Experimental study on the coupling effects of feedstock properties and catalyst type on the distribution of phosphorus and nitrogen during hydrothermal conversion of municipal sewage sludge into liquid biofuels

The role of catalysts in the hydrothermal liquefaction (HTL) process has already been established in the literature, but it is largely considered in the context of process efficiency and bio-oil quality, without attempting to determine their effect on the redistribution of some essential biogenic elements. The study involved HTL batch conversion of two selected municipal sewage sludge samples (MSS1 and MSS2) of different origin and characteristics. The experimental campaign involved 12 HTL runs for each MSS. Both homogeneous catalysts, i.e. Na₂CO₃, Li₂CO₃, K₂CO₃ and Ba(OH)₂ and selected heterogeneous catalysts, i.e. Fe₂O_3, CeO_2, NiO/MoO₃/Al₂O₃ (Ni-Mo), MoS₂, Ni/NiO/Al₂O₃ (Ni-Ni), SnO₂ and FeS were used in the study. HTL process was executed under subcritical conditions (350 °C) for a 30 min residence time and in an inert atmosphere of nitrogen. Investigating the coupling effects of feedstock compositions and catalyst type on the distribution and migration pathways of phosphorus (P) and nitrogen (N) into liquid (bio-oil), solid (char) and aqueous HTL fractions was the main objective of the study. The results confirmed almost complete migration of P into solids (94.62–99.51 % for MSS1 and 96.04–99.92 % for MSS2) and, to a large extent, migration of N into the aqueous phase (76.99–82.47 % for MSS1 and 68.07–77.86 % for MSS2) over the entire range of the catalysts used. The coupling effects of the HTL feedstock and catalyst on the distribution of the studied biogenic elements were proven.