HU Xun，FAN Mengjiao

School of Materials Science and Engineering， University of Jinan， Jinan 250022， China

Abstract

Objective Pyrolysis is an important way to convert biomass into char， but the complex reaction network of the pyrolysis process makes it challenging to control the properties of the resulting char. Studies have shown that temperature, among all the parameters, is the main factor affecting pyrolysis reactions, followed by residence time. This study focuses on the correlation between char properties and pyrolysis temperature and time by conducting pyrolysis experiments on cotton fiber and peach wood at temperatures from 250 to 750 ℃ and at 350 ℃ for various residence times.

Methods The synthesis of char was conducted in a fixed-bed reactor within a tube furnace under a nitrogen (N₂) atmosphere, with a flow rate maintained at 60 mL/min. 2 g of biomass was placed in a quartz tube. Before heating, N₂ was introduced to establish an inert environment within the reactor. The tube furnace was heated to 250, 350, 450, 550, 650, and 750 ℃ at a temperature ramping rate of 10 ℃/min, and then held at the target temperature for 30 min. Upon completion of the heating process, the reactor was immediately removed from the furnace and quenched. In addition, to further investigate the effects of residence time, the pyrolysis time was extended to 60, 90, 120, and 180 min at 350 ℃.

Results and Discussion The cracking of cellulose was observed at 350 ℃, while the rigid structure of lignin stayed stable at this temperature. X-ray diffraction (XRD) results showed that the char generated from the pyrolysis of cotton fiber and peach wood at both 350 ℃ or 650 ℃ was primarily amorphous carbon. The char derived from peach wood had a higher degree of graphitization. The original biological structure could be preserved even after extended exposure at 350 ℃, whereas increasing the temperature to 650 ℃ led to the fracture of the structural framework. Elemental analysis and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results indicated that high temperatures enhanced aromatization by promoting dehydration, dehydrogenation, and deoxygenation reactions. The abundance of —OH and aliphatic C—H groups reached a maximum at 300~400 ℃, then decreased monotonically via dehydration and dehydrogenation. This process accelerated the formation of C＝O, C—H, C＝C, and aromatic C—O—C bonds.

Conclusions and Prospects Extending the residence time at 350 ℃ can accelerate the carbonization process by promoting the cracking of oxygen-containing functional groups in cellulose. However, the temperature is insufficient to drive further condensation to form aromatic ring structures. Although 350 ℃ is not high enough to destroy the rigid structure of lignin, prolonging the exposure time at this temperature enhances the dehydrogenation reaction. Higher temperatures accelerate both carbonization and aromatization processes. This results in a continuous increase in the abundance of C＝C and C—O—C bonds at the expense of the —OH， C—H， and C＝O functional groups. The study provides theoretical guidance for customizing char with specific functional groups.

Keywords：biomass derivatives; char; pyrolysis temperature; residence time

DOI：10.13732/j.issn.1008-5548.2025.04.013