A gas mixture preheated to high temperatures using an oxy-fuel, an oxygen-enriched air-fuel or an air-fuel burner is used to devolatilize and partially oxidize carbonaceous feedstock, thereby producing an active residual char that can be used in applications utilizing activated carbon. Use of hot gas and ground carbonaceous feedstock allows the equipment to be minimized, thereby allowing the activated carbon to be produced at or near points of use, for example the production of activated char at or near utility boilers for use in the reduction of mercury emissions from flue gas streams.