FUELS & FUELING METHODS Used in the Blast Furnace for Production of Pig Iron with special emphasis on ALTERNATE FUELING METHODS.
The various fuels used to charge the blast furnace may be enlisted as follows:
Natural Gas/ Coke Oven Gas
Liquid mixture of Oil-Coal in the form of Slurry
Addition of Gaseous Fuels through Stack Injection
Coke is a universal fuel used in the blast furnace.
It satisfies two fundamental working criteria of the blast furnace, namely:
It acts as a major reductant.
It acts as a supplier of heat.
Applications/ Functions of Coke in the B.F.
Acts as a fuel by providing for the thermal requirements in the furnace.
2C + O 2 = 2CO : ∆H o = -2300kcal/kg.C
Provides CO for reduction of iron oxides.
Reduces the oxides of metalloids, such as, Mn, Si, P, etc.
Carburizes the iron and lowers its melting point.
Provides permeability and mechanical support to the large charge column.
Physical properties of coke
Coke Size : Coke size must not be less than 40mm (40-60mm or 40-80mm) although the burden material may be in the range 10-30mm.
Coke Strength: It is the quality cohesion that prevents the coke from collapsing and tends to avoid the formation of small particles. There are various methods evolved for testing of coke strength . The most widely used are ‘ shatter ’ for impact and ‘ drum ’ test for abradability.
Chemical properties of coke
Carbon: The calorific value of the coke and the carbon content should be as high as possible. More importantly, carbon content should not vary much. If it does, it becomes difficult to control the quality of iron.
Moisture: For a lower coke rate and uniform running of the furnace, the moisture content of coke should be minimum and constant. According to experimental evidence, for every 1% change in moisture, the blast temperature increase needed is 25-50 0 C.
Coke Ash: It consists mostly of Silica and Alumina. A high ash content is very unfavorable for uniform furnace operation because the bosh slag becomes highly basic. Depending on the ore quality, the ash content of coke should not exceed 10-15 percent.
Sulfur: Sulfur removal through slag depends upon the slag bulk and basicity.
Good quality coke is becoming scarce, therefore inferior coke is being used in a modified manner to feed the blast furnace.
The coke is blended, mixed with peach as a bonding agent, briquetted and carbonized in coke-oven.
Such form cokes can be used as part replacement of the usual coke.
Benefits of using Form Cokes
Usage of upto 30% of form coke in the charge feed shows no detrimental effect in the final product as compared to 100% usual coke.
The contribution of form coke to the resistance to the flow of the gases is much lower than the usual ore.
With use of 100% form coke, the coke rate somewhat increases but the output doesn’t decrease since the cole consumption intensity increases.
The changing price structure and availability of liquid and gaseous fuels are making them competitive with coke.
Fuel oil can be supposed to consist of hydrocarbons of general formula (CH 2 ) n
Natural Gas has almost 100% methane and Coke-oven gas has about 25% methane.
The fuel gases may be injected in the blast furnace during the operation as well through various pipes provided in the stack region of the furnace, such a process is called stack injection.