A STUDY OF MEQUINENZA LIGNITE

The sulfur constituents in a Mequinenza lignite (Spain) were character ized using a variety of analytical techniques. The investigation was p erformed to provide information to scientists and engineers attempting to design methods to remove sulfur from coal and lignite, test new an alytical methods directed at characterization of sulfur constituents i n fuel, and provide basic analytical information concerning the sulfur constituents in coal and lignite. Such information is particularly us eful during the evaluation of coal desulfurization processes. The natu re of the lignite and the sulfur components were explored using standa rd ASTM methods; petrographic techniques; mineral analysis using micro scopy and X-ray diffraction; low-voltage, high-resolution mass spectro metry (LVHRMS); sulfur L-edge X-ray absorption near-edge structure spe ctroscopy (XANES); sulfur K-edge XANES; electron paramagnetic resonanc e (EPR); and controlled atmosphere programmed temperature oxidation (C APTO). The Mequinenza material is a lignite containing 13.6 % sulfur ( maf) and 21.6 % (dry) ash. The sulfur is primarily organic sulfur (12, 80 %) and contains smaller amounts of pyrite and very little sulfate. Microscopic analysis showed that clay minerals were the most abundant minerals and that the lignite consisted mainly of the macerals huminit e and liptinite. A small amount of elemental sulfur was also detected (L mu g/g) in a tetrachloroethylene extract. LVHRMS analysis of a pyri dine extract showed that molecular formulas consistent with thiophenes , aryl sulfides, their corresponding oxidized forms, and disulfides we re present. Results from sulfur L-edge and K-edge XANES spectroscopy o n the whole lignite were in quantitative and qualitative agreement wit h LVHRMS results on the pyridine extract with respect to thiophenes. S ulfur L-edge XANES indicated that 40 % of the sulfur was present as th iophenes and 20 % as aryl sulfides. CAPTO reports the sum of these two sulfur forms as 60.4 wt %. The CAPTO and sulfur L-edge XANES results are in excellent agreement. The L-edge XANES indicated that 20 % of th e sulfur was present as aliphatic sulfides and 20 % as disulfides. Res ults of the K-edge XANES analysis indicated that nonpyritic sulfur was distributed as follows: 10 % disulfide, 33 % sulfide, 42 % thiophenic , 9 % sulfoxide, and 5 % sulfate. CAPTO results indicated that 34.4 wt % of the sulfur was present as nonaromatic sulfur. The W-band EPR spe ctrum was unusually rich in features compared to other coals. These si gnals are attributed to aromatic radicals that are either purely hydro carbon or heteroatomic in nature.