Stages of Coal Liquefaction

Prior to liquefaction, coal is often washed to remove inorganic minerals and dried. This process sometimes changes the structure and assemblages of coal macromolecules, which profoundly influences the reactivity of coal as described in Section XI. 

In the preheater, coal with or without catalysts is rapidly heated to reaction temperature in the presence of solvent and pressurized hydrogen. Ex- 

The preheated coal slurry (essentially liquefied) is sent to the reactor, where thermal and catalytic cracking, hydrogenation, and hydrocracking take place. These reactions occur rather slowly because fewer reactive bonds are involved in this stage, which produces distillate range small molecules. 

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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. 

In this paper, a number of low-severity liquefaction regimes are considered. The influence of different H-donor and non-donor solvents on primary conversions without a hydrogen overpressure is discussed in the light of other recent work (10-131. Also, it is demonstrated that oil yields broadly increase with decreasing coal rank in both H-donor extraction and dry catalytic hydrogenation provided that retrogressive reactions are avoided in the initial stages of coal dissolution. 

A study of catalysts for the first stage of coal hydrogenation has disclosed conditions for efficient liquefaction using cheap and readily available catalysts (27,34). 

Hence, the catalysts and catalyses of coal liquefaction are reviewed with reference to the above stages. 

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 difficult task of examining the role of catalysis in coal liquefaction has been taken on by Mochida and Sakanishi. They show the catalytic requirements in various stages of coal conversion and the many complex interactions of the catalyst with coal constituents. They also point out directions for future catalysis research needed for more economical coal liquefaction, a commendable feature for processes requiring a long lead time. 

Catalytic hydrogenation of phenanthrene to the octahydro-stage produces both sym- and asym-isomers, although the former predominate Additionally, interconversion of the two forms tends to occur at coal liquefaction conditions. Since the... 

If the mobile phase is present in a significant concentration, as suggested by the results of solvent extraction studies (1,8), the practical meaning of the mobile phase to coal conversion processes may be profound. In coal liquefaction, two stage processes emphasizing the mobile phase and the macromolecular structure separately could well be most economical. In devolatilization kinetics, at least two sets of kinetic parameters are necessary to model the devolatilization phenomena associated with the mobile phase and the macromolecular structure respectively since the mobile phase components devolatilize at much lower temperatures than the macromolecular structure components 0. In addition, the mobile phase appears to have a significant influence on the thermoplastic properties of coal (0 and thereby on coke quality. 

Kinetic studies ( ) of such systems Indicate that the Initial stages of liquefaction Involve conversion of the coal Into a more or less completely pyrldlne-soluble solid and thereafter Into a benzene-soluble material which Is gradually transformed Into a viscous liquid as increasing amounts of hydrogen combine with It. This process can be catalyzed by, e.g., cobalt molybdate, but proceeds rapidly even in the absence of catalysts. At 775 F (A00°C), total py-solubi1ity (and vSO per cent solubility In benzene) can be attained within less than 10 minutes. 

Multistage coal liquefaction has been proposed to consist of the following stages, as shown in Fig. 1 [this scheme is the basis for promising processes... 

Catalysts for coal liquefaction require specific properties. Catalysts of higher hydrogenation activity, supported on nonpolar supports, such as tita-nia, carbon, and Ca-modified alumina, are reasonable for the second stage of upgrading, because crude coal liquids contain heavy polar and/or basic polyaromatics, which tend to adsorb strongly on the catalyst surface, leading to coke formation and catalyst deactivation. High dispersion of the catalytic species on the support is very essential in this instance. The catalyst/support interactions need to be better understood. It has been reported that such interactions lead to chemical activation of the substrate 127). This is discussed in more detail in Section XIII. 

All of these problems are related to the performances of the catalysts used in coal liquefaction. Very active, durable, recoverable, and regenerable catalysts are most wanted in the primary liquefaction stage, where catalyst poisons from asphaltenes and minerals are most severe. Multifunctional catalysts should be designed by selecting supports with specific functions, such as strong but favorable interactions with catalytic species, resistance to poisons, and improved properties to allow easy recovery, while maintaining high activity. 

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