Study on CO2 gasification of biochar in molten salts: Reactivity and structure evolution

To study the influence of molten salts on structure evolution and reactivity of biochar, CO2 gasification of biochar in molten salts (Na2CO3-K2CO3) was conducted in a fixed bed reactor. The results indicated that the reactivity of biochar was significantly enhanced with the introduction of molten salts, and the apparent activation energy reduced greatly from 145 kJ/mol for C750 (char gasification without molten salts) to 46 kJ/mol for C1MS1 (mass ratio of molten salts to char was 1:1). However, the presence of molten salts would weaken the endothermic feature of Boudouard reaction with rising gasification temperature and the enhancement of reactivity decreased gradually with mass ratio of molten salts to char increasing. Respected to the structure of char, the presence of molten salts improved biochar pore structure with the increase of external pore volume. The Raman spectra showed that molten salts could promote the decomposition of large aromatic ring systems (no less than 6 rings) into small aromatic ring systems (with 3 to 5 rings). Moreover, the microcrystalline structure of biochar in molten salts contained lower aromaticity, larger microcrystalline interlayer spacing, and smaller crystallite size. The addition of molten salts improved the biochar structure and further enhanced its gasification reactivity.