Once coal is excavated it comes into contact with atmospheric oxygen and begins to undergo low temperature oxidation. The mechanism by which the molecular oxygen interacts with the coal macromolecule is suggested to occur in several steps. These steps primarily involve O₂ diffusion to the surface where physical adsorption followed by chemical adsorption takes place. The chemical adsorption forms several types of oxides that can subsequently react to form several products, primarily CO₂. It has also been suggested that some of these oxidation mechanisms might involve radical reactions. As the previous studies were conducted under conditions where significant structural changes occur it is possible that in the low temperature range (T < 100 °C) the oxidation mechanism is different. Several different rank (lignite–subbituminous–bituminous) coals were isothermally heated at 95 °C in an air atmosphere for a period of up to 6 months and samples were collected at two week intervals. The radical concentration of each sample was measured by Continuous Wave Electron Paramagnetic Resonance (CW-EPR). It is apparent that there are distinct differences between the lower rank (lignite) and the higher rank (subbituminous, bituminous) coals. The lower rank coals exhibited only carbon centered radicals with an adjacent oxygen atom and the higher rank coals exhibited only carbon centered radicals. Interestingly, the lower rank coals exhibited no change in radical concentration due to the long term oxidation treatment while the higher rank coals showed a distinct increase in the radical concentration. These findings shed new light on the complex heterogeneous low temperature oxidation reactions occurring at the coal surface.

Coal is at present a major fuel source for power generation worldwide and will remain as such in the near future. The most important property of coal that determines its price is its calorific value. However, volatiles, ash, and moisture content are also very important properties needed for the quality control (QC) of the coal used to maintain an optimal operation of coal combustion in a boiler. The determination of these properties is carried out via well-established ASTM/DIN methods, which are slow and time-consuming. This study uses combined thermogravimetric analysis (TGA)/differential thermal calorimetry (DSC) instrumentation as a tool to evaluate the reactivity patterns of the aliphatic versus aromatic content of coals, which is correlated to the volatile content of coals. Two coals, bituminous (American Baily Pittsburgh No. 6) used in Israeli utilities and lignite (brown coal Hambach) used in German power plants, have been investigated in this study. The results show that the combined TG/DSC method can provide a much better understanding of the chemical reactivity of coals in the combustion process.

We report the discovery that a flow of CO₂, N₂ or He can sufficiently reduce the spin–spin interactions of specific stable carbon centered radicals by displacing the molecular oxygen in the atmosphere enabling their detection via electron paramagnetic resonance (EPR). This finding unlike other reported effects on carbon radicals occurs under STP conditions and is reversible.

Measuring radical concentration in varied coals, as a function of reaction with O ₂ . Dependency of radical formation within this process.