Enhancement of Gasification Reactivity of Low-Rank Coal through High-Temperature Solvent Treatment

We have recently proposed a degradative solvent extraction method to upgrade and fractionate various types of low-rank coals by a nonpolar solvent at temperatures below 350 °C. The main products obtained are solvent-soluble fractions (extracts) and an insoluble fraction, which we call “upgraded coal (UC)”. The UC samples were prepared from eight low-rank coals and one bituminous coal in this work. It was found that the gasification reactivities of the UC chars prepared from seven coals of the eight low-rank coals were larger than those of the raw coal chars. The CO₂ gasification rates of the UC chars prepared from an Australian brown coal, Loy Yang (LY/UC), and an Australian bituminous coal, Gregory (GR/UC), were surprisingly larger than the gasification rates of the corresponding raw coal chars by 2.4–1.9 times. The mechanism of gasification reactivity enhancement was examined from the viewpoints of the catalytic effect of coal inherent minerals, pore surface area, and carbon structure of the chars. The reactivity enhancement of GR/UC was well-explained by the increase of the pore surface area through the degradative solvent extraction. On the other hand, neither the catalytic effect of minerals nor the pore surface area was responsible for the gasification reactivity enhancement for LY/UC. X-ray diffraction (XRD) measurement and elemental analysis showed that LY/UC had a more disordered carbon structure than raw coal char. The reactive surface area estimated by the transient gasification method was judged to be a good index to explain the significant gasification enhancement for LY/UC. Although more detailed examination is required to elucidate the mechanism of the gasification enhancement, the proposed degradative solvent extraction method of low-rank coal can be one of the ways to enhance the gasification reactivity of coal char without using a catalyst.