A flame exists because a chemical reaction is taking place.
It is converting some things to other things.
Some things are being converted to light and heat. Which are the same things in different wavelengths.
Other things are being converted do different combinations of molecules, that you may not feel as heat or see as light, but you might smell, taste, or get on you.
If fire was converting the various fuel completely to energy, then it would also convert you, unless you were a very long way away. That would be like a nuclear bomb sort of fire.
A flame is a reaction that radiates energy as it converts things to other things. But has limits to that conversion process.
Okay, in reality a fire is just the physical manifestation of the conversion of carbon and oxygen to carbon dioxide. Infrared and other forms of light are produced. If light and energy are equivalent, then I suppose so, but I will leave that to the better educated.
Visible flame is due to tiny particles of soot being heated to incandescence before being completely burned away to carbon dioxide. This is demonstrable in a number of ways:
[ul]
[li]holding a cold object such as a metal can full of cold water in a flame will cause soot to deposit on it[/li][li]a fuel source burned with a good enough supply of air will produce little or no visible flame. In fact yellow, orange or red flame is a sign of inefficient combustion.[/li][li]Alcohol lamps produce almost no carbon soot when they burn, and hydrogen produces none at all, resulting in nearly invisible combustion; while charcoal which is already nearly pure carbon doesn’t produce any intermediate products that can be carried away, burning directly into carbon monoxide and carbon dioxide.[/li][/ul]
And things being “purely energy” is Star Trek science. “Pure energy” is as fictional as “pure roundness”. Where there is energy, there is something else manifesting it.