Pyrolysis of sludge briquettes for the preparation of cylindrical-shaped biochar and comparison between CO2 and steam activation

Abstract

Sludge briquettes were prepared through briquetting (under the pressure of 25 MPa) of half-dried sewage sludge with moisture content of 30 wt%, and the cylindrical-shaped biochar (CSB) was obtained by subsequent pyrolysis (600 °C) and CO2/steam activation (650–950 °C). Briquetting process showed great influence on the pyrolysis of sewage sludge. The temperature range for the organic decomposition increased from 150 to 500 °C for the sludge without briquetting to 150–700 °C for the sludge briquette. The mechanical performance of CSBs was greatly influenced by activation temperature. The axial compressive strength of the CO2/steam activated CSBs were in the range of 17.8–22.2 kg·cm‑2 at the activation temperature of 650 °C and 800 °C, but dropped markedly when the activation temperature increased to 950 °C. CSBs were negatively charged and rich in oxygen/nitrogen-containing functional groups like –OH, –COOH, amines and amides, and the specific surface area of the optimal steam and CO2-activated CSB were 123.2 and 113.5 m2·g‑1, respectively. The prepared CSBs were applied in methylene blue (MB) adsorption, and the maximum adsorption capacity of 513.81 and 419.29 mg·g‑1 could be achieved by the CO2 and steam-activated CSB, respectively. Both physical and chemical adsorptions played important role in the MB adsorption. Finally, the regeneration test showed that optimal samples of the CO2 and steam-activated CSB still retained around 90 % of its original adsorption capacity after three cycles of adsorption-regeneration tests, but the mechanical performance dropped by 58.1 % and 47.8 %, respectively.