Investigation on oxygen-controlled sewage sludge carbonization with low temperature: from thermal behavior to...

A comprehensive study was conducted on the characteristics of oxygen-controlled carbonization process of sewage sludge (SS) using thermogravimetric analysis and lab-scale carbonization experiment. Reaction temperature of SS carbonization was varied between 250 and 650 °C in carrier gas with different O₂ contents. The thermal process of SS in low oxygen could be divided into three stages: dehydration (below 160 °C), devolatilization (160-380 °C), stubborn volatile decomposition and fixed carbon combustion (380-600 °C). Based on Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) methods, the reaction activation energy (E) of SS carbonization process in 10% O₂ was the lowest, with values of 98.50 kJ mol^-1 (KAS) and 103.49 kJ mol^-1 (FWO). The properties of the obtained char, tar, and gas products were analyzed by FTIR and GC-MS. With the increase of carbonization temperature, char yield decreased and gas yield increased. The highest yield of tar was 27.76% (N₂) and 27.04% (10% O₂) at 450 °C. Low-oxygen atmosphere at the same temperature did not change the yield of char but increased the fixed carbon content and its aromaticity. Oxygen would participate in secondary cracking in tar and promote gas generation above 350 °C. It was found that the presence of oxygen not only increased the concentration of H₂, CO, and CH₄ in gas product, but also improved the quality of tar in terms of high aromatic content and low nitrogen-containing compounds.