Energy recovery from secondary pulp/paper-mill sludge and sewage sludge with supercritical water treatment

Secondary pulp/paper-mill sludge (SPP) and sewage sludges (primary, secondary, and digested sewage sludges) were treated in supercritical water at temperatures ranging between 400 °C and 550 °C over 20–120 min for energy recovery. Low temperature and short reaction time favored the formation of heavy oil (HO) products, which were mainly composed of a variety of phenol and phenolic compounds, as well as some nitrogen-containing compounds, long-chain alkenes and alcohols, etc., with high gross calorific values (>36 MJ/kg). By contrast, the formation of synthetic gases, a mixture of hydrogen, carbon monoxide, carbon dioxide, methane, and other light hydrocarbons, were not significantly affected by reaction time but greatly enhanced with increasing temperature. The highest gas yield was obtained at 550 °C, where 37.7 wt.% of the SPP (on dry basis) was converted into gases, with hydrogen yields as high as 14.5 mol H2/kg SPP (on a dry basis). In comparison to sewage sludges, SPP exhibited a greater capability for the production of HO and gases owing to its higher contents of volatiles and alkali metals, indicating a prospective utilization potential for SPP as a source of bio-energy.