Abstract
The chemical structures of pyrolytic chars prepared from 32 kinds of Chinese coals were investigated with micro-Raman spectroscopy in this study. Both first-order and second-order Raman spectra of the chars were curve-fitted and analyzed. The effects of the parent coal properties, including coal rank, volatile, fixed carbon, and ash content, on the pyrolytic char structures were detailed discussed and the correlations between these coal properties and pyrolytic char chemical structures were set up. Multiple-factor analysis was done to propose a comprehensive coal property index that relates well to the pyrolytic char chemical structure. The results indicate that the aromatization degree is the key distinguishable structure of pyrolytic chars prepared from coals with various rank, and the alkyl C−H and aryl C−H structures have no significant difference. The aromatization degree of pyrolytic char decreases with the increase of coal rank, while it increases with the increase of the fixed carbon content in parent coals. The high content of moisture in parent coal can induce condensation of the pyrolytic char, but the inorganic composition probably prevents the condensation of the char. Limited correlations between the coal rank, fixed carbon, moisture and ash content, and the aromatization degree of pyrolytic chars were found. A comprehensive coal property index: (fixed carbon content + moisture content)/(volatile content + ash content) (in air dry basis) combining the coal properties together relates well to the aromatization degree of pyrolytic char and can act as a good indicator for the pyrolytic char chemical structure. This study reveals the effects of the parent coal properties, including coal rank, fixed carbon, moisture, and ash content, on the pyrolytic char chemical structure, and provides a new comprehensive coal property index to predict the pyrolytic char chemical structure.
Highlights
The results indicate that char chemical structures are determined by the heating condition, including the heating rate, temperature, and time, but are strongly dependent on the parent coal property [3,8]
Many studies have reported that parent coal rank can affect the pyrolytic char structures [1,4]
The correlations between the coal rank represented by volatile content in dry-ash-free basis (Vdaf) and the pyrolytic char chemical structures are firstly discussed
Summary
Pyrolysis, gasification, and combustion are the main ways for coal utilization [2,3]. In all these processes, coal pyrolysis is the first step, releasing volatiles and forming pyrolytic char, and the pyrolytic char undergoes further conversion reactions [3,4,5,6]. Because the rate of char conversion reaction is significantly slower than that of devolatilization, the char conversion process is the rate-limited step during coal conversion [3,4,5,6]. In addition to the catalysis of inorganic minerals and reaction channels supplied by pores, the char chemical structure, including the carbon matrix and functional groups, significantly influences the char reactivity [1,2,4,5,6,8,9,10]
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