Isabel Bommer

UPM Kymmene is the world's third largest pulp and paper producer. The decision was taken to increase capacity at the Wisaforest pulp mill in Pietersaari, Finland, by building the world's largest ever black liquor boiler. For this project the Finnish company Pohjolan Voima Oy (PVO), Helsinki, supplied the biggest single-cylinder turbine-generator ever to be installed in a pulp and paper mill. With the plant operating at 100 % steam generating capacity some of the electricity produced is supplied to the grid.

The steam turbine, a single-cylinder backpressure machine with an electrical output of 150 MW is equipped with a multiple shifting nonautomatic extraction. It is operated with main steam at 100 bar / 505°C.  Read on to find out more.

Some 110 kilometers east of Dresden on the Polish border lies the picturesque town of Görlitz - known for its architectural treasures dating back more than 900 years and for one of the most high-tech production plants for industrial turbines anywhere in the world. Having acquired this location steeped in tradition and invested more than €75 million in the manufacturing plant, Siemens Power Generation (PG) is putting its new special machinery to use there in the manufacture of high-quality steam turbines to generate power and steam for the production industry. The world's largest turbine-generator destined for the pulp and paper industry is almost ready for delivery. The 150-MW backpressure turbine is on order for the Wisaforest pulp mill's new power plant in Pietarsaari in northwest Finland.

Siemens PG secured the order worth more than €10 million from Wisapower Oy, a fully owned subsidiary of the Helsinki-based Finnish utility Pohjolan Voima Oy (PVO), that will own and operate the power plant. It is part of a €250 million development project designed to raise the pulp mill's annual capacity by around 25% to 800,000 tons. Significant efforts will be put into optimizing reclamation processes for chemicals and the conversion of residues and waste products into useful energy. "We already have one power plant up and running on site," says Timo Rajala, President and CEO of PVO, "That made us a natural partner to cooperate with for the second one. We now want to make the most of the synergy effects resulting from their physical proximity." In specific terms, this includes efficiency improvements based on an optimized fuel supply: bio fuels such as wood offcuts, sawdust or residues from the paper mill (e.g. black liquor) can vary enormously in quality. By working together, says Timo Rajala, the power plants can improve the quality and mix of materials to their mutual benefit. Efficiency will thus also improve.

At the same time this is a particular challenge for power and steam generation: The fluctuating quality of the fuel produces varying load conditions in the turbine. Moreover, to begin with - until ultimate project completion in ten years' time - only 110 MW of the available 150 MW of power will be extracted. "However, in order to meet all of the process requirements for pulp production at all times with regard to correct temperatures and steam pressures we have equipped the Wisaforest turbine with a five-fold shifting nonautomatic extraction instead of the more common double or triple one," explains Dr. René Umlauft, Head of the Turbosets Subdivision at Siemens Power Generation. This same turbine will therefore be able to handle an increased quantity of steam following the planned increase in the capacity of the plant. In operation the Siemens turbine will have maximum main steam conditions of 100 bar and 5050C, with the steam being expanded to a backpressure of 4.4 bar.

Focus on flexibility, customized design and rapid availability

The turbine's high operating flexibility was one of the main planning criteria. "We want to generate as much electricity as possible here," explains Bjarne Jåfs, project manager at UPM Kymmene, owner of the Wisaforest mill. "This means that the steam must remain in the turbine for as long as possible and that we extract it at the latest possible moment to suit the various production processes. "The shifting nonautomatic extraction helps to optimize this process. Together with the new chemicals reclamation plant and the world's largest combustion boiler, the Siemens turbine and generator should be highly efficient in operation, said Jåfs, and produce more than enough electricity to power both the pulp and paper mill and processing plant in Pietarsaari - and still be doing so in ten years' time.

"Besides flexibility, the important thing in our business is for us to identify our customers' specific requirements and find individual solutions while nevertheless making the requisite products available quickly and at competitive prices," explains Siemens turbine-generator expert Umlauft. To help them tackle this challenge, the engineers in Görlitz aim to achieve maximum standardization, modularization and system integration during planning and fabrication. "With standardized products, technical descriptions and other documents are available quickly and in detail early on in the process so helping to cut design and planning costs.

Also, if elements are designed for series production, outage, operating and maintenance costs are reduced because the manufacturer's learning curve is better and faster due to constant repetition." Modularization and system integration should allow a flexible approach to product design to suit customer requirements - and help to reduce the time and costs involved in the planning and design phase. Installation of the turbine-generator at Wisaforest was completed under Siemens management in early 2004.