Development of Metal Supported SOEC for Carbon Recycling Iron Making System

Abstract

A new solid oxide electrolysis cell (SOEC)—a metal-supported SOEC (MS-SOEC) where the SOEC is structured on a metal support and is capable of extending the cell surface area to a wider extent than a conventional ceramics base SOEC—was designed for carbon dioxide (CO₂) recycling in the ironmaking process. The MS-SOEC demonstrated CO₂ reduction and CO and oxygen production properties. The possibility of carbon cycling was examined with the CO₂ resource utilization technology using MS-SOEC and its application to the iron-making process. The required cell area of MS-SOEC for an iACRES, which combines an active carbon recycling energy system (ACRES) and a steelmaking process, was estimated using experimental results. By improving the performance of the cell, MS-SOEC was expected to be applied to a carbon recycling ironmaking system that could contribute to the establishment of a zero-emission CO₂ iron making system.

1. Introduction

The United Nations Intergovernmental Panel on Climate Change (IPCC) regards carbon dioxide (CO₂) emissions as a cause of global warming, and curbed CO₂ emissions for temperature rises of ≤ 2.0°C in 2014 and 1.5°C or less in 2018.¹⁾ In response to the IPCC, the Japanese government proposed a CO₂ emission reduction of 80% in 2050 and a roadmap for carbon recycling technoloies.²⁾ The iron and steel industry and manufacturing sector in Japan have achieved the world’s highest energy savings and CO₂ emission control. However, further activities are required to further reduce carbon in response to the demands of the times.³⁾ It is expected that this achievement will require a discontinuous innovative technological response rather than an extension of conventional technology.

Carbon has been a valuable energy source for humankind since ancient times to the present day, and it is an important energy source in the secondary industry. Therefore, the carbon cycle (carbon recycling) has been studied in various fields for several years as a means of industrially using carbon and suppressing CO₂ environmental emissions. The iron-making industry is leading the way in reducing carbon, and there are even pioneering efforts in Japan, Europe, and countries around the world.⁴⁾

The authors have proposed an Active Carbon Recycling Energy System (ACRES)⁵⁾ and have studied a steelmaking process (smart ironmaking based on the ACRES, iACRES)⁶⁾ to which the ACRES is applied. To establish a carbon cycle system, CO₂ is reduced with non-fossil primary energy, and carbon resources such as carbon monoxide (CO) are recycled and reused. It is necessary to establish a rational CO₂ resource recycling and energy reduction technologies such as CO₂ electroreduction,^7,8,9) recycling of plastic waste,^10,11) utilization of scrap metal,^12,13,14) and waste heat recovery^15,16) in this system. To establish the iACRES, we have been studying CO₂ reduction technology using solid oxide electrolysis cells (SOECs).¹⁷⁾ SOECs have been made of ceramics in the past, and it was difficult to increase the area and capacity, and most importantly, it was difficult to perform large-scale CO₂ treatment required by the iACRES.¹⁸⁾ Here, we proposed a metal-supported SOEC (MS-SOEC).¹⁹⁾ Since MS-SOEC is supported by metal, the area can be made larger and it can be stacked. Therefore, it is expected to achieve compaction and large-scale CO₂ reduction. In this study, MS-SOEC was built, and its CO₂ electrolysis was experimentally demonstrated. The possibility of a carbon cycle was examined by showing the example of CO₂ resource utilization technology by MS-SOEC and its application to the iron-making process.

2. CO₂ Recycling for the Carbon Cycle

There were various proposals for carbon recycling by recycling CO₂. Thermodynamic and industrial rationality should be the basis for selecting the method, and the carbon cycle using CO was considered as one candidate.

2.1. Active Carbon Cycle Energy System

The Active Carbon Recycling Energy System (ACRES) has been proposed as a technology that captures CO₂ as a resource, returns it, recycles it, and reuses it. A conceptual model applied to the iron-making process (Smart Ironmaking process based on an ACRES, iACRES) is shown in Fig. 1. By retrieving CO₂ emitted from the steelmaking process, reducing it to CO and others, and recycling these reduced substances as an energy carrier for iron reduction, it is possible to save carbon resources and reduce CO₂ environmental emissions.²⁰⁾ Non-fossil primary energy is essential for reduction, and renewable energy and nuclear power are alternatives worldwide. In addition, the ACRES concept can be applied not only to the iron-making process but also to the energy-consuming industrial process, and is expected to be one of the innovative methods of reducing carbon in the industry.

Fig. 1.

Structure of the iACRES system with a carbon recycling process.⁵⁾ (Online version in color.)

2.2. CO of Recycled Carbon Material

There are various candidates for carbon energy materials for the carbon cycle, and it is necessary to select a material suitable for this purpose. Figure 2 shows the relationship between the exergy ratio of carbon-based energy materials and hydrogen (η = ΔG/ΔH) (ΔG: change in reaction Gibbs free energy, ΔH: change in enthalpy) (HHV standard). η is a portion of ΔG in the material ΔH and can also be understood as the ratio of the required electric energy to the required reaction heat. The rest of ΔH is a portion of TΔS (T: temperature, ΔS: change in entropy) and thermodynamic heat loss. The η of CO is 97%, which is almost equivalent to electricity (100%), and can be said to be a high-quality energy carrier that exceeds the η (82%) of hydrogen (H₂). This system is established when the conversion loss from electricity to CO (TΔS = 0.03) is allowed and CO is more useful as an energy medium than electricity. From this figure, CO is a candidate for the circulating carbon medium. The water (H₂O)/H₂ system is the most popular system for circulating energy materials and energy carriers. Here, the CO₂/CO system and H₂O/H₂ system were reconfirmed by comparing the reaction heats.   

$$\mathrm{C}{\mathrm{O}}_2\to \mathrm{C}\mathrm{O}+1/2{\mathrm{O}}_2,\mathrm{\hspace{0.17em} }\mathrm{\Delta }H=+283\mathrm{\hspace{0.17em} }\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$(1)

  

$${\mathrm{H}}_2\mathrm{O}(\mathrm{g}) \to {\mathrm{H}}_2+1/2{\mathrm{O}}_2,\mathrm{\hspace{0.17em} }\mathrm{\Delta }H=+242\mathrm{\hspace{0.17em} }\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$(2)

Fig. 2.

Exergy ration (ΔG/ΔH) for carbon materials and hydrogen (HHV). (Online version in color.)

CO has a higher enthalpy of formation than hydrogen and a high energy density per amount of substance. For example, the reduction of iron by hydrogen is endothermic, whereas reduction by CO is exothermic and proceeds spontaneously. This also makes CO the first choice for carbon recycling materials. Pure carbon (C) is also suitable because of its high energy density since C is solid, it is easy to store and has affinity with steel processes. However, CO is easier to operate than C in distribution system. Taking into account the practical operation cost, we do not use C, and further discuss CO utilization.

2.3. Carbon Cycle with CO as a Hub

CO can be used as a basic material for carbon resource materials as well as a reducing agent. Figure 3 shows a carbon circulation system that uses electricity and hydrogen with CO as a hub. With the completion of this system, it will be possible to achieve a low-carbon, large-scale carbon cycle and resource recovery for carbon-consuming industries.

Fig. 3.

Carbon neutral system with carbon monoxide as a hub (a system combined with CO₂ electrolysis and FT synthetic method processes in series). (Online version in color.)

A hydrogenation process is indispensable for recycling CO₂ into methane, methanol, olefins, and chemical products. The methanol synthesis reaction by CO₂ direct hydrogenation is as follows.   

$$\text{CO}{}_2+3{\text{H}}_2\to {\text{CH}}_{\text{3}}\text{OH}+{\text{H}}_2\text{O,}\mathrm{\hspace{0.17em} }\mathrm{\Delta }H=-\text{49}\text{.8}\mathrm{\hspace{0.17em} }\text{kJ/mol}$$(3)

However, the direct hydrogenation of CO₂ resulted in a low reaction yield (Fig. 4). This is the problem of CO₂ resource utilization. To solve this problem, when CO₂ is reduced to CO, CO is hydrogenated (Eq. (4)) (CO hydrogenation), the reaction yield improves as shown in Fig. 4.   

$$\text{CO}+{\text{2H}}_2\to {\text{CH}}_{\text{3}}\text{OH,}\mathrm{\hspace{0.17em} }\mathrm{\Delta }H=-\text{91}\text{.0}\mathrm{\hspace{0.17em} }\text{kJ/mol}$$(4)

Fig. 4.

Equilibrium conversion for methanol synthesis from CO₂ and CO.²¹⁾ (Online version in color.)

For example, in the production of methanol at approximately 240°C under 10 MPa as the initial total pressure, the reaction yield of CO₂ direct hydrogenation is approximately 30%, while CO hydrogenation is approximately 80%. In addition, in the case of methanol production, hydrogen consumption can be reduced to 2/3 compared to CO₂ direct hydrogenation. This is because it is not necessary to capture the separated oxygen as water by a mole of H₂ (Eq. (3)) through the method that separating the generated O₂ from Eq. (1). The production methods of CO include CO₂ hydrogenation and electrolysis. Since electrolysis can produce CO with a higher selectivity than hydrogenation, and CO and oxygen can be produced independently from the cathode and anode sides without a separation process, it is suitable for practical use of Eq. (1). Renewable energy and non-fossil power, such as nuclear power, can be used as electrolysis power. Once CO production begins, it is possible to utilize the existing hydrogenation technology of the Fischer–Tropsch method (FT) for highly efficient carbon resource utilization. By recycling the emitted CO₂ into CO, a carbon recycling system that does not emit CO₂ outside the system can be formed. Therefore, it can be said that the system in Fig. 3 is a carbon circulation system combined with CO₂ electrolysis and FT synthetic method in series. Figure 3 is considered to be the basic configuration for the ultimate use of CO₂, which has high social implementation. Electricity is required for the system, however, in comparison with CO₂ direct hydrogenation one, the system can save reaction amount of H₂ which is produced generally by electrolysis, and produce hydrocarbons with higher yield, then, electricity consumption also can be saved relatively than the direct system. The CO manufacturing process is the technical key to the establishment of Fig. 3, and its development is important.

2.4. MS-SOEC

2.4.1. SOEC

The reduction of CO₂ to CO is possible with an electrolysis cell having the structure of a fuel cell. Electrolysis at higher temperatures is more efficient, and SOECs are candidates for high-temperature electrolysis.⁴⁾ SOEC applies a reverse electric potential on a cell with a solid oxide fuel cell (SOFC) structure. SOEC consists of two electrodes and a solid electrolyte. The decomposition of CO₂ into CO occurs at the cathode.   

$$\mathrm{C}{\mathrm{O}}_2+2{\mathrm{e}}^{-}\to \mathrm{C}\mathrm{O}+{\mathrm{O}}^{2-}$$(5)

The generated oxygen ions (O²⁻) diffuse and move into the solid electrolyte layer and are oxidized into O molecules at the anode.   

$${\mathrm{O}}^{2-}\to 1/2{\mathrm{O}}_2+2{\mathrm{e}}^{-}$$(6)

To build this system, non-carbon primary energy that replaces fossil fuels is required. Nuclear power is assumed to be a candidate in terms of quantity and stable output. The high temperature gas cooled reactor (HTGR) has a high output temperature and is useful as a high-quality heat source. One of the HTGRs, the high-temperature engineering test reactor (HTTR) is operated at the Japan Atomic Energy Agency (Oarai Town, Ibaraki Prefecture) to achieve the world’s highest reactor outlet gas temperature of 950°.²²⁾ HTGR is currently the most valuable technology in the world. Since the SOEC consumes both high temperature and electricity, HTGR is suitable as a primary energy source. High-temperature output of 95°C from HTGR is supplied to the SOEC as reaction heat, electric power is generated at medium temperature heat after reaction use, and high-temperature electrolysis can be performed by sending electric power to the SOEC.

2.4.2. Increasing the MS-SOEC Cell Area

To apply SOEC to a carbon recycling system, increasing the cell area is a technical issue that needs to be overcome. SOECs, such as SOFCs, use ceramics as an electrolyte and thus generally have problems in thermal shock resistance and stacking. In SOFCs using ceramic cells, which have been researched in advance, cylindrical cell aggregates (diameter less than 3 cm) are often used, and increasing the volumetric density of cells is an issue. Flat plate stacking is ideal for high density as in polymer electrolyte fuel cells, but for ceramic flat plate cells, the maximum size of one plate is approximately 10 cm², and the stacking cell-layer number should be in tens or less. In many cases, it is difficult to increase the cell area for large-scale electrolysis on the megawatt order. Moreover, because yttria stabilized zirconia (YSZ) is expensive, reducing the amount used is desirable. However, in order to maintain cell strength, the lower limit of thinning the YSZ layer is around 300 μm, which incurs a high cost. Additionally, it is difficult to form a stack of cells.

Therefore, a metal-substrate-supported SOEC (MS-SOEC) has been proposed to increase the cell area of the SOEC.¹⁹⁾ MS-SOEC is expected to have high durability against thermal and mechanical stress, and it enables large-scale cell formation and cell stacking to enable large-scale CO₂ electrolysis. This technology is expected to lead to carbon recycling steelmaking.

3. MS-SOEC Demonstration

Figures 5 (a) and 5(b) present a schematic cross-section and a sectional image of the developed MS-SOEC. The MS-SOEC has a multi-layer structure: metal substrate, metal mesh, diffusion barrier (La_0.6Sr_0.2Ca_0.2CrO₃: LSCC), cathode (NiO–YSZ), electrolyte (4.5 mol% Y₂O₃ stabilized ZrO₂: YSZ), and anode (La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ: LSCF). Both the metal substrate and metal mesh were made of SUS430. First, the metal mesh was welded onto the metal substrate by Nippon Seisen Co., Ltd. Then, additional layers were fabricated on the metal mesh in the order of LSCC, NiO-YSZ, YSZ, and LSCF by TOCALO Co., Ltd. using an atmospheric plasma spraying process. The thicknesses of the produced layers were approximately 50, 50, 300 or 500, and 50 μm, respectively, for LSCC, NiO-YSZ, YSZ, and LSCF. Figures 6(a) and 6(b) show images of the anode and cathode sides of the developed MS-SOEC, which is shaped as a coin with a diameter of 20 mm. Notably, the metal substrate was designed with seven hexagonally arranged small holes with a diameter of 3 mm to provide stability to the substrates. Figure 6(c) shows an image of metal mesh of MS-SOEC, which has a diameter less than 10 μm. In response to the thickness of electrolyte (300 or 500 μm), MS-SOEC explored were named MS300 and MS500.

Fig. 5.

(a) A schematic cross-section and (b) a sectional image of the developed MS-SOEC. (Online version in color.)

Fig. 6.

Images of (a) anode side, (b) cathode side, and (c) mesh section of MS-SOEC. (Online version in color.)

To investigate the performance of MS300 and MS500, a CO₂ decomposition experiment was conducted at 800°C using the two-terminal method. The results are shown in Fig. 7, which shows the relationship between current density and voltage. The ratio of gases fed to the cathode was CO₂:H₂:N₂ = 10:1:9, while only N₂ gas was supplied to the anode side. The total flow rate was set to 40 mL/min for both electrode sides. Despite the thickness of the electrolyte, the current started to increase from around 0.7 V. When the voltage continued to increase from 0.7 to 4 V, the current gradually increased and reached 156 and 125 mA cm⁻² at 4 V for MS300 and MS500, respectively. Improvement of the cell performance was caused by a decrease in the cell internal resistance due to the electrolyte thickness.

Fig. 7.

I–V curve of MS300 and MS500 for CO₂ electrolysis at 800°C Cathode feed gas: 20 mL min⁻¹ of CO₂, 2 mL min⁻¹ of H₂, and 18 mL min⁻¹ of N₂ Anode feed gas: 40 mL min⁻¹ of N₂. (Online version in color.)

For further investigation of the cell performance, gas analyses were conducted on both electrode sides using gas chromatography. Figure 8 shows the CO and O₂ production rates of the cells on the cathode and anode side at 800°C. Notably, the total production rate included both the production rate from CO₂ decomposition via electrolysis (CO₂ → CO + 1/2 O₂) and the reverse shift reaction (CO₂ + H₂ → CO + H₂O). The CO production rate from the electrolysis was estimated on the basis of the amount of H₂O measured, since H₂O can be produced only through the reverse shift reaction. As shown in Figs. 8(a) and 8(b), the CO production was found even when the current density was zero, and O₂ was not detected on the anode side of the cell. Therefore, it is attributed to the effect of the reverse shift reaction. When the current continued to increase, O₂ was detected indicating that CO₂ decomposition from electrolysis proceeded. Similarly, CO production from electrolysis increased with increasing current density. For instance, the highest CO production rates from electrolysis of 0.678 and 0.430 μmol cm⁻² s⁻¹ were attained for MS300 and MS500 at current densities of 165 and 123 mA cm⁻², respectively. Based on these results, it was suggested that a thin electrolysis layer is desirable from an industrial perspective.

Fig. 8.

CO and O₂ production rate of (a) MS300 and (b) MS500 in cathode and anode side at 800°C. (Online version in color.)

The Faraday efficiency for CO production was calculated using Eq. (7).   

$$\eta =r_{electrolysis}\frac{nF}{i},$$(7)

where η [%], r_electrolysis [mol cm⁻² s⁻¹], n [–], F [C mol⁻¹], and i [A cm⁻²] are the Faraday efficiency, CO production rate from the electrolysis, number of electrons transferred, Faraday constant, and current density, respectively. The Faraday efficiency estimated for CO production is shown in Fig. 9. Compared to MS500, MS300 exhibited higher Faraday efficiency for CO production, and its value was nearly 80% regardless of the current density. From a practical point of view, it is necessary to increase the cell area and stack large-area cells while overcoming issues such as electrolyte densification and performance improvement of the cell. A detailed discussion on efficiency improvement will be included in our next work. It is expected that the establishment of this technology will accelerate the social implementation of the carbon cycle system centered on CO.

Fig. 9.

Faraday efficiency for CO production of MS-SOEC at 800°C. (Online version in color.)

The utilization of MS-SOEC for the iACRES was considered based on the experimental results. In general, a large blast furnace (BF) has an internal volume of over 5000 m³ and produces approximately 10000 tons of hot metal per day.^23,24,25,26) The gas emission from the BF is 1533 Nm³ per tonne of hot metal, and the CO₂ concentration of the gas is 22.8 vol% (Table 1). Thus, the emission rate of CO₂ from the BF can be calculated as 3.5 × 10⁶ Nm³ per day. It is assumed that 30% of CO₂ produced from the BF decomposes into CO with the help of MS-SOEC. In accordance with Table 2, the required cell areas of MS300 and MS500 were estimated to be 8.00 × 10⁴ and 1.26 × 10⁵ m², respectively. Therefore, it is expected that the metal-supported SOEC can achieve a larger cell area and large-capacity CO₂ recycling for industrial usage.

Table 1. Large blast furnace specifications.²³⁾

Internal volume of BF [m3]	5200
Production rate of hot metal from BF [tonnehot metal day−1]	10000
Gas emission from BF [Nm3 $\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{n}{\mathrm{e}}_{\mathrm{h}\mathrm{o}\mathrm{t}\mathrm{\hspace{0.17em} }\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}}^{-1}$]	1533
CO2 concentration in the emission gas [vol%]	22.8
Emission rate of CO2 from BF [Nm3 day−1]	3.50 × 106

Table 2. MS-SOEC specifications (based on experimental results).

	MS300	MS500
Operation temperatre [°C]	800
Applied voltage [V]	4.0
Current density [mA cm−2]	165	123
CO production rate [μmol cm−2 s−1]	0.678	0.430
Faraday efficiency for CO production [%]	79.2	66.0

4. Conclusions

The possibility of an ACRES as the ultimate CO₂ utilization technology is described. Recycling carbon energy material by the decomposition of CO₂ is an important technical element. As a recycled carbon energy material, CO is the first choice, followed by pure carbon because of its high energy density. Renewable energy and HTGR are options for non-fossil primary energy.

In this study, MS-SOEC was built using an atmospheric plasma spraying process. CO₂ electrolysis was experimentally demonstrated, and CO₂ electrolysis by MS-SOEC was demonstrated for CO production. As an example, the required cell area of MS-SOEC for an iACRES, that combined the ACRES and conventional blast furnace process for decomposition of 30% of produced CO₂ into CO, was estimated to be approximately 8.00 × 10⁴ m², which is based on the experimental results of MS300.

In order to put SOEC into practical applications, large cell area and stack large-area cells are required.

The metal-supported SOEC is expected to achieve a larger cell area and large-capacity CO₂ recycling for industrial use.

Acknowledgement

The results of this research are closely related to the research activities of the Carbon Recycling Steel Research Group (2011–2013) and Smart Steel System Research Group (2015–2017), the Japan Iron and Steel Institute. We would like to thank everyone who supported the activities of the study groups. Furthermore, some of the results were supported by JSPS Kakenhi 16H04644.

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