Simultaneous multi-stage co-hydrothermal liquefaction of fecal sludge and rice husk with low-cost magnetite-induced in-situ catalytic upgrading of biocrude

Abstract

Disclosed herein is a process for the conversion of high-moisture wastewater sludge (fecal sludge) in combination with lignocellulosic biomass (rice husk) into biocrude through co-hydrothermal liquefaction (co-HTL). The process employs single-stage (SS), multi-stage (MS), and catalytic multi-stage (Cat_MS) hydrothermal liquefaction under subcritical water conditions. In the present invention, the single-stage process is conducted at approximately 320 ºC for 60 min. In comparison, the multi-stage process comprises sequential heating stages at approximately 180 ºC, 280 ºC, and 350 ºC, with corresponding retention times of about 15, 25, and 20 minutes, respectively. Magnetite (Fe3O4) catalyst is incorporated at 5 wt% relative to the dry feedstock mass in the Cat_MS process to facilitate simultaneous co-liquefaction and in-situ catalytic upgrading within a single reactor system. At first, fecal sludge and rice husk are co-processed at a feedstock ratio of approximately 1:1 and a solids concentration of about 15%. The staged heating profile promotes progressive biomass conversion, including hydrolysis and depolymerization at lower temperatures, followed by liquefaction and upgrading-related reactions at elevated temperatures. The process produces biocrude, aqueous phase, gaseous products, and biochar. The Cat_MS process increases the combined biocrude yield up to 47.3%, compared to 43.3% for MS and 39.5% for SS conditions. The Cat_MS process produces biocrude with higher heating values (HHV) of approximately 33.5-34.2 MJ / kg, indicating improved overall fuel characteristics. FTIR analysis indicates reduced O-H and C=O functional groups with enhanced C-H stretching intensity, suggesting dehydration and hydrodeoxygenation-related reactions during catalytic co-HTL. TGA indicates redistribution toward middle-distillate-rich biocrude, with the first extraction biocrude containing approximately 25.9% diesel-range and 18.5% jet-fuel-range fractions, while the second extraction biocrude exhibits approximately 19.7% diesel-range and 22.2% jet-fuel-range fractions suitable for transport fuel applications. The process is also associated with reduced levels of oxygen and sulfur-containing compounds compared to SS and MS conditions, indicating partial in-situ upgradation of the biocrude during the Cat_MS co-HTL process.