Co-spontaneous combustion of coal and gangue: Thermal behavior, kinetic characteristics and interaction mechanism

• The co-spontaneous combustion process of coal and gangue is investigated. • The interaction of coal and gangue is affected by microstructure and reaction stage. • Promotion dominates the co-spontaneous combustion process of coal and gangue. • Co-combustion stage of mixtures of coal and gangue follows order reaction. • The C b is greatly affected by gangue content while the S c depends more on coal. The co-spontaneous combustion of coal and gangue widely exists in the world and threatens the safety of coal mines. In this paper, the microstructure of coal and gangue were tested and the thermal behavior during the co-spontaneous combustion process of coal and gangue with different mass ratios were investigated by using synchronous thermal analyzer, and the combustion characteristic indices and kinetic parameters were calculated. The results indicated the carbon content of gangue is much lower than that of coal, but the content of oxygen-containing functional groups and sulfides in gangue are far higher than that in coal. Moreover, gangue possesses more developed pores and specific surface area. With the increase of coal mass fraction, the combustion index of ignition of mixed samples increases linearly, while the combustion index of burnout and synthetical combustion grows exponentially. The mixtures of coal and gangue follows the interfacial reaction at the oxygen adsorption stage and the order reaction at the moisture evaporation and combustion stages. The practical apparent activation energy at the moisture evaporation and combustion stages is higher than the theoretical value. However, it shows the opposite result at the oxygen adsorption stage. Coal and gangue inhibit each other at the moisture evaporation and later combustion stages, but promote each other at the oxygen adsorption and earlier combustion stages. On the whole, the co-spontaneous combustion process of coal and gangue is dominated by promotion. As a result, the practical values of mass loss and heat release in the whole reaction process are greater than the theoretical ones. Therefore, the danger of spontaneous combustion is higher when coal and gangue coexist.