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The study on CO2 gasification behavior helps to promote the clean and efficient utilization of high-alkali fuels. The high-alkali fuels feature high contents of alkali metals, and sometimes they are also rich in alkali earth metals and iron. Though the influences of minerals on gasification characteristics have been extensively studied, few researchers have selected minerals based on the ash composition of high-alkali fuel. Moreover, the complex interactions among minerals could lead to inaccurate conclusions. In this paper, the CO2 gasification behavior of chars originated from one high-alkali biomass and three high-alkali coals was studied. The effects of principal minerals related to the high-alkali fuels, which were determined through the new plasma ashing method and traditional muffle ashing method, were also further evaluated via synthetic coal char. The synthetic coal char was free from any intrinsic minerals. The results indicate that the gasification reactivities of chars from high-alkali fuels are positively associated with the reaction temperature (900–1200 °C). The biomass char possesses the highest gasification reactivity, and three coal char samples are inferior to various degrees. The related chemicals offer their catalytic activities at 1000 °C in the sequence of K/Na-containing chemicals > Fe-containing chemicals > Ca-containing chemicals. The shrinking core model (SCM) and two-dimensional growth of nuclei model (2DGM) were chosen to conduct the kinetic analysis. The reaction constants of chars from different high-alkali fuels agree with their gasification behavior. Generally, the 2DGM is more suitable than SCM to predict the conversion of selected char samples. In most cases, the additions of chemicals increase the reaction constants. When Fe2O3 and CaCO3 are added into synthetic coal char, the SCM is more accurate to describe the gasification behavior at 900 °C, but the addition of Na2SO4 makes the 2DGM a better one. [ABSTRACT FROM AUTHOR]