Thermodynamic Feasibility of Chemical Looping CO Production from Blast Furnace Gas Based on Fe-Ca-Based Carriers

Abstract

In this paper, a novel process for synergistic carbon in situ capture and the utilization of blast furnace gas is proposed to produce CO via chemical looping. Through thermodynamic analysis, this process was studied in terms of the carbon fixation rate, CO yield, in situ CO₂ utilization rate, CH₄ conversion rate and energy consumption. It provides valuable insights for achieving efficient CO₂ capture and in situ conversion. FeO and CaO are used as the oxygen carrier and the carbon carrier, respectively. Under the conditions of reaction temperature of 400 °C, pressure of 1 bar and FeO/CO ratio of 1, the carbon capture rate of blast furnace gas can reach more than 99%. In the carbon release reactor, the CO yield is lower than that in the original blast furnace gas (BFG) if no reduction gas is involved. Therefore, methane is introduced as a reducing gas to increase CO yield. When the reaction temperature is increased to 1000 °C, the pressure level is reduced to 0.01 bar and the CH₄/C ratio is 1:1 (methane to carbon), the CO yield is four times that of the initial blast furnace gas. Under the optimal conditions, the energy consumption of the system is 0.2 MJ/kg, which is much lower than that of the traditional process. This paper verifies the feasibility of the new process from the perspective of thermodynamics.

1. Introduction

The blast furnace–converter route is the main process route of steelmaking. China produces about 1 billion tons of crude steel every year, and nearly 90% of crude steel comes from the blast furnace–converter process. And with that, about 1.8 trillion Nm³ BFG is produced. BFG is primarily composed of CO (20–28 mol%), CO₂ (17–25 mol%), N₂ (50–55 mol%) and trace H₂, O₂ and other gases, causing its low calorific value. This perspective restricts the direct application of BFG as fuel. Commonly, more BFG is mixed with natural gas or coke oven gas, which is used as gas fuel in steel rolling heating furnaces, boilers and even blast furnaces themselves. However, the majority of BFG is not utilized reasonably. For example, there is still a massive amount of BFG combusted directly at the top of blast furnace and then discharged into the atmosphere, leading to significant potential energy wastage and severe environmental implications [1]. CO has important and extensive applications in the chemical and metallurgical industries, so the production of CO from the rich BFG is of great concern. Mature methods include low temperature separation [2], pressure swing adsorption [3], Cosorb adsorption [4], copper ammonia solution [5], membrane utilization [6,7,8] and so on. However, these physical separation and purification methods have their shortcomings. It is common that other BFG components, especially greenhouse CO₂, are not used, and the energy consumption is also high. China’s steel industry accounts for about 15% of total national CO₂ emissions, and approximately 70% of the total CO₂ emission of the steel industry can be ascribed to the blast furnace process. Therefore, to achieve carbon neutrality and reduce the environmental pressure of the steel industry, low-carbon and high-quality utilization of BFG is a current necessity [9]. To convert CO₂ in BFG to CO is a promising approach, which can not only increase CO yield but also reduce CO₂ emissions.

CO₂ can be captured and converted into CO via photochemical reduction, electrochemical reduction, microwave desorption [10], radio frequency heating [11], chemical chain and other methods, namely carbon capture and utilization (CCU) technology [12,13,14]. Photocatalysis using light can reduce the temperature and pressure of CO₂ hydrogenation to produce different chemical products by assisted catalysis [15,16]. He et al. [17] found that the CO yield rate over the Cu/ZnO catalyst can reach 300 μmol g⁻¹ h⁻¹, which was higher than the sum of single light irradiation and single thermal catalysis, but there was still a limitation with extremely low conversion efficiency. Electrochemical reduction of CO₂ is also an effective way to reduce CO₂ to other high-value-added chemicals [18]. However, the slow mass transfer process during the CO₂ reaction leads to a small current density and a low reaction rate [19]. The development and utilization of high-efficiency catalysts can promote CO₂ conversion. Noble metal catalysts such as Au, Ag and Pd have good selectivity, but their scarcity makes them unable to be applied to large-scale industrial yield [20].

Chemical looping technology, making use of a solid intermediate(s) to break down a target reaction into two or more steps wherein the reaction and regeneration of the intermediate(s) occur cyclically, can realize the inherent separation of products with higher exergetic efficiencies [21,22]. Thus, chemical looping has been concerned widely in the processes of CO₂ separation and capture via calcium looping [23], combustion [24,25], gas yield (O₂ [26,27], NH₃ [28,29,30], H₂ [31,32], etc.), pollutant removal (desulfurization [33], dechlorination [34], etc.) and so on. Recent research reported different chemical looping methods to enhance the value of BFG. Sun et al. [35] proposed a system for the synergistic decarbonization and desulfurization of BFG via double simultaneous chemical chains with MgO-MnO₃ as a carbon–sulfur carrier, which focuses CO₂ separation and capture from BFG, as well as the sulfur-containing components, along with H₂ yield via the water gas shift reaction between CO in BFG and added steam, but the obtained H₂ is diluted by N₂ from BFG. Orlando et al. [36] proposed a BFG decarbonization system. Firstly, CO₂ is separated from the BFG by a calcium ring. Subsequently, CO₂ was released by an iron-based carrier (Fe₂O₃↔FeO) and converted into H₂/CO syngas through a thermochemical decomposition cycle with H₂O. Despite CO₂ separation from BFG, the remaining H₂ and CO still address the high dilution of N₂. Singh et al. [22] proposed a chemical looping CO yield process concept from BFG, with Fe₃O₄ and CaO as the oxygen carrier and the carbon carrier. In the first step, CO and H₂ in BFG is oxidized to CO₂ and H₂O with the reduction of iron valence state; meanwhile, the formed CO₂ along with the original CO₂ in BFG is captured by CaO, with N₂ discarded. In the next step, the adsorbent is decarbonized and the released CO₂ is reduced to CO with the regeneration of the iron-based material. The whole cycle is performed by temperature swing under atmospheric pressure. Further, Marian et al. [37] developed a pressure-swing chemical looping process to produce CO from BFG, also with iron-based oxygen storage materials and calcium-based CO₂ adsorbents used as solid intermediates. The above BFG chemical looping CO process can overcome the thermodynamic limitations of reverse water–gas conversion and the energy loss caused by the separation of other components from the product flow. However, the theoretical amount of CO produced is only equal to the total amount of CO and H₂ in the initial BFG, while almost equal CO₂ remains unconverted.

The integration of carbon dioxide capture and utilization (ICCU) is an emerging research hotspot in CCU [38]. This process captures CO₂ from the atmosphere, flue gas or other industrial gases, and then converts CO₂ in situ into high-value-added gas. Compared with traditional CCU technology, ICCU reduces the energy consumption from capture to conversion and mitigates the risk of transportation leakage. In the CO₂ capture step, calcium-based materials are still common CO₂ sorbents. In the subsequent conversion step, the captured CO₂ is converted with the addition of reducing gases (H₂, CH₄, etc.), and catalysts are required to improve the conversion efficiency, such as Ni, Ru, etc. Usually, the catalyst and the carbon carrier are integrated, which are the bifunctional material. Xie et al. [39,40] used CaO as an adsorbent to enhance the high-quality utilization of reforming coke oven gas for hydrogen production. Farrauto et al. [41] synthesized bifunctional materials using calcium oxide as an adsorption component and ruthenium as a catalytic component to capture CO₂ from flue gas and then convert it into CH₄ by injecting H₂ as a reduction gas. Kim et al. [42] immobilized CaO and Ni on MgO-Al₂O₃ to create a bifunctional material, with methane serving as the reducing gas to convert CO₂ into CO via methane dry reforming. Jin et al. [43] physically mixed CaCO₃ and Fe₂O₃ as CO₂ carriers and oxygen carriers, respectively, to achieve comprehensive CO₂ capture and utilization in one system. Zhao et al. [44] use Fe\Cu as oxygen carrier and CaO\MnO as a carbon carrier to pass into CH₄ for CO₂ chemical chain capture and utilization to prepare H₂ and CO.

According to the above, we develop a novel chemical looping CO production process from BFG coupled with ICCU, which is illustrated in Figure 1. This process integrates two chemical chains simultaneously, carbon carrier looping (Me_xO↔Me_xCO₃) for carbon capture and release, and oxygen carrier looping (MO_y↔MO_y−n) for conversion between CO and CO₂. The process integrates two chemical chains simultaneously: the oxygen carrier cycle (MO_y↔MO_y−n) for the conversion between CO and CO₂, and the carbon carrier cycle (Me_xO↔Me_xCO₃) for carbon capture and release. First, in the carbon fixation process, CO input into the BFG is oxidized to CO₂ by the oxygen carrier MO_y, which is reduced to MO_y−n, and the resulting CO₂ is captured by the Me_xO carbon carrier along with the CO₂ already in the BFG. In this step, all the carbon in the BFG is immobilized in the carbon carrier. During the carbon release process, the CO₂ released from the carbon carrier Me_xCO₃ at high temperature reacts with the reducing gas passed through to produce syngas. At this time, the oxygen carrier and the carbon carrier exist in the form of MO_y−n and Me_xO, respectively, and the carbon carrier completes the cycle. To realize the circulation of oxygen carriers, it is necessary to introduce an oxidation reactor, in which the oxygen carriers are regenerated under oxygen-poor conditions. Compared with the traditional BFG separation and purification technology [45], the novel process can realize the complete decarbonization of BFG with all carbon being able to convert to CO.

In this paper, the feasibility of this new process is investigated via thermodynamic analysis. First, the reaction equilibrium characteristics of the carbon fixation step and the carbon release step are emphasized, as well as the effects of reaction temperature, pressure, and the addition of carbon carrier, oxygen carrier, and reducing gas. The energy consumption of the whole process is then investigated and compared with conventional CO₂ production processes. This work provides the theoretical feasibility for the practical application of the chemical cycle CO₂ production process from BFG to ICCU.

2. Methodology

2.1. Process Model Description

In this paper, the feasibility of the practical application of the chemical looping CO production process from BFG coupled with ICCU is explored from the perspective of thermodynamics, based on the Gibbs energy minimization principle. The composition of BFG is shown in

Table 1. Composition of BFG.

Component	N2	CO	CO2	H2
Content (mol%)	50	23	25	2

. Oxygen carriers and carbon carriers play a crucial role in chemical looping cycles, determining the primary reaction performance and degree. Considering the wide application of iron-based oxygen carriers and calcium-based carbon carriers, Fe/FeO and CaO/CaCO₃ are used as oxygen and carbon carriers in this paper.

The reaction equilibrium products of each step in this novel process are calculated by using the component equilibrium module of HSC 6.0 software, with 100 kmol BFG as research object. Firstly, for the carbon fixation process, the effects of temperature, pressure, CaO/C ratio (the mole ratio of the CaO addition to the total carbon in BFG including CO and CO₂) and FeO/CO ratio (the mole ratio of the FeO addition to the CO in BFG) are explored to obtain the product distribution under the optimal reaction conditions. Secondly, the equilibrium components from the fixed carbon process are introduced into the carbon release process to explore the effects of different reaction conditions (temperature, pressure, etc.) on the CO yield. Significantly, in this step, the oxygen carrier reduced in the last step cannot convert the total CaCO₃ or CO₂ released from CaCO₃. Therefore, methane as reducing gas is introduced into the carbon release process for methane dry reforming in order to convert all the carbon sources from BFG to CO [41]. Therefore, the cases without and with CH₄ introduction are compared, and the different CH₄ additions are also discussed. The main chemical reactions of carbon fixation and carbon release processes are shown in Table 2.

In order to verify the feasibility and superiority of the new process, the energy consumption under optimum reaction conditions was explored and was also compared with the conventional BFG adsorption separation process and CH₄ dry reforming process for CO production. The material and energy relationships in the novel CO production system are shown in Figure 2. Throughout the reaction process, energy input is required to heat the reaction feedstock and to maintain the set reaction temperatures. In order to minimize the energy input, the high-temperature gas produced in the system is cooled to recover the waste heat. The total energy consumption of the system (Q_Total) mainly includes four parts: carbon fixation reactor energy consumption (Q_R), carbon release reactor energy consumption (Q_O), oxidation reactor energy consumption (Q_C) and waste heat recovery (Q_Re). A positive sign in Q_Total, Q_R, Q_O and Q_O indicates that heat needs to be supplied to it, and a negative sign indicates that heat is given off.

$$Q_{T\mathrm{o}tal}=Q_R+Q_O+Q_C-Q_{R\mathrm{e}}$$

(1)

Table 2. The main chemical reactions of carbon fixation reactor and carbon release reactor.

Reaction Equation	${∆\mathit{H}}_{273.15\mathit{K}}^{\mathit{\theta }}$/kJ·mol−1	Numbering
The main chemical reactions in the carbon fixation reactor
FeO + CO(g) = Fe + CO2(g)	−15.262	(2)
3FeO + CO2(g) = Fe3O4 + CO(g)	−30.410	(3)
2FeO + CO2(g) = Fe2O3 + CO(g)	−5.341	(4)
4FeO = Fe + Fe3O4	−45.671	(5)
CaO + CO2(g) = CaCO3	−178.255	(6)
The main chemical reactions in the carbon release reactor
CaCO3 = CaO + CO2(g)	+178.255	(7)
CO2(g) + Fe = CO(g) + FeO	+15.262	(8)
3CO2(g) + CH4(g) = 4CO(g) + 2H2O(g)	+328.476	(9)
CO2(g) + CH4(g) = 2CO(g) + 2H2(g)	+245.931	(10)
CH4(g) = 2H2(g) + C	+73.843	(11)
C + CO2(g) = 2CO(g)	+172.084	(12)
3FeO + CH4(g) = 3Fe + CO(g) + 2H2O(g)	+282.687	(13)
3Fe3O4 + CH4(g) = 9FeO + CO(g) + 2H2O(g)	+419.703	(14)

Table 2. The main chemical reactions of carbon fixation reactor and carbon release reactor.

Reaction Equation	${∆\mathit{H}}_{273.15\mathit{K}}^{\mathit{\theta }}$/kJ·mol−1	Numbering
The main chemical reactions in the carbon fixation reactor
FeO + CO(g) = Fe + CO2(g)	−15.262	(2)
3FeO + CO2(g) = Fe3O4 + CO(g)	−30.410	(3)
2FeO + CO2(g) = Fe2O3 + CO(g)	−5.341	(4)
4FeO = Fe + Fe3O4	−45.671	(5)
CaO + CO2(g) = CaCO3	−178.255	(6)
The main chemical reactions in the carbon release reactor
CaCO3 = CaO + CO2(g)	+178.255	(7)
CO2(g) + Fe = CO(g) + FeO	+15.262	(8)
3CO2(g) + CH4(g) = 4CO(g) + 2H2O(g)	+328.476	(9)
CO2(g) + CH4(g) = 2CO(g) + 2H2(g)	+245.931	(10)
CH4(g) = 2H2(g) + C	+73.843	(11)
C + CO2(g) = 2CO(g)	+172.084	(12)
3FeO + CH4(g) = 3Fe + CO(g) + 2H2O(g)	+282.687	(13)
3Fe3O4 + CH4(g) = 9FeO + CO(g) + 2H2O(g)	+419.703	(14)

2.2. Process Evaluation

In this paper, the CO yield, in situ CO₂ conversion, CH₄ conversion rate, C distribution and Fe distribution of syngas are studied, and the relevant definitions are as follows:

Carbon fixation reactor:

(1) C distribution, D_C, the molar ratio of each C-containing substance to the total amount of C in the carbon fixation reactor.

$$D_C={\displaystyle \frac{n_{C-i}}{n_{C-total}}}\times 100\%$$

(15)

where n_C-i is the molar amount of each carbonous substance (CO, CO₂, etc.) in the reactor, kmol and n_C-total is the molar amount of C entered, kmol. When C_i is CaCO₃, the carbon distribution (D_C) is the carbon fixation rate (F_C).

(2) Fe distribution, D_Fe, the molar ratio of Fe in each Fe-containing substance to the total amount of Fe in the reactor.

$$D_{F\mathrm{e}}={\displaystyle \frac{n_{Fe-i}}{n_{Fe-total}}}\times 100\%$$

(16)

where n_Fe-i is the molar amount of Fe in each Fe-containing substance (Fe, FeO, Fe₂O₃, Fe₃O₄) in the reactor, kmol and n_Fe-total is the amount of Fe input, kmol.

Carbon release reactor:

(1) CO yield rate, Y_CO, the molar ratio of CO produced in the carbon release process to the initial input C.

$$Y_{CO}={\displaystyle \frac{n_{co}}{C_{total}}}\times 100\%$$

(17)

where n_co is the produced CO molar amount, kmol and C_total is the amount of total C (CO₂, CO, CaCO₃, etc.) in the system, kmol.

(2) In situ CO₂ utilization rate, U_CO2, the molar ratio of the CO₂ converted into syngas to the initial input C.

$$U_{CO2}={\displaystyle \frac{n_0-R_{CO2}-R_{CaCO3}}{n_0}}\times 100\%$$

(18)

where n₀ is the initial input CaCO₃ molar amount, kmol and R_CO2 and R_CaCO3 are, respectively, the molar amounts of CO₂ and CaCO₃ remaining after carbon conversion, kmol.

(3) CH₄ conversion rate, C_CH4, and the molar ratio of the CH₄ converted to the input CH₄.

$$C_{CH4}={\displaystyle \frac{n_{CH4}-R_{CH4}}{n_{CH4}}}$$

(19)

where n_CH4 is the initial input CH₄ molar amount, kmol and R_CH4 is the molar amount of CH₄ after carbon conversion, kmol.

In the absence of reduction gas, the definition of the distribution of carbon is consistent with that in the carbon fixation reactor (Equation (15)). However, when CH₄ is utilized as a reducing gas, carbon deposition occurs, and the rate of carbon deposition (CD_C) equates to Equation (16) where C_i represents solid C.

3. Results and Discussion

3.1. Product Characteristic of Carbon Fixation Step

3.1.1. Effect of CaO/C

Under different CaO/C ratios, the effect of reaction temperature on the F_C is shown in Figure 3. The F_C change trend is the same under different CaO/C. At low temperatures (≤400 °C), the carbon fixation rate is maintained at about 1. Then, F_C decreases with increasing temperature, attributed to CaCO₃ decomposition at high temperature. At this time, the F_C increases with the increase in CaO/C ratio. The increase in the CaO is conducive to Equation (6), but the carbon fixation characteristics at low temperatures are almost the same. Considering the economy and availability, the optimal CaO/C ratio was determined to be 1.

3.1.2. Effect of Temperature

The effect of reaction temperature on the distribution of carbon fixation products is shown in Figure 4. Under a different FeO/CO ratio, the change trend of F_C with temperature is consistent. At low temperature, there is basically no residual CO₂ and CO in the system, and the F_C is close to 1, as shown in Figure 3. At 400 °C, the F_C is 99.1%. As the temperature increases, CaCO₃ decomposes and the fixed carbon content decreases. Simultaneously, higher temperatures inhibit the exothermic Equation (2), leading to a gradual decrease in Fe yield and a gradual increase in FeO yield. When the temperature exceeds 1050 °C, carbon primarily exists in the form of CaO and all the adsorbed CO₂ is released. Therefore, it can be concluded that the optimal reaction temperature for the fixed carbon step is 400 °C.

3.1.3. Effect of FeO/CO and Pressure

From Figure 3, at 400 °C, the F_C reaches approximately 1 under various FeO/CO ratios. However, an increase in the FeO/CO ratio leads to the formation of other iron oxides. Figure 5 demonstrates the impact of FeO/CO and pressure on F_C. Across different pressures, F_C gradually increases with the increase in FeO/CO, but increasing FeO has little effect on F_C. At the FeO/CO ratio of 1, the yield of Fe can reach over 99%. Considering the utilization efficiency of raw materials comprehensively, the optimal FeO/CO ratio was determined to be 1. With increasing pressure, F_C exhibits minimal change. However, increasing the pressure increases the energy consumption correspondingly, so the optimal reaction can be determined to be 1 bar.

3.2. Product Characteristic of Carbon Release Step

3.2.1. With Absence of Reducing Gas

Effect of Temperature

Figure 6 shows the variation of D_C and D_Fe with temperature in the carbon release reactor. At low temperatures (≤600 °C), C mainly exists in the form of CaCO₃, and no CO is produced in the system. At this time, the reverse reaction of Equation (5) occurs, and the yield of FeO gradually increases. As the temperature continues to rise, the decomposition of CaCO₃ accelerates, promoting Equation (6), and Y_CO gradually increases. When the temperature reaches 900 °C, the decomposition of CaCO₃ is complete, and the yield of CO and CO₂ has little change as the temperature continues to rise. Therefore, the optimum temperature for releasing carbon is 900 °C. However, at this time, the Y_CO is only 37.91%, which is even lower than the original CO of BFG.

Effect of Pressure

Figure 7 shows the variation of D_C and D_Fe with pressure in the carbon release reactor. With the decrease in pressure, Y_CO increases gradually. This is because low pressure is more conducive to the decomposition of CaCO₃, and the generated CO₂ promotes Equation (8). When the reaction pressure is 0.1 bar, there is almost no CaCO₃ residue in the reactor. However, the yield of Y_CO is only 37.91%, which is still lower than the original Y_CO of BFG. At this time, there is still a large amount of CO₂ in the reactor, which needs to be further converted.

3.2.2. With Presence of Reducing Gas

It can be seen that the yield of CO could not meet the requirements of separation and purification, so it is necessary to further increase the yield of CH₄, which has strong reducing properties and produces only the impure H₂O in the reduction process, which can be removed by a simple condensation process. Therefore, the reduction gas CH₄ is introduced to reduce the remaining CO₂ to increase the yield of CO in the carbon release step.

Effect of Temperature

The effects of temperature on D_C and D_Fe, yield and conversion rate in carbon release reactor are shown in Figure 8 and Figure 9. Under different CH₄/C ratios, Y_CO, U_CO2 and C_CH4 gradually increase, while CD_C first increased and then decreased. At low temperatures, Y_CO and U_CO2 do not change significantly, while CD_C and C_CH4 increase gradually. This is because CaCO₃ is difficult to decompose at low temperatures, and CH₄ is gradually converted into solid carbon. Further increasing the temperature, the CO₂ produced by the decomposition of CaCO₃ promotes Equations (10) and (12), resulting in an increase in Y_CO and U_CO2 but a decrease in CD_C. When the temperature reaches 1000 °C, increasing the temperature has little effect on Y_CO, U_CO2 and C_CH4. With the increase in temperature, FeO yield first increased and then decreased. This is because at low temperatures, small amounts of Fe and Fe₃O₄ are converted to FeO. With the increases in temperature and CH₄, the reaction rates of Equations (10) and (13) are accelerated, and the Fe yield gradually increases. However, as the CH₄/C ratio increases, carbon deposition increases. Therefore, the CH₄/C ratio should not be too large.

Effect of CH₄/C and Pressure

The effects of reaction pressure on U_CO2, Y_CO and CD_C under different CH₄/C ratios are shown in Figure 10. Under different pressures, U_CO2 and Y_CO gradually increase with the increase of CH₄. This is because increasing the CH₄/C ratio promotes the conversion of CH₄ to CO. When CH₄/C is 1:1, Y_CO reaches the maximum value, U_CO2 is above 90% and CD_C gradually decreases to 0 with the reduction in pressure. With the increase of the CH₄/C ratio, U_CO2 and Y_CO did not change significantly but CD_C increased. Therefore, the optimal CH₄/C ratio is 1. Reducing pressure is beneficial to the reforming Equation (10), but reducing pressure increases energy consumption. Considering the energy efficiency and economy comprehensively, the reaction pressure is 0.01 bar.

In summary, the optimum reaction temperature in the carbon release step is 1000 °C, the optimum reaction pressure is 0.01 bar and the optimum ratio of CH₄/C is 1:1. The C in the BFG was almost completely converted, and the CO yield reached 99%, which was higher than the CO stream rich in 62.2 mol% CO and 37.8 mol% CO₂ produced using the PS-RWGS process [37]. However, at this time, the yield of FeO in the reactor is close to zero, and almost all of it is converted to Fe. Therefore, it can be considered to oxidize Fe to FeO by introducing oxidizing gas.

3.3. System Energy Consumption

Through the above analysis, the best carbon release reaction parameters were obtained: The carbon release reaction temperature was 1000 °C, the reaction pressure was 0.01 bar, and the CH₄/C ratio was 1: 1. The raw material temperature is set at 25 °C, without considering heat loss. At higher temperatures (T ≥ 575 °C), iron is converted to FeO in a low-pressure oxygen environment (air atmosphere). It is assumed that all Fe is oxidized to FeO, and the reaction temperature is 575 °C.

The total energy consumption of the system is presented in

Table 3. Total energy consumption of CO₂ in situ capture and conversion process system.

QR	QO	QC	QRe	QTotal
−2.35 MJ/kg	11.06 MJ/kg	−2.06 MJ/kg	6.45 MJ/kg	0.2 MJ/kg

. Q_R represents the total heat input required to heat the BFG and the heat released by the exothermic reaction in the reactor. Similarly, the energy consumption Q_O of the carbon release reactor was calculated to be 11.06 MJ/kg, including heating oxygen carrier, carbon carrier and reducing gas CH₄. The energy consumption of various endothermic reactions is discussed. Q_C is the sum of the heat of oxidation reaction and the heat required to heat the raw material (25 °C). Qtv_Re refers to the sum of heat recovered by cooling the syngas discharged from the carbon fixation reactor (400 °C) and the carbon release reactor (1000 °C) to 200 °C. It can be seen that under the optimal reaction parameters, the total energy consumption of the system is 0.2 MJ/kg. The process is equivalent to the BFG pressure swing adsorption separation, the purification of CO and methane dry reforming. According to the reference [46,47], the energy consumption of CO production by pressure swing adsorption is 2.88 MJ/kg. The energy consumption per unit mass CO₂ via methane dry reforming is 1.24 MJ. The total energy consumption of the new process is superior to that of the traditional process, which has advantages and application feasibility.

4. Conclusions

Aiming at the new chemical ring-forming CO process coupled with BFG and ICCU proposed in this paper, the process of carbon release and carbon fixation is explored from the thermodynamic point of view. The results show that when the fixed carbon temperature is 400 °C, the pressure is 1 bar and the FeO/CO ratio is 1, the F_C can reach 99%. In order to obtain higher Y_CO, the reducing gas CH₄ was introduced. When CH₄/C = 1, the reaction temperature is 1000 °C and the pressure is 0.01 bar. The C in the BFG is almost completely converted, and the Y_CO is as high as 99%, which is approximately four times that of the initial. Under these conditions, the evaluation of the total energy consumption shows that the system is more energy efficient than the traditional separation and purification process. Therefore, the research in this paper effectively verifies the feasibility of the new process. However, it should be noted that the total energy consumption calculated in this study is based on theoretical calculations. At the same time, the principles of material saving and energy saving are considered.