We've done that too! We used a small barrel inside a large one. The small one has the stuff you want to turn to biochar. You build a fire in the space between them and put a tight lid on it with a chimney to let the gases out. It worked really well. We sprinkled the biochar in the chicken yard, which helped the smell. The charcoal soaked up all the nitrogen in the fresh chicken poop. Then we tilled it into the garden where it will slowly release the nitrogen without burning the plants. Super cool.