Porous solids by the pyrolysis of residue obtained after NaOH extraction of lignite

To utilize the lignite residues after humic acid extraction, experimental studies on pyrolysis of Xianfeng lignite residue (XLR) were conducted with a fixed-bed reactor under nitrogen atmosphere in the temperature range of 500–900 °C. The product gas yield and composition, and the properties of solid product were investigated and compared with those obtained from pyrolysis of Xianfeng lignite (XL). The results showed that the gas yield of dry-ash-free XLR was higher than that of XL and the content of CO in the product gas increased rapidly with increasing temperature. The surface area of the char from XLR pyrolysis increased as the pyrolysis temperature increased, which reached a high value of 612 m²/g at 900 °C compared to only 10 m²/g for the char obtained from XL pyrolysis. The porosity development in the former was due to the activation of carbon in the char matrix with Na-containing groups leading to the formation of micropores and mesopores in the solids, which could be potentially useful as adsorbents.