Key Takeaways & Executive Findings
- •• • Pingshuo coal ash has Si+Al >85% and Si/Al <1.5, with FT >1550 °C, necessitating flux addition for entrained-flow gasification. • • Adding 20% calcium-sodium composite flux (CaO/Na2O = 3:7) reduces FT to 1377 °C and 1279 °C for two coal ashes, outperforming single CaO or Na2O additions. • • Composite flux induces formation of low-melting minerals (nepheline, albite, gehlenite) and inhibits mullite, enabling low-temperature eutectic melting. • • MD simulations show that below 1600 K, Na+ enhances atomic diffusion (higher MSD) and disrupts silicate networks, facilitating Ca2+ coordination with [AlO4]5- and further breaking Si-O-Si bonds.
Abstract
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.
1. Introduction
Coal gasification, particularly entrained-flow technology, demands ash with a flow temperature (FT) below 1400–1500 °C to ensure smooth slag tapping. However, many Chinese coals, such as Pingshuo coal, possess high ash fusion temperatures (AFTs) exceeding 1550 °C due to their high silicon-aluminum content (Si+Al >85%) and low Si/Al ratio (<1.5). This renders them unsuitable for direct use in liquid-slag gasifiers, necessitating the addition of fluxing agents to lower the melting point. Traditional single fluxes like CaO or Na2O have limited efficacy, often requiring high dosages that increase costs and operational complexity.
This study addresses this bottleneck by systematically investigating the synergistic effect of calcium-sodium composite flux on the ash fusibility of Pingshuo coal. By combining experimental characterization (XRD, FactSage) with molecular dynamics simulations, the research elucidates the mineral transformation pathways and atomic-scale mechanisms that enable superior fluxing performance. The results demonstrate that a 20% addition of composite flux at a CaO/Na2O ratio of 3:7 reduces FT to below 1400 °C, offering a cost-effective solution for utilizing high-AFT coals in gasification processes.
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ZHANG Qihui, GAO Longfei, GUO Zhenxing, KONG Lingxue, LI Huaizhu, CAO Yang, WANG Kun, BAI Jin, BAI Zongqing, LI Wen (2026). Effect of Calcium-Sodium Composite Flux on Ash Fusibility and Mineral Transformation of Pingshuo High Ash Fusion Temperature Coal. Journal of Fuel Chemistry and Technology. https://doi.org/10.3724/2097-213X.2025.JFCT.0029
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Frequently Asked Questions
What is the optimal calcium-to-sodium ratio for the composite flux, and how does it affect the flow temperature?
The optimal CaO/Na2O ratio is 3:7, which reduces the flow temperature (FT) of Pingshuo coal ash to 1377 °C and 1279 °C for the two tested coals, respectively, when added at 20% total flux. This is significantly lower than the FT achieved with single CaO or Na2O additions, demonstrating a synergistic effect.
How does the composite flux influence mineral transformation compared to single fluxes?
XRD and FactSage analyses show that the composite flux promotes the reaction of quartz with Na2O and CaO to form low-melting-point minerals such as nepheline, albite, and gehlenite, while inhibiting the formation of high-melting mullite. This leads to the formation of low-temperature eutectics, which is not observed with single fluxes.
What is the mechanistic role of Na+ in enhancing ash melting?
Molecular dynamics simulations reveal that Na+ exhibits high diffusivity, disrupting the local ordered structure of the silicate network. This induces Ca2+ to preferentially coordinate with [AlO4]5- tetrahedra, further breaking Si-O-Si bonds. This synergistic disruption enhances atomic diffusion, as evidenced by higher mean square displacement (MSD) values below 1600 K.
Can the composite flux be applied to other high-AFT coals with different Si/Al ratios?
The study indicates that the effectiveness of the composite flux depends on the coal ash composition. For Pingshuo 4# coal (Si/Al = 0.94), even with 20% composite flux, FT could not be reduced below 1400 °C, whereas for Pingshuo 11# coal (Si/Al = 1.16), FT was successfully lowered. Therefore, the flux formulation must be optimized based on the specific ash chemistry.
What are the practical implications for industrial gasification operations?
The use of calcium-sodium composite flux at 20% addition can reduce FT to below 1400 °C, enabling the use of high-AFT coals in entrained-flow gasifiers. This not only expands the feedstock flexibility but also potentially reduces operational costs by avoiding the need for blending with low-AFT coals or using higher flux dosages.
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