Effects of Vitrinite in Low-Rank Coal on the Structure and Combustion Reactivity of Pyrolysis Chars

Pyrolysis is a highly promising technology for the efficient utilization of low-rank coal. The structure of coal plays an important role in its utilization. In this paper, the evolution of the char structure during heat treatment (200–800 °C) of Naomaohu coal and its different vitrinite-rich fractions was studied. The functional group structure, aromatic ring structure, and crystallite size of chars were determined by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and X-ray diffraction (XRD) spectroscopy, respectively. The results indicated that minerals inhibit the condensation reaction of aromatic rings during pyrolysis. The high vitrinite content in coal is conducive to the formation of larger char crystallite average sizes (L_a). The relationship between L_a (1.69–3.10 nm) and the Raman band area ratio A_(GR+VL+VR)/A_D or A_D/A_all was established. In addition, the combustion performance and kinetics of chars were also investigated. The results showed that the char from high contents of the liptinite fraction has lower combustion reactivity, and demineralization treatment has significantly reduced the combustion reactivity of char.