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If you dismiss hydrogen as fuel made from renewable electricity, and frankly you should on density and safety basis, the two most practical options are methane

by brohee 3y ago

If you dismiss hydrogen as fuel made from renewable electricity, and frankly you should on density and safety basis, the two most practical options are methane and ammonia. Ammonia got an easy electrity to fuel process, easier than methane that starts with hydrogen. Ammonia is kinda nasty but used plenty (for example in cooling) without a terrible safety record. Not sure that safety is a better argument against it than the lack of distribution network. I think ammonia will find its niche in shipping, but I just can't see it on the road.

FooBarBizBazz 3y ago
Yeah, I'll grant that there are problems with ammonia, but I think this parallel quest to come up with synthetic fuels is a good one. A car is just one part of a car-fuel-road system, and if you can move complexity out of the car and into a chemical plant, that might be good -- particularly if that solves the range and recharging issues. Fuel cells are close cousins of batteries anyway. And there are even redox flow batteries that are even more intermediate. The energy density isn't high enough (these things are actually used for static energy storage, with vats the size of buildings), but imagine a "fuel station" that exchanged Vanadium in different oxidation states with a car, or that swapped some hydrocarbon that can store energy like ATP in biology. Heck, including the atmosphere in the system could be ok too (e.g., CO2 capture), so long as everything balances to zero and the thermodynamics work out (even just mixing and unmixing CO2 with oxygen and nitrogen is an expensive trip up and down in entropy though). I'm quite happy for this kind of work to continue. And like you say, people are putting big money towards running container ships on ammonia. It's sure better than bunker oil. And we're also definitely not going to use big batteries. So, good on Toyota. Somebody needs to keep exploring alternatives.
Turing_Machine 3y ago
Some argue that liquid nitrogen is a decent option for short range. It's cheap, can be contained in relatively low-cost tanks (compared to hydrogen, certainly), and can be refilled quickly. The LN2 doesn't actually burn. It's more like a steam engine, where the pressure from the vaporizing nitrogen drives pistons or turbines. The nice thing here is that the LN2 comes straight from the atmosphere and goes straight back into it. Net zero emissions from the car (obviously producing the energy to compress the nitrogen in the first place isn't going to be net zero, but that's basically the same issue as with hydrogen, electricity, or this ammonia system).
- robryk 3y ago
  That seems to have energy density lower than even batteries though. When you vaporize LN2, you apparently need 0.2MJ/kg of heat. The amount of mechanical energy you extract out of vaporizing that (when heat is provided by the environment) can't exceed the heat required to do so (because condensing the nitrogen back into LN2 is exothermic). Apparently current Li-ion batteries have ~250 Wh/kg = ~0.9MJ/kg energy density, which is significantly higher than the upper bound above. Am I mistaken somewhere?
jfengel 3y ago
If you're making methane and ammonia, why not go further and make hydrocarbons? I'm sure you lose some energy in the process, but you gain the ability to re-use all of your existing infrastructure unchanged.
- brohee 3y ago
  Synfuel for now is super expensive, the one use case that makes sense that I have seen is an aircraft carrier making jet fuel from nuclear fuel.
- ephbit 3y ago
  I think the motivations to produce ammonia instead of hydrocarbons are: For one, energy efficiency. For hydrocarbons you need carbon. Either you take carbon from existing renewable hydrocarbon waste streams (biomass) or you pull CO2 from the air. The first doesn't make much sense because those hydrocarbons would better be used to produce chemicals. The second (direct air capture DAC of CO2) is - as everyone knows - hugely energy intensive, because CO2 comes in concentration of 420 ppm. Yes, I know the Haber-Bosch process to produce ammonia is hugely energy intensive as well. But if you regard ammonia synthesis as well as synfuel synthesis as fundamentally the same processes - both reduce elements in their oxidized forms (C as in CO2, H as in H20, N as in N2) to store energy in them - you need one extra heavily energy intensive step to get synthetic hydrocarbons compared to the sythesis of ammonia: pulling CO2 from the air where it's got a really low concentration (420 ppm). Compare that to nitrogen which is the main component of our atmosphere (780,000 ppm) and thus much less energy intensive to isolate. So neither source of carbon to produce CxHy makes sense in the long term IMO. Also, as soon as you use/burn the synthesized hydrocarbons, the carbon ends up back in the atmosphere (at least in mobile applications where you can hardly capture it) which goes against the main goal of putting less CO2 in there. Ammonia on the other hand requires nitrogen which is abundant in the atmosphere and thus relatively easy to get (compared to the carbon from DAC). The hydrogen that's needed is also readily available in many places, in the worst case from sea water. As the goal is a reduction of the amount of CO2 emissions, in the really long-term, hydrocarbons will probably just be the less efficient option compared to ammonia. And since all existing infrastructure does get exchanged/altered in the long-term anyway, slowly switching to ammonia won't mean wasting this infrastructure.

by brohee 3y ago

FooBarBizBazz 3y ago

Yeah, I'll grant that there are problems with ammonia, but I think this parallel quest to come up with synthetic fuels is a good one. A car is just one part of a car-fuel-road system, and if you can move complexity out of the car and into a chemical plant, that might be good -- particularly if that solves the range and recharging issues. Fuel cells are close cousins of batteries anyway. And there are even redox flow batteries that are even more intermediate. The energy density isn't high enough (these things are actually used for static energy storage, with vats the size of buildings), but imagine a "fuel station" that exchanged Vanadium in different oxidation states with a car, or that swapped some hydrocarbon that can store energy like ATP in biology. Heck, including the atmosphere in the system could be ok too (e.g., CO2 capture), so long as everything balances to zero and the thermodynamics work out (even just mixing and unmixing CO2 with oxygen and nitrogen is an expensive trip up and down in entropy though). I'm quite happy for this kind of work to continue. And like you say, people are putting big money towards running container ships on ammonia. It's sure better than bunker oil. And we're also definitely not going to use big batteries. So, good on Toyota. Somebody needs to keep exploring alternatives.

Turing_Machine 3y ago

Some argue that liquid nitrogen is a decent option for short range. It's cheap, can be contained in relatively low-cost tanks (compared to hydrogen, certainly), and can be refilled quickly. The LN2 doesn't actually burn. It's more like a steam engine, where the pressure from the vaporizing nitrogen drives pistons or turbines. The nice thing here is that the LN2 comes straight from the atmosphere and goes straight back into it. Net zero emissions from the car (obviously producing the energy to compress the nitrogen in the first place isn't going to be net zero, but that's basically the same issue as with hydrogen, electricity, or this ammonia system).

robryk 3y ago
That seems to have energy density lower than even batteries though. When you vaporize LN2, you apparently need 0.2MJ/kg of heat. The amount of mechanical energy you extract out of vaporizing that (when heat is provided by the environment) can't exceed the heat required to do so (because condensing the nitrogen back into LN2 is exothermic). Apparently current Li-ion batteries have ~250 Wh/kg = ~0.9MJ/kg energy density, which is significantly higher than the upper bound above. Am I mistaken somewhere?

jfengel 3y ago

If you're making methane and ammonia, why not go further and make hydrocarbons? I'm sure you lose some energy in the process, but you gain the ability to re-use all of your existing infrastructure unchanged.

brohee 3y ago
Synfuel for now is super expensive, the one use case that makes sense that I have seen is an aircraft carrier making jet fuel from nuclear fuel.
ephbit 3y ago
I think the motivations to produce ammonia instead of hydrocarbons are: For one, energy efficiency. For hydrocarbons you need carbon. Either you take carbon from existing renewable hydrocarbon waste streams (biomass) or you pull CO2 from the air. The first doesn't make much sense because those hydrocarbons would better be used to produce chemicals. The second (direct air capture DAC of CO2) is - as everyone knows - hugely energy intensive, because CO2 comes in concentration of 420 ppm. Yes, I know the Haber-Bosch process to produce ammonia is hugely energy intensive as well. But if you regard ammonia synthesis as well as synfuel synthesis as fundamentally the same processes - both reduce elements in their oxidized forms (C as in CO2, H as in H20, N as in N2) to store energy in them - you need one extra heavily energy intensive step to get synthetic hydrocarbons compared to the sythesis of ammonia: pulling CO2 from the air where it's got a really low concentration (420 ppm). Compare that to nitrogen which is the main component of our atmosphere (780,000 ppm) and thus much less energy intensive to isolate. So neither source of carbon to produce CxHy makes sense in the long term IMO. Also, as soon as you use/burn the synthesized hydrocarbons, the carbon ends up back in the atmosphere (at least in mobile applications where you can hardly capture it) which goes against the main goal of putting less CO2 in there. Ammonia on the other hand requires nitrogen which is abundant in the atmosphere and thus relatively easy to get (compared to the carbon from DAC). The hydrogen that's needed is also readily available in many places, in the worst case from sea water. As the goal is a reduction of the amount of CO2 emissions, in the really long-term, hydrocarbons will probably just be the less efficient option compared to ammonia. And since all existing infrastructure does get exchanged/altered in the long-term anyway, slowly switching to ammonia won't mean wasting this infrastructure.