Jon S. Gethner

Thermal and oxidation chemistry of coal at low temperatures

The chemical reactions which occur when Illinois No. 6 (hv C) and Rawhide (SBB C) coals are thermally pretreated at 100°C and when Illinois No. 6 coal is subsequently oxidized at 100°C with O2 have been studied using in-situ FT-i.r. differende spectroscopy. Significant spectroscopic changes were seen. Vacuum drying at 100°C resulted in the decomposition of carboxylic acid species to form a variety of new carbonyl species (in Rawhide) and decarboxylated or decarbonylated coal (Illinois No. 6). Oxidation of predried Illinois No. 6 coal leads to the formation of new carbonyl species. The assumption that drying does not alter the chemical composition of coal may not be correct. Thus, overall spectroscopic (and chemical) changes observed in moderate temperature reaction studies may depend upon sample pretreatment, drying and storage. In addition, the time/temperature profile used in a reaction study may affect the overall changes observed by altering the relative contribution of the different reactions.

Kinetic study of the oxidation of Illinois No. 6 coal at low temperatures: Evidence for simultaneous reactions

Abstract The changes which occur during the low-temperature (25–100 °C) oxidation of thin sections of Illinois No. 6 coal were observed using time-resolved in-situ FT-i.r. difference spectroscopy. Graphical visualization of the spectral changes clearly demonstrates that multiple reactions occur. The presence of at least three interrelated reactions is inferred from the temporal behaviour of the absorbance changes. Consequently the overall oxygen-to-carbon ratio of the final oxidized coal will depend critically on both time/temperature conditions of the oxidation as well as the pretreatment and sample handling history of the coal.

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.

The determination of the void structure of microporous coals by small‐angle neutron scattering: Void geometry and structure in Illinois No. 6 bituminous coal

The access of solvents and reactants to the microvoid volume in porous materials such as coal plays an important role in determining the overall chemistry which takes place during a variety of chemical transformations including oxidation, combustion, and pyrolysis. The structure and surface composition of these voids were studied using small‐angle neutron scattering techniques to examine selectively the subset of the overall void volume distribution which comprises the microvoid volume. Powdered Illinois No. 6 coal containing approximately 20% void volume was slurried in several different aqueous and cyclohexane solutions. The solutions used had various hydrogen‐to‐deuterium ratios in order to contrast match most of the open pore volume thereby making the microvoid volume visible. The microvoid volume observed is characterized as elongated voids having a fairly well‐defined diameter and surface composition. The scattering intensity from the microvoid volume shows a well‐defined Porod region, indicating th...

Coal physical structure: porous rock and macromolecular network

Abstract This paper is a review of current knowledge of coal physical structure and the various new approaches used to investigate it. It is shown that in its native state coal is a porous solid, the pore structure being unstable in subbituminous and lower rank coals. Coals behave like cross-linked macromolecular networks, and can be swollen with appropriate organic solvents. Coal physical structure may be described as a porous macromolecular network having either plastic or elastic properties depending upon treatment.

Observation of hydroxyl hydrogen in coal by in-situ FTIR difference spectroscopy

Direct studies of the chemical and thermal reactivity of complex materials such as coal are difficult due to their high optical density and chemical heterogeneity. To overcome these difficulties, highly sensitive semi-micro in-situ techniques have been developed which employ Fourier transform i.r. difference spectroscopy of optically thin sections of coal. These novel techniques are illustrated in studies of the intrinsic Bronsted type hydrogen bonding in coal at room temperature. Hydroxyl groups in coal are isotopically labelled with deuterium by exposure to gaseous D2O. The difference spectrum obtained by comparing the deuterium labelled coal to the unlabelled coal provides a ‘fingerprint’ of Bronsted type hydrogen bonding. The exchanged hydroxyls are spectroscopically identified as hydrogen bonded phenolic-OH (5 per 100 carbon atoms) and 0.03% tightly bound water. The high intrinsic sensitivity of in-situ i.r. difference spectroscopy combined with the spectral simplification resulting from isotopic substitution provides a rapid and direct method for the study of the effects of chemical and thermal treatment of coal.

FT-IR studies of the low-temperature-induced chemical changes in Rawhide coal

Control of the water in the porous structure of coal is desirable both in reaction studies and in order that one be able to rationally compare the composition and processing behavior of different coals. Experimental studies of coal routinely use coal which has been dried under standard conditions (frequently vacuumed at approximately 100°C). It is generally accepted that such drying produces little or no significant chemical or structural change other than volatilization of the water. Recently developed in-situ FT-IR difference spectroscopy techniques, which are extremely sensitive to chemical changes, are used to examine the effect of 100°C vacuum drying on Rawhide (subbituminous) coal. Complex spectroscopic changes of the coal are found to occur which might be indicative of changes both in internal hydrogen-bonded interactions in the coal and in the number and types of strong “crosslinking” bonds. Reactions involving carbon-oxygen functionality, probably as carboxylic acid groups, are observed. A significant change is observed in the composition of the acidic functionality contained in the coal. The substantial IR absorptivity changes in both static and kinetic experiments imply that significant perturbation of the organic coal matrix takes place upon drying. These data imply that mechanistic interpretations of low-temperature chemistry may not be correct. While it is quite conceivable that low-temperature thermal chemistry might not affect the behavior of coal in a much higher temperature pyrolysis, liquefaction or gasification procedure, it is likely to influence the results of some analytical procedures and interpretation of coal structural studies, low-temperature oxidation studies, and low-temperature degradation chemistry. Standardized drying procedures in any particular laboratory might have unknowingly changed the coal structure. Thus, the anomalous results of subsequent experiments could not be fully interpreted because the thermal history of the sample was altered.

The Graphical Analysis of In-situ FTIR Studies of Coal Reactions

High resolution graphical analysis techniques of FTIR spectra permit correlations between spectral features to be easily visualized. The techniques are illustrated with an analysis of FTIR difference spectra showing the kinetics of coal drying.