Reaction Characteristic Investigation of the Combined Template-Method-Made CaSO4-Mn3O4 Mixed Oxygen Carrier with Lignite

Application of the disposed CaSO4 waste of different industrial processes as a potential oxygen carrier in chemical looping combustion (CLC) has attracted great attention as a result of its high oxygen content, abundant reserve, and low cost. However, both the inferior reactivity and deleterious SO2 gas emitted via the CaSO4 side reactions hinder its wide application in CLC. To heighten the reactivity of CaSO4 and increase its potential to inhibit the emission of harmful sulfur species, the mixed CaSO4 oxygen carrier (OC) decorated with Mn3O4 was produced using the tailored synthesis route by combining the template method with the sol-gel combustion synthesis (SGCS) method developed in our group before. Reaction of the as-prepared CaSO4-Mn3O4 mixed OC with lignite (abbreviated as YN below) was systematically evaluated, and the remarkably enhanced reactivity was reached for YN coal oxidation through sequential oxygen transfer from Mn3O4, recovered oxygen from reduced Mn3O4 via metastable CaSO4 at the interface, and CaSO4. Thus, oxygen involved was effectively used. The solid products formed after the mixed CaSO4-Mn3O4 reaction with YN coal were collected and further characterized. Morphological observation of the solid products indicated the good sintering resistance for the prepared CaSO4 Mn3O4 OC in CLC. Furthermore, both comprehensive analysis of the gaseous and solid sulfur species by integration of the experimental means with the thermodynamic simulation indicated that the main solid sulfur compounds were dominated as CaS, MnS, and MnSO4 with their total fraction above 98%, and the optimized content of Mn3O4 in the prepared mixed OC was around 30-50% for effective fixation of the gaseous sulfur emission. Overall, the combined template-method-made CaSO4-Mn3O4 mixed OC not only obtained the better reactivity for coal conversion but also owned the potential to control the release of sulfur in the CaSO4 side reaction, which is quite attractive in future CLC application.