Methods of improving the adsorption capacity of biochar (14). Initial biochar (14) is charged in a closed vessel (12). The biochar (14) is then exposed to a processing temperature and oxygen level below that of vapor space luminous combustion, and at the same time the oxygen level in a vapor space surrounding the biochar (14) is controlled, so as to promote oxidation reactions that generate additional heat. The biochar (14) is held at the processing temperature by removal of heat by conduction through the vessel (12) walls, uptake of specific heat by solids and vapors in the vessel (12), and the endothermic requirements of converting any biomass present in the vessel (12) into char. The biochar (14) is maintained at the processing temperature and oxygen level for sufficient duration such that the adsorption capacity of the final biochar (14) is improved, as measured by ASTM D-5742 or the equilibrium uptake of R134a at 100 degrees Celsius.