Stable radicals formation in coals undergoing oxidative weathering

Coals stored under open air for long periods, undergo gas/solid surface reactions defined as Low Temperature Oxidation (LTO). These weathering processes results in self heating of the piles and decrease the calorific value of the coal and also different gasses such as carbon oxides (CO, CO₂), water, hydrogen (H₂) and low molecular weight organic gases (C_1–5) are released as products. In extreme cases, fire eruptions, have been reported.^1,2

The mechanism by which the molecular oxygen interacts with the coal macromolecule, is suggested to occur in several steps in which radicals are formed. Two types of radicals are observed: aromatic and aliphatic radicals, and also in some cases adjacent to an oxygen atom.³ The main concept is that a chain of radical reactions is taking place, but the exact mechanism is not clear yet. We have succeeded to identify various radical species resulting from these LTO processes and measure its spin concentration (#spins/ml) in different atmospheres and temperature range. Several methods utilizing EPR spectroscopy coupled with MATLAB (for simulations quantities parameter) and Origin (for spin concentration), have been used.⁴