Coking properties

Seam correlations, measurements of rank and geologic history, interpretation of petroleum (qv) formation with coal deposits, prediction of coke properties, and detection of coal oxidation can be deterrnined from petrographic analysis. Constituents of seams can be observed over considerable distances, permitting the correlation of seam profiles in coal basins. Measurements of vitrinite reflectance within a seam permit mapping of variations in thermal and tectonic histories. Figure 2 indicates the relationship of vitrinite reflectance to maximum temperatures and effective heating time in the seam (11,15). 

Coking coal is cleaned so that the coke ash content is not over 10%. An upper limit of 1—2 wt % sulfur is recommended for blast furnace coke. A high sulfur content causes steel (qv) to be brittle and difficult to roU. Some coal seams have coking properties suitable for metallurgical coke, but the high sulfur prevents that appHcation. Small amounts of phosphoms also make steel brittle, thus low phosphoms coals are needed for coke production, especially if the iron (qv) ore contains phosphoms. 

A coherent plastic layer from a few mm to 2—3 cm thick separates the semicoke and coke from the unfused coal in the coke oven. Coking properties are assessed in Russia and some other countries by a measurement of the thickness of this plastic layer. A standardized test widely used in eastern Europe is the best known of this type (6) and involves a penetrometer used to measure the thickness of the plastic layer in a column of coal heated from the bottom. The various standard tests give results that are similar but do not give close correlations with each other. 

The surface of the coal particles oxidizes or weathers resulting in cracks, finer particles, and reduced agglomeration all of which may destroy coking properties. If spontaneous heating takes place, the calorific value of the coal is reduced. Hot spots must be carefully dug out and used as quickly as possible. Without spontaneous heating and with good compaction, calorific value losses below 1%/yr have been recorded. 

Backateinofen, m. brick kiln, bickt, pr. S sing, (of backen) bakes, etc. Back-Termdgen, n. baking ability, coking property, etc. (see backen). -werk, n. pastry. Wert, m. baking value, -zahn, m. molar tooth, molar. 

Fixed carbon is the combustible residue left after the volatile matter is driven off. It is not all carbon. Its form and hardness are an indication of fuel coking properties and, therefore, serve as a guide in the selection of combustion equipment. Generally, fixed carbon represents that portion of fuel that must be burned in the solid state. 

In the present work, a mild thermal hydrogenation treatment was used to revive the coking properties of an oxidized bituminous coal. The coal studied was a severely weathered high volatile bituminous coal from eastern Canada. 

A severely weathered bituminous coal from eastern Canada was treated by thermal hydrogenation under various reactor conditions. The coking properties of this coal were found to be restored under appropriate hydrogenation conditions. The semi-coke of the hydrogenated coal exhibited an anisotropic coke structure. The size of the anisotropic domains in the semi-coke was found to depend on reactor temperature and hydrogen pressure during hydrogenation. 

Correlation of Coke Properties. This discussion has given qualitative trends in coke properties. More quantitative relations have been obtained from a series of cokes prepared in the laboratory from a constant feed material. Figure 4 summarizes the results. The friability (units are arbitrary), the per cent soluble in trichloroethylene, and the per cent hydrogen all increase as the per cent volatile matter in the coke increases (7). 

The vitrinite without any sheets appears to correspond to what Brown, Cook, and Taylor (2) have called vitrinite A that with sheets appears to correspond to vitrinite B. The existence of this kind of laminar structure helps to explain the observation that vitrinite B has a higher volatile matter yield than vitrinite A although some of the coking properties of the two vitrinite types seem fairly similar. Many of the coking properties in fact appear to be primarily related to the matrix vitrinite. 

The classes of coal are subdivided into groups according to their coking properties, as reflected in the behavior of coals when heated rapidly. A broad correlation exists between the crucible swelling number and the Roga index (ISO methods), and either of these may be used to determine the group number of a coal. 

Coals classified by class and by group are further subdivided into subgroups, defined by reference to coking properties. The coking properties are determined by either the Gray-King coke type of assay or the Audibert-Amu dilatometer test (ISO methods). These tests express the behavior of a coal when heated slowly, as in carbonization. 

Zimmerman, R. E. 1979 Evaluating and Testing the Coking Properties of Coal. Miller Freeman, San Francisco, CA. 

Weathering action of air and water on coal in surface stockpiles, causing size reduction, oxidation, and decreases of any caking or coking properties. 

There is a complex and little understood relationship between coke content, catalyst activity, and the chemical nature of the coke. For instance, the H/C ratio of coke depends on how the coke was formed its exact value will vary from system to system (Cumming and Wojciechowski, 1996). And it seems that catalyst decay is not related in any simple way to the hydrogen-to-carbon atomic ratio of the coke, or to the total coke content of the catalyst, or any simple measure of coke properties. Moreover, despite many and varied attempts, there is currently no consensus as to the detailed chemistry of coke formation. There is, however,... 

During this period, useful reviews were published. White (15) and Zimmer and White (16) describe the disclinations present in mesophase microstructures and their relation to coke properties. 

Inert material in pitch, the QI material, is not an inert constituent and its behaviour should be noted seriously, both with regard to the improvement and to the detriment of coke properties. 

Petroleum coke calcining is a process whereby green or raw petroleum coke is thermally upgraded to remove associated moisture and volatile combustible matter (VCM) and to otherwise improve critical physical properties, e.g., electrical conductivity and real density (JL ) The calcining process is essentially a time-temperature function the most important variables to control are heating rate, VCM to air ratio and final temperature. To attain the calcined coke properties necessary for its end use by the amorphous carbon or graphite industries, the coke must be heat treated to temperatures of 1200-1350°C (2200-2500°F), or higher, to refine its crystalline structure. 

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