Energy carrier hydrogen
Energy carrier hydrogen Material selection Fig. 2: Test setup H 2 permeation test and cost reasons. The H 2 permeation coefficient varies considerably between the ASTM classes (elastomer material groups) and there are significant differences between the materials within an ASTM class as well. VMQ (silicone), for example, has a very poor permeation coefficient, EPDM has a much better one and FKM (fluororubber) has the best value in comparison. The temperature has a significant influence on the result as well. A value determined at 23 °C may, for example, exhibit a factor of +5 for EPDM materials and a factor of +3 to over 16 for FKM at 80 °C. Tested materials are therefore clearly recommended in H 2 applications. The field of application itself can also be an important selection criterion. Seals for natural gas containing hydrogen (in distribution/transport, for example) have to meet the requirements of the DVGW: Practical example – hydrogen electrolyser Permeability is not always the decisive selection criterion. A manufacturer of AEM (anion exchange membrane) electrolysers for hydrogen production experienced major problems with the elastomer seals. The chosen NBR material failed after a short time. The medium in the electrolyser was 5 % caustic potash solution (KOH) at max. 65 °C. The seal manufacturer COG suggested a peroxide cross-linked EPDM as a suitable material. Surprisingly this too failed after approximately 100 hours. Exposure tests in caustic potash solution (KOH 5 %) at 65 °C did not result in any significant material changes. It was therefore presumed that the material incompatibility was related to the materials used in the electrolyser itself. AEM electrolysis requires a catalyst and nickel was used in this case. Nickel is known to be “toxic” to rubber. Ethylene propylene diene monomers (EPDM) are terpolymers made of ethylene, propylene and diene. Dienes contain two carbon-carbon double bonds (C=C double bonds). Nickel attacks precisely those double bonds in diene and destroys the rubber. COG then proposed using an ethy lene propylene copolymer (EPM). This rubber does not contain diene and therefore has no double bonds in the polymer. Its resistance to caus- • Gases according to DVGW worksheet G 260 (max. hydrogen content 10%) • DIN-DVGW certification of the sealing material according to DIN EN 549 and/or DIN EN 682 • Typical operating temperature ranges: • DIN EN 549: -20°C to +80°C (class B2) • DIN EN 682: -15°C to +50°C (type GBL) • Typical pressure ranges: • Up to 5 bar (DIN EN 549) • Up to 100 bar (DIN EN 682) A comprehensive survey of the application process is essential for the selection of materials. Fig 3: Permeation test evaluation – “Vi 840” as high-quality FKM comparison material 18 GREEN EFFICIENT TECHNOLOGIES 2023
Energy carrier hydrogen Material selection 281 Ncm 3 mm m -2 day -1 bar -1 on average. Its H 2 impermeability is therefore considerably better compared to what is normally expected for FKM compounds. A high chemical resistance and a broad operating temperature range from -10 to +200°C complete the material profile. The newly developed EPDM AP 208 also passed the H 2 permeation test with very convincing values for an EPDM material (hydrogen permeation coefficient: 1317 Ncm 3 mm m -2 day -1 bar -1 ). With a compression set of
