Reaction pathways and functional group transformation in CO/CO2 generation from low-rank coal at low temperature oxidation

Carbon monoxide (CO) and carbon dioxide (CO 2 ) are the primary gases generated during the low-temperature oxidation (LTO) of coal. This study explores their generation mechanisms in low-rank coal through a combination of LTO experiments, in-situ Fourier Transform Infrared (FTIR) spectroscopy, and density functional theory (DFT) calculations. Results show that CO formation begins at 30 °C and increases exponentially above 70 °C, primarily due to the decomposition of aldehyde and carboxyl groups. In contrast, CO 2 production is initially delayed by gas desorption and gradual oxidation of carboxyl groups, but accelerates rapidly above 100 °C due to thermal decomposition. DFT calculations confirm that both gases are produced via temperature-sensitive free radical chain reactions: CO through a two-step aldehyde decomposition and CO 2 via high-energy oxidation of carboxyl groups. These findings highlight the pivotal role of temperature and functional group evolution in governing gas generation pathways during coal oxidation.