Coke is a solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatiles are driven off by heating without oxygen at temperatures as high as 1,000 °C (1,832 °F) so that the fixed carbon and residual ash are liquefied and fused together. Metallurgical coke is used as a fuel and as a reducing iron ore to iron in the blast furnace. The product is cast iron and is too rich in dissolved carbon, and so the carbon is reduced next in the steelmaking process.

The coke must be strong enough to resist the weight of overburden in the blast furnace, or cupola in a foundry application which is why coking coal is so important in making hot metals by the conventional route. The trend is the alternative to coke or DRI (direct reduced iron) which is typically higher in sulfur causing the rising demand of low sulfur carbons (anthracite coal) in the remaining steelmaking process.  Coke from coal is grey, hard, and porous and has an extremely high heating value.  Some cokemaking processes produce valuable by-products that include coal tar, ammonia, light oils, and “coal gas”.

The most commonly used metallurgical coke is “met-coke breeze” for foundries and steel plants. Met coke breeze is the result of screening Metallurgical coke so as to reduce the restriction of air flow through the lumps when used in the blast furnace or cupola and to ensure the cokes ability to become hot incandescent red and melt or reduce the iron ore or scrap present in the furnace.

Coke breeze is very similar to anthracite coal in chemistry but has lower volatiles and slightly higher fixed carbon values.  One of the drawbacks of coke breeze to anthracite is that met-coke is more abrasive and may cause wear problems in the elbow areas of pneumatic conveying systems.  This is typically overcome by blending the met-coke with other carbons to reduce the wear considerably.