Experiment and simulation study on mechanism and solution of ash agglomeration in supercritical water gasification of coal for hydrogen production

Supercritical water gasification (SCWG) technology shows huge advantages to achieve efficient and clean utilization of coal. Ash agglomeration caused by K₂CO₃ addition makes ash discharging process more difficult and inhibits gasification reaction of carbon in agglomerated ash, which prevents the constant operation of the system and decreases gasification efficiency. This study investigated the formation mechanism of ash agglomeration in potassium carbonate-catalyzed SCWG of coal and find a solution to inhibit this problem. Experiments were conducted in an autoclave to figure out the effect of K₂CO₃ (0 wt%-10 wt%) and Al₂O₃ (0 wt%-20 wt%) on ash agglomeration level at 750 °C. After every single experiment, solid residue was dried and classified into different sizes (0–100 μm, 100–1000 μm, >1000 μm). The ash agglomeration characteristics was examined by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDS). Results indicated that K₂CO₃ reacted with clay and quartz in raw coal, then formed K₂Si₂O₅, KAlSiO₄, KAlSi₂O₆ and KAlSi₃O₈. K₂Si₂O₅ plays a cohesive role between different ash particles. Adding Al₂O₃ can effectively solve this problem by forming KAlSiO₄ instead of K₂Si₂O₅. Besides, Al₂O₃ enhances carbon gasification efficiency by increasing heating rate of feedstock temperature. Simulation work was also done to investigate the boundary condition for ash agglomeration.