Everyone wants green hydrogen, some sooner than others. The areas of application for the eco-gas in the process industry are well known; so are the methods of production - centralized or decentralized, domestic or foreign. But between production and application lies transportation. Among the numerous media suitable for this purpose, ammonia stands out above all. Why is that? What are the chemical's strengths, and what are its risks and side effects as a hydrogen transport medium? An overview.
Basically, a distinction is made between four types of hydrogen transport:
Gaseous in high-pressure containers or pipelines.
Liquid in deep-freeze containers at -233 °C
Chemically stored in solids (especially metal hydrides)
Chemically stored in liquids (methanol, ammonia, LOHC etc.)
Pressure vessels are still by far the most common transport medium. The alternatives either lack the appropriate infrastructure or are not yet economically viable - the cooling of liquid hydrogen in particular is considered extremely cost-intensive.
But transport in a high-pressure tank is not optimal, especially for longer distances. Put simply, even large tanks can only store a relatively small amount of energy. The tanks are also often transported by truck, which reduces the climate-friendly effect of hydrogen use. The production of the tanks, which are mostly made of carbon fiber-reinforced plastics (CFRP), is also not climate-neutral.
The chemical formula of ammonia (NH3) suggests it: In an ammonia molecule, there are three hydrogen atoms for every nitrogen atom. This means that large quantities of hydrogen can be transported in comparatively small volumes of ammonia. In addition, the gas can already be liquefied at >8 bar or <-33 °C - i.e., with significantly less energy input than is required for the liquefaction of pure hydrogen.
Ammonia has an advantage over newer substances such as LOHC in that it is one of the most produced chemicals in the world: 185 megatons (Mt) were produced worldwide in 2020, according to Hydrogen Europe. Fertilizer producers are the largest customer. So global trade in ammonia is already happening on a large scale; countless producers could ramp up their production if demand increases.
And what is missing from "NH3"? That's right, "C." Unlike alkanols such as methanol, ammonia does not contain carbon and therefore does not run the risk of emitting additional greenhouse gases into the atmosphere. Nevertheless, its production is not environmentally friendly.
How fossil-free is ammonia?
In 1908, the Haber-Bosch process, which is still used today, was patented; it has been carried out on an industrial scale since BASF was founded in 1913. In the Haber-Bosch process, hydrogen is converted into ammonia with nitrogen from the ambient air at high heat and pressure. The process has been optimized over decades and is considered extremely energy-efficient. Nevertheless, it is a source of CO2, which is due to hydrogen of all things: Today, this is generally still gray, meaning it comes from the classic steam reforming of fossil fuels.
In order to remedy this situation, many manufacturers have been using plants for the production of "green ammonia" for some years now. The term simply means that the hydrogen for ammonia synthesis was produced without CO2 emissions. In other words, ammonia can only be called fossil-free if it is made from green hydrogen. From this point of view, therefore, a market ramp-up of green hydrogen would also be a way of driving forward the decarbonization of ammonia production.
Project 1: Ammonia synthesis at Han-Ho H2 Australia
A recent example: at the beginning of August, Thyssenkrupp's plant engineering subsidiary Uhde announced that it is conducting a feasibility study for an ammonia project called "Han-Ho H2" in Queensland on behalf of the Australian energy and hydrogen producer Ark Energy. The aim of the project is to export green hydrogen produced in Australia to South Korea in the form of ammonia. A volume of up to 1.8 million t per year is planned.
Uhde's study will now examine the factors under which the ammonia synthesis plant can operate economically. The Dortmund-based company will run through various scenarios to identify an optimum overall concept. In the best-case scenario, Uhde would then also take over the engineering of the plant based on Uhde's proprietary ammonia synthesis technology.
Why not ammonia?
So what else should speak against ammonia? First of all, it is still a toxic substance that can cause serious damage to health and nature in both liquid and gaseous form. But more problematic from the point of view of hydrogen ramp-up is that recovering hydrogen from ammonia is still too expensive.
Transporting hydrogen by means of ammonia has something of alchemy about it: first, hydrogen is transformed into ammonia so that it can be transported to its place of use. Then, however, it usually has to be converted back into pure hydrogen. This reverse reaction is known in chemistry as ammonia reformation. Like synthesis, reformation requires high temperatures (>450 °C), but unlike synthesis, it is most efficient at low pressures.
To keep the energy input of on-site reforming as low as possible, so-called catalysts are required. Metals such as iron, nickel or cobalt are just as suitable for this as the disproportionately more expensive precious metals. The problem is that research into suitable catalysts is still in its infancy. This means that the reforming of ammonia – commonly referred to as "cracking" – is almost as intensive as its synthesis. The good news is that several research projects are working on optimizing catalysts. One example is AmmoRef, a subproject of the TransHyDE hydrogen lead project funded by the German Federal Ministry of Education and Research.
Example 2: Ammonia reforming in the port of Antwerp
What do ammonia reformers look like today? Many plants are still at the project stage, with new ones being added regularly. By their very nature, they are usually located near major ports. In mid-March, the French gas company Air Liquide announced that it would build a pilot plant for reforming ammonia on an industrial scale in the port of Antwerp. This will involve the use of a new type of technology.
The so-called "ammonia cracker" – it would be the first plant of its kind in Belgium – is expected to emit less CO2 by using more efficient technology. The pilot plant, which combines the novel process with proprietary technologies, is expected to be operational as early as 2024. The Flemish government has pledged financial support for the project through VLAIO (Flemish Agency for Innovation and Entrepreneurship).
Ammonia has potential. As is often the case, however, the "ifs" must be considered: Next to high-pressure pipelines, it is arguably the most promising medium for hydrogen transport at present, if it is derived from green hydrogen, if transport is safe and environmentally sound, and if both synthesis and recovery are energy efficient.
The biggest advantage is probably that ammonia can be produced wherever green hydrogen is also produced in large quantities, for example in sun- and wind-rich, remote regions such as the deserts of North Africa. From here, it can then be transported comparatively easily by ship to Europe and elsewhere. If efficient cracking plants are then available in the ports, it can be quickly cracked again there so that the hydrogen can then be transported quickly and inexpensively - via a pipeline network that has yet to be built - to industrial consumers.
At the end of May, the Port of Rotterdam investigated how much ammonia could be converted there in perspective. The answer: 1 million tons – per year. The study concludes that ammonia is a "realistic and safe method for large-scale hydrogen transports", and that a central cracking plant on the port site is more efficient than several decentralized ones.
The fact is: if the hydrogen boom also leads to an ammonia boom, companies would be well advised to look into the relevant technologies in a timely manner.
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