Energy recovery from secondary pulp/paper-mill sludge and sewage sludge with supercritical water treatment q Linghong Zhang a , Chunbao (Charles) Xu b, * , Pascale Champagne a,c a Department of Civil Engineering, Queen’s University, Kingston, ON, Canada K7L 3N6 b Department of Chemical Engineering, Lakehead University, Thunder Bay, ON, Canada P7B 5E1 c Department of Chemical Engineering, Queen’s University, Kingston, ON, Canada K7L 3N6 article info Article history: Received 17 June 2009 Received in revised form 12 November 2009 Accepted 30 November 2009 Available online 30 December 2009 Keywords: Secondary pulp/paper-mill sludge Sewage sludge Supercritical water Synthetic gas Heavy oil abstract 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 H 2 /kg SPP (on a dry basis). In comparison to sewage sludges, SPP exhibited a greater capabil- ity for the production of HO and gases owing to its higher contents of volatiles and alkali metals, indicat- ing a prospective utilization potential for SPP as a source of bio-energy. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Secondary pulp/paper-mill sludge (SPP) is the residue produced in the biological treatment process of wastewaters from the pulp/ paper industry. It is a liquid suspension that contains approxi- mately 2 wt.% solids, and is composed of cells, biodegradable and recalcitrant organic compounds (e.g., carbohydrates and lignin), as wells as ashes. SPP can also be considered a waste biomass that can potentially be recovered into a series of solid (e.g., charcoal), liquid (e.g., bio-oils), and gaseous (e.g., biogas and synthetic gas) bio-fuels using various biological and thermo-chemical processes, contributing little or no net greenhouse gas emissions to the envi- ronment because of its carbon–neutral lifecycle (Metcalf and Eddy, 2003; Fytili and Zabaniotou, 2008). Supercritical water (SCW) is a special water phase where the pressure and temperature of water are increased to or above their respective critical points (22.1 MPa and 374 °C). As a superior reac- tion medium, SCW is featured as having a high diffusivity, a low viscosity, and a high solvating ability for organic compounds (Gloyna et al., 1994). In addition, water is also an active reactant in steam reforming and water–gas shift reactions (shown below in Reactions 1 and 2) in the supercritical region. Therefore, SCW is able to transform biomass feedstock into hydrogen (H 2 )-rich gas- eous products with reduced tar and coke formation, and this pro- cess is commonly regarded as supercritical water gasification (SCWG) (Guo et al., 2007; Demirbas, 2004; Osada et al., 2006; Ya- nik et al., 2007; Kruse et al., 2003). Steam reforming reaction: CH x O y þð1  yÞH 2 O ! CO þ 1  y þ x 2   H 2 ð1Þ Water–gas shift reaction: CO þ H 2 O $ CO 2 þ H 2 ð2Þ For most conventional thermo-chemical treatment processes, such as combustion, pyrolysis, and gasification, a pre-dried feed- stock is typically required (Furness et al., 2000). Considering the extremely high water content (>90% on wet mass basis) of sludges, the pre-drying operation would hence be energy and capital inten- sive. However, sludge can be treated directly with SCW, eliminat- ing the need for drying. In comparison to anaerobic digestion (biomethanation), a con- ventional biological method for the methane (CH 4 )-rich biogas product generation from sludge, SCWG can be considered to be more advantageous in terms of providing a higher treatment 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.11.106 q Part of this work has been presented and collected in the Proceedings of the 8th World Congress of Chemical Engineering, Montreal, Quebec, Canada, August 23–27, 2009. * Corresponding author. Tel.: +1 807 343 8761; fax: +1 807 343 8928. E-mail address: cxu@lakeheadu.ca (C. Xu). Bioresource Technology 101 (2010) 2713–2721 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech