Why Use Coke for Blast Furnace Ironmaking not Use Coal Directly?
Coke is an important raw material in the smelting process of the blast furnace, accounting for about 45%. Others include pellets, sinter, lump ore, limestone and so on. Regarding coke, some people may have doubts. Coke and coal are all high-heating fuels. Even some anthracites have higher calorific value than coke and are cheaper than coke. Why does the blast furnace not use coal directly?
1. Q1: What? What is coke? Where is better than coal? Is it the same as the charcoal commonly used at home?
Generally speaking, coke is the product of coal purification. Compared with coal, coke is less water and gas, and it also separates the gas that is useful and the tar that is harmful to iron making.
The daily coal is generally referred to as coal, and the commonly used charcoal is generally charcoal.
2. Q2: How? How is coke processed?
Put the coal isolated air into the “oven” (coke oven) for high-temperature baking. As the temperature rises, the coal is first dehydrated and degassed, and after 300 °C, it becomes soft and oily (tar), and then the semi-finished semi-coke (also It is called Lan Tan) and it can be burned directly. The temperature is raised to about 1000 ° C, and the semi-coke is then matured into coke by polycondensation.
Coke comes out of the “oven”. When the water is “sprayed”, the water is quenched. When it is placed in a “box” with inert gas, it is dry and quenched.
3. Q3: Why? Why use coke? Can’t you use coal directly?
Let me talk about the use of coke. In the blast furnace ironmaking, coke has the functions of column skeleton, reduction, carbon supply, and heat supply.
Column skeleton: The coke heap density in the blast furnace charge is the smallest, the gap is large, and the volume accounts for 35%-50% of the total volume of the charge. Coke is relatively strong, as long as it remains blocky above the tuyere area, which enables the column to have good gas permeability and keep the furnace indirect condition.
Reduction: The coke carbon content is above 85%. The high-temperature CO2 gas reacts with the carbon in the coke to form CO, and the CO further reduces the iron ore to form molten iron.
Carbon supply: The carbon content of pig iron is about 4%, all from blast furnace coke, accounting for 7%-9% of coke carbon content.
Heating: The combustion of coke, injected fuel and external hot air provide all the heat needed for blast furnace smelting, of which coke combustion heat supply accounts for up to 75%-80%.
The reaction in the blast furnace occurs between “one liter and one drop”, that is, the rise of the gas and the drop of the charge, which requires the column to have good gas permeability so that the raw material can be fully reduced. In blast furnace ironmaking coal is not as strong as coke, and crushing will cause a decrease in the permeability of the blast furnace. The calorific value and carbon content of some anthracite coals are higher than coke, but there are more harmful impurities such as phosphorus and sulfur in coal. As the combustion enters molten iron, the quality of subsequent steel will be affected. Comparing the two phases, coke is not only a fuel or a raw material in the blast furnace, but the coal can replace the coke in the heating performance. This can increase the proportion of coal injection, but the reduction reaction and the skeleton function still depend on coke. There is currently no better alternative solution.
