Control Technique of Coke Rate in Shaft Furnace by Controlling Coke Reactivity

Abstract

The shaft furnace, which is a scrap melting furnace, plays an important part as an energy supplier in a steel works because the shaft furnace produces high calorie gas. The shaft furnace is required either to reduce the coke rate or to increase the exhaust gas calorie, corresponding to the energy balance in the steel works. Because the coke rate and exhaust gas calorie are determined by coke gasification reactivity, the reactivity control technique is very important. In this study, the coke surface was coated with CaCO₃, Fe₂O₃ and SiO₂ to control coke reactivity, and the gasification rates were measured at 1573–1773 K. As the results, the gasification rate was accelerated by CaCO₃ and Fe₂O₃ and decelerated by SiO₂. The acquired gasification rates were applied to a one-dimensional mathematical model of the shaft furnace. The shaft furnace operation with controlled coke reactivity was simulated, and the effects of coke reactivity on the coke rate and exhaust gas calorie were estimated.

1. Introduction

The ironmaking process, which is the first upstream process in a steel manufacturing process, is both the largest energy consumer and the largest energy supplier for the downstream processes in a steel works. The shaft furnace (cupola) produces molten iron by melting scrap by the heat of combustion of coke, and also plays the role of an energy supplier because it discharges a high calorie CO-rich gas. Since the energy balance in a steel works changes depending on productivity and operating conditions, the shaft furnace is also required to respond flexibly to those changes. Concretely, when energy is in short supply in the works, the shaft furnace should discharge a large amount of high calorie gas, which is achieved by increasing the coke rate, and when the works energy supply is sufficient, the coke rate should be decreased to reduce production costs.

The outline of the shaft furnace is shown in Fig. 1. First, CO₂ is generated by the combustion reaction between coke and the oxygen in the blast. Next, CO is generated by the coke gasification reaction. Because reduced iron (scrap) is used as the raw material, the CO gas generated by the gasification reaction is discharged from the furnace without being consumed. Since the gasification reaction is endothermic, when the gasification rate is increased, although the coke rate increases, the exhaust gas calorie also increases due to increased CO generation. Conversely, when the gasification rate decreases, both the coke rate and exhaust gas calorie decrease. Thus, if it is possible to control the gasification rate, the coke rate and exhaust gas calorie of the shaft furnace can also be controlled.

Fig. 1.

Outline of shaft furnace.

Addition of catalysts is an effective method for accelerating the gasification rate of coke. In past studies, methods in which the gasification rate was accelerated by adding Ca or Fe were reported.^1,2,3,4,5,6) However, many of those studies were carried out at temperatures around 1273 K, which is near the starting temperature of gasification, since the purpose of the studies was to reduce the temperature of the thermal reserve zone in the blast furnace. On the other hand, the gasification reaction in the shaft furnace occurs actively in the temperature region of 1773 K and higher,⁷⁾ but knowledge of this temperature region is still inadequate. Moreover, there is virtually no existing knowledge in connection with methods for decelerating the gasification rate.

This paper investigates a method of coating the coke surface as a technique which makes it possible to control the gasification rate simply by changing the coating material. When the coke surface is coated with a catalyst, the gasification rate is accelerated, and conversely, when it is coated with an inert substance with a high melting point, contact between the CO₂ gas and coke is prevented, and the gasification rate is decelerated. In this paper, the gasification rate was measured and formulated in the temperature region up to 1773 K by using CaCO₃ and Fe₂O₃ as reaction accelerators and SiO₂ as a reaction retardant. The obtained rate equations were applied to a mathematical model, and the effects of gasification rate control on shaft furnace operation were evaluated.

2. Coke Gasification Experiment

2.1. Experimental Method

2.1.1. Evaluation of Effects of Various Compounds on Activation Energy

First, the effect of simple contact between coke and various chemical compounds (CaCO₃, Fe₂O₃, SiO₂) on the activation energy of the gasification reaction was evaluated. Each material was pulverized to 150 μm or smaller and mixed, and thermogravimetry (TG) was performed. The properties of the coke used here are shown in Table 1. As specimens for the experiment, 10 mg of coke was mixed with 5 mg of the respective compounds. Three types of reaction resistance exist in the gasification reaction, namely, the chemical reaction, intraparticle diffusion and diffusion through the gas film. The gasification rate is controlled by the chemical reaction rate at low temperatures and by the diffusion rate through the gas film at high temperatures.⁸⁾ In the present measurements, the test conditions were set so that the chemical reaction would be rate-determining in order to evaluate the change in activation energy. First, the specimens were preheated to 1073 K at the heating rate of 40 K/min under N₂ gas at the flow rate of 1 NL/min. After reaching 1073 K, the gas was switched to a CO/N₂ (=30/70) mixture at 1 NL/min, and the weight change of the specimens was measured while continuing heating at a constant rate of 1, 2 or 5 K/min.

Table 1. Characteristics of coke.

	Ash	VM	F.C	S	SiO2	Al2O3
in Coke	11.5	0.9	87.6	0.47	–	–
in Ash	–	–	–	–	55.2	26.4

2.1.2. Measurement of Reaction Rate with Coke Surface Coating

Next, the effect of coating the coke surface with CaCO₃, SiO₂ or Fe₂O₃ on the gasification rate was evaluated. Coke which had been crushed and classified to 10–11 mm was used as specimens. The respective compounds, which had been classified to 150 μm or smaller, were suspended with the concentration of 33% in water. The coke surface was coated by immersing and mixing the coke in the respective suspensions, followed by natural drying for 24 h. The gasification rate measurement apparatus is shown in Fig. 2. The specimens were set on layers of alumina balls with the diameter of 10 mm and refractory bricks in a SSA-S core tube with the inner diameter of 80 mm. A thermocouple was installed in the center of the packed bed of the specimens. Argon gas was then blown from the furnace bottom, and the specimens were heated to the specified temperature. After holding at that temperature for 30 min, the gas was switched to a CO₂/N₂ (=15/85) mixture, and the gasification reaction occurred at a constant temperature. The gasification conditions in the shaft furnace were reproduced within the limits of this experimental apparatus. The conditions were temperatures of 1573, 1673 and 1773 K and the gas superficial velocity of 0.27 Nm/s. The gasification rate was obtained from the weight change during the reaction.

Fig. 2.

Experimental apparatus.

2.2. Results and Discussion

2.2.1. Effect of Compounds on Activation Energy

The relationship between the temperature and the fractional reaction measured under the respective conditions is shown in Fig. 3. The fractional reaction is defined as the weight loss ratio to the initial weight of C, but the weight loss due to the decomposition reaction when CaCO₃ was mixed was excluded. As shown in Fig. 3, the reaction rate was nearly the same as in the case of simple coke when SiO₂ was mixed, but increased with mixing of CaCO₃ and Fe₂O₃. Based on these results, the activation energies were calculated by the Friedman-Ozawa method.^9,10) In this method, the slopes of the linear approximation lines of the relationship between ln(dα/dt) and 1/T at each heating rate correspond to the activation energy E_a, according to Eq. (2), which takes the natural logarithms of both sides of the reaction rate equation shown in Eq. (1).   

$$\frac{d\alpha }{dt}=A\times e^{-\frac{E_a}{RT}}f(\alpha )$$(1)

  

$$ln\frac{d\alpha }{dt}=ln(Af(\alpha ))-\frac{E_a}{R}\frac{1}{T}$$(2)

Fig. 3.

Relationship between temperature and carbon fractional reaction in (a) 1 K/min, (b) 2 K/min and (c) 5 K/min.

Figure 4 shows the relationship of ln(dα/dt) and 1/T when the respective compounds were mixed. The activation energies obtained from the slopes were 2.33×10⁵ kJ/kmol for simple coke and 2.36×10⁵ kJ/kmol for the SiO₂ mixture. Although there was not a large difference between the activation energies with these two substances, when CaCO₃ was mixed, the activation energy decreased to 2.19×10⁵ kJ/kmol. This is considered to be due to the catalytic effect of Ca. On the other hand, when Fe₂O₃ was mixed, the activation energy increased to 2.94×10⁵ kJ/kmol. Conventionally, it was thought that Fe is the most active state of Fe-based catalysts, and activity decreases in the oxide state.¹¹⁾ However, it has also been reported that the gasification rate was accelerated even with Fe₃O₄ as a result of measurements under various oxidation states of Fe depending on the gas composition.¹²⁾ In the present experiment, when Fe₂O₃ was mixed, the weight was more decreased than in the case of simple coke by approximately 0.2 mg in the process of preheating to 1073 K. Because the weight loss when all Fe₂O₃ is reduced to Fe₃O₄ by direct reduction with C is about 0.3 mg, it is estimated that the larger part of this Fe₂O₃ had already been reduced to Fe₃O₄ before the start of gasification. Since the final weight change was virtually the same as in the case of simple coke, it can be inferred that Fe₃O₄ was a stable chemical composition in the present experiment, and the gasification rate was accelerated by the oxidation-reduction reactions shown in Eqs. (3) and (4).   

$$\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4+\mathrm{C}\to 3\mathrm{F}\mathrm{e}\mathrm{O}+\mathrm{C}\mathrm{O}$$(3)

  

$$3\mathrm{F}\mathrm{e}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2\to \mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4+\mathrm{C}\mathrm{O}$$(4)

Fig. 4.

Relationship between temperature and reaction rate.

It is estimated that the reaction shown in Eq. (3) is rate-determining for these reactions. However, because this reaction is accelerated by increasing temperature, it can be assumed that the gasification rate was also accelerated as the temperature increased, and as a result, the apparent activation energy increased.

2.2.2 Reaction Rate with Coke Surface Coating

The weight ratios of CaCO₃, Fe₂O₃ and SiO₂ on the coke obtained from the difference in the weights of the specimens before and after coating were 9, 4 and 3 wt%, respectively. These compounds are presumed to exist mainly on the coke surface or in coarse pores near the surface. Figure 5 shows the temporal change in the fractional reactions of the coated coke at each temperature. Here, the reaction rate constant k is obtained assuming that the measured reaction rate dα/dt is proportional to the product of the CO₂ concentration and the ratio of unreacted carbon (1-α), as shown in Eq. (5).   

$$\frac{d\alpha }{dt}=k\cdot x_{\mathrm{c}{\mathrm{o}}_2}(1-\alpha )$$(5)

Fig. 5.

Temporal change in fractional reaction in (a) 1573 K, (b) 1673 K and (c) 1773 K.

Figure 6 shows the values of k obtained from the measurement results. The plots are the experimental results, and the lines are the calculation results (discussed below). In the case of coating with CaCO₃, as in the above-mentioned TG results, the gasification rate increased. With Fe₂O₃ coating, the gasification rate increased at 1673 K and higher, but at 1573 K, it was approximately the same as in the uncoated state. This is considered to be because the shielding effect, by which the coating material prevents contact between the coke and CO₂ gas, offset the reaction accelerating effect. With SiO₂ coating, the gasification rate decreased, and this is attributed to the shielding effect.

Fig. 6.

Relationship between temperature and reaction rate.

Because these experimental conditions are close to the conditions in which the diffusion rate through the gas film is rate-determining, the reaction rate was analyzed by using the overall rate equation⁸⁾ shown in Eq. (6).   

$$R_s=k\cdot x_{\mathrm{C}{\mathrm{O}}_2}=\frac{\pi d_p{}^2{\varphi }^{-1}{N}_C\cdot 273P{x}_{\mathrm{C}{\mathrm{O}}_2}/22.4T}{(1/k_f+6/d_p{E}_f{k}_s)}$$(6)

For the chemical reaction rate k_s in this equation, various equations such as the Langmuir-Hinshelwood type have been reported in the past.¹³⁾ Here, however, an Arrhenius type equation was used for simplicity. The values obtained by the above-mentioned TG were used for the activation energy E_a, and the calculation results obtained with Eq. (6) were fitted to the experimental results by adjusting the frequency factor A as a fitting parameter. The values of k_s after fitting are shown in Eq. (7).   

$$\begin{array}{l}\hspace{1em}\hspace{1em}\hspace{0.5em}{k}_s=6.2\times {10}^{13}\mathrm{\hspace{0.17em} }exp(-2.33\times {10}^5/RT)\\ k_{s,\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3}=8.0\times {10}^{13}\mathrm{\hspace{0.17em} }exp(-2.19\times {10}^5/RT)\\ \hspace{0.17em}{k}_{s,\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3}=1.2\times {10}^{16}\mathrm{\hspace{0.17em} }exp(-2.94\times {10}^5/RT)\\ \hspace{0.5em} k_{s,\mathrm{S}\mathrm{i}{\mathrm{O}}_2}=4.4\times {10}^{13}\mathrm{\hspace{0.17em} }exp(-2.36\times {10}^5/RT)\end{array}$$(7)

The results of calculations of the overall reaction rate k by substituting Eq. (7) into Eq. (6) are shown by the respective lines in Fig. 6. The calculation results were in good agreement with the experimental results.

3. Simulation of Shaft Furnace Operation

3.1. Calculation Method

In order to evaluate the effect of controlling the coke gasification rate on shaft furnace operation, a simulation was performed using a one-dimensional mathematical model.¹⁴⁾

First, as a case study, a sensitivity analysis of the coke rate and gas utilization ratio η_CO for the gasification rate was performed by changing the parameter γ, assuming that the gasification rate R_s’ is defined as R_s’ = γR_s. Next, the coke rate and exhaust gas calorie when the gasification rate was accelerated or decelerated by coating the coke were calculated by applying the gasification rates obtained by the experiment to the model. Here, as the coating materials, CaCO₃ was used for acceleration of the gasification rate, and SiO₂ was used for deceleration. It is possible to substitute these materials for limestone and silica, which are used as submaterials for adjustment of slag basicity in a shaft furnace.

3.2. Calculation Results

Figures 7 and 8 show the changes in the coke rate and gas utilization ratio η_CO when the parameter of the gasification rate is changed. As the gasification rate becomes larger, the coke rate increases and η_CO decreases. However, as η_CO approaches zero, the increase in the coke rate also becomes moderate. While γ = 1 is the case of normal coke, it is possible to control the coke rate and η_CO by controlling the gasification rate. Next, Figs. 9 and 10 show the change in the coke rate and the exhaust gas calorie when the gasification rate was controlled by coating the coke. The coke rate is decreased by 5 kg/t by coating with SiO₂, and the exhaust gas calorie is increased by 0.33 GJ/t by coating with CaCO₃. These results suggest the effectiveness of this gasification control technique.

Fig. 7.

Effect of gasification rate on coke rate.

Fig. 8.

Effect of gasification rate on gas utilization ratio.

Fig. 9.

Effect of gasification control on coke rate.

Fig. 10.

Effect of gasification control on exhaust gas calorie.

4. Conclusions

A method for controlling the coke gasification rate in the shaft furnace by coating the coke surface was studied with the aim of controlling the coke rate and exhaust gas calorie and thereby responding flexibly to changes in the energy balance in the steel works. The following knowledge was obtained.

(1) The activation energy of the gasification reaction was virtually unchanged when SiO₂ was mixed with the coke. However, the activation energy decreased in the case of mixing CaCO₃ and increased in the case of mixing Fe₂O₃.

(2) The gasification rate was decelerated with SiO₂ coating, accelerated with CaCO₃ coating, and accelerated with Fe₂O₃ coating at high temperature. The experimental results were reproduced by using an overall reaction equation by fitting the frequency factor of the reaction rate constant.

(3) It was estimated by a simulation of shaft furnace operation that the coke rate decreased by 5 kg/t with SiO₂ coating, and the exhaust gas calorie increased by 0.33 GJ/t with CaCO₃ coating, suggesting the effectiveness of gasification rate control by this coke coating method.

Symbols

α: fractional reaction (-)

t: time (s)

A: frequency factor (1/s)

E_a: activation energy (J/mol)

R: gas constant (J/ K/mol)

T: temperature (K)

k: overall reaction rate constant (1/s)

R_s: overall reaction rate (kmol/m³(bed)/s)

d_p: particle size of coke (m)

ϕ: shape factor of coke (-)

N_C: number of coke particles per unit of volume (1/m³(bed))

k_f: gas film mass transfer coefficient (m/s)

E_f: reaction effectiveness factor (-)

k_s: chemical reaction rate constant of coke (1/s)

k_s,CaCO3, k_s,Fe2O3, k_s,SiO2: chemical reaction rate constants of coke coated with CaCO₃, Fe₂O₃ and SiO₂, respectively (1/s)

P: pressure in bed (atm)

x_CO2: mole fraction of CO₂ (-)

γ: gasification parameter (-)

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