Separation and characterization of different types of residual carbon in fine slag from entrained flow coal gasification

A large amount of coal gasification fine slag (CGFS) with substantial carbon is produced in the entrained flow coal gasification process. Understanding the type and activity of the residual carbon in the slag is essential for its efficient use. Coal undergoes a series of complex reaction in a gasifier. The residue carbon contains not only incompletely reacted semi-char, but also soot formed by the condensation of volatile fractions. In this study, the ultrasound-assisted anhydrous ethanol method was used to separate the different types of carbon in the fine slag. Light carbon deposit (LCD) and heavy carbon residue (HCR) were obtained from the top and bottom of the centrifuge tube respectively. The results showed a dry basis yield of 47 % for LCD and 30 % for HCR. LCD particle size is concentrated in the range of 0.2 μm to 5 μm, and the specific surface area is 116 m²/g after de-ashing, in the form of flocs. HCR particle size is concentrated in the range of 20 μm to 100 μm, and the specific surface area is 179 m²/g after de-ashing, composed of irregular particles. The TPO results of the residual carbon in the fine slag were fitted by peak splitting, and three main types of residual carbon were found: polymerization soot, amorphous carbon and graphitization carbon, corresponding to maximum CO₂ release peaks of 596.3 °C, 626.8 °C and 705.6 °C respectively. LCD is composed of polymerization soot. HCR is composed of amorphous carbon and graphitization carbon. Raman characterization reveals that LCD has more active sites than HCR, thus leading to their higher reactivity. This study is an innovative approach to the separation of residual carbon in fine slag. It helps to understanding the formation mechanism of residue carbon in the gasifier.