When I were a little lad... (RUN for the hills now, before he gets going...)we had a coal cellar with a trap in the pavement and I remember the coal being delivered (horse & cart). You had to watch and count the bags because it was alleged the coal man would try to cheat you by banging an empty sack on the chute if you only counted by ear!
When the cellar was "empty" we did indeed sweep up the dust etc (we called it "slack" in Birmingham) which was used to bank up the fire at night. When I was in S.Wales my landlady used to call it "small coal".
RtS (OK come out now, the old fart's stopped rambling)