) the plant concept is designed for a net efficiency of 45.6 per cent based on the lower heating value. Key levers for improving Lünen’s overall plant efficiency have already been addressed in the article: high-steam parameters, optimized processes, and highly efficient energy conversion in key plant components.

Optimizing the cold end of the water/steam cycle also shows some potential for improvement. Once again this is a trade-off between capital expenditures and fuel costs (efficiency), and needs to be evaluated for the given boundary conditions. At Lünen, the cooling water flows in series through the condensers of the two LP turbines. In a parallel configuration the cooling water mass flow rate is equally split between the two condensers. Both LP turbines expand to the same condenser pressure since the temperature profiles in the condensers are identical.

In the case of a serial arrangement, the total cooling water mass flow rate will pass through each condenser, so different condenser pressures are achieved. Assuming equal mass flow rates of for the cooling water in both configurations, the average condenser pressure in the serial arrangement is lower. In general, the lower the condenser pressure, the higher the efficiency of the overall plant. Exhaust losses of the LP steam turbines for given last stage blades, pressure losses on the cooling water side and investment costs of a larger heat transfer area in the condensers need to be carefully evaluated before a decision for a serial condenser configuration shall be taken. For Lünen, it turned out to be the most economic solution.

The authors would like to thank their colleagues at Siemens Energy Sector, Trianel Kohlekraftwerk Lünen and Trianel Boiler Consortium Lünen for their suppport in the preparation of this article.