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.     

Coprocessing of a Turkish lignite with a cellulosic waste material: 2. The effect of coprocessing on liquefaction yields at different reaction pressures and sawdust/lignite ratios

Most of the research works done for alternative energy sources have shown that, in general, coprocessing of coal with biomass-type wastes has a positive effect on the liquefaction yields and these materials are increasingly studied as coliquefaction agents for the conversion of coal to liquid fuels. Addition of biomass waste materials to coal is known to be synergetic in that it improves the yields and quality of liquid products produced from coal under relatively mild conditions of temperature and pressure. This paper reports the coprocessing of a Turkish lignite with sawdust in the category of biomass-type waste material. The experiments have been conducted in a stainless-steel reactor, and temperature and tetralin/(lignite+sawdust) ratio were kept constant at 350 °C and 3:1 (vol/wt), respectively. This is the first time that the influence of reaction pressures on coliquefaction yields was investigated. In addition, the influence of the sawdust/lignite ratios on coprocessing conversion and product distribution was also investigated under the same reaction conditions. The runs were carried out at 10, 25, 40, 55, and 70 atm initial cold hydrogen pressure values and at 0.5:1, 0.75:1, 1:1, 1.25:1, and 1.5:1 sawdust/lignite (wt/wt) ratio values.     

Direct liquefaction of sawdust under syngas

Liquefaction of sawdust under syngas was performed in an autoclave and the effects of temperature, initial syngas pressure and reaction time on the product distribution of sawdust liquefaction were studied. The results using different solvents and atmospheres were also compared by product distribution and analyses of GC-MS, TG, IR and GPC. It was found that hydrogen donor solvent showed remarkable effect than either non-hydrogen donor solvent or without presence of solvent and its hydrogenation ability was much higher than gaseous hydrogen. Among various atmospheres H₂ displayed higher activity than syngas and both of them were better than Ar and CO, while CO did not give the favorable influence. With increasing temperature and reaction time (10–30 min) the oil yield increases, while less effect with increasing initial syngas pressure. The thermal decomposition of sawdust to form preasphaltene and asphaltene (PA + A) is a fast step, while longer reaction time is necessary for conversion of PA + A to oil as the 2nd step. The results also indicated that syngas can replace hydrogen in sawdust liquefaction.     

Low temperature—long time,high temperature-short time liquefaction of coal: The Hokudai process

A Japanese cola was hydrogenated in wash oil with fine iron dust and sulphur as catalyst under a reaction pressure of 12–13 MPa at 420 °C for 2 h and then at 500 °C for 0–20 min. The liquid yield boiling up to 600 °C amounted to 55–66 wt%. Pyridine conversion was ≈ 100 wt%, benzene conversion 82–90 wt% and n-hexane conversion 53–70 wt%. Compared with direct hydrogenation at 500 °C for 10 min the low temperature-long time plus high temperature-short time liquefaction process (the Hokudai process) is a very effective method for obtaining high liquid yield under relatively low hydrogen pressure without coking, using disposable catalyst and non-donor solvent.     

Solvolytic liquefaction of coals with a series of solvents

Solvolytic liquefaction of coals of different rank was studied with a variety of solvents at 370–390 °C under nitrogen in order to elucidate the role of solvent in coal liquefaction of this kind and to find a suitable solvent for the highest yields of liquefaction. The yield was found to depend strongly upon the nature of the coal as well as the solvent under these conditions. Pyrene and a SRC-BS pitch were excellent solvents for Miike coal, which was fusible with high fluidity at these temperatures. However, the former was less efficient for Itmann and Taiheiyō coals which were fusible at a higher temperature and non-fusible, respectively. The mechanism of solvolytic liquefaction is discussed, including nature of coal and solvent at reaction temperatures, in order to understand the properties required for high yields with non-fusible coals in solvolytic liquefaction. It is found that for liquefaction with a high yield if the coal is non-fusible, solvolytic reaction should take place between solvent and coal, so giving a liquid phase of low viscosity at the reaction temperature. The solvolytic reaction may be one of hydrogen transfer when SRC-BS is used as the solvent.     

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.     

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.     

Nonsolvent coal hydrogenation at short contact time

Yields and properties of products on hydrogenation of Japanese and Australian coals have been studied using an autoclave equipped with a magnedrive device at temperatures of 500–600 °C and hydrogen pressure 4.9–14.7 MPa (gauge) in the absence of solvent. Optimum contact time, at which maximum extraction yield was observed, shifted from 15 s to a few seconds with increasing reaction temperature and hydrogen pressure. The extracts derived from both coals reveal similar structural parameters.     

Carbon foam production from bio-based polyols of liquefied spruce tree sawdust: Effects of biomass/solvent mass ratio and pyrolytic oil addition

One of the next-generation structural materials is carbon foam. Porous materials have become an intriguing alternative material to traditional ones in many utilizations based on their light weight and incomparable properties. Coal or fossil oils are conventionally used to produce pitch, phenolic resin, and polyurethane as carbon foam precursor. Biomass liquefaction is a developing technique to convert biomass resources into the industrial chemicals. In this study, spruce tree sawdust was liquefied under mild conditions with different solvent type (phenol or phenol + bio-oil mixture). The unique aspect of this work is the synthesis of bio-polyol when pyrolytic oil is used as an alternative to phenol in the solvolysis reaction and its evaluation in carbon foam production with multilayer graphene sheets. Therewithal, the ratios of biomass to solvent were 1/3 as well as 1/5, and the comparison of product characteristics is another originality of the study. Slow pyrolysis of spruce tree sawdust was performed under static atmosphere and bio-oil was characterized with elemental analysis and various chromatographic and spectroscopic techniques. The effect of mass ratio of biomass/solvent on the characteristics of porous resin foams synthesized from liquefaction product. Obtained resin foams were carbonized at 400 °C, and then activated at 800 °C under nitrogen atmosphere. Structure evaluation of resin foams, carbonized foams, and activated carbon foams from liquefied spruce tree sawdust was investigated by using elemental analysis, x-ray diffraction, nitrogen adsorption/desorption isotherms, scanning electron microscopy, true/bulk density, and compressive strength tests. Although the surface area values decreased when bio-oil was added as a solvent, it was determined that the compression strengths of the produced carbon foams (up to 1.080 MPa) were higher than that of conventional phenolic foams. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47185.     

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.     

Effect of pressure and temperature on the reaction of coal with alcohol-alkali

The ethanol-NaOH reaction of Taiheiyo coal (C, 77.5; H, 6.3 wt%) was examined under a pressure of 0.1–8 MPa nitrogen or hydrogen, at 300 °C for 1 h. Almost all of the products are extracted with pyridine for the entire pressure range and the extraction yield with ethanol increases with pressure, even under nitrogen. The yield of the products extracted with ethanol is higher when the coal is reacted under hydrogen than when reacted under nitrogen. The explanation for these results is that, under pressure, the hydrogen produced from the reaction of alcohol and NaOH is enclosed for a longer period in the solvent, thus accelerating the hydrogenation reaction of the coal, also under hydrogen pressure, the reaction is particularly accelerated because the hydrogenation takes place with the hydrogen gas. At 300 °C, the ethanol-extraction yield is much higher than the benzene-extraction yield, but the latter increases rapidly and approaches that of the ethanol-extraction yield as the temperature rises to 400 °C. This is because the polar groups, e.g. as hydroxyl groups which are rich in the low-temperature products, decrease with the temperature rise.     

Comparison of effect of ferric sulfide catalyst and nickel sulfide on coliquefaction of coal and biomass

Coliquefaction of coal and sawdust in the presence of catalysts was studied under cold hydrogen pressure from 2.04 to 4.76 MPa in the temperature range of 300–400 °C. In terms of overall conversion, yields and increment values from comparisons, investigations were made on catalysts of the sulfides of Fe, Ni and Cu and the methods of catalyst preparation. As economics of the technological process is concerned, a catalyst of ferric sulfide is preferred, apart from its advantages over the catalyst of nickel sulfides in catalytic effects under three sets of reaction conditions from our experiments. The optimal operation conditions were concluded for the preferred catalyst FeS_x.     

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.     

Free radicals in coal and coal conversions. 8. Experimental determination of conversion in hydroliquefaction

The results of conversion determinations on the products from Powhatan No.5 coal liquefied in an autoclave and in a high-pressure, high-temperature e.s.r. cavity are reported. Oil, asphaltene and preasphaltene yields, and overall conversion have been determined for Powhatan No.5 coal samples liquefied in tetralin, SRC-11 heavy distillate, and naphthalene at temperatures from 400 to 480 ° C in both reactor systems. The concept of reaction severity is introduced and used to formalize the relation between the effect of temperature and reaction time on oil yield and conversion. Oil is the predominant product in liquefaction in tetralin or naphthalene, asphaltene is the major product of liquefaction in SRC-II heavy distillate. Retrogressive reaction (THF-insoluble product formation) becomes severe when SRC-II heavy distillate is the liquefaction solvent and residence time of >10 min are used at temperatures >450 °C. Preasphaltenes appear to be the only intermediate species in Powhatan No.5 liquefaction.     

Coal liquefaction by hydrogen produced from methanol: 3. Examination of reaction conditions

Coals were liquefied in a 50 ml autoclave using hydrogen produced from methanol with decomposition and hydrogenation catalysts under various conditions. The conversion increased with increasing reaction temperature and time. This reaction is more suitable for the liquefaction of lower-rank coals. The hydrogen pressure had little effect on the conversion in the range of the ratio of methanol to coal of 2 g/g. The use of a solvent is very effective in increasing the liquefaction, especially at short times.     

Superacid coal chemistry. 2. Model compound studies under conditions of HF:BF3:H2 catalysed mild coal liquefaction

Selected model compounds representing coal structural entities were studied under the conditions of HF-BF₃-H₂ catalysed mild coal liquefaction. Bibenzyl and diphenylmethane gave near quantitative conversion at room temperature without added hydrogen. Biphenyl, however, required hydrogen pressure at 150 °C and gave a conversion of only ≈30%. Among the model compounds containing ether linkages, dibenzyl ether and benzyl phenyl ether gave quantitative conversion at room temperature without added hydrogen. Diphenyl ether in contrast was converted (≈70% yield) only under hydrogen pressure at 155 °C. Sulphur- and nitrogen-containing model compounds were also studied. At 95 °C in the absence of hydrogen, benzyl phenyl sulphide and dibenzyl sulphide gave over 95% conversion. On the other hand diphenyl sulphide and diphenyl disulphide required hydrogen pressure at 150 °C to give conversions of ≈95%. Quinoline gave a conversion of ≈20% under hydrogen pressure at 150 °C. The formation of condensation products in these conversion processes could be suppressed by the use of a good hydrogen donor, such as isopentane.     

Approach for promoting liquid yield in direct liquefaction of Shenhua coal

Single and multi-stage liquefaction of Shenhua (SH) bituminous coal and re-liquefaction of its liquefaction residue (SHLR) were carried out in an autoclave reactor to investigate the essential approach for promoting oil yield and conversion in SH coal direct liquefaction (SHDL). The multi-stage liquefaction includes pretreatment, keeping the reactor at 250 °C for 40 min before heating up to the reaction temperature, and two-stage liquefaction processes consisting of low temperature stage, 400 °C, and high temperature stage, 460 °C. The results show that the pretreatment has slight effect on oil yield and conversion of SHDL, especially for liquefaction at 460 °C. There is a positive function of two-stage liquefaction in shortening reaction time at high temperature. Increasing ratio of solvent to SHLR can promote the oil yield and abate reaction condition in SHLR re-liquefaction, that is, it can promote the conversion from preasphaltene and asphaltene to oil. The primary factor to inhibit coal liquefaction is the consumption of hydrogen free radical (H·) from solvent or H₂ and condensation of free radicals from coal pyrolysis after a period of reaction. So the essential approach for increasing oil yield and conversion of SHDL is to provide enough H· to stabilize the free radicals from coal pyrolysis.     

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.     

Free radical chemistry of coal liquefaction: role of molecular hydrogen

Model compounds containing the types of carboncarbon bonds thought to be present in coal were pyrolyzed in the presence of tetralin and molecular hydrogen at 450 °C. The relative rates of conversion of the model structures are predictable from the bond dissociation energies of the compounds. Conversion of dibenzyl in the presence of both tetralin and molecular hydrogen or in the presence of hydrogen alone proceeds along two parallel reaction paths. Toluene is produced by a thermal cracking reaction in which the rate-controlling step is the thermal cleavage of the β-bond in dibenzyl. Benzene and ethylbenzene are produced by a hydrocracking reaction. The rate of the hydrocracking reaction is directly proportional to the hydrogen pressure. The strong bond in diphenyl is hydrocracked in a system containing both molecular hydrogen and a source of free radicals. These studies on model coal structures offer firm evidence that molecular hydrogen can participate directly in free radical reactions under coal liquefaction conditions. Under some conditions molecular hydrogen can compete with a good donor solvent to stabilize the thermally produced free radicals. Molecular hydrogen can also promote some hydrocracking reactions in coal liquefaction that do not occur to an appreciable extent in the presence of only donor.     

Efficiency of hydrogen transferring liquefaction of a subbituminous coal using hydrogenated fluoranthene for short contact time at high temperature and low pressure

An Australian subbituminous coal (Wandoan) was effectively liquefied at 490 and 510 °C under nitrogen pressure of 2.5 MPa for 1.0–7.5 min using 1, 2, 3, 3a-tetrahydrofluoranthene (4HFL) as a hydrogen-donating solvent. The yields of oil and asphalthene could be as high as 58 and 24 wt%, respectively. The content of 4HFL was very influential on the oil yield although under appropriate liquefaction conditions, a considerable amount remained after reaction. The kinetics of the reaction and analytical study of the products and the solvent suggest consecutive as well as instantaneous depolymerization in the process. The coking or recondensation reaction was very rapid after 4HFL was consumed, confirming the efficacy of the short contact time liquefaction.