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.