lsayre wrote:What is the minimum slope for the walls in order for the coal to reliably run down hill to the auger on its own?
35 degrees +/- depending on coal size and the coefficient of friction of the bottom. I would make sides at 45 degrees to be safe. This would ensure a clean dump of the coal bin.
Attached are plans for a fertilizer bin I found on the internet. It would make a interesting coal bin with a little more elevation. The sides are at a 45 degree angle. Powdered fertilizer has a angle of repose of about 40 degrees.