Goodfellow and list:
[RWL: There may be sites near you where charcoal has been sold by middlemen. There could be piles of charcoal dust - generally unusable by housewives for cooking - but could be perfect (and low cost) for use as biochar.
RWL: the biochar community tries to avoid the word “burning” with making char - we use “pyrolyze, gasify and carbonize”.
Yes, crop residues can be turned into char. Probably many of your customers are already burning corn stalks for cooking (using 3-stone fires). There are stoves that can use corn stalks and make (not use) charcoal.
Most everyone on this list would rather use the pyrolysis gases than vent or flare them. Cookstove users can make money - not spend it - while cooking. (With wood as well as corn stalks). There is a companion list for this stove topic.