If I heat up one of my big stainless steel pans on the stove, above a gas buner, like "NASA HOT" and pour a can of beer on it, the alcohol will boil off 1st. As it boils off it will burn (chef pours alcohol in a frying pan, bursts into flames
) The water part of the beer will contine to boil off. So... The beer itself has to heat up quick enough to have all the alcohol "boil off" per-say at the same time. If you heat cold beer up slowly, the alcohol boil off slowly, and you won't be able to light it at all.
-SF