Conversion of CO₂ Gas to CO Gas by the Utilization of Decarburization Reaction during Steelmaking Process

Abstract

Dissolved carbon in molten iron can be decarburized by CO₂ gas instead O₂ gas commonly used in a basic oxygen furnace, in which CO₂ gas is reduced to CO gas. Produced CO gas can be returned to ironmaking process and as a whole carbon works as energy transfer medium. However, the precise control of the process is required to utilize the decarburization reaction by CO₂–O₂ gas mixture because of its significant endothermic reaction. The present study has focused on the decarburization reaction of molten Fe–C alloy by CO₂–O₂ or H₂O–CO₂–O₂ gas mixtures and the effects of gas composition and temperature on temperature change of molten alloy and exhaust gas compositions were studied by means of thermodynamic calculation. Repeated equilibrium calculations between existing molten alloy and newly introduced reacting gas enabled the estimation of molten alloy temperature and exhaust gas composition changes. Results were discussed in terms of molten alloy temperature change and conversion ratio of CO₂ to CO or H₂O to H₂.

1. Introduction

More than 100 Mt of crude steel is annually produced in Japan¹⁾ and the emission of CO₂ gas from steel industries accounts for around 12.5% of total domestic CO₂ emission (FY2012).²⁾ Therefore, the reduction in CO₂ emission from integrated steel production process is responsible for the dramatic curtailment of an environmental load and the establishment of a sustainable steel production process. Various technologies for the recovery and storage of CO₂ gas have been widely studied and pilot-scale plant tests have been conducted, one of which is COURSE50 project conducted by Japanese steelmaking companies with the support of New Energy and Industrial Technology Development Organization and the Japan Iron and Steel Federation.³⁾ If recovered CO₂ gas could be utilized effectively by circulating in ironmaking and steelmaking processes, CO₂ gas could become an energy transfer medium. However, recovered CO₂ gas has nearly no value as is and thus some sort of energy must be adequately given. One of such methods is the conversion of CO₂ gas to valuable CO gas by the reduction reaction. The value of CO gas is relatively high as a fuel in steel plants or a reducing agent for newly developed various ironmaking processes. CO gas is also required in various petroleum refinery processes and chemical plants.

Many processes to reduce CO₂ gas to CO gas have been proposed, such as electrolysis, hydrogen reduction, and thermo chemical reduction. In the present study, the possibility of the reduction of CO₂ gas to CO gas in the steelmaking process has been considered, one of which is the utilization of CO₂ gas as an oxidizing gas for the decarburization process. Usually O₂ gas is used to decarburize hot metal to produce molten steel, which reaction is expressed by Eq. (1).   

$$\begin{array}{l}\text{C}\mathrm{\hspace{0.17em} }\left(\text{in Fe}–\text{C liquid},\text{ mass\%}\right)+1/2{\text{ O}}_2\left(\text{g}\right)=\text{CO}\left(\text{g}\right)\\ \mathrm{\Delta }G°=-139\mathrm{\hspace{0.17em} }310–41.73T\hspace{1em}\text{J}/\text{mol}\end{array}$$(1) ⁴⁾

As shown above, the reaction is a large exothermic reaction and therefore molten steel can be heated up to around 1933 K at the blow-end of BOF steelmaking process.

On the contrary, the decarburization reaction by CO₂ gas is expressed as follows.   

$$\begin{array}{l}\text{C}\mathrm{\hspace{0.17em} }\left(\text{in Fe}–\text{C liquid},\text{ mass\%}\right)+{\text{CO}}_2\left(\text{g}\right)=2\text{CO}\left(\text{g}\right)\\ \mathrm{\Delta }G°=144\mathrm{\hspace{0.17em} }700–129.5T\hspace{1em}\text{J}/\text{mol}\end{array}$$(2) ⁵⁾

Standard Gibbs energy change of the above reaction is negative at steelmaking temperature range and thus the decarburization reaction takes place, while it is a largely endothermic reaction. Therefore, there would be a limitation in CO₂ gas utilization as a decarburization agent substituting O₂ gas.

In the present study, thermodynamic estimations of decarburization reaction by using CO₂ gas were conducted with various operation conditions and the effects of such conditions on the decarburization reaction behavior, degree of the reduction of CO₂ gas to CO gas and temperature variation were studied.

2. Conditions for Thermodynamic Calculation

In the present study, thermodynamic calculation software FactSage 6.3.1 was used for estimation of equilibrium state between molten Fe–C alloy and CO₂-containing gas.

Table 1 shows the detail of calculation conditions. Firstly, 1 kg of molten Fe–3.5 mass%C alloy was prepared at 1573 K. Subsequently the prepared molten Fe–C alloy was equilibrated with O₂, CO₂–O₂ or CO₂–H₂O–O₂ gas in various compositions and temperatures. In the equilibrium calculation between molten Fe–C alloy and gas, 87 kinds of pure compounds (pure gaseous species in ideal behavior: 43, pure liquid species: 16 and pure solid species: 28) and a solution considering 3 species were considered in the equilibrium calculation as final candidates. For the estimation of change in temperature and composition of molten Fe–C alloy and exhaust gas with proceeding of decarburization reaction, 1.0 L of reacting gas at predetermined gas temperature was equilibrated with molten Fe–C alloy. The amount, temperature and composition of molten Fe–C alloy after equilibrium calculation were input as the initial alloy conditions in the next calculation, and 1.0 L of new reacting gas was equilibrated again. The above equilibrium calculation between molten Fe–C alloy and gas was repeated by decreasing carbon content of Fe–C alloy less than 0.1 mass%. Change in alloy and exhaust gas compositions and system temperature with process of reaction between molten Fe–C alloy and reacting gas were estimated. The algorithm of equilibrium calculation is schematically shown in Fig. 1.

Table 1. Conditions for calculations by FactSage 6.3.1.

Molten Fe–C alloy preparation
Reaction	Fe (s) + C (s) → Fe–C (l)
Amount	Total: 1 kg
Number of compounds	0
Number of solutions	1 (Species: 2)
	FSstel-LIQU (Metal-liquid)
Temperature	Fixed (1573 K)
Composition	C: 3.5 mass%
Alloy – gas equilibrium
Reaction	Fe–C (l) + H2O–CO2–O2 (g) → Fe–C (l) + H2O–H2–CO2–CO–O2 (g)
Amount	Molten Fe–C alloy + 1.0 L of gas (1 calculation)
Number of compounds	Total: 87 (Gas(ideal): 43, Liquid: 16, Solid: 28)
Number of solutions	1 (Species: 3)
	FSstel-LIQU (Metal-liquid)
Gas temperature	300–1500 K
Gas composition	P(CO2): 0.0–0.8 atm, P(H2O): 0.0–0.2 atm
Temperature	Change to satisfy 80% thermal efficiency

Fig. 1.

Algorithm of equilibrium calculation.

3. Results and Discussion

3.1. Determination of Thermal Efficiency

As explained previously, one of objectives in the present study is the estimation of temperature change of molten steel during decarburization reaction by CO₂-containing gas. However, the present calculation method using thermodynamic equilibrium has a limitation for the consideration of dynamic change of BOF steelmaking process such as scrap melting or slag formation. Therefore, the present method simply calculated the equilibrium between molten Fe–C alloy and blowing gas and thus thermal energy is excess compared to an actual operation. To estimate reasonable temperature change during calculation, heat utilization efficiency was preliminary determined. Figure 2 shows the effect of thermal efficiency on temperature change of the system during equilibrium calculation between molten Fe–C alloy and pure O₂ gas introduced at 300 K, in which 100% thermal efficiency means that the equilibrium calculation is processed in the adiabatic condition. However, the adiabatic condition increased the system temperature above 2000 K when molten Fe–C alloy was decarburized from 3.5 mass%C to 0.1 mass%C, which is unrealistic. As shown in the figure, molten steel temperature reaches around 1920 K when thermal efficiency is 80%, which is close to molten steel temperature at the blow-end. Therefore, the thermal efficiency in following all calculations was set as 80%, which means that 80% of thermal energy generated by decarburization reaction is used to increase the system temperature and 20% is simply lost.

Fig. 2.

Effect of thermal efficiency on temperature change of the system during decarburization reaction of molten Fe–C alloy.

3.2. Effect of CO₂ Composition of Introduced Gas

Equilibrium calculation between molten Fe–C alloy and CO₂–O₂ gas introduced at 300 K was conducted by changing CO₂ gas composition. Figure 3 shows the effect of partial pressure of CO₂ gas in CO₂–O₂ gas on temperature change of the system during decarburization of molten Fe–C alloy. Temperature change became smaller with increasing CO₂ content of introduced gas. Temperature and carbon content of molten Fe–C alloy reached the liquidus of the Fe–C system in the case CO₂ partial pressure more than 0.3 atm, in which solid Fe was precipitated during decarburization reaction. Molten alloy temperature even decreased with decreasing carbon content by CO₂–O₂ gas containing more than 60% of CO₂. Therefore, CO₂ content in CO₂–O₂ gas must be lower than 20% to avoid any solid steel precipitation.

Fig. 3.

Effect of partial pressure of CO₂ on temperature change of the system during decarburization reaction of molten Fe–C alloy.

Figure 4 shows the change in partial pressures of CO and CO₂ in exhaust gas with carbon content of molten Fe–C alloy. Partial pressure of CO gas is close to 1 atm and the effect of CO₂ partial pressure in introduced gas was not clearly seen. On the contrary, partial pressure of CO₂ gas gradually increased with the decrease of carbon content. The increasing trend became significant at carbon content below 1 mass%. Decrease in carbon content of molten Fe–C alloy decreases activity of carbon and thus the reduction degree of CO₂ gas became lower at lower carbon content region as expected from the equilibrium constant of reaction (2).

Fig. 4.

Effect of partial pressure of CO₂ on the relationship between partial pressures of CO and CO₂ and carbon content in molten Fe–C alloy.

Instantaneous and integrated conversion ratios of CO₂ gas to CO gas were calculated by Eqs. (3) and (4), respectively.   

$$\begin{array}{l}\text{Instantaneous}\mathrm{\hspace{0.17em} }\text{conversion}\mathrm{\hspace{0.17em} }\text{ratio}\mathrm{\hspace{0.17em} }\text{of}\mathrm{\hspace{0.17em} }{\text{CO}}_2=\\ \frac{\text{Amount}\mathrm{\hspace{0.17em} }\text{of}\mathrm{\hspace{0.17em} }{\text{CO}}_2\mathrm{\hspace{0.17em} }\text{in}\mathrm{\hspace{0.17em} }\text{the}\mathrm{\hspace{0.17em} }\text{gas}\mathrm{\hspace{0.17em} }\text{after}\mathrm{\hspace{0.17em} }\text{equilibrium}}{\text{Amount}\mathrm{\hspace{0.17em} }\text{of}\mathrm{\hspace{0.17em} }{\text{CO}}_2\mathrm{\hspace{0.17em} }\text{in}\mathrm{\hspace{0.17em} }1.0\mathrm{\hspace{0.17em} }\text{L}\mathrm{\hspace{0.17em} }\text{introduced}\mathrm{\hspace{0.17em} }\text{gas}}\end{array}$$(3)

  

$$\begin{array}{l}\text{Integrated}\mathrm{\hspace{0.17em} }\text{conversion}\mathrm{\hspace{0.17em} }\text{ratio}\mathrm{\hspace{0.17em} }\text{of}\mathrm{\hspace{0.17em} }{\text{CO}}_2=\\ \frac{\text{Total}\mathrm{\hspace{0.17em} }\text{amount}\mathrm{\hspace{0.17em} }\text{of}\mathrm{\hspace{0.17em} }{\text{CO}}_2\mathrm{\hspace{0.17em} }\text{in}\mathrm{\hspace{0.17em} }\text{exhaust}\mathrm{\hspace{0.17em} }\text{gas}}{\text{Total}\mathrm{\hspace{0.17em} }\text{amount}\mathrm{\hspace{0.17em} }\text{of}\mathrm{\hspace{0.17em} }{\text{CO}}_2\mathrm{\hspace{0.17em} }\text{in}\mathrm{\hspace{0.17em} }\text{introduced}\mathrm{\hspace{0.17em} }\text{gas}}\end{array}$$(4)

Figure 5 shows change in conversion ratio of CO₂ gas to CO gas with (a) carbon content of molten Fe–C alloy, or (b) introduced CO₂ gas amount in mole which makes the calculation of generated amount of CO gas easier. Instantaneous conversion ratio decreased drastically at carbon content below 1 mass%, while that was almost constant at carbon content above that and hence integrated conversion ratio was still more than 95% at the blow-end. Increase in initial CO₂ gas partial pressure by 0.1 atm decreased the instantaneous conversion ratio approximately 1%, while integrated CO₂ gas conversion ratio was still above 97% when CO₂ gas was introduced as 0.2 atm CO₂–O₂ gas, in which the amount of totally introduced CO₂ gas was 0.18 mol larger compared to that in 0.1 atm CO₂–O₂ gas. Energy required to further reduce CO₂ gas to CO gas was compensated as the temperature decrease by about 50 K at the blow-end.

Fig. 5.

Change in conversion ratio of CO₂ to CO with (a) carbon content and (b) introduced CO₂ amount during decarburization reaction of molten Fe–C alloy.

3.3. Effect of Introduced Gas Temperature

Equilibrium calculation between molten Fe–C alloy and 0.2 atm CO₂–O₂ gas was conducted by changing introduced gas temperature. Figure 6 shows the relationship between carbon content of molten Fe–C alloy and the system temperature. Molten Fe–C alloy temperature at the blow-end increased by 60 K from 1820 K to 1880 K by increasing gas temperature from 300 K to 1500 K.

Fig. 6.

Effect of introduced gas temperature on temperature change of the system during decarburization reaction of molten Fe–C alloy.

Figure 7 shows the effect of introduced gas temperature on the change in partial pressures of CO and CO₂ in exhaust gas. Effect of introduced gas temperature was insignificant but the temperature increase slightly decreased the equilibrated CO₂ partial pressure.

Fig. 7.

Effect of introduced gas temperature on the relationship between partial pressures of CO and CO₂ and carbon content in molten Fe–C alloy.

Figure 8 shows change in conversion ratio of CO₂ gas to CO gas with (a) carbon of molten Fe–C alloy or (b) introduced CO₂ gas amount in mole. Effect of introduced gas temperature on CO₂ gas conversion ratio was insignificant. From above results, the effect of preheating of CO₂–O₂ gas before blowing on temperature increase of molten steel at the blow-end is limited and the positive effects on CO₂ gas conversion ratio was not obviously observed.

Fig. 8.

Change in conversion ratio of CO₂ to CO with (a) carbon content and (b) introduced CO₂ amount during decarburization reaction of molten Fe–C alloy.

3.4. Effect of H₂O Gas Addition

Similar to CO₂ gas, H₂O gas (water vapor) can also decarburize molten Fe–C alloy which reaction is expressed as Eq. (5).   

$$\begin{array}{l}\text{C}\mathrm{\hspace{0.17em} }\left(\text{in Fe}–\text{C liquid},\text{ mass\%}\right)+{\text{H}}_2\text{O}\left(\text{g}\right)=\text{CO}\left(\text{g}\right)+{\text{H}}_2\left(\text{g}\right)\\ \mathrm{\Delta }G°=112\mathrm{\hspace{0.17em} }600–100.1T\hspace{1em}\text{J}/\text{mol}\end{array}$$(5) ⁶⁾

This reaction is also endothermic reaction and thus the effects similar to CO₂ gas on decarburization behavior are expected. Therefore, the effects of H₂O gas addition on the decarburization reactions were examined.

Equilibrium calculation between molten Fe–C alloy and x atm H₂O – (0.2−x) atm CO₂ – O₂ gas (0 ≤ x ≤ 0.2) at 1500 K was conducted by changing partial pressure of H₂O gas. Figure 9 shows the effect of H₂O gas addition on temperature change of the system during decarburization reaction. Temperature change with decreasing carbon content in the case of H₂O–CO₂–O₂ gas is similar to that of CO₂–O₂ gas and temperature slightly increased with increasing partial pressure of H₂O gas. This is because of slightly smaller enthalpy change of Eq. (5) compared to that of Eq. (2). Temperature increase at the blow-end is however only 10 K when 0.2 atm CO₂ gas is fully substituted by 0.2 atm H₂O gas.

Fig. 9.

Effect of H₂O addition into introduced gas on temperature change of the system during decarburization reaction of molten Fe–C alloy.

Change in partial pressures of CO, CO₂, H₂ and H₂O with carbon content in molten Fe–C alloy is shown in Fig. 10. Partial pressure of CO₂ is similar to that in the case of CO₂–O₂ gas equilibrium as shown in Fig. 7 and the effect of H₂O gas addition is negligibly small. On the contrary, partial pressures of H₂O and H₂ obviously increased with increasing H₂O partial pressure of introduced gas. Similar to CO₂ partial pressure change, H₂O partial pressure gradually increased with decreasing carbon content and the partial pressure ratio of H₂ to H₂O drastically decreased when carbon content decreased below 1 mass%.

Fig. 10.

Effect of H₂O addition into introduced gas on the relationship between (a) partial pressures of CO and CO₂ and carbon content in molten Fe–C alloy, and (b) partial pressures of H₂ and H₂O and carbon content in molten Fe–C alloy.

Instantaneous and integrated conversion ratios of H₂O gas to H₂ gas were calculated by Eqs. (6) and (7), respectively.   

$$\begin{array}{l}\text{Instantaneous}\mathrm{\hspace{0.17em} }\text{conversion}\mathrm{\hspace{0.17em} }\text{ratio}\mathrm{\hspace{0.17em} }\text{of}\mathrm{\hspace{0.17em} }{\text{H}}_2\text{O}=\\ \frac{\text{Amount}\mathrm{\hspace{0.17em} }\text{of}\mathrm{\hspace{0.17em} }{\text{H}}_2\text{O}\mathrm{\hspace{0.17em} }\text{in}\mathrm{\hspace{0.17em} }\text{the}\mathrm{\hspace{0.17em} }\text{gas}\mathrm{\hspace{0.17em} }\text{after}\mathrm{\hspace{0.17em} }\text{equilibrium}}{\text{Amount}\mathrm{\hspace{0.17em} }\text{of}\mathrm{\hspace{0.17em} }{\text{H}}_2\text{O}\mathrm{\hspace{0.17em} }\text{in}\mathrm{\hspace{0.17em} }1.0\mathrm{\hspace{0.17em} }\text{L}\mathrm{\hspace{0.17em} }\text{introduced}\mathrm{\hspace{0.17em} }\text{gas}}\end{array}$$(6)

  

$$\begin{array}{l}\text{Integrated}\mathrm{\hspace{0.17em} }\text{conversion}\mathrm{\hspace{0.17em} }\text{ratio}\mathrm{\hspace{0.17em} }\text{of}\mathrm{\hspace{0.17em} }{\text{H}}_2\text{O}=\\ \frac{\text{Total}\mathrm{\hspace{0.17em} }\text{amount}\mathrm{\hspace{0.17em} }\text{of}\mathrm{\hspace{0.17em} }{\text{H}}_2\text{O}\mathrm{\hspace{0.17em} }\text{in}\mathrm{\hspace{0.17em} }\text{exhaust}\mathrm{\hspace{0.17em} }\text{gas}}{\text{Total}\mathrm{\hspace{0.17em} }\text{amount}\mathrm{\hspace{0.17em} }\text{of}\mathrm{\hspace{0.17em} }{\text{H}}_2\text{O}\mathrm{\hspace{0.17em} }\text{in}\mathrm{\hspace{0.17em} }\text{introduced}\mathrm{\hspace{0.17em} }\text{gas}}\end{array}$$(7)

Calculated conversion ratios of CO₂ and H₂O are shown in Figs. 11(a) and 11(b) as a function of carbon content in molten Fe–C alloy, and also shown in Figs. 12(a) and 12(b) as a function of introduced CO₂ or H₂O amount in mole. As shown in Figs. 11(a) and 12(a), conversion ratio of CO₂ decreases drastically at lower carbon content range. Instantaneous conversion ratio decreased to less than 90% at the blow-end. As mentioned previously, the oxidation of Fe to produce FeO is not taken into consideration in this calculation. Therefore, CO₂ conversion ratio would increase to some extent due to the oxidation of Fe in the practical condition.

Fig. 11.

Change in conversion ratio of (a) CO₂ to CO and (b) H₂O to H₂ with carbon content during decarburization reaction of molten Fe–C alloy by H₂O–CO₂–O₂ gas.

Fig. 12.

Change in conversion ratio of (a) CO₂ to CO with introduced CO₂ amount and (b) H₂O to H₂ with introduced H₂O amount during decarburization reaction of molten Fe–C alloy by H₂O–CO₂–O₂ gas.

On the contrary, conversion ratio of H₂O to H₂ was not considerably affected with changing H₂O partial pressure of introduced gas and more than 93% of instantaneous conversion ratio was maintained until the blow-end as shown in Figs. 11(b) and 12(b). Integrated conversion ratio was significantly large, more than 99%. Simultaneous addition of CO₂ and H₂O to oxidizing gas would be feasible to diminish the decrease of temperature of molten steel and also to produce CO and H₂ gas simultaneously, in which especially almost complete reduction of H₂O gas to H₂ is expected.

4. Conclusions

In the present study, the conversion process of CO₂ gas to CO gas utilizing thermal and chemical energy of molten iron was considered and the effect of the substitution of O₂ gas to CO₂–O₂ gas or H₂O–CO₂–O₂ gas on decarburization behavior of molten Fe–C alloy and conversion efficiency was examined by thermodynamic calculation.

Due to largely endothermic reaction of decarburization by CO₂ gas, considerable difference in temperature change was observed after the change of O₂ gas to CO₂–O₂ gas. The maximal substitution ratio of O₂ gas to CO₂ gas was 20% to avoid the precipitation of solid Fe during decarburization. Integrated conversion ratio of CO₂ to CO was more than 95%, while instantaneous conversion ratio of CO₂ to CO decreased drastically when carbon content decreased below 1 mass%.

Preheating of introduced gas from 300 K to 1500 K was not significantly effective to improve temperature decrease of molten alloy, which increased molten alloy temperature at the blow-end by about 60 K.

Addition of H₂O gas as a substitute of CO₂ slightly increased molten alloy temperature at the blow-end, approximately 10 K, when 0.2 atm H₂O–O₂ gas was introduced at 1500 K. This slight temperature increase is due to smaller enthalpy change of decarburization reaction by H₂O gas than that by CO₂ gas. Conversion ratio of H₂O to H₂ was more than 99% and thus the simultaneous addition of CO₂ and H₂O to oxidizing gas would be feasible to diminish temperature drop of molten steel and to produce CO and H₂ gas simultaneously.

Acknowledgment

This research was partially supported by the Arai Science and Technology Foundation. Authors greatly appreciate the financial support.

References

• 1)  The Japan Iron and Steel Federation: Series Statistics for Japanese Steel Production, http://www.jisf.or.jp/en/statistics/production/TimeSeries.html, (accessed 2014-07-02).
• 2)  Greenhouse Gas Inventory Office of Japan, Center for Global Environmental Research and National Institute for Environmental Studies, ed. by National Greenhouse Gas Inventory Report of Japan 2014, NIES, Tokyo, (2014), 2.
• 3)  COURSE50 official website: http://www.jisf.or.jp/course50/index.html, (accessed 2014-07-02).
• 4)  The 19th Committee on Steelmaking, The Japan Society for the Promotion of Science ed.: Steelmaking Data Sourcebook, Gordon and Breach Science Publishers, New York, (1988), 59, 279.
• 5)  Ibid., 59.
• 6)  Ibid., 59, 117.