We've constructed a simple closed retort kiln using an insulated firebrick enclosure designed for a 200 liter dry-goods steel barrel with a clamp-on cover as a retort. The barrel is filled with split wood or other biomass feedstock, covered, and heated from below with a separate wood fire until it reaches pyrolysis temperatures (over about 320 C). If the moisture content of the wood is low, very little if any additional heat is needed once pyrolysis begins, as the reaction gives off heat.