Chemical looping combustion of sulfur paste to SO2 by phosphogypsum oxygen carrier for sulfur acid production

Sulfur paste is the side product of coal chemical engineering, with its main component being elemental sulfur. Sulfuric acid production via Chemical Looping Combustion (CLC) of sulfur paste is a novel technology to generate SO2 that are highly concentrated in combustion products, by means of circulating oxygen carrier particles. The objective of the present work is investigating reaction kinetics and mechanism of CLC application using sulfur paste as fuel by a phosphogypsum oxygen carrier, which is also one kind of solid waste with large stock. Thermodynamics analysis, experiments and reaction kinetics model validation were performed to evaluate the sulfur paste conversion behavior. Then, based on density functional theory and periodic structure model, the microscopic reaction mechanism of the interaction between S2 and CaSO4(0 1 0) oxygen carrier surface was studied at the molecular level. CaSO4 has the strong chemical affinity with sulfur to produce SO2, but thermodynamic limitation of S conversion to SO2 with CaSO4 oxygen carrier exits in the process. The intrinsic reduction kinetics of phosphogypsum oxygen carrier with sulfur paste focusing on the dominant reaction CaSO4 + 2S = CaS + 2SO2 was described well by the Anti-Jander function model, which agrees well with the experimental data. The formation of two SO2 molecules needs to overcome the reaction energy barrier of 124 kJ/mol.