Molecular Composition and Hydrotreatment Effects in Sewage Sludge Pyrolysis Biocrude Revealed by Supercritical Fluid Chromatography-Mass Spectrometry

Sewage sludge, a continuously available municipal waste, offers a renewable alternative to first-generation biofuels without competing with arable land. However, biocrude derived from the pyrolysis of sewage sludge contains high nitrogen and oxygen levels, making it immiscible with fossil fuels without further upgrading. This study systematically investigates the chemical composition of biocrude (both feeds and hydrotreated products) produced in a catalytic two-reactor system under varying processing conditions. Supercritical fluid chromatography (SFC) coupled with high-resolution mass spectrometry (HRMS) was employed for chemical characterization, including feature detection, filtering, and statistical analysis. Nontarget screening analysis identified compounds and homologous series affected by hydrotreatment, with 44 compounds confirmed using standards, including fatty acid amides (FAA), fatty acids, quinolines, carbolines, indoles, hydroxylated polycyclic aromatic hydrocarbons, imidazoles, and benzimidazoles. At mild conditions (reactor temperatures below 200 °C), fatty acids decreased, while FAA, imidazoles, indoles, quinolines, carbolines, and hydroxylated polycyclic aromatic hydrocarbons remained stable or increased, suggesting intermediate formation or ionization efficiency changes due to matrix effects and ion competition in electrospray ionization. Higher second-reactor temperatures reduced nitrogen, oxygen, and sulfur content and the aforementioned compound groups. However, the reduction of pyridines and anilines required pressures of 120 bar and temperatures of 360 °C. The SFC-HRMS platform, with minimal sample preparation and fast analysis time, provides a powerful tool to monitor the chemical composition of biocrudes under varying hydrotreatment conditions, facilitating the production of compatible biofuels from sewage sludge.