Decarbonisation Technology - November 2022

1-20 bar(g) saturated steam

Boiler feed water

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Vacuum steam generator

Gearbox

Boiler feed water

Cooling water

Multisectional steam compressor

Electrolyser

Figure 7 A smart combination of low-pressure steam generator and multistage steam compressor can upcycle electrolyser waste heat to usable levels

75%, while most advanced designs reach up to 75% Similar to many other technical and thermodynamic processes, hydrogen electrolysers become more efficient with rising operational temperatures. This is due to the fact that the water molecules already contain a higher inherent thermal energy. The added electrical energy required to split it into hydrogen and oxygen is consequently lower. Efficiencies higher than 80% are possible with high-temperature solid-oxide electrolysers. And the high-operational temperature allows the direct integration of the resulting waste heat into existing process plants in the form of pressurised steam. However, while this technology is a promising development for high-temperature electrolysers in the years to come, the race is already on today for installing megawatt-scale hydrogen electrolysers using mature PEM (polymer electrolyte membrane) or alkaline electrolysis technology. The predominant PEM technology achieves 60-75% efficiency at operational temperatures below 100°C. The name-giving polymer layer in the cell stack will degrade more rapidly if exposed to higher temperatures, preventing, in many cases, the useful utilisation of the unavoidable waste heat. As global green hydrogen production ramps up, so will waste heat. It is noteworthy that – in the case of green electricity – this heat is carbon free. In industry in particular, upcycling this stream represents an easy way to exploit the full potential of the hydrogen economy: on the one hand, replacing fossil fuels with hydrogen

where really necessary (such as in furnaces, cement, and metallurgy); and on the other hand, replacing fossil-fuel generated steam with hydrogen ‘green steam’ from the waste heat of the electrolysis. With waste-heat temperatures of PEM electrolysers just high enough, the technology of Atlas Copco Gas and Process can efficiently provide the required lift that turns the waste into valuable process energy: pressurised steam (see Figure 7 ). Surprisingly, the process of turning the waste heat into pressurised steam starts at below atmospheric pressure. Due to this steam generation process under vacuum, it is possible to turn even such low temperatures, such as that produced by a PEM electrolyser, into the compressor technology, used for multistage steam compression, upcycles the given low- temperature waste heat up to the desired steam pressure. The oftentimes feared ingress of air into a steam system is prevented by advanced shaft-sealing systems and decades of experience in the design and production of steam compressors with below-atmospheric suction conditions. Due to individual compressor stages, mounted on the integral gearbox, multiple interstage steam cooling (also called desuperheating) by water injection is possible. This reduces the thermal stress on components, increases the steam mass flow, and supports a more efficient compression process. Even systems with steam extraction on intermediate pressure levels can be realised due to the possibility of fitting every input stream of a steam compressor. The proven integrally geared radial