Coke is the product that remains after the organic matter in coking coal decomposes when heated above 850°C in a boiler. As the temperature rises, volatile products are released, leaving behind the non-volatile residue known as coke. This includes combustible fixed carbon and combustion residues—ash. During combustion, it emits a brief blue flame and releases a significant amount of heat. The combustion process is slow and prolonged, and it is prone to forming an ash shell on the surface. To ensure complete combustion of the coke, it is necessary to remove the ash shell in a timely manner. The physical properties of coke, such as its cohesiveness, have a considerable impact on the operation of a fluidized bed furnace.
Coke production accounts for approximately 75% of coking products. It is primarily used in iron smelting, constituting about one-half to one-third of the production cost of pig iron. The blast furnace smelting process is essentially a reduction process for iron ore, with coke serving as both a reducing agent and a source of heat.
Coke can also be utilized in the fertilizer industry, where it reacts with steam and air to produce semi-water gas, which is then combined with hydrogen and nitrogen from the air to generate ammonia. Additionally, coke is a key reagent in the production of acetylene, calcium cyanamide, carbon disulfide, and electrodes, and it is an important raw material in the urban gas industry. Analyzing the distribution of coke production in China reveals an uneven geographical distribution of coking enterprises, primarily concentrated in North China, East China, and Northeast China.
The main component of coke is fixed carbon, with minimal volatile matter, resulting in smokeless combustion. Its calorific value ranges from approximately 25104 kJ/kg to 31380 kJ/kg. It appears silvery-white or gray-black, has a metallic luster, and is hard and porous. Large pieces are referred to as lump coke or metallurgical coke, while smaller pieces are called crushed coke, and powdered forms are known as coke dust. In the production of ferroalloys, coke is used as a reducing agent, with higher fixed carbon content and lower ash content being preferable. The particle size significantly affects smelting. Larger coke particles have lower electrical resistance and better conductivity, but they can complicate electrode insertion and increase thermal losses in the electric furnace. Conversely, smaller coke particles have higher electrical resistance and larger contact surfaces, facilitating deeper electrode insertion and reducing thermal losses; however, excessively small particles can decrease the permeability of the charge, leading to issues such as fire stalling.
Therefore, coke should have a suitable particle size. The size of the particles is related to the capacity of the furnace; larger electric furnaces require larger coke particles, while smaller electric furnaces require smaller particles. For example, in a furnace with a capacity of 12500 kVA, the coke particle size should be between 5 mm and 18 mm; for furnaces with capacities ranging from 400 kVA to 1800 kVA, the coke particle size should be between 1 mm and 8 mm, with particles sized 1 mm to 3 mm not exceeding 20%. During the smelting process, it is essential to regularly check the moisture content in the coke and adjust the charge ratio to maintain normal furnace conditions.