Hot Coke Strength in CO₂ Reaction

Abstract

This study aims to investigate the strength of coke at high temperature in reaction atmosphere. A uniaxial compression test measured the fracture stress of coke at three conditions: (i) room temperature and air atmosphere; (ii) 1100°C and N₂ atmosphere; (iii) 1100°C and CO₂ atmosphere. The drum index (DI) and coke strength after reaction (CSR) were also measured. The DI and CSR of coke produced by a lump of caking coal were higher than those of coke produced by non- or slightly caking coal. The compression tests showed that the fractured stress at room temperature in the air atmosphere was lower than that at 1100°C in the N₂ atmosphere. Also, the fractured stress at 1100°C in the N₂ atmosphere was higher than that at 1100°C due to the CO₂ reaction. The DI, CSR, and hot coke strength of coke produced by caking coal was higher than those of coke produced by non- or slightly caking coal.

1. Introduction

Strength is an important quality of metallurgical coke. Many possible factors have been investigated to determine factors decreasing coke strength because coke is a porous material, and pores and cracks occur in carbonization. Patrick and Stacey¹⁾ measured the porosity of coke using a microscope and showed that the coke strength decreased with increasing porosity. Sakai et al.²⁾ evaluated the fracture strength of coke by diametral compression tests based on material mechanics and showed that the fracture strength was related to the drum index (DI). The drum index is used for industrial evaluation, and the diametral-compression test is used for academic evaluation. Here, the coke samples in the tests are evaluated at room temperature.

Coke reacts in a blast furnace, and its strength decreases. In other words, coke is exposed to high-temperature CO₂ and degraded due to the chemical reaction. For industrial evaluation, coke strength after reaction (CSR) is also used as a strength index. In the CSR, coke strength after CO₂ reactions at high temperature is evaluated in air atmosphere at room temperature. Ghosh et al.³⁾ examined the effect of the wall thickness of coke on CSR and showed that the wall thickness affects the strength of coke and reactivity with CO₂. Pusz and Buzko⁴⁾ showed that coke crystallinity affects CSR. Numazawa et al.^5,6,7) investigated the degradation mechanism of coke due to gasification using numerical analyses. The effect of gasification by H₂O on the degradation of coke strength was investigated.⁸⁾ On the other hand, Xing et al.⁹⁾ compared the tensile strength of coke samples with CO₂ at high temperature and showed that the coke strength decreases by CO₂ gasification. In this way, many studies have been investigated the degradation of coke due to gasification. Thus, the coke strength at high temperatures is important both industrially and academically. However, to our knowledge, coke strength during the CO₂ reaction is unknown because coke strength is measured at room temperature even if coke and CO₂ are reacted at high temperature. In the present study, to investigate the difference in coke strength between at room temperature and at high temperature, we measured the coke strength in high-temperature CO₂ reaction atmosphere. That is, the strength of hot coke was measured. In addition, the DI and CSR of coke were measured.

2. Experimental

2.1. Preparation of Samples

Coal A (caking coal) and Coal C (non-or slightly caking coal) were used as raw materials. Table 1 shows the proximate analysis and ultimate analysis of the coals. Each coal was crushed to 3 mm or less, inserted into a test furnace with a bulk density of 850 kg/m³, and carbonized for 18.5 h. The resulting cokes were labeled Coke A and Coke C. The lump coke and measurement data were provided by Nippon Steel Corporation.

Table 1. Characterization of raw materials.

Coal name	Coal category	Proximate analysis [wt% d.b.]			Ultimate analysis [wt% d.b.]
		Ash	VM	FC	C	H	N	S
Coal A	Coking coal	9.14	24.97	65.89	80.41	4.61	1.48	0.60
Coal C	Non-or slightly-coking coal	8.51	34.02	57.47	77.82	5.01	1.83	0.41

where VM is volatile matter, FC is fixed carbon.

2.2. Measurement of Coke Strength Index

The drum index (DI) was measured according to JIS K2151, and DI¹⁵⁰₁₅ was calculated. To measure the coke strength after reaction (CSR), 200 g of coke with a diameter of 20 ± 1 mm was reacted with CO₂ at the temperature of 1100°C for 2 hours. The flow rate of CO₂ was 5 NL/min. And then, the reacted coke samples were placed on an I-type drum. The diameter and length of the I-type drum were 130 mm and 1700 mm. The samples were rotated 600 times by the drum and then sieved. The coke strength after reaction (CSR) was calculated as the ratio of the weight of coke with a diameter of 9.52 mm or more to that of coke before the test. The measurement data was provided by Nippon Steel Corporation.

2.3. Uniaxial Compression Test in CO₂ Reaction Atmosphere at High Temperature

A uniaxial compression test was performed at high temperature to measure the strength of coke in CO₂ reactions. The coke samples were carved from the head of the coke lump. The specimen was a columnar sample with a diameter of 10 mm and a height of 10 mm. The size of the specimen is the same as the previous study.¹⁰⁾ A universal testing machine (Shimadzu Corp., Autograph AG-X) equipped with an electric furnace was used shown in Fig. 1(a). As shown in Fig. 1(b), a coke sample was placed in the reaction vessel, and gas was introduced through one of the holes. A weight (1400 g) was placed on the sample, and the electric furnace heated the sample. Here, the vessel was not sealed, as shown in Fig. 1(c). This void is clearance for thermal expansion of the metal of the vessel. In this study, gas was constantly supplied (gas pressure is 0.15 MPa), and pressure in the vessel would be positive. Thus, the airtightness would not affect the results. If the temperature of the vessel rises in the CO₂ atmosphere, reacted coke collapses just by touching it. However, coke samples maintained their shape even after the test shown in Fig. 2. Therefore, it is considered that coke reacted by CO₂ introduced.

Fig. 1.

A uniaxial compression test in high-temperature reaction. (a) Experimental apparatus, (b) Sample in the reaction vessel, (c) The vessel and a weight, (d) Compression direction (Image). (Online version in color.)

Fig. 2.

Overview of fractured coke samples with CO₂ reaction: (a) CokeA-React-01; (b) CokeC-React-01. Here, 01 is sample number. (Online version in color.)

In this study, the temperature of the furnace was set to 1100°C. This is the way that the temperature condition is the same as that used in the CSR evaluation. The heating rate of the furnace was 35°C/min, and nitrogen gas flowed during the heating. When the furnace temperature reached 1100°C, the furnace was left for 10 minutes, and the introducing gas was switched to CO₂ gas. After a specified time, the uniaxial compression test was conducted. The compression rate is 2 mm/min, and the load cell is 20 kN. The coke sample was laid down for stabilization, as shown in Fig. 1(d). After the test, nitrogen gas was supplied, and the sample was rapidly quenched to room temperature.

The target cokes are Coke A and Coke C. We measured under three conditions: the air atmosphere at room temperature (RT), the inert atmosphere at high temperature (Inert), the reaction atmosphere at high temperature (React). Since coke is a brittle material and porous material, coke strength varies greatly depending on samples. In the previous studies,^2,11,12) several samples were evaluated by the Weibull plot, but the measurable conditions are limited because the test at high temperatures requires a lot of machine time. Therefore, the number of samples used in each condition was 2. The reaction time of Coke A was 80 minutes, and that of Coke C was 60 minutes. Since the CO₂ reaction decreased the weight of coke, the weight of the sample would be decreased. Unfortunately, the weight of fine particles generated by the fracture and chemical reactions cannot be measured. Thus, the weight of measurable lumpy samples was measured in evaluating the weight difference before and after the experiments. The measurement was outsourced to JAPAN TESTING LABORATORIES, Inc.

3. Results and Discussion

The strength index and fracture stress listed in Table 2 are discussed. The values of the drum index (DI¹⁵⁰₁₅) and coke strength after reaction (CSR) of Coke A are larger than those of Coke C. This is because Coke A is made from caking coal, and Coke C is made from non- or slightly caking coal. These results are consistent with past studies. Focusing on the value of fracture stress, the value decreased in the order of CokeA-Inert, CokeA-RT, and CokeA-React. In addition, the weight difference of coke increased in the order of CokeA-RT, CokeA-Inert, and CokeA-React. These tendencies are the same for Coke C.

Table 2. Coke strength.

Sample name	DI15015 [–]	CSR [–]	Fracture stress [MPa]	Difference in weight [%]
CokeA-RT	85.7	73.2	24.3	0.4
CokeA-Inert			44.1	7.2
CokeA-React			31.3	12.8
CokeC-RT	66.0	24.4	11.7	1.0
CokeC-Inert			16.5	8.9
CokeC-React			11.0	27.2

where the fraure stress and the differene in weight are averaged values.

Comparing CokeA-RT and CokeA-Inert, the fracture stress of CokeA-Inert is larger than that of CokeA-RT. Focusing on the stress-strain curve shown in Fig. 3, the stress increased with an increase in strain in both the case of CokeA-RT and CokeA-Inert, and its fracture occurred at a certain point. The strain at fracture of CokeA-Inert is larger than that of CokeA-RT. In general, when a load is applied to a material, elongation occurs, creep deformation is likely to occur at high temperatures, and its strain increases until fracture. However, the fracture stress of general materials may not increase significantly at the creep fracture. Although the creep deformation would occur at high temperatures, the mechanism of increase in fracture stress of coke is not discussed in this paper. The fracture stress of not only Coke A but also Coke C increased. These results suggest that coke may have higher strength at high temperatures. We quantitatively revealed that the coke strengths at room temperature in air atmosphere and at high temperature in inert atmosphere are different. The weight of CokeA-Inert is decreased. This would be because only the weight of lumpy samples fractured was measured.

Fig. 3.

Stress-strain curve of coke samples: (a) Coke A; (b) Coke C. 01 and 02 are sample numbers. (Online version in color.)

Focusing on CokeA-Inert and CokeA-React, the fracture stress decreased by 30%. This is because the coke matrix was lost due to the reaction with CO₂. Numazawa et al.⁶⁾ showed that the CO₂ reaction reacts outside the coke sample, and the coke matrix can disappear from the outside. Therefore, the reaction proceeded from the outside of the sample, and the strength would be decreased as the reaction progressed. This tendency was the same in CokeC-Inert and CokeC-React. From these facts, we quantitatively revealed that the coke strength decreases due to the CO₂ reaction.

In general, coke strength decreases due to the chemical reaction. In the DI and CSR tests, a rotational impact was applied to coke samples at room temperature. In the uniaxial compression test, the coke sample carved from the coke head was fractured. In every case, the strength of Coke A is larger than that of Coke C. The results of DI, CSR, and hot coke strength are consistent in Coke A and Coke C. Here, we focused on the fracture stress of Coke-RT and Coke-React. The fracture stress at high temperatures in the reaction atmosphere was larger or equal to that at room temperature in the air atmosphere. We found that the fracture behavior differs greatly depending on the temperature.

4. Conclusion

Coke strength was evaluated using the drum index (DI), the coke strength after reaction (CSR), and the uniaxial compression test with the furnace. The uniaxial compression test measured the fracture stress of coke at three conditions: room temperature and air atmosphere; 1100°C and N₂ atmosphere; 1100°C and CO₂ atmosphere. When the coke samples were fractured at high temperature, the fracture stress of coke at high temperature in inert atmosphere was higher than that at room temperature in air atmosphere. This is thought to be due to creep deformation. Furthermore, the coke strength decreased due to the reaction with CO₂. This is because the coke matrix that supports the coke structure disappears due to the CO₂ reaction. Coke strength at high temperature in the reaction atmosphere is larger than or equal to that at room temperature in the air atmosphere. This suggests that coke strength should be evaluated at the same temperature field. Further, there is a possibility that coke that is judged to be defective in the DI and CSR measurements is suitable for high-temperature conditions.

Acknowledgments

This work was supported by the 29th ISIJ Research Promotion Grant. Coke samples and a part of measurement data were supplied by Nippon Steel Corporation.

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