I was able to order a 1.5 tons of Donson/Coal Contractor's bagged coal, and tried my first 5 bags over last couple of days.
The Kimmel's coal has been very wet and frozen like a brick during this cold weather, and the fines have been clogging the back pressure holes in the upper feed tube.
It appears the Donson coal is burning better. The bags of Donson coal are not frozen solid like the Kimmel bags are even though the coal in the Donson bags is still a bit wet. I was able to dump the bags of Donson coal directly into my coal bin without having to melt/dry it in another bin first as I do with the Kimmel's coal.
Hopefully, this coal will be better for the DF520. Still having cold weather so just in time to try it.