Effect of H₂ and CO as pyrolysis atmosphere on chemical structure of char by XRD and Raman methods

XRD and Raman spectroscopy were used to study chemical structure evolution of Shenmu coal in the main pyrolysis temperature range (450-750℃) and three pyrolysis atmospheres (N₂, H₂-containing and CO-containing). Correlation of structural parameters obtained by the two methods was compared. The results show that the char prepared by pyrolysis of raw coal in N₂ has a continuous increase in the size of carbon crystallites in the transverse direction, a gradual increase of spacing in the longitudinal direction, and a sharp change of stacking height around 650℃. Raman parameter A_D1/A_G increases, while A_G/A_all decreases, indicating a decrease in proportion of the ordered char structure. The H₂-containing atmosphere promotes the longitudinal development of the carbon crystallite structure, increases the conversion of small molecule groups and the ordered degree of char. The influence of CO-containing atmosphere on the carbon crystallite structure parameters is less than that of the H₂ atmosphere. But below 700℃, the dense carbon particles produced by CO-containing atmosphere due to carbon deposition are entrapped on the char surface, resulting in an increase in the ordered degree of char carbon. There is a certain correlation between L_c and A_G/A_all, and d₀₀₂ and A_D1/A_G of char; L_a has a good positive correlation with A_D1/A_G.