MOLECULAR ACCESSIBILITY IN OXIDIZED AND DRIED COALS

Changes in physical and chemical structure of the micropore system in eight solvent swelled Argonne Premium Coal Sample (APCS) coals upon weathering were studied using the EPR spin probe method. Spin probes, which are allowed to diffuse into the coal structure during swelling, are trapped when the swelling solvent is removed. Excess spin probes are removed from the coal surface and larger pores so that only the presence of spin probes trapped in pores which closely approximate the size of the spin probe are detected. Detailed explanations and illustrations of the experimental procedure used are given. Careful examination of the weathering process on coal as a function of rank was accomplished using the EPR spin probe method. The retention of spin probes in eight APCS coals provided valuable insight into both the loss of water and the oxidation which occur during the weathering process. The results could be explained in terms of the autoxidation process observed in other polymeric systems. It was shown that initial oxidation of coal can result in increased cross-linking in the coal structure. As the oxidation process continued, both the covalent and hydrogen bonded character of the coal were significantly altered. The retention character of some coals during oxidation was shown to change by as much as three orders of magnitude. Experiments were performed to study the effects of short term oxidation and dehydration on coal structure by exposing the coal samples to argon or oxygen for time periods up to five minutes. The results indicate that the structure of coal is extremely sensitive to environmental changes and exhibits significant changes in as little as 30 seconds. Exposure of Illinois No.6 coal to argon or oxygen for 30 seconds caused a decrease in the retention of polar spin probes by as much as an order of magnitude. The studies presented here suggest that the structure of coal is dynamic in nature, and has an intimate relationship with the nature of its environment. This method has been shown to be very sensitive to structural changes brought about in coal by oxidation and dehydration, and can be used to follow changes in coal during the swelling process. Additional ideas for future studies using the EPR spin probe method are also discussed