Product characteristics and nitrogen evolution pathways of two-stage hydrothermal liquefaction of municipal sludge

Hydrothermal liquefaction (HTL) is an effective method for converting municipal sludge (MS) into biocrude oil. However, the high nitrogen (N) content in the bio-oil produced by conventional HTL (namely, direct HTL [D-HTL]) is a big drawback that limits its application as a biofuel. Two-stage hydrothermal liquefaction (T-HTL) shows promise for efficient nitrogen removal. In this study, the product distribution and properties of MS treated by D-HTL, consecutive T-HTL (CT-HTL), and separated T-HTL (ST-HTL) under different temperatures and reaction durations were investigated. The advantages and disadvantages of ST-HTL were examined, with a focus on the distribution and migration of nitrogen. Furthermore, the migration and transformation pathways of nitrogen during ST-HTL were revealed. The results showed that although ST-HTL produced the lowest bio-oil yield, it significantly reduced the nitrogen and sulfur contents in the oil. Especially for the second stage, the nitrogen content decreased by up to 37%. Moreover, ST-HTL improved the total content of desirable products such as hydrocarbons, alcohols, and esters, while reducing the proportion of N-containing compounds. After the first stage of ST-HTL, the aqueous phase contained the highest proportion of nitrogen-containing compounds (65.08%) and the highest total nitrogen (TN) concentration (2,850 mg/L), indicating that most nitrogen-containing substances were transferred to the aqueous phase.