Effect of Local Oxygen-enrichment Ways of Oxygen-coal Double Lance on Coal Combustion

Abstract

The extent of coal combustion within the tuyere and raceway region is one key factor affecting the maximum pulverized coal injection (PCI) rate. Oxygen enrichment, especially local oxygen enrichment, is the most effective way to increase the PCI rate. In this study, a three-dimensional numerical model was developed to simulate the lance-blowpipe-tuyere-raceway of a blast furnace. In the study, the characteristics of oxygen-coal combustion are investigated under the single oxygen-coal and oxygen-coal double lances. Under local oxygen enrichment ways, the oxygen content around the coal particles increases significantly, benefiting coal combustion. However, the cooling effect of room-temperature oxygen delays coal combustion. Therefore, the way by which the oxygen flows should not be neglected. The results indicate that the increase in the burnout is quite different under different lance patterns. The burnout had the maximum increase of 2.17% under the coaxial oxygen-coal lance. The burnout had the highest increase of 12.84% under the oxygen-coal double lance.

1. Introduction

Blast furnaces remain the main facilities for hot metal production due to their superiority in productivity and heat utilization.^1,2,3) During blast furnace operation, the hot blast of 1100°C to 1250°C is blown into the furnace through tuyeres, and reacts with coke in the raceway region to generate heat and reducing gas for hot metal production. However, due to high coke prices and pollution in coke-making, some cheaper auxiliary fuels are injected into the blast furnace as the substitutes of partial coke. Because of its low price and widespread distribution, coal has been widely used in the operation of blast furnace injection. Therefore, improving the coal ratio has been a main problem of ironmaking for a long time.⁴⁾ However, the extent of coal combustion in the raceway region is the main factor affecting the maximum pulverized coal injection (PCI) rate. Practically, the burnout can be increased to a certain degree by adjusting some operating parameters, such as oxygen enrichment, high blast temperature, coal bend, and so on.^5,6) Oxygen enrichment, especially local oxygen enrichment is the most effective way to solve this problem.^7,8,9,10)

There are two main ways of oxygen enrichment, namely, conventional oxygen enrichment (stove oxygen enrichment) and local oxygen enrichment. For local oxygen enrichment, the coaxial oxygen-coal lance is the popular way of local oxygen enrichment. In this method, room-temperature oxygen flows through the annulus of the lance, and this lance has been adopted by many blast furnaces.^11,12,13,14) However, the information regarding the operation of oxygen enrichment in PCI remains insufficient.

Because of the high temperature and harsh environment of the blast furnace, it is not practical and expensive to carry out a trial of coal injection in a real blast furnace. And due to the limitation of many conditions, the results may be not reliable.¹⁵⁾ The development of computational fluid dynamics (CFD) and its application in coal combustion provide a new research method for blast furnace PCI. Many studies have been carried out on blast furnace PCI using CFD. Wijayanta et al.^16,17) investigated pulverized biochar injection in blast furnace using CFD. Shen et al.^6,18,19,20) carried out many studies on the flow and combustion behaviors of pulverized coal in blast furnace using CFD, and the validation of the models is validated by a pilot-scale test rig. Many CFD investigations on pulverized coal injection have demonstrated the reliability of this approach.

To further recognize oxygen-coal combustion characteristics and optimize local oxygen enrichment ways, in this study, a three-dimensional numerical model was developed to simulate the lance-blowpipe-tuyere-raceway of a blast furnace using CFD. The model aims to describe the flow and combustion behaviors of pulverized coal along the coal plume under different local oxygen enrichment ways. In this study, the coaxial oxygen-coal and oxygen-coal double lances are designed. The oxygen-coal combustion and flow characteristics are investigated under different lance patterns. The oxygen content around the coal particles increases greatly under local oxygen enrichment, benefiting coal combustion. However, the cooling effect of room-temperature delays coal combustion. Therefore, if the local oxygen-enrichment way is inappropriate, the burnout may decrease. The simulation result is able to provide a useful insight into the coal combustion within the tuyere and raceway region and some guidance for the practical operation of oxygen enrichment in PCI.

2. Model Description

The gas-particle flow and coal combustion in the tuyere and raceway region were calculated based on the framework of software package ANSYS–FLUENT. The mathematical formulation is described below.

2.1. Basic Equations

In this model, the gas phase is treated with an Eulerian frame and described by the steady-state Reynolds-averaged Navier–Stokes equations closed by a k–ε turbulence model.²¹⁾ The governing equations for the gas phases are summarized in Table 1.

Table 1. Governing equations for the gas phase.

Mass	$\nabla \cdot \left(\rho U\right)=\sum _{n_P}\dot{m}$
Momentum	$\begin{array}{l}\nabla \cdot (\rho UU)-\nabla \cdot \left[(\mu +{\mu }_t)\left(\nabla U+{(\nabla U)}^T\right)\right]\\ =-\nabla \left(P+\frac{2}{3}\rho k\right)+\sum _{n_P}{f}_D\end{array}$
Energy	$\nabla \cdot \left[\rho UH-\left(\frac{\lambda }{C_P}+\frac{{\mu }_t}{{\sigma }_H}\right)\right]=\sum _{n_P}q$
Gas species	$\nabla \cdot \left[\rho U{Y}_i-\left({\mathrm{\Gamma }}_i+\frac{{\mu }_t}{{\sigma }_{Yi}}\right)\right]=W_i$
Turbulent kinetic energy	$\nabla \cdot \left[\rho Uk-\left(\mu +\frac{{\mu }_t}{{\sigma }_k}\right)\nabla k\right]=P_k-\rho \epsilon$
Turbulent dissipation rate	$\nabla \cdot \left[\rho U\epsilon -\left(\mu +\frac{{\mu }_t}{{\sigma }_{\epsilon }}\right)\nabla \epsilon \right]=\frac{\epsilon }{k}\left(C_1{P}_k-C_2\rho \epsilon \right)$

Particles of pulverized coal are treated as discrete phase, modeled using the Lagrangian method, where the trajectories of the discrete particles are determined by integrating Newton’s second law of motion. The drag force (f_D) and turbulence dispersion are included. Full coupling of mass, momentum, and energy of particles with the gas phase is implemented. The change of particle temperature is governed by three physical processes: convective heat transfer, latent heat transfer associated with mass transfer, and radiative heat transfer. The governing equations for the particle phase are summarized in Table 2.

Table 2. Governing equations for the particle phase.

Mass	$\frac{d{m}_p}{dt}=-\dot{m}$
Momentum	$\begin{array}{l}{m}_p\frac{d{u}_p}{dt}=-f_D\\ \hspace{0.5em}-f_D=\frac{1}{8}\pi d_p^2\rho C_D\left|U-U_p\right|\left(U-U_p\right)\end{array}$
Energy	$\begin{array}{l}{m}_p{C}_p\frac{d{T}_P}{dt}=-q\\ \pi d_p\lambda N_{\mu }\left(T_g-T_p\right)+\sum \frac{d{m}_p}{dt}{H}_{reac}+A_p{\epsilon }_p\left(\pi I-{\sigma }_B{T}_p^4\right)\end{array}$

2.2. Combustion Process

Coal combustion is regarded as a multistage overlapping process: (i) preheating, (ii) the devolatilization of raw coal, (iii) the volatiles combustion, (iv) the oxidation/gasification of the residual char.

2.2.1. Release of Volatiles

The devolatilization process releases volatiles (C_αH_βO_γN_δ) and char (C(s)). The release of volatiles is simulated using the so-called two-competing-reactions model.²²⁾ A pair of reactions with different rates (k₁,k₂) and volatile yields (α₁,α₂) compete to pyrolyse the raw coal.   

$$\mathrm{R}\mathrm{a}\mathrm{w}\mathrm{\hspace{0.17em} }\mathrm{c}\mathrm{o}\mathrm{a}\mathrm{l}{\mathrm{\hspace{0.17em} }}_{\searrow }^{\nearrow }\begin{array}{l}\mathrm{\hspace{0.17em} }{\alpha }_{1}V{M}_1+(1-{\alpha }_1)C_1\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\hfill \\ \mathrm{\hspace{0.17em} }{\alpha }_{2}V{M}_2+(1-{\alpha }_2)C_2\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\hfill \end{array}\begin{array}{l}(\mathrm{L}\mathrm{o}\mathrm{w}\mathrm{\hspace{0.17em} }\mathrm{T}\mathrm{e}\mathrm{m}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e})\hfill \\ (\mathrm{H}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{\hspace{0.17em} }\mathrm{T}\mathrm{e}\mathrm{m}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e})\hfill \end{array}$$

The rate constants k₁ and k₂ are in the Arrhenius format:²³⁾   

$$k=Aexp(-E/T)$$(1)

Where A represents the Arrhenius rate constant, E represents the activation energy. A and E are 3.7×10⁵ s⁻¹ and 18000 K at low temperature, and 1.46×10¹³ s⁻¹ and 30189 K at high temperature.

The rate of volatiles production is given by the following:   

$$\frac{dw(VM)}{dt}=\left({\alpha }_1{k}_1+{\alpha }_2{k}_2\right)C_0$$(2)

2.2.2. Gas Combustion

The volatiles are treated as one fuel gas (C_αH_βO_γN_δ), and its combustion is simplistically represented by two overall reactions as follows:   

$$\left({\mathrm{C}}_{\alpha }{\mathrm{H}}_{\beta }{\mathrm{O}}_{\gamma }{\mathrm{N}}_{\delta }\right)+\mathrm{a}{\mathrm{O}}_2\to \mathrm{b}\mathrm{C}\mathrm{O}+\mathrm{c}{\mathrm{H}}_2\mathrm{O}+\mathrm{d}{\mathrm{N}}_2$$R₁

  

$$\text{CO}+\text{0}{\text{.5O}}_{\text{2}}\to {\text{CO}}_{\text{2}}$$R₂

For the gas combustion, in addition to the combustion of volatiles, the combustion of hydrogen is included:   

$${\text{H}}_{\text{2}}+\text{0}{\text{.5O}}_{\text{2}}\to {\text{H}}_{\text{2}}\text{O}$$R₃

In turbulent flow, the gas reaction mechanisms are represented by finite rate/eddy dissipation model. In this model, the reaction rates of Arrhenius model, R_g,arr and the Eddy Break-Up turbulence chemistry interaction model, R_g,EBU,R for reactants and R_g,EBU,P for products, are calculated. The net reaction rate is taken as the minimum of these three rates. They are expressed as follows:   

$$R_{g,Arr}=\left({\nu }_p-{\nu }_R\right)M_w\left(k_{r,f}\prod _R{C}_R^{{\alpha }_R}-k_{r,b}\prod _P{C}_P^{{\alpha }_P}\right)$$(3)

  

$$R_{g,EBU,R}={\nu }_R{M}_{w}A\rho \frac{\epsilon }{k}\underset{R}{min}\left(\frac{Y_R}{{\nu }_R{M}_{w,R}}\right)$$(4)

  

$$R_{g,EBU,P}={\nu }_p{M}_{w}AB\rho \frac{\epsilon }{k}\frac{\sum _P{Y}_P}{\sum _P{\nu }_P{M}_{w,p}}$$(5)

where ν, M_w, Y, and C are stoichiometric coefficient, molecular weight, mass fraction and molar concentration [kmol/m³] of the corresponding species, respectively. α is rate exponent for reactants and products. ρ represents the density [kg/m³]. A and B are the empirical parameters. The Arrhenius kinetic rate of reaction is defined as:   

$$k_r=A_r{T}^{{\beta }_r}{e}^{-E_r/RT}$$(6)

where, A_r, E_r and β_r represents the Arrhenius rate constant, the activation energy and the temperature exponent, respectively.

2.2.3. Oxidation/gasification of the Residual Char

For the char reactions, the heterogeneous surface reactions model is used. The solid reactions include the following processes.   

$$\text{C(s)}+\text{0}{\text{.5O}}_{\text{2}}\to \text{CO}$$R₄

  

$$\text{C(s)}+{\text{CO}}_{\text{2}}\to 2\text{CO}$$R₅

  

$$\text{C(s)}+{\text{H}}_{\text{2}}\text{O}\to \text{CO}+{\text{H}}_{\text{2}}$$R₆

The reaction rate is expressed as follows:   

$$R_{j,r}=A_p{\eta }_r{Y}_{j}P\frac{k_r{D}_{0,r}}{D_{0,r}+k_r}$$(7)

where A_p and η_r indicate the particle surface area [m²] and effectiveness factor, respectively. Y_j is the mass fraction of species j. P represents the partial pressure [Pa]. The diffusion rate coefficient, D_0,r, for reaction r is computed as:   

$$D_{0,r}=C_{j,r}\frac{{\left[\left(T_{\text{p}}+T_{\text{g}}\right)/2\right]}^{0.75}}{d_{\text{P}}}$$(8)

where C_j,r is the molar concentration of species j in reaction r. d_p represents particle diameter [m]. The kinetic parameters to determine k_r using the Arrhenius expression are summarized in Table 3.

Table 3. Kinetic reaction parameter.

Reaction	Ar	βr	Er [J/kmol]
R2	2.2×1011	0	1.67×1018
R3	6.8×1015	0	1.68×1018
R4	1.36×106	0.68	1.3×108
R5	6.78×104	0.73	1.63×108
R6	8.55×104	0.84	1.4×108

2.3. Geometry and Operating Condition

An oxygen blast furnace of 120 m³ will be built in a steel corporation in China. Therefore, some investigations are carried out based on a commercial blast furnace of 120 m³ with 8 tuyeres. The main operational parameters of the blast furnace are summarized in Table 4. The properties of coal used in this model are shown in Table 5.

Table 4. Main parameters of the blast furnace.

Blast Volume (Nm3/h)	Blast Temperature (K)	Volume (m3)	Coal Ratio (kg/t)
19766.78	1473	120	150

Table 5. Properties of the coal.

Proximate analysis (wt%)		Ultimate analysis (wt%)		Size distribution
Moisture	1	C	88.78	90 μm: 5%
Volatiles	20.02	H	4.54	63 μm: 25%
Ash	8.32	O	4.68	45 μm: 55%
Fixed Carbon	70.66	N	2	20 μm: 15%

The model simulates the lance-blowpipe-tuyere-raceway of a practical blast furnace of 120 m³. The blast furnace was dissected in 2007. The results of blast furnace dissection show that the depth of the raceway was about 700 mm. The detailed parameters of this model are shown in Fig. 1. For the base case and coaxial oxygen-coal lance, the coal lance tip is located on the centerline of the raceway. The base case is used for the investigation of the effect of conventional oxygen enrichment on coal combustion. The coaxial oxygen-coal and oxygen-coal double lances are used for the investigation of the effect of local oxygen enrichment on coal combustion. The main focus was the flow and combustion behaviors of pulverized coal in the coal plume formed, which plays a significant role in determining the permeability of the birdnest. The raceway was designed as a tube of 700 mm long with a divergence angle of 3° referring to others.¹⁸⁾ Because this geometric setting could avoid the formation of flow recirculation, so that this simulation mainly focuses on coal flow and combustion behaviors of pulverized coal in the coal plume formed.

Fig. 1.

Geometry of the model and configurations of the lances. (Online version in color.)

3. Results and Discussion

3.1 Model Validation

The combustion behavior of pulverized coal in blast furnace has been investigated by some researchers using CFD, and the model has been validated by experimental data or measurements in a real blast furnace.^{17,18,24,25,26)} Some of the results obtained in this study have been compared with others.^18,24,25) The results and phenomena are similar to others. Therefore, the validation of this model is acceptable.

3.2. Coal Combustion Characteristics under Conventional Oxygen Enrichment

Figure 2 shows the effect of oxygen content in hot blast on the coal burnout of the raceway outlet. The coal burnout increases from 73.16% to 79.31%, an increase of 6.15%, when oxygen content increases from 21% to 27%. The results are similar to the data reported by Shen.¹⁸⁾ This is because at a high oxygen content level, more oxygen is available around the coal particles. The conventional oxygen enrichment way can partly increases the coal burnout, but it is not obvious. This is because the oxygen content directly related to the coal combustion does not increase significantly. Compared to conventional oxygen enrichment, the local oxygen enrichment way can increase the oxygen content around the coal particles significantly, benefiting coal combustion.

Fig. 2.

Effect of oxygen content in hot blast on coal burnout of the raceway outlet.

3.3. Coal Flow and Combustion Characteristics under Coaxial Oxygen-Coal Lance

Figure 3 shows the effect of oxygen content on the coal burnout of the raceway outlet under coaxial oxygen-coal lance. When oxygen content is 22–24%, the burnout is higher than that of the base case. When oxygen content is 23%, the burnout reaches the maximum value of 75.33%, an increase of 2.17%. When oxygen content is 25–27%, the burnout is lower than the base value. Particularly, when oxygen content is 27%, the burnout is only 67.42%, a decrease of 5.74%. The reasons causing aforementioned phenomena are as follows: The oxygen content around the coal particles increases significantly, benefiting coal combustion. However, the cooling effect of room-temperature oxygen delays the release of volatiles, and the entire coal combustion process is delayed. Moreover, the oxygen flowing through the annulus of the lance limits the dispersion of the coal particles. Therefore, the coal burnout does not clearly increase under the coaxial oxygen-coal lance. The results and phenomena are similar to that reported by Du.²⁴⁾

Fig. 3.

Effect of oxygen content on coal burnout of the raceway outlet under coaxial oxygen-coal lance.

To further recognize the coal flow and combustion behaviors under the coaxial oxygen-coal lance, the burnout distributions of the raceway outlet at different oxygen content is investigated, as shown in Fig. 4. Each point represents one coal particle, and the color indicates the value of the coal burnout. For the base case, the coal particles are mainly concentrated in the lower part of the raceway. The coal particles around the coal plume are more dispersed and combust completely. The burnout of the coal particles in the center is less due to the lack of oxygen. Furthermore, the coal particles in the center are less dispersed. For the coaxial oxygen-coal lance, the coal plume moves downward, and the coal particles are less dispersed compared to the base case. Furthermore, the effect becomes more obvious with the increase of oxygen content. With the increase of oxygen content, the coal particles of less-burnout around the coal plume decrease. The results indicate the limitation of oxygen flowing from the annulus of the lance on coal particles dispersion and volatiles release is the main factor affecting coal combustion.

Fig. 4.

Coal burnout distributions of the raceway outlet under coaxial oxygen-coal lance. (Online version in color.)

3.4. Coal Flow and Combustion Characteristics under Oxygen-Coal Double Lance

To eliminate the effect of oxygen flowing through the annulus of coaxial oxygen-coal lance on dispersion of the coal particles, the oxygen-coal double lance is designed, as shown in Fig. 1(c). In the setting of oxygen-coal double lance, the oxygen lance tip is located on the centerline of the tuyere. The coal lance tip moves upstream of the blowpipe by 20 mm compared to the base case and coaxial oxygen-coal lance. Moreover, in the investigation and optimization of the oxygen-coal double lance the coal lance is fixed, and the oxygen lance moves along the horizontal or vertical direction.

Figure 5 shows the effect of oxygen content on the coal burnout of the raceway outlet under oxygen-coal double lance. When oxygen content is 22–25%, the burnout is higher than the base value. When oxygen content is 23%, the burnout reaches the maximum value of 76.59%, an increase of 3.43%. When oxygen content is ≥24%, the burnout decreases with more oxygen is added. When oxygen content is 27%, the burnout is only 68.11%, a decrease of 5.05%. Compared to the coaxial oxygen-coal lance, the burnout slightly increases at the same oxygen content. This is because the limit of oxygen on the dispersion of coal particles decreases. However, the cooling effect of room-temperature oxygen on the coal combustion does not decrease obviously.

Fig. 5.

Effect of oxygen content on coal burnout of the raceway outlet under oxygen-coal double lance.

The burnout does not clearly increase under the aforementioned oxygen-coal double lance. This is mainly because the cooling effect of room-temperature oxygen delays the coal combustion. To overcome this problem, the effect of the distance between oxygen and coal lance tips in the horizontal direction on the burnout of the raceway outlet is investigated, as shown in Fig. 6(a). In the setting, the coal lance is fixed, and the oxygen lance tip moves along the horizontal direction.

Fig. 6(a).

Effect of distance between oxygen and coal lance tips in the horizontal direction on coal burnout of the raceway outlet. (Online version in color.)

The burnout increases with the increase of the distance between the coal and oxygen lance tips in the horizontal direction. When the distance is ≥100 mm, the coal burnout is higher than the base value, as oxygen content is 22–27%. This is because the preheating distance of the coal particles is extended, and the cooling effect of room-temperature oxygen decreases with the increase of the distance. Furthermore, the oxygen content around the coal particles increases significantly, benefiting coal combustion. Particularly, when the distance is 200 mm, the burnout significantly increases, as shown in Fig. 6(b). When oxygen content is only 22%, the burnout is 83.64%, an increase of 10.48%. When oxygen content is 24%, the burnout reaches the maximum value of 86%, an increase of 12.84%. When more oxygen is added, the burnout slightly decreases. Although when the oxygen content is 27%, the burnout can reach the value of 83.82%, an increase of 10.66%. When oxygen content is ≥22%, the coal burnout is less affected by oxygen content. This is because the cooling effect of room-temperature oxygen increases with the increase of oxygen content. To further recognize the flow and combustion characteristics of pulverized coal, the gas temperature distributions on different cross-section planes when the distance is 200 mm are investigated, as shown in Fig. 7.

Fig. 6(b).

Effect of oxygen content on coal burnout of the raceway outlet at the distance of 200 mm.

Fig. 7.

Gas temperature distributions on different cross-section planes along the raceway. (Online version in color.)

When oxygen content is 21%, the high-temperature zone is mainly concentrated in the bottom of the raceway, and the highest temperature of the raceway outlet is about 3000 K. This is because the coal particles are mainly concentrated in the lower part of the raceway under the influence of the gas flow. Moreover, Fig. 4 shows that the coal particles at the bottom of the raceway combust completely. Under the oxygen-coal double lance, the high-temperature zone moves upward. This is because the coal lance moves upward, and the coal plume moves upward. Moreover, the high-temperature zone enlarges. Figure 4 shows that the coal particles of low-burnout are mainly concentrated in the lower part of the raceway. When the coal lance moves upward, the coal plume moves upward, and the oxygen stream flows into the low-burnout region of the coal plume. Moreover, the coal particles are more dispersed under the impact of the oxygen stream. Therefore, the coal burnout increases significantly, and the combustion region enlarges. However, the highest temperature of the raceway outlet decreases. This is because room-temperature oxygen absorbs parts of heat, reducing the gas temperature. Moreover, the coal particles are more dispersed, and the heat is not concentrated in some region. The highest temperature of the raceway outlet increases with the increase of oxygen content. When oxygen content is 23%, the highest temperature is about 2400 K. When oxygen content is 25%, the highest temperature of the raceway outlet is about 2500 K. When oxygen content is 27%, the highest temperature of the raceway outlet is about 2600 K. This is because the cooling effect of room-temperature oxygen delays the entire coal combustion process, and the effect increases with the increase of oxygen content. The coal combustion is concentrated at the end of the raceway, releasing much heat and causing a higher temperature at the raceway outlet.

Figure 8 shows the coal burnout distributions of the raceway outlet under the oxygen-coal double lance, when the distance between the two lance tips in the horizontal direction is 200 mm. Compared to the base case, the coal plume moves upward, and the coal particles are more dispersed under the influence of oxygen flow. Under the oxygen-coal double lance, a high-burnout zone is observed in the center of the coal plume. The reasons are as follows: Under the base case, the coal particles of lower-burnout are mainly concentrated in the center of the coal plume, where the oxygen content is the main factor affecting coal combustion. Under the oxygen-coal double lance, the coal plume moves upward, and the room-temperature oxygen flows into the center of the coal plume. The oxygen concentration in the center of the coal plume increases significantly, and the burnout rapidly increases. Furthermore, the coal particles are more dispersed with the increase of oxygen content. This is because the coal particles move around the coal plume under the influence of the oxygen flow, and the effect increases with the increase of oxygen content.

Fig. 8.

Coal burnout distributions of the raceway out under oxygen-coal double lance. (Online version in color.)

To further optimize the oxygen-coal double lance, the distance between the coal and oxygen lance tips are kept as 200 mm in the horizontal direction. The effect of the position of the oxygen lance tip in the direction of Y axis on coal combustion is investigated. The values of Y-coordinate of oxygen lance tip are set as 0 mm, −15 mm and −30 mm, respectively, and the effect on the burnout of the raceway outlet is shown in Fig. 9. The burnout decreases, when the oxygen lance tip moves downward. The reasons are as follows: Under the oxygen-coal double lance, the coal plume moves upward, and the coal particles at the bottom of the raceway are less. With the oxygen lance tip moving downward, the room-temperature oxygen is mainly concentrated in the lower part of the raceway, and the coal particles at the lower part of the coal plume are mainly affected. However, the coal particles at the bottom of coal plume combust more completely, less affected by oxygen content. Moreover, less coal particles are affected by the room-temperature oxygen with the oxygen lance tip moving downward.

Fig. 9.

Effect of Y-coordinate of oxygen lance tip on coal burnout of the raceway outlet. (Online version in color.)

4. Conclusions

In this study, a three dimensional coal combustion model was developed. The coal flow and combustion behaviors under the coaxial oxygen-coal and oxygen-coal double lances are investigated. The results provide some references and guidance for the operation of oxygen enrichment PCI in a real blast furnace. The main conclusions are as follows.

(1) For conventional oxygen-enrichment, the burnout of the raceway outlet increases from 73.16% to 79.31%, an increase of 6.15%, when the oxygen content increases from 21% to 27%. The burnout does not obviously increase. This is because the oxygen concentration around the coal particles does not increase significantly.

(2) For coaxial oxygen-coal lance, the maximum increase in the coal burnout of the raceway outlet is only 2.17%. When the oxygen content is 27%, the coal burnout has a decrease of 5.74%. This is because the cooling effect of room-temperature oxygen delays the coal combustion. Moreover, the oxygen flowing through the annulus of the lance limits the dispersion of the coal particles.

(3) The coal burnout of the raceway outlet is very different under the aforementioned oxygen-coal double lance patterns. The maximum increase in the coal burnout is 12.84% under the aforementioned oxygen-coal double lances. At this time, the oxygen flows into the center of the coal plume, where the burnout is lower due to the lack of oxygen. The oxygen concentration around these particles increases significantly, and the burnout rapidly increase. Moreover, the coal particles are more dispersed under the influence of the oxygen flow, benefiting coal combustion.

Acknowledgements

The authors gratefully acknowledge the financial support from National Key Research and Development Program (No. 2016YFB0601304).

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