Development and Application of the Operating Line for the CHORSF Process

Abstract

To achieve carbon emission reduction in the long ironmaking process with blast furnace-basic oxygen furnace (BF-BOF), the Hebei Iron & Steel Group and Northeastern University have jointly developed the Reduction Smelting Furnace with Carbon-Cycling, Hydrogen-Rich, and Pure-Oxygen (CHORSF) ironmaking process. This new process employs advanced technology to overcome the hydrogen enrichment limitation of traditional BFs and the problems of “hot at the lower part and cold at the upper part” in all-oxygen BFs. This paper establishes an operating line for the CHORSF ironmaking process, systematically analyzes the influence mechanisms of key smelting parameters on CHORSF, and provides guidance for optimizing the process. The results show that the slopes of the operating lines in the indirect reduction zone can characterize the reducing gas consumption under actual conditions; under the smelting conditions of this study, the reducing gas consumption falls within a specific range. The slope of the operating line in the softening–melting–dripping zone can be used to quantify the coke ratio. Furthermore, increasing the metallization ratio at the bottom of the indirect reduction zone leads to a slight increase in reducing gas consumption, while a 1% increase in the same metallization ratio results in a notable decrease in the coke ratio.

1. Introduction

Under the guidance of the dual-carbon goals, metallurgy, as a high energy-consuming and high-emission industry [1], urgently requires green transformation. At present, the blast furnace–basic oxygen furnace (BF-BOF) long process remains the main process for steel production in China and even the world. There are mainly two kinds of low-carbon technologies for BF production: one is hydrogen-rich injection into the BF [2,3] and the other is to achieve the recycling of top gas by using a pure oxygen blast [4,5]. However, both of these technological paths face their own challenges in industrial application. Due to the small molecular size and high diffusion coefficient of H₂, it exhibits superior gas–solid mass transfer capabilities compared to CO, resulting in a higher reaction rate with iron oxides [6]. However, the reduction reaction of H₂ with iron oxides Fe₃O₄ and FeO is an endothermic reaction [6], which limits the proportion of hydrogen-rich injection in traditional BFs [7,8]. As for the pure-oxygen blast furnace using the top gas cycling technology, it will cause the problem of “hot at the lower part and cold at the upper part” [9,10,11].

In order to overcome the aforementioned problems, Northeastern University and the Hebei Iron & Steel Group have comprehensively adopted carbon reduction methods such as “Hydrogen-rich injection” and “Top gas cycling + CCUS” and proposed the Reduction Smelting Furnace with Carbon-Cycling, Hydrogen-Rich, and Pure-Oxygen (CHORSF) ironmaking process in this paper. The core equipment of this process is the reduction melting furnace, which has a clearly defined Indirect Reduction Zone (IRZ) and Softening Melting Dripping Zone (SMDZ). However, the industrial entity of the CHORSF ironmaking process is not yet in operation and still faces challenges in its promotion and industrialization. Therefore, a tool is needed that can visually characterize the intrinsic relationship between CHORSF’s raw materials and reducing gas consumption. This tool can study the impact of different smelting parameters on the coke ratio and reducing gas consumption, providing a theoretical basis for operational optimization of CHORSF.

The Rist operating line, based on the three elements Fe–O–C, is a tool that can intuitively reflect the intrinsic relationships among ore reduction, carbon consumption, and gas utilization, and it has been widely applied in the analysis and optimization of the BF ironmaking process. With the development of hydrogen metallurgy, researchers have incorporated hydrogen into the operating line system by establishing the Fe–O–C–H system [12,13,14]. The following is the previous application of the traditional Fe–O–C–H system operating line: Ono [15] and Ujisawa [16] studied the carbon reduction effect of injecting CH₄ in BF based on the operating line and Manuel [17] modified the traditional Rist operating line to characterize the oxygen BF using “top gas cycling” technology and analyzed its energy-saving and carbon-reduction potential using the modified operating line. The Rist operating line plays a significant role in modern ironmaking theory and engineering practice; Zhang [18] analyzed the potential of saving coke through pulverized coal and natural gas injection into the BFs using the hydrogen-rich operating line. Sun [19] established a Rist operating line for hydrogen-rich BFs, and the results indicated that hydrogen had a significant impact on the slope of the operating line. However, the traditional Rist operation line is mainly used to characterize the traditional BF ironmaking process, and lacks applicability to new smelting reduction processes with a high hydrogen injection ratio and distinct functional zones (e.g., COREX). Therefore, it is necessary to refine the traditional Rist operating line.

The COREX process consisting of a pre-reduction shaft furnace and a melting gasifier, can provide a reference for the CHORSF process. Previous researchers used the method of establishing the operating line in different zones to study and optimize it. Srishilan et al. [20] established a Rist operating line suitable for the COREX process and analyzed the influence of different metallization rates on the slope of the operating line. Du et al. [21] used the Rist operating line established by zone to study the influence of the metallization rate and gas composition entering the furnace on the COREX shaft furnace. Zhan et al. [22,23] proposed the measures of increasing the DRI metallization rate by using the Rist operating line established by zone.

In this paper, an operating line suitable for the CHORSF is established by zone. This operating line can characterize CHORSF’s key smelting parameters (such as the coke ratio, reducing gas consumption). Furthermore, this paper analyzed the impact of different metallization ratios at the bottom of the IRZ on coke ratio and reducing gas consumption, providing a theoretical basis for process operation optimization and carbon emission reduction potential assessment.

2. The CHORSF Ironmaking Process and Its Mass–Energy Balance Model

2.1. The CHORSF Ironmaking Process

The all-oxygen hydrogen-rich low-carbon reduction melting furnace ironmaking process [24] is shown in Figure 1. This process employs technologies such as hydrogen-rich reducing gas injection into the upper furnace, significantly increasing the metallization rate of iron-bearing materials in the IRZ to 85–90% while reducing direct reduction and consumption of coke. The use of pure oxygen not only greatly enhances the smelting intensity of the furnace but also enables carbon recycling through top gas circulation, ultimately achieving over 40% CO₂ emission reduction.

Figure 1. Schematic of CHORSF ironmaking process.

The COURSE50 ironmaking process, proposed by Japan in 2008 [25] has a carbon reduction pathway that focuses on two core strategies: first, internal decarbonization within the BF, primarily by injecting hydrogen to replace part of the coke, thereby reducing coke consumption; second, CO₂ capture from the top gas, which can subsequently be utilized for storage (CCS, Carbon Capture and Storage) or resource recovery (CCU, Carbon Capture and Utilization). The process aims to ultimately achieve a 30% reduction in total CO₂ emissions across the entire production chain.

Compared with this process, the CHORSF ironmaking process proposed in this study not only integrates “hydrogen-rich” and “top gas recycling” technologies but also features a unique reactor design that allows higher hydrogen injection ratios and more flexible control of gas composition. This design is expected to enable deeper carbon reduction beyond the levels achievable with COURSE50.

2.2. The Mass–Energy Balance Model of CHORSF Ironmaking Process [24]

The mass–energy balance model of the ironmaking process is developed based on the constraints of the mass–energy balance equations. This model includes: the mass balance and heat balance model of the CHORSF, the mass balance and heat balance model of the CHORSF by zone, and the parameter matching model of the entire process. This mass–energy balance model is suitable for operating conditions with a high proportion of hydrogen. The operating conditions corresponding to the cases mentioned in the following article are all greater than 40%, which meets the requirements of the process model. The following cases are completed under the conditions that the corresponding metallization rate can be achieved. The limitations include raw material conditions, injection position, furnace body height, operating conditions, etc., and further design optimization can be carried out through kinetic research in the future.

According to the constraint conditions of the model, the results satisfying the process balance of the whole process under different operation parameters are calculated, and through iterative calculations, the results of the mass and heat balance for the IRZ and SMDZ inside the CHORSF are obtained. This model can output data such as fuel consumption, furnace body reducing gas injection volume, and furnace top gas composition, providing the foundation for plotting the operating line of the CHORSF.

Our team previously conducted sensitivity analyses on the MR_IRZ, the volume of reducing gas injected into the shaft, and the gas temperature, and published the relevant findings in 2023 [24]. The results show that with every 1% increase in MR_IRZ, coke consumption decreases by 5.95 kg/tHM, while the volume of reducing gas injected into the shaft increases by 7.13 Nm³. With every 1% increase in the CO proportion of the shaft reducing gas, CO₂ emissions increase by approximately 7.53 kg/tHM, and energy consumption increases by 2.68 MJ/tHM. Regarding gas temperature, the temperature of the injected reducing gas has a certain impact on CO₂ emissions at low CO concentrations, but this effect diminishes as the CO concentration increases. In view of the CHORSF industrial entity that is not yet in operation, this process draws on industrial data from mature processes for segmented verification.

2.3. Data Required for Model Calculation

The compositions of the raw materials in this paper are listed in Table 1, Table 2, Table 3 and Table 4.

Table 1. Composition of ore (mass fraction), %.

Composition	TFe	FeO	CaO	MgO	SiO2	Al2O3	MnO	P2O5	TiO2
ore	65.91	0.55	2.63	0.3	2.2	0.39	0.15	0.07	0.065

Table 2. Fe oxides in minerals (mass fraction), %.

Composition	TFe	FeO	Fe2O3	Fe3O4
Mass Fraction	65.91	0.55	93.55	0

Table 3. Composition of flux (mass fraction), %.

Composition	TFe	CaO	MgO	SiO2	Al2O3	MnO	SO3	P2O5	CO2
Limestone	0	55.36	0.9	1	0.14	0	0.1	0	44.49

Table 4. Composition of coke (mass fraction), %.

Composition	C(Fix)	Ash					H2O	Volatile
		Fe2O3	CaO	MgO	SiO2	Al2O3		H2	CO2	N2	CH4
Coke	87.301	0.762	0.451	0.113	5.321	4.027	0.43	0.12	0.28	0.64	0.02

The following assumptions are based on similar process models (e.g., COREX) and can be applied to all operating conditions:

(1)

The volume utilization coefficient of CHORSF is set as 2.5 t/(m³·d) [26];

(2)

Sulfur in the CHORSF goes into the top gas, hot metal, and slag. The distribution ratio of sulfur in the top gas ($w_{\mathrm{s}\_\mathrm{top}}$) is 5%, the content of sulfur in the hot metal is 0.025%, and the others enter the slag [27];

(3)

The methane production ratio (${\phi }_{{\mathrm{CH}}_4}$) is 1% of the total carbon consumption in the SMDZ;

(4)

The temperature of the hot metal and slag are set to 1500 °C and 1550 °C, respectively;

(5)

In order to ensure the fluidity of the slag, the binary basicity (R₂) of the slag is set to 1.15 [28];

(6)

According to the operation experience of the shaft furnace, the reduction potential (E) of the top gas is not less than 1.22 [29].

2.4. Calculation Results of the Mass–Energy Balance Model

The smelting parameters required for plotting the operating line of CHORSF, generated by the mass–energy balance model of CHORSF under specific process conditions, are listed in Figure 2 and Table 5, Table 6, Table 7 and Table 8.

Figure 2. CHROSF ironmaking process mass and energy data.

Table 5. Composition of hot metal (mass fraction), %.

Composition	Fe	C	Si	Mn	P	S
Hot Metal	94.73	4.63	0.5	0.086	0.02	0.025

Table 6. Composition of top gas (volume fraction), %.

Composition	CO	CO2	H2	H2O
Top Gas	28.76	23.43	32.05	15.76

Table 7. Composition of the reducing gas into the IRZ (volume fraction), %.

Composition	CO	H2
Reducing Gas	52	48

Table 8. Additional parameters required for plotting the CHORSF operating line.

Item	Unit	Value
Mass of Carbon Combusted in the Tuyere	kg/t HM	203.47
Oxygen Consumption	Nm3/t HM	189.90
Gas Produced by Coke Combustion	Nm3/t HM	427.32
Temperature of Gas Entering the IRZ from Outside	°C	950
Oxygen Heat Entering the Furnace	kJ/m3	37.42
Heat Carried Away by CO Generated from Combustion in the SMDZ	kJ/m3	1403.86
Slag Ratio	kg/t HM	120
Sulfur Mass Fraction in Slag	%	1.31
Metallization Rate of the Ore at the Bottom of IRZ (MRIRZ)	%	90

3. Establishment the Operating Line of the CHORSF

The reduction smelting furnace of this process has a clearly defined IRZ and SMDZ. First, the operating line for IRZ and SMDZ are established respectively, and finally the two models are connected to construct the operating line for the CHORSF.

3.1. Operating Line for the IRZ

(1)

Determine the coordinates of key characteristic points.

According to the results of the mass and heat balance for the IRZ, the coordinates of key characteristic points are determined via the following steps to establish the operating line for the IRZ:

a.

Point T

According to the composition of the reducing gas (${\phi }_i^1$) and the MR_IRZ, the coordinates of point T are as follows:

$$X_{\mathrm{T}}=1+\left({\phi }_{{\mathrm{CO}}_2}^1+{\phi }_{{\mathrm{H}}_2\mathrm{O}}^1\right)$$

(1)

where ${\phi }_{{\mathrm{CO}}_2}^1$ denotes the volume fraction of CO₂ content (%) and ${\phi }_{{\mathrm{H}}_2\mathrm{O}}^1$ denotes the volume fraction of H₂O (%) at the bottom of IRZ.

$$Y_{\mathrm{T}}=1.056X_{{\mathrm{Fe}}_x\mathrm{O}}=1.056\times (1-{\mathit{MR}}_{\mathrm{IRZ}})$$

(2)

where $X_{{\mathrm{Fe}}_x\mathrm{O}}$ denotes the mass fraction of Fe_xO in ore at the bottom of the IRZ and MR_IRZ denotes metallization rate of the ore at the bottom of the IRZ.

b.

Point A

According to the top gas composition of the furnace (${\phi }_{i\_\mathrm{top}}$) and the content of charged minerals, the coordinates of point A are as follows:

$$X_{\mathrm{A}}=1+\left({\phi }_{{\mathrm{CO}}_2\_\mathrm{top}}+{\phi }_{{\mathrm{H}}_2\mathrm{O}\_\mathrm{top}}\right)$$

(3)

where 2. is the furnace top gas (%) and ${\phi }_{{\mathrm{H}}_2\mathrm{O}\_\mathrm{top}}$ denotes the volume fraction of H₂O in the furnace top gas (%).

$$Y_{\mathrm{A}}={\displaystyle \frac{\left[w\left({\mathrm{Fe}}_2{\mathrm{O}}_3\right)\times 0.3+w\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)\times 0.276+w\left(\mathrm{FeO}\right)\times 0.222\right]/16}{w\left(\mathrm{Fe}\right)/56}}$$

(4)

where $w\left({\mathrm{Fe}}_2{\mathrm{O}}_3\right)$, $w\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)$, $w\left(\mathrm{FeO}\right)$, and $w\left(\mathrm{Fe}\right)$ denote the mass fraction of Fe₂O₃, Fe₃O₄, FeO, and Fe in the total feed ore (%), respectively.

c.

Chemical Equilibrium Limit Point W

In CHORSF, the two chemical reactions for the reduction of wüstite in the IRZ are as follows:

$$\mathrm{FeO}+\mathrm{CO}\to \mathrm{Fe}\left(\mathrm{s}\right)+{\mathrm{CO}}_2; \lg K_{{\mathrm{p}}_1}={\displaystyle \frac{688}{T}}-0.9$$

(5)

$${\mathrm{FeO}+\mathrm{H}}_2\to \mathrm{Fe}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}; \lg K_{{\mathrm{p}}_2}=-{\displaystyle \frac{1225}{T}}+0.845$$

(6)

where $K_{{\mathrm{p}}_1}$ and $K_{{\mathrm{p}}_2}$ denote the equilibrium constants of the above two reactions at temperature T. Wüstite is a non-stoichiometric crystal with cation Fe²⁺ vacancies, denoted as Fe_xO, where x ranges from 0.88 to 0.95. The boundary between the wüstite reduction and indirect reduction lies within the range of 900–1000 °C, at which temperature x ≈ 0.95.

For point W, the ordinate $Y_{\mathrm{W}}$ is determined by the oxygen content of wüstite at the corresponding temperature, yielding ($Y_{\mathrm{w}}=n(\mathrm{O})/n(\mathrm{Fe})\approx 1/0.95\approx 1.056$). Based on the above conclusion, the coordinates of the chemical equilibrium limit point W [23,30] can be determined as follows:

$${n^{\prime }}_{{\mathrm{CO}}_2}=K_{{\mathrm{p}}_1}\times {n^{\prime }}_{\mathrm{CO}}$$

(7)

$${n^{\prime }}_{{\mathrm{H}}_2\mathrm{O}}=K_{{\mathrm{p}}_2}\times {n^{\prime }}_{{\mathrm{H}}_2}$$

(8)

$$X_{\mathrm{W}}=1+{\displaystyle \frac{{n^{\prime }}_{{\mathrm{CO}}_2}+{n^{\prime }}_{{\mathrm{H}}_2\mathrm{O}}}{{n^{\prime }}_{\mathrm{C}}+{n^{\prime }}_{{\mathrm{H}}_2}}}$$

(9)

$$Y_{\mathrm{W}}=1.056$$

(10)

where ${n^{\prime }}_{{\mathrm{CO}}_2}$ and ${n^{\prime }}_{{\mathrm{H}}_2\mathrm{O}}$ denote the moles of CO₂ and H₂O in the furnace top gas under ideal conditions (mol); ${n^{\prime }}_{\mathrm{CO}}$ and ${n^{\prime }}_{{\mathrm{H}}_2}$ denote the moles of CO and H₂ consumed in the direct reduction reaction under ideal conditions (mol); ${n^{\prime }}_{\mathrm{C}}$ and ${n^{\prime }}_{{\mathrm{H}}_2}$ denote the total moles of C and H₂ participating in the direct reduction reaction (mol); and $K_{{\mathrm{p}}_1}$ and $K_{{\mathrm{p}}_2}$ denote the equilibrium constants of the direct reduction reactions of CO and H₂ at temperature T.

d.

Point ${\mathrm{A}}^{\prime }$

Based on the coordinates of points W and T, the coordinates of point ${\mathrm{A}}^{\prime }$ can be determined using the following formulas:

$$Y_{{\mathrm{A}}^{\prime }}=Y_{\mathrm{A}}$$

(11)

$$X_{{\mathrm{A}}^{\prime }}={\displaystyle \frac{Y_{{\mathrm{A}}^{\prime }}-b}{k_{\mathrm{WT}}}}$$

(12)

where $Y_{{\mathrm{A}}^{\prime }}$ denotes the ordinate of point A; $k_{\mathrm{WT}}$ denotes the slope of the linear equation for WT; and b denotes intercept of the linear equation for WT.

(2)

Plotting of the Operating Line for IRZ.

Based on the results in Section 2.4, the coordinates of the characteristic points A, T, W, and ${\mathrm{A}}^{\prime }$ on the operating line of IRZ have all been determined. Connecting AT and ${\mathrm{A}}^{\prime }\mathrm{T}$, the operating line diagram for the IRZ of CHORSF is plotted in Figure 3.

Figure 3. Operating line diagram for IRZ.

In Figure 3, the slope of line AT ($k_{\mathrm{AT}}=3.550$) represents the actual consumption of reducing gas required for reducing iron oxides per mole of Fe and the slope of line A′T ($k_{{\mathrm{A}}^{\prime }\mathrm{T}}=2.217$) represents the theoretical minimum consumption required for reducing iron oxides per mole of Fe. The corresponding required volume of furnace shaft reducing gas can be calculated based on the AT slope. Under the smelting conditions described in Section 2.3, the reducing gas volume calculated by the slope of the actual operating line in the IRZ is 1372.1 Nm³/tHM, with a relative error of approximately 1.5% compared to the calculated value of 1351.3 Nm³/tHM from the mass–energy balance model, verifying the accuracy of the operating line for the IRZ.

The mechanism of the IRZ in the CHORSF process is basically consistent with the shaft furnace process, utilizing both CO and H₂ as reducing agents for gas–solid reduction reactions, which can achieve over 90% MR_IRZ [31]. Among them, the gas volume of the COREX process shaft furnace is approximately 1400 Nm³/tHM [32]. The gas volume of the MIDREX process is about 1400 Nm³/tDRI [33], while that of the HYL-III [34] process ranges from 1300 to 1400 Nm³/tDRI. The HYBRIT process [35], which uses pure hydrogen, requires a higher gas volume of approximately 1700 Nm³/tDRI. The gas volume of the CHORSF process also falls within the range of about 1300–1400 Nm³/tHM.

3.2. Operating Line for the SMDZ

The steps for establishing the operating line for the SMDZ are as follows:

(1)

Determine the coordinates of key characteristic points

a.

Point U

$$X_{\mathrm{U}}=0$$

(13)

Y_U denotes the sum of the oxygen content introduced by the direct reduction in minor elements in the ore and by the CO₂ released from the flux.

$$Y_{\mathrm{U}}=-y_{\mathrm{f}}=-\left(y_{\left[\mathrm{Si}\right]}+y_{\left[\mathrm{Mn}\right]}+y_{\left[\mathrm{P}\right]}+y_{\left(\mathrm{S}\right)}\right)$$

(14)

where $y_{\left[\mathrm{Si}\right]}$ denotes the oxygen content introduced by the direct reduction of Si in hot metal; $y_{\left[\mathrm{Mn}\right]}$ denotes the oxygen content introduced by the direct reduction of Mn in hot metal; $y_{\left[\mathrm{P}\right]}$ denotes the oxygen content introduced by the direct reduction of P in hot metal; and $y_{\left(\mathrm{S}\right)}$ denotes the oxygen content introduced by slag desulfurization.

$y_{\left[\mathrm{Si}\right]}$ can be determined from the reaction equation ${\mathrm{SiO}}_2+2\mathrm{C}\to \mathrm{Si}+2\mathrm{CO}$. Other elements can also be determined using the direct reduction reaction formula with C, where the calculation equation for $y_{\left[\mathrm{Si}\right]}$ is as follows:

$$y_{\left[\mathrm{Si}\right]}={\displaystyle \frac{10\left[\mathrm{Si}\right]\times 2/28}{10\left[\mathrm{Fe}\right]/56}}=4\left[\mathrm{Si}\right]/\left[\mathrm{Fe}\right]$$

(15)

where [Fe] and [Si] denote the mass fraction of Fe and Si in the target hot metal (%). $y_{\left(\mathrm{S}\right)}$ can be calculated using the following equation:

$$y_{\left(\mathrm{S}\right)}=0.00175u\times \left(\mathrm{S}\right)/\left[\mathrm{Fe}\right]$$

(16)

where u denotes the slag ratio (kg/t HM) and $\left(\mathrm{S}\right)$ denotes the S content in the slag (%).

b.

Point B

The x-coordinate of point B is 0, and the y-coordinate is $Y_{\mathrm{B}}=-\left(Y_{\mathrm{b}}+y_{\mathrm{f}}\right)$, where

$$Y_{\mathrm{b}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{b}}/12}{10\left[\mathrm{Fe}\right]/56}}$$

(17)

where $Y_{\mathrm{b}}$ denotes the oxygen input from the blast (mol/mol Fe); ${\mathrm{C}}_{\mathrm{b}}$ denotes the mass of carbon combusted in the tuyere (kg/t HM); and $\left[\mathrm{Fe}\right]$ denotes the mass fraction of Fe in the target hot metal (%).

c.

Point V

The x-coordinate of point V is 1, and the y-coordinate must be determined by calculating the effective heat per kilomole. The effective heat per kilomole ($Q_{\mathrm{b}}$) is calculated as $Q_{\mathrm{b}}=Q_1-Q_2$. Where $Q_1$ denotes heat input in SMDZ, $Q_2$ denotes heat carried away by the CO. The specific calculations are as follows:

$$Q_{\mathrm{b}}=12\times \left(Q_{\mathrm{C}}+v_{\mathrm{b}}\times {\overline{C}}_{\mathrm{pb}}\times t_{\mathrm{b}}-v_{\mathrm{g}}\times {\overline{C}}_{\mathrm{pg}}\times t_{\mathrm{g}}\right)$$

(18)

where Q_C denotes the combustion heat of carbon, with a value of 9800 kJ/kg; v_b denotes the blast volume required for burning per kilogram of carbon in the tuyere (Nm³/kg); and $v_{\mathrm{g}}$ denotes the amount of CO produced per kilogram of carbon during combustion in the SMDZ. (Nm³/kg); ${\overline{C}}_{\mathrm{pb}}$ denotes specific heat capacities for oxygen, (kJ/( Nm³·°C)); ${\overline{C}}_{\mathrm{pg}}$ CO denotes the specific heat capacities for CO, (kJ/(Nm³·°C)); and $t_{\mathrm{b}}$ and $t_{\mathrm{g}}$ denote the temperature of the oxygen entering the furnace and the temperature of the CO produced by combustion in SMDZ, (°C). The formula for calculating $v_{\mathrm{b}}$ is as follows:

$$v_{\mathrm{b}}={\displaystyle \frac{V_{\mathrm{b}}}{C_{\mathrm{b}}}}$$

(19)

where $V_{\mathrm{b}}$ denotes the blast volume in the tuyere (Nm³/tHM) and $C_{\mathrm{b}}$ denotes the mass of carbon combusted in the tuyere (kg/tHM).

The heat expenditure in the SMDZ is divided into two parts: the heat for direct reduction of wüstite ($Q_{\mathrm{d}}$), and other heat consumption ($Q$). Where $Q_{\mathrm{d}}=$152,190 kJ/kmol Fe, $Q$ represents the sum of the heat carried away by the CO, the heat carried away by the hot metal, the heat carried away by the slag, and the system heat losses.

Based on the thermal equilibrium of the SMDZ, the following equation can be determined:

$$Q=Y_{\mathrm{b}}\times Q_{\mathrm{b}}-Y_{\mathrm{T}}\times Q_{\mathrm{d}}$$

(20)

Since

$$Y_{\mathrm{b}}=\mathrm{BU}$$

(21)

Which is equivalent to the following:

$$\mathrm{BU}\times Q_{\mathrm{b}}=Y_{\mathrm{T}}\times Q_{\mathrm{d}}+Q$$

(22)

The equation is transformed to the following:

$$\mathrm{BU}={\displaystyle \frac{Q_{\mathrm{d}}}{Q_{\mathrm{b}}}}\times \left(Y_{\mathrm{T}}+{\displaystyle \frac{Q}{Q_{\mathrm{d}}}}\right)$$

(23)

From the above equation, the ordinate of point V is determined as $Q/Q_{\mathrm{d}}$. Y_V denotes the fraction of other effective heat consumption in the SMDZ of CHORSF, relative to the heat consumption of direct reduction.

(2)

Plotting of the operating line for the SMDZ.

Based on the data in Section 2.4, the coordinates of characteristic points T, B, U, and V on the operating line of the SMDZ have all been determined. Connecting TB and UV, the operating line diagram for the SMDZ of CHORSF is plotted as shown in Figure 4.

Figure 4. Operating line diagram for the SMDZ of CHORSF.

This paper is an Fe–O–C–H system and the slope of the TB line is given by $k=Y/X={\displaystyle \frac{n\left({\mathrm{O}+\mathrm{H}}_2\right)}{n\left(\mathrm{Fe}\right)}}/{\displaystyle \frac{n\left({\mathrm{O}+\mathrm{H}}_2\right)}{n\left({\mathrm{C}+\mathrm{H}}_2\right)}}={\displaystyle \frac{n\left({\mathrm{C}+\mathrm{H}}_2\right)}{n\left(\mathrm{Fe}\right)}}$. According to the operating characteristics of CHORSF, the circulating gas injection position is located in the furnace shaft, the lower part is coke combustion, and the product is CO. The difference between CHORSF and traditional BFs is that there is no pulverized coal burning in the lower part, so the main source of hydrogen in the furnace is above the SMDZ. Since no H₂ enters or is generated in the SMDZ of CHORSF, the slope of the TB line ($k_{\mathrm{TB}}$) in Figure 4 can be regarded as the carbon consumption per 1 mol of Fe produced (mol/mol Fe). After accounting for the carburization of molten iron and the percentage of fixed carbon in coke, the calculated coke ratio is 317.66 kg/t HM. Compared with the result of 317.25 kg/tHM obtained from the mass–energy balance model, the relative error is 0.13%. This indicates that the $k_{\mathrm{TB}}$ in the SMDZ can accurately reflect the consumption of coke.

The fuel consumption of the COREX process is approximately 950–1050 kg/tHM [36]. Among them, coal consumption accounts for about 80–85% for comparison, and coke consumption is about 20–15%. Under the smelting conditions described in Section 2.3, the solid fuel of the CHORSF process is entirely coke, and the fuel consumption is only 317.25 kg/tHM, which is significantly lower than the fuel consumption of the COREX process.

The intersection point of UV and TB in the diagram represents the thermal pitch point P of CHORSF. According to the principle of the Rist operating line, the following can be determined:

$${\displaystyle \frac{\left|\mathrm{PU}\right|}{\left|\mathrm{PV}\right|}}={\displaystyle \frac{Q_{\mathrm{d}}}{Q_{\mathrm{b}}}}$$

(24)

$Q_{\mathrm{b}}$ = 152,190 kJ/kmol Fe denotes the theoretical heat consumption for direct reduction per kilomole of Fe and $Q_{\mathrm{d}}$ denotes the effective heat released by the combustion of per mole of C in the tuyere (kJ/kmol). The coordinates of point P characterize the relationship between the effective heat from tuyere carbon combustion and the total heat demand in the SMDZ. The higher the PU to PV ratio, the steeper the slope of the TB line, indicating higher coke consumption in the process [22]. Given that $Q_{\mathrm{d}}$ is a constant, increasing $Q_{\mathrm{b}}$ can reduce $\left|\mathrm{PU}\right|/\left|\mathrm{PV}\right|$, as shown in the following formula:

$${\displaystyle \frac{\left|\mathrm{PU}\right|}{\left|\mathrm{PV}\right|}}={\displaystyle \frac{Q_{\mathrm{d}}}{Q_{\mathrm{b}}}}\propto {\displaystyle \frac{1}{Q_{\mathrm{b}}}}$$

(25)

Therefore, the coke consumption of the CHORSF process can be reduced through measures such as pulverized coal injection and increasing the temperature of the gas fed into the furnace.

3.3. Operating Line for CHORSF

The core equipment of this process is the reduction melting furnace, which has clearly defined IRZ and SMDZ. Connecting the operating line of IRZ and SMDZ at point T yields the complete operating line for CHORSF. Point T is determined by the reducing gas composition and MR_IRZ. It marks the endpoint of indirect reduction and the starting point of direct reduction. By investigating the effects of factors such as the MR_IRZ and the composition of reducing gas injected into the furnace on point T, a theoretical basis can be provided for optimizing the CHORSF ironmaking process parameters. The operating line diagram of CHORSF plotted according to the corresponding parameters is shown in Figure 5.

Figure 5. Operating line diagram of CHORSF.

4. Application Analysis of the Operating Line for the CHORSF

The metallization rate is a critical indicator for evaluating reduction efficiency. The increase in MR in IRZ can reduce the direct reduction of Fe in the SMDZ, thereby reducing the consumption of coke [37]. Studies have demonstrated that when the H₂ content in the reducing gas falls below 80%, the reduction rate and efficiency of Fe are constrained, resulting in an MR below 93% [38]. Our team has established one-dimensional [39] and two-dimensional kinetic models [40] for CHORSF, based on the kinetic parameters of ore under a hydrogen-rich reducing gas atmosphere [41]; these studies show that the MR_IRZ of CHORSF can reach over 90%. Thus, the MR_IRZ were set at 85%, 87.5%, and 90% to investigate the specific effects of the MR_IRZ in CHORSF.

The operating line diagrams under three different MR_IRZ are compared in Figure 6.

Figure 6. Comparison of operating lines for CHORSF under different MR_IRZ. (a,b) comparisons between the operating line in the IRZ and the SMDZ of CHORSF under different MR_IRZ conditions. (c) comparison of the whole CHORSF operating line under different MR_IRZ condition.

Figure 6a,b respectively show comparisons between the operating line in the IRZ and the SMDZ of CHORSF under different MR_IRZ conditions, and Figure 6c presents the comparison of the whole CHORSF operating line under different MR_IRZ conditions. The key parameters under different MR_IRZ conditions are shown in Table 9.

Table 9. Comparison of key parameters under different MR_IRZ.

MRIRZ	${\mathit{k}}_{\mathbf{AT}}$	${\mathit{k}}_{\mathbf{TB}}$	Coke Ratio (kg/t HM)	CO2 Emissions (kg/t HM)	CO2 Utilization and Storage (kg/t HM)
85%	3.395	1.266	347.35	922.94	706.20
87.5%	3.408	1.203	331.75	871.25	663.99
90%	3.413	1.138	317.66	834.53	633.57

As shown in Table 9, when the MR_IRZ increases from 85% to 90%, the slope of the actual operating line in the CHORSF IRZ ($k_{\mathrm{AT}}$) increases by 0.018 mol/mol Fe, corresponding to an increase of 7.2 Nm³ of reducing gas consumption per ton of hot metal. This indicates that a higher MR_IRZ requires more reducing gas to achieve a higher FeO conversion rate in the IRZ. In addition, for every 1% increase in the MR_IRZ, the slope of the TB operating line in the SMDZ decreases by 0.0256 mol/mol Fe, corresponding to a reduction of 5.94 kg/t HM in the coke ratio. With the increase in MR_IRZ, the amount of FeO entering the SMDZ decreases, resulting in reduced coke consumption in the SMDZ. This result indicates that the coke ratio of CHORSF can be reduced by increasing the MR_IRZ appropriately. From an industrial perspective, the MR_IRZ is typically in the range of 80% to 90%, so its influence is relatively significant.

As for the trade-off between reducing gas consumption and coke rate, it should be determined according to the carbon emission regulations of different countries, while also seeking optimization and balance between the economy and the environment by considering the costs of hydrogen production and coke.

Under MR_IRZ conditions of 85–90%, the CO₂ emissions per ton of iron from the CHORSF process and the CO₂ captured and stored by CCUS are summarized in Table 9. The CO₂ emissions per ton of iron from BFs are approximately 1.4–2.08 t CO₂/tHM [24]. Compared with BFs, the CHORSF process can reduce CO₂ emissions by 34–60% without CCUS; with CCUS, the reduction increases to approximately 40–70%.

5. Conclusions

In this paper, an operating line suitable for the CHORSF is established by the zone. Through analyzing its process characteristics, the purpose of optimizing and evaluating the carbon-saving potential of the process is achieved. The main conclusions are as follows:

(1)

The slope of the AT line in the IRZ of CHORSF ($k_{\mathrm{AT}}$) represents the actual consumption of reducing gas required for reducing iron oxides per mole of Fe. The slope of the A′T line of IRZ in the CHORSF ($k_{{\mathrm{A}}^{\prime }\mathrm{T}}$) represents the theoretical minimum reducing gas required for reducing iron oxides per mole of Fe. Under the smelting conditions in this study, the reducing gas consumption of CHORSF is 1200–1400 Nm³/tHM.

(2)

The slope of the TB operating line in the SMDZ of CHORSF ($k_{\mathrm{TB}}$) can be used to quantify the coke ratio; the coke consumption of CHORSF can be reduced through measures such as pulverized coal injection and increasing the temperature of the gas entering the furnace (CO+H₂).

Under the smelting conditions described in this study, when the MR_IRZ increases from 85% to 90%, the $k_{\mathrm{AT}}$ in the CHORSF IRZ increases by 0.018 mol/mol Fe, corresponding to an increase of 7.2 Nm³/t HM in reducing gas consumption per ton of hot metal. In addition, for every 1% increase in the MR_IRZ, the coke ratio decreases by 5.94 kg/t HM. Thus, increasing the MR_IRZ enables the simultaneous reduction in the coke ratio.