Following successful commissioning, Poland’s Patnow II power station has been handed over for commercial operation. With a nominal power output of 464 MW, the plant is now described as the largest, most efficient and cleanest brown coal fired unit in the country. The supercritical steam power plant has an electrical efficiency of 42 per cent, making it one of the most competitive plants in the liberalized Polish power market.

The Patnow, Adamow, Konin power complex, located some 200 km west of Warsaw, is an important group of power stations in the Polish power market. Patnow provides around ten per cent of generation and is located near to major cities such as Poznan, Plock and Wloclawek.

The Patnow, Adamow, Konin power complex by night, with the new Patnow II unit in the foreground
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Over seven years ago, the new owner of the power complex took the decision to modernize the Patnow plant by replacing two of the old, subcritical 200 MW units with a state-of-the-art supercritical unit known as Patnow II. Finally, after a chequered period of development, the plant has finally been commissioned to become Poland’s most efficient brown coal fired power plant.

Supercritical design

Alstom provided the main technology for Patnow II power station, delivering on a turnkey basis, the turbine island with a state-of-the-art STF40 steam turbine and generator. As consortium leader, Alstom also designed and manufactured the supercritical pulverized coal fired boiler. The boiler has a spiral wall, tower design and uses advanced once-through technology. The boiler is equipped with low-emissions firing system to reduce nitrogen oxides (NOx) levels to below current environmental requirements. This, combined with high plant efficiency, and therefore economic use of fuel, will result in lower carbon dioxide emissions compared to conventional coal fired plants.

The Patnow II unit burns lignite delivered from nearby mines. The coal is delivered to two crusher stations, which sieve and classify it before it is then transported by a belt conveyor to the bunker area. There are eight bunkers, one for each of the beater wheel mills. The lignite has a calorific value in the range of 7.6-9.8 MJ/kg, with a typical ash content of 9.5 per cent and a sulphur content of 0.8 per cent.

Although lignite has a high volatile content, it also has a high moisture content, so the fuel needs to be dried before being ignited. The drying takes place together with crushing in the beater wheel mills. The heat necessary for the drying process is provided by sucking the flue gas from the top of the furnace to the mills. This mixture of dried lignite, vapour and recirculated flue gas forms an inert atmosphere and is transported via short coal ducts to the burners.

The pulverized lignite is fed through ducts to the furnace of a once-through supercritical boiler. There is one burner for each mill giving a total of eight lignite burners. Light fuel oil is used for starting the boiler, which also has eight oil start-up burners. The boiler can operate up to about 35 per cent load on oil.

Overfire air is used to control NOx levels in the boiler. The original specification required NOx to be limited to less than 350 mg/Nm3. However, a subsequent request was made for NOx emissions to be kept below 200 mg/Nm3, so a second level of overfire air was required.

The furnace has a spiral wall design up to approximately 56 m, where it then changes to a vertical wall design. The heating surface contains three superheater sections, two reheat sections and an economizer. Flue gas leaving the boiler is fed via a duct to the regenerative air heater before flowing to an electrostatic precipitator.

The boiler, with its once-through design, has no control valve between the boiler and feedwater entry in the economizer. The feedwater pumps provide the required pressure and flow for the boiler. Steam generated from the boiler is fed to a turbine that operates in sliding pressure mode, with the pressure changing in a linear fashion with the steam mass flow.

Alstom’s state-of-the-art STF40 steam turbine contributes to Patnow II’s greater efficiency
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The steam turbine consists of four casings – one high pressure (HP), one intermediate pressure (IP) and two double low pressure (LP). The supercritical temperature and pressure require the use of higher quality materials in the boiler and steam turbine, so alloy steel components were used as opposed to carbon steel. Furthermore, the quality of welding also had to be much higher.

High efficiency & low emissions

The turbine island can provide more power than the power station is contracted to deliver to the grid. The nominal power between the power station and the Polish power grid is 464 MW. However, the power output at the generator at maximum continuous rating is 479 MW and the maximum power output with the valve wide open is 488 MW.

The plant demonstrates an efficiency of around 42 per cent. Compared to the old units, which had an efficiency of around 33 per cent, this will mean less fuel is burned to generate the same power, and thus will have a more positive impact on the environment.

Patnow II is said to be Poland’s largest, most efficient and most environmentally clean brown coal fired power station
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Emissions of sulphur oxides (SOx) and NOx are in line with European Union requirements: SOx (as SO2) and NOx (as NO2) emissions from the boiler are less than 200 mg/Nm3 (at six per cent O2). In the trial run which took place between 8 April 2008 and 8 May 2008, basic thermal analysis showed that Patnow II met all of its efficiency and all environmental requirements.

In addition, there is a two-year warranty period during which the owner can check any of these parameters. In the first half-year of operation, Patnow II together with Alstom will also make warranty parameter measurements.

Construction challenges

The new project essentially replaces two old units that were demolished at the beginning of 2001. The machine hall of the new building has a relatively lightweight construction, so the overhead crane is relatively small with a lifting capacity of 115 tonnes. This meant that one of the most challenging issue during erection was the placement of the generator stator, which weighs 300 tonnes.

Alstom contracted a specialist company to carry out the placement of the generator stator on the turbine foundation. It was the most complicated part of the erection phase, especially in terms of logistics. The stator was transported to the site by railway, which called for careful planning – containers capable of transporting stators of this size and weight are few and far between.

Delivery of the stator had to be well timed because the special container had to be quickly released. The whole task was made even more difficult because it took place during winter, when the conditions were less than ideal. Nevertheless the task was completed on schedule and without any major problems.

The turbine foundation was also part of the scope of supply. Erection of the turbine foundation was divided into two phases. There is a physical split in the foundation, with the lower table and columns forming one part, with springs between this and the upper turbine table.

The lower part of the foundation with the columns was erected first. The condenser, which weighs over 700 tonnes, was then built and placed on the lower foundation. This was then followed by the erection of the upper plate of the turbine foundation.

The site of the new plant also presented challenges. Patnow II is located close to a lake, so there is little space between the lake and the power plant. The proximity of the lake means the quality of the soil is not ideal, so a large number of piles had to be placed for the turbine and the boiler foundations. The plant also had to be built in the space left by the old units. This presented a challenge in arranging the equipment and piping in what was a fairly narrow space.

Project schedule

Originally, the project was scheduled to be executed in 36 months but a delay occurred because of changes to how the project was be financed.The project resumed in October 2005, with most of the key components such as the turbine, generator, turbo-pump, electrical feedwater pumps ready to be delivered in just a few months.

Before the equipment could be dispatched from Alstom’s factories, however, some minor maintenance work was required. All the major components were therefore delivered during the first quarter of 2006.

Despite not being the EPC contractor, Alstom, as plant integrator, was able to minimize delays. Alstom in Poland and in Germany, being the designer and supplier of the turbine and boiler, respectively, could effectively decide and agree on the interfaces between the boiler and turbine. During the guarantee period, it also gave the customer a single point of contact.

Successful Commissioning

The trial run began in April 2008 and lasted 720 hours, ending in May 2008. During this time a number of design parameters had to be demonstrated. There were approximately 28 tests requested by the plant owner’s advisor to demonstrate the reliability of the plant and the flexibility of operation. For example, the plant had to run for 14 days at nominal power at an availability of 80 per cent. It achieved an availability of more than 90 per cent.

With regard to operational capability, Alstom had to demonstrate load rejections – from full load to house load i.e. 460+ MW down to 20 MW, and from full load to idle where the turbine is kept running at nominal speed without house load.

Load ramps also had to be demonstrated where load had to be increased by increments of six per cent per minute. Meeting potential load ramps, as may be required by the grid can present a challenge for such a large boiler but the plant successfully achieved the required load profiles.

With all tests successfully completed, Patnow II was taken over by the customer at the end of June 2008. Although a long time in the making, Patnow II is now amongst the country’s most efficient brown coal fired plants – a huge benefit to both the owner and to Poland’s environment.