The sea- coal collected at Hartlepool and to a lesser extent at Redcar was the residue from the pits, further north, after the coal had been washed and the resulting sludge jettisoned into the sea.
The incoming tide left drifts of fine black coal on the beach and this was raked up when the tide went out.
As a child I remember my father coming home with bags.
The coal was very slow burning and was used to bank up the fire at night so that (particularly in winter) the fire would burn very slowly ovenight and need not be lit every morning.
This also meant that the kitchen was always warm and the Winter fuel bill was considerably reduced.