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• The gangue micropores are converted into mesopores and macropores. • The content of various functional groups increased and reached the maximum at S60. • Dry cracking temperature increases, other characteristic temperatures decrease. • The combustion properties all following the chemical reaction mechanism(n = 3). A large amount of coal gangue (CG) produced in coal mining accumulates to form CG hills, and the combustion properties change under the effect of leaching, increasing the risk of spontaneous combustion. The study was mainly carried out by nitrogen adsorption, infrared spectroscopy and thermogravimetry on the physicochemical properties of the CG under different immersion times (30, 60, 90 days). And the activation energy of the immersion CG at different oxidation or combustion states was calculated. The results show that the pyrite contained in gangue is oxidized after immersion, and the heat released by oxidation and the acid solution formed have an obvious effect on the pore expansion of gangue. The mesoporous and macroporous increase by 2.61% and 0.95% respectively, which strengthens the physical and chemical adsorption of oxygen for CG. The immersion process also destroys the structure of the gangue, transforming stable functional groups into active ones, especially the most significant increase in hydroxyl and oxygen-containing functional groups. The characteristic temperature changed under the action of water immersion, the cracking temperature increased, and other characteristic temperatures decreased, among which the ignition temperature decreased by about 20℃. In addition, the water immersion process inhibits the low-temperature oxidation stage of CG and promotes the high-temperature combustion stage, and the risk of spontaneous combustion is the strongest after 60 days of immersion in water. The results can provide guidance for the prevention of spontaneous combustion in drenched CG hills. [ABSTRACT FROM AUTHOR]

• The cyclic model of free radicals R1 ~ R9 during coal self-heating was proposed. • Most reactions in free radicals are exothermic with a maximum heat release of 167.113 kJ/mol. • The initial reactions (R1 and R2) can accumulate heat and generate R-O-O and OH. • Both OH and R play vital roles in consuming active groups in coal continuously. • R R-O-O and OH should be eliminated for terminating the cyclic chain reaction. Reactions of free radicals play essential roles in the evolution of the spontaneous combustion of coal. However, their elementary reaction pathways and details during this process have not been revealed by far. This paper established reaction pathways of free radicals during coal spontaneous combustion based on typical active free radicals by quantum chemistry method. The specific structural parameters and thermodynamic data for reactions (R1 ~ R9) are calculated to reveal the reaction process. The cyclic model of free radicals during coal self-heating has been proposed for the first time. The results indicate that most of the reactions involved in free radicals are exothermic reactions with heat release ranging from 7.568 ~ 167.113 kJ/mol. The initial reactions (R1 and R2) can accumulate heat and generate active peroxide free radical (R-O-O) as well as hydroxyl radical (OH). Both of them can occur easily at the normal temperature. The heat release increases the ambient temperature, which causes non-spontaneous reactions to proceed successively. The decomposition of peroxide free radicals triggers the reaction pathways of the cyclic model. The hydroxyl radical can lead to the reaction with active sites in coal, which can accumulate plenty of heat and supply the R for initiating reaction R(1) in the cycle. Both hydroxyl radicals and hydrocarbon radicals play vital roles in consuming active groups in coal and oxygen continuously. The results demonstrate that to fundamentally prevent coal spontaneous combustion, it is necessary to eliminate the key active free radicals (R , R-O-O , and OH) to terminate the cyclic chain reaction. [ABSTRACT FROM AUTHOR]

• R 3 -CH structure especially the H atom is the most active site in coal self-heating with oxygen. • The free energy barrier of oxygen attacking R 3 -CH site is 8.28 kJ/mol. • The aromatic hydrocarbon radicals and hydroxyl radicals are significant for coal self-heating. • The carbon radical combining with an O plays a vital role in initial coal spontaneous combustion. R 3 -CH structure is a typical active group during coal self-heating. In this paper, Ph-CH(CH 3)–CH 2 -Ph was selected to analyze reaction activity as well as mechanism between R 3 -CH structure and oxygen in coal self-heating by Multiwfn and Gaussian calculation. The results demonstrate that R 3 -CH structure especially the H atom is the most active site in coal self-heating with oxygen. The oxygen attacks R 3 -CH site in the Ph-CH(CH 3)–CH 2 -Ph, which results in the breaking of C–H bond and formation of O–H bond. The free energy barrier of this oxidation reaction is 8.28 kJ/mol and the heat release of the process is 86.89 kJ/mol, which indicates that it can easily occur at room temperature. From the perspective of products, the aromatic hydrocarbon radicals and hydroxyl radicals related to the initial spontaneous combustion of coal are generated, which is of great significance for the further development of coal self-heating. Subsequently, the carbon radical is combined with a single oxygen atom to generate oxygen-containing free radical which plays vital roles in initial coal spontaneous combustion. [ABSTRACT FROM AUTHOR]