Apparatus and processes are provided for combusting hydrocarbon-containing waste fuel with an oxidizer, preferably liquid oxygen, and producing a substantially clean exhaust gas and a separate high purity steam product (e.g., superheated steam) that can be used to generate electrical power. The apparatus includes a plurality of combustion chambers in series with means to control the combustion temperature while facilitating complete combustion. The apparatus and method are useful for transforming waste materials, particularly solid and liquid wastes, such as the rubber from scrap tires into non-hazardous materials and energy.