Use of Alloy 59 in flue gas desulfurization

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The flue gas desulfurization plant built as part of Syncrude Canada Ltd.’s expansion project is one of the biggest of its kind in the world. In the Syncrude project, alloy 59 makes an important contribution to securing energy supplies and protecting the environment thanks to its universal corrosion resistance. Around 700 metric tons of this nickel-base alloy have been used in the flue gas desulfurizer.

Dr. Helena Alves

Syncrude is the world’s biggest producer of oil from oil sands and aims to raise its annual production from 90 to an amount of 130 to 135 million barrels by 2005. The flue gas desulfurization plant built as part of this expansion project also sets new standards. The plant is 94.5 m high and features an approximately 21 m diameter absorber sump, a 15 m diameter absorber and a roughly 6 m diameter stack. ThyssenKrupp VDM supplied 700 t of its nickel-base alloy Nicrofer 5923 hMo – alloy 59 for this flue gas desulfurizer. It is one of the biggest orders the company has ever received for this application.
Producing oil from oil sands involves several steps. After mining of the oil sands, bitumen is extracted which is then processed into the clean, liquid Syncrude Sweet Blend (SSB). The utilities which produce electricity and steam for these processes emit acidic gases which then have to be scrubbed in a flue gas desulfurization plant.
Wet scrubbing
Flue gas desulfurizers generally form part of coal-fired power stations or waste incineration plants, where they are used to minimize emissions of the sulfur dioxide produced during incineration. This is done in a wet scrubbing process, normally using a limestone slurry. The end product of this process is gypsum. During flue gas scrubbing, sulfuric acid may be formed which, at the high temperatures involved, creates an extremely corrosive atmosphere. In addition to this, the plant materials may also be subjected to pitting or crevice corrosion from chlorides. The corrosion loads are particularly high beneath scale deposits, which enrich the corrosive materials.
In the Syncrude project, the desulfurization process is based on Marsulex technology, which uses ammonia rather than limestone for scrubbing. This produces ammonium sulfate, which can be used as an artificial fertilizer. A minimum design life of 20 years and a run length of 3 years between maintenance and inspection shutdowns were specified for the absorber.
High nickel content
For applications such as flue gas desulfurization plants, so-called C-alloys have proved to be the most suitable materials, even in comparison with nonmetallic alternatives. In addition to a high nickel content (over 50 %), these alloys also contain 16 to 23 % chromium and 13 to 16 % molybdenum. The high chromium content gives them good anti-corrosion properties in oxidizing media, while the molybdenum content also protects them from corrosive attack in reducing media. The high nickel content makes them insensitive to stress corrosion cracking.
The first alloy of this type was marketed under the name C in 1930 in the USA. Since then there have been seven additions to the alloy family, with ThyssenKrupp VDM taking out a patent on alloy 59 in the early 1990s. Alloy 59 holds a special position in this family as the purest of the nickel-chromium-molybdenum alloys. Whereas the other materials contain 2 to 6 % iron and tungsten or copper in addition to the main alloying elements, alloy 59 is made up of nickel, chromium and molybdenum with an iron content of less than 1.3 %. When it came to selecting the material for the Syncrude project, alloy 59 faced competition from four other C-alloys from American suppliers. Tests showed that alloy 59 was far superior to the other materials in terms of both corrosion resistance and thermal stability.
Successful testing
In the selection process, the materials were subjected to a test under crevice conditions in a solution prepared by the Syncrude project team to match the plant medium. A further test addressed corrosion performance in a hot solution referred to as “green death” containing 11.4 % sulfuric acid, 1.2 % hydrochloric acid and 1 % each of copper and iron chloride. This model solution was developed to simulate corrosion conditions in flue gas desulfurization plants, and specifically to test the pitting and crevice corrosion performance of the C-alloys. Alloy 59 displayed the highest resistance under these conditions.
Very good processability
In addition to corrosion resistance, the main criterion for material selection was processability. Both properties are strongly influenced by thermal stability. For example, welding of the highly corrosion-resistant materials must not lead to the formation of carbides or intermetallic phases in the heat-affected zone which could result in corrosion. This effect is called sensitization and can occur during multi-pass welding of thicker sections, such as in the Syncrude project, or during hot forming and heat treatment processes. Standardized tests, such as test G28-A of the American Society for Testing and Materials, showed that at a temperature of around 870 °C it took roughly two hours before alloy 59 started to show signs of sensitization. A key factor in this is alloy 59’s low iron content and the fact that it has no additional elements such as tungsten. Competing materials showed signs of sensitization after just a few minutes. For the Syncrude project, alloy 59 is being used in the form of solid plate, roll-clad plate and as a welding filler.
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