hate to bust in on a nice party, and I know nothing about the AHS. Just what I have read here in 41 pages....
However, how do you push enough coal to satisfy a call for heat at 50%??? If I cut my VF3000 back 50%, it wouldn't heat my garage let alone the house.
Because the coal is always there, held within the column of the 6" tube (Anthratube for AA's). It's merely a matter of where within the tube it is burning. High is safe (to a point, unless it goes high enough to burn in the hopper) and low (heading down toward the ash grate) causes puff-backs.
AA and AHS boilers are not your typical stokers. For the AHS, picture a round hand fired stove that is a scant 6" in diameter, and with a huge 250 lb. capacity gravity hopper on top of the 6" round stove, and which has periodic automatic ashing via a grate 'shaker' (sort of) that is driven by an electric motor. Surround this 6" tube stove in water, and now you have a boiler. The ashing causes the hopper coal to enter the tube and eventually it gets down to where the fire is. These boilers stoke by gravity from above via ashing from below. Everything takes place within the 6" diameter tube. Only about a 4 nuggets high fire exists, and the remaining 250 lbs. of coal don't ignite due to oxygen starvation.
Ashing more slowly (the ash grate has a knife edge that cuts the ash off the bottom of the tube as it moves back and forth) doesn't deprive the tube of coal, but rather deprives the ashing knife from cutting off too much ash over one time interval (an interval of fan induced firing of the tube via air drawn in from below via the fan). Cutting away the ash at half the speed just means that instead of ashing roughly every 4th fan running cycle, it will probably ash every 2nd fan cycle, but only cutting off half as much ash each cycle, so the net effect is the same amount of ash cut away for every 4 fan cycles.
The net effect is that the 4 nugget high fire zone within the tube also can only fall half as far down toward the ashing grate with each ashing cycle, but a full load of coal from above the tube is always there. Nothing separates or divides or cuts off the gravity hopper from the fire containing anthratube. Everything is always wide open.