Numerical Analysis of Blast Furnace with Injection of COREX Export Gas After Removal of CO₂

Abstract

Blast furnace (BF) injection of COREX export gas after removal of CO₂ (CEG) displays many ecological and environmental advantages. A static model of BF operation of CEG was developed according to mass and heat balance. The effect of CEG injection on the raceway adiabatic flame temperature (RAFT), the amount and composition of bosh gas, and the shape of raceway were studied. The acceptable injection volume of CEG under different thermal compensation measures was investigated. The results show that under no thermal compensation, with the increase of CEG injection, the RAFT decreases but the volume of bosh gas increases. The content of CO and H₂ increases with the increase of CEG injection. Based on the standard of maintaining the RAFT and volume of bosh gas, addition of oxygen, reducing blast humidity and increasing blast temperature are effective measures of thermal compensation to increase the quantity of CEG injection. The characteristics of high temperature zone of BF under different suitable CEG injection volumes were also studied. The findings of this work can be used as a theoretical basis to guide plant operations for CEG injection in BF.

1. Introduction

The iron and steel industry is the largest energy consuming manufacturing sector in the world as well as one of the most important sources of greenhouse gas (GHG) emissions.¹⁾ The “Blast furnace (BF)-LD Converter” process is the dominant steelmaking route in the world, which produced about 70.8% of the global crude steel in 2018.²⁾ Among this process, the BF occupied the 50% of energy consumption and 60% of GHG emission in the iron and steel sector.^3,4) Conservation of energy and reducing CO₂ emission is the main direction of the green development of the whole steel industry. Recently, various new iron-making technologies, such as COREX process and oxygen blast furnace, have been developed in recent decades to reduce the energy consumption and CO₂ emission.^5,6,7,8)

COREX process has realized the largest plant C-3000 (named according to the production capacity of COREX process) at Baosteel in China.⁹⁾ One of the C-3000 module in Baosteel was moved from Shanghai to Xinjiang in a bid to utilize local coal resources.¹⁰⁾ COREX export gas, with CO content of 60–65% and H₂ content of 15–20% after removal of CO₂,^11,12) is produced as a by-product (In the later, CEG refers the COREX export gas after removal of CO₂). The CEG has been used as a reducing agent in the direct reduction of iron in MIDREX process in Saldanha, South Africa.^13,14) In Xinjiang, owing to the COREX process is adjacent to the traditional blast furnace ironmaking plant, the CEG can be utilized as auxiliary fuel or reducing agent in BF process for improving the economic efficiency of CEG and reducing the solid fuel consumption in BF.

BF injection of reducing gas, such as natural gas and coke oven gas (COG), has been extensively studied and applied. For example, the America and former Soviet Union have carried out fruitful practices in injecting natural gas into blast furnaces.^15,16) The industrial-scale practice and experiment of blowing COG in BF has also been widely reported around the word.^17,18,19) For example, the COURSE 50 carried out COG and reformed COG injection operation trials at LKAB’s experimental BF. These successful applications provide a technical guidance for BF injection CEG. Actually, the injection of reducing gas into BF directly influences on the raceway conditions and indirect reduction. Guo et al. analyzed the characteristics of blast furnace raceway with COG injection.²⁰⁾ The combustion reactions of COG in the tuyere of a BF with different lance types were also investigated.²¹⁾ The effects of COG with oxygen enrichment injection into blast furnace on theoretical flame temperature, direct reduction rate and so on are investigated through the heat and mass balance calculation.^22,23,24) Multi-fluid models were developed to study the inner characteristic of blast furnace operation with nature gas or COG injection. Castro et al. developed a multi-phase mathematical model to simulate the BF operation with multiple injection of pulverized coal and natural gas with the blast enriched by oxygen.²⁵⁾ The inner characteristics of BF operation with COG injection were studied though a multi-fluid BF model,²⁶⁾ and the effect of COG injection in combination with top gas recycling were also investigated.²⁷⁾ These works are useful for understanding reducing gas injection in blast furnace, whereas the difference properties between natural gas/COG and CEG makes it impossible for the latter to draw experience directly from natural gas/COG injection. The CEG, with less CH₄ content, has a weaker cooling effect on the raceway zone, and is easier to realize the thermal compensation measures.

In fact, the CEG injection in BF can be regarded as a kind of top gas recycling (TRG) technology in ironmaking field. Previous researchers have established static mathematical model to basic study the inner characteristics and carbon emission in gas-solid countercurrent ironmaking reactors. For example, Zhang J. L. et al. used a mathematical model to study the optimum process parameters of two kinds of nitrogen free blast furnaces (NF-BF) which use different methods to inject recycling top gas.²⁸⁾ Zhang W. et al. developed a static model study the medium oxygen enriched blast furnace with top gas recycling.²⁹⁾ Jin et al. developed a comprehensive model to investigate the energy consumption and carbon emission for the TGR-OBF process.³⁰⁾ Liu et al. developed a static model to analyze the performance of gas- based shaft furnace with top gas recycling and oxygen blowing.³¹⁾ Yao et al. established a mathematical model of COREX process with top gas recycling.³²⁾ These works, especially the static model method, provide an effective reference for the theoretical exploration of CEG injection in BF. However, as the CEG injection in BF is not operation with full oxygen or high oxygen enriched supply,^33,34) the phenomenon in BF with CEG injection are different from that in oxygen BF with top gas recycling, and the information about the CEG injection in BF are rare. Actually, the CEG injection in the shaft zone of BF has been studied,³⁵⁾ while the information about the characteristics of the BF processes such as the temperature in the hearth, the dynamic conditions of the bosh gas have not been reported.

Thus, we firstly developed a static model of BF operation of CEG injection. The influences of CEG injection with no thermal compensation on the raceway adiabatic flame temperature (RAFT), the amount and composition of bosh gas, and the shape of raceway were firstly studied. Then, the appropriate injection volume of CEG under different thermal compensation measures was investigated. The characteristics of high temperature zone of BF under different suitable CEG injection volumes were also studied. The findings of this work lay a foundation for the further research of BF operation of CEG injection, and provide theoretical basis for guiding the actual industrial production and application. The influence of CEG injection on the coke ratio and carbon emission has also been studied by the static model and will be report elsewhere.

2. Establishment of the Model

2.1. Model Calculation

The schematic diagram of COREX export gas and BF operation with CEG injection is shown in Fig. 1. Based on the mass and heat balance, the RAFT, volume and composition of bosh gas, as well as the shape of raceway are calculated.

Fig. 1.

The schematic diagram of COREX export gas (a), and BF operation with CEG (b). (Online version in color.)

2.1.1. Raceway Adiabatic Flame Temperature (RAFT)

  

$$t_{\text{f}}\text{=}\frac{Q_{\text{C}}\text{+}{Q}_{\text{F}}\text{+}{Q}_{\text{R}}\text{+}{Q}_{\text{G}}-Q_{\text{d}}-Q_{\text{x}}}{V_{\text{g}}{C}_{\text{g}}}$$(1)

where, Q_c is the heat released by carbon combustion in raceway, kJ/t; Q_F is the physical heat taken in by hot air, kJ/t; Q_R is the physical heat taken in by CEG, kJ/t; Q_G is the sensible heat taken in by the coke and pulverized coal, kJ/t; Q_d is the decomposition heat of pulverized coal, kJ/t; Q_x is the heat of decomposition of water, kJ/t; V_g is the volume of generated gas, m³/t; C_g is the specific heat capacity of the reducing gas at t_f, kJ/(m³·°C).

2.1.2. Volume and Composition of Bosh Gas

  

$$V_{\text{CO}}\text{=}\frac{22.4}{12}{m}_{\text{C}}\text{+}{V}_{\text{R-CO}}+V_{{\text{R-CO}}_{\text{2}}}$$(2)

  

$$V_{{\text{H}}_{\text{2}}}\text{=}{V}_{\text{F}}\times \phi \text{+}{m}_{\text{f}}\times \left[\omega {\text{(H}}_{\text{2}}\text{)+}\omega {\text{(H}}_{\text{2}}\text{O)}\right]\times \text{(22}\text{.4/2)+}{V}_{{\text{R-H}}_{\text{2}}}$$(3)

  

$$V_{{\text{N}}_{\text{2}}}\text{=}{V}_{\text{F}}\times \text{(}1-\phi \text{)}\times \left[1-\omega {\text{(O}}_{\text{2}}\text{)}\right]\text{+}{V}_{{\text{R-N}}_{\text{2}}}$$(4)

  

$$V_{\text{BG}}\text{=}{V}_{\text{CO}}\text{+}{V}_{{\text{H}}_{\text{2}}}\text{+}{V}_{{\text{N}}_{\text{2}}}$$(5)

where, V_BG, V_CO, V_H2 and V_N2 are the volume of bosh gas and its components, m³/t; V_F is the volume of blast, m³/t; m_c is the carbon combustion in raceway, kg/t; m_f is injection amount of pulverized coal, kg/t; φ is blast humidity, %; ω(H₂) is H₂ content of pulverized coal, %; ω(H₂O) is H₂O content of pulverized coal, %; ω(O₂) is the mass fraction of oxygen in dry air, %; V_R-i is the volume of gas i in CEG, m³/t.

2.1.3. Shape of the Raceway

The calculation of the shape of raceway is based on the previous works.^36,37) The depth, width, height and volume of raceway are calculated through analysis of the blast force, coke gravity and the reverse force of the raceway wall.

2.2. Condition of Calculation

The calculation is carried out on the practical operation conditions of a 2580 m³ blast furnace. The hearth diameter is 11.2 m, the tuyere diameter is 118 mm, the number of tuyere is 30 and the total tuyere area is 0.327 m². The composition of the fuel and CEG are shown in Tables 1 and 2. The blast parameters and thermal compensation measures discussed in this work are shown in Table 3.

Table 1. Chemical compositions of fuel.

	C	Volatile						Ash
		∑	CO	CO2	CH4	H2	N2	∑	SiO2	CaO	MgO	Al2O3	P2O5	FeS
Coke	87.28	1.18	0.398	0.415	0.059	0.105	0.199	11.38	4.89	0.73	0.17	3.00	0.40	2.37
Coal	62.46	23.86		16.89		5.6	1.36	12.46	5.01	1.73	0.58	2.31	0.03	0.16

Table 2. Chemical compositions of COREX top gas.

Parameters	CO2	CO	H2	CH4	N2
COREX export gas	35	45	10	0.4	9.5
CEG	5	62	14.3	0.6	13.6

Table 3. Blast conditions of BF.

Parameters	Value
Blast temperature/°C	1120 (Base), 1150, 1180, 1210
Oxygen flow rate/m3·min−1	848.8 (Base), 874.2, 899.7, 925.2
blast humidity/g·m−3	12 (Base), 9, 6, 3
coal injection volume/kg·t−1	100

3. Results and Discussion

3.1. CEG Injection with no Thermal Compensation

Under the condition of maintain the blast parameters, such as the blast temperature, amount of coal injection and blast humidity, the RAFT and volume of bosh gas after CEG injection is shown in Fig. 2. It can be seen that the RAFT is 2120°C and volume of bosh gas is 1692 m³/t with no CEG injection. With the increase of CEG injection, the RAFT decreases but the volume of bosh gas increases. For every 10 m³/t increase of CEG injection volume, the RAFT is reduced by 9.68°C and the volume of bosh gas is increased by 9.96 m³/t. Previous work has found that the RAFT decreased by 1.2–1.5°C for 1 m³ COG injected into BF to replace coal.²³⁾ In our model, the influence of COG injection on the RAFT also studied. As shown in Fig. 2, the RAFT decreases with the increase of COG volume. The slop of the reduction is 1.18°C/(m³/t). This result is consistent with the previous calculation data. Compared with COG injection, the RAFT of CEG injection has a less reduction. That is due to the CEG containing less CH₄, has less decomposition heat absorption. Therefore, the thermal compensation of CEG injection may be easier to realize than COG injection.

Fig. 2.

Effect of injection volume on the RAFT and volume of bosh gas. (Online version in color.)

Figure 3 shows the change of composition of bosh gas with the CEG injection volume. It can be seen the content of CO and H₂ increases but the content of N₂ decreases with the increase of CEG injection. The concentration of total reducing gas increases significantly. It can be predicted that with the injection of CEG, the content of reducing gas in BF increases, which is conducive to promoting the development of indirect reduction in the furnace and reducing the degree of direct reduction, thereby contributing to the reduction of coke consumption in BF. A detail investigation about the effect of CEG injection on the degree of direct reduction and coke ratio will be reported in our other work.

Fig. 3.

Change of composition of bosh gas with CEG injection volume. (Online version in color.)

Effect of CEG injection on the size of raceway is shown in Fig. 4. On the base condition with no CEG injection, the depth, width, height and volume of the raceway are 1.43 m, 0.71 m, 1.1 m and 0.59 m³ respectively. With the increase of CEG injection, the volume flow rate and the kinetic energy of gas increase, resulting in a significant increase in the depth of raceway and corresponding the volume of the raceway zone. For every 100 m³/t increase in CEG injection volume, the depth of the raceway is increased by 0.105 m and the volume is increased by 0.068 m³.

Fig. 4.

Effect of reducing gas injection on the size of raceway. (Online version in color.)

3.2. CEG Injection with Thermal Compensation

Without taking thermal compensation measures, the CEG injection in BF makes the RAFT lower. In order to maintain the fluidity of the slag and iron in hearth and ensure the smooth operation, it is necessary to achieve an appropriate RAFT, and keep the heat in the high temperature zone. In this work, the heat balance in the high temperature zone under thermal compensation is consist with the base case with no CEG injection. Since the gas composition of the bosh gas is CO, H₂, and N₂, their specific heats are approximately the same by the volume method.¹⁹⁾ Therefore, it can be considered that the heat in the high-temperature region is approximately equal when the RAFT and the amount of bosh gas of the furnace remain the same before and after CEG injection. Effects of thermal compensation on the CEG injection volume and characteristic about the high temperature zone will be discussed.

3.2.1. Addition of Oxygen

In the base case with no CEG injection, the oxygen flow rate is 848.8 m³/min, and the oxygen enrichment rate is 0 (O₂ concentration is 21%). In this study, the thermal compensation measure considers increasing the total oxygen content by 3%, 6% and 9% respectively, that is, the flow rates are 874.2, 899.7, 925.2 m³/min, respectively. Figure 5 shows the acceptable quantities of CEG under different blast oxygen volumes. The RAFT increases with the increase of oxygen content. The total oxygen content increases by 1 m³/min, the RAFT increases by 1.33°C. When the RAFT is consists with the base case with no CEG injection, the acceptable quantities of CEG under different oxygen content are 72.7, 142.4 and 212.9 m³/t, respectively.

Fig. 5.

Acceptable quantities of CEG under different blast oxygen volumes. (Online version in color.)

In order to keep the volume of bosh gas constant, when the total oxygen content increases, it should reduce the amount of N₂ blown in, thereby reducing the blast volume. Figure 6(a) shows the blast volume under different blast oxygen volumes. If the oxygen flow rate increases by 1 m³/min, the blast volume decreases by 2.55 m³/min. For different oxygen levels, the blast volume is 3754, 3418, 3079 m³/min when the appropriate volumes of CEG are injected. The oxygen enrichment under different blast oxygen volumes is shown in shown in Fig. 6(b). At different total oxygen levels, when the appropriate amount of CEG gas is injected, the amount of oxygen that needs to be increased relative to the base condition is 111.9, 221, 330.8 m³/min, and the corresponding oxygen enrichment rates are 2.29%, 4.80%, and 7.66% respectively.

Fig. 6.

Blast volume (a) and oxygen enrichment (b) under different blast oxygen volumes. (Online version in color.)

Figure 7 shows the volume fraction of reducing gas in bosh gas under different blast oxygen volumes. As descripted in Fig. 3, with the increase of CEG injection, the content of CO and H₂ increases. Especially when the total oxygen content is increased, the concentration of reducing gas is further increased. At different total oxygen levels, when the appropriate amount of CEG gas is injected, the volume fraction of reducing gas is 43.56, 47.72, 51.93%, which are respectively 4.31, 8.47, 12.68% higher than the volume fraction in base case with no CEG injection. This increase in the concentration of reducing gas is conducive to promoting the development of indirect reduction in the BF, thereby reducing the coke ratio. However, an increase in the total oxygen amount will cause an increase in the coke consumption in the raceway. Detail about this information will be reported elsewhere.

Fig. 7.

Volume fraction of reducing gas under different blast oxygen volumes. (Online version in color.)

The depth of raceway under different blast oxygen volumes is shown in Fig. 8. For the same oxygen content, with the increase of the CEG injection volume, the depth of raceway gradually decreases. The main reason is that with a fixed amount of bosh gas, the RAFT decreases with the increase of CEG injection. If the total oxygen content increases, the blast volume decreases more, and the kinetic energy of the air flow in the tuyere decreases, resulting a decrease of the raceway depth. At different oxygen levels, the depth of raceway is 1.371, 1.318 and 1.263 m when the appropriate volumes of CEG are injected.

Fig. 8.

Depth of raceway under different blast oxygen volumes. (Online version in color.)

3.2.2. Reducing Blast Humidity

Reducing humidity can effectively reduce the heat of coke solution loss with H₂O, which is one of the main measures to provide thermal compensation. Figure 9 illustrates the acceptable quantities of CEG under different blast humidity. With decrease the humidity, the RAFT increases. The main reason is the reducing of the heat of decomposition of water. The RAFT could increase by 0.891°C when the humidity decreases by 1 g/m³. Considering that the RAFT is constant, the acceptable quantities of CEG under different humidity are 22, 40.3 and 58.9 m³/t, respectively.

Fig. 9.

Acceptable quantities of CEG under different blast humidity. (Online version in color.)

Figure 10(a) shows the blast volume under different humidity. Under the same CEG injection condition, the blast volume increases with the decrease of humidity. The blast volume increases by 2.58 m³/t for every 1 g/m³ decrease of humidity. For different humidity, if the acceptable quantities of CEG are injected in the BF, the corresponding blast volumes are 4040, 3994, 3948 m³/min. The oxygen enrichment under different humidity is shown in Fig. 10(b). The oxygen enrichment decreases with the decrease of humidity content. At different humidity levels, under each acceptable CEG injection volume, the amount of oxygen that needs to be increased relative to the base condition is 11, 18.4, 25.8 m³/min, and the corresponding oxygen enrichment rates are 0.22%, 0.36%, and 0.51% respectively.

Fig. 10.

Blast volume (a) and oxygen enrichment (b) under different blast humidity. (Online version in color.)

The volume fraction of reducing gas under different humidity is shown in Fig. 11. Since the coke solution loss with H₂O generates H₂ and CO, the volume of reducing gas in bosh gas increases with the increase of blast humidity, and the change range is 0.0579%/(g·m⁻³). At different humidity levels, under each acceptable CEG injection volume, the volume fractions of reducing gas are 39.69, 39.96, 40.23%, respectively.

Fig. 11.

Volume fraction of reducing gas under different humidity. (Online version in color.)

Figure 12 shows the depth of raceway under different humidity. With the decrease of humidity, the depth of raceway increases. For every 1 g/m³ reduction of blast humidity, the depth of raceway increases by 0.0015 m. For different humidity, when the acceptable volume of CEG is blasted, the depths of raceway are 1.433, 1.438, 1.442 m.

Fig. 12.

Depth of raceway under different humidity. (Online version in color.)

3.2.3. Increasing Blast Temperature

Improving blast temperature is one of the best thermal compensation measures for injecting reducing gas into BF. Figure 13 shows the acceptable quantities of CEG under different blast temperatures. It can be seen that with the increase of blast temperature, the RAFT increases by 0.67°C/°C. When the blast temperature rises from the based 1120°C to 1150, 1180, 1210°C, the corresponding suitable CEG injection volume is 45.4, 85.5, 123.3 m³/t, respectively.

Fig. 13.

Acceptable quantities of CEG under different blast temperatures. (Online version in color.)

Figure 14 shows the blast volume and oxygen enrichment ratio under different blast humidity. As the increase of blast temperature leading to an increase of CEG injection volume, it is necessary to reduce the blast volume to maintain the constant bosh gas volume. When the appropriate CEG injection volume is 45.4, 85.5, and 123.3 m³/t, the blast volume is reduced to 3925, 3769, and 3622 m³/min compared with the reference 4102 m³/min. At the same time, the blast oxygen enrichment rate increased from based non-oxygen enrichment to 0.84%, 1.62%, and 2.41%, respectively. However, it should be pointed out that, under the condition of increasing blast temperature, although the oxygen enrichment rate increases, the total oxygen content in the raceway is consistent. In this way, the influence of coke consumption in the raceway caused by oxygen change can be reduced, thereby avoiding fluctuations in the BF.

Fig. 14.

Blast volume and oxygen enrichment rate under different blast humidity. (Online version in color.)

Figure 15 shows the bosh gas compositions under different acceptable quantities of CEG. The CO and H₂ contents in the raceway increased with the increase of the injection volume of CEG. The main reason is the increase of reducing gas brought by CEG. In addition, as the blast volume decreases with the increase of CEG injection, the proportion of N₂ carried by the blast decreases correspondingly, which makes the N₂ concentration of the gas in the bosh decrease. For every 10 m³/t CEG injection, the total reducing gas content in the bosh gas increases by 0.46%.

Fig. 15.

Bosh gas compositions under different acceptable quantities of CEG. (Online version in color.)

Figure 16 shows the raceway sizes under different acceptable quantities of CEG. With the increase of CEG injection volume, the measures of reducing air and enriching oxygen are adopted, and the depth, width, height and volume of the raceway are reduced to some extent. When the suitable CEG injection volume is 123.3 m³/t, the depth and volume of the gyre are reduced by 0.05 m and 5.19% respectively.

Fig. 16.

Raceway sizes under different acceptable quantities of CEG. (Online version in color.)

4. Conclusions

The characteristics of BF operation of CEG were investigated by means of a static model. The effect of thermal compensation measures on the CEG injection was studied. The following conclusions could be drawn:

(1) Under no thermal compensation, with the increase of CEG injection, the RAFT decreases but the volume of bosh gas increases. The content of CO and H₂ increases but the content of N₂ decreases with the increase of CEG injection.

(2) Based on the standard of maintaining the RAFT and volume of bosh gas, the acceptable quantity for CEG injection is 72.7, 142.4 and 212.9 m³/t after the total oxygen content is increased by 3%, 6% and 9%. At different oxygen levels, with the increase of acceptable quantities of CEG, the blast volume and depth of raceway decrease but the oxygen enrichment and volume fraction of reducing gas increase.

(3) The acceptable quantity for CEG injection is 22, 40.3 and 58.9 m³/t after the blast humidity decreases to 9, 6, 3 g/m³. At different humidity levels, the oxygen enrichment, volume fraction of reducing gas and depth of raceway increase with the increase of acceptable quantities of CEG. The blast volume shows an opposite tendency.

(4) When the blast temperature rises from the based 1120°C to 1150, 1180, 1210°C, the corresponding suitable CEG injection volume is 45.4, 85.5, 123.3 m³/t, respectively. The CO and H₂ contents in the raceway increased with the increase of the injection volume of CEG. With the increase of CEG injection volume, the raceway size decreases.

Further simulation will be carried out to investigate the multi-phase transport in BF with CEG injection through a CFD model. This will also take into account the influence of different reducing gases on the inner characteristics of BF. The comparison with the research results previously published will also be discussed. Work on these aspects is in progress and will be reported hopefully in the near future.

Acknowledgments

The authors would like to thank the National Natural Science Foundation of China (Grant Number: 51904023, 51804027), the Fundamental Research Funds for the Central Universities (Grant Number: FRF-TP-19-035A2), the project of State Key Laboratory of Advanced Metallurgy (Grant Number: KF20-07) and Australian Research Council (DP180101232) for their financial supports.

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