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A significant problem with ammonia-as-fuel is that it combusts to produce NOx, already a major pollutant and public health issue in many parts of the world. As

A significant problem with ammonia-as-fuel is that it combusts to produce NOx, already a major pollutant and public health issue in many parts of the world. As with diesel engines, the exhaust can be scrubbed with AdBlue selective catalytic reduction, but this adds to the cost and complexity.

jessaustin 6y ago
This is exactly wrong. Ammonia and its precursor urea are the chemicals used in SCR to reduce NOx.
- Reason077 6y ago
  4 NH3 + 5 O2 → 4 NO + 6 H2O A subsequent reaction leads to NO2: 2 NO + O2 → 2 NO2
  - jessaustin 6y ago
    Per wikipedia [0], the combustion equation is: 4 NH3 + 3 O2 → 2 N2 + 6 H2O That's nitrogen and water. Apparently, your scary equation requires the presence of appropriate catalysts, which catalysts could certainly be excluded from any particular engine. [0] https://en.wikipedia.org/wiki/Ammonia#Combustion https://en.wikipedia.org/wiki/Ammonia#Combustion
    - Reason077 6y ago
      Yes, but if you read just a little further in that article: “The combustion of ammonia in air is very difficult in the absence of a catalyst” Practically speaking, NOx formation is difficult to avoid in combustion applications and would require safeguards/mitigations just as it is in Diesel engines. Another issue is that the whole concept of green ammonia relies on having sufficient green hydrogen feedstock to produce it with in the first place. I do recommend reading the Royal Society policy paper linked in the parent post!
mhandley 6y ago
These issues are discussed in the report - see pages 28/29 for example. Another approach discussed in the context of transport fuels is to use ammonia as an effective storage medium for hydrogen - cracking ammonia back to hydrogen before feeding it to a PEM fuel cell. That also has some challenges, notably in minimizing ammonia in the hydrogen, which isn't good for fuel cells. In the end, I do think we'll end up with liquid synthetic fuels being essential for decarbonization - the question is not if, but which?

jessaustin 6y ago

This is exactly wrong. Ammonia and its precursor urea are the chemicals used in SCR to reduce NOx.

Reason077 6y ago
4 NH3 + 5 O2 → 4 NO + 6 H2O A subsequent reaction leads to NO2: 2 NO + O2 → 2 NO2
- jessaustin 6y ago
  Per wikipedia [0], the combustion equation is: 4 NH3 + 3 O2 → 2 N2 + 6 H2O That's nitrogen and water. Apparently, your scary equation requires the presence of appropriate catalysts, which catalysts could certainly be excluded from any particular engine. [0] https://en.wikipedia.org/wiki/Ammonia#Combustion https://en.wikipedia.org/wiki/Ammonia#Combustion
  - Reason077 6y ago
    Yes, but if you read just a little further in that article: “The combustion of ammonia in air is very difficult in the absence of a catalyst” Practically speaking, NOx formation is difficult to avoid in combustion applications and would require safeguards/mitigations just as it is in Diesel engines. Another issue is that the whole concept of green ammonia relies on having sufficient green hydrogen feedstock to produce it with in the first place. I do recommend reading the Royal Society policy paper linked in the parent post!

mhandley 6y ago

These issues are discussed in the report - see pages 28/29 for example. Another approach discussed in the context of transport fuels is to use ammonia as an effective storage medium for hydrogen - cracking ammonia back to hydrogen before feeding it to a PEM fuel cell. That also has some challenges, notably in minimizing ammonia in the hydrogen, which isn't good for fuel cells. In the end, I do think we'll end up with liquid synthetic fuels being essential for decarbonization - the question is not if, but which?