Abstract: A method of converting a combustor from hydrocarbonaceous fuel to carbonaceous fuel characterized by the following multi step process. Oxygen-enriched gas is provided and the carbonaceous fuel is burned in a primary combustor in the presence of the oxygen-enriched gas to produce a high temperature, combustible product gas comprising carbon monoxide and hydrogen and produce a molten slag in the bottom that contains materials that would normally polute the atmosphere in a low temperature combustion process. The slag is withdrawn from the primary combustion chamber. Inert gas is employed to isolate and cool the explosive product gas and transfer the heat to a second combustion-supporting gas, such as air being supplied to the combustor. The less hot product gas is passed to the combustor and burned in the heated air. The waste gases that are produced by burning the product gas of carbon monoxide and hydrogen have much lower concentrations of pollutants therein.