coking
DESCRIPTION
CokingTRANSCRIPT
Cost contribution from coke
• Production of coke constitute 60% of the hot metal cost
• Approximately 50% of the finished steel cost is due to cost of coke
• World figure of 53.8% is the cost of coal against the finished steel cost
Estimation of coke consumption
• Coke rate ( coke consumption in kg per ton of hot metal)
VSP - 530 Kg / thm
• Coal to hot metal ratio ( Coking coal consumed for production of one ton of hot metal)
VSP – 800 KG / thm
Why Coke making• 60% of steel produced in
the world still under conventional route of coke oven and BF.
• Still Blast Furnace route of iron making is dominant
• Added advantage of gas yield, ease of operation control in spite of higher cost of production
• With increased focus on increasing the productivity of BF, supply of higher quality coke is a priority
WHAT IS COKE
It is a product of carbonization of coal in a oven heated to 1200 OC, without the presence of air. It is a re-crystallized carbon form incorporating mineral matter and other tramp material.
Role played by coke in blast furnace
• Thermal Role Provide heat for smelting of iron ore
• Chemical role1. Provide reductant such as CO to reduce iron
ore
2. Direct reduction of various oxides
Physical Role1. Coke supports weight of burden
2. Coke plays a significant role in gas distribution
3. Coke acts as a spacer below the cohesive zone providing a permeable bed
• In high capacity and low coke rate furnace physical role of coke is critically influence the technological limit of the process such as;
Maximum Driving Rate
Fuel utilization
Campaign life
Coking behavior of coalTesting of plastic property in Plastometer
TEMPERATURE
PLA
ST
ICIT
Y (
DD
PM
)
5 ddpm 5 ddpm
Initial Softening Temp. (IST)
Final Softening Temp. (FST)
Maximum Fluidity
Fluidity Range
What is coke making Coke is a product of heating of
coking coal in absence of air at around 1000 OC by which time coal loses all its volatile matter and also the solid residue gets recrystalised in to a hard mass, called coke. This process of conversion of coal to coke is called carbonization. The solid product of carbonization is strong and can resists degradation inside the blast furnace caused by burden weight as well as abrasion.
Four facets of coke making
There are four major steps in producing metallurgical coke suitable to blast furnace. They are:
1. Coal quality and blend designing
2. Pre-carbonization treatment
3. Carbonization factors4. Post carbonization
factors
SCHEME OF COKING
Physical weakening of bond
Devolatalization
Formation of metaplast / solvolysis
Agglutination and assimilation of inerts
Recrystalization in to specific carbon order(Semi coke)
Formation of final coke texture
Scheme of Pyrolysis of coal
sR
OHO
CH3
C2H5
OH
CH3
Cra
ckin
g
R’
Aromatization orCondensation
s R
OH
+
CH3
CH3
CH3
OH
R’ + H2O
OH
s
R
CH3
CH3
OH
R’
+ C2H6+2H2
Pyrolysis & Formation of Metaplast
• Pyrolysis of coal consists of two parallel reaction;
Cracking reaction
Condensation reaction • Cracking reaction forms mare
tarry liquid which peptize coal particles and form plastic mass
• Higher mataplast formation is good for coking
• Predominance of cracking reaction necessitates higher hydrogen content in coal
• Hydrogenation of coal and macerals rich in hydrogen content contribute higher plasticity
Rank of coal & its effect on plasticity
• Hydrogen & oxygen content in coal strongly influence the formation of metaplast during pyrolysis
• At lower rank coal richer in hydroxyl group does not peptize to form metaplast
• As rank increase reduction in hydroxyl group facilitate initial cracking followed by condensation. This help in coking
• At very high rank cracked product no longer remain in fulid state.
Effect of coal elementary composition on coking
• Coal at same rank will exhibit difference in plasticity due to its elementary composition
• Higher hydrogen in coal facilitate higher plasticity
• Higher oxygen in coal has the opposite effect
• Role of Sulfur is similar to that of the oxygen
• Higher H/O index in coal having same vitrinite reflectance will give higher plasticity
Condensation & resolidification
• When coal under pyrolysis get depleted with hydrogen, condensation reaction predominates
• More C-C bonds are formed with joining of aromatic rings
• Onset of resolidification starts with increase viscosity of the melt due to higher condensation reaction
• Occurrence of condensation reaction necessitates presence of hydroxyl groups
Coal to coke transformation
Time
Tem
pera
ture
Wet coal
Water Driven off
Dry coalHeat up
Plastic phaseTar and oil Driven off
Semi cokeGas drivenoff
Coke textureH2 driven
Precarbonization treatment
• Selective crushing
• Oil addition in charge
• Humidification of charge
• Preheating of charge
• Hot briqueting
• Stamp charging
• Blending of coal
Carbonization factors
• Heating rate or the rate of carbonization
• Charging methodology
• Oven width
• Oven height
Post carbonization factors
• Dry cooling of coke
Coke Dry cooling
• Coke stabilization