COKING COAL

Definition : Coking coals are those varieties of coal which on heating in the absence of air (process known as Carbonisation) undergo transformation into plastic state, swell and then re-solidify to give a Cake. On quenching the cake results in a strong and porous mass called coke.

Primary/Medium/Semi/Week Coking Coal : Coking coal is divided into 3 sub-categories namely, Primary Coking Coal (Low ash , low Volatile, High Coking property) Medium Coking Coal (low ash, medium volatile, low caking index) and Semi/ Weak Coking Coal: (low ash, high volatile, very low caking index).

Characteristics of Coking coal for BF Coke : Coking coal for production of BF Coke (which is the right type of fuel/reductant needed for a BF) is characterised by certain specific properties in terms of appropriate composition (viz low Ash (10% max ), Volatile Matter ( 20-26%), and very low sulphur and phosphorous content, appropriate Rank of coal (1— 1.3), good rheological properties, wide range of fluidity , low inert content etc.

Indian Coking Coal : Indian Coking Coal found in Gondwana belt (Bihar &West Bengal region) has very high ash (17% or more) and poor rank and other properties, which results in lower productivity and higher coke consumption in blast furnace. Assam coking coals though, are low in ash have very high sulphur which limits their use in iron making in blast furnace.

Washing of Coal: Since ash content in Indian coal is very high, washing is resorted to to lower the ash content to some extent. However, Indian coals are notorious with respect to its Washability because the ash /inerts are fairly and finely distributed in the coal matrix thereby rendering washing difficult.

Blending of coal : Because of limited availability of good quality coking coal, the Indian Steel plants use a optimal Blend of the 3 or more varieties of coking coal to compensate for the lack of individual coals with the necessary properties. Another important consideration in selecting a coal blend is that it should not exert a high coke oven wall pressure and should contract sufficiently to allow the cake/coke to be pushed out from the oven.