Coal is formed through the chemical and physical alteration of peat through compaction, decay and heat over time. Peat occurs in waterlogged areas, such as swamps or bogs, where plant matter is accumulated and buried by sediment, then compacted. Due to the increased heat and pressure, peat transition into lignite, sub-bituminous coal, bituminous coal, anthracite coal, and finally to graphite, a mineral comprised of pure carbon takes place.
The degree of ‘metamorphism’ or coalification undergone by a coal, as it matures from peat to anthracite, has an important bearing on its physical and chemical properties, and is referred to as the ‘rank’ of the coal. As the rank of the coal is increased, its moisture content and the volatile matter (VM) content is reduced and its carbon content and the energy content is increased. Coking coal is a type of bituminous coal. Different types of coal and their uses are given in Fig 1.
Fig 1 Types of coal and their uses
The term ‘coking coal’ is used to designate certain types of bituminous coals which, when heated at high temperatures (over 1,000 deg C) in the absence of air (carbonization), soften, liquefy, and then re-solidify into a hard but porous mass known as coke, used mainly in the production of hot metal in a blast furnace. Coking coals have specific properties which allow for the formation of coke. Only bituminous coals possess such properties, and in varying degrees. Coke strength is an indicator of physical strength of a coke made from a particular coal.
Coke is required to be strong, with as little degradation as possible, to support the iron ore and coke mix above it in the blast furnace. The larger the blast furnace the higher the strength of the coke is needed to support the load. As coke being the major raw material fed into the blast furnace, coking coals are to be of high quality to provide high thermal efficiency and metal reduction.
Coking coal is also referred as ‘metallurgical (met) coal’. In fact, metallurgical coal is a wider term since it also includes all those coals which are used in steelmaking and foundry. Coking coal is a naturally occurring sedimentary rock found within the Earth’s crust. Categories of coking coal include hard coking coal, semi-hard coking-coal or semi-soft coking coal, and coal for pulverized coal for injection (PCI). These apply to the different quality grades of the coking coal, all of which are used in ironmaking. Coking coal typically contains more carbon, less ash and less moisture than thermal coal, which is used for the generation of power.
Hard coking coal is used in the coke ovens. It is a necessary input in the production of strong coke. The semi-soft coking coal (also known as medium/weak coking coal) is used in the coal blends along with hard coking coal. Coal for PCI is used for its heat value and injected directly into the blast furnace as a replacement for coke. Coal for PCI reduces the consumption of coke per ton of hot metal as it replaces coke as a source of heat and thus reduces the requirement of higher quality, higher cost coking coal.
Blends of the metallurgical coals are generally formulated from a variety of different ranks, types and grades of coals sourced from different sources with the purpose of producing the highest quality coke at the lowest possible cost while protecting the coke ovens in which those blends are being carbonized.
Characteristics of coking coals
Coking coals are to be prepared for their charging into the coke ovens. There is substantial cost attached to the coal preparation. The ‘Hardgrove grindability index’ (HGI) is a good indicator for the expected crushing behaviour of the coal. The high HGI of a soft coal allows a crusher to be operated at a higher throughput with the same or lower power requirement. The size distribution of the coal is important since it can impact on the coal handleability in the bins/silos and in transfer conveyors.
Petrographic analysis of coking coals is a major tool for predicting their coking properties. Prediction through petrography is done in three steps. First step involves the identification of macerals which are describes as microscopically distinct organic entities in coal. Then, the macerals are grouped into reactive, semi-inert, and inert categories. These categories are partially based on maceral behaviour during carbonization. Lastly, the rank of coal is determined by measuring the reflectance and calculating the strength of the binding materials created through carbonization of reactives. Using these characteristics, the strength (rank) and inert indices of the coking coals are calculated. Coke of high stability can be produced from coal blends which have an optimum ratio of reactive components to inert components. The reactive components contribute fluidity to the coal and act as binders, while the inert components (either organic or inorganic) act as fillers in the formation of the coke structure.
Macerals are the microscopically recognizable individual organic constituents of coal. They are recognized on the basis of their reflectance and morphology. A given maceral can differ significantly in composition and properties from one coal to another. For some macerals, the variation depends mainly on the rank of the coal. Bituminous coal, of which the coking coal is a part, is generally comprised of three major maceral groups namely (i) vitrinite, (ii) exinite, and (iii) inertinite.
Vitrinite is the predominant maceral constituent in nearly all coals, originating from the woody tissue of plants. It is the most abundant of the macerals and matures the most uniformly throughout the coalification process. The reflectance of vitrinite in plane polarized light is frequently used as the decisive indicator of coal rank. Vitrinite is quantitatively the most important maceral, accounting for probably 60 % to 80 % of most of the worked coals, and it is the material primarily responsible for the characteristic coking behaviour of higher rank bituminous coals. In terms of coking properties of coals, vitrinite is predominate reactive binder forming the wall and pore structure of coke and acts as the adhesive necessary to assimilate and bond the aggregate, which originates with the inertinite group. The other important characteristics of the coking coal are described below.
Maceral analysis – Maceral analysis is important parameter since vitrinite and exinite are more reactive than the other species of coal. Maceral analysis is obtained by the microscopic examination of coal and is a volumetric distribution of macerals in a coal sample. Fine coal (not pulverized) is set into a small block of epoxy type material and one face is polished. A number of different points on the polished face (normally at least 500) are examined by the microscope and the maceral species observed are recorded. The maceral is the result of these observations presented in percentage.
Vitrinite reflectance – It is also known as mean maximum reflectance (MMR) and is used to determine the coal rank. It is measured by the percentage of light reflected off the vitrinite maceral at 500x magnification in oil immersion. The MMR values for the coking coals in case of low volatile coals are normally in the range of 1.42 % to 1.75 %, in case of medium volatile coals normally in the range of 1.05 % to 1.4 %, and in case of high volatile coals normally in the range of 0.70 % to 1.02 %.
Rank – It is measured through a process called petrography whereby the amount of light which is reflected off the coal is quantified as its ‘reflectance’ (R). The higher is the reflectance, the higher is the coal rank.
Hardness – The hardness of the coke produced increases the value of the coal. Coal can be classified as weak, semi-soft coking, semi-hard and hard coking coal.
Calorific value – It is the amount of chemical energy stored in a coal which is released as thermal energy upon its combustion. It is measured in kilo-calories. It is directly related to the coal rank.
Moisture content – It is determined by heating an air-dried coal sample at 105 deg C to 110 deg C under specified conditions until a constant weight is obtained.
Ash – It is left over inorganic residue after the coal is completely burnt. Increased ash (or more strictly mineral matter) decreases coke yield, increases slag volume in the blast furnace, and consumes more coke during the smelting operation in the blast furnace.
Ash fusion temperature – Ash fusion temperature (AFT) is the temperature which characterizes the behaviour of ash as it is heated. The temperature is determined by heating cones of ground, pressed ash in both oxidizing and reducing atmospheres.
Volatile matter – Volatile matter (VM) consists of substances other than moisture which are given off as gas and vapour during combustion. VM is measured by heating the coal and measuring the loss of weight at 105 degrees, when the moisture content is removed, then again at 950 degrees, when the VM is burned off. VM is generally an indicator of coal rank, but also an indicator of the amount of volatile substances in the coal which gets gasified and given off during the coking process thus impacting the coke yield. The higher VM in the coal results in lower yield of coke after its carbonization. The coal with a lower VM has a higher rank.
Plasticity –It refers to the melting and bonding behaviour of the coal. It is the ability for coal to soften and become plastic when heated, and then to re-solidify into a coke. It is an indication of the initial softening, chemical reaction, gas liberation, and re-solidification process within the coke oven. It is an important requirement in the coal blend and is required for the strength of the end product coke. The fluidity of the plastic stage is a major factor in determining what proportions of a coal is used in a blend.
Fluidity – Fluidity of coking coal is measured by Gieseler plastometer. In this test fine coal (not pulverized) is heated slowly and as it melts and passes through its plastic range, its fluidity is measured. Results are expressed as maximum fluidity in ‘dial divisions per minute’ (ddpm). Characteristics temperatures recorded are initial softening temperature, maximum fluidity temperature, and re-solidification temperature. The plastic range, which is the temperature range during which the coal is in plastic state, is also important. All coking tests are sensitive to oxidation but the Gieseler plastometer test is by far the most sensitive.
Maximum dilatation – Maximum dilatation test measures the expanding and contracting characteristics of the coal. The test is carried out in Audibert-Arnu dilatometer. Finely crushed coal is compressed into a pencil, which is heated slowly and as the coal passes through its plastic range, the pencil initially gets shorter (contracts) and then gets longer (expands). Measurements taken are the ‘maximum contraction’ and ‘maximum dilation (expansion)’, both expressed as a percentage of initial pencil length, such that maximum contraction is always positive, and the maximum dilation is positive when the pencil increases in length from the initial length and negative when the pencil decreases in length. Temperature of initial softening (first indication of the pencil contraction), and maximum dilation is also recorded. Results from this test are very sensitive to oxidation of the coal being tested.
Gray-King coke type – The test determines the coking properties. It is a simple coking test. Finely crushed coal is heated in a glass retort and the shape, texture, and appearance of the coke residue is compared to standards to give a letter, which is the Gray-King coke type. Values range from ‘A’ (no coking characteristics at all) to ‘G’, then ‘G1’ to ‘G9’ (superior coking properties.
Free swelling index – Free swelling index (FSI) also tests the plastic properties of coal. It is also known as crucible swelling number (CSN). However, its value as a rheological test is limited since it is more of a threshold test having little quantitative value. The test involves heating a gram of coal in a crucible to 800 deg C and then visually comparing the resulting coke button to a standard chart of shapes and sizes to determine the FSI value on a scale of one to nine (Fig 2). FSI values of hard coking coal are in more than 4 (normally in the range of 4-6), that of medium coking coal in the range of 2-4 and that of weak coking coal in the range of 1-2.
Fig 2 Standard chart of sizes and shape for FSI determination
Other rheological tests – These include the Roga index, the G caking index, and the Sapozhnikov plastometer. The sole-heated oven test, the pressure oven test, and the movable oven wall test are technically classified as rheological tests but are intended to measure the performance of formulated blends in terms of contraction away from the oven walls, the pressure exerted against the oven walls, and the quality of coke expected to be produced respectively.
Quality of coking coals
The quality of the coking coal is determined in terms of chemical elements, ash content, moisture, coking properties, and strength. The most important chemical elements of the coking coal consist of sulphur, phosphorus, and alkalis (such as sodium and potassium). Coking coals are to have low percentage of these elements.
Parameters used to evaluate the coking properties can be classified in three major characteristics namely (i) parameters describing quantity and quality of liquid matter during the process of coking, including maximum plastic layer thickness, maximum fluidity, and maximum dilatation, (ii) parameters focusing on coke profiles and geometry, such as free swelling index, and Gray-King index, and (iii) parameters regarding the coal caking ability to combine inertinite matter. The relationship between coking coal, rank, and fluidity is shown in Fig 3.
Fig 3 Relationship between coking coal, rank, and fluidity
Typical property requirements of the coking coals are given in Tab 1.
|Tab 1 Typical property requirements of coking coal|
|1||Total moisture||10 % max as received||High moisture creates handling problem and lowers available carbon|
|2||Ash||10 % max air dried||High ash reduces BF productivity and increses coke rate|
|3||Volatile matter (VM)||20 % to 35 % air dried||High volatile matter reduces BF coke yield|
|4||Sulphur||0.6 % max as dried||Part of sulphur goes to hot metal|
|5||Phosphorus||0.1 % as dried||Phosphorus goes to hot metal creating difficulties in dephosphorization during steel making|
|6||Free swelling index (FSI)||2 min – 6 max||Hard coking coals have higher FSI while medium coking coals have low FSI|
|7||Maximum dilatation||55 % min||Depends on other blend components|
|8||Maximum Fluidity||600 ddpm min||Higher fluidity gives better flowability in the coke ovens|
|9||Alkalis (Na2O, K2O)||2 % max in ash||High alkalis are not desirable in BF|
|10||Ash fusion temperature (AFT)||1450 deg C min||AFT is to be higher than coking temperature|
|11||Gray king coke type||G 5 min||For medium coking coal the limit is G min|
|12||Mean maximum reflectance (MMR)||0.85 % – 1.35 %||Medium coking coals have MMR in the lower range|
|13||Vitrinite||50 % min||For medium coking coals the limit is 45 %|
|14||Vitrinite distribution (V9 – V 14)||70 % min|
|Note: max means maximum and min means minimum|