Mechanisms of coal liquefaction

Coal liquefaction basically follows the stages described below, regardless of the type of process  

Mixing finely-crushed coal with a suitable solvent, 

Hydrocracking the hot reaction mixture under pressure with molecular or transferable hydrogen (from donor solvents) using catalysts 

Depressurising the reaction product and separating the residue from the distillable portion and 

In the first stage of coal liquefaction, finely-crushed coal (particle size 0.1 mm) is slurried with a solvent, to render the coal flowable and pumpable. The choice of solvent is particularly important, since it must be suitable to stabilize the coal fragments and to dissolve the smaller disintegrated molecular moieties. Due to their similarity in chemical nature, coal-derived oils are particularly efficient. Anthracene oil from coal-tar processing was therefore preferred as a solvent, when coal hydrogenation was being developed. 

---

The effects of various reaction conditions on the reaction rate and the mechanism of coal liquefaction were investigated.Conclusions are summarized as follows ... 

Under the same reaction conditions, the reaction rate are depend on the mechanism of coal liquefaction and kinds coal and catalyst. The reaction rate is in the following order Morwell> Bukit... 

The object of this paper is to draw attention to the possible importance of concerted molecular reactions, of the type termed pericyclic by Woodward and Hoffman (1), in the mechanism of coal liquefaction. 

The preceding experiments offer preliminary support to our notion that pericyclic pathways might be intimately involved in the mechanism of coal liquefaction. More specifically, the results indicate that pericyclic group transfer reactions constitute a plausible pathway for the transfer of hydrogen from donor solvents to coal during liquefaction. 

Fully-deuterated xetralin was used to study the mechanisms of coal liquefaction. Experiments were conducted with xetralin-di2, deuterium g s and bituminous coal at 400°C and at 15.2-20.7 MPa. The recovered solvent and solvent-fractionated coal products were analyzed for total deuterium content and for deuterium content in each structural position. 

In an earlier presentation, we reported on a deuterium tracer method for investigating the mechanisms of coal liquefaction Q). 

The material presented herein was meant to provide an indication of the utility of ESR and ENDOR spectrometries as probes to characterize the free radicals in coals and coal-derived materials. The results suggest that, in addition to their use in characterization studies, the techniques may have potential in helping to elucidate the mechanism of coal liquefaction. They further suggest that coals may be heat-treated directly in the cavity of an ESR spectrometer with the assurance that reliable data will be obtained provided simple precautionary measures are taken to avoid unwanted effects. Similar in situ ESR measurement of coals under liquefaction conditions remains a possibility, although considerable... 

Chemical reactions such as pyrolysis, depolymerization, and condensation could be clarified. Moreover, application of the technique seems to be promising in the mechanism of coal liquefaction as well as in that of mesophase formation. 

In spite of the large volume of work, it must be said that the mechanism of coal liquefaction is not clearly understood due to both the complexity of products and the vast differences found among coals of similar rank—in other words, the pronounced heterogeneity of coal. Furthermore, the heterogeneity of coal has not deterred investigators devising kinetic models for the liquefaction of bituminous and subbituminous coals in tetralin and anthracene oil. The issue is whether or not such work and models bear any relationship to the realty of the coal liquefaction process. 

Szladow, A. J. "Some Aspects of the Mechanism and Kinetics of Coal Liquefaction , Ph.D. Thesis, Pennsylvania State University, 1979, 172 pp. 

Various mechanisms and kinetics of coal liquefaction have been proposed and examined by many investiga tors(l,2,4-8). As a general kinetic model of coal lique-action, scheme 1 was assumed. The reaction rate of every reaction step in the scheme assumed to be first order with respect to reacting species and dissolved hydrogen. A few typical cases of a general kinetic model and the general characteristics for their cases are illustrated on Table 3. When compared these typical figures, the curves are apparently different in shape. 

The difficulty in the recovery of catalysts from unreacted coal and minerals and the poor regenerability of used catalysts forces one to use disposable catalysts, especially in the primary stage. This increases the cost of coal liquefaction considerably. This section reviews the mechanism of catalyst deactivation, design of recoverable catalysts in the primary stage, and catalyst deactivation in the secondary stage. 

The chemistry of coal liquefaction is not very well understood, even after more than two decades of research into the kinetics and mechanism of the process. There have been a number of models for conversion proposed, most of them focused on the several liquefaction products, including preasphaltenes, asphaltenes, oils, and gases. A survey of some of the models has been presented (1 ), and a common feature among them is the multiplicity of paths connecting all of the components. 

At the present time, few, if any, details of chemical reaction mechanisms in coal conversion are known with certainty. This situation is particularly distressing in the areas of coal liquefaction and pyrolysis where chemical kinetics may strongly influence process efficiency and product quality. To improve this situation, in recent years a number of research groups have been performing chemical studies of coal and "model" compound reactions. 

This paper presents a brief state of the art review of direct coal liquefaction. The review Includes Important pilot scale processes available for the liquefaction and a brief description of the structure of coal and the chemistry, mechanism and available lumped kinetic models for the liquefaction process. It also Includes some discussions on the role of catalysts during coal liquefaction and on the use of model compounds for the understanding of coal liquefaction kinetics. Reactor design aspects are covered In a separate paper and will not be repeated here. 

Data for the kinetics of coal liquefaction have been published in the literature (1-11). A review of the reported studies has recently been given by Oblad (12). The reported data were mostly obtained in bench-scale reactors. Guin et al. (7) studied the mechanism of coal particle dissolution, whereas Neavel (7), Kang et al. (8), and Gleim (10) examined the role of solvent on coal liquefaction. Tarrer et al. (9) examined the effects of coal minerals on reaction rates during coal liquefaction, whereas Whitehurst and Mitchell (11) studied the short contact time coal liquefaction process. It is believed that hydrogen donor solvent plays an important role in the coal liquefaction process. The reaction paths in a donor solvent coal liquefaction process have been reviewed by Squires (6). The reported studies examined both thermal and catalytic liquefaction processes. So far, however, very little effort has been made to present a detailed kinetic model for the intrinsic kinetics of coal liquefaction. 

Most studies on the mechanism and kinetics of solvent extraction have necessarily used only a limited selection of coals. In a commercial environment where coals with widely varying properties are available, it is necessary to develop a generalised system of grading the coals with respect to their suitability for liquefaction. 

Neavel, R.C., MCoal Plasticity Mechanism Inferences from Liquefaction Studies , Proceedings of the Coal Agglomeration and Conversion Symposium, Morgantown, W.Va., May 1975 (.published April 1976). 

A considerable effort has been emended in the past few years by many researchers in attempts to better understand the mechanism by which coal is liquefied. From this work has emerged the concept of short residence time coal liquefaction which promises potential process advantages, small reactor, minimum hydrogen flow, and the efficient utilization of hydrogen for a particular product slate. 

In outline of what follows we will begin by brief reference to previous work on coal liquefaction. The present approach will then be motivated from considerations of coal structure and hydro-gen-donor activity. A theoretical section follows in the form of a pericyclic hypothesis for the coal liquefaction mechanism, with focus on the hydrogen transfer step. Experiments suggested by the theory are then discussed, with presentation of preliminary results for hydrogen transfer among model substrates as well as for the liquefaction of an Illinois No. 6 coal to hexane-, benzene-, and pyridine-solubles by selected hydrogen donors. 

The present approach to the coal liquefaction mechanism evolved from contemporary knowledge of coal structure (e.g. 11,... 

Direct Liquefaction Kinetics Hydrogenation of coal in a slurry is a complex process, the mechanism of which is not fully understood. It is generaly believed that coal first decomposes in the solvent to form free raclicals which are then stabilized by extraction of hydrogen from hydroaromatic solvent molecules, such as tetralin. If the solvent does not possess sufficient hydrogen transfer capability, the free radicals can recombine (undergo retrograde reactions) to form heavy, nonliquid molecules. A greatly simplified model of the liquefaction process is shown below. 

Much of the research pursued by the authors of this paper and by their associates has involved studies of the catalytic hydrogenation of coals in the absence of solvent. The technique has been used to elucidate the mechanisms of catalytic coal liquefaction and to provide simultaneously some insight into the structure of coals. Peter Given was directly instrumental in providing the incentive for this research which has extended since 1983. Previous findings were disseminated through several publications (4-8. In this paper, some of the earlier data have been collated with more recent results (9) to provide an account of the relevance of these studies to the two-component concept. 

---