I switched the nitrogen stem over to C02 on my regulator as it was cheaper than getting the adapter fitting. Check out the local welding gas supplier for help.
I also want to force carbonate a corny and have no refrigeration . I checked a Carbonnation Chart and for 70F @ 32 psi the volume of CO2 is 2.7
But how long would it take carbonate the keg? How long could the beer be stored @70F?