Structural ordering of coal char during heat treatment and its impact on reactivity

摘要

The effect of heat treatment on the structure of an Australian semi-anthracite char was studied in detail in the 850-1150deg C temperatue range using XRD, HRTEM, and electrical resistivity techniques. It was found that the carbon crystallite size in the char does not change signficantly during heat treatment in the temperature range studied, for both the raw cooal and its ash-free derivative obtaiend by acid treatment. However, the fraction of the orgnaized carbon in the raw caol chars, determined by XRD, increased with increase of heat treatment time and temperature, while that for the ash-free caol chars remained almost unchanged. This suggests the occurrence of catalytic ordering during heat treatment, supported by the observation that the electrical resistivity of the raw coal chars decreased with heat treatment, while that of the ash-free coal chars did not vary signficantly. Further confirmatory evidence was provided by high resolution transmission electron micrographs depicting well-organized carbon layers surrounding iron particles. It is also found that the fraction of organized carbon does not reach unity, but attains an apparent equilibrium value that increases with increase in temperature, providing an apparent heat of ordering of 71.7 kJ mol(~-1) in the temperature range studied. Good temperature-independent correlation was found between the electrical resistivity and the organized carbon fraction, indicating that electrical resistivity is indeed structure sensitivie. Good correlation was also found between the electrical resitivity and the reactivity of coal char. All these results strongly suggest that the thermal deactiviation is the result of a crystlalite-perfecting process, which is effectivively catalyzed by the inorgnaic matter in the coal char. Based on kinetic interpretation of the data it is concluded that the process is diffusion controlled, most likely involving transport of iron in the inter-crystallite nanospaces in the temperature range studied. The activtion energy of this transport process is found to be very low, at about 11.8 kJ mol~(-1), which is corroborated by model-free correlation of the temporal variation of orgnaized carbon fractin as wel as electrical resistivity data using the superposition method, and is suggestive of surface transport of iron.