Numerical Study of Combustion and Air Supply Characteristics and Structural Optimization of Top Combustion Hot Blast Stoves

Abstract

The high-temperature hot air of hot blast stoves has an important effect on blast furnace ironmaking; it is one of the crucial factors that are used to assess the performance of hot blast stoves. In this study, a three-dimensional fluid flow heat transfer model combining turbulence, heat transfer, combustion, heat radiation, and heat exchange models was developed to assess the combustion and air supply characteristics of a new type of top combustion hot blast stove. The results indicate that nozzles that are alternately arranged in the same layer of the new hot blast stove caused rapid combustion reactions. In addition, it caused the high-temperature flue gas in the pre-combustion chamber to accelerate toward the combustion chamber, thereby eliminating the “eccentric swirl” of the traditional hot blast stove and improving the heat transfer efficiency and heat storage capacity. However, the “attachment effect” of the fluid still occurred in the new stove type, which led to an unreasonable temperature distribution inside the combustion chamber and regenerator. Therefore, an improved design of a top combustion hot blast stove was proposed in this paper. Using the developed numerical model, the performance of the new design was evaluated and compared with the original one.

1. Introduction

Hot blast stoves are important heat exchange facilities in the blast furnace ironmaking process; they continuously and steadily provide high temperature hot air to blast furnaces. The working principle is as follows: fuel and air are mixed and burned to produce high-temperature flue gas, which heats up the regenerator. The cold air that is subsequently passed through the regenerator becomes high-temperature hot air for the blast furnace. With the continuous development of the blast furnace ironmaking technology toward large-scale and high-efficiency applications, the high blast temperature has become one of the most important indicators for assessing the technical level of hot blast stoves; the blast temperature has an important influence on the output, quality, efficiency, and energy consumption of blast furnace ironmaking. Therefore, increasing the hot air temperature is a topic of concern in the research field of hot blast stoves.^{1,2,3,4,5,6,7,8)}

The hot air temperature is affected by many factors: for example, the structure of the hot blast stove, fuel composition, combustion efficiency, and thermal physical parameters of the regenerator materials.^{9,10,11,12,13)} It is well known that the combustion and air supply process of hot blast stoves involve complex physical and chemical phenomena, including turbulent flow, combustion reactions, heat and mass transfer, convection, and radiation. In addition, the huge structures of hot blast stoves put a limit on experimental research. With the rapid development of computational technology and simulation software, numerical simulations of hot blast stoves have become an effective and popular research method. Numerical studies have been conducted to simulate the performance of specific parts of hot blast stoves. Regarding burner simulations, mainly the effects of the fluid uniformity at the nozzle, number of nozzles, and radial angle on the combustion characteristics have been discussed.^14,15,16) The simulations of the combustion chamber have focused on the combustion process, gas flow, and heat and mass transfer characteristics.^17,18,19,20) The study on the numerical model of regenerators built on the early work of simplified models. Recent work mainly involved the heat transfer characteristics of the regenerator, and the impact of physical parameters of lattice brick on the distribution of temperature and the capacity of heat storage.^21,22) The numerical simulations of the entire hot blast stove have made some progress. For instance, Kimura et al.²³⁾ studied the phenomenon of the heat and mass transfer in a hot blast stove with a three-dimensional mathematical model and designed the layout and operation conditions of lattice bricks for a new stove. Zeterholm et al.²⁴⁾ developed a mathematical model with the finite-difference approximation to evaluate the heat transfer performance of a hot blast stove and simulated the influence of a new oxygen fuel technology and the cycle time on a hot blast stove. Moreover, Jiang et al.²⁵⁾ established the transient model of a three-dimensional temperature field of a top combustion ball-type hot blast stove and simulated the change in the temperature field of the heat storage ball during the burning process. The theoretical value obtained by merging the heat conduction of the regenerative ball with that of porous media was close to the measured value.

In summary, the extant literature focuses on the combustion characteristics of the combustion chamber, the heat transfer of the regenerator, and the effects of the mass and heat transfer of the hot blast stove on the combustion process. However, there is little research on how to build a mathematical model for the entire hot blast stove and how to simulate the combustion and air supply process, in particular, with a structural optimization for improving the performance of the hot blast stove. This paper fills the gap and the contribution is threefold. Firstly, we are the first to simulate systematically the whole process of combustion and air supply of a new type of top combustion hot blast stove. Secondly, we point out that the structure of the new stove changes the flue gas flow pattern, thus eliminating the “eccentric swirl” (an unsymmetrical swirl) of the traditional hot blast stove, and explain the reason for the “attachment effect” (a high edge velocity and a low center velocity) of the fluid. Thirdly, we apply the nozzle principle to the reconstruction of the neck of hot blast stove to resolve the “attachment effect”. By increasing the velocity of the fluid and changing the motion pattern of the fluid, the “attachment effect” in the new stove is eliminated, and the heat transfer performance of the hot blast stove is improved remarkably.

2. Mathematical Model

A top combustion hot blast stove mainly consists of four parts: pre-combustion chamber, combustion chamber, regenerator, and grill; each part except the regenerator can be used as fluid domain in the model owing to the sole existence of a fluid. The regenerator (composed of a porous lattice brick) can be modelled as a porous-medium domain. In the operation of a hot blast stove, various complex physical and chemical phenomena occur. Therefore, corresponding transient models were established based on the commercial computational fluid dynamics package ANSYS FLUENT for the different parts.

2.1. Fluid Flow Model

The fluid motion within the hot blast stove follows three basic laws of physical conservation: mass, momentum, and energy conservation. Therefore, the governing equations of the fluid flow process can be established as follows:

Mass conservation equation:   

$$\frac{\partial \rho }{\partial t}+\nabla \cdot (\rho \mathit{v})=0,$$(1)

Momentum conservation equation:   

$$\begin{array}{l}{\displaystyle \frac{\partial }{\partial t}}(\rho \mathit{v})+\nabla \cdot (\rho \mathit{v}\mathit{v})=-\nabla p\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}+\nabla \cdot \left[\mu \left(\nabla \mathit{v}+\nabla {\mathit{v}}^{\mathrm{T}}\right)-{\displaystyle \frac{2}{3}}\nabla \cdot \mathit{v}\right]+\rho \mathit{g},\end{array}$$(2)

where ρ represents the fluid density, v the velocity vector of the fluid, p the pressure, μ the dynamic viscosity, g the gravity acceleration vector, and t the time.

Energy conservation equation:   

$$\frac{\partial }{\partial t}(\rho E)+\nabla \cdot [\mathit{v}(\rho E+P)]=\nabla \cdot \left(k_{eff}\nabla T-\sum _i{h}_i{\mathit{J}}_i\right),$$(3)

where E represents the specific heat capacity, E=h−p/ρ+v²/2, h the enthalpy, $h=\sum _i{Y}_i h_i$, Y_i the mass fraction of species i, J_i the diffusion flux of species generated by concentration gradient, k_eff the effective thermal conductivity of fluid, and T the thermodynamic temperature.

The fluid motion in the hot blast stove is turbulent flow. To model the fluid motion rule accurately, the realizable k − ε turbulence model was employed; it is relatively stable and computationally efficient. The calculation equations are as follows:^26,27)   

$$\frac{\partial }{\partial t}(\rho k)+\nabla \cdot (\rho k \mathit{v})=\nabla \cdot \left[\left(\mu +\frac{{\mu }_t}{{\sigma }_k}\right)\nabla k\right]+G_k-\rho \epsilon$$(4)

and   

$$\begin{array}{l}{\displaystyle \frac{\partial }{\partial t}}(\rho \epsilon )+\nabla \cdot (\rho \epsilon \mathit{v})=\nabla \cdot \left[\left(\mu +{\displaystyle \frac{{\mu }_t}{{\sigma }_{\epsilon }}}\right)\nabla \epsilon \right]\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}+C_{1\epsilon }{\displaystyle \frac{\epsilon }{k}}{G}_k-C_{2\epsilon }\rho {\displaystyle \frac{{\epsilon }^2}{k}},\end{array}$$(5)

where k represents the turbulence kinetic energy, ε the turbulent dissipation rate, G_k the turbulence kinetic energy generated by average velocity gradient. C_1ε and C_2ε are constants, C_1ε = 1.44 and C_2ε = 1.9 in the present model, σ_k = 1.0 and σ_ε = 1.2 are the Prandtl numbers for k and ε, respectively.

2.2. Chemical Reaction Model

The blast furnace gas and air are mixed in the pre-combustion chamber. To deal with the mixing and exchange of various chemical components, the species transport model was selected for describing the transport and mixing characteristics of the gas and air mixture. The general form of the transport equations for species are as follows:   

$$\frac{\partial }{\partial t}\left(\rho Y_i\right)+\nabla \cdot \left(\rho \mathit{v}{Y}_i\right)=-\nabla \cdot {\mathit{J}}_i+R_i$$(6)

and   

$${\mathit{J}}_i=-\left(\rho D_{i,m}+\frac{{\mu }_t}{S{c}_t}\right)\nabla Y_i,$$(7)

where R_i represents the net rate of production species i by chemical reaction, D_i,m the mass diffusion coefficient, μ_t the turbulent viscosity, Sc_t the turbulent Schmidt number (the default value of 0.7).

The fluid in the combustion chamber undergoes rapid combustion reactions in the turbulent state. To simulate the combustion process of the fluid, the eddy-dissipation combustion model was used. The model assumes that the chemical reaction rate of the species is much higher than the turbulent mixing rate. Therefore, the determination of the combustion rate is on the turbulent mixing-time scale; namely, the reaction rate depends on the turbulence fluctuation decay rate. The final rate of reaction is determined by the mass fractions of the fuel, oxidant, and product, and the rate can be controlled by the model automatically. The net rate R_i,r of component i produced by reaction r depends on the smaller one of the following two expressions:   

$$R_{i,r}=v_{i,r}^{\prime }{M}_{w,i}A\rho \frac{\epsilon }{k}{min}_R\left(\frac{Y_R}{v_{R,r}^{\prime }{M}_{w,R}}\right)$$(8)

and   

$$R_{i,r}=v_{i,r}^{\prime }{M}_{w,i}AB\rho \frac{\epsilon }{k}\frac{\sum _P{Y}_P}{\sum _j^N{v}_{j,r}^{″}{M}_{w,j}},$$(9)

where $v_{i,r}^{\prime }$ and $v_{i,r}^{″}$ represent the stoichiometric coefficients of the reactant and product in chemical reaction r for component i, M_w,i the molecular weight, ρ the mixture density, N the number of chemical species in the system, Y_R and Y_P the mass fractions of the reactant R and the product P, respectively. Empirical constants A = 4.0 and B = 5.0. The chemical reaction rate is controlled by the large eddy mixing time scale k/ε. Turbulence (k/ε>0) leads to combustion; hence, no ignition source is required.

2.3. Radiation Model

The combustion products of blast furnace gas contain asymmetric polyatomic molecules (e.g., CO₂ and H₂O), which have great radiation and absorption capacities. The radiation energy of fluid combustion and movement must be included in the energy conservation equation in the combustion simulation of a hot blast stove. In this study, the discrete ordinates (DO) radiation model was adopted to simulate the radiant heat exchange between the combustion gases and internal surface of the hot blast stove. One of the advantages is that the DO model tracks radiation rays over the entire solid angle with discrete ordinates in each computational grid. Therefore, the model can be applied to complex geometries. In the DO model, the radiative transfer equation (RTE) in direction s is used as a field equation:^28,29)   

$$\begin{array}{l}\nabla \cdot (I(\mathit{r},\mathit{s})\mathit{s})+\left(a+{\sigma }_S\right)I(\mathit{r},\mathit{s})\\ =a{n}^2{\displaystyle \frac{\sigma T^4}{\pi }}+{\displaystyle \frac{{\sigma }_S}{4\pi }}{\int }_0^{4\pi }I\left(\mathit{r},{\mathit{s}}^{\prime }\right)\mathrm{\Phi }\left(\mathit{s},{\mathit{s}}^{\prime }\right)d{\mathrm{\Omega }}^{\prime },\end{array}$$(10)

Moreover, ANSYS FLUENT allows the modeling of non-gray radiation with a gray-band model. The RTE for the spectral intensity I_λ(r,s) can be expressed as follows:   

$$\begin{array}{l}\nabla \cdot \left(I_{\lambda }(\mathit{r},\mathit{s})\mathit{s}\right)+\left(a_{\lambda }+{\sigma }_S\right)I_{\lambda }(\mathit{r},\mathit{s})\\ =a_{\lambda }{I}_{b\lambda }+{\displaystyle \frac{{\sigma }_S}{4\pi }}{\int }_0^{4\pi }{I}_{\lambda }\left(\mathit{r},{\mathit{s}}^{\prime }\right)\mathrm{\Phi }\left(\mathit{s},{\mathit{s}}^{\prime }\right)d{\mathrm{\Omega }}^{\prime },\end{array}$$(11)

where λ represents the wavelength, a_λ the spectral absorption coefficient, I_bλ the black body intensity given by the Planck function, σ_S the scattering coefficient, n the refraction coefficient, r the position vector, s the direction vector, s′ the scattering direction vector, Φ the phase function, Ω′ the solid angle. σ_S, Φ and n are assumed independent of wavelength.

The non-gray DO model divides the radiation spectrum into a series of wavelength bands, which does not need to be contiguous or equal. The wavelength intervals, supplied by the user, correspond to values in vacuum (n = 1). The RTE is integrated over each wavelength interval to generate two quantities: the transport equations for the quantity I_λΔλ, and the radiant energy contained in the wavelength band Δλ. The black body emission in the wavelength band per unit solid angle is:   

$$\left[F\left(0\to n{\lambda }_{2}T\right)-F\left(0\to n{\lambda }_{1}T\right)\right]n^2\frac{\sigma T^4}{\pi },$$(12)

where F(0→nλ₂T) is the fraction of radiant energy emitted by a black body in the wavelength interval 0 to λ at temperature T in a medium of refractive index n; λ₁ and λ₂ are the wavelength boundaries of the band.

2.4. Porous Media Model

In this study, the porous-media model was used to simulate the flow and heat transfer process in the regenerator. For a single phase flowing through porous media, the mass, momentum, and energy conservation equations can be written as follows:

Mass conservation equation:   

$$\frac{\partial (\gamma \rho )}{\partial t}+\nabla \cdot (\gamma \rho \mathit{v})=0,$$(13)

Momentum conservation equation:   

$$\begin{array}{l}{\displaystyle \frac{\partial }{\partial t}}(\gamma \rho \mathit{v})+\nabla \cdot (\gamma \rho \mathit{v}\mathit{v})+\gamma \nabla p\\ -\nabla \cdot \left[\gamma \mu \left(\nabla \mathit{v}+\nabla {\mathit{v}}^{\mathrm{T}}\right)-{\displaystyle \frac{2}{3}}\gamma \nabla \cdot \mathit{v}\right]-\left({\displaystyle \frac{\mu }{\beta }}+{\displaystyle \frac{C_2\rho }{2}}\left|\mathit{v}\right|\mathit{v}\right)=0,\end{array}$$(14)

Energy conservation equation:   

$$\begin{array}{l}{\displaystyle \frac{\partial }{\partial t}}\left(\gamma {\rho }_f{E}_f+(1-\gamma ){\rho }_S{E}_S\right)\\ +\nabla \cdot \left[\mathit{v}\left({\rho }_f{E}_f+P\right)\right]=\nabla \cdot \left(k_{eff}\nabla T-\sum _i{h}_i{\mathit{J}}_i\right),\end{array}$$(15)

where γ represents the porosity, β the viscous resistance coefficient, C₂ the inertia resistance coefficient, E_f the total fluid energy, E_S the total solid medium energy, ρ_f and ρ_S are the densities of the fluid and solid.³⁰⁾

3. Computational Methods and Conditions

3.1. Computational Domain and Mesh Generation

The three-dimensional computational model of the new type of top combustion hot blast stove in Fig. 1 is composed of the essential stove parts: the burner, pre-combustion chamber, combustion chamber, regenerator, and furnace grill. The height of the stove is 46.9 m, and the bottom diameters of the pre-combustion and combustion chambers are 3.7 and 8.8 m, respectively. The diameter and height of the regenerator are 8.3 and 26 m, respectively. Moreover, the gas and air nozzles (30 in total) in the burner are arranged alternately in the same layer with a horizontal angle of 12° between them (Fig. 1(c)). The arrangement of the nozzles is the main structural difference between the new and traditional stove types with two layers gas and two air nozzles (showing up and down), respectively.²⁰⁾ The new structure enables rapid mixing of the two gases after ejection and preheating combustion.

Fig. 1.

Grids of the three-dimensional computational model of hot blast stove (a), burner (b), gas and air loop and nozzles (c) and grill (d) in the new stove type.

Based on the structure and physicochemical characteristics, the hot blast stove was divided into three computational domains: the pre-combustion and combustion chambers were one fluid domain; the regenerator was a porous medium domain, and the grill was a fluid domain. A hexahedral grid was employed to divide the regenerator, and a tetrahedral grid was used to divide the rest of the hot blast stove. The mesh of the burner nozzles was encrypted to capture the local flow structure and combustion process. In addition, a hybrid structured and unstructured mesh was generated in the entire computational domain of the hot blast stove. According to the grid independence analysis, the total grid nodes of the final mesh were 742010.

3.2. Boundary Conditions

All boundary conditions were based on the actual operating parameters of a plant. The fuel gas was blast furnace gas, and its composition is summarized in Table 1. In the combustion period, the fuel gas was supplied at a flow rate of 11.83 × 10⁴ Nm³/h at 423 K, and air was supplied at a flow rate of 7.74 × 10⁴ Nm³/h at 473 K. At the stove bottom, the static pressure of the two exhaust gas outlets was set to 0 Pa. Moreover, the regenerator was composed of two kinds of checker bricks: the upper layer was silicon brick, and the lower layer was clay brick. The detailed parameters of the checker bricks are listed in Table 2. In the air supply period, the cold air inlet was considered the speed inlet with a flow rate of 33.42 × 10⁴ Nm³/h at 473 K. The hot-air outlet was used as a pressure outlet with a static pressure of 0 Pa, and all surface walls were considered nonslip walls.

Table 1. Gas composition of blast furnace.

Component	CO	CO2	H2O	H2	N2
Percentage composition	0.24	0.17	0.023	0.01	0.557

Table 2. Parameter of checker brick.

Checker brick	Density/kg·m−3	Thermal conductivity/W·(m·K)−1	Specific heat/J·(kg·K)−1
Silica brick	1900	0.93+0.0007 t	794.0+0.251 t
Clay brick	2070	0.84+0.0052 t	836.8+0.263 t

Because of the periodic operation characteristics of hot blast stoves, multiple periodic cycles had to be conducted after setting the initial conditions of the calculation. In each cycle, the air supply occurred under the same furnace burning conditions, and the resulting state was used as the heat carrier for the next combustion–air supply cycle. When the temperature difference of the hot air outlet after two air supply cycles is below 5 K, the typical numerical simulation results can be obtained.

4. Results and Discussions

4.1. Characteristic Analysis of Combustion Period of New Stove Type

4.1.1. Velocity Field in New Stove Type

The fuel gas and air nozzles of the new stove type are alternately arranged on the same floor. In the combustion process, the fuel gas and air are mixed and ignited immediately after exiting the nozzles. The disturbing current produced by combustion further intensifies the mixing of the two gases, which promotes the combustion reaction. Combustion increases the temperature and pressure in a narrow pre-combustion chamber. Depending on the spray angle of the nozzle, the flue gas forms a swirling flow and accelerates to enter the combustion chamber. Figure 2 shows the velocity vector distribution of the pre-combustion and combustion chambers of the new stove type. As depicted in Fig. 2(a), the high-temperature flue gas exits the neck between the pre-combustion and combustion chambers at a high speed. The highest velocity of the fluid occurs near the surrounding wall rather than in the center of the combustion chamber; this is called the “attachment effect”. This phenomenon is confirmed by Figs. 2(c)–2(e) (Sections 2–4).

Fig. 2.

The velocity vector of pre-combustion and combustion chamber of the new stove type on the vertical section (a) and Sections 1-5 (b)–(f), respectively.

The “attachment effect” can be explained by analyzing the geometrical structure of the combustion chamber and characteristics of the gas flow. When the high-speed flue gas enters the combustion chamber from the neck, it expands with the increasing space, and the flow rate decreases. The low-velocity fluid particles near the wall have a restraining effect on the adjacent high-velocity fluid particles, which increases the viscous resistance; simultaneously, the swirling flow of the high-temperature flue gas produces a centrifugal movement, which causes the “attachment effect”. Consequently, the flame does not flow directly to the lattice brick along the central area of the combustion chamber; it flows down along the wall surface.

When the flue gas approaches Section 5 on the regenerator top surface (Fig. 2(f)), its flow rate increases in the central region owing to the backflow of the flue gas caused by the blockage of the lattice brick. In addition, according to Figs. 2(b)–2(f), the velocity distribution of each horizontal section is centrally symmetrical in the entire combustion area, which is in sharp contrast to the velocity “eccentric swirl” of the traditional stove. The explanation lies in the distribution of velocity, which is mainly determined by the arrangement of nozzles in the new stove type. The distribution of velocity leads to an intense combustion reaction, increases the momentum of the flue gas in the vertical downward direction, counteracts the imbalance force of the rotation direction, and therefore eliminates the “eccentric swirl” of the traditional hot blast stove. The influence of the flow field on the temperature distribution will be discussed in the next section.

4.1.2. Temperature Field in New Stove Type

Figure 3 shows the temperature distribution inside the new stove type at the end of the combustion process. To analyze the relative temperature distribution of each cross-section, the local temperature coordinate was adopted for Sections 1–6 (Figs. 3(c)–3(h)).

Fig. 3.

The temperature distribution of the new stove type on the wall (a), the vertical section (b) and Sections 1-6 (c)–(h), respectively. (Online version in color.)

As shown in Fig. 3(b), the fuel gas and air begin to burn violently in the pre-combustion chamber, which increases the temperature to over 1292 K in this area. However, in the traditional stove, the temperature in the pre-combustion chamber is between 500–600 K, and the intense combustion occurs at the neck and below.²⁰⁾ This is due to the nozzle structure, which enables the fuel gas and air to be mixed rapidly in the new stove, thereby causing the combustion reaction to increase the temperature and pressure of the pre-combustion chamber; this improves the velocity of the flue gas. In the combustion chamber, the combustion process continues to develop fully, thereby leading to the highest temperature of 1664 K near the dome of the hot blast stove. In the regenerator area, the temperature is stratified from top to bottom. Evidently, the upper layer of the regenerator is close to the heat source, thereby forming a high-temperature area. In the deep part of the regenerator, the temperature decreases gradually with the distance from the heat source. This high-temperature gradient is maintained until the end of the combustion period. In general, the depth and temperature distribution of the high-temperature zone reflect the amount of heat stored in the regenerator, which directly affects the next air supply temperature and duration. Therefore, the temperature and its distribution in the regenerator are usually the key indexes for evaluating the hot blast stove.

Regarding the temperature distribution of each section, as depicted in Figs. 3(c) and 3(d), Sections 1 and 2 are in the combustion zone. In each section, the high-temperature zone is distributed in the surrounding ring area, and the low-temperature zone is in the central area. This is consistent with the law of the “attachment effect” in the previous velocity field. Thus, there is a relationship between the velocity and temperature field distributions. In the combustion period, the high-temperature flue gas generated by combustion flows downward to realize energy transfer, and the fluid is the energy carrier. Evidently, in the high-velocity region, the higher the energy density, the higher is the corresponding temperature. Sections 3–5 (Figs. 3(e)–3(g)) of the regenerator exhibit a temperature distribution similar to those in the combustion zone. Hence, the temperature distribution of the combustion chamber is directly related to that of the regenerator; that is, the unreasonable speed of the high-temperature flue gas from the combustion chamber in horizontal direction results in an unreasonable temperature distribution in the regenerator. Compared with those of the traditional stove, the temperature distribution of each section in the new stove type is still unreasonable; however, the temperature difference inside the section is significantly reduced. The total temperature of the hot blast stove is increased, and the high-temperature area and heat storage capacity are improved.

4.2. Characteristic Analysis of Air Supply Period of New Stove Type

In the air supply process, cold air is blown into the bottom of the hot blast stove, which then enters the heated checker brick channel through the grill. The cold air transforms into hot air after the heat exchange with the lattice brick, which flows through the hot-air outlet in the upper part of the stove into the blast furnace. In engineering practice, the standard for evaluating the performance of hot blast stoves is investigating its effective air supply duration and temperature distribution during this period under certain conditions. The effective air supply duration refers to the time required for the air supply temperature to reach 1273 K (1000°C) after the start of the air supply. The longer the effective air supply duration or the higher the temperature during the air supply, the better is the performance of the hot blast stove. Figure 4 presents the temperature curve of the air supply simulation with the end state of the combustion period as the heat carrier; Figs. 4(a) and 4(b) present the temperature distributions of the two points a and b at the beginning and end of the air supply, respectively.

Fig. 4.

The temperature curve of the new stove type in the air supply period. For the points a and b on the curve, the temperature distribution is shown in (a) and (b), respectively. (Online version in color.)

As shown in Fig. 4, at the beginning of the air supply, the maximum air temperature is 1610 K, and the air temperature changes slowly (Fig. 4(a)). In the middle stage of the air supply, the temperature decreases faster due to the mass output of thermal energy. At the end of the air supply, the thermal energy stored in the regenerator is greatly reduced and not able to provide enough heat for the cold air at the inlet continuously. As a result, the air-supply temperature decreases sharply until reaching the lower limit of the temperature required by the project (1273 K, point b), which marks the end of the air-supply period.

4.3. Structure Optimization

According to the numerical simulation of the combustion and air supply process of the new stove type, the problem of speed eccentricity in the traditional stove has been solved, the rationality of the temperature distribution has been improved, and the thermal efficiency of the combustion and heat storage capacity has been enhanced to a certain extent by adopting the same layer–nozzle structure in the new stove type. However, the new stove type exhibits velocity “attachment effect” and an unreasonable temperature distribution. The consequences caused by this temperature distribution are evident: first, the temperature in the center of the combustion chamber and regenerator is relatively low, which directly affects the heat transfer efficiency, heat storage capacity, and air supply in the later stage; second, the wall temperature is relatively high, which accelerates the damage of the refractory materials and reduces the safety of the furnace body; in addition, it increases the wall heat dissipation and reduces the thermal efficiency of the hot blast stove. To solve these problems, the nozzle principle is used for a local stove optimization; thereby, the “attachment effect” and unreasonable temperature distribution can be mitigated by increasing the fluid speed or changing the flow pattern, which enhances the stove performance.

4.3.1. Optimization Scheme of Hot Blast Stove

The nozzle is a device that can accelerate the airflow by changing the geometric shape of the inner wall of the pipe section without the addition of power devices. To optimize the previously presented stove structure, an improved hot blast stove was proposed: the equal-diameter cylinder form of the nozzle is changed to a tapered nozzle structure with a thick top and thin bottom. According to the nozzle theory, the suitable inward inclination is generally between 10° to 12°; an inclination angle of 10° was adopted in this optimization scheme. Figure 5 shows the schematic of the neck region before and after the change. Four values of the large diameter of the tapered nozzle were considered, i.e., D = 2.8, 3.0, 3.2, 3.4 m. Owing to space limitations, only the best result (D = 3.0 m) is presented. Other than the neck, the hot blast stove design remains identical to the new stove design. The performance of the improved hot blast stove was investigated by using the mathematical model and boundary conditions presented before. A final mesh with 741555 total grid nodes was used for the numerical simulations based on the grid independence analysis.

Fig. 5.

The schematic of the neck region before (a) and after (b) the optimization.

4.3.2. Velocity Field in Improved Stove

As previously mentioned, the high-speed flue gas in the new stove enters the combustion chamber in swirls and flows the lattice brick of the regenerator. This flow mode promotes the mixing of gas and air and prolongs the residence time of the flue gas in the pre-combustion chamber, which promotes the combustion reaction. However, the swirling flow weakens the penetration of the flue gas into the porous media, which is undesirable for the heat transfer.

The numerical simulation was conducted based on the optimized design. According to the velocity vector distribution in Fig. 6, the velocity field in the combustor chamber changes fundamentally after the neck is transformed into a tapered nozzle. The flow pattern of the fluid in the combustion chamber changes from “swirl down” to “vertical down”, and the flow rate at the nozzle outlet (the thinnest neck) was 66.5 m/s, which is 29.6% higher than that of the model before optimization. This means that the velocity of high temperature flue gas in the vertical flow direction is increased. At the same time, due to the shearing effect at the nozzle outlet, the high-speed fluid is mainly distributed in the center of combustion chamber, which effectively blocks the “attachment flow”. The rapid flow of high temperature flue gas to the regenerator significantly increases the energy density of the lattice brick and improves the efficiency of heat transfer.

Fig. 6.

The velocity vector of pre-combustion and combustion chamber of the improved stove type on the vertical section (a) and Sections 1-5 (b)–(f), respectively.

4.3.3. Temperature Field in Improved Stove

Figure 7 shows the predicted temperature distribution in the improved stove. As depicted in Fig. 7(b), under the same combustion conditions, more area experiences high temperatures (i.e., over 1500 K) than in the original design (Fig. 3(b)). This confirms that the improved hot blast stove can effectively enhance the heat exchange energy between the high-temperature flue gas and lattice brick, which improves the heat transfer efficiency.

Fig. 7.

The temperature distribution of the improved stove type on the wall (a), the vertical section (b) and Sections 1-6 (c)–(h), respectively. (Online version in color.)

Regarding the temperature distribution in each section, as shown in Figs. 7(c) and 7(d), the higher temperature area is found in the center of each section rather than in the surrounding area in Sections 1 and 2 located in the combustion zone. Sections 3–5 in the regenerator exhibit a similar trend (Figs. 7(e)–7(g)). Thus, the distribution characteristics of the velocity and temperature in the combustion chamber continue to exist in the regenerator, and the central area of the lattice brick becomes the active heat exchange area between the high-energy fluid and porous lattice brick, which improves the heat transfer performance of the hot blast stove.

4.3.4. Performance Comparison of New Stove Type and Improved Version

After the optimization of the hot blast stove, the velocity and temperature fields are effectively improved. The influence of the new design on the performance of the hot blast stove can be estimated based on a comparison with the original design. Because the effect of the flue gas on the heat transfer performance is reflected by the temperature distribution of the regenerator, the performances of the two models can be assessed by comparing the temperature distribution of the regenerator and temperature variation curve when the regenerator is used as the heat carrier for the air supply at the end of the combustion period.

(1) Comparison of Temperature Distributions Along Vertical Direction of Regenerators of New Stove Type and Improved Version

Figure 8 shows the predicted temperature distribution of the two hot blast stoves along the design centerline of the regenerator and the vertical bisector from the center to the wall. The trends in Figs. 8(a) and 8(b) are similar: in general, the temperature increases with the height. At the top of the regenerator, the temperature difference is not big (the temperature of the improved version is slightly higher). As the height decreases, the temperature difference becomes evident, and the temperature of the improved hot blast stove is always higher than that of the new version. The higher the temperature in the central region, the greater is the energy storage, which improves the energy transport.

Fig. 8.

The distribution of temperature over the vertical direction of the regenerator along the design centerline of the stove (a), and the vertical bisector between the center line and the wall (b), for the improved stove type (red circle markers) and the new one (black square markers). (Online version in color.)

(2) Comparison of Temperature Variation Curves of Hot Air Outlets of New Stove Type and Improved Version During Air Supply

Figure 9 presents the predicted temperature variation curves in the air supply period of the two models under the same air supply conditions. There is no big difference between the two air supply temperatures at the initial stage. At the beginning of the middle period, the air supply temperature of the improved stove is considerably higher than that of the new version (maximal temperature difference of 78 K). During this period, the air supply times exhibit similar trends at equal air supply temperatures: the effective air supply time of the improved hot blast stove is 604.0 s longer than that of the original one. This result corresponds to the temperature distribution of the regenerator before the air supply of the two stoves. Hence, the temperature and duration of the air supply are determined by the temperature and heat storage capacity of the regenerator.

Fig. 9.

The temperature distribution of hot air outlet in the air supply period, for the improved stove type (red circle markers) and the new one (black square markers). (Online version in color.)

5. Conclusion

Based on a new type of top combustion hot blast stove, a three-dimensional fluid flow heat transfer model combining turbulence, combustion, heat radiation, and heat exchange models was developed to study the combustion and air supply characteristics of hot blast stoves. The results are as follows:

(1) The nozzles of the new type of hot blast stove are alternately arranged in the same layer. Thus, the gas and air can be quickly mixed for an intense combustion reaction. The high-temperature and high-pressure flue gas formed in the pre-combustion chamber accelerates to flow toward the combustion chamber, thereby eliminating the “eccentric swirl” of the traditional hot blast stove.

(2) Compared with that of the traditional hot blast stove, the heat transfer efficiency and heat energy storage capacity of the new hot blast stove are improved to a certain extent. Nevertheless, the “attachment effect” of the fluid still exists in the new stove type; it has a negative effect on the temperature distribution, mass transfer, and heat transfer of the regenerator.

(3) An improved optimal design for a hot blast stove was proposed, and its performance was evaluated and compared with that of the new hot blast stove with a developed numerical model. The results show that the flow pattern of the fluid changed after the neck was reformed based on the nozzle principle, and the outlet velocity of the neck increased by 29.6%. The “attachment effect” was eliminated, and the temperature distribution was more reasonable. Furthermore, the temperature of the improved design along the vertical direction of the regenerator was evidently higher than that of the original design; the maximal values of the air supply temperature and effective air supply time of the former were increased by 78 K and 604 s compared with those of the latter in the air supply period, respectively.

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