Howdy Timberwolf !
And happy New Years to you and yours !
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Well, lets see...
a typical 20gal propane tank(100lb'er) is tested to 300psi that is for Propane however
if you put 300psi of air in it it will more than likely explode on you...as air compresses a great deal
and in doing so it stores a considerable amount of power.
at the very highest I'd only go to 150psi if it were me.
I have had a propane tank as a supplemental air tank for my air compressor charged up to 100psi but I admit I was getting nervous !<grin> but it held that pressure with no complaints...hehhehe
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I have not experimented with a Cornish generator as of yet... though I am still looking for a paint pressure pot NOT made of Aluminum! and have yet to find one !
... As you know, you can generate Hydrogen gas (with other gasses mixed in with it, that are quite lethal)by simply pouring a strong mixture of KOH on to aluminum scraps.... the electrolyte will dissolve the aluminum and in doing so it puts of a copious amount of hydrogen gas...
I believe KOH will do this but it may take Hydrochloric Acid.... I'm not sure about that!
so you see the need of your Pressure pot Not being made out of aluminum ! cast iron or steel would suffice just fine even better if you could find one in stainless steel !
... Charging the propane tank with 150lb of Hydroxy gas would be Dangerous to put it mildly....even more so than charging a 300psi tank of propane... because propane is much less volitial
yet a small tank of it can level a 3 story building ! think of what a 20gal tank charged to 150psi of Hydroxy gas could do..... the question becomes one of "a crater of course, but how big of one!"

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Anyway, a 5 gal propane tank pressurized up to 150psi with Hydroxy gas , ran through at least 2 blubbers as a safety precaution could fire a large flame for some time ! how long is anyone's guess
because that depends on the size of the orface emitting the gas to make the flame...
I would venture out on a limb and guess at about 2hrs for a really hot fire or as much as 12 hrs for a small fire.
My GUESS may well be way off the mark because I have no Idea really ! I am groping in the dark here!
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you can use an old refrigerator compressor to suck up the hydroxy gas and compress it into a tank
it will take a long time to do that and may indeed be dangerous to do it I dunno !
I got a 20gal propane tank up to 120psi with an old refrigerator compressor once....left it on and it blew the plastic line at 120psi... I went running out to un plug it when I heard the Hisssss! forgot it was on ! .... OOOOPS ! it ran my air tools for about 2 minutes ! that's when I decided I needed a air compressor!
so as you can see 120~150psi isn't Much in a 20 gal tank... but its how much you use of the stored component that matters..... my air grinder really sucks the CFM ! LOL !
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Bob......