Heterogeneous catalytic upgrading of biocrude oil produced by hydrothermal liquefaction of microalgae: State of the art and own experiments

This paper reviews the literature on upgrading of biocrude oil obtained by hydrothermal liquefaction of microalgae. It analyses the influence of several parameters (e.g., temperature, operation mode, reaction time, catalyst) on the yields and properties of the upgraded product. Some own experiments are performed based on the review outcomes, comparing for the first time the application of heterogeneous catalysis to biocrude oil obtained from two different algae species: Scenedesmus almeriensis (freshwater) and Nannochloropsis gaditana (marine). The conditions applied were 4 to 8 MPa hydrogen atmosphere in 10 mL microautoclaves at 400 °C and 4 h. The influence of two catalysts (Pt/Al₂O₃ and HZSM-5) and the effect of water addition to the reaction medium were investigated. Many of the effects were found to be caused by the temperature applied, rather than by the catalyst. The conditions applied improved the quality of the algal biocrude oil, viz. by reducing its amount of heteroatoms, saturating and cracking its molecules, and increasing its volatility. The degree of upgrading to a transportation biofuel in the experiments herewith reported, as well as in the literature studies, is however still insufficient and demands further developments of the upgrading techniques.