Insights into hydrochar’s physicochemical properties evolution and nitrogen migration mechanism during co-hydrothermal carbonization of microalgae and corn stalk with FeCl3/NH4Cl/melamine addition

Co-hydrothermal carbonization (co-HTC) of microalgae and lignocellulosic biomass can boost co-hydrochar yield and nitrogen retention rate, but the co-hydrochar still suffers from low nitrogen content and porosity. Accordingly, in this study, FeCl₃, NH₄Cl and melamine were employed as additives to intensify the co-HTC process of microalgae (CP) and corn stalk (CS), and their impacts on the co-hydrochar yield, physicochemical properties and nitrogen transformation pathways were firstly investigated comprehensively. The results indicated that adding FeCl₃ increased co-hydrochar’s specific surface area, but the oxidation power of Fe³⁺ facilitated nitrogen into the aqueous-phase products, leading to the nitrogen distribution in aqueous-phase products reaching up to 83.51 %. Melamine incorporation increased the nitrogen content of co-hydrochar (6.95 %) and oil-phase products, while reduced co-hydrochar’s porosity. NH₄Cl was the most effective additive for nitrogen-doped hydrochar production, simultaneously increasing the yield (40.24 %), nitrogen content (8.93 %) and specific surface area (9.12 m² g⁻¹) of co-hydrochar. The nitrogen transformation mechanism during NH₄Cl-assisted co-HTC process could be divided into two stages. At the first stage (200 ℃–240 ℃), the NH₄⁺ in NH₄Cl could react with hydrolysis intermediates (Maillard and Mannich reactions) to generate heterocyclic nitrogen compounds entering the oil-phase, which would further react with the oxygen-containing functional groups to facilitate the nitrogen enrichment in the co-hydrochar. At the second stage (240 ℃–280 ℃), a portion of hydrochar would undergo secondary degradation and formed water-soluble nitrogen-rich small molecules, which were transferred to the aqueous-phase products again. This study demonstrated the high feasibility of additive-assisted co-HTC for producing high-quality nitrogen-rich carbon materials.