Molecular reaction dynamics simulation of pyrolysis mechanism of typical bituminous coal via ReaxFF

A reasonable and effective macromolecular model of bituminous coal was established. The molecular dynamics method based on reactive force field (ReaxFF) was used to simulate the pyrolysis process of typical bituminous coal in the range of 1400-2600 K. The distribution of products and evolution of intermediate radicals were analyzed. Calculation results showed that with increase of pyrolysis temperature, yield of char firstly increased and then decreased, while the trend in tar production was opposite. Yield of pyrolysis gas increased monotonously with increasing temperature. The pyrolysis of coal at low temperature mainly experienced primary reaction with formation of tar free radical fragments and small molecular gases. At high temperature, the secondary reaction of tar fragments was remarkable, and char with more content but less quantity and small molecular gas with more content and quantity were produced. The temperature turning point from the primary reaction to the second one was 2000 K. Under the high temperature pyrolysis conditions, C and H in coal gradually migrated into char and tar, while oxygen-containing functional groups were more active, resulting in migration of O to pyrolysis gases. In the secondary reaction stage, comparing chemical properties of the three elements C, H and O, O was the most active, H was the second, and C was the most stable. H₂O was firstly released during pyrolysis. NH₃ mainly came from secondary reactions during which H₂S was consumed and converted into other products. Yield of H₂ was the highest, and increased with increasing pyrolysis temperature. A large amount of H₂ was generated in secondary reactions, which was mainly from collision of hydrogen radicals generated from pyrolysis and condensation of aromatic structures. Based on ReaxFF simulation results, the weightless activation energy of coal pyrolysis was 39.45 kJ/mol.