coking

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