Boiler tubes: protect and serve

Gary Heath explains how to deploy advanced coatings to extend the lifetime of boiler tubes in waste and coal-fired power plants

The typical power generation boiler represents an aggressive environment in terms of erosive wear and high temperature corrosion for the heat exchanger tube materials. The many forms, sizes, designs, makes, etc. of boilers, together with the diversity of fuels used (coal, oil, waste, biomass, etc.) and operating regimes gives rise to a broad and complex range of wear/corrosion problems.

The erosion or corrosion can lead to a thinning of the tube wall cross-section of what is a pressure vessel. When the tube wall thickness becomes too thin, the tube bursts and operations have to be closed to replace it.

There are various philosophies to reduce tube wastage to acceptable levels, which include design, operating temperature and gas flow modifications (which can reduce boiler efficiency) or tube material changes (which can be expensive and usually mean sacrificing other properties). Methods available include co-extrusion, calorising, nitriding, chromising, boronising, pre-oxidising, weld-overlay and thermally sprayed coatings.

Today, the overwhelming technology used against corrosion is weld overlay (usually 2mm thick) of a nickel-based complex alloy (usually Inconel 625, 622).

Against erosion, thermal sprayed coatings are used that typically are hard, thin (0.3-1.0mm), fast to apply and can be applied like paint. The spray processes that have been used in boiler protection are: flame combustion (with powder or wire), electric arc wire, air plasma (APPS), high velocity oxygen fuel (HVOF), spray and fuse (S&F), and painted coatings of inorganic ceramic-based compounds (slurry coatings).

In Europe and also increasingly Asia, the move to alternative energy producing sources and demands for lower NOx and CO2 have lead to stricter boiler operations plus the use of alternative fuels (municipal waste, biomass). These trends plus the burning of low-grade coal have increased the amount of corrosion and wear that is being seen in the boiler tubes.

Coating quality and performance, coating cost, reduction in downtime to apply coats on site, new tubes versus repairing on site worn tubes, etc. are now becoming critical issues in the economics of many power plants. The addition of the USA-based company, WherTec, to the Castolin Eutectic group in 2014 brought 16 years of boiler coating experience as well as innovative laser cladding and slurry coatings technologies to the portfolio.

Densification and laser cladding

Castolin Eutectic has developed a thermos chemical process that ‘densifies’ the thermally sprayed coating. This densification process coupled to the arc spray process (using the BTC and TubeArmor coating product ranges) is well suited to coat waterwalls in waste-to-energy and biomass boilers where corrosion is an important wear mechanism.

Meanwhile, high power diode laser (HPDL) technology has enabled the previously promising, but expensive, laser cladding technology to become a cost-effective corrosion solution for large boiler surface areas in the company's workshops. Inconel type materials with thicknesses down to 0.7mm are produced and give better properties (lower dilution, smaller heat affected zone, finer structure, smooth coating surface) over traditional weld overlay boiler coating. In the USA, several hundred square meters of laser-clad boiler tube are installed in boilers.

Real-world case studies

At Europe's largest 5,053MW lignite-fired power station in Poland, where erosion wear in superheater tubes was prevalent, trials with various thermal spray coating over a period of eight years identified a clear winner, Eutronic Arc 595 coating sprayed with Castolin Eutectic equipment at the firm's service workshop. Arc 595 is a very hard Fe-based alloy and it extended the superheater tube lifetime by the up to seven years compared to the original naked steel tubes. Over 46km of tubes and elbow were coated for each of the six boilers.

A major French waste company tested the 53606 S&F alloy in its laboratory-accelerated corrosion reactor, which simulates the typical corrosion mechanism observed on the superheater tubes used in its waste to energy boiler plants. Many different alloys and coating systems were tested, including Inconel 625 weld overlay. The waste company reports that Castolin Eutectic's 53606 alloy outperformed the other alloys and deposition systems.

In many biomass and waste boilers, there is a mixed erosion/corrosion attack of the boiler tubes. An Italian power station encountered a severe corrosion problem when adding 10% of waste to the coal usually burned in its boiler. The wire BTW 66 (erosion-corrosion) proved successful when field-tested on panels, and the customer consequently ordered this solution for a much larger waterwall surface.

For more information, visit www.engineerlive.com/ipe

Gary Heath is with Castolin Eutectic.