Ammonia for Energy
Hydrogen, the dream carbon-free fuel that releases no emissions when burnt. But we only wish it was this simple. There are many obstacles that prevent us from using hydrogen widely, including the high expenses and challenges of storage and transportation. But there are alternate solutions. A very interesting one would be to use other substances such as ammonia as a medium for hydrogen. Ammonia is a promising hydrogen carrier due to its high hydrogen density, ease of liquefaction at ambient temperature, and low production cost.
Hydrogen is a very good energy vector, but the difficult storage conditions make the use of hydrogen challenging. H2 is a very small substance and therefore it is very difficult to confine. Special material and expensive methods must be used to ensure no leakage happens. Expensive storage conditions, low energy density, and high production costs are the key obstacles in the advancement of hydrogen technologies. To be able to profit from hydrogen, we need to be able to store it more densely. There are three main ways that this can be done. Hydrogen can be compressed, liquefied, or chemically combined in other substances. In this article, we are focusing on hydrogen stored in ammonia. Ammonia containing 17.5 wt% hydrogen is an ideal carbon-free fuel for fuel cells. We will further discuss the advantages and challenges of hydrogen stored in ammonia.
Strengths and Opportunities of Ammonia
Hydrogen is not energy-dense, and therefore, To make good use of this substance, we need to store it at very low temperatures or very high pressures. Hydrogen is 3.2 times less energy-dense than natural gas and 2700 times less energy-dense than gasoline[1]”Hydrogen for transport and the B&E report,” [Online]. Available: https://planetforlife.com/h2/h2swiss.html.. The raw energy density of ammonia is 11.5 MJ/L, which is higher than the 8.491 MJ/L for liquid hydrogen [2]R. Lan and S. Tao, “Ammonia as a suitable fuel for fuel cells,” 2014.. Compared to compressed or cryogenic hydrogen, ammonia is easier to transport and remains much more energy efficient. The cost to produce, store, and deliver hydrogen from NH3 is also considerably lower [3]R. Lan and S. Tao, “Ammonia as a suitable fuel for fuel cells,” 2014.. Ammonia can be produced from any raw energy source, including all renewable, fossil, and nuclear sources. Also, it is cost-competitive with gasoline as a transportation fuel. The infrastructure for ammonia already exists, while for hydrogen new fueling stations have to be built, which is a large investment. And finally, NH3 contains no carbon and therefore it will not release CO2 when used in fuel cells or combustion.
However, as perfect as ammonia seems, it has its imperfections.
The Challenges, Weaknesses & Threats of Ammonia
Although ammonia is a very promising fuel, its vapors are a fire and explosion hazard at concentrations between 16% and 25%. However, mixtures involving ammonia contaminated with lubricating oil from the system may have a much broader explosive range. Also, it is a toxic gas under ambient conditions that is harmful to the eyes and skin. In the liquid phase, in case of contact with skin, it can cause frostbites. [4]”Hazards of Ammonia Releases at,” United States Environmental Protection Agency, 2001.[5]Walter S. Kessler, “The Good, the Bad, and The Ugly of Using Anhydrous Ammonia Refrigerant in The Process Industries”.
Ammonia is toxic but in contrast with hydrogen, it’s detectable by human sense. The ammonia released from an ammonia tank during a car accident may cause potential safety problems but this can be solved through the application of metal amines with low ammonia partial. All in all, NH3 would be as safe as the use of gasoline as a transportation fuel[6]A. Klerke, Claus Hviid Christensen, J. K. Nørskov and T. Vegge, “Ammonia for hydrogen storage: challenges and opportunities,” 2008.
Besides ammonia being toxic, it causes problems when used in fuel cells. For example, in Proton Exchange Membrane Fuel Cells (PEMFC), proton-conducting materials based on acidic properties cannot be used as electrolytes for ammonia fuel cells due to the reaction between ammonia and acids. A thorough analysis of the problems in fuel cells is left to another article.
References & Notes
| ↑1 | ”Hydrogen for transport and the B&E report,” [Online]. Available: https://planetforlife.com/h2/h2swiss.html. |
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| ↑2, ↑3 | R. Lan and S. Tao, “Ammonia as a suitable fuel for fuel cells,” 2014. |
| ↑4 | ”Hazards of Ammonia Releases at,” United States Environmental Protection Agency, 2001. |
| ↑5 | Walter S. Kessler, “The Good, the Bad, and The Ugly of Using Anhydrous Ammonia Refrigerant in The Process Industries”. |
| ↑6 | A. Klerke, Claus Hviid Christensen, J. K. Nørskov and T. Vegge, “Ammonia for hydrogen storage: challenges and opportunities,” 2008 |
