Trapped organic compounds and aromatic units in coals

Some insight into the chemical nature of coals and the coalification process was obtained by detailed analyses of the organic constituents of three coals — a lignite, a bituminous, and an anthracite coal. Organic compounds trapped in the coal matrix, residuals and products of the original coalification process, were isolated by vacuum distillation and solvent extraction. The macromolecular material which constitutes the bulk of coals was degraded by a series of selective oxidations to smaller units which could be identified and measured. The essential aromatic character of coals was demonstrated, with condensation of aromatic rings increasing with increasing rank.     

A kinetic investigation of coal liquefaction at short reaction times

Coal liquefaction kinetics have been studied at very short reaction times (less than 250 seconds) in order to emphasize the initial underlying physical and chemical processes involved. These studies were made possible by the use of a continuous flow stirred tank reactor (CSTR) which avoids the problems of slow heat up and cool down associated with the massive equipment required for running high-temperature and high-pressure liquefaction reactions. Preliminary physical (NMR and ESR) and chemical analytical results are presented on the coal liquids and reaction residues from Illinois No. 6 hv bituminous and Wyodak Black Thunder subbituminous coals.

ESR results showed that radical concentration in the solid residue changed during coal liquefaction. These changes were accompanied by changes in the NMR-derived aromaticity. The rate of decrease of organic-based radicals was different for Wyodak Black Thunder and Illinois No. 6 coals, perhaps indicating a different mechanism for the quenching of radicals in these bituminous and subbituminous coals. NMR spectra of the liquid products indicated that the initially produced material was relatively aromatic, and that subsequent products had lower aromatic content. This is consistent with secondary hydrogenation of the primary liquefaction products. Finally, the total oxygen contents of the coal residues decreased gradually during the first three minutes of coal liquefaction at 390°C. A corresponding decrease in the hydroxyl content of these residues was also noted.     

The action of solvents on coal at low temperatures: 2. Medium- and high-rank coals

The dissolution behaviour of three medium-rank and three high-rank coals in various solvents has been studied. Aqueous KOH is a non-solvent up to 180 °C. Dissolution of medium-rank coals into aliphatic amines and pyridine is not influenced by mass transfer effects in the 120–180 °C temperature range-although it is at room temperature-and does not involve the breaking of chemical bonds. This sets them apart from low-rank coals, where ester bonds have to be broken before dissolution can take place. An extraction limit of ≈50% seems to exist. Of the solvents studied, nonoethanolamine is the best for low-rank bituminous coals but rather poor for higher-rank coals, while the reverse is true of pyridine. Extract yields in hexylamine and ethylenediamine remain approximately constant in the medium-rank range. Low-volatile bituminous coals and semi-anthracites still contain some soluble matter, the extraction of which is diffusion-limited, at least up to 180 °C. From the results a hypothesis concerning the basic physical constitution of coal is derived and a model of the coalification process rationalizing the conjectured changes in constitution with rank is suggested.     

Steric requirements for coal swelling by amine bases

A set of six coals ranging in rank from lignite to hvA bituminous was swollen with a series of alkyl-substituted pyridines and a smaller set of 4-alkylanilines. The size and branching of the alkyl groups was varied and the effect of this variation on the dissolution of the amines in the coal and the resulting coal swelling was measured volumetrically. In a few cases, substituents which hindered the amine nitrogen were studied. The lignite and subbituminous coal have a much higher tolerance to branched, bulky groups than do the bituminous coals. The presence of tertiary groups in a solute strongly inhibits their dissolution in bituminous coals. Bituminous coals behave as if extensive parallel packing of structures occurs. Often, they can accept very large planar groups but have a low capacity for branched groups.     

Interrelationship of chemical characterization and rheological behaviour of coal liquefaction bottoms

In the development of the Exxon Donor Solvent (EDS) process, bituminous and subbituminous coals have been processed in a one ton-per-day coal liquefaction pilot plant using the feed coal slurried with solvent and with or without bottoms recycle. The liquefaction bottoms from both once-through and bottoms recycle operation exhibit non-Newtonian viscometric behaviour. The recycled bottoms, however, are more viscosity/shear dependent, less viscosity/temperature dependent and more thermally stable than the once-through bottoms. Chemical characterization of these bottoms reveals that alkyl and phenoxy groups are important functional groups responsible for the viscometric behaviour of bottoms. The increase of bottoms viscosity is postulated to involve the elimination/condensation of methylene units are phenolic functional groups to form the crosslinkages of large aromatic clusters.     

Solubilization of coals by reductive alkylation

Anthracite, bituminous and subbituminous coal when treated with naphthalene anion in tetrahydrofuran added negative charges to form the corresponding coal anions. Alkylation of bituminous and subbituminous coal anion with ethyl iodide resulted in the addition of 16 and 14 ethyl groups per 100 carbon atoms. The alkylated coals were 88 and 45% soluble in benzene. The molecular weights of the benzene-soluble portions of the bituminous and subbituminous coal were respectively 2000 and 700. An attempt to add alkyl groups to anthracite anion was not successful.     

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.     

Acidic groups in coal and coal-derived materials

The application of non-aqueous titrations to coal and coal-derived liquids is described to distinguish and determine carboxylic and phenolic groups in these materials. Such titrations have been applied to two coals, a subbituminous and a bituminous coal. For the subbituminous coal the amount of phenolic groups estimated from non-aqueous titrations is only about 30% of that estimated from acetylation studies, whereas both these methods give similar results for the bituminous coal. The lower values obtained by non-aqueous titrations, for the subbituminous coal, are due to proximal phenolic groups which are only partially titratable and alcohol groups which are non-titratable.     

Ignition behaviour of different rank coals in an entrained flow reactor

An experimental study to determine the temperature and mechanism of coal ignition was carried out by using an entrained flow reactor (EFR) at relatively high coal feed rates (0.5 g min⁻¹). Seven coals ranging in rank from subbituminous to semianthracite, were tested and the evolved gases (O₂, CO, CO₂, NO) were measured continuously. The ignition temperature was evaluated from the gas evolution profiles, and it was found to be inversely correlated to the reactivity of the coal, as reflected by the increasing values of the ignition temperature in the sequence: subbituminous, high volatile bituminous, low volatile bituminous and semianthracite coals. The mechanism of ignition varied from a heterogeneous mechanism for subbituminous, low volatile bituminous and semianthracite coals, to a homogeneous mechanism for high volatile bituminous coals. A thermogravimetric analyser (TGA) was also used to evaluate coal ignition behaviour. Both methods, TGA and EFR, were in agreement as regards the mechanism of coal ignition. From the SEM micrographs of the coal particles retrieved from the cyclone, it was possible to observe the external appearance of the particles before, during and after ignition. The micrographs confirmed the mechanism deduced from the gas profiles.     

Liquefaction of subbituminous coals with a basic nitrogenous model process solvent

Liquefaction reactions in a tubing-bomb reactor have been carried out as a function of coal, coal sampling source, reaction time, atmosphere, temperature, coal pre-treatment, SRC post-treatment and process solvent. Pyridine as well as toluene conversions ranging from 70 to > 90 wt% involving both eastern bituminous and western subbituminous coals are obtained. 1,2,3,4-Tetrahydroquinoline (THQ) has been extensively used as a process solvent under optimized liquefaction conditions of 2:1 solvent: coal, 7.5 MPa H₂, 691 K and 30 min reaction time. Comparisons of THQ with other model process solvents such as methylnaphthalene and tetralin are described. Liquefaction product yield for conversion of subbituminous coal is markedly decreased when surface water is removed from the coal by drying in vacuo at room temperature prior to liquefaction. The effect of mixing THQ with Wilsonville hydrogenated process solvent in the liquefaction of Wyodak and Indiana V coals is described.     

Extraction of coals with CS2-N-methyl-2-pyrrolidinone mixed solvent at room temperature Effect of coal rank and synergism of the mixed solvent

Coals (from lignite to anthracite) were extracted at room temperature with CS₂-N-methyl-2-pyrrolidinone (MP) mixed solvent (1:1 by volume), which was found to be a very efficient solvent for the extraction of bituminous coals in a previous study. High yields of 30–66% (daf) were obtained for 29 of the 49 bituminous coals (C%76.9–90.6% daf) examined. The anthracite, subbituminous coals and lignites did not give high yields. The results of the characterization of the raw coals, extracts and residues suggest that reactions between the coals and the solvents do not occur to a significant extent during the extraction. The synergistic effect, i.e. the large increase in yield and rate for the mixed solvent compared with those for CS₂ and MP alone has been explained by increasing solubility and diffusibility of the extracts and increasing swelling of the coals, in the mixed solvent. The mixed solvents of CS₂ with quinoline, pyridine and THF gave lower extraction yields than the CS₂-MP mixed solvent.     

Ignition characteristics of coal blends in an entrained flow furnace

Ignition tests were carried out on blends of three coals of different rank - subbituminous, high volatile and low volatile bituminous - in two entrained flow reactors. The ignition temperatures were determined from the gas evolution profiles (CO, CO₂, NO, O₂), while the mechanism of ignition was elucidated from these profiles and corroborated by high-speed video recording. Under the experimental conditions of high carbon loading, clear interactive effects were observed for all the blends. Ignition of the lower rank coals (subbituminous, high volatile bituminous) enhanced the ignition of the higher rank coal (low volatile bituminous) in the blends. The ignition temperatures of the blends of the low rank coals (subbituminous-high volatile bituminous) were additive. However, for the rest of the blends the ignition temperatures were always closer to the lower rank coal in the blend.     

Solvent treatment of coals: 1. Effects on microporosity at ambient temperature

The effects of extraction with thirteen solvents and three solvent mixtures at ambient temperature on the surface areas of three bituminous coals were investigated. The total surface area of each coal, measured by CO₂ at 298 K, was substantially increased by extraction with pyridine, ethylenediamine, methylpyrrolidinone and THF. The maximum surface area attained was nearly the same for the three coals—evidence for a similar, highly crosslinked macromolecular network type of structure in all three coals. Given enough time, the mixed solvents were also effective in increasing the total surface area when the ‘solubility parameter’ of the coal was equal to the ‘solubility parameter’ of the mixture. None of the solvents increased the surface area measured by nitrogen significantly, indicating that the pores opened by the extraction of soluble pore material were < 0.5 nm in size. All the effective solvents satisfied the donor and acceptor number requirements for extraction of pore material based on the donor-acceptor model of coal extraction.     

Reactivity of hydrolysed coals in liquefaction

A hvA bituminous, a subbituminous and a lignite coal have been hydrolysed by 20–30% aqueous caustic solution at 100–300 °C and total pressure from ambient to 8.3 MPa (1200 psi). Reactivity of these pretreated coals toward liquefaction has been examined. The conversion to benzene-soluble material (BS) and oil increases, and the preasphaltene and char residue decreases after pretreatment. Improvement in the conversion to the BS fraction is only marginal for the pretreated bituminous coal, but substantial for the low-rank coals. For the subbituminous coal, the liquefaction reactivity (conversion to BS) increases with the severity of hydrolysis pretreatment. Analyses of chemical compositions, ¹H n.m.r. nuclei distributions and hydroxyl concentrations of the acid-insoluble hydrolysis coal extracts indicate that both O and S are enriched in the extracts with half of the oxygen atoms being in hydroxyl forms. The hydroxyl concentrations of the extracts (acid-insoluble) are ≈2 to 3 times higher than their parent coals. Coal activation by this alkali pretreatment is explained by the hydrolytic attacks on ether C–O linkages, and the removal of some constituents rich in oxygen functional groups which are responsible for poor liquefaction behaviour.     

Short contact-time liquefaction of subbituminous coal: Directions for improving coal conversion

This study was undertaken to find ways to solubilize subbituminous coal in high yield at short contact times. Short contact-time hydroliquefaction runs were carried out in a continuous unit using Belle Ayr subbituminous coal with a solvent rich in hydrogen-donor molecules derived from the Lummus ITSL process, and also with solvents lower in donor concentration derived from the SRC processes. Catalysis by pyrite, molybdenum and supported cobalt-molybdenum was also studied. It was found that pyrite and other hydrogenation catalysts enhanced solubilization and also increased production of distillate oil. Solvent quality seemed to have no effect on the solubilization of Belle Ayr coal. The availability of H₂S also appeared to have no effect. Provided that pyrite was present, reaction could be carried out at severities high enough to give insoluble organic matter yields equivalent to those obtained with bituminous coals (5–10 wt% daf coal). At equivalent levels of solubilization, however, hydrocarbon gas and distillate yields were invariably higher for the subbituminous coal. Implications for two-stage processing of Belle Ayr coal are discussed.     

Characterization and classification of Rocky Mountain coals by Curie-point pyrolysis mass spectrometry

A set of 102 coal samples from the Rocky Mountain coal province, selected from the Penn State Coal Sample Bank, was analysed by Curie-point pyrolysis mass spectrometry in combination with computerized pattern recognition techniques. The spectra obtained were shown to be quite representative for the coal seams with characteristic differences often present between different seams, fields or regions within the Rocky Mountain Coal province. In general, the spectra were found to be dominated by homologous ion series, e.g., representing dihydroxybenzenes, phenols, naphthalenes, benzenes, alkenes, dienes, alkyl fragments and sulphur compounds with varying degrees of alkyl substitution. The relative abundances of dihydroxybenzenes and naphthalenes were shown to correlate closely with differences in rank, whereas those of phenols, aliphatic hydrocarbons and sulphur compounds appeared to correlate more closely with differences in depositional environment. Different spectra -dominated by aliphatic compound series -were obtained from several samples of a boghead coal (Cannel King seam). Moreover, spectra of two of these boghead coal samples, known to be severely weathered, showed markedly increased CO₂⁺ and C₆H₆⁺ signals, indicating the presence of benzenecarboxylic acids. Factor analysis of pyrolysis m.s. data revealed the two main underlying chemical tendencies to be a shift from heteroatomic compounds to hydrocarbon series with increasing rank and a difference in degree of aromaticity corresponding primarily to differences in depositional environment. The dominant rank-related factor exhibited a clear coalification break between the ASTM hvC bituminous and hvB bituminous ranks and appears to represent a significant shift in coalification mechanisms. It was demonstrated that rank-dependent differences in the pyrolysis mass spectra enable correct classification of the spectra into four ASTM rank classes (subbituminous, hvC, hvB and hvA bituminous) in 90% of all cases. Moreover, the discovery of a marked aliphaticity/aromaticity factor in the data could be useful for the direct measurement of aromaticity (f_a) from pyrolysis mass spectra of whole coals.     

Liquefaction of partially dried and oxidized coals: 2. Coal characteristics

Samples of Belle Ayr (WY) subbituminous and Powhatan No. 5 mine (Pittsburgh Seam) bituminous coals were dried with gases including nitrogen, air and nitrogen/air mixture at temperatures essentially from ambient to 200 °C as part of a study to measure liquefaction behaviour. While the oxygen uptake of the subbituminous coal was up to 3 wt% and that of the bituminous coal was up to 8 wt%, the physical and chemical characteristics of both coals appeared to undergo only minor changes during treatment. However, CPAA oxygen analysis showed an apparent reaction of water with the coal, and FT—i.r. spectral results showed the formation of carbonyls and carboxylic acids with an indication of the formation of ethers.     

Fate of aliphatic groups in low-rank coals during extraction and pyrolysis processes

Information on the nature of aliphatic groups in some bituminous coals and lignites was obtained by determining their fate during extraction and pyrolysis processes of differing severity. Aromatics (neutral oils) and asphaltenes from supercritical gas and hydrogen-donor solvent extracts and from pyrolysis and hydropyrolysis tars have been characterized by an n.m.r.-based structural analysis method which identifies hydroaromatic, methyl and long alkyl (?C₈) groups. The results indicate that methyl and other alkyls account for about half of the aliphatic carbon, long alkyl chains being the major aliphatic group in the lignites. There is evidence to suggest that some of the long alkyl chains are joined to aromatic structures. Hydroaromatic groups are small consisting of only 1–2 rings and account for less of the aliphatic carbon in bituminous coals than previously thought. Their concentrations and those of long alkyl chains in the aromatics and asphaltenes generally decrease with increasing process severity.     

Chemical structure of heavy oil from coal hydrogenation 1. Hydrogenation with zinc chloride catalyst

Oil product from the hydrogenolysis of a high-volatile bituminous coal was separated by solubility, fractionated by gel permeation chromotography and characterized by structural analysis. The average structural unit in the hexane-soluble, aromatic oil fraction consists of 1–3 aromatic rings with 0.3-0.5 of the ring carbons substituted by alkyl groups and oxygen containing groups. Molecular weights vary from 200 to 500. The larger molecular weight fractions have longer alkyl chains and lower carbon aromaticities. The molecules are mainly of single unit structures. The average structural units in asphaltene fractions contain from 2.5-4 aromatic rings, are of higher carbon aromaticities and contain shorter alkyl groups. The asphaltene molecules consist of two or more structural units, crosslinked together, and have molecular weights of 300–1400. The oxygen content of the fractions decreases with decreasing molecular weight. Increasing the amount of ZnCl₂ catalyst during hydrogenolysis resylts in an increased yield of lower-molecular-weight material, but no change in the structural properties of the product. This is interpreted to mean that ZnCl₂ is active in the scission of covalent bonds between structural units during liquefaction and that the hydrogenolysis reaction is mostly cleavage of crosslinks between structural units with minimal reaction of the units themselves.     

The associative nature of lower rank coal

Initial volumetric swelling in tetrahydrofuran of pyridine-unextracted parts from subbituminous coal and lignite showed no dependence of their concentration, and was smaller than that of their pyridine extracts. These results are opposite to those obtained from high volatile bituminous coals and coincide with predictions for the cross-linked network model of coal. However, when ionic forces in these coals were reduced by acid washing or O-alkylation, these coals showed the same associative nature as did high volatile bituminous coals. Swelling kinetics were analysed on the basis of associative equilibria controlled by the ionic forces. It was concluded that solvation of the ionic forces was the rate determining step of volumetric swelling of lower rank coal rather than solvent diffusion into the coal, although diffusion has been proposed to be the most important factor in swelling.