Study on the Formation Mechanism of the Pyrolysis Products of Lignite at Different Temperatures Based on ReaxFF-MD

The internal part of coal that is not in contact with oxygen will undergo pyrolysis reaction due to heat conduction, and the active groups generated can reverse-aggravate the degree of coal spontaneous combustion. At present, a few studies have been conducted on the pyrolysis mechanism of coal at different temperatures by using mutually validated experimental and simulation methods, resulting in the mismatch between the microscopic mechanism and macroscopic characteristics. In this paper, DH lignite is taken as the research object, and its macromolecular model is established. The pyrolysis reaction of lignite is studied by the experimental methods of coal pyrolysis index gas collection and detection experimental and thermogravimetric analyses and the simulation method of ReaxFF-MD. The influence of temperature on lignite pyrolysis is explored by analyzing the distribution of products at different temperatures and the formation mechanism of typical products, so as revealing the microscopic mechanism of lignite pyrolysis. The results show that 110-500 K of experimental temperature corresponds to 1400-2400 K of simulation temperature. CO2 and C2H4 are the main gas products during pyrolysis simulation. Carboxyl and ester groups are the main source of CO2, which gradually increases with the rise of temperature. Since CO2 can be reduced to produce CO, H2O, and C2H2O at high temperatures, the yield decreases when the temperature is higher than 2000 K. C2H4 is derived from the decomposition of long-chain aliphatic hydrocarbons, and its yield fluctuation rises with the rise of temperature. The formation of H2O and H-2 mainly occurs in the secondary pyrolysis stage. When 1400 K < T < 2100 K, the primary pyrolysis is the main reaction, where the weak bridge bonds and macromolecular structure undergo cleavage to form gas products and tar free radical fragments. When T > 2100 K, the secondary pyrolysis reactions were significant. Tar free radicals and char undergo decomposition, hydrogenation, and polymerization reaction, gas products and tar free radicals increase, and the char yield decreases compared with the primary pyrolysis stage, so 2100 K is the key temperature of the pyrolysis reaction. The research is of great importance in improving the accurate control of coal spontaneous combustion.