We used to call that "slack" at Cedar Point, and if you used too much blower it would go right up the stack,usually before it caught. Those engines really don't get worked hard, so there isn't much danger of the exhaust sucking that stuff up, would go straight through the grates into the ash pan. When we dumped the fire at closing the stuff would catch tho. We would routinely wind up dealing with this stuff when we used up the reserve coal pile and the delivery truck was late.