Experimental study on the effects of residual carbon on ash fusion characteristics of high-alkali Zhundong coal under deep oxygen-staged condition

Due to the deep stage of oxygen, the pulverized coal cannot be burned completely in the primary combustion zone, resulting in a high content of residual carbon. The residual carbon in ash has a potential effect on the melting characteristics of ash, while there is a lack of specific research on the effects of residual carbon on ash fusion characteristics of high-alkali coals. In the present study, synthetic ash was blended with residual carbon to study the impacts of residual carbon content, graphitization level of the carbon, and atmospheric condition on ash fusion characteristics. Thermogravimetric analyzers and ash melting point testers were used to study the thermogravimetric behavior of ash and the characteristic temperatures of ash fusion, and then analyze the changes in the mineral composition and micromorphology of slag using X-ray diffractometers and scanning electron microscopy. The experimental results reveal that the change in residual char content exerts a minor influence on the type of crystalline minerals within the ash, while it affects the relative content of various minerals. The softening temperature of the ash blended with activated carbon and graphite are 1234°C and 1242°C, respectively. The blending of activated carbon greatly promotes the generation of gehlenite (Ca2Al2SiO7, 1593°C) and inhibits the generation of merwinite (Ca3MgSi2O8, 1550°C), increasing the ash melting temperature. The ash fusion characteristic temperatures of the pure synthetic ash decrease with the enhancement of the reductive atmosphere. However, the ash fusion characteristic temperatures of the ash blended with carbon in strong reducing atmosphere are increased compared to those in weak reducing atmosphere because part of the carbon acts as a skeleton or reacts to form a carbon-silicon compound. The present study can offer improved knowledge to provide fundamental support for the combustion and utilization of high-alkali Zhundong coal under deep oxygen-staged conditions.