COKE

Coke is the residual solid product (carbon and earthy matter) obtained upon carbonisation (after the volatile constituents are driven off by heat in closed spaces) of coking coal.

Depending upon property, coke is known as Hard Coke, Soft Coke and Metallurgical Coke.

Metallurgical Coke: Not all coke can be used in metallurgical operations for which good quality coke made from specific blend of coking coal is essential. Such coke is classified as Met. Coke.

Blast Furnace (BF) Coke: The term is used to refer to such Met Coke which are used for iron making in BF. BF coke fulfills 3 main functions in the blast furnace operation :

It acts as a fuel providing heat for all reactions
It acts as a reductant provicing Carbon dioxide gas anc carbon for readuction of iron ore ,and
It provides the required permeability for movement of gases through the bed of iron ore, coke and limestone inside the blast furnace.

BF Coke is characterised by the following parameters :

Specified Size Range (25/40-80mm),
High Fixed Carbon (80-85%),
Low Ash (10- 15% ash),
Low Volatile Matter (2% Max),
Low Alkalies,
Low Sulpher(0.7%Max),
Low Phosphorous(0.3% max),
High Strength/Abrasion Resistance( measured in terms of Micum Index(namely M10 value 10% Max and M40 value 75/80% Min),
Reasonable Coke Strength after Reaction (CSR : 55-60), and
Appropriate Reactivity (CRI : below 25).

These characteristics depends not only on the coal properties but also on the coking technology/parameters as well as pre-carbonisation & postcarbonisation techniques adopted thereof.

Adverse effects of Ash: Ash has highly adverse effect on the productivity of BF and on consumption of coke in the BF. An increase in ash content by 1% over a critical limit results in increase in coke consumption by about 45% and decrease in BF productivity about 3-6%.

Indian Integrated Steel Plants normally use high ash coke produced inhouse, at the cost of productivity, energy consumption etc. The Mini Blast Furnace units however use mainly imported low ash Met coke from China and other sources.