Co-pyrolysis of Hami coal and mixed plastic: An interaction between primary volatiles study via in-situ Py-TOF-MS

Abstract

Online detection of pyrolysis volatiles is essential for exploring the mechanism of coal and plastics co-pyrolysis. In-situ pyrolysis time-of-flight mass spectrometry (Py-TOF-MS) equipped with double ionization sources, vacuum ultraviolet photoionization (VUVPI) and electron ionization (EI), was used to analyze the release of primary volatiles during the pyrolysis and co-pyrolysis of Hami coal (HM) with mixed plastic (PP/LDPE and PP/LDPE/PS, with ratios of 3:2 and 12:8:1, respectively). Meanwhile, the interaction mechanism was discussed at the molecular level. According to the thermogravimetric (TG) results and the total ion currents (TIC) of primary volatiles with temperature, the inclusion of plastic shortens the overall reaction time. In-situ Py-TOF-MS results indicate that the volatiles produced from HM pyrolysis at lower temperatures induce the decomposition of PP and promote further dehydrogenation of long-chain C–C. Besides, the addition of PP visibly facilitates the transportation of hydrogen, which reacts with phenoxy from HM pyrolysis to produce diphenol. The further addition of LDPE after PP enhances the inhibitory effect of molten plastic on HM pyrolysis, resulting in a decrease in the content of diphenol, such as benzenediol (from 1.34% to 0.71%), methyl-benzenediol (from 1.12% to 0.68%) and dimethyl benzenediol/ethyl benzenediol (from 0.68% to 0.50%). The co-pyrolysis of HM with PP/LDPE/PS generates more aromatic products compared to that of theoretical value, such as styrene (from 0.29% to 3.16%), 1-ethenyl-3-methyl benzene (from 0.07% to 0.31%) and 1-butene-2,4-diphenyl (from 0.19% to 0.65%). This indicates that co-pyrolysis strengthens the breakage of Car-Cβ bond in the PS backbone. The addition of PS to the mixture plastic also promotes intramolecular and intermolecular hydrogen transfer in the system. For the evolution of inorganic gases, the addition of mixed plastics promotes the decomposition of aliphatic and aromatic carboxylic acids in coal at lower temperatures, resulting in a decrease in CO2.