C/H₂O Reaction under Supercritical Conditions and Their Repercussions in the Preparation of Activated Carbon

Abstract

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 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.

Physicochemical and microtextural characterization of activated carbons produced from water steam activation of three bamboo species

“…In some cases, this carbonaceous material seems to tube-like structures as result of the elimination of organic and volatile compounds during the thermal decomposition and activation process. This distinctive morphology has been observed in other lignocellulosic materials [37].…”

“…6c). The presence of silica, identified by XRPD and observed in the TEM examinations, was attributed to the lignocellulosic nature of the bamboo, as occur in other carbon materials produced from natural precursors [37]. The thermal treatment has permitted the elimination of low boiling point compounds from the bamboo structure, but from inorganic species due to their high melting and boiling points.…”

Biomass‐Derived Porous Carbon Materials: Synthesis and Catalytic Applications

“…Salvador et al. developed a high‐temperature HTC process for the production of activated carbon materials from oak wood and anthracite 20b. The carbonization process was performed under both supercritical water (SCW) and steam.…”

Porous Carbohydrate‐Based Materials via Hard Templating

“…Although known since 1913, when Bergius first reported on the hydrothermal transformation of cellulose into coal-like materials, [20] this approach has recently been rediscovered and employed by several authors to produce a wide range of carbonaceous materials from natural precursors in an efficient energy-saving manner, importantly, without CO 2 release. [21][22][23][24][25][26][27] Compared with slow, fast, or flash pyrolysis, biochar from the HTC process has the highest char yield, although it is less resistant to decomposition. Furthermore, the HTC process is both energy-and atom-economical: [28] 1) it releases one-third of the combustion energy through dehydration, 2) the wet conditions avoid the predrying process, and 3) the carbon efficiency is close to 1 after adequate reaction time under proper conditions.…”