Influence mechanism of Na2O on the network structure depolymerization of ZD coal ash silicate under high temperature

Abstract

Na2O is the basic alkali metal oxide in Zhundong (ZD) coal, the content of Na2O directly affects the ash fusion temperatures (AFTs) of coal ash. Fourier transform infrared (FTIR) spectroscopy and molecular dynamics (MD) simulation were used to study the structural characteristics of synthetic coal ash under Na2O concentration of 1–13 mol%. Both FTIR experiments and MD simulation show that Na+ attacks the tri-coordinate oxygen (Ot) and the bridge oxygen (Ob) formed by [SiO4]4− and [AlO4]5− tetrahedron structure, generates more non-bridge oxygen (Onb), the shelf-like network structure(Q4) in fused coal ash is depolymerized to form layered silicate structure (Q3), chain or cyclic silicate structure (Q2, Q1). O(Al, Al, Si) and O(Al, Al, Al) in Ot decreased their trend by 3% each, Si–O–Si was reduced from 12% to 7%. Al–O–Si was reduced from 36% to 29%. The intricate structures are gradually depolymerized into plain structures, which leads to the decrease of AFTs.