Hello F Frantz, welcome to the forum. It sounds like you are burning your coal stove to the limit. If you are filling the firebox to the top of the firebrick then you are using the maximum amount of coal in the stove. Most stoves will have the coal slope down in the front when filled to the top of the brick in the back.
If you are only getting 12 hours of burn out of your stove, you must be running pretty hot. Do you have a thermometer on the stove or the flue pipe?? Do you need all the heat you are currently getting from the stove?? Are you running a fan on the stove to wash the heat off the stove body and into the room?
If you have burnt the coal down to just ash in 12 hours, then there is nothing to do but add more coal more often. Or maybe just turn down the air inlet control to a lower level to get a few more hours of burn time.
I do not advocate a manual hand damper or 'pipe draft' without using a manometer to read the draft on the stove body ANYTIME the manual draft is closed AT ALL. These coal stoves make Carbon Monoxide, and the chimney MUST pull anough draft to not only pull air from the fire, but pull a slight vacuum on the stove body and piping. If a manual draft is closed and the draft falls too low, then there is a risk of Carbon Monoxide in your house....
How will the draft fall too low?? By reducing the hot gas flow up the chimney, the weather warms up, or the winds drop, or even turning on the clothes dryer and bathroom fan in the house. Any of these can cause less draft. So I believe in NOT using any manual method of blocking the chimney without a draft gauge on the stove measuring draft over the fire.
And of course at least one or two Carbon Monoxide detectors in the stove's room