What is charcoal and why is it possible for it to burn for such a long time?
Its carbon, in an amorphous form that has plenty of surface area which enables air to make contact with enough carbon to sustain a reaction ( once the ignition energy has been supplied.) The carbon burns slowly because, in this amorphous form the carbon is a poor conductor of heat ( both diamond and graphite are good conductors of heat ). The outer layer of charcoal burns away, but doesn’t transfer much heat to the inside. As the outer layer reacts to form CO2, the inner layers are exposed to the air and burn in their turn, but only after the outer layer combustion has finished. This process makes for quite slow burning.
