Coal behavior

Berkowitz, 1979). Since coal is a porous material, porosity can have a large influence on coal behavior during mining, preparation, and utilization operations. 

In contrast to proximate analysis, ultimate (elemental) analysis, and the physical properties, the mechanical properties of coal have been little used and are not often reported in the scientific literature. This is a serious omission because these properties are of importance and should be of consideration in predicting coal behavior during mining, handling, and preparation. 

If this book helps toward a better understanding of the criteria for determining the properties of coal, leading to an understanding of coal behavior, it will have served its purpose. 

Since these relationships only apply to specific coal types their application is limited, and it is unfortunate that composition and coal behavior do not exist in the form of simple relationships. In fact, classification by means of elemental composition alone is extremely difficult. Nevertheless,... 

Seyler s attempt at coal classification should not be ignored or discredited as it offered an initial attempt at an introspective look at coal behavior. 

Plasticized cell polymerization FIGURE 9.14 Coal behavior during heating through the plastic range. 

That coal contains low-molecular-weight extractable species is a fact. That these species may form a mobile phase within the macromolecular network of coal is effective in explaining some of the many facets of coal behavior, including physical phenomena such as porosity and solvent diffusion (Rodriguez and Marsh, 1987 Hall et al., 1992). However, that the constituents of this network can be extracted (unchanged) by thermal means or by solvent treatment after exposure of the coal to high temperatures where the stability of many organic species is suspect and that only the weak bonds are broken is open to question. 

An alternative choice is the representation of coal as a two-component system, thereby abandoning the concept of individual structures (Figure 10.35) (Haenel, 1992). Obviously during the use of such a model, the details of any chemical transactions may be missing (but they should always be borne in mind and diminished or ignored) and the model might be convenient to explain many, if not all, of the physicochemical aspects of coal behavior. A very worthy accomplishment, indeed. 

Acceptance of these premises would presumably (or, at least, hopefully) facilitate a better understanding of the concepts of coal behavior during utilization, such as in beneficiation, combustion, and gasification processes as well as in liquefaction processes. However, the behavior of coal does not and cannot be represented by an average structure. At best, determining an average structure for a heterogeneous material such as coal is a paper exercise that may bear little relationship to reality. 

Solvent extraction in coal research (Chapter 10) has been used for isolation and characterization of both soluble and insoluble coal fractions (Van Krevelen, 1957) and fall into four general areas (1) improvement in extraction yields or selectivity, (2) correlation of solvent swelling and extraction behavior to structural models for the insoluble organic portion of coal, (3) analyses of extracts to identify and quantify organic compounds in the raw coal, and (4) use of solvent extraction to predict or influence coal behavior in some other process such as liquefaction. 

But it needs to be emphasized that caution must be exercised in drawing too many strict (and/or inflexible) conclusions from activation energy data. This, of course, must also throw some doubt on, and add caution to, the inconsiderate use of model compounds as materials from which to project coal behavior during coal pyrolysis. However, on a positive note, the use of model compounds does offer valuable information about pyrolysis mechanism it is the means by which the conclusion is drawn with respect to coal that can hurt the effort. Finally, the concept of induced bond scission (McMillen et ah, 1989) also opens up the area of coal pyrolysis to the additional concept of selective bond breaking by addition of suitable reagents. 

Another aspect of coal behavior in relation to liquefaction that has also received (and is stiU receiving) some attention is the relationship of liquid yield to petrographic composition (Chapter 4) (Gagarin and Krichko, 1992). For example, vitrinite can be converted to liquid products readily as can exinite, but fusinite is quite resistant to liquefaction conditions and, thus, the petrographic composition of coals (whatever the rank) may be an important variable in determining the yields of liquid products. 

Petrography A branch of coal petrology specifically deals with the analysis of the maceral composition and rank of coal and therefore plays an essential role in predicting coal behavior. 

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