Conversion of CO 2 Gas to CO Gas by the Utilization of Decarburization Reaction during Steelmaking Process

, pp. 413 418 Conversion of CO 2 Gas to CO Gas by the Utilization of Decarburization Reaction during Steelmaking Process Hiroyuki MATSUURA* and Fumitaka TSUKIHASHI Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5-501 Kashiwanoha, Kashiwa, Chiba, 277-8561 Japan. (Received on August 11, 2014; accepted on November 3, 2014) Dissolved carbon in molten iron can be decarburized by CO 2 gas instead O 2 gas commonly used in a basic oxygen furnace, in which CO 2 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 CO 2 O 2 gas mixture because of its significant endothermic reaction. The present study has focused on the decarburization reaction of molten Fe C alloy by CO 2 O 2 or H 2O CO 2 O 2 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 CO 2 to CO or H 2O to H 2. KEY WORDS: reduction of CO 2 gas; decarburization reaction; steelmaking; H 2O; H 2; thermodynamic calculation. 1. Introduction * Corresponding author: E-mail: matsuura@k.u-tokyo.ac.jp DOI: http://dx.doi.org/10.2355/isijinternational.55.413 More than 100 Mt of crude steel is annually produced in Japan 1) and the emission of CO 2 gas from steel industries accounts for around 12.5% of total domestic CO 2 emission (FY2012). 2) Therefore, the reduction in CO 2 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 CO 2 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 CO 2 gas could be utilized effectively by circulating in ironmaking and steelmaking processes, CO 2 gas could become an energy transfer medium. However, recovered CO 2 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 CO 2 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 CO 2 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 CO 2 gas to CO gas in the steelmaking process has been considered, one of which is the utilization of CO 2 gas as an oxidizing gas for the decarburization process. Usually O 2 gas is used to decarburize hot metal to produce molten steel, which reaction is expressed by Eq. (1). C ( in Fe C liquid, mass% ) + 1/ 2 O2 ( g) CO( g)... (1) 4) ΔG 139 310 41. 73T J/ mol As shown above, the reaction is a large exothermic reaction and therefore molten steel can be heated up to around 1 933 K at the blow-end of BOF steelmaking process. On the contrary, the decarburization reaction by CO 2 gas is expressed as follows. C ( in Fe C liquid, mass% ) + CO2 ( g) 2CO( g)... (2) ΔG 144 700 129. 5T J/ mol 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 CO 2 gas utilization as a decarburization agent substituting O 2 gas. In the present study, thermodynamic estimations of decarburization reaction by using CO 2 gas were conducted with various operation conditions and the effects of such conditions on the decarburization reaction behavior, degree of the reduction of CO 2 gas to CO gas and temperature variation were studied. 413 2015 ISIJ

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 CO 2-containing gas. Table 1 shows the detail of calculation conditions. Firstly, 1 kg of molten Fe 3.5 mass%c alloy was prepared at 1 573 K. Subsequently the prepared molten Fe C alloy was equilibrated with O 2, CO 2 O 2 or CO 2 H 2O O 2 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. 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 CO 2-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 O 2 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 2 000 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 1 920 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. 3.2. Effect of CO 2 Composition of Introduced Gas Equilibrium calculation between molten Fe C alloy and CO 2 O 2 gas introduced at 300 K was conducted by changing CO 2 gas composition. Figure 3 shows the effect of partial pressure of CO 2 gas in CO 2 O 2 gas on temperature change of the system during decarburization of molten Fe C alloy. Temperature change became smaller with increasing CO 2 content of introduced gas. Temperature and carbon content of molten Fe C alloy reached the liquidus of the Fe C system in the case CO 2 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 CO 2 O 2 gas containing more than 60% of CO 2. Therefore, CO 2 content in CO 2 O 2 gas must be lower than 20% to avoid any solid steel precipitation. 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 (1 573 K) Composition C: 3.5 mass% Alloy gas equilibrium Reaction Fe C (l) + H 2O CO 2 O 2 (g) Fe C (l) + H 2O H 2 CO 2 CO O 2 (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 1 500 K Gas composition P(CO 2): 0.0 0.8 atm, P(H 2O): 0.0 0.2 atm Temperature Change to satisfy 80% thermal efficiency 2015 ISIJ 414

Fig. 1. Algorithm of equilibrium calculation. Fig. 4. Effect of partial pressure of CO 2 on the relationship between partial pressures of CO and CO 2 and carbon content in molten Fe C alloy. Fig. 2. Effect of thermal efficiency on temperature change of the system during decarburization reaction of molten Fe C alloy. expected from the equilibrium constant of reaction (2). Instantaneous and integrated conversion ratios of CO 2 gas to CO gas were calculated by Eqs. (3) and (4), respectively. Instantaneous conversion ratio of CO2 Amount of CO2 in the gas after equilibrium... (3) Amount of CO2 in 10. L introduced gas Integrated conversion ratio of CO2 Total amount of CO2 in exhaust gas... (4) Total amount of CO2 in introduced gas Figure 5 shows change in conversion ratio of CO 2 gas to CO gas with (a) carbon content of molten Fe C alloy, or (b) introduced CO 2 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 CO 2 gas partial pressure by 0.1 atm decreased the instantaneous conversion ratio approximately 1%, while integrated CO 2 gas conversion ratio was still above 97% when CO 2 gas was introduced as 0.2 atm CO 2 O 2 gas, in which the amount of totally introduced CO 2 gas was 0.18 mol larger compared to that in 0.1 atm CO 2 O 2 gas. Energy required to further reduce CO 2 gas to CO gas was compensated as the temperature decrease by about 50 K at the blow-end. Fig. 3. Effect of partial pressure of CO 2 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 CO 2 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 CO 2 partial pressure in introduced gas was not clearly seen. On the contrary, partial pressure of CO 2 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 CO 2 gas became lower at lower carbon content region as 3.3. Effect of Introduced Gas Temperature Equilibrium calculation between molten Fe C alloy and 0.2 atm CO 2 O 2 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 1 820 K to 1 880 K by increasing gas temperature from 300 K to 1 500 K. Figure 7 shows the effect of introduced gas temperature on the change in partial pressures of CO and CO 2 in exhaust gas. Effect of introduced gas temperature was insignificant but the temperature increase slightly decreased the equilibrated CO 2 partial pressure. Figure 8 shows change in conversion ratio of CO 2 gas to 415 2015 ISIJ

Fig. 7. Effect of introduced gas temperature on the relationship between partial pressures of CO and CO 2 and carbon content in molten Fe C alloy. Fig. 5. Change in conversion ratio of CO 2 to CO with (a) carbon content and (b) introduced CO 2 amount during decarburization reaction of molten Fe C alloy. Fig. 6. Effect of introduced gas temperature on temperature change of the system during decarburization reaction of molten Fe C alloy. CO gas with (a) carbon of molten Fe C alloy or (b) introduced CO 2 gas amount in mole. Effect of introduced gas temperature on CO 2 gas conversion ratio was insignificant. From above results, the effect of preheating of CO 2 O 2 gas before blowing on temperature increase of molten steel at the blow-end is limited and the positive effects on CO 2 gas conversion ratio was not obviously observed. Fig. 8. Change in conversion ratio of CO 2 to CO with (a) carbon content and (b) introduced CO 2 amount during decarburization reaction of molten Fe C alloy. 3.4. Effect of H 2O Gas Addition Similar to CO 2 gas, H 2O gas (water vapor) can also decarburize molten Fe C alloy which reaction is expressed as Eq. (5). ( ) ( ) ( )+ ( ) C in Fe C liquid, mass% + H O g CO g H g ΔG 112 600 100. 1T 2 2 6) J/ mol... (5) 2015 ISIJ 416

This reaction is also endothermic reaction and thus the effects similar to CO 2 gas on decarburization behavior are expected. Therefore, the effects of H 2O gas addition on the decarburization reactions were examined. Equilibrium calculation between molten Fe C alloy and x Fig. 9. Effect of H 2O addition into introduced gas on temperature change of the system during decarburization reaction of molten Fe C alloy. atm H 2O (0.2 x) atm CO 2 O 2 gas (0 x 0.2) at 1 500 K was conducted by changing partial pressure of H 2O gas. Figure 9 shows the effect of H 2O gas addition on temperature change of the system during decarburization reaction. Temperature change with decreasing carbon content in the case of H 2O CO 2 O 2 gas is similar to that of CO 2 O 2 gas and temperature slightly increased with increasing partial pressure of H 2O 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 CO 2 gas is fully substituted by 0.2 atm H 2O gas. Change in partial pressures of CO, CO 2, H 2 and H 2O with carbon content in molten Fe C alloy is shown in Fig. 10. Partial pressure of CO 2 is similar to that in the case of CO 2 O 2 gas equilibrium as shown in Fig. 7 and the effect of H 2O gas addition is negligibly small. On the contrary, partial pressures of H 2O and H 2 obviously increased with increasing H 2O partial pressure of introduced gas. Similar to CO 2 partial pressure change, H 2O partial pressure gradually increased with decreasing carbon content and the partial pressure ratio of H 2 to H 2O drastically decreased when carbon content decreased below 1 mass%. Instantaneous and integrated conversion ratios of H 2O gas to H 2 gas were calculated by Eqs. (6) and (7), respectively. Fig. 10. Effect of H 2O addition into introduced gas on the relationship between (a) partial pressures of CO and CO 2 and carbon content in molten Fe C alloy, and (b) partial pressures of H 2 and H 2O and carbon content in molten Fe C alloy. Fig. 11. Change in conversion ratio of (a) CO 2 to CO and (b) H 2O to H 2 with carbon content during decarburization reaction of molten Fe C alloy by H 2O CO 2 O 2 gas. 417 2015 ISIJ

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 CO 2 and H 2O to oxidizing gas would be feasible to diminish the decrease of temperature of molten steel and also to produce CO and H 2 gas simultaneously, in which especially almost complete reduction of H 2O gas to H 2 is expected. Fig. 12. Change in conversion ratio of (a) CO 2 to CO with introduced CO 2 amount and (b) H 2O to H 2 with introduced H 2O amount during decarburization reaction of molten Fe C alloy by H 2O CO 2 O 2 gas. Instantaneous conversion ratio of H2O Amount of H O in the gas after equilibrium 2 Amount of H2O in 10. L introduced gas Integrated conversion ratio of H2O Total amount of H2O in exhaust gas Total amount of H2O in introduced gas... (6)... (7) Calculated conversion ratios of CO 2 and H 2O 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 CO 2 or H 2O amount in mole. As shown in Figs. 11(a) and 12(a), conversion ratio of CO 2 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, CO 2 conversion ratio would increase to some extent due to the oxidation of Fe in the practical condition. On the contrary, conversion ratio of H 2O to H 2 was not considerably affected with changing H 2O partial pressure of 4. Conclusions In the present study, the conversion process of CO 2 gas to CO gas utilizing thermal and chemical energy of molten iron was considered and the effect of the substitution of O 2 gas to CO 2 O 2 gas or H 2O CO 2 O 2 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 CO 2 gas, considerable difference in temperature change was observed after the change of O 2 gas to CO 2 O 2 gas. The maximal substitution ratio of O 2 gas to CO 2 gas was 20% to avoid the precipitation of solid Fe during decarburization. Integrated conversion ratio of CO 2 to CO was more than 95%, while instantaneous conversion ratio of CO 2 to CO decreased drastically when carbon content decreased below 1 mass%. Preheating of introduced gas from 300 K to 1 500 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 H 2O gas as a substitute of CO 2 slightly increased molten alloy temperature at the blow-end, approximately 10 K, when 0.2 atm H 2O O 2 gas was introduced at 1 500 K. This slight temperature increase is due to smaller enthalpy change of decarburization reaction by H 2O gas than that by CO 2 gas. Conversion ratio of H 2O to H 2 was more than 99% and thus the simultaneous addition of CO 2 and H 2O to oxidizing gas would be feasible to diminish temperature drop of molten steel and to produce CO and H 2 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/time- Series.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. 2015 ISIJ 418