Studies on steel slag as an oxygen carrier for chemical looping combustion

Chemical looping combustion (CLC) has been suggested as an energy-efficient method for clean combustion with inherent capture of carbon dioxide. The development of commercially available, less-costly, thermally-stable and good kinetics oxygen carriers is the first propriety in the CLC process. This study is the first to explore the use of steel slag, a typical metallurgical waste, as an oxygen carrier in CLC. The primary advantage of steel slag is its low cost because of its nature of the solid waste. Steel slag mainly contains two interested substances for oxygen transfer in CLC, iron oxides and calcium sulfate (after sulfation). The thermochemical analysis on pre-sulfating steel slag was conducted by the Aspen Plus software, and the experiments of the reduction of pre-sulfating steel slag were carried out using a thermogravimetric analyzer (TGA). Characterizations of used steel slag were carried out by X-ray diffraction (XRD). Results indicated that the pre-sulfating steel slag can be a proper and excellent oxygen carrier, whose reduction kinetics using model fuel gas (CO, carbon monoxide) was rapid at medium temperatures. Further investigations revealed that it is Ca-containing components (CaSO4) in pre-sulfating steel slag that was mainly involved in the redox process using CO. Moreover, the reaction rate has increased by nearly 15 times as compared to that of the pure CaSO4 at the same temperature. The conversion of CaSO4 to CaS can be nearly complete. Higher temperatures were adverse to the stability of CaSO4, leading the decomposition of CaSO4, the degradation of its oxygen transfer capability and the increase of sulfur emissions. However, the use of steel slag rationally control the optimized reduction temperature to be 750 degrees C, when its kinetics can be maintained rationally at 100% conversion within 30 min and a stable oxygen transfer capability at 10.63 wt% with the minimized sulfur emissions. This finally validated steel slag as a promising oxygen carrier candidate.