Study on the pyro-hydrolysis of CaCl2 with steam and CO2 by experiments and DFT calculation

Abstract

The pyro-hydrolysis method is a crucial way to treat calcium chloride (CaCl2)-containing solid waste to achieve environmental friendliness and efficient utilization. In this paper, we have conducted both DFT (density functionary theory calculation) calculations and experiments by flowing carbon dioxide (CO2) and steam (H2O) on the CaCl2 in a fluidized bed. The thermogravimetric analysis reveals that the pro-hydrolysis reaction can proceed more vigorously at temperatures ranging from 300 to 500 °C. The structure characterization of residues collected from the pyro-hydrolysis of CaCl2 was characterized using X-ray diffraction (XRD), X-ray Photoelectron Spectrometer (XPS), and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). The XRD patterns show that a small amount of CaCl2 was decomposed into calcium chloride hydroxide (Ca(OH)Cl) in a fluidized bed with O2 at temperatures above 450 °C. Nevertheless, with the presence of CO2, CaCl2 can convert into calcium carbonate (CaCO3) at a lower temperature. The SEM-EDS displays a swelling structure that appears on the CaCl2 particles surface after the thermal decomposition with CO2, and the content of Cl element at a specific position is significantly reduced. DFT calculations is shown that when CO2 and H2O are bonded to form a carbonic acid complex adsorbed on the surface, the energy barrier for the rupture of the H-OH bond and Ca-Cl bond is dramatically decreased, and the dissociated H+ combined with the Cl- to form hydrogen chloride (HCl). The overall reaction pathway only adsorbs 1.099 eV energy. This research is useful for promoting knowledge of the chlorine evolution mechanism for pyro-hydrolysis of CaCl2 in CO2 atmosphere.