By separating low-temperature pretreatment from high-temperature liquefaction, the team demonstrates that nitrogen can be redirected away from the oil phase at an early stage, yielding a cleaner bio-oil with improved chemical composition and greater potential for sustainable energy applications.
Municipal sludge, a rapidly growing by-product of wastewater treatment, has considerable energy potential but remains challenging as a biofuel feedstock due to its high nitrogen content, which complicates refining and increases emissions. As urbanization accelerates, sludge production continues to rise, placing growing pressure on conventional disposal methods such as landfilling, incineration, and composting—options that are often costly, inefficient, and environmentally burdensome. Hydrothermal liquefaction (HTL) offers a promising alternative by directly converting wet biomass into bio-oil without energy-intensive drying. However, sludge-derived bio-oil typically contains nitrogen-rich compounds from proteins that poison catalysts and elevate nitrogen oxide emissions. Although two-stage HTL has emerged as a potential low-severity solution, systematic process comparisons and mechanistic understanding of nitrogen migration remain limited.
A study (DOI:10.48130/een-0025-0017) published in Energy & Environment Nexus on 16 January 2026 by Donghai Xu’s team, Xi'an Jiaotong University, shows that two-stage HTL strategy can dramatically reduce nitrogen in sludge-derived bio-oil while improving its chemical composition, offering a promising pathway toward cleaner, more usable biofuels from urban waste.
The study systematically evaluated three HTL configurations—direct HTL (D-HTL), consecutive two-stage HTL (CT-HTL), and separated two-stage HTL (ST-HTL)—to examine how reaction temperature and residence time influence product yields, bio-oil quality, and nitrogen migration from municipal sludge. Product distributions were quantified under varying thermal severities, while elemental analysis, heating value calculations, Van Krevelen diagrams, and GC–MS characterization were employed to elucidate changes in oil composition. Solid residues were further analyzed using proximate analysis and X-ray photoelectron spectroscopy to resolve nitrogen speciation, and aqueous phases were examined by GC–MS alongside total nitrogen (TN) and total organic carbon (TOC) measurements to track nitrogen partitioning. The results show that increasing temperature in D-HTL enhanced bio-oil yield from 5.38 wt.% at 200 °C to 17.37 wt.% at 325 °C by promoting macromolecular decomposition, while solid yields declined correspondingly. CT-HTL produced slightly lower oil yields due to limited high-temperature residence time. In contrast, ST-HTL generated lower overall oil yields but substantially improved oil quality, particularly in terms of nitrogen reduction. Elemental analysis revealed that ST-HTL reduced nitrogen content in bio-oil by up to 37% compared with D-HTL, while maintaining high heating values up to 37.18 MJ kg⁻¹. Van Krevelen analysis indicated lower N/C ratios for ST-HTL oils, confirming effective nitrogen diversion during the low-temperature pretreatment stage. GC–MS results further showed that ST-HTL decreased the proportion of nitrogen-containing compounds by up to 20.45%, while enriching hydrocarbons, alcohols, and esters. Analysis of solids demonstrated progressive dehydration, decarboxylation, and deamination with increasing temperature, accompanied by the transformation of ammonia nitrogen into heterocyclic and quaternary forms. Aqueous-phase analysis confirmed that more than 65% of nitrogen was captured as nitrogen-containing compounds during the first stage of ST-HTL, with TN concentrations reaching ~2,850 mg L⁻¹, while only 1.45–5.47% of total nitrogen ultimately migrated into the oil phase. Together, these results demonstrate that separated two-stage HTL effectively redirects nitrogen away from bio-oil by modifying reaction pathways, thereby enabling the production of cleaner, lower-nitrogen bio-oil from municipal sludge.
By producing lower-nitrogen bio-oil without catalysts or extreme conditions, separated two-stage HTL addresses one of the most significant barriers to sludge-based biofuels. The cleaner oil could reduce upgrading costs, improve combustion performance, and lower nitrogen oxide emissions. Meanwhile, nitrogen-rich aqueous streams could potentially be recovered for nutrient recycling, supporting circular economy goals.
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References
DOI
10.48130/een-0025-0017
Original Source URL
https://doi.org/10.48130/een-0025-0017
Funding information
This work was supported by the Projects from National Natural Science Foundation of China (Grant No. 52576227), the Fundamental Research Funds for the Central Universities (Grant No. ND6J018), and the National Key Research and Development Program of China (Grant No. 2021YFE0104900).
About Energy & Environment Nexus
Energy & Environment Nexus is a multidisciplinary journal for communicating advances in the science, technology and engineering of energy, environment and their Nexus.