I'm sure you have read the 'sticky thread' at the top of the hand fired forum titled 'minor explostion ... ' Minor Explosion In Coal Stove
In every case the explosion is the result of FRESH coal added to a hot coal base.. the FRESH coal has the volitiles that escape as the coal is heated, and without a flame to ignite the volitiles, they can accumulate untill a small flame makes it through the coal bed and ignites the volitiles all at once..
The blue flames you see are volitlles and gasses being burnt.. so once you have blue flames above the coal, you are OK to shut off the over fire air.. and let only under-fire air feed the fire.
To safely add fresh coal, you want to 'bank' the fire,, add fresh coal either only on one side or one end of the firebox.. leaving the exsisting redhot coals exposed on part of the firebed.. then, as the FRESH coal starts to warm up and give off combustible gasses, there is a flame from the redhot coals to ignite the gasses in a slower, controled manner.. then once the fresh coal has had time to get started burning, and you can see blue flames above the fresh coal, then you can add more coal and cover the rest of the firebox. This method if done with patience, over say 30-40 minutes will pretty much eliminate puff-backs.. the larger the quantiy of fresh coal added, the longer it will take to cook off the volitiles..
I would not be at all concerned about a puff back in a firebox that has visible red coals showing through the coal bed and blue flames dancing above.. this is a normal, healthy coal fire.. relax and enjoy the heat.. I'd monitor the heat output and flue temps and furnace body temps.. you don't want to create too much heat, it is just wasting fuel.. turn down the air once the house is warm..
BUT be patient !! wait at least an hour after any air adjustments to see the effect on the fire.. coal is slow to react, unlike wood..
Hope this helps.. Greg L