Mechanism Insight into the Conversion between H₂S and Thiophene during Coal Pyrolysis: A Theoretical Study

Abstract

Clean coal technology is the important thrust to the achievement of "carbon neutralization"; clearing the transformation mechanism of thiophene, the dominant organic sulfur species in coal, is conducive to promoting the development of sulfur removal technology. DFT calculations were performed, and 28 reaction paths were proposed in this research, clarifying the decomposition mechanism of thiophene and the fixation mechanism of H 2 S. Thiophene is pyrolyzed mainly through the hydrogentransfer reaction, which occurs at above 2000 K rather than 800 K. The hydrogen transfer between the C−C bond rather than the C−S bond causes the ring opening. Hydrogen promotes the decomposition of thiophene, which happens at 800 K, with a molar ratio of hydrogen to thiophene of 5. Therefore, thiophene is decomposed at 800 K mainly through the hydrogenation reaction that occurs at para carbons and the C−S bond, the H 2 S elimination reaction, and the generation of ethane. Furthermore, H 2 S can be converted into thiophene through the addition reaction with unsaturated hydrocarbon, or the dehydration reaction with hydroxyl or carboxyl groups. The combination between H 2 S and the aliphatic compound occurs at 800 K, which is mainly influenced by the species of the functional group rather than the composition and morphology of the carbon chain. Meanwhile, the conversion of aromatic compounds tends to the generation of aryl mercaptan rather than thiophene at around 800 K.

Mechanism insight into the conversion between COS and thiophene during CO2 gasification of carbon-based fuels

“…The decomposition of thiophene will generates various gaseous products 6 , 14 , which will also be converted into COS. The secondary conversion reactions of gaseous products also need to be investigated.…”

“…During CO 2 gasification of carbon-based fuel, thiophene will be decomposed into gaseous sulfur, meanwhile, gaseous sulfur will also be captured by nascent char to form thiophene in char 31 , while thiophene is the dominate solid sulfur in char 32 , indicating the re-fixation of the dominate gaseous sulfur (COS) will be converted into thiophene. The introduction of CO 2 leads to the generation of carboxyl functional groups in char, and H 2 S will be captured by carboxyl functional groups through dehydration reaction 14 . Generated solid sulfur with poor thermal stability will further be converted into thiophene, through aromatization reaction.…”

“…Similarly, Lu 8 studied the desulfurization mechanism of thiophene compounds in supercritical water, and found out that the introduction of H 2 O promoted the breakage of C-S bond, and thus promoting the generation of H 2 S. Zhang 9 compared the decomposition pathways of 2-methylthiophene during inert and oxidizing atmospheres, and reported that the desulfurization of 2-methylthiophene is thermodynamically more favorable in oxidizing atmosphere. Valuable conclusions have been obtained in published researches, H 2 evolution and its relationship with the released sulfur have also been studied 10 – 13 , while the re-fixation of H 2 S has also been investigated in previous research 14 ,the conversion mechanism of thiophene into H 2 S in different atmospheres have been elucidated. However, the decomposition mechanism of thiophene into COS have rarely been reported, while COS is the dominate gaseous sulfur during CO 2 gasification of carbon-based fuels 15 , and the conversion mechanism of COS to thiophene in CO 2 atmosphere is not yet clear.…”

Analysis of the Formation Mechanism of Hydrogen Sulfide in the 13# Coal Seam of Shaping Coal Mine

“…Hydrogen sulfide gas (H 2 S) is a common toxic and harmful gas in coal mines. It is heavier than air and often accumulates in low depressions. − H 2 S is extremely soluble in water, and its aqueous solution is acidic and highly corrosive to metal equipment in the mine. H 2 S concentration of 100 ppm, 15 min will cause human olfactory paralysis; at a concentration of 1500 ppm, the human body immediately smells like a dead body.…”