I have only had a couple fires in the "lively" style forge I built, plan on giving it its first true workout this weekend. I'm using clay and made a fire in it just to dry and harden it for use.
That being said the one thing I noticed immediately was that on the second handful of charcoal I put into it something started sparking like crazy. I actually have a little raspberry behind my right ear where one of them landed, funny but not fun.
Anyway, because its trench shaped and all the sparking was at one end, I isolated the cause pretty easily. There was a piece of half charcoal half bark in the end. once I pulled that out the sparking went away.
After that I started watching as I was digging charcoal out of the bag, bark = sparks every time. Without that I have almost none. Hope this helps