Structure evolution characteristic of hydrochar and nitrogen transformation mechanism during co-hydrothermal carbonization process of microalgae and biomass

Co-hydrothermal carbonization (co-HTC) of microalgae and lignocellulosic biomass has high potential for producing nitrogen-rich carbon materials (hydrochar) with relatively high yield. The synergistic effect, hydrochar structure evolution and nitrogen transformation mechanism during the co-HTC process were depicted in depth. The results of the co-HTC process showed that the positive synergy between biomass and microalgae could improve the hydrochar yield and facilitate incorporation of nitrogen into the hydrochar aromatic heterocycles structures. Moderate reaction conditions were beneficial for co-HTC hydrochar nitrogen enrichment and porosity increment, and the optimal nitrogen content and specific surface area reached 3.50% and 5.91 m2/g in co-HTC hydrochars at 240 °C and 1 h. Severe reaction conditions could enhance the formation of stable heterocyclic nitrogen species (reaching 54% at 300 °C). The hydrochar structure evolution and nitrogen transformation mechanism during co-HTC process could be generally divided into two stages. The first stage took place from 180 to 260 °C, and the nitrogen-rich secondary chars could be formed by the interaction of the hydrolysis intermediates. The second stage occurred from 260 to 300 °C, and the previously formed secondary char would be gradually decomposed. This study provides theoretical guidance for regulating the co-HTC process to produce nitrogen-rich carbon materials.