Although green hydrogen consumed in Europe is economically and sustainably best produced locally within Europe, many European countries are lagging behind national targets for renewable electricity production and grid expansion. The import of hydrogen from non-European countries with great potential for the production of green hydrogen is therefore an important complementary option for achieving the climate targets. One possible form of import is transportation in the form of the hydrogen derivative ammonia. This article and the associated fact sheet are intended to provide a general overview of the opportunities and challenges of ammonia as an energy carrier and its transportation options.
Opportunities and challenges as a green energy carrier
Currently, 183 million tonnes of ammonia are produced worldwide, a large proportion of which is used for the production of fertilizers . Therefore, there are many years of extensive experience in both the production and global transportation of industrial quantities of ammonia. The conventional method for producing ammonia is the Haber-Bosch process. In this process, hydrogen reacts with nitrogen at temperatures between 300 and 550 °C and a pressure of 20-35 MPa in the presence of an iron oxide catalyst to produce ammonia . Although most of the hydrogen required for this currently comes from fossil sources, the Haber-Bosch process is nevertheless an established process that can be carried out in the future on the basis of green hydrogen with slight adjustments to the process configuration. In contrast to the conventional process, the electrolytically produced hydrogen must be compressed for further synthesis. In addition, the nitrogen required for the synthesis gas mixture no longer occurs directly in the hydrogen production process step, but must be obtained via an air separation plant .
The advantages of transporting hydrogen in the form of ammonia lie primarily in its high volumetric energy density, which means low energy consumption for transportation. In its liquid state, which is reached at ambient pressure at temperatures below -33°, ammonia contains 121 kg H2/m3 , which is 1.7 times more hydrogen per cubic meter than liquid hydrogen, which requires temperatures of -253°C for liquefaction . But it is not only the high volumetric density of ammonia that offers advantages for transportation. In the future, ammonia itself can also be used as a fuel for ship transportation instead of heavy fuel oil, thus contributing to the reduction in emissions from hydrogen imports. At the ports, existing LNG infrastructure can also be converted to the import of ammonia by ship with moderate adjustments. .
The imported ammonia can either be used directly, for example in fertilizer production, or converted back into hydrogen. The synthesis of hydrogen to ammonia and the conversion back to hydrogen by cracking are shown in Figure 1 for the example of 1 kWh of hydrogen at atmospheric pressure. It can be seen that a loss of 43% is recorded for the conversion processes alone. However, if the electrolysis of hydrogen takes place directly at the ammonia synthesis site where the hydrogen is available at around 30 bar, some of the compression work is eliminated, which leads to an improvement in the efficiency of the process chain. The splitting of ammonia to hydrogen in particular requires high temperatures in the range of 500 to 1000 °C, depending on the catalyst used . Commercial cracking plants are currently limited to capacities of around 20 GWh of hydrogen per year . Cracking plants with large capacities of up to 2 TWh of hydrogen production per year are still at the development stage . Depending on the application, purification with additional energy input is also necessary after the reconversion of ammonia to hydrogen .
Due to the toxic effect of ammonia on humans and the environment, it is classified as a hazardous substance. The transportation of ammonia is therefore subject to strict regulations, such as the use of specially approved rail tank wagons  .
Ammonia can be transported by truck on the road, by rail, by ship and via pipelines. While the sea route is the most commonly chosen option for global transportation over long distances, ammonia is currently mainly transported by rail within Europe. In Europe, pipeline transportation is only used for short distances of up to 12 km in industrial areas , while in the United States there is an extensive pipeline network of around 5000 km .
Figure 1 (top center) compares the energy required for the transport options (calculations based on  ). According to this, the comparatively highest energy requirement of 1.65 kWh for the transportation of one MWh of ammonia is for truck transport by road for the transport distance of 100 km. The transport volume is limited to around 190 MWh of ammonia per trip, meaning that the transportation of large quantities of ammonia over long distances cannot be represented efficiently by this option .
By rail, the energy requirement for a distance of 100 km is 0.95 kWh per MWh of ammonia transported. Transport volumes of around 60 GWh of ammonia per trip are possible . This form of transportation requires the availability of a rail network and therefore offers little flexibility. On the other hand, import ports are generally already equipped with an existing rail infrastructure that can be used for ammonia transportation .
For large transport volumes and distances, transportation by ship and pipeline are the most suitable options. The specific energy consumption for ship transportation is slightly higher than for pipeline transportation. However, significantly less energy is required at the terminals than for compression in pipeline transportation. For a more precise assessment of the two long-distance transport options, Figure 1 (center) compares the transport costs depending on the transport distance (calculations based on ). Both investment costs and operating costs over the entire lifetime are taken into account. Accordingly, from a transport distance of around 500 km, transportation by ship is the cheaper alternative. For shorter distances of less than 500 km, pipeline transportation is more attractive. Other sources even cite a distance of 1000 km, at which pipeline transportation is the more economical option . The costs for pipeline construction are primarily dependent on the transport capacity and therefore on the costs of the pipe material for the required diameter and pumps.
Centralized vs. decentralized reconversion into hydrogen
In addition to the direct use of ammonia, e.g. for fertilizer production or in ammonia burners, there is also the possibility of converting it back into hydrogen through so-called cracking. The hydrogen obtained can be used, for example, at industrial sites, filling stations or to generate electricity. When importing hydrogen in the form of ammonia, there are therefore two options for the further distribution of the containing hydrogen:
- The imported ammonia is cracked into hydrogen centrally at the ports of import and the hydrogen is transported in a gaseous state via pipelines to the consumption sites.
- The imported ammonia is transported by pipeline and cracked into hydrogen decentrally at the consumption locations.
As part of the TransHyDE-Sys research project, FfE is currently developing an optimization model for the expansion of hydrogen infrastructure in Europe. This model aims to minimize the infrastructure costs for hydrogen and ammonia. The calculated systemic optimum of the model provides insights for each ammonia import port under consideration as to which option causes the lower overall costs: the centralized variant with cracking at the port and subsequent hydrogen transport or the transport of ammonia to the consumption centers with decentralized cracking to hydrogen.
Within the TransHyDE project, AmmoRef is researching the industrial feasibility of cracking processes. The project partners are testing an adapted management system for high- and low-pressure processes on a pilot plant scale. New catalysts developed by the project partners will also be used. These catalysts will be passed on to the TransHyDE project CAMPFIRE, where they will be tested on a larger scale. The aim of CAMPFIRE is to demonstrate the entire transport chain for hydrogen based on green ammonia.
Further information on the TransHyDE lead project can be found at https://www.wasserstoff-leitprojekte.de/leitprojekte/transhyde.
- TransHyDE-Sys – Systemanalyse zu Transportlösungen für grünen Wasserstoff – FfE
- TransHyDE-Sys: Aspekte der Systemanalyse in einem Videobeitrag erklärt – FfE
- Importe von grünem Wasserstoff und seinen Derivaten nach Deutschland in einer klimaneutralen Zukunft – FfE
- Beitragsreihe Wasserstoff: Wie wird Wasserstoff transportiert? – FfE
- InfraInt – Infrastrukturausbau-Modell für Wasserstoff und CO₂ – FfE
- Wie gelangt grüner Wasserstoff zum Verbraucher? Potenzielle Infrastruktur für Wasserstoff in Deutschland – FfE
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