Improvement in coal liquefaction solvent quality by dewaxing

Recycle oils from the Integrated Two-Stage Liquefaction (ITSL), H-Coal and Solvent Refined Coal (SRC) processes were dewaxed by variants of commercial dewaxing processes—the ketone and the urea adduction techniques — yielding up to 47 wt % ‘wax’. Feed oils and product fractions were characterized by elemental analysis, ¹H n.m.r. and gas chromatography. The clean waxes were nearly pure mixtures of n-paraffins. The dewaxed oils were substantially better coal liquefaction solvents than the original (non-dewaxed) oils in batch liquefaction tests. For example, in one case, dewaxing improved the conversion of a bituminous coal to tetrahydrofuran-solubles under standard reaction conditions from 71 wt% (dafb) with the original oil to 87 wt % (dafb). These data provide a direct indication of the inimical effect of paraffinic components on solvent quality. The impact of solvent quality is particularly relevant to liquefaction processes in which thermal reactions proceed in a recycle solvent. In addition, the results indicate the technical feasibility of dewaxing coal liquefaction recycle oils by commercially available technology to improve solvent quality and to produce a useful by-product. Dewaxing could be applied in any liquefaction process that uses a deasphalted (preferably distillate) recycle stream.     

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.     

Composition and performance of distillate recycle solvents from the SRC-I process

Recycle distillates (‘solvents’) from the Wilsonville, Alabama, SRC-I pilot plant have been analysed by a variety of methods, and the composition data related to the performance of the solvents in both batch and continuous unit coal liquefaction. Because of its low distillate yield, solvents in the SRC-I process evolve rather slowly from the ‘anthracene oil’ start-up solvent to steady-state, process-derived distillates. As the steady-state solvent evolves it increases in saturates and mono-aromatic hetero molecules (primarily phenols and indanols), and its quality as a liquefaction medium declines. Because it is low in condensed aromatics, the steady-state SRC-I solvent cannot be greatly improved by hydrogenation. Other analytical data are provided which describe the composition of SRC-I recycle distillates. The analytical methods used in this work are of adequate simplicity that they could serve as routine monitoring methods in SRC-I liquefaction.     

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

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

Regulation of radicals by hydrogen-donor solvent in direct coal liquefaction

Radicals are important intermediates in direct coal liquefaction. Certain radicals can cause the cleavage of chemical bonds. At high temperatures, radical fragments can be produced by the splitting of large organic molecules, which can break strong chemical bonds through the induction pyrolysis of radicals. The reaction between the formation and annihilation of coal radical fragments and the effect of hydrogen-donor solvents on the radical fragments are discussed in lignite hydrogenolysis. Using the hydroxyl and ether bonds as indicators, the effects of different radicals on the cleavage of chemical bond were investigated employing density functional theory calculations and lignite hydrogenolysis experiments. Results showed that the adjustment of the coal radical fragments could be made by the addition of hydrogen-donor solvents. Results showed that the transition from coal radical fragment to H radical leads to the variation of product distribution. The synergistic mechanism of hydrogen supply and hydrogenolysis of hydrogen-donor solvent was proposed.     

Determination of transferable hydrogen in coal liquefaction solvents by spectroscopic methods

One of the most important properties of solvents used in direct coal liquefaction processes is their ability to release hydrogen for stabilizing radicals formed by thermolysis. The proposed mechanisms usually involve direct hydrogen transfer reactions between coal fragments and hydroaromatic compounds, which are thereby converted to aromatic structures and/or a hydrogen shuttling reaction. In the latter case, solvent molecules (e.g. PNA) can exchange hydrogen without changing their nature. This paper discusses the n.m.r. and m.s. techniques in relation to a new low voltage mass spectrometric method (l.v.-m.s.) that has been developed.     

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.     

溶剂对依兰煤液化反应特性的影响

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

Investigation of free radicals produced during coal liquefaction using ESR

Coal liquefaction experiments are described which were carried out to dissolve a subbituminous coal in low and high hydrogen content solvents, in which liquid samples were periodically withdrawn and examined by electron spin resonance spectroscopy. The objectives of this study were to gain information on the free radical processes involved in coal liquefaction, to determine the relative effectiveness of two solvents (H-donor and non-donor) in influencing the radical processes involved in coal liquefaction and to investigate changes in radical concentration on storage.     

Effects of solvent pretreatment and solvent incorporation on coal liquefaction

Yields of short-contact-time liquefaction product can be increased by pretreating the coal-solvent slurry at below normal liquefaction temperatures to permit solvent incorporation and/or swelling of the coal before liquefaction. An external pretreatment and an in situ pretreatment produce similar results. A coal-vehicle adduct was isolated from the pretreated coal and had liquefaction characteristics similar to the original coal. The beneficial effect of the solvent pretreatment is therefore believed to be the result of physical solvent incorporation with the coal, which causes solvent-aided liquefaction, in contrast with the thermal decomposition that can occur if some of the coal reaches liquefaction temperatures before it is contacted by vehicle. Pretreatment allows vehicle to be present (in contact with coal) at the reactive site in order to react with, and cap, coal free radicals.     

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.     

Hydroliquefaction of Wyodak coal using bottoms recycle

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

Coal liquefaction solvents: An n.m.r. analysis of a recycle oil over successive passes

Recycle distillates from a coal liquefaction continuous pilot plant, which used prehydrogenated anthracene oil as a start-up solvent and up to seven successive passes over an operation time of 800 h, were analysed by ¹³C and ¹H n.m.r. to give average structure and molecular parameters. The fraction of donatable hydrogen steadily decreased in the successive recycle solvents, by ≈ 3 wt% over each pass, as a result of a slow degradation of benzonaphthenic rings into saturates and alkyl-substituted aromatics or five-membered hydroaromatics.     

Free radicals in coals and coal conversion. 2. Effect of liquefaction processing conditions on the formation and quenching of coal free radicals

Free radicals are generally accepted as playing an important role in the liquefaction of coal. In a continuing series of studies, we are attempting to determine the relation between the nature and properties of free radicals and process variables such as residence time in a reactor, heating rates to achieve highest temperature and the role of various solvents and gases in stabilizing free radicals in lower-molecular-weight moieties. The free radical concentration after thermal treatments correlates well with the propensity of a given solvent to donate its hydrogen.     

Coal solvolysis with a series of coal-derived liquids

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

Pyrolysis of coal liquids

The pyrolysis of process recycle solvent derived from Western Kentucky coal via the SRC-II coal liquefaction process was investigated to ascertain the effect of residence time and temperature on the production of olefins. The study was made using an alonized transfer line reactor operating at temperatures of 650 and 730 °C, essentially atmospheric pressure, and residence times up to 0.13 s. A comparison is made with previously published results for the pyrolysis of hydrotreated COED light and heavy coal liquids (subsequently referred to as COED light and heavy oils, respectively) derived from Western Kentucky coal and steam pyrolysis of a hydrogenated fraction of SYNTHOIL derived from Western Kentucky coal. Results indicate that in each case the preferential pyrolysis of the saturate fraction occurs under convential pyrolysis conditions. Ethylene, propylene, and methane were the dominant gas products in all cases. The liquid pyrolysates from the COED oils and SRC-II recycle solvent had lower $\text{H}\text{C}$ ratios and heating values than their respective feedstocks. Mass spectroscopic analysis of the liquid pyrolysates in each case revealed the presence of polycyclic aromatics that were not present in the individual feedstocks. This trend which increased with temperature is indicative of cyclization and/or recombination of free radicals during pyrolysis. It is therefore surmized that the yields of light olefins from primary coal liquefaction products can be improved by partially hydrogenating them prior to pyrolysis. Alternatively, sufficient hydrogen can be provided in the vicinity of cracking to suppress retrogressive reactions which lead to the formation of coke. The pyrolysis of COED oils and SRC-II recycle solvent was found to follow first-order irreversible kinetics. The activation energy for the pyrolysis of the COED light and heavy oil was found to be 76.1 and 70.85 kJ g-mol^?1, respectively.     

Wasserstoff-Erzeugung und Kreislaufgas-Zerlegung für die Kohleverflüssigung

Hydrogen generation and recycle gas separation for coal liquefaction . Just as 40 years ago, low-temperature processes are now again being considered for recycle gas separation in today's large scale coal liquefaction projects. Hydrogen can be generated by gasification of heavy residues and by steam reforming of ethane. Alternatives for recycle gas separation into hydrogen and gas products are butane and methane absorption or low-temperature condensation processes at high or medium pressure. These processes employ several additional absorption and adsorption steps for gas purification. They are proven on a large technical scale, and fulfill the requirements of environmental legislation.     

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.     

煤炭直接液化溶剂的研究

讨论了煤炭直接液化过程中溶剂的特点、作用及质量要求,煤液化溶剂具有一般溶剂的功能,同时还具有良好的供氢和传递氢的功能特点,起到溶解、分隔煤裂解生成的自由基的作用,溶剂必须具有一定的分子结构和分子大小。初步讨论了表征煤液化循环溶剂供氢性的指标,指出普通溶剂如四氢萘和二氢萘等部分饱和的芳香化合物可直接用作煤液化溶剂,多环芳烃含量较高的煤焦油和石油系重质油,经过预加氢处理提高溶剂的供氢性后,可作为煤液化过程的起始溶剂或替代溶剂。