Chemical looping gasification of phosphogypsum as an oxygen carrier: The Ca and S migration mechanism using the DFT...

Chemical-looping gasification (CLG) is a novel process for syngas generation from solid fuels that shares the same basic principles as chemical-looping combustion (CLC). This method also uses oxygen carriers (mainly metal oxides and calcium sulfate) to transfer heat and oxygen to the fuel. In this work, we used phosphogypsum (PG) as the oxygen carrier in CLG with lignite fuel. Based on experimental and density functional theory (DFT) theoretical calculation analysis, the Ca and S migration characteristics in PG and the reaction mechanism between lignite fuel molecules and the oxygen carrier PG in CLG were explored. The results show that there is a series of tandem and competitive reactions during this process, and the optimal temperature range of the fuel reactor in the chemical looping gasification of PG oxygen carriers is 1173 K-1223 K. The only gaseous sulfide detected is H₂S, and the solid sulfide CaS and a small amount of unreacted CaSO₄ are detected at a temperature of 1173 K. DFT calculation shows that the presence of S⁺⁶ atoms from SO₄^2- on the top of the CaSO₄ surface is more favourable than Ca²⁺ for C, CO, and H₂ oxidation at a temperature of 1173 K. During oxidation, lattice O consumption mainly occurs around the S⁺⁶ atoms stepwise from S⁺⁶ → S^-2 at a temperature of 1173 K. In addition, CO and H₂ oxidation occur more easily than that of C at a temperature of 1173 K. The experimental results and the calculated results show good consistency, providing valuable information regarding the reactivity of the oxygen carrier PG and the C, CO, and H₂ oxidation over the CaSO₄ surface at an atomic level.