The structural alignment of coal and the analogous case of Argonne Upper Freeport coal Jonathan P. Mathews a,⇑ , Atul Sharma b a The John and Willie Leone Family Department of Energy & Mineral Engineering, and The EMS Energy Institute, The Pennsylvania State University, 126 Hosler Building, University Park, PA 16802, United States b Advanced Fuel Group, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan article info Article history: Received 27 August 2010 Received in revised form 21 December 2011 Accepted 23 December 2011 Available online 5 January 2012 Keywords: Coal structure HRTEM Coalification Anisotropy Argonne Premium Coals abstract It has long been recognized that coal is somewhat aligned. Multiple techniques imply a structural align- ment but its quantification has been challenging. Moreover, discrepancies exist among techniques as to whether low-rank coals are aligned. The extent of structural alignment for the rank range was quantified directly via image analysis of high-resolution transmission electron micrograph lattice fringes. Alignment was quantified, for each coal, by the contribution to the total fringe length within the prominent 45° of orientation over random orientation (1/4 of the possible orientations). It was evident that there is structural alignment across the rank range. Thus it is time for the community to desist from making the erroneous statement that: low-rank coals are randomly oriented. The slight orientation was similar for low-rank Beulah-Zap lignite and Illinois No. 6 bituminous coals (24% and 22%) with Pocahontas (lvb) coal showing slightly greater (39%) alignment with extensive alignment (65%) in the case of an anthracite coal. The degree of ordering is illustrated with the aid of false-color lattice fringe images and Rose diagrams. The fringe contribution 90° opposed to the maximum length contribution had the minimum or near minimum percentage length contribution for all coals except Upper Freeport and to a lesser degree Illinois No. 6. For the Upper Freeport coal the alignment is lower than expected given its mvb rank (14% over random) and is attributed to a variant of T-stacking for the small aromatic moieties sited perpendicular and between horizontal displaced fringes. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction There are many analytical techniques that infer a structural alignment of coal. These include: vitrinite reflectance anisotropy (birefringence) [1], orientated static solid-state NMR [2], electrical resistance anisotropy [3], gas transportation anisotropy through crack-free coal [4], thermal expansion anisotropy [5], shrinkage during thermal drying [6,7], solvent swelling anisotropy [8–11], and gas-induced swelling anisotropy [12–17]. It is likely that this behavior is related to preferential alignment of the coal structure but this is difficult to quantify. Fortunately with advances in high-resolution transmission electron microscopy (HRTEM) and in image processing a direct measure of the orientation across the rank range is now possible. Here, we directly quantify the struc- tural ordering of coal, covering the rank range of lignite to anthra- cite, via image analysis of lattice fringes extracted from HRTEM micrographs. The HRTEM-based structural analysis of these coals has already been reported [18]. For bituminous coals the bedding plane is typically visible, often horizontal, and delimitated by obvious lithotype- and mineral- bands. Structural ordering can be influenced by inherent alignment (phytal), tectonic stresses, and or overburden and tectonic stresses [19]. Tectonic stresses are evident on the orientation of reflectance maxima/minima in strongly folded or faulted zones [20]. These processes impart an obvious orientation into the structure of many coals. What is uncertain is the quantification of this structural or- der over the rank range. It is generally accepted that low-rank coals have a ‘‘more or less random ordering’’, bituminous coals have some orientation, and anthracite coals are well ordered [21]. These Van Krevelen [21] statements were discussing the Hirsh X-ray scatter- ing data [22], along with birefringence values. Unfortunately, this interpretation has been widely adopted but shortened to the erro- neous statement that low-rank coals are ‘‘randomly orientated’’. A more accurate statement from the original X-ray diffraction based interpretation is ‘‘in all cases, including the lower rank coals, some preferred orientation was detected, the degree of preferred orientation is small in the low-rank coals and increases with rank’’ [22]. Other observations such as anisotropy of methane diffusion in solid, crack-free coal [4] further support structural alignment in bituminous coals (diffusion being faster parallel to the bedding 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.12.046 ⇑ Corresponding author. E-mail address: jmathews@psu.edu (J.P. Mathews). Fuel 95 (2012) 19–24 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel