Deep structure analysis on coal slags with increasing silicon content and correlation with melt viscosity

Abstract

In an entrained flow gasifier, the viscosity of molten slag along the wall is critical for continuous operation. Molten coal slag belongs to silicate-containing melts and slag properties are intrinsically affected by structure transformation. In this article, the structure of coal slags with different SiO2 compositions ranging from 48% to 63% wt is investigated by solid-state nuclear magnetic resonance (SS-NMR) spectroscopy and molecular dynamics (MD) simulation. 29Si, 27Al spectra were obtained and the contents of Qn species were analyzed. Radial distribution functions, proportions of bridging oxygen (Ob) and non-bridging oxygen (Onb) were calculated. Viscosities of slags were measured by a high temperature viscometer. Both NMR measurement and MD simulation show that an increase of silicon can lead to a high degree of polymerization. Most alumina is in form of the four-coordinate structure like the tetrahedral Si. The chemical bond of Si-O is the most stable compared to other bonds such as Al-O, Ca-O etc. Therefore, an increase of silicon can promote the [SiO4] formation and form more Ob while lowering the amount of Onb. The increased polymerization degree with higher silicon leads to a high slag viscosity and a linear relationship is found between the measured viscosities and fractions of Q4.