High Temperature Stability of Coke in Relation to its Bulk Permeability

Abstract

In the blast furnace, the initial size grading of the feed coke and subsequent changes in its lump size distribution can markedly affect the permeability of the stock column and hence furnace performance. A realistic assessment of the role of coke as a burden spacer requires measurements of its mechanical size stability at conditions simulating the chemical environment and thermal stresses of the smelting process, using bulk samples of the coke as charged.The thermo-mechanical size stability of a wide range of cokes has been evaluated by a test based on the rapid reheating of 70 kg samples to 1400 0 C in an inert atmosphere, followed by cooling to ambient temperature and micum-drum testing.The results have shown that the bulk thermal stability of coke depends on the rank of the parent coals and blend composition, the oven charge preparation and carbonisation conditions, and the lump size of the cokes produced. In general, the thermal stabiljty of coke cannot be predicted from tests at ambient temperature alone.Complementary measurements of the bulk reactivity and post-reaction strength of coke have been carried out by reacting, initially, 25 kg samples with carbon dioxide at lOOOoC to a weight loss of up to 60%. Some results are also included of a separate, complementary study of the bulk chemical stability of coke. BULK THERMAL STABILITY OF COKE Method of measurementThe development and details of the CRE bulk thermal stability test (4-7) have been described in previous papersThe basic aim has been that coke of the size grading fed to blast furnaces should be heated to temperatures and at rates approaching those encountered in the bosh but that it should not undergo any significant chemical reaction with its environment.Accordingly, the method developed involves the rapid reheating of 70 kg of coke to about 1400 0 C in an oven-furnace purged with nitrogen. After a heat -111 -soak at l400 0 C followed by a slow cooling to ambient temperature, the coke is tested mechanically in a half-micum drum.The irreversible nature-of the thermal effects renders the mechanical testing of coke in the hot state unnecessary. Affected coke propertiesAn example of average changes in coke properties upon reheating to 1400 0 C is shown in Table 1. With a net loss in weight of 5~ there were contributory reductions in volatile matter, ash and sulphur contents. In addition to a densification of the coke substance, coke lump size was also reduced and mechanical testing in the micum drum markedly increased the size difference between the original and reheated coke samples. Clearly, Finishing temperatureTests on cokes produced at a constant (standard) flue temperature in a 500 kg pilot oven confirmed the importance of adequate finishing in the oven on the cokes' thermo-mechanical size stability. Figure 4 indicates that although the ambient size stability of the cokes was virtually unaffected by increasing the final charge-centre temperature from 900 0 C to nearly 1150 o C, the thermal stability (residual harmonic mean size) increased progr...

Evaluation of Characteristics of Coke Degradation after Reaction in Different Conditions

“…2,3) However, it has been pointed out that solution loss reaction in the NSC-tests, especially of highly reactive coke, might be overestimated, compared with that in an actual blast furnace where CO 2 /CO + CO 2 of ascending gas is varied between 0 and 0.5. Moreover, Goleczka et al 4) and Barnaba 5) assumed that weight loss percentages due to solution loss in actual blast furnaces were about 20-30% and 25%, respectively.…”