Simulated investigation of chemical looping combustion with coal-derived syngas and CaSO₄ oxygen carrier

Compared to metal oxides， CaSO₄ adopted as oxygen carrier （OC） in chemical looping combustion （CLC） presents several advantages such as low cost， easy availability and superior oxygen transfer capacity， whilst the SO₂ emission and solid sulfur deposit in the process could be a big concern. In this study， thermodynamic simulations were conducted to investigate the sulfur distribution in a CLC system with CaSO₄ as OC and syngas derived from coal as the fuel. Several findings were attained:（i） On the main products and reaction pathways in the fuel reactor （FR）， at the low temperature of 100℃~400℃， the main sulfur species and carbon deposit were H₂S and CaCO₃ via the methanation of CO with H₂ coupled with the shift reaction of CO with H₂O（g） and the ensuing thermochemical sulfate reduction （TSR）. Then at 400℃~915℃， CaS and CO₂ were the main products through the reaction of CaSO₄ with H₂ or CO， and both products increased with increasing FR temperature. Furthermore， at the FR temperature higher than 915℃， due to the initiation of the solid side reaction between CaS and CaSO₄， the percentage of CaS declined. In contrary， the percentages of CaO， H₂ and CO increased possibly due to the consumption of part of CaSO₄ in the side reaction and thus not enough lattice oxygen available. In the air reactor（AR）， the oxidization of CaS by air into CaSO₄ was always dominant. Besides at Ф_AR below 0.8， both the solid side reaction of CaSO₄ with CaS and the oxidization of CaS into CaO were simultaneously in effect.（ii） In the FR， the optimized condition was suggested as at around 915℃， atmospheric condition and carefully controlled Ф_FR around unity.（iii） In the AR， sufficient supply of air was important for the oxidization of CaS， and Ф_AR ≥1 would ensure the full oxidization of CaS into CaSO₄ and prevent the emission of SO₂ and formation of CaO as well. Overall， this study provided the most suitable conditions of using CaSO₄ as OC in CLC of syngas with the minimal SO₂ emissions and CaO formation.