The main "why" is a carbon-free fuel (when produced by solar) in a relatively dense form. Liquid ammonia has higher energy density than liquid hydrogen, is easier to store, and is already used a bulk chemical around the world. It also is more energy efficient than liquifying hydrogen.

If you imagined what oil would look like without carbon, ammonia is pretty close. The toxicity probably prevents cars and trucks from using ammonia, but for marine power or grid energy, it seems feasible.

Also, the co-combustion fuel can be supplied on-site by cracking the ammonia to hydrogen.

https://www.kallanishenergy.com/2020/07/08/air-products-to-b...

I'm confused. If we're stipulating carbon-neutral liquid fuel from renewable energy, what precisely is wrong with depolymerizing organic matter into ordinary hydrocarbons? Sure it generates CO2 at the point of use, but it's offset by the photosynthesis that made the organic matter. As long as the carbon cycle is a closed loop there shouldn't be problem, right?

I don't think the closed loop argument is sound. We care about the total amount of carbon in the loop, we care specifically about the amount in the atmosphere. That's the equilibrium point, and there's a lot you can do to push around the equilibrium point in a closed system.

Consider - we've been digging up ancient, sequestered carbon and putting it in the atmosphere on a mass industrial scale for hundreds of years. I don't think that anything we can do within the confines of a closed loop system is going to remotely compare with that.

Or, to put it another way - we've been treating the atmosphere as a sink. But if we stop digging up carbon, we'll still need carbon, and it will become a source instead. By buffering that resource into fuel stocks and plastics and everything else we use carbon for, the only direction CO2 levels can go is down.

Cargo/freight shipping could be a prime target for a cleaner ammonia base fuel as most of the ships currently run dirty semi-processed petroleum, “bunker fuel”. It’s a hyper-cost sensitive market so without regulation I’d doubt ammonia would compete. Still it could be a feasible option to reduce pollution. Some Western countries might accept ammonia or similar diesel replacement, as it’d be a sort of natural tariff, but likely China would lobby hard against such a measure. There’s have to be a very strong push from a few nations, which I’ll doubt we’ll see anytime soon.

What is the energy return of energy invested on getting hydrogen from solar and then turning into ammonia?

It's really poor, you need at least hydrogen for seasonal energy storage.

Are you counting the energy cost of preventing or mitigating climate change?

It's not as good as plugging ibto the mains or using batteries, but it could well be the best option for industrial hydrogen, large boats and planes.

So we need to add into EROEI all the energy needed to deactivate/retrofit de old technology used in planes and large boats. If I don't do this exercise I will be as in denial as the ones that don't believe in climate change. We need to reduce air and maritime travel. We need to invest in trains and also avoid unnecessary travel. Fuel taxes in the previously mentioned sectors may be a good option because at the moment they don't have taxes on externalities. We have only a few years to prevent climate change getting out of hand and there are simple and effective solutions. We do not have a carbon budget for solutions that are not yet viable on paper.

> Liquid ammonia has higher energy density than liquid hydrogen

Why not rockets then?

Methane is better than ammonia (less toxic, similar performance), while hydrogen is also still better (less toxic, higher performance), and hydrazine is also better (still toxic, but hypergolic with certain oxidizers).

It has been used in rockets, but there's usually better choices.

I'd be suspicious of anything people would prefer hydrazine over. ;-)

It has to be mixed with diesel in order to function in compression ignition engines, otherwise known as "diesel engines". So, this "criticism" could be leveled at gasoline and other fuels as well. Spark plugs will ignite ammonia just fine. There is a single reference to this in a different post on the site. [0] My understanding is that most "hobbyist" ammonia fuel setups (i.e. "my pickup runs on ammonia now!") also use spark ignition. I suspect they're investigating the diesel engine possibilities because they would require fewer modifications to existing large marine engines.

[0] https://www.ammoniaenergy.org/articles/wartsila-tests-intern...

Knowing nothing about this, broadly speaking, the enthalpy of formation of the diatomic nitrogen molecule is very high, and you might expect to achieve high energy density. Also it does not produce any CO2.

So for context -- I was working on this ten years ago. I don't have much to say about this particular example other than to note that almost all ammonia today is made using natural gas as the hydrogen source.

Using ammonia is a neat idea, if made renewably, but it will continue to be a challenge to use in cars because it is pressurized, corrosive, and toxic even compared to gasoline.

Obviously the bigger market for renewable ammonia should be fertilizers because right now we're feeding our crops petrochemicals in order to get high enough yields to feed us. That seems like a bad situation to put yourself in if there's any other choice.

It's technologically trivial to make renewable hydrogen from clean electricity and non-potable water, under the right economic incentives (Carbon tax)

Indeed.

I would put "making transportation fuels" at about the bottom of the list in terms of the best use of electricity until such time as all electricity is renewable.

It just doesn't make sense until the entire grid or close to it is renewable already because if you use 1J of electricity to make 0.95J of hydrogen that gets turned into 0.9J of ammonia you'd have been way better off just using 0.9J of gasoline in the first place because that 1J of electricity probably was generated from 3J of fossil fuels.

So I'd say the same regarding fertilizers -- I'd rather they didn't use natural gas as a hydrogen feedstock, but generating hydrogen from electricity generated at a natural gas plant is a very Rube Goldbergy way to generate ammonia that still uses natural gas.

That's changing, but it's nowhere near true yet. Generating ammonia from excess renewable electricity that the grid can't sink does make more sense to me.

I'm not sure I'd agree about ordering. Solar power seems to be getting cheaper faster than batteries are. We might be in the position in the not too distant future where we have more electricity during sunny days than we know what to do with but still need some natural gas power plants to get us through cloudy months. Depending on capital costs "make H while the sun shines" might be a reasonable business.

Yeah, I am also on board with diverting excess electricity into transportation fuels if it literally can't fit on the grid.

But every joule of electricity generated from renewables that makes it to the grid is a joule of electricity that wasn't generated with petrochemicals at the moment because natural gas peaker plants are still a thing.

I had to once write a grant proposal for essentially the kind of thing you're talking about. It's exceptionally hard to make a good overall systems efficiency case for taking electricity and turning it back into fuels. I hope you agree that displacement of petrochemical electricity sources is far preferable to generating chemicals as a byproduct if the grid does have room for more electrons.

The only way I was able to find to make it work economically was to take excess electricity and convert it into fairly high value chemicals -- but even there, just turning petrochemicals into those same chemicals would in almost every case be more efficient.

I look forward to the day when we have so much renewable generation that we have no petrochemical electric plants left to displace. Hopefully it is not too far off, I like your optimism about solar and agree it may be warranted.

The spot price of electricity has gone negative in certain times and places. So power from then could potentially have not have been generated from petrochemicals (though, it might have been from a oil or coal fired steam plant).

If we could change the natural gas peaker plants to ammonia powered peaker plants, we could also use plants that are more base-load(nuclear), and use the ammonia as a long term power storage.

With 80%-90% losses -- Haber-Bosch is inefficient and so is combustion. I just have a hard time picturing that beating batteries. If it made a high value chemical maybe economically, but not on a purely energy basis, the efficiencies just can't add up.

Haber-Bosch requires compression to ~100bar or greater to see much of any conversion and has something like 15% yield per cycle. It is not pretty.

I find it impossible to imagine transitioning engines and fueling stations to ammonia before we get batteries light and reliable enough to meet the majority of people's needs. We put many power plants worth of petrochemicals into ammonia right now, replacing it would be great but it just doesn't seem like a low fruit to grab at.

What I think is a lot of this is put out there as an attempt sideline workable technologies to keep the fossil fuel party going. 'Clean coal' 'Carbon Capture' 'Next Generation Reactors' 'Hydrogen economy' all unworkable trash.

I agree, I won't name and shame but the grant I was working on was clearly funded by some coal lobby or something because the premise was desperate on the face of it.

I practically said "this won't work efficiently, you should just not burn the coal" because nothing else could be justified energetically even if it was maybe economical. I tried my best but I can't change thermodynamics.

You can also make it with natural gas and capture the CO2 produced.

If they had a FAQ I'd be looking for "How does ammonia power handle the many significant dangers of ammonia?" Except when extremely diluted that stuff makes diesel look like milk.

Maybe it'd work for large trucks in very cold environments. Not much around in an accident and it handles lower temperatures.