Hydrothermal liquefaction (HTL) of microalgae for biofuel production: State of the art review and future prospects Diego Lo ´pez Barreiro a, *, Wolter Prins a , Frederik Ronsse a , Wim Brilman b a Department of Biosystems Engineering, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, B9000 Ghent, Belgium b Thermo-Chemical Conversion of Biomass Group, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands article info Article history: Received 4 July 2012 Received in revised form 17 December 2012 Accepted 19 December 2012 Available online 8 February 2013 Keywords: Microalgae Hydrothermal liquefaction Biofuel production Biomass conversion Thermochemical abstract Among the various types of biomass, microalgae have the potential of becoming a signifi- cant energy source for biofuel production in the coming years. Currently, research is mainly focusing on optimization of the cultivation methods and the conversion of just a single microalgae fraction (lipids for biodiesel production). Hydrothermal liquefaction is a method for thermochemical conversion of wet microalgae, producing a liquid energy carrier called ‘bio-oil’ or ‘biocrude’, next to gaseous, aqueous and solid by-products. A review of the available literature is presented here, analyzing the influence of parameters such as temperature, holding time and catalyst dosage on the yield and properties of the different product fractions. Also, the strain selection and the status of the technology for hydrothermal processes are analyzed. Finally, based on the findings obtained from the literature review, directions for future research are suggested. ª 2012 Elsevier Ltd. All rights reserved. 1. Introduction Biomass can be converted by several processes to interme- diate solid, liquid or gaseous biofuels that are said to have the potential of supplying 30% of the global fuel demand without affecting the food production and being respectful to the environment [1]. Microalgae are microscopic photosynthetic organisms that can be considered as versatile biological cell factories for renewable biofuel production, with higher photosynthetic efficiency, faster growth rates and higher area- specific yields than terrestrial biomass. They have simple growth needs (water, light, a carbon source and nutrients), and their main constituents are proteins, carbohydrates and lipids. Another fraction, the algaenans, has gained recently attention as source of biofuels [2,3]. These algaenans are insoluble non- hydrolysable biomacromolecules, resistant to drastic chem- ical treatments, in the form of long aliphatic chains that can be found in the outer wall of the microalgae cells [4], mainly from freshwater strains [5]. Cultivation of microalgae brings along some advantages [6,7]: they can be used in waste water treatment while using NH þ 4 , N0  3 or PO 3 4 from the aqueous waste streams as nutrient sources; they can be grown in areas unsuitable for agriculture uses, thus not competing with food production; they can be used for the production of valuable co-products (such as proteins) and they can reduce CO 2 emissions from industrial * Corresponding author. Tel.: þ32 (0)9 264 61 90; fax: þ32 (0)9 264 62 35. E-mail address: Diego.LopezBarreiro@UGent.be (D. Lo ´ pez Barreiro). Available online at www.sciencedirect.com http://www.elsevier.com/locate/biombioe biomass and bioenergy 53 (2013) 113 e127 0961-9534/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biombioe.2012.12.029