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Metallurgical coke is an indispensable raw material in iron and steel production and is vital to the blast furnace ironmaking process. In a low-oxygen environment, coal is heated to 900–1100 °C and then dry distilled to create it (including coke breeze). The coal releases volatiles and moisture at this high temperature, and the remaining solid part is transformed into a porous carbon material called coke. Coke is not only the basis of ironmaking but also a vital driver of the entire ironmaking process.
In the blast furnace, coke provides a carbon source for the reduction reaction of iron ore and supports the stove’s structure to keep it stable. Coke reacts with oxygen in the air to produce carbon monoxide, which reacts with iron oxide in the iron ore to reduce it to metallic iron and release carbon dioxide.
Coke not only acts as a reducing agent, but its strong structure also supports iron ore and limestone, ensuring that everything in the furnace usually operates. The porous structure of the coke helps the gas flow penetrate and increases the efficiency of the reaction. The fine-grained structure of the coke (about 1.5 microns) optimizes the reaction environment inside the blast furnace, making the reduction reaction more efficient.
Specialized coke ovens generally carry out coke production. Construction teams typically build the coke ovens to a height of 6 meters, a depth of 15 meters, and a width of 0.5 meters, stacking multiple units to form a large-scale production system. The coal coke charge is heated to about 1000°C inside the coke oven by heating the walls.
During this process, volatile substances in the coal, such as gases and tars, are released and recycled to be converted into coke. Each coke oven can handle 15 to 30 tons of coal, gradually forming during a charring process of about 18 hours.
Volatiles are recovered as by-products and often used in producing fertilizers or chemicals, allowing coke production to go beyond meeting ironmaking needs and promoting efficient use of resources.
Several process parameters have an impact on the quality and productivity of coke. First, the strength of the coke is critical; it must be strong enough to support the iron ore load in the blast furnace. Secondly, they have to control the reactivity of the coke to ensure that it can efficiently reduce the iron oxidesFurther, the pore structure of the coke is equally critical; it must have good airflow permeability to ensure that the reduction reaction proceeds smoothly.
The fine-grained mosaic structure of the coke is an important quality indicator, which improves the reactivity of the coke and promotes the rate of the reduction reaction. Therefore, operators must strictly control parameters such as coal coke ratio, coke oven heating temperature, and carbonization time during production. These factors directly affect the final quality and production efficiency of coke. These factors directly affect the final quality and production efficiency of coke.
Metallurgical coke must resist crushing and abrasion to work stably in the blast furnace. Its pore structure also dramatically influences the gas flow, ensuring the reaction can proceed smoothly. Coke’s thermal conductivity, density, volatile matter, and moisture will also affect its performance.
The main components of metallurgical coke are carbon, hydrogen, oxygen, nitrogen, and sulfur, and the ratio of these elements directly determines its performance. Standard combustion methods to measure carbon and hydrogen content are high temperature, carbon into carbon dioxide, hydrogen into water vapor, and gas through absorbent absorption. Then, they calculate the weight gain to get the ratio. They generally analyze nitrogen content by converting it to ammonia, while they measure sulfur by high-temperature combustion. The simple difference method, which subtracts 100% from the contents of the other elements, extrapolates oxygen content.
Metallurgical coke is essential for blast furnace ironmaking, providing a carbon source for iron ore. As the demand for iron and steel increases, so does the need for met coke. It provides a carbon source as a reducing agent and helps gas flow penetrate through its pore structure, ensuring the blast furnace reaction can proceed efficiently.
However, the production of metallurgical coke also has environmental problems. For every 1,000 tons of coke produced, the process generates 1,000 cubic meters of highly hazardous wastewater. This results in the consumption of large amounts of water resources, a challenge the steel industry must now face.
Producing metallurgical coke will require specific chemical and physical properties. These properties will include the coke’s mechanical strength, pore structure, density, and thermal conductivity. Regarding mechanical strength, the focus will be on the coke’s compressive strength, crushing resistance, and abrasion resistance. These properties are essential because the coke’s ability to withstand loads and friction prevents it from crushing at high pressures.
Coke has a good pore structure, such as its porosity and the proper distribution of pore diameters, which have an essential impact on the permeability of the gas flow and improve the efficiency of the blast furnace reaction. Moreover, an essential factor that affects the pore structure is related to the coke’s density.
The critical factors affecting the pore structure are associated with the density of coke because the density will reflect the tightness of the pore structure of coke. An in-depth understanding of the density will find that it also affects the mechanical strength of coke, so the higher the density of coke, the better its mechanical strength.
Finally, the thermal conductivity in the physical properties also impacts the uniform distribution of the blast furnace temperature, and the thermal conductivity of coke affects the thermal conductivity of the blast furnace.
The chemical property requirement mainly focuses on Coke’s carbon, hydrogen, oxygen, nitrogen, and sulfur content. For example, the process substantially reduces the high carbon content of coke, affecting its reduction reaction ability in the ironmaking process. The sulfur content is also strictly controlled because a high sulfur content will affect the ironmaking process and reduce the reactivity of coke. The operators must ensure that the high content of these chemical elements does not affect the ironmaking process, and they must pay attention to harmful elements in coke like phosphorus and vanadium. By controlling their levels, technicians can prevent a reduction in coke reactivity.
Metallurgical coke and foundry coke belong to the category of coke, a solid fuel. However, 90% of met coke is primarily used in blast furnaces for iron making, which is called blast furnace coke. Casting coke, used explicitly in melting iron furnaces, is the primary fuel for this process.
The coke’s reaction rate, porosity, mechanical strength, and other properties reflect their differences. For example, metallurgical coke used in the blast furnace has a higher reaction rate to support the reduction process in the blast furnace. However, foundry coke requires heat, so the reaction rate of coke is lower. Metallurgical coke has a large porosity (35%–50%).
Foundry coke needs to provide better heat transfer efficiency and requires a more minor porosity to make the coke more compact. In addition, met coke needs a higher degree of mechanical strength. Foundry coke needs a higher pseudo-density to increase the calorific value and high temperature resistance.
Metallurgical and casting coke significantly differ in production and performance requirements. Still, they have the same standard regarding ash and sulfur content control.
Petroleum coke is a combustible solid product different from metallurgical coke in composition, physical properties, and application areas.
First, metallurgical coke’s main component is carbon, which contains a certain amount of volatile matter, ash (10%), and sulfur. Carbon is the main component of petroleum coke, with a low ash content (5%), resulting from removing many magazines from raw materials after coking.
Physically, metallurgical coke has high mechanical strength and large porosity (35%–50%). Petroleum coke has a dense particle structure and honeycomb, a large porosity, and an oval shape.
Metallurgical coke is mainly used in blast furnace and converter processes for field applications. It is also an essential raw material for the iron and steel industry, whose primary role is participating in metallurgical reactions to produce molten iron.
Petroleum coke’s primary use is to manufacture graphite electrodes, and it is also widely used in electric arc furnace ironmaking, creating highly conductive graphite materials.
Recently, metallurgical coke prices in the Chinese market have been on an overall downward trend, with dry quenched coke prices now standing at 295 USD/ton and wet quenched coke at 280 USD/ton.
Metallurgical coke and coking coal prices are still under downward pressure in the short term, significantly as the supply of metallurgical coke and coking coal exceeds demand and is expected to continue to show a steady downward trend.
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