Study of non-isothermal pyrolysis mechanism of lignite using ReaxFF molecular dynamics simulations

Compared with the isothermal pyrolysis, non-isothermal pyrolysis is more accurate and efficient, and closer to the actual reaction process. To better understand the pyrolysis mechanism of lignite, a series of ReaxFF molecular dynamics (ReaxFF-MD) simulations were performed within the temperature range of 300-3000 K at different heating rates. The results indicated that the secondary reaction of tar took place when the temperature was above 2400 K at a low heating rate (2 K/ps). In comparison with light tar, more heavy tar was converted to gas or char through further decomposition or condensation reaction. The influence of heating rate on pyrolysis reaction was investigated at the heating rates of 2, 10, 50 and 100 K/ps. In the low heating rate range (below 10 K/ps), the heating rate exhibited a prominent influence on the product distribution. Moreover, more tar and pyrolysis gas were generated at low temperatures (below 2400 K) compared to those at high heating rates. The reaction mechanisms of typical lignite pyrolysis products (C2H4 and phenols) were explored. The formation of C2H4 was attributed to the presence of large amounts of methylene carbon in lignite. Possible formation and transformation pathways of phenols have been proposed, and the simulation results were found to be consistent with the evolution of gases.