Gasification Reactivity and Structure Evolution of Metallurgical Coke

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Gasification Reactivity and Structure Evolution of Metallurgical Coke under H2O/CO2 Atmosphere Runsheng Xu, Bowen Dai, Wei Wang, Johannes Schenk, Anrin Bhattacharyya, and Zhengliang Xue Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03023 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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Gasification Reactivity and Structure Evolution of Metallurgical Coke under H2O/CO2 Atmosphere Runsheng Xu†*, Bowen Dai, Wei Wang†*, Johannes Schenk‡, Anrin Bhattacharyya‡, Zhengliang Xue†

†State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China; Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China. ‡Chair of Ferrous Metallurgy, Montanuniversität Leoben, Franz-Josef-Straße 18, A-8700 Leoben, Austria * Corresponding author; E-mail:[email protected]; [email protected]

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KEYWORDS: H2O; coke gasification; pore structure; microcrystalline structure; energy barrier

ABSTRACT: The metallurgical properties and the microstructure of coke after gasification reaction with pure H2O and pure CO2 were investigated in this study. Moreover, the first principles calculation was conducted to study the reaction process of the carbon with pure H2O and pure CO2. The results show that the CRI (coke reaction index) increases sharply and the CSR (coke strength after reaction) decreases sharply, when the cokes are gasified with H2O as compared to CO2. The scanning electronic microscopy (SEM) images and the coke panoramagrams show that H2O more easily leads to the generation of large pore (>500µm) and destroys the coke structure than CO2. The X-ray diffraction (XRD) results indicate that the arrangement of carbon atoms of coke becomes regular and the ordered degree of coke increases after reaction with CO2 and H2O; however, after being gasified with H2O, the cokes have a higher ordered degree than with CO2. The results of the first principles calculation show that the H2O molecule is more likely to react with carbon as compared to the CO2 molecule due to the lower energy barriers of H2O adsorption and H2 formation. The M2→FS reaction process is the controlled step of the C-H2O reaction process, as well as in the C-CO2 reaction system.


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1. INTRODUCTION Metallurgical coke is a porous carbon material used in the ironmaking blast furnace. It serves a significant important role as a stock column skeleton, heat supply, carburizing agent and reducing agent [1,2]. The function of heat supply, carburizing and reducing can be partly replaced by other fuels, such as coal, oil, biomass char, and so on. But the role of providing support to the ore burden cannot be substituted by any fuels, because coke is the only material that remains a solid phase in the high temperature zones in blast furnace. Therefore, the coke is required to keep adequate strength to support the furnace burden and resist fracture, minimizing coke powder generation with consequent improvement of burden permeability [3]. In the industry, two indexes, CRI (coke reaction index) and CSR (coke strength after reaction), are widely used for determination of post reaction properties of coke in a blast furnace. It is known that high CSR and low CRI of coke are more desirable for the stable operation of blast furnace [4,5]. To save energy and reduce greenhouse gases, many innovations have been applied in the ironmaking process of blast furnace, such as high oxygen injection, mixed injection of pulverized coal and hydrogen-rich gas (natural gas, coke oven gas and artificial gas) and even hydrogen injection, and so on[6-9]. New ironmaking technology will not only decrease the requirement amount of coke consumption, but also lead to thinner “coke window” for the passage of reduction gas and molten liquid, less coke amount to support the burden. In general, the injection of hydrogen fuel will greatly increase the content of H2O and CO2 in blast furnace. However, the high content of H2O and CO2 will accelerate the coke solution loss reaction, and then seriously destroy the structure and metallurgical properties of coke[10]. Therefore, the effect mechanism of H2O and CO2 on coke gasification reaction and post reaction strength should be


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clarified due to its great importance on the determination of the feasible of hydrogen fuel injection. A large body of studies were carried out to investigate the gasification of coke with CO2 and its effect on the strength and microstructure

[11-19]

, but the gasification of coke with H2O are

rarely discussed. Iwanaga et al. examined the effects of gas type, coke characters, particle size and reaction temperature on the coke’s degradation, and revealed that the reacted coke with H2O generated more powders at a higher reaction rate compared to the reacted coke with CO2[20-21]. Shin et al. investigated the effect of coke gasification with CO2 or H2O on the coke porosity and strength in the temperature range of 1100-1500℃, and showed that the change of porosity of coke gasified with H2O was quite different from the case of coke gasification by CO2. The difference in the reaction mode of coke under CO2 or H2O caused an obviously different coke strength and powder formation [22]. According to the study of Wang et al., the reaction of coke with H2O occurred more strongly at the exterior compared to the reaction with CO2. Furthermore, the coke strength after reaction (CSR) with H2O was lower than with CO2 at 9501100°C, but higher at 1200°C[23,24]. In order to clarify the difference of the coke gasification reaction with CO2 and H2O, Guo et al. conducted the kinetic analysis of gasification of coke under CO2 or H2O. On the basis of the results, the reaction rate of coke with H2O is about 2-4 times faster than that with CO2 at the same temperature. The gasification reaction control mechanism of coke varied with the gas type and the reaction temperature[25,26]. Beside above studies on the coke gasification with pure CO2 or pure H2O, some experiments were also designed to ascertain the coke gasification under mixed gas (CO2 and H2O)[27] or simulated blast furnace shaft gas atmospheres[28,29]. The results indicated that the increase of H2O further increased coke reactivity, but it had no obvious effect on the threshold temperature of coke


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gasification in the simulated blast furnace shalt gas atmosphere. When the H2O was introduced to the gas atmosphere, the correlation between coke reactivity and industry CRI was weak [29]. In previous studies, it is clear that the gasification reaction mode and strength degradation of coke is quite different when the CO2 is replaced by the H2O or a blended gas with CO2 and H2O, but few studies have explored the difference of the microstructure evolution and the molecular reaction mechanism of coke under H2O and CO2 atmosphere. Therefore, the current study aim to illustrate the change of coke properties (CRI and CSR) and microstructure after gasified with pure CO2 or pure H2O. The features of pore structure and crystallite structure of reacted coke were identified through the analysis of SEM (Scanning Electron Microscope) images, panoramagrams and XRD (X-Ray Diffraction) spectrums. Furthermore, the differences between the carbon reaction with H2O and CO2 were revealed from the molecular level using the first principles calculation, which clarified why H2O was more likely to react with carbon than CO2. These results in paper enrich the understanding of the gasification mechanism and degradation behaviors of coke gasified with H2O and CO2, and have directive significance to the utilization and preparation of coke in the blast furnace with the injection of hydrogen fuels. 2. EXPERIMENTAL 2.1. Raw materials and experimental apparatus The cokes used in the experiments were obtained from steel companies. The proximate analyses of the cokes are tested according to Chinese standard GB/T212-2008, and the results are shown in Table 1. Coke A and Coke B are top charging cokes, while Coke C and Coke D are tamping cokes.


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Table 1. Proximate analysis of coke samples (in air dry base wt.%). Samples

Moisture

Ash content

Volatile matter

Fixed carbon

Coke A

1.00

13.16

2.60

83.24

Coke B

0.59

12.86

1.42

85.13

Coke C

0.43

12.84

1.47

85.26

Coke D

0.32

12.97

1.81

85.90

The apparatus (SYD-T224M) jointly developed by Wuhan University of Science and Technology and Anshan Xingyuanda Science and Technology co. LTD was employed as the experimental apparatus, as shown in Figure 1. The composition and flow rate of the gas (N2, CO2 and H2O) were controlled by a gas control instrument. Particularly, the water steam was generated by a vaporizing stove and provided to the reactor steadily.

Figure 1. Schematic diagram of gasification experimental apparatus. (1-electric furnace, 2reactor, 3-electronic balance, 4-computer measurement and control system, 5-vaporizing stove, 6-water steam flow control instrument, 7-CO2 and N2 flow control instrument, 8-the inlet pipe, 9the outlet pipe, 10-nitrogen, 11-carbon dioxide.) 2.2. Experimental procedures and analysis methods 2.2.1. Gasification experimental procedure


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In the gasification experiments, the cokes (approximately 200 g) with a size of 23-25 mm were put into the reactor (according to Chinese standard GB/T 4000-2008). Before the experiment, the samples were placed in an oven for drying at 180°C for 2 h to remove the water in the coke. Then, they were placed in the apparatus to heat to the experimental temperature (1100°C) under nitrogen conditions with a flow rate of 0.8 L/min, and that temperature was held for 10 min. Next, N2 was replaced by the reaction gas with the flow rate shown in Table 2, and the coke was gasified with the gas for 2 h according to Chinese standard GB/T 4000-2008. At last, the samples were cooled down to ambient temperature under N2 (3.0 L/min). Table 2. Flow rate of the reaction gas for different experiments. Reaction gas

CO2/(L/min)

H2O/(L/min)

Pure CO2

5

0

Pure H2O

0

5

2.2.2. CRI and CSR testing The coke reactivity index (CRI) indicates the ability of the coke reaction with carbon dioxide and water steam, and the coke strength after reaction (CSR) indicates the ability of coke to resist fracture and abrasion under mechanical force. The residual samples after gasification were weighed and this weight marked as G1. Subsequently, the residual samples were placed into an Itype rotating drum (Φ130 mm×700 mm) and rotated for 600r at a rate of 20 r/min. After the test, the samples over 10 mm were screened and weighed, and the weights of the particles were defined as G2. The CRI and CSR can be calculated by formulas (1) and (2). CRI（%）=

G0 − G1 × 100% G0

(1)

CSR （%）= G2 / G1 × 100%

(2)


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Where, G0, G1 and G2 donate the weight of raw coke, the weight of reacted coke after gasification and the weight of reacted coke with a size greater than 10 mm after rotating, respectively. 2.2.3. Characterization of coke The samples were measured using a TTR Ⅲ powder diffractometer made in Japan. All experimental samples were crushed to fine powder of less than 0.074 mm in size before packing them into a holder. Copper Kα radiation (30kV, 30mA) was used as the X-ray source. The XRD spectra were obtained by scanning over an angular range (2θ) of 10° to 90° in a step of 2°/min. The microcrystalline parameters of coke were calculated by the following formulas (3-5) [30-32]: d (002) =

Lc =

N=

λ 2sin θ (002)

(3)

0.94λ β( 002) ⋅ cosθ( 002)

(4)

Lc

(5)

d ( 002 )

where λ represents the wavelength of X-ray, λ = 1.5418Å; d(002) is the average spacing of crystallite layer, Å; θ(002) is the scanning angle of (002) peak, rad; β(002) is the full width at half maximum (FWHM) of (002) peak, rad; Lc is the average stacking height of crystallite, Å; N is the average stacking layers of the microcrystalline. The micro morphologies of reacted and unreacted samples were examined by a scanning electron microscope (Nova 400 Nano), and the microscopic panorama was established by a three-dimensional digital microscope (Olympus DSX510). Meanwhile, the pore area of coke was counted by the Image-Pro-Plus software.


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2.2.4. Analysis procedure of pore size

Firstly, a series of microscopic images of samples were obtained by the digital microscope. About 225 (15×15) images with a size of 1280×960 pixels were obtained for each sample. Subsequently, the Photoshop software was adopted to splice these images into a panoramagram based on the principle of partially overlapping of the adjacent images. And then, the Image-proplus software was used to calibrate the scale plates of the panoramagrams. Finally, the pores were picked out of the panoramagram using threshold segmentation and the numbers as well as the average diameters of pores were counted by the Image-pro-plus software. 3. RESULTS AND DISCUSSION 3.1. Coke reactivity and coke strength after gasification reaction The cokes were gasified with pure H2O and pure CO2 at the temperature of 1100°C, and the coke reactivity index (CRI) and the coke strength after the reaction (CSR) of different coke samples are shown in Figure 2 and Figure 3, respectively.

Figure 2. CRI of cokes gasified with pure CO2 or pure H2O (1100℃).


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Figure 3. CSR of cokes gasified with pure CO2 or pure H2O (1100℃). As shown in Figure 2, it is obvious that the CRI of cokes gasified with pure CO2 is relatively low; however, the CRI increases by 20% when cokes are gasified with pure H2O, although the CRI of cokes vary among each other. This finding indicates that the consumed carbon of the

cokes reacting with pure H2O is more than that reacting with pure CO2. It can be seen from Figure 3 that the CSR of reacted cokes with pure CO2 is relatively high, but the CSR sharply decreases when the cokes are gasified with pure H2O. For example, the CSR of coke A decreases from 60.6% to 39.6%, and the CSR of coke B decreases from 71.3% to 50.3%. The above changes of CRI and CSR of reacted cokes are consistent with the founding of Wang et al.

[24]

,

indicating that the damage of H2O on cokes is larger than that of CO2, which is mainly due to the faster reaction rate of cokes with H2O. The faster gasification rate will lead to a more consumption of carbon matrix at a certain time and then cause greater serious damage to the coke structure

[33,34]

. Thus, the coke becomes loose and is easy to breakdown during the drum test,

which may be the main reason for the sharply decrease of CSR. The following chapter will investigate the structure features of gasified coke with H2O and CO2.


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3.2. Effect of H2O or CO2 on reacted coke structure 3.2.1. Effect of H2O or CO2 on the micro morphology of reacted coke

Figure 4. SEM images of cokes after different reaction conditions: (a) is the unreacted coke A; (b) is the coke A that reacted with pure CO2; (c) is the coke A that reacted with pure H2O; (d) is the unreacted coke C; (e) is the coke C that reacted with pure CO2; (f) is the coke C that reacted with pure H2O. From the CRI and CSR testing results of four cokes, it can be found that the H2O has stronger damage to coke compare to CO2 no matter what kind of coke is used. Therefore, two typical cokes (a top charging coke A and a tamping coke C) were chose for the following detail investigation. Figure 4 is the morphology of reacted cokes observed by SEM. It can be found that the pore wall and pore’s distribution of raw cokes and reacted cokes have clear differences. Particularly, the strong changes occur in the case of the coke gasification with H2O. The pore walls of reacted coke were eroded, and many minerals concentrated in the matrix or struck to the pore wall

[35-37]

. Furthermore, more large pores appeared in the reacted coke

[22,23,38]

. It was


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notably that there were palpable differences in coke structures when cokes were gasified with pure H2O or pure CO2. The pore size of coke after gasified with pure H2O was expanded more rapidly, and the pore depth was further enlarged, even leading to the generation of cracks. Moreover, the pore wall became thinner or even damaged, resulting in the connection of pores, which formed the passage channel of the reaction gas. Thus, it can be learned that H2O is more destructive to coke structure than CO2, which results in the appearance of a large number of big pores in the coke matrix and the collapse of the structure of coke. The loose structure and crack are easy to fall off under the mechanical shock, which is one of the reasons for the sharp decline in the strength of coke. 3.2.2. Effect of H2O or CO2 on the pore structure of reacted coke

The panoramagrams of coke A and coke C are shown in Figure 5. It is obvious that panoramagrams (a) and (d) represent a relatively dense structure, which shows that the pores in unreacted coke are relatively small with almost no connection pores, and coke C has a more compact structure than coke A. The pore size is enlarged when the cokes are reacted with pure CO2, as shown in panoramagrams Figure 5 (b) and (e). It is observed that a small number of connected pores appear, which indicates that CO2 has a certain destructive effect on the coke structure and leads to a thinner pore wall and larger pore size. When the cokes are gasified with pure H2O, as shown in panoramagram Figure 5 (c) and (f), it is intuitive that the structure of coke is seriously damaged, the pore wall collapses seriously, and the pores are connected with each other to form larger pores. These findings agree with the observation results using SEM.


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Figure 5. Panoramagrams of coke under different conditions: (a) unreacted coke A, (b) reacted coke A with pure CO2, (c) reacted coke A with pure H2O, (d) unreacted coke C, (e) reacted coke C with pure CO2, (f) reacted coke C with pure H2O In order to quantify the evolution of pore structure of coke after the gasification, the area and the size of pores were counted using the Image-pro-plus software by extruding the panoramagrams. In order to facilitate statistical analysis, pore with size of 0-100 µm is defined as small pore, pore with size of 100-500µm is defined as medium pore, and pore with size greater than 500µm is defined as large pore. The statistical results are shown in Table 3. It is observed that the unreacted cokes have a small number of small pores and are mainly comprised of medium and large pores. When the cokes are gasified with pure CO2 or pure H2O, the proportions of small pore and medium pore sharply decrease, while the proportion of large pore drastically increases, which indicates that the small pores expand to the medium pores and the


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medium pores expand to the large pores, resulting in an increased pore size. In addition, the proportion of large pore of reacted coke with pure H2O is about 96.83-98.53%, which is much higher than that of reacted coke with pure CO2 (about 86.15-87.7%). The above analysis shows that the effect of water steam on coke pore size is greater than that of carbon dioxide, resulting in the looser structure of the reacted coke with water steam. Table 3. Area percentage of pores with different sizes for coke samples (%).

Samples

Coke A

Coke C

Reaction conditions

Small pore

Medium pore

Large pore

unreacted

5.22

40.67

54.11

pure CO2

1.09

11.09

87.73

Pure H2O

0.60

2.57

96.83

unreacted

12.92

57.06

30.02

pure CO2

2.71

11.14

86.15

Pure H2O

0.38

1.09

98.53

3.2.3. Effect of H2O or CO2 on microcrystalline structure of reacted coke

As shown in Figure 6, the XRD diffraction patterns of cokes before and after gasification were obtained. The height of (002) peak of unreacted coke is relatively low and the full width at half maxima β(002) of (002) peak is the largest. When the cokes are gasified with pure CO2 or pure H2O, it is clear that the (002) peak becomes higher, and β(002) is smaller compared with that of the unreacted cokes. In addition, it can be learned from Table 4 that the stacking height Lc of the coke gasified with pure CO2 or pure H2O is larger than that of the unreacted cokes. Although the average spacing for the crystallite layer d(002) changes little, the average stacking layer of the microcrystalline N increases after the gasification reaction. It is known that Lc reflects the ordered degree of aromatic nucleus in carbon materials, i.e., the higher the Lc is, the higher the


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ordered degree of crystallization is. Therefore, this finding indicates that the arrangement of carbon atoms becomes regular after the cokes are gasified with pure CO2 or pure H2O. Furthermore, β(002) of the cokes that reacted with pure H2O is the smallest and Lc is the highest, which indicate that after being gasified with pure H2O, the cokes have the highest ordered degree of carbon microcrystal. Because the amorphous carbons in coke have better reactivity with the CO2 and H2O compared to the aromatic carbons, so they will prefer to be consumed in the reaction

[39,40]

. Furthermore, the reaction rate of coke with H2O is faster than that with CO2

[25]

.

Therefore, more amorphous carbons will be consumed by H2O compared to CO2 at the same reaction time, and the less disordered amorphous carbon in the coke will contribute to its higher ordered degree.

Figure 6. XRD curves of coke A and coke C. It is known that the reaction of carbon with CO2 and H2O mainly depends on the activated carbon atoms, which are more active than the graphitized carbon atom and mainly exist in the isotropic structure[41]. Therefore, when the cokes are gasified with CO2 and H2O, the isotropy of the cokes would be consumed preferentially to anisotropy. As shown in Figure 7, it is obvious that the isotropic structure is more severely damaged, while the anisotropy maintains a relatively


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compacted structure. Moreover, the ordered degree of the anisotropic structure is higher than that of the isotropic structure[41,42]. Therefore, the increasing proportion of the anisotropy of the coke after gasification reaction contributes to its higher ordered degree. In addition, comparing with the CO2 molecules, the H2O molecules are more likely to enter the space of the crystallite structure and react with carbon atoms due to their smaller molecule size[43], which results in the multiple consumption of the isotropy and increased ordered degree. Table 4. Calculated results of microcrystalline parameters of cokes. Samples

Coke A

Coke C

Reaction conditions

2θ(002)/°

β(002)/°

d(002)/Å

Lc/Å

N

unreacted

26.28

5.37

3.39

15.04

4.44

pure CO2

26.22

4.16

3.40

19.41

5.71

Pure H2O

26.35

4.03

3.38

20.51

6.07

unreacted

26.07

4.93

3.42

16.75

4.90

pure CO2

26.33

4.04

3.38

20.46

6.04

Pure H2O

26.28

3.39

3.39

24.36

7.19

Figure 7. Microstructure of isotropy in coke A before (a) and after (b) reaction. 4. FIRST PRINCIPLES CALCULATION OF THE GASIFICATION OF COKE WITH H2O OR CO2


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At present, however, research on the reaction mechanism of H2O and CO2 with coke is deficient, and few researchers have studied the reaction mechanism at the molecular level. The energy barrier (the production energy minus the reaction energy) and the transition state of the reaction of coke that gasifies with H2O and CO2 are calculated in this study based on the density functional theory to illustrate the difficulty degree of the two gasification reactions. The larger the energy barrier is, the more difficult the reaction is. Previous results showed that the carbon in coke was mainly graphite like carbon[42,44], so the six membered ring structure of coke was used in the calculation model[45-49]. The Dmol3 module of the Materials Studio software was used for all calculation, and the PW91 functional in generalized gradient approximation (GGA) was used. A Fermi smearing of 0.1 eV was used to speed up convergence. The cutoff energy, convergence tolerance energy, max force and max displacement were 340eV, 1.0×10-5eV/atom, 0.03eV/Å and 0.001Å, respectively[50]. The reaction of carbon with CO2 or H2O consists of the following steps. First, the CO2 or H2O molecule is chemically adsorbed on C atoms, and the bonds of CO2 or H2O molecules extend long enough to break into a CO perssad and an O perssad or an OH perssad and a H perssad, which are adsorbed on two adjacent C atoms. For reaction of carbon with CO2, the absorbed CO perssad and C atom form a ketene radical while the O perssad and C atom form a ketone radical, and then the ketene radical is first dissolved to form a CO molecule due to its poor stability. For reaction of carbon with H2O, the two hydrogen bonds breakdown to form a H2 molecule, leaving an O atom and a C atom to form a ketone radical. After that, the ketone radical will be dissolved to form the second CO molecule, and this step will destroy the six-membered ring structure of carbon and generate a new carbon ring[50-51]. The specific reaction steps are shown in equation (6) to equation (11).


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2C + CO2 = C(Oabsorb) + C(COabsorb)

(6)

C(COabsorb) = C + CO

(7)

C(Oabsorb) = Cx−1 + CO

(8)

2C + H 2O = C(OHabsorb) + C( H absorb)

(9)

C(OHabsorb) + C( H absorb) = C + C(Oabsorb) + H 2

(10)

C(Oabsorb) = Cx−1 + CO

(11)

The Cx-1 represents a new carbon ring formed by the removal of a C atom from the initial carbon ring. According to the above reaction procedure, the energy changes for carbon gasified with CO2 and H2O were calculated separately, the reactants and the products of each step were optimized to achieve the energy minimization structure. The structure of the intermediates and transition states are shown in Figure 8 and Figure 9. In the figures, IS, TS1 and M1are the initial state, transition state and final state of the equation (6) and (9); TS2 and M2 are the transition state and final state of the equation (7) and (10); TS3 and FS are the transition state and final state of the equation (8) and (11). The red ball is the oxygen atom, white ball is the hydrogen atom, and the grey ball is the carbon atom. The reaction starts from IS, and then passes through the transition state TS1 to reach the intermediate state M1, while the intermediate state M2 is generated through the transition state TS2. Finally, the final state FS is generated by passing over the TS3 transition state. Assuming that the potential energy of the initial reactant is zero, the potential energies of the intermediates and the transition states in the reactions are shown in Table 5. The active energy (energy barrier) and reaction energy (bond dissociation energy) for simple system can be defined by equation (12) and (13): Energy barrier=Energy of transition state-Energy of reactants

(12)

Reaction energy=Energy of products-Energy of reactants

(13)


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Energy & Fuels

Table 5. Potential energy of each intermediate and transition state of the carbon gasification (kcal/mol). IS

TS1

M1

TS2

M2

TS3

FS

C+CO2

0

61.875

-43.173

-3.396

-3.744

84.265

55.054

C+H2O

0

53.954

-47.430

-17.979

-18.278

69.731

40.020

Figure 8. Schematic diagram of the intermediates and transition states of carbon reacting with CO2.

Figure 9. Schematic diagram of the intermediates and transition states of carbon reacting with H2O.


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It can be learned from Table 5 that the energy of the reaction system decreases sharply from the state (IS) to the state (M1) for the CO2 and H2O molecules, which indicates that it is an exothermic process that gives off a lot of heat. However, the adsorption energy barrier (IS→M1) for H2O molecule (53.954 kcal/mol) is lower than that for CO2 molecule (61.875 kcal/mol), which indicates that H2O molecules are more easily adsorbed on the surface of carbon than CO2 molecules. It can be seen from Figure 8 and Figure 9 that both the adsorption reactions of CO2 and H2O are accompanied by bond breaking and formation. However, the energy of the hydrogen bond (4.539 kcal/mol) is lower than that of the carbon oxygen double bond (179.211 kcal/mol). Thus, when the transition state TS1 of the C-H2O reaction system is generated, the energy that needs to be overcome is smaller, and the energy barrier is lower compared with that of the C-CO2 reaction system. From the M1 state to the M2 state, the ketene radical formed from the absorbed CO2 generates a CO molecule with an energy barrier of 39.777 kcal/mol, and the absorbed H2O molecule generates a H2 molecule with an energy barrier of 29.451 kcal/mol. It can be concluded that, from the M1 state to the M2 state, the energy barrier of the C-H2O reaction system is lower than that of the C-CO2 reaction system. Thus, H2O molecule is more likely to react with carbon to form H2 molecule as compared to CO2 molecule. When the reaction is carried out to the M2 state, the generated CO molecule and H2 molecule will leave the carbon surface, and only one O atom is adsorbed on the C atom to form the ketone radical. Therefore, for the reaction from the intermediate state M2 to the final state FS, the reaction process of the C-CO2 reaction system is same as that of the C-H2O reaction system, which is the decomposition of the ketone radical to form a CO molecule, accompanied by the rupture of the intrinsic carbon ring and the formation of the new carbon ring. The energy barrier of these two-reaction systems is 88.009 kcal/mol.


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Energy & Fuels

Furthermore, it can be observed that the energy barrier of the reaction process M1→M2 is 39.777 kcal/mol when carbon reacts with CO2, and the energy barrier of the reaction process M2 →FS is 88.009 kcal/mol. Therefore, the M2→FS reaction process is the controlled step for the C-CO2 reaction process. When the carbon reacts with H2O, the energy barrier of the reaction process M1→M2 is 29.451kcal/mol, and the energy barrier of the M2→FS reaction process is 88.009 kcal/mol. Therefore, as in the C-CO2 reaction process, the M2→FS reaction process is the controlled step of the C-H2O reaction system. In addition, for the total reaction process, the reaction energy of the C-H2O reaction system (40.020 kcal/mol) is lower than that of the C-CO2 reaction system (55.054 kcal/mol), which indicates that the H2O molecule is easier to react with carbon compared to CO2 molecule. From above calculation result, it reveals that the higher reaction rate of coke with H2O molecule may be attributed to its less energy barriers of H2O adsorption and H2 formation. 5. CONCLUSIONS Experiments were carried out in this study to illustrate the differences of the CRI, CSR and microstructure of cokes gasified with pure H2O and pure CO2. The first principles calculation was conducted to illustrate the imparity of the reaction between carbon with H2O and CO2. The following conclusions were obtained: (1) According to the experimental results, it is obvious that the CRI of coke gasified with pure H2O is higher than that with pure CO2, and the CSR sharply decreases when the cokes are gasified with pure H2O. The damage of H2O on coke is larger than that of CO2. (2) After the gasification of coke with CO2 or H2O, the small pores (500μm). However, the pore size of coke after gasifying with pure H2O expanded more strongly compared to the reacted coke


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Page 22 of 26

with CO2. Furthermore, more than 96% large pore (>500μm) were generated and the pore depth was further enlarged, potentially leading to the appearance of cracks. The looser structure of reacted coke with H2O leaded to its less CSR value. (3) The results of XRD indicate that the gasification reaction can increase the ordered degree of coke due to the consumption of active carbon atoms. The ordered degree of carbon crystal in the reacted coke with H2O is higher than that in the reacted coke with CO2. Furthermore, the isotropy of the cokes has a priority to be consumed as compared to the anisotropy, and H2O can consume more isotropy than CO2 due to its small molecule size and fast reaction rate. (4) It can be obtained from the first principles calculation that H2O molecule is more likely to react with carbon compared with CO2 molecule due to the lower energy barrier of H2O adsorption and H2 formation. The M2→FS reaction process is the controlled step of the C-H2O reaction process as well as the C-CO2 reaction process. For the total reaction process, the reaction energy of the C-H2O reaction system (40.020 kcal/mol) is lower than that of the C-CO2 reaction system (55.054 kcal/mol), which contributes to the greater CRI of the reacted coke with H2O. ACKNOWLEDGEMENTS Authors acknowledge the ﬁnancial support for the National Natural Science Foundation of China (U1760101, 51704216, 51474164) and the ﬁnancial support for the China Postdoctoral Science Foundation (No. 2016M602378). REFERENCES [1] Li, K.; Khanna, R.; Zhang, J.; Liu, Z.; Sahajwalla, V.; Yang, T.; Kong, D. Fuel 2014, 133, 194-215.


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Energy & Fuels

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Gasification Reactivity and Structure Evolution of Metallurgical Coke

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