Chemistry of solvents in coal liquefaction: Quantification of transferable hydrogen in coal-derived solvents

Hydrogen was evolved as hydrogen sulphide when coal-derived solvents for liquefaction were heated with sulphur (dehydrogenation method) and their naphthene contents were quantified by titration and ¹³C n.m.r. analysis to estimate the amount of transferable hydrogen from hydroaromatics present in the solvent. Examination of synthetic solvents consisting of model compounds confirmed the validity of both approaches. The content of transferable hydrogen, thus measured, in the various solvents correlated well with their liquefaction activities using Morwell brown coal. This suggests that the sufficient stabilization of radical fragments derived thermally from the coal at the initial stage of its liquefaction leads to high conversion. It was also shown that the dehydrogenation method was applicable to non-distillable heavy fractions of coal-derived liquids such as SRC which are difficult to measure by n.m.r. because of their limited solubility.     

Hydroliquefaction of Wyodak coal using bottoms recycle

Wyodak coal has been liquefied using recycle solvents consisting of blends of Wyodak coal-derived distillates and SRC or SRC oils, asphaltenes and oils plus asphaltenes. Whilst the quality of the distillate portion of the bottoms recycle is maintained by hydrogenation and distillation in the Exxon Donor Solvent (EDS) process, no reported efforts have been made to hydrogenate the nondistillable portion of the EDS bottoms recycle solvent nor the bottoms recycle solvent in the SRC-II process. As hydrogenation of the distillate portion of the recycle solvent in the EDS process increased Wyodak coal distillate yields, this study was initiated to determine whether hydrogenation of the nondistillable portions of Wyodak coal-derived bottoms recycle solvent would show similar beneficial effects. Results suggest that distillable liquid yields in the range of 55–60 wt% of dry Wyodak coal can be obtained using mildly hydrogenated SRC or SRC oils plus asphaltenes as a bottoms recycle solvent component. This result can be compared to distillable liquid yields of 40 wt% of dry, Wyodak coal obtained from the EDS process using bottoms recycle. Further, the unhydrogenated, SRC-derived oil and asphaltene portions of the recycle solvent also appear to be effective solvent components. However, the most effective solvents were obtained using hydrogenated SRC or SRC-derived oils plus asphaltenes.     

Effects of solvent composition on coprocessing coal with petroleum residua

The effect of solvent composition on coprocessing of coal and petroleum solvents is examined under a variety of reaction conditions. The effects of solvent modification procedures on enhancing methylene chloride/methanol (MeCl/MeOH) soluble coal conversion and pentane soluble oil production are studied. Solvent modification procedures performed prior to coprocessing reactions include pentane deasphalting, catalytic hydrotreatment, H-donor addition, and blending of coal-derived liquids with petroleum solvents. Oil production and coal conversion were variously affected by the different solvent modifications. Prior hydrotreatment of petroleum solvents generally enhanced coal conversion, as did H-donor addition. The presence of a hydrotreating catalyst exerted a leveling effect on the effects of solvent modification. Blending of coal liquids and petroleum solvents resulted in complex and not readily predictable interaction. Solvents yielding the highest MeCl/MeOH soluble coal conversion were not generally the optimal solvents for pentane soluble oil production.     

Solvent degradation during coal liquefaction in a flowing-solvent reactor

We have previously observed that primary coal extracts recovered from the flowing solvent reactor appeared to be of large molecular mass. Short residence times in the reaction zone following the solubilisation of the extracts tended to limit their thermal degradation. This observation offered the vista of detailed characterisation and analysis of most (∼80-90%) of the coal mass in solution-and in a state relatively free of thermal degradation. Point of Ayr coal was extracted with tetralin and with non-donor solvents, quinoline and NMP at 350 and 450 °C. Structural evaluations have been carried out using size exclusion chromatography, UV-fluorescence spectrometry, GC-MS and probe-MS. Little coal derived material could be found in the pentane-soluble part of the reaction mixture; solvent dimers and trimers were prominent and coal-derived components, such as alkanes, were only minor components. In attempting to characterise the main, pentane-insoluble fraction of the coal extract, the level of interference from solvent-derived material emerged as the decisive parameter for the success or failure of the general method. However, the predominantly pentane-insoluble coal extracts from each solvent were contaminated by solvent polymerisation products. For all three solvents, the level of contamination of extracts with solvent derived material tended to interfere with the detailed characterisation/analysis of material extracted from the coal.     

Size and shape of a coal asphaltene studied by viscosity and diffusion coefficient measurements

Hydrodynamic properties, such as intrinsic viscosity and diffusivity, were measured to investigate the solution state characteristics of coal-derived materials in three organic solvents: toluene, pyridine, and N-methyl-2-pyrolidinone. A coal-derived asphaltene was prepared using coal liquefaction residue and was used as the solute. The translational diffusion coefficient of the coal-derived asphaltene was determined using the Taylor dispersion technique. The sphere equivalent hydrodynamic radius of the coal asphaltene was estimated to be 0.6–0.8 nm, depending on the solvent. The combined intrinsic viscosity and diffusion coefficient data suggest that the shape of coal asphaltene can be modeled as a prolate ellipsoid.     

Coal solvolysis with a series of coal-derived liquids

A systematic study of the effectiveness in coal conversion of two coal-derived solvent series, hydrogenated creosote oils and SRC recycle solvents, is characterized by spectroscopic measurement of their hydrogen distribution. Optimum ranges are found for the type and amount of hydrogen contained in the solvents. The results indicate that there are solvent properties, other than those studied in this paper, which may have an important effect on the effectiveness of a particular solvent for coal dissolution.     

The co-processing of coal with heavy feedstocks

When coal is processed with bitumen, heavy oils and petroleum residua there is a technological and economical advantage over conventional liquefaction processes. “Current coal” liquefaction concepts use, as vehicle, a coal-derived oil to react with coal, but because two-thirds of the coal-derived oil must be recycled, the throughput efficiency of these processes is low. In contrast, using bitumen, heavy oils and residua as vehicles to process coal is a “once-through” process. The data show that bitumen, heavy oils, residua and related liquids can be used to liquefy coal, and that the extent of liquefaction and the yields of distillable oils vary and depend upon the type of solvent and process conditions. Some of the co-processing options involve catalysts made of transition metals, such as nickel, cobalt, iron and molybdenum. However, the development of better catalysts and additives should be pursued to increase the production of high quality liquid fuels.     

Fate of solvents in the non-catalytic depolymerization of Kentucky bituminous coal

To study the fate of the solvents in the non-catalytic liquefaction of Kentucky bituminous coal, a number of batch liquefaction experiments have been carried out using hydrophenanthrenes and coal-derived hydrogenated recycle solvents. Filtrates from the first runs have been rehydrogenated and used as solvents for additional liquefaction experiments. The reactions have been followed by detailed analysis of the feeds and products.At constant temperature and reaction time, the conversion levels are affected by the amount of transferable hydrogen, H_s (up to a value of about 0.6 wt.% of the feed), by the coal concentration and, to a lesser extent, by the pressurizing gas.The solvents are depleted by isomerization, cracking, and adduction-polymerization reactions. Only about 50% of the initial solvent is available for use in the second-pass runs. In addition, the quality of the second-pass solvents is lower than that of the initial solvents due to a decrease of 6-membered hydroaromatic rings and an increase of 5-membered hydroaromatic rings which are poor hydrogen donors.     

Stability studies of coal-derived liquids

The ageing characteristics of liquid products derived from the hydrogenation of coal have been studied. Viscometric, spectrometric, ultimate analysis, solvent separations and oxygen consumption measurements have been employed to monitor the storage properties of the coal-derived liquids under a variety of environmental conditions. These studies show that the coal-derived liquids are very susceptible to oxidative degradation. Rates of this degradation process, based on viscosity measurements, are related to the initial viscosity of the coal-derived liquid, storage temperature and concentration of oxygen in the gaseous environment over the liquids. Analysis of aged samples show the components of higher molecular weight with functional components are most susceptible to the oxidation process. Although the total coal-derived liquids are more susceptible to oxidative degradation than the petroleum-derived boiler feedstocks, removal of the most reactive components from the coal-derived liquids, by solvent separation, results in a fuel that has analogous stability characteristics as the petroleum products.     

Dissolution of subbituminous coal in tetrahydroquinoline

Two different samples of Wyodak subbituminous coal from the Powder River Basin in Wyoming were liquefied in a two dm³ batch reactor using 1,2,3,4 tetrahydroquinoline, THQ, as a solvent. Sufficient sample was produced to determine product boiling ranges by distillation and to measure THQ distribution in the product. Product distillation showed that even at cyclohexane conversions greater than 50%, net distillate yields produced using THQ as a solvent were negative. In some cases, high boiling, coal-derived resid yields were greater than the dry coal charged to the reactor. These observations have been attributed to THQ losses resulting from dimerization of the THQ and reactions between THQ and coal derived components.     

Reactivity of cycloalkanes during the solvent extraction of coal

The reactivity of cycloalkanes, either alone or as part of a solvent mixture, during the solvent extraction of coal at 430 °C has been studied. When used with polyaromatic compounds (? 3 rings) as solvents, cycloalkanes participate in hydrogen donation reactions giving rise to high extraction yields of the coal.     

Characterization of catalytically hydrotreated coal liquid produced by solvolytic liquefaction in petroleum asphalt

A hydrotreated coal liquid produced by solvolytic liquefaction of a feed originally consisting of 66% petroleum asphalt and 34% coal liquid was characterized. The hydrotreatment was carried out over a CoMo catalyst in order to upgrade moderately and selectively the coal-derived fraction. The present process was found very effective to depolymerize rather selectively the coal-derived fraction, originally benzene insoluble, to be soluble in benzene in the dominant presence of petroleum asphalt, while 20% of the asphalt was converted to be distillable with α-methylnaphthalene (the hydrotreatment solvent). Structural analyse of the fractions before and after hydrotreatment were compared to reveal what kinds of chemical reactions had taken place during hydrotreatment. The major reactions of the coal derived fraction are hydrodeoxygenation and hydrodealkylation, both of which are effective for depolymerization. No significant hydrogenation on aromatic rings was observed. Hydrotreated liquid was further pyrolyzed to obtain oils and cokes of high quality.     

Solubility characterization of coal-derived products using pressure filtration

Studies on the solubility characterization of coal-derived products by a pressure-filtration method have been applied to SRC-I and ‘SRC-II Mode’ centrifuged liquid product — using cyclohexane, pentane or toluene as solvent. Reproducible analytical results can be obtained for these crude coal-derived products; thus the majority of coal-derived products can be classified by the pressure-filtration solubility method. In developing the procedure, tetrahydrofuran was found to be a crucial solvent in initially dispersing/dissolving the sample. Results using the above solvents demonstrated that the method is versatile, rapid, precise and gives solubility values comparable to those from Soxhlet extraction. Standard deviations of the solubility values with cyclohexane ranged from 0.4 to 0.6%; with pentane, from 0.5 to 0.7%; and with toluene, from 0.3 to 0.4%.     

Difference molecular ion spectrum method for reaction analysis of complex mixtures

A difference molecular ion spectrum method based on field-ionization mass spectrometry is proposed for the study of chemical reactions of complex mixtures such as coal liquefaction, hydrogenation treatment of coal-derived liquids, etc. The method allows reaction schemes to be elucidated and an evaluation of the activities of catalysts and the degree of hydrogenation of the liquefaction solvents.     

Effect of Wyodak coal properties on hydroliquefaction reactivity

The effect of Wyodak coal properties on liquefaction reactivity as measured by distillate yield and cyclohexane conversion has been investigated. Spot samples of four Wyodak subbituminous coals from the Anderson and Canyon coal seams in the Powder River Basin of NE Wyoming were liquefied in microautoclave and batch reactor experiments. Runs were made using two different Wyodak coal-derived solvents. Emphasis in this work was directed toward correlation of C₄-700 K distillate yield and cyclohexane conversion as functions of measurable physical, chemical and petrographic properties of the feed coal. Reactivity rankings were found to be the same using either measure of coal reactivity. However, the data indicated that distillate yields were a function of both solvent quality and feed coal properties. For each solvent studied, selected coal properties, including carbon content, total and organic sulphur content, vitrinite content and total reactive maceral (vitrinite plus exinite) content, were found to give statistically significant correlations with distillate production and cyclohexane conversion. Pyritic and sulphate sulphur contents did not appear to enhance liquefaction yield or conversion at the reaction conditions studied. However, any catalytic effects due to pyrite or sulphate sulphur may have been masked by the use of two high quality liquefaction solvents.     

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.     

溶剂对依兰煤液化反应特性的影响

不同的溶剂对煤直接液化过程产生不同的影响，其组成和性质决定了反应进行的途径。本文介绍了进行该项研究的试验装置和工艺过程，从试验结果得出了三种不同溶剂即ＨＡＯ，ＤＡＯ和循环溶剂对煤液化产物及各项收率的影响。     

催化裂化（FCC）油浆作煤直接液化溶剂的研究进展

煤制油工艺等煤炭清洁高效转化技术是能源化工领域的研究热点,溶解性好、提供/传递氢能力强且热稳定性高,其溶剂选择、使用是影响煤制油工艺经济运行的关键。本文以煤液化溶剂作用为基础,通过对液化自身产物、废塑料及FCC油浆等煤直接液化溶剂的组成、性质及作用效果的综合评述,指出煤、溶剂、氢气间的混合并非理想混合,与煤H/C适宜、极性相近的溶剂在共处理过程表现出良好的协同作用,液化过程的转化率、轻质产物选择性明显提高。分析表明,协同作用的大小取决于煤、溶剂的组成、性质匹配。煤-重质烃共处理工艺利用富芳烃油浆溶解性好、提供/传递氢能力强的特点强化了煤热解加氢反应的进行,同时煤加氢液化产生的多孔残煤具有吸附性强的特点,有助于重质烃改质,使共处理转化率显著提高、轻质产物选择性增大。最后指出,煤-重质烃共处理的协同作用为改善煤、中质/重质芳烃的综合利用提供了可能。     

Liquefaction of partially dried and oxidized coals: 3. Liquefaction results

Samples of partially dried and oxidized Belle Ayr subbituminous coal were liquefied in a recycle donor solvent (SRC-ll heavy distillate) to observe the effect of coal pretreatment on conversion. Because subbituminous coals have moisture contents typically > 25%, it would appear useful to dry these coals prior to liquefaction; however, the drying of Belle Ayr coal, either in nitrogen or oxygen-containing gases, resulted in a significant decrease in yields of liquefied coal products. The liquefaction residues recovered from these runs were examined by optical microscopy and were found to contain high levels of coke. This coke appeared to have formed by polymerization of coal-derived liquid products.     

Thermal cracking of coal-derived materials to BTX and ethylene

The vapour-phase thermal cracking (i.e. non-catalytic) of a range of coal-derived materials and pure compounds using laboratory scale reactors is described. Up to 27% benzene, toluene and xylenes (BTX) and 24% ethylene were obtained by cracking hydrogenated coal extract, compared with less than 4% of each from unhydrogenated extract, anthracene oil and coal, confirming the importance of naphthenes (cyclic aliphatics) as BTX and ethylene precursors. Experiments with pure compounds showed that product yields could be predicted closely from the cracking pattern of the components and the composition of the mixtures.