Investigating Secondary Pyrolysis Reactions of Coal Tar via Mass Spectrometry Techniques

Abstract

Coal pyrolysis tars produced from a United States bituminous coal in two reaction confinements of different aspect ratios (0.11:1.0 and 2.0:1.0) and at heating rates of 1, 3, and 10 °C min −1 were analyzed by laser desorption/ionization− time-of-flight mass spectrometry (LDI−TOF), thermogravimetric−gas chromatography (TG−GC), and gas chromatography− mass spectrometry (GC−MS). The reaction confinement served to extend the volatile residence time in the pyrolysis environment by 350 ms. The goal of this study was to determine the effect of extended residence times on the tar product distribution. LDI−TOF and GC−MS analyses combined suggest that tars exposed to an extended residence time in the pyrolysis environment were lower in average molecular weight, had a lower H/C ratio, and produced more expansive speciation of nitrogen and sulfur species. The heating rates were able to vary the average tar molecular weight by 15−32 amu, which is, at least in part, attributed to variations in the volatile residence time between 110 ms and 1 s. These findings are evidence of tar cracking during the extended residence times of nominally 350 ms. TG−GC of coal tar shows that 78% of the remaining tar products vaporize before 400 °C, while the remaining 22% of the tar decomposes by 1000 °C, producing secondary tar products and without producing secondary gas or coke products. The thermally treated tar was collected from a condenser system and analyzed by LDI−TOF, which suggested that secondary tar had an overall lower average molecular weight but a higher presence of both light (<220 and >100 amu) and heaver (>267 and <567 amu) molecules.