Opening the air vent on the loading door is to help prevent a minor 'explosion' if you load a huge amount of fresh coal in the 'wrong' way.
Read this thread: http://nepacrossroads.com/viewtopic.php?t=2130
A chimney fire will not happen when burning only coal. A chimney fire is caused by the buildup of creosote in a chimney from the partial burning of wood. A hot wood fire, burning dry wood with a hot chimney will not deposit very much if any creosote in the chimney.
But if the fire is damped down by reducing the air, or the wood is damp, or the chimney is cold, then creosote, which is unburnt but very flamable byproducts of wood combustion, will accumulate on the inside of the the chimney. Then if enough heat, oxygen and flame are applied, the creosote starts to burn very hot very fast and often with catastrophic results.
When burning coal in a stove that has been burning wood, the creosote often drys out, and flakes off, falling into the clean out or tee or stove.
Creosote is why wood burners often have their chimney cleaned several times a season. With coal it is unnesessary. Coal creates fly ash, an inert dry ash that can't burn.
so to prevent chimney fires, clean your chimney, burn only very dry wood, with lots of air, and keep it burning hot. When you limit the air, then lots of smoke is created, the chimney cools down, and creosote is deposited in the chimney.
Hope this makes sense.