The Migration Mechanism and Distribution Characteristics of Sodium and Potassium during Catalytic Gasification with Alkali Carbonate Additive

Catalytic gasification is a promising technology for clean and efficient utilization of coal. The migration and distribution of alkali metals on carbon are crucial for the effectiveness of catalytic gasification when alkali salts are added. This paper investigated the catalytic gasification behaviors and migration characteristics of sodium and potassium by designing three types of contact between alkali carbonates and carbon: totally mixed type, complete isolation, and isolated at varying distances. The different distribution characteristics of alkali carbonates on three types of carbon: biochar carbon, coal-char carbon and graphitic carbon are investigated by thermogravimetric differential scanning calorimetry (TG-DSC), scanning electron microscopy (SEM), and heating stage microscope. The results indicate that there are two distinct gasification behaviors depending on the distance from potassium to coal-char, corresponding to the two modes of potassium migration: migration by volatilization without carbon interaction, migration by carbon promoting volatilization with alkali-carbon interaction. With potassium–carbon interaction, the volatilization amount of potassium swiftly increases, and the migration distance of potassium can reach over 1000 μm. When alkali carbonates migrates to different carbon matrix, there is a selective distribution of alkali carbonates on different carbon matrix. The distribution ability of alkali on carbon matrix follows this order: coal-char carbon > biochar carbon > graphitic carbon.