RE: Green Energy Boom # EUA230 Dec 2021 10:05
Recently the concept of an ammonia economy has gained eminence [1]. Like hydrogen, ammonia is carbon free and can be produced from any energy resource. However there are also some significant advantages in terms of storage and transport. Ammonia can be liquefied at room temperature at pressures of 8-10 bar and stored in a similar manner to propane, whereas hydrogen requires expensive cryogenic storage. In addition, ammonia allows for safer handling and distribution than hydrogen. Although it is toxic, its smell can be detected even at safe concentration levels (< 1 ppm). Ammonia has a narrower flammable range than hydrogen and is actually considered nonflammable when being transported, whereas hydrogen burns with an invisible flame. Ammonia is the second most widely produced commodity chemical in the world (second to sulfuric acid), with over 100 million tons per year being transported [2], and as such its worldwide distribution system is well established. Such is not the case for hydrogen. In fact, one major drawback with hydrogen technologies is the fact that the necessary hydrogen infrastructure does not presently exist. Essentially the ammonia economy can achieve the same benefits of a hydrogen economy, but using infrastructure that already exists.
Ammonia provides a source of hydrogen for fuel cells. It contains 17% hydrogen by weight, which can be extracted via thermal catalytic decomposition or electro-oxidation. Alternatively ammonia may be oxidized directly in fuel cells without the need for a separate reactor. Table 1 compares the storage capabilities of various fuels based on their higher heating value (HHV) [3]. Hydrogen has a very low energy density (per volume) because of its low density. Ammonia’s energy density is comparable to that of compressed natural gas (CNG) and methanol, but lower than gasoline and liquefied propane gas (LPG). Per unit volume, the cost of hydrogen energy is lower than that of ammonia energy, but hydrogen has less energy stored per volume than ammonia. Per unit energy, ammonia is the cheapest energy source listed in Table 1 – estimated at US$13.3/GJ. Note that these values are based on the HHV of the fuel and do not account for conversion of this energy to useful forms. The life cycle production cost of energy from ammonia is estimated at US$1.2/kWh compared to US$3.8/kWh for methanol and US$25.4/kWh for hydrogen [4]. Thus ammonia presents a very viable and cost effective fuel for fuel cells.
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