ISSN 1068364X, Coke and Chemistry, 2012, Vol. 55, No. 8, pp. 300–303. © Allerton Press, Inc., 2012. Published in Russian in Koks i Khimiya, 2012, No. 8, pp. 15–18.

COKE

PostReaction Strength of Coke under Various Conditions1 R. Guo, L. Sun, and Q. Wang University of Science and Technology Liaoning, China email: [email protected] Received May 29, 2012

Abstract—The postreaction strength of coke was studied at different reaction temperatures in CO2 and mixed gas atmospheres. The influence of solution loss conditions on postreaction strength of coke, with dif ferent levels of reactivity, was studied in this paper. DOI: 10.3103/S1068364X12080030

1. EXPERIMENT

1

The properties of metallurgical coke are some of the most important factors affecting the economic efficiency of blast furnaces. The coke solution loss reaction is able to provide a reducing agent that is required for iron ore reduction. The reaction is as fol lows: C + CO2 = 2CO.

(1)

The solution loss reaction is an important reason for why coke degradation occurs in blast furnaces [1]. In the early 1970s, a method for measuring the coke reactivity index (CRI) and coke strength after reaction (CSR) was developed by Nippon Steel Corporation (NSC). Since then many standard tests have been derived from the NSC methods, such as ISO18894:2006, ASTM D534199(2010) e1, GB/T40002008, etc. For many years, it was consid ered that coke with a low CRI and high CSR would be of greatest benefit to the blast furnace process, not only for maintaining permeability but also for increas ing efficiency of gas utilization. However, Negro et al. [2] and Cheng [3] proposed that the total amount of coke solution lost in a blast furnace is mainly deter mined by the carbon dioxide concentration, which itself is controlled by the availability of oxygen from iron oxides. Goleczka, et al. [4] and Barnaba [5] assumed that coke gasification in blast furnaces, due to solution loss, is about 20–30% and 25%, respectively. In blast furnaces coke reacts with carbon dioxide from mixed gas and this is accompanied by an increase in temperature. There are some differences between the CSR test and conditions in blast furnaces. In this paper, postreaction strengths of three representative cokes were studied after undergoing various reaction conditions. This will provide a new methodology for evaluating the capability of coke to resist the solution loss reaction.

1.1. Samples Three typical cokes were used for the experiment. All three cokes had good mechanical strength (M40 and M10) but different CRI and CSR indexes. The properties of the cokes are shown in Table 1. Grain sizes of the coke samples were 21.0–25.0 mm. 1.2. Apparatus The coke solution loss reaction was carried out in a supporting thermogravimetric apparatus (temperature range: ambient1500°C with a heating rate from 1 to 20°C/min, weighing capacity: 20 kg, accuracy: 0.1 g), as shown in Fig. 1. The reaction tube is inserted into a gland cover combined with a bracket. The reaction gas enters the reaction tube through the entrance at the bottom of the gland cover. After flowing through a ven tilate firebrick and spherepacking, the reaction gas contacts the sample. Using the entrance at the bottom of the gland cover a thermocouple is inserted into the sample layer. 1.3. Procedure The postreaction strength of coke was determined using four methods. The reaction conditions used are shown in Table 2. The coke sample (200 ± 0.1 g) was heated to the solution loss temperature in a nitrogen Table 1. Properties of coke, % Coke

Ash

Volatiles

M40

M10

CRI

CSR

Coke A Coke B Coke C

11.88 12.85 11.45

1.09 0.98 1.22

81.9 81.5 82.8

6.2 5.7 6.7

15.9 38.6 21.5

73.5 38.9 66.0

Note: M40 and M10 are the mechanical strengths of coke (GB/T 2006–2008).

1 The article is published in the original.

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POSTREACTION STRENGTH OF COKE UNDER VARIOUS CONDITIONS

301

11

2

12

3

13

4 5 14

6 1

7

8 9

10

Fig. 1. Experimental apparatus. (1) The lift furnace; (2) the blast furnace; (3, 11) the thermocouple; (4) the sample; (5) the spherepacking; (6) the ventilate firebrick; (7, 12) the reaction tube (corundum); (8) the support of the reaction tube; (9) the elec tronic balance; (10) the supporting tube; (13) the gland cover; (14) the gas entrance.

atmosphere (5 L/min). When the temperature was reached, the atmosphere was changed to the reaction gas. When the reaction degree or time arrived, the reaction gas was changed to N2 (5 L/min), in which the sample was cooled. The postreaction strength of 10 the coke sample was evaluated by I 600 (percentage of coke mass retained on a sieve with 10 mm apertures to the mass of the original coke sample after 600 revolu tions at a rate of 20 r/min in an IType tumbler). The test method had good experimental repetition. The pre 10 cision of I 600 was ±1.5%. Table 2. Reaction conditions Reaction Tem perature

Reaction Degree (or time)

100% CO2, 5 L/min

1100°C

120 min

No. 2

100% CO2, 5 L/min

1100°C

weight loss 25%

No. 3

100% CO2, 5 L/min

Conditions

Reaction Gas

No. 1 (NSC)

No. 4

10

I 600 of the cokes, from different reaction condi tions, are shown in Table 3. The CSR indexes (No. 1, NSC standard condition) of the three cokes are quite different, as the weight loss percentage of the three cokes was different under constant reaction times. 10

The differences in the I 600 between the different coke samples have been reduced when determined by the same solution loss degree, at the temperature of 10 1100°C (No. 2 condition). I 600 (No. 2 condition) of coke C is close to the CSR (first, NSC standard con dition) due to normal reactivity. Under the second and third sets of conditions, the coke had the same weight loss percentage, but from a different reaction temper ature, as under the third set of conditions, so the post reaction strength of the coke was different due to the different solution loss kinetics behaviors. At low tem 10

Table 3. I 600 of coke under different reaction conditions, %

1000~1250°C, weight loss 5°C/min 25%

CO2 : CO : N2 = 1000~1250°C, weight loss 1 : 3 : 6, 2°C/min 25% 10 L/min

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Coke A Coke B Coke C

No. 1 (CSR)

No. 2

No. 3

No. 4

73.5 38.9 66.0

60.4 50.1 65.1

59.8 57.1 60.3

62.6 59.7 64.8

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GUO et al.

Table 4. Gas composition for coke gasification N2, % CO, % CO2, % % CO/(% CO + % CO2) 1000°C 1100°C 1150°C 1200°C 1250°C

60 60 60 60 60

27.3 28.6 29.2 29.7 30.1

12.7 11.4 10.8 10.3 9.9

0.682 0.715 0.729 0.742 0.754

peratures, the reaction gas diffuses into the inner parts of the coke. At high temperatures, the solution loss reaction was primarily with the surface of the coke. On increasing the temperature (No. 3 condition), 10 the I 600 of coke B (high reactivity) was much better than the constant temperature (No. 2 condition), as the reaction is primarily with the surface rather than the interior. Compared with coke B, the surface reac % CO/(% CO + % CO2)

1.0 Fe

0.8

a

0.6 FeO

0.4 Fe3O4

0.2

b

0 400

600

800

1000

1200 T, °C

Fig. 2. Equilibrium between CO–CO2 gas mixture and iron oxides.

tions of coke C (normal reactivity) and coke A (low reactivity) under the third set of conditions were not as vigorous. So the postreaction strengths of cokes C and A under the third set of conditions were not better than under the second set of conditions. Compared with other conditions, the fourth set were the ones closest to blast furnace conditions. When gas flows through the coke layers in the blast fur nace, the concentration of carbon monoxide increases, as the solution loss reaction produces car bon monoxide. When gas flows through the iron ore layers, the concentration of carbon dioxide increases as iron ore reduction produces CO2. As shown in Fig. 2, the condition of FeO can be reduced to Fe, as long as the concentration of carbon monoxide is between “line a” and “line b”. The gas composition of the early stage coke layer is close to “line b”. The gas composition of “line b” was calculated from thermo dynamic data, as shown in Table 4. So the gas compo sition of the fourth set of conditions is CO2 : CO : N2 = 1 : 3 : 6. Under the fourth set of conditions, carbon monox ide, in the mixed gas, restrained the solution loss reac tion, in addition, diffusion of carbon dioxide decreased. The particle distribution of the coke after reactiontumbling is shown in Fig. 3. The percentage of coke B >18 mm from the fourth set of conditions was less than under the second and third sets of condi tions, and the percentage of coke B >10 mm and <1 mm was close to that from under the third set of conditions. This means that solution loss from high reactive coke under mixed gas conditions is greater than under the other conditions, but the general 10 destruction degree (evaluated by I 600 ) form the fourth and third sets of conditions were close to each other.

Particle distribution, %

100 Percentage, mm <1

80

1–3 3–5

60

5–10 10–15

40

15–18 20

0

>18

No. 2

No. 3 CokeА

No. 4

No. 2

No. 3 Coke B

No. 4

No. 2

No. 3 No. 4 Coke C

Fig. 3. Particle distribution of coke after reactiontumbling. COKE AND CHEMISTRY

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POSTREACTION STRENGTH OF COKE UNDER VARIOUS CONDITIONS

The percentage of coke C >18 mm and coke A under the fourth set of conditions was close to that from the third set of conditions, but the percentage >10 mm and <1 mm was closer to that from under the second set of conditions. This means that solution loss depth of normal and low reactive cokes under mixed gas conditions is similar to the third set of conditions, but 10 the general destruction degree (evaluated by I 600 ) from the fourth and second sets of conditions were close to each other. 3. CONCLUSIONS The characteristics of postreaction strength coke under various conditions can be summarized as follows. 10

(1) Normal reactive coke: CSR, I 600 from the sec ond set (100% CO2; 1100°C; weight loss⎯25%) of 10 I 600

conditions and from the fourth set of conditions are close to each other (CO2 : CO : N2 = 1 : 3 : 6; 1100–1250°C; weight loss⎯25%). 10

(2) High reactive coke: I 600 from the third set (100% CO2; 1100°C; weight loss⎯25%) of conditions 10

and I 600 from the fourth set (CO2 : CO : N2 = 1 : 3 : 6; 1100–1250°C; weight loss⎯25%) of conditions are similar to each other and, much better than CSR and 10 I 600 from the second set (100% CO2; 1100°C; weight loss⎯25%) of conditions. 10

(3) Low reactive coke: I 600 under the second set (100% CO2; 1100°C; weight loss⎯25%) of conditions 10

and I 600 under the fourth set (CO2 : CO : N2 = 1 : 3 : 6; 1100–1250°C; weight loss⎯25%) of conditions are similar to each other and much lower than CSR. CRI and CSR are widely accepted, but there are some objections. Testing of CSR and CRI, in the lab oratory, under NSC standard conditions and experi mental blast furnace (EBF) conditions has been car ried out by Lundgren et al. [6]. The test result shows

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that the postreaction strength of the coke tested from the EBF was much better than from the NSC test. Nomura et al. [7] considered that CSR was not suit able for evaluating the postreaction strength of highly reactive cokes. They thought that the solution loss reaction should be stopped with a weight loss of 20%. The reaction temperature should be adjusted to main tain the same reaction time. CSR is the strength of coke after a reaction at 1100°C for a constant period (120 min). It is a useful and effective index for blast furnace operation [8], as long as coke with a normal reactivity is used. But post reaction strength of coke with a special reactivity should be evaluated by a special method. REFERENCES 1. Ida, S., Nishi, T. and Nakama, H., Behaviour of Bur den in the Higashida No. 5 BlastFurnace, Fuel Soc of Japan, 1971, vol. 50, pp. 645–654. 2. Negro, P., Steiler, J.M., Beppler, E., et al.., Assessment of Coke Degradation in the Blast Furnace from Tuyere Probing Investigations, 3rd European Ironmaking Con gress Proceedings, 1996, pp. 20–27. 3. Cheng, A., Coke Quality Requirements for Blast Fur naces, Ironmaking and Steelmaking, 2001, vol. 28, no. 8, pp. 78–81. 4. Goleczka, J. and Tucker, J., Coke quality and its assess ment in the CRE laboratory, Ironmaking Conference Proceedings, 1985, vol. 44, pp. 217–232. 5. Barnaba, P., A New Way for Evaluating the High Tem perature Properties of Coke, Coke Making Interna tional, 1993, vol. 5, no. 2, pp. 47–54. 6. Lundgren, M., Ökvist, L.S., and Björkman, B., Coke Reactivity under Blast Furnace Conditions and in the CSR/CRI Test, Steel Research International, 2009, vol. 5, no. 6, pp. 396–401. 7. Nomura, S., Naito, M., and Yamaguchi, K., Post Reaction Strength of CatalystAdded Highly Reactive Coke, ISIJ International, 2007, vol. 47, no. 6, pp. 831– 839. 8. Nakamura, N., Togino, Y., and Tateoka, M., Behavior of Coke in a Large Blast Furnace, Ironmaking and Steelmaking, 1978, vol. 5, pp. 1–7.