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.