Note: The graph assumes H2 use by the end user. LH2, CH2 and ammonia can in many cases also directly serve as input or feedstock for the end user without prior transformation to H2.
Source: adapted from Notteboom, T., Haralambides, H., 2023, Seaports as green hydrogen hubs: advances, opportunities and challenges in Europe, Maritime Economics and Logistics, 25 (1), 1-27
Most of the available techniques to transport hydrogen transport over long distances require the conversion of wind or solar energy to hydrogen carriers in or near the exporting port and the transport of a suitable hydrogen carrier to importing areas. The most commonly considered supply chain solutions for the import of green hydrogen produced overseas include :
- Hydrogen can be transported in liquid form (LH2) at an extremely low temperature in its pure form, but cooling to below -252.87 °C consumes a lot of energy. A wide range of large-scale hydrogen liquefaction methods and approaches exist (see for an overview Aasadnia and Mehrpooya, 2018);
- Hydrogen can also be compressed in hydrogen tanks at very high pressures to compressed hydrogen (CH2 or CGH2);
- Hydrogen can be packed in ammonia (NH3). Reacting green hydrogen with nitrogen forms green ammonia. This allows hydrogen to be efficiently and safely transported in large volumes. Ammonia synthesis is traditionally achieved via the Haber-Bosch process, converting nitrogen (N2) to ammonia by a reaction with hydrogen using a metal catalyst under high temperatures and pressures. In recent years, a lot of research has been dedicated to explore sustainable synthesis of ammonia as an alternative to the capital- and often carbon-intensive fossil-fuel-driven Haber–Bosch process. The ammonia can then be stored and converted again to green hydrogen, but turning ammonia back to hydrogen requires decomposition (cracking) at high temperatures. Green ammonia is immediately usable as CO2-free fuel, for example in shipping, or as a raw material in the production of fertilizers;
- Hydrogen can be transported by coupling to other Liquid Organic Hydrogen Carriers (LOHCs). These are organic compounds that can absorb and release hydrogen through chemical reactions. A good example is methylcyclohexane (MCH), which is a liquid obtained from the chemical reaction of hydrogen and toluene. After the initial hydrogenation step, MCH can be transported by ship, truck, or tank wagon. Dehydrogenation ensues, followed either by direct use of the obtained hydrogen, or its conversion back into electricity. The byproduct toluene can be returned to the hydrogenation plant for reuse.
The above processes lead to various possible green hydrogen value chains. Investigating the technical and economic feasibility of such value chain solutions is crucial for the sustainable development of hydrogen-based energy production and consumption.