Retention and transformation of nitrogen in hydrochars from co-hydrothermal carbonization of fiberboard and glucose

•N in fiberboard tended to be dissolved during the hydrothermal carbonization.•Part of N in liquid phase was re-combined into hydrochars at high temperatures.•The effectiveness of glucose in retaining N in hydrochars was clearly demonstrated.•Heterocyclic-N was the dominant N-containing functional groups in hydrochars.•Amine-N was re-formed when N from the aqueous phase re-solidifies into hydrochars. The retention and transformation of nitrogen in hydrochars prepared from co-hydrothermal carbonization (Co-HTC) of fiberboard (FB) and glucose were mainly investigated in this study. This study showed that more than 90 wt% nitrogen has transferred into aqueous solution at 180–220 °C during the hydrothermal carbonization (HTC) of FB. When the HTC temperatures were higher than 220 °C, some dissolved nitrogen recombined into hydrochars due to the enhancement of nitrogen fixation reactions (Maillard reactions). The addition of glucose could significantly increase nitrogen retention rate in hydrochars, which was due to abundant oxygen-containing intermediates (5-HMF, furfural, etc.) formed in aqueous phase by the hydrothermal decomposition of glucose. Besides, the hydrochars with more microspheres formed on the surface could apparently enhance nitrogen fixation due to the increased contacting area. In Co-HTC, the temperature range of 220–280 °C favored the nitrogen retention in hydrochars. Additionally, nitrogen retention rate increased significantly with increasing residence time and mixture ratio of glucose in a certain extent. While the heterocyclic-N was the main nitrogen functional group in hydrochars and it transformed from pyridinic-N, pyrrolic-N to quaternary-N with higher HTC temperatures and longer residence times, additionally, some amine-N was clearly formed when the nitrogen returned from the aqueous phase into hydrochars.