Blast furnace coke size is a critical factor in controlling the availability of hot blast for steel production. The size of the coke affects the permeability of the coke bed, the ability of the coke to breathe, the intensity of the air flow results from the increased pressure from the outside air and the resistance which the air has to penetrate through the coke bed.
The coke size is determined by the type of furnace and its required operating parameters like fuel rate, heat, wind pressure and the size of the charging units installed. In general, the bigger the size of the coke, the more difficult it is to control the burning rate and the moisture and carbon reactivity levels of the furnace.
It is not uncommon to find that the size of the coke found in the lower part of the blast furnace stack is closer to the size of the coke entering the furnace, while the coke size in the upper stack is much smaller. The smaller size coke is required to reduce the pressure drop and to disperse the air and hot gas to the furnace in the most economical way.
The most commonly used size for blast furnace coke is between 6 and 8 millimeters for top charged furnaces and between 10 and 12 millimeters for bottom charged furnaces. The coke should also have a uniform size to ensure that it burns evenly throughout the furnace. The size of the coke must also be larger than the size of the charging unit to ensure that the desired lump size can be achieved.
The size of the coke can be adjusted by using different types of sieves or metal screens to select the right size granules. The size can also be adjusted by using metal sleepers placed on the grid plates which support the coke bed. The metal sleepers provide a higher resistance than the coke particles and can help in controlling the coke size in the furnace.
Coke size is an important parameter which affects the production rate of the furnace and it should be closely monitored by furnace operators. The right size of the coke can improve the uniformity of the burning rate, the production rate of the furnace and the quality of the end product. In addition, the right sized coke will also ensure that there is adequate permeability of the coke bed for the hot blast to be produced.
