2024 Volume 64 Issue 9 Pages 1486-1492
The iron and steelmaking industry must focus on neutralizing CO2 emissions. One solution involves using hydrogen as a reducing agent for iron ore. However, carbon is an essential element as primary steel is produced by refining molten carbon-saturated iron (hot metal). Ironmaking processes applying CO2 capture and utilization have been suggested; however, they are limited to the reduction process. To satisfy the demand for primary steel production with net-zero CO2 emissions, a new carbon recycling ironmaking process capable of producing hot metal must be considered. This study proposes a carbon recycling ironmaking process using deposited carbon-iron ore composite (CRIP-D). In the CRIP-D process, hot metal is produced by using the solid carbon recovered by reforming exhaust gas as reducing and carburizing agents. Moreover, using the recovered solid carbon, iron oxides are reduced more rapidly, and reduced iron is melted at a lower temperature than that using fossil fuel-derived carbon. This means carbon-neutral steel can be produced more efficiently than conventional ironmaking processes. Using proven technologies, following hot metal production, primary steel can be produced while minimizing the burden on the steel mills for converting equipment. Thus, true carbon-neutral primary steel is feasible using the proposed CRIP-D process.
At approximately 1.9 billion tons worth of production per year, steel is the third most abundant material on earth, after cement and timber. Its high strength, ease of processing, and relatively low cost render its complete substitution less likely in the foreseeable future. Approximately 70% of global crude steel is produced within a blast furnace-basic oxygen furnace (BF-BOF). The remainder is produced within an electric arc furnace (EAF).1) The BF process consumes considerable amounts of coking coal, which is equivalent to approximately 16% of the global demand for coal. Consequently, the high dependence on coal has resulted in the iron and steelmaking sector accounting for 2.6 gigatonnes of CO2 emissions annually, which is 7% of the global total.1) With the recent strong demand for achieving a carbon-neutral society, net-zero CO2 emission in the iron and steelmaking industry has been an important issue.
Steel is an alloy of iron and carbon. Thus, the use of carbon is inevitable in the process of steelmaking. Carbon performs three important chemical roles in the BF process. First, it functions as a heat source. Second, it is used as a reducing agent for iron ores. Third, it functions as a carburizing agent for reduced metallic iron. The melting point of pure iron (1537°C) is decreased to 1150°C by dissolving carbon at a saturated concentration (4.3 mass%). Molten carbon-saturated iron (hot metal) can be separated from molten impurities such as sulfur, phosphorus, and silicon based on the difference in densities. Consequently, the hot metal is refined into primary steel by injecting oxygen in the BOF. Further, the dissolved carbon is consumed as a heat source and as sacrificial protection from re-oxidization of iron. Carbon content in the primary steel is typically reduced to below 0.25 mass% via the refining process. However, the consumed carbon is emitted as CO and CO2.
Hydrogen produced by electrolysis of water using fossil-free electricity can be used as a substitute for carbon as a reducing agent. Consequently, solid direct reduced iron (DRI) can be produced without emitting CO2 by using the electrolytic hydrogen.2,3,4) The hot metal is produced by melting the DRI and carburizers in an electric smelting furnace (ESF). The DRI-ESF process is under investigation as an alternative to the BF process.5,6,7) Carbon-neutral hot metal can be produced when carbon-neutral electricity is used for hydrogen production and as energy for the ESF operation. In Sweden, the HYBRIT project has already begun operation of a pilot plant for DRI production using renewable energies and electrolytic hydrogen in August 2020;8) the aim is to create a demonstration plant by 2025. However, the DRI-ESF process would necessitate significant changes to steel mills as it has to be separated into two furnaces: reducing and melting furnaces. Furthermore, the emission of CO2 is inevitable in the oxygen refining process. As the use of carbon is essential, biomass utilization for carburization and the application of the concept of CO2 capture and utilization (CCU) for iron and steelmaking are necessary for achieving net-zero CO2 emissions.
Carburization trials using biomass have been conducted in Sweden and Australia.9,10,11) The substitution of 33% of the standard anthracite carbon charge in the smelting process with biochar exhibited no differences with the conventional process.9) However, biochar substitution can be commercialized in only those regions having sufficient source of biomass owing to the limited sustainable supply. To achieve full substitution of a conventional carbon source with biochar in China, which produced more than half of the world’s steel in 2022,12) approximately 20% of annual fixed carbon by forest biomass is required as input in the smelting process as a carburizing agent.13) However, this value is unrealistic as primary steel mills are far from forest abundant regions.
The conversion of CO2 captured from the ironmaking process into CO and CH4 for reuse as reducing agents has been considered. JFE Steel Corporation, Japan, has reported that more than 30% of the CO2 emitted from the conventional BF process can be reduced using converted CH4.14) However, certain issues remain in the carburization and melting processes owing to the CCU process being limited to the reduction reaction of iron ores. The carburization proceeds via primarily two routes. One involves direct contact between solid carbon and metallic iron.15,16) The other involves the absorption of high carbon activity gas on metallic iron.17) The iron carburized by the former route melts more than 1000 times rapidly than that in the case of the latter at 1250°C.18) The hot metal can be produced via the conventional BF process as the reduced iron contacts with solid carbon frequently. To extend the CCU process up to the smelting process of the hot metal, CO2 should be converted to solid carbon to carburize the reduced iron. However, the main focus of CCU research has been on conversion to hydrocarbon gases and organic compounds,19,20,21,22) with little consideration given to conversion to pure solid carbon and its utilization. This is because the solid carbon recovered from the gas phase is a very fine particle and considered difficult to utilize.
This study proposes deposited carbon-iron oxide composite (DCIC) as a new iron-bearing raw material. The conceptual diagram of carbon recycling ironmaking process using DCIC (CRIP-D) is shown in Fig. 1. DCIC is a composite comprising fine iron ores and carbon deposited by the exhausted gas from iron and steelmaking processes. The proposed CRIP-D comprises four processes: (1) gas reforming, (2) porous iron production, (3) carbon deposition, and (4) reduction and melting of DCIC. The mixture of the exhausted gas from each process (Gas A) and hydrogen is converted into high carbon activity gas (Gas B) by promoting a reverse water gas shift reaction (CO2+H2→CO+H2O). Porous iron is used as a catalyst for the reaction. In addition, a Gas C is produced by dehydrating gas B using a condenser.
The porous iron is continuously produced by reducing fine iron ores mixed with charcoal particles. The iron ore particles reduced by CO at approximately 900°C grow to form a whisker shape.23) The heat is provided by fossil-free electricity. Murakami et al. successfully produced porous iron via a carbothermic reduction of carbon and iron oxide composite at 1000°C.24)
Furthermore, the porous iron is repurposed as the substrate for depositing carbon. The following carbon deposition reactions (Eqs. (1), (2), (3)) are performed at approximately 700°C using gas C.
| (1) |
| (2) |
| (3) |
Gas D, which was obtained after the carbon deposition reaction, contained CO and H2. Thus, gas D could be reused as a heat source and as a reducing agent for subsequent processes.
The retrieved deposited carbon was agglomerated with fine ores as DCIC. A typical carbon-iron ore composite (CIC) has high self-reducibility as the gasification reaction of carbon and reduction reaction of iron ores are accelerated owing to close contact with each other.25,26,27,28) Thus, fine deposited carbon is rather desirable as a carbon raw material. Several types of furnaces can produce hot metal with CIC. By inputting 2% of the total input iron ores as CIC in BF, hot metal production increases by 1.9%, whereas lump coke consumption decreases by 4.1%.29) Forno de Auto-Reducao furnace (FAR) can feed all iron ores as CIC. Plant trials have demonstrated 16 times faster hot metal production than that using the BF process.30) Rotary hearth furnaces (RHF) can also produce pig iron with CIC. A commercial plant using the RHF was in operation for a while in America.30) The hot metal was refined into primary steel in BOF, similar to that in the case of the BF process. However, the output carbon from CRIP-D was only that dissolved in the steel provided the emitted gas from both the reduction refining processes could be reutilized for the gas reforming process.
The novelty of this process is to produce “true” carbon-neutral primary steel. As long as carbon use is essential, it is impossible to eliminate CO2 emissions itself, even if CO2 emissions can be reduced using hydrogen. Thus, the current “apparent” carbon-neutral steel focuses on such CO2 reductions distributed by the mass balance approach.31) On the contrary, the proposed CRIP-D can achieve carbon-neutrality comprehensively from raw materials to final products.
This study aims to confirm the principle feasibility of CRIP-D. The reduction and melting behaviors of DCIC compared with typical CIC were investigated as the first step.
Hematite reagent (1 μm size, purity 99.9%) and biomass char (53–150 μm size) were well mixed to obtain a molar ratio of fixed carbon in the char to oxygen in the hematite, which is a C/O ration of 0.85. The result of proximate analysis for the char on air dried basis is presented in Table 1. A schematic of the apparatus used for the production of porous iron is shown in Fig. 2. A magnesia crucible (inside diameter: 53 mm, outside diameter: 63 mm) filled with the mixture was set in an electric furnace. Porous iron was obtained by conducting a carbothermal reduction of the mixture by heating it to 950°C for 75 min under a nitrogen atmosphere. The gas flow rate was set at 15 L/min. The metallization degree of the porous iron obtained from quantitative evaluation by Rietveld analysis was 99.1%. The remained carbon in the obtained porous iron was below 0.6%. These results confirms that the porous iron was almost a single phase of ferrite. Fine iron ore (150 μm size) was pelletized using bentonite (150 μm size) at a mixing rate of 99.5:0.5 (mass%). The chemical compositions are presented in Table 2. The green pellet was reduced under N2-50%H2 atmosphere for 200 min at 830°C. The obtained metallic iron pellet had a diameter of approximately 12 mm with 57.4% porosity and showed 99.6% of reduction degree.
| F. C | V. M. | Ash | |
|---|---|---|---|
| Biomass char | 95.2 | 3.9 | 0.9 |
| Non-coking coal | 55.5 | 36.1 | 8.4 |
| Graphite | 99.9 | – | – |
| Fe2O3 | FeO | SiO2 | Al2O3 | CaO | MgO | Na2O | P | S | |
|---|---|---|---|---|---|---|---|---|---|
| Iron ore | 94.12 | 1.17 | 1.40 | 0.36 | 0.18 | 0.03 | – | 0.45 | 0.05 |
| Bentonite | 3.89 | 0.06 | 68.96 | 11.05 | 1.27 | 1.71 | 2.17 | 0.03 | 0.13 |
Figure 3 shows a schematic of the apparatus used for the carbon deposition experiment. N2, H2, and CO were supplied from the bottom of the reaction tube made of a Ni-based alloy. An electronic balance was installed at the bottom of the apparatus. Furthermore, fused silica tubes used for sample positioning were placed on the balance. The sample holder made of magnesia had 34 evenly spaced vent holes measuring 4 mm in diameter on its bottom for the gas flow. Prepared porous iron was cut out into cylindrical blocks measuring 18 mm in diameter. Four iron whisker blocks or metallic iron pellets were placed on the alumina ball layer in the holder, which was set inside the furnace. The sample was heated to 600°C at 10°C/min under N2 gas flow rate of 15 L/min. Subsequently, the supplied gas was switched to N2-10%H2 gas to remove the remaining oxide layer of the sample by holding the temperature for 30 min. The carbon deposition reaction was conducted for 100 min after purging N2 gas for 30 min by switching the supplied gas to CO at 4 L/min. The carbonization degree (C.D.) was defined as Eq. (4).
| (4) |
where Wt and Wbase are the weights of the sample during and before the carbon deposition experiment measured by the balance, respectively, and
The deposited carbon obtained from the carbon deposition experiment using porous iron was ground and sieved to a particle size of 106 μm. Its carbon content was 19.3 mass%, measured via an infrared absorption method after combustion. DCIC was prepared by press-shaping the mixture of a hematite reagent and the deposited carbon under a pressure of 90 MPa into a cylindrical shape measuring 10 mm in diameter. The mixing ratio C/O, which was defined as the molar ratio of the fixed carbon in carbonaceous material to oxygen in iron oxide, was 1.0.
The fossil fuel derived CIC was also prepared using non-coking coal (45–105 μm size) and graphite (150–250 μm size) as reducing and carburizing agents, respectively.32) The results of a proximate analysis of carbonaceous materials on air dried basis are presented in Table 1. The carbonaceous particles were well mixed with the hematite reagent such that the C/O values for the reducing and carburizing agents were 0.8 and 0.2, respectively. CIC was created via press-shaping the mixture, following the same method as that for preparing DCIC.
The composite sample was set in the experimental apparatus, as shown in Fig. 4. Following the evacuation of air from the chamber, Ar-5%N2 gas was introduced at a rate of 0.5 L/min under atmospheric pressure. The sample was heated to 1300°C at a heating rate of 10°C/min using an infrared image furnace and then cooled by turning off the power. The temperature at 1 mm above the surface of the sample was measured using an R-type thermocouple. The concentrations of CO, CO2, H2O, and N2 in the outlet gas were measured during the experiment at 1.5 min intervals via gas chromatography. N2 gas was used as a tracer to estimate the amount of gas generated from the sample. The reduction degree (R.D.) of the sample was calculated using Eq. (5).
| (5) |
where MCO,
To confirm the reproducibility, the reduction experiment was conducted twice for DCIC and CIC, respectively.
Figure 5(a) shows the changes in carbonization degree (defined as Eq. (4)) obtained for the porous iron and iron pellets with carbon deposition reaction time. The carbonization degree reached 1 at 16 and 66 min using porous iron and iron pellets, respectively. Thus, porous iron carbonized approximately 4 times faster than the iron pellets. This is attributed to the negligible diffusion resistance of the carburizing gas owing to its high porosity. Figures 5(b) and 5(c) show the appearances of porous iron as observed via a scanning electron microscope before and after following the carbon deposition reaction. The produced iron exhibited approximately 95% porosity by intertwining the iron whisker. Free carbon was deposited inside the pores of the carbonized iron substrate, as shown in Fig. 5(d). This will prevent carbon loss during transportation.
Figure 6(a) shows the changes in the reduction degree obtained for DCIC and CIC with temperature. The reduction experiment of DCIC at a laboratory scale exhibited a rapid reduction reaction at approximately 800°C. This facilitated DCIC in completing its reduction at 300°C lower than that when using CIC. Figures 6(b) and 6(c) show the appearances of DCIC and CIC heated to 1300°C, respectively. Iron nuggets were observed in DCIC. Thus, reduced iron in DCIC was melted at a lower temperature than that in CIC. Figure 7 shows a three-dimensional image of the internal structure of the reduced DCIC. Grown iron nuggets were observed in the composite. Sato et al.33) have reported that molten Fe–C decomposed from Fe3C is a supplier of carbon to reduced iron. Thus, the molten iron derived from Fe3C inside the DCIC may grow by merging with the surrounding reduced iron. Thus, CRIP-D is expected to produce hot metal more rapidly than conventional ironmaking processes such as BF.
These results mean that the proposed DCIC has a higher reactivity for both the reduction and melting than in case of a conventional CIC. The deposited carbon used in this study is derived from pure CO gas. However, in order to achieve carbon neutrality, solid carbon should be recovered from CO2. There are many trials for converting CO2 into CO, such as applying the reverse water gas shift reaction (RWGS).34,35,36,37) The typical components of reformed gas by RWGS are CO, CO2, and H2, except for H2O. It is reported that more carbon deposits occur by using a CO–CO2–H2 mixture gas than pure CO gas.38,39) Thus, CRIP-D is expected to produce hot metal more rapidly without emitting CO2 from the steel mill than conventional ironmaking processes such as BF by applying RWGS for exhausted gas.
The production of carbon-neutral primary steel is the primary advantage of CRIP-D. In addition, there are other advantages as well. For instance, CO2 need not be separated from the exhaust gas. Typical CCU processes require the capture of CO2, and the separation process restricts the pressure and temperature conditions of the system.40) However, CRIP-D exhibits greater flexibility in process design as the major exhausted gas is used as raw material for each process except for H2O.
In addition, two other benefits are obtained owing to continuous porous iron production and carbon deposition. First, the catalyst degradation is minimized. The catalytic performance decreases with longer usage time owing to pollution. However, the catalyst used in CRIP-D is continuously produced and consumed as raw material for DCIC. This material flow facilitates the use of new catalysts for gas reforming and carbon deposition reactions. The second benefit is the effective use of sensible heat. RWGS proceeds efficiently due to immediate placement in hot porous iron immediately following production into the gas reforming process as the reaction is an endothermic reaction. Although the porous iron is cooled slightly with the conduction of the gas reforming process, the carbon deposition reaction catalyzed by metallic iron proceeds significantly at 600–700°C.41) Thus, input energy into the porous iron production can be effectively used up to the carbon deposition process.
The proven technologies are easily adaptable to the CRIP-D process. For example, this process offers the advantage of being an easy step-wise transition from the conventional BF-BOF process because CIC is already used partially as a raw material for the BF process. An immediate reduction in CO2 emission is expected by inputting DCIC into the BF even in case of a small amount. Consequently, a carbon-neutral primary steelmaking process can be achieved while minimizing the burden on the steel mills by gradually converting to a process that increases the amount of DCIC.
Our group has suggested a new carbon recycling ironmaking process (CRIP) using deposited carbon-iron oxide composite (DCIC) (CRIP-D) as a new ironmaking process for hot metal production. In this study, the objective is to confirm the principle feasibility of CRIP-D. The reduction and melting behaviors of DCIC compared with typical CIC were investigated as the first step. The following results were obtained.
• The reduction experiment of DCIC at a laboratory scale exhibited a rapid reduction reaction at approximately 800°C. This facilitated DCIC in completing its reduction at 300°C lower than that when using CIC.
• Iron nuggets were observed in only DCIC heated to 1300°C. Thus, reduced iron in DCIC was melted at a lower temperature than that in CIC.
The proposed CRIP-D process has great potential for producing carbon-neutral primary steel. The findings of this study are evidence of the principle feasibility and technical advantages of CRIP-D, including its ability to produce hot metal more rapidly at lower temperatures. Thus, considering the contradiction between the inevitable use of carbon and the requirement for being carbon-neutral, the technology of the proposed CRIP-D process should aid in facilitating true carbon-neutral primary steel making processes. In this study, the raw material for carbon is recovered from pure CO. To increase the feasibility of the proposed process, the case for application of RWGS should be verified: using CO–CO2–H2 gas mixtures.
This work was supported by JSPS Grant-in-Aid for JSPS Fellows Grant Number JP22KJ0282, Steel Foundation for Environmental Protection Technology, ISIJ Research Promotion Grant and JFE 21st Century Foundation. The synchrotron radiation experiment was performed at the BL28B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2023B1692).
