Study on the reactivity of oxygen-containing functional groups in coal with and without adsorbed water in low-temperature oxidation

Abstract

The form of water adsorbed in coal was analyzed, and the effect of adsorbed water on the low-temperature oxidation of coal was investigated. A dynamic vapor sorption analyzer and a specific surface and pore analyzer was used to analyze water diffusion/sorption processes and oxygen adsorption, respectively. Quantum chemical calculations were carried out to analyze H abstraction from oxygen-containing functional groups by O2. With increasing relative pressure, the manifestations of adsorbed water in coal are as follows: sequential adsorption at primary sites (44%), adsorption at secondary sites (34%), the formation of water clusters (5%), and micropore filling (17%). The moisture content in coal reaches a critical value with the maximum adsorption of oxygen when the air humidity is approximately 55%. Reactions of hydroxyl, carbonyl, and carboxyl groups with oxygen need to overcome energies of 70.89, 68.26, and 84.02 kJ/mol, have associated heat releases of 86.64, 128.65, and 170.66 kJ/mol, and rate constants of 4.622 × 104, 1.924 × 103, and 3.334 s−1m−1, respectively. When water participates in these reactions, the energies to overcome are increased to 97.14, 76.14, and 120.77 kJ/mol, the associated heat releases are 154.91, 123.40, and 322.94 kJ/mol, and the rate constants are 6.668 × 102, 6.641 × 10−4, and 4.169 × 10−7s−1m−1, respectively. The respective functional groups show lower reactivity with adsorbed water. Therefore, the adsorption of water increases the reaction of aliphatic structures/carbon free radicals with oxygen to promote free radical chain reactions, and also plays a role through the heat of wetting in the initial stage of the spontaneous combustion of coal.