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

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.

[1]  Paul C. Painter,et al.  Fourier transform infrared study of the variation in the oxidation of a coking coal , 1980 .

[2]  V. Marinov.  Self-ignition and mechanisms of interaction of coal with oxygen at low temperatures. 1. Changes in the composition of coal heated at constant rate to 250 °C in air , 1977 .

[3]  Thomas D. Smith,et al.  Free radical involvement in the oxidation of Victorian brown coal , 1983 .

[4]  N. Berkowitz.  CHAPTER 5 – CHEMICAL PROPERTIES , 1979 .

[5]  Thomas D. Smith,et al.  Free-radical involvement in the drying and oxidation of victorian brown coal , 1984 .

[6]  R. E. Jones,et al.  The oxidation of coal , 1949 .

[7]  Joseph L. Walter,et al.  The Infrared Spectra of Complex Molecules , 1982 .

[8]  D. G. Evans,et al.  The brown-coal/water system: Part 1, The effect of temperature on the evolution of water from brown coal , 1971 .

[9]  R. Jenkins,et al.  Liquefaction of partially dried and oxidized coals: 3. Liquefaction results , 1984 .

[10]  R. Bouwman,et al.  Low-temperature oxidation of a bituminous coal. Infrared spectroscopic study of samples from a coal pile , 1980 .

[11]  S. Mraw,et al.  Water in coal pores: The enthalpy of fusion reflects pore size distribution , 1982 .

[12]  Paul C. Painter,et al.  Further studies of coal oxidation , 1983 .

[13]  Vassil N. Marinov,et al.  Self-ignition and mechanisms of interaction of coal with oxygen at low temperatures. 2. Changes in weight and thermal effects on gradual heating of coal in air in the range 20–300 °C , 1977 .

[14]  Glen B. Brons,et al.  Oxidative weathering of Illinois No.6 coal , 1983 .

[15]  Vassil N. Marinov,et al.  Self-ignition and mechanisms of interaction of coal with oxygen at low temperatures. 3. Changes in the composition of coal heated in air at 60 °C , 1977 .

[16]  L. Petrakis,et al.  Electron spin resonance spectrometric study of free radicals in coals , 1978 .

[17]  David Evans,et al.  Low-temperature oxidation of brown coal. 3. Reaction with molecular oxygen at temperatures close to ambient , 1979 .

[18]  M. Rockley,et al.  Photoacoustic Infrared Spectra (IR-PAS) of Aged and Fresh-Cleaved Coal Surfaces , 1980 .

[19]  R. Friedel,et al.  Electron spin resonance in American coals , 1968 .

[20]  J. Gethner.  FT-IR Studies of the Low-Temperature-Induced Chemical Changes in Rawhide Coal , 1985 .

[21]  D. Ingram,et al.  The investigation of free radicals trapped in low temperature carbons , 1958 .

[22]  D. Schlyer,et al.  Liquefaction of partially dried and oxidized coals: 1. Coal drying and oxidation , 1983 .

[23]  C. Morterra,et al.  The Nature of the 1600 cm−1 Band of Carbons , 1982 .

[24]  P. Walker,et al.  Low Temperature Air Oxidation of Caking Coals: Fourier Transform Infrared Studies , 1981 .

[25]  P. Painter,et al.  Concerning the 1600 cm−1 region in the i.r. spectrum of coal , 1983 .

[26]  R. Hazlett,et al.  Coal liquefaction. 2. Controlled oxidations and thermolysis of coal in pyridine , 1980 .