Hydrotreatment of Sewage Sludge-Derived Hydrothermal Liquefaction Biocrude. Part I: Experimental

• Published in: Energy & fuels 2024-07, Vol.38 (13), p.11793-11804
• Main Authors: Browning, Barbara, Batalha, Nuno, Costa Gomes, Margarida, Laurenti, Dorothée, Lebaz, Noureddine, Geantet, Christophe, Tayakout-Fayolle, Melaz
• Format: Article
• Language: English
• Subjects: Bioenergy, Biofuels, and Biorefinery; Catalysis; Chemical Sciences
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/acs.energyfuels.4c00883

Hydrothermal liquefaction (HTL) has the potential to produce biofuels from waste, such as sewage sludge. However, the biocrude obtained from hydrothermal liquefaction requires further processing to remove heteroatoms, particularly O, N, and S, and achieve the boiling ranges (Teb < 350 °C) compatible with standard petroleum-issued fuels. Here, sewage sludge-derived HTL biocrude was upgraded via hydrotreatment (HDT) under batch conditions, 100 bar of hydrogen, and over durations of 0–5 h and at 350, 375, and 390 °C to allow the study of the upgrading reaction kinetics. The sewage sludge biocrude and the HDT reaction products were fully characterized by a combination of analytical techniques, enabling a fine description of the evolution of products with operating conditions and the construction of a complete description of the upgraded sewage sludge HTL oil reaction product families as carbon number distributions. As expected, the concentration of heteroatoms in the HDT liquid product decreased progressively with the reaction time and was more significant at 390 °C, with 96% of biocrude S and O and 79% of N being removed after 5 h of reaction. The high concentration of N in the liquid product, e.g., 1.4 wt % at 390 °C and 5 h reaction, was attributed to denitrogenation-resistant compounds, like carbazoles and indoles, still observed in the liquid after HDT reaction. The amount of liquid product in the range of liquid fuels attained the higher yield, i.e., 73 wt %, under the harshest conditions. The fuel range products were mainly composed of aliphatic, monoaromatic, and diaromatic hydrocarbons, together with indoles, carbazoles, and monoaromatic-nitrogenated compounds, e.g., amines. The significant concentration of nitrogenated compounds in the fuel range suggests that a subsequent HDT stage, mainly focused on denitrogenation, is necessary for the liquid product to be used as a drop-in fuel.