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Materials

Alkaline electrolyzers operate within a temperature range of 80 – 90 ° C , with a design temperature typically closer to 120 ° C . The widely used standard stainless steel 304L can provide satisfactory service life in caustic environments such as the pulp and paper industry . However , in the case of alkaline electrolyzers , it is necessary to eliminate or minimize corrosion to keep the systems clean and ensure a long service life with minimal maintenance and part replacement . In strongly caustic solutions , chromium is the key element in stainless steel to resist corrosion . Duplex grades are generally higher alloyed in chromium than austenitic grades and have proven to perform very well in caustic service at high temperatures . This makes them suitable materials for alkaline electrolyzers , especially in heat exchangers and piping that are not exposed to high-pressure hydrogen gas .

By Jonas Höwing and Angela Philipp , Technical Marketing – EMEA , Alleima

In the industrial transition towards more sustainable and less carbon dioxide-intensive operations , the use of clean hydrogen is seen as a means to reduce the reliance on fossil fuels and enable energy storage . Water electrolysis is the most viable method for large-scale hydrogen production , excluding fossilbased technologies such as steam methane reforming ( SMR ). When hydrogen is produced using electricity from renewable sources , it is classified as ‘ green hydrogen ’. Certain industrial processes , such as steel production , ammonia-based fertilizers , chemicals , and renewable fuels , are specifically targeted for utilizing green hydrogen due to their potential for significant environmental benefits . There are three major technologies for water electrolysis : polymer electrolyte membrane ( PEM ) electrolysis , alkaline electrolysis , and high-temperature solid oxide electrolysis cell ( SOEC ). Of these , alkaline electrolysis is the oldest and most mature technology , which has been commercially used for over a century . The different techniques have their advantages and disadvantages , with PEM being compact and efficient but expensive , alkaline electrolysis being low-cost and robust but bulky and less efficient , and SOEC being highly efficient but operating at high temperatures and suffering from material degradation and slow startup and shutdown .

Alkaline electrolyzers This article focuses on alkaline electrolyzer technology , which today is the most used or considered technology for large-scale hydrogen plants . In alkaline electrolyzers , as suggested by the name , the electrolyte is a caustic solution , commonly 30 % potassium hydroxide ( KOH ). The water in the electrolyte is split into oxygen and hydrogen through the reactions summarized below :

The electrolyzer is commonly operated at 80 – 90 ° C and has a working pressure of 30 bar at the electrodes , although the pressure is lower in other parts of the system . As water is consumed in the process , it must be continuously replenished . Furthermore , the added water must be of high purity as any impurities added through the water will accumulate in the electrolyzer system . Over time , this can cause scaling in piping systems and on electrodes and can also poison the catalysts on the electrodes . The alkaline electrolyzer generally consists of the cell stack , where the electrolysis occurs , gas separators , tanks for electrolyte and water replenishment ( as water is consumed in the process ), and piping for electrolyte and gases . There are also heat exchangers for cooling and balance of plant , where the cooling media will largely depend on the electrolyzer plant ’ s location . If the plant is located inland or far from an open water source , a closed-loop system with cooling towers will most likely be used . Seawater or freshwater will most likely be used for the cooling system in coastal areas or close to open water sources . This will put different demands on the metallurgy used for the heat exchangers in the cooling system . The material of construction ( MOC ) for alkaline electrolyzers has generally been standard austenitic stainless steel such as 304L or 316L , which in many cases provides a suitable service life . However , any corrosion in

Table 1 . The corrosion resistance of some stainless-steel grades in NaOH solutions at different temperatures . Testing was performed in atmospheric conditions . www . heat-exchanger-world . com Heat Exchanger World September 2023

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