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GRAEME RODDEN
A new release from the Nickel Institute (www.nickelinstitute.org) was the focus of a special panel discussion during the PEERS conference in Portland, OR, in late 2018.
Stainless Steels and Specialty Alloys for Pulp, Paper and Biomass Conversion Mills is the second edition of the manual, which provides an update on recommended construction materials for process equipment. It is the first update in 18 years.
The three PEERS panelists—Andrew Garner, Dave Bennett, and Douglas Singbeil—represent decades of experience in the pulp and paper industry. Each had a hand in contributing to the new manual and Garner was the overall editor. The manual is free and available from the Nickel Institute in print or electronic form. In his remarks, Garner said the revised manual was driven by the efforts of TAPPI’s Corrosion and Engineering Committee. It is a “practical guide,” he added.
RECENT DEVELOPMENTS
Garner went over some of the most recent developments, such as advanced automation and the adoption of duplex stainless steel (four levels of duplex). For example, lean duplex is used throughout the kraft liquor system; 2205 is used for pressure levels. “Duplex is much more resistant to stress corrosion cracking and has better tensile strength,” Garner said.
In total, 11 authors contributed to the manual, which contains 21 chapters. They look at topics such as general characteristics, chemical recovery, sulfite processing, bleaching and stock prep, paper machines, threaded fasteners, welding, abrasion, corrosion, risk-based inspection, biomass conversion, and biorefineries.
In his handout to delegates, Bennett listed the top five things mills can learn from the revised manual:
• Lean duplex stainless steel (SS) makes the 300 series austenitic SS “basically obsolete” in alkaline pulping and brownstock processes including oxygen delignification. This is because of its superior strength and wear resistance as well as its corrosion resistance, especially with regard to chloride stress corrosion cracking in normal process conditions.
• In acidic oxidizing bleaching (D stage), super duplex SS grades (25Cr) match or surpass the localized corrosion resistance of super austenitic SS at substantially lower cost—up to 50 percent less on a comparative strength basis.
• In many power and recovery boilers, with reducing and oxidizing gas environments, 12Cr and 18Cr ferritic grades (and LDSS grades) resist corrosion up to significantly higher temperatures than carbon steel. They can also resist sulfide stress corrosion cracking thermal fatigue better than austenitic grades.
• Carefully designed sheet linings of duplex grade can cost-effectively replace corrosion-resistant linings on carbon steel in many applications: for example, 309 SS weld overlay in continuous digesters (12 Cr, LDSS); 312 SS and 309 SS weld overlay in batch digesters (LDSS); 304/316 sheet lings in flash tanks, evaporators, dissolving tanks, and causticizers (LDSS, 12Cr); and corrosion resistant masonry in acid bleach service.
• 12 Cr SS is substantially more cost-effective over the long term than galvanized and painted carbon steel in all mill atmospheric exposures where the “time of wetness” exceeds 15 percent.
Bennett also spoke about a chapter he contributed on risk-based inspection. Fixed equipment will benefit from more cost-effective maintenance and greater safety, comparable to the care given rotating equipment. The industry needs detailed knowledge of what particular failure mechanism could most readily affect the specific material from which the equipment is built.
MATERIALS FOR THE BIO-BASED ECONOMY
Co-editor Douglas Singbeil, FPInnovations, capped off the session with a look at how material selection could affect the bio-based economy. He co-wrote the chapter on the bio-economy and admitted it is still an “unfinished story.”
After going over the issues driving the growth of the bioeconomy, Singbeil discussed how the selection of construction materials could be affected. The chapter relates mostly to fuels and chemicals. The course a mill chooses is important. Is it a biothermal conversion or a biochemical one? “New technology brings unexpected issues,” Singbeil said.
There is a substantial gap between the feedstock used in pilot trials and the real attributes of forest biomass. “We need to understand this,” Singbeil added.
In new biomass handling and feed systems, damage mechanisms are generally abrasive, mechanical impact or general corrosion. Carbon steel is generally used, but duplex would be better. The biochemical conversion can present greater challenges; the pretreatment stage often presents the greatest risk of corrosion.
There are four methods of biochemical conversion: combustion, hydrothermal liquification, fast pyrolysis, and gasification. Some are well documented, Singbeil said, but others have little data available. Each method faces different challenges. For example, in biomass combustion, dewpoint corrosion is often encountered and it is usually caused by condensing hydrochloric acid.
The production of biomaterials (CNC, CNF, lignin) presents very challenging material selection problems. “This is the next chapter,” according to Singbeil.
Materials selection needs to start early in the process development cycle. The common misconception is that materials selection for new biochemical conversion platforms is well established by conventional processes. Singbeil said that conventional pulp and paper processes are a “good starting point, but…”
In summary, the editors agree that this new and free edition of Stainless Steels and Specialty Alloys for Pulp, Paper and Biomass Conversion Mills should be readily at hand for every mill maintenance engineer. Visit the Nickel Institute website for this free download. (https://www.nickelinstitute.org/library/articles/stainless-steels-and-specialty-alloys-for-pulp-paper-and-biomass-conversion )
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