If you look at charcoal under a high powered microscope it is extremely porous.  The surface area of one gram of our charcoal is ~250 m2.  That's a LOT of surface area.

If those fibers have moisture trapped in them, you will hear the snap, crackle, and pop just like a log on a fire.  As the charcoal heats up, those fibers will open up even more releasing the moisture, rather than flash boiling it.  With activated charcoal, they steam it or use a chemical bath to open up those pours and get everything out of there, boosting the surface area as high as 2000 m2/g.

Either way, this is why high carbon lump can get so much freakin hotter than briquettes......it's burning all the way through at once.  A briquette is compressed and has to burn from the outer layer in.  Think about burning a seasoned piece of kindling vs. a piece of Masonite.

As far as the sparks, I think this is more from the dust.  When the you hear the pops, it blows a piece off, and thus you have some dust that goes with it which creates the sparks.

Btw, don't take any of this as the bible.  The numbers above are lab proven, but I'm concluding rest on my personal observations based off humidity, moisture, etc in my own testing.

Google image search "charcoal under a microscope" and you'll see a bunch of photos like this.