Fuel - Coke

There are many type of coal present in world that is charcoal, rock coal, beehive coke, by product coal and anthracite. But due the weak in bond structure and high volatile matter these are not sufficient for blast furnace, where anthracite is pure from of coal till is not good for blast furnace because of its soften quality.
Coke arrangements in blast furnace
Coke arrangements

● Therefore the good grade (metallurgical coal) coal are coked and made in to coke, which is the suitable for blast furnace. Coke is obtained by heating metallurgical coal out of contact with air, when the volatile matter gets removed and carbon particles join each other to form a porous cellular mass with sufficient strength, is called as coke having negligible moisture, volatile matter, good strength and porous structure which enables to react with oxygen for burning.

Function of Coke :

The three major function of coke are-
1. It is a fuel providing heat for meeting the endothermic requirements of chemical reactions and melting of slag and metal.

Coke function in blast furnace
Coke function

2. It produces and regenerate reducing gases for the reduction of iron oxides. As a regenerator of reducing gases and a heat producer it should be high reactivity with oxygen , carbon dioxide and water vapour.

3. It provides an open permeable bed through which slag and metal pass down in to hearth and hot reducing gases pass upwards. This permeability of the charge particularly in the bosh region, where everything are either plastic or molten except coke. Which is maintain as a nazzow size and suffer minimal breakdown in its paggeage to and thought the furnce until it burns on the thyere level.

Quality Requirements of the Coke :

The efficiency of the blast furnace is directly depends on the permeability of the charge in the furnace which is directly depends on the quality of coke.
1. Chemical composition as determined in terms of the contents of fixed carbon, ash, sulphur, phosphorus and other deleterious impurities.
2. Reactivity as determined by its physical structure that is cellular structure and porosity.
3. Size range.
4. Thermal stability at high temperature.
5. Strength and abrasion resistance.

Chemical composition :

The banalce of coke made up of ash, volatile matter and other impurities etc, as the ash content increase the available carbon of coke decrease. The amount and quality of ash of coke has a considerable bearing on the blast furnace operation.

● Ash contain usually oxides like SiO2, Al2O3, CaO etc, where phosphorus and sulphur are present in as inorganic compounds. An increase of 1% ash from coke over a critical limit results in decrease in production 3 to 6% and improve the coke consumption of about 4 to 5%. All the phosphorus in coke, nay, or in entire burden joins pig iron.

● The ash contain if the coal blend for coke oven is therefore adjusted by blending or washing to produce coke of low ash and most economic coke.

Reactivity :

● It may be defined as the rate of reaction between coke and oxygen or any other gaseous phase capable of reacting with coke.
Example : carbon dioxide in the case of blast furnace. 

● The rate of burning if coke in a way controls the rate of production From a blast furnace. The data up the rate of burning is directly propersional to.
1. The area of fuel
2. The temperature and pressure of the blast.
3. The affinity of the particular type of carbon for oxygen.

Thermal Stability :

● In the furnace coke is heated about 1500 -1600℃ before it burns in front of tuyeres. This is done in coke oven in carbonisation process. The temperature gradient in an individual piece of coke sets in differential contraction and expansion, which means smaller size is the better for the thermal stability due to resulting high local stress and it's consequences tendency to degradation increases. The following are the favour of high thermal stability.

1. Absence of large lumps in feed coke.
2. Uniformity of coke texture.
3. Minimum inert inclusion of large sizes.
4. High carbonisation temperature and heat soak.
5. Prior mechanical conditioning.
6. Low chemical reactivity.

Strength and Abrasion Resistance :

1. Quality of coal used.
2. Blending procedures adopted
3. Carbonisation temperature.
4. Carbonisation time
5. Quenching procedure and subsequent handling.
6. Moisture content of coal mix vice and versa bulk density.
7. Extent of crushing and degree of oil addition.
8. Mechanical compaction used.

Size :

● The size of the coke generally choosen to match the size of other raw material and much bigger than then the size of the iron bearing materials to ensure maximum bed permeability for smooth furnace operation.The degradation of coke from the original size, does occur when finally coke descends in to tuyeres regions

● In order to maintain permeability in the bosh region the original size should be enough to accommodate this degradation and still end up with adequate permeability in the bosh region.

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