Corrosion-resistant heat exchangers

21st February 2013

AlWaFlon pressure tubes supplied by Wallstein Ingenieur GmbH of Recklinghausen/Germany are made with DuPont Teflon polytetrafluoroethylene (PTFE). This enables them to resist even the highly corrosive media present at 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.  Such mixtures are comprised of SO2, SO3, HF and HCl, which are formed as flue gases cool and then condense on the heat exchanger surfaces along with ash particles.
The superior performance of these 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 620 MW and is operated by PGNiG S.A. It comprises two flue gas desulphurization lines, including a heat displacement system, fitted by RAFAKO SA, 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 23 MW of energy that would otherwise have to be sourced from superheated steam. This is equivalent to an 8600kg/h reduction in CO2 emissions.

In the flue gas desulphurization plant of the Siekierki coal-fired power station situated near Warsaw/Poland, large-size heat exchangers equipped with AlWaFlon pressure tubes made with DuPont Teflon PTFE prove resistant to the attack of highly corrosive mixed acids. The AlWaFlon heat exchanger system is key to achieving major primary energy savings, resulting in substantial reduction in CO2 emissions. This primary energy would otherwise be needed for reheating the cleaned exhaust gas so as to allow it to be discharged through the stack.

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