Powder Technology 223 (2012) 27–38

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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c

CFD study of in-furnace phenomena of pulverised coal injection in blast furnace: Effects of operating conditions Y.S. Shen a,⁎, A.B. Yu a, P.R. Austin b, P. Zulli b a b

Laboratory for Simulation and Modelling of Particulate Systems, School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia BlueScope Steel Research, BlueScope Steel, P.O. Box 202, Port Kembla, NSW 2505, Australia

a r t i c l e

i n f o

Available online 23 July 2011 Keywords: Pulverised coal injection CFD Blast furnace

a b s t r a c t A three-dimensional mathematical model of the flow and combustion of coal powder in a coke packed bed has been developed to simulate the complicated in-furnace phenomena of pulverised coal injection (PCI) in the raceway and the surrounding coke bed in an ironmaking blast furnace. The model includes the flow–thermal–chemical behaviours of two fuels: (i) coal powders; and (ii) coke solids. The typical in-furnace phenomena of the base case are obtained by means of this model in terms of flow and combustion behaviours of coal powder. An array of parametric studies are carried out to evaluate the effects of operating conditions under practical conditions, including blast conditions and types of cooling gas. The results show that these operating conditions play a limited role in improving the burnout at the endpoint of the raceway but a notable role in the recirculation region, i.e. over the raceway surface. Gas composition is sensitive to oxygen enrichment and use of oxygen as cooling gas. The results are then compared to the previous studies. The underlying mechanisms are explored. It is indicated that this study can provide more useful information for optimising the PCI operation in full-scale blast furnaces. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The ironmaking blast furnace plays a dominant role in an integrated steel work in terms of energy consumption and CO2 emission. Pulverised coal injection (PCI) technology is widely used in this process [1,2]. In this operation, coal powder is injected with gas into the blast furnace via a lance through a tuyere, and combusts in the raceway cavity and then in the surrounding packed bed of coke solids (Fig. 1) [3]. This technology can offer various benefits, including reducing expensive coke consumption and thus lowering carbon dioxide emission in coke-making, and adjusting furnace stability. As a result, the rate of PCI is increasingly high in practice. Under high PCI rate operation, the injected coal powders cannot burn completely inside the raceway and the unburnt coal powders may accumulate in the boundary of raceway, affecting the coke bed permeability and thereby the furnace stability (Fig. 1) [4–6]. Therefore, high burnout of coal powder is desired and necessary for high PCI rate operation. On the other hand, an appropriate distribution of gas composition is favourable for furnace stability. In practice, under given raw materials conditions (i.e. coal/coke properties), adjusting operating conditions (i.e. blast conditions and cooling gas selection) in real time is the key countermeasures to optimise/control the furnace operation [7,8]. Therefore, it is of great necessity to understand the complicated flow and combustion behaviours of coal powders in the lower part of blast furnace. Especially, understanding the in-furnace phenomena under

⁎ Corresponding author. Tel.: + 61 2 93854443; fax: + 61 2 93856565. E-mail address: [email protected] (Y.S. Shen). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.07.020

various operational conditions is important for providing direct guidance for process control and optimisation. PCI technology has been studied at different scales by different methods. Industry-level investigations of these operating conditions are actual and direct but extremely difficult due to the severe in-furnace environment and could affect the furnace stability. Laboratory- and pilot-scales experimental studies can replicate the PCI operation to a certain degree but is hard to replicate the in-furnace phenomena; moreover, it is rather expensive in terms of time and investment. As a result, only a few such attempts have been reported in the literature mainly on gas compositions along the tuyere axis [9]. Alternatively, mathematical modelling, supported by physical experiments, provides an effective way to conduct such parametric studies. However, to date, few such parametric studies of operating conditions are found for the region of raceway–coke bed. Dong et al. [5] reported a parametric study of gas–powder flow using a lab-scale two-dimensional model without heat and mass transfer. Takeda and Lockwood [10] reported a parametric study of various operating conditions using a two-dimensional model, such as oxygen enrichment and different lance designs. However, threedimensional (3D) integrated modelling is needed for more practical problems. Nogami et al. [11] reported a parametric study of blast temperature using a 3D transient-state model for a laboratory-scale test rig, where the reactions of both coal and coke were considered. The socalled discrete element method (DEM) was used for coke solids movement so that the raceway structure could be predicted directly. This approach is generally difficult to apply to a practical system where the number of particles is huge and thus the computation will be extremely expensive. For this reason, most PCI models are based on

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Y.S. Shen et al. / Powder Technology 223 (2012) 27–38

Blast

Cohesive Zone

Active Coke Zone/ Dripping Zone

PC lance

Zone of PCI Influence

Char particles swept into the coke bed

Tuyere Deadman

Raceway

Fig. 1. Schematic of pulverised coal injection technology in a blast furnace [3].

continuum approach. Shen et al. [12,13] reported parametric studies of these operating conditions using the 3D models of a pilot-scale test rig for the raceway region. All these model studies in lab- or pilot-scales could offer qualitative results to a certain degree. However, in these previous models, the flow and combustion features of coal powder in PCI operation were only investigated along the tuyere centreline and did not consider the coke bed region, which will affect the predictions significantly [14]. The results obtained in such lab- or pilot-scales about the effects of the operating conditions might be misleading for infurnace phenomena under practice conditions. To overcome these deficiencies, in this study, a recently developed 3D in-furnace model of coal powder flow and combustion [14] is used for investigating the effects of operating conditions on in-furnace phenomena of PCI operation under practical conditions. The investigated parameters include blast temperature, oxygen content in blast, cooling gas types and PCI rate. The investigated key flow and combustion characteristics include gas–powder flow, evolutions of coal temperature and burnout, and gas composition distributions. Such parametric study based on the in-furnace model is considered

superior to the previous studies for its three dimensionality, inclusion of coke bed and the subsequent recirculation region in the raceway. The results are then compared to the previous studies with the underlying mechanisms explored.

2. Model description In this CFD model, one computational domain is used covering lance, blowpipe, tuyere, raceway and coke bed, so that the in-furnace phenomena of PCI operation can be studied under various conditions. The blowpipe–tuyere–raceway region is treated as a cavity. The coke bed is treated as a porous media. The model includes the following physical and chemical processes associated with PCI operation in the region of raceway and coke bed: (1) turbulent gas– coal particle flow in the raceway cavity and coke bed; (2) coal combustion (devolatilisation, volatile combustion, and char reactions); (3) coke combustion and gasification; and (4) heat transfer between the gas–coal powder–coke bed.

Table 1 Governing equations for the gas and particle phases. For the gas phase Mass Momentum Energy Gas species i Turbulent kinetic energy Turbulent dissipation rate

∇⋅ðρUÞ = ∑ m˙   np    2 ∇⋅ðρUUÞ−∇⋅ ð μ + μt Þ ∇U + ð∇UÞT = −∇ p + ρk + ∑ f D 3 np     λ μt + ∇⋅ ρUH− ∇H = ∑ q CP σH np     μt ∇⋅ ρUYi − Γi + ∇Yi = Wi σY i     μt ∇⋅ ρUk− μ + ∇k = ðPk −ρε Þ σk     μt ε ∇⋅ ρUε− μ + ∇ε = ðC1 Pk −C2 ρε Þ k σε

For a particle in the coal powder phase

Momentum

dmp = − m˙ dt dUp = −f D mp dt
1 2 f D = πdp ρCD j U−Up j U−Up 8

Energy

mp Cp

Mass

dTp = −q dt

 
dmp −q = πdp λNu Tg −Tp + ∑ Hreac + Ap εp πI−σB Tp4 dt k2 ; Pk = (μ + μt) ∇ U ⋅ (∇ U + (∇ U)T). ε CD = max (24 (1+ 0.15Re0.687)/Re, 0.44); i = O2, CO2, CO, VM, H2, H2O.

μ t = Cμ ρ

Y.S. Shen et al. / Powder Technology 223 (2012) 27–38

29

Table 2 Reactions of coal considered and their rate expressions (for one particle). Reactions Coal = VM + Char (devolatilisation)

Models Two-competing model

VM + O2 = CO2 + H2O (gaseous combustion)

Eddy dissipation model

Char + O2 = CO + CO2 (char oxidation) Char + CO2 = 2CO (char gasification) Char + H2O = CO + H2 (char gasification)

Gibb model

Reaction rate expressions

Rate constants [13,15]

dVM = ðα1 k1 + α2 k2 ÞCO dt k = A exp(− E/Tp)

A1 = 3.7 × 105 s−1, E1 = 18,000 K A2 = 1.46 × 1013 s−1, E2 = 30,189 K

α1 = VM (daf.); α2 = 1.25α12 + 0.92α1   ½i ri = CA κε min νi′

CA = 4.0

  −1 2ðϕ−1Þ dmc 3ϕ MC ρ∞  −1 Ts −1 = As exp − =− k1 + ðk2 + k3 Þ mC , 1−e MO2 ρc 2−ϕ dt Tp  α 2 Tp + Tg D kc D k1 = 2 , D = ρref , k2 = ð1−eÞ , k3 = kcTp(β coth β − 1)/β a, 2Tref rp rp  0:5 kc kc = AcTp exp(− Tc/Tp), β = R DP ea

2.1. Gas–coal powder flow in raceway cavity The model formulation of gas–coal powder flow has been detailed elsewhere [14–16]. It is outlined below for completeness. The gas phase is described by a set of 3D, steady-state Reynolds averaged Navier–Stokes equations closed by the standard k–ε turbulence model equations. The variables in the governing equations solved for the gas phase include mass (m), momentum (u, v, w), turbulence kinetic energy (k), turbulence dissipation rate (ε), enthalpy (H) and a number of species (Yi), including O2, CO2, CO, H2, H2O and volatiles. Coal powders are treated as a dispersed phase and modelled using the Lagrangian method. Coal particles are tracked along discrete particle trajectories without considering interaction between particles. Newton's second law is used to calculate their movements, considering gas drag force and turbulence dispersion are considered. The change of particle temperature is determined by three heat transfer modes: convective heat transfer, latent heat transfer associated with mass transfer, and radiative heat transfer. Full coupling of mass, momentum and energy of particles with the gaseous phase is carried out. Coal combustion is regarded as a multi-stage process: 1) preheating; 2) devolatilisation of raw coal, modelled using the two-competing model [17]; 3) gaseous combustion, modelled using the eddy dissipation model [18]; 4) the oxidation and gasification of residual char, modelled using the Gibb model [19]. The governing equations for the gas and particle phases are summarised in Table 1. The coal reaction models have been detailed in our previous work [13,15,20]. The reactions of coal powders and their reaction rate expressions are summarised in Table 2. The model is developed based on the framework of the software package ANSYS-CFX. 2.2. Gas flow in coke bed

Ac = 14 m s−1 K−1 Tc = 21,580 K Ac = 20,230 m s−1 K−1 Tc = 39,743 K Ac = 606.9 m s−1 K−1 Tc = 32,406 K

and previous DEM modelling [22], only a limited number of coke particles are moving in the raceway cavity, which would not affect the coal combustion much. Similar to the raceway cavity, the gas flow field in coke bed is described by a set of 3D, steady-state Reynoldsaveraged Navier–Stokes equations, closed by the standard k–ε turbulence model equations. The general form of these equations is

∇⋅ðργUΦÞ−∇⋅ðΓγ∇ΦÞ = γSΦ

ð1Þ

a

Coke bed

Raceway

Tuyere Lance Blowpipe

b X

The coke bed is treated as a porous media for computational efficiency. This is because based on the experiment observation [21]

Table 3 Dimensions of the geometry of lance–blowpipe–tuyere–raceway–coke bed region. Blowpipe:

Tuyere:

Radius: 90 mm Length: 800 mm

Radius: 75 mm/90 mm Length: 135 mm

Raceway:

Coke bed:

Depth: 1600 mm Height: 1000 (925 + 75) mm Width: 710 mm

Depth: 3700 mm Height: 4500 mm Width: 1000 mm

Cooling gas

Z

Coal+ Conveying gas 17 21.3 28.6 35 Fig. 2. Model geometry: (a), whole model (for blowpipe, radius: 90 mm, and length: 800 mm; for tuyere, radius: 75 mm/90 mm, and length: 135 mm; for raceway, depth: 1600 mm, height: 1000 mm (925 + 75), and width: 710 mm; and for coke bed, depth: 3700 mm, height: 4500 mm, and width: 1000 mm.); and (b) cross-section area of the lance tip in mm.

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Table 4 Boundary conditions and operating conditions. Operating conditions

Proximate analysis (ad.)

Working volume Productivity Tuyere number Reference pressure

2749 m3 2.4 tHM/m3 day 28 461.0 kPa

Boundary conditions O2 enrichment in blast Blast (22.9% O2) Cooling gas (100% O2) Conveying gas (100% N2) Coal

6000 Nm3/h 300,000 Nm3/h 5000 Nm3/h 1317 Nm3/h 35 t/h (127.3 kg/tHM)

1200 °C 327 °C 45 °C 45 °C

Moisture, % Volatile matter, % Ash, % Fixed carbon, % Gross specific energy, MJ/kg

3.2 32.5 9.8 54.5 30.1

Ultimate analysis (daf.) C, % H, % N, % S, % O (by diff), %

83.5 5.3 1.95 0.6 8.6

a b

c

d

e

Fig. 3. Flow pattern of gas-powder flow in the base case: (a), streamlines of gas flow in the raceway; (b), particle trajectories in the raceway coloured by particle diameter; (c), particle trajectories coloured by particle travelling time (particle residence time); (d) streamline of gas flow in the coke bed; and (e) particle trajectories in the coke bed coloured by particle diameter.

Y.S. Shen et al. / Powder Technology 223 (2012) 27–38

γ is the volume porosity (γ = 1 for cavity). SΦ is calculated in the similar way shown in Table 1. In this model, the momentum source through the coke packed bed is obtained by Ergun equation 2

∇P =

150μ ð1−γ Þ 1:75ρð1−γ Þ U+ jUjU γdp γ 2 d2p

ð2Þ

In real blast furnaces, the temperature of the coke bed is much lower than the raceway due to the effects of various complicated phenomena, such as FeO–Coke reaction (highly endothermic), solid– liquid heat transfer, and coke solids flow. These phenomena are difficult to be included in the model to avoid the complexity. In this study, in order to account for these effects, a heat sink is used, allowing for the consideration of more coke bed properties. For details, see Ref. [23]. 2.3. Reactions of coke solid and gas composition The important coke reactions are considered in the porous media region where the consumption of coke is refilled continuously to give an unchanged simulation domain. The Field model [24] is used for coke reactions in the coke bed, including coke solution loss and coke combustion. The overall rate at the surface of a coke particle is determined by a combination of chemical reaction and diffusion of reacting gas. This is controlled by the smaller one of the rates kd and kc.   dmcoke P −1 −1 −1 = kd + kc ½i4πrcoke2 PA dt kd =

    Dref Tcoke + Tg 0:75 PA T ; kc = Ac exp − c rcoke 2Tref P Tcoke

ð3Þ

ð4Þ

where [i] is the molar fraction of the reacting gas specie i (i = O2 for coke combustion, and CO2 for coke solution loss reaction).

31

The composition of gas species (O2, CO, CO2, H2, H2O and N2) is the consequence of the reactions of coal and coke at respective reaction rates. The rate constants come from the literature [15]. The gas compositions are obtained by solving the governing equations of each gas species.

3. Simulation conditions The model geometry is set up based on a practical blast furnace. The main dimensions of the model geometry are listed in Table 3. Note that the dimensions of lance–blowpipe–tuyere region are real based on BlueScope Steel Port Kembla No. 6 furnace. The co-axial lance is introduced into the blowpipe at an angle of 10° with its tip on the centreline. Three gas streams (conveying gas, cooling gas and hot blast) are introduced into the domain (Fig. 2). The Rosin–Rammler distribution is used for the particle size distribution of pulverised coal. So corresponding to the current practice, the particles have a wide size distribution where the mean size is 65 μm. Other coal properties are listed in Table 4. The raceway is designed in the shape of a ‘balloon’. In the previous studies, the shape of raceway was determined by means of experiments [25], continuum approach [26,27], and DEM coupled with computational fluid dynamics (CFD) [6,11,22]. Under the present conditions (Table 4), a constant raceway profile is assumed and used as shown in Fig. 2, determined based on the CFD–DEM simulations using a model similar to Feng et al. [22] and previous experimental/ practical observations [21]. Note that different operational conditions may give different raceway shapes. This aspect will be studied in the future. The accumulation of unburnt char particles as static holdup in the coke bed is simulated by a low local porosity based on the measurement [28] and simulations [5], as discussed in our previous study [14]. In addition, the computational domain considered in this model is a small slice of the lower part of a blast furnace, only 1/56 of

a

b

c

d

Fig. 4. Combustion characteristics of coal powder in the base case: (a), temperature field; (b), oxygen distribution; (c), coal burnout along particle trajectories in the raceway; and (d), coal burnout along particle trajectories in the coke bed.

Y.S. Shen et al. / Powder Technology 223 (2012) 27–38

4. Results and discussion The model has been validated against the measurements in terms of gas–powder velocities, coal burnout and gas composition, respectively [14,15]. Specifically, the model validity of gas–powder flow is confirmed against the measurements from a pilot-scale combustor [29]. The model validity of coal burnout is confirmed against the measurements from pilot-scale experiments [15]. The model validity of gas composition has been confirmed using the measurements in Newcastle No. 4 blast furnace in Australia [30,31]. These confirm the validity of the model in terms of gas–powder flow, coal burnout and gas composition, so that the in-furnace phenomena of these combustion behaviours can be investigated under different conditions. 4.1. Typical in-furnace phenomena The typical in-furnace phenomena of the base case are outlined below in order to provide some background information for the latter discussion. In connection with our previous study [14], the phenomena are presented in terms of gas–powder flow–thermal–chemical characteristics. They provide background information for understanding the effects of different operating variables. Fig. 3(a) shows the gas streamline in the raceway cavity. The flow pattern can be divided into two parts: a high-speed jet and a largescale recirculation. This is because, the blast stream, together with the inclined low-speed gas flow of conveying stream and cooling stream from the inclined lance, is accelerated through the tuyere. As a result, a high-speed jet of up to 220 m/s forms along the tuyere axis to the end of the raceway. Subsequently, after reaching the raceway boundary of low porosity, the gas flow is reduced to 20–30 m/s; at the same time, the gas flow starts a large-scale recirculation above the main gas flow jet in the raceway. Fig. 3(b) shows the particle trajectories of coal powder in the raceway, it is found that corresponding to the gas flow, the coal trajectories have two different flow patterns: (i) a main coal plume located along the lower part of the raceway, where fine particles leave the main coal plume before reaching the end of the raceway; and (ii) a large-scale recirculation of the fine particles of up to 70 μm around the raceway centre. Subsequently, the coke bed also shows two flow patterns of coal particles accordingly: (i) the main coal plume (relatively large particles of around 100 μm) penetrates deep across the bed (the so-called deadman zone); (ii) the recirculating fine particles exit mainly from the top of the raceway and then move upward into the upper part of the coke bed (the so-called dripping zone). This is because the large particles tend to maintain their initial momentum and the fine particles are easier to be affected by the turbulence and then dispersed more widely. Fig. 3(c) shows the particle trajectories coloured by travelling time. The residence time of coal powders along the main coal plume is around 10–50 ms before reaching the end of the raceway, while the recirculating coal powders may be up to 0.9 s in the raceway. On the other hand, compared with the raceway, in the coke bed, the travelling time of the particles penetrating the coke bed is quite long, around 1.0 s. In the deadman zone (Fig. 3(c,d,e)), the gas flow is slower; the particle size is larger; and the travelling time is even longer, compared to the dripping zone. Fig. 4(a) shows the temperature field of the raceway and surrounding coke bed. The main coal plume and recirculation region show a great difference in temperature. Along the main coal plume, a high temperature field of up to 2900 K forms at the downstream of the coal plume and the nearby coke bed. Especially, an annular hightemperature zone, the so-called flame front, is observed at the surface of coal plume in front of tuyere. In the recirculation region, the

Table 5 Operating conditions investigated in this study. Operating conditions

Values

Blast temperature, °C O2 mass fraction in blast, % Type of cooling gas PCI rate, kg/tHM

1000 23 Oxygen 127

1200 30 Air 200

1400 35 Methane 255

302

temperature is decreased to ~ 2000 K. This temperature difference results from the gas–solid flow in the raceway and its subsequent heat releases from chemical reactions. Fig. 4(b) shows the oxygen distribution in the raceway. Due to the coke bed, the oxygen injected is quickly converted to CO by reacting with the surrounding coke bed. Similar to the temperature field, the oxygen distribution in the raceway also shows two patterns (Fig. 4(b)): depleted along the main coal plume and a small amount in the large-scale recirculation. The coal burnout is defined according to the ash balance, Burnout =

    ma;0 1− = 1−ma;0 ma

ð5Þ

It represents the total weight loss of the coal due to devolatilisation and char reactions. Fig. 4(c,d) shows the particle trajectories coloured by burnout. The recirculation region shows a higher burnout than

a

1000 oC 1200 oC

80

1400 oC 60

Burnout, %

360°. It is very small compared to the whole blast furnace and thus can be simplified as rectangular shape for computational efficiency.

40

20

0 0

0.2

0.4

0.6

0.8

1

1.2

Distance from lance tip, m

b

1000 oC

2800

1200 oC

Particle temperature, K

32

1400 oC

2300

1800

1300

800

300 0

0.2

0.4

0.6

0.8

1

1.2

Distance from lance tip, m Fig. 5. Effect of blast temperature on coal combustion characteristics along the tuyere axis in the raceway: (a), burnout; and (b), particle temperature.

Y.S. Shen et al. / Powder Technology 223 (2012) 27–38

100

a

33

raceway raceway bottom

Burnout,.%

95

90

85

80 800

1000

1200

1400

1600

Blast temperature, oC

b

Fig. 6. Effect of blast temperature on coal combustion characteristics over the raceway surface: (a), burnout; and (b), particle temperature.

4.2. Effects of operating conditions The effects of operating conditions on coal/coke combustion are investigated in terms of gas–powder flow, gas composition and coal burnout, respectively. Table 5 summarises the operating conditions investigated in this study. The effect of one specific parameter is quantified by fixing the rest at their base values. This study will be focused on the effects of operating conditions (i.e. variables in relation to blast furnace operation) only. In particular, variables considered are blast conditions (temperature and composition), cooling gas and PCI rate. The effects of other factors, such as coke bed properties, tuyere and lance setting, raceway shape, liquid dripping and so on, will be investigated in the future. For example, the effect of tuyere inclination angle has been studied by others [32]. The results indicate the proper downward inclination of tuyere can stabilise the raceway. This effect can also be studied by means of this in-furnace model, linked to the formation and flow of a raceway which is a challenging topic. It has to be studied in our future work. Note that in this study, the burnout is examined in two ways: along the tuyere axis (centreline) and over the raceway surface. The former is used in the previous studies and can only represent coal burnout at one point; whereas superiorly, the latter can represents coal combustion efficiency of coal particles leaving the entire raceway region, which is regarded as more useful and reliable [31]. To demonstrate this, the findings about these effects will be compared with the previous findings obtained using the

previous pilot-scale model [12] and subsequently the underlying mechanisms for these comparisons will be explored. 4.3. Blast temperature Fig. 5 shows the effect of blast temperature on coal combustion characteristics along the tuyere axis, in terms of burnout and particle temperature. A higher blast temperature causes an earlier and faster increase of the coal burnout, but reaching the same burnout level

0.6 0.5

Gas mass fraction

the final burnout at the endpoint, ~85% vs. ~ 60%, resulting from the different residence time and oxygen distribution in the raceway. Therefore, the burnout at the endpoint of raceway along the tuyere centreline cannot describe the unburnt coal entering the coke bed accurately in quantity. It is necessary to use a better indicator to represent the unburnt coal entering the coke bed over the entire raceway surface when investigating the operating variables.

0.4 0.3 0.2 0.1 0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Distance from lance tip, m O2, 1000K CO2, 1000K CO, 1000K

O2, 1200K CO2, 1200K CO, 1200K

O2, 1400K CO2, 1400K CO, 1400K

Fig. 7. Effect of blast temperature on gas composition along tuyere axis.

Y.S. Shen et al. / Powder Technology 223 (2012) 27–38

4.4. Oxygen enrichment Fig. 8 shows the effect of the oxygen enrichment into blast on coal combustion characteristics along the tuyere axis. The blast of higher oxygen content could give a slightly earlier and faster burnout increase after 0.4 m, followed by the same final burnout as other oxygen contents. This is not consistent with the previous studies [12], where oxygen enrichment was considered to be able to improve the coal burnout. This is because, in the previous study using a pilot-scale model, the coke bed and its reactions is not considered; whereas in this in-furnace model, the coke bed is included and thus the fuel is excessive, leading to much lower oxygen content along the tuyere centreline (Fig. 4(b)). Therefore, oxygen enrichment cannot overwhelm a large amount of excessive coke filling at the endpoint of the raceway. On the other hand, the higher oxygen content blast could cause a higher particle temperature in the raceway. That is, the

a 23%

80

30%

.

35%

Burnout, %

finally. The release of volatile matter (VM) under different blast temperatures are all completed, which is considered responsible for the similar final burnout level at the endpoint of raceway along the tuyere axis. However, the higher blast temperature still can give a higher particle temperature and a higher gas temperature, which can help compensate for the cooling effect from the decomposition of coal volatiles. Fig. 6 shows effect of blast temperature on coal combustion characteristics over the raceway surface, in terms of burnout and particle temperature. As the blast temperature is increased from 1000 to 1400 °C, although the burnout over the entire raceway surface increases slightly by 2% (absolute), the burnout along the raceway bottom is increased by 6% (absolute). Fig. 6(b) shows the effect of blast temperature on particle temperature in the entire raceway. It is found that the particle temperature of 1400 °C blast is much higher than that of 1000 °C blast. Such large difference in particle temperature can be reflected by the difference in coal burnout over the raceway surface but not at the endpoint of the raceway. Therefore, the burnout over the raceway bottom surface is regarded as a more useful way to represent coal combustion efficiency in the raceway. In the previous studies, e.g. a numerical study by Shen et al. [12] and an experimental study by Mathieson et al. [3], high blast temperature is regarded to play a limited role in improving the final burnout. However, using this in-furnace model, it is indicated that the burnout along the tuyere axis is consistent with the findings in the previous studies. Moreover, this in-furnace model can offer additional information over the raceway surface, that is the high blast temperature indeed improves the coal burnout over the raceway surface, as expected in the practice [1]. Thus, the in-furnace model can confirm the finding in the previous model and also develops additional findings over the raceway surface because this model includes not only the tuyere centreline but also the recirculation region for the raceway. These explain why there are different views on the effects of operating conditions on PCI operation by different investigators. The gas composition can be analysed over the entire 3D model of raceway–coke bed region. In this study, gas composition is analysed along the tuyere axis in the raceway and coke bed. Fig. 7 shows the effect of blast temperature on gas composition along the tuyere axis. As the higher temperature blast is used, in the raceway, the O2 depletion becomes faster and accordingly the CO2 concentration starts to increase earlier at a considerably faster rate due to the extra heat compensation from higher temperature blast. Beyond the raceway, the different blast temperatures show a similar conversion rate of CO2 to CO and reach a similar level finally, since the heat compensation from high blast temperature is overwhelmed by the treatment of the coke bed temperature. It is indicated that the gas composition is sensitive to the change of blast temperature inside the raceway but insensitive in the coke bed.

60

40

20

0

0

0.2

0.4

0.6

0.8

1

1.2

Distance from lance tip, m

b 2800

Particle temperature, K

34

23% 30%

2300

35%

1800

1300

800

300

0

0.2

0.4

0.6

0.8

1

1.2

Distance from lance tip, m Fig. 8. Effect of oxygen content in the blast on coal combustion characteristics along the tuyere axis: (a), burnout; and (b) particle temperature.

oxygen enrichment plays a significant role in heat compensation in the raceway. Fig. 9 shows the effect of oxygen enrichment on coal combustion characteristics over the raceway surface. Different from the final burnout along the tuyere axis, the average burnout over the entire raceway surface is increased by 3.5% (absolute) and the average burnout along the raceway bottom is increased by 7% (absolute), as the oxygen content increases from 23 to 35%. This is because different from the tuyere centreline (the oxygen-depleted zone), the largescale recirculation region still has some oxygen due to strong gas flow recirculation located above the main coal plume (Fig. 4(b)). Fig. 9(b) shows the effect of oxygen content in blast on particle temperature over the entire raceway. It is found that high oxygen content in the blast affects the particle temperature along tuyere axis more than the large-scale recirculation region. Fig. 10 shows the effect of oxygen enrichment on gas composition along the tuyere axis. Under the condition of the higher oxygen enrichment into the blast, inside the raceway, the O2 is converted to CO2 at a faster rate; whereas inside the deadman zone the conversion of CO2 to CO is considerably faster. It is indicated that the gas composition in both raceway and deadman zone is sensitive to the oxygen enrichment. This confirms and explains an argument about the observations [33] that oxygen enrichment plays an important role as a means of controlling gas flow in the furnace rather than controlling pulverised coal combustion along the tuyere axis.

Y.S. Shen et al. / Powder Technology 223 (2012) 27–38

35

100

a Burnout,.%

95

90

85

raceway raceway bottom

80 20

25

30

35

40

Oxygen content, %

b

Fig. 9. Effect of oxygen content in the blast on coal combustion characteristics over the raceway surface: (a), burnout; and (b), particle temperature.

4.5. Type of cooling gas In practice, different gases could be adopted as cooling gas inside the lance. Fig. 11 shows the effect of various types of cooling gas on coal combustion along the tuyere centreline. Compared with pure oxygen and air, by using the methane as cooling gas, it is found that the strong combustion of methane happens in front of tuyere, which then could increase the temperature and trigger an earlier devolatilisation and burnout rise inside the raceway. But near the end of

raceway, the burnout is found similar with the other two gases. This finding is different from the previous study [13], where oxygen gives the highest burnout in the endpoint of raceway along the tuyere centreline. This is because of the oxygen-depleted and fuel-rich region in the endpoint of the raceway, similar to the reasoning discussed for the oxygen enrichment in above. Fig. 12 shows the effect of types of cooling gas on coal combustion characteristics over the raceway surface. It is shown that by using the methane as cooling, the burnout is lowered slightly by 2% (absolute) in terms of both final burnout at the endpoint of the raceway along the tuyere axis and burnout over the raceway surface/bottom. This is because the methane combustion consumes a large amount of oxygen and thereby the oxygen available to the coal combustion becomes less, leading to lower burnout. On the other hand, the different cooling gas affects little on particle temperature (Fig. 12(b)). Fig. 13 shows the effect of types of cooling gas on gas composition along the tuyere axis. A considerable effect on gas composition is found in front of tuyere due to the strong methane combustion. Beyond this zone, this effect becomes insignificant due to the completion of methane combustion in the raceway. Therefore, it is indicated that using methane as cooling gas is favourable to increase the gas temperature inside the raceway and thereby compensate for the heat, although affecting the burnout and gas composition slightly. 4.6. PCI rate (coal powder rate)

Fig. 10. Effect of oxygen content in blast on gas composition along the tuyere axis.

Fig. 14(a) shows the effect of PCI rate on the evolution of burnout along the tuyere axis. The predicted burnout along the tuyere axis is insensitive to the increase of PCI rate. In order to better quantify the amount of unburnt char entering the coke bed, the burnout over the raceway surface is investigated (Fig. 14(b)), it is shown that as the PCI rate increases from 127 to 302 kg/tHM (kg per tonne hot metal), the average burnout over the entire raceway boundary is decreased by ~30% and the average burnout along the raceway bottom is decreased

36

Y.S. Shen et al. / Powder Technology 223 (2012) 27–38

a

Methane Air

80

Burnout, % .

Oxygen 60

40

20

0 0

0.2

0.4

0.6

0.8

1

1.2

Distance from lance tip, m

b Methane

Particle temperature, K

2800

Air Oxygen

2300

1800

1300

800

300 0

0.2

0.4

0.6

0.8

1

1.2

Distance from lance tip, m Fig. 11. Effect of types of cooling gas on coal combustion characteristics along the tuyere axis in the raceway: (a), burnout; and (b), particle temperature.

by ~ 20%. It is indicated that in contrast to the insensitivity of burnout along the tuyere axis, the burnout over the raceway surface is sensitive to the increase of PCI rate. Fig. 15 shows the effect of PCI rate on gas composition along the tuyere axis. It is shown that under higher PCI rate, the conversion rate of O2 to CO2 is faster and the conversion rate of CO2 to CO becomes slower. Such difference can be reflected by the burnout over the raceway surface but cannot be reflected by the burnout along the tuyere axis, where different PCI rate gives similar burnout predictions. That is, the burnout along the tuyere axis cannot represent the actual amount of unburnt coal and the average burnout over the raceway surface is a more reliable indicator to represent the mount of unburnt coal entering the coke bed and can better characterise in-furnace phenomena of PCI operation. 5. Conclusions By means of a recent developed model of pulverised coal flow/combustion [14] for simulating the in-furnace phenomena in the raceway–coke bed region of a blast furnace, the effects of some key operating conditions on coal/coke flow–thermal–chemical behaviours are investigated, such as gas–powder flow, gas composition, coal particle temperature and coal burnout. The following conclusions can be drawn from this study. • The large-scale recirculation region of the raceway shows significant differences from the tuyere centreline. These include lower-speed

recirculation of smaller-size coal powders in the gas–powder flow, lower temperature, slight higher oxygen content, and much higher burnout in the recirculation region. • These operating conditions play a limited role in improving the burnout at the endpoint of raceway but a notable role at the recirculation region i.e. over the raceway surface. Gas composition distribution is sensitive to oxygen enrichment and use of oxygen as cooling gas in both raceway and coke bed region, and sensitive to blast temperature only in the raceway region but not in the coke bed. • Compared with the previous studies in pilot-scale [12], this infurnace model confirms some findings from the previous studies and develops some further findings obtained over the raceway surface. The underlying mechanisms of these differences are explored: the inclusion of coke bed and the subsequent recirculation region are responsible for the differences. These explain why there are different views on the effects of operating conditions on PCI operation by different investigators. Based on these comparisons, it is indicated that the burnout over the raceway surface is regarded as a more useful indicator to describe the amount of unburnt char entering the coke bed. It is necessary to consider the coke bed and the subsequent large-scale recirculation of the raceway in the future PCI investigations. List of symbols A1, A2 pre-exponential factors of devolatilisation reactions, s −1 Ac pre-exponential factors in Gibb model, m s −1 K −1 Ap particle area, m 2 As constant in Gibb model, 0.0004 a exponent in Gibb model, 0.75 C0 mass of raw coal, kg C1, C2 turbulent model constants CD drag coefficient Cp particle heat capacity, J kg −1 K −1 D external diffusion coefficient of oxygen in Gibb model, m 2 s −1 Dref reference dynamic diffusivity in Gibb model, 1.8e5 kg m −1 s −1 daf. dry and ash free dp particle diameter, m e void fraction of char particles E1, E2 activation energy of devolatilisation reactions, K fD drag force from a particle, N H enthalpy, J kg −1 Hreac reaction heat, J kg −1 I radiation intensity on particle surface, W m −2 [i] molar concentration of component i k turbulent kinetic energy, m 2 s −2 k1, k2 devolatilisation rate constant, s −1 k1 rate of external diffusion in Gibb model, s −1 k2 rate of surface reaction rate in Gibb model, s −1 k3 rate of internal diffusion and surface reaction in Gibb model, s −1 kc carbon oxidation rate in Gibb model, m s −1 ˙ m mass transfer rate from a particle, kg s −1 mc mass of char, kg ma ash mass fraction ma,0 original ash mass fraction Mc molecular weight of carbon MO2 molecular weight of oxygen molecule np particle number per unit volume, m −3 Nu Nusselt number p pressure, Pa PA atmospheric pressure, Pa q heat transfer from a particle, W rp particle radius, m

Y.S. Shen et al. / Powder Technology 223 (2012) 27–38

37

a

Burnout,.%

100

raceway raceway bottom

95 91.8

90.6

92.6

90 85 81.1

83.4

82.1

80 methane

air

oxygen

Types of cooling gas

b

Fig. 12. Effect of types of cooling gas on coal combustion characteristics over the raceway surface: (a), burnout; and (b), particle temperature.

ri Re T Tblast Tc Tref Ts U u, v, w

reaction rate of gas species i, mol m −3 s −1 Reynolds number temperature, K blast temperature, K activation energy in Gibb model, K reference temperature in Gibb model, 293 K constant in Gibb model, 6240 K mean velocity of gas, m s −1 gas velocity components, m s −1

VM vi Wi

Greek letters α volume/internal surface area ratio in Gibb model volatile yield α1, α2 molecular diffusivity of species i, kg m −1 s −1 Γi ε turbulent dissipation rate, m 2 s −3 εp particle emissivity λ thermal conductivity, W m −1 K −1 ρ density, kg m −3 μ dynamic viscosity, Pa s turbulent viscosity, Pa s μt turbulence model constant σk, σε Stefan–Boltzmann constant, 5.67 × 10 −8 W m −2 K −4 σB ϕ mechanism factor in Gibb model

0.6

Gas mass fraction

0.5 0.4 0.3 0.2

Subscripts c char g gas p particle

0.1 0.0 0.0

volatile matter of coal stoichiometric coefficient of species i. reaction rate of species i (per unit volume), kg m −3 s −1

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Distance from lance tip, m O2, oxygen CO2, oxygen CO, oxygen

O2, air CO2, air CO, air

Acknowledgements

O2, methane CO2, methane CO, methane

Fig. 13. Effect of cooling gas on the gas composition along the tuyere axis.

The authors wish to thank the Australian Research Council and BlueScope Steel for their support of this project. Dr. Baoyu Guo (UNSW) and Dr. Daniel Maldonado (BlueScope Steel) are acknowledged for useful discussion during model development.

38

Y.S. Shen et al. / Powder Technology 223 (2012) 27–38

a

References

127kg/tHM

80

200kg/tHM 255kg/tHM

Burnout, %

60

40

20

0 0

0.2

0.4

0.6

0.8

1

1.2

Distance from lance tip, m

b 100 raceway

Burnout,.%

90

raceway bottom

80

70

60

50 100

0

200

300

400

PCI rate, kg/tHM Fig. 14. Effect of PCI rate on coal burnout: (a), along the tuyere axis; and (b), over the raceway surface.

0.6

Gas mass fraction

0.5 0.4 0.3 0.2 0.1 0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Distance from lance tip, m O2, 127kg/tHM O2, 302kg/tHM CO2, 255kg/tHM CO, 200kg/tHM

O2, 200kg/tHM CO2, 127kg/tHM CO2, 302kg/tHM CO, 255kg/tHM

O2, 255kg/tHM CO2, 200kg/tHM CO, 127kg/tHM CO, 302kg/tHM

Fig. 15. Effect of PCI rate on gas composition along the tuyere axis.

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