Relative reactivity of hydrogen donor solvent in coal liquefaction

Hydrogen transfer from donor solvent to coal must involve reactions such as hydrogen donation to free radicals and hydrogenation of aromatic structures. The relative reactivities of five typical hydrogen donor solvents, more reactive than tetralin, were determined using a competing elimination reaction in the liquefaction of a bituminous coal at 400 °C and a brown coal at 350 °C. 9,10-Dihydroanthracene, 9,10-dihydrophenanthrene and 1,2,3,4-tetrahydroquinoline exhibited outstanding hydrogen donating ability. Further, the relative reactivities of five mild hydrogen donor solvents such as acenaphthene and indan were determined by a similar elimination reaction using a bituminous coal at 450 °C.     

Behaviour of tetralin in coal liquefaction: Examination in long-run batch-autoclave experiments

The decomposition of tetralin in the presence and absence of coal was investigated in batch-autoclave experiments. The effect of temperature, atmosphere and reaction time on tetralin dehydrogenation, isomerization and hydrocracking was studied. At 400 and 450 °C, coal accelerates the formation of 1 - methylindan and n-butylbenzene (as primary products) changing the tetralin into compounds with reduced hydrogen donor capacity. The 1 -methylindan and n-butylbenzene are subsequently (hydro)-cracked to smaller products. At low hydrogen pressure the conversion of tetralin into naphthalene and hydrogen becomes considerable, making uncertain the calculation of hydrogen transfer from the tetralin to the coal on the basis of tetralin/naphthalene ratios.     

Pyrolysis of hydrocarbon mixtures characteristic of coal: Application to dibenzyl mixture

Thermal cracking of dibenzyl dissolved in two solvents, tetralin and decalin, has been studied in a flow reactor, in the presence of steam, under atmospheric pressure and at temperatures between 600 and 750 °C. The nature of the products obtained depends upon the structure of the hydrogen-donor agent, but is independent of the structure of dibenzyl. Valuable products such as ethylene and a benzene, toluene and xylene (BTX) mixture, obtained by a β-scission reaction with a monomolecular mechanism, are predominant when decalin is used as solvent. The dehydrogenation of tetralin to naphthalene precedes cracking reactions of the bimolecular type, which lead to significant production of hydroaromatics such as indene. Cracking of dibenzyl, followed by hydrogen transfer from the solvent to the radicals formed, leads to toluene irrespective of the chemical nature of the hydrogen donor.     

Liquefaction mechanism of Wandoan coal using tritium and 14C tracer methods: 2. Liquefaction using 3H labelled gaseous hydrogen

Tritium labelled gaseous hydrogen was used to clarify the role of gaseous hydrogen in coal liquefaction. Wandoan coal was hydrogenated under 5.9 MPa (initial pressure) of ³H-labelled hydrogen and in unlabelled solvents such as tetralin, naphthalene and decalin at 400 °C and for 30 min in the presence or absence of NiMoAl₂O₃ catalyst. Without a catalyst, liquefaction proceeded by addition of the hydrogen from donor solvent. The NiMoAl₂O₃ catalyst enhanced both hydrogen transfer from gas phase to coal and hydrocracking of coal-derived liquids. With NiMoAl₂O₃ catalyst, liquefaction in naphthalene solvent proceeded through the hydrogen-donation cycle: naphthalene → tetralin → naphthalene. The amount of residues showed that this cycle was more effective for coal liquefaction than the direct addition of hydrogen from gas phase to coal in decalin solvent. The ³H incorporated in the coal-derived liquids from gas phase was found to increase in the following order: oil < asphaltenes < preasphaltenes < residue.     

The chemical and physical structure of hydrogenation residues of maceral concentrates

The chemical structure of the residues from hydrogenation of maceral concentrates of an Australian bituminous coal have been investigated using nuclear magnetic resonance and infrared spectroscopic techniques and microscopy. It is shown that even before coal morphology is modified, aromatic substitution reactions result in loss of ether and/or phenolic groups from the coal. The results also demonstrate that the structure of residues depends markedly on the hydrogen source available for reaction. Tetralin, 1-methylnaphthalene and molecular hydrogen provide hydrogen to cap radicals, the order of efficiency being tetralin > molecular hydrogen > 1-methylnaphthalene. Although molecular hydrogen and 1-methylnaphthalene are insufficiently reactive to cause liquefaction, they reduce the degree of cross-linking of aromatic rings, thereby enhancing the plasticity of the residues.     

Effects of solvent and iron-compounds on the liquefaction of brown coal

Hydrogen-donor solvents such as hydrophenanthrene are the most effective aromatic solvents for the liquefaction of brown coal. The hydrogen-donating ability of the solvent is more important for brown coals than for bituminous coals, because the thermal decomposition and subsequent recombination of the structure of the brown coals occurs rapidly. Three-ring aromatic hydrocarbons are more effective solvents than two-ring aromatics, and polar compounds are less effective solvents with brown coals than with bituminous coals. The thermal treatment of brown coal, accompanied by carbon dioxide evolution at temperatures > 300°C, in the presence of hydrogen-donating solvent did not affect the subsequent liquefaction reaction. However, thermal treatment in the absence of solvent strongly suppressed the liquefaction reaction, suggesting that the carbonization reaction occurred after the decarboxylation reaction in the absence of hydrogen donor. To study the effect of various iron compounds, brown coal and its THF-soluble fraction were hydrogenated at 450°C in the presence of ferrocene or iron oxide. The conversion of coal and the yield of degradation products are increased by the addition of the iron compounds, particularly ferrocene, and the yield of carbonaceous materials is decreased.     

Effect of coal minerals on the thermal treatment of aromatic ethers and carbonyl compounds

To elucidate the effect of mineral matter in coal on coal liquefaction, the thermal decomposition of some model compounds of coal structure, aromatic ethers and carbonyl compounds, has been carried out in tetralin solvent and in the presence of coal ash obtained by low temperature combustion. The conversion of benzyl phenyl ether and dibenzyl ether was considerably enhanced; alkylated products such as benzyltetralin were obtained. The conversion of phenoxyphenanthrene and phenoxynaphthalene also was increased to some extent in the presence of coal ash. These effects can be attributed to the acidic components of coal minerals, because silica-alumina has shown the same effect, which is suppressed by quinoline. The addition of coal ash increases the yield of hydrocarbon from the corresponding aromatic carbonyl compounds by reduction. This effect is attributable to iron sulphide.     

Solvolytic liquefaction of coal in the presence of poly (vinylcyclohexane)

Coal liquefaction in the presence of poly(vinylcyclohexane) (PVCH) was studied at 400 °C under an atmosphere of nitrogen. PVCH promoted coal conversion better than tetralin (27.8% for tetralin, 47.2% for preheated PVCH). The effects of the initial molecular weight of PVCH, of free radicals formed during a decrease within molecular weight, and of changes in molecular structure with decomposition were examined. Conversion increased with increased initial molecular weight between 112 amu (ethylcyclohexane) and 700 amu (low molecular weight PVCH); no effect was found for molecular weights greater than 700. The best conversion was obtained when preheated PVCH was used, resulting from formation of allylic hydrogens. Aromatic structures were not found in the preheated PVCH. It is concluded that PVCH with molecular weights of ≈ 700 amu with allylic hydrogens present promoted the liquefaction of coal better than tetralin.     

Mechanisms leading to process improvements in lignite liquefaction using CO and H2S

Optimum distillate yields from US lignites can be as high on a dry, ash-free basis as those obtained from bituminous coals, but only if the vacuum bottoms are recycled. Lignites are more readily liquefied if the reducing gas contains some carbon monoxide and water, which together with bottoms recycle has proven to yield the highest conversions and the best bench-unit operability. The recycle solvent in the reported tests consisted of unseparated product slurry, including coal mineral constituents. Variability in coal minerals among nine widely representative US low-rank coals did not appear to correlate with liquefaction behaviour. Addition of iron pyrite did, however, improve yields and product quality, as measured by hydrogen-to-carbon ratio. Future improvements in liquefaction processes for lignite must maintain high liquid yields at reduced levels of temperature, pressure, and reaction time whilst using less reductant, preferably in the form of synthesis gas (CO + H₂) and water instead of the more expensive pure hydrogen. Understanding the process chemistry of carbon monoxide and sulphur (including H₂S) during lignite liquefaction is a key factor in accomplishing these improvements. This Paper reviews proposed mechanisms for such reactions from the viewpoint of their relative importance in affecting process improvements. The alkali formate mechanism first proposed to explain the reduction by CO does not adequately explain its role in lignite liquefaction. Other possible mechanisms include an isoformate intermediate, a formic acid intermediate, a carbon monoxide radical anion, direct reaction with lignite, and the activation of CO by alkali and alkaline earth cations and by hydrogen sulphide. Hydrogen sulphide reacts with model compounds which represent key bond types in low-rank coal in the following ways: (1) hydrocracking; (2) hydrogen donor; (3) insertion reactions in aromatic rings; (4) hydrogen abstraction, with elemental sulphur as a reaction intermediate; and (5) catalysis of the water-gas shift reaction. It appears that all of these reaction pathways may be operative when catalytic amounts of H₂S are added during liquefaction of lignite. In bench recycle tests, the addition of H₂S as a homogeneous catalyst reduced reductant consumption as much as three-fold whilst maintaining high yield levels when the reaction temperature was reduced by 60°C. Attainment of the high distillate yield at 400°C was accompanied by a marked decrease in the production of hydrocarbon gases, which normally is a major cause of unproductive hydrogen consumption and solvent degradation via hydrocracking. Processing with synthesis gas and inherent coal moisture using bottoms recycle and H₂S as a catalyst appears to be the most promising alternative combination of conditions for producing liquids from lignite at reduced cost.     

Aspects of the chemistry of donor solvent coal dissolution. The role of phenol in the reaction

The relative reactivity of phenol in exchange reaction with tetralin-d₁₂ and naphthalene-d₈ indicates that hydrogen exchange occurs by a free radical process rather than by an electrophilic substitution reaction at 427 °C. Substances such as benzyl phenyl ether that initiate free radical reactions increase the rate of the exchange reaction between phenol and tetralin-d₁₂ whereas weak acids have only a modest influence on the exchange. Phenol and 1 -naphthol accelerate the rates of decomposition of some ethers and amines in tetralin. These results suggest that phenolic compounds enhance thermal coal dissolution reactions in donor solvents via their influences on the rates of the homolytic cleavage of carbon-oxygen and carbon-nitrogen bonds.     

Aspects of the chemistry of donor solvent coal dissolution: Promotion of the bond cleavage reactions of diphenylalkanes and the related ethers and amines

The influences of Illinois No. 6 coal, benzyl phenyl sulphide, 9, 10-anthraquinone, tetracene, phenol and benzoic acid on the rates of decomposition of 1,2-diphenylethane, 1,3-diphenylpropane, 1, 4-diphenylbutane, benzyl phenyl ether, dibenzyl ether, N-benzylaniline, and dibenzylamine in tetralin have been investigated. Radical initiators such as benzyl phenyl sulphide enhance the rate of decomposition of the compounds that form benzylic radicals which can decompose via β-scission reactions. Weak organic acids enhance the reaction rates of the compounds, for example, benzyl phenyl ether and N-benzylaniline, that undergo the homolysis reaction via polar transition states. Among the reactive initiators considered in this Paper, benzyl phenyl sulphide is the only compound which, in small quantities, accelerates the conversion of Illinois No. 6 coal to pyridine-soluble products in tetralin at 400 °C. The facile reactions of Illinois No. 6 coal with tetralin and the accelerating effects of benzyl phenyl sulphide, anthraquinone, tetracene and phenols on the decomposition of the diphenylalkanes and the related heteroatom analogues are discussed.     

Synergistic effects between light and heavy solvent components during coal liquefaction

As part of research to examine coal conversion in solvents containing high-boiling-point components, experimental studies were carried out with model compound solvents. The dissolution of bituminous and subbituminous coals was investigated in pyrene-tetralin and 2-methylnaphthalene-tetralin mixtures. The effects of donor level, gas atmosphere, hydrogen pressure and conversion temperature were determined. At 400 °C, in the presence of hydrogen gas, pyrene-tetralin solvent mixtures show synergism in coal conversion. At donor concentrations as low at 15 wt%, the degree of conversion was almost as high as in pure tetralin. This phenomenon was not apparent in 2-methylnaphthalene-tetralin mixtures. The relative ease of reduction of pyrene and its ability to shuttle hydrogen is considered to be a principal reason for this difference in behaviour. Conversion in pure pyrene and in pyrene-tetralin mixtures at low donor concentrations increased with increasing hydrogen pressure. At 427 °C, bituminous coal conversion was higher in a 30 wt% tetralin-70 wt% pyrene mixture than in either pure compound. It was found that in the absence of coal pyrene can be hydrogenated by H-transfer from tetralin as well as by reaction with hydrogen gas. This can provide a means to increase the rate of transfer of hydrogen to the dissolving coal through the formation of a very active donor (dihydropyrene). During coal liquefaction, several pathways appear to be available for hydrogen transfer for a given coal, the optimal route being dependent upon the solvent composition and the conditions of reaction.     

The influence of hydrogen donors on the pyrolysis of a bituminous coal as studied by in situ e.s.r. spectroscopy

The effect of hydrogen gas, a hydrogen donor solvent (tetralin) and a non-donor solvent (decane) on the pyrolysis (to 500 °C) of a bituminous coal, before and after extraction with chloroform, has been studied by in situ e.s.r. in a flowing gas cell at atmospheric pressure. It was found that hydrogen gas at 1 bar had an insignificant effect on the course of the reaction, as determined by free radical population measurements, compared with nitrogen gas. In contrast, both tetralin and decane change the free radical populations developed during pyrolysis, and the extent of the induced change varies upon chloroform extraction of the coal. These results are discussed with reference to current coal liquefaction models, and are interpreted in terms of the chemical and physical interactions of the solvent with the coal.     

The nature of the synergistic effect of binary tetralin-alcohol solvents in Kansk-Achinsk brown coal liquefaction

Kansk-Achinsk brown coal hydrogenation and swelling in tetralin, in low molecular alcohols, in other solvents and in binary mixtures were studied. Tetralin was found to be the most effective liquefaction solvent, but methanol and ethanol were the active ones in coal swelling. Synergistic effects were observed when the mixtures of tetralin and methanol or ethanol were used for liquefaction and swelling. The effect of binary solvents was shown to be due to the ability of alcohol components to cause brown coal to swell improving the availability of the fragments of coal matter for the reactive hydrogen donor tetralin molecules.     

Catalytic hydrocracking of primary maceral concentrate extracts prepared in a flowing solvent reactor

Differences between the behaviour of coal macerals during liquefaction and catalytic hydrocracking were investigated. The liquefaction experiments were carried out in tetralin, using a flowing solvent reactor. The extracts were catalytically hydrocracked in a micro-bomb reactor, using a commercial catalyst. Extracts and hydrocracked products were characterised by size exclusion chromatography (SEC), UV-fluorescence spectroscopy (UV-F), probe-mass spectrometry and thermogravimetric analysis. Conversions of the vitrinite and the liptinite concentrates during liquefaction were high (∼89%), while inertinite samples yielded a little over 20% extract. For inertinites, the emerging picture was consistent with high cross-link density. Liptinite was extracted less completely at lower temperatures and more slowly at high temperatures compared to corresponding vitrinites and vitrinitic coals. Long chain aliphatics released from the liptinite concentrate between 340 and 390°C appeared likely to have originated in lower-molecular mass material occluded in the sample matrix and dissolving in tetralin prior to the onset of massive covalent bond scission. SEC chromatograms showed material of larger MMs in liptinite and vitrinite extracts than in the inertinite extract. The molecular mass distributions broadened with increasing extraction temperature. Catalytic hydrocracking experiments were carried out in a micro-bomb reactor for 10 and 120 min at 440°C, under 190 bar of hydrogen. In hydrocracking, the liptinite was the slowest extract to react at short reaction times (∼10 min). However, at longer reaction times, its products showed the smallest MM-distribution. Smaller differences were observed between the chromatograms of the 10 and 120 min hydrocracked products of the inertinite extract. Differences between spectra of the three extracts would strongly suggest the presence of larger (and apparently irreducible) polycyclic aromatic ring systems, in the hydrocracked products of the inertinite extract.     

Hydroliquefaction of low-sulfur coals using iron carbonyl complexes—Sulfur as catalysts

Hydroliquefaction of low-sulfur Australian coals (Wandoan and Yallourn) was studied using iron carbonyl complexes as catalyst. The addition of Fe(CO)₅ (2.8 wt% Fe of coal) increased coal conversion from 48.6 to 85.2% for Wandoan coal, and from 36.7 to 69.7% for Yallourn coal in 1-methylnaphthalene at 425°C under an initial hydrogen pressure of 50 kg cm^?2. When molecular sulfur was added to iron carbonyls (Fe(CO)₅, Fe₂(CO)₉ and Fe₃(CO)₁₂), higher coal converions ( > 92%) and higher oil yields (>46%) were obtained, along with an increase in the amount of hydrogen transferred to coal from the gas phase (0.2 to 2.8%, d.a.f. coal basis). In the liquefaction studies using a hydrogen donor solvent, tetralin, Fe(CO)₅S catalyst increased the amount of hydrogen absorbed from the gaseous phase and decreased the amount of naphthalene dehydrogenated from tetralin. The direct hydrogen transfer reaction from molecular hydrogen to coal fragment radicals seems to be a major reaction pathway. Organic sulfur compounds, dimethyldisulfide and benzothiophene, and inorganic FeS₂ and NiS were found to be good sulfur sources to Fe(CO)₅. From X-ray diffraction analyses of liquefaction residues, it is concluded that Fe(CO)₅ was converted into pyrrhotite (Fe_1?xS) when sulfur was present, but into Fe₃O₄ in the absence of sulfur.     

Direct liquefaction of hydrolytic eucalyptus lignin in the presence of sulphided iron catalysts

The liquefaction of hydrolytic eucalyptus lignin has been studied in hydrogen donor and non-donor solvents in the presence of ferrocene and of ferrocene associated with sulphur or carbon disulphide. With the exception of tetralin, all reactions were carried out at supercritical conditions of the solvent. It was found that the yields of heavy oils increase significantly with the increase in hydrogen pressure in the non-donor solvents. The oil yields also increase with the density of the solvent but level out at densities higher than 0.30 g/ml. The consumption of molecular hydrogen is significant in the non-donor solvents; in the donor solvents the hydrogen is mostly transferred from the solvent itself. In the non-donor solvents, the oil yields depend strongly on the efficiency of the catalyst, but not in the donor solvents.     

煤液化油中低级酚生成的影响因素研究

为了探究反应温度、反应压力、催化剂添加量以及供氢溶剂对褐煤直接液化油中低级酚生成的影响以及低级酚生成的机理,利用模型化合物邻苄基苯酚在煤直接液化条件下进行加氢反应。实验结果表明:邻苄基苯酚在液化条件下主要发生桥键断裂反应生成低级酚,苯环不易被加氢饱和。温度升高对促进邻苄基苯酚桥键断裂有利;压力升高则不利于其桥键断裂;铁系催化剂添加量的增加会促进桥键断裂;供氢溶剂四氢萘相比十氢萘会抑制低级酚的生成。邻苄基苯酚加氢液化的产物以苯酚与甲苯为主,邻甲酚与苯相对较少。     

Reactivity of some organic compounds with supercritical water

This study was initiated to determine some of the chemical reactions that occur during the supercritical fluid extraction of coal, using model compounds to simulate molecular structures found in coal. Water was chosen as the fluid because of its unique chemical and physical properties at critical conditions. Two primary functions of coal processing are the removal of heteroatoms and the depolymerization of larger molecules, thus the reactions of quinoline and isoquinoline were extensively examined, with very brief studies made of benzonitrile, aniline, tetralin, dihydroanthracene and ethylbenzene. In addition, since ZnCl₂ has been used as a hydrogenation catalyst and in the hydrocracking of aromatics, it was added in some experiments to increase the reactivity of some of the compounds. It was found that the compounds studied were more reactive and reached by different mechanisms in the presence of supercritical water than when undergoing inert pyrolysis. The product distributions from the two quinolines indicated that they reacted by very different mechanisms; possible reaction schemes for these are discussed. In addition, it was found that about 70% of the nitrogen from the consumed quinolines and aniline was removed in the water layer as ammonia, that alkyl sidechains on aromatics were somewhat reactive and that some carbon atoms are oxidized by the water thus providing a source of hydrogen for the formation of other products.     

Analysis of solvent extracts from coal liquefaction in a flowing solvent reactor

Point of Ayr coal has been extracted using three solvents, tetralin, quinoline and 1-methyl-2-pyrrolidinone (NMP) at two temperatures 350 and 450 °C, corresponding approximately to before and after the onset of massive covalent bond scission by pyrolysis. The three solvents differ in solvent power and the ability to donate hydrogen atoms to stabilise free radicals produced by pyrolysis of the coal. The extracts were prepared in a flowing solvent reactor to minimise secondary thermal degradation of the primary extracts. Analysis of the pentane-insoluble fractions of the extracts was achieved by size exclusion chromatography, UV-fluorescence spectroscopy in NMP solvent and probe mass. With increasing extraction temperature, the ratio of the amount having big molecular weight to that having small molecular weight in tetralin extracts was increased; the tetralin extract yield increased from 12.8% to 75.9%; in quinoline, increasing extraction temperature didn't have an effect on the molecular weight of products but there was a big increase in extract yield. The extracts in NMP showed the enhanced solvent extraction power at both temperatures, with a shift in the ratio of larger molecules to smaller molecules with increasing extraction temperature and with the highest conversion of Point of Ayr coal among these three solvents at both temperatures. Solvent adducts were detected in the tetralin and quinoline extracts by probe mass spectrometry; solvent products were formed from NMP at both temperatures.