Extending the Life of Old Incinerators
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THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS |
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WASTE BURNED AT THIS PLANT in Haderslev, Denmark, is only moderately agressive. However, to reduce maintenance downtime, nickel alloy N06625 has been used to protect critical components.
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As more plastic enters municipal waste steams, nickel alloys are being used to extend the
operating life of old incineraters
By Virginia Heffernan
Nickel Magazine, July 2005 -- Ongoing research at a Danish university is showing that N06625, a nickel-chromium-molybdenum alloy, remains the best choice of alloy to combat the increasingly corrosive environment of waste-to-energy (WTE) incineration plants.
Since landfilling is discouraged in Denmark, almost all the country's municipal waste ends up at WTE plants built primarily from low-alloyed carbon steel. But as this waste becomes more corrosive, possibly as a result of increasing contents of polyvinyl-chlorides which leads to the formation of hydrochloric acid when incinerated. Therefore demand for corrosion-resistant alloys is rising.
Meanwhile, European plants are being modernized 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, N06625 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 it has sold about 5,000 square metres of nickel-protected heating surfaces to about 30 separate WTE plants up to 2004. For comparison, the leading weld overlay applicator in the U.S. has applied over 30,000 square metres.
But in a recent study presented at the annual conference of NACE International, researchers from the Technical University of Denmark tested the performance of other nickel-based alloys similar to N06625. They wanted 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 N06625 in 1998 when corrosion rates became unacceptably high.
The N06625 performed as expected, but the researchers were interested in testing similar alloys, namely N06022 and N06686, 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 one to two years for results.
Their conclusion was that N06625 showed similar corrosion resistance to N06686 and is not as susceptible to dendritic attack as N06022. Therefore, because N06625 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 N06625 is the best alloy for the job," says Melanie Montgomery, lead researcher on the project and a scientist at the Department of Manufacturing Engineering. "It is easily available, compared with N06022 and N06686, and 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.
Virginia Heffernan is a Toronto-based science writer.
PHOTOS: Technical University of Denmark
Melanie Montgomery |
