I started working with agrichar/biochar early last year. I've made about three 55 gal. drums. Excellent results in the garden but watch the soil pH.

I don't worry about grinding it too fine. I fill a 5-gallon pail half full and work it with a garden spade. Once it's down to marble size, that's enough. Time and rototilling will handle the rest.

Doing this creates a lot of very fine material mixed with the bigger stuff. There's no reason you couldn't sift it through a coarse screen -- secure it over the top of the pail, invert, and shake baby shake.

There's no clean way to handle finely ground carbon. Be prepared to resemble a chimney sweep. Oi, guv'nor!