Certain lumps that didn't burn well reduced to very white lumps of ash. As opposed to normal coal that disintegrated as it fell to ash. We called those lumps "bats".
When I was installing a weighing machine at the Grimethorpe Colliery they scooped up the dust on wet days and sold it to power station for their fluidised beds. Such power stations had to crush coal into small pieces anyway. Fluidised beds were coal infused with air. It made it easier to transport to the furnace and with the extra surface surrounded by air area it burned more efficiently.