In-depth understanding of high temperature and low residence time on the corn straw rapid pyrolysis char structure evolution

Abstract

The degree of effective utilization of biomass char depends on the rate of conversion of the elemental carbon in it, which is influenced by the physical and chemical structure of the biomass itself. Clarifying how physical and chemical structure properties of biomass char change during pyrolysis is a key issue in determining its subsequent utilization efficiency. In order to investigate the evolution of char structure during the high-temperature rapid pyrolysis of corn straw. In this paper, a corn straw high-temperature rapid pyrolysis system was designed and constructed, and the evolution of corn straw char at different temperatures and residence times was investigated by precisely controlling the pyrolysis parameters. The physicochemical structure of corn straw char was characterized by scanning electron microscopy and Raman spectroscopy (RAMAN), and the changes of char yield, pore development, chemical functional groups and carbon structure were explored. The results showed that the corn straw char yield decreased rapidly with increasing temperature and residence time, and the residence time of 5 s at 1300 ℃ led to the fusion of surface ash to form attached molten ash balls. With the increase of pyrolysis temperature from 900 ℃ for 5 s to 1300 ℃ for 5 s, the specific surface area of corn straw char increased from 1.50 m²/g to 293.13 m²/g, and finally to 588.51 m²/g at 1300 ℃ for 13 s. The pores were mainly distributed in the range of 2–10 nm. The high temperature and long residence time resulted in a more ordered char structure and increased concentration of aromatic rings. The increase in temperature breaks the C-O bond, –OH, leading to the release of oxygen functional groups and graphitization of the carbon skeleton.The C–C/C–H content increases from 80.25 % at 900 ℃ for 5 s to 91.65 % at 1300 ℃ for 5 s, whereas the C-O, C=O, and COO– content decreases. With the increase of pyrolysis temperature, the average number of aromatic rings increased and the number of surface methylene groups decreased. This study can provide a reference for the evolution of the structure of corn straw char and its subsequent utilization.