Solvent extraction of coal

Several methods can be employed to convert coal mto liquids, with or without the addition of a solvent or vehicle. Those methods which rely on simple pyrolysis or carbonization produce some liquids, but the main product is coke or char Extraction yields can be dramatically increased by heating the coal over 350°C in heavy solvents such as anthracene or coal-tar oils, sometimes with applied hydrogen pressure, or the addition of a catalyst Solvent components which are especially beneficial to the dissolution and stability of the products contain saturated aromatic structures, for example, as found in 1,2,3,4 tetrahydronaphthalene Ilydroaromatic compounds are known to transfer hydrogen atoms to the coal molecules and, thus, prevent polymerization 

Further details and specialized information on the mechanisms, product qualities, and processes applied to the heating of coal in solvents can be found in the abundant literature [20], What follows arc some of the results of research conducted at West Virginia University, where investigations of the conversion of coal into pitches suitable for graphite production have been carried out 

This section summarizes the coals used for the project, some of their characteristics, the preparation of extract and pitch materials, as well as the 

Three West Virginia coals were supplied by the West Virginia Geological Survey (WVGS). The particular coals were chosen on the basis of rank, petrographic composition, and mineral matter content The coals were limited to the bituminous rank since these coals are the most amenable to the NMP solvent extraction process and are mdigenous to the Appalachian region. Some of the coal characteristics are listed in Table 2. 

The extraction results are summarized in Table 3 The percent yield is  

In order to process larger quantities of coal, an extraction reactor was constructed. The extractor was essentially a standing metal cylmder sealed on the bottom, approximately 30 cm in diameter and 84 cm in length. An electrical belt-driven stirring mechanism was fitted through the top of the reactor to provide mixmg, and three elechical band heaters were wrapped around the outside to supply process heat. A water-cooled condenser was also installed to condense NMP durmg reflux The system could routinely extract 1 to 2 kg of coal with 20 to 25 L of NMP. 

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The Pott-Broche process (101) was best known as an early industrial use of solvent extraction of coal but was ended owing to war damage. The coal was extracted at about 400°C for 1—1.5 h under a hydrogen pressure of 10—15 MPa (100—150 atm) using a coal-derived solvent. Plant capacity was only 5 t/h with an 80% yield of extract. The product contained less than 0.05% mineral matter and had limited use, mainly in electrodes. 

Hubert Tschamler They were derived either from mean values observed for model compounds of known structure or from the properties of solvent extracts of coals. In the latter case an accurate value for %H i can be derived by measuring proton spin resonance in solution and this, combined with the optical densities observed in the infrared spectra, gives the specific extinction coefficients. (See J. F. M. Oth, E. de Ruiter and H. Tschamler, Brennstoff-Chem. 42, 378 (1961) also Ref. 7). 

Solvent extraction of coal can be explained by a series of independent first-order reactions, the residue of one reaction being the reactant for another reaction. It can be formalized as follows ... 

Figure 12. Variation of fraction extraction with time for the second stage of two-stage solvent extraction of coal first stage at 250°C. for 8 hours not shown...

Nondestructive solvent extraction of coal is the extraction of soluble constituents from coal under conditions where thermal decomposition does not occur. On the other hand, solvolysis (destructive solvent extraction) refers to the action of solvents on coal at temperatures at which the coal substance decomposes and in practice relates in particular to extraction at temperatures between 300 and 400°C (572 and 752°F). In the present context (i.e., the solvents extraction of coal), the solvent power of the extracting liquid appears to be determined solely by the ability of the solvent to alter the coal physically (by swelling). In this respect, the most effective solvents are aromatics, phenol derivatives, naphthol derivatives, anthracene, and phenanthrene. 

For the purposes of this chapter, the solvent extraction of coal is limited to those investigations and test methods that are separate form the high-temperature treatment of coal with solvents in which the production of liquid products (liquefaction) is the goal. 

Early work on the solvent extraction of coal was focused on an attempt to separate from coal a coking principle (i.e., the constituents believed to be responsible for coking and/or caking properties). But solvent extraction has actually been used to demonstrate the presence in coal of material that either differed from the bulk of the coal substance or was presumed to be similar to the bulk material. 

Coal rank has a considerable influence on the nature and amount of extracts obtained by the solvent extraction of coal (Kiebler, 1945). In addition, the soluble products of the extraction, whether they be called extracts or (incorrectly) bitumen, vary according to the means by which they are obtained. 

Thus, it is possible to deduce several preferred options in the solvent extraction of coals using the so-called specific solvents. For example, the yield of extract usually decreases (but may, on occasion, increase) with an increase in the carbon content of the coal over the range 80 to about 87% carbon. However,... 

In contrast to the previously reported work utilizing supercritical solvent extraction of coal, the major objective of our research effort is to develop a desulfurization process that will result in a solid product suitable for combustion in existing coal fired utility boilers. 

In summary, the results from this and other studies (30, 31, 32) unambiguously demonstrate that simple solvent extraction of coal liquids does not yield chemically well-defined fractions. Consequently, detailed molecular analysis is a prerequisite for an in-depth analysis/prediction of the production and/or upgrading of coal liquids and for the correct evaluation of process effectiveness. In addition, implementation of routine process control/monitoring schemes employing fractions obtained from separation of products/reactants necessarily requires calibration by detailed molecular analysis. Finally, the separation method(s) should produce fractions possessing chemical significance. 

In addition, the occurrence of picene derivatives as well as similar polynuclear aromatic systems (White, 1983) in the solvent extracts of coal might be cited as evidence for the inclusion of sterane-type materials in the precursor material to coal. This does assume that the skeletal structure remains intact throughout the maturation process and that there has been no degradation or even formation of new skeletal systems. Indeed, the biosynthesis of aromatic compounds is well documented, including the synthesis of aromatic species from nonaromatic precursors (Weiss and Edwards, 1980 Lee et al., 1981). 

The purpose of many solvent extraction investigations is to develop continuous processes for solvent extraction of coal for the production of carbon products, which include materials used in metals smelting, especially in the aluminum and steel industries, as well as porous carbon structural material (carbon foam and carbon fibers). 

When certain bituminous coals are heated, they soften and become fluid commensurate with the evolution of gas and tar (Van Krevelen et al., 1956 Waters, 1962). The plastic behavior is transient and the mass eventually thickens, swells, and fuses to form a porous solid or coke. This phenomenon is of the utmost importance to the production of metallurgical coke, and in other processes sensitive to caking and agglomeration of coal. Because of the impact of plastic behavior on industrial processes, the solvent extraction of coal has been studied in the past in an attempt to isolate the coking principle from the coal (Burgess and Wheeler, 1911 Illingworth, 1922 Dryden and Pankhurst, 1955). 

It was concluded that these nuclei occur in bituminous coal but are linked by more readily oxi-dizable structures. An examination of the solvent extracts of coal has produced evidence for the presence of short methylene chains which were part of neither an aliphatic chain nor an alicyclic ring. It was noted that a series of tetralin derivatives (or indane derivatives) and higher kata-condensed aromatics were prevalent in the extracts of this same coal. 

Furthermore, from a more practical viewpoint, the solvent extraction of coal has been used as a means of coproducing clean liquid transportation fuels as well as solid fuels for gasification. Coal solvents are created by hydrogenating coal tar distillate fractions to the level of a fraction of a percent, thus enabling bituminous coal to enter the liquid phase under conditions of high temperature (above 400°C [750°F]). The pressure is controlled by the vapor pressure of the solvent... 

Thus, the solvent extraction of coal, far from being a theoretical study, is, in fact, very pertinent to the behavior of coal in a variety of utilization operations. For example, the liquefaction of coal (Chapters 18 and 19) often relies upon the use of solvent and, in addition, the thermal decomposition of coal can also be considered to be an aspect of the solvent extraction of coal. The generation of liquid products during thermal decomposition can be considered to result in the exposure of coal to (albeit coal-derived) solvents with the result that the solvent materials being able to influence the outcome of the process. And there are many more such examples. 

The solvent extraction of coal is, in essence, a mild form of chemical conversion because in addition to the pnrely solvent action, there may also be molecular alterations that are definite and irreversible. Coal-solvent interactions are complex (Szeliga, 1987) but, in more general terms, extraction is usually enhanced by temperature in addition, the presence of hydrogen will significantly alter the molecular changes. 

In addition, coal tar fractions are effective solvents while paraffinic hydrocarbons are ineffective for the solvent extraction of coal. The former concept (i.e., the utilization of solvents similar to coal tar materials) is a feature of commercial hydrogenation recycle. Partially reacted liquid is recycled to form a paste with the ground coal fuel. The coal is partially dissolved and reacted while the recycle oil is also reacted to a further stage of conversion. 

Generally, with careful attention to coal history, solvent history, and solvent properties, the solvent extraction of coal may be a useful technique for application to coal science. Studies of the extraction process itself and the related solvent swelling can provide insights into coal matrix structure only with careful, well thought out, and reproducible analyses. The data may also provide detailed and valuable information about the chemical species present in both extract and raw coal. 

In addition, the solvent extraction of coal can be improved with various types of pretreatment, one of which is thermal pretreatment. For example, when certain coals were heated at 200°C-400°C (390 F-750 F) in an inert atmosphere, cooled, and extracted with solvents, the yield of extract is often higher than the yield of extract obtained with untreated coals for a specific type of solvent. 

The presence of oxygen during the solvent extraction of coal decreases the yield for most solvents. The presence of moisture in coal has an unfavorable effect on extraction yield for solvents that are miscible with water. The moisture content of coal is extremely important, especially for any extraction process in which the solvent would be recycled. 

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