Variation of the coal chemical structure and determination of the char molecular size at the early stage of rapid pyrolysis

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Rapid pyrolysis (∼104K/s) experiments investigating lignite, bituminous coal and anthracite were conducted to study the effect of the thermal process on the coal structure. The interest was focused on the chemical structure variation occurring at the early stage of pyrolysis (0–500ms) and high temperature (1173–1773K), which includes the mass loss ratio, volatile content, element content and organic groups. A series of parameters was used to characterize the chemical structure, CH2/CH3 ratio, fraction of aromatic hydrogen and aromaticity (fa). The char’s molecular size was determined based on both ring condensation degree (R/C)u and equivalent diameter of the aromatic monomer (DC). This result demonstrated that these five chemical structural parameters (CSPs) increase with the pyrolysis time and temperature. The comparison of three types of coals reveals that the CSPs of lignite are lower and increase faster than those of bituminous coal and anthracite.