Decarbonisation Technology - May 2023 Issue

Renewable electricity

Oxygen

Green ammonia

Methanol synthesis

Ammonia synthesis

SOE

e-methanol

Water

Carbon dioxide

Steam for e-methanol distillation

Steam

Figure 7 Solid oxide electrolysis for energy-efficient e-fuels production

ammonia, at circa 20 million t/y, represents about 11% of worldwide production. One hundred seventy ammonia tankers sail the world’s oceans shipping these merchant ammonia volumes across 120 portside ammonia terminals. A typical ammonia tanker can transport 60,000 tonnes of liquid ammonia, and the terminal would typically be built to store twice this capacity. The maturity of the ammonia transportation infrastructure is an attractive reason for the use of green ammonia as a traded energy vector. Merchant grey ammonia pricing is influenced by natural gas costs and supply vs demand balance. Significant underutilised ammonia production capacity exists in China. However, this is land-locked inland production and is not available to international markets, so utilisation of internationally tradeable ammonia capacity has been high. The pricing volatility of grey hydrogen and the lack of availability of excess capacity for international trade are key drivers for the development of new green ammonia capacity to supplement existing grey ammonia production. Traditionally, ammonia plants have introduced air to the ammonia synthesis loop to bring in the required nitrogen. Using pure nitrogen instead of air means no oxygen is circulating in the ammonia synthesis loop. This minimises energy losses and reduces the required plant size, resulting in Opex and Capex reductions. In recent years, small to medium ammonia plants have tended to use a pure nitrogen feed from an on-site nitrogen generator.

The inputs required to operate a nitrogen generator are electrical power, cooling water, and ambient air. Using renewable electricity, the operation of a nitrogen generator can be fully sustainable. For the future generation of green ammonia projects, nitrogen generators will be ideal for supplying the required green nitrogen to react with green hydrogen to produce green ammonia. Solid oxide electrolysis for Haber-Bosch process integration Green electrons are highly valuable and result from significant infrastructure investment in wind and solar parks or hydro dams. Using them to produce green ammonia is essential to optimising project economics and reducing the cost of the energy transition. Using a solid oxide electrolyser or SOEC is highly efficient if hydrogen is to be converted to green ammonia (see Figure 7 ). Steam generated by the Haber-Bosch ammonia synthesis reactor can supply up to 70% of the steam required for solid oxide electrolysis. Methanol synthesis is also exothermic. However, the heat liberated by the reaction of syngas to form methanol is generally used to purify the methanol using distillation. Therefore, alkaline, PEM or SOEC electrolysis technologies can be equally efficient for methanol production.

Stephen B. Harrison sbh@sbh4.de