Decarbonisation Technology - February 2024 Issue

membrane. Alkaline and solid oxide electrolysers also have sensitivities to poisons in the water. Ultrapure water for solid oxide electrolysis and ammonia production Ammonia production (see Figure 2 ) also requires a vast amount of boiler water to generate superheated steam. The reaction between hydrogen and nitrogen to make ammonia is highly exothermic, and the reaction heat is recovered as steam to ensure energy efficiency. For each tonne of ammonia produced, there are approximately four tonnes of ultrapure boiler water required. The equipment used to generate this boiler feed water is similar to that required to generate the water for electrolysis, and economies of scale and synergies can be achieved by combining the equipment. Most of the steam is used to drive the compressor that feeds the nitrogen and hydrogen gas mixture to the Haber-Bosch ammonia synthesis loop. Additional steam is used to drive the refrigeration compressor that is required to recycle incondensable gases from the ammonia liquefier. It is common to use steam turbines on the drives of these machines to avoid the losses of converting the steam to power. After expansion in these turbines, some lower-grade steam is available for export. The solid oxide electrolysis (SOEC) technology

(see Figure 3 ) is well aligned to green ammonia projects. The reason is related to process integration potential and energy efficiency. Waste heat from the Haber-Bosch ammonia synthesis loop can be used to reduce the electrical power input requirement to the SOEC electrolyser, which operates at high temperatures and is fed with steam, not water. When fed with steam, the SOEC requires about 20% less power than a PEM or alkaline electrolyser to generate the same amount of hydrogen. The catalysts that are used in the cells within an SOEC are highly sensitive to sulphate ions as well as silicates, siloxanes, and aluminium oxides. These impurities must be reduced to less than five or 10 parts per billion to avoid degradation of the SOEC performance. The typical feed for an electrolyser is referred to as ‘Grade 2’ in the ISO 3696:1987 standard, with a maximum conductivity of 0.1 mS/m – identical to ASTM D1193-06(2018) ‘Type 2’ water. Both the ASTM and ISO standards have Silica specifications, but the permitted amount differs by almost a factor of 10 between the two standards. Also, neither standard explicitly specifies total sulphur. These points highlight the importance of researching the precise ultrapure feed water specification that the electrolyser manufacturer requires and designing a suitable ultrapure water treatment facility.

Condenser

Water feed

Preheater

Steam superheater

Water outlet

Sweep air feed

Oxygen heat recovery

Vapouriser

Compressor

Air & oxygen exhaust

Expander

SOE stack + –

Rectier

Transformer

Electricity

Figure 3 The solid oxide electrolysis process for hydrogen generation