Depolymerization mechanism of MgO on a silicate microstructure under different CaO contents: a theoretical and experimental study

Abstract

The content of alkaline earth metals in Zhundong (ZD) coal ash has a great effect on its melting characteristics. Herein, the influence of MgO content on a silicate microstructure was studied using molecular dynamics simulation under the conditions of 40 mol% high calcium and 20 mol% low calcium. Moreover, the theoretical results were verified by the Fourier transform infrared (FTIR) spectroscopy analysis of synthetic coal ash. In the MgO–CaO–Al₂O₃–SiO₂ system, the results showed that the average bond lengths of Si–O and Al–O in the system are 1.61 and 1.75 Å, respectively, under the high calcium condition. With an increase in MgO content, Q⁴ with a high degree of polymerization was depolymerized and changed into Q¹, Q² and Q³ with a low degree of polymerization; O_t decreased, O_b increased first and then decreased, and O_nb increased. Under the low calcium condition, the average bond lengths of Si–O and Al–O were 1.61 and 1.76 Å, respectively. When the content of MgO increased, Q⁴ was depolymerized into Q² and Q³, O_t decreased, O_nb increased, and O_b only increased by 1%. Compared with Si⁴⁺, Mg²⁺ easily attacks Al³⁺, and the degree of damage to the [AlO₄]⁵⁻ tetrahedron is greater. By comparing the distribution of oxygen and the degree of polymerization in the high calcium and low calcium systems, it was concluded that MgO more easily destroys the silicate networks of ash under the condition of high calcium CaO content of 40 mol%, which changes the viscosity and combustion characteristics of the system.