Influence of Vacancy Defects of the Calcium Oxide Surface on the Nonequilibrium Phase Transition of Alkali Metal Salts

Molten alkali metal salt effectively promotes the performance of calcium looping (CaL). Deep insight into the nonequilibrium phase-transition characteristic of alkali metal salt is better for the control of the temperature in CaL, which not only ensures the complete melting of metal salt but also prevents the reaction from inhibiting caused by higher temperatures. In this work, therefore, the molecular dynamics simulation method is used to explore the nonequilibrium phase-transition characteristic of Na2SO4. The results show that the equilibrium melting temperature of nanosodium sulfate on the calcium oxide surface is 810 K, which is lower than the macroscopic melting temperature. Meanwhile, the high heating rates led to the atoms in Na2SO4 unable to break through the thermal stability limit, resulting in overheating of the crystal. Both the surface premelting and overheating melting temperature of the crystal are increased. When the heating rates are 0.25, 0.5, and 1.0 K/ps, the overheating melting temperatures are 845, 885, and 930 K, respectively. More than that, the surface defects enhance the interaction between CaO and Na2SO4 because of the surface being charged. The increases in the interaction not only effectively break the stability of the crystal lattice of Na2SO4 on the defective surfaces but also promote the energy transport inside Na2SO4. Therefore, as the defect concentration increases from 0 to 3% and 5%, the overheating melting temperature of Na2SO4 gradually decreases from 845 to 836 and 815 K.