Thermal conduction mechanism based on microstructural transformations of molten slag: The role of calcium oxide

摘要

The built up of the slag layer is a character of the entrained-flow gasifier, and its thickness distribution and heat transfer characteristics affect the stable operation of gasification, which is closely related to the thermal conductivity of molten slag. A non-stationary hot wire experimental method and a reverse non-equilibrium molecular dynamics simulation method were combined to investigate the thermal conductivity of the SiO_2-Al_2O_3-CaO-Fe_2O_3-MgO molten slag with a mass ratio of CaO ranging from 5 to 25 wt.%. The slag microstructure information of the distribution of oxygen and tetrahedron Q_n was calculated by molecular dynamics simulation. A good agreement within the deviation of 18% between the measured and calculated thermal conductivity over the entire operating temperature range is observed, and one can find that thermal conductivity decreases significantly as the CaO content increased. The principal pathway for CaO modifying the network structure is the connection between two tetrahedral structures by generation of the Si-O-Ca and Al-O-Ca through the fracture of Si-O-Si and Si-O-Al. The rise of CaO promotes the formation of non-bridging oxygen (NBO) and the decomposition of bridging oxygen. The existence of NBOs leads to an enhancement of anharmonicity and shortens the interatomic distance in the tetrahedral structure, which aggravates phonon scattering to reduce phonon mean free path, thereby decreasing the thermal conductivity. Since the phonon propagation is restricted to short-range tetrahedron separated by structural disorder, the Q_4 structure is the most sensitive to the addition of CaO. An equivalent molar concentration of the Q_4 tetrahedron (X_Q4) is defined and the approximate positive linear relationships between the thermal conductivities and X_Q4 are obtained.