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  • Ammonia as a means of transport

    When people talk about the "ramp-up of the hydrogen economy" today, ammonia does not necessarily immediately come to mind. Yet ammonia has been sufficiently tested as a transport medium for hydrogen and is already being used as such - and on an industrial scale.

    Transport can take place in containers at normal temperature and low pressures, which significantly improves the energy balance compared to transporting liquefied hydrogen in pressure tanks. For this purpose, ammonia is produced, transported and split into nitrogen and hydrogen at the point of consumption.

    If the EU Commission's goals in the Green Deal are to be achieved, there will be no way around the mass use of green ammonia and its import into Europe. Germany is following this path with its National Hydrogen Strategy. The focus here is on decarbonizing the chemical industry on the one hand and steel production on the other. Both need hydrogen as a basic material for other products or as a reducing agent to dissolve oxygen from pig iron. In short, the use of hydrogen - especially in its green variant - is simply without alternative for both industries in the future if they want to achieve their climate targets. Yet the demand for hydrogen is considerable: to date, the German chemical and pharmaceutical industries use the equivalent of 137 terawatt hours of natural gas annually, according to the German Chemical Industry Association (VCI). Out of a total of 1,000 terawatt hours of natural gas consumed in Germany per year, this is still just under 14 percent, with about a quarter of this being used as a raw material for hydrogen and nitrogen production. These 34 terawatt hours will have to be covered by green hydrogen in the future. This cannot be done without imports. Consequently, ways and means are needed to manage this transport - and this is where ammonia comes into play.

    What is ammonia?

    The chemical formula of ammonia is NH3. At normal pressure and room temperature, ammonia is gaseous; when cooled below -33 °C or compressed to just under 9 bar, the level of liquid gases such as propane and butane, it becomes liquid. Its density is 0.73 kg/m³ in gas form and 0.68 kg/l in liquid form. Under 9 bar pressure, it becomes liquid at 20 °C. Its combustion produces nitrogen and water, and regasification, such as hydrogen transport, produces nitrogen and hydrogen.

     

    Industrial importance
    Around 200 million metric tons of ammonia are produced worldwide every year. This accounts for 2 percent of global energy production. 75 percent of ammonia is used for fertilizer production.

    Ammonia is also used, for example (but not only) as "Ad Blue" in the exhaust gas cleaning of diesel vehicles - here in dissolved form as urea. Ammonia itself could also be used as a fuel. However, with an energy density of 17.2 MJ/kg or 5.2 kWh/kg, it has only half the energy density of gasoline or one-sixth of that of liquid hydrogen, which is why its use has not yet been considered.

     

    Production of ammonia
    Ammonia is synthesized from nitrogen and hydrogen using the so-called Haber-Bosch process. In this process, nitrogen and hydrogen react at 200 bar and 450 °C using an iron catalyst according to the formula N2 +3H2 → 2NH3. When hydrogen is recovered from ammonia, the reverse formula applies.

     

     

    Research is currently being conducted into a CO2-free variant of production. This is because the emissions from the conventional production process, at 1.9 metric tons of CO2 per ton, are immense, particularly due to the energy-intensive extraction of nitrogen from the air and the steam reforming of natural gas (methane, CH4) into hydrogen and carbon dioxide. Only if this can be achieved can ammonia be considered as a "green", because climate-neutral, carrier of hydrogen.

    A carbon-neutral process would use renewable energy sources and is called power to ammonia (PtA). Since this would require the availability of large quantities of inexpensive green power sources for both the electrolysis of water to hydrogen and the production of nitrogen, the only possible production sites would be sun- and wind-rich regions of the world, such as North Africa, South America and the Middle East. From there, Europe could easily be supplied by ship.

    Storage and transport capability of hydrogen
    Ammonia can also act as a transport medium for hydrogen. The advantages are obvious: Hydrogen has to be expensively cooled and transported in refrigerated tanks, either in liquid form at 20 °C and a volume-specific energy density of 2.4 kWh/liter or in gaseous form at 700 bar in CFRP pressure cylinders with 1.35 kWh/liter. The energy consumption here is immense and corresponds to about 30 percent of the calorific value of the transported hydrogen in the case of liquefaction and 12 percent in the case of pressurized storage.

    For ammonia, either pressure tanks or refrigerated tanks are sufficient. Pressure tanks are common because of their lower energy consumption.

    Adsorption or metal hydride storage would also be possible. But research into these is still in its infancy. So far, the disadvantages outweigh the benefits, such as low storage capacity and slow buildup or release of hydrogen. The biggest competition for ammonia as a transport medium for H2 is currently organic carrier substances, since they are already liquid under normal pressure and completely non-toxic. Above all, the so-called LOHC (liquid organic hydrogen carriers) and in particular dibenzyltoluene (DBT) are suitable for this purpose.

    Ammonia, on the other hand, has a great deal of experience in handling the compound, as it is already one of the world's most traded and transported chemicals. It can be transported safely and split into nitrogen and hydrogen at temperatures around 700 °C at the point of use.

    Does that make sense?

    Inevitably, ammonia plays a major role in hydrogen transport when large quantities and long transport distances are involved. So far, it is the only means of transport that can perform this task. Production regions for green hydrogen and thus for green ammonia using the PtA method would be the Middle East, North Africa, Australia and Chile. The ammonia produced there can only reach Europe by ship.

    Summary

    Ammonia is currently and probably for the foreseeable future the most energy-efficient solution for transporting green hydrogen produced by electrolysis from the producing countries to Europe by water, but also by land. The great advantage lies in the vast experience with the transport of this chemical, for which a complete infrastructure already exists. This can be expanded specifically for the needs of hydrogen consumption. For all other possible transport technologies, there is neither comparable experience nor a corresponding infrastructure. This would first have to be installed at great expense.