**4. Catalysts used in SCWG of black liquor**

To improve the gasification efficiency and lower the reaction temperature, the addition of catalysts can be considered. Catalysts for SCWG can be classified into alkali salts, metals, and metal oxides [24]. They have different advantages in SCWG of biomass due to their different catalytic properties. In particular, alkali salts are effective catalysts for SCWG of biomass [25]. However, some catalysts, including alkali salts, may be inapplicable in SCWG of black liquor due to its unique composition and properties (**Table 1**).
