J. of Supercritical Fluids 69 (2012) 57–63 Contents lists available at SciVerse ScienceDirect The Journal of Supercritical Fluids jou rn al h om epage: www.elsevier.com/locate/supflu Oxidative gasification of olive mill wastewater as a biomass source in supercritical water: Effects on gasification yield and biofuel composition Ekin Kıpc ¸ ak, Mesut Akgün ∗ Chemical Engineering Department, Yıldız Technical University, Davutpasa Campus, No. 127, 34210 Esenler, Istanbul, Turkey a r t i c l e i n f o Article history: Received 22 December 2011 Received in revised form 14 May 2012 Accepted 23 May 2012 Keywords: Biofuels Gasification Olive mill wastewater Partial oxidation Supercritical water a b s t r a c t In this study the partial oxidation of a biomass source, namely olive mill wastewater (OMW) was inves- tigated in supercritical water. OMW is a by-product obtained in olive oil production and it has a very complex nature that is characterized by a high content of polyphenols and organic compounds. The par- tial oxidation experiments were carried out at five different reaction temperatures (400, 450, 500, 550 and 600 ◦ C) and five different oxygen concentrations (9.11, 18.22, 45.55, 91.10 and 182.20 mmol O 2 /L) with a reaction time of 30 s, under a pressure of 25 MPa. In addition, various OMW concentrations were tested in order to comprehend the effects on biofuel yield and composition. The gaseous products were generally composed of hydrogen, carbon monoxide, carbon dioxide and C 1 –C 4 hydrocarbons like methane, ethane, propane and propylene. The maximum yield of the obtained gaseous product was 5.55 m 3 /kg carbon in OMW with feed total organic carbon (TOC) concentration of 2500 mg/L, at a temperature of 550 ◦ C. At these reaction conditions, the aforementioned product composition was 7.93% for hydrogen, 28.91% for methane, 5.85% for ethane, 0.47% for propane, 0.55% for propylene, 54.04% for carbon dioxide and 0.16% for minor components such as n-butane, i-butane, 1-butene, i-butene, t-2-butene, 1,3-butadiene and nitrogen. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Supercritical water gasification (SCWG) has recently received much attention as a potential alternative to energy conversion methods applied to aqueous/non-aqueous biomass sources [1–3] or fossil fuels such as coal [4,5]; due to the unique physical properties of water above its critical conditions (i.e. 374.8 ◦ C and 22.1 MPa). At temperatures of near-critical and supercritical point, both H 3 O + and OH - ions are formed due to the self-dissociation of water. Therefore, water can behave as a catalytic precursor for acidic or basic reactions. Moreover, since organic compounds have complete miscibility and a high solubility in supercritical water, chemical reactions with high efficiencies and without interfacial transport limitations can be obtained in the case of water–organic mixtures [6–8]. Therefore, supercritical water offers a control mechanism depending on solubility, excellent transport properties based on its high diffusion ability, a low viscosity and new reaction alternatives for hydrolysis or oxidation [9]. With the current shortage of fossil fuels, a major strategic impor- tance is attributed to not only using biomass but also to utilizing the huge amount of waste for high-value energy. Biomass energy ∗ Corresponding author. Tel.: +90 212 383 4759; fax: +90 212 383 4725. E-mail address: akgunm@yildiz.edu.tr (M. Akgün). is among the green energy sources, which has the potential to provide the increasing energy demand of the world. Being an environmental friendly and a renewable energy source, it can be utilized with the use of various energy conversion technologies. One of these technologies, which have shown a great poten- tial for the conversion of biomass with high moisture content to produce hydrogen and other gases, is the oxidative or thermal gasification under supercritical conditions. Extensive investiga- tions have been conducted in the recent years on this topic, which included model compounds such as glucose [10–12], cel- lulose [13–15], lignin [16,17] and some real biomass compounds [1,3,18]. OMW is a by-product obtained during the production of olive oil. It is generally composed of the olive fruit’s water content, soft tissues from the olive pulp, water used to wash and process the olives and a very stable oil emulsion [1]. The typical OMW compo- sition by weight is 83–96% water, 3.5–15% organic compounds and 0.5–2% mineral salts. The organic fraction includes sugars, lipids, organic acids, tannins, pectins, nitrogen compounds, polyalcohols and polyphenols [1]. OMW has a significant pollution potential because of its high organic and polyphenol content, with its dark color. It is responsible for several biological effects, as phenolic com- pounds of low molecular weight show toxicity on seed germination, aquatic organisms and bacteria. All of these characteristics, com- bined with the high amount of its organics content can make OMW 0896-8446/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.supflu.2012.05.005