Nitrogen diffusion and transformation during hydrothermal carbonization of xylan, cellulose, and lignin in aqueous solution recycled from hydrothermal treatment of fiberboard

The fate of nitrogen during the thermochemical carbonization of biomass is crucial for the preparation of bio-fuels and bio-materials. This study examines the nitrogen diffusion and transformation during the hydrothermal carbonization (HTC) of xylan, cellulose, and lignin in a nitrogen-rich aqueous solution recycled from the hydrothermal treatment of fiberboard. The results show that the HTC of xylan in nitrogen-rich aqueous solution promotes the nitrogen diffusion from the aqueous phase to hydrochars in a broad temperature condition, while the HTC of cellulose in similar conditions mainly influences the nitrogen evolution when the temperature exceeds 220 °C. However, the HTC of lignin has no significant effect on the nitrogen conversion. Heterocyclic-N was the predominant form of nitrogen-containing structure in the hydrochars, and it converted to a more stable structure with increasing temperature. During the HTC, the ammonium ion was the predominant nitrogen species in the aqueous phase, which was readily converted to organic-N during HTC of xylan, while it was temperature dependent in the case of the HTC of cellulose. The results provide an in-depth understanding of nitrogen transfer and transformation during the HTC of nitrogen-rich lignocellulose biomass.