Abstract—Hydrothermal liquefaction (HTL) is a technique for obtaining clean biofuel from biomass in the presence of heat and pressure in an aqueous medium which leads to a decomposition of this biomass to the formation of various products. A role of operating conditions is essential for the bio-oil and other products’ yield and also quality of the products. The effects of these parameters were investigated in regards to the composition and yield of the products. Chlorellaceae microalgae were tested under different HTL conditions to clarify suitable conditions for extracting bio-oil together with value-added co-products. Firstly, different microalgae loading rates (5-30%) were tested and found that this parameter has not much significant to product yield. Therefore, 10% microalgae loading rate was selected as a proper economical solution for conditioned schedule at 250 o C and 30 min-reaction time. Next, a range of temperature (210-290 o C) was applied to verify the effects of each parameter by keeping the reaction time constant at 30 min. The results showed no linkage with the increase of the reaction temperature and some reactions occurred that lead to different product yields. Moreover, some nutrients found in the aqueous product are possible to be utilized for nutrient recovery. Keywords—Bio-oil, Hydrothermal Liquefaction, Microalgae, Aqueous co-product. I. INTRODUCTION HERMOCHEMICAL conversion (TCC) is a chemical amending process of biomass production under heated, generally pressurized, and oxygen deprived conditions in which long-chain organic compounds (solid biomass) are broken into short-chain hydrocarbons, such as syngas or oil [1]. TCC also includes gasification, pyrolysis, and liquefaction [1], [2]. Hydrothermal liquefaction (HTL) engages in direct liquefaction of biomass, with the presence of water and with or without a catalyst that directly transforms the biomass into liquid oil, which contains higher energy content than syngas or alcohol, with a temperature lower than 400 o C for the reaction [1]. HTL is different from biomass gasification and pyrolysis, which require dried feedstock and an environmental temperature higher than 600 o C to encourage the process and therefore consume a larger amount of energy [1], [3]. Water is an important factor in HTL since its property is changed when temperature increases. First, its relative permittivity decreases rapidly, then the dissociation of water dramatically increases. Therefore, water becomes a good solvent for hydrocarbon at a M. Tantiphiphatthana, L. Peng, and K. Yoshikawa are with the Yoshikawa laboratory, Tokyo Institute of Technology, Yokohama 226-8502, Japan (phone: 045-924-5507; fax: 045-924-5518; e-mail: tantiphiphatthana.m.aa@ m.titech.ac.jp, plinjp@gmail.com, yoshikawa.k.aa@m.titech.ac.jp). R. Jitrwung is with the Energy Technology Department, Thailand Institute of Scientific and Technological Research, Pathumthani 12120, Thailand (phone: 02-577-9500; e-mail: rujira_j@tistr.or.th). high temperature, typically non-polar under standard environmental conditions [1], [2], [4]. HTL, under high temperature and pressure (generally carried out at 200-370 o C and 5-25 MPa), is ideal for energy recovery from high moisture containing biomass since the water is still in a liquid state and acts as a reactant and catalyst and has been extensively studied [2], [5]-[10]. HTL involves an application of heat and pressure to biomass in an aqueous medium, therefore, high energy efficiency in terms of obviating biomass dewatering and drying is its distinct merit. Feedstock containing about 80% of water is subjected to subcritical temperature (250-350 o C) to create a hydrophilic bio-oil with a reduction of 10-18% of oxygen content when compared to the parent material [11]. This bio-oil can be used directly as a heavy petroleum oil replacement, for co-firing with coal, and is a candidate for upgrading to high quality distillate fuels (e.g., diesel and gasoline) [11]. Thereby, HTL processing does rely on the unique properties of water at high temperature and pressure. At elevated temperatures, hydrogen bonding of the water is diminished then the water dielectric constant is reduced, and thus its ion product is increased. As a consequence, many organic compounds become completely miscible in the high temperature water [1], [2], [4], [11]. Dielectric constant is a ratio of a permittivity of a substance to a permittivity of a free space. The water does have a very high dielectric constant of 80.10 at 20°C (as depicted in Fig. 1), because a dipole moment of the water molecule and so the water can be polarized. The large dielectric constant means that substances whose molecules contain ionic bonds will tend to dissociate in water yielding solutions containing ions [11]. Thereby water becomes a very good solvent. Liquid biofuel from microalgae has drawn many attentions and beaten other biomasses due to it has been identified as a promising feedstock for scaling up to industrial-scale production of carbon-neutral biodiesel [1]-[4]. Moreover, HTL has been emphasized due to energy saving on dewatering process and economical viability on production of value-added co-products along with bio-oil. Microalgae is a sunlight-driven cell that efficiently converts solar energy, water and carbon dioxide into large amount of lipids, proteins and carbohydrates as membrane components, storage products, metabolites and sources of energy, in a shorter period of time compared to other terrestrial plants [12]- [15]. Algae contain about 2-40% of lipids per weight [12], whereas microalgae hold about 15-77% of oil contents per weight [15]. This difference is because the entire cell surface of algae can be involved in the photosynthesis process. As a Hydrothermal Treatment for Production of Aqueous Co-Product and Efficient Oil Extraction from Microalgae Manatchanok Tantiphiphatthana, Lin Peng, Rujira Jitrwung, Kunio Yoshikawa T World Academy of Science, Engineering and Technology International Journal of Energy and Power Engineering Vol:9, No:5, 2015 503 International Scholarly and Scientific Research & Innovation 9(5) 2015 scholar.waset.org/1307-6892/10001294 International Science Index, Energy and Power Engineering Vol:9, No:5, 2015 waset.org/Publication/10001294