NMR Imaging of Heterogeneous Coal Macromolecular Networks

Over the past few years, we have been developing new NMR imaging strategies for probing the chemical and physical structure of coal. Early experiments focused on proton NMR ima ing techniques for spatially resolving individual macerals within a solid coal specimen5 Distinction of individual macerals was achieved by taking advantage of differences in proton density or in spin-lattice relaxation rates, which were shown to vary significantly between the two maceral types. With the implementation of multipulse proton line narrowing in the presence of large static magnetic field gradients, together with backprojection reconstruction NMR imaging methods *, we were able to image and distinguish macroscopic resinite and vitrinite regions within a 2-mm X 2-mm X 1.4-mm specimen of Utah Blind Canyon (APCS No. 6) coal to an in-plane resolution of about 150 p. More recent efforts have focused on obtaining information concerning solvent accessibility in coals and maceral domains b proton NMR imaging of mobile proton distributions resulting from solvent swelling3 Imaging in the presence of deuterated solvents illuminated proton distributions of mobile phases within the coal macromolecular structure preferentially. Images with resolutions of 40 to 80 ! . UTI were obtained using protic solvents, and showed the presence of different macerals, mineral cleats, and microfractures. Regions which differed with regard to pyridine accessibility were also contrasted in the images. We also were prompted by the initial investigation to elucidate the details of transport behavior of pyridine into coal during swelling. In that study: time-resolved proton NMR images of pyridine penetrating a homogeneous sample of high volatile bituminous A rank vitrain (APCS No. 4) that had been pre-extracted with pyridine were used to descibe the uptake behavior. In addition to coal swelling in pyridine, we investigated transient state imaging results from three polymer-solvent systems for comparison: polyethylmethacrylate swollen in methanol, polymethylsilicone swollen in hexafluorobenzene, and isobutyl rubber swollen in toluene. A model for case 11 transport in coals was described. The purpose of this presentation is to review these earlier studies and to provide an overview of the present and future applications of NMR imaging for coal structure determination.