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Continued on page 29 G FEATURE ARTICLE Testing Materials for Corrosion in an Icelandic Geothermal Environment Study Determines Best Stainless Steel for Hydrogen Sulfide Abatement System Kathy Riggs Larsen, Editor Geothermal power stations, which use heat energy from the Earth's core, are similar to other power sta- tions that use heat from a fuel source to create steam that turns turbines to generate electricity. Because these power plants use steam from a renewable en- ergy source—hot water located below the Earth's sur- face—they are considered to be environmentally friendly. In Iceland, geothermal power is the country's sin- gle largest source of energy. 1 Geothermal steam, how- ever, contains noncondensable gases (gases that are not easily condensed by cooling), such as carbon diox- ide (CO 2 ), hydrogen sulfide (H 2 S), hydrogen (H 2 ), nitro- gen (N 2 ), methane (CH 4 ), and argon (Ar), that are con- sidered to be either greenhouse, corrosive, or toxic gases. A recent Icelandic regulation with a stricter guideline on atmospheric concentration of H 2 S, which took effect in 2014, has compelled the country's geo- thermal industry to take actions that will reduce H 2 S emissions into the air. 2 Through a collaboration of universities and Ice- landic power companies, an injection abatement project, CarbFix-SulFix, is being implemented at the Hellisheiði geothermal power plant operated by ON Power (Reykjavík, Iceland), a subsidiary of Reykjavík Energy. The Hellisheiði geothermal plant is one of the largest geothermal power plants in the world, with a production capacity of 303 MW of electricity and 133 MW of thermal energy. It is located in one of Iceland's biggest geothermal zones, the Hengill area, which is linked to three volcanic systems. 1 The project captures CO 2 and H 2 S emissions from the Hellisheiði power plant in a gas abatement plant that separates them from the noncondensable gas stream using a simple scrubbing process, and then reinjects them in water into reactive basaltic rock at a depth of around 800 m or greater. If these gases were not captured and reinjected, the Hellisheiði power plant would emit ~40,000 tons of CO 2 and ~12,000 tons of H 2 S. 3 Steam ejector vacuum systems are used to extract the noncondensable gases from the geothermal tur- bine's exhaust steam. The noncondensable gas stream is sent to the gas abatement plant, which includes a gas separation station that isolates the H 2 S and CO 2 from the other noncondensable gases such as H 2 , N 2 , and CH 4 . The H 2 S and CO 2 are separated by dissolving them in condensed water, says NACE International member Sigrún Nanna Karlsdóttir, associate profes- sor in the Industrial Engineering, Mechanical Engi- neering, and Computer Science Department at the University of Iceland (Reykjavík, Iceland). The process pumps the compressed noncondens- able geothermal gases (CO 2 , H 2 S, H 2 , N 2 , CH 4 , and Ar), which are contaminated with a small concentration of oxygen gas (O 2 ), into the bottom of a ~20-ft (6-m) tall absorption tower, explains Karlsdóttir. Cold con- densed water (68 °F [20 °C]) from the power plant is simultaneously pumped to the top of the absorption tower. As the gases flow upward, the water trickles downward through the separation equipment, which facilitates the dissolution of the gases into the water that is reinjected into the basaltic bedrock. The colder the water, she notes, the more soluble the H 2 S. 26 MARCH 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 3