Macromolecular Structure Of Coal Research Articles

Liquefaction behaviour of solvent-swollen coals

Batch autoclave experiments show that pre-swelling of bituminous and lower rank coals with a suitable solvent, followed by removal of the swelling agent, significantly enhances conversion and product quality during subsequent liquefaction. The enhancement depends on coal rank and swelling agent and is a maximum for a bituminous coal. The beneficial effect of swelling may be due to the expansion of the coal macromolecular structure, making it more accessible to hydrogen donor solvent. The enhanced accessibility may enable prompt capping of the coal free radicals with concomitant reduction in retrograde reactions and facilitate solvent-induced bond cleavage reactions. These effects are reflected in high liquefaction yield and improved product quality. In addition to enhancement in liquefaction yields, swelling also obviates the need for fine grinding of coal, thus reducing grinding costs. For example, without any pre-swelling the − 325 fraction of a subbituminous coal gives higher conversion and product quality than the 8 × 70 fraction. In contrast, after pre-swelling both 8 × 70 and − 325 mesh samples gave the same conversion and product quality.

Anomalous Volume-Strain Relationships in Bituminous Coals

Most bituminous coals are optically anisotropic, a phenomenon that results from reversible strain of coal's macromolecular structure. Although this strain is reversible, it is known to be locked in place via intermolecular hydrogen-bonding interactions between phenolic and carboxylic acid functional groups. Thus, appropriate agents are necessary to relax the strained macromolecular structure. Such agents typically are basic organic solvents capable of neutralizing the acidic oxygen containing functional groups. Within coal-forming sedimentary basins, the nature of the external stress field responsible for the strain of coal is simple. Assuming the tectonic stresses are minimal, the stress regime is uniaxially compressive. Under such a stress field, a reversibly strained elastic body naturally is expected to lose volume. Therefore, upon relaxation, uniaxially strained coals would be expected to regain their original volume. When lower rank bituminous coals are relaxed in appropriate solvents, however, the net volume change is negative and they lose volume. This seemingly anomalous larger (strained) molar volume was presumably sustained in the presence of the lithostatic stress field by the presence of volatile (light hydrocarbons, CO{sub 2}, and/or water vapor) trapped within the macromolecular network. Data on the volume-strain relationships of two coal suites from different sedimentary basins and acrossmore » regional coalification gradients aids in illustrating potential mechanisms of volatiles storage as well as possible causes leading to the release of trapped volatiles.« less

Aspects of sporinite chemistry

With the recent advent of the ability to separate coal into maceral concentrates of high purity, the individual constituents of coal can now be analyzed separately, without their mutual interference, giving a much better understanding of the macromolecular structure of coal. The sporinites from two Pennsylvanian age coal samples (Illinois Basin, U.S.A.) were studied, one from a vitrinite-rich high-volatile bituminous coal, the other from a liptinite-rich high-volatile butuminous coal of slightly higher rank. Sporinites were isolated from each coal by density gradient centrifugation. The sporinite of the vitrinite-rich coal was compared chemically and petrographically with the parent coal and with the sporinite of the liptinite-rich coal. The fluorescence spectrum of the sporinite from the liptinite-rich coal is shifted to the red end of the spectrum, which may be accounted for by the somewhat higher rank of the sample and/or by differences in the original assemblage of spores. The lack of chemical differences between the extracts of the sporinite and its whole coal reinforce the concept of bitumen as an homogeneous mobile phase pervading the coal. Thus, extract chemistry seems an unsuitable technique for distinguishing between macerals from the same coal. Hopane and sterane distributions in the sporinite and parent coal pyrolyzates are very similar, but the two materials can be readily distinguished by the distribution of tetracyclic diterpanes of the phyllocladane type, which are biological marker compounds derived from higher plant material. Overall, the sporinite is considerably more paraffinic in character and has a greater preponderance of straight-chain alkane moieties than the coal as a whole. In the case of the vitrinite-rich coal, the whole-coal structure appears significantly more polyaromatic than the sporinite. The distributions of thiophenic compounds differ in the pyrolyzates of the two materials. The sporinite from the liptinite-rich coal is even less polycondensed than the sporinite from the vitrinite-rich sample. The chemical and petrographic differences of the two sporinites probably reflect the different assemblages of spores in the original peats and their different diagenetic histories.

Influence of cross-linking on tar formation during pyrolysis of low-rank coals

In this paper the pyrolysis behaviour of two Spanish low-rank coal in the 275–600° C range is studied. Gases evolved from pyrolysis were analyzed by on-line Fourier transform infrared spectroscopy (EGA-FTIR) and gas chromatography and the changes in the macromolecular structure of coal by a solvent swelling technique. The studied coals cross-link very early and the differences found in their tar yields can be explained in terms of cross-linking reactions. Cross-linking of chars correlates with the evolution of CO 2 and pyrolysis water and the extent of the cross-linking reactions is related to the coal's initial content of labile functional groups, namely carboxyl and hydroxyl groups.

石炭の分子構造に基づく急速熱分解モデル

Recently statistical models of flash pyrolysis based on the macromolecular coal structure have been proposed. While traditional models based on lumping kinetics have little flexibility for the variety of coal ranks and reaction conditions, statistical models can predict the variation of tar yield and molecular weight with pressure and heating rate. However, the relation among the coal structure and chemical reactions (degradation and crosslinking reaction) is not fully understood. Advanced pyrolysis models which provide enough information to control pyrolysis reaction of coal should be created in near future.

Effect of demineralization on the macromolecular structure of coals

It appears that HCl/HF demineralization has little, if any, effect on the macromolecular structure of coals apart from those due to ion exchange and the removal of inorganics. Six coals varying in carbon content between 69% and 86% were demineralized in HCl/HF, dried, and extracted with pyridine, and the amount and molecular weight distribution of the pyridine extracts were determined. The observed alterations in the amounts and molecular weight distributions of the extracts from these three coals due to demineralization are probably due to solubility changes caused by converting acid salts to acids by ions exchange

Macromolecular and chemical changes induced by air-oxidation of a medium volatile bituminous coal

A medium volatile bituminous coal was oxidized in air at 200 °C. The oxidized coal was characterized by its swelling properties, infrared spectra, thermogravimetric analysis, and optical microscopy. Examination of the oxidized coal indicated that carboxyl and hydroxyl groups are formed during oxidation of alkyl and cycloalkyl cross-linkages which have been identified as largely responsible for the macromolecular structure of coal. Treatment of the oxidized coal with pyridine broke weak hydrogen bonds, but bases with higher pK b values were needed to attack stronger hydrogen bonds. Extraction of the oxidized coal with dilute sodium hydroxide yielded characteristic coal humic acids that contained no pyrite. Yields of humic acids, rates of humic acid formation, some properties of the humic acids and results of examination under optical microscopy of the acids and the oxidized coal are reported.

Solvent induced swelling of coals to study macromolecular structure

The swelling properties of a rank range of eight coals were studied using two series of solvents, in terms of electron donor number and pK b values. It was found that swelling properties could be correlated well with pK b values, the electron donor number values were not related to coal behaviour. The swelling data are consistent with the conclusion that the mechanism of swelling of the macromolecular structure of coals in basic solvents is one of breakage of hydrogen bonds.

Stepwise chemical degradations of a UK bituminous coal

Organic materials extracted from a UK high volatile bituminous coal, its solvent extracted O-alkylation product and its solvent extracted O-alkylated, LiAlH 4 reduction product have been analysed using g.c.-m.s. to determine changes in the distributions of extractable components with increasing extent of solubilization. Changes in the distribution of functional groups in the insoluble residues have been followed after each step using FT-i.r. Although the range of compounds released at each step is similar, the distribution of extractable lipid classes and their individual components varies. Hopanes (αβ and βα), acyclic isoprenoids, n-alkanes and alkyl phenanthrenes are present in all extracts, with n-alkanes predominating. Maturity parameters based on hopane, phenanthrene and n-alkane distributions give conflicting results for the relative maturities of the three extracts, possibly due to the differential association of individual components within the coal matrix. The organic components extracted after O-alkylation and after LiAlH 4 reduction are likely to arise from the release of physically trapped material as a consequence of cleavage of specific bonds within the coal macromolecular structure.

Structure in Wandoan coal from analysis of mild hydrogenation products:: 2. Analysis of first- to third-stage hydrogenation products

Wandoan subbituminous coal (C 76.8%), after exhaustive extractions by benzene/ethanol, has been liquefied step by step under mild conditions to obtain n-hexane-soluble H1, H2 and H3 fractions. The acidic and basic free fractions were separated into saturated, aromatic and polar fractions. Further, the polar fraction has been vacuum-distilled into distillable and non-distillable polar fractions. It has been found that the corresponding subfractions (acidic, basic, saturated, aromatic and distillable polar) in H1H3 contain similar components, but the yields of the heavier components increased as the hydrogenation progressed from H1 to H3. This result suggests that the composition of coal could be non-homogenous. It is also suggested that mono- and multi-alkyl phenols may have a dominant role in the cross-linked macromolecular structure of coal.

Transport of penetrants in the macromolecular structure of coals: 7. Transport in thin coal sections

The macromolecular network structure of the organic phase of coal was investigated by dynamic penetrant swelling using pyridine vapour in thin sections of coal. The effect of the macromolecular relaxation time and characteristic diffusion time on the phenomenological and mechanistic behaviour of pyridine uptake in coal was investigated by varying the coal sample thickness. The relative importance of the relaxational and diffusion processes in describing the transport kinetics for pyridine in coal was determined using a semiempirical, exponential expression which relates its exponent, termed the diffusion exponent, to the mechanism of penetrant transport. Penetrant studies with thin coal sections from the same coal coal sample but with different thicknesses show that within the range 150–1500 μm the transport mechanism of pyridine in the macromolecular coal network was purely relaxation-controlled.

Transport of penetrants in the macromolecular structure of coals: 4. Models for analysis of dynamic penetrant transport

Mathematical models are presented which can be used to describe Fickian (Case-I) diffusion and Case-II transport in thin coal slabs, spheres and cylinders. Useful short-time approximations for each of these models are also presented. From heuristic arguments, based on these short-time solutions, a semi-empirical equation is developed, which is useful in defining anomalous transport behaviour in the macromolecular network structure of coal. The utility of this empirical equation is considered.

Macromolecular structure of coals. IV. Equilibrium swelling studies of coal networks subjected to various treatments

The macromolecular network structure of the organic phase of bituminous coals was investigated with equilibrium swelling experiments at 35° and 60°C. It was determined that an oxygen-methylation technique led to cleavage of hydrogen bonds and significant increase of swelling. Pyridine preextraction led to significant differences in swelling due to partial incorporation in the network structure.

Macromolecular structure of coals: 2. Molecular weight between crosslinks from pyridine swelling experiments

The effective macromolecular structure of ten coals was investigated by application of a modified Gaussian network equation to the results of equilibrium vapour-swelling studies using pyridine as the penetrant. Using data at 35 °C it was found that the effective number N of repeating units between crosslinks varied from 7.0 to 8.5, corresponding to a molecular weight M ̄ c between crosslinks of 990 to 1280, over the coal rank range 69.9–82.4 wt%C (dmmf). At 86.0 wt%C, N fell to 4.6, corresponding to a molecular weight between crosslinks of 780, and continued to decline to 2.2, corresponding to M ̄ c = 814 , at 91.5 wt%C. An increase in swelling temperature from 35 to 60 °C had little effect on the analysis; however, at 80 °C substantially increased equilibrium swelling values were obtained, which indicated a change in coal structure, increased thermodynamic interaction or attack on the hydrogen-bonded structure of the coal by pyridine.

Macromolecular structure of coals: 8. Viscoelastic behaviour of coal networks determined by thermomechanical analysis

The viscoelastic behaviour of certain American coals was investigated by thermomechanical and thermogravimetric analysis. Thin coal sections were tested in creep under continuous stress ranging from 0.158 to 0.790 M Pa during heating from 35 to 350 °C at l0 K min −1. Additional samples were tested in creep at constant temperature and stress, and also under cyclic stressing. The results show that the compressive strain of coals is a function of the temperature, carbon content and number of creep-recovery cycles. It is concluded that the mechanical (viscoelastic) behaviour is time-dependent (at constant temperature) and that it is related to the glassy-rubbery transition temperature and the subsequent network degradation at temperatures significantly higher than T g .

Transport of penetrants in the macromolecular structure of coals: VIII. Transport mechanism as a function of temperature

Pyridine transport in 600–850-μm coal microparticles, pre-extracted with pyridine and redried, was studied as a function of time and temperature from 35 to 300 ° C. Simultaneous thermogravimetric studies were run to determine the net weight loss due to thermal degradation. The true pyridine uptake in coals at high temperatures was thus established.

Macromolecular structure of coals, 7. Mass spectrometry of pyridine extracts and depolymerized coal

AbstractElectron impact mass spectrometry and mass ion kinetic energy spectrometry were used to analyze: (i) the structure of uncrosslinked, predominantly aliphatic chains obtained by pyridine extraction of bituminous coals; and (ii) the identity of compounds found in depolymerized coal samples produced by the technique of Hooker et al. (1986).

Transport of penetrants in the macromolecular structure of coals. V. Anomalous transport in pretreated coal particles

Dynamic swelling studies were performed on well-characterized coal particles treated in various ways. Pyridine was the penetrant and all studies were performed at 35°C. The transport behavior was compared for untreated, pyridine-extracted, oxidized and alkylated samples. Data analysis using an exponential equation of the penetrant uptake as a function of time and a relaxation-controlled analysis showed that the pyridine transport mechanism was non-Fickian or Case-II transport, and that it was only slightly affected by the pretreatment of the coal samples.

Transport of penetrants in the macromolecular structure of coals. 6. Amine transport mechanisms

Etude du transport dynamique du n-propylamine, n-butylamine, diethylamine et dipropylamine sur des echantillons de charbons non traites et extraits a la pyridine

Macromolecular structure of coals. 9. Molecular structure and glass transition temperature

Determination de la temperature de transition vitreuse d'echantillons de charbon de differentes teneurs en carbone, par calorimetrie differentielle a balayage. Les echantillons sont utilises tels quels ou extraits a la pyridine