Kinetic study of coal slurry electrolysis-oxidation and desulfurization of Illinois No. 6 coal by aqueous ferric chloride

The kinetics of oxidation and desulfurization of Illinois No. 6 coal with a ferric chloride solution was extensively studied by monitoring the concentration-time profiles of Fe(II) and sulfate ions formation for the temperature range 333 to 363 K. The formation rates of both ions increased significantly with increasing temperature, and were independent of coal particle size. Increasing the initial concentration of Fe(III) ion caused an increase in the formation rates of both Fe(II) and sulfate ions, while addition of Fe(II) ion retarded the rates.A kinetic model was proposed assuming that coal matrix involves two different reactive sites which are oxidized consecutively. The rate expression derived in the Langmuir-Hinshelwood form provided excellent agreement with experimental results. Desulfurization kinetics were also modeled to fit the experimental data.     

Electrolytic pretreatment of Illinois No. 6 coal

Electrolysis of Illinois No. 6 coal in acidic as well as basic electrolytes accomplishes significant amount of sulfur and ash removal under moderate reaction conditions. Electrolysis at 1.4 V vs. SCE (saturated calomel electrode) in 2 M NaCl + 9% HCl electrolyte accomplishes 62% sulfur removal and leaves behind a clean coal with the sulfur to heating value ratio of 1.44 lb S/million Btu (0.619 kg S/GJ) while coal slurry oxidation at 3.0 V vs. SCE results in 72% ash removal. Coal electrolysis in basic electrolytes accomplishes a clean residue with relatively low oxygen content. The sulfur to heating value ratio of 2.11 lb S/million Btu (0.907 kg S/GJ) is observed for coal electrolyzed in 2 M NaOH at 3.0 V vs. SCE. Impurity removal from coal is simultaneously accompanied by clean hydrogen gas production at the cathode at Faradaic coulometric efficiencies of over 95%. Hydrogen gas is produced by the depolarization of water by mineral impurities present in coal. A relatively small amount of H₂ is produced due to water splitting caused by the carbonaceous part of the coal. Model reaction pathways for coal cleaning are discussed. More work is in progress on the types of sulfur forms removed from coal.     

Oxidative weathering of Illinois No.6 coal

The exposure of freshly-mined Illinois No.6 bituminous coal (Monterey Mine No.1) to atmospheric oxygen at ambient conditions resulted in a slow oxidation reaction which appeared to be complete within two months to produce an oxidized coal product with ≈26% more organically bound oxygen than the fresh coal. An oxygen functional group analysis was performed to determine the carboxylic acid, hydroxyl and ether group content of the coal before, during and after the reaction. Infrared analysis showed no carbonyl production; however, ether functionality was being produced. O-methylation reactions which employed isotopically labelled methyl groups (both C-13 and deuterium) were used to identify and quantify O-H and CO₂H sites in the weathering coal samples. It was found that these acidic groups were not the products of oxidative weathering. Essentially all of the chemically incorporated oxygen eventually formed ether linkages. In a related study on this coal, hydroperoxide was detected as a transient intermediate in the early stages of the oxidation. A mechanism is proposed which involves the simultaneous formation under mild temperatures of a hydroperoxide and a carbon radical that undergoes a radical displacement producing the ether product directly. Solvent swelling studies revealed that these ethers act as cross-linking agents in the coal structure. The more highly cross-linked coal structure that resulted from weathering was responsible for the destruction of the plastic properties of this bituminous coal.     

Flash hydrogenation of coal 2. Yield structure for Illinois No. 6 coal at 100 atm

Hydrocarbon yields are mapped for a mine-mouth sample of Illinois No. 6 coal in 100 atm of flowing hydrogen, brought to reaction temperature at 650 °C/s. The influence of reac tion temperature was explored from 620 to 980 °C with a vapour-product residence time of 0.6 s. The effect of increasing residence time was explored at 700 °C. The only light products observed in more than trace amounts (above 1%) were methane, ethane, propane, and BTX (benzene, toluene, and xylene). Carbon balances show little if any heavier material in the product at temperatures beyond 850 °C at 0.6 s vapour-residence time or beyond a residence time of 3 s at 700 °C.     

Thermal and oxidation chemistry of coal at low temperatures

The chemical reactions which occur when Illinois No. 6 (hv C) and Rawhide (SBB C) coals are thermally pretreated at 100°C and when Illinois No. 6 coal is subsequently oxidized at 100°C with O₂ have been studied using in-situ FT-i.r. differende spectroscopy. Significant spectroscopic changes were seen. Vacuum drying at 100°C resulted in the decomposition of carboxylic acid species to form a variety of new carbonyl species (in Rawhide) and decarboxylated or decarbonylated coal (Illinois No. 6). Oxidation of predried Illinois No. 6 coal leads to the formation of new carbonyl species. The assumption that drying does not alter the chemical composition of coal may not be correct. Thus, overall spectroscopic (and chemical) changes observed in moderate temperature reaction studies may depend upon sample pretreatment, drying and storage. In addition, the time/temperature profile used in a reaction study may affect the overall changes observed by altering the relative contribution of the different reactions.     

Dihydroaromatic structure of Illinois No. 6 Monterey coal

Coal from the Monterey mine (Illinois No. 6 seam) has been oxidatively fragmented by trifluoroperoxyacetic acid. The major products were acetic, malonic, and succinic acids; benzene di, tri, tetra; and penta-carboxylic acids; and two compounds provisionally identified as the epoxides of ethylene tri- and tetra-carboxylic acids. These products are in best accord with a structure for Monterey coal in which dihydrobenzene units are frequently interspersed in a polymer composed of condensed aromatic chains and clusters. The methyl and hydroxyl substituents complete the dominant pattern.     

Ruthenium tetroxide catalysed oxidation of Illinois No. 6 coal: The formation of volatile monocarboxylic acids

Ruthenium tetroxide selectively oxidizes activated aromatic compounds under mild conditions and has been used for the catalytic oxidation of several different coals. The quantities of the volatile monocarboxylic acids with 2–6 carbon atoms have been determined by an isotope dilution technique. The amount of ethanoic acid ranges from 1 to 3mol/100C and the quantities of propanoic acid, and butanoic acid exceed 0.1 mol/100C. Methylpropanoic, pentanoic, 2-methylbutanoic, 3-methylbutanoicand hexanoic acid are formed in lesser amounts. The outcome of the experiment depends upon whether or not the coal has been extracted prior to oxidation. The results for Illinois No. 6 coal expressed in moles of ethanoic acid produced/l00C illustrate this feature: raw whole coal, 1.9; coal extracted by the method of Hayatsu and co-workers, 1.0. Dehydrogenation with benzoquinone prior to oxidation increases the yield of ethanoic acid to $\text{sol1.5mol}\text{100C.}$ The amounts of ethanoic acid obtained from the other extracted coals are 1.0 for Texas lignite, 1.2 for Rawhide subbituminous, 3.4 for Pittsburgh No. 8, 3.3 for a higher ranking bituminous coal (PSOC 726) and 0.7 for an anthracite (PSOC872). The results obtained in this study are compared with related information concerning the distribution of alkyl groups within coal by other methods.     

Carbonylation studies with Illinois No. 6 bituminous coal

Sodium metal and carbon dioxide were used to carbonylate Illinois No. 6 coal in a 1 l, high-pressure, autoclave in anhydrous tetrahydrofuran from 150 to 350 °C. Solubility of the reaction products in tetrahydrofuran and water, and the sodium uptake in the insoluble residue were determined. The reduction and subsequent carbonylation of coal produced a limited amount of solubility in these solvents, and the uptake of sodium in the insoluble residue reached a maximum of 6.29% at 250 °C with no carbon dioxide added. This corresponded to a ratio of sodium atoms per 1000 carbon atoms of 62.9, a 69-fold increase over the unreacted dried coal. Total acidity and carboxyl group content were calculated for the insoluble residues and unreacted bituminous coal. The per cent increase of carboxyl groups reached a maximum of 1.37 meq g^?1 at 2000 °C which represents a 174% increase over the unreacted coal.     

Reductive alkylation of Illinois No. 6 coal in liquid ammonia

The reductive alkylation of Illinois No. 6 coal was investigated using alkali metals and alkyl halides in liquid ammonia. Potassium is the most effective reducing agent and butyl iodide is the most effective alkylating agent for the preparation of coal alkylate that is soluble in tetrahydrofuran. The overall yield of soluble product is often improved through the reaction of the tetrahydrofuran-insoluble portion of the initial reaction products with an alkylating agent in the presence of tetrabutylammonium hydroxide. The infrared spectra of these materials suggest that the phase transfer agent catalyses the esterification of residual carboxylic acid functions. The intermolecular interactions between such acid groups and acceptor groups markedly restrict the solubility of the coal alkylate. The gel permeation chromatograms of the soluble reaction products are essentially featureless with only modest maxima at short and long elution volumes. The proton and carbon nuclear magnetic resonance spectra of the reductive methylation products, prepared using methyl-¹³C iodide, suggest that carbon alkylation exceeds oxygen alkylation and that the alkylation of phenolic groups is the dominant O-alkylation reaction. The spectra also suggest that fewer ethers are cleaved in the reaction in liquid ammonia than under the conditions of the Sternberg reaction.     

Calorimetric investigation of chemical additives affecting oxidation of coal at low temperatures

Interactions in the system “coal-chemical additive-oxygen” were studied at the temperature 30 °C with the aim of recognising inhibitors of the coal oxidation process that are able to modify the coal surface in a chemical way. Three coals of different ranks were tested. As possible inhibitors, 14 aqueous solutions (10 wt.%) of both inorganic (chlorides, sulphates, nitrates, phosphates, and sulphites) and organic (formates, acetates, urea, and thiourea) substances were examined. The action of additives on coal was evaluated from the heat effect during immersion of the coal in the additive solution. It was found that most of the studied substances proved immersing heats quite comparable with that of pure water. However, rather a chemical action was ascertained for sodium sulphite, phenol, urea and/or thiourea with heat effects several times exceeding heats for pure water. To quantify oxyreactivity of original coals as well as additive-treated ones, oxidation heats were measured by the pulse flow calorimetric method. Urea was found to be the chemically acting additive with the most significant inhibiting efficiency (up to 70%) of coal oxidation at low temperatures.     

Catalytic upgrading of coal liquid vacuum residue derived from Illinois No. 6 coal

Two-stage hydrotreatment under appropriately optimized conditions, including the selection of catalysts and solvents, is very effective for the upgrading of heavy coal liquid residues. A catalyst of high hydrogenation activity and anti-coking nature is desirable for the first, lower temperature stage. Support is to be carefully designed in terms of pore structure and acidity. The coke produced during the hydrotreatment tends to plug intergranular pores of the extruded catalyst. Grinding enhances the activity of the extruded catalyst. The removal of coke precursor and minerals by solvent extraction is also helpful for maintaining catalyst activity.     

Effects of coal cleaning on the short contact time liquefaction of Illinois No. 6 coal

This study was carried out to determine the effect of coal cleaning by oil agglomeration and sink-float methods on yields from short contact time liquefaction of Illinois No. 6 coal. The runs were made in a continuous unit using SRC-II distillates as process solvent. Measured yields included hydrogen (consumption), hydrocarbon gas, distillate oil, SRC (the pyridine-soluble portion of the residue) and insoluble organic matter, the pyridine-insoluble organic residue. The solubility of product SRC in hexane, toluene and pyridine was also determined. The principal finding was that coal cleaning by density methods reduced the yield of IOM obtained in subsequent liquefaction and this is attributed to the removal of inert components from the feed coal. In addition, cleaning which significantly reduced pyrite content of the feed coal also reduced the yield of distillate oil and tended to give a less soluble SRC during liquefaction. Deep cleaning by gravity methods gave the lowest IOM, but reduced pyrite content to the point where distillate oil was consumed rather than produced. Oil agglomeration reduced total ash to 50% of that in the run-of-mine coal, but left the pyrite level in the coal high. The relevance of these results to two-stage liquefaction is discussed.     

Supercritical toluene and ethanol extraction of an Illinois No. 6 coal

The effect of temperature, pressure, solvent/coal ratio and reaction time on supercritical toluene dissolution of an Illinois No. 6 are reported in this Paper. The experiments were conducted in a batch autoclave operating at temperatures ranging between 350 and 450 °C and pressures between 5.89 and 16 MPa. Low liquid yields in the range of 5–20% were obtained during toluene extraction. The liquid yields increase with temperature and total pressure, and only slightly with hydrogen partial pressure. Ethanol extraction gives better yields but significant solvent losses also occurred. The low liquid yield obtained can be attributed to the nature of the coal and the contacting batch mode used.     

Comparison of fractions of pyridine extract and solvent-refined coal from Illinois No. 6 coal

A pyridine extract of Illinois No. 6 coal and a solvent-refined coal from the same coal have been fractionated. Parallels between the two sets of fractions in composition and molecular weight and some experiments on phenol-pyridine interactions lead to the conclusion that solvent refining cleaves ether links and removes much of the organic 0 and S, without much change in acidic and basic groups. Gel-permeation chromatograms of the fractions indicate that they have moderately narrow molecular weight distributions. A plot of number-average molecular weights against retention times for both sets of fractions gives a smooth curve that is consistent with relations for polystyrene and poly(ethylene oxide).     

Thermal decomposition of solid oxygenated complexes formed by coal oxidation at low temperatures

Solid oxygenated complexes formed by coal oxidation play an important role in low-temperature oxidation of coal. Using an isothermal-flow reactor, the decomposition behaviour of solid oxygenated complexes was examined under pure nitrogen, at temperatures between 60 and 110 °C. The production of CO₂ and CO during thermal decomposition of the complexes was quantified by an on-line dual-column micro GC. Experiments show that the production rates of CO₂ and CO depend on temperature, but are independent of the particle size of the samples, indicating that the thermal decomposition process is dominated by chemical kinetics rather than diffusion. It was also found that the rates of formation of carbon oxides follow the Elovich equation and the activation energies for the production of CO₂ and CO are 52.1±6.3 and 72.0±5.8 kJ/mol, respectively, indicating two separate reaction pathways proceeding in the decomposition of solid oxygenated complexes.     

Reactivity of O-methylated Illinois No. 6 coal towards alkyllithiums/methyl iodide

An O-methylated Illinois No. 6 vitrain was treated repeatedly with a series of C-H indicator bases of known pK_a and quenched with ^13,14C-methyl iodide. The extent of ¹⁴CH₃ incorporation depended strongly upon base strength, increasing in the order 9-phenylfluorenyllithium < fluorenyllithium < trityllithium. A significant number of methyl groups ($\text{0.7–1.2}\text{100}$ coal carbons) were introduced with fluorenyllithium and trityllithium as the base. Additional methyl groups were added upon a second and third treatment with base and alkylating agent. After three treatments, the number of added methyl groups had doubled. The extent of base reagent incorporation was established by parallel experiments using ¹⁴C-enriched bases. CP/MAS ¹³C n.m.r. of the first coal alkylation product using trityllithium and ¹³C-enriched methyl iodide was consistent with predominantly C-alkylation. From these results, a unified picture for coal structure emerges which contains a distribution of reactive C-H sites, distinguishable on the basis of their acid-base properties. Possible reactive sub-structures are considered.     

The preparation and pyrolysis of O- and C-benzylated Illinois No. 6 coal

Illinois No. 6 coal was modified by selective benzylation reactions and the O- and C-benzyl, benzyl-d₇ and benzyl-1-¹³C products were pyrolysed in a wire screen reactor at 500–1000 °C. The char, tar and gas yields were measured and the distribution of the isotopic labels in the lower molecular weight gaseous products was determined. The results suggest that the modified coals are more reactive than the natural coal. First, the modifications increase the concentration of reactive radicals and of hydrogen donor groups. Second, the exchange patterns also suggest that the energetically more favourable reactions occur reversibly and that radical addition and recombination reactions compete favourably with fragmentation and radical substitution reactions. Third, the non-random distribution of the isotopic labels in the products indicates that the reactions in the coal particle are kinetically controlled even at temperatures near 850 °C. Fourth, the distribution of the labels in the water and ethene implies that non-radical processes contribute. Thus, theories of pyrolysis that are based exclusively on radical processes may be seriously misleading.     

Sulphur distribution in Illinois no. 6 coal subjected to different oxidation pre-treatments

The effect of different oxidation pre-treatments on the sulphur distribution in Illinois No. 6 coal (IBC-101) was studied using Atmospheric Pressure-Temperature Programmed Reduction (AP-TPR). Oxidation pre-treatments were carried out on demineralized coal with peroxyacetic acid (PAA) as a function of reaction time. For the longest reaction time the effect of PAA on the coal was compared to the effect of several other oxidants. AP-TPR results also show a clear effect of LiAlH₄ on the coal. Not only is pyrite removed, but there is also a decrease in other sulphur forms. Subsequent PAA oxidation causes an overall decrease of the AP-TPR signal, signifying the attack on organic as well as inorganic sulphur forms. With other oxidants, under the same conditions, only minor differences could be found after different treatments.     

Action of solvents on coal at low temperatures: 1. Low-rank coals

The dissolution behaviour of brown coals (67–75% C, daf) in pyridine, primary amines and aqueous KOH has been studied. The solubility in the last two solvents greatly depends on temperature, but in the first it is relatively temperature-independent. Pretreatment of the brown coals with aqueous HCl or with sodium ethanolate in ethanol leads to enhanced solubility. It is concluded that ester-bond breaking is necessary before extensive dissolution can take place. The solubility of brown coals in amines and aqueous KOH is found to increase with increasing carboxylic-acid group concentration in the coal. The solubility of Morwell brown coal in n-alkylamines at 180 °C increases with increasing length of the alkylchain in the solvent. The class of good solvents for brown coals is restricted to strong bases, because: 1. ester bonds have to be broken, 2. the acidic coal fragments have to be solubilized. Because of their capacity to break ester bonds these are so-called reactive solvents. Complete solvent recovery is impossible in the case of amines.     

The ruthenium(VIII)-catalysed oxidation of Illinois No. 6 bituminous coal: An application of g.c.-FT-i.r. spectroscopy for structural analysis

The oxidation of Illinois No. 6 coal proceeds readily with ruthenium(VIII) to provide a mixture of aliphatic and aromatic carboxylic acids. The acids were converted into their methyl esters for analysis by g.c.-m.s. and g.c.-FT-i.r. spectroscopy. Mono-, di-, tri- and tetracarboxylic acids are the principal products of the oxidation of this bituminous coal. The g.c.-FT-i.r. approach has enabled the detection of several different lactones that have not been observed previously. The results imply that cyclic ethers are present in low abundance in Illinois coals.