liveforphysics said:
Can you tell me what Im seeing incorrectly?
In a word, "chemistry." Please reread my previous post. I'll repeat the key items here.
NOx emissions, should any exist after ammonia combustion, are
neutralized by residual ammonia already in the exhaust stream (the best way to do it), or by ammonia leaked into the exhaust stream after the fact (harder to do correctly).
Let me repeat, a properly tuned ammonia engine will not exhaust more NOx than a gasoline engine. In fact, the ammonia provides a way to make even
lower NOx emissions than a gasoline engine would itself provide. This is
not a scrubbing operation. This is
catalytic conversion of a mixed gas stream.
Wikipedia states it another way (emphasis mine):
Combustion
The combustion of ammonia to nitrogen and water is exothermic:
4 NH3 + 3 O2 → 2 N2 + 6 H2O (g) (ÃŽâ€Hºr = –1267.20 kJ/mol)
The standard enthalpy change of combustion, ÃŽâ€Hºc, expressed per mole of ammonia and with condensation of the water formed, is –382.81 kJ/mol. Dinitrogen is the thermodynamic product of combustion: all nitrogen oxides are unstable with respect to nitrogen and oxygen, which is the principle behind the catalytic converter. However, nitrogen oxides can be formed as kinetic products in the presence of appropriate catalysts, a reaction of great industrial importance in the production of nitric acid: ...
You really do need that catalyst to make ammonia go to NO. There is an intrinsic reaction
temperature and
kinetic pathway difference between the homogeneous (flame) combustion and heterogeneous (surface/catalytic) combustion of ammonia. In short, I mean that the surface reaction is much more controlled (fewer intermediates and side reactions) and may be better controlled to yield one preferred product over another by choosing catalyst type and reactant ratios appropriately. If we could make significant NO by burning ammonia in a flame, we would! It's much cheaper to have a torch than a precious metal like Pt supported on a ceramic. Nitric acid factory on wheels? If only it were so easy to make commercially valuable NO.
You seem to have accurately grasped a couple of things that I like to stress about the nature of ammonia combustion. First, partially cracking the ammonia or adding a combustion promoter to it in small amounts is done in order to get flame speed up, not to tune exhaust composition. Second, ammonia is indeed a high octane fuel just as is methanol, and the higher allowed compression ratios will garner more efficiency at the wheel.
As far as exhaust or pollution, you are focusing on a problem that doesn't exist. You haven't been around an ammonia engine before, so it's understandable that you might want to guess at exactly how combustion proceeds in the chamber and to worry that huge amounts of excess NO, NO2, N2O4, or whatever else might be formed. What isn't understandable is your claim that a NO/NO2 issue exists in an ammonia engine when you have zero experience with them.
And you don't need to take my word for it. The Hydrogen Engine Center, who made up the presentation I referenced in my last post, installed and ran an ammonia fuel-based irrigation engine in California which ran for almost a full year in 2008, constantly, during a demonstration project for ammonia fuel as a replacement for petro diesel. It was part of an effort by the California Air Resources Board to trim the CO2 pollution from agricultural applications. Some intermediate results are prepared
here. The engine was tested not just for providing sufficient pump power but also for emissions, as the Board was very interested in those aspects as well. Tests (results on file with HEC) showed
negligible NOx emission. And this is the standard of a state that requires all kinds of special emission equipment on regular cars.
Ted disassembled the engine block at the end of the project (late 2008) and found no abnormal wear.
If you'd like, I can put you in touch with Ted and you can discuss crevice areas or whatever else comes to mind. He has far more experience making engines run on alternative fuels than you or I ever will.
Again, the bottom line... these engines are being built and they run. They don't kill everyone nearby with a massive cloud of brown nitrogen dioxide. We have annual conferences where several groups will present their results on ammonia operation in spark and compression ignition engines, and occasionally gas turbines. This isn't a matter of which of us has the best hunch about what might possibly happen. It's already out there. I'd no sooner claim to you that light can't travel through a vacuum.
liveforphysics said:
If you dont like my 5-10% energy in vs energy out efficiency estimation, work your own numbers.
Easy, now. I don't like it when people throw numbers out there for no good reason (and neither should you). All I'll say is that it's likely that the "well to wheel" efficiency for ammonia and gasoline is likely quite close (per liter, slightly less so per kilogram). You will note that Paul was talking about fertilizer manufacture, rather than fuel. In that case, it'll be a $/ton of ammonia that is important, and with dominating dependence on electricity cost (which is regional).
You're a smart guy Luke, you know that in the past that I've respected your advice on electromechanics a great deal. Public discussion forums are a great place to learn and talk about ideas that are new, or at least new to some, but there's no need to try and be an expert in everything. Nobody here (or anywhere!) is.