The Mudcat Café TM
Thread #102071 Message #2066381
Posted By: JohnInKansas
02-Jun-07 - 05:33 AM
Thread Name: BS: Can We Afford Biofuels?
Subject: RE: BS: Can We Afford Biofuels?
CarolC -
If you'll check your numbers, I'll check mine, but I rather doubt you'll find many CNG or LPG tanks in mobile use above a low few hundred psi. Both gases are fully liquid, and "incompressible" well below 1,000 psi, and compressing them further would contribute nothing to storage capacity, although it would add significantly to the hazards of material failure.
It's extremely rare for either gas to be stored at more than a maximum (limited by industry specifications and regulations at least in the US) of about 870 psia (60 atm), in fixed-base storage. Most mobile applications rely on self cooling via venting, and cryogenic insulation, to maintain pressures at less than a couple of hundred psia.
At the pressures cited for the "Air Car" it's probably a gross error to refer to it as "compressed air," since the pressure indicated is above the critical point (triple point) for either oxygen or nitrogen, so it's actually being stored as a "liquid air." The intensely low temperatures just from boiloff of the liquid in the event of an accidental release is an additional hazard not mentioned in the adspeak for the car, since virtually all polymers and most metals become brittle and structurally useless at temperatures easily observed in case of a failure. (And one deep breath of the vapor makes you DEAD by freezing your lungs.)
It should also be noted that cryogenic temperature "air" does not spontaneously rise and dissipate, as LNG or CNG do, but may "lay on the ground" in case of a leak, producing a horridly hazardous condition for anyone arriving on the scene of an accident (not to mention anyone in the accident).
If "plain air" is in fact compressed into the tanks, nearly pure oxygen will boil off first, causing an intense flame/explosion hazard if any other "burnable" materials are present, followed by evaporation of a cloud of inert, nearly pure, nitrogen that might well suffocate all life forms in the vicinity.
The difficulties can be "managed," perhaps; but I'd want to see something without the advertising, and with some sound engineering data before becoming enthused with this.
John