C/H2O reaction under Supercritical conditions and their repercussions in the preparation of activated carbon

Two chars prepared by carbonization of oak wood and anthracite were used to perform a comparative study of the gasification with supercritical water (SCW) and with steam. This work reports the effects of the type of char, the activating agent, temperature, flow rate, and particle size employed on the kinetics, mechanism of reaction, and the characteristics of the activated carbons obtained. The results show that the reactivity of the two chars is much higher with SCW than with steam. Although this increase can be explained in terms of the greater penetration of SCW and diffusional effects in the pore structure of the chars, some aspects suggest a possible change in the mechanism of reaction favored by the formation of clusters in SCW. The evolution of porosity was also found to differ when the char was gasified with SCW and with steam, being governed strongly by the starting material. When the oak char was activated with SCW, the smallest microporosity was broadened from the very first moments due to its very open pore structure, providing carbons with larger micropores and some mesoporosity. In contrast, in the case of the anthracite char, with 0 a narrower pore structure, the evolution of the porosity was slower and less uniform, favoring external gasification of the particle. Accordingly, the carbons had a broader distribution of micropores, and mesoporosity was scarce.