Coke is a solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents are driven off by baking in an oven without oxygen at temperatures as high as 1,000 °C (1,832 °F) so that the fixed carbon and residual ash are fused together. Metallurgical coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. The coking coal should be low in sulphur and phosphorus so that they do not migrate to the metal. The product is cast iron and is too rich in dissolved carbon, and so must be treated further to make steel.
Coke must be strong enough to resist the weight of overburden in the blast furnace, which is why coking coal is so important in making steel using the conventional route. However, the alternative route is to directly reduced iron, where any carbonaceous fuel can be used to make sponge or pelletized iron. Coke from coal is grey, hard, and porous and has a heating value of approx 7050kcal/kg and above. Some coke making processes produce valuable by-products that include coal tar, ammonia, light oils, and "coal gas".
Petroleum coke is the solid residue obtained in oil refining, which resembles coke but contains too many impurities to be useful in metallurgical applications.
For Coke we analyze Coke Strength After Reaction (CSR) and Coke Reactivity Index (CRI).
Coke Strength After Reaction:
A laboratory test designed to give an 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 carbon dioxide atmosphere of the coke reactivity test, is tested in a tumbler device to determine its strength.
Coke Reactivity Index:
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 950C in an inert atmosphere and held at that temperature in an atmosphere of carbon dioxide. The coke is cooled down under the inert atmosphere and the loss in weight expressed as a percent is the reactivity.
- M40 – the percentage material remaining +40mm round hole after 100 revolutions in a drum
- M10 – the percentage material –10mm round hole after 100 revolutions in a drum