Ongoing research at a Danish university is showing that Inconel 625, a nickel-chromium-molybdenum alloy, remains the best choice of alloy to combat the increasingly corrosive environment of waste-to-energy (WTE) incineration plants.
Because landfilling is discouraged in Denmark, almost all of the country’s waste ends up at WTE plants built from low-alloyed carbon steel. But as this waste becomes more corrosive, possibly as a result of increasing contents of PVCs that contain heavy metal chlorides, demand for corrosion-resistant alloys is rising.
Waste-to-energy plant
Meanwhile, European plants are being updated not only to adhere to stricter EU legislation, but also to increase efficiency and reduce maintenance and operational costs to be competitive in a common market.
Over the past decade, Inconel 625 has become a popular solution to corrosion in European WTE plants, usually in the form of a weld overlay on the existing structure. The alloy is known for its strength at high temperatures, excellent corrosion resistance, and resistance to intergranular attack and stress-corrosion cracking.
Denmark-based Babcock & Wilson Volund, one of the main suppliers to European plants, reports that it has sold about 5,000 square metres of Inconel-protected heating surfaces to approximately 30 separate WTE plants up to 2004.
But in a recent study presented at Corrosion, an annual conference hosted by NACE International, researchers from the Technical University of Denmark tested the performance of other nickel-based alloys similar to Inconel 625 to see if there was a better alternative that could handle the increasingly corrosive waste at Haderslev, a WTE plant commissioned in Denmark in 1993.
Haderslev has two parallel furnace and boiler lines. The boiler has three passes consisting of two empty radiation passes and one convection pass. Three-quarters of the first pass is refractory lined to protect the membrane walls from corrosion. The remaining quarter was weld overlaid with Inconel 625 in 1998 when corrosion rates became unacceptably high.
The Inconel 625 preformed as expected, but the researchers were interested in testing similar alloys, namely Inconel alloys 622 and 686, to see if they could do a better job. To do so, they installed a wall panel made up of various types of weld alloy into the original rear wall and waited 1-2 years for results.
Their conclusion was that 625 showed similar corrosion resistance to 686 and is not as susceptible to dendritic attack as 622. Therefore, because 625 is the least expensive of the three alloys, it remains the most practical choice for corrosion resistance in WTE plants.
“From the results presented at NACE, we think that Inconel 625 is the best alloy for the job,” says Melanie Montgomery, the lead researcher on the project, from the Department of Manufacturing Engineering. “It is easily available compared to 622 and 686 and is not as expensive.”
The environment at Haderslev is only moderately aggressive, so Montgomery has moved the research project to Mabjerg, another WTE plant believed to have a more aggressive environment.This time, she expects to be able to collect more information on the role of additional elements in alloys, such as molybdenum and niobium, in corrosion resistance.