Reduction of hazardous SO2 into elemental sulphur over chicken eggshells-derived calcium-based redox agent: A systematic step-by-step thermodynamic analysis and process validations

Abstract

Current work demonstrated the thermodynamic feasibility of SO2 reduction into elemental S via catalytic reaction-regeneration loop over chicken eggshells-derived Ca-based redox agent, while the experimental results were presented subsequent to thermodynamics analysis for validating purpose. Ideally, CaS, engineered from chicken eggshells, would facilitate the SO2 reduction upon oxygen-transferring process, resulting in CaSO4 and sulphur solid. Thermodynamic investigations via minimization of Gibb’s Energy (ΔGmin) were initiated with the development of Ca-based redox agent from modelled chicken eggshells, followed by pre-treatment of redox agent and finally the SO2 reduction reaction. Based on the thermodynamic analysis, Chicken eggshell, modelled by CaCO3, is mainly converted into functionalized CaS through dual-sulphurization process, favorably at mild temperature of 623–1023 K. At optimum 1023 K, the obtained product thermodynamically consists of CaSO4 (74.97%) and CaS (24.79%), which required further treatment to maximize CaS content prior to SO2 reduction reaction. H2- and CO-reductions indicate that conversion into 100% of desirable CaS are thermodynamically feasible, with H2-reduction preferred due to its lesser exothermic behavior. As for the SO2 reduction, CaS exhibits an excellent activity in reducing SO2 to elemental S (S2–S8) at relatively low temperature of 723–973 K, with exceptionally high yield (>99%). Upon increasing temperature (>1173 K), adverse effects to both SO2 conversion and sulphur yield were observed, implying the impracticality in laboratory executions. Simple, yet powerful experiments were conducted, subsequently validated the thermodynamic estimations in current study. This systematic study comprehensively investigated and justified the feasibility of entire process, hence providing provisional guidelines for the future execution of laboratory works.