I have a Keystoker KAA-2 boiler burning rice and there is a fair amount of unburned coal in the ash. I'm new to a stoker, but I expected more efficiency than my handfired. Not really true, but far more convenient.
I hadn't considered if it was practical to attempt to recover any of it. Rather, I've been trying to minimize it. When the stoker runs flat-out, the ash is almost ceramic and fused, breaking into clumps as it falls into the pan. Otherwise it does drop a suspicious (to my inexperienced eye) amount of coal. I haven't figured out why. I've played with more air/less air, more stoker/less stoker, but I haven't nailed it.
The unit otherwise seems to work well. Much ado about nothing or a chance to squeeze out a few more BTU?