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Numerical simulation and optimization of pulverized coal injection with enriched oxygen into blast furnace
Applied Thermal Engineering 67 (2014) 72e79
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Numerical simulation and optimization of pulverized coal injection with enriched oxygen into blast furnace Yongqing Li a, Xiaohui Zhang b, *, Jiayuan Zhang a, Jiemin Zhou a, Hongjie Yan a a b
School of Energy Science and Engineering, Central South University, Changsha 410083, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China
h i g h l i g h t s We have established a mathematical model of combustion process in blast furnace. The gas velocity distribution and motion characteristics of pulverized coal in raceway and coke bed were obtained. We have developed the optimal parameters combination in purpose of increasing burnout rate.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 September 2013 Accepted 23 February 2014 Available online 15 March 2014
The combustion characteristics of pulverized coal in the blast furnace raceway is an important factor which determines the performance of injection of pulverized coal with enriched oxygen as well as the energy consumption of blast furnace. Research was carried out to investigate processes including the gas and solid flow, particle movement, drying and pyrolysis of pulverized coal and combustion of carbon residue and coke in a blast furnace with standard ke3 model, discrete phase model, evaporation and diffusion model, dual parallel competition model and diffusion-kinetic combustion model respectively in Fluent 6.3. The accuracy and reliability of the numerical model was verified with a semi-industrial experiment. Moreover, 7 parameters were optimized by the orthogonal experiments, taking the burnout rate as the evaluation indicator. Results show that the burnout rate under the optimal condition can be increased by 110.8% comparing to that under the standard condition. The importance of parameters influencing the evaluation indicator is found in the order which is the type of the pulverized coal, the pulverized coal injection rate, the oxygen enrichment rate, the blast temperature, the spraying gun type, the blast volume and the angle. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Oxygen enrichment Burnout rate Numerical simulation Orthogonal experiment
1. Introduction The pulverized coal injection (PCI) with enriched oxygen into a blast furnace is an important technology to reduce the cost and maintain a steady production in an iron and steel industry [1]. In the past long period, the pulverized coal used for injection was all anthracite in China. However, as raw coal resources decrease continuously while the combustion performance of mixture of anthracite with bituminous coal keeps improving, the mixed coal injection is more often adopted [2]. The combustion characteristic in the raceway is an important factor for the performance of oxygen-enriched coal injection technology as well as the energy consumption of the blast furnace. Most research of oxygen-
* Corresponding author. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.applthermaleng.2014.02.062 1359-4311/Ó 2014 Elsevier Ltd. All rights reserved.
enriched coal injection for blast furnace are focused on the fluid flow, the heat and mass transfer process, the relationship between the oxygen enrichment rate and the coal injection ratio, and the combustion characteristics of blended coal in the blast furnace and coke bed. For example, Shen et al. [3e5] developed a mathematical model to investigate the fluid flow and combustion of pulverized coal in the raceway and the coke bed, thus influences of operating parameters on the pulverized coal burning rate and relationship between the burning rate and the oxygen enrichment rate are obtained. Wijayanta et al. [6] got the relationship between the oxygen enrichment rate and the combustion performance of carbon powder mixed with biomass by comparing the combustion characteristic of two kinds of biomass carbon powder in the blast furnace. Khairil et al. [7] studied the relationship between the oxygen enrichment rate and the smelting intensity, tuyere and raceway temperature and pulverized coal burnout in a blast furnace by experiments. Zhao et al. [8] and Kong et al. [9] also carried out some
Y. Li et al. / Applied Thermal Engineering 67 (2014) 72e79
Nomenclature Ap Bs Char C0 Cp D dp Fi fi gi H Hreac I k l mp Ri R1, R2
surface area of particle [m2] pre-exponential factor residual carbon mass of pulverized coal [kg] specific heat of particle [J/kg K] diffusion coefficient [m2/s] particle size [m] additional force of particles in direction i [N] body force of fluid in direction i[N] gravitational acceleration in direction i, [m/s2] total enthalpy [kJ/mol] reaction heat [J/kg] radiation intensity [W/m2] turbulent kinetic energy frequency factors particle mass [kg] generation/vanishing rate of species i low/high temperature reaction
experimental studies on various mixed coal injection and combustion characteristics. Ziebik et al. [10] and Xu et al. [11] discussed how differently the coal injection ratio and the oxygen enrichment rate influence the internal heat distribution and the pulverized coal combustion process in a blast furnace by experiments and numerical simulation respectively. In order to analyze the flow and combustion in the raceway and to assist the improvement of the burner design, Takeda [12] developed a two-dimensional mathematical model of the pulverized coal combustion. All these studies investigate the combustion characteristics of the pulverized coal in the blast furnace mainly from aspects of the oxygen enrichment rate and the fuel characteristics, however not take the influence of operating parameters into considerations. The effect of oxygen-enriched PCI technology is mainly on the combustion characteristics of the pulverized coal in furnace. Meanwhile, operation parameters of the blast furnace will also have certain effects on the combustion characteristic. In order to improve the burnout rate of the pulverized coal, it is particularly necessary to optimize the operation parameters such as the oxygen enrichment rate and the coal injection ratio. In this paper, a threedimensional mathematical model is developed for the combustion of the pulverized coal in blast furnace, and an optimization is carried out for the oxygen enrichment for the blast furnace injection based on the numerical simulations of the pulverized coal combustion process in the raceway.
S T u VM Yi Yox Ys Yss Yw,g Yw,s
sji G a b 3 3p
sB u r 4
source term temperature of gas/particle [K] speed of gas/particle [m/s] volatile of pulverized coal mass fraction of species i mass fraction of oxygen in gas mass fraction of gas phase composition mass fraction of oxidant on particle surface mass fraction of moisture in gas mass fraction of moisture in particle surface force of fluid in direction i [N] heat/species transfer coefficient content of dry and ash free basis in volatiles equivalence ratio dissipation rate of turbulent kinetic energy particle emission rate StefaneBoltzmann constant reaction rate density [kg/m3] porosity
Therefore, the computational domain included all the three parts and was divided into the raceway, the dripping region and the dead stock region (as shown in Fig. 1). The coke bed was assumed as a porous media zone [13,14]. Further simplification was made for the simulation of the pulverized coal combustion process in the raceway as follows [15,16]. (1) The granularity of the pulverized coal and the coke are welldistributed, and all particles are assumed to be spherical. (2) The crushing and the coagulation of both the pulverized coal and the coke particles are ignored. (3) The flow of liquid in the cavity is ignored. (4) The shape of the raceway keeps unchanged under the modeling conditions. A physical model was developed according to the actual size of the blast furnace in an iron and steel plant, and the size of the raceway was calculated with the empirical formula [17]. Due to the symmetric structure of the blast furnace, only half of the furnace was taken in the physical model, as shown in Fig. 2. The mesh of the model is shown in Fig. 3.
2. Numerical simulation 2.1. Physical model A physical model is developed taking a 600 m3 blast furnace in an iron and steel company as its prototype. The diameter and the height of the hearth are 6150 mm and 3300 mm, the diameter and the angle of the bosh are 7150 mm and 80 , the height of the tuyere is 2900 mm, the diameter of the straight tube is 120 mm, the diameter and the length of the tuyere are 120 mm and 340 mm, the diameter of the spray gun is 20 mm, and the angle between the spray gun centerline and the straight tube is 11. To understand the burning process of the coal along the border of raceway, the combustion of the pulverized coal in both the straight tube and the tuyere needs to be taken into account.
73
Fig. 1. The diagram of bottom area in blast furnace.
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Y. Li et al. / Applied Thermal Engineering 67 (2014) 72e79
Fig. 3. The diagram of grid structure.
v v v ruj H ¼ ðrHÞ þ vt vxj vxj
v Gh H vxj
! þ
vp þ Sh vt
(6)
The species transport equation: Fig. 2. The diagram of physical model.
vrYi v v ruj Yi ¼ þ vxj vxj vt
v Gi Yi vxj
! þ R i þ Si
(7)
2.2. Mathematical model 2.2.1. Governing equations The governing equations describe the heat and mass transfer of the pulverized coal combustion process in the blast furnace include equations of the gasesolid two-phase continuity, momentum, energy and species transport. All equations are shown below [18]. The continuity equation:
2.2.2. Combustion and chemical reaction model The pulverized coal combustion process includes stages of drying (water evaporation), pyrolysis (volatile separating out) and combustion (residual carbon burning).
vr v ruj ¼ 0 þ vt vxj
CmHn is set as the volatile of pulverized coal. The combustion reaction of CmHn is as follows.
(1)
Cm Hn þ ðm þ n=4ÞO2 /mCO2 þ nH2 O=2
The momentum equation:
v v v ruj ui ¼ s þ fi ðrui Þ þ vt vxj vxj ji
(2)
The coke bed is considered as isotropic porous media [19] in the model. Then it is necessary to consider the influence of the porous media on the viscosity and the inertial of the fluid. Therefore, a source term Si, which is consisted of two parts, i.e. the viscous loss term and the inertial loss term [20], must be added to the momentum equation. For a homogeneous porous media, the source term is described as follows [20].
m 1 Si ¼ vi C2 rvmag vi a 2
150 ð1 4Þ2 ¼ 2 a dp 43
(4)
3:5 ð1 4Þ dp 43
(5)
C2 ¼
The energy equation:
(8)
The reaction rate of formula (8) is mainly affected by the diffusion rate in the raceway and the kinetic rate in the dead stock and the dripping region. Then the EBU-Arrhenius model is used to describe the gas-phase chemical reaction rate [22], which are as follows.
us ¼ minðuE ; uA Þ
(9)
3
uE ¼ cE r minðYs ; Yox =bÞ
(10)
uA ¼ Bs r2 Ys Yox expðE=RTÞ
(11)
k
(3)
The ErguneBlakeeKozeny equation is used to calculate the viscous resistance coefficient (1/a) and inertial resistance coefficient (C2) [21].
1
(1) Gas-phase chemical reaction model
(2) Pulverized coal combustion model A droplet evaporation diffusion model is used to describe the drying process [23], and the evaporation rate is:
Yw;s Yw;g mw ¼ pdp NuDr ln 1 þ 1 Yw;s
(12)
A double parallel competition model is used to describe the pyrolysis process [3], that is:
aVM1 þ ð1 aÞChar1 ðR1 Þ 1:5aVM2 þ ð1 1:5aÞChar2
ðR2 Þ
The formation rate of the volatile is:
(13)
Y. Li et al. / Applied Thermal Engineering 67 (2014) 72e79
dVM ¼ ðl1 þ 1:5l2 ÞaC0 dt
(14)
The diffusion-kinetic combustion model is used to describe the combustion process of the carbon residue and coke [24]. The burning rate is:
mh ¼
X
E RTp
pd2p rp Yss Bs exp
(15)
As the radiation must be considered for the combustion reaction in the blast furnace, the P-1 model is used in this study to describe the radiation process. (3) Coke combustion model The heterogeneous surface chemical reaction model is used to describe the coke gasification reaction. The coke is assumed as elemental carbon, and the reactions are assumed as the first-order reactions. The surface chemical reactions are as follows.
C þ O2 /CO2
(16)
2C þ O2 /2CO
(17)
C þ H2 O/CO þ H2
(18)
The finite-rate model is used to simulate above reactions, and the reaction rates are determined by the Arrhenius expressions. 2.2.3. Discrete phase model The mass concentration of pulverized coal is lower in the straight tube and the raceway, so the pulverized coal can be modeled as discrete solid particles distributed in the gaseous phase, and the RosineRammler distribution function is used to describe the particle size distribution. The trajectory and temperature of pulverized coal particles are described by the discrete phase model based on Euler-Lagrange method [25]. (1) Motion equation The motion equation of the pulverized coal particles of unit mass is:
gi rp r dup ¼ FD u up þ þ Fi rp dt
(19)
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Table 1 Ultimate and proximate analysis of pulverized coal. Volatiles (daf)
Ash (daf)
C (daf)
H (daf)
O (daf)
19.2
10.0
91.7
4.83
3.50
The hot air inlet and the spray gun are set as mass flow inlets, and their flow rate, static pressure, temperature and gas composition are set to be the average of the actual value in a stable operation condition of the blast furnace. The outlet of the raceway is set as a pressure outlet. The wall of the straight tube, the bosh and the hearth are set to be non-slip walls, and the value of the outlet pressure and wall temperature is measured during the industrial test. The contour of the gaseous temperature in the computational domain is shown in Fig. 4. The temperature distribution in the center and the bottom of the raceway are different. The maximum temperatures in these two areas are about 1800 K and 2416 K, as shown at 1.4e1.5 m in Fig. 5. The velocity of the pulverized coal decreases because the viscous and inertial resistance increase when they move to the border of the raceway, so the residence time of the pulverized coal increases as well. All these indicate that the combustion of the pulverized coal mainly take place in this region. Fig. 6 shows the diagram of flow distribution. As shown in Fig. 6(a), most of the gas from the pipe changes its direction when it approaches the border of the raceway, then it circulates in the cyclotron zone. For the air flow left the cyclotron area, most moves upward to the coke bed, with its velocity decreasing quickly. The air velocity in the coke bed is nonuniform, for which it is bigger in the dripping region and smaller in the dead stock, because the resistance to the gas flow is smaller when the porosity is greater. In Fig. 6(b), it can be found that particles with larger size (>70 mm) move upward in the raceway when approaching the border and enter into the coke bed. Particles with smaller size (<70 mm) mainly circulate in the raceway with the cyclotron currents. In the coke bed, small particles move upward to the dripping region after escaped from the raceway. Larger particles (>100 mm) are mainly concentrated in the dead stock because they are less easily influenced by the gas flow. Fig. 6(c) illustrates that the pulverized coal particles stay in the cyclotron for a short time while they stay in the coke bed for a longer time. Moreover, their dwell time in the dripping region is shorter than that in the dead stock, which is considered to be mainly related to both the gas flow rate and the particle size. Fig. 7 shows the burnout rate of the pulverized coal of different sizes changing from the straight tube to the border of raceway.
(2) Energy equation The energy equation of the pulverized coal particles is as follows:
mp Cp
X dmp dTp ¼ pdp lNu Tg Tp þ Hreac dt dt
þ Ap 3 p pI sB Tp4
(20)
2.3. Simulation results and analysis Oxygen-enriched PCI combustion process in the raceway is simulated by Fluent 63. The basic and operating parameters of the blast furnace are determined in a field test. Physical parameters of the pulverized coal are shown in Table 1, and the average particle size of the coal is 40 mm.
Fig. 4. Gas temperature distribution in simulation region.
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Fig. 5. Gas temperature along the axis of straight tube.
The burnout rate is mainly determined by the volatilizing rate and the combustion rate of the carbon residue. As shown in Fig. 7, the smaller the pulverized coal is, the greater the volatilizing rate and the carbon residue combustion rate are. This indicates that the particle size has a great effect on the burnout rate. The reason is that the smaller the particle size is, the quicker the particle surface temperature rises and the shorter time it requires to achieve the pyrolysis temperature and the carbon residue ignition point. Fig. 8 shows how the temperature of particles of different sizes varies along the distances from the outlet of the spraying gun. It can be seen that the smaller the coal particle is, the faster the temperature rises and the greater the temperature difference between the pyrolysis process and the carbon residue combustion process is. The particle temperature reaches the maximum at the end of the residual char combustion state, followed by temperature decreasing and keeping stable. The highest temperatures of most particles appear between the distance of 1.4 and 1.6 m. Meanwhile, the smaller the particle size is, the earlier the highest temperature appears, and the highest temperatures of small particles are usually greater than those of larger particles. This indicates that the small pulverized coal particles are more effective in providing good dynamic condition for the pulverized coal combustion. 3. Experimental verification The gas composition and the pulverized coal burnout rate were measured by a half-industrializated experiment equipment, as shown in Fig. 9. The physical model established according to the experiment equipment for simulation is shown in Fig. 10 (As it is a symmetrical structure, only a half is taken in the simulation.). The coal injection inlet is located in the center of the cylinder, while the oxygenenriched air at high temperature comes from the outer ring. The initial simulation condition is in consistent with the experimental condition, which is shown in Fig. 10. Boundary conditions are as follows: the inlet is a velocity inlet; the outlet is set to be an outflow; the wall is adiabatic and set to be a no-slip boundary. The distances from the test points to the center of the coal injection inlet are 0.330 m, 0.626 m and 1.030 m (A, B, C) respectively. The average particle size of the pulverized coal is 40 microns. The physical parameters of the pulverized coal are shown in Table 1. After the pulverized coal combustion becomes stable, sampling analysis are carried out at the three test points by a flue gas analyzer for multiple measurements. At each test point, average concentration of the flue gas composition is obtained. Coal particles are collected at each relevant test point as well, so that the burnout rate of the pulverized coal can be calculated.
Fig. 6. The diagram of flow distribution.
Y. Li et al. / Applied Thermal Engineering 67 (2014) 72e79
77
Fig. 9. The schematic diagram of pulverized coal combustion experimental equipment.
Fig. 7. Burnout rate of pulverized coal.
Table 2 gives the simulation and tested results of the mass fraction of O2 and CO2. It can be seen that the maximum deviation of the mass fraction of O2 and CO2 are 8.8% and 7.1% respectively, and the change tendencies of simulation and tested value are consistent. Table 3 shows the simulation and tested results of the burnout rate. The deviation decreases with the distance from the straight tube inlet. The change tendencies of the simulation and the tested value are consistent too. Comparison between the simulation and the test shows that the maximum deviation is less than 9%, and the change tendencies are consistent. The model developed in this paper is proved to be correct and reliable. 4. Orthogonal experimental 4.1. Parameters and levels To study the influence of injection modes and operation parameters on the burnout rate of the pulverized coal injected with oxygen-enriched air, numerical simulation and orthogonal experiment are used to optimize the combination of various factors to achieve a maximum burnout rate. Taking the burnout rate as the evaluation index, factors include the pulverized coal types, the spray gun types, the oxygen enrichment rates, the angles, the coal injection ratios, the blast volumes and the blast temperatures are studied. The angle (factor A) refers to the angle between the centerline of the spray gun and the straight tube. The injection angles of the
Fig. 10. The diagram of physical model of pulverized coal combustion.
spray gun are 11 and 0 in most cases, hence only 2 different levels are used for the factor. Since the injection ratio responses sensitively to the burnout rate according to the practical experience, the selected coal injection ratio (factor B) levels should not be too big. Levels of 100 kg/t, 150 kg/t and 125 kg/t are selected according to the actual coal injection condition in the study. According to the basic design condition of a hot blast stove, the maximum and the average value of the dry air flow are 2150 Nm3/ min and 2050 Nm3/min respectively; thus levels of 1950 Nm3/min, 2050 Nm3/min and 2150 Nm3/min are selected for the blast volume (factor C). The blast temperature (factor D) is usually designed to be more than 1423 K. For a hot blast stove, it is important to take into account not only the requirement of a high air temperature and thermal efficiency, but also some other factors such as its service life. So the blast temperature should not be too high, and levels of 1423 K, 1473 K and 1523 K are selected. Studies [26] revealed that the increase of oxygen enrichment (factor E) is beneficial to the improvement of the burnout rate when the coal injection ratio is less than 200 kg/t and the oxygen enrichment rate is less than 5%, but when oxygen enrichment rate is more than 5%, the effect of oxygen enrichment rate on enhancing of the burnout rate is weakened, and the economic benefit reduces. Therefore the selected oxygen enrichment rate level should be limited within 5%, and level of 1.5%, 3.0% and 5.0% are selected, in which 1.5% is the benchmark. The pulverized coal types (factor F) used for the blast furnace injection are anthracite, bituminous coal and blended coal. In this paper, low volatile anthracite coal (CA), high volatile bituminous coal (CB) and blended coal of the former two types (ratio of 1:1, and Table 2 Comparison of simulation and tested results of mass fraction. Gas component
Test point
Simulated value (%)
Tested result (%)
Deviation (%)
O2
A B C A B C
4.8 5.5 7.3 13.4 14.5 15.1
5.1 5.2 8.0 13.8 15.6 14.1
5.9 5.8 8.8 2.9 7.1 7.1
CO2 Fig. 8. Temperature of pulverized coal particle change with distance.
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Y. Li et al. / Applied Thermal Engineering 67 (2014) 72e79
Table 3 Comparison of simulation and tested results of burnout rate. Evaluation index Burnout rate
Test point A B C
Table 7 Orthogonal experimental results.
Simulated value (%)
Tested result (%)
Deviation (%)
26.2 33.8 39.8
28.3 32.4 39.9
7.4 4.3 0.3
k1 (%) k2 (%) k3 (%) Ri (%)
Table 4 Parameters and values in different levels.
A
B
C
D
E
F
G
65.62 65.35
69.23 65.53 61.69 7.55
66.66 65.27 64.52 2.14
63.48 65.75 67.22 3.74
62.98 65.34 68.13 5.15
46.16 70.05 80.25 34.10
64.26 65.11 67.08 2.82
0.27
Table 8 Significance test results.
Parameters
Level 1
Level 2
Level 3
Si
Si/fi
F
11 100 1950 1423 1.5 CA SG
0 125 2050 1473 3.0 CC OG
Variation source
fi
A ( ) B (kg/t) C (Nm3/min) D (K) E (%) F G
150 2150 1523 5.0 CB DG
A B C D E F G
1 2 2 2 2 2 2
0.31 171.03 14.11 42.55 79.65 3675.28 25.02
0.31 85.52 7.06 21.28 39.83 1837.64 12.51
0.07 24.50 2.02 6.10 11.41 526.54 3.58
CC) are chosen as three levels of the injection coal by considering the actual coal injection situation in this company. The spray gun types (factor G) include the single coal spray gun (SG), the double coal spray gun (DG) and the oxygen coal spray gun (OG). Table 4 shows the all above factors and their corresponding levels. The standard operation condition is A1B1C2D1E1F1G1, and its corresponding burnout rate is 43.37%. It can be found in Table 4 that the factor A is set to be 2 levels, and the other 6 factors are set to be 3 levels. According to the standard design of the orthogonal experiment, a mixed orthogonal table of L18 (2 36) was adopted, as shown in Table 5.
4.2. Results and analysis The evaluation index is the average burnout rate of the pulverized coal at the endpoint of raceway (1.4 m in Fig. 1). Simulation Table 5 Design of orthogonal experiment. Experimental no.
A ( )
B (kg/t)
C (Nm3/min)
D (K)
E (%)
F
G
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
11 11 11 11 11 11 11 11 11 0 0 0 0 0 0 0 0 0
100 100 100 125 125 125 150 150 150 100 100 100 125 125 125 150 150 150
1950 2050 2150 1950 2050 2150 1950 2050 2150 1950 2050 2150 1950 2050 2150 1950 2050 2150
1423 1473 1523 1423 1473 1523 1473 1523 1423 1523 1423 1473 1473 1523 1423 1523 1423 1473
1.5 3.0 5.0 3.0 5.0 1.5 1.5 3.0 5.0 5.0 1.5 3.0 5.0 1.5 3.0 3.0 5.0 1.5
CA CC CB CC CB CA CB CA CC CC CB CA CA CC CB CB CA CC
SG OG DG DG SG OG OG DG SG OG DG SG DG SG OG SG OG DG
FA
Significance
F0.25(2,15) ¼ 1.52 F0.1(2,15) ¼ 2.70 F0.05(2,15) ¼ 3.68 F0.01(2,15) ¼ 6.36
** [*] * ** ** (*)
results of all the experimental conditions given in Table 5 are listed in Table 6. Results of the orthogonal experiments were analyzed through the visual analysis method and shown in Table 7. ki in Table 7 is the average value of the burnout rate at level i. The larger the ki is, the greater the burnout rate at level i is. So the best combination of various factors is A1B1C1D3E3F3G3, which means the centerline angle between the spray gun and the straight duct is 11, the coal injection ratio is 100 kg/t, the blast volume is 1950 Nm3/min, the blast temperature is 1523 k, the oxygen enrichment rate is 5.0%, the coal injected is bituminous coal and the spray gun is the double coal spray gun. The burnout rate in this case is calculated to be 91.43%, which is 110.8% higher than the standard condition. Ri is the difference of the burnout rate between the best and worst level, it represents the influence of parameter i on the experimental results. The bigger the value of Ri is, the greater the degree of influence is, and vice versa. The results of Ri in Table 7 are in sequence of RF > RB > RE > RD > RG > RC > RA. Thus, the significance degree of parameters are as follows: the pulverized coal type, the coal injection ratio, the oxygen enrichment rate, the blast temperature, the spray gun type, the blast volume and the angle. The results of the variance analysis are shown in Table 8, in which the sum of squares Se ¼ 17.14, the total degrees of error freedom fT ¼ 17, the degree of error freedom fe ¼ 4. According to the value F in Table 8, the significance degree of parameters are in a sequence of the pulverized coal type, the coal injection ratio, the oxygen enrichment rate, the blast temperature, the spray gun type, the blast volume and the angle, which is in consistent with the results of visual analysis. Significance results reveal that to the burnout rate, the pulverized coal type, the coal injection ratio and the oxygen enrichment rate have a great significant effect, the blast temperature has a significant effect, the spray gun type has a relative significant effect, the blast volume has a tiny but not significant effect, and the angle has no significant effect.
Table 6 Simulation results of burnout rate. Experimental no.
1
2
3
4
5
6
7
8
9
Burnout rate (%) Experimental no.
46.02 10
88.12 11
88.12 12
69.06 13
81.68 14
44.52 15
74.34 16
46.21 17
66.10 18
Burnout rate (%)
79.02
81.43
46.32
53.19
67.13
77.62
78.33
40.67
64.46
Y. Li et al. / Applied Thermal Engineering 67 (2014) 72e79
5. Conclusions (1) A three-dimensional mathematical model of the oxygenenriched PCI combustion process in the blast furnace was developed, and the reliability of the model was proved through a comparison of the simulation and the tested value. (2) It is found that air circulates in the cyclotron zone. Most of the air flows into the coke bed after leaving the raceway and its velocity reduces rapidly. The air velocity in the dripping region is bigger and it is smaller in dead stock region. Larger size particles get into the coke bed after leaving the raceway and smaller size particles mainly flow as a circular in the cyclotron. In the coke bed, particles in smaller size mainly stay in the dripping region and bigger ones mainly stay in the dead stock region. (3) The smaller the particle size is, the larger the volatilization rate and residual carbon burnout rate are. The smaller the particle size is, the faster they reach the highest temperature, and the highest temperature of the smaller particles is greater than that of larger particles. (4) The optimal combination of parameters is obtained through an orthogonal optimization experiment. The burnout rate in the optimal condition is increased by 110.8% compared to the standard condition. The significance degree of parameters is obtained through the visual and variance analysis.
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[7] Khairil, K. Daisuke, N. Katsuya, N. Ichiro, Fundamental reaction characteristics of pulverized coal at high temperature, ISIJ Int. Iron Steel Inst. Jpn. 41 (2) (2001) 136e141. [8] X.M. Zhao, J.L. Zhang, T.J. Ren, F.S. Kong, J. Hu, Performance analysis of blast furnace with mixed coal injection, in: National Iron Production Technologies and Ironmaking Conference, Wuxi, China, 2004, pp. 748e751 (in Chinese). [9] D.W. Kong, J.L. Zhang, X.H. Lin, Y.N. Bai, X.B. Wu, Combustion characteristics and kinetic analysis of blended coal in blast furnace, J. Iron Steel Res. 23 (11) (2011) 4e9 (in Chinese). [10] A. Ziebik, K. Lampert, M. Szega, Energy analysis of a blast-furnace system operating with the Corex process and CO2 removal, Energy 33 (2) (2008) 199e 205. [11] Y. Xu, B.S. Hu, Proper combination of oxygen enrichment percentage with large amount of coal powder injection in blast furnace, J. Iron Steel Res. 18 (1) (2006) 6e8 (in Chinese). [12] K. Takeda, F.C. Lockwood, Integrated mathematical model of pulverized coal combustion in a blast furnace, ISIJ Int. 37 (5) (1997) 432e440. [13] H. Nogamih, H. Yamaoka, K. Takatani, Raceway design for the innovative blast furnace, ISIJ Int. 44 (12) (2004) 2150e2158. [14] J.G. Liang, H.J. Yan, J.Y. Zhang, Numerical simulation and optimization of gasparticle turbulent flow and pulverized coal combustion in the lower zone of blast furnace, Res. Iron Steel 40 (4) (2012) 14e20 (in Chinese). [15] Y.S. Shen, B.Y. Guo, A.B. Yu, P. Zulli, A three-dimensional numerical study of the combustion of coal blends in blast furnace, Fuel 88 (2) (2009) 255e263. [16] C. Moon, Y. Sung, S. Ahn, T. Kim, G. Choi, D. Kim, Thermochemical and combustion behaviors of coals of different ranks and their blends for pulverizedcoal combustion, Appl. Therm. Eng. 54 (1) (2013) 111e119. [17] S.W. Ma, G.B. Qiu, X.W. Lv, Q.Y. Deng, H. Wang, Theoretical research on the effects of oxygen enrichment and pulverized coal injection on performance of tuyere raceway zone in blast furnace, Iron Steel Vanadium Titanium 33 (4) (2012) 62e67 (in Chinese). [18] W. Yang, C. Ryu, S. Choi, E. Choi, D. Lee, W. Huh, Modeling of combustion and heat transfer in an iron ore sintering bed with considerations of multiple solid phases, ISIJ Int. 44 (3) (2004) 492e499. [19] J.Y. Jang, Y.W. Chiu, 3-D transient conjugated heat transfer and fluid flow analysis for the cooling process of sintered bed, Appl. Therm. Eng. 29 (14) (2009) 2895e2903. [20] FLUENT 6.3 User’s Guide, Fluent Inc., Lebanon, USA, 2005. [21] S. Ergun, Fluid flow through packed columns, Chem. Eng. Prog. 48 (2) (1952) 89e94. [22] L.L. Zhang, S.J. Zhou, J.H. Chen, Numerical simulations of three-dimensional gaseparticle turbulent flow and pulverized coal combustion in blast furnace, J. Nanjing Univ. Sci. Technol. 31 (2) (2007) 163e167 (in Chinese). [23] L.X. Zhou, L. Li, R.X. Li, J. Zhang, Simulation of 3-D gas-particle flows and coal combustion in a tangentially fired furnace using a two-fluid-trajectory model, Powder Technol. 125 (2) (2002) 226e233. [24] T. Asotani, T. Yamashita, H. Tominaga, Y. Uesugi, Y. Itaya, S. Mori, Prediction of ignition behavior in a tangentially fired pulverized coal boiler using CFD, Fuel 87 (4e5) (2008) 482e490. [25] C.W. Chen, Numerical analysis for the multi-phase flow of pulverized coal injection inside blast furnace tuyere, Appl. Math. Model. 29 (9) (2005) 871e 884. [26] Z.Y. Xiang, X.L. Wang, Blast Furnace Design: The Theory and Practice of Ironmaking Process Design, Metallurgical Industry Press, Beijing, China, 2007 (in Chinese).
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Numerical simulation and optimization of pulverized coal injection with enriched oxygen into blast furnace Yongqing Li a, Xiaohui Zhang b, *, Jiayuan Zhang a, Jiemin Zhou a, Hongjie Yan a a b
School of Energy Science and Engineering, Central South University, Changsha 410083, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China
h i g h l i g h t s We have established a mathematical model of combustion process in blast furnace. The gas velocity distribution and motion characteristics of pulverized coal in raceway and coke bed were obtained. We have developed the optimal parameters combination in purpose of increasing burnout rate.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 September 2013 Accepted 23 February 2014 Available online 15 March 2014
The combustion characteristics of pulverized coal in the blast furnace raceway is an important factor which determines the performance of injection of pulverized coal with enriched oxygen as well as the energy consumption of blast furnace. Research was carried out to investigate processes including the gas and solid flow, particle movement, drying and pyrolysis of pulverized coal and combustion of carbon residue and coke in a blast furnace with standard ke3 model, discrete phase model, evaporation and diffusion model, dual parallel competition model and diffusion-kinetic combustion model respectively in Fluent 6.3. The accuracy and reliability of the numerical model was verified with a semi-industrial experiment. Moreover, 7 parameters were optimized by the orthogonal experiments, taking the burnout rate as the evaluation indicator. Results show that the burnout rate under the optimal condition can be increased by 110.8% comparing to that under the standard condition. The importance of parameters influencing the evaluation indicator is found in the order which is the type of the pulverized coal, the pulverized coal injection rate, the oxygen enrichment rate, the blast temperature, the spraying gun type, the blast volume and the angle. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Oxygen enrichment Burnout rate Numerical simulation Orthogonal experiment
1. Introduction The pulverized coal injection (PCI) with enriched oxygen into a blast furnace is an important technology to reduce the cost and maintain a steady production in an iron and steel industry [1]. In the past long period, the pulverized coal used for injection was all anthracite in China. However, as raw coal resources decrease continuously while the combustion performance of mixture of anthracite with bituminous coal keeps improving, the mixed coal injection is more often adopted [2]. The combustion characteristic in the raceway is an important factor for the performance of oxygen-enriched coal injection technology as well as the energy consumption of the blast furnace. Most research of oxygen-
* Corresponding author. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.applthermaleng.2014.02.062 1359-4311/Ó 2014 Elsevier Ltd. All rights reserved.
enriched coal injection for blast furnace are focused on the fluid flow, the heat and mass transfer process, the relationship between the oxygen enrichment rate and the coal injection ratio, and the combustion characteristics of blended coal in the blast furnace and coke bed. For example, Shen et al. [3e5] developed a mathematical model to investigate the fluid flow and combustion of pulverized coal in the raceway and the coke bed, thus influences of operating parameters on the pulverized coal burning rate and relationship between the burning rate and the oxygen enrichment rate are obtained. Wijayanta et al. [6] got the relationship between the oxygen enrichment rate and the combustion performance of carbon powder mixed with biomass by comparing the combustion characteristic of two kinds of biomass carbon powder in the blast furnace. Khairil et al. [7] studied the relationship between the oxygen enrichment rate and the smelting intensity, tuyere and raceway temperature and pulverized coal burnout in a blast furnace by experiments. Zhao et al. [8] and Kong et al. [9] also carried out some
Y. Li et al. / Applied Thermal Engineering 67 (2014) 72e79
Nomenclature Ap Bs Char C0 Cp D dp Fi fi gi H Hreac I k l mp Ri R1, R2
surface area of particle [m2] pre-exponential factor residual carbon mass of pulverized coal [kg] specific heat of particle [J/kg K] diffusion coefficient [m2/s] particle size [m] additional force of particles in direction i [N] body force of fluid in direction i[N] gravitational acceleration in direction i, [m/s2] total enthalpy [kJ/mol] reaction heat [J/kg] radiation intensity [W/m2] turbulent kinetic energy frequency factors particle mass [kg] generation/vanishing rate of species i low/high temperature reaction
experimental studies on various mixed coal injection and combustion characteristics. Ziebik et al. [10] and Xu et al. [11] discussed how differently the coal injection ratio and the oxygen enrichment rate influence the internal heat distribution and the pulverized coal combustion process in a blast furnace by experiments and numerical simulation respectively. In order to analyze the flow and combustion in the raceway and to assist the improvement of the burner design, Takeda [12] developed a two-dimensional mathematical model of the pulverized coal combustion. All these studies investigate the combustion characteristics of the pulverized coal in the blast furnace mainly from aspects of the oxygen enrichment rate and the fuel characteristics, however not take the influence of operating parameters into considerations. The effect of oxygen-enriched PCI technology is mainly on the combustion characteristics of the pulverized coal in furnace. Meanwhile, operation parameters of the blast furnace will also have certain effects on the combustion characteristic. In order to improve the burnout rate of the pulverized coal, it is particularly necessary to optimize the operation parameters such as the oxygen enrichment rate and the coal injection ratio. In this paper, a threedimensional mathematical model is developed for the combustion of the pulverized coal in blast furnace, and an optimization is carried out for the oxygen enrichment for the blast furnace injection based on the numerical simulations of the pulverized coal combustion process in the raceway.
S T u VM Yi Yox Ys Yss Yw,g Yw,s
sji G a b 3 3p
sB u r 4
source term temperature of gas/particle [K] speed of gas/particle [m/s] volatile of pulverized coal mass fraction of species i mass fraction of oxygen in gas mass fraction of gas phase composition mass fraction of oxidant on particle surface mass fraction of moisture in gas mass fraction of moisture in particle surface force of fluid in direction i [N] heat/species transfer coefficient content of dry and ash free basis in volatiles equivalence ratio dissipation rate of turbulent kinetic energy particle emission rate StefaneBoltzmann constant reaction rate density [kg/m3] porosity
Therefore, the computational domain included all the three parts and was divided into the raceway, the dripping region and the dead stock region (as shown in Fig. 1). The coke bed was assumed as a porous media zone [13,14]. Further simplification was made for the simulation of the pulverized coal combustion process in the raceway as follows [15,16]. (1) The granularity of the pulverized coal and the coke are welldistributed, and all particles are assumed to be spherical. (2) The crushing and the coagulation of both the pulverized coal and the coke particles are ignored. (3) The flow of liquid in the cavity is ignored. (4) The shape of the raceway keeps unchanged under the modeling conditions. A physical model was developed according to the actual size of the blast furnace in an iron and steel plant, and the size of the raceway was calculated with the empirical formula [17]. Due to the symmetric structure of the blast furnace, only half of the furnace was taken in the physical model, as shown in Fig. 2. The mesh of the model is shown in Fig. 3.
2. Numerical simulation 2.1. Physical model A physical model is developed taking a 600 m3 blast furnace in an iron and steel company as its prototype. The diameter and the height of the hearth are 6150 mm and 3300 mm, the diameter and the angle of the bosh are 7150 mm and 80 , the height of the tuyere is 2900 mm, the diameter of the straight tube is 120 mm, the diameter and the length of the tuyere are 120 mm and 340 mm, the diameter of the spray gun is 20 mm, and the angle between the spray gun centerline and the straight tube is 11. To understand the burning process of the coal along the border of raceway, the combustion of the pulverized coal in both the straight tube and the tuyere needs to be taken into account.
73
Fig. 1. The diagram of bottom area in blast furnace.
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Y. Li et al. / Applied Thermal Engineering 67 (2014) 72e79
Fig. 3. The diagram of grid structure.
v v v ruj H ¼ ðrHÞ þ vt vxj vxj
v Gh H vxj
! þ
vp þ Sh vt
(6)
The species transport equation: Fig. 2. The diagram of physical model.
vrYi v v ruj Yi ¼ þ vxj vxj vt
v Gi Yi vxj
! þ R i þ Si
(7)
2.2. Mathematical model 2.2.1. Governing equations The governing equations describe the heat and mass transfer of the pulverized coal combustion process in the blast furnace include equations of the gasesolid two-phase continuity, momentum, energy and species transport. All equations are shown below [18]. The continuity equation:
2.2.2. Combustion and chemical reaction model The pulverized coal combustion process includes stages of drying (water evaporation), pyrolysis (volatile separating out) and combustion (residual carbon burning).
vr v ruj ¼ 0 þ vt vxj
CmHn is set as the volatile of pulverized coal. The combustion reaction of CmHn is as follows.
(1)
Cm Hn þ ðm þ n=4ÞO2 /mCO2 þ nH2 O=2
The momentum equation:
v v v ruj ui ¼ s þ fi ðrui Þ þ vt vxj vxj ji
(2)
The coke bed is considered as isotropic porous media [19] in the model. Then it is necessary to consider the influence of the porous media on the viscosity and the inertial of the fluid. Therefore, a source term Si, which is consisted of two parts, i.e. the viscous loss term and the inertial loss term [20], must be added to the momentum equation. For a homogeneous porous media, the source term is described as follows [20].
m 1 Si ¼ vi C2 rvmag vi a 2
150 ð1 4Þ2 ¼ 2 a dp 43
(4)
3:5 ð1 4Þ dp 43
(5)
C2 ¼
The energy equation:
(8)
The reaction rate of formula (8) is mainly affected by the diffusion rate in the raceway and the kinetic rate in the dead stock and the dripping region. Then the EBU-Arrhenius model is used to describe the gas-phase chemical reaction rate [22], which are as follows.
us ¼ minðuE ; uA Þ
(9)
3
uE ¼ cE r minðYs ; Yox =bÞ
(10)
uA ¼ Bs r2 Ys Yox expðE=RTÞ
(11)
k
(3)
The ErguneBlakeeKozeny equation is used to calculate the viscous resistance coefficient (1/a) and inertial resistance coefficient (C2) [21].
1
(1) Gas-phase chemical reaction model
(2) Pulverized coal combustion model A droplet evaporation diffusion model is used to describe the drying process [23], and the evaporation rate is:
Yw;s Yw;g mw ¼ pdp NuDr ln 1 þ 1 Yw;s
(12)
A double parallel competition model is used to describe the pyrolysis process [3], that is:
aVM1 þ ð1 aÞChar1 ðR1 Þ 1:5aVM2 þ ð1 1:5aÞChar2
ðR2 Þ
The formation rate of the volatile is:
(13)
Y. Li et al. / Applied Thermal Engineering 67 (2014) 72e79
dVM ¼ ðl1 þ 1:5l2 ÞaC0 dt
(14)
The diffusion-kinetic combustion model is used to describe the combustion process of the carbon residue and coke [24]. The burning rate is:
mh ¼
X
E RTp
pd2p rp Yss Bs exp
(15)
As the radiation must be considered for the combustion reaction in the blast furnace, the P-1 model is used in this study to describe the radiation process. (3) Coke combustion model The heterogeneous surface chemical reaction model is used to describe the coke gasification reaction. The coke is assumed as elemental carbon, and the reactions are assumed as the first-order reactions. The surface chemical reactions are as follows.
C þ O2 /CO2
(16)
2C þ O2 /2CO
(17)
C þ H2 O/CO þ H2
(18)
The finite-rate model is used to simulate above reactions, and the reaction rates are determined by the Arrhenius expressions. 2.2.3. Discrete phase model The mass concentration of pulverized coal is lower in the straight tube and the raceway, so the pulverized coal can be modeled as discrete solid particles distributed in the gaseous phase, and the RosineRammler distribution function is used to describe the particle size distribution. The trajectory and temperature of pulverized coal particles are described by the discrete phase model based on Euler-Lagrange method [25]. (1) Motion equation The motion equation of the pulverized coal particles of unit mass is:
gi rp r dup ¼ FD u up þ þ Fi rp dt
(19)
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Table 1 Ultimate and proximate analysis of pulverized coal. Volatiles (daf)
Ash (daf)
C (daf)
H (daf)
O (daf)
19.2
10.0
91.7
4.83
3.50
The hot air inlet and the spray gun are set as mass flow inlets, and their flow rate, static pressure, temperature and gas composition are set to be the average of the actual value in a stable operation condition of the blast furnace. The outlet of the raceway is set as a pressure outlet. The wall of the straight tube, the bosh and the hearth are set to be non-slip walls, and the value of the outlet pressure and wall temperature is measured during the industrial test. The contour of the gaseous temperature in the computational domain is shown in Fig. 4. The temperature distribution in the center and the bottom of the raceway are different. The maximum temperatures in these two areas are about 1800 K and 2416 K, as shown at 1.4e1.5 m in Fig. 5. The velocity of the pulverized coal decreases because the viscous and inertial resistance increase when they move to the border of the raceway, so the residence time of the pulverized coal increases as well. All these indicate that the combustion of the pulverized coal mainly take place in this region. Fig. 6 shows the diagram of flow distribution. As shown in Fig. 6(a), most of the gas from the pipe changes its direction when it approaches the border of the raceway, then it circulates in the cyclotron zone. For the air flow left the cyclotron area, most moves upward to the coke bed, with its velocity decreasing quickly. The air velocity in the coke bed is nonuniform, for which it is bigger in the dripping region and smaller in the dead stock, because the resistance to the gas flow is smaller when the porosity is greater. In Fig. 6(b), it can be found that particles with larger size (>70 mm) move upward in the raceway when approaching the border and enter into the coke bed. Particles with smaller size (<70 mm) mainly circulate in the raceway with the cyclotron currents. In the coke bed, small particles move upward to the dripping region after escaped from the raceway. Larger particles (>100 mm) are mainly concentrated in the dead stock because they are less easily influenced by the gas flow. Fig. 6(c) illustrates that the pulverized coal particles stay in the cyclotron for a short time while they stay in the coke bed for a longer time. Moreover, their dwell time in the dripping region is shorter than that in the dead stock, which is considered to be mainly related to both the gas flow rate and the particle size. Fig. 7 shows the burnout rate of the pulverized coal of different sizes changing from the straight tube to the border of raceway.
(2) Energy equation The energy equation of the pulverized coal particles is as follows:
mp Cp
X dmp dTp ¼ pdp lNu Tg Tp þ Hreac dt dt
þ Ap 3 p pI sB Tp4
(20)
2.3. Simulation results and analysis Oxygen-enriched PCI combustion process in the raceway is simulated by Fluent 63. The basic and operating parameters of the blast furnace are determined in a field test. Physical parameters of the pulverized coal are shown in Table 1, and the average particle size of the coal is 40 mm.
Fig. 4. Gas temperature distribution in simulation region.
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Y. Li et al. / Applied Thermal Engineering 67 (2014) 72e79
Fig. 5. Gas temperature along the axis of straight tube.
The burnout rate is mainly determined by the volatilizing rate and the combustion rate of the carbon residue. As shown in Fig. 7, the smaller the pulverized coal is, the greater the volatilizing rate and the carbon residue combustion rate are. This indicates that the particle size has a great effect on the burnout rate. The reason is that the smaller the particle size is, the quicker the particle surface temperature rises and the shorter time it requires to achieve the pyrolysis temperature and the carbon residue ignition point. Fig. 8 shows how the temperature of particles of different sizes varies along the distances from the outlet of the spraying gun. It can be seen that the smaller the coal particle is, the faster the temperature rises and the greater the temperature difference between the pyrolysis process and the carbon residue combustion process is. The particle temperature reaches the maximum at the end of the residual char combustion state, followed by temperature decreasing and keeping stable. The highest temperatures of most particles appear between the distance of 1.4 and 1.6 m. Meanwhile, the smaller the particle size is, the earlier the highest temperature appears, and the highest temperatures of small particles are usually greater than those of larger particles. This indicates that the small pulverized coal particles are more effective in providing good dynamic condition for the pulverized coal combustion. 3. Experimental verification The gas composition and the pulverized coal burnout rate were measured by a half-industrializated experiment equipment, as shown in Fig. 9. The physical model established according to the experiment equipment for simulation is shown in Fig. 10 (As it is a symmetrical structure, only a half is taken in the simulation.). The coal injection inlet is located in the center of the cylinder, while the oxygenenriched air at high temperature comes from the outer ring. The initial simulation condition is in consistent with the experimental condition, which is shown in Fig. 10. Boundary conditions are as follows: the inlet is a velocity inlet; the outlet is set to be an outflow; the wall is adiabatic and set to be a no-slip boundary. The distances from the test points to the center of the coal injection inlet are 0.330 m, 0.626 m and 1.030 m (A, B, C) respectively. The average particle size of the pulverized coal is 40 microns. The physical parameters of the pulverized coal are shown in Table 1. After the pulverized coal combustion becomes stable, sampling analysis are carried out at the three test points by a flue gas analyzer for multiple measurements. At each test point, average concentration of the flue gas composition is obtained. Coal particles are collected at each relevant test point as well, so that the burnout rate of the pulverized coal can be calculated.
Fig. 6. The diagram of flow distribution.
Y. Li et al. / Applied Thermal Engineering 67 (2014) 72e79
77
Fig. 9. The schematic diagram of pulverized coal combustion experimental equipment.
Fig. 7. Burnout rate of pulverized coal.
Table 2 gives the simulation and tested results of the mass fraction of O2 and CO2. It can be seen that the maximum deviation of the mass fraction of O2 and CO2 are 8.8% and 7.1% respectively, and the change tendencies of simulation and tested value are consistent. Table 3 shows the simulation and tested results of the burnout rate. The deviation decreases with the distance from the straight tube inlet. The change tendencies of the simulation and the tested value are consistent too. Comparison between the simulation and the test shows that the maximum deviation is less than 9%, and the change tendencies are consistent. The model developed in this paper is proved to be correct and reliable. 4. Orthogonal experimental 4.1. Parameters and levels To study the influence of injection modes and operation parameters on the burnout rate of the pulverized coal injected with oxygen-enriched air, numerical simulation and orthogonal experiment are used to optimize the combination of various factors to achieve a maximum burnout rate. Taking the burnout rate as the evaluation index, factors include the pulverized coal types, the spray gun types, the oxygen enrichment rates, the angles, the coal injection ratios, the blast volumes and the blast temperatures are studied. The angle (factor A) refers to the angle between the centerline of the spray gun and the straight tube. The injection angles of the
Fig. 10. The diagram of physical model of pulverized coal combustion.
spray gun are 11 and 0 in most cases, hence only 2 different levels are used for the factor. Since the injection ratio responses sensitively to the burnout rate according to the practical experience, the selected coal injection ratio (factor B) levels should not be too big. Levels of 100 kg/t, 150 kg/t and 125 kg/t are selected according to the actual coal injection condition in the study. According to the basic design condition of a hot blast stove, the maximum and the average value of the dry air flow are 2150 Nm3/ min and 2050 Nm3/min respectively; thus levels of 1950 Nm3/min, 2050 Nm3/min and 2150 Nm3/min are selected for the blast volume (factor C). The blast temperature (factor D) is usually designed to be more than 1423 K. For a hot blast stove, it is important to take into account not only the requirement of a high air temperature and thermal efficiency, but also some other factors such as its service life. So the blast temperature should not be too high, and levels of 1423 K, 1473 K and 1523 K are selected. Studies [26] revealed that the increase of oxygen enrichment (factor E) is beneficial to the improvement of the burnout rate when the coal injection ratio is less than 200 kg/t and the oxygen enrichment rate is less than 5%, but when oxygen enrichment rate is more than 5%, the effect of oxygen enrichment rate on enhancing of the burnout rate is weakened, and the economic benefit reduces. Therefore the selected oxygen enrichment rate level should be limited within 5%, and level of 1.5%, 3.0% and 5.0% are selected, in which 1.5% is the benchmark. The pulverized coal types (factor F) used for the blast furnace injection are anthracite, bituminous coal and blended coal. In this paper, low volatile anthracite coal (CA), high volatile bituminous coal (CB) and blended coal of the former two types (ratio of 1:1, and Table 2 Comparison of simulation and tested results of mass fraction. Gas component
Test point
Simulated value (%)
Tested result (%)
Deviation (%)
O2
A B C A B C
4.8 5.5 7.3 13.4 14.5 15.1
5.1 5.2 8.0 13.8 15.6 14.1
5.9 5.8 8.8 2.9 7.1 7.1
CO2 Fig. 8. Temperature of pulverized coal particle change with distance.
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Y. Li et al. / Applied Thermal Engineering 67 (2014) 72e79
Table 3 Comparison of simulation and tested results of burnout rate. Evaluation index Burnout rate
Test point A B C
Table 7 Orthogonal experimental results.
Simulated value (%)
Tested result (%)
Deviation (%)
26.2 33.8 39.8
28.3 32.4 39.9
7.4 4.3 0.3
k1 (%) k2 (%) k3 (%) Ri (%)
Table 4 Parameters and values in different levels.
A
B
C
D
E
F
G
65.62 65.35
69.23 65.53 61.69 7.55
66.66 65.27 64.52 2.14
63.48 65.75 67.22 3.74
62.98 65.34 68.13 5.15
46.16 70.05 80.25 34.10
64.26 65.11 67.08 2.82
0.27
Table 8 Significance test results.
Parameters
Level 1
Level 2
Level 3
Si
Si/fi
F
11 100 1950 1423 1.5 CA SG
0 125 2050 1473 3.0 CC OG
Variation source
fi
A ( ) B (kg/t) C (Nm3/min) D (K) E (%) F G
150 2150 1523 5.0 CB DG
A B C D E F G
1 2 2 2 2 2 2
0.31 171.03 14.11 42.55 79.65 3675.28 25.02
0.31 85.52 7.06 21.28 39.83 1837.64 12.51
0.07 24.50 2.02 6.10 11.41 526.54 3.58
CC) are chosen as three levels of the injection coal by considering the actual coal injection situation in this company. The spray gun types (factor G) include the single coal spray gun (SG), the double coal spray gun (DG) and the oxygen coal spray gun (OG). Table 4 shows the all above factors and their corresponding levels. The standard operation condition is A1B1C2D1E1F1G1, and its corresponding burnout rate is 43.37%. It can be found in Table 4 that the factor A is set to be 2 levels, and the other 6 factors are set to be 3 levels. According to the standard design of the orthogonal experiment, a mixed orthogonal table of L18 (2 36) was adopted, as shown in Table 5.
4.2. Results and analysis The evaluation index is the average burnout rate of the pulverized coal at the endpoint of raceway (1.4 m in Fig. 1). Simulation Table 5 Design of orthogonal experiment. Experimental no.
A ( )
B (kg/t)
C (Nm3/min)
D (K)
E (%)
F
G
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
11 11 11 11 11 11 11 11 11 0 0 0 0 0 0 0 0 0
100 100 100 125 125 125 150 150 150 100 100 100 125 125 125 150 150 150
1950 2050 2150 1950 2050 2150 1950 2050 2150 1950 2050 2150 1950 2050 2150 1950 2050 2150
1423 1473 1523 1423 1473 1523 1473 1523 1423 1523 1423 1473 1473 1523 1423 1523 1423 1473
1.5 3.0 5.0 3.0 5.0 1.5 1.5 3.0 5.0 5.0 1.5 3.0 5.0 1.5 3.0 3.0 5.0 1.5
CA CC CB CC CB CA CB CA CC CC CB CA CA CC CB CB CA CC
SG OG DG DG SG OG OG DG SG OG DG SG DG SG OG SG OG DG
FA
Significance
F0.25(2,15) ¼ 1.52 F0.1(2,15) ¼ 2.70 F0.05(2,15) ¼ 3.68 F0.01(2,15) ¼ 6.36
** [*] * ** ** (*)
results of all the experimental conditions given in Table 5 are listed in Table 6. Results of the orthogonal experiments were analyzed through the visual analysis method and shown in Table 7. ki in Table 7 is the average value of the burnout rate at level i. The larger the ki is, the greater the burnout rate at level i is. So the best combination of various factors is A1B1C1D3E3F3G3, which means the centerline angle between the spray gun and the straight duct is 11, the coal injection ratio is 100 kg/t, the blast volume is 1950 Nm3/min, the blast temperature is 1523 k, the oxygen enrichment rate is 5.0%, the coal injected is bituminous coal and the spray gun is the double coal spray gun. The burnout rate in this case is calculated to be 91.43%, which is 110.8% higher than the standard condition. Ri is the difference of the burnout rate between the best and worst level, it represents the influence of parameter i on the experimental results. The bigger the value of Ri is, the greater the degree of influence is, and vice versa. The results of Ri in Table 7 are in sequence of RF > RB > RE > RD > RG > RC > RA. Thus, the significance degree of parameters are as follows: the pulverized coal type, the coal injection ratio, the oxygen enrichment rate, the blast temperature, the spray gun type, the blast volume and the angle. The results of the variance analysis are shown in Table 8, in which the sum of squares Se ¼ 17.14, the total degrees of error freedom fT ¼ 17, the degree of error freedom fe ¼ 4. According to the value F in Table 8, the significance degree of parameters are in a sequence of the pulverized coal type, the coal injection ratio, the oxygen enrichment rate, the blast temperature, the spray gun type, the blast volume and the angle, which is in consistent with the results of visual analysis. Significance results reveal that to the burnout rate, the pulverized coal type, the coal injection ratio and the oxygen enrichment rate have a great significant effect, the blast temperature has a significant effect, the spray gun type has a relative significant effect, the blast volume has a tiny but not significant effect, and the angle has no significant effect.
Table 6 Simulation results of burnout rate. Experimental no.
1
2
3
4
5
6
7
8
9
Burnout rate (%) Experimental no.
46.02 10
88.12 11
88.12 12
69.06 13
81.68 14
44.52 15
74.34 16
46.21 17
66.10 18
Burnout rate (%)
79.02
81.43
46.32
53.19
67.13
77.62
78.33
40.67
64.46
Y. Li et al. / Applied Thermal Engineering 67 (2014) 72e79
5. Conclusions (1) A three-dimensional mathematical model of the oxygenenriched PCI combustion process in the blast furnace was developed, and the reliability of the model was proved through a comparison of the simulation and the tested value. (2) It is found that air circulates in the cyclotron zone. Most of the air flows into the coke bed after leaving the raceway and its velocity reduces rapidly. The air velocity in the dripping region is bigger and it is smaller in dead stock region. Larger size particles get into the coke bed after leaving the raceway and smaller size particles mainly flow as a circular in the cyclotron. In the coke bed, particles in smaller size mainly stay in the dripping region and bigger ones mainly stay in the dead stock region. (3) The smaller the particle size is, the larger the volatilization rate and residual carbon burnout rate are. The smaller the particle size is, the faster they reach the highest temperature, and the highest temperature of the smaller particles is greater than that of larger particles. (4) The optimal combination of parameters is obtained through an orthogonal optimization experiment. The burnout rate in the optimal condition is increased by 110.8% compared to the standard condition. The significance degree of parameters is obtained through the visual and variance analysis.
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