I am attaching a link that explains better than I can the rational for making Good bio-char- otherwise you're just making charcoal !
While I know of one story of fabulous Yams /Sweet potatoes grown on a mound of charcoal left over from a Syn-gas experiment, Their is a difference
in Just Charcoal(S) and Bio-Char(s)
A quick backstory after the hurricanes that Smacked the New Orleans and Huston areas Rice Univ. was given the lead in finding ways to deal with huge
mountainous piles of woody debris from those storms
In a Permaculture like way they stacked functions trying to create something out of the acres of materials.
In the link below is a primer on producing Great Bio-char :
http://news.rice.edu/2012/03/22/cooking-better-biochar-study-improves-recipe-for-soil-additive/
Further information is no more than a google search away-
I would be remiss if I didn't mention the Research at Cornell Univ. and the Cornell Co-operative Extension Services ! For the Good of the Crafts ! Big AL