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In this work, the characterization of the non-covalently bonded compounds present in a set of perhydrous coals of different age and geographical location (Cretaceous coals: UCV and TCV and Jurassic coals: AJV, PGJV, WJVh and WJVl) was carried out by means of a combination of the analyses of the material soluble in chloroform and the thermal extract. The extract in chloroform was studied through GC/MS and NMR and the thermovaporized fraction was obtained by means of flash pyrolysis at the Curie temperature of 350°C and quantified by on-line GC-MS. The results obtained for the Cretaceous coals confirm that the substances responsible for the hydrogenation are those covalently bonded to the vitrinite network as a result of the modifications undergone by the botanical precursors. Despite the striking similarity in the global characteristics of these two coals (TCV and UCV) significant differences between the material non-covalently bonded to their coal matrices were found. These differences are attributed to the type of resins present in the coals and/or to their different degree of evolution. With respect to the Jurassic coals, the present study allows the process by which the hydrogenated substances were assimilated into their structure to be established. The characterization of the assimilated substances in the WJVl coal reveals an unexpectedly high incorporation of alkanes given the humic origin of this sample. From these results the assimilation of hydrogen-rich substances from the decomposition of the organic remains in the sedimentary environment in which WJVl precursor were deposited, is proposed. The incorporation of products derived from the primary decomposition of organic material is not evident for the WJVh and AJV coals. The substances assimilated into the coal matrices show a higher aromaticity, the aromatic structures of WJVh being more condensed than those found in AJV. The compositional differences between them probably arise from the different source of the hydrogen-rich material. In the case of AJV coal the source was the adsorption of hydrocarbons generated and migrated from the Pliensbachian source-rocks whereas in WJVh, assimilated compounds come from the material generated by the thermal transformation (coalification) of the adjacent organic rocks of similar age. Finally, PGJV shows two types of non-covalently bonded compounds. Solvent extractable material is mainly composed of the first type of compounds, which is predominantly aliphatic in nature with a preponderance of alkanes. The second type of compounds is more aromatic and they are probably located in the close porosity of this coal. They are not accessible to chloroform but they are the most abundant in the thermal extract., The financial support for this work was provided through a contract with the European Community (No. 7220/EC-769).

A study of coal products obtained from Gray-King pyrolysis at 550 °C was carried out in order to obtain information about the chemical modifications of the huminite/vitrinite structure resulting from different processes of natural hydrogen-enrichment. The high oil yield obtained, the similarity between the infrared spectra of the generated oils and those of the parent vitrinite as well as the parallelism between the compositional data deduced for these oils and those obtained by means of Curie point pyrolysis-gas chromatography/mass spectrometry justify the use of this thermal approach. Gray-King pyrolysis is preferable to other techniques because it is possible to make an accurate mass balance, a thorough study of the solid residues and a full characterization of both the volatile and non-volatile fractions of the degradation products. From a study of selected perhydrous coals, mainly composed of the vitrinite maceral group, only in the low reflectance Jurassic coal from the Whitby (England) basin (WJVl) was the incorporation of lipoidal material evident, due to the significant contribution of long-chain alkyl aromatics in its oil. The structure of the vitrinite in the other Jurassic coals, the high reflectance sample from the Whitby basin (WJVh) and the coals from the Asturias (Spain) and Portugal basins (AJV and PGJV, respectively), is probably similar to that described for non-perhydrous vitrinites in the subbituminous coal rank. However, the presence of assimilated hydrogenated substances of a secondary nature, in particular for AJV and PGJV, hinders the cross-linking and condensation processes typical of the transit to the bituminous coal rank. In Cretaceous coals, from Teruel (Spain) and Utah (EEUU), (TCV and UCV, respectively), whose perhydrous character is mainly due to the specific hydrogenated nature of their botanical precursors, the incorporation of aliphatic structures via covalent oxygen linkages is proposed. The results obtained may be useful for establishing accurate structural models for perhydrous vitrinites. At the same time, the data obtained will contribute to explain the involvement of the high hydrogen content in the suppression of vitrinite reflectance.

The vitrain from Teruel (North-Eastern Spain) is a good example of a vitrinite whose perhydrous character and anomalous properties are caused by the presence of terpene-type resin. The sample used in this work was isothermally pyrolysed in an open-medium system at temperatures ranging from 250 to 500 °C. A mass balance, a petrographic and geochemical characterisation of the pyrolysates and compositional and structural analyses (GC and NMR) of the generated oils were performed. The results confirm the association of huminite-resinous substances through covalent bonds and also point to the possible incorporation of aliphatic material via covalent oxygen bonds. The side-chains that serve as linkers between the phenolic subunits derived from the lignin contain more labile thermal bonds than those present in other vitrinites not affected by resin saturation. Consequently, this type of vitrinite undergoes a more intense degradation at lower temperatures than non-perhydrous and other hydrogen-rich vitrinites. The vitrinite network itself due to the incorporation of resin-like substances (cycloparaffinic in nature) may serve as a hydrogen donor, enhancing the hydrogenation processes during pyrolysis with respect to the polymerisation and recombination reactions. The higher ability of hydrogen to stabilise the free radicals formed during pyrolysis explains the higher conversion to liquids at lower temperatures than in the case of non-perhydrous vitrinites. At the same time, this ability limits the growth of the aromatic structures. The evolution of the vitrinite modified by the presence of terpene-type resin is, thus, retarded with respect to non-perhydrous vitrinites, although in both cases the trend followed is very close. In contrast the trend of this type of vitrinite differs considerably from that of perhydrous vitrinites which are affected by hydrocarbon impregnation. Only for the most severe treatments (450/500 °C) was some degree of structural uniformity observed in the solid residues.

This work analyses a set of perhydrous coals (mainly composed of huminite/vitrinite maceral group) in order to determine the inter-relations between the hydrogen content and the modifications in the coal structure at a molecular level. The study involves the direct solid state characterisation of the coal combined with the analysis of representative fragments of the coal network obtained through flash-pyrolysis. The perhydrous character of the coals is not reflected either in the aliphatic hydrogen concentration (from FTIR data) or by the presence of straight-chain aliphatic moieties in the pyrolysates. This structural study shows that perhydrous coals contain mainly aromatic structures with 1–2 rings and a very small concentration of aromatic rings of large size. In agreement with this, phenol and alkyl phenols are the most prominent degradation products whereas other aromatic compounds (mainly benzene and naphthalene derivatives) are minor and probably evaporative compounds. The major structural elements in the samples studied are simple phenols with a preponderance of substituted para alkyl. The results obtained show that the processes of hydrogen enrichment affect the reactions of aromatisation and condensation. During the natural evolution of the perhydrous coals the transformations of the oxygenated functionalities in the lignin precursor seem to have taken place without the parallel structural reorganisation of the lignin framework responsible for the formation of polycyclic aromatic systems. As a result, the chemical structure of perhydrous vitrinites in coals is substantially modified with respect to that described in ‘normal' coals. The results obtained also indicate that the source of hydrogen content and the effect that it has during the subsequent evolution process of the coals, affects the chemical structure of the perhydrous vitrinite and hence its properties and behaviour., Financial support for this work was provided through a contract with the European Community (No. 7220/EC-769).

The present work analyses the changes in the chemical structure of a perhydrous coal during its thermal evolution at different temperatures in an open-medium pyrolysis system. The results obtained were compared with those described for non-perhydrous coals in order to establish the effect of the substances assimilated by the coal structure (hydrocarbon/oil-like substances) on the thermal evolution of the coal. The transformation ratio at each stage of thermal treatment was determined and the chemical-structural characterisation of the resultant products was performed. Changes in textural and microtextural properties associated with structural modifications during the evolution were also tested. The results obtained show that this perhydrous coal develops a specific evolution pathway different from that followed by non-perhydrous coals with a normal H/C ratio. The substances assimilated by the perhydrous coal cannot be easily and totally released from its structure so that they can be only partially removed after thermal treatment. Thus, the treatment debilitates the interactions between the substances and the coal matrix in addition to weakening and cracking the matrix during the thermal process. The increase in temperature also leads to the conversion of some of the heavy assimilated substances into lighter compounds. However, the assimilated substances are present even at high temperatures of the thermal process, providing hydrogen which has the effect of stabilising the radicals originated during the pyrolysis and improving the fluidity properties in the reacting medium., Financial support for this work was provided through a contract with the European Community (No. 7220/EC-769).