Metallurgical coke or Met coke in short is a hard carbon material produced in the process of the “destructive distillation” of various blends of bituminous coal. It is produced by carbonization of coal at high temperatures (1100°C) in an oxygen deficient atmosphere in a coke oven.
A good quality coke is generally made from carbonization of good quality coking coals. Coking coals are defined as those coals that on carbonization pass through softening, swelling, and re-solidification to coke. One important consideration in selecting a coal blend is that it should not exert a high coke oven wall pressure and should contract sufficiently to allow the coke to be pushed from the oven. The properties of coke and coke oven pushing performance are influenced by following coal quality and battery operating variables: rank of coal, petrographic, chemical and rheologic characteristics of coal, particle size, moisture content, bulk density, weathering of coal, coking temperature and coking rate, soaking time, quenching practice, and coke handling. Coke quality variability is low if all these factors are controlled.
The coal-to-coke transformation takes place as follows: The heat is transferred from the heated brick walls into the coal charge. From about 375°C to 475°C, the coal decomposes to form plastic layers near each wall. At about 475°C to 600°C, there is a marked evolution of tar, and aromatic hydrocarbon compounds, followed by re-solidification of the plastic mass into semi-coke. At 600°C to 1100°C, the coke stabilization phase begins. This is characterized by contraction of coke mass, structural development of coke and final hydrogen evolution. During the plastic stage, the plastic layers move from each wall towards the center of the oven trapping the liberated gas and creating in gas pressure build up which is transferred to the heating wall. Once, the plastic layers have met at the center of the oven, the entire mass has been carbonized. The incandescent coke mass is pushed from the oven and is wet or dry quenched prior to its shipment to the blast furnace.
In case of Non recovery or heat recovery coke plants the coal is carbonized in large oven chambers. The carbonization process takes place from the top by radiant heat transfer and from the bottom by conduction of heat through the sole floor. Primary air for combustion is introduced into the oven chamber through several ports located above the charge level in both pusher and coke side doors of the oven.
The water content in coke is practically zero at the end of the coking process, but it is often water quenched so that it can be transported to the blast furnaces. The porous structure of coke absorbs some water, usually 3-6% of its mass. In some of the coke plants dry quenching of coke is practiced.
Met coke is normally available in 3 varieties. These are coke breeze (size – 10 mm), nut coke (size +10 mm to – 25 mm) and blast furnace (BF) coke (+25 mm to – 80 mm). BF coke is shown in Fig 1.
Fig 1 BF Coke
Blast furnace coke has three major roles in iron making process: thermal, chemical and physical. The thermal role of blast furnace coke is being a source of fuel which provides the heat needed to melt iron and slag and for endothermic reactions inside the iron making blast furnace. The chemical role of blast furnace coke is producing and regenerating the reducing gases which are needed to reduce iron oxides; it’s also carburizing molten iron. The physical role of blast furnace coke is supporting mechanically the charge column and the permeable bed below the cohesive zone.
Metalurgical coke properties
BF coke has a porous, open morphology and in some cases it may appear glassy. BF coke has hardly any volatile content; however the “ash” constituents, which were the part of the original feed coal remains entrapped in the resultant BF coke. The bulk density of coke is typically around 0.78.
High quality coke is characterized by a definite set of physical and chemical properties that can vary with in narrow limits. The coke properties can be grouped into following two groups: a) Physical properties and b) Chemical properties.
Measurement of physical properties aids in determining coke behavior both inside and outside the blast furnace. The physical properties are given below.
- Mean coke size– It is the arithmetic mean size of the coke determined by hand sizing the coke over a specified series of screens. Normally the larger the size of the coke it is better. A narrow size distribution of coke is also desirable.
- Coke reactivity index (CRI) – It is measured by a laboratory test designed to simulate the loss of coke through reaction in the reducing atmosphere, as the coke makes its way down the blast furnace. Coke is heated up to 950 deg C in an inert atmosphere and held at that temperature in an atmosphere of CO2. The coke is cooled down under the inert atmosphere and the loss in weight expressed as a percentage is the CRI value of the coke. CRI measures the ability of coke to withstand breakage at room temperature and reflects coke behavior outside the blast furnace and in the upper part of the blast furnace.
- Coke strength after reaction (CSR) – This gives indication of the strength of coke after being exposed to the reducing atmosphere of the blast furnace. Coke, after exposure to the high temperature and CO2 atmosphere of the coke reactivity test, is subjected to a tumbler test to determine the CSR. CSR measures the potential of the coke to break into smaller size under a high temperature CO/CO2 environment that exists throughout the lower two-thirds of the blast furnace.
- Drum test – The test is based on Japanese standard JIS K2151. A 10 Kg representative sample of + 50 mm square hole coke is placed in the specified tumbler drum and rotated for 30 revolutions, removed, screened and replaced in the drum and subjected to a further revolution of 150 revolutions. The drum contains lifters that raise the coke and allow it to fall so that it undergoes a large number of impacts with the drum walls. The indices reported are percentages of material remaining on +15 mm square hole after 30 revolutions and the same after 150 revolutions. The larger is the value the higher is the coke strength.
- ASTM Tumbler test- In this test a 10 Kg representative sample of the – 75 mm +50 mm square hole coke is placed in a specified tumbler drum and rotated for 1400 revolutions. The test is based on American standard ASTM D3402. The coke stability is reported as the percentage of coke + 25 mm after 1400 revolutions and the hardness as the percentage of coke + 6.3 mm after 1400 revolutions. Higher values of these indices indicate the strength of the coke.
- Combined half Micum/Irsid test – In it a representative sample of +25 mm round hole coke is placed in the specified tumbler drum and rotated for 100 revolutions. The coke is removed, screened and replaced in the drum and subjected to a further 400 revolutions in the drum. The test is based on the international standard ISO 556. The following values are reported.
i) M40 – It is the percentage of coke remaining on the +40 mm round hole after 100 revolutions
ii) M10- It is the – 10 mm round hole coke after 100 revolutions.
iii) I 20 – It is the percentage of coke remaining on the +20 mm round hole after 500 revolutions
iv) I 10 – It is the – 10 mm round hole coke after 500 revolutions.
|Larger values of M40 and I 20 and smaller value of M 10 and I 10 normally indicate coke with higher strength.|
The most important chemical properties are moisture, fixed carbon, ash, sulfur, phosphorus, and alkalis. Fixed carbon is the fuel portion of the coke; the higher the fixed carbon, the higher the thermal value of coke. The other components such as moisture, ash, sulfur, phosphorus, and alkalis are undesirable as they have adverse effects on energy requirements, blast furnace operation, hot metal quality, and/or refractory lining. The percentage of ash and sulfur content in coke is linearly dependent on the coal used for production.
Uses of Met coke
Besides being used in blast furnace, sinter plant, steel making furnaces and ferro – alloy production, metallurgical coke has many more applications. It is used where a tough and resilient, high quality wearing carbon is needed. Met coke’s applications include for example: friction materials, conductive flooring, foundry coatings, corrosion materials, foundry carbon raiser, reducing agents, drilling applications, ceramic packing media, heat-treatment, oxygen exclusion and electrolytic processes. Met coke can be also used as a filler coke for the poly-granular carbon products.