Macromolecular structures of coals

For the above reaction to occur, the aromatic nuclei in compound 1 should carry activating groups, such as hydroxyl, alkoxy, or fused ring aromatics". As a result of reaction with BF3 and phenol, the macromolecular structure of coal should undergo rupture at the aliphatic bridges, and these bridges are transferred to phenol molecules to produce bisphenols. Analysis of the bisphenols should provide information on the aliphatic bridges present in coal structure. 

Lucht, L. M., Larson, J. M., and Peppas, N. A. (1987). Macromolecular structure of coals. 9. Molecular structure and glass transition temperature. Energy Fuels 1, 56-58. 

Figure 2.28. A two-dimensional model for the macromolecular structure of coal illustrating (i) the network, (ii) surface functionality and (iii) porosity (Shinn, 1984).

Line-broadening is now explained, for coals and non-graphitic carbons (usually of H IT > 1200 °C), as resulting from structures made up of associations, roughly parallel, of hydrocarbon (polycyclic) moieties (for the macromolecular structure of coal) and of quite defective, non-planar but roughly parallel associations of carbon atoms as summarized in Figure 2.38, and termed the defective micro-graphene layers. That is, there is no planarity, only strain and defects. 

Figures 5-7 are the spectra exhibiting the thermally extracted mobile phase components over different temperature intervals. All five coals mentioned in Figures 5-7 show distinct mass spectra in the mobile phase compared to the spectra of the macromolecular structure (25) and contain alkylsubstituted naphthalenes in the mobile phase although their relative amounts are dependent on coal characteristics. In general, (the temperature where the maximum rate occurs) in Py-FIMS was in the range of 430-470 C for bituminous rank coals. Around T, the macromolecular structure of bituminous rank coals is decomposed to yield FI spectrum showing the dominant peaks of alkylsubstituted phenols. Detailed FI spectra of the macromolecular structure for the ANL-PCS coals mentioned in Figures 5-7 are illustrated elsewhere (25).

In many ways, the molecular models that we have used as the basis for our CAMD studies describe coal structure very well. However, none of the models investigated thus far contains explicit three-dimensional covalent cross-links. Actualfy, the models we have studied are primarily constructed of long chains of one-dimensionally-linked clusters with a number of short side-chains. However, it has been established on the basis of solvent swelling studies (11.15.16) that bituminous coal is primarily made up from a three-dimensional network of clusters held together by covalent bonds and by an even higher density of hydrogen bonds. These macromolecular models of coal, which are less concerned with the molecular structure than with the ways that clusters are bonded to one another, provide a complementary way of describing coal structure. 

As a side note, throughout this chapter, the term structure of coal will be employed to indicate the structure of the higher-molecular-weight species, in fact the macromolecular part, of coal to the exclusion of those lower-molecular-weight materials that can be extracted according to the methods outlined in the preceding section. 

On the molecular level, coal is an extremely complex substance and in spite of extensive research in this area the exact structure of coal has yet to be determined (Chapter 10). Different authors have proposed various coal models from time to time. Broadly it is agreed that coal consists of heterogeneous polyaromatic clusters in a complex array resulting in a highly cross-linked macromolecular gel structure. Another opinion that coal has a highly associated structure also exists. 

Figure 2.27. A computer-generated three-dimensional model of the macromolecular structure of vitrinite in coal showing porosity between coal substance (Marsh, unpublished results).

One of the dominant issues in coal structure to re-emerge in the past decade is the two-phase concept of coal structure. A very spirited discussion of this topic was a feature of the 1989 symposium (48-52V Peter Given played a central role in the recent work on the two-phase concept, particularly in fostering the usage of the terms "mobile phase" and "macromolecular network" (23.53-551 and in organizing the "debate in print" (54V which has become a landmark papers in coal structure. In particular, the debate in print (24) was cited by all of die contributors to the 1989 discussion of the mobile phase (48-52V Given s work on the mobile phase was a... 

The hypothesis that coals can be considered to consist of two component phases has its origins in observations of coal behaviour as well as deriving from the analysis of coals and attempts to define their structure. The results of extensive studies of untreated, preheated and hydrogenated coals, using analytical and microscopic techniques, have allowed some insight into the association between the so-called mobile phase and macromolecular network, and have provided information upon differences in their chemical properties. 

If the mobile phase is present in a significant concentration, as suggested by the results of solvent extraction studies (1,8), the practical meaning of the mobile phase to coal conversion processes may be profound. In coal liquefaction, two stage processes emphasizing the mobile phase and the macromolecular structure separately could well be most economical. In devolatilization kinetics, at least two sets of kinetic parameters are necessary to model the devolatilization phenomena associated with the mobile phase and the macromolecular structure respectively since the mobile phase components devolatilize at much lower temperatures than the macromolecular structure components 0. In addition, the mobile phase appears to have a significant influence on the thermoplastic properties of coal (0 and thereby on coke quality. 

Fusion of the llptlnlte macerals In bituminous coals commences at lower temperatures and reaches a much greater extent than that of the aromatic macerals (Figures 4 and 5). The greater thermal stability Indicated by the much higher fusion temperatures of the bituminous llptlnltes compared to brown coal llptlnltes can be explained In terms of these materials having a more highly crossllnked macromolecular structure than the llptlnltes In the brown coals. This Increase with coallflcatlon could be the consequence of In situ crosslinking of material or the selective loss of llptlnlte fractions that are less crossllnked and therefore less Inherently stable ... 

Of particular interest in this study is the nature of the non-aromatic structures in the three main maceral groups. It should be noted that the exinites in both the coals separated by float-sink are 90% sporinite. It has been theorized that small molecules, especially the aliphatics, are fairly mobile at some period during the formation of coal (5,6). The studies which support this theory were done on coals that are very rich in exinites and some contained alginite. Two of the coals chosen in the present work (PSOC 828 and 1103) have a more normal distribution of macerals and yet the pyrolysis results indicate that migration of molecules from the exinites to vitrinite and then incorporation into the macromolecular structure might have occurred. 

Important theoretical and experimental considerations of the use of macromolecular theories for the description of coal network structures have been recently analyzed (1). Relevant equations describing the equilibrium swelling behavior of networks using theories of modified Gaussian distribution of macromolecular chains have been developed by Kovac (2 ) and by Peppas and Lucht (3) and applied to various coal systems in an effort to model the relatively compact coal network structures (1 4). As reported before (1), Gaussian-chain macromolecular models usually employed in the description of polymer networks (such as the Flory... 

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