Waste Heat Recovery from Converter Gas by a Filled Bulb Regenerator: Heat Transfer Characteristics

Abstract

The iron and steel industry is a high-energy consumption and high-pollution industry. Energy recovery in its process is of great significance. Converter gas is an important by-product in the process of iron and steel production. It is difficult to take effective measures to recover waste heat from converter gas due to its flammability. In this paper, a new technology is proposed based on waste heat recovery by a filled bulb regenerator. The mathematical model of heat transfer and flow in the compound regenerator for converter gas containing dust is established. The effects of the diameter and concentration of dust to heat transfer and flow are discussed. The results show that the temperature of converter gas declined to 132 °C, and recovery efficiency was above 90% using this technology system. Resistance loss increased by 25% due to the dust; with the increase in the diameter and initial concentration of dust, the heat transfer rate in the regenerator was reinforced. At the entrance of the regenerator, dust’s effects are more obvious, and the efficiency of heat transfer is increased by 4.5–8.5%. The results can provide a theoretical basis for a new converter gas waste heat recovery method.

1. Introduction

Converter gas contains a large amount of CO (>60%), which is the most important by-product of converter steel-making. In the existing converter gas recovery process, the gas is initially cooled by a vaporization cooling flue, and the temperature is reduced to 800~1000 °C [1]. If the sensible heat of converter gas in the medium temperature section is recovered continuously, it will be at risk of explosion [2,3]. Thus, one can take the risk of explosion to recover the waste heat of converter gas. Therefore, water spray or spray mist is used for quenching (cooling to about 200 °C) and removal of coarse dust. At present, two kinds of typical converter gas purification technologies, wet method (OG) and dry method (LT) purification and recovery technology, are widely used in the world [4,5]. These two dedusting methods have the defects of large water consumption and unused waste heat of converter gas in the medium temperature section (<800 °C).

With the aggravation of energy and environmental problems, people gradually attach great importance to the waste heat recovery of converter gas. Scholars began to try to recover the waste heat effectively. Based on coal gasification and waste heat recovery of BOF off-gas, Wang [6] proposed a novel technology in which pulverized coal is injected into a converter evaporating flue and efficiently reacts with the CO₂ of flue gas utilizing its high-temperature sensible heat to convert CO₂ into valuable CO. Furthermore, Zhou [7] proposed to inject pulverized coal into the vaporization cooling flue and conducted a thermodynamic analysis of the improvement of converter gas. Indeed, theoretically, these two methods can absorb the heat of converter gas through chemical reactions. However, these two methods only stay in the experimental stage, and the input pulverized coal will further aggravate the energy consumption. From the perspective of improving the utilization ratio of gas, Wu [8] also proposed to use the released gas by chemical chain combustion. However, waste heat cannot be recovered efficiently with this method.

Based on the critical aperture flameout theory and the analysis of gas combustion and explosion [2,9,10], a new dry purification technology to safely and effectively recover the waste heat in the medium temperature section of converter gas using a regenerator is proposed by the author [11,12], as shown in Figure 1. The dedusting purification and waste heat recovery system in the medium temperature section of converter gas shall be carried out according to the following steps:

(1)

Preliminary dedusting of a dedusting sedimentation chamber

First, 1450~1650 °C high-temperature dusty gas is produced in the converter smelting process. After vaporization and cooling, the temperature drops to 800~1000 °C and then enters the dust removal sedimentation chamber to remove large dust particles in the gas.

(2)

Heat exchange and coarse dust removal of the regenerator

After preliminary dust elimination, the high-temperature dusty gas enters one of the regenerators of the regenerative heat exchange device through the high-temperature reversing valve. After the high-temperature dusty gas releases heat, the temperature drops to about 100 °C, and the regenerator stores the heat released by the gas and filters out part of the dust in the gas; at the same time, the other group of regenerators is filled with low-temperature (100 °C) and purified inert flue gas in the opposite direction. When the flue gas passes through the regenerator, it is heated into high-temperature inert flue gas (700~900 °C), the regenerator is cooled, and the dust is removed when the gas flows through the regenerator and blows the dust using inert flue gas. Most of the dust will fall into the ash hopper at the lower part of the regenerator, and a small part will be carried away by inert flue gas.

(3)

Reversing of gas and inert gas in the regenerator

If the best reversing time is set, the high-temperature reversing valve group and low-temperature reversing valve group act at the same time to reverse the flow direction of inert flue gas and gas in the regenerator. That is, the low-temperature inert flue gas is reversely introduced into the regenerator with high-temperature dusty gas before switching, and the high-temperature dusty gas is reversely introduced into the regenerator with low-temperature inert flue gas before switching.

(4)

Gas purification and recovery: Inert flue gas purification and waste heat recovery

The gas leaving the regenerative heat exchanger enters the gas holder for recovery after passing through the low-temperature three-way reversing valve group, the low-temperature gas dust collector, the rotary water seal valve, and the V-type water seal valve in turn. The high-temperature inert flue gas leaving the regenerative heat exchanger enters the waste heat boiler through the high-temperature reversing valve group to recover the waste heat and generate steam. At the same time, the high-temperature inert flue gas is cooled below 100 °C and then returns to the regenerator through the induced draft fan and low-temperature three-way reversing valve group after passing through the low-temperature flue gas dust collector for continuous recycling. When the system recovers converter gas normally, steps (2), (3), and (4) are repeated.

Using this technology, during the heating period, the gas heats the regenerator, and the smoke and dust accumulate in the regenerator. During the cooling period, the regenerator heats the cooling gas (N₂), and the smoke and dust are blown back and cleaned. The heat transfer and flow characteristics of gas in the regenerator need to be further investigated.

The converter gas medium temperature dry purification and waste heat recovery system uses the regenerator technology to overcome the above disadvantages and has the following advantages: (1) dry treatment and anhydrous dust removal. The converter gas is completely treated by the dry method, which overcomes the problems of water consumption and pollution caused by the existing water spray or steam spray cooling converter gas; (2) Waste heat recovery, high efficiency and energy saving. Using the regenerator as the heat exchange equipment, the temperature of the heat exchange medium obtained by heat exchange is high, the converter gas waste heat between 900~100 °C can be recovered efficiently, the heat recovery rate is high, and the generated steam can be directly used for other processes or for power generation.

In the process of recovering the waste heat of converter gas in the regenerator, the gas carries out heat transfer and flow in the regenerator, and dust particles are collected at the same time. The heat transfer of converter gas with dust belongs to gas–solid two-phase heat transfer, and the dust particles affect the gas-phase convective heat transfer. At present, the research on the heat exchange theory of regenerators mainly includes the Shack theory and Hausen theory [13,14]. Both heat exchange theories do not involve the particle dust removal process. Over the years, many scholars have established gas–solid two-phase flow heat transfer models, but they have not analyzed the gas–solid two-phase flow heat transfer characteristics in the process of particle dust removal [14].

In this paper, the mathematical model of heat transfer and flow of dusty converter gas in a regenerator is established, and the effects of soot particle size and soot concentration on gas-phase convective heat transfer and dust removal efficiency are investigated. The changes in temperature and resistance loss in the regenerator in one cycle are calculated. The research results can provide a theoretical basis for the purification and waste heat recovery of medium-temperature converter gas in the regenerator.

2. Mathematical Model of Heat Transfer and Flow Process

2.1. Mathematical Model of Heat Transfer Process in Regenerator

The heat transfer process during heating and cooling can be expressed as follows, and the heat transfer process contains the following steps:

(1)

Flue gas heat absorption or heat release process;

(2)

Smoke retention heat release or blowing heat absorption;

(3)

Convective heat transfer process between gas and regenerator surface;

(4)

Radiation heat transfer process between gas and regenerator surface;

(5)

Unsteady heat conduction, heat storage, and heat release process in the regenerator.

A schematic diagram of the regenerator grid division is shown in Figure 2. In order to simplify the mathematical model of heat transfer, the following assumptions are made:

(1)

There is no heat conduction between the filled balls (compared with the convective heat transfer coefficient, the thermal conductivity between spheres is very small).

(2)

The porosity of the regenerator is equal everywhere, and the gas velocity distribution is uniform on any cross-section of the regenerator.

(3)

The gas composition and flow rate do not change with time.

(4)

Ignore the impact of commutation (mixing of hot and cold fluids, retention in regenerator during commutation).

(5)

The heat loss between the regenerator and the environment is ignored.

➀ Gas

$$W_f\left(\tau ,x\right)C_f\left(\tau ,x\right)\frac{\partial t_f\left(\tau ,x\right)}{\partial x}=-a{A}_0\alpha \left(\tau ,x\right)\left[t_f\left(\tau ,x\right)-t_m\left(\tau ,x,R\right)\right].$$

(1)

➁ Surface of fill ball

Heating period:

$$\left[C_d\frac{M_1\left(x\right)}{a{A}_{0}dx}+\alpha \left(\tau ,x\right)\right]\left[t_f\left(\tau ,x\right)-t_m\left(\tau ,x,R\right)\right]={\lambda }_m\frac{\partial t_m\left(\tau ,x,r\right)}{\partial r}\left|\left(\tau ,x,R\right).\right.$$

(2)

Cooling period:

$$\left[C_d\frac{M_2\left(x\right)}{a{A}_{0}dx}+\alpha \left(\tau ,x\right)\right]\left[t_f\left(\tau ,x\right)-t_m\left(\tau ,x,R\right)\right]={\lambda }_m\frac{\partial t_m\left(\tau ,x,r\right)}{\partial r}\left|\left(\tau ,x,R\right).\right.$$

(3)

➂ Fill ball

$$\frac{1}{a_m}\frac{\partial t_m\left(\tau ,x,r\right)}{\partial \tau }=\frac{2}{r}\frac{\partial t_m\left(\tau ,x,r\right)}{\partial r}+\frac{{\partial }^2{t}_m\left(\tau ,x,r\right)}{\partial r^2}.$$

(4)

➃ Center of fill ball

$${\lambda }_m\frac{\partial t_m\left(\tau ,x,r\right)}{\partial r}\left|\left(\tau ,x,0\right)\right.=0,$$

(5)

where W_f is the gas flow, m³/s; C_f is the specific heat of gas, J/(m³·°C); A0 is the sectional area of regenerator, m²; α is the coefficient of convective heat transfer, W/(m²·°C) [15,16]; t_f is the temperature of the gas, °C; t_m is the temperature of the filled ball, °C; a is the specific surface area, m²/m³; C_d is the specific heat of smoke and dust, J/(kg·°C); M₁(x) and M₂(x) are the mass flow of smoke and dust in the gas in the heating period and cooling period, respectively, kg/s; β is the dust content of flue gas, g/m³; W_f0 is the gas volume flow at the inlet, m³/s; C_m is the specific heat of the filled ball, J/(kg·°C); ρ_m is the sphere density of the filled ball, kg/m³; ε is the regenerator porosity;  ${\lambda }_m$  is the thermal conductivity of the filled ball, W/(m·K); subscript 1 indicates the heating period, and 2 indicates the cooling period.

Definite solution conditions: during normal operation, the inlet temperature of flue gas and cooling gas is a fixed value. The temperature of the heat storage body at the end of the heating period is the temperature of the heat storage body at the beginning of the cooling period.

➀ Initial condition

$$t_f\left(0,x\right)=f_1\left(x\right),$$

(6)

$$t_m\left(0,x,r\right)=f_2\left(x,r\right).$$

(7)

➁ Boundary condition

$$t_f{}_{,g}\left(\tau ,0\right)=C_1,$$

(8)

$$t_{f,}{}_c\left(\tau ,0\right)=C_2.$$

(9)

➂ Commutation condition

$$t_{m_1}\left(0,x,r\right)=t_{m_2}\left(P,H-x,r\right),$$

(10)

$$t_{m_2}\left(0,x,r\right)=t_{m_1}\left(P,H-x,r\right),$$

(11)

where  $\tau$  is time, s; x is the coordinates of height direction in the regenerator, m; H is the regenerator height, m; P is the cycle time of commutation, s.

2.2. Treatment of Soot Filtration Process in the Model

The content of smoke and dust in converter gas is 60–140 g/Nm³, and the average diameter of smoke and dust is about 50–60 μm. During the heating period, the regenerator is in the smoke and dust capture stage, and the main filtration process is internal filtration. In the cooling period, the regenerator is in the soot purging stage, and the soot purging process is the reverse process of the accumulation process. The calculation formula of the particle layer filtration process of the regenerator is as follows:

$${\eta }_k=1-(1-{\eta }_l{)}^N,$$

(12)

$${\eta }_l=1.209\epsilon {(1-\epsilon )}^{\frac{2}{3}}{\eta }_s,$$

(13)

$${\eta }_s=1-\left(1-{\eta }_R\right)\left(1-{\eta }_I\right)\left(1-{\eta }_D\right),$$

(14)

where,  ${\eta }_k$  is the dust removal efficiency passing through the N-layer regenerator layer;  ${\eta }_l$  is the dust removal efficiency of a single-layer regenerator;  ${\eta }_s$ is the dust removal efficiency of the isolated sphere;  ${\eta }_R$  is the interception efficiency;  ${\eta }_I$  is the inertial collision efficiency;  ${\eta }_D$  is the diffusion efficiency.

$$M_1\left(x\right)=\left({\eta }_{\mathrm{k},\mathrm{j}}-{\eta }_{\mathrm{k},\mathrm{j}-1}\right)M_s{W}_f,$$

(15)

$$M_2\left(x\right)={\eta }_k{M}_s{W}_f,$$

(16)

where M_s is the dust content of smoke and dust, kg/m³.

2.3. Effect of Dust Particles in Gas on Convective Heat Transfer

Solid dust particles strengthen the convective heat transfer coefficient. With the increase in the concentration of solid dust particles in the gas flow, the collision times of dust particles on the wall surface of the heat storage body in the bed increase. On the one hand, when the particles hit the heating surface each time, they conduct part of the heat to the heating surface through the collision process. On the other hand, when dust particles pass through the transition layer from the core area and enter the laminar bottom layer, they disturb the boundary layer and reduce the thickness of the laminar bottom layer so as to reduce the thermal resistance and strengthen the fluid heat transfer. Figure 3 shows the effect of particles on the boundary layer. The main factors affecting enhanced heat transfer are dust particle concentration and dust particle size.

The empirical formula of the enhanced heat transfer of the dusty gas scouring wall is as follows [16].

$$\frac{{\alpha }_c}{{\alpha }_c{}_0}=1+0.788W_f{}^{-0.31}{d}_{\mathrm{ps}}{}^{-0.233}{M}_s{}^{0.72},$$

(17)

where  ${\alpha }_c$  and  ${\alpha }_c{}_0$  are heat transfer coefficients of multiphase flow and pure gas flow, W/(m²·°C); d_ps is the particle size of dust particles, mm.

2.4. Radiation Heat Transfer in Gas

In the cooling cycle of the regenerator, the main component of flue gas is inert gas (N₂), and its radiation heat transfer part can be ignored. During the heating period, the initial temperature of the gas is above 800 °C, and the main components are CO (60–80%), CO₂ (15–20%), and N₂ (10–20%). CO (asymmetric diatomic gas) and CO₂ have radiation characteristics, and the radiation heat transfer cannot be ignored. Therefore, the radiation heat transfer coefficient is converted into an equivalent convective heat transfer coefficient. The gas heat transfer coefficient in the regenerator is the sum of convection and radiation heat transfer coefficients.

α = α_c + α_r,

(18)

α_r = q_r/(t_f − t_m).

(19)

2.5. Flow Resistance Loss of Gas in Regenerator

Dust particles in gas are treated as pseudo-fluid. Gas is a mixture of gas phase and solid phase dust particles. Resistance loss is as follows [17].

$$\frac{d\left(\Delta p\left(x\right)\right)}{dx}=k_1\frac{{(1-\epsilon )}^2}{{\epsilon }^3}\frac{\mu \left(x\right)W_f\left(x\right)}{d_p{}^2}+k_2\frac{1-\epsilon }{{\epsilon }^3}\frac{{\rho }_f\left(x\right)W_f{(x)}^2}{d_p},$$

(20)

where  $\Delta p\left(x\right)$  is resistance loss, Pa;  $\mu \left(x\right)$  is the dynamic viscosity of gas, Pa·s;  ${\rho }_f$  is gas density, kg/m³;  $k_1$  and  $k_2$  are constant and they are 229 and 1.96, respectively.

2.6. Solve Steps of Equations

The numerical calculation process of heat transfer and flow in the regenerator can be divided into the following six steps:

(1)

Parameter input and initialization;

(2)

Mesh division and step calculation;

(3)

From the beginning of the heating period, according to the initial temperature of the regenerator and the flue gas, the temperature and pressure of the regenerator in the heating period are calculated in turn with the time step method;

(4)

According to the commutation conditions, the initial conditions of the temperature field in the cooling period are determined from the temperature field in the heating period, and the temperature and pressure in the cooling period regenerator are calculated in turn with the time step method;

(5)

After the end of the heating period and the cooling period, judge whether the result reaches the iteration calculation number. If it meets the number, enter (6); otherwise, use the temperature field at the end of the cooling period as the initial temperature field of the heating period, enter (3);

(6)

Calculate the average gas outlet temperature, temperature efficiency, and thermal efficiency of the regenerator.

3. Results and Discussion

3.1. Effects of Soot Particle Size on Gas Phase Heat Transfer and Dust Removal Efficiency

The main factors affecting the enhanced heat transfer of soot on the gas phase are soot concentration and soot particle size. Figure 4 shows different particle sizes (50, 55, and 60 μm) when the inlet soot concentration is constant (100 g/Nm³). The dust removal efficiency is 90%, 94%, and 96%, respectively. The larger the soot particle size, the higher the dust removal efficiency. Figure 5 is the variation curve of the relative value of smoke and dust with different particle sizes on the enhancement of the heat transfer coefficient along the height of the regenerator. The smaller the soot particle size, the more the soot particles, the stronger the disturbance to the boundary layer on the surface of the heat storage body, the thinner the laminar bottom layer, the smaller the thermal resistance, and the stronger the strengthening effect. At the inlet, the heat transfer coefficient increases by 6.5% when there is smoke disturbance.

3.2. Effect of Initial Soot Concentration on Gas Phase Heat Transfer and Dust Removal Efficiency

Initial soot concentration affects gas phase heat transfer and dust removal efficiency. Figure 6 is the variation curve of smoke and dust concentration in the regenerator with different initial concentrations. The dust removal efficiency of the regenerator is about 90% under the five initial smoke concentrations. Figure 7 is the influence curve of smoke and dust with different initial concentrations on heat transfer in the regenerator. The greater the initial concentration of soot, the more soot particles and the stronger the heat transfer enhancement effect. The inlet soot increases the heat transfer coefficient by about 4.5–8.5%. As shown from the curves, the dust concentration and heat transfer index in smoke decrease gradually with the increase in the regenerator height. When the height is higher than 2.5 m, the concentrations change slowly. In other words, the height of the regenerator should be higher than 2.5 m from the perspective of heat transfer and dust removal efficiency.

3.3. Heat Transfer and Flow in the Regenerator in a Commutation Cycle

Take a 100 t steel-making converter as an example: the gas inlet flow is 37,360 nm³/h, the inlet temperature is 800 °C, the soot concentration is 100 g/N m³, and the soot particle size is about 50 μm. The designed height of the regenerator is 3 m, and the diameter is 4 m.

Figure 8 is the variation curve of gas temperature and heat storage temperature with the height of the regenerator during the heating period. Figure 9 is the variation curve of cooling gas and heat storage temperature with the height of the regenerator during the cooling period. As shown from the curves, the temperature of converter gas and heat storage materials change gradually with the increase in the regenerator height. In a commutation cycle, the heat exchange in the regenerator is an unsteady heat transfer process. The temperature of the gas and regenerator changes with time and along the height of the regenerator. Due to the high-efficiency heat exchange performance of the regenerator, the average temperature of the gas in the heating period can be reduced from 800 °C to 132 °C, and the average temperature of the cooling gas in the cooling period can be increased to 558 °C. Figure 10 is the variation curve of resistance loss with regenerator height in the heating period and cooling periods. The dotted line is the change curve of resistance loss in gas without soot. The existence of smoke and dust in gas increases the resistance loss in the regenerator, and the resistance loss increases by about 25%. Moreover, the slope of the resistance loss curve is greater than that of other parts due to the high soot concentration at the inlet of the regenerator at 0–0.7 m.

4. Conclusions

According to the characteristics of the two-phase flow heat transfer of converter gas in the medium temperature section, a mathematical model of converter gas heat transfer and flow in the regenerator was proposed in this paper. The effects of particle size and concentration of dust in the regenerator on heat transfer and dust removal characteristics were investigated through the model, and the changes in temperature and resistance loss in the regenerator in a cycle were calculated. The conclusions are as follows:

(1)

Smoke and dust strengthen the heat transfer process in the regenerator. With the increase in soot particle size, the enhancement of the heat transfer process is weakened, and the dust removal efficiency is increased.

(2)

With the increase in the initial concentration of soot particles, the enhancement of the heat transfer process is strengthened, and the dust removal efficiency is reduced. At the entrance of the regenerator, the strengthening effect of smoke and dust is the strongest, and the heat transfer coefficient increases by 4.5–8.5%.

(3)

In a reversing cycle, the temperature of converter gas can be reduced to about 132 °C after flowing through the regenerator, and the dust removal efficiency in the regenerator can reach 90%.

(4)

The existence of smoke and dust in gas increases the resistance loss in the regenerator, and the resistance loss increases by about 25%. Moreover, due to the high smoke concentration at 0–0.7 m in the regenerator, the change speed of resistance loss is greater than that in other parts.