Numerical and experimental investigations of converter gas ...

Numerical and Experimental Investigations of Converter Gas Improvement inside a Flue Using Its Waste Heat and CO2 by Pulverized Coal Injection Bao Wang ,a,b Jian-An Zhou,a,b Jian-Bo Xie,a,b Zhong-Qiu Liu,c Hua Zhang,a,b and Lan-Hua Zhoud a The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science &Technology, Wuhan, Hubei, 430081, China; [email protected] (for correspondence) b The Key Laboratory of Ferrous Metallurgy and Resources Utilization, Ministry of Education, Wuhan University of Science & Technology, Wuhan, Hubei, 430081, China c School of Metallurgy, Northeastern University, Shenyang, 110819, China d School of Resources and Environmental Engineering, Panzhihua University, Panzhihua, Sichuan, 617000, China Published online 00 Month 2017 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.12812 To explore how to use the waste heat and reduce the CO2 emissions of converter flue gas, a new process is proposed in this paper, performed by injecting pulverized coal into the hot gas using its waste heat to generate CO and further to achieve a secondary recovery of flue gas. Numerical simulations and industrial investigations are conducted. By comparing the injecting rate, the particle size, and the injecting velocity of the pulverized coal, an optimized scheme was obtained. The testing results show that, with increasing the injecting rate, the O2 and CO2 contents in the flue gas are reduced, while the CO and H2 contents are increased. When the injecting rate is 20 kg min21, the O2 and CO2 contents are decreased by 72.16% and 50%, respectively, yet the CO and H2 contents are raised by 21.08% and 171.8%, respectively. The gas recovery time is extended by 10.12% and the carbon conversion is increased as much as 55%. Thus, the new process will be of great benefit to reduce CO2 emissions C 2017 American and increase the energy of converter gas. V Institute of Chemical Engineers Environ Prog, 00: 000–000, 2017 Keywords: converter gas, flue, pulverized coal, waste heat, CO2 emission INTRODUCTION

The steel manufacturing process is one of the most energy-intensive industrial sectors. Large quantities of waste heat are discharged into the atmosphere or exhaust water during steel production processes. It is estimated that as much as 20% to 50% of the thermal energy consumed by metallurgical industry is lost as waste heat, mostly in the form of high-grade waste heat (>923 K) [1,2]. With the growing awareness of environmental issues, increasing energy costs, and tightening emission controls, increasingly importance has been attached to improving energy efficiency and recovering waste heat for steel plants. Converter steelmaking is the dominant method for worldwide steel plants, which nearly occupies 74% of the total C 2017 American Institute of Chemical Engineers V

amount of the produced crude steel [3]. Converter gas, the by-product of converter steelmaking, is rich in 50–80% carbon monoxide and regarded as a significant secondary energy source [4]. During BOF steelmaking process, large number of high temperature (1473–1773 K) gas produced by carbon-oxygen reaction immediately leaves converter mouth, and it is quickly cooled down to a temperature below 1273 K though the evaporating flue [5]. Then it flows into the evaporative cooler for the secondary temperature dropping to 373–473 K and primary dust removal. Approximately 70% of the sensible heat is recovered by the evaporation cooler for generating high-pressure steam, which is a state of the art waste heat recovery system and widely used in steel industry [6]. Generally, the recovered sensible heat is only 16.5% of the total heat value of 8.8 MJ Nm23 exhaust gas [7]. More energy is recovered as latent heat from the CO-rich converter gas, widely by combustion method or noncombustion method [8]. In combustion type system, CO in off-gas is combusted with air at BOF mouth and converted to CO2, and then the off-gas after heat recovered is flared and released. About 80% of total heat can be recovered in a waste heat boiler to generate steam. In non-combustion system, the CO-rich converter gas after cooled and treated for the dust removal is recovered and stored in the gas holder or mixed with blast furnace gas and coke oven gas, which can be used for electricity generation, methanol production [9], and direct reduction of lump iron ore [10]. However, with either waste heat recovery approach, the off-gas produced by carbon-oxygen reaction in converter typically contains 14–20% of carbon dioxide in-situ measured by Sandl€ obes et al. [11]. High concentration CO2 significantly affects the heat value of converter gas, and it is also ultimately discharged into the ambient air as greenhouse gas. A series of methods in which CO2 is captured and stored physically or chemically have been developed to reduce CO2 emissions. Pinto et al. designed a route of CO2 capture and its conversion into CaCO3 in means of Solvay process utilizing steel slag and industry waste heat [12]. PSA (pressureswing adsorption) and MEA-based (monoethanolamine)

Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep

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Figure 1. Schematic for injecting pulverized coal into the evaporating flue. (a) New process of converter gas recovery (b) Injecting gun. [Color figure can be viewed at wileyonlinelibrary.com]

chemical absorption are applied for development of high purity CO gas recovery [13,14]. Nevertheless, few effective measures have been taken to reduce the content of CO2 in converter gas, most methods are in the light of raw materials, air imbibed quantity, and intensity of oxygen blowing during BOF steelmaking [15]. Based on coal gasification and waste heat recovery of BOF off-gas, the authors [16] proposed a novel technology that pulverized coal is injected into converter evaporating flue and efficiently reacts with CO2 of flue gas utilizing its high temperature sensible heat, to convert CO2 into valuable CO. The conversion conditions refer to parameters of pulverized coal injection, flue gas flow and temperature, and distribution of CO2 concentration in evaporating flue. Therefore, how to inject the pulverized coal into the flue is the key to the new recovery process. Consequently, the aims of present work are: to investigate the influences of pulverized coal injecting parameters on the flow of gas-solid phases in evaporating flue by CFD numerical simulation; design of the industrial pilot test schemes by the results of simulations; optimize the injecting process of pulverized coal and verify the implementation effects. CONVERTER GAS RECOVERY PROCESS

Method Description Converter gas and waste heat recovery is the key energysaving process for BOF steelmaking process, making the BOF process with minus energy consumption and lowering CO2 emissions [17]. The conventional process of gas recovery is described here. After the hot metal and scrap steel are poured into the furnace, an oxygen lance is lowered into the melt bath and blowing pure oxygen for inclusions removal (e.g., C, P, Si, Mn, and S) along with a significant amount of lime injected into the vessel [18]. The carbon in the hot metal is removed as CO and CO2 during the O2 blow. Usually, the movable hood of converter is lowered and the gas recovery valve is opened after blowing oxygen for 3–4 minutes, when the CO content reaches over 20% and the O2 content is less than 2% in the flue gas [15]. Then the high temperature offgas with great amount of dust is caught by moveable hood and cooled through the evaporating flue to a temperature approximately 1273 K. Subsequently the CO-rich gas is cooled again, cleaned and stored into the gas holder for further use. But during the start and end of blowing, because of 2 Month 2017

high O2 content and low CO in the off-gas, to ensure the safety of off-gas recovery, the movable hood is raised, the off-gas is instead flared and discharged after sensible heat recovery and dust removal [19]. It is estimated that the specific direct CO2 emission is approximately 180 kg per tonne of produced steel during BOF steelmaking process [20]. The new technology for gas recovery utilizing the waste heat, CO2 in the flue gas to convert pulverized coal into CO, was established to improve converter gas recovery efficiency, security and reduce CO2 emission, as shown in Figure 1a. When blowing oxygen, the pulverized coal delivered from powder dispenser with N2 protection is synchronously injected into the evaporating flue by injecting guns, which extend into both the feeding ports of the flue, as shown in Figure 1b. If the gas components reach the recovery level (O2 20%), the recovery valve would be opened. Within recovery process, the amount of pulverized coal injection can be adjusted dynamically by the injection control system according to the online dynamic varying analysis of the gas components. Finally, stop to inject pulverized coal once the oxygen blowing is completed. In this method, the anthracite, bituminous coal or the mixed can be chosen as the injection carbon materials, its particle diameter is about 149 lm (100 meshes) to 48 lm (300 meshes). Reaction Thermodynamics BOF off-gas mainly consists of CO (20–80%), CO2 (10– 30%), and O2 (1–6%), and varies greatly in temperature, carbon monoxide and nitrogen concentrations. During the pulverized coal injecting, the probable reactions between the coal and gas in the evaporating flue and its corresponding Gibbs free energy change (DG) are depicted by Eq. (1) to Eq. (4) below. 2CðsÞ 1 O2 5 2CO DG1 522288002171:5T kJ mol21

(1)

CðsÞ 1 O2 5 CO2 DG2 5239535020:54T kJ mol21

(2)

2CO 1 O2 5 2CO2 DG3 525653901175:17T kJ mol21

(3)

CðsÞ 1 CO2 5 2CO DG4 51665502171T kJ mol21

(4)

where, T represents the thermodynamic Kelvin temperature, K. According to the equations above, the Gibbs free


Figure 2. Gibbs free energy changes with temperature. [Color figure can be viewed at wileyonlinelibrary.com] Figure 3. Scheme grids of the evaporating flue with pulverized coal injection. energy variation of each reaction is in a linear fashion with temperature as shown in Figure 2, which can be used to predict the feasibility of reactions. For a reaction to be feasible, the value of DG has to be less than zero. The results clearly show that the Gibbs free energy changes of above reactions are below zero from 1000 K to 1800 K, means that pulverized coal can spontaneously react with O2, CO2 to generate CO in high temperature gas of 1473–1773 K. Generally, the small amount of O2 in flue gas is rapidly depleted by the coal. Thus, the gasification rates of pulverized coal with CO/CO2 atmosphere in converter flue greatly depends on the reaction kinetic factors, such as injection velocity, particle size, flow rate and so on. More details are discussed by CFD simulation and pilot tests as follows. CFD NUMERICAL SIMULATIONS

Mathematical Models In the present work, a numerical study on the mixed situation of gas-solid two-phase mixtures in the flue is carried out. The simulated object is a 35-ton top-blowing oxygen converter. The computation domain is the part above the converter mouth, about 6.92 m long and 1.48 m of diameter. The domain is meshed by a combination of hexahedral and individually discretized numerical sub-grids, as shown in Figure 3. The total mesh grids include approximately 1 066 000 elements. This modeling employs the standard k-E model for gas turbulence. The Lagrange particle tracking model [21] is used to describe the motion of the particle. In addition, the random Migration model [22] is also applied to consider the effects of the turbulence and vortexes on the moving tracks of the particles. The governing equations for the flue in the system are established as follows: The continuity equation can be expressed by: oq 1rðqui Þ50 ot

(5)

The Naiver-Stokes equation can be expressed by: oðqui Þ 1r qui uj 52rp1leff r2 ui 1qg1Fi ot

(6)

The turbulent kinetic energy equation can be expressed by:

oðqjÞ oðqjui Þ o 1 5 ot oxi oxj

l1

lt oj 1Gk 2qE rk oxj

(7)

The turbulent dissipation equation can be expressed by: oðqEÞ oðqEui Þ o 1 5 ot oxi oxj

l1

lt oE C1E E E2 1 Gk 2C2E q k rE oxj k

(8)

The particle transport equation can be expressed by: p dvp qp dp3 5Fg 1Fb 1Fd 1Fs 1Fp 6 dt

(9)

The density of the flue gas mixture (O2, CO, CO2, and N2) is defined by the incompressible ideal-gas. The actual inlet velocities of the flue gas is measured at 17.6 m s21, considered as a constant. The injection velocity of pulverized coal out of injecting guns is controlled from 100 to 250 m s21. Injecting rate of the pulverized coal is in range of 5–20 kg min21. Outlet pressure is a standard atmospheric pressure. The fore-end inlet of injecting gun is set to escape boundary and the pulverized coal uniformly distribute at the inlet. The exit of the injecting gun is regarded as the boundary condition for the pressure and also set as escape boundary. When the particle reaches the exit of the calculated domain, it escapes the system. The no-slip boundary condition is employed at all the wall boundaries. When the pulverized coal hits the wall, it is bounced off to the flow field. CFD Results and Discussions Effects of Particle Size on the Mixed Distribution

To explore an improved injecting effect, several simulated conditions are investigated. Combined with practical measured values at the inlet of the gas hood during the steelmaking process, the first simulated condition is described as follows: the inlet velocity of the gas is set to 17.6 m s21, the injecting velocity of the pulverized coal is set to 100 m s21 and the injecting rate is 5 kg min21, while the particle diameter of the pulverized coal is in the range of 100 meshes to 250 meshes. The effects of the diameters of the pulverized coal and the resulting distributions are shown in Figure 4. When the pulverized coal is injected into the fore-end of the evaporating flue, and its fixed carbon reacts with multicomponents of gas to form CO and H2 produced by


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Figure 4. Effects of the particle diameter on the movement of the pulverized coal. (a) 100 meshes, (b) 200 meshes, (c) 300 meshes, (d) 400 meshes.

Figure 5. Effects of the injecting rate on the mixed state of the pulverized coal. (a) 5 kg min21 (b) 10 kg min21 (c) 15 kg min21 (d) 20 kg min21.

Figure 6. Effects of injection velocities on the movement of the pulverized coal. (a) 100 m s21 (b) 150 m s21 (c) 200 m s21 (d) 250 m s21.

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Figure 7. Effects of the inserted horizontal length on the movement of pulverized coal. (a) 0 mm (b) 300 mm (c) 600 mm.

Figure 8. Transient movement of the pulverized coal with the time. (a) 0.1 s (b) 0.2 s (c) 0.3 s (d) 1.0 s.

Table 1. Optimal injection parameters from simulation results.

Cass No. 1-1 1–2 1–3 1–4 2-1 2-2 2–3 2–4 3-1 3-2 3-3 3–4 4-1 4-2 4-3

Gun inserted horizontal length (mm)

Injecting velocity (m s21)

Injecting rate (kg min21)

Particle size (meshes)

0

100

5

0

100

0

100 150 200 250 200

5 10 15 20 5

100 200 300 400 200

0 300 600


5

200

200

Mixed effects Worst Best Middle Middle Worst Middle Middle Best Worst Middle Best Middle Middle Best Worst

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velocity on the movement of the pulverized coal are shown in Figure 6. A homogenous distribution is therefore expected to be observed under the various injection velocity conditions. The results shows that a greater injecting velocity was illustrated to produce a more dense mixed distribution in the evaporating flue. When increasing the injection velocity from 100 to 150 m s21, gas-solid mixture obviously changes from a scare to a dense state, because of the increased turbulence and collisions between the gas-solid two phases. However, when the injection velocity of the pulverized coal reaches to 250 m s21, the mixed state has no significant changes, as shown in Figure 6d. It is therefore demonstrated that the pulverized coal with an injection velocity of 150–200 m s21 should be applied to the experiment. High velocity gas flow collision can form the turbulence effect, resulting in a strong mixture of gas-solid two-phase flows.

decomposition and oxidation under high temperatures (1573–1773 K) conditions. Meanwhile, the high velocity inlet gas quickly and strongly impacts the particles of pulverized coal, resulting in a dispersed gas-solid mixture [23]. When the particle diameter is 100 meshes, the two-phase flow shows separate motions. The pulverized coal moves in a path by firmly holding together while the gas flow moves in another separate path, which means that the reactions in this situation doesn’t happen fully because of the poor gassolid contact. This can be explained that the larger particle diameter results in worse mixing because the gas flow cannot efficiently change the moving state of the pulverized coal with larger diameters. By reducing the pulverized coal size to 200 meshes, the mixed state is apparently improved, which is beneficial for fostering the gas-solid reactions from full mixed. However, when the diameter of the pulverized coal is superfine over 200 meshes, the mixture gradually flows near the wall of flue, especially for 400 meshes as shown in Figure 4c,d. It is therefore demonstrated that the pulverized coal with a diameter of 200 meshes is the better choice taking into account grinding costs and reaction effects.

Effects of Horizontal Length of Injecting Guns on the Mixed Distribution

When the inlet velocities of the gas and pulverized coal is 17.6 m s21 and 200 m s21, respectively, injection rate is 5 kg

Effects of Injection Rate on the Mixed Distribution

In practical the inlet flow rate of pulverized coal from injecting gun can be adjusted according to the CO2 concentration in flue gas. With an inlet gas velocity of 17.6 m s21, injecting velocity of the pulverized coal of 100 m s21, and size of 200 meshes pulverized coal, the effects of the injection rates on the movement of the pulverized coal are simulated and shown in Figure 5. From Figure 5 we can see that, when the injecting rate is in the range of 5 kg min21 to 20 kg min21, the pulverized coal becomes more uniform with high concentration of coal in gas flue. In addition, the greater impact results in a more scattered and discretized mixture distribution, which is beneficial to foster the mixed reactions. However, the reactions last approximately 2 s between the interval distances whose temperature is in range of 1823 K to 1273 K. Once the mixture travels the distance of the tube, the reactions are complete because the temperature decreases. If too much pulverized coal was injected, it cannot be converted completely within less than 2 s, so that the remaining carbons will leave the flue to the first dust removal tower with a lower temperature (