A STUDY OF COAL EXTRACTION WITH ELECTRON-PARAMAGNETIC-RESONANCE AND PROTON NUCLEAR-MAGNETIC-RESONANCE RELAXATION TECHNIQUES

An electron spin and proton magnetic relaxation study is presented on the effects of the solvent extraction of coal on the macromolecular ne twork of the coal and on the mobile molecular species that are initial ly within the coal. The eight Argonne Premium coals were extracted at room temperature with a 1:1 (v/v) N-methylpyrrolidinone (NMP)-CS2 solv ent mixture under an inert atmosphere. As much solvent as possible was removed from extract and residue by treatment under vacuum oven condi tions (approximately 10(-2) Torr at 145-150-degrees-C) until constant weight was achieved. The extraction, without further washing with othe r solvents, results in substantial uptake of NMP, apparently by H-bond ing or acid-base interactions. The NMP uptake tends to be higher, and the NMP tends to be more tightly bound in coal matter with higher hete roatom (N, 0, S) content. The molecular material in the medium rank bi tuminous coals is more aromatic and heteroatom-poor than the macromole cular material and is mobilized by the extracting solvent. Likewise, t he more aromatic and heteroatom-poor molecular free radicals are also extracted. However, mobilization of the molecular free radicals by sol vent and the exposure of free radicals in the macromolecular matrix to solvent or species dissolved in the solvent result in preferential re actions of the more aromatic and heteroatom-poor free radicals. Greate r losses of extract free radicals, being the more aromatic, occur than residue free radicals. As a consequence, the surviving extract radica ls exhibit a greater heteroatom content than the original whole coals, as determined from EPR g value changes. The electron paramagnetic res onance (EPR) spin-lattice relaxation (SLR) of these coal free radicals has previously been inferred (Doetschman and Dwyer, Energy Fuels 1992 , 6, 783) to be from the modulation of the intramolecular electron-nuc lear dipole interactions of the CH groups in a magnetic field by motio ns of the radical in the coal matrix. Such a modulation depends on the flexing amplitude and frequency and to a lesser extent upon the elect ron spin density at the CH groups in the radical. The observed EPR SLR rates decrease with coal rank in agreement with the smaller spin dens ities and the lower rocking amplitudes that are expected for increasin g aromaticity with rank and increasing polycondensation at the highest ranks. The EPR SLR rates are found to be generally faster in the extr acts (than residues) where the molecular species would be expected to be smaller and more flexible than in the cross-linked, polymeric, macr omolecular matrix of the residue.