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.     

Additive effect of cobalt-exchanged coal on the liquefaction of subbituminous coal

In comparison with the liquefaction (420 °C, with tetralin and elemental sulfur) of the proton-exchanged Adaro subbituminous coal with complex (metallocene or metal carbonyl) of cobalt, nickel or iron, the coal liquefaction was enhanced significantly on the liquefaction of cation (Co²⁺, Ni²⁺ or Fe²⁺) exchanged Adaro coal. However, for several kinds of subbituminous coal, metal cations hardly exchange due to the small content of carboxyl group in coal. Therefore, the simultaneous liquefaction of subbituminous coal and cobalt-exchanged coal were carried out in this study. Further, the simultaneous liquefaction of lower rank and subbituminous coals with Co₂(CO)₈ was carried out to compare the catalytic effect of cobalt derived from either the exchanged coal or Co₂(CO)₈. The catalytic effect of cobalt in the exchanged Morwell brown coal was appeared as the decrease of residue yield on the liquefaction with Yilan subbituminous coal. On the other hand, there was little enhancement on the simultaneous liquefaction of cobalt-exchanged Adaro and Yilan subbituminous coals. Accordingly, it is clarified that the hydroliquefaction of subbituminous coal, which was exchanged cation hardly, was enhanced by the addition of cobalt-exchanged brown coal.     

Catalytic liquefaction of coal with supercritical water/CO/solvent media

This paper describes studies of the catalytic activity of cobalt molybdenum sulphide, cobalt molybdenum oxide, iron sulphate, iron acetate dibasic and H₂S during the reaction between supercritical CO and water, and during liquefaction of coal using supercritical CO-water-solvent mixtures. The kinetics of the water gas shift reaction was studied first and was found to be first order in all the catalysts studied. The activity of the catalysts decreased in the following order: cobalt molybdenum sulphide > cobalt molybdenum oxide > iron salts. The presence of toluene, tetralin, and THQ decreased the CO conversion on the cobalt catalysts but increased CO conversion in the presence of iron salts catalysts. Moderate coal conversion of toluene soluble products (25%–35%) were obtained in the presence of supercritical water and water/CO mixtures. Addition of organic solvents to a supercritical water/CO medium increased conversion of toluene soluble products to 70–80% for THQ, to 50–60% for tetralin, and to 35–40% for toluene. Addition of H₂S to the solvent/water/CO medium increased conversion to toluene soluble products even further. In the presence of H₂S/solvent/water/CO, the presence of catalysts had only a minor effect on coal conversion and were not required to achieve high coal conversions. The optimum operating conditions for an Illinois No. 6 coal were obtained using a H₂S/THQ/CO/water medium at 3600 psi, and 400 °C. Higher conversions were attained with a subbituminous Wyodak coal. These studies clearly demonstrate that high conversions to soluble products can be attained using a supercritical water/Co/solvent medium.     

Relative activity of transition metal catalysts in coal liquefaction

The catalytic activity of transition metals in coal liquefaction was studied and compared. Impregnation of coal with transition metals significantly increased oil production and asphaltene and preasphaltene conversion in coal liquefaction. Overall, coal conversion increased marginally and hydrocarbon gas production decreased slightly with metals. Iron impregnation was more active than cobalt, nickel, and molybdenum in preasphaltene conversion, whereas the other metals were more active than iron in asphaltene conversion. Hydrogen consumption decreased with all metals. The quality of generated solvent decreased with iron, but increased with other metals. Significant benefits were observed by using iron and molybdenum together; simultaneous impregnation of coal with iron and molybdenum significantly increased coal, asphaltene, and preasphaltene conversion, as well as oil production compared to individual metals. In addition, a mixture of iron and molybdenum decreased hydrocarbon gas production and increased hydrogen consumption and the quality of generated solvent over iron alone.     

Iron/sulfur-catalyzed coal liquefaction in the presence of alcohol and carbon monoxide

The activities of several iron-based catalyst precursors towards the liquefaction of various kinds of coals, ranging from brown to bituminous, were examined in alcohol–carbon monoxide systems. Pentacarbonyliron (Fe(CO)₅) with or without sulfur, or synthetic pyrite were found to be excellent catalyst precursors. Primary alcohols (ethanol and 1-propanol)–CO acted as an effective hydrogen source, whereas branched alcohols were less effective. In the Fe(CO)₅/sulfur catalyzed liquefaction of Yallourn coal at 375°C for 120 min, a high conversion (99.5%) was achieved in the presence of ethanol and CO (7.0 MPa/cold). The two-staged reaction (375°C, 60 min+425°C, 60 min) further improved the oil yield to 59.1% with a slight decrease in the coal conversion. The uptake of alcohol into asphaltene and preasphaltene fractions was distinctly observed, especially for Illinois No. 6 coal. The infrared analyses of the asphaltene fractions from each coal showed absorption at around 1705 cm⁻¹, characteristic for those obtained in the linear alcohol–CO systems. According to the characterization of the products by NMR and the preliminary study using a model compound, alkylation as well as the hydrogenolysis seem to contribute to the dissolution of coals.     

Catalytic effects of iron compounds and the role of sulfur in coal liquefaction and hydrogenolysis of SRC

Effective iron catalysts in coal liquefaction have been explored. Reduced Fe, Fe(CO)₅, and ferrocene have shown a considerably high catalytic activity in the liquefaction of Yallourn coal at 400°C. Degree of reduction has been shown to be one of the most important factors determining the catalytic activity of the iron catalyst. In the hydrogenolysis of SRC derived from Wandoan coal, water formed in situ has been shown to exhibit a negative effect due to reoxidation of the iron catalyst, but addition of elemental sulfur is greatly effective in preventing such deleterious influence of water. This can be interpreted on the basis of the sulfidation of reduced Fe by elemental sulfur, competing with reoxidation of the iron catalyst by water. In the systems with elemental sulfur, active and oxidation-proof pyrrhotite is formed, and the degree of reduction of the remaining iron oxide is maintained higher than that in the absence of elemental sulfur.     

The behavior of n-paraffins under coal liquefaction conditions

In order to evaluate the concentration and the distribution of n-paraffins in recycle solvent of coal liquefaction processes, the behavior of n-paraffins under coal liquefaction conditions was investigated. Four coals (Wandoan, Taiheiyo, Wyodak, Illinois No. 6) were liquefied and n-paraffins produced were analyzed. For example, n-paraffins, produced from liquefaction of Wandoan coal at 450°C for 1 h, contain approximately 5.2 wt.% (dry coal base) hydrocarbons in the range C₁₀C₃₆.Furthermore, cracking reactions of n-paraffins were carried out and their behavior under coal liquefaction conditions was analyzed. The cracking conversions of n-paraffins increased with increasing carbon numbers of n-paraffins, and the rate constants for cracking of n-paraffins were directly proportional to carbon numbers. The product distribution in the cracking of n-paraffins was evaluated by using bond dissociation energy. On the basis of these results, the concentrations of n-paraffins in the recycle solvents were calculated and these calculated values agreed well with those observed in the coal liquefaction process.     

Iron sulfate/sulfur-catalyzed liquefaction of Wandoan coal using syngas–water as a hydrogen source

Water-soluble iron sulfate/sulfur-catalyzed coal liquefaction using three kinds of hydrogen sources including syngas–water has been investigated. The liquefaction of Wandoan coal, an Australian subbituminous, with iron sulfate/sulfur as a catalyst precursor using syngas–water or carbon monoxide–water afforded higher coal conversions and oil yields than those using pressurized hydrogen gas. The pretreatment at relatively low temperature (200°C) was indispensable to achieve the high coal conversion. In the two-staged liquefaction (400°C, 60 min+425°C, 60 min), the use of syngas–water as a hydrogen source afforded higher coal conversion of 90.1% together with a high oil yield of 46.2% than those using pure hydrogen, and almost comparable to those using carbon monoxide–water, indicating the presence of synergistic effects of two hydrogen sources. At the early stage of the reaction, the contribution of carbon monoxide–water was predominant, whereas hydrogen gas significantly took effect at the latter stage. The XRD and XPS study revealed the formation of pyrrhotite, a possible active species, covered with a small amount of sulfate species.     

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.     

Coal hydroliquefaction using iron pentacarbonyl as a catalyst precursor

Hydroliquefaction of several coals, Taiheiyo (Japanese), Mi-ike (Japanese), Wandoan (Australian), and Illinois No.6 (American), was carried out using iron pentacarbonyl(Fe(CO)₅) at 425–460°C under a hydrogen pressure of 4.9MPa in a non-hydrogen donating solvent, 1-methylnaphthalene. With the addition of iron pentacarbonyl coal conversion increased substantially for all of the coals used. Lighter fraction (oil) also increased, by ≈ 10–17 wt%, in the presence of the catalyst. The addition of Fe(CO)₅ suppressed coking, resulting in high values of coal conversion and oil fraction even at 460°C. The amounts of hydrogen transferred from the gas phase increased by 2–4 times with Fe(CO)₅. A process involving direct hydrogen transfer to coal fragment radicals is proposed.     

Catalytic liquefaction of various coals using a mixture of carbon monoxide and water

In a batch-autoclave, twenty coals were liquefied using a cobalt-molybdenum oxide catalyst with a mixture of CO and H₂0 at 400 °C with or without vehicle oil. Furthermore, lignite and peat were liquefied on tungsten oxide catalyst at 300 °C in the absence of CO. The reactivity of coal in this liquefaction is found to depend strongly on its rank. The vehicle oil significantly influences the extent of the water-gas shift reaction, especially when bituminous coals are liquefied, by dissolving such coals. Liquefaction of coal by this process is considered to take place via three routes: hydrogenolysis by the nascent hydrogen produced from the shift reaction; dissolution of coal into the vehicle oil which is an initial stage of hydrogenolysis; and a solvolytic reaction with H₂O, such as hydrolysis.     

An investigation of the role of hydrogen sulphide in coal liquefaction catalysis by high-temperature and high-pressure e.s.r.

The role of hydrogen sulphide (H₂S) as a coal liquefaction catalyst was tested using two typical coals having different ash content and mineral matters by means of high-temperature and high-pressure e.s.r. For Illinois No. 6 coal having pyrite as minerals, a slight increase in radical concentration in a coal/H₂S system was observed at about 500 K and then drastic increase in radical concentration at above 620 K. On the other hand, in a coal/N₂ system, the increase in radical concentration was much smaller.The effect of H₂S on the change of radical concentration for Yallourn coal which has little ash content was not remarkable. It was concluded that H₂S is a promoter of liquefaction in presence of iron, but has little inherent catalytic function itself. High-temperature and high-pressure e.s.r. is found to be a useful tool to understand the synergistic effect of H₂S-iron sulphide in a coal liquefaction catalyst.     

Oil agglomeration as a pretreatment for coal liquefaction

Laboratory experiments demonstrated that a variety of distillate coal liquefaction recycle oils were satisfactory agents for cleaning Illinois no. 6 bituminous coal by oil agglomeration. Ash rejection up to 41% with 98% organic recovery was attained with conventionally cleaned coal, and ash rejection up to 67% with 90% organic recovery with run-of-mine coal. Agglomerates of > 1 mm average diameter were produced under a variety of conditions. Similar results were obtained in the scaled-up production of 268 kg of agglomerates. Oils with lower hydrogen aromaticities and higher hydrogen contents performed better than more aromatic oils. Fe, Ti and Mg were selectively enriched in the ash of the product coal, while Ca, Si, and Al were selectively rejected. The mineral pyrite was rejected only ≈ 30–40% as extensively as the bulk of the ashforming minerals. The coal cleaned by oil agglomeration performed similarly to the feed coal in batch donor liquefaction tests. In continuous hydroliquefaction tests, run-of-mine coal cleaned by oil agglomeration performed substantially better than coal cleaned to the same ash level by conventional means, because of the selective enrichment of catalytic iron minerals.     

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.     

Differences in the behaviour of US bituminous and subbituminous coals during short contact time liquefaction

The short contact time (SCT) liquefaction of Belle Ayr subbituminous coal has been compared with that of Illinois No. 6 and Pittsburgh seam bituminous coals. Each bituminous coal was highly solubilized (90 wt%, daf coal) in 3–4 min at 450 °C and 13–16 MPa hydrogen pressure. More than 80 wt% of each coal was converted to solvent-refined coal (SRC, pyridine-soluble residuum), with only small quantities of distillate oil and C₁–C₄ gas being formed. A longer reaction (up to 30 min) gave only a small increase in total conversion, but gas and distillate yields increased significantly. Iron sulphides did not appear to catalyse coal solubilization. By contrast, only 65 wt% of the Belle Ayr coal dissolved rapidly in SCT liquefaction and pyrite addition catalysed the conversion of the remaining insoluble organic matter (IOM). With an equivalent amount of pyrite present the Belle Ayr coal also gave more C₁–C₄ gas and substantially more distillate in SCT liquefaction than the bituminous coals. These differences in product distributions obtained from bituminous and subbituminous coals in SCT liquefaction can be rationalized on the basis of differences in the structures of the starting coals. However, the origin of high IOM yields with the Belle Ayr coal remains unclear.     

Liquefaction of bituminous coal at moderate temperatures

Coal hydrogenation was investigated in the temperature range 275 to 325 °C in order to minimize the number of thermal side reactions that take place. Gas-phase hydrogen was used in batch experiments without an added donor solvent, to avoid the additional analytical complexities introduced by such a solvent. It was found that significant oil yields (up to 72% of the daf coal) can be obtained from the hydrogenation of bituminous coal at 325 °C. Furthermore, at this temperature, the data indicate that cleavage of certain C---O bonds may have an important role in oil formation. The metal surfaces of the liner and impeller of the autoclave had a strong catalytic effect on the liquefaction reactions under these conditions. The oil yield was 48% when the metallic surfaces were exposed and only 19% when these components were coated with glass. Catalysis by nickel, applied as nickel acetate impregnated into the coal, gave higher overall conversion, lower oil yield, and a more saturated oil product than catalysis by the autoclave surfaces.     

Oxidative coupling of methane over transition-metal-substituted strontium hydroxyapatite

Lead-substituted strontium hydroxyapatite (Sr_10-xPb_x(OH)₂(PO₄)₆) showed remarkably enhanced catalytic performance for the oxidative coupling of methane (OCM) when compared with the unsubstituted strontium hydroxyapatite. Other substituted transition metals such as zinc, cobalt and nickel were not so effective for improving the catalytic performance for the OCM. The Ni-substituted catalyst exhibited quite different catalytic behavior: CO and hydrogen were the major products instead of the C₂ products. The catalyst with the extent of Pb substitution(x) of 0.2 showed the highest C₂ selectivity and yield (about 47% and 17% at 1,023 K, respectively) and also exhibited quite stable behavior.     

Catalytic effects of MoCl3- and NiCl2-containing molten salts in hydroliquefaction of Akabira bituminous coal with and without hydrogen-donor vehicle

Catalytic effects of MoCl₃-LiCl-KCl and NiCl₂-LiCl-KCl molten salts in hydroliquefaction of Akabira bituminous coal were studied. In the absence of solvent, both catalysts showed high coal conversion activity and high selectivity for the formation of hexane-soluble oil product. Oil yields from the catalytic runs were 3.4–3.0 times that from a non-catalytic run. Addition of hydrogen-donor tetralin considerably increased the oil yield and conversion and reduced the total hydrogen consumption. About 95 and 91 wt% daf coal was converted into pyridine-solubles and 59 and 54 wt% into oil with relatively low total hydrogen consumption (3.5 and 3.1 wt% daf coal) with the MoCl₃ and NiCl₂ catalysts respectively, in the presence of tetralin. Thermogravimetric analysis indicated that these catalysts enhanced the depolymerization of the coal organic matrix. Analysis of the liquefaction products suggested that the catalysts effectively catalysed the hydrocracking of polyaromatic structures contained in heavy products to hydroaromatics with relatively small ring sizes, explaining the high oil selectivity.     

Co-liquefaction of lignite and sawdust under syngas

Individual and co-liquefaction of lignite and sawdust (CLLS) under syngas was performed in an autoclave and the effects of temperature, initial syngas pressure, reaction time and ratio of solvent to coal and biomass on the product distribution of CLLS were studied. Sawdust is easier to be liquefied than lignite and the addition of sawdust promotes the liquefaction of lignite. There is some positive synergetic effect during CLLS. In the range of the experimental conditions investigated, the oil yield of CLLS increases with the increase of temperature, reaction time (10-30 min) and the ratio of the solvent to the feedstock (0-3), but varies little with the increase of initial syngas pressure. Accordingly, the total conversion, the yield of preasphaltene and asphaltene (PA + A) and gas, changes by the difference in operation conditions of liquefaction. The gas products are mainly CO and CO₂ with a few C₁-C₄ components. The syngas can replace the pure hydrogen during CLLS. The optimized operation conditions in the present work for CLLS are as follows: syngas, temperature 360 °C, initial cold pressure 3.5 MPa, reaction time 30 min, the ratio of solvent to coal and sawdust 3:1. Water gas shift reaction occurs between CO in the syngas and H₂O from coal and sawdust moisture during the co-liquefaction, producing the active hydrogen which increases the conversion of liquefaction and decreases the hydrogen consumption.     

Liquefaction mechanism of Wandoan coal using tritium and 14C tracer methods: 1. Liquefaction in 3H and 14C labelled solvent

Liquefaction of Wandoan coal using a ³H labelled tetralin solvent which contains a small amount of ¹⁴C labelled naphthalene has been studied at 400 °C under an initial hydrogen pressure of 5.9 MPa, in the presence or absence of NiMoAl₂O₃ catalyst. The amounts of ³H and ¹⁴C transferred from the solvent to the products were measured as liquefaction progressed. The reaction pathways in the presence and absence of the catalyst were discussed and their reaction rate constants were calculated. According to the mass balances of hydrogen and ³H, in the absence of catalyst, tetralin provided coal with hydrogen atoms, and the degree of hydrogen exchange between coal and solvent was small. The catalyst decreased the hydrogen addition from solvent to coal and increased that from gas to coal.