Main mineral melting behavior and mineral reaction mechanism at molecular level of blended coal ash under gasification condition

The main mineral melting behavior and mineral reaction mechanism at molecular level of Chinese blended coal ash under gasification condition (30% H₂, 66% CO, 4% CO₂) from 1073 K to 1573 K were studied through the ASTM test, X-ray diffraction (XRD), ternary phase diagram system and quantum chemistry calculation with ab-initio calculations. The results show that with increasing blending mass fraction of low ash fusion temperature (AFT) ash (ash B), the location of blended ash in ternary systems is transferred from the mullite region to the anorthite region, as the dominant crystal mineral of blended ash at around DT (XRD analysis) is also transferred from mullite to anorthite. The calcium-bearing minerals, such as anhydrite, calcite etc., can react with mullite and the precursors of mullite (metakaolinite etc.), which is one of the main refractory minerals in high AFT ash (ash A), and is converted into low-melting minerals (anorthite, gehlenite, and fayalite etc.) in the temperature range between 1273 K and 1403 K. The reaction between mullite and CaO to form anorthite plays a significant role in decreasing AFTs of blended coal ash A/B. It is because the chemical activity of the highest occupied molecular orbits (HOMO) in mullite cluster is stronger than that of the lowest unoccupied molecular orbits (LUMO) in mullite cluster, the Ca²⁺ as electron acceptor can easily enter into the crystal lattice of mullite mainly through O (7) and O (12) and cause the rupture of bonds Al (1)-O (13) (in the [AlO₆]⁹⁻-octahedron) and Al (8)-O (13) (in the [AlO₄]⁵⁻-tetrahedron), which are weaker than any other bonds in crystal lattice of mullite. Finally, the entrance of Ca²⁺ can force mullite to transform to anorthite by the effect of Ca²⁺, and the entered Ca²⁺ is located in the center of [SiO₄]⁴⁻-tetrahedron ring in the anorthite crystal lattice. Taking the [SiO₄]⁴⁻-tetrahedron, which is composed of Si (70), O (78), O (48), O (91), O (86) as an example, the Ca²⁺ can capture the partial electronics of O (86) and cause the bond length (B.L.) of bond Si (70)−O (86) to become longer and unstable.