Reactivity of Cokes with Gases Under Blast Furnace Conditions

The NSC reactivity test is often criticised for not being able to accurately predict performance of cokes in the blast furnace. One explanation commonly proposed for this lack is that the gas used in the NSC test, pure carbon dioxide, is different to the gas that coke is exposed to in blast furnaces, which is a complex mixture including carbon monoxide, carbon dioxide and nitrogen. Evidence has been presented in the literature to support this. The aim of this work was to see to what extent different cokes behaved differently under different gases, and if there was any connection between these differences and the composition of the ash.

Nine Australian coals, used in coking blends were selected to cover a wide range of rank, maceral composition and elemental composition of the mineral matter. These coals were coked and their relative reactivities in a series of gas mixtures were compared. The time for the reaction of the coke in a 30% CO₂/70% CO mixture was set to eight hours to give about the same weight loss as two hours exposure to 100% CO₂.

The main conclusion of this study was that gas composition (using mixtures of CO, CO₂ and N₂) had little effect on the relative rate of gasification of cokes over a wide compositional range of gas and of coke (although of course the absolute reaction rate decreased with decreasing CO₂ levels). There was no effect found of the amount of any element in the ash on the relative rates of burnoff. But the difference in gas composition did result in different amounts of the different forms of iron in the reacted coke ? metallic iron, for example, was rarer in the coke reacted with pure CO₂ than with the gas mixture. The cokes made from lower rank coals did appear to be relatively more reactive than cokes made from high rank coals in 30% CO₂/70% CO compared to 100% CO₂. The previous studies that suggested changes in gas composition affects the relative reaction rate of cokes were misleading because they performed the studies at constant burnoff time (2 hours) rather than ensuring the cokes were reacted to about the same weight loss.

Thus any differences between the behaviour of cokes in the NSC test and in the blast furnace are not due to differences in gas composition between the two.

Another finding was that the CSR value was found to be a combination of strength and reactivity: for the data in this study, CSR could be entirely explained by a two-component fit involving CRI and the I600/10 index (the strength of the unreacted coke as measured by the CSR tumble test).