Ammonia is a chemical compound consisting of nitrogen and hydrogen with the formula NH₃. This colorless gas has a characteristic pungent odor. It liquefies at -33°C or at a pressure of about 8 bar. Its high hydrogen content of 17.8 percent makes ammonia an important energy carrier in the hydrogen economy.
In its liquid form, ammonia has a high energy density of 3.3 kWh/l (by comparison: liquid hydrogen at −253 °C: 2.4 kWh/l; compressed hydrogen at 1,000 bar: 1.7 kWh/l). In addition, existing infrastructure can be used to transport the chemical.
Manufacturing
Ammonia is produced primarily using the Haber-Bosch process. This process, which is still in use today, was patented in 1908; it has been carried out on an industrial scale since BASF was founded in 1913. In this process, nitrogen is extracted from the air and combined with hydrogen under high pressure and at high temperatures.The process has been optimized over decades and is considered extremely energy-efficient. Nevertheless, it is a source of CO₂, primarily due to the hydrogen: Today , this hydrogen is generally still “gray ,” meaning it is produced via conventional steam reforming of fossil fuels.
The production of green ammonia is therefore becoming an increasingly important aspect. Unlike conventional production methods, this process uses renewable electricity to electrolyze water and produce hydrogen. This green hydrogen is then combined with nitrogen from the air to form ammonia. Another way to produceCO2-neutral ammonia is through an electrochemical process (“electrochemical ammonia synthesis”). In this process, the hydrogen produced by the electrolysis of water is made to react directly with nitrogen in the presence of catalysts and membranes to form ammonia.
Areas of Application
Ammonia is one of the most widely produced chemicals in the world: According to Hydrogen Europe, 185 megatons (Mt) were produced globally in 2020. Fertilizer manufacturers are the largest consumers. However, its applications are diverse. In addition to fertilizer production, the chemical is also used in the chemical industry and is being considered as a fuel for ships.
Challenges
Ammonia is a toxic substance that, in both its liquid and gaseous forms, can cause serious harm to human health and the environment. The toxicity of this substance therefore requires strict safety measures.
In addition, the energy required for production is rather high, and the conversion back to hydrogen is still inefficient and expensive. In chemistry, this reverse reaction is known as ammonia reforming. Like the synthesis process, reforming requires high temperatures (>450 °C); however, unlike the synthesis process, it is most efficient at low pressures. To keep the energy consumption 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 purpose as the far 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 energy-intensive as its synthesis.
Video
This video was created as part of the OTH Regensburg Hydrogen Atlas project and illustrates the basic principle of ammonia synthesis:
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