Coal liquefaction catalyst role

In coal liquefaction, highly dispersed, disposable, catalysts are needed because maximal contact between coal and catalysts is essential. It is assumed that one of the roles of the coal liquefaction catalyst is to assist in the rehydrogenation of the donor solvent (e.g. tetralin) by facilitating the hydrogen transfer from the gas phase.36,37... 

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. 

We have our work divided into process engineering, process chemistry, catalysis, and support technology. As an example, one of the indirect liquefaction projects, tube wall reactor, deals with the design and operation of high thermal efficiency catalytic reactors for syn-gas conversion. Other activities are coal liquefaction properties of coal minerals, the role of catalysts, coal liquid product stability, and environmental impact—to name a few. 

In recent years, it has been realized that mineral matter plays an important role in coal liquefaction (9-11), similar to the role of the added catalyst in the Bergius process. Several experimental techniques have been used to study the effects of minerals on coal liquefaction and to identify the specific catalytic phase (12). Most studies (12-14) strongly imply that the iron sulfides are the roost active species, and the other minerals appear to have little effect on enhancement of liquid yield or quality. 

The specific role of pyrite (FeS2) as a catalyst has been under investigation since pyrite was identified as the most active inherent mineral for coal liquefaction. Under liquefaction conditions, FeS2 is transformed into a nonstoichiometric iron sulfide, Fei-x (0 X 0.125). Thomas et al. (15) studied the kinetics of this decomposition under coal liquefaction conditions, and concluded that the catalytic activity of FeS2 is associated with radical initiation resulting from the... 

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... 

Solids play different roles in the different processes. In direct coal liquefaction, a part of the solid is dissolved in liquid (mainly in the preheater) and a part (i.e. mineral matter) may act as a catalyst for the hydrogenation reactions. In Fischer-Tropsch slurry processes, solids are catalysts. Finally, in chemical cleaning of coal, only a part of solid (i.e. sulfur) takes part in the reaction following the shrinking core diffusion/ reaction mechanism. The role of solids in the design and scaleup of the reactors for the three processes is therefore different. 

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. 

In preceding sections, fundamental coal chemistry, liquefaction mechanisms, solvent and catalyst characteristics were summarized briefly. In the following three sections, the roles and improvements in solvents and catalysts in multistage liquefaction processes are reviewed in more detail on the basis of recent progress in this area. 

While many studies indicate that pyrrhotites are probably involved in the liquefaction process, the exact mechanism by which pyrrhotite catalyzes the conversion of coal to oil is not clear. Based on the works of Thomas et al. (1 ) and Derbyshire et al., (13) one can suggest that a possible role of pyrrhotite is as a hydrogenation catalyst. However, more work is necessary on the surface properties of the pyrrhotites and the interaction with model compounds before a definite catalytic mechanism can be proposed. 

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