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Companhia Vale do Rio Doce

COKEMAKING PROCESS

Uses of Coal in Cokemaking Process

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Companhia Vale do Rio Doce

PART X

IRONMAKING COURSE

March 12th, Belo Horizonte, Brazil

Uses of Coal

Uses of Coal and the Future of Coke

Heating Value

LOW RANK COAL48%

HARD COAL52%

LIGNITE20%

SUB-BITUMINOUS28%

HIGH

BITUMINOUS51%

ANTHRACITE1%

STEAMTHERMAL COAL

METALLURGICALCOKING COAL

Electricity generationCement makingIndustrial uses

Mainly for electricitygeneration

Pig iron andsteel making

Industrial use andas smokeless fuel

HIGH

Moisture

USES OF COAL(WORLD COAL

INSTITUTE)

Uses of Coal and the Future of Coke

The main function of the coke in the blast furnace is as “process permeabilizer”. Without that function it would not be possible the iron-making in the blast furnace. Worldwide the production of hot metal and crude steel via the blast furnace/converter route is regarded as the dominant process line also in the next two or three decades (there's no metal-production system that can beat the blast furnace). In other words, the coke still has an on-life at least equal.

Coke in the Production of Hot Metal (I)

The principle of iron-making in a blast furnace is that iron ore is reduced by carbon from coke to form metallic iron and carbon dioxide. The production of hot metal in an integrated steel plant involves three basic units: coke oven, sintering and blast furnace.

Coke Oven Gas and By-ProductsBlast Furnace Top Gas

Fine Iron Ore

Coking Coal

AnthraciteSinter

Pulverised Coal

Blast Furnace

Breeze

Fluxes

Hot MetalSlag

Coke Oven

Sintering

Coke and Small Coke

Lump Iron Ore

Coke in the Production of Hot Metal (II)By-Product Cokemaking. The process is developed in a wet-charge, by-product coke

oven and is comprised of the following steps: i) the coals are reclaimed from stockyards, crushed in hammer or impact mills to about 85% under 7 mesh and mechanically blended according to a formula of a multi-component coal blend, iii) the coal blend is charged into a number of slot ovens wherein each oven shares a common heating flue with the adjacent oven and iv) coal is coked at a temperature of about 1250°C for about 18 hours in the absence of air and the off-gas is collected via the ascension pipes and crossover mains to the by-products area, where it is cooled, scrubbed, and many valuable by-products are extracted.

Coke in the Production of Hot Metal (II)

Coal-to-Coke Transformation Steps: i) the heat is transferred from the heating walls (indirect heat is applied by means of gas firing) into the coal charge, ii) from about 350°C to 475°C, the coal decomposes to form plastic layers near each wall (during the plastic stage, the plastic layers move towards the centre of the oven trapping the evolved gas and creating a gas pressure build up which is transferred to the heating wall and is traditionally known as coking pressure), iii) at about 475°C to 600°C, there is a marked evolution of tar, and aromatic hydrocarbon compounds, followed by solidification of the plastic mass into semi-coke, iv) at 600°C to 1100°C there is a contraction of coke mass, structural development of coke and final hydrogen evolution. The incandescent coke mass is pushed from the oven and is wet or dry quenched and then transported to the blast furnace.

Coke in the Production of Hot Metal (II)

Coke in the Production of Hot Metal (III)Non-Recovery Coke-Making. Coal is carbonised in large oven chambers and the

coking process takes place from the top by radiant heat transfer and from the bottom

by heat conduction through the sole floor. Primary air for combustion is introduced

into the chamber through ports located above the charge level in both pusher and

coke side oven doors. The volatile compounds that are produced during the

carbonisation of coal are oxidised in the oven chamber and partially combusted

gases exit the top chamber through "down comer" passages in the oven wall and

enter the sole flue, thereby heating the sole of the oven. Combusted gases are

collected in a common tunnel and exit via a stack which creates a natural draft in the

oven. Since the by-products are not recovered, the process is called non-recovery

coke-making (if the waste gas exits into a waste heat recovery boiler which converts

the excess heat into steam for power generation it is named non-recovery-heat

recovery). In-oven combustion of hydrocarbons appears to virtually eliminate

hydrocarbon emissions and also the negative-pressure ovens should not leak

appreciable amounts of emissions.

Coke in the Production of Hot Metal (III)

Sintering is a process in which iron ore fines (in a mixture with several recycled

materials, fluxes and solid fuel) is agglomerated forming a porous mass (sinter) that

it is fed in the blast furnace as component of the mineral burden. In the process the

mixture is ignited by a gas burner and then moved along a travelling grate machine

until partial melting and agglomeration of iron ore particles occur. The burning of

coke breeze resulting from the process of blast furnace coke calibration and sized in

rod mill to below 4.76 mm (in mixture with other solid fuels as anthracite, wood

charcoal or green pet coke) provides the heat for the process.

Coke in the Production of Hot Metal (IV)

Coke in the Production of Hot Metal (IV)

Coke in the Production of Hot Metal (IV)

The purpose of a blast furnace is to chemically reduce and physically convert iron oxides into liquid iron called “hot metal or pig iron”. Blast furnace is a chemical reactor which requires certain physical conditions such as permeability to fluid flows in order to produce hot metal efficiently. From the tuyere level and above, it is a counter-current reactor. The descending solids are processed by the ascending gas which carries heat and reducing agents, H2 e CO. Coke is used as reducing agent, fuel and permeability provider.

Coke in the Production of Hot Metal (V)

The main blast furnace product is iron in melted state that it is called hot metal or pig iron. Hot metal is sent for the steel plant of an integrated steelmaker and fed in converters. Pig iron is iron liquid solidified in metallic ingot molds (pig iron is called like this since when the liquid iron was drained through a channel in the soil to flow in molds and whose arrangements resembled with avid newly born pigs to suck).

Coke in the Production of Hot Metal (VI)

Iron bearing materials (iron ore, sinter, pellets), coke and fluxes are charged into the top of the furnace. A blast of pre-heated air enriched with oxygen and also, in most cases, a gaseous, liquid or powdered fuel (for instance, pulverised coal) are introduced through openings (tuyeres) at the bottom of the furnace just above the hearth crucible. Coke and mineral burden are charged in separate and alternate layers and nowadays all steelmakers are mixing small coke or nut coke with the mineral burden (iron bearing materials + fluxes). The heated air burns the injected fuel and most of the coke charged in front of tuyeres to produce the heat required by the process and to provide reducing gas that removes oxygen from the ore. The reduced iron melts and runs down to the bottom of the hearth. The flux combines with the impurities in the ore to produce a slag which also melts and accumulates on top of the liquid iron in the hearth. The melted iron and slag are both tapped periodically of the furnace. The total furnace residence time is about 6 to 8 hours.

Coke in the Production of Hot Metal (VI)

Carbon Blast Furnace Reactions

Carbon oxidised by hot air:

Carbon(s) + oxygen(g) → carbon dioxide(g)

C(s) + O2(g) → CO2(g)

Carbon oxidised by carbon dioxide:

Carbon(s) + carbon dioxide(g) → carbon monoxide(g)

C(s) + CO2(g) → 2CO(g)

Carbon monoxide reduces iron (III) oxide

Carbon monoxide(g) + iron (III) oxide(s) → iron(l) + carbon dioxide(g)

3CO(g) + Fe2O3(s) → 2Fe(l) + 3CO2(g)

After extinction coke is transported to the blast furnace in a handling system constituted by belts conveyors, transfer chutes, screens and bunkers (the handling system in the figure proceed is from Brazilian Usiminas steelmaker). In this system coke suffers mechanical stabilisation due to the drops that is submitted (few companies have coke cutters for size reduction, even if its stabilisation in the handling system is not enough). Coke is classified in two or three products that are used in the blast furnace and sintering. As an example i) coke breeze under 10 mm is used after rod mill size preparation in sintering, ii) small coke (10 - 25 mm) or nut coke (25 - 40 mm) is used as component of metallic burden in the blast furnace and iii) lump coke (25 - 75 mm or 40 - 80 mm) in the blast furnace coke bed (coke layers) as main responsible for the furnace permeability.

Coke in the Production of Hot Metal (VII)

Coke in the Production of Hot Metal (VII)

AF3

AF2

AF1

Bat

eria

2

Ram

pa

Bat

eria

1

Ram

pa

Bat

eria

4

Ram

pa

Bat

eria

3

Ram

pa

K 203-1

K 207

K 208

K 210

K 211

K 211r

K 1

03-1

K 1

03-2

K 1

05

K 106r

K 104-r

C 2

C 4

E 4

E 2

C 1

E 1

C 3

E 3

K 202r K 102r

CokeBunker

S1

C2

C1

C3Finos para

Sinterização

CT C-4

CT C-3

CT C-5Finos para

Sinterização

Skip

CT C-4

CT C-3

CT C-5

Skip

Finos para

Sinterização

Peneiras

K 203-2

K 204-r

K 105-A

Hot metal production in the blast furnace is linked with coke and its availability. A blast furnace cannot be operated without coke for physical reasons and the coke is generally the most expensive blast furnace burden material. The blast furnace operator will therefore always try to reduce coke consumption to the lowest level technically possible by injecting coal or other reducing agent. But for this they need to place more rigid quality requirements on coke.

Coke Role in the Blast Furnace (I)

Coke Role in the Blast Furnace (I)

The quality of the coke can be defined as being its capacity to fill out the requirements demanded in the blast furnace and for an appropriate definition it is necessary to know i) the coke roles in the blast furnace and ii) the factors that act on the coke inside the blast furnace. The coke, besides having low contents of contaminants of the hot metal (S and P) and/or of operation disturbing elements (Na, K and Zn), should be capable to perform three main roles in the blast furnace: a thermal, a chemical and a physical role.

Role 1: ThermalProvision of fuel for combustion in the raceway region.

Role 2: ChemicalReacting with CO2 it provides the reducing gases for the reduction of iron.

Role 3: Physical (coke is the great provider)

Provides a permeable bed at the top of the furnace for gas to pass through.

Parts the heavier, denser and less permeable layers of mixed pellets, lump

ores, sinter and fluxes (nowadays the small coke or nut coke is part of those

layers).

Provides a permeable matrix (windows) in the lower part of the furnace

through which liquids can drip and hot gas pass.

Support the weight of stock.

Provides a permeable bed for iron and slags to flow to the tap holes of the

blast furnace.

Coke Role in the Blast Furnace (II)

Coke Role in the Blast Furnace (II)

Coke can be replaced to a large degree in the first two roles by, for instance, by pulverised coal.The physical role is dependent on the size, size distribution, shape and superficial irregularity of the coke.The maintenance of a good permeability in the several zones of the blast furnace is dependent of the strength, coke strength after reaction, amount of recycled alkalis inside the blast furnace etc.