Hi Rich, what you are experiencing is exactly what I do when I burn nothing but Bituminous coal.
Bituminous cooks off a lot of volitiles, which is the black sooty smoke you see. You can burn some of this with hot secondary air, but it needs to be hot air, and this requires some engineering and fabrication. Just opening the loading door air vent will probably not burn off much of the soot.
The large sheet of fused black gooey coal is clasic Bituminous. I can make a similar sheet of coal on my firebox. I do what you are describing, let it fuse, then break it up an hour later to expose the edges of many chunks of coal to the flame and heat instead of just the bottom of the fused sheet or 'bridge' of coal over the fire.
The big 'bang' is also pretty much a regular occurance if you load the way you describe. To reduce or prevent most of the exposive gas/fumes ignition, leave an area of the underlying fire open when loading on fresh coal. The open hot coal area will act like a pilot light and ignite the gasses sooner and with less of an explosion. If you don't have the exposed 'pilot light' the gasses build up to large volume and wait for enough heat and a flicker of flame to ignite, with enthusiasm!!
Read this thread: http://nepacrossroads.com/viewtopic.php ... sc&start=0
Then once the load of new coal is burning, add a few more shovels over the 'pilot light' area to fill it in.
Your Bituminous has a lot of bitumin [tars] and therefore sticks together. When it is burnt hot, it probably will fuse the ash together too. It's just the nature of Bituminous coal. These issues are why I spend the extra dollars and burn Anthracite as well as Bituminous.
Check your PM's