Performance improvement and the mechanisms of red mud oxygen carrier in chemical looping gasification using strontium doping strategy

Abstract

Red mud, a kind of solid waste from aluminum industry, is rich in Fe₂O₃, which may serve as oxygen carrier in chemical looping gasification process. However, its inborn properties have brought about inevitable disadvantages, such as low reactivity. In order to enhance performance of red mud, strontium doping strategy was adopted and the mechanisms of promotive effects of strontium were investigated in this work. The results indicate that the temperature required for triggering gasification reactions was lowered by strontium. Meanwhile, strontium addition suppressed the reactions between lattice oxygens and CO. Therefore, syngas yield showed a clear upward trend with high strontium content, reaching up to 122.42 mmol/g at maximum. Gas product distribution was significantly influenced by reaction temperature, found to be optimal at 1050 °C. As the quantity of oxygen carrier added increased, tar cracking was promoted. Nevertheless, syngas underwent deep oxidation with O/C ratio exceeding three. Regarding mechanisms, after strontium was doped, Fe element mainly existed in the form of SrFeO₃ and SrFe₁₂O₁₉. Sr-Fe-O solid solution was formed within red mud and it induced local stress, leading to that fissure progression to deeper levels and facilitating lattice oxygen migration. Moreover, strontium increased specific surface area and pore volume of oxygen carrier while reducing pore size. H₂-TPR curves of red mud exhibited two obvious peaks corresponding to separate reduction of Fe₂O₃ → FeO → Fe. In contrast, oxygen carrier doped with strontium only displayed a single peak owing to direct reduction of SrFeO₃ and SrFe₁₂O₁₉ to Fe. To conclude, strontium doping strategy is proved to be feasible for the complex reaction systems represented by red mud to achieve control of chemical looping gasification process.