Free radicals in coal and coal conversions. 5. Methodology for the in-situ investigation of free radicals in coal depolymerization under SRC-II preheater/reactor conditions

In a continuing series of publications, a systematic investigation of the free radicals during coal depolymerization under SRC-II preheater/reactor conditions has been made. A uniquely designed electron spin resonance (e.s.r.) cavity allows the monitoring of free radicals and how they are affected by residence time, temperature and pressure changes. This paper summarizes the methodology that has been developed to study systematically free radical formation and behaviour in-situ during coal liquefaction. The methodology involved is to be used extensively in subsequent papers that examine in detail the manifold parameters that affect coal depolymerization. Also, results are given on the dependence of free radicals on temperature, pressure and gases in the presence of tetralin. The free radical concentration can be measured 3–4 min after the initiation of the heating process to an accuracy of ± 20%, while the process variables may cause several-fold changes in the free radical concentration.     

Free radicals in coal and coal conversions. 6. Effects of liquefaction process variables on the in-situ observation of free radicals

To determine the effects and relative importance of process variables in coal liquefaction, a uniquely designed and fabricated high-pressure/high-temperature electron spin resonance (e.s.r.) apparatus is used to monitor the in-situ formation and behaviour of free radicals, which are generally assumed to be the key factor. It is concluded that the temperature is the most significant single process variable that affects free radical formation; for Powhatan No. 5 coal there is a 9-fold increase in going from 400 to 460 °C. At 460 °C the other process variables tested can affect significantly the free radicals significantly, but at 400 °C these variables have essentially no effect on free radicals formation. The next most significant effect is due to the combination of solvent nature and residence time. Tetralin and the SRC-II heavy distillate quench the free radicals from Powhatan No. 5 to the same extent with one significant difference. In tetralin the maximum concentration is observed shortly after the slurry achieves its highest temperature, whereas in the SRC-II heavy distillate experiments the concentration is still increasing, at 460 °C, even after 1 h of reaction. The heating time, pressures and types of gas used affect the free radical concentration to a much smaller extent. The conversions obtained in the in-situ e.s.r. experiments using SRC-II heavy distillate as the solvent are somewhat lower than those obtained with tetralin as the solvent. The corresponding oil yields with tetralin are considerably higher than with SRC-II heavy distillate.     

Coal dissolution in high boiling process solvents

The liquefaction of a bituminous coal has been studied using distillate and non-distillable, coal-derived solvents. Different classes of solvent components can interact non-additively during reaction with coal to either promote or retard the process of coal dissolution. Above certain concentrations, polyfunctional compounds in heavy coal-derived liquids effect a measurable reduction in coal conversion and product selectivity by accentuating the degree of condensation reactions. Substantial removal of these compounds can be achieved by precipitation in a paraffinic solvent. Retrogressive reactions are reduced and positive synergistic interactions can be realized by the blending of heavy solvents with selected components, such as polycondensed aromatics, and with lower boiling, hydrogen-donor species.     

Free radicals in coals and coal conversion. 4. Investigation of the free radicals in selected macerals upon liquefaction

This study of the nature and role of free radicals in coals and coal conversion considers free radicals from selected macerals in the liquefaction process. High-purity macerals were liquefied to different extents at high temperature (425–480 °C) in the presence of naphthalene or tetralin and hydrogen or nitrogen. The initial concentrations of free radicals as well as the residual concentrations have been measured. The dependence of free radical concentrations on solvents, gases, and temperature is reported. Resinites show the least number of free radicals. They also liquefy almost completely under all conditions used. Fusinites have the highest concentration of residual free radicals and they are unaffected by solvent, gas or temperature changes: The vitrinites show intermediate free radical concentrations that depend very strongly on the solvent used, and on the temperature, but they are unaffected by the gas used. A correlation has been established between free radical concentration and degree of conversion.     

Extents and models of adduction of hydrocarbon and nitrogen-containing solvents in coal liquefaction

Coal liquefaction experiments have been carried out to determine the tendency for adduction of several hydrocarbon- and nitrogen-containing structures typical of coal. Principal conclusions are: that even good hydrogen donors such as tetralin and octahydrophenanthrene are adducted readily; that benzyl radicals adduct primarily by radical-radical reaction, yielding thermally labile products; and that quinoline adducts to coal-derived liquids primarily by hydrogen bonding with little formation of C-C or C-N bonds.     

Free radical involvement in the oxidation of Victorian brown coal

Electron spin resonance spectroscopy has been used to investigate free radical involvement in the oxidation of Victorian brown coal. Fresh, wet Yallourn brown coal exhibits two superimposed e.s.r. signals due to organic free radicals. The free radicals responsible for the narrow signal interact reversibly with atmospheric oxygen while the broad signal changes intensity dramatically during prolonged exposure to air. Coal:oxygen complex formation may explain these observations. Results also suggest that the free radical content of brown coal increases as a consequence of vacuum drying.     

Chemistry of solvents in coal liquefaction: Quantification of transferable hydrogen in coal-derived solvents

Hydrogen was evolved as hydrogen sulphide when coal-derived solvents for liquefaction were heated with sulphur (dehydrogenation method) and their naphthene contents were quantified by titration and ¹³C n.m.r. analysis to estimate the amount of transferable hydrogen from hydroaromatics present in the solvent. Examination of synthetic solvents consisting of model compounds confirmed the validity of both approaches. The content of transferable hydrogen, thus measured, in the various solvents correlated well with their liquefaction activities using Morwell brown coal. This suggests that the sufficient stabilization of radical fragments derived thermally from the coal at the initial stage of its liquefaction leads to high conversion. It was also shown that the dehydrogenation method was applicable to non-distillable heavy fractions of coal-derived liquids such as SRC which are difficult to measure by n.m.r. because of their limited solubility.     

Hydrogenation of brown coal: 5. A study of coal-derived materials as vehicles for brown coal hydroliquefaction reactions

A series of batch autoclave experiments has been conducted in which the product boiling in the range 220–450°C was recycled several times to test the suitability of this liquid as a vehicle for the hydroliquefaction of Victorian brown coal. The product liquid was used at the end of each cycle as a vehicle for an independent small-scale coal hydrogenation and repetitive hydrogenations of the parent solvent were also carried out to help facilitate understanding of the role and chemical changes associated with the vehicle. It has been shown that the distillable coal-derived liquid, even without recycling part of the bottoms, has a hydrogenation potential in the same range as tetralin and hydrogenated anthracene oil, despite its relatively poor quality. However, the amount of liquid produced was less than the amount of solvent used.     

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 functional groups of coal-derived liquids by n.m.r. and elemental analysis

A new method of structural analysis was applied to a group of hydroliquefied coal samples. The method uses elemental analysis and n.m.r. data to estimate the concentrations of functional groups in the samples. The samples included oil and asphaltene fractions obtained in a series of hydroliquefaction experiments, and a set of nine fractions separated from a coal-derived oil. The structural characterization of these samples demonstrates that estimates of functional group concentrations can be used to provide detailed structural profiles of complex mixtures and to obtain limited information about reaction pathways.     

Effects of solvent composition on coprocessing coal with petroleum residua

The effect of solvent composition on coprocessing of coal and petroleum solvents is examined under a variety of reaction conditions. The effects of solvent modification procedures on enhancing methylene chloride/methanol (MeCl/MeOH) soluble coal conversion and pentane soluble oil production are studied. Solvent modification procedures performed prior to coprocessing reactions include pentane deasphalting, catalytic hydrotreatment, H-donor addition, and blending of coal-derived liquids with petroleum solvents. Oil production and coal conversion were variously affected by the different solvent modifications. Prior hydrotreatment of petroleum solvents generally enhanced coal conversion, as did H-donor addition. The presence of a hydrotreating catalyst exerted a leveling effect on the effects of solvent modification. Blending of coal liquids and petroleum solvents resulted in complex and not readily predictable interaction. Solvents yielding the highest MeCl/MeOH soluble coal conversion were not generally the optimal solvents for pentane soluble oil production.     

基于Py-GC/MS研究热解反应中自由基的捕获反应

在500℃下对类煤模型化合物1-萘甲醇与同位素示踪剂的共热解进行了机理研究。利用Py-GC/MS进行快速产物检测,同时联合自由基捕获剂来推断自由基的反应过程。结果表明,在500℃和0.2 min的条件下,1-萘甲醇的热解产物相对单一,但是自由基反应的引发、交换和湮灭极其快速,现有检测方法是对某一时间节点的平衡反应结果的分析。通过D₂O和H₂¹⁸O同位素示踪,成功捕获到了自由基交换后的D取代产物和¹⁸O取代产物。揭示了1-萘甲醇经过1-萘甲基自由基转化为1-萘甲醛的转化过程,发现萘环上的取代基在1-萘甲醇的热解过程中起着重要作用。同时对自由基及产物进行了定性和半定量分析,发现在加入自由基捕获剂后产物的丰度较之前降低了一个数量级,说明了自由基捕获剂抑制了产物的生成。     

Stability studies of coal-derived liquids

The ageing characteristics of liquid products derived from the hydrogenation of coal have been studied. Viscometric, spectrometric, ultimate analysis, solvent separations and oxygen consumption measurements have been employed to monitor the storage properties of the coal-derived liquids under a variety of environmental conditions. These studies show that the coal-derived liquids are very susceptible to oxidative degradation. Rates of this degradation process, based on viscosity measurements, are related to the initial viscosity of the coal-derived liquid, storage temperature and concentration of oxygen in the gaseous environment over the liquids. Analysis of aged samples show the components of higher molecular weight with functional components are most susceptible to the oxidation process. Although the total coal-derived liquids are more susceptible to oxidative degradation than the petroleum-derived boiler feedstocks, removal of the most reactive components from the coal-derived liquids, by solvent separation, results in a fuel that has analogous stability characteristics as the petroleum products.     

Solubilization of Illinois bituminous coal: the critical importance of methylene group cleavage

The extent of solubilization of an Illinois bituminous coal achieved by hydroliquefaction at various temperatures with and without catalyst was determined. The coal was also solubilized by treatment in suspension in THF with potassium in the presence of naphthalene. The original coal was also extracted with THF without prior treatment. After removal of solvent the soluble fractions were examined by ¹H n.m.r. spectroscopy. It is concluded that there are two temperature regimes which affect coal conversion: in the 350–400 °C region, hydroliquefaction proceeds principally by ether cleavage; whereas in the 400–450 °C range, hydroliquefaction results in additional cleavage of methylene groups joining aromatic or heteroaromatic units. Comparison of solubilization by hydroliquefaction with solubilization achieved with potassium in THF seems to indicate the initial degradation of coal by both techniques proceeds by ether cleavage and that further degradation is achieved in both cases by attack at the methylene bridge. The n.m.r. studies on hydroliquefaction products indicate that Illinois bituminous coal is structurally quite homogeneous because the product composition is largely independent of the degree of solubilization. Thus, solubilization results from molecular weight degradation rather than from preferential-degradation to different structural units.     

Free radicals in coal and coal conversions. 9. Statistical correlative models of the effect of process variables on hydroliquefaction products

Previous papers in this series have reported the degree of coal conversion in autoclave and e.s.r. cavity liquefaction experiments and free radical measurements from the e.s.r. liquefaction experiments. Here a correlative model for the liquefaction products is discussed in terms of temperature,solvent reactor type, pressure and other process variables, similar in form to that used for the free-radical concentration measurements in an earlier paper in this series. Solvent-time interactions were found to be the most significant term in modelling the yields. Based on the strong similarity in the functional form of the radical and product yield models, a model of conversion and the various yields as a fraction of radical concentration and other process variables was constructed. Free-radical concentration and its interaction with solvent were found to be very significant.     

Transformations of iron minerals during coal oxidation

Oxidative transformations of iron minerals in weathered Illinois No. 6 coal were investigated by Mössbauer spectroscopy and X-ray powder diffraction. Temperature ranges were determined for the conversion of pyrite to ferrous sulphate and ferric hydroxysulphate and of ferric hydroxysulphate to ferric oxysulphate and ferric oxide. Most pronounced mineral transformations occurred at temperatures at which the concentration of thermally produced organic free radicals was a maximum and at which extensive weight loss of the organic coal matter began. These results suggest that the oxidation of iron minerals and the free radical oxidation of the organic matter in coal may be associated with each other.     

Upgrading of syncrude from coal

The processing of coal-derived liquids to produce marketable products is being jointly investigated by Ruhrkohle AG and VEBA Oel. Experimental work was performed in process development units to find optimum process parameters for hydrotreating, hydrocracking and reforming. The removal of heteroatoms, hydrogen consumption, conversion rates and gas formation have been investigated under various process conditions. The range of applications of the different products has been studied.     

Generation of free radicals in partial oxidation of coal

Pyrolytic and oxidative treatments of two coals, Illinois No. 6 and Montour, have been performed between 140 and 400 °C. Free radical concentrations were followed by electron paramagnetic resonance as a function of time of treatment at a given temperature. Initial increases in the organic free radical concentrations were observed at all temperatures, but at the higher temperatures termination reactions caused the increase to be transient. Variations in the free radical concentrations of weathered and unweathered coal showed that the former produced a larger number of radicals. Possible effects due to native mineral matter are suggested.