How woodburners work
It's a 2-stage process. First, if the wood is hot enough, the combustible creosotes and resins will evaporate out of the timber and will be burnt as gases (if these gases are not burnt completely the result is smoke).
The evaporation of the gases turns the wood to charcoal, which then burns easily and cleanly - and produces most of the heat.
To burn as cleanly as possible, the fire needs to be as hot as possible. It also needs the right amount of air to support the combustion. Too much air cools the fire and smoke is produced. Not enough air has the same effect.
Modern woodburners burn efficiently because the firebox is lined with firebrick material, making a hotter fire. The combustion air is carefully admitted to give the most complete combustion possible. The resulting efficiency (conversion of the fuel energy into heat in the room) is around 65 percent.
Contrast this to the traditional open fire, which admits far too much air, cooling the fire and giving an efficiency of 15 to 20 percent. Some even have negative efficiency – they draw cool air into the house, warm it and send it up the chimney!