Impact of the Temperature, Pressure, and Particle Size on Tar Composition from Pyrolysis of Three Ranks of Chinese Coals

Tar compositions from pyrolysis of three ranks of Chinese coals of Hailaer lignite, Fugu sub-bituminous coal, and Liulin bituminous coal under different temperatures and pressures were analyzed via a pyrolyer combined with gas chromatography/mass spectrometry (Py–GC/MS). A nuclear magnetic resonance (NMR) analysis was performed to characterize the coals. It was found that the relative content of polycyclic aromatic hydrocarbons (PAHs) is not as sensitive to temperature as monocyclic aromatic hydrocarbons (MAHs), which increased with increasing the pyrolysis temperature. A decreasing trend of the content of acids/esters was observed for all three ranks of coals. However, the three coals exhibited quite different trends with the temperature in terms of the relative content of aliphatic hydrocarbons. MAHs and PAHs were predominant components in the tar from Liulin bituminous coal. The tars from Hailaer lignite and Fugu sub-bituminous coal were rich in phenolics, which can be detected at a low temperature of 400 °C. In contrast, the phenolics were much less in the tar from Liulin bituminous coal and can merely remarkably be detected at a higher temperature of 600 °C. The contents of phenolics and aliphatic hydrocarbons decreased with increasing the pressure for all three coals, and the contents of MAHs and PAHs showed inverse trends with the pressure. This could result from the enhanced reactions of cyclizations of aliphatics and scissions of the C–OH bond in phenolics by elevated pressure. C₁₉ alkane and p-cresol as model compounds of aliphatics and phenolics were pyrolyzed under high pressures to verify this point. For a larger particle size of Fugu sub-bituminous and Liulin bituminous coals, the contents of aliphatics decreased remarkably, while the contents of MAHs and PAHs were distinctly augmented. This could be attributed to the intraparticle secondary reactions.