Y'all who burn nut or larger prolly know what I'm talking about. I dunno if rice in a stoker also does the snap, crackle, & pop that I'm referring to.
Anyways......My twice a day routine is to shake my hand-fired inert really well, scoop out the ash & such, sit in front of her with the ash door open to get the coal bed really cookin', and then throw a bucket of fresh coal on top.
When I do that.....I'm rewarded with pretty blue licks of flame wandering around the black mound of coal and a frenzy of snaps, crackles, and pops from the new coal.
My guess is that it's not moisture in the coal expanding as we usually see (hear) when burning wood, but rather it's the extremely fast expansion of the outer area of the coal nugget versus the stodgy "unwilling to expand" cooler center of the nugget. That creates tension within the nugget which causes the hot expanding shell to shear from the stubborn cooler center.
I dunno how much coal/carbon actually expands with heat, but even just a little oughtta cause shear & thus popping.
Believe it or not....I'm not even drinking when I come up with these thoughts while sitting in front of the fire!
Chime in and tell me if I'm on track or in outer space!