For what it's worth,.....
Because of the top cap being to restrictive, I always ran my fireplace hot, or any wind gusts would overpower the weaker draft and I'd get smoke puffs into the living room. That running it hot, plus using well seasoned wood, never let the creosote build up to a wet and thick gooey layer. The next season it would flaked off easily with a brush. There's never been any coal burning appliance hooked into that chimney.
The same happened in my range.
Last Spring, I took Rev Larry's creosote rust prevention advice and burned a couple of firebox loads of wood as the last fire. The creosote build up throughout the stove's flues was black, thin, and dry.
When I started the range two weeks ago on coal, all of the creosote layer that was not close enough to the firebed to get hot enough to be burned off by the coal fire, had scaled off shortly after starting the stove due to the heat expansion. I was able to easily brush off whatever didn't flak off and vac it all out of the flues.
I saw nothing that would lead me to believe the coal flue gases removed any of the creosote. Just burned it off in the flues closest to the firebox where the temps run 400F and above, and in cooler places it flaked off from temperature changes expansion/contraction.