Leon Petrakis

13C Nuclear magnetic resonance characterization of coal macerals by magic-angle spinning

Abstract 13C cross-polarization and magic-angle spinning (CP/MAS) nuclear magnetic resonance (n.m.r.) experiments are reported on two fusinites, three vitrinites, one micrinite, one sample of attrite, one exinite, one resinite and one lignite. With one exception, the maceral spectra show no more fine structure than the CP/MAS n.m.r. results for whole coals. The maceral spectra are discussed in relation to each other and to the corresponding spectra of whole coals.

Free radicals in coals and coal conversion. 3. Investigation of the free radicals of selected macerals upon pyrolysis

The nature and concentration of free radicals in natural and pyrolysed samples of several high-purity resinites, vitrinites and fusinites have been determined by electron spin resonance (e.s.r.) spectrometry. Careful measurements of the g-values, linewidths and integrated intensities have been made. The natural concentration of free radicals in the various macerals was measured and found to vary by approximately 5 orders of magnitude. Of the three macerals, the free radical concentration of vitrinites was found to have the greatest dependence upon pyrolysis temperature. The g-values measured offer insight into the chemical nature of the free radicals and how they are altered upon pyrolysis. Interpretation of the data is based upon earlier work on pure macerals and coal pyrolysis. The implications of the nature and behaviour of free radicals in macerals in coal-conversion processes are discussed.

Free radicals in coal and coal conversions. 7. An in-depth experimental investigation and statistical correlative model of the effects of residence time, temperature and solvents

Abstract Previous papers in this series have described the methodology for in-situ studies and the relative effect of various process variables on the concentration of free radicals from a Powhatan No. 5 coal. This paper describes the role of temperature and solvents for the same coal. In addition, a correlative mathematical model has been developed that fits the data and is used to examine the sensitivity of free radical formation to the various process variables. E.s.r. measurements have been made at various temperatures from 400–480 °C. The solvents that have been used are tetralin, naphthalene and SRC-II heavy distillate. The main conclusions from this work are: 1. (1) in general, the effectiveness in quenching the coal free radicals increases in the order: naphthalene, SRC-II heavy distillate, tetralin; 2. (2) the free radical concentration of the liquefaction slurry generally increases with increasing temperature; and 3. (3) the rate of quenching of free radicals by SRC-II heavy distillate is closer to that of tetralin at lower temperatures and approaches that of naphthalene at the higher temperature. From the results of the full correlative model, it has been found that changes in temperature, nature of solvent and residence time, and their interactions, account for approx. 90% of the changes noted in the free radical concentration. The spin concentration is dependent upon reaction time, and this dependence is also influenced by solvent and temperature. The activation energies for quenching the free radicals are distinctly different for the different solvents.

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

Abstract 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.

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

Abstract 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.

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

Abstract 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.

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. 10. Kinetics and reaction pathways in hydroliquefaction

Abstract The development is reported of a point rate model for the solubility fractions obtained from Powhatan No.5 coal liquefaction in terms of free-radical concentration. Stepwise regression procedures were used to determine rate constants for general hypothetical rate models. Chemical principles were used at each stage to eliminate terms and to fix others. Rate constant expressions were obtained by fitting In k to the inverse absolute temperature for each solvent. Generalized reaction paths were determined from these individual reactions by combining various reactions. These reactions described the conversion of material between solubility fractions via free-radical interactions. In all three solvents progressive liquefaction reactions that did not involve measured radicals (conventional reaction terms) predominated in the rapid break-up of coal, while free-radical reactions are predominant in retrogressive changes. At higher radical concentrations retrogressive reactions predominated. More reactions of all types became significant with increasing temperature.

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

Abstract 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.

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

Abstract 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.