Most flames work by combining molecules with carbon (C) in them
with oxygen (O2) from the air to make carbon dioxide (CO2) that's
released back into the air.
Luminous flames are the yellow ones. Although they can get at some
oxygen, they can't get as much as they need to turn all of the carbon
that's being burnt up into CO2. This is why this kind of flame produces
soot - since it can't release all of the carbon as CO2, some of it gets
released as the black stuff in smoke (soot).
Non-luminous flames are the ones that burn blue. These flames have
access to as much oxygen as they could possibly use, so they can burn
very efficiently. All of the carbon that gets used can be turned into
CO2, so there's actually no soot.
Because luminous flames don't burn as efficiently as non-luminous
ones, they don't produce as much energy. This means that the
non-luminous flames have a lot more energy than luminous ones, and
their flames are actually hotter. This is why the luminous ones look
yellow and the non-luminous ones look blue. Hotter flames burn blue and
(relatively) cooler ones burn yellow.
So, getting back to your question, the reason that we have both
types of flames is simply that not all situations are the same. Either
the flame can get enough oxygen or it can't, so it's either
non-luminous or luminous.
(republished on 07/24/06)