Kinetic Behaviors of Coke Gasification with CO₂ and H₂O

Abstract

In the temperature range of 1173–1573 K, the constant temperature weight-loss experiment of coke reaction with CO₂ and H₂O was carried out by thermogravimetry. The gasification kinetic behaviors of coke reaction with CO₂ and H₂O were compared and analyzed, and the mechanism of the difference of the kinetic behaviors was discussed. The results show that the internal diffusion condition and interfacial reaction condition of coke gasification in H₂O are better than those in CO₂, and the difference of diffusion property is greater than that of interface reaction property. The activation energies of internal diffusion and interface reaction of coke gasification in H₂O are 80.36 kJ/mol and 36.97 kJ/mol lower than that in CO₂, respectively. In H₂O, the controlling region of interfacial reaction is larger than that in CO₂, and the effect of temperature on controlling region of the interfacial reaction is also greater than that in CO₂. The energy required for the chemical adsorption of H₂O on the coke surface to form a stable intermediate configuration is 40.22 kJ/mol less than that of CO₂, and the energy barrier that needs to cross during the coke gasification in H₂O is 29.11 kJ/mol less than that in CO₂. The chemisorption capacity of CO₂ on coke surface is weaker than that of H₂O.

1. Introduction

Coke is an irreplaceable fuel in blast furnace (BF) ironmaking, which plays an important role in the BF smooth operation, energy consumption, CO₂ emission and other important indicators.¹⁾ The high temperature strength of coke directly affects the gas permeability and liquid permeability of BF, and the gasification behavior of coke is an important factor affecting its high temperature strength. In the process of coke gasification, the fixed carbon consumption and the diffusion degree of reaction gas comprehensively affect the reduction degree of coke strength after reaction.^2,3) Therefore, researchers have done a lot of research on the process of coke gasification. According to the study, the coke gasification mainly occurs in the temperature range of 1173–1573 K in the BF.⁴⁾ The coke gasification belongs to gas-solid reaction, and the reaction process is as follows:^5,6,7) 1) external diffusion: the reaction gas diffuses to the external surface of coke through the stagnation boundary layer; 2) internal diffusion: the reaction gas diffuses to the gas-solid reaction interface through pores and cracks; 3) interface reaction: the reaction gas occurs at the reaction interface in the process of reaction gas adsorption-chemical reaction-product gas desorption; 4) internal diffusion: reaction product gas passes through pores and cracks from reaction interface of coke to coke surface; 5) external diffusion: reaction product gas is separated from coke surface. If the controlling factor in the process of coke gasification is interface chemical reaction, the coke damages as a whole. That is, the interface reaction rate is slow, and more reaction gas diffuses into the coke for internal reaction, thus damaging the coke matrix structure. If the controlling factor is internal diffusion, the gasification reaction occurs more on the coke surface, the coke matrix structure is protected, and the coke strength will not be large decrease in magnitude. Therefore, the degradation degree of coke in BF depends not only on the gasification rate of coke, but also on the gasification reaction behavior of coke,⁸⁾ which has also been proved by experiments.⁹⁾ This makes researchers pay more attention to the kinetics of coke gasification. Cui et al.⁷⁾ studied the reaction kinetics of coke with CO₂ at different temperatures, finding that the apparent activation energy and effective diffusion activation energy of the reaction are 124.50 kJ/mol and 642.40 kJ/mol, respectively. Zamalloa¹⁰⁾ studied the gasification reaction of coke with different particle size and CO₂, finding that the gasification reaction in the range of 1173–1573 K does not involve the influence of internal diffusion. Xue⁶⁾ also studied the kinetics of coke gasification in CO₂, finding that the effective diffusion activation energy of coke gasification reaction is greater than the apparent activation energy, the controlling region of the interfacial reaction increases with increasing reaction temperature.

In order to reduce the emission of CO₂ in ironmaking system, it is an extremely effective solution to use hydrogen as a fuel and chemical reactant in production. In the world, a series of plans have been put forward to use hydrogen for low-carbon ironmaking,^11,12,13) and the necessity and application value of developing hydrogen-rich smelting technology of BF have been fully recognized. With the increase of H₂ content in BF, the volume fraction of reduction product (H₂O) increases, which changes the gasification reduction process of coke. According to the study,^14,15) the reaction temperature of coke gasification in H₂O is lower than that of in CO₂, and the reaction rate is higher than that in CO₂. Xu et al.¹⁶⁾ studied the gasification reaction and structure change of metallurgical coke in the atmosphere of H₂O/CO₂, finding that the destruction of coke structure by H₂O is greater than that by CO₂. Zhang¹⁷⁾ pointed out that the addition of H₂ can obviously promote the coke gasification and reduce the apparent activation energy. Wang et al.¹⁸⁾ pointed out that, compared with the CO₂, the H₂O mainly reacts on the surface of coke, when coke reacts with CO₂ and H₂O, the large pores on the edge increases by 66.98% and 94.01% respectively, and the influence of H₂O on coke strength is less than that of CO₂. Guo¹⁹⁾ found that H₂O has stronger selectivity to the optical tissue with lower reactivity, and promotes the gasification of the optical tissue with strong resistance to gasification.

The above analysis shows that the influences of CO₂ and H₂O on coke structure are quite different. Based on the influences of CO₂ and H₂O on the kinetic process of coke gasification, this paper analyzes the difference of its kinetic process by high temperature gasification experiment, explores the mechanism of its chemical reaction process by quantum chemical calculation, and comprehensively analyzes the influences of CO₂ and H₂O on the kinetic behaviors of coke gasification, so as to provide theoretical guidance for the gasification behaviors of coke under the hydrogen-rich smelting of BF.

2. Experimental Methods

The coke used in the experiment was taken from the BF production site. First, the coke was crushed, and then the crushed coke was ground to the shape of a ball with a diameter of 25 mm. Put the coke into the oven, dried it for 2 h under the condition of 378 ± 5 K, and then sealed it for standby. The industrial analysis and ash composition analysis of coke are shown in Table 1.

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

Name	Industrial analysis			Ash compositions analysis
	Ash	Volatile	Fixed carbon	SiO2	CaO	MgO	FeS	Al2O3	K2O	Na2O	Fe2O3
Coke	11.76	1.24	86.08	50.43	3.36	1.09	0.20	39.55	0.75	0.21	4.41

The experimental process was carried out in MoSi₂ high temperature furnace, and the schematic diagram of high temperature furnace is shown in Fig. 1. The temperature of the high temperature furnace was automatically controlled by a computer program. The N₂ and CO₂ used in the experiment were supplied by gas cylinder, and their purity is 99.99%. The H₂O used in the experiment was supplied quantitatively by peristaltic pump, gasified into the high temperature furnace in the heating mixing tank. The change of coke quality during the whole experiment was recorded in real time by a thermobalance.

Fig. 1.

Schematic of the experimental apparatus.

Put 60 g coke with particle size of 25 mm into the basket and hanged it on the thermobalance. Under the protection of N₂ (3 L/min), the coke was heated to the predetermined temperature at the heating rate of 10 K/min. In order to reduce the temperature fluctuation, the sample was kept at a constant temperature for 20 min after reaching the predetermined temperature. After the temperature was stable, N₂ was switched to CO₂ or H₂O respectively for constant temperature gasification weight-loss experiment of coke. The flow rate of reaction gas was 3 L/min. The gasification reaction temperature of coke was 1173 K, 1273 K, 1373 K, 1473 K and 1573 K, respectively. And the reaction time was 120 min. At the end of the experiment, the reaction gas was switched to N₂, and the coke real-time weight-loss data recorded by the thermobalance was analyzed.

3. Results and Discussion

3.1. Effect of CO₂ and H₂O on Coke Gasification

The relationships between gasification rate (X) and reaction time (t) of coke gasification with CO₂ and H₂O were shown in Fig. 2. Where, X can be obtained from Eq. (1):   

$$X=\frac{m_{\text{0}}-m_{\text{t}}-m_{\text{b}}}{m_{\text{0}}\cdot w\text{(C)}}\times \text{100\%}$$(1)

where m₀ is the initial weight of the sample (g), m_t is the weight of the sample after t min (g), m_b is the weight-loss value of coke at the beginning of the gasification reaction (g), and w(C) is the fixed C content.

Fig. 2.

Relationship between X and t under different conditions. (Online version in color.)

As shown in Fig. 2, the X of coke gasification with CO₂ at various temperatures is lower than that of coke gasification with H₂O. After 120 min of reaction, the X in CO₂ is 1.53%, 8.90%, 21.03%, 37.52 and 59.43% respectively, the X in H₂O reaches 6.36%, 27.85%, 46.75%, 64.09 and 78.97% respectively. The X increases by 4.83, 18.95, 25.72, 26.57 and 19.54 percentage points respectively, with the largest difference at 1473 K. The coke gasification is endothermic reaction. With the increase of temperature, the reaction rate increases. The initial reaction temperature of coke gasification in CO₂ is higher than that in H₂O, but the reaction rate is lower than that in H₂O.^14,15) When the temperature is lower than 1473 K, the reaction rate of coke with CO₂ and H₂O is relatively low. The influence of temperature on the reaction rate of coke gasification in H₂O is higher than that of coke gasification in CO₂, which makes the difference increases with the increase of temperature. However, when the temperature is over 1473 K, the reaction rate of coke gasification in H₂O has been at a high level, making the difference gradually reduce, which is also consistent with the conclusion of literature.²⁰⁾

3.2. Analysis of Coke Gasification Rate

The process of coke gasification is gas-solid reaction, and the schematic diagram of the reaction process is shown in Fig. 3. There is no product layer in the gasification process of coke. As the fixed carbon in coke is continuously gasified, a ash layer gradually forms. Reaction gas and product gas diffuse in the ash layer and coke reaction layer and form diffusion resistance. The gasification process of coke can be described by unreacted nuclear shrinkage model,^6,7) and the internal diffusion process and interface chemical reaction process are calculated and analyzed by Eqs. (2) and (3), respectively.   

$$t=\frac{r_0^2{\rho }_C}{6D_e(C^0-C^{*})}\left[1-3{(1-X)}^{\raisebox{1ex}{$2$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}+2(1-X)\right]$$(2)

  

$$t =\frac{{\rho }_C{r}_o}{k_{+}(C^0-C^{*})}\left[1-{(1-X)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}\right]$$(3)

where C⁰−C^* is the difference in the equilibrium concentration of the reaction gas (mol/m³); ρ_C is the fixed carbon content (mol/m³); r₀ is the initial radius (m); and D_e and k₊ are the effective internal diffusion coefficient and the interfacial reaction rate constant, respectively.

Fig. 3.

Schematic diagram of coke gasification process. (Online version in color.)

Take the data in Fig. 2 into Eqs. (2) and (3), respectively. Do t linear fitting on [1−(1−X)^1/3] and [1−3(1−X)^2/3+2(1−X)], and solve D_e and k₊ from the slope of the straight line obtained by linear fitting. The results of fitting are shown in Table 2. The solution results of D_e and k₊ are shown in Table 3.

Table 2. Fitting results for the controlling equations.

No.	Controlling steps	Parameter	T/K
			1173 K	1273 K	1373 K	1473 K	1573 K
coke gasification in CO2	Internal diffusion	R2	0.939	0.930	0.928	0.917	0.895
		Slope	1.433×106	4.074×104	6.910×103	1.973×103	6.364×102
	Interfacial reaction	R2	0.999	0.999	0.999	0.998	0.990
		Slope	2.350×104	3.925×103	1.592×103	8.335×102	4.488×102
coke gasification in H2O	Internal diffusion	R2	0.914	0.932	0.930	0.954	0.971
		Slope	7.963×104	3.890×103	5.16×103	5.328×102	2.900×102
	Interfacial reaction	R2	0.996	0.999	0.999	0.999	0.997
		Slope	5.545×103	5.01×103	1.191×103	4.035×102	2.802×102

Table 3. Values of D_e and k₊.

T/K	coke gasification in CO2		coke gasification in H2O
	De (m2/min)	k+(m/min)	De (m2/min)	k+(m/min)
1173	1.71×10−7	0.005	3.08×10−6	0.021
1273	6.53×10−6	0.033	6.83×10−5	0.107
1373	4.15×10−5	0.086	2.39×10−4	0.216
1473	1.56×10−4	0.177	5.77×10−4	0.366
1573	5.16×10−4	0.351	1.13×10−3	0.563

As shown in Table 3, the D_e and k₊ of coke gasification in CO₂ are lower than that in H₂O at various temperatures, indicating that the internal diffusion condition and interface reaction condition of coke gasification in H₂O are better than that of coke gasification in CO₂. At 1173 K, the D_e and k₊ of coke gasification in CO₂ are 1.71×10⁻⁷ m²/min and 0.005 m/min respectively, while the D_e and k₊ of coke gasification in H₂O are 3.08×10⁻⁶ m²/min and 0.021 m/min respectively. The D_e and k₊ increase by 18.01 and 4.2 times, respectively. At 1573 K, the D_e and k₊ of coke gasification in CO₂ are 5.16×10⁻⁴ m²/min and 0.351 m/min respectively, while the D_e and k₊ of coke gasification in H₂O are 1.13×10⁻³ m²/min and 0.563 m/min respectively. The D_e and k₊ increase by 2.19 and 1.60 times, respectively. The results show that the difference of diffusion property is larger than that of interface reaction property during the coke respectively reacts with CO₂ and H₂O.

According to the data in Table 3, the effective diffusion activation energy (E_D) and apparent reaction activation energy (E_a) of coke gasification are calculated, see Eqs. (4), (5), where A and D₀ are pre-exponential factors.   

$$ln{D}_{\mathrm{e}}=-\frac{E_{\mathrm{D}}}{RT}+ln{D}_0$$(4)

  

$$ln{k}_{+}=-\frac{E_{\mathrm{a}}}{RT}+lnA$$(5)

During the coke gasification in CO₂, E_D and E_a are 299.46 kJ/mol and 158.80 kJ/mol, respectively. During the coke gasification in H₂O, E_D and E_a are 219.10 kJ/mol and 121.83 kJ/mol, respectively. The activation energy of internal diffusion and interfacial reaction of coke gasification in H₂O are lower than that of coke gasification in CO₂. The difference of E_D is 80.36 kJ/mol, and the difference of E_a is 36.97 kJ/mol. The lower the activation energy is, the more favorable the reaction of coke gasification occurs. The reduction of E_D is greater than that of E_a, which also proves that H₂O can improve the internal diffusion condition more than the interface reaction compared with CO₂.

3.3. Coke Gasification Resistance

In order to further analyze the controlling factor in the gasification reaction process, the internal diffusion resistance (η_i) and the interface reaction resistance (η_C) were calculated from Eqs. (6), (7), and the relative resistance (ξ_i and ξ_C) were calculated from Eqs. (8), (9), and the larger one in ξ_i and ξ_C was the controlling factor of the gasification reaction.   

$${\eta }_{\mathrm{i}}=\frac{r_0}{D_{\mathrm{e}}}\left[{(1-X)}^{-\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}-1\right]$$(6)

  

$${\eta }_{\mathrm{C}}=\frac{1}{k_{+}}{(1-X)}^{-\raisebox{1ex}{$2$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}$$(7)

  

$${\xi }_{\mathrm{i}}=\frac{{\eta }_{\mathrm{i}}}{{\eta }_{\mathrm{i}}+{\eta }_{\mathrm{C}}}$$(8)

  

$${\xi }_{\mathrm{C}}=\frac{{\eta }_{\mathrm{C}}}{{\eta }_{\mathrm{i}}+{\eta }_{\mathrm{C}}}$$(9)

The relationships between ξ_i and X and between ξ_C and X are shown in Fig. 4. As shown in Fig. 4, the controlling region of the interfacial reaction of coke gasification in H₂O is larger than that of coke gasification in CO₂ at each temperature. During the coke gasification in CO₂, when the temperature increases from 1173 K to 1573 K, the intersection point of ξ_i and ξ_C increases from 0.82% to 35.61%, i.e., by 34.79%. During the coke gasification in H₂O, when the temperature increases from 1173 K to 1573 K, the intersection point of ξ_i and ξ_C increases from 3.52% to 48.88%, i.e., by 45.36%. The results show that the influence of temperature on the controlling region of the interfacial reaction in the reaction of coke with H₂O is greater than that in the reaction of coke with CO₂. At the same time, the controlling region of the interfacial reaction increases with the increase of temperature. In the process of coke gasification, there is no product layer, and only one ash layer with good diffusion condition exists. When the interface reaction rate increases, the reaction gas is consumed more on the coke surface, and the amount of gas diffuses to the inside reduces, so that the resistance of internal diffusion of reaction gas and the resistance of desorption and separation of product gas from coke reduces, which greatly reduces the resistance of internal diffusion of coke gasification process, making the controlling region of the interfacial reaction increases with increasing temperature.^21,22,23)

Fig. 4.

Relationship between reaction ratio and relative resistances at different temperatures. (Online version in color.)

4. Discussion on Influence Mechanism

According to the above experimental study, the internal diffusion rate and the interface chemical reaction rate of coke gasification in H₂O are higher than those of coke gasification in CO₂. The internal diffusion rate is mainly determined by the size of molecules. H₂O molecule is smaller than CO₂ molecule, and its diffusion rate is fast, which makes its internal diffusion rate in the process of coke gasification greater than CO₂. However, the interfacial reaction rate is mainly determined by the change of atomic structure and reaction potential energy. Therefore, in order to further explore the difference of chemical reaction ability of coke with CO₂ or H2O, the ab initio method of quantum chemistry was used to study the reaction paths, reaction potential energy, and Mulliken charge distributions of the reaction mechanism expressions (Eqs. (10) and (11)^14,24,25,26)), respectively. Quantum chemical calculations have been widely used in the field of coal and coke reaction mechanisms.^27,28) The ab initio method of quantum chemistry is an effective method to study the mechanisms of chemical microreactions.^29,30)   

$${\text{C}}_f+{\text{CO}}_{\text{2}}\leftrightarrow {\text{C}}_f(\text{O})+\text{CO}$$(10)

  

$${\text{C}}_f+{\text{H}}_{\text{2}}\text{O}\leftrightarrow {\text{C}}_f(\text{O})+{\text{H}}_{\text{2}}$$(11)

In this paper, GaussView software was used to construct the molecular models, and all theoretical calculations were completed with Gaussian 09 software.³¹⁾ The graphite structure is mainly composed of 12–25 carbon clusters (3–7 benzene rings).^28,32) A graphite structure of five benzene rings was selected to represent coke for theoretical calculations and edge carbon atoms were saturated with H atoms. The B3LYP method was used to optimize the stable structures of all reactants (R), intermediates (IM), transition states (TS) and products (P) with the 6-31G (d) basis set.³³⁾ The intermediate is a relatively stable structure with no imaginary frequency, and the transition state has only one imaginary frequency. To obtain the correct transition-state structure, IRC (Intrinsic Reaction Coordinate) detection of the transition state was necessary. The single-point energy of each structure was calculated with the 6-311G (d, p) basis set, and the zero-point energy (ZPE) was corrected.

The reaction potential energy surfaces of Eqs. (10) and (11) are shown in Fig. 5. The atomic structures of intermediates and transition states in the reaction process of Eqs. (10) and (11) are shown in Fig. 6. As shown in Fig. 5(a), the reaction path of Eq. (10) is R→IM1→TS→IM2→P. It is necessary to absorb 56.53 kJ/mol of heat for the chemical adsorption of CO₂ molecules on the coke surface to form stable IM1. The transition from IM1 to IM2 requires crossing an energy barrier of 82.62 kJ/mol. The energy increases from 49.89 kJ/mol to 83.80 kJ/mol when IM2 changes to P. Equation (10) is an endothermic reaction with an endothermic energy of 83.80 kJ/mol. As shown in Fig. 5(b), the reaction path of Eq. (11) is R→IM→TS→P. The chemical adsorption of H₂O on the coke surface to form stable IM needs to absorb 16.31 kJ/mol of heat, which is 40.22 kJ/mol less than that of CO₂. The transition from IM to P requires crossing an energy barrier of 53.51 kJ/mol, and the energy of TS is less than that of TS in Eq. (10). The energy of P is only 1.02 kJ/mol. Equation (11) is an endothermic reaction, and the endothermic value is only 1.02 kJ/mol, which is 82.78 kJ/mol less than that of CO₂.

Fig. 5.

Reaction path and potential energy surfaces of Eqs. (10) and (11).

Fig. 6.

Atomic structures of intermediates and transition states of Eqs. (10) and (11). (Online version in color.)

The Mulliken charge layouts of CO₂ and H₂O adsorbed on the coke surface are shown in Table 4. As shown in Table 4, the number of gain electrons in C1, C2, C6 and C7 increase after CO₂ and H₂O are adsorbed on the coke surface, while the number of loss electrons in C3, C4 and C5 increase. Based on a comparison of the Mulliken charge layouts of Eq. (10)-IM1 and Eq. (11)-IM, the numbers of gain and loss electrons of C1, C2, C3 and C4 in Eq. (10)-IM1 are higher than those in Eq. (11)-IM, and the numbers of gain and loss electrons of C5, C6 and C7 in Eq. (10)-IM1 are lower than those in Eq. (11)-IM. The number of electrons transferred from C4 to O1 in Eq. (10)-IM1 is lower than that in Eq. (11)-IM, indicating that the chemical adsorption capacity of CO₂ on the coke surface is lower than that of H₂O. Bu et al.³⁴⁾ studied the reaction mechanism of bituminous coal in CO₂/H₂O atmosphere, finding that H₂O can preferentially adsorb on the active sites of bituminous coal surface, which also proves that H₂O molecule has stronger adsorption capacity on coke surface.

Table 4. Mulliken charge layout.

Mulliken charge	C1/e	C2/e	C3/e	C4/e	C5/e	C6/e	C7/e	O1/e
Coke	−0.074	−0.031	0.046	−0.005	0.046	−0.031	−0.074	—
Eq. (10)-IM1	−0.162	−0.054	0.116	0.179	0.147	−0.060	−0.165	−0.461
Eq. (11)-IM	−0.171	−0.122	0.131	0.193	0.097	−0.048	−0.164	−0.627

In conclusion, based on a comparison of coke gasification in CO₂ and in H₂O, the high diffusion rate of H₂O improves the internal diffusion condition of coke gasification process and promotes the reaction. At the same time, due to the strong adsorption capacity and the low energy requirements in the process of coke gasification in H₂O, the interface reaction rate greatly increases, the reaction layer narrows, and the movement speed of the reaction layer to the inside of coke increases. Compared with CO₂, the internal diffusion condition and the interface reaction condition of coke gasification in H₂O have been greatly improved, but the improvement degree of the internal diffusion condition is greater than that of the interface reaction condition, which makes the controlling region of the interfacial reaction increase.

5. Conclusion

(1) The internal diffusion condition and interface reaction condition of coke gasification in H₂O are better than that of coke gasification in CO₂, and the difference of diffusion property is larger than that of interface reaction property. The activation energies of internal diffusion and interface reaction of coke gasification in H₂O are lower than that of coke gasification in CO₂. The difference of E_D and E_a are 80.36 kJ/mol and 36.97 kJ/mol, respectively.

(2) The controlling region of interfacial reaction of coke gasification in H₂O is larger than that in CO₂, and the effect of temperature on controlling region of the interfacial reaction is also greater than that in CO₂.

(3) The chemical adsorption of H₂O on the coke surface to form of stable intermediate configuration need to absorb 16.31 kJ/mol of heat, which is 40.22 kJ/mol less than that of CO₂. And the energy barrier that needs to be crossed during the coke gasification in H₂O is 29.11 kJ/mol less than that in CO₂. The chemisorption of CO₂ on coke surface is weaker than that of H₂O.

Acknowledgement

The authors are grateful for financial support from the Key Program of the National Natural Science Foundation of China (U1360205, 51674122) and Natural Science Foundation of Hebei Province (E2019209424).

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