Investigation of ash fusion characteristics and migration of sodium during co-combustion of Zhundong coal and oil shale

Zhundong coal, which has high sodium content, induces severe fouling and slagging during its combustion in a boiler. Co-combustion is an effective method for mitigating these processes. In this study, oil shale is selected as an additive during combustion in order to study the properties of mixed fuel. The study results reveal that water-soluble sodium is the predominant component of Zhundong coal, which originates from the salt in seawater. With an increase in the oil shale blending ratio, the ash fusion temperatures of the mixed coal and the difference between softening temperature and deformation temperature first decrease and then increase. The maximum sodium retention of 86.01% occurs at a blending ratio of 10% and a final temperature of 815 °C. With an increase in temperature, aluminosilicates, such as albite and nepheline, form in the ash. Therefore sodium gradually migrates from the water-soluble phase to the aluminosilicate phase. With an increase in the SiO2/Al2O3 ratio, the ash fusion temperature of mixed coal decreases gradually, which is attributed to the effects of the SiO2 and Al2O3 contents on the melting point. Additionally, a mathematical model for sodium volatilization in the coal is constructed by MATLAB R2014a for fitting and optimization.