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

•Effect of additives on gas products and ash size were investigated.•Formation mechanism of ash agglomeration was revealed by XRD and SEM-EDS analysis of coal ash.•The ternary phase diagram K2O-Al2O3-SiO2 was calculated to obtain the boundary parameters for ash agglomeration.•The formation and inhibition pathway of coal ash agglomeration were concluded.•The experimental and simulated methods of this study can be extended to the ash agglomeration research of other coals. 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.