Physical and chemical characterization of two kinds of coal-derived soot

Abstract

The chemical composition, oxidation reactivity, nanostructure, and particle size of two coal-derived soot were experimentally investigated. The soot samples were collected from Yinni (YN) and Datong (DT) single coal combustion flames through a nitrogen-quenched sampling tube. The collected soot samples were then tested on an X-ray photoelectron spectroscopy (XPS) and a thermogravimetric analyzer (TGA) to obtain their surface chemistry and oxidation reactivity. XPS analysis showed that extensive C and O, as well as trace other elements, including S, Na, Fe, and Si, etc., appeared on the surface of the two coal-derived soot. The sp2/sp3 content of YN soot was higher than that of DT soot, indicating the better reactivity of YN soot. The results of TGA experiments showed that the oxidation reaction constant of YN soot was indeed larger than DT soot. Interestingly, the two coal-derived soot contained a portion of unburned ash after the TGA oxidation experiments.The nanostructure and particle size distribution function of soot generated in YN and DT single coal combustion flames were also studied by a self-designed thermophoresis sampling system combined with scanning electron microscopy (SEM) and an online dilution probe in conjunction with scanning mobility particle sizer (SMPS). From the SEM images collected from a relatively low height of the single coal flames, a small-sized individual nascent soot and a large-sized and liquid-like drops were observed. With the increase of flame height, the primary particle size of the two kinds of coal-derived soot aggregates decreased due to oxidation, while the primary particle number increased because of agglomeration. The soot aggregates sampled from higher heights of the two single coal flames had a relatively larger particle size but lower concentration.