S-XANES analysis of thermal iron sulfide transformations in a suite of Argonne Premium Coals: A study of particle size effects during pyrolysis

A suite of four bituminous Argonne Premium Coal Samples, namely Pittsburgh#8 (P8), Blind Canyon (BC), Upper Freeport (UF), and Illinois #6 (IL6), were pyrolyzed according to the Easy Ro kinetic model (Burnham and Sweeney, 1989) to Ro = 4.3 and iron sulfide thermal transformations were tracked by the use of S-XANES (Sulfur X-ray Absorption Near Edge Structure.) It was shown that the pyrite transformed first to pyrrhotite by Ro = 1.5, and then started to transform to troilite by Ro = 2.4. Some Argonne Coals displayed evidence of structural instability. In addition, particle size effects were examined. Pyrolysis was performed on not-ground (large-particled) coal samples, which were subsequently ground to micron-size particles before data collection. S-XANES was also collected for the not-ground post-pyrolysis IL6 coal to show the effect of the extent of reaction on the surface of the particles as opposed to the bulk. It was found that the pyrite-to-pyrrhotite transformation in large particles of IL6 coal proceeded from the surface of the particle and progress inward, consistent with the shrinking core model. A scheme for determining particle size based on organic sulfur content was also developed for a coal model consisting of a 50/50 mol% mixture of pyrite and Maya petroleum vacuum resid asphaltene for a range of known particle sizes. Lastly, the behavior of both marcasite (a polymorph of pyrite) and pyrite in a coal model was investigated for large (~ 100 μm) and small (~ 5 μm) particles. The marcasite proved to be less structurally stable than pyrite for the large particles, with an abrupt transformation to a mixture of pyrrhotite and troilite, and an abrupt drop in aliphatic sulfur content, indicating consequent H2S generation at Ro = 2.4. This transformation is much less pronounced for pyrite at the same point in pyrolysis.