Study on the Evolution Pattern of the Aromatics of Lignin during Hydrothermal Carbonization

Waste straw contains a large amount of lignin, and its resource utilization is not only in line with the national double carbon development strategy, but also to alleviate environmental pollution. Hydrothermal carbonization is a new thermochemical conversion technology, which has attracted much attention because it can directly transform carbon containing waste raw materials with high moisture content and low energy density. To investigate the physicochemical properties and aromatization changes of lignin hydrochar, hydrothermal carbonization experiments were carried out at 290 degrees C and a solid-liquid ratio of 1:20 for 0.00, 0.25, 0.50, 1.00, 1.50, 2.00, 4.00, 8.00 h, respectively. The experimental results shows that hydrothermal carbonization can increase the combustion quality of lignin. Physical and chemical properties analysis shows that with the increase of hydrothermal carbonization time from 0 to 2 h, the hydrochar content increased from 21.21% to 26.02% and the HHV of hydrochar increased from 20.01 MJ/Kg to 26.32 MJ/Kg. When the holding time exceeded 2 h, the carbon content and calorific value of hydrothermal tended to be stable. With the increase of holding time, FTIR analysis and XRD analysis show that the free hydroxyl groups in water-soluble lignin were easily combined with intramolecular and intermolecular hydrogen bonds, thus forming an ordered crystal arrangement. Subsequently, the crystal structure formed a well-arranged long chain through a strong hydrogen bond network, forming a ring structure in the process of aromatization. Aromatic ring structure accumulated, aromatization wave peak increased with holding time and aromatization intensified. Hydrochar crystal particles became larger and arranged in order. At the same time, the surface functional group detection and degree of crystallization were almost unchanged when holding time exceeded 2 h. The surface morphology of hydrochar was observed by SEM as follows: when the hydrothermal carbonization reaction of lignin entered the insulation stage, the microsphere structure began to aggregate and then became larger. When the holding time reached 2 h, the growth rate of carbon microspheres noticeably slowed. Therefore, the optimal hydrothermal carbonization time of lignin is 2 h, and hydrochar fuel has the best performance and aromatization.

Automated summary

Waste straw, which contains a large amount of lignin, can be effectively utilized through hydrothermal carbonization, a new thermochemical conversion technology. Experimental results showed that hydrothermal carbonization improved the combustion quality of lignin. With increasing hydrothermal carbonization time, the content and heating value of hydrochar increased, and the crystalline structure and aromaticity intensified. Optimal hydrothermal carbonization time for lignin was determined to be 2 hours, resulting in hydrochar fuel with the best performance and aromatization.