HVTS cladding proves effective against biomass corrosion

Some operators build new biomass-fired boilers, while others retrofit old coal-fired boilers to burn biomass. Alkali- and sulphide-induced corrosion, however, has become an issue for many biomass- fuelled boilers. Integrated Global Services (IGS) (headquartered in Virginia, USA) has developed a novel HVTS (High Velocity Thermal Spray) metal alloy cladding technology to address the biomass corrosion problem. Among successful adopters of this technology, which IGS describes as “groundbreaking”, has been E.ON in Sweden.

IGS performed a High Velocity Thermal Spray application on a steam boiler (Boiler No 5) at E.ON Va¨rme Sverige AB A°byverket, O¨rebro, Sweden in 2017. The boiler is a 170 MW biomass-fired CFB (circulating fluidised bed) unit built in 1988 by Go¨taverken- Generator (now Valmet Technologies Inc). The steam conditions are 150 bar/540°C.

A°byverket wanted to upgrade the previously installed boiler erosion protection in preparation for a fuel change.

The new fuel mix is: stemwood, 10%; peat, 20%; creosote, 30%; sawdust, 20%; wood branches, 5%; dry wood chips, 15%.

Characteristics of the new fuel can be summarised as follows:.

Cl and S content: Cl, 0.3 g/kg; S, 1.3 g/kg.

Chemical composition of particulate emissions: SO3, 7.5-41.6%; Cl, 0-5%; K2O, 11-21%

Chemical composition of fly ash: Cl, 0.3% ; S, 1.6% ; Si, 19%; Ca, 12.5%

pH of flyash: 12.5

The boiler revamp included the replacement of 20 m high waterwalls on all four boiler walls.

Biomass fuel can be derived from various sources, including virgin wood, plants or animal residues, as well as recycled materials, such as demolition wood. Depending on the type of biomass, the combustion can generate more or less corrosion and erosion. The type of boiler is also an important factor: fluidised bed boilers optimise combustion but will also generate erosion. Various factors influence the choice of erosion/corrosion protection.

According to IGS subject-matter expert on waste to energy and biomass boiler coatings, Cyril Narjoz: “Some boiler operators believe that weld overlay is the only solution to protect boiler walls from biomass-induced corrosion safely.”

This might have been true many years ago, he notes. “But now HVTS has proven its performance in several independent tests and practical applications. With its on-site application being much faster and easier than weld overlay, HVTS has become a favoured alternative for corrosion protection.”

For the E.ON Va¨rme Sverige AB project, IGS applied 993.5 m2 (in a 20 m band) of HVTS cladding during a three week period, working 24/7 with eight spray units.

Annual inspections

In August 2018, IGS conducted its first annual inspection of the cladding, including thickness mapping. The result was as expected: no deviation in the HVTS cladding thickness.

A similar inspection was carried out the following year and the report stated: “No black areas were detected (bleed through), the cladding turned out to be completely intact and without material loss.

Measurements taken showed results similar to those from the year of application.”

Avoiding panel replacement

As the range of biomass fuels widens, boiler engineers will face more arduous corrosion conditions. HVTS cladding can provide an effective barrier for existing pressure parts, which will prevent further need for boiler panel replacement.