Investigation on nitrogen migration and transformation during hydrothermal conversion of glucose and xylose in N-rich solution recycled from hydrothermal treatment of fiberboard

Nitrogen migration and transformation have been extensively studied during the thermochemical conversion of biomass, particularly N-rich biomass, due to concerns about nitrogen oxides pollution or the preparation of N-doped carbon materials. Nitrogen in fiberboard was extracted into an aqueous solution through low-temperature hydrothermal treatment to prepare the recycled aqueous solution. Subsequently, hydrothermal conversions of glucose and xylose were conducted in the recycled solution. The migration and transformation of nitrogen between hydrochars and aqueous phase were investigated. The results showed that the nitrogen retention rates of glucose hydrochars were higher than those of xylose hydrochars at low temperatures. As the temperature increased, the nitrogen retention rates of glucose and xylose hydrochars tended to converge, reaching their highest value at 220 °C. In hydrochars, nitrogen had a high content and was widely distributed on the surface of the carbon microspheres. The nitrogen content did not change significantly with increasing temperatures, but the nitrogen functional groups tended to become more thermostable. In addition, the char yields of glucose and xylose in fiberboard recycled solution were significantly higher than that in the ultrapure water at low temperatures, which indicates that the introduction of nitrogen promotes the formation of hydrochars. These findings favored the study on the nitrogen migration and transformation during hydrothermal conversions of lignocellulose in a N-rich environment.