Enhanced hydrothermal denitrogenation of tobacco stem: novel strategies and mechanistic insights

Tobacco stem (TS), a nitrogen-rich lignocellulosic biowaste, presents challenges for clean thermal utilization because its significant fuel-bound nitrogen converts into NO x pollutants during combustion. While hydrothermal carbonization (HTC) offers a promising pretreatment for source-level denitrogenation, existing HTC methods remain inadequate, typically removing less than 70% of nitrogen and leaving high residual nitrogen in hydrochar. Here, we propose an alkali-assisted HTC strategy employing potassium carbonate (K 2 CO 3 ) as a promoter to enhance nitrogen removal from TS. Under optimized conditions (210 °C, 5 g/100 mL solid-liquid ratio, 30 min), adding only 3 wt% K 2 CO 3 achieved a remarkable nitrogen removal efficiency exceeding 90%. reducing the nitrogen content in the resulting hydrochar to as low as 0.70 wt%, comparable to that of conventional woody and herbaceous biomass. Mechanistic insights revealed that CO 3 2− ion plays a crucial role by enhancing protein hydrolysis, facilitating C N bond cleavage, promoting heterocyclic-N deamination, and suppressing nitrogen-fixation pathways such as Maillard reactions. Despite this effective denitrogenation, the fuel properties of the resulting hydrochar were not superior to those of conventional hydrochar, exhibiting a calorific value of 14.68 MJ·kg −1 , a fuel ratio of 0.19, a comprehensive combustion index of 12.37 × 10 −7 , and a combustion activation energy of 55.53 kJ·mol −1 . These characteristics are attributed to the limited carbonization degree (H/C: 1.77, O/C: 0.84), ash enrichment (ash density: 124.34%), and the catalytic effect of K + . This work successfully demonstrates an efficient catalytic hydrothermal route for converting nitrogen-rich tobacco waste into low-nitrogen hydrochar suitable for use as a solid biofuel, while also highlighting the need for further optimization of hydrochar quality within the carbonate-assisted HTC framework.