Characteristics of Chars Prepared from Various Pulverized Coals at Different Temperatures Using Drop-Tube Furnace

Abstract

Char samples were prepared from five Australian black coals at different temperatures using a well-characterized drop-tube furnace. The characteristics of resultant chars and their parent coals were determined by such techniques as chemical analysis, XRD (X-ray diffraction), FESEM (field emission scanning electron microscopy), and the newly developed FIB (focused ion beam) miller. Some conventional particle analysis techniques were also used. The atomic structure, physical structure, and chemistry of chars were investigated as a function of pyrolysis conditions including pyrolysis temperature and coal type. Pyrolysis temperature is one of the key parameters influencing the char atomic structure. In general, the chars become more ordered and condensed with increasing pyrolysis temperature. This could be seen in their crystallite size (L11), aromaticity (fa), and interlayer spacing (d002/dγ). After being treated at 1200 °C, the L11 value increased from 5.9−6.5 Å for raw coals to 7.8−10 Å for the chars, and fa increased from 58−72% for raw coals to 75−82% for chars. Meanwhile d002 decreased from 3.5−3.59 Å for raw coals to 3.46−3.53 Å for chars, and dγ decreased upon charring from 4.72−5.01 to 4.33−4.74 Å. The char atomic structure is also dependent on coal type. However, the strong dependence on volatile matter observed for raw coals is diminished for the chars. Although all chars, except chars from coal AC-5, have similar spherical morphologies and surface areas, chars from different coals demonstrate extremely different pore structure. Some chars are very porous and light, while the others are solid and dense. The chemical analysis of chars shows that the atomic ratios H/C and O/C significantly decrease with increasing pyrolysis temperature. At pyrolysis temperatures of 1200 °C or greater, all the obtained chars have similar H/C and O/C ratios regardless of their origins. The decrease in H/C ratio is in agreement with the disappearance of γ band and increasing aromaticity, which were observed in char XRD spectra with increasing pyrolysis temperature.