Pyrolysis and Pyrolysis Reoxidation Combustion Characteristics of Lignite

Coal, as the main energy source for building heating in northern cities of China, features large reserves, low cost, and high stability in heat supply. Low-temperature pyrolysis reactions are prone to occur within the coal pile; the coal after low-temperature pyrolysis is more prone to spontaneous combustion. From the point of chemical change, there is little discussion on the oxidation and combustion characteristics of coal affected by heat conduction. This paper takes the lignite of Xilin Gol League in Inner Mongolia as the research object and prepares mixed large-particle coal samples, raw coal samples, and pyrolysis samples. The pyrolysis and reoxidation characteristics of the samples are analyzed using experimental equipment such as a TG, tubular furnace, GC, and detection methods such as elemental analysis, 13C-NMR, XPS, and in situ FTIR. The results show that the thermal weight loss of the raw coal sample heated to 900 °C in an N2 atmosphere is 33.74%. The activation energy and pre-exponential factors of pyrolysis are calculated by a one-dimensional diffusion model. The non-isothermal pyrolysis can be divided into heat conduction, drying, active decomposition, and pyrolysis equilibrium stages. The concentration of characteristic gases has a quasi-exponential relationship with temperature, and the trend conforms to the characteristics of the secondary pyrolysis stage. The content of oxygen-containing functional groups (OCFGs) decreased during pyrolysis, and the active sites generated new OCFGs after contacting oxygen, which increased the risk of coal spontaneous combustion. The combustion performance and activation energy at 200 °C are lower than those of other samples, which are more prone to combustion reactions and have a greater tendency for spontaneous combustion. This paper explores the coupling relationship between macroscopic aspects such as stage division and gas release patterns during the thermal decomposition of lignite, as well as microscopic structures such as functional group evolution and aromatic condensation. It reveals the quantitative impact of the thermal decomposition reaction on the characteristics of reoxidation combustion, providing theoretical guidance for preventing and reducing the occurrence of coal spontaneous combustion accidents.