Effect of Calcium-Sodium Composite Flux on Ash Fusibility and Mineral Transformation of Pingshuo High Ash Fusion Temperature Coal
Pingshuo coal ash, characterized by high silicon-aluminum content (Si+Al >85%) and low Si/Al ratio (<1.5), exhibits ash fusion temperatures (AFTs) exceeding 1550 °C, rendering it unsuitable for entrained-flow gasifiers. This study investigates the effect of calcium-sodium composite flux on ash fusibility and mineral transformation. X-ray diffraction (XRD) and FactSage thermodynamic simulations were employed to analyze mineral evolution, while molecular dynamics (MD) simulations revealed the underlying melting mechanism. Results show that adding 20% composite flux (CaO/Na2O) lowers AFTs more effectively than equivalent additions of CaO or Na2O alone, indicating a synergistic effect. At a CaO/Na2O ratio of 3:7, the flow temperatures (FT) of two Pingshuo coal ashes decreased to 1377 °C and 1279 °C, respectively. The composite flux promotes reactions between quartz and Na2O/CaO, forming low-melting-point minerals such as nepheline, albite, and gehlenite, while inhibiting mullite formation. Additionally, Na+ disrupts the silicate network, inducing Ca2+ to preferentially coordinate with [AlO4]5- tetrahedra, further breaking Si-O-Si bonds. MD simulations show that atomic diffusion, quantified by mean square displacement (MSD), is significantly enhanced below 1600 K with composite flux addition compared to single fluxes. These findings provide a mechanistic basis for optimizing flux formulations to enable efficient gasification of high-AFT coals.