Catalytic hydrothermal liquefaction of Enteromorpha prolifera for bio-oil upgrading through in-situ hydrogenation

Biomass energy is increasingly recognized as a promising substitute. Algae can be transformed into high-quality biofuels (rich in 5-methylfurfural) through the process of hydrothermal liquefaction. In this study, the lumped reaction kinetics approach is employed to investigate the reaction mechanism of hydrothermal liquefaction. The results indicate that during the initial stage of HTL, Enteromorpha prolifera (EP) is predominantly converted into water phase substances with activation energy value (E) of 15.0 kJ/mol which subsequently undergo further transformation into bio-oil (E = 87.2 kJ/mol). To further enhance the quality of bio-oil, two distinct approaches are applied for in-situ hydrogenation: (1) employing various types and volume fractions of alcohol-water solutions (methanol or ethanol) as hydrogen donors, and (2) utilizing polypropylene (PP, medical plastic waste mainly consists of substances) as a hydrogen source for in-situ hydrogenation. The effects of different mass ratios (1:3, 1:2, 1:1, 2:1 and 3:1) of EP and PP on in-situ hydrogenation performance are revealed. When an alcohol-water solution is used for in-situ hydrogenation, the maximum 5-methylfurfural content reaches 50.2 % using a 40 % ethanol-water solution higher than that with 40 % methanol (46.4 %). Besides, the 5-methylfurfural content is 48.1 % at a mass ratio of 1:1 between PP and EP, in which PP is employed as the hydrogen source. The highest content of 5-methylfurfural reaches 52.5 % used 5 wt% Ni/HT-1.0. The response surface analysis results indicate that the loading amount of Ni has the greatest impact on the content of 5-methylfurfural. The research of this study can provide solutions to the increasingly severe energy problems and environmental issues caused by medical plastic waste and EP, thereby providing a scientific basis for its energy utilization. • EP is converted into aqueous products and then transformed into bio-oil through HTL. • The addition of alcohols promotes the dehydration of biomass to form 5-methylfurfural. • The highest content of 5-methylfurfural reaches 48.1 % at a ratio of EP to PP of 1:1. • The highest content of 5-methylfurfural reaches 52.5 % used 5 wt% Ni/HT-1.0. • Ni loading is the most influential parameter affecting the 5-methylfurfural content.