Reaction Mechanism of Coal Chemical Looping Process for Syngas Production with CaSO4 Oxygen Carrier in the CO2 Atmosphere

Chemical looping combustion process for gaseous and solid fuel has been investigated widely. Recently, particular attention is paid to syngas and hydrogen generation from natural gas or solid fuels. CaSO4 has been proved to be a promising oxygen carrier for the chemical looping process with the merit of low price and environmental friendliness. The reaction mechanism of coal and pure CaSO4 for syngas production in the CO2 atmosphere was investigated using the simultaneous thermal analyzer in this paper. First, the thermodynamic analysis of reaction between coal and CaSO4 with different mass ratios was carried out respectively in N2 and CO2 atmospheres. It predicted that the CO2 can promote the reactions, while the coal-CaSO4 mass ratios affected the fate of sulfurous gaseous species greatly. Subsequently, thermogravimetric experiments were conducted by the peak fitting technique. It concluded that the drying and pyrolysis of the coal were main reactions before 800 °C, while the complex reactions, including the reaction between CaSO4 and coal char, gasification of coal char, and the decomposition of CaSO4, occurred during 800–1100 °C. The reaction kinetics and types of the reaction between coal and CaSO4 for syngas in the CO2 atmosphere were explored by isoconversional method. It indicated that the complex processes were controlled by the diffusion of gas–solid or solid–solid first, followed by parallel competing reactions. Finally, the reaction residues between coal and pure CaSO4 with different mole ratios were analyzed using scanning electron microscopy and energy dispersive spectrometer (SEM-EDS).