Submitted by Hypx t3_10mdti7 in Futurology
BoredCop t1_j63cgpz wrote
Reply to comment by Hypx in Cryo-compressed hydrogen, the best solution for storage and refueling stations? by Hypx
Liquid fuels are nowhere near as explosive as compressed hydrogen gas.
For one thing, hydrogen has much wider explosion limits than gasoline so explosions can happen in a wider range of fuel/air mixtures.
The main difference, however, is that liquids are liquid while compressed gasses are compressed gases. By which I mean, if you spill gasoline on the ground it merely forms a very non-explosive puddle and slowly evaporates. It's the mix of vapour and air that is potentially explosive. If you get a hydrogen leak, the pressure makes it rapidly expand and mix with air so you get nearly complete mixing with air almost instantly. A liquid leak just drips down under gravity. A compressed gas leak jets out at high velocity and turbulently mixes with air. Big, big difference in explosion hazard.
Hypx OP t1_j63d0et wrote
Liquid fuels are considered more dangerous because they burn in place. It is not just a "non-explosive puddle." It is a carpet of flames if it ignites. This is a very dangerous situation since it can trap someone in a car during a fire.
A gas, especially one that is much lighter than air, will quickly dissipate. It will have a much lower chance of fire and any fire that does happen will not stay in place. The danger is only during the moments when gas is leaking.
BoredCop t1_j63ejdy wrote
Tell me you've never responded to a fire, without telling me you've never responded to a fire.
I'm a cop, I have seen the immediate aftermath of many fires including ones that involve flammable liquids and gases. And I've been around many a wrecked car with gasoline leaking out. The only times on my watch that we've had real ka-Boom explosive fires involved pressurised gases, such as propane. Conversely, we've had arsonists intentionally starting fires with gasoline only to have the fire self extinguish after using up the available oxygen in a room.
A gas that completely mixes with air in a second or two is much more likely to ignite than a puddle of fuel on the ground. Especially so when the gas in question has a very wide range of ignitable fuel-air mixtures, as is the case for hydrogen.
Sure, the hydrogen fire will "not stay in place". Nice euphemism for "blow up the whole garage".
Hypx OP t1_j63g264 wrote
Hydrogen isn't propane FYI. It is a lighter than air gas. Propane is heavier than air.
BoredCop t1_j63k2ed wrote
I'm fully aware of that. Makes no difference to degree of risk, only to wether it will accumulate along the floor or along the ceiling in a garage or a tunnel.
There are reasons why an old name for hydrogen in some languages is "knallgass" or variants of same, it translates as "bang gas" and refers to its explosive tendencies.
There are reasons for why you're supposed to connect and disconnect the charger or jumper cables in a certain order when dealing with a car that has a flat lead-acid battery, and make the final connection to chassis ground away from the battery. Such batteries can create and leak hydrogen via electrolysis, and explosions are a known hazard. The act of connecting jumper cables can create enough of a spark to set it off, as many people have experienced.
Basically, the explosive hazard of hydrogen gas is well known and has been for over a century. Your claiming otherwise cannot change the facts.
Hypx OP t1_j63kvb5 wrote
And yet it is fact that it is safer than gasoline. This is not actually a debate here. People have set both on fire and gasoline is a lot more dangerous:
BoredCop t1_j63nx60 wrote
That first one is laughable, it is from a biased source and is intentionally rigged to produce the desired result. They present it as if the overpressure relief valve is the only probable leak point, and set it up to immediately ignite before any real volume of gas has time to mix with air. It's like lighting a gas range in a kitchen immediately after opening the valve, and claiming this proves it would be safe to let gas leak out for half an hour and then light a match in the same kitchen. Of course you get no explosion when you carefully make sure the conditions needed for an explosion don't occur, by igniting immediately at the leakage site.
The second one is more interesting. Here, they do in fact allow the gas to mix with air for a few seconds before ignition. However, these tests used liquid hydrogen at a much lower temperature than the gaseous phase cold compressed hydrogen we are discussing here. Liquid hydrogen is seriously cold, and only boils off to gas phase at a slow rate depending on how much heat energy it can absorb from the surroundings. In that sense liquid hydrogen it is rather similar to liquid gasoline; it's the gas that burns not the liquid. You'll note none of the spill experiments in that video involved pressure vessels rupturing or pressurised fuel lines breaking, it was all liquid hydrogen at atmospheric pressure. That makes a huge difference in how rapidly it mixes with air. The very low temperature also slows down the reaction when it is ignited, compared to igniting gas that's a hundred degrees warmer.
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