Study of the microtextural transformation of coal char during supercritical water activation

• Oxygen functional groups are hydrolyzed to form aromatic crystallites in SCW.
• Aromatic crystallites are active units in SCW and composed of meso-pores.
• Cross-linked unit and large aromatic rings are composed of micro-pores.

Low-rank coal chars with different structures were prepared in a fixed-bed pyrolyzer. These coal chars were activated in a semi-continuous tubular supercritical water (SCW) reactor to produce activated carbon. Raman spectra data showed that the amount of aromatic carbon increased with pyrolysis temperature. After SCW activation, smaller aromatic ring units gradually disappeared in SCW, while large aromatic rings in the carbon matrix evolved into the pore structure. Corresponding to the results of nitrogen adsorption, different structures had different reaction rates as a result of SCW exposure, which led to the production of different pore textures. One of the possible pathways in the SCW activation process is proposed. Although there is a strong association between the ratio of cross-linked structures and BET surface area at higher pyrolysis temperatures, the texture of activated carbon is mainly related to the degree of coal conversion achieved.

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