Coal-Fired Power Generation Handbook. James G. Speight

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СКАЧАТЬ coal as an essential fuel in Britain from the time of the Industrial Revolution has led to the development of a system of classification in which code numbers are used to denote the different types of coal. The coal survey system (developed by the National Coal Board of the United Kingdom) is based on the coke-forming characteristics of the various coals as well as on the types of coke produced by a standard of coking test (the Gray-King carbonization assay). The system also employs the amount of volatile matter produced thermally by the various types of coal (Table 2.7). In this classification system, a three-figure code number is used to describe each particular coal (Table 2.7) – the lower numbers are assigned to the higher rank coal, i.e., anthracite. However, because of the various divisions of this particular system, such approximations have to be made with extreme caution and with extremely careful cross-checking.

      Although the English system does appear to have some merit because of the dependence on two simple physical parameters (i.e., the volatile matter content of the coal and the Gray-King carbonization assay), there are, nevertheless, disadvantages to the method, not the least of which is the susceptibility of the Gray-King assay data to oxidation (weathering) of the coal and, apparently, the time required to conduct the assay.

Table 2.6 Differentiation of coal rank, coal type, and coal grade.

Rank

Indicative of the degree of metamorphism (or coalification) to which the original mass of plant debris (peat) has been subjected during its burial history.

Dependent on the maximum temperature to which the proto-coal has been exposed and the time it has been held at that temperature.

Also reflects the depth of burial and the geothermal gradient prevailing at the time of coalification in the basin concerned.

Type

Indicative of the nature of the plant debris (proto-coal) from which the coal was derived, including the mixture of plant components (wood, leaves, algae) involved and the degree of degradation before burial.

The individual plant components occurring in coal, and in some cases fragments or other materials derived from them, are referred to as macerals.

The kind and distribution of the various macerals are the starting point for most coal petrology studies.

Grade

Indicative of the extent to which the accumulation of plant debris has been kept free of contamination by inorganic material (mineral matter), before burial (i.e., during peat accumulation), after burial, and during coalification.

A high-grade coal is coal, regardless of its rank or type, with a low overall content of mineral matter.

      2.3.5 International System

      The International System of coal classification came into being after the Second World War as a result of the greatly increased volume of trade between the various coal-producing and coal-consuming nations. This particular system, which still finds limited use in Europe, defines coal as two major types: hard coal and brown coal. For the purposes of the system, hard coal is defined as a coal with a calorific value greater than 10,260 Btu/lb (5700 kcal/ kg) on a moist, but ash-free basis. Conversely, brown coal is defined as coal with a calorific value less than 10,260 Btu/lb (5,700 kcal/kg). In this system, the hard coals (based on dry, ash-free volatile matter content and moist, ash-free calorific value) are divided into groups according to their caking properties (Chapters 5, 6). These latter properties can be determined either by the free swelling test and the caking property is actually a measure of how a coal behaves when it is heated rapidly (Speight, 2013). The coal groups are then further subdivided into subgroups according to their coking properties (which may actually appear to be a paradox since the coking properties are actually a measure of how coal behaves when it is heated slowly).

Briefly, coking coal is a hard coal with a quality that allows the production of coke suitable to support a blast furnace charge. On the other hand, steam coal is all other hard coal not classified as coking coal. Also included are recovered slurries, middlings, and other low-grade coal products not further classified by type. Coal of this quality is also commonly known as thermal coal.

Table 2.7 National coal board (UK) system of coal classification.

СКАЧАТЬ

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Class	Volatile matter* (% w/w)	Comments
101	< 6.1	Anthracite
102	3.1 - 9.0
201	9.1 - 13.5	Dry steam coal	Low volatile steam coal
202	13.6 - 15.0
203	15.1 - 17.0	Cooking steam coal
204	17.1 - 19.5
206	19.1 - 19.5	Low volatile steam coal
301	19.6 - 32.0	Prime cooking coal	Medium volatile coal
305	19.6 - 32.0	Mainly heat altered coal
306	19.6 - 32.0
401	32.1 - 36.0	Very strongly coking coal	High volatile coal
402	> 36.0
501	32.1 - 36.0	Strongly coking coal
502	> 36.0
601	32.1 - 36.0	Medium coking coal
602	> 36.0
701	32.1	Weakly coking coal