Thanks for the warm welcome!
For those who are curious, Faye is likely using something like this here welder
. It's about as portable as they come, plugs into a standard 120V single-phase outlet, and has enough power to do all but the largest jobs efficiently. Dora's electric bill is going to go up, and the Coffee O' Doom is going to smell real weird for a while, but there's no reason it wouldn't work otherwise.
As for the person saying the grey bottle is CO2, well, yes CO2 bottles are grey, but that bottle is standard argon green, as someone else pointed out previously. Also, in the first panel she's holding a TiG torch, plus the title of the comic is rather leading.
To the blowtorch theory, arc welders don't work that way. They only generate heat when they arc through something both solid and conductive. If Faye is making a drink with that thing, let's see... Well I suppose she could be using the welder to pump heat into a metal cup with a beverage in it, but that is so, so silly.
My conjecture, based on the fact that Faye is rather crafty, is that she was making/modifying something to either, A) help out around the shop, B) spruce the place up, or C) murder/maim Pintsize/customers.
EDIT: The rule about having flammable materials around is, "DON'T DO IT!".