The role of co-solvents in improving the direct transesterification of wet microalgal biomass under supercritical condition Hamed Abedini Najafabadi a,1 , Manouchehr Vossoughi a,b,⇑ , Gholamreza Pazuki c a Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran b Institute for Biotechnology and Environment (IBE), Sharif University of Technology, Tehran, Iran c Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran highlights  In situ supercritical methanol conversion was used for algal biodiesel production.  Several co-solvents were used to improve the performance of the process.  Hexane was the most efficient co-solvent in enhancing biodiesel production.  Biomass water content has slight effect on decreasing the performance of the process.  In situ supercritical conversion outperforms the conventional extraction method. graphical abstract article info Article history: Received 10 April 2015 Received in revised form 9 June 2015 Accepted 10 June 2015 Available online 15 June 2015 Keywords: Microalgae Wet biomass Supercritical Methanol Co-solvent abstract In this research, direct conversion of wet algal biomass into biodiesel using supercritical methanol was studied. In this process, microalgal lipids simultaneously was extracted and converted to biodiesel under high pressure and temperature conditions without using any catalyst. Several experiments have been performed to optimize the methanol amount and it has been revealed that the best performance was achieved by using methanol/wet biomass ratio of 8:1. The effect of using various co-solvents in increasing the efficiency of the supercritical process was investigated. It has been shown that hexane was the most effective co-solvent and its optimal ratio respect to wet biomass was 6:1. The results indicated that com- pare to conventional extraction plus transesterification reaction, fatty acid methyl esters (FAMEs) yield was slightly higher in the direct conversion process. Moreover, increasing the moisture content up to 80% has no significant effect on reducing the performance of this process. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction In recent decades, interest in using microalgae as a nonedible biodiesel feedstock has grown considerably due to its substantial amount of lipids (Mata et al., 2010), high growth rate (Rawat et al., 2013), high actual photosynthetic yield (Lardon et al., 2009), and cultivation capability in saline water as well as nonagri- cultural lands (Wijffels, 2008). However, various technological and economical aspects obstruct the industrial-scale production of http://dx.doi.org/10.1016/j.biortech.2015.06.045 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran. Tel.: +98 21 66164140; fax: +98 21 66005417. E-mail address: vosoughi@sharif.edu (M. Vossoughi). 1 Current address: Swammerdam Institute for Life Sciences, University of Amster- dam, Science Park 904, 1098 XH, Amsterdam, Netherlands. Bioresource Technology 193 (2015) 90–96 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech