Gradient separation of ⩾300 °C distillate from low-temperature coal tar based on formaldehyde reactions

•Gradient separation of ⩾300 °C distillate (300DF) from coal tar reacted with formaldehyde.•H-oil was separated with MB and THF into MBS, THFS and THFR.•MBS contains long chain alkanes and PAHs, e.g., C4 alkyl phenanthrene with the content of 8.45 wt%.•The final mass loss order is 300DF > THFS > THFR at a pyrolysis temperature of 800 °C.•PAHs in 300DF are mainly mono- and multi- alkyl substituted compounds.

The high-temperature distillate of low-temperature coal tar (LTCT) is constituted of polycyclic aromatic hydrocarbon (PAHs) with a wide molecular distribution and many different functional groups. This distillate is a sort of complicated material, which is difficult to be identificated and characterized. The objective of this paper is to get more information on PAHs in the high-temperature distillate from LTCT. ⩾300 °C distillate (300DF) of heavy tar (H-tar) from LTCT was reacted with formaldehyde under the condition of alkaline. After reaction, light oil (L-oil) and heavy oil (H-oil) samples were collected. H-oil was gradient separated with toluene (MB) and tetrahydrofuran (THF). MB soluble (MBS), THF soluble (THFS) and THF insoluble (THFR) were collected. L-oil, MBS and THFS were analyzed by gas chromatography–mass spectroscopy (GC–MS). The results show that MBS contains long chain hydrocarbons and PAHs, especially C4 alkyl phenanthrene with the content of 8.45 wt%. There is no detectable matter by GC–MS in THFS. The pyrolysis processes of THFS and THFR using thermogravimetric analyzer coupled with Fourier transform infrared spectroscopy (TG-FTIR) are mainly divided into 5 stages and 3 stages, the total weight losses are 56.00 wt% and 38.00 wt%, respectively. The corresponding volatiles of FTIR show that the maximum temperature of CH4 emission in THFS is higher than that in THFR. Peaks at 882 cm−1 are due to the decomposition of mono- and multi-substituted compounds of the benzoic ring in THFR. THFR is a polymer which is mainly composed of alkyl PAHs and the –CH2– bridge.

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