Variation characteristics of active groups and macroscopic gas products during low-temperature oxidation of coal under the action of inert gases N2 and CO2

Abstract This research is aimed at studying the different effects and mechanisms of CO2 and N2 on inhibiting coal spontaneous combustion (CSC) and providing theoretical guidance for the prevention and control of CSC in the goaf. The variations of free radicals and functional groups and the generation of indicator gas CO during low-temperature oxidation of coal under the conditions of dry air (DA), dry air-nitrogen mixture (DA + N2) and dry air-carbon dioxide mixture (DA + CO2) were investigated based on electron spin resonance spectrometer, Fourier transform infrared spectroscopy and gas chromatograph. The experimental results reveal that the reaction of free radicals and the transformation of functional groups occur in the low-temperature oxidation process. In the whole oxidation heating process (30–230 °C), the concentrations of free radicals (Ng) increase by 4.04 × 1017 spin/g, 2.24 × 1017 spin/g and 1.90 × 1017 spin/g under the conditions of DA, DA + N2 and DA + CO2, respectively. The variation ranges of peak areas of active functional groups (–C-O–, –C O, –CH2–, –CH3 and –OH) under the action of inert gases (CO2 and N2) are notably smaller than those in the DA environment. The time and amount of CO generation both exhibit an obvious “hysteresis effect”, indicating that the presence of N2 and CO2 inhibits coal oxidation and conduces to CSC prevention. A comparison between the values of inhibition effect parameters R (RCO, RNg, R-C-O- and R-C O) and P (P-CH2-, –CH3, P-OH) discloses that CO2 boasts a superior ability to inhibit CSC than N2. This study presents a comparison between N2 and CO2 in CSC prevention and control based on the variation characteristics of indicator gas and active groups. It provides an experimental and theoretical basis for developing and improving the technology of fire prevention and extinguishing by inert gases.