You're best bet is to build a kiln using corrugated iron to both reflect the heat back in & restrict the oxygen. You still need to quench at the end but it produces a lot more biochar. Simply cut lengths of corrugated iron to 1.2m (4ft) & bolt/screw them together to form a cube that's open at both ends. Star pickets wired to the top hold the general shape & support it when you throw long branches in. Feed the fire when the wood burning starts to turn to charcoal. Only the top layer of wood will be combusting because of the restricted oxygen.