The classification of coal in China has been adopted as one of China’s national standards appearing as Guo Biao # GB5751-1986.
China classification of coal was borne out of a necessity in the 1950s to support Chairman Mao’s urgent mandate to develop the steel industry. The Chinese engineers’ and scientists’ only access to information then was the Soviet Union and Poland, which for the latter was based on the German system. By the late 1950s, the system that emerged and still stands was tailored to classify Chinese coal for the unique requirements of iron and steel making and other metallurgical applications. The Chinese system has a distinct German-Polish imprint, as evidenced by the terminology used to designate the different classes of coal.
The Chinese classification is far more detailed and has a finer gradation than those in the West. For example, in the U.S. and Canada, coal falls into four classes: the highest rank anthracite, followed by bituminous, sub-bituminous, and then the lowest rank lignite.
The Chinese classification, however, grades the Chinese bituminous and subbituminous coal into 12 different categories, and within a few categories, there are even more sub-categories, for a total of 24 sub-categories. Each of the 24 sub-categories is structured around volatile matter and a caking index, as a way to help distinguish their suitability for making coke, producing coal by-products, coal gasification, pulverized coal injection (“PCI”) and a number of other applications. In general, higher rank coal has lower volatiles and lower rank coal the reverse. Selecting the right coal as a coking coal to produce coke is still one of the most important. However, as iron and steel manufacturers will confirm, the selection of the appropriate coking coal, and the appropriate mix or blend of coals to make the right coke for particular steel or metallurgical application is both a science and an art. The chemistry of coke making is complex and the attributes for what makes a high-quality, desirable coke are many.
Besides volatile matter, as with the Germans, the Polish and the Russians, the Chinese honed in on two other parameters: the caking properties and the plasticity of the coal to help in quantifying and classifying its coal types. Instead of relying on indices in common use in the West, the Chinese developed its own Caking Index, G, and its own Plasticity Index, Y.
The Chinese Caking Index, G, is a variation of the Roga Index. The index is the result of a laboratory test to measure the caking capacity of a sample of a blend of coal and anthracite to ascertain how well the materials would bind or fuse together. A test sample is heated to a very high temperature, under a small load, for a certain period of time, and the resulting coke button is screened in a drum tumbler. The percentage of coarse material remaining on the screen is the index. Higher indexed G indicates a coal with higher caking capacity. The Chinese test uses a smaller-size sample and one fewer turn of the drum tumbler than the Roga test. The Chinese test has been accepted by the International Organization for Standardization as ISO-15585 “Hard Coal-Determination of Caking Index”.
To make coke, coal is heated to a very high temperature and becomes soft and passes from a solid to a fluid plastic state, as volatiles are driven off, and the plastic mass fuses together and then re-solidifies to become a solid, but porous solid, which is coke. The Chinese Plasticity Index, Y, is a measure of the maximum thickness of the plastic mass at its peak and before it re-solidifies.
There are no direct parallel to the Chinese Plasticity Index in the tests employed in the West but there are similarities. The most commonly used in the West is the test for crucible swelling number, which measures how well a coal sample swells after heating. Another would be the Giessler plastometer test, which monitors of the fluidity and the viscosity of the plastic mass and the difference between the initial softening and reconsolidation. Still another is the Audibert-Arnu dilatometer test, which monitors the volume change of a heated sample of coal in the absence of air as a proxy measure of how well coals will blend, among other factors.
In practice, by 1986, when the current coal classification standard was introduced, the Chinese coal engineering community had determined the Chinese Plasticity Index, first in use in the 1960s, was not, by itself, a fine tool to help distinguish and classify the bituminous and certainly not the sub-bituminous coals.
Since then, reliance also has been placed on the use of the Caking Index G as a classification tool. Nevertheless, the Plasticity Index, Y, remains as a measure in the standard.