Reaction pathways of hydroxyl groups during coal spontaneous combustion

Abstract

Hydroxyl groups are one of the key factors for the development of coal self-heating, although their detailed reaction pathways are still unclear. This study investigated the reaction pathways in coal self-heating by the method of quantum chemistry calculation. The Ar-CH 2 -CH(CH 3 )-OH was selected as a typical structure unit for the calculation. The results indicate that the hydrogen atoms in hydroxyl groups and R 3 -CH are the active sites. For the hydrogen atoms in hydroxyl groups, they are directly abstracted by oxygen. For hydrogen atoms in R 3 -CH, they are abstracted by oxygen at first and generate peroxy-hydroxyl free radicals, which abstract the hydrogen atoms in hydroxyl groups later. The reaction of R 3 -CH contains three elementary reactions, i.e., the hydrogen abstraction of R 3 -CH by oxygen, the conjugation reaction between the R 3 C▪ and oxygen atom, and the hydrogen abstraction of -OH by hydroxyl free radicals. Then, the microstructure parameters, IRC pathways, and reaction dynamic parameters were respectively analyzed for the four reactions. For the hydrogen abstraction of -OH by oxygen, the enthalpy change and activation energy are 137.63 and 334.44 kJ/mol, respectively, which will occur at medium temperatures and the corresponding heat effect is great. For the reaction of R 3 -CH, the enthalpy change and the activation energy are −3.45 and 55.79 kJ/mol, respectively, which will occur at low temperatures while the corresponding heat influence is weak. They both affect heat accumulation and provide new active centers for enhancing the coal self-heating process. The results would be helpful for further understanding of the coal self-heating mechanism.Résumé : Les groupes hydroxyle constituent l'un des principaux facteurs à l'origine du chauffage spontané du charbon. Toutefois, les mécanismes réactionnels précis de ce phénomène sont encore mal compris. La présente étude s'est penchée sur les mécanismes de réaction en cause dans le chauffage spontané du charbon en utilisant la méthode de calcul quantique de modélisation moléculaire. Nous avons choisi le Ar−CH 2 −CH(CH 3 )−OH comme unité structurale de référence pour le calcul. Les résultats indiquent que les atomes d'hydrogène des groupes hydroxyle et de la position R 3 −CH sont les centres réactifs. Les atomes d'hydrogène des groupes hydroxyle sont directement arrachés par l'oxygène. Quant aux atomes d'hydrogène de la position R 3 −CH, ils sont d'abord arrachés par l'oxygène et génèrent ensuite des radicaux libres peroxohydroxyle, qui arrachent à leur tour des atomes d'hydrogène des groupes hydroxyle. La réaction du R 3 −CH comprend trois réactions élémentaires : l'arrachement par l'oxygène de l'atome d'hydrogène de la position R 3 −CH, la réaction de conjugaison entre le radical R 3 C▪ et l'atome d'oxygène et l'arrachement de l'atome d'hydrogène du groupe -OH par les radicaux libres hydroxyle. Ensuite, nous avons analysé les paramètres microstructuraux, les voies de la coordonnée intrinsèque de réaction et les paramètres de dynamique de réaction ...

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