Here is a link to the relevant article.
http://www.straightdope.com/classics/a1_205b.html
Part way down it mentions the ‘wick effect’ and smouldering.
I was always under the impression that free flame fire could only exist within a certain concentration of oxygen.
Most of the ‘spontaneous combustion’ episodes ( the overwhelming majority) have taken place in enclosed spaces where the victim is also unable to move, due to frailty, drunk, or unconscious due to some accident.
One article I have read claims if the person becomes lit due to some source, the flames and fumes rapidly incapacitate the victim.
The flames flare and wither very rapidly, due to lack of oxygen which also either kills the person directly or ensures they remain unconscious.
The victims clothing then smoulders, since no room is completely airtight, there is enough airflow to maintain this smouldering.
Just like a candle wick, fat is drawn from the victim and burned off.
The parts of the victim that tend to remain are the less fatty parts such as a foot or a hand.
Although the local temperature of the smouldering is high enough to completely destroy tissue and bone, it does not render the air temperature high enough to destroy the surroundings.
Heat layers show a very marked stratification, and this causes very specific damage at differant heights within the enclosed space.
Things such as light fittings may be damaged but nearby flammable materials on the floor are hardly marked, and windows are often drenched with water, which is a by product of fat combustion.
In one scene a portable tv case on a kitchen worktop has deformed due to the heat and yet plastic bowls on the floor are intact.