A method is provided for thermally converting non-radioactive combustible wastes to a substantially non-hazardous, non-leachable, sintered particulate carbon-less ash by-product in a kiln having a plurality of reaction zones. The kiln including first and second ends and a body provided between the first and second ends that defines a cavity having a refractory lining that provides resistance to heat conduction. A processor and flow rate controllers are provided that control a flow rate through the body of waste that enters at the first end of the kiln and the flow rate of oxidant gas that enters at the second end of the kiln, the second end being opposite to the first end. The body may be positioned substantially horizontal and may include a length-to-diameter ratio and a resistance to heat conduction that provides a temperature gradient within the cavity to forms separate reaction zones during operation.