Synergistic mechanism of CO2 and active functional groups during low temperature oxidation of lignite

The rate of oxygen consumption and the release of CO2 were studied using a low-temperature oxidation simulation system. Six model compounds with different functional groups and pore structures were used in the experiment. The results showed that the oxygen consumption rate of six model compounds was mainly controlled by pore diffusion; the release of CO2 for the carbon nanotube (CNT) model compounds CNTs-Mac and CNTs-Mes was controlled by chemical adsorption of oxygen-containing functional groups, while the CNTs-Mic compounds was by diffusion resistance in channels. The carboxyl group, where CO2 adsorbed, was the main adsorption site; this had a higher oxygen consumption rate than the hydroxyl group. Two different methods were employed in further studies to explore the relationship between CO2 and oxygen-containing functional groups: a quantum chemical simulation for basic structural units of lignite, and measuring the activation energy of two types of lignite pre-treated by different CO2 concentrations. Experimental results revealed that the O-H bond of the carboxyl was longer than that of the hydroxyl, and the negative surface potential of the carboxyl was stronger than that of the hydroxyl. The non-covalent bond between the reactive group and coal macromolecule could be weakened due to the presence of CO2. Finally, there was a synergistic effect between 30% CO2 and active functional groups to promote low-temperature oxidation of lignite.