Continuous Hydrothermal Liquefaction for Biofuel and Biocrude Production from Microalgal Feedstock Abdolmajid Lababpour [1], * www.ChemBioEngRev.de ª 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ChemBioEng Rev 2018, 5, No. 2, 1–15 1 Abstract Microalgal biomass processing by continuous hy- drothermal liquefaction (HTL) has recently received much interest and can be used to convert microalgal biomass to biocrude oil and drop-in fuels. This re- view focuses on the conversion of microalgae bio- mass to energy production through continuous HTL processes. In addition, processes for upgrading feedstocks and biocrude are also discussed. The feed of the biomass slurry is treated according to size and moisture regulations. Following separation of the gaseous, liquid, and solid phases, the determina- tion of elemental, physical, and chemical fragments, yield of energy, and other properties of the various products is carried out. Homogeneous or heteroge- neous catalysts improve the biocrude yield and quality as does higher moisture content. The high percentage of N 2 and O 2 in the feedstock results in lower quality products. This may be resolved through pre-processing of the biomass, controlling of HTL processes conditions, and post-upgrading processing of the HTL products. The promising potential of microalgae biomass for biocrude and drop-in fuels may have an important contribution to the world’s renewable energy sources through HTL in the near future. Keywords: Energy conversion, Hydrothermal liquefaction, Microalgae biomass, Renewable energy sources, Wet biomass Received: September 13, 2017; revised: January 11, 2018; accepted: January 15, 2018 DOI: 10.1002/cben.201700017 1 Introduction 1.1 Potential and Availability of Microalgae as Energy Source Various types of microalgae biomass are being used as bio- renewable potential feedstock for biocrude production by being converted into near-perfect substituents for petroleum prod- ucts. This is known as drop-in fuel. Drop-in fuels can serve as direct replacements or supplements for existing gasoline, diesel, and jet fuels, without any changes to the existing fuel distribu- tion networks or engines [1–3]. Recent reports have indicated an intensive focus on the hydrothermal liquefaction (HTL) of microalgae biomass [4, 5], especially on continuous HTL pro- cesses [6–10]. There are a variety of thermochemical, chemical, and bio- chemical methods outlining the conversion of microalgae bio- mass to energy, with HTL having special advantages [11–13] which make them at an interesting possibility. Higher overall fuel yields were reported for HTL methods with lower esti- mated costs [5, 10]. In addition, a wide range of gas, liquid, and solid fuel options are possible by HTL, depending on the fuel market conditions [10]. HTL has the flexibility for processing various categories of biomass feedstock such as wastes, aquatic, lignocellulosic, herbaceous, food processing wastes, fungus [8, 14], cyanobacteria as well as the biochemical wastes resulted from macro and microorganisms [7, 14–21]. This review focuses on microalgal feedstock, despite diverse HTL feed- stocks beeing reported in the literature [14, 22–25]. The main differences between microalgae-to-crude oil natural processes and the ones with biomass HTL probably lies in the timeframe. While the former takes place in million years, the latter takes only minutes in the laboratory [26]. In addition, using micro- algae as a source for biocrude may reduce greenhouse gas emis- sions for a safer environment [11, 20, 27–32]. 1.2 Microalgae Hydrothermal Liquefaction Microalgae hydrothermal liquefaction (MiA-HTL) may simply be described as a thermal process that takes place at high tem- perature near the critical point of water (374 °C, 22 MPa), with or without catalysts or a mixture of water and solvents. As water consumes high energy to reach its critical point, various organic solvents with lower critical values and dielectric con- stants as well as lower oxygen content compared to water have been adapted for MiA-HTL. Among such organic solvents are ————— [1] Dr. Abdolmajid Lababpour Faculty of Engineering, Shohadaye Hoveizeh University of Tech- nology, P.O. Box 64418-78986, Susangerd, Iran. E-Mail: lababpour@shhut.ac.ir These are not the final page numbers! &&