Coking coal is a bituminous coal that can be used in the production of coke which in turn is used in the blast furnace in the production of pig iron. Coking coals are able to be softened, liquefied and resolidified into hard and porous lumps when heated in the absence of air.
Two main types of coal are used to make coke for steelmaking.
Hard coking coal (HCC) This type of coal is used to make coke, by heating in the absence of oxygen.
HCC is structurally very strong and is layered in the blast furnace between iron ore to provide the energy to melt the iron and also an escape valve for gasses in the furnace. The volume of HCC required in a furnace depends on the size of the furnace, with more HCC required in larger furnaces as the weight of the burden is relatively higher.
Semi-soft coking coal This type of coal is similar to HCC but does not have the same physical strength, and is thus unable to hold up the burden in a blast furnace. It can be blended with HCC however, and can be used in a larger proportion in smaller furnaces where the structural properties are not so important. SSCC is used in coking coal blend but results in a weaker coke and more impurities.
Coking coal when converted into coke, yields a fuel with high carbon content.
High carbon content is what iron/steel manufacturers look for in a coke for a cost-effective reduction of iron ore. Besides high carbon content, a coke must be physically strong to withstand the violent reactions and the massive weight of materials above it in an oven. A coke must also be sufficiently porous to allow full reactivity with the oxygen in the hot air, while it is itself being consumed as fuel. Finally, a coke needs to have few, if any impurities. In the selection of coal as candidates as a coking coal, coke producers look for coal with low sulfur and low ash.
Besides carbon, volatile matter and moisture content, sulfur and ash are the other two principal constituents in a coal. Not all bituminous coal qualifies as a good coking coal. Chinese engineers and scientists have sub-classified bituminous and sub-bituminous coal into 24 subcategories. Only three of them, designated at Jiaomei JM Type 15, 24, and 25, can be used pretty much by itself as raw material for coke making. Different types of coals can be blended to make coke. When coal is heated to a very high temperature, the heated material begins to soften and it passes from a solid state to a fluid, plastic state. As volatiles are being driven off, the material swells. With the gases liberated, the plastic mass begins to reconsolidate to become a solid material, which is coke. If coal has too much volatile material, the resulting coke would not be strong enough. The walls in a much too porous coke would be thin and weak. If the coal has too little volatiles, the resulting coke would not be porous enough to provide the necessary environment to allow a robust interaction between the carbon in the coke and oxygen in the hot air. In a traditional coke oven, it has been found that blending only works for a limited number of coal types. Higher volatile coal generally is not suitable in a traditional oven technology.
High quality Jiaomei JM, Type 15, 24, 25 is harder to come by and is expensive. Industry looked for alternatives. Blending different types of coal has been an accepted practice. Blending is both an art and a science. In China, engineers have commercialized the technology of utilizing a wide variety of blended coals in a no-byproduct, heat recovery oven to produce coke. In the latter, instead of being captured, volatiles are being burned off in an oven. The technology and design are substantially different from traditional coke oven. The difference lies in part in the oven design and in part in the heat transfer mechanism to allow a full combustion of the volatiles. To compensate for the loss of revenue from giving up the high-value by-products, heat is recovered instead for use in producing electricity.
Coal is plentiful on earth. It is combustible. It has a relatively high energy density. It is transportable. Therefore, one of the most widespread uses for coal is as a fuel. Over 95% of the world’s total coal production is consumed for purposes as below:
1) To produce electricity Coal is burned in large industrial-size boilers lined on the inside perimeter with water-filled steel tubes to create high-pressure, superheated steam. The steam is passed to and moves through a series of large steam turbine generators. The steam would lose its pressure and would expand in volume. In the process, the thermal energy would be converted into mechanical energy, which would turn the blades in the turbine generator to make electricity.
2) To produce cement Limestone and a mixture of certain raw materials are heated to a very high temperature in a kiln to produce a substance known as clinker. Clinker is then mixed with gypsum and ground to a fine powder to make cement. Cement, when mixed with water, sand and gravel becomes concrete and is the basic building material in the construction industry worldwide.
3) To produce coke, which in turn is used to make iron and steel.
The first two commercial uses of coal (producing electricity and cement) select the type of coal to use primarily based on its heat content. Such coals are commonly referred to as either steam coal or thermal coal.
Lower rank lignite and sub-bituminous coals with lower sulfur and other objectionable mineral contents are generally the coal of choice. Some lower quality bituminous coal is also used to raise steam for power generation. Steam coal prices are always indexed to its heat content.
To produce coke for making iron and steel, coal’s thermal energy is only a small part of the decision in choosing the type of coal to use. The type and quality of the coal are the more important considerations.
Good quality coke is highly sought after by iron and steel producers. In recent years, the market price for good quality coke and therefore good quality coal has escalated and buyers are paying a substantial premium for it. An understanding of coke and its role in iron and steel making would be necessary to explain the qualities to look for in a coal suitable for coke making.
Materials that may be extracted from coal:
Lump Coke metallurgical coke, copper smelting, iron smelting, lead smelting and iron and steel casting.
Calcium Carbide acetylene chemicals.
Water Gas heating homes and industry chemical processing.
Industrial chemical processing, lime burning, beet sugar refining, manufacturing of mineral wool.
Screenings or Breeze iron-ore agglomeration, chemical processing, steam generation
Coal Tar carbolic acids, pharmaceuticals, cresole, lysol, photo developer, plastics, phenols, detergents, drugs, dyes, food preservatives, perfumes, rubber chemicals, weed killer.
Tar Bases pyridine bases, antiseptics, disinfectants, paint thinner, pyridine, clothes water proofing, sulfa drugs, synthetic vitamins.
Napthaline insecticides, fungicides, plastic dolls, explosives, moth balls, synthetic fibres. Heavy Oil dyes, embalming fluid, laxatives, wood preservatives.
Pitch electrodes, insulating, paving, roofing, storage batteries, water proofing.
Benzene synthetic fibres, nylon, aniline dyes, food preservatives, motor fuel, plastics, synthetic rubber, tanning fluids.
Toluene antiseptics, fingernail polish, printing ink, saccharine, TNT explosives, aviation gas, detergents.
Xylene motor fuel, gasoline solvents, herbicides.
Solvent Naptha rubber solvent, electrical - insulation, linoleum, varnish.
Coke is a solid, hard, porous and nearly 100% pure carbon material which is derived from heating coal in a coke oven to temperature as high as 20000C in order to drive out or liberate all the volatile constituents held inside the coal.
Volatile matter exists in coal primarily as organic material derived from the ancient fossilized vegetation. When liberated, the volatile material first appears as coal gas, light oil, and coal tar, which can be refined into coal chemicals as high-value by-products (e.g., toluene, benzene, phenol and numerous others).
Coke production is technology dependant. The traditional coke oven favored in commercial use captures the volatiles as by-products. The types and quality of coal that can be used are limited to a few. In the past 10 years, China has taken the lead to innovate and commercialize the no-byproduct, heat recovery coke-oven technology. One of the major advantages is opening up the possibilities of the type and quality of coal that can be blended for use as raw material for making coke.
Coke, produced in a traditional by-product recovery oven, has a typical relative density of ~0.78, is about 44% lighter than coal. Coke, produced from a no-by-product, heat-recovery coke oven, has a relative density of ~ 1.1 and about21% lighter than coal. Coke serves two main purposes: as a reducing agent to refine iron ore in a blast furnace to produce pig iron, and as a fuel to help to make it happen in the furnace.
Coke, limestone flux, and iron ore, and in that order, is filled or charged into from the top of the blast furnace, a tall, chimney-like structure. Charging the furnace with these raw materials is a very important step to control gas flow and chemical reactions inside the furnace. Hot air is blasted or blown into the bottom of the furnace to help the heating and combustion. Temperature in a blast furnace can reach as high as 1,3000 C to1,4000 C. Pig iron is the golden-color, hot, liquid metal that pours out of the bottom of a blast furnace. Pig iron is only an intermediate step to making other grades of iron (e.g., cast iron and wrought iron) and steel (e.g., carbon steel and alloy steel). Pig iron with its relatively high carbon content (~ 3.5 to 4.5%) is too brittle to be of much commercial use.
The entire process to produce pig iron, coke it is very important to possesses the right kind of physical and chemical properties. The words “thermally unstable” or “poor thermal stability” are often used to describe the characteristics of such poor quality coke. Coke also has to have little or no other impurities. To minimize any undesirable reactivity that may develop in the blast furnace, buyers look for coke with low sulfur, low ash, and low phosphorous. Depending on the specific applications, buyer may also try to avoid coke with other chemical constituents in the non-combustible portion of the ash.
In the commercial market place, coke is usually produced and sold as either foundry coke or metallurgical coke. In the trade, foundry coke refers to larger sizes of at least 80 mm and greater. Metallurgical coke is smaller in size; typically 45 mm to 60mm.
Foundry coke being larger is slower in reactivity as contrasted with the faster reactive metallurgical coke. Their usage depends on the end user’s applications. To produce coke with the above physical and chemical properties, the process begins with selecting the right kind of coal. In the trade, coal used to make coke is coking coal. In short; coke is one of important raw materials that is needed to produce pig iron in a furnace. Pig iron is used to make steel. Coke is used to extract iron from iron ore in a high-temperature furnace or oven. To make that happen, coke must be sufficiently porous and physically strong to withstand the violent chemical reactions in the furnace or oven, while coke itself is being consumed as fuel. And coke must be virtually free of impurities. The process all begins with selecting the right kind of coal as a coking coal in the first place.
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.