Catalytic Effect of Alkali Metals on the Gasification Dissolution Reaction and Deep Reaction Behavior of Metallurgical Cokes

In this paper, the deep reaction behaviors of tamping and top-charging cokes with different K2CO3 and Na2CO3 contents were investigated and the evolution of the functional group structure and the carbon structure of coke with the extension of the deep reaction was clarified. The results showed that the deep reaction gasification of coke steadily increased with the K2CO3 and Na2CO3 content. However, the catalytic effect of different contents of K2CO3 seemed to be stronger than that of different contents of Na2CO3. Meanwhile, the catalytic effect of alkali metals on the gasification reaction of tamping coke was more significant. The gasification dissolution of coke gradually decreased from the outside to the inside of the particle with the extension of the deep reaction, while the catalytic effect of K2CO3 and Na2CO3 seemed to be more dramatic on surface of the coke particle. In contrast, the gasification dissolution reaction on the surface of tamping coke was more severe; consequently, the surface pore area of tamping coke was much higher than that of top-charging coke. According to the variations in the functional group structure of cokes, aromatic hydrocarbon gradually became the dominant functional group in the coke structure with the extension of the deep reaction. The above variation led to a gradual decrease in the reactivity of coke, though the decomposition reactions of oxygen-containing functional groups in coke were promoted to some extent by the addition of K2CO3. With the extension of the deep reaction of coke, amorphous carbon was gradually transformed into sequential carbon.


INTRODUCTION
Although the blast furnace process is confronting the problems of high fuel consumption and greenhouse gas emissions, the blast furnace process will remain the dominant ironmaking method for a long time. The price of metallurgical coke has gradually increased over the past few decades due to the complexity of the coking process and the resource condition of coking coal; consequently, it is essential to reduce the consumption and improve the metallurgical properties of coke. 1−5 The quality of coke directly determines the anterograde, output, coke ratio, and blast furnace fuel ratio of the blast furnace and other vital indicators, which directly affect the economic benefits of steel enterprises. As a new technology, tamping coke can reduce the reliance of ironmaking process on high-quality coking coal, since the cost of coke has been constantly increased in the past decades. Based on the cost of molten iron and the demand for sustainable development, the large-scale use of tamping coke in blast furnaces instead of conventional top-charging coke has become a trend for blast furnace ironmaking. 6−8 However, due to differences between the production process for tamping coke and that for top-charging coke, the degradation behavior of these two types of coke in the blast furnace is quite different. 9,10 Alkali metals exist in different parts of the blast furnace in various forms during blast furnace smelting. Alkali metals mainly exist in the form of oxides, carbonates, silicates, aluminosilicates, cyanide, and interlayer compounds (e.g., C8K, C60K, etc.). 11 −15 In order to further clarify the influence of alkali compounds on the gasification dissolution reaction and the deep reaction of cokes in a CO 2 atmosphere, several studies were carried out in recent years. 16,17 Wang et al. 18,19 studied the catalytic effect of alkali metals on the coke gasification reaction by studying the adsorption of alkali solution vapor on pores of coke, and the results showed that the addition of alkali compound promoted the gasification reaction and consequently the degradation of coke. Gornostaye et al. 20 and Zhou et al. 21 studied the effect of alkali compounds on the coking behavior of coking coals, and the results showed that presence of excess alkali compounds influenced the formation of the coke colloid layer, which might greatly weaken the physical and chemical properties of metallurgical coke. The mechanical strength and thermal strength of coke was significantly reduced as the amount of K and Na increased. Liu et al. 22 used the sodium leaching method to test the reactivity and postreaction strength of coke with different added alkali compounds to clarify their effect on coke gasification performance at different temperatures. The results indicated that the reactivity of coke gradually increased and the strength of coke gradually deteriorated with as the alkali compound content increased after gasification under different temperatures. Zhao et al. 23 and Yu et al. 24 found that both K and Na vapors might promote the gasification and dissolution reaction of coke; however, the catalytic effect of K on the gasification and dissolution reaction of coke was stronger than that of Na.
The above investigations showed that alkali compounds might deteriorate the quality of coke, but few studies have been conducted on the variability of the radial dissolving loss of coke particles during alkali-rich coke gasification reactions. The catalytic effect of alkali metals on the deep reaction of coke and the corresponding influence on cracks and pores of coke particles has not been sufficiently investigated, and differences in the microstructures of the gasification dissolution reaction of top-charging coke and tamping coke under different alkali metal concentrations have yet to be thoroughly investigated. In this paper, the influence of addition quantities of K 2 CO 3 and Na 2 CO 3 on the deep reaction of different cokes and the variation in the functional group structure and carbon structure of coke with the extension of the gasification dissolution reaction were analyzed. Meanwhile, the difference in the structure and relative density of different depths of top-charging coke and tamping coke particles during gasification dissolution process were compared.

Materials and Preparation.
Three tamping cokes (A, B, and C) and three top-charging cokes (X, Y, and Z) were selected as the samples for this study, the proximate analysis and CRI and CSR indexes of which are listed in Table 1. The proximate analysis results of tamping and top-charging cokes were quite close. In comparison, the CRI values of tamping cokes seemed to be higher than those of top-charging coke; consequently, the CSR values of top-charging cokes were higher than those of tamping cokes.

Gasification Dissolution Reaction of Coke. 2.2.1. K and Na Enrichment on Coke.
In order to study the influence of K and Na on the gasification reaction and the deep reaction of different types of cokes, tamping coke C and top-charging coke X with similar thermal properties were selected for comparison; this additionally prevented the influence of the gasification performance of the coke itself on the alkali metal catalysis results. Both cokes were polished into cylindrical samples with a diameter of 25 mm and height of 25 mm, and the samples were subsequently dried at 115°C for 8 h to eliminate the influence of moisture. The prepared columnar coke samples were soaked in K 2 CO 3 or Na 2 CO 3 solutions with various concentrations (3%, 5%, and 7%) 25 for 24 h, then these K-enriched or Na-enriched coke samples were dried at a constant temperature of 115°C for 12 h. The region division of a cylindrical coke sample is shown in Figure 1.

Deep Reaction of Cokes.
K-or Na-enriched coke samples were loaded into a corundum crucible. The crucible was subsequently suspended under an electronic balance and heated to 1100°C at a constant heating rate of 5°C/min in a N 2 atmosphere with a flow rate of 2 L/min. When the coke samples reached the target temperature, N 2 was changed to CO 2 with constant flow rate of 5 L/min. After a 2 h gasification dissolution reaction, CO 2 was switched back to N 2 with a constant flow rate of 5 L/min until the coke samples cooled to ambient temperature. The real-time weight data of coke samples were recorded by a computer connected to the electronic balance. The configuration of the deep reaction device is illustrated in Figure 2.

FTIR Measurement.
Infrared spectroscopy was used to clarify variation in the functional group structure of the coke with the extension of the gasification dissolution reaction. Coke samples were crushed and dried at a constant temperature of 105 ± 5°C for 8 h to eliminate the influence of moisture. The dried coke samples were mixed with KBr in a ratio of 1:100, then the samples were ground for 10 min to mix them thoroughly. An Agilent Technologies Cary 630 FTIR infrared spectrum analyzer was used to perform infrared spectroscopy detection on a thin slice of sample at ambient temperature. The infrared scanning range was 450−4000 cm −1 , and the scanning resolution was 2 cm −1 . Before the each test, a background scan without sample was acquired.
2.4. Determination of Functional Group Structure Parameters. The peak positions and areas of characteristic peaks were obtained by the deconvolution integral peak fitting to calculate the functional group structural parameters of coke sample, including the apparent aromaticity f a , the aromaticity parameter I, the degree of condensation DOC, and othe xygencontaining functional group parameter "C".
Aromaticity is the ratio of aromatic carbon atoms to the total number of carbon atoms in a structural unit in a coke sample. The aromaticity value can be determined as follows: 26 In the above formula, H al and H ar respectively represent the content of hydrogen in aliphatic hydrocarbons and aromatic hydrocarbons, A 2800−3000 is the peak area in the wavenumber range between 2800 and 3000 cm −1 , A 700−900 is the integrated area of peak to 700−900 cm −1 , and a al and a ar are fixed values (a al = 746 and a ar = 686).  (5) Here H al /H is the ratio of aliphatic (H al ) hydrogen atoms to total hydrogen atoms (H); C al /C is the ratio of aliphatic carbon to total carbon; H/C is the ratio of aliphatic hydrogen to aliphatic carbon atoms as determined by the ultimate analysis; H al /C al , with a fixed value of 1.8, is the ratio of hydrogen to carbon in aliphatic group; and A 2800−3000 is the integrated area in the wavenumber range of 2800−3000 cm −1 , which is used to estimate the total aliphatic content (CH 3 , CH 2 , and CH).
Parameter I refers to the relative abundance of aromatic and aliphatic structures and can also characterize the length of the aliphatic chains. The value of I can be calculated using the following formula: 2800 3000 (6) DOC represents the repetition rate of the aromatic structures and can be used to estimate the activity of aromatic structures.
The DOC value can be determined using the following equation: where A 1600 is the area of absorption peak located at the wavenumber of 1600 cm −1 . "C" indicates the maturity of the coke and represents the ratio of C�O to C�C, the value of which can be calculated as follows: 1650 1800 1600 (9) where A 1650−1800 is the area of the peak located between 1650 and 1800 cm −1 .

Raman Spectroscopy Inspection.
A high spectral resolution Raman spectrometer (LabRAM HR Evolution) was used to analyze the carbon structure characteristics of coke samples. The coke samples were tested in the wavenumber range of 100−2500 cm −1 at an ambient temperature of 20−25°C with a wavelength of 532 nm. 31−33 The resolution of the test results was better than 0.65 cm −1 .

Calculation of Fixed and Nonfixed Carbon.
The parameters of I D1 /I G , I D3 /I G , I G /I all , and L a represent the degree of graphite-like structure in the graphite layer, the amorphous carbon fraction, the content of the graphite-like crystal structure, and the size of crystallite, respectively. 34,35 L a can be calculated as follows: 36,37 where C(λ) is the correction factor for the wavelength, C 0 is −12.6 nm, C 1 is 0.033 nm, and the effective wavelength range is 400−700 nm. The wavelength used in the paper is 532 nm.

K 2 CO 3 and Na 2 CO 3 Absorption of Coke Samples.
The adsorption capacity of K 2 CO 3 and Na 2 CO 3 in coke samples is shown in Figure 3.
As shown in Figure 3, the adsorption capacity of cokes increased with the concentration of the soaking solution, while the adsorption capacity of K 2 CO 3 was slightly higher than that of Na 2 CO 3 . As shown in Table 2, the adsorption capacity of sample X for the K 2 CO 3 solution was slightly larger than that for the Na 2 CO 3 solution when the concentration was 3% but the gasification catalysis of Na 2 CO 3 was smaller than that of K 2 CO 3 , which indicates that the difference in the gasification reaction was not due to the concentration. The difference may be caused by the different coke structures.

Experimental Results and Analysis.
The gasification dissolution reaction curves of C and X coke samples with various amounts of added K 2 CO 3 or Na 2 CO 3 are respectively shown in Figures 4 and 5. The effect of different alkali metal contents on the coke gasification dissolution is shown in Table 2. It can be seen from above data that the catalytic effect of K 2 CO 3 on C coke seemed to be much stronger than that on X coke during the gasification dissolution reaction, while the catalytic effect on both coke samples was stronger with higher amounts of added K 2 CO 3 .
The catalytic effect of Na 2 CO 3 on C and X coke samples was very close at low Na 2 CO 3 addition conditions, but the gasification dissolution reaction of C coke was significantly stronger when concentration of the Na 2 CO 3 solution was higher than 5%. Besides, for the deep reaction of coke, the catalytic effect of Na 2 CO 3 on coke seemed to be slightly weaker than that of K 2 CO 3 , and the difference in the catalytic effect increased with the solution concentration.

Influence of the Alkali Metal Microstructure on the Coke Structure. 3.3.1. Microscopic Morphology of Coke.
The structure of coke is a critical factor for the gasification of coke, which is of great importance for evaluating coke quality. Hence, the radical differences of coke samples at different stages of the gasification dissolution reaction were observed by SEM, and the proportion of the pore area at different radical positions was analyzed to clarify the variation in the pore structure of coke with the extension of the deep reaction. The variations of the microstructure at different radical positions of coke after the gasification dissolution reaction for different K 2 CO 3 addition conditions are shown in Figures 6−9. The depth range of 0−7 mm from the surface of the coke particle is defined as the edge, the depth range of 8−14 mm from the surface of the coke particle is defined as the middle, and the depth range of 15−25 mm from the surface of the coke particle is defined as the center.
As can be seen, the dissolution of pore structures for both C and X coke samples gradually developed at the centers of the coke particles with the extension of the deep reaction, while the number of pores on the surface of the coke particle gradually decreased. Due to the limitation of CO 2 diffusion, the gasification reaction on the surface of the coke particle was more violent. Consequently, the dissolution on the surface structure was found to be more severe; the superficial pore structure seemed to be loose, and the surface of coke particle became rough after the gasification test. In addition, the dissolution degree of C coke was more severe compared to that of X coke. The pore walls at the edges of coke particles became thinner, and the perforation of large internal pores was also noticed. As a result, many internal pores were connected with the extension of the deep reaction, and the superficial porosity of the coke particle increased significantly. As the amount of added K 2 CO 3 or Na 2 CO 3 increased, the degree of carbon dissolution on the surface of the coke particle intensified significantly and the superficial damage in the microstructure became more severe. It was also observed that the dissolution degree in the middle part of coke was the relatively weaker compared to that at the surface. It is worth noting that the carbon dissolution in middle part of C coke was significantly higher than that of X coke. The dissolution degree in the middle part of coke particle increased under K 2 CO 3 or Na 2 CO 3 addition conditions, which led to the formation of cracks in microstructures of the cokes. These formed cracks provided an ideal channel for the diffusion of CO 2 and consequently resulted in severe dissolution in the middle part of the coke structure.

ACS Omega
http://pubs.acs.org/journal/acsodf Article Image-Pro Plus software was applied to analyze the SEM pictures of different coke samples in order to quantitatively compare the area proportions of the pores at different depths of the coke structure, and the results are shown in Figure 10. It can be seen that the proportion of the surface pore area increased with the amount of added K 2 CO 3 and Na 2 CO 3 . The surface pore area proportion under K 2 CO 3 addition conditions was slightly higher than that of under Na 2 CO 3 conditions, while the proportion of the surface pore area gradually decreased as the depth increased and increased with the amount of added K 2 CO 3 and Na 2 CO 3 . This was possibly due to the catalytic effect of the alkali metal and the diffusion of CO 2 into internal parts of the coke particles. On the basis of the above analysis results, the gasification reaction mechanism of the coke particle is summarized in Figure 11.

Functional Group Structural Evolution in Coke Samples during Gasification.
The gasification experiments of coke samples were conduted under various K 2 CO 3 and Na 2 CO 3 addition conditions, and the functional group structures of coke samples at different stage of gasification were analyzed. The infrared spectra of different coke samples are shown in Figure 12. All spectra were smoothed and baselinecorrected using OMINIC software. The absorption peaks at different wavenumber positions are related to the relative   amount of the corresponding functional groups in the coke sample. The original infrared spectra were deconvolutioned into several independent peaks to calculate the functional group structures of the coke samples, and the deconvolution process is illustrated in Figure 13. The infrared absorption peaks, peak areas, and other data can be obtained through this process. The affiliation relationship between absorption peaks at different wavenumber positions and functional groups is shown in Table  3.
It can be seem from Figure 12 that similarities there are similarities in the spectra of the selected samples in the wavenumber range between 3000 and 3750 cm −1 , indicating the abundance of −OH and −CH 3 functional groups in the selected coke samples. The intensities of the above-mentioned absorption peaks in the X sample were noticed to be significantly higher than those in the C sample after the gasification reaction, which indicated the high proportion of −OH and −CH 3 functional groups in the X sample. The absorption peak between 900 and 1040 cm −1 represents the ash content of the coke sample, which was noticed to be much higher in the C sample than the X sample. Meanwhile, the absorbance of coke gradually increased with the extension of the deep reaction, indicating the production of ash in coke samples with the consumption of the carbon matrix.
The functional group structural parameters of coke samples were calculated according to the data of independent peaks from deconvolution; the detailed calculation method was described previously. The functional group structural parameters results of different coke samples are listed in Table 4. It can be seen from the data that the f a and I values both increased after the gasification dissolution reaction, which indicated that the aromatic functional groups were relatively stable during the gasification of coke. Meanwhile, the degree of condensation of the coke samples was significantly higher after the reaction index test, indicating that the aromatic hydrocarbons gradually became more stable with the extension of the gasification dissolution reaction. Under K 2 CO 3 and Na 2 CO 3 addition conditions, the f a and I values both decreased as the amount of added K 2 CO 3 and Na 2 CO 3 increased after gasification, while the DOC value decreased as the amount of alkali metals increased. The above phenomenon implies that the both K 2 CO 3 and Na 2 CO 3 have an obvious catalytic effect on −OH and −CH 3 groups and that the catalytic intensity has a positive relationship with the amount of added K 2 CO 3 and Na 2 CO 3 , while the effect of K 2 CO 3 and Na 2 CO 3 on the reaction behavior of aromatic functional groups was insignificant. The catalytic effect of K 2 CO 3 and Na 2 CO 3 on the C coke sample seemed to be higher than that on the X coke. The higher A(CH 2 )/A(CH 3 ) value of C suggests that the length of the carbon chain in the structure of C was longer than that in X, that is, the instability of C during the chemical reaction. As the amount of added K 2 CO 3 and Na 2 CO 3 increased, the collapse of the carbon chain in the C coke sample intensified significantly and the A(CH 2 )/A(CH 3 ) value of C sharply decreased. However, the A(CH 2 )/A(CH 3 )  value of X was observed to be quite stable with different K 2 CO 3 and Na 2 CO 3 addition conditions. Besides, the number of oxygen-containing functional groups in both coke samples was noted to be slightly lower after the gasification dissolution reaction. The value of "C" sharply decreased with as the amount of added K 2 CO 3 and Na 2 CO 3 increased, indicating the strong catalytic effect of the alkali metal on the decomposition of oxygen-containing functional groups; additionally, the catalytic intensity increased with the alkali metal content. 38,39 3.5. Raman Spectroscopy Analysis. The carbon structures of C and X coke samples after reaction index test under various K 2 CO 3 and Na 2 CO 3 addition conditions were analyzed by Raman spectroscopy, and the test results are shown in Figure  14. The deconvolution process and the affiliation of independent peaks are respectively shown in Figure 15 and Table 5. The structural parameters of coke samples were respectively calculated according to the peak areas of independent peaks, and the results are shown in Table 6.
The variation in carbon structural parameters I D1 /I G , I D3 /I G , I G /I all , and L a of C and X coke samples after the gasification reaction under different K 2 CO 3 and Na 2 CO 3 addition conditions is shown in Table 6. The I D1 /I G and I D3 /I G values decreased with the extension of the deep reaction, which indicated an increase in the proportion of graphite-like carbon in  the coke sample. Meanwhile, the decrease in the fraction of nongraphitic crystalline carbon indicated that the irregular carbon in coke was transformed into graphite-like carbon over the course of the gasification reaction; the increase of the I G /I all value also proved that both the proportion of the graphite-like crystal structure and the grain size increased during gasification.
According to the changes in carbon structural parameters, the existence of both K and Na led to a reduction in graphitization, grain size, and the graphite-like crystal structure content and an increase in fraction of amorphous carbon in coke samples. The above phenomena indicate that the addition of K and Na might facilitate the gasification dissolution reaction process of coke, as it inhibits the conversion of amorphous carbon to ordered carbon during gasification; for example, most active sites had amorphous carbon structures during gasification. Meanwhile, the addition of K and Na would promote the decomposition of stable aromatic rings into small hydrocarbons, consequently preventing the graphitization of coke during the gasification reaction. Besides, the catalytic effect of K was noticed to be stronger than that of Na, especial for the condition of tamping coke in comparison to top-charging coke. The fraction of amorphous carbon was higher due to the lower degree of  graphitization in tamping coke, which was possibly the reason for the stronger catalytic effect. A higher resistance of topcharging coke to the alkali metal was noticed; hence, the metallurgical property of top-charging coke in a blast furnace is thought to be superior.

■ CONCLUSIONS
In this paper, the gasification dissolution reaction and the deep reaction between coke with different K 2 CO 3 and Na 2 CO 3 contents and CO 2 were studied and the variation of the pore microstructure, functional group structure, and carbon structure with the extension of gasification and dissolution reaction and the deep reaction were analyzed. The results can be summarized as follows: (1) As the K 2 CO 3 and Na 2 CO 3 content in the coke sample increased, the gasification degree in the deep reactions of both tamping coke and top-charging coke gradually increased. No significant difference was noticed in the catalytic effect of K 2 CO 3 and Na 2 CO 3 when the concentration of soaking solution was increased from 3% to 5%. However, the catalytic effect of K 2 CO 3 was seemed to be much stronger than that of Na 2 CO 3 when the concentration of the applied solution was 7%. (2) With the extension of gasification dissolution, the difference in the gasification dissolution extent between the exterior and interior of the coke samples became more significant. The catalytic effect of K 2 CO 3 and Na 2 CO 3 seemed to be stronger on the exterior of the coke particle. (3) Due to the stability of the aromatic structure, the proportion of aromatic hydrocarbons in coke gradually increased with the extension of the gasification dissolution reaction. The addition of K 2 CO 3 may significantly    catalyze the decomposition of oxygen-containing functional groups in the coke, and the catalytic effect seems to increase with the concentration of the soaking solution. (4) The gasification dissolution reaction of coke could be catalyzed by the addition of K 2 CO 3 and Na 2 CO 3 , as the presence of the alkali metal might inhibit the conversion of amorphous carbon to ordered carbon during gasification. The catalytic effect of K 2 CO 3 and Na 2 CO 3 on the gasification dissolution reaction of tamping coke seemed to be stronger than that of top-charging coke, which was possibly due to the lower degree of graphitization and the higher fraction of amorphous carbon in tamping cokes.