A method and system for continuous production of contaminant free and size specific biochar using downdraft gasification of variable quality feedstock. The system and process of the present invention includes the transfer of biochar from a gasifier after gasification to a temperature-controlled cooling screw conveyor, into a drum magnet for ferrous metal removal into multiple diverters to separate and remove ungasified materials and non-ferrous metal contaminants, then transferred into a granulator for grinding and screening the biochar to a pre-selected size. By directly attaching a novel and continuous product treatment process to the biochar stream as it exits the gasifier, the particle size, moisture content, carbon content and yield of a contaminant free biochar product can be narrowly controlled and improved to meet strict product quality specifications required by specialty applications.