Coal reaction, structural change

Once thermal reactions begin, the coal undergoes structural changes through spontaneous bimolecular reactions between coal constituents or solvent species (75) or through catalytic reactions accelerated by added catalysts or the inherent mineral components (76). Hence, the reactivity changes with the progress of these reactions. 

Structural change of solvents and coals after the liquefaction reaction. To analyse structural change of solvents and coals in the liquefaction reaction, the solvent and coal should be separated. 

Now, since each respective fraction shows a structural change along with the reaction course, it may be considered at the same time that it contains products from a higher molecular fraction. In other words, for instance i r A2 is produced from coal 2 which changes by hydrogenolysis and at the same time, the residue resultinq from the reaction from Fr Ai to Fr also appears in this fraction. 

When a coal or coal char containing volatile matter is initially subjected to an elevated temperature, a series of complex physical and chemical changes occur in the coal s structure, accompanied by thermal pyrolysis reactions which result in devolatilization of certain coal components. The distribution of the evolved products of the reactions, which initiate at less than 700°F and can be considered to occur almost instan-... 

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy has been proven to be an excellent means of characterizing coals and related materials. This report is devoted to the evaluation of the technique as a method for situ monitoring of the chemical structural changes wrought in reactions of coal with fluid phases. This technique does not require a supporting medium (matrix) which can contain chemical artifacts which inherently serve as a barrier for access to the solid coal. The rapid response of the Fourier transform infrared technique is further beneficial for kinetic studies related to combustion, liquefaction, gasification, pyrolyses, etc. Experimental equipment and techniques are described for studies over wide ranges of pressure (10 5 Pa to ca 1.5 x 10 kPa) and temperature (298 K to 800 K). 

The structural parameter changes of products of coal reduction under various reducing reaction conditions were followed up, and a discussion of the reaction mechanisms involved was made and the following conclusion were obtained. 

Short-Contact-Time Coal Liquefaction Products and the Initial Structure of Coal. Our own data and those from the literature indicate that no major changes take place in the different elements of coal structure under short-contact-time reaction conditions and at relatively low temperature. It is obvious, however, that fragmentation occurs by... 

More-complex models have been proposed to link the coal structure with volatile decomposition. The model, which considers the changes of functional groups and reactions of char-forming repolymerization (cross-linking), can predict gas species and tar in volatiles.34 36 Another model applies concepts of polymer decomposition to describe the release of tar fragments from the coal macromolecule.37 The models will not be summarized here since they are more complex than the single-step, two-step, and multiple-step models. Interested readers should consult the appropriate literature for more details. 

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