Screening and Optimization of Microalgae Biomass and Plastic Material Coprocessing by Hydrothermal Liquefaction

Abstract

In the past decade, microalgae biomass has been attracting considerable interest in valuable biocomponents and biofuel production. Meanwhile, plastic waste handling has become one of the most pressing global environmental concerns. Coprocessing of plastic waste and biomass has previously been reported to produce good quality fuel oil and high-value chemicals. In this study, we examined a coliquefaction process (co-HTL) of 2 microalgae, Chlorella vulgaris (Cv) and Nannochloropsis gaditana (Ng), with nine types of common plastics. In a first step, the co-HTL process was conducted in microautoclave reactors with a fixed algae/plastic mass ratio (50:50) at a temperature of 350 °C and a pressure of 16 MPa for a holding time of 15 min. Among the different types of plastics, positive synergistic effects between polycarbonate (PC), polystyrene (PS), and microalgae have been observed: (1) Plastics showed greater decomposition. (2) HTL crude oil yields were increased. Ng algae exhibits a higher interaction ability with plastics. Then, PC and PS were coprocessed with Ng algae using the response surface methodology to optimize the effects of temperature (300–400 °C), algae/plastic mass ratio (20:80–80:20), and holding time (5–45 min) on HTL crude oil yield. Software-based data analysis of the co-HTL experiments were conducted, and the optimal parameters were proposed, which were verified by the experiment results; Ng+PC (20:80 wt %) exhibits the highest crude oil yield of 67.2% at 300 °C with a 5 min holding time, while Ng+PS (80:20 wt %) generates 51.4 wt % crude oil yield at 400 °C and a 25 min holding time. Finally, the analytical results of elemental analysis, FTIR, 1H NMR, GPC, GC-MS, and TGA on the crude oil produced from pure microalgae HTL and co-HTL were compared, indicating that Ng+PC crude oil is more suitable for aromatic chemicals production and Ng+PS crude oil could be more favorable for biofuel applications.