Martin Brück, DuPont, Switzerland
The 12 existing generating units at Europe’s largest lignite fired power station in Belchatów, Poland, are to be supplemented by a new 858 MW rated unit (Figure 1). Following its scheduled completion in October 2010, the plant will be one of the country’s most efficient lignite fired power stations, with a rating of approximately 42 per cent, as well as one of the world’s most modern installations. As a consequence of this high rating, both fuel consumption and emissions will be reduced.
Figure 1: Construction as of the end of April 2009 of the new 858 MW unit at Belchatów in Poland
The prime contractor for the specification, construction and commissioning of the new power plant is Alstom, while the customer and future operator of the plant is the power company PGE Elektrownia Belchatow SA.
A significant contribution to the new power plant’s high efficiency rating are two innovative flue gas coolers. Each is equipped with five heat exchanger modules, incorporating approximately 500 000 metres of AlWaFlon pressure hose, they reduce the temperature of the flue gas by about 50 K. The out-coupled heat resulting from this process (two x 32 MW) contributes towards the feed water heating at the power plant, while the surplus steam is used for energy production, thereby significantly improving the overall energy balance of the plant.
Integrated pressure hose concept
Wallstein Ingenieurs-Gesellschaft of Recklinghausen, Germany, is the developer and supplier of the entire heat transfer system, including its associated systems engineering, housing and piping. It is also responsible for its on-site assembly, which was commenced at the beginning of 2009. When selecting the material for the heat exchanger pressure hoses, the company chose the AlWaFlon fluoropolymer system over the dominant corrosion-resistant nickel-based alloys. This was because although metals may allow better heat transfer they are unable unlike polytetrafluoroethylene (PTFE) to resist the levels of corrosion exposure associated with this application.
AlWaFlon is defined by the interaction of an almost entirely chemically inert fluoropolymer, designed for use in highly corrosive and reactive gases, and the use of complementary processing methods.
Its joint developers are Wallstein, DuPont of Switzerland, as manufacturer of the base material Teflon® PTFE, and Fluortubing of the Netherlands, as the producer of the assembly-ready pressure hoses for integration in the heat exchanger housing (Figure 2).
Figure 2: A fully assembled air gas cooler is 13 metres long and approximately 2 metres wide and high
Ideal for Demanding Applications
The AlWaFlon concept combines cost-efficient processing and application benefits, with exceptional property consistency over long production periods, from charge to charge and for the entire hose length.
Compared to many other fluoropolymer grades, the attributes of AlWaflon include higher upper service temperatures of a maximum of 260 °C and a significant increase in the Vicat softening temperature to between 130 °C and 140 °C, as well as high flexural fatigue strength and stress cracking resistance.
The overall result is long-term resistance in demanding conditions. Decisive for Wallstein was the ability of AlWaFlon pressure hoses to resist the extremely aggressive media that occur in the highly corrosive environments encountered during energy production, when the temperature of flue gases is cooled to below the acid dew point.
In such cases, contaminants such as sulphur dioxide (SO2), a high proportion of which is present in the flue gas emitted from the Belchatów lignite fired plant, sulphur trioxide (SO3), hydrogen fluoride (HF) and hydrochloric acid (HCl) all mix together with water to form corrosive mixed acids.
These condense on the colder surfaces of the heat exchanger pressure hoses and bond with dust contained within the gas to form solid coatings. Corrosion-resistant steels, even nickel-based alloys, can only resist such attacks for a limited time.
The heat exchangers deployed at Belchatów use pressure hoses with an outer diameter of 12 mm and a wall thickness of 1 mm. Further benefit is gained from the distinctive non-stick properties and self-cleaning effect of the fluoropolymer. In comparison to metal hoses, considerably fewer deposits are formed on the exterior and those that do form can be removed both quickly and thoroughly, and therefore in a cost-efficient manner. This helps extend maintenance intervals and increases the overall availability of the plant.
The AlWaFlon pressure hoses, manufactured on the basis of the almost entirely chemically inert PTFE, provide long-term resistance to the combination of internal pressure, high temperatures and corrosive media. This has been confirmed by internal long-term hydrostatic strength tests according to DIN 16 887, carried out by a BASF Ludwigshafen testing laboratory in Germany, which compared the results from earlier tests with other polymers conducted under the same conditions.
The results of 10 000-hour tests, which, according to DIN ISO 9080, can be extrapolated to a time period of 50 years, are convincing. This can be seen Figure 3, which shows the long-term behaviour of AlWaFlon pressure hoses at different temperatures. While Figure 4 shows the higher performance of AlWaFlon when compared to other perfluoroalkoxy (PFA) and PTFE polymers used in power stations to date. Ongoing, long-term testing based on DuPont’s Teflon PTFE Fine Powder, conducted by the German TÜV Süd since the 1960s, confirms the accuracy and admissibility of the extrapolation in such cases.
Figure 3: Behaviour of AlWaFlon pressure hoses at different temperatures during long-term hydrostatic strength tests
Figure 4: Comparison of AlWaFlon pressure hoses and equivalent products in internal long-term hydrostatic strength tests
On the basis of their high, long-term resistance to internal pressure and tensile strength, AlWaFlon pressure hoses can be used at lower wall thicknesses than pressure hoses made, for example, from the equally corrosion resistant fluoropolymer PFA, whilst providing the same levels of safety and being exposed to the same mechanical stress. Accordingly, larger amounts of heat energy can be out-coupled at the same length, or alternatively the number/length of pressure hoses used can be reduced to achieve the same level of heat exchange performance.
Basis for Continued Success
Wallstein has conducted pioneering work in the area of heat exchanger production and has developed a system that combines high cost efficiency with long maintenance intervals, while also making a significant contribution to energy saving.
In addition, DuPont, in cooperation with the TÜV Süd, has attained certification of AlWaFlon for use as a pressure hose at high temperatures. Such a range of benefits has been a convincing argument for additional suppliers of heat exchangers for power stations. Indeed, Wallstein has managed to win further large European projects where AlWaFlon pressure hoses will again be used. These include the power station sites Siekierki in Poland and Ledvice in the Czech Republic.
The author would like to thank Peter Dittmann of Wallstein Ingenieurs-Gesellschaft, Germany and Alex Soeterbroek of Fluortubing, the Netherlands for their invaluable contribution to the article.