The molecular representations of coal – A review

Fuel 96 (2012) 1–14

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Review article

The molecular representations of coal – A review Jonathan P. Mathews a,b,⇑, Alan L. Chaffee c a

The John and Willie Leone Family Department of Energy & Mineral Engineering, The Pennsylvania State University, 126 Hosler Building, University Park, PA 16802, USA The EMS Energy Institute, The Pennsylvania State University, 126 Hosler Building, University Park, PA 16802, USA c School of Chemistry, Monash University, Vic 3800, Australia b

a r t i c l e

i n f o

Article history: Received 14 June 2011 Received in revised form 6 November 2011 Accepted 8 November 2011 Available online 30 December 2011 Keywords: Coal structure Coal molecular modeling Coal models

a b s t r a c t Between 1942 and 2010 there were >134 proposed molecular level representations (models) of coal. While they spanned the rank range, bituminous representations are the bulk, with far fewer lignite, and very few subbituminous or anthracite representations. They have transitioned from predominantly 2D pen and paper drawings into 3D computational structures, and have recently increased in complexity, and to a limited degree, in scale. Advances in analytical techniques as well as modeling software, and computation power have resulted in improved partial representations of coal structure. Computer aided design has helped to overcome some of the challenges in model construction for a few models. Yet generally it is the capturing of the constitution of coal that remains elusive. Evaluation of physical parameters and behavior observations has aided our confidence in the representations but models are typically generated for a specific use. No model has faced the gambit of ‘‘tests’’. Ó 2011 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Coal models historic overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Model construction strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Molecular representations of coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. The models of lignite/brown coals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.2. The models of subbituminous coals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.3. The models of bituminous coals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.4. The models of anthracite coals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5. Concluding comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1. Introduction The coal literature, over the last 66 years, has generated a surprisingly large number (>133) of molecular level representations of coal (or coal extracts): 1940s [1–3], 1950s, 1960s [4–9], 1970s [10–13], 1980s [14–33], 1990s [34–61], and >2000s [62–82]. A few are very well known such as Given [5], Wiser [22], Wender [11], Solomon [19], and Shinn [23] models, but far more are obscure. While excellent reviews of coal structure exist [83–88] and a recent kerogen review [89], that includes coal models, their focus ⇑ Corresponding author. E-mail addresses: [email protected] (J.P. Mathews), [email protected]. edu.au (A.L. Chaffee). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.11.025

has mainly been on coal structure. This paper provides a dedicated review of the history and advances in the structural representations of coal. The usefulness and application of coal molecular representations was recently evaluated [90]. 2. Coal models historic overview Figs. 1–4 show the selected models of coal grouped according to rank. The first coal model was generated at the State College of Pennsylvania (now the Pennsylvania State University) in 1942 [1]. While this model set the stage for further advancements the generation of a 2D structure was accompanied with a ‘‘real’’ van der Waals representation. The 3rd dimension (space filling models) in coal representations would not appear again until the Spiro [20],

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Spiro and Kosky [21] and Meyers [18] models of the 1980s and the first computational views generated by Carlson in the early 1990s [35]. Through the 1940s models were proposed by Gillet [2,3]. Major improvements were made in the work of Given (also at Penn State) in the 1960s after a decade lull in which no models were generated. The well-known Given model first appeared in the journal Nature [91] before being reproduced in the journal Fuel [5] (Fig. 3b). It was slightly refined with a tweaking of the hydroaromatic structure in 1961 [92]. While the structure was not a ‘‘model’’ per se, its intended function was to show the types of hydrogen structures in the vitrain-rich bituminous coal, it has been widely adopted as a structural representation. A secondary, lesser known, variant of a ‘‘typical structure’’ of a bituminous coal followed [92]. The change was a move away from a 9,10-dihydroanthracene to a 9,10-dihydrophenanthrene configuration of fused hydroaromatic rings (Fig. 3c). An additional adapted model from the earlier work was also published [8] as was a simplified 3D model consisting of glued flat and bent hexagonal disks [9]. The Hill high-volatile bituminous model of 1962 was a significant improvement in scale (C597H1428O81N14S6), and appeared on the journal cover in a white representation over a black background, with ‘‘COAL’’ in large red lettering (Fig. 3h) [7]. Yet it is one of the many relatively obscure representations. The 1970s brought the well known Wiser bituminous model (Fig. 3i) [22] along with the now rejected polyadamantane postulation of Chakrabartty and Berkowitz [10]. Their ‘‘structural alternative’’ carbon skeletal representations had an increasing scale representing the rank transition from 76% C to 90% C (wt. basis). This injected passion into the coal structure debate, but was almost immediately rejected as a structural entity [93–96]. The noted coal scientists Wender [11] and Pitt [12] also produced their structures at the end of the decade. The Wender models represented an early attempt to illustrate the structural differences as a function of rank with four very different partial-structure entities (Figs. 1a and 5a and b). The Pitt models also captured coalification from 80% to 90% C for a vitrain coal by alteration of the 80% C model (functional group modification, oxygen removal, and aromatization) [12].

Heredy and Wender [14] generated their bituminous model in 1980 with the now familiar model format: smaller aromatic ring structures with cross-links. Previous models tended to be based on undulating (non-condensed) ring catenation, as in the case of Pitt, or hydroaromatic and aromatic ring catenation, as seen in the Fuchs model (Fig. 3a). Oberlin et al. [15] generated a Kuckersite model (a rare type II kerogen that is ‘‘bituminous’’ in nature) with artistic flair, for the first time representing a coal as an assemblage of smaller fragments with strong relationship to their organic precursors. It also captured the phenomenon of aromatic (p–p) stacking which Meyers also captured [18]. Whitehurst et al. [16] developed small skeletal models for the rank range as an aid for discussion. Iwata et al. [17] generated simple molecular representations of three Japanese bituminous coals. The largest of these ‘‘average’’ structures contained 45 atoms. Meyers [18] generated a 3D ‘‘stick’’ model variant of the Wiser [22] model. The well known Solomon (Fig. 3g) [19] bituminous model was also produced around this time. His three-fragment model included hydrogen bonding explicitly. Spiro [20] generated 3D space-filling models of four of the well known models (Wiser, Given, Solomon, and Herdy and Wender (model constructed but not shown in the paper)). To enable 3D space-filling structures, three of the structures required alteration due to steric interferences. He concluded that the third dimension should be included with other parameters for model generation for more appropriate model structures. Later Spiro and Kosky [21] extended the rank range with low-intermediate and high-rank molecules in 3D representations and also attempted ‘‘eureka’’ type volume displacement measurements, the first evaluation of a coal model physical parameter. The Shinn [23] bituminous model (Fig. 3j) is perhaps the most comprehensive 1980s representation and captured the raging debate concerning an extractable/mobile phase [97,98] with his 2D bituminous coal model. This was created at a larger scale (10,000 amu) and contained three relatively small unconnected molecular entities, based on liquefaction products, held within a larger ‘‘encasement’’ molecule.

Fig. 1. The molecular representations of lignite and brown coals: (a) adapted from Wender (* indicating connectivity points) [11], (b) Philip et al. [24], (c) Wolfrum [25], (d) Millya and Zingaro [26], (e) Tromp and Moulijn [30], (f) Huttinger and Michenfelder [28], (g) Kumagai et al. [58], and (h) Patrakov et al. [68]. Structures are reprinted with permission of the copyright holders.

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Fig. 2. The molecular representations of subbituminous coal: (a) Shinn [51], (b) Nomura et al. [59], and (c) Hatcher [34]. Structures are reprinted with permission of the copyright holders.

Several lignite/brown coal models appeared in 1984 book ‘‘The chemistry of low-rank coals’’ [24–26]. Another early brown coal model was generated in 1987 by Huttinger and Michenfelder [28] and included cations (calcium, potassium, iron, aluminum, and sodium) as a structural entity, but not water (Fig. 1f). The high-vitrinite coking coal model of Lazarov and Marinov [29] was a large molecule with two additional hydrogen-bound moieties and intra molecular hydrogen-bonding. Along with traditional hydrogen-bonding, hydrogen-bonding to nitrogen structures was also indicated in this 2D structure. The only semianthracite model was generated along with a small lignite model by Tromp and Moulijn [30] in the same year (Fig. 5d). The semianthracite model had a range of aromatic ring sizes up to six condensed rings. Hatcher et al. [31,34,37,43] generated a range of models from lignite to subbituminous coal. These models emphasized coalification and the relationship between coal and lignin and were strongly influenced by the prior Adler lignin model [99]. These models specifically account for demethoxylation, catechol formation, aryl-ether bond cleavage, and carbon cross-links. The first application of computational 3D representations to coal models were by Carlson [35] (1992), but the molecular modeling of aromatic stacking in pitch, by Vorpagel and Lavin [100] in the same year, is also applicable to coal [101]. This permitted comparison and relative contribution of bonding and non-bonding interaction energies of previously published models of Wiser (Fig. 3i), Solomon (Fig. 3g), and Shinn (Fig. 3j). The computational approach allowed calculation of a physical parameter, simulated helium density, for the first time since the Spiro and Kosky [21]

water displacement evaluation. Following the Carlson work generating 3D computational representations became common. Nomura et al. [38] introduced the Akabira bituminous model also with a range of ring sizes and long chain aliphatics based on extract data, Curie point pyrolysis and with cross polarization magic angle spinning (CPMAS) NMR data. The 3D model was manually assembled via a reverse engineering approach similar to that of Shinn. Murata et al. [39] also used previously generated [17] small structural entities (simplified coal models) representing a rank range of Japanese coals, that were treated as oligomers for successful agreement with experimental versus density calculations. In following papers they (Dong et al. [40]) utilized computational aided design to examine the density of five variants of the earlier model to discern crosslinking (model flexibility) applicability. Models without extensive covalent cross-linking were more consistent with density calculations better agreeing with physical measurements. They utilized periodic boundary conditions and molecular dynamics/mechanics to generate ‘‘semistable conformers’’. Faulon et al. [41] contributed to computer aided design in coal model construction with the SIGNATURE program designed to generate and assemble molecular representations based on assembling (flash-pyrolysis) fragments via designated cross-linking/hydroaromatic cluster formation to be consistent with elemental, NMR, and other parameters within allowed limits. Multiple solutions could be generated and constructed. They also followed up with physical evaluation of density and pore size distribution [45]. The POR program [45] immersed models in a volume of 1 A3 cells and determined if the cells were within van der Walls radii or in inaccessible or accessible pore

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Fig. 3. The molecular representations of bituminous coal: (a) Fuchs and Sandoff structure [1], (b) Given [5], (c) adapted from Given [8], (d) Meyers [18], (e) Cartz and Hirsch [4], (f) Ladner (as printed in Gibson) [133], (g) Solomon [19], (h) Hill and Lyon [7], (i) adapted from Wiser [22], (j) Shinn [23]. Structures are reprinted with permission of the copyright holders.

space. Others have used computer-based stochiastic generation approaches to aid overcoming the issues of scale-limited diversity. While the approach is not common for generating representations,

it is more commonly employed to obtain a large-scale distribution of structural features usually coupled to reactive studies. The approach has been used for asphaltenes, kerogen [102], and coal [50].

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Fig. 4. Continuation of the molecular representations of bituminous coal: the space-filling representation of Spiro (a–f) [20] (a) Wiser model, (b) Given model, (c) Solomon model, (d) Wiser model, (e) Solomon model, (f) Solomon model globular configuration, (g) Lazarov and Marinov [29], (h) Zao Zhuang coal model (Nomura, Artok, Murata, Yamamoto, Hama, Gao, and Kidena), (i) Takanahashi et al. Upper Freeport model, (j) Narkiewicz and Mathews [73], k-l) inertinite-rich and vitrinite-rich models of Van Niekerk and Mathews [82]. Structures are reprinted with permission of the copyright holders.

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Fig. 5. The molecular representations of anthracite coal, a and b) adapted from Wender (* indicating connectivity points) [11] (c) Spiro and Kosky [21], (d) semianthracite model of Tromp and Moulijn [30], (e) Fragments of an 85% and 75% aromatic carbon ‘‘anthracite’’ by Vishnyakov et al. [55], (f) representation of Jeddo Anthracite, adapted from Pappano et al. [60]. Structures are reprinted with permission of the copyright holders.

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Coal-specific modeling advances followed with the CS2/NMP extract work of Iino group, specifically with extract fractions being produced from the acetone insoluble sub-fractions with both pyridine soluble and insoluble models being produced (Takanohashi et al. [46]). A more complex model of Zao Zhuang Chinese bituminous coal followed utilizing a similar extraction process but with a more extensive fractionation to generate a model with five cuts, some with multiple fragments, that were recombined into an associated structure (‘‘anisotropic model’’) [49]. Shinn produced a subbituminous model in 1996 (Fig. 2a) [51]. Nomura et al. [52] generated low-rank Australian and Indonesian coals models. Ohkawa et al. [53] utilized computer aided construction to the Akabira bituminous coal with connections between nodes being selected based on lower steric hindrance as determined by energy evaluations for enhanced construction efficiency. Ibayashi et al. [103] and Tanaka et al. [104] also utilized an efficient constructional approach based on a ‘‘genetic algorithm’’ to determine lowest energy configurations. Based on their earlier work Takanohashi et al. [54] generated an associated version of the Upper Freeport Argonne Premium coal extracts. The model consisted of three CS2/NMP sub-fractions: acetone soluble, pyridine soluble, and pyridine insoluble. They examined the stability of associated molecular structures and the interaction with various solvents, including swelling [61,105]. Vishnyakov et al. [55] generated anthracite models for use in examining methane adsorption and phases transitions in slits of various widths. Jones et al. [57] created a model of Pittsburgh No. 8 bituminous coal for utilization in a char formation study. Kumagai et al. [58] studied a Yallourn low-rank coal and specifically modeled the role of water, a first in the computational and non-computational coal models. They were successful in simulating irreversible volumetric changes with moisture release [58]. Pappano [63] generated four models of anthracite structures, some of which contained 80 or so rings in multiple stacks, only one of these models is available outside of the thesis [60]. At the turn of the 21st century, the state-of-the-art in coal representation was computational 3D models of limited scale, and one could argue, limited applicability beyond their intended use. The adaptation of computational approaches in the early 1990s limited the scale to that available via software and or computational limitations. These were, in many cases, smaller than the models generated in 2D without the aid of computers. Advancements to the Upper Freeport associated model continued with residue inclusion and NMR simulation resulting in extracts and residue model modifications [65,67]. This was the first whole coal model that had contributions from all of the fractions. Later this model was utilized to determine relaxation behavior of the associated structure and concurrent volume change by simulated heat treatment captured with molecular dynamics [106]. Mathews et al. [64] also generated molecular structures at this time for Upper Freeport and Lewiston–Strockton vitrinites. Both models were single molecular entities that ignored molecular weight distribution. They were utilized in char model generation simulating the devolatilization process [107,108]. A HyperCoal extract model was generated by Takanohashi et al. [66] used computational approaches (molecular dynamics) to identify physical cross-links; these being hydrogen bonded and cation bridging between carboxylic groups. The first liptinite-rich (Barzas) coal molecular representation was generated by Patrakov et al. [68], this structure was large (10,260 amu) consisting of copious quantities of condensed-linked (but not cross-linked) hydroaromatic rings (Fig. 1h). Vu et al. [69] developed a 3D model of a low-rank coalified wood (Australian). This model also included water as a structural entity, and examined coal–water, coal–cation interactions. Molecular dynamic simulations demonstrated restricted mobility of

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water in the pores due both to the confining nature and coal–water hydrogen bonding. This was enhanced with the presence of cations. Domazetis et al. [70,71,76,78,109] also generated low-rank coal models to examine inorganic interactions with the coal with higher level modeling approaches. A leap in scale occurred with the Narkiewicz and Mathews [73] model of Pocahontas No. 3 coal. This Argonne Premium coal has been well studied and a structural review was available [110]. This model contained >20,000 atoms and contained an extensive molecular weight distribution and was ‘‘squashed’’ preferentially to induce structural alignment. It was later used to aid in visualizing issues of interest in CO2 sequestration with CO2, CH4, and H2O being ‘‘loaded’’ into the model [111]. The large-scale effort of van Van Niekerk and Mathews [82] also contained a molecular weight distribution, solvent-swelling, [74] and solubility calculations [112] for the molecular entities for inertinite-rich and vitrinite-rich South African coals. Advancements in scale were also made with an automated aromatic structure generation approach directly from HRTEM lattice fringe images (Fringe3D) [113,114]. With this approach, >20,000 atom non-cross-linked slice model representations can be generated easily that includes the orientation, stacking and those distributions for the aromatic moieties of coals. Scripting populated the structure with heteroatoms and aliphatic structures to meet desired levels. The latest coal model (Morwell brown coal) has been generated in accordance with 2D NMR and FTIR data [80] and used to follow thermal maturation via a reactive (bond-breaking and bond-forming) molecular dynamics approach (ReaxFF) [115].

3. Model construction strategies There are a variety of construction approaches used in the generation of coal models. While it is not implicitly stated, the early models were likely constructed with little more than intellect, paper, and pencil with the model draft being converted into a 2D drawing with the aid of chemistry stencils. The 3D van der Waals or Drieding physical models [18,20,21] were initially, simply extensions of this approach. Later models were then constructed in a similar manner, or an earlier model was manipulated to represent the new coal of interest. This latter approach is common in the low-rank models where lignin models [99] are commonly adapted thus taking advantage of its known chemistry. The scale of coal models are heavily influenced by the intended use, with the few larger scale models being generated for use in following processes such as pyrolysis or liquefaction. Small-scale models were for coalification transitions or to simply aid in the illustration of structural differences. The early models were based solely on chemistry and to a limited extent on expected behaviors. Spiro [20] introduced the first of the physical parameters, steric hindrance and altered the structures of several of the early models to better account for the third dimension. This approach was expanded once the computational representation appeared and density was also considered [35,39,40,44,45,47,116]. It is the construction of models that is the time consuming and challenging undertaking. The first computer aided design (elucidation) and construction for coals appeared in the early 1990s [37,41] and several different groups [53,103,104] proposed elucidation and constructional approaches. Initially the constraints of the software and hardware restricted the scale of the model to sizes smaller than the largest pen and paper models. Consideration of structural isomers and statistical sampling further expanded the realization that many representations could be constructed [44] with the relatively simple constraints of average values such as elemental composition and limited NMR parameters (sometimes only aromaticity). Later models capitalized on the NMR advances with proton NMR of an expanded

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data set from dipolar dephasing and spectral peak deconvolution. Structural fragments were gleaned by destructive techniques such as flash-pyrolysis [37,41,64] or wet chemistry approaches that imposed necessary limitations on the component molecules. Structural diversity however, was limited by model scale. There were behavior evaluations, such as solvent-swelling, were performed with simple [54,61,105] and complex models [74] demonstrating that the extent of swelling with water [52] and common solvents could be captured. In the case of a lignite coal, the structural collapse with water removal beyond the gel-point [52] aided in the confidence that structural models could capture salient structural features and basic behaviors. Other more novel validation approaches have included NMR parameter/spectra predictions [65,67,117], HRTEM simulation [73], theoretical solubility distribution [74,112], and wide angle X-ray scattering [118]. Significant advances in scale were required for meaningful incorporation of molecular weight. The largest of the coal models currently are >20,000 atoms. Concurrently, advances are being made in the representations of kerogens, humic acids, and carbon materials with Reverse Monte Carlo construction approaches [119–123] being some of the many significant advances. No model has yet captured the structural features of the macerals that comprise the coals structure although representations of vitrinite [41,64], inertinite-rich [82], and ‘‘liptinite-rich’’ [68] have been generated. Humic and aromatic portions were included in both the Wolfrum [25] and the Domazetis–James brown coal models [71]. Further advances with a ‘‘supermodel’’, an association of small molecular components comprising solubility fractions of the Upper Freeport coal, improved the quality of representations as did higher-level computer simulation capturing cations [71,75] and simulations including water [69,81,111] as structural factures. Recently, model construction directly from HRTEM lattice fringes has been attempted to directly capture the aromatic stacking and alignment of the aromatic moiety of coal [113]. This approach has great potential to enable better representations of those structural features at scale and with significantly reduced construction and elucidation effort. After all, it is the effective use of coal models, rather than their construction, where more significant scientific contributions can be made.

4. Molecular representations of coal 4.1. The models of lignite/brown coals The development of structural understanding of lignite lagged behind that of bituminous coals and the first explicit model of a lignitic coal was published, only in 1976 by Wender (Fig. 1a) [11]. Drawing on US experience, Wender put forward a series of ‘representative partial structures’ for coals of different rank, but qualified them deliberately by commenting: ‘The structures shown are not coal models. But they represent a convenient way of cataloguing representative chemical structures so that the reactions of the various ranks of coals can be understood, at least in a preliminary way. They are useful as an aid to memory and a basis for prediction; they are, in short, frames of reference.’ Wender’s model (Fig. 1a) contained only 92 atoms with a molecular formula C42H40O10 and he presents very little discussion as to its development. Nevertheless, it captures a number of essential features of lignite structure. For example, it consists of single aromatic rings linked and cross-linked by aliphatic side chains. The concept of cross-linking also is explicitly captured. The oxygen is in a variety of forms (carboxylic acid, ketone, phenol, alcohol, ether, furan). The model also incorporates an aryl-methoxy group and some C3-aliphatic side chains. The latter are known to be common building units of lignin, one of the major inputs of organic matter to coal. In a similar vein, the group

of Iwata et al. [17] developed representative models for Japanese coals of various rank. The model for Tempoku lignite proposed by Iwata [124] was based on a repeating structural unit (C21H20O5). By virtue of its size, it does not incorporate the structural heterogeneity inherent in the Wender model, but it does conceive lignite as a polymeric entity. By the mid-1980s, as lignite liquefaction work was well underway in response to the oil crisis [125], chemists were actively thinking about lignite structure and a number of models emerged. A number of these were first presented at an American Chemical Society symposium, later published as an ACS book ‘The Chemistry of Low-Rank Coals’ [126]. A model put forward by Philip et al. [24] proposed an average structural unit (C115H125O17NS) based on a chain of benzofuran units, together with hydroaromatic and aliphatic side chains (Fig. 1b). The model was created after consideration of liquefaction products of Texas lignite and is understood as being derived from cellulose, lignin and other plant components by dehydration and deoxygenation of the original organic matter. This model introduces a number of further important structural features of lignite for the first time; specifically the heteroatoms N (as indole) and S (as thiol), the presence of an esterified aliphatic side chain, and the involvement of H-bonding as a mechanism of holding structural sub-units together. The model by Wolfrum [25] (Fig. 1c) was developed following detailed analytical studies of Rhenish brown coal. Being larger (C227H183O35N4S3CaFeAl), it incorporates more structural heterogeneity. Aromatic rings are present as well as condensed aromatic (and some hydroaromatic) of up to seven rings in size are linked by small aliphatic groups. Heteroatoms are present in a number of functional forms – amide, amine, hydroxylamine and indole for N; thiol, thiophene and thioether for S. The coordination of metal atoms (presumably as cations, though this is not what is shown) to polar functional groups, are now explicitly recognized. Interestingly the Wolfrum model does not incorporate any long aliphatic chains or include any ester linkages. The model (C122H146O23N2SM4) by Millya and Zingaro [26], pertinent to Wilcox lignite, does incorporate long aliphatic chains linking smaller hydro aromatic units (Fig. 1d). Consequently this provides the highest H/C ratio (1.25) of any model in the literature. This is the first model that depicts lignite as a composite of discrete fragments and that then further suggests how the fragments may be bound together – that is by metal cations, which act as ‘coordinating bridges’, principally by bridging between phenol and carboxylic acid sites. In 1987, Tromp and Moulijn [30] published a model of lignite (C161H185O48N2S1M4, Fig. 1e) as an aide to understanding its pyrolysis behavior. In this model there is a clear prevalence of structural units containing an isolated aromatic ring (C6), usually with methoxy and/or hydroxyl substitution, and connected to a C3 aliphatic side chain (C6AC3 unit). This is the characteristic building block of lignin. The model also contains ester linked aliphatic side chains, mono- and multi-valent cations (M) exchanged at carboxylic acid sites, together with a small number of heteroaromatic entities. This model is, arguably, a significant conceptual advance in as much as it brings together knowledge of the lignite’s origins with that from reaction behavior to formulate an improved more appropriate model. The model by Huttinger and Michenfelder [28] (C270H240O90 N3S3M10, Fig. 1f), from the same year, was the largest (for lignite/ brown coal) put forward to that time. The relation to a lignin precursor structure is not as clear, but it does contain a mixture of aliphatic chains and hydroaromatic ring systems condensed to various degrees. The significance of exchanged cations to the structural integrity of lignite is again recognized in this model. Prior to this model, there had been little concern about the essential three dimensional nature of the structures being proposed, but these

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authors comment: ‘To examine the structure in three dimensions a space filling model was made (which was possible without any steric problems). This model showed a satisfying space filling, especially with respect to the extension of the structure into the third dimension.’ The authors present this model as a single entity (no potential crosslinking sites are inferred), but note that this leaves a fundamental question as to how the linkages between several structural units may be realized. Hatcher and colleagues, in a series of papers starting from the late 1980s [31,34,43], proposed a strong relationship between the structure of the woody component lignite (which can often be isolated by hand sampling) and lignin as its principal evolutionary precursor. Using Adler’s lignin model [99] as a starting point, they provided a series of models, that demonstrated consistency with experimental (solid state 1H-NMR) data through the sequence lignin ? brown coal (lignite B) ? lignite A ? subbituminous coal. They also proposed a series of relatively straightforward chemical reactions that could account for these observed chemical changes. The preservation of the C6AC3 motif throughout the evolution of these ‘fossil wood’ models is clearly evident (Fig. 2c). A slightly modified Hatcher lignite model also appears in a paper debating the structure of coal [98]. Nomura et al. (1997) also generated two 3D models for an Australian (Yallourn) and an Indonesian brown coal [52]. By the mid 1990s, as computational chemistry began to emerge as a distinct specialist discipline, interest in the molecular representation of coal was rekindled as coal scientists began to sense its potential utility in understanding the behavior of coal containing systems in a truly 3D context. Kumagai et al. [58], who first applied this to Yallourn brown coal, created a 3D periodic cell consisting of one tetramer plus one pentamer based on the unit structure C21H20O7 (total molecular weight of 3464 amu). A 2D representation of the unit structure, constructed on the basis of the data from elemental analysis and 13C-NMR spectroscopy, is shown in Fig. 1g. Clearly the model is simplistic in the sense that it fails to incorporate a cross-section of functional groups, polycondensed hydroaromatic ring systems, heteroatoms, cations, etc. – all of which had been embodied in previous models. However it represents a significant advance on two fronts. First, the model incorporates water. Since Yallourn brown coal contains ca. 60% water by weight [127] – or, in other words, more water than ‘coal’ – this inclusion is very appropriate and significant for structural investigations. Second, the ability to calculate the relative energy of the structure and iteratively reduce it to a minimum value (i.e., geometrically optimize the structure) facilitated the comparison of 3D structures on a quantitative basis for the first time. For this purpose, Kumagai et al. [58] used a molecular mechanics (force field) approach, in which the energy of a total system was evaluated from the sum of the energies due to bonded interactions (bond: Eb; angle: Ea; torsion: El, inversion: Ei) plus the energies due to non-bonded interactions (van der Waals: EvdW; electrostatic: Eel; hydrogen bond: Ebb), making use of the Drieding [128] force field:

E ¼ ðEb þ Ea þ El þ Ei Þ þ ðEv dW þ Eel þ Ebb Þ Since they used a 3D periodic cell, the calculated energies apply over an extended 3D (infinite) context, as opposed to vacuum. They were thus were able to investigate conformational and volumetric changes in the macromolecular structure of the model as water was progressively removed from the structure. Vu et al. [69] proposed a structural unit (C100H80O2) for coal derived fossil wood. This was based on 13C-solid-state NMR, ultimate and functional group analytical data taken for a sample of Podocarpus sp. collected from the Loy Yang mine, Latrobe Valley, Victoria. This model is, it can be argued, pertinent to just one component of the complex heterogeneous array of morphologically distinct components that make up lignite. The model is based on an

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11-mer of (different) degraded lignin subunits, three of which were then assembled with water into a 3D periodic cell (27.3  27.3  27.3 Å3), so as to achieve a density consistent with the coal sample. This group investigated the time development (molecular dynamics) of a brown coal model system, so as to characterize for the first time molecular motion and average structural properties (e.g., molecular diffusion and average bond distances) at ambient conditions (298 K, 1 atm). This is an improvement over standard molecular mechanics methods since it incorporates the inherent structural motion of matter at the conditions of interest. This work demonstrated that the diffusion of water molecules in the vicinity of the lignite (degraded lignin) is substantially reduced compared to pure water, largely as a result of hydrogen bonding interactions. Cation exchanged (Na- and Ca-) forms of the model were also investigated and, in this case, the mobility of water molecules was further reduced. The liptobiolith model of Patrakov et al. [68] is about the only representation of a cuticular coal (85% C). This Siberian brown coal was constructed based on non-isothermal liquefaction products, was large (C727H790N2S4O36) and also constructed in 3D. The structure is shown in Fig. 1h. The hydroaromatic fragments comprising the structure vary between 1 and 8 rings, with more rings being aliphatic than aromatic. Domazetis and coworkers [70,71,75,129] have published a series of papers that focus on the interaction between inorganic cations and brown coal. This group prepared a model (C258H256N2O78 S) based on the structural data from the literature [71]. Rather than using a molecular mechanics approach, this group applied semi-empirical and ab initio methods to compare the structures of the plain model versus those of derivative Na, Ca, Mg salts prepared by modification of the original model. They investigated simple addition of salt to the model [Coal(Na+Cl)], þ ion-exchange of cations on the model ½CoalðCO 2 ÞðNa Þ and the effects of further hydration of these models þ ½CoalðCO 2 ÞðNa Þ  10H2 O. For most cases, the authors reported increased stability when salts and/or water [76] were added to the basic coal model. For Fe (and Ni) this group considered a range of more complex ion configurations, with the metal assuming mono-dentate, multi-dentate and bridging configurations, for   þ3 þ3 example: ½CoalðCO 2 Þ2 ðFe Þ3 ðOH Þ7 ðH2 OÞ, ½CoalðCO2 Þ3 Fe  and    þ3 ½CoalðCO2 Þ2 ðFe Þ2 l  ðOH Þ2 ðOH Þ2 ðHOPhÞ2 ðH2 OÞ2 , respectively. The latter case was simulated by the co-addition of phenol groups (HOPh) to the model. It was found that polymeric multi-nuclear complexes based on Fe and Ni were generally energetically favored to form in coal [128], as was water adsorption up to at least 20% [76]. Further ab initio studies by this group concluded that the interaction between metal complexes and brown coal becomes important in coal pyrolysis [130] and char formation [107] as well as for catalysis of steam gasification reactions [77]. Recently, Salmon et al. [80] created a macro-model for an Angiosperm wood isolated sample from Australian Morwell brown coal. Using an approach similar to Hatcher et al. [31], the model was constructed to match experimental data via manipulation of an angiosperm lignin structure in the literature [131]. A geopolymer consisting of five C232H212O94 units was used in reactive force field (ReaxFF) simulations to investigate thermal transformations as a means of simulating coal maturation [115]. 4.2. The models of subbituminous coals The subbituminous coal models are few in number and, along with anthracite, are the poor cousins of the far more abundant bituminous models. Shinn who is better known for his bituminous model [23] generated, with a similar approach, a subbituminous model in a conference paper in 1996 [51]. The model (shown in Fig. 2a) consists of mostly small ring structures (<4), with limited hydroaromatic structures, lots of cross-linking, and multiple

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examples of oxygen functionality within two macromolecules. Comparison between the subbituminous and bituminous models was helpful in visualizing structural differences. He also presented the emerging vision of how the computational advances could impact coal molecular modeling: computer construction of molecular connectivity, 3D energy minimized structures, reaction pathway predictions with quantum mechanical approaches, incorporation of greater molecular detail, and Monte-Carlo construction approaches to display multiple potential structures. It would take several decades before these were realized, and of these approaches, the use of coal models in a predictive capacity advanced the least. Nomura et al. [59] generated a sizable ‘‘structural unit’’ subbituminous coal model (522 carbon atoms) for Taiheiyo coal (Fig. 2b), but it was not widely cited being published in a report, rather than a peer-reviewed journal. The work of Hatcher [34] also captured subbituminous structure through careful coalification evaluations based on an improved coal sampling, selection of obvious coalified wood, and the application of solid state 13C NMR, and flash-pyrolysis GCMS. This model captured lignin structure with appropriate coalification transitions (Fig. 2c). Takanohashi et al. generated a much smaller scale HyperCoal model capturing model information from raw and extracted Wyodak coal [66]. The model was an associated model of three fragments (containing 3–4 hydroaromatic rings) bond together via a phenolic hydrogen bond and a calcium–oxygen ionic bond. Molecular dynamics was used to probe any dissociation with temperature, but none was observed at 300 °C in agreement with expectations. Given the current importance of subbituminous coal to electricity generation in the US with low sulfur emissions, the large reserve base worldwide, and a re-emerging desire for coal-to-liquids it seems likely that the subbituminous coal omissions will be re-evaluated by the coal structural modeling community. For completeness, there are also two small molecular representations for subbituminous coal used in visualizing coalifcation structural differences [16,17]. 4.3. The models of bituminous coals The bituminous coals are the best represented of the coal rank in the model representation arena. The first bituminous model was also the first coal model. The Fuchs model [1] at first glance appears to be closer to a structure we would expect for an anthracite coal or char. The 43 conjoined rings actually contain only 24 conjugated aromatic systems in three separate islands interconnected with hydroaromatic, cyclopentane ethers, and diphenylene ketone oxide. The model was utilized to explain pyrolysis behavior and was compatible with ’’the proximate and ultimate analyses of bituminous coals, with the results of group determinations, and with the experiences of oxidation, reduction, and thermal decomposition.’’ Interestingly, the model (84.3% C) was presented in 3D space, together with a 2D drawing (Figs. 3a), and was utilized to exemplify a process, an impressive feat in 1942! A few of the structural features, such as diphenylene ketone oxide appear in later but often lower-rank structures. The Gillet models consisted of small molecular entities (for example hexacene and perylene-like structures along with more complex multicyclic dibenzofuran-peroxide molecules for a coking coal [2,3]. Of these, perylene and its various derivatives of were favored as ‘‘hypothetical structural formulae’’. While not a molecular level representation, the schematic of Hirsch [132] appeared in 1954 and proposed the 3D nature of ‘‘open’’, ‘‘liquid’’, and ‘‘anthracitic’’ structures crossing the rank range and presented conjugated aromatic structures of a benzopyrene (5-rings), coronene (7-rings) and a 30-ring structure for aromatic lamellae diameters for 80%, 89%, and 94% carbon coals. This influential work was based on landmark X-ray scatting data. The highly cited Given model first appeared in the journal Nature

in 1959 [91] and later in Fuel in 1960 (Fig. 3b) [5]. The model was tweaked in 1961 with a change in the orientation of the hydroaromatic structures from dihydro-9,10-anthracene to dihydro-9,10-phenanthracene type structures as the hydroaromatic linkages [92]. An additional similar, yet unique coal, model appeared in 1962 and was slightly modified in 1964 (Fig. 3c) [8]. However, these structures are not well cited. The well known Given ‘‘model’’ was highly influential and proposed structures we recognize today in many of the models ‘‘Thus a molecule in coal apparently consists of a number of rather small aromatic systems, say 1–3 fused rings, highly substituted by aliphatic groupings that serve mainly to link together the ordered regions and mostly do not terminate in methyl groups.’’ [91] The structure presented was a 20-ring aromatic, hydrocylic, and highly-linear structure with a 4-ring appendage adding the third dimension to an otherwise nearly planar molecule. The 1960 model of Cartz and Hirsch [4] fleshed out a 2D hydroaromatic representation for an 84.5% carbon based on extensive X-ray diffraction analyses determined in earlier work (Fig. 3e) [132]. The presence of steric hindrance in the model was presented by a 2D drawing with out-of-plane bonds. The structure was a combination of 13 rings including 5-membered and hydroaromatic structures. The oxygen functionality was simpler, as expected with a higher rank coal, and included phenolic structures. Ladner created his distillate model to recognize the NMR-determined contribution of tertiary CH components that increased the hydroaromatic cyclic components over methylene linkages [6]. A larger Ladner structure, modified from the Given model, appears in a later Gibson paper [133] as a personal communication (Fig. 3f). A leap in model creativity was generated by Hill and Lyon [7]. Their model for a high-volatile bituminous coal was a significant increase in size and structural diversity with an overall molecular weight approaching 10,000 amu; five times larger than previous models (Fig. 3h). Ring sizes varied from benzene to a dibenzo-ovalene structure. The oxygen functionality included carboxylic acid, phenolic, aryl-O cross-links, aliphatic ethers, furan analogs, and quinone analogs. With modern analytical approaches many of these functionalities are no longer supported in coals of this rank, but their inclusion was indicative of the growing realization of the complexity of coal and the movement away from polymeric structures. Functional diversity was also present with S and N forms: pyridine, pyrrole, thiophene, thiol, amines, analine, and their analogs. The representation also has a molecular weight distribution with two small non-connected molecules, the first occurrence of this feature in coal model literature. Given created a model for ‘‘bright’’ (vitrain) bituminous coal that was ‘‘lashed together by chemical bonds’’ [134], as an apparent precursor to his well recognized model [5] although other slight variants of the model exist [91,92]. A ‘‘classic model’’, it is very different than the Chakrabartty and Berkowitz polyadamantanes postulation [10] discussed earlier. The other classic models are the well cited papers of Solomon (Fig. 3g) [19], Wiser (Fig. 3i) [22], and Shinn (Fig. 3j) [23]. These classic models also had the distinction of being the first computationally modeled coal structures [35] although ‘‘real’’ 3D models had previously been constructed (Given, Wiser, Solomon, and Heredy and Wender [14]) with van der Wall spheres (space-filling models) [20] and, also, for the original molecular representation of coal by Fuchs and Sandoff [1]. There are however, many, many, more bituminous models that are worthy of recognition. Wender [11] generated his frames of reference coalification models as an aid for viewing the ‘‘Catalytic Synthesis of Chemicals from Coal’’ (paper title), a familiar trend for many of the bituminous coal models. Oka et al. [135] also generated similar sized (small) models with a computer aided design approach for components with an overall formula of C24.7H23.05S0.85N0.44.

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The Bartle et al. [13] small model structures of supercritical-gas extract fractions are also note worthy due to the careful chemical speciation. The Pitt [12] model was ‘‘snake like’’ undulating pericondensed 25 ring structure (aromatic, hydroaromatic, and 1,4benzoquinone) for an 80% C coal. The 90% C variant having greater aromaticity, no benzoquinone, and some catacondensed rings. It was the last of the coal models to have an extended mostly pericondensed single molecule representations. Models following this period tended to have the familiar cross-linked, mostly singular molecular, representations. The Heredy and Wender [14] model is one such example with 4 molecules (naphthalene, phenanthrene, and fluorene hyroaromatic derivates) cross-linked together. The Merrick [27] model is also a similar example of this style of model. Other small representations such as the two bituminous models of Iwata et al. [17] are typical of small molecular representation showing compositional differences. Whitehurst et al. [16] generated a rank range of small-scale skeletal structures for an aid in liquefaction discussions. The Wiser [22] model (Fig. 3i), larger and more complex, was drawn for similar reasons and, along with the Shinn (Fig. 3j) [23] and Solomon (Fig. 3g) [19] models, are perhaps those that are most respected and relevant bituminous models from this era of pen and paper construction. Meyers [18] generated 3D structures with the Drieding molecular modeling kit. These metal rods interconnected and generated structures that when suspended in 3D space could easily be confused with computationally generated structures that still utilize this name to describe that molecular view. The photograph of the arrangement is shown in Fig. 3d. In this manner stacking was captured with two free aromatic structures forming a three aromatic stack describing the ‘‘crystalline structure’’ coupled with a cross-linked component also. A polymeric chain segment was also generated. Also of note for innovative structural generation, is the glued together bent and flat hexagon disks of Francis [9] (1961), for a simplified 3D version of the Given model. Three dimensional space-filling structures appeared for the Wiser, Given, Solomon, and Heredy models (Fig. 4a–f) [20]. The inclusion of the third dimension demonstrated strain in the models, and all but one were altered. The Solomon model was also presented in a stacked ‘‘globular configuration’’ (Fig. 5f). This work continued with a later paper extending the rank range (low-rank and a high-rank (anthracite) coal models [21]. The bituminous coal model has subsequently been generated in 3D space with computational modeling approach and utilized in CO2 sequestration study [136]. Solomon [19] (Fig. 3g) created his hypothetical bituminous model to aid in the understanding of thermal decomposition for a Pittsburgh seam coal. It contained multiple components crosslinked or associated with hydrogen bonds. The ‘‘cracking’’ of the molecules generating light gases, tars and char. In comparison, the Wiser model (Fig. 3i) did not show hydrogen bonding and there was a single molecule comprising cross-linked entities with ether and aliphatic linkages. The intent of the model was to indicate ‘‘what must be done to bituminous coal to convert it to the liquid state’’. It has been pointed out that there are similarities between coal-pyrolysis and coal-liquefaction in primary conversions [137,138]. Thus, similarities are expected in the use of coal models to show the initial stages of these processes. The Shinn [23] model continues in a similar vein, showing product structures for both single and two-stage liquefaction from fragmenting the crosslinked initial model (initial model is shown in Fig. 3j). Lazarov and Marinov [29] generated an ‘‘assembled’’ coking coal model with similar approach based on product analysis of soluble fractions of a butylated (to increase extraction yield) coal to create a three-molecule model with cross-linked relatively large, and mostly pericondensed, aromatic and hydroaromatic components including hydrogen bonding (Fig. 4g). It is easy to imagine that a

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few additional cross-links and some aromatization would generate large aromatic rafts responsible for aligned coke structures. They also produced butlyated extract models and delved in statistical considerations for the number of models that could be generated from that data [29,139]. The 1990’s were the beginning of the computational approaches to construction and structural representation. Initially these necessarily small structures, due to software and computational constraints, added little to the existing capability to represent and utilize coal representations [37]. However, it was soon evident that there was great potential for computation studies accompanied by increased computation access and capability [45,46,50,54]. Carlson [35] demonstrated and quantified the importance of van der Waal interactions and hydrogen bonding, along with density measurements on the 3D models. He also performed aromatic stacking experiments and observed, with the minimization approach used, that the ring pairs stacked in a parallel, overlapping fashion but usually with some ring rotation relative to the other aromatic structure. Strained molecules and van der Waal stabilization was also investigated. An expanded modeling study of polyaromatics [100] is also worthy of note; although it was pitch-related and not coal specific, it is relevant to the high-rank coals [101]. Computational models of Upper Freeport and Lewiston–Stockton bituminous vitrains were generated [64] along with a model for Pittsburgh No. 8 coal model [57]. A series of models for Akabira coal [38–40,53,116] explored density, cross-linking approaches, and construction. The very long chain aliphatics, reminiscent of terpene chemistry, (one chain contained 26 carbon atoms) found in the original larger-scale models are relatively rare in the modeling literature. These components likely come from the liptinite group and the careful selection of vitrinite-rich coals and coalified logs in the bituminous work may be partially responsible for their omission. However, their inclusion throughout the structure may be misleading, for behavioral studies, if they exist in discreet associated components. This complex nature of coals is one of the reasons that their structural elucidation is so challenging. Recent large-scale modeling efforts have adapted this associated approach [82] for the long aliphatic chains that are evident from rapid-pyrolysis mass spectra. The inclusion of physical evaluations via computational approach [45] was a significant aid to modeling efforts as it further constrained the representations and yielded more reasonable porosity values. Long chains were omitted from bituminous vitrinite (coal scrapings obtained from obvious coalified trees from underground mines) models [64]. There are few representations of non-vitrain coals. Also there are remarkably few of the bituminous coal models that address the structures of coking coals, besides Pittsburgh coal [19,72]. There are a few exceptions either considering the coal [29,56] or the thermoplastic transformations [140,141]. Notable in its construction strategy utilizing methylene chloride soluble, pyridine-soluble and pyridine-insoluble fractions (Fig. 4h) for a coking Zao Zhung coal, Nomura et al. generated a structure showing these three separate components with considerable attention to detail in the structural evaluation [56]. Further gains came from similar careful consideration of solubility fractions and the exploration of swelling, and the associated models for Upper Freeport coal via Takanohashi and co-workers [54,61,62,65,67,105,106,142]. The super-molecular structure of Upper Freeport coal is shown in Fig. 4i [67]. Modifications, based on simulated NMR spectra, further refined the CS2/NMP insoluble portion’s structure [67]. These models however are still relatively small. While useful for illustrative purposes, behavioral studies would are limited in models of this scale. The era of the larger-scale pen and paper bituminous models was started by Hill & Lyon (Fig. 3h) [7], Wiser (Fig. 3i) [22], and Shinn (Fig. 3j) [23] between 1962 and 1984. It was much later in the start of the new century

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before large-scale computational models were constructed for Pocahontas coal with the implicit goal of incorporating a molecular weight distribution [73]. This Narkiewicz and Mathews model (Fig. 5j) was a much larger structure (C13,781H8,022O140N185S23), 20 times the scale of the Shinn model. It was also used to visualize CO2 sequestration-relevant issues [111]. This model was also preferentially aligned, via a squashing protocol to force alignment, another first for bituminous coal molecular representations. The reason for the larger scale was due to the desire to incorporate a significant molecular weight distribution (215 separate molecular entities, ranging between 78 and 3286 amu) for improved representation and utility. The slightly smaller van Niekerk and Mathews models for inertinite-rich (Fig. 4k) and vitrinite-rich (Fig. 4l) South African coals have also been constructed at scale >14,000 carbon atoms [82] for similar reasons and for consideration of solvent interactions [143] and solvent extraction [112]. The scale however needs to be expanded further if we wish to capture even a miniscule portion of the continuum that is coal structure. Recent work utilizing Fringe3D, to obtain a distribution of PAH ring sizes from HRTEM lattice fringe images, coupled with scripting for aliphatic and heteroatom inclusion and a semi-automated automated cross-linking approach has enabled a model for a subbituminous coal [81] and the Illinois No. 6 coal to be generated at even greater scale (50,789 atoms within 728 cross-linked aromatic clusters) [144]. Interestingly, the expansion of scale has caused visualization issues that are being solved with a novel reactive-coarse-graining approach that generates 2D lattice structures (reminiscent of the Bathe lattice coal network representations) from complex 3D atomistic representations [145,146].

imparting sympathetic curvature (due to non-bonding interactions) in adjacent sheets.

4.4. The models of anthracite coals

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.fuel.2011.11.025.

The anthracite models are, like the subbituminous models, far less frequent than the bituminous coal examples. Limited representations that essentially capture the increase in ring numbers with coalification are numerous however, such as the seminal Hirsch [132] X-ray scattering work which included the now famous coal rank schematic representation (open, liquid, and anthracite structures) and the Wender [11] ‘‘frame of reference’’ representation (relevant structures shown in Fig. 5a and b). The anthracite representation of Spiro and Kosky [21] was a 2D (Fig. 5c) and a space-filling model similar to the low- and intermediate-rank (bituminous) models. A semianthracite model has been proposed by Tromp and Moulijn (Fig. 5d) [30] but, like many models presented here, its construction was not explained beyond extending the Shinn [23] model. Anthracite models were also generated for a methane simulation by Vishnyakov et al. [55] but the construction details are vague. Two fragments of their model are shown in Fig. 5e. The Pappano [63] models are the only anthracite atomistic representations which capture the stacking and large aromatic sheets. His four models represent then still mined coal sampled from the Pennsylvania anthracite fields. As all of the commercially mined US anthracite comes from these fields [147] it is not surprising that beyond this Penn State work and the somewhat obscure Russian work [55], little anthracite modeling has been performed. This is despite its importance in other countries and the expectation that it is, perhaps, the easiest of the coal ranks to model as there are: less maceral structural divergence, less oxygen, and less hydrogen enabling more simplistic generation of highly aromatic carbon rich coals. One of the models (Jeddo) is shown as a 3D representation in Fig. 5f [60,63]. The model was generated with laser desorption ionization mass spectroscopy, X-ray diffraction data the aid of the SIGNATURE [44] and POR programs [45] for construction and physical evaluation. Provocatively, this structure strayed from the expected linear form of the graphic sheets with curvature in one sheet

5. Concluding comments There are an abundance of structures that capture, to a certain degree, the structural features of coal. With >134 structures being generated over the last 70 years the field has been active, yet only a few structures are well known. The field has been dominated by the representations of bituminous coal. Far fewer lignite and brown coal models exist. Surprisingly, given their importance to US coal production, very few subbituminous models are available. The limited anthracite models however are expected given its mostly very regional nature and a historic importance. Models have moved from pen-and-paper constructs to models generated with computational aids. Many of the structures have been small (100’s of atoms) with progress, only recently, in generating structures at greater scale for improved representation quality and applicability in following behaviors. Progress is being made in both analytical techniques and the computational power coupled with software advances that are enabling improved structural representation. Yet no coal model exists that represents maceral contributions (although several maceral-rich structures exist). This is likely to change as construction strategies enable more rapid model building approaches to larger scale models that can accommodate multiple simulation directions. Appendix A. Supplementary data

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