Behavior of Alkali Accumulation of Coke in the Cohesive Zone

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Behavior of Alkalis Accumulation of Coke in Cohesive Zone Zhiyu Chang, Kexin Jiao, Xiaojun Ning, and Jianliang Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02214 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 22, 2018

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Behavior of Alkalis Accumulation of Coke in Cohesive Zone Zhiyu Chang, *† Kexin Jiao, † Xiaojun Ning,† Jianliang Zhang†,‡ †School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China ‡School of Chemical Engineering, The University of Queensland, St Lucia, QLD 4072, Australia

Abstract The behavior of alkalis accumulation of coke in cohesive zone is confirmed, and the kalsilite crystals observed on the coke pore walls are presented in the hexagonal prism and sphere morphology. With closer to the lower edge of the cohesive zone, the amount of potassium adsorbed in coke decreases and the amount of sodium adsorbed increases. The content of K2O in cohesive zone coke ash is approximately 19-34 times that of feed coke while the corresponding Na2O content is about 7-14 times that of feed coke.

1. INTRODUCTION

In the blast furnace ironmaking process, reducing coke consumption is a critical approach to decrease greenhouse gas emissions and energy consumption.1,2 Currently, under the pressures of economic and environmental protection, pulverized coal injection (PCI) has been widely used by many steel enterprises to replace the expensive coke so as to improve economic and environmental efficiency.3,4 Especially in China, the high pulverized coal injection (PCI) ratio has become a pursuit target of some steel companies. As we know, coke plays multiple roles in a blast furnace by providing heat energy and reducing agent, carburization source for hot metal, permeability for liquid phase drainage and upward flow of BF gases and also structural support for burden.5,6

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Nevertheless, due to the lower coke ratio and higher PCI ratio, both the thickness of coke layer in the stack and the amount of coke that plays role in mechanical support for materials column reduce.7 Accordingly, the coke performance such as the Coke Strength after Reaction (CSR) which reflects the mechanical strength of coke at high temperature becomes increasingly important.3,6 On the one hand, it has been verified that the high PCI rate will aggravate the degree of coke degradation to some extent.8,9 On the other hand, mechanical stress, solution loss reaction as well as accumulation of alkalis and zinc have important effects on the reactivity and high-temperature strength of coke.9-14 Wherein, alkali attack is considered to be one main mechanism for coke degradation, which is of great concern.13,15 Therefore, it’s of great significance to further understand the alkalis accumulation behavior of coke in the high temperature zone of a blast furnace to optimize the BF operation and energy utilization. Over recent years, there have been some studies about the effect of alkalis on the degradation of metallurgical coke. Chan et al.

15

and Li et al.

16

have sought to establish the mechanism of coke

degradation by alkalis under laboratory conditions, and the results showed that the degradation of coke quality was mainly attributed to the high stress generated by the expansions of coke carbon matrix and lattice. Gupta et al 6. studied the coke samples from tuyere level of a working blast furnace and found the potassium content of tuyere cokes increased significantly due to recirculating alkalis. The work done by Dong at al.

17

showed that the “dead man” coke and coke in the rear of the “bird’s nest” were

found to have much higher alkali concentration than the coke samples in the bosh and raceway. However, owing to the complexity and severity of environment in the BF lower zone, it’s difficult to extract representative coke samples from operating blast furnaces to conduct the study. Consequently, very little is known as yet about the behavior of coke in the cohesive zone, where the coke is exposed to high temperature and participates in various solid-gas, solid-liquid and solid-solid reactions. It’s an


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exciting field of interest to study the coke samples collected from the cohesive zone, and it will help to further clarify the behavior of coke degradation in the cohesive zone. In this work, the behavior of alkalis accumulation of coke in the cohesive zone of a commercial blast furnace was investigated by XRD (X-ray diffraction) analysis and SEM (Scanning Electron Microscope) equipped with (EDS) Energy Dispersive X-ray spectrometer.

2. EXPERMENTAL SECTION 2.1. Samples Selection and Preparation

In the present study, the coke samples of the cohesive zone were obtained from a medium-sized blast furnace that was quenched by water during the overhaul process in October, 2017. The blast furnace was put into operation in October, 2006 and the overall operated smoothly at approximately 3.42 t/m3·d, although furnace protection with titanium ores was taken during the production at late stage of the campaign. The effective volume of the blast furnace was 1050 m3 and was equipped with 21 tuyeres as well as 2 tapholes. In this campaign, the average coke ratio and pulverized coal injection (PCI) ratio are 350 kg/t and 155 kg/t respectively while the blast temperature is around 1175 ℃. After the blowing-out and water-quenching of the blast furnace, the hearth was dissected and manually sampled from different positions of the cohesive zone. The sampling position and sample morphology are shown in Figure 1.


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Figure 1. Sketch map of sampling position and sample morphology.

2.2. X-ray Diffraction Measurements

The surface of the sample was removed by a thickness of about 2 mm, and the powder particles were collected and prepared for X-ray diffraction detection analysis. All the prepared specimens were crushed to fine powders of less than 74 µm in size before packing them into an aluminum holder. The XRD patterns were obtained by recording the scattering intensities of powder samples using a Rigaku Diffractometer (UltimaⅣ, Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation (40 kV, 40 mA) as the X-ray source; the scanning angles were in the range from 10 to 90 deg (2θ) at a scan rate of 20 deg/minutes.

2.3. SEM-EDS Analysis

The pieces of about 20 mm in length, 15 mm in width and 5 to 10 mm in thickness were cut from the selected coke samples under dry conditions, and then were mounted in an epoxy slow setting resin in plastic molds (25 mm diameter). Surfaces were ground on four different grades of silicon carbide paper


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(500, 800, 1500, and 2000 grit) with distilled water and polished with four different grades of papers with diamond paste of particle sized of 15 µm, 9 µm, 3 µm, and 1 µm. Lubrication fluid was used during the polishing. The polished coke samples in the current research were coated with gold according to the standard procedure of the SEM/EDS study. Then the coated samples were examined with a ZeissEVO-18 Scanning Electron Microscope (SEM) equipped with Energy Dispersive X-ray spectrometer (EDS) for chemical analysis and mapping.

3. RESULTS AND DISCUSSIONS 3.1. XRD Analysis of Coke

Figure 2 is the XRD patterns of the coke samples, the sharp (002) carbon peaks can be clearly observed from the diffraction patterns, which indicates the different degrees of ordering of carbon structure as well as graphitization in cohesive zone cokes. The characteristic peaks of kalsilite, magnetite, sodium aluminum oxide and nepheline were also detected in the XRD patterns, although the intensities of some peaks are relatively weak. The presence of kalsilite, sodium aluminum oxide and magnetite was prevailing in all coke samples from cohesive zone while the nepheline was only detected in sample A. In addition, it should be noted that except for the feed coke, SiO2 was not detected in these coke samples. Even for the feed coke, the characteristic peak of SiO2 is not obvious. This may be understood from the following aspects. First, some SiO2 in cokes would be involved in the formation of aluminosilicate compounds of potassium-sodium or may react with carbon to form silicon carbide that was found to be presented in the bosh and raceway cokes.18 Secondly, the identification of the mineral phases in coke is usually difficult because of the high percentage of carbon (about 90%), which produces peaks with high intensity and reduces the intensity of the mineral phases through dilution.19


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Besides, as the characteristic peak of SiO2 is too close to the carbon peak, they may also be overwrote by the broad peak of carbon. Hence, it is difficult to distinguish these two phases especially when they are at low concentrations.

Figure 2. X-ray diffraction patterns of the coke samples: Particles taken from the surface of samples A, B, C and feed coke As for the formation of kalsilite, nepheline and sodium aluminum oxide, it is well known that illite is a common coal mineral that contains potassium and iron species, which can be transformed to potassium aluminosilicates in cokes.3 Furthermore, it has been proved that SiO2 and Al2O3 are the main minerals in cokes20, and the kalsilite can be synthesized by the mixed reagents of SiO2, Al2O3 and K2CO3 at about 1000℃.21 Therefore, the kalsilite, nepheline and sodium aluminum oxide may be formed when the potassium-sodium vapor ascends with the gas flow and condenses on the cokes in the cohesive zone. Wherein, the possible chemical reaction equation for generation of kalsilite in the cohesive zone coke can be expressed as follows.

2K(g)+2SiO 2 (s)+Al 2 O 3 (s)+CO(g)=2KAlSiO 4 (s)+C(s)

(1)

The Gibbs free energy of the chemical reaction can be calculated by subtracting the Gibbs free energy of formation of products from that of reactants. The calculation formula can be written as below.

Gr (T ) = 2 f GKAlSiO (T ) − 2 f GSiO (T ) − f GAl O (T ) − f GCO (T ) 4

2

2


3

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= H r (T ) − T Sr (T ) Where T is the absolute temperature, K; reactants and products, kJ/mol;

f

(2)

Gr (T ) is the variation of Gibbs free energy between the

GKAlSiO (T ) , 4

f

GSiO (T ) , 2

f

GAl O (T ) , 2

3

energy of formation of KAlSiO 4 , SiO 2 , Al 2 O 3 and CO , kJ/mol; between the reactants and products, kJ/mol;

f

GCO (T ) is the Gibbs free

H r (T ) is the enthalpy change

Sr (T ) is the entropy change between the reactants and

products, kJ/mol·K. Among the above parameters, the Gibbs free energy of formation of the reactants and products could be obtained from the FactSage database.

Gr (T ) and

H r (T ) of the total

reaction at different temperatures could be calculated by FactSage commercial software under the pressure of 1 atm, as shown in Table 1.

Table 1 Thermodynamic Parameters of Chemical reaction for the Generation of Kalsilite(kJ/mol) Parameters

1000℃

1200℃

1400℃

1600℃

H (T )

-791.67

-785.50

-779.42

-773.73

G (T )

-348.52

-279.39

-211.05

-143.43

It can be seen from Table 1 that with the increase of temperature, the amount of heat released by the reaction decreases. In the temperature range of interest, the variation of Gibbs free energy between the reactants and products are all far less than zero. Although the calculation conditions are somewhat different from the real environment in the blast furnace, the reaction can occur easily in cohesive zone from a thermodynamic point of view. Additionally, the primary slag formed in the cohesive zone may also adsorb the rising potassium-sodium vapor during the downward flow and enter the cracks and pores of the coke to generate the aluminosilicate of alkalis. Despite the different views on the influence of potassium and sodium vapor on the degradation of coke16,22, there is no doubt that the volume expansion of alkalis minerals in cokes is an important factor for the coke degradation.23 It may be noted from Figure 2 that


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the Fe3O4 was also detected in the coke samples of cohesive zone. As is known to us, there is a considerable amount of FeO in the primary slag of cohesive zone. Thus, it can be speculated that during the blowing-out of the blast furnace, FeO-containing liquid slag adsorbed on the surface of the coke or in its pores may be oxidized to transform to Fe3O4. Nevertheless, it should be pointed out that the hypothesis needs to be approached with caution because of no solid evidence.

3.2. SEM-EDS Analysis of Coke

For the convenience of comparison, the SEM images of feed coke are demonstrated in Figure 3, and the SEM images and EDS maps of coke sample A are shown in Figure 4. It can be seen from Figures 4(a) and (c) that a significant amount of macro pores are distributed in the coke, and many pores are filled with gray phases. Compared with the feed coke in Figure 3, it’s obvious that many pore walls of the coke are consumed and thinned, and some pores interconnect with each other to form larger pores with the diameters of about 150 µm, as shown in Figure 4(b). The presence of these pores reflects the violent gasification reaction of coke in the cohesive zone. The EDS results show the striking enrichment of elements such as Si, Al, K and Na in the gray phases of coke pores. It appears that the gray phase mainly consists of amorphous potassium nepheline and some aluminosilicate of potassium and sodium. Combined with XRD analysis, it's possible that these gray phases are mainly derived from the reactions of primary slag in cohesive zone or coke ash with potassium-sodium vapor. The possible formation pathways of aluminosilicate compounds of alkalis can be described as follows. (Ca − Mg)-Al − Si − O (melt) +O (gas) +K (gas) → (Ca − Mg) − Al − Si − K − O (melt)

(3)

(Ca − Mg) − Al − Si − O (melt) +O (gas) +K (gas) +Na (gas) → (Ca − Mg) − Al − Si − K − Na − O (melt)

(4)

The EDS maps in Figure 4 seem to indicate that besides the recirculating and accumulation of alkalis


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found in the tuyere cokes6,22, it’s also proved the evident enrichments of K and Na in the coke of cohesive zone. As a result, the role of coke in structural support, the permeability and the distribution of gas flow will be seriously affected by the alkalis due to its negative effect on the cokes in cohesive zone. In addition, spinel crystals were also found on the matrix of the gray phase. The EDS spectrum obtained from the area near the center of the crystal plane gave Mg:Alat ratio close to 1:2 that is in accordance with spinel stoichiometry. In comparison with the octahedral and dodecahedral spinels found in the tuyere cokes24-25, the spinel crystal occurred as hexagonal plate-like of about 15 µm in diameter which may be related to its formation conditions. Previous research shows spinel formation from a mixture of periclase and corundum is accompanied by a volume expansion of 8.0% as calculated from the density values.26 Therefore, the clearly visible cracks on the gray phase matrix where the spinel grows may be caused by the sharp edges and volume expansion of the spinel. The formation mechanism of spinel has exceeded the scope of this study. However, it can be speculated that the spinel that grows on the coke matrix or pore wall will probably cause the formation of coke cracks, leading to a weakening of its strength upon crystal growth.25 There are some signs and indirect comparisons that may favor such a conjecture, although no spinels that grow on a coke matrix were found in the present work currently.


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Figure 3. SEM images of feed coke.


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Figure 4. SEM images and EDS maps of coke sample A. The distribution and morphology of the gray phases and pores in the coke sample B are presented in Figure 5. As can be seen from Figures 5(a) and (b), similar to the sample A, the coke is also distributed with many macro pores of different sizes and some of them are filled with gray phases. Compared to the coke pores shown in Figure 4, the sizes of these coke pores are obvious larger, but the pores thinning or pore walls disappearance of the coke seems to be less pronounced. In the context of the widespread use of hydrogen-rich fuels, the existence of the pores increases the reaction rate of coke with CO2 and water vapor because of the increased the gas-solid reaction areas provided by the large number of pore walls. Moreover, the reaction will be further exacerbated by the catalysis of alkalis.16 Not only that, but some coke pores provide effective channels for the diffusion of CO2 and water vapor as well as potassium-sodium vapors into the coke. Eventually, the degradation of the coke is aggravated as a consequence of the thinning or disappearance of pore walls caused by the gasification reaction.


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What's more, it can also be speculated that during the downward movement of some cohesive zone cokes, the pores of the surface layer will be filled with the primary slag or final slag. Previous studies have revealed that at different periods, the coke bed in the hearth of a blast furnace either floats in the iron bath or sits on the bottom of the furnace, and the coke beds of smaller furnaces is more likely to be in a floating state compared with the large blast furnaces.27-28 Thus, when the coke with the slag phases filled comes into contact with the furnace bottom, these slag components may react with the refractories on the furnace bottom and cause the refractory to be dissolved and spalled, thereby increasing the corrosion of the refractory. From Figures 5(c) and (d), a large amount of aluminosilicate compounds of potassium and sodium are distributed on the coke pore walls. The spheroidal particles are likely to be undeveloped potassium nepheline crystals. And it may be noted that the kalsilite crystals with ideal hexagonal prism are observed on the pore walls, which is in correspondence with the results of XRD. The EDS results further indicate that the gray phases in the coke consisted of mainly Si, Al, K, Na and O, which reveals the accumulation of alkalis.


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Figure 5. SEM images and EDS maps of coke sample B. The SEM images and EDS maps of coke sample C are provided in Figure 6. Compared with coke samples A and B, the distribution of coke pores is not uniform and the size as well as crowding level of the pores are rather less, as shown in Figure 6(a). It seems that the coke was not reacted as evidently as the coke samples A and B. It can be clearly seen in Figure 6(b) that a large number of filamentous or spotty gray phases are distributed in the coke matrix and pores, which may be the aluminosilicates of K and Na formed by the reaction of the alkalis vapor with coke ash. At the same time, the appearance of these filamentous or spotty gray phases is likely to be resulted from the scour of intense gas flow. This also appears to suggest that the alkalis not only penetrates the coke through cracks and pores, but also invades the coke matrix. From the results shown in Figures 6 (c) and (d) and their EDS maps, one can see that the gray phase is uniformly distributed with K, Na, Al, Si, O and other elements, while the white phase is evenly distributed with Fe, O. Unlike the coke sample B, it seems that the accumulation


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of sodium in the gray phase of the coke is also relatively obvious from the color of EDS mapping. In a word, the compositions of gray phases in the cokes are similar, despite distinctive various features of the three coke samples. As discussed before, the gray phases are supposed to mainly amorphous potassium nepheline or aluminosilicate of potassium and sodium, while the white phase is mainly the oxides of iron or iron. It is interesting to note that the iron oxides or iron are either aggregated or distributed around the aluminosilicate of potassium and sodium. One possible reason for the presence of the iron oxides or iron is the reduction of Fe2O3 in coke ash.


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Figure 6. SEM images and EDS maps of coke sample C. As shown in Table 2, ten EDS points were selected respectively to calculate the average chemical


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compositions of the grey phases in Figures 4(d), 5(a) and 6(d). From Table 2, in the gray phases of the three specimens A, B, and C, the mass fraction of K decreases in turn, while the mass fraction of Na and Fe increases sequentially. This seems to indicate that the closer the coke to the lower edge of cohesive zone, the lower the amount of K adsorption and the greater the amount of Na adsorption. As for the change of the mass fraction of Fe element, it may be the more reduction amounts of iron liquid or low-valent iron oxides at the position near the lower end of the cohesive zone, so the iron element adsorbed by the coke increases. Moreover, the comparison of the ash chemistries of three coke samples and the feed coke are presented in Table 3. It’s obvious that the change rules of K2O, Na2O and Fe2O3 contents in the ash of the three coke samples are similar to the EDS results in Table 2. It may be note that the content of alkalis in the coke ash of the cohesive zone is significantly higher than that in the feed coke. The content of K2O in the coke ash of the cohesive zone is about 19-34 times that in the feed coke, while the corresponding Na2O content is 7-14 times that in the feed coke. The above results further confirm the significant enrichment of alkalis in the coke of cohesive zone. What’s more, it seems that no pronounced zinc enrichment was found in the cohesive zone coke from the current study.

Table 2. Elemental Chemical Compositions of Gray Phase in Coke Samples (Weight Percent) Sample

Ca

Mg

Al

Si

O

K

Na

Fe

A

0.80

0.40

19.91

26.69

22.65

27.46

2.01

0.09

B

0.48

0.07

25.80

25.72

28.32

16.58

2.75

0.29

C

0.17

0.23

22.23

22.57

28.47

14.49

10.31

1.53

Table 3. Chemical Composition of Coke Ash Indicated as Oxides (Weight Percent) Coke Type

CaO

MgO

Al2O3

SiO2

K2O

Na2O

Fe2O3

P2O5

SO3

TiO2

MnO

ZnO

Feed coke

4.02

0.92

35.72

47.09

0.44

0.67

6.27

0.53

2.37

1.75

0.21

0.01

Sample A

3.91

0.48

26.66

35.21

14.79

4.66

6.69

0.58

5.60

0.94

0.47

0.01

Sample B

4.69

0.83

25.82

33.49

13.11

4.87

9.13

0.52

5.63

0.98

0.76

0.17

Sample C

2.84

0.74

26.95

33.49

8.39

9.19

12.07

0.31

4.34

0.96

0.63

0.09


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As mentioned before, the effect of alkalis on coke performance is primarily achieved through the combined action of mechanical and chemical effects. The formation of aluminosilicate compounds of alkalis in the coke pores or matrix is accompanied by volume expansion, which may promote the formation of cracks. On the other hand, the alkali vapors may exacerbate the gasification reaction of coke because of its strong catalytic effect, thereby leading to the severe degradation of coke quality. The accumulation of alkalis in the cohesive zone coke is of great significance for understanding the degradation and energy utilization of coke in blast furnace. However, further study about the mechanism of the evolution of alkalis in the cohesive zone and its comprehensive influence on the degradation behavior of cohesive zone coke are required.

4. Conclusions

In this study, the cokes of the cohesive zone were examined by XRD and SEM-EDS to investigate the micro morphology and elemental distribution as well as the behavior of alkalis accumulation of the cokes. The main conclusions can be drawn from the results and observations as follows. 1.

The pores thinning or pore walls disappearance observed in the cohesive zone cokes reveals the severe gasification reaction of coke in cohesive zone. Both the size and crowding level of coke pores decrease with the position closer the lower edge of the cohesive zone.

2.

The accumulation of alkalis of the cokes in the cohesive zone was verified by the results of XRD and SEM-EDS. The vapors of K and Na can not only enter the coke through coke pores and cracks, but also penetrate into the coke matrix. The amorphous potassium nephelines or aluminosilicates of potassium and sodium in the coke pores or matrix are mainly derived from the reactions of primary slag in cohesive zone or coke ash with potassium-sodium vapor. Besides, the crystals of


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kalsilite (KAlSiO4) and spinel (MgAl2O4) were also observed in the cokes. 3.

The accumulation of K in the cokes is more obvious than that of Na. As the sampling position is closer to the lower edge of the cohesive zone, the adsorption amount of K decreases while the adsorption amount of Na increases. Meanwhile, the content of K2O in the coke ash of the cohesive zone is about 19-34 times that of the feed coke, while the corresponding Na2O content is 7-14 times that of the feed coke.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors appreciate the National Science Foundation for Young Scientists of China (No.51704019) for the financial support.

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Behavior of Alkali Accumulation of Coke in the Cohesive Zone

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