Effects of functional groups on the energetics of CO and CO2 formation during secondary oxidation of coal following pre-treatment

Abstract

To investigate how functional groups introduced in coal during pre-oxidation influence the release behavior of CO and CO₂ during subsequent secondary oxidation, experimental characterization was conducted to identify the types and relative abundances of functional groups formed at varying pre-oxidation temperatures. Concurrently, the apparent activation energy (E_α) for CO and CO₂ evolution during secondary oxidation was determined via kinetic analysis. A series of multiple linear regression models were then developed to quantitatively assess the relationships between E_α values for CO/CO₂ generation and specific functional group contents – or their combinations-in pre-oxidized coal, across distinct temperature intervals of secondary oxidation. The results indicate that, under all pre-oxidation conditions examined, the E_α for CO evolution during secondary oxidation exceeds that for CO₂. However, this gap narrows progressively with increasing pre-oxidation termination temperature. Notably, when pre-oxidation is terminated below 140 °C, the E_α for CO formation during secondary oxidation is lower than that observed during initial (unprioritized) coal oxidation – whereas the E_α for CO₂ formation remains consistently higher than in the initial oxidation stage. Further, within the secondary oxidation temperature range of 40–170 °C, elevated concentrations and synergistic interactions between aliphatic –CH₂/–CH₃ groups and carbonyl (CO) groups significantly reduce the E_α for both CO and CO₂ evolution. However, as the secondary oxidation temperature interval is refined into narrower subranges, the dominant functional groups-and their interaction patterns-exhibit marked variation, underscoring the context-dependent nature of functional group reactivity.