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Comment by mhandley | Hacker News Reader

One thing the article misses is that green hydrogen may in many cases be better used as the feedstock for other products such as green ammonia. Other than its use in fertilizer, ammonia can also be burned as a fuel. Using green ammonia as a fuel is one of our best bets for decarbonizing shipping, for example. And it has the additional advantage that we already have a fairly large scale distribution infrastructure for ammonia, so there's less of a chicken-and-egg bootstrap problem for early adopters. This Royal Society report is a pretty good introduction to the subject: https://royalsociety.org/-/media/policy/projects/green-ammon...

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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 with diesel engines, the exhaust can be scrubbed with AdBlue selective catalytic reduction, but this adds to the cost and complexity.

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

4 NH3 + 5 O2 → 4 NO + 6 H2O

A subsequent reaction leads to NO2:

2 NO + O2 → 2 NO2

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

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!

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?

That looks interesting, now I wonder how dangerous/polluting a tank leak can be for ammonia compared to diesel and hydrogen

Well, hydrogen tanks are kept at crazy crazy pressures, so if you stand next to an active leak it will literally cut you in half - high pressure tanks are pretty scary stuff. On the other hand, pure ammonia will kill you straight away if you breathe it in. Diesel is not great, but short of drinking it it won't kill you, and it doesn't really like exploding either. I'm not really trying to make an argument against hydrogen/ammonia, I just find this stuff interesting.

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