Mass and Heat Balance Model and Its Engineering Application for the Oxygen Blast Furnace Smelting Process of Vanadium–Titanium Magnetite

Abstract

The oxygen blast furnace (OBF) process presents a promising low-carbon pathway for the smelting of vanadium–titanium magnetite (VTM). This study develops an innovative mathematical model based on mass and heat balance principles, specifically tailored to the OBF smelting of VTM. The model systematically investigates the effects of key parameters—including pulverized coal injection ratio, recycling gas volume, hydrogen content in the recycling gas, and charge composition—on furnace productivity, hearth activity, and the tuyere raceway zone. The results show that increasing the pulverized coal injection ratio slightly reduces productivity and theoretical flame temperature: for every 25 kg/tHM increase in the coal ratio, the theoretical flame temperature decreases by 21.95 °C; moreover, indirect reduction is enhanced and the heat distribution within the furnace is significantly improved. A higher recycling gas volume markedly increases productivity and optimizes hearth thermal conditions, accompanied by enhanced blast kinetic energy and an expanded tuyere raceway zone, albeit with a notable drop in combustion temperature. Increased hydrogen content in the recycling gas promotes productivity, but may weaken blast kinetic energy and reduce the stability of the raceway zone. Furthermore, a higher titanium content in the charge increases the difficulty of iron oxide reduction, resulting in lower CO utilization and reduced productivity.

1. Introduction

Vanadium–titanium magnetite (VTM) is rich in valuable metals such as iron, vanadium, and titanium, and has high comprehensive utilization value [1,2,3,4,5]. It has become one of the important strategic resources for the future [6,7,8,9,10]. Currently, its smelting process mainly relies on the traditional blast furnace (TBF) process. However, due to the high TiO₂ content and complex mineral composition of VTM, TBF generally faces a series of technical challenges during the smelting process [11,12]. These include the easy formation of high-melting-point phases such as TiC and TiN, high slag viscosity, the frothy slag phenomenon, and difficulties in effectively separating slag and hot metal [1,13,14,15,16,17]. To ensure stable furnace conditions, the current industrial practice often involves blending VTM with ordinary iron ore for smelting, which makes it difficult to achieve efficient utilization of 100% VTM [18,19]. This results in the inability to economically recover TiO₂ from the slag. These issues severely restrict the efficient development and comprehensive utilization of vanadium–titanium resources.

The oxygen blast furnace (OBF) process, as an emerging low-carbon smelting technology [20,21,22,23,24], offers several process advantages suitable for the smelting of VTM [25]. First, the use of pure oxygen instead of traditional oxygen-enriched air can significantly reduce the formation of high-melting-point phases such as TiC and TiN. This, in turn, lowers slag viscosity, improves slag and hot metal separation behavior, and enhances hearth stability. Combined with top gas recycling and hydrogen-rich gas injection, this process not only strengthens the indirect reduction reaction and improves the utilization efficiency of carbon, but also helps reduce coke ratio and pollutant emissions [25,26,27,28,29,30,31]. It can also significantly increase production, with studies [25,26] showing potential increases of 30–200%. Based on these advantages, the OBF holds promise for achieving 100% smelting of VTM and the economic recovery of TiO₂ from slag [25].

Although the OBF process has a certain theoretical and experimental basis in ordinary iron ore smelting [32,33,34,35,36,37,38], research on the smelting of VTM is still in its infancy. There is currently a lack of a complete mathematical model for the smelting of VTM in OBF worldwide, making it difficult to provide effective theoretical support for process optimization and equipment design. Therefore, there is an urgent need to develop a mathematical model tailored for the smelting conditions of VTM in OBF to deeply analyze the impact of process parameter changes on smelting behavior.

Based on this foundation, this study specifically focuses on the OBF process for smelting vanadium–titanium magnetite (VTM). A comprehensive mathematical model is developed, grounded in the principles of mass and heat balance. This model integrates several key operational features, including the smelting of 100% VTM pellets, the use of 100% oxygen blast conditions, and the injection of hydrogen-rich recycled gas. The study analyzes the impact of different coal ratios, recycling gas volume, hydrogen content, and charge composition on OBF productivity, hearth heat state, and the structure of the raceway zone. The research findings provide theoretical support for the industrial practice of the OBF smelting process of VTM.

2. Mathematical Model Development

2.1. Process Characteristics of the Oxygen Blast Furnace

The OBF is an advanced ironmaking process evolved from the TBF, in which the conventional hot blast, typically air enriched with limited oxygen, is replaced with high-purity oxygen or highly oxygen-enriched air. This modification significantly intensifies combustion reactions and elevates process efficiency. The OBF process represents a major innovation in ironmaking technology and is typically coupled with top gas recycling (TGR) and carbon capture, utilization, and storage (CCUS) technologies to drastically reduce carbon dioxide emissions while improving energy utilization and productivity.

The OBF process is characterized by complex physicochemical phenomena. As shown in Figure 1, at the raceway zone, coke, pulverized coal, and hydrogen-rich recycled gas undergo vigorous combustion with oxygen, generating a high-temperature gas stream rich in CO, and providing both the heat and the reducing atmosphere required for smelting. Simultaneously, VTM introduced from the furnace top descends countercurrently against the ascending reducing gas. During descent, the iron oxides in the ore are progressively reduced to metallic iron, while titanium and vanadium oxides undergo various transformations depending on the temperature and gas atmosphere. The final products are liquid hot metal and slag, which accumulate at the hearth. A key feature of the OBF system is the partial recycling of top gas. After CO₂ removal, the cleaned top gas, supplemented with additional hydrogen, is injected into the furnace through the tuyeres, forming a carbon recycling system. This process not only reduces fuel consumption, but also enhances the utilization of reducing agents.

To quantitatively evaluate the effects of key operational parameters—such as smelting conditions, charge composition, and blast conditions—on furnace productivity and hearth dynamic behavior, it is crucial to develop comprehensive mass and heat balance models tailored to the OBF smelting of VTM. The model serves as a theoretical basis for process optimization, reactor design, and reduction in OBF.

2.2. Methodology and Calculation Conditions

2.2.1. Assumptions and Boundary Conditions

The developed model quantifies the material mass and thermal relationships based on the production of 1 ton of hot metal (tHM). In this study, the OBF process is assumed to utilize 100% VTM pellets as the iron-bearing charge. The reductants include coke, pulverized coal, and hydrogen-enriched recycled top gas. To ensure smooth slag tapping and maintain appropriate slag fluidity, fluxes are added to adjust the slag basicity to 1.05, consistent with the operational conditions of industrial-scale vanadium–titanium blast furnaces.

The model outputs products include hot metal, slag, and top gas. The chemical compositions of raw materials used in this study are based on data provided by a steel plant in China. The main chemical composition of VTM pellets is listed in

Table 1. Chemical composition of vanadium–titanium magnetite pellets (wt%).

Component	TFe	FeO	SiO2	CaO	MgO	Al2O3	TiO2	V2O5	MnO	S	P
wt%	54.97	1.06	3.54	0.67	2.97	3.41	9.73	0.69	0.42	0.01	0.01

. The chemical composition of coke and pulverized coal, as well as the fluxes, is detailed in

Table 2. Chemical composition of fuel and fluxes (wt%).

Component	C	S	CaO	SiO2	MgO	Fe2O3	Al2O3	H2O	Volatile
Coke	84.84	0.77	0.61	7.00	0.16	0.66	3.94	0.88	1.14
Coal	67.98	0.16	1.75	4.35	0.23	0.48	2.21	1.27	21.56
Flux	—	—	47.94	1.69	2.48	—	—	—	47.89

.

The simulation parameters adopted in this work are derived from OBF’s practical operating data in China. The oxygen blast volume is set at 214.80 m³ per ton of hot metal (tHM), while the supplemental hydrogen-rich recycled top gas is introduced at a rate of 750 m³/tHM. The temperature of hot metal is maintained at 1500 °C, ensuring adequate fluidity and separation from slag. The temperature of the circulating top gas before reinjection is assumed to be 1200 °C. Meanwhile, the furnace top gas is assumed to exit the system at a temperature of 200 °C. These parameters provide the basis for establishing a comprehensive mass and heat balance model, enabling the quantitative evaluation of material conversion and energy distribution under various smelting conditions.

2.2.2. Calculation Method of Key Parameters

• Utilization of CO and H₂

Following hydrogen-rich gas injection into the OBF, the utilization efficiencies of CO and H₂ serve as key indicators for assessing the degree of chemical energy utilization within the furnace. The CO utilization efficiency can be expressed as

$${\eta }_{CO}={\displaystyle \frac{{\phi }_{C{O}_2}}{{\phi }_{CO}+{\phi }_{C{O}_2}}}$$

(1)

Similarly, the H₂ utilization efficiency can be expressed as

$${\eta }_{H2}={\displaystyle \frac{{\phi }_{H_{2}O}}{{\phi }_{H_2}+{\phi }_{H_{2}O}}}$$

(2)

When the H₂/CO ratio in the indirect reduction zone remains constant, various operational parameters tend to exert similar effects on both CO and H₂ utilization. As such, a proportional relationship exists between the utilization rates of CO and H₂. This correlation can be approximated using the following empirical formula [39]:

$${\eta }_{H_2}=0.88{\eta }_{CO}+0.1$$

(3)

2.

Hot Metal Productivity

The hot metal productivity is primarily determined by the consumption of reducing gases within the furnace. First, the effective volume of reducing gas participating in the reaction is calculated based on the total gas input and its utilization rate. Then, according to reduction reaction principles, the amount of oxygen removed by these gases is used to estimate the quantity of iron reduced from iron oxides. Finally, the total amount of iron generated via indirect and direct reduction is summed to determine the iron yield over time, from which the productivity per ton of hot metal can be derived.

3.

Blast Kinetic Energy

In an OBF, parameters such as the flow rate and composition of top recycled gas, as well as the tuyere diameter, significantly affect the distribution of blast kinetic energy. The blast kinetic energy can be expressed as

$$E_b={\displaystyle \frac{1}{2}}{\rho }_g{\displaystyle \frac{V_g}{60}}\left[{\displaystyle \frac{4V_g}{60\pi \cdot n{d}^2}}\cdot {\displaystyle \frac{T_g{P}_0}{T_0{P}_g}}\right]={\displaystyle \frac{1}{1800}}\cdot {\displaystyle \frac{{\rho }_g{V}_g^2}{\pi n{d}^2}}\cdot {\displaystyle \frac{T_g{P}_0}{T_0{P}_g}}$$

(4)

where E_b is the blast kinetic energy (J), V_g is the volumetric flow rate of the hot blast (m³/min), ρ_g is the density of the hot blast (kg/m³), P₀ is standard atmospheric pressure (kPa), P_g is the hot blast pressure (kPa), n is the number of tuyeres, d is the tuyere diameter (m), T₀ is the ambient air temperature (K), and T_g is the hot blast temperature (K).

4.

Raceway Zone

Further investigation reveals that the spatial characteristics of the tuyere raceway zone in the hearth are strongly correlated with the blast kinetic energy. Specifically, the kinetic energy governs the flow regime and turbulence intensity of the gas in the hearth, thereby directly influencing the shape and size of the raceway zone. The depth of the tuyere raceway zone can be estimated using the following expression:

$$l_R=0.118\times {10}^{-3}{E}_b+0.77$$

(5)

The height of the raceway zone is given by

$$H_R=l_R/K_R$$

(6)

where K_R denotes the raceway zone shape coefficient. For large blast furnaces, a smaller value of K_R is typically used, while a larger value is used for small blast furnaces. In this study, a value of K_R = 1 is assumed, indicating a spherical shape with equal height and depth for the raceway zone.

Under these conditions, a mathematical model is established based on mass and heat balance principles. This model is further employed to investigate the influence of smelting conditions, titanium content in the charge, and blast parameters on OBF productivity, hearth dynamic behavior, and the tuyere raceway zone.

3. Results and Discussion

3.1. Effect of OBF Conditions on Productivity

3.1.1. Effect of Pulverized Coal Ratio on Productivity

Pulverized coal injection (PCI) technology is now a well-established and effective method for controlling the thermal regime of the blast furnace. Considering the practical operational parameters, the pulverized coal rate was set in the range of 100–200 kg/tHM. The recycled gas volume was fixed at 750 m³/tHM, based on the operating parameters of the OBF in China. To maintain furnace balance, the coke ratio was adjusted accordingly. The effect of coal ratio on productivity and top gas volume in the OBF is illustrated in Figure 2.

With the increase in the pulverized coal ratio in the OBF, the productivity tends to decrease, although this impact is relatively minor. Concurrently, the volume of top gas in the OBF correspondingly rises. After the coal ratio is increased, there is a reduction in the theoretical flame temperature and insufficient heat in the hearth. This, in turn, affects the reduction reactions and results in a decline in productivity. Moreover, as the coal ratio increases and the coke ratio decreases, the permeability within the OBF deteriorates, and the utilization rate of gas also declines, thereby further reducing production efficiency. The increase in top gas volume is primarily due to the gasification of pulverized coal at the tuyere front after injection, which generates more top gas. Additionally, the more hydrocarbons contained in the coal powder, the more hydrogen gas is produced, leading to an increase in the volume of top gas.

Although an increase in the coal ratio may lead to a slight decrease in blast furnace productivity, in practical terms, raising the coal ratio is advantageous for blast furnace smelting. On one hand, increasing the coal ratio reduces the coke ratio, thereby conserving coking resources and mitigating the environmental pollution associated with the coking process. On the other hand, a higher coal ratio increases the volume of gas in the hearth. The abundant hydrocarbons in pulverized coal decompose during combustion to produce a large amount of hydrogen, thereby strengthening the reducing atmosphere and particularly facilitating indirect reduction reactions. Moreover, the increase in coal ratio leads to a lower hearth temperature, allowing more heat to be transferred to the upper low-temperature zones. This helps address the “cold upper and hot lower” issue commonly found in OBF smelting. In summary, employing an appropriate coal ratio is beneficial for the OBF smelting of VTM.

3.1.2. Effect of Recycling Gas Injection Volume on Productivity

Based on the production data from the OBF in China [30], the range of recycling gas volume is set to 300–1000 m³/tHM in this model. The relationship between the injected recycling gas volume and OBF productivity is shown in Figure 3.

It can be observed that OBF productivity exhibits a growth trend with increasing recycling gas volume. However, as the injected recycling gas volume continues to increase, the rate of productivity growth gradually slows down. Increasing the recycling gas volume helps promote indirect reduction and reduce the coke ratio, thereby enhancing the economic benefits. In this model, the injected recycling gas is a hydrogen-enriched gas mixture composed of treated top gas. This hydrogen-enriched recycling gas is more effective in promoting indirect reduction than ordinary gas.

After the recycling gas is injected into the blast furnace, it increases the volume of top gas and the concentrations of CO and H₂ in the top gas, thereby enhancing the reducing potential of the gas. The increase in gas volume also expands the tuyere raceway zone, improves the activity of the hearth, and enhances the permeability of the lower part of the furnace.

Additionally, when recycling gas is injected at the hearth, the increased gas volume allows the gas to carry away more heat, thereby raising the temperature of the gas flow and increasing the heat input to the upper part of the OBF. This effectively addresses the “cold upper” issue in OBF smelting. The temperature of the recycling gas injected at the hearth is 1200 °C, which is significantly lower than the flame temperature in the hearth. The absorption of heat from the hearth by the injected recycling gas also helps to mitigate the “hot lower” issue in OBF smelting.

3.1.3. Effect of Hydrogen Content in Recycling Gas on Productivity

Hydrogen-rich smelting also offers numerous advantages, such as reducing carbon consumption, lowering carbon emissions, and improving indirect reduction efficiency. However, the industrial application of hydrogen-rich technology in the blast furnace smelting of VTM still faces many pressing challenges. Therefore, the research and development of hydrogen-rich technology are of great practical significance for OBF smelting of VTM. In this model, the recycling gas volume is fixed at 750 m³/t, and the coal ratio is set at 110 kg/t. The relationship between OBF productivity and the H₂ content in the recycling gas after calculating the blast furnace productivity under different H₂ contents is shown in Figure 4.

As can be seen from Figure 4, OBF productivity increases significantly with the increase in H₂ content in the recycling gas. From a thermodynamic perspective, the Gibbs free energy (ΔG) of the H₂ reduction reaction decreases more significantly with temperature compared to that of CO. This characteristic makes the reduction capacity of hydrogen higher than CO at high temperatures (above 810 °C). From a kinetic standpoint, H₂, with its smaller molecular size and lower viscosity, exhibits a higher diffusion coefficient, enabling more efficient kinetic reactions. Consequently, as the H₂ content increases, the productivity of the OBF also rises.

3.1.4. Effect of Titanium Content in Charge on Productivity

The OBF smelting of VTM exhibits certain unique characteristics compared to TBF smelting. By analyzing the titanium content in the composite charge and the reaction behavior of TiO₂ during the smelting process, the specific features of the OBF smelting of VTM can be explored. In this model, the effects of different TiO₂ contents (with corresponding changes in iron grade) in the composite charge on CO utilization and blast furnace productivity are analyzed.

The results are shown in Figure 5. It can be seen that both CO utilization and blast furnace productivity decrease with the increase in the titanium content of the composite charge. The primary reason for the decrease in CO utilization is that, as the titanium content in the composite charge increases (from 2–12 wt%, with an interval of 2%), more iron oxides combine with titanium oxides in the form of solid solutions, increasing the difficulty of reducing iron oxides and thereby reducing the utilization of CO.

The main reason for the decrease in blast furnace productivity is that the increase in TiO₂ content leads to a lower iron grade in the composite charge, which, in turn, directly affects blast furnace productivity. Additionally, the decline in CO utilization reduces the reduction efficiency of iron oxides. Within the same smelting time, fewer iron oxides are reduced, indirectly affecting blast furnace productivity.

3.2. Effect of Injection Parameters on Theoretical Flame Temperature and Heat Distribution

3.2.1. Effect of Recycling Gas Injection Volume on Theoretical Flame Temperature and Heat Distribution

As shown in Figure 6, the theoretical flame temperature decreases significantly with the increase in recycling gas volume, and the rate of decrease gradually diminishes. Meanwhile, the heat in the high-temperature zone correspondingly increases. Specifically, for every 100 m³/tHM increase in recycling gas volume, the heat in the hearth zone increases by 0.17 GJ. When the recycling gas volume reaches 400 m³/t, the heat in the hearth zone is 2.82 GJ, with a theoretical flame temperature of 2498.52 °C. When the recycling gas volume increases to 1000 m³/t, the heat in the hearth zone further rises to 3.84 GJ, while the theoretical flame temperature drops to 1912.86 °C.

The decline in theoretical flame temperature is primarily attributed to three factors. First, the gas components dilute the oxygen concentration in the combustion zone, thereby inhibiting the complete combustion of pulverized coal and coke. Additionally, the calorific value of H₂ and CO in the recycling gas is significantly lower than that of the complete combustion of coke. Moreover, when the 1200 °C recycling gas is injected into the raceway zone, it absorbs a substantial amount of sensible heat to increase its temperature. The combustion products (H₂O and CO₂) also have a higher heat capacity, and these factors collectively contribute to the reduction in theoretical flame temperature. As the recycling gas volume continues to increase, the composition of combustion products gradually stabilizes, resulting in a decreasing trend in the rate of temperature drop.

The increase in heat is primarily attributed to the optimization of the reduction process. The use of recycling gas significantly enhances the proportion of indirect reduction while substantially reducing the highly endothermic direct reduction reactions. This shift not only improves the utilization rate of the gas, but also minimizes heat loss. Additionally, the 1200 °C recycling gas carries a significant amount of heat into the furnace, markedly increasing the overall heat input. Under the combined effect of increased total heat input and optimized heat utilization efficiency, the hearth heat is effectively enhanced. This thermodynamic rebalancing allows the blast furnace to maintain a good hearth heat state even at a lower theoretical flame temperature.

3.2.2. Effect of Recycling Gas Temperature on Theoretical Flame Temperature and Heat Distribution

As shown in Figure 7, both the theoretical flame temperature and the heat in the high-temperature zone increase with the rise in recycling gas temperature. Specifically, for every 50 °C increase in recycling gas temperature, the theoretical flame temperature rises by approximately 33.50 °C, and the heat in the high-temperature zone increases by about 0.06 GJ. This phenomenon can be attributed to the fact that the theoretical flame temperature is primarily influenced by the heat released from the combustion reactions in the raceway zone and the heat absorption capacity of the gas. When the recycling gas temperature increases, it means that more physical heat is brought into the hearth by the blast, which directly participates in the combustion reactions, raising the temperature of the combustion products and thereby increasing the theoretical flame temperature. Higher blast temperatures facilitate more complete combustion of the fuel, enhancing the combustion efficiency of coke and pulverized coal. This leads to an increase in heat release, which, in turn, raises the theoretical flame temperature and the heat in the high-temperature zone.

Therefore, the increase in blast temperature not only directly raises the theoretical flame temperature but also, through various means such as accelerating the combustion of pulverized coal, further increases the heat in the high-temperature zone. While this enhances the efficiency of OBF smelting, it also necessitates optimized tuyere operations to prevent excessively high temperatures in the hearth from affecting production stability.

3.2.3. Effect of Pulverized Coal Ratio and Coke Ratio on Theoretical Flame Temperature and Heat Distribution

As shown in Figure 8, the theoretical flame temperature decreases with an increase in the coal ratio and a decrease in the coke ratio, while the heat in the high-temperature zone exhibits an opposite trend of increase. Specifically, for every 25 kg/tHM increase in the coal ratio, the coke ratio decreases by 20 kg/tHM, resulting in a decrease of approximately 21.95 °C of the theoretical flame temperature and an increase of about 0.02 GJ of the heat in the high-temperature zone.

The primary reason for the decrease in theoretical flame temperature is that the injected pulverized coal typically has a temperature below 200 °C. During the processes of heating, combustion, and decomposition, the pulverized coal absorbs a significant amount of heat. As the coal ratio increases, more pulverized coal participates in the reactions, absorbing more heat to complete these processes, thereby lowering the theoretical flame temperature. Additionally, an increase in the coal ratio leads to a decrease in the coke ratio, meaning less heat is generated from coke combustion, which is one of the main heat sources in the high-temperature zone. Therefore, a reduction in the coke ratio results in a lower theoretical flame temperature.

On the other hand, as the coal ratio increases, the production of CO and H₂ from coal combustion also increases. These gases release a significant amount of additional heat when they undergo secondary combustion or participate in reduction reactions within the OBF, leading to the accumulation of heat in the high-temperature zone. Moreover, a decrease in the coke ratio implies a reduction in the coke framework, which decreases the permeability within the furnace. This hinders the transfer of heat from the hearth and bosh to the lower-temperature zones, thereby increasing the heat in the high-temperature zone. Consequently, despite the decrease in theoretical flame temperature, the overall heat in the high-temperature zone still rises.

By comparing the effects of recycling gas volume, coal ratio, and recycling gas temperature on the theoretical flame temperature and heat input in the high-temperature zone, it is found that recycling gas volume has a more significant impact on both the theoretical flame temperature and the heat input in the high-temperature zone. Given the unique characteristics of VTM smelting, blindly increasing the temperature in the lower part of the blast furnace should be avoided, as it may lead to increased TiC formation, higher slag viscosity, and exacerbated frothy slag phenomena. Therefore, selecting an appropriate blast regime and reasonably adjusting the recycling gas volume, coal ratio, and recycling gas temperature can not only provide a suitable theoretical flame temperature to ensure efficient blast furnace smelting, but also achieve the goals of reducing coke ratio and carbon emissions.

3.3. Effect of Injection Parameters on Blast Kinetic Energy and Raceway Zone Structure

3.3.1. Effect of Recycling Gas Injection Volume on Blast Kinetic Energy and Raceway Zone Structure

As shown in Figure 9, both the blast kinetic energy and the structure of the tuyere raceway zone increase significantly with the increase in recycling gas volume, and the trends of these two parameters are highly consistent. When the recycling gas volume increases from 300 m³/tHM to 1000 m³/tHM, the blast kinetic energy rises from 1086.74 J to 9875.38 J, and the diameter of the tuyere raceway zone increases from 0.9 m to 1.94 m. This phenomenon occurs primarily because blast kinetic energy is determined by factors such as gas density and the square of the blast velocity. The increase in recycling gas volume significantly increases the total gas flow rate. Under the condition of a fixed tuyere cross-sectional area, the blast velocity increases accordingly, and the kinetic energy grows quadratically with the velocity. Meanwhile, the enhanced momentum carried by the high-velocity gas drives the coke layer at the front of the tuyere towards the central region, promoting its combustion and thereby expanding the diameter of the raceway zone. Therefore, with the increase in recycling gas volume, both blast kinetic energy and the size of the tuyere raceway zone increase significantly.

In industrial production, high-titanium slag has high viscosity and is prone to frothing. The increase in recycling gas volume, by raising the partial pressure of CO, inhibits the excessive reduction of TiO₂ to Ti(C, N), thereby reducing the generation of frothy slag and maintaining the stability of the raceway zone structure. At the same time, the expansion of the raceway zone enhances the permeability of the coke layer, alleviating the negative impact of high-titanium slag on permeability and forming a positive feedback loop of “increased kinetic energy—expanded raceway zone—improved permeability”.

3.3.2. Effect of Hydrogen Content in Recycling Gas on Blast Kinetic Energy and Raceway Zone Structure

Figure 10 illustrates the impact of hydrogen content in recycling gas on blast kinetic energy and the structure of the tuyere raceway zone. As shown in Figure 10, both blast kinetic energy and the diameter of the tuyere raceway zone decrease significantly with an increase in the hydrogen content of the recycling gas. This phenomenon is closely related to changes in gas density and momentum. The molecular weight of hydrogen (2 g/mol) is significantly lower than that of carbon monoxide (28 g/mol). According to the formula for blast kinetic energy, when the proportion of hydrogen increases, the average density ρ of the recycling gas decreases. The increase in the explosion velocity v is not sufficient to compensate for the effects brought about by the reduction in density, resulting in a significant reduction in total kinetic energy. The insufficient kinetic energy weakens the penetration ability of gas through the coke layer, making it difficult to form a strong recirculating gas flow and causing the diameter of the raceway zone (the coke fluidization area in front of the tuyere) to shrink. Although the low viscosity of hydrogen may slightly increase the flow velocity, the momentum reduction dominated by its density is still the key factor.

Moreover, the reduction of iron oxides by hydrogen is an endothermic reaction, which may lead to local temperature decreases. This, in turn, indirectly affects the fluidity of liquid slag and hot metal, increasing the resistance to gas flow. Additionally, under a fixed recycling gas volume flow rate, an increase in the hydrogen proportion reduces the total mass flow rate (due to the lower density), further decreasing the kinetic energy input and leading to the contraction of the tuyere raceway zone. To maintain a balanced state within the furnace, it is necessary to optimize the hydrogen blending ratio and blast parameters to ensure stable and efficient blast furnace smelting operations.

3.3.3. Effect of Tuyere Diameter on Blast Kinetic Energy and Raceway Zone Structure

Figure 11 illustrates the impact of tuyere diameter on blast kinetic energy and the structure of the tuyere raceway zone. The results indicate that an increase in tuyere diameter leads to a significant decrease in both blast kinetic energy and the diameter of the tuyere raceway zone. This is primarily because the increase in tuyere diameter results in a substantial reduction in blast velocity. According to the formula for blast kinetic energy, when the tuyere diameter increases, the blast velocity v decreases due to the larger tuyere diameter and cross-sectional area A, assuming a constant recycling gas volume flow rate. Since kinetic energy is proportional to the square of the velocity, the decrease in velocity leads to a sharp reduction in blast kinetic energy.

The reduction in blast kinetic energy decreases the penetration depth and turbulence intensity of the gas flow within the coke layer, causing the boundary of the raceway zone to contract. Additionally, the lower velocity may shift the combustion zone towards the tuyere, reducing the concentration of local high-temperature areas and thereby suppressing the intensity of coke gasification reactions. This indirectly weakens the kinetic conditions necessary for the formation of the raceway zone. Moreover, a larger tuyere diameter can lead to uneven gas flow distribution along the furnace wall, with the excessive development of edge gas flow and insufficient central gas flow, exacerbating the asymmetry of the raceway zone.

Therefore, in practical production, it is essential to optimize tuyere size and blast parameters (such as increasing blast pressure) to maintain sufficient kinetic energy. This ensures the stability of the raceway zone and the appropriate permeability of the coke bed, thereby enhancing blast furnace smelting efficiency.

4. Conclusions

This study systematically explored the effects of different injection strategies on the smelting behavior of vanadium–titanium magnetite in an oxygen blast furnace (OBF), revealing the impact mechanisms of key process parameters such as coal ratio and recycling gas volume on blast furnace productivity, hearth activity, and the tuyere raceway zone. The main conclusions are as follows:

• As the coal ratio increases, blast furnace productivity decreases slightly, but the overall benefits are significant. On the one hand, it effectively conserves coking resources and reduces environmental pollution. On the other hand, the increased gas volume in the hearth promotes indirect reduction and significantly improves the “cold upper and hot lower” condition in OBF. However, for every 25 kg/tHM increase in the coal ratio, the theoretical flame temperature decreases by 21.95 °C, while the heat in the high-temperature zone increases.
• As the recycling gas volume increases, blast furnace productivity shows an upward trend, though at a gradually slowing rate. Additionally, an increase in recycling gas volume leads to a significant decrease in theoretical flame temperature, while the heat in the hearth zone rises markedly, and both blast kinetic energy and the diameter of the tuyere raceway zone increase significantly, which is conducive to maintaining the stability of the raceway zone and improving permeability. It is recommended that in industrial production, the recycling gas volume should be ≥600 m³/tHM.
• As the hydrogen content increases, blast furnace productivity rises significantly. However, an increase in hydrogen content in the recycling gas leads to a decrease in blast kinetic energy and the size of the tuyere raceway zone, which may adversely affect the stability of the blast furnace smelting process. Therefore, it is necessary to optimize the hydrogen content in the recycling gas. It is suggested that the hydrogen content be controlled at ≤20%. In addition, with the increase in titanium content in the charge, both CO utilization and blast furnace productivity show a downward trend.
• In the future, research should focus on further verifying the feasibility of smelting VTM in OBF through laboratory investigations and industrial-scale trials.