Thermodynamic and kinetic behaviors of coke gasification in N2COCO2H2H2O

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Thermodynamic and kinetic behaviors of coke gasification in N2eCOeCO2eH2eH2O Chenchen Lan a, Qing Lyu b,*, Xiaojie Liu b, Maofa Jiang a, Yana Qie b, Shuhui Zhang b a b

School of Metallurgy, Northeastern University, Shenyang, 110819, China College of Metallurgy and Energy, North China University of Science and Technology, Tangshan, 063009, China

article info

abstract

Article history:

Changes in the gas balance components, carbon gasification rate and reaction enthalpy of

Received 4 July 2018

carbon in reaction with CO2 and H2O were analyzed by thermodynamic calculations under

Received in revised form

different temperatures and pressures. The dynamic behavior of coke was simulated using

22 August 2018

a thermogravimetric method in N2eCOeCO2eH2eH2O, and the gasification reaction rate of

Accepted 31 August 2018

coke and the controlling factors in the gasification process were analyzed. The results

Available online xxx

show that the effect of H2O on coke gasification is stronger than that of CO2. In the low-

Keywords:

and the reaction enthalpy of C and CO2 gradually becomes higher than that of C and H2O

Coke

with increasing temperature. Increasing pressure can block the C gasification. Increases in

temperature region, the reaction enthalpy of C and H2O is higher than that of C and CO2,

Gasification

temperature and in H2O volume fraction promote coke gasification; furthermore, the

Thermodynamics

effective internal diffusion coefficient and the interfacial reaction rate constant of coke

Kinetics

gasification gradually increase, and the improvement of the internal diffusion conditions is

Reaction resistance

clearly greater than that of the interfacial reaction. In the process of coke gasification, with increasing temperature and H2O volume fraction, the gasification reaction region controlled by the interfacial reaction gradually increases. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Coke is an irreplaceable fuel in blast furnace (BF) ironmaking. The high-temperature strength of coke directly affects BF permeability, liquid permeability, the coke ratio, the coal injection ratio and the quality of molten iron [1]. The gasification reaction of coke in the BF is the most important factor affecting the high-temperature strength of coke [2]. The gasification reaction of coke mainly refers to the reaction of carbon with CO2 and H2O, which are produced by indirect reduction of iron oxides in the BF with CO and H2. The

gasification reaction causes coke to be dissolved and damaged, thereby reducing coke's metallurgical strength, and the production index of the BF is deteriorated [3,4]. Therefore, the study of coke gasification process in the BF is consistently an important research direction. In recent years in particular, to reduce CO2 gas emissions and the consumption of coke in BF operation, some researchers have proposed technologies for hydrogen-rich smelting, including oxygen BF [5e7], gasinjection BF [8,9], coke oven gas-injection BF [10,11], and natural-gas-injection BF [12e14] technologies. In these technologies, the H2 content of the bosh gas can reach 14e20%

* Corresponding author. E-mail address: [email protected] (Q. Lyu). https://doi.org/10.1016/j.ijhydene.2018.08.216 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Lan C, et al., Thermodynamic and kinetic behaviors of coke gasification in N2eCOeCO2eH2eH2O, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.216

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[9,10,15], the content of H2O in the BF increases continuously, and the gasification effect on coke also strengthens. Therefore, it is particularly important to understand the gasification behavior of coke after increasing the volume fraction of H2O in the gas. The gasification process of coke is a gas-solid reaction. The reaction process is as follows [16,17]: 1) External diffusion: CO2 and H2O diffuse to the coke surface; 2) internal diffusion: CO2 and H2O diffuse through the pore structure and cracks of coke to the interior of coke; and 3) interface reaction: CO2 and H2O continuously react with the carbon in contact with it during diffusion. Dynamic analysis is an important method for studying the reaction process of coke gasification [18,19]. This method can be used to analyze changes in diffusion and the interfacial reaction in the reaction process and to establish the gasification reaction law of coke in the BF. Some studies have reported on the influence of H2O on the kinetic behavior of coke gasification. Wang [20e23] et al. studied the differences between the reaction behavior of coke gasification in pure CO2 and pure H2O, finding that the gasification reaction rate in H2O is approximately 1.27e3.16 times greater than that in CO2 and that the initial temperature in H2O is 37 K lower than that in CO2. They also found that higher temperatures result in a greater degree of reaction on the surface of coke and that coke in H2O reacts more on the surface than coke in CO2. Guo [18] et al. also studied the different kinetic behaviors of coke in pure CO2 and pure H2O, finding that the internal diffusion resistance in CO2 is far higher than that in H2O. In CO2, during the early stage between 1173 K and 1273 K, coke gasification is controlled by the interfacial reaction and internal diffusion, while in the final stage at temperatures above 1273 K, coke gasification is controlled by the interfacial reaction; and coke gasification in H2O at 1173e1573 K is almost entirely controlled by interfacial reaction. The aforementioned studies focused on comparisons of coke gasification behavior in pure CO2 and pure H2O. These conditions are quite different from those associated with the actual atmosphere of the BF and cannot realistically reflect the dynamic behavior of coke in the BF. This paper is based on hydrogen-rich smelting in the BF. Changes in the gas balance components, carbon loss rate and reaction enthalpy of carbon in reaction with CO2 and H2O were compared and analyzed by thermodynamic calculations; the dynamic behavior of coke in BF was simulated using a thermogravimetric method and by simulating the actual atmosphere (N2eCOeCO2eH2eH2O) of a hydrogen-rich BF. We expect the results to provide a reliable theoretical basis for hydrogen-rich smelting in a BF.

Thermodynamic behavior of coke gasification The main gas components in the BF include N2, CO, CO2, H2 and H2O, among which N2 is mainly driven by hot air from the BF tuyere and does not participate in the reactions in the BF; instead, the gas dilutes the concentrations of the reaction gases. The gasification reactions of coke mainly refer to the reactions of coke with CO2 and H2O, whose equations are as follows, respectively: 1

CO2 ðgÞ þ C ¼ 2COðgÞDr Gqm ¼ ð166550  171TÞ J$mol

(1)

1

H2 O þ C ¼ COðgÞþH2 ðgÞr Gqm ¼ ð133100  141:63TÞ J$mol

(2)

To analyze the thermodynamic behavior of the C gasification reactions with CO2 and H2O, this paper establishes two reaction systems for Eqs. (1) and (2), namely, reaction system (1) (1 mol Cþ1mol CO2) and reaction system (2) (1 mol Cþ1mol H2O). The gas equilibrium components of reaction system (1) and reaction system (2) were calculated under different pressure conditions using thermodynamics software (HSC Chemistry). HSC Chemistry was developed by Outokumpu in Finland. The thermodynamic equilibrium calculation is carried out using the Equilibrium Compositions Component, which is based on the system's total Gibbs free energy minimum method [24,25]. The results are shown in Fig. 1. As shown in Fig. 1, with increasing pressure, the equilibrium concentration curves of each gas in reaction system (1) and reaction system (2) move to the hightemperature zone. For example, as the pressure increases from 100 kPa to 400 kPa, the temperature at the equilibrium concentration of CO2 at 50% in reaction system (1) increases from 942 K to 1010 K, and the temperature at the equilibrium concentration of H2O at 50% in reaction system (2) increases from 750 K to 819 K. These findings indicate that the increase in pressure is not conducive to the occurrence of the reactions in Eqs. (1) and (2). Comparing Fig. 1(a) and (b) reveals that the temperature at which the CO2 equilibrium concentration begins to decrease is higher than that in reaction system (2), when the H2O equilibrium concentration begins to decrease. This behavior indicates that the initial reaction temperature of coke and H2O is lower than that of coke and CO2. In Fig. 1(b), some CO2 gas appears and its concentration increases gradually with increasing temperature, reaching a maximum at approximately 900 K. The main reason for the appearance of CO2 gas is that Eq. (3) occurs in this temperature range. As the temperature continues to increase, the CO2 gradually disappears, mainly due to the occurrence of Eq. (4) [26]. With increasing pressure, the temperature value corresponding to the highest concentration of CO2 does not change, indicating that pressure has no effect on the reactions in Eqs. (3) and (4): 1

COðgÞþH2 OðgÞ¼ CO2 ðgÞþH2 ðgÞDr Gqm ¼ ð36571 þ 33:51TÞ J$mol

(3) 1

CO2 ðgÞþH2 ðgÞ ¼ COðgÞþH2 OðgÞDr Gqm ¼ ð36571  33:51TÞ J$mol

(4) The gas equilibrium components of Eq. (4) are shown in Fig. 2. The equilibrium components of CO2 and H2 gradually decrease with increasing temperature, while the equilibrium components of CO and H2O increase gradually. Eq. (4) can promote the occurrence of reaction system (2). Fig. 3 shows the gasification rate of C in reaction systems (1) and (2). As shown in Fig. 3, the gasification rate of C in reaction system (2) is higher than that in reaction system (1), indicating that H2O has a stronger gasification effect on coke than CO2 does. With increasing pressure, the gasification rates of C in reaction systems (1) and (2) decrease. For example, as the pressure increases from 100 kPa to 400 kPa, the

Please cite this article in press as: Lan C, et al., Thermodynamic and kinetic behaviors of coke gasification in N2eCOeCO2eH2eH2O, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.216

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(a)

(b)

100

100 kPa

H2O(g)

CO2 80

Gas volume fraction(volume%)

Gas volume fraction (volume%)

100

100 kPa 60

200 kPa

300 kPa 40

400 kPa 20

CO 0 600

800

1000

1200

1400

200 kPa

80

300 kPa 400 kPa

60

40

CO

H2 20

CO2 0

1600

400

600

800

1000

1200

1400

1600

T/K

T/K

Fig. 1 e Gas equilibrium components of reaction systems (1) and (2). (a) Reaction system (1); (b) Reaction system (2).

gasification rate of C in reaction system (1) decreases from 55.3% to 31.5% at 1000 K, and the gasification rate of C in reaction system (2) decreases from 73.9% to 54.4% at 1000 K. These findings indicate that increasing the pressure is beneficial to reducing C gasification.

Gas volume fraction(volume%)

50

CO2

40

H2

30 20

H2O

10

CO 0 200

400

600

800

1000

1200

1400

1600

1800

T/K Fig. 2 e Gas equilibrium components of Eq. (4).

Fig. 4 shows the reaction enthalpy of reaction systems (1) and (2). As shown in Fig. 4, in the low-temperature zone, the reaction enthalpy of reaction system (2) is higher than that of reaction system (1). With increasing temperature, the reaction enthalpy of reaction system (1) is higher than that of reaction system (2), and the difference gradually increases. Additionally, increased pressure is beneficial to reducing the reaction enthalpy of C gasification. For example, as the pressure increases from 100 kPa to 400 kPa, the reaction enthalpy in reaction system (1) decreases from 139.80 kJ to 99.61 kJ at 1000 K, and the reaction enthalpy in reaction system (2) decreases from 135.42 kJ to 105.2 kJ at 1000 K. Fig. 5 shows the gasification rate of C for coke with different H2O contents in N2eCOeCO2eH2eH2O. As shown in Fig. 5, with increasing H2O content, the gasification rate of C increases at all temperatures and the extent of the increase in the C gasification rate increases with increasing temperature. As the concentration of gas that can react with C increases in the mixed gas, the gasification rate of C increases when the reactions reach equilibrium. The thermodynamic analysis of coke solves only the possible limits of C loss in N2eCOeCO2eH2eH2O, and the kinetic conditions of coke gasification play an important role in the coke gasification process of BF production. Studying the dynamic behavior of coke gasification can provide a reliable theoretical basis for reducing coke consumption in BFs. 300

100 kPa 200 kPa 300 kPa 400 kPa

80

60

40

100 kPa 200 kPa 300 kPa 400 kPa

250

C+H2O(g) C+CO2

20

Reaction Enthalpy/ kJ

C gasification rate (mass%)

100

200

C+CO2

150

C+H2O(g)

100 50

0

0

400

600

800

T/K

1000

1200

1400

Fig. 3 e Gasification rate of C in reaction systems (1) and (2).

600

800

1000

1200

1400

T/K

Fig. 4 e Reaction enthalpy of systems (1) and (2).

Please cite this article in press as: Lan C, et al., Thermodynamic and kinetic behaviors of coke gasification in N2eCOeCO2eH2eH2O, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.216

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Results and discussion

V(H2O)=0%

80

Influence of temperature on coke gasification under different atmospheres

V(H2O)=10% V(H2O)=20%

60

Fig. 7 shows the gasification rate of coke (X, mass%) after t min at different temperatures; X can be obtained by Eq. (5) [18,27]:

40

X¼ 20

600

800

1000

1200

1400

(5)

where m0 is the initial weight of the sample (g), mt is the weight of the sample after t min (g), mb is the weight-reduction value of coke at the beginning of the gasification reaction (g), and C is the fixed C content. As shown in Fig. 7, with increasing temperature, X increases significantly in all atmospheres. When the volume fraction of H2O (V(H2O)) is 0%, 10% and 20% and the temperature increases from 1173 K to 1573 K, X increases from 0.24% to 27.04%, 0.35%e 42.64%, and 1.11%e59.18%, respectively. With increasing V(H2O), X increases at all temperatures. When V(H2O) increases from 0% to 10%, X increases by factors of 1.46, 1.58, 1.95, 1.80 and 1.58 at 1173 K, 1273 K, 1373 K, 1473 K and 1573 K, respectively. When V(H2O) increases from 0% to 20%, X increases by factors of 4.63, 2.67, 2.81, 2.11 and 2.19 at 1173 K, 1273 K, 1373 K, 1473 K and 1573 K, respectively. The increases are significant. When V(H2O) increases, the reactant concentration of coke gasification increases, which promotes the coke gasification.

0 400

m0  mt  mb  100% m0 $C

1600

T/K Fig. 5 e The C gasification rate with different H2O contents in N2eCOeCO2eH2eH2O.

Kinetic behavior of coke gasification Experimental method The coke used in the experiments was sourced from a steel company. The coke was ground to the shape of a sphere with a diameter of 25 mm and subsequently dried at 378 ± 5 K for 2 h. The results of the industrial analysis and ash composition analysis of the coke are shown in Table 1. The equipment used in the experiment is shown in Fig. 6. The high-temperature furnace was equipped with MoSi2 heating elements, reaching a maximum temperature of 1873 K with a temperature fluctuation range of ±3 K. The gases (N2, CO, CO2 and H2) used in the experiments were 99.99% pure and were supplied by cylinders. The H2O used in the test was provided quantitatively by a peristaltic pump, gasified in the heating mixing tank, and mixed with other gases to enter the high-temperature furnace. The weight-reduction values of the samples were recorded in real time by a thermobalance. In the experiments, 50 g samples were placed in the reactor. The samples were heated at a rate of 10 K/min to the required temperature and subsequently held for 20 min to minimize temperature fluctuations. During the entire process of heating and holding, N2 was flowed into the reaction tube as the shielding gas at a rate of 3 L/min. Subsequently, the gas mixtures were used in the gasification with a flow rate of 3 L/ min. The reaction time was 120 min. To study the effect of the volume fraction of H2O on the gasification reaction, the experimental schemes were designed according to the temperature range (1173e1573 K) of the gasification reaction of the BF. The specific experimental schemes are shown in Table 2.

Gasification rate of coke The gasification process of coke involves a gas-solid reaction. It was assumed that the reaction resistance includes external diffusion resistance, interfacial reaction resistance and internal diffusion resistance; that when the reaction reached equilibrium, the reaction rates of the various steps were equal; that the reaction belonged to a first-order irreversible reaction; and that the shape and density remained unchanged. Based on the aforementioned assumptions, the coke gasification reaction was described using the unreacted nuclear shrinkage model [16,17]. The external diffusion condition of gas is superior, and the external diffusion resistance is ignored. The equations of the controlling steps for the internal diffusion and interfacial reactions are shown in Eqs. (6) and (7) [18,27], respectively: t¼

h i 2 r20 rC  1  3ð1  XÞ 3 þ2ð1  XÞ o  6De CA  CA

(6)

t¼

h i 1 r r
C o  1  ð1  XÞ 3 kþ CoA  CA

(7)




=

C gasification rate (mass%)

N2-CO-CO2-H2-H2O

=

100

where C0A  C*A is the equilibrium concentration difference of the reaction gas (mol/m3); rC is the fixed C content (mol/m3); r0

Table 1 e Industrial analysis and ash compositions analysis of coke (mass%). Name

Coke

Industrial analysis

Ash compositions analysis

Aad

Vad

Cad

Mad

SiO2

CaO

MgO

FeS

Al2O3

K2O

Na2O

Fe2O3

11.76

1.24

86.08

0.92

50.43

3.36

1.09

0.20

39.55

0.75

0.21

4.41

Please cite this article in press as: Lan C, et al., Thermodynamic and kinetic behaviors of coke gasification in N2eCOeCO2eH2eH2O, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.216

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9

5

Fig. 6 e Schematic of the experimental apparatus. 1) computer; 2) thermocouple; 3) H2O; 4) peristaltic pump; 5) thermobalance; 6) corundum tube; 7) sample; 8) heater; 9) corundum ball; 10) heating pipe; 11) heating mixture tank; 12) N2; 13) CO; 14) CO2; 15) H2.

T/K

N2

CO

CO2

H2O(g)

H2

45 45 45 45 45 35 35 35 35 35 25 25 25 25 25

30 30 30 30 30 30 30 30 30 30 30 30 30 30 30

15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

0 0 0 0 0 10 10 10 10 10 20 20 20 20 20

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

1173 1273 1373 1473 1573 1173 1273 1373 1473 1573 1173 1273 1373 1473 1573

is the initial radius (m); De and kþ are the effective internal diffusion coefficient and the interfacial reaction rate constant, respectively. Linear fitting of the t-[1-(1-X)1/3], t-[1e3 (1-X)2/ 3 þ 2 (1-X)] term was used to solve for De and kþ, respectively. The fitting results are shown in Table 3. C0A  C*A was obtained from Eqs. (8) and (9). CoA  CA ¼

ð40  41 ÞP RT

1 0 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  1 @ K1 K1 41 ¼ 1   þ þ pK1 A P 2 4

(8)

Resistance of coke gasification In the process of coke gasification, the internal diffusion resistance hi and the interfacial reaction resistance hC are calculated using Eqs. (12) and (13). The relative resistances are calculated using Eqs. (14) and (15) [18,27]. hi ¼

(10)

(11)

where 4CO2 and 4H2 O are the volume fractions of CO2 and H2O, respectively; K1, K2 and K4 are the equilibrium constants of Eqs. (1),(2) and (4), respectively; and K12 represents the comprehensive equilibrium constant of Eqs. (1) and (2). The values of De and kþ are shown in Table 4. As shown in Table 4, with increasing temperature the values of De and kþ for coke gasification gradually increase, indicating that the gasification rate of coke increases. The variation in De with the temperature is obviously greater than that of kþ, showing that the improvement of the internal diffusion conditions is obviously better than that of the interfacial reaction. With increasing V(H2O), both De and kþ increase. When the V(H2O) increases from 0% to 20%, De increases by a factor of 9.92, 3.39, 3.75, 2.21 and 2.57 at each temperature, while kþ increases by a factor of 2.2, 1.30, 1.31, 1.02 and 1.13 at each temperature, respectively. The increase in De is greater than that of kþ, which indicates that the increase in V(H2O) can effectively promote the improvement of the internal diffusion conditions and improve the gasification rate of coke.

(9)

where 40 and 41 are the initial and equilibrium volume fractions of the reaction gas, respectively; R is the gas constant; T is temperature (K); and P is the total pressure of system (100 kPa). K' indicates the comprehensive equilibrium constant of mixed gas, which can be obtained by Eqs. 10 and 11 [8]: K' ¼ minðK12 ; K4 Þ

K12 ¼ 4CO2 K1 þ 4H2 O K2

i 1 r0 h ð1  XÞ 3 1 De

2 1 ð1  XÞ 3 kþ hi xi ¼ hi þ hC

=

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Gas composition (volume%)

=

Table 2 e Experimental schemes. No.

hC ¼

xC ¼

hC hi þ hC

(12)

(13) (14)

(15)

Please cite this article in press as: Lan C, et al., Thermodynamic and kinetic behaviors of coke gasification in N2eCOeCO2eH2eH2O, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.216

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50

30

(b)

(a) 25

1173K 1273K 1373K 1473K 1573K

15

1173K 1273K 1373K 1473K 1573K

30

X (mass%)

X (mass%)

20

40

10

20

10

5

0

0 0

20

40

60

80

100

120

0

t/min

40

60

80

100

120

t/min 60

(c) 1173K 1273K 1373K 1473K 1573K

50 40

X (mass%)

20

30 20 10 0 0

20

40

60

80

100

120

t/min

Fig. 7 e Relation of X and t at different temperatures. (a) V(H2O) ¼ 0%; (b) V(H2O) ¼ 10%; (c) V(H2O) ¼ 20%.

Table 3 e Fitting results. No.

Controlling steps

1e5

Internal diffusion

Parameter

Temperature 1173 K

R2 Slope R2 Slope R2 Slope R2 Slope R2 Slope R2 Slope

Interfacial reaction 6e10

Internal diffusion Interfacial reaction

11e15

Internal diffusion Interfacial reaction

0.965 6.39  0.979 1.58  0.901 2.86  0.985 1.07  0.941 2.75  0.997 3.18 

1273 K 0.927 4.50  0.997 1.33  0.897 1.67  0.983 7.82  0.926 5.70  0.994 4.43 

107 105 107 105 106 104

105 104 105 103 104 103

1373 K 0.918 3.83  0.994 3.75  0.912 9.41  0.998 1.81  0.929 4.37  0.999 1.23 

1473 K 0.929 7.68  0.999 1.66  0.922 2.15  0.998 8.52  0.945 1.49  0.999 7.02 

104 103 103 103 103 103

1573 K 0.925 3.97  0.999 1.18  0.925 1.44  0.998 6.87  0.963 6.61  0.997 4.51 

103 103 103 102 103 102

103 103 103 102 102 102

Table 4 e Values of De and kþ. V(H2O) ¼ 0%

T/K

De (m2/min) 1173 1273 1373 1473 1573

2.57 3.95 5.01 2.68 5.52

8

 10  106  105  104  104

V(H2O) ¼ 10%

kþ (m/min) 0.005 0.064 0.245 0.593 0.889

De (m2/min) 3.44 6.39 1.22 5.74 9.15

    

8

10 106 104 104 104

V(H2O) ¼ 20%

kþ (m/min) 0.004 0.065 0.306 0.695 0.920

De (m2/min) 2.55 1.34 1.88 5.91 1.42

    

7

10 105 104 104 103

kþ (m/min) 0.011 0.083 0.321 0.602 1.001

Please cite this article in press as: Lan C, et al., Thermodynamic and kinetic behaviors of coke gasification in N2eCOeCO2eH2eH2O, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.216

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100

100

(a)

(b)

Interfacial reaction Internal diffusion

Re l a t i v e r e s i s t a n c e s / %

Relative resistances/%

1173K 1273K

60

1373K

1473K

1573K

40

Internal diffusion

1273K

60

1373K

1573K

1473K

1173K

40

20

20

0

Interfacial reaction

80

80

0

2

4

6

8

10

12

X/%

14

16

18

0

20

0

3

6

9

12

15

18

21

24

27

30

X/%

100

(c)

Interfacial reaction Internal diffusion

Relative resistances/%

80

60

1273K

1473K

1573K

1373K

1173K

40

20

0

0

5

10

15

20

X/%

25

30

35

40

Fig. 8 e Relationship between reaction ratio and relative resistances at different temperatures. (a) V(H2O) ¼ 0%; (b) V(H2O) ¼ 10%; (c) V(H2O) ¼ 20%. where xi and xC are the relative resistance of the internal diffusion and interfacial reactions, respectively. The plots of xi and xC vs. X for the process of coke gasification are presented in Fig. 8. With a continuous increase in the coke gasification rate, xC gradually decreases, while xi gradually increases. This behavior is mainly due to the gradual thickening of the ash layer in the process of coke gasification, which hinders contact between the reaction gas and C and makes the internal diffusion resistance increase significantly. In the process of coke gasification, with increasing reaction temperature the gasification reaction region controlled by the interfacial reaction increases. In Fig. 8(a), when the temperature increases from 1173 K to 1573 K, the intersection point between xi and xC increases from 0.12% to 14.95%. In Fig. 8(b), when the temperature increases from 1173 K to 1573 K, the intersection point between xi and xC increases from 0.18% to 24.05%. In Fig. 8(c), when the temperature increases from 1173 K to 1573 K, the intersection point between xi and xC increases from 0.58% to 34.85%. This

comparison reveals that the controlling region of the interface reaction increases gradually with V(H2O) and that a higher V(H2O) corresponds to a greater increase in the range of the controlling region of the interfacial reaction with the increase in temperature.

Comparison with previous work The literature contains reports on the kinetics of the metallurgical coke gasification reaction, and the main investigations involve the gasification kinetics of coke in pure CO2 and pure H2O atmospheres. The specific results (Table 5) show that the coke is gasified in pure CO2 and pure H2O atmospheres and that, with increasing temperature, the controlling region of the interfacial reaction increases gradually, which is in accordance with the law of this paper. However, the range of changes is large. For example, in the literature [18], when the temperature is greater than 1473 K in a pure CO2 atmosphere, the whole reaction process is controlled by the

Table 5 e Comparison with previous work. Author

Atmosphere

N2eCOeCO2eH2eH2O(0%) N2eCOeCO2eH2eH2O(10%) N2eCOeCO2eH2eH2O(20%) Guo W. T [18]. Pure CO2 Pure H2O Xue Z. L [17]. Pure CO2

This paper

Intersection point between xi and xC

Particle size F25 F25 F25 F17 F17 F23

mm mm mm mm  17 mm mm  17 mm mm  17 mm

1173 K

1273 K

1373 K

1473 K

1573 K

0.12% 0.18% 0.58% 10% >90% 3%

1.47% 2.31% 3.96% 15% No intersection 10%

4.90% 9.87% 14.32% 29% No intersection 25%

10.87% 19.89% 24.01% No intersection No intersection e

14.95% 24.05% 34.85% No intersection No intersection e

Coke gasification process.

Please cite this article in press as: Lan C, et al., Thermodynamic and kinetic behaviors of coke gasification in N2eCOeCO2eH2eH2O, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.216

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9

Fig. 9 e Coke gasification process. interfacial reaction; when the temperature is 1173 K in a pure H2O atmosphere, the intersection point between xi and xC is greater than 90%; as the temperature continues to rise, the whole reaction process becomes controlled by the interfacial reaction. In this paper, changes in the intersection point between xi and xC due to temperature are relatively small, mainly because of the different experimental atmospheres. Fig. 9 is the process of coke gasification. In the process of coke gasification, increases in temperature and the concentration of reactants can increase the interfacial reaction rate and gas diffusion rate. As the interfacial reaction rate increases, more of the reaction gas is consumed on the coke surface, and the amount of gas that diffuses to the interior decreases. Moreover, the reaction layer becomes narrow. The internal diffusion resistance of the reaction gases and the desorption resistance of the product gases from the coke surface during the gasification process will decrease, which greatly reduces the internal diffusion resistance of the coke gasification process. Therefore, when the temperature and concentration of reactants are increased, the gas diffusion resistance decreases greatly and the controlling region of the interfacial reaction increases. The gasification process of coke directly influences the changes in coke strength and grain size in the BF. Therefore, in the future, the gasification behavior of coke should be further studied in combination with changes in coke size and structure and the evolution law and mechanism of coke in N2eCOeCO2e H2eH2O should be analyzed and mastered.

Conclusions The starting temperature of coke and H2O is lower than that of coke and CO2. The effect of H2O on coke gasification is stronger than that of CO2. In the low-temperature region, the reaction enthalpy of C and H2O is higher than that of C and CO2, and the reaction enthalpy of C and CO2 gradually becomes greater than that of C and H2O with increasing temperature. Increasing pressure can block the C gasification. The increase in temperature and H2O volume fraction promote the gasification reaction of coke. With increasing temperature and H2O volume fraction, the effective internal diffusion coefficient and the interfacial reaction rate constant for coke gasification gradually increase; the improvement of the internal diffusion conditions is clearly

greater than the improvement of the interfacial reaction conditions. In the process of coke gasification, with increasing temperature and H2O volume fraction, the gasification reaction region controlled by the interfacial reaction gradually increases. The higher H2O volume fraction corresponds to a greater increase in the range of the controlling region of the interfacial reaction with the increase in temperature.

Acknowledgements The authors are grateful for financial support from the Key Program of the National Natural Science Foundation of China (U1360205) and the North China University of Science and Technology Distinguished Youth Scholars Fund (JP201508).

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Please cite this article in press as: Lan C, et al., Thermodynamic and kinetic behaviors of coke gasification in N2eCOeCO2eH2eH2O, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.216