Coking coals because of their special characteristics are suitable for carbonizing to produce blast furnace (BF) coke. Coking coal is also known as metallurgical coal. Fig 1 shows coking coal.
Fig 1 Coking or Metallurgical coal
Important properties required technically to produce good coke are
- Good coking and caking properties such as fluidity, dilatation and crucible swelling number etc.
- Appropriate rank indicated by reflectance value (MMR)
- Properties important for commercial as well as BF operation such as total moisture, ash etc.
- Properties which affects the properties of hot metal and steel such as percentage of sulphur and phosphorus.
Modern coke making practice involves carbonization of a blend of coking coals. Coking coals in blended form are carbonized in a coke oven battery to produce metallurgical grade coke which is suitable for the operation of a blast furnace. Each plant uses its own blend based on the facilities available. In some of the Japanese plants coal blends consists of up to 20 different coals ranging in quality from coals with very high coking properties to coals with almost zero coking properties. The blended coking coal should have desirable coking properties for producing coke of required properties for BF operation.
Quality requirement of coking coals
- Total moisture – It is limited to 10% maximum in as received condition. High moisture creates handling problem and lowers available carbon.
- Ash – It is limited to 10% maximum in air dried condition. High ash content reduces BF productivity and increases coke rate in the furnace.
- Volatile matter (VM) – The volatile matter in coking coals ranges from 20% to 35% in air dried sample. High volatile matter reduces the yield of metallurgical coke in coke oven battery but improves the coke oven gas generation.
- Sulphur – In coking coals sulphur is to be limited to 0.6% maximum in as dried condition. Higher sulphur in coking coals increases the sulphur content of hot metal in blast furnace.
- Phosphorus – Phosphorus content of coking coals is to be limited to 0.1% maximum in as dried condition. Phosphorus goes to hot metal in blast furnace creating difficulties in dephosphorisation during steel making.
- Crucible swelling number (CSN) – The range of CSN in coking coals is 3 minimum to 6 maximum. Prime coking coals have higher CSN values while soft coking coals have low CSN values.
- Maximum dilatation – The value of maximum dilatation is 55% minimum. It depends on the value of maximum dilatation of the coal blend components.
- Maximum fluidity – The value of maximum fluidity is 600 ddpm minimum in coking coals. Higher fluidity gives better flowability in the coke ovens.
- Alkalis (Na2O, K2O) – The alkalis in coking coals is to be controlled at 2% maximum in coal ash. High alkali content is not desirable in BF and also affects the BF lining adversely.
- Ash fusion temperature (AFT) – In coking coals higher AFT is needed and it should be 1450ºC minimum. AFT in coking coals is to be higher than coking temperature.
- Gray king coke type – The value of the Gray king is to be G 5 minimum in coking coals. For soft coking coal the limit of Gray king value is G min.
- Mean Maximum Reflectance (MMR) – The value of MMR in coking coals vary in the range of 0.85% – 1.35%. Soft coking coals have lower MMR while hard coking coals have higher MMR.
- Vitrinite – The value of vitrinite in coking coals is 50% minimum. For soft coking coals the limit is 45%.
- Vitrinite distribution (V9 – V14) – The value of vitrinite distribution in coking coals is 70% minimum.