The acid test for sprayable coating on flue gas duct

Even after five years, the effects of acid, corrosion and intense heat have had minimal effect on a flue gas duct at Henkel power plant in Düsseldorf.  The reason for this resilience is the ceramic-reinforced, epoxy surface coating – LOCTITE^® PC 7255 – that was applied to the inside surface of the duct in 2015 to protect it against aggressive sulphur and nitrogen compounds as well as condensation water.  

The performance of the coating was assessed during a recent two-week inspection of and as Operations’ Assistant, Axel Olschinski, confirmed: “LOCTITE PC 7255 met the demands of this endurance test and still shows very good chemical resistance.”

LOCTITE PC 7255 is a low friction epoxy that is sprayed onto the substrate and is therefore especially good for coating large surface areas and complex structures.  It creates an ultra-smooth and high gloss finish that is resistant to both acids and alkalis and also protects against, abrasion, corrosion and, in particular, chemical attack.

In 2015, two coats of this product, each at least 250 micrometres thick, were applied by Henkel maintenance engineers to the flue gas duct and within 24 hours the system was returned to full operation.

Protection against warm and humid exhaust gases

The coal-fired power plant provides the entire Düsseldorf production site with energy. As one of its steam boiler systems is operated with hard coal, the flue gas contains - among other things - sulphur and nitrogen compounds that must be scrubbed out in several steps.

The resultant warm, humid and purified flue gases are deposited as an extremely corrosive condensate on the surface of the flue gas duct. So, the aim of applying LOCTITE PC 7255 was to protect the duct to extend its service life, minimise maintenance and underpin production.

Spraying instead of coating

Before LOCTITE PC 7255 was used, it was common practice to shield the flue gas duct with sandwich panels. This method was both complicated and costly. The process of rubberizing the surface with bromobutyl rubber is time-consuming and expensive as the material needs to be adapted to suit the design of the duct. This is precisely where LOCTITE PC 7255 proves to be particularly efficient and user-friendly: the protective layer can be sprayed on easily and quickly.

Even complex designs involving bends or coils pose no problem. A further advantage is the weight of the product: conventional methods require coatings of up to five millimetres, whereas a coating thickness of 500 micrometres is all that is required for LOCTITE PC 7255. 

Minimum changes in coating thickness

To determine, precisely, the resistance of the coating after five years, changes in colour were examined as a visual indication and the coating’s thickness and Shore hardness measured. Engineers were delighted to record that even with long term exposure to extreme corrosion, the thickness of the coating and Shore hardness had changed only minimally over time. 

“With LOCTITE PC 7255, we create a sacrificial layer that prevents wear and tear of the base material. Thanks to this process, we cut down on costly maintenance work”, explained Hans-Dieter Krause, Business Development Manager for Maintenance, Repair and Overhaul in Western Europe. “This method also allows us, for example, to dispense with costly FRP coatings”. 

He concluded: “Having the option of simply spraying the coating onto a surface and its resistance to adverse conditions saves costs and extends the service life. The plant can be restarted more quickly and remains efficient in the long term. LOCTITE PC 7255 has certainly passed our five-year endurance test and displayed an impeccable performance.”