Study on pyrolysis characteristics of Ningxia high rank bituminous coal composite macerals based on ReaxFF molecular dynamics simulation

Taking the typical high rank bituminous coal in Western China as the research object,the internal relationship between chemical bond information and thermochemical transformation behavior in coal chemical structure was explored from the micro perspective. A series of constant temperature simulations were carried out at 1 000-3 000 K using ReaxFF-MD. The main thermal decomposition mechanism of a high-order molecular model containing 5 765 atoms was studied,and the distribution of pyrolysis products at different final temperatures and heating rates was discussed. The simulation results show that the potential energy and the number of molecules of the system have no obvious change with the reaction time at 1 000 K. With the increase of reaction temperature,the potential energy and molecular number of the system increase with the increase of reaction time,the content of C40+ component decreases, and the contents of gas component and C5 - C15 component increase. The content of C16 - C40 increases at first and then decreases,reaching a maximum of 13.68% at 2 500 K. The results show that the pyrolysis reaction is an endothermic process. At 1 500 K,the pyrolysis reaction begins to take place,the reaction rate increases with the increase of pyrolysis temperature. In the initial stage of pyrolysis reaction, the non - chemical bond, weak bridge bond and side chain structure are broken, including hydrogen bond, π … π stacking, Cal ― O, Cal ― Cal and other chemical bonds,which leads to the aggregation state of vitrinite and inertinite molecules entangled each other and begins to depolymerize and decompose. Then the pyrolysis reaction is initiated by free radicals and small molecular fragments in the initial stage of pyrolysis,including Cal ― O,Cal ― Cal, Car ― O,Cal ― H and other chemical bonds. When the simulation temperature is 2 500 K and 3 000 K, the decomposition and polycondensation are the main reactions in the model,and the secondary reaction of tar is the main one. At different heating rates,the content of products produced by pyrolysis is different,the content of C40+ increases significantly with the increase of heating rate. The contents of C1 -C4 and non-hydrocarben gases decrease with the increase of heating rate. The smaller the heating rate,the longer the time needed to reach the same pyrolysis temperature. The longer the residence time of the sample at the pyrolysis temperature, the sample has enough time to absorb heat to make the pyrolysis reaction more sufficient. The more the amount of volatiles in the sample, the less the residual mass. However, when the heating rate is high,the shorter the residence time of the sample at the pyrolysis temperature,the less the heat absorbed by the sample,the insufficient pyrolysis reaction and the low pyrolysis efficiency. Therefore,a high heating rate is harmful to the pyrolysis reaction. © 2021 China Coal Society. All rights reserved.