Corrosion-resistant heat exchangers feature pressure tubes made from DuPont PTFE
03 July 2012
AlWaFlon pressure tubes supplied by Wallstein Ingenieur GmbH of Recklinghausen, Germany are made with DuPont Teflon polytetrafluoroethylene (PTFE). This enables them to resist the highly corrosive media present within the flue gas desulphurisation plants of coal-fired power stations. Unlike heat exchangers made of nickel-base alloys, these systems withstand a long-term exposure to aggressive acid mixtures.
The superior performance of systems made with Teflon PTFE is confirmed by a growing number of applications, the oldest of which have been in service since the early 1980s. One current example is the heat displacement system employed at the hard coal fired power plant at Siekierki near Warsaw, Poland. Here the energy recovered is used as a substitute for primary energy in reheating the cleaned flue gas.
One benefit of this technology is that it prevents corrosion in the ducting and flue gas stack while also avoiding droplet formation once the gas is discharged from the stack. The system has been continuously in use for two years, and no corrosion issues or other problems have been encountered.
Heat exchangers incorporating AlWaFlon pressure tubes contribute significantly to the sustainable operation of coal-fired power plants. They are suitable for service in temperature ranges in which metal-based heat exchanger systems can no longer be used due to attack by a mixture of acids.
As a result, fossil fuel input and CO2 emissions can be reduced even in highly corrosive applications. Moreover, more efficient cleaning cycles can be implemented thanks to the anti-stick properties of Teflon fluoropolymers. An on-line cleaning system is also in use at the Siekierki plant.
The CHP (combined heat and power) generating station at Siekierki has a rated output of 620MW and is operated by PGNiG SA It comprises two flue gas desulphurisation lines, including a heat displacement system, fitted by RAFAKO S.A., the Polish equipment building company which refers to itself as Europe's largest boiler manufacturer.
The systems were sourced from Wallstein and comprise 12 flue gas heat exchanger modules featuring AlWaFlon pressure tubes, plus 42 clean gas heat exchanger modules of which 14 likewise incorporate AlWaFlon pressure tubes. Line 2 has been in service since November 2010; Line 1 was commissioned half a year later.
Each of these lines saves 23MW of energy that would otherwise have to be sourced from superheated steam. This is equivalent to an 8,600kg/h reduction in CO2 emissions.
The AlWaFlon system was jointly developed through partnership among Wallstein Ingenieur GmbH, DuPont, and Fluortubing B.V. Indeed, the AlWaFlon trademark stands for the combined benefits of DuPont Teflon PTFE and a special processing method tailored to this material. The product is characterised by an exceptionally high maximum usage temperature of 260°C and a similarly high Vicat softening point of 130 °C to 140 °C, combined with both a high flexural fatigue strength and stress cracking resistance.
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