Study on Char Surface Active Sites and Their Relationship to Gasification Reactivity

Abstract

Char gasification reactivity was considered to be proportional to the number of active sites in the char. Therefore, in this study, char surface active sites (including carbon active sites and catalytic active sites) were first measured with the help of the chemisorption process of CO2 at 300 °C, using a thermogravimetric apparatus. It was found that strong chemisorption (Cstr) and weak chemisorption (Cwea) of CO2, which relate to the presence of active inorganic components and organic matter of char, respectively, existed in this reaction procedure. A higher pyrolysis temperature and slower heating rate induced a decrease of both Cstr and Cwea. Then, char structure evolution was systematically investigated with multi-techniques, such as N2 adsorption isotherm, elemental composition, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and energy-dispersive spectrometry (EDS) analysis, and attempts were made to correlate the measured structure parameters with CO2 chemisorption parameters. The results showed that a higher pyrolysis temperature and slower heating rate improved coal char C/H and C/O mole ratios, promoted the growth of the char crystalline structure and significant loss of functional groups, reduced the active mineral matters in the char, and consequently, resulted in a decrease of char surface active sites. Moreover, for both slow chars (SC) and rapid chars (RC), the activation energy decreased linearly with the increase of CO2 chemisorption parameters: Cstr, Cwea, and Cstr + Cwea. Hence, CO2 chemisorption parameters were better than the Brunauer–Emmett–Teller (BET) surface area to correlate with char reactivity.