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“Inherently safer than kerosene and synfuels” - No way. Or at least: I don’t think we truly know yet, until they’ve flown for a few years. I used to think hydr

by audunw 3y ago

“Inherently safer than kerosene and synfuels” - No way. Or at least: I don’t think we truly know yet, until they’ve flown for a few years. I used to think hydrogen was extremely safe. It’s super light right, so it should quickly vent away if there’s a leak? Then a hydrogen gas station nearby exploded. Didn’t look like it was very airtight to me, but somehow enough gas collected for a big explosion. It’s extremely hard to prevent hydrogen leaks. The cause of the explosion was a fitting that wasn’t tightened to just perfectly the right torque. Supposedly the procedure to fix this is to have every fitting double checked by a second person. Doesn’t sound inherently safe to me. There’s no warning if there’s a hydrogen leak. There’s no smell or visible gas. The right mixture of hydrogen and air will practically self ignite. There was no source of fire at the gas station. The tiniest invisible spark can set it off. I was also surprised by how loud the explosion was. The explosion is fast. It will rip things apart. On the positive side, it’s over very quickly. You’re not going to have a lingering fire from fuel that has sprayed everywhere like with Kerosene. I think hydrogen CAN become perfectly safe with the right engineering. I think the solution will be that every single piece of tube or valve that has hydrogen flowing through it needs a double tubing around it with active ventilation, and sensors where it vents out to check that it has the expected air flow and no hydrogen gas is detected. But inherently safe? No way

foobarbecue 3y ago
> There’s no smell or visible gas. FWIW, this is also true of methane. An odorant is added to the stuff that's put in gas pipelines. Couldn't they add the odorant to H2 as well?
- merek 3y ago
  H2 molecules are small and thus prone to leaking through small breaches in the containment. Odorant molecules are much larger and less likely to escape.
- c_o_n_v_e_x 3y ago
  At least methane burns with a visible flame in daylight.
Intralexical 3y ago
IIRC, hydrogen gas will basically detonate at any mixture ratio in oxygen or in air. Quick search says 4% to 75% to burn in air, and 18% to 60% to fully detonate. By contrast it looks like kerosene will only burn at between 0.7% to 5% in air, so that's a much narrower range that's dangerous. And of course that's just burning, lots of heat and light, but no brisant supersonic shockwave to shred everything around like you get with hydrogen. Plus apparently if you end up with atomic hydrogen somehow, it will embrittle and structurally ruin a lot of common metals on contact. It also seems generally just not very practical even ignoring the tendency to leak and explode, needing either cryo or high pressure to be stored at any reasonable density, and even in that case taking up over three times as much space as hydrocarbon fuels by volume, despite also packing three times as much energy by mass. The British designs for Skylon are basically just one massive hydrogen fuel tank with wings and engines.
brutusborn 3y ago
Hydrogen should be odourised, so lack of smell isn’t an issue https://en.m.wikipedia.org/wiki/Hydrogen_odorant https://en.m.wikipedia.org/wiki/Hydrogen_odorant But yes, inherently safe hydrogen either describes an impressive engineering feat or a bad joke.
reacharavindh 3y ago
A silly curious question.. if contact with air is what we are worried about, could it make a great fuel in space? Or is it already what we use?
p1mrx 3y ago
> I was also surprised by how loud the explosion was. The explosion is fast. Here's an example: https://www.youtube.com/watch?v=l9CI6KSV560&t=1625s https://www.youtube.com/watch?v=l9CI6KSV560&t=1625s
svnt 3y ago
One of the things that seems to have been kept very quiet about hydrogen is its ability to self-ignite when leaking from high pressure into air. The jury is out on the exact mechanism but most seem to think it creates a wavefront that has high enough energy content to auto ignite. This seems to be why automotive hydrogen refueling is relatively slow. At very high pressures there is no reason why refueling should take longer than gasoline refueling. They made the nozzle so small (less than a millimeter) because if a valve fails open and the hydrogen vents to air, it will limit the length of the invisible flame that probabilistically results to a meter or two. The temperature limit of the tank is one reason to rate-limit as well, but I don’t think it explains the whole picture.
light_hue_1 3y ago
People would be shocked by how hard it is to ignite Jet A (the fuel that they're seeking to replace). You can take a blowtorch to it and it won't ignite. In many accidents it spills everywhere but it still doesn't ignite. It only becomes seriously flammable when it's atomized. No way is hydrogen safer! That's just an outright lie.

by audunw 3y ago

foobarbecue 3y ago

> There’s no smell or visible gas. FWIW, this is also true of methane. An odorant is added to the stuff that's put in gas pipelines. Couldn't they add the odorant to H2 as well?

merek 3y ago
H2 molecules are small and thus prone to leaking through small breaches in the containment. Odorant molecules are much larger and less likely to escape.
c_o_n_v_e_x 3y ago
At least methane burns with a visible flame in daylight.

Intralexical 3y ago

IIRC, hydrogen gas will basically detonate at any mixture ratio in oxygen or in air. Quick search says 4% to 75% to burn in air, and 18% to 60% to fully detonate. By contrast it looks like kerosene will only burn at between 0.7% to 5% in air, so that's a much narrower range that's dangerous. And of course that's just burning, lots of heat and light, but no brisant supersonic shockwave to shred everything around like you get with hydrogen. Plus apparently if you end up with atomic hydrogen somehow, it will embrittle and structurally ruin a lot of common metals on contact. It also seems generally just not very practical even ignoring the tendency to leak and explode, needing either cryo or high pressure to be stored at any reasonable density, and even in that case taking up over three times as much space as hydrocarbon fuels by volume, despite also packing three times as much energy by mass. The British designs for Skylon are basically just one massive hydrogen fuel tank with wings and engines.

brutusborn 3y ago

Hydrogen should be odourised, so lack of smell isn’t an issue https://en.m.wikipedia.org/wiki/Hydrogen_odorant https://en.m.wikipedia.org/wiki/Hydrogen_odorant But yes, inherently safe hydrogen either describes an impressive engineering feat or a bad joke.

reacharavindh 3y ago

A silly curious question.. if contact with air is what we are worried about, could it make a great fuel in space? Or is it already what we use?

p1mrx 3y ago

> I was also surprised by how loud the explosion was. The explosion is fast. Here's an example: https://www.youtube.com/watch?v=l9CI6KSV560&t=1625s https://www.youtube.com/watch?v=l9CI6KSV560&t=1625s

svnt 3y ago

One of the things that seems to have been kept very quiet about hydrogen is its ability to self-ignite when leaking from high pressure into air. The jury is out on the exact mechanism but most seem to think it creates a wavefront that has high enough energy content to auto ignite. This seems to be why automotive hydrogen refueling is relatively slow. At very high pressures there is no reason why refueling should take longer than gasoline refueling. They made the nozzle so small (less than a millimeter) because if a valve fails open and the hydrogen vents to air, it will limit the length of the invisible flame that probabilistically results to a meter or two. The temperature limit of the tank is one reason to rate-limit as well, but I don’t think it explains the whole picture.

light_hue_1 3y ago

People would be shocked by how hard it is to ignite Jet A (the fuel that they're seeking to replace). You can take a blowtorch to it and it won't ignite. In many accidents it spills everywhere but it still doesn't ignite. It only becomes seriously flammable when it's atomized. No way is hydrogen safer! That's just an outright lie.