The small explosion that you experienced was not caused by a build up of pressure that was then released. It was caused by the sudden ignition of the right mixture of wood gas and air in a confined space. You should not have holes on the sides or the top of the interior can. By doing that you created a path for oxygen to get into the inner vessel during the process. When you get the right mix of wood gas, air, and heat you generate a sudden explosion. By putting holes only on the very bottom of the can, gas can find it's way out but air won't get back in. It works a little like a check valve. Those are the conditions that we want in order to make biochar. Heat in. Gas out. No air in to the inner can.
