Hydrothermal Liquefaction of Microalgae for the Production of Biocrude and Value-added Chemicals

Abstract

Hydrothermal liquefaction (HTL) is a thermochemical process used to convert biomass with high moisture content, like lignocellulosic material and aquatic biomass, into biocrude and value-added chemicals. It occurs in water subcritical conditions. The main advantages in relation to other thermochemical processes is the possibility of using wet biomass, avoiding the high cost of drying processes. The microalgae tested grew in a fertilizer industry effluent. Growing microalgae has a significant cost in the production process of liquid biofuels. Thus, the use of industrial effluents has economic and environmental advantages. Effluents from fertilizers industries are rich in different forms of nitrogen that can be assimilated by growing microalgae. HTL is dependent on experimental conditions, namely temperature, reaction time and biomass/water ratio. Reaction temperatures from 300 to 350 °C and biomass ratio from 1/5 to 1/20 were studied. Four product fractions were obtained: gases, aqueous and organic (biocrude) fractions and solid. All these fractions were characterized. The main components in biocrude were long chain hydrocarbons and aromatic ring type structures like phenols or nitrogen heterocyclics such as indole or pyrrole. The increases in the reaction temperature decreased the water-soluble products, increasing the other products yields. However, the biocrude yield increased with temperature until 325 ºC, and then decreased slightly at 350 ºC (from 56 % to 48 %, dry basis). This paper will analyse the effect of operation conditions on biocrude yield and composition to select the best conditions for microalgae conversion into biofuels or valuable chemicals.