The Mechanism of the Low-Temperature Oxidation of Coal by O2: Observation and Separation of Simultaneous Reactions Using In Situ FT-IR Difference Spectroscopy

Abstract

Coal is complex, predominantly organic-containing porous solid which is important both as an energy and a chemical source material. The physical and chemical properties of most coals are extremely sensitive to air oxidation. There is no generally accepted mechanism for the oxidation process, in spite of past interest. Using in situ FT-IR difference spectroscopy of 0.4 μm thin sections of coal, we have examined the mechanism of the low-temperature oxidation of Illinois No. 6 bituminous coal by O2. The overall oxidation with O2 is found to be comprised of three separate chemical reactions. Two of the reactions involve O2 addition to reactive species in the coal. One is predominant at temperatures close to or slightly above room temperature and apparently involves the reversible binding of O2 to a free radical site, followed by reaction. The other oxidation is predominant by 100°C and proceeds by the formation and subsequent decomposition of hydroperoxides. The third reaction is a thermolysis which is important at temperatures between 25°C and 100°C and is competitive with the lower-temperature oxidation. It results in a partial decarbonylation and decarboxylation of the coal. Since three separate reactions contribute to the overall oxidation, the chemical and physical changes resulting from oxidation are dependent upon the oxidation conditions. Control of experimental conditions is critical in order for one to obtain reproducible results. Some of the possible implications of these results on the technologically important process of spontaneous ignition of coal are discussed. Results of previous oxidation studies are discussed in view of the present results. The large variations reported in oxidation studies are likely to be the consequence of ill-defined or poorly controlled experiments. We interpret the correlation between the present study and number of other studies to indicate that oxidation chemistry is the same in most coals, with the principal differences between coals being due to the different relative proportions of the reactive species in the starting coal.