Decarbonisation Technology - May 2023 Issue

Nitrogen for safe electrolyser operations The lower flammable limit (LFL) of hydrogen in pure oxygen at atmospheric pressure and 20°C is 4%. However, electrolysers generally operate at 80°C; at this temperature the LFL reduces to 3.8%. Pressurised alkaline and PEM electrolysers operate at between 15 and 30 bar. At 20 bar and 20°C, the LFL of hydrogen in oxygen increases to 5%. Both the operating pressure and temperature influence the flammability limit of hydrogen in pure oxygen (see Figure 5 ). Electrolysis splits water to create oxygen in addition to hydrogen. In the PEM and anion exchange membrane (AEM) electrolyser types, these two gases are separated by a membrane. In pressurised and low-pressure alkaline electrolysers, both gases are present together, dissolved in the lye electrolyte that circulates around the electrolyser. During operation with variable renewable power, as the electrolyser ramps down with reduced power availability, the oxygen concentration in the hydrogen gas increases. Safety management systems in the electrolyser prevent the hydrogen concentration in the oxygen from rising to a dangerous level. When activated, these safety systems flood the electrolyser with inert nitrogen gas. For a GW-scale green hydrogen alkaline electrolyser system working with a variable or intermittent renewable power source, there

could be several occasions per year where the system may need to shut down and initiate a nitrogen purge. At GW-scale operation, the amount of nitrogen required in these events may justify an on-site nitrogen generator. Nitrogen generators use a cryogenic process that is a simplified version of a full air separation unit (ASU). Unlike the ASU, nitrogen generators are not designed to produce oxygen and argon as co-products. The focus on nitrogen reduces the specific power requirement, simplifies operation, and reduces the plant capital cost compared to an equivalent nitrogen flow derived from the more complex ASU. Nitrogen feedstock for green ammonia Ammonia is readily liquefied and, in this state, has a volumetric energy density 50% higher than liquid hydrogen. The reduced shipping costs of liquid ammonia, compared to liquid hydrogen, mean that Capex and Opex savings from shipping can be directed to the ammonia conversion facility. For long distances, such as the Australia to Europe route, liquid ammonia is the most cost-effective mode of green hydrogen transportation (see Figure 6 ). One of the attractions of using ammonia as a fungible energy vector is that it is already a globally produced and traded commodity. Worldwide grey ammonia production is typically 185 million t/y. The global merchant market for traded grey

Repurposed natural gas pipeline

1.5

New hydrogen pipeline

Liquid H shipping

1.0

Compressed gas H shipping

LOHC shipping

Ammonia shipping

Liquid H road tankers

0.5

100

1,000 Compressed gas H Road tube trailers

5,000

10,000

Distance (km)

Figure 6 Hydrogen transport options when considering volume and distance