Thermal reactions, coal liquefaction

A New Outlook on Coal Liquefaction Through Short-Contact-Time Thermal Reactions Factors Leading to High Reactivity... 

In catalytic coal liquefaction processes, reaction temperatures must be high in order to insure that thermal reactions disrupt the coal structure to the point that the catalyst can act on the products. 

To improve selectivity and conservation of hydrogen over present liquefaction technology in the conversion of coal to high quality liquids, we believe that thermal reactions should be kept as short as possible. Catalytic processes must be used for upgrading but should be used in a temperature regime which is optimal for such catalysts. 

A question then arises as to whether the CSD recovery is being limited by the preasphaltene content produced from direct products of coal liquefaction or whether by low liquefaction severity a more thermally sensitive product is produced resulting in retrogressive reactions of liquefaction products to "post-asphaltenes." There is some indication that "virgin" preasphaltenes, primary products of coal dissolution, are more easily recovered via CSD as shown in Table VII however, "postasphaltenes" made from thermal regressive reactions are not. 

We have studied the thermal decomposition of diaryl ether in detail, since the cleavage of ether linkage must be one of the most responsible reactions for coal liquefaction among the various types of decomposition reaction and we found that the C-0 bond of polynucleus aromatic ethers is cleaved considerably at coal liquefaction temperature. 

Fundamental studies of coal liquefaction have shown that the structure of solvent molecules can determine the nature of liquid yields that result at any particular set of reaction conditions. One approach to understanding coal liquefaction chemistry is to use well-defined solvents or to study reactions of solvents with pure compounds which may represent bond-types that are likely present in coal [1,2]. It is postulated that one of the major routes in coal liquefaction is initiation by thermal activation to form free radicals which abstract hydrogen from any readily available source. The solvent may, therefore, function as a direct source of hydrogen (donor), indirect source of hydrogen (hydrogen-transfer agent), or may directly react with the coal (adduction). The actual role of solvent thus becomes a significant parameter. 

The yields of the reaction of maceral concentrates with pyridine and iodine show some interesting trends and are given in Table V. Unlike the results from the thermal reactions such as vacuum pyrolysis (Table IV) or short contact time liquefaction (29), the vitrinites are more reactive than the spori-nites. The inertinites are less reactive but the magnitude of the difference in the comparison with the other maceral groups from the Indiana and Kentucky coals is much less than what has been found for the yields from the thermal reactions. 

Coal liquefaction studies with Illinois No. 6 coal in supercritical water/CO systems have demonstrated that a suitable model for liquefaction includes a branch point. Thus coal is partitioned between reducing (i.e. liquefying) steps, and steps where strictly thermal reactions consume convertible sites and yield unconvertible char. 

Absorption of carbon dioxide in a suspension of lime and thermal coal liquefaction are examples of Type I reactions. In the first example, calcium carbonate is produced by carbonation of suspensions of lime, whereas, in the second example, coal is liquified in the presence of hydrogen and oil to produce a host of products. These and several other examples of this type of reaction are summarized in Table 1-1. 

The process of converting coal to liquid products involves at least two, often overlapping, steps viz. coal depolymerization and product upgrading, the latter involving hydrogen transfer and heteroatom removal. Both processes employ catalysts, although considerable depolymerization does occur even under thermal conditions as discussed above. The role of the catalysts is to facilitate reactions, prevent retrograde reactions, and improve product selectivity. Thomas has reviewed direct coal liquefaction processes and... 

As already noted, the exact chemistry of coal liquefaction is, at best, difficult to define but there is the suggestion that the first reaction step in the direct liquefaction of coal is the thermal rupture of... 

Parulekar, S.J. and Y.T. Shah. "Steady State Thermal Behavior of an Adiabatic Three Phase Fluidized Bed Reactor-Coal Liquefaction Under Slow Hydrogen Consumption Reaction Regime." Chem. Eng. J. (1981). in press. 

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. 

A more complex reaction model was proposed from the results of a kinetic study of thermal liquefaction of subbituminous coal. Data were obtained over a temperature range of 673 to 743 K (752 to 878°F) at 13.8 MPa (2000 psia) by using two solvents, hydrogenated anthracene oil (HAO), and hydrogenated phenanthrene oil (HPO), at a coal-solvent ratio of 1 15. Results were correlated with the following model ... 

Carbonaceous solids appear as a result of retrogressive reactions, in which organic thermal fragments recombine to produce insoluble semi-cokes (59,65). Coke formation is observed during liquefaction of all coals and its extent can vary widely according to the coal, the reaction solvent, and reaction conditions. The predominant inorganic species produced during the process of coal... 

Conversion of coal to benzene or hexane soluble form has been shown to consist of a series of very fast reactions followed by slower reactions (2 3). The fast initial reactions have been proposed to involve only the thermal disruption of the coal structure to produce free radical fragments. Solvents which are present interact with these fragments to stabilize them through hydrogen donation. In fact, Wiser showed that there exists a strong similarity between coal pyrolysis and liquefaction (5). Recent studies by Petrakis have shown that suspensions of coals in various solvents when heated to 450°C produce large quantities of free radicals (. 1 molar solutions ) even when subsequently measured at room temperature. The radical concentration was significantly lower in H-donor solvents (Tetralin) then in non-donor solvents (naphthalene) (6). 

If the initial reactions of coal are purely thermal, one might expect that the H-donor level will be of minor importance if times are kept short. In fact, all coals contain a certain portion of material that is extractable by pyridine. On heating coals to liquefaction temperatures, some additional material also becomes soluble in even non-donor solvents. Thus, there is a portion of all coals which can be solubilized with little dependence on the nature of the solvent. 

Conclusions When coal is contacted with a non-donor supercritical fluid a part of the coal instantaneously dissolves in the. supercritical fluid. The dissolved coal undergoes liquefaction reactions which are thermal in nature resulting in toluene soluble products being formed from coal. These products can subsequently undergo retrogressive reactions yielding insoluble material. Hence the toluene solubles show a maxima in conversion with time. 

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