Numerical studies of pulverized coal combustion in a tubular coal combustor with slanted oxygen jet☆

Numerical studies of pulverized coal combustion in a tubular coal combustor with slanted oxygen jet☆

Fuel 82 (2003) 893–907 www.fuelfirst.com Numerical studies of pulverized coal combustion in a tubular coal combustor with slanted oxygen jetq Y.C. Gu...

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Fuel 82 (2003) 893–907 www.fuelfirst.com

Numerical studies of pulverized coal combustion in a tubular coal combustor with slanted oxygen jetq Y.C. Guoa, C.K. Chanb,*, K.S. Lauc b

a Department of Engineering Mechanics, Tsinghua University, Beijing 100084, People’s Republic of China Department of Applied Mathematics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, People’s Republic of China c Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, People’s Republic of China

Received 3 September 2002; revised 4 November 2002; accepted 8 November 2002; available online 11 December 2002

Abstract A pure two-fluid model for turbulent reacting gas-particle flow of coal particles is developed using a unified Eulerian treatment of both the gas and particle phases. The particles’ history caused by mass transfer due to moisture evaporation, devolatilization and char reaction is described. Both velocity and temperature of the coal particles and the gas phase are predicted by solving the momentum and energy equations of the gas and particle phases, respectively. A k – 1 – kk two-phase turbulence model, EBU – Arrhenius turbulent combustion model and fourflux radiation heat transfer model are incorporated into a comprehensive model. The above comprehensive mathematical model is used to simulate two-dimensional gas-particle flows and pulverized coal combustion in a newly designed tubular oxygen – coal combustor of blast furnace. Predicted results of isothermal gas-particle flows are in good agreement with those obtained by measurements. The results also show that the proposed tubular oxygen – coal combustor prolongs the coal particle residence time and enhances the mixing of coal and oxygen. Results indicate that smaller coal particles of 10 mm diameter are heated and devolatilized rapidly and have volatile combustion in the combustor, while larger coal particles of 40 and 70 mm in diameter are heated but not devolatilized, and combustion of such particles does not occur in the tubular combustor. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Coal combustion; Gas-particle flows; Oxygen–coal combustor; Blast furnace

1. Introduction Blast furnace is used worldwide in the modern iron and steel manufacturing industry. With increasing coke prices and the problem of pollution caused by coke ovens, pulverized coal injection has received widespread attention in recent years. Coal injection has many advantages such as reduced coke consumption, increased productivity and also improved control of raceway adiabatic flame temperature [1]. These characteristics are important in the commercialization of blast furnaces. In the process of pulverized coal injection into the blast furnace, pulverized coal is injected into the blowpipe by a lance with pre-heated air at 1000 – 1200 8C with a high velocity of 150– 250 m s21. Due to the high temperature produced, the heating rate is around 105 – 106 K s21. On the other hand, the residence time of coal particles in the blowpipe-tuyere-raceway system is very * Corresponding author. Tel.: þ 852-2766-6919; fax: þ 852-2362-9045. E-mail address: [email protected] (C.K. Chan). q Published first on the web via Fuelfirst.com –http://www.fuelfirst.com

short (about 10 – 20 ms) [2] due to the high velocity. In addition, the incomplete combustion of coal causes poor gas emission and disrupts the operation of the blast furnace. In order to increase coal combustion efficiency and to consume most of the pulverized coal in the blowpipe-tuyere-raceway system within the short residence time, many investigations have been carried out, such as using oxygen-enriched air, very small coal particles or high combustion air pre-heated temperatures [3]. In addition, different types of oxygen – coal lances for blast furnaces have been developed, such as co-axial lance with swirling oxygen jet [4], a coupled coal and oxygen lance system [5] and multi-hole co-axial oxygen –coal lance [6]. However, most of these designs are either very complex in their structure, such as requiring a lot of changes to the blowpipe and water-cooling system or very good coating at the lance exit. The high operating temperature also affects the lifespan of the lances. In this paper, a new oxygen –coal tubular combustor with slanted oxygen jet is proposed. The combustor is characterized by co-axial tubes with an expanding exit in the inner

0016-2361/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 6 - 2 3 6 1 ( 0 2 ) 0 0 3 6 7 - 8

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Nomenclature a g, a k B Bk Cd C1, C2, Cm Cmp D Eb h k LW M n W Subscripts Arr c,k daf

absorption coefficient for gas phase and particle phase, m21 pre-exponential factor, s21 transfer number drag coefficient between gas and one particle empirical constant of gas phase k – 1 turbulence model empirical constant of particle phase turbulence model diffusivity, m2 s21 emissivity of black body, J m22 enthalpy, J kg21 gas phase turbulent kinetic energy, m2 s22 latent heat of water evaporation, J kg21 molecular weight, kg kg mol21 particle phase number density, m23 reaction rate, kg s21 Arrhenius reaction model char of k-group coal particle phase dry and free coal

tube. Numerical simulations describing the major features of flow, combustion, and radiation heat transfer of gas and coal particles in the combustor are conducted. Results indicate that the slanted oxygen jet reduces the primary jet velocity, enhance the mixing between the coal particles and the oxygen jet and also reduce the temperature of the lance. During the operation of the blast furnace, it is difficult to investigate the blowpipe-tuyere-raceway system experimentally and very limited information about the lifetime of the lance or coal/coke combustion efficiency can be obtained. In addition, detailed information about the hydrodynamics of coal and hot blast flow, coal and oxygen mixing and pulverized coal combustion process are not available. Therefore, mathematical modeling and numerical simulation have the great advantage in evaluating and predicting the optimum conditions for blast furnace coal injection. Suzuki et al. [3] used a one-dimensional model developed by Smith and Smoot [7] to predict the overall characteristics of pulverized coal combustion under a wide range of conditions in a test furnace similar to the blowpipetuyere-raceway system. Jamaluddin et al. [8] proposed a quasi-two-dimensional model for simulating pulverized coal combustion under very high heating rates and very high temperature similar to a blast furnace and found that devolatilization plays an important role in coal particle burn-off. In this paper, mathematical models based on Eulerian descriptions are used to describe the gas-particle flow. The standard k – 1 model is used for the gas phase turbulent flow,

dk EBU g i, j k p r s T wk Greek symbols bCA, bCB, bCC

bk m y yk sp sY

daf coal of k-group particle phase EBU turbulent combustion model gas phase co-ordinate direction k-group particle phase particle phase radiation heat transfer gas phase species turbulence moisture of k-group particle phase stoichiometric ratio of char surface reaction gas-particle flow drag coefficient, kg m23 s21 viscosity, kg m21 s21 gas phase kinematic viscosity, m2 s21 particle phase turbulent kinematic viscosity, m2 s21 turbulent Schmidt number for particle phase turbulent Schmidt number for gas phase species

eddy breakup model is used to quantify the turbulent combustion, a diffusion model is used for the moisture evaporation rate and the four heat flux model is used for the radiation heat transfer. Predictions of the gas phase and coal particle phase flow field, gas species concentration and temperature distribution along the tubular combustor are presented.

2. Mathematical model 2.1. Governing equations for gas phase As provided by Guo and Chan [9], the governing equations for steady-state reacting gas-particle flows are listed as follows. Continuity equation X › › ðruÞ þ ðr rvÞ ¼ 2 nk m _ k: ›x r ›r k

ð1Þ

Axial momentum equation

› › ðruuÞ þ ðr rvuÞ ›x r ›r      ›p › ›u › ›v ›u þ2 r me þ me ¼2 þ ›x ›x ›x r ›r ›x ›r X X 2 bk ðu 2 uk Þ 2 u nk m _ k þ rgx : k

k

ð2Þ

Y.C. Guo et al. / Fuel 82 (2003) 893–907

Radial momentum equation

› › ðruvÞ þ ðr rvvÞ ›x r ›r     ›p › ›v › ›v v þ r me me ¼2 þ2 2 2m e 2 ›x ›r ›x r ›r ›r r   X X › ›u þ me bk ðv 2 vk Þ 2 v nk m _ k þ rgr : 2 ›x ›r k k ð3Þ Turbulent kinetic energy equation

› › ðrukÞ þ ðr rvkÞ ›x r ›r     › me ›k › m ›k ¼ r e þ þ G k 2 r1 þ G p þ G R : › x sk › x r › r sk › r ð4Þ › › ðru1Þ þ ðr rv1Þ ›x r ›r     › me ›1 › m ›1 1 r e ¼ þ þ ½C1 Gk 2 C2 r1 ›x s1 › x k r › r s1 › r ð5Þ

Thermal enthalpy equation

› › ðruhÞ þ ðr rvhÞ ›x r ›r     › me ›h › me ›h ¼ r þ þ Ws Qs ›x sh › x r › r sh › r 0 1 X X X þ 2ag @ qrj 2 2Ebj A 2 nk Qk 2 nk m _ k Cpk Tk : ð6Þ j

k

k

Species mass fraction equation

› › ðruYs Þ þ ðr rvYs Þ ›x  r ›r    › me ›Ys › m ›Y r e s ¼ þ › x sY › x r ›r s Y ›r X _ k: 2 W s 2 as n k m

2. the coal particle phase and gas phase co-exist and interact with each other, each having its own velocity and temperature; 3. apart from mass, momentum and energy interaction with the gas phase, each coal particle has its own turbulent fluctuation resulting in coal particle turbulent transport of mass, momentum and energy. Such coal particle fluctuations are determined by convection, diffusion, production and interaction with gas phase turbulence; 4. effective transport coefficient of the coal particle phase is determined by turbulent viscosity described by the kk model; 5. with large particle-to-gas density ratio ðrp =rg . 1000Þ; effects of static pressure gradients, virtual mass, Basset, Saffman, and Magnus forces are neglected; 6. effect of particle – particle collision is also neglected. Under these assumptions, each coal particles has a set of governing equations describing its mass, momentum and energy. The time-averaged coal particle phase governing equations can be written as follows. Number density equation

Turbulent kinetic energy dissipation rate equation

1 þ C1 ðGp þ GR Þ: k

895

› › ðn u Þ þ ðrn v Þ ›x k k r ›r k k ! ! › y k ›nk › y k ›nk ¼ þ r : ›x sp ›x r ›r sp ›r

ð8Þ

Raw coal bulk density equation

› › ðr u Þ þ ðr r v Þ ›x k k r ›r k k ! ! › y k ›rk › y k ›rk þ r þ nk m ¼ _ k: ›x sp ›x r ›r sp ›r

ð9Þ

Daf coal bulk density equation

ð7Þ

k

› › ðr u Þ þ ðr r v Þ ›x dk k r ›r dk k ! ! › y k ›rdk › y k ›rdk þ r þ nk m ¼ _ dk : ›x sp ›x r ›r sp › r

ð10Þ

2.2. Governing equations for coal particle phase In a pure Eulerian model, the coal particle phase is considered as a pseudo-fluid interacting with the gas phase. The major assumptions are: 1. each coal particle phase is identified by its initial size distribution having continuous velocity and temperature profiles;

Moisture bulk density equation

› › ðr u Þ þ ðr r v Þ ›x wk k r ›r wk k ! ! › y k ›rwk › y k ›rwk þ r þ nk m ¼ _ wk : ›x sp › x r ›r sp › r

ð11Þ

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Y.C. Guo et al. / Fuel 82 (2003) 893–907

Axial momentum equation

› › ð rk uk uk Þ þ ðr r v u Þ ›x r ›r k k k      › ›uk › ›uk ›v k r mk þ m ¼2 þ ›x k ›x r ›r ›r ›x ! › y ›r k uk k þ bk ðu 2 uk Þ þ unk m þ2 _k ›x sp › x "  # › yk ›rk ›rk r þ vk þ u þ rk gx : r ›r sp k › r ›x

Axial direction radiation heat flux   d 1 dqrx dx a þ s dx ¼ ag ðqrx 2 Ebg Þ þ

d B B rdr @ ð12Þ

› › ð rk uk v k Þ þ ðr r v v Þ ›x r ›r k k k       › ›v k ›u › ›v k þ k r mk mk ¼ þ2 ›x ›x ›r r ›r ›r ! › y ›r rvk k k þ bk ðv 2 vk Þ þ vnk m þ2 _k r ›r sp › r "  # › yk ›rk ›rk 2mk vk þ uk þ v þ rk gr : ð13Þ 2 ›x sp k › x ›r r2

dqrr C C 1 dr A aþsþ r s ¼ ag ðqrr 2 Ebg Þ þ ðqrr 2 qrx Þ þ ak ðqrr 2 Ebk Þ; 2 r

ð17Þ

where a ¼ ag þ ak ; s ¼ sg þ sk ; ak ¼ Qak p=4nk dk2 and sk ¼ Qsk p=4nk dk2 : Qak and Qsk are efficiency factors for absorption of particle radiation and scattering of particles, respectively. 2.4. Drag force between gas and coal particles The terms related to the gas-particle drag coefficients are given as

bk ¼

3 rl~v 2 ~vk l C ; 4 D dk

ð18Þ

where CD is the drag coefficient based on different Reynolds number given by 8 ! 0:6667 > 24 Re > k < for Rek , 1000; 1þ 6 CD ¼ Rek ð19Þ > > : 0:44 for Re $ 1000;

Thermal enthalpy equation

› › ðr u h Þ þ ðr r v h Þ ›x k k k r ›r k k k     › mk ›hk › m ›h r k k þ Qrk þ nk Qck ¼ þ › x sh › x r ›r sh › r

k

ð14Þ

Turbulent kinetic energy equation

› › ðr u k Þ þ ðr r v k Þ ›x k k k r ›r k k k ! ! › mk ›kk › mk ›kk þ r þ Gpk þ Ggk ¼ ›x skp ›x r ›r skp ›r ! ! › y k ›rk › y k ›rk k þ rkk : þ ›x k s p ›x r ›r sp › r

ð16Þ

Radial direction radiation heat flux 0 1

Radial momentum equation

þ nk Qk þ hk nk m _ k:

s ðq 2 qrr Þ þ ak ðqrx 2 Ebk Þ: 2 rx

dk is the diameter of k-group coal particles, and Rek is the Reynolds number of particle-gas relative motion, defined as Rek ¼

l~v 2 ~vk ldk : y

ð20Þ

2.5. Turbulence terms for gas phase and coal particle phase

ð15Þ

Eqs. (8) – (11) are solved in order to obtain the coal particle mass ðmk ¼ rk =nk Þ; daf coal mass ðmdk ¼ rdk =nk Þ and moisture mass ðmwk ¼ rwk =nk Þ in each computational cell as they describe the mass change due to moisture evaporation, daf coal devolatilization and char combustion. 2.3. Governing equations for radiation heat flux

The gas phase turbulent viscosity is determined by the k – 1 turbulence model [11], and the kk model [12] is used to model particle phase turbulence. The terms related to the gas and particle phase turbulence models are expressed as follows

me ¼ m þ mT ; mT ¼ Cm rk2 =1; ( " Gk ¼ mT 2

ð21Þ ð22Þ 2  2  2 #  2 ) ›u ›v v ›u ›v þ þ ; þ þ r ›x ›r ›r ›x ð23Þ

For radiation heat transfer, the four heat flux model [10] is used. The governing equation for the radiation heat fluxes for the axial and radial directions are given as

X 2rk k pffiffiffiffi Gp ¼ ðC kkk 2 kÞ; trk p k

ð24Þ

Y.C. Guo et al. / Fuel 82 (2003) 893–907

GR ¼ 2k

X

nk m _ k;

897

ð25Þ

k

mk ¼ Cmp rk kk0:5 k1:5 =1; ð26Þ ( " 2  2  2 #   ) ›uk ›v k vk ›uk ›v k 2 þ Gpk ¼ mk 2 ; þ þ þ ›x ›r r ›r ›x ð27Þ Ggk ¼

pffiffiffiffi 2rk k pffiffiffiffi ðC kkk 2 kk Þ þ ð2Cpk kkk 2 kk Þnk m _k trk p     1 m _ k yk ›rk ›rk þ ðv 2 vk Þ þ þ ðu 2 uk Þ : ð28Þ trk mk sp ›x ›r

2.6. Turbulent combustion model for gas phase The present model is applied to simulate volatile fuel of methane (CH4) and carbon monoxide (CO) combustion. During the combustion process, global single step reaction of volatile fuel (CH4) is assumed to form carbon dioxide and water vapor. In the species mass fraction transport equations, equal effective turbulent mass diffusion coefficients for fuel, oxygen and productions of carbon dioxide and water vapor are used. The commonly used eddy breakup (EBU) turbulent combustion model in Magnussen – Hjertager form [13] is used to quantify the turbulent combustion rates of volatiles and carbon monoxide. The reaction rate is determined as Ws ¼ minðWs;EBU ; Ws;Arr Þ; where   k Y ð29Þ Ws;EBU ¼ CR r min YF ; ox ; 1 b   E Ws;Arr ¼ Bs r2 YF Yox exp 2 s : ð30Þ RT

2.7. Heat transfer between coal particles and the gas phase In the case where the coal particle temperature is different from that of the gas phase, the so-called 1/3 Law is used to calculate the thermal conductivity ls and specific heat Cps around the coal particles. Heat transfer between single reacting coal particle and gas phase is calculated as Qk ¼ pdk Nuk ls ðT 2 Tk Þ Bk ¼

Bk ; expðBk Þ 2 1

2m _ k Cps ; pdk Nuk ls

Nuk ¼ 2 þ 0:5Re0:5 k :

ð31Þ ð32Þ ð33Þ

2.8. Single coal particle mass change model A single reacting coal particle is considered to consist of dry and free (daf) coal, moisture, char and ash as shown in

Fig. 1. Schematic diagram of the mass change of a single coal particle.

Fig. 1. Char reaction rates are known to be coal-specific and ash content does have some influences on char burning levels. As the ash content increases, the combustibility of pulverized coal combustion is suppressed due to the increase of heat capacity of the increased ash [14]. In this paper, an intrinsic combustion model is used, in which reaction occurs inside the particle, so that the particle density changes with unchanged particle size. With moisture evaporation, coal devolatilization and char combustion, the particle density would be reduced. When the ash content is high, the particle density remains large giving rise to the high heat capacity accordingly. In our study, heat capacity of a single coal particle is determined by four parts, namely moisture, daf coal, char and ash described as follows Cpk ¼

mv;k m m m Cpw þ d;k Cpd þ c;k Cpc þ a;k Cpa : mk mk mk mk

ð34Þ

It can be seen that the heat capacity of a single coal particle will increase with increasing ash content, indicating that the char burning rate modeled in the present study reflects the influence of ash content. Apart from ash, which is an inert material without mass change, the other three parts change due to moisture evaporation, devolatilization and char combustion. The total mass change of a single coal particle can be obtained as m _k ¼ m _ wk þ m _ v;k þ m _ c;k ;

ð35Þ

where m _ wk is moisture evaporation rate, m _ v;k is volatile fuel releasing rate, and m _ c;k is char reaction rate. These mass transfer rates are described by the sub-models outlined in the following sections. 2.8.1. Moisture evaporation rate The moisture evaporation rate is calculated by a diffusion model in a way similar to droplet evaporation rate [15]. Assuming that the moisture in a coal particle diffuses to the surface of the coal particle to form a liquid film and treating this liquid film as a surface layer of a water droplet with the same diameter, the moisture evaporation rate can be

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Y.C. Guo et al. / Fuel 82 (2003) 893–907

determined as 8 ! > YH2 O;s 2 YH2 O;g > > ; ðTk , Tb Þ > < 2pdk Nuk Ds rs ln 1 þ 1 2 YH O;s 2 m _ wk ¼ ;   > > Cps ðT 2 Tk Þ ls > > ; ðTk $ Tb Þ : 2pdk Nuk C ln 1 þ 1 2 L ps w ð36Þ where YH2 O;s is the mass fraction of vapor at the surface of the coal particles, given by YH2 O;s ¼ Bw expð2Ew =RTk Þ; and YH2 O;g is the mass fraction of vapor surrounding the coal particle, which is equal to the vapor concentration in the calculation grid. 2.8.2. Daf coal devolatilization rate The coal devolatilization rate is simulated by a competing-reaction kinetic model [16]. Assuming that there are two competing reactions with one reaction having an advantage at lower temperature and the other reaction having an advantage at higher temperature. After the devolatilization process, the original daf coal changes to volatiles V1 and V2, and residual char of R1 and R2. The two competing reactions are given as * 1 2 ða1 ÞR1 þ a1 V1 ; reaction ð1Þ ; daf coal ð1 2 a2 ÞR2 þ a2 V2 ; reaction ð2Þ where a1 takes the value of volatile matter percentage obtained in proximate analysis of coal, and a2 is given the value of 0.8 to reflect the characteristics of devolatilization at high temperature. The daf coal devolatilization rate is proportional to the mass of daf coal and taking the first-order kinetic model, the volatile release rate can be given as     Ev1 Ev2 m _ v;k ¼ 2a1 md;k Bv1 exp 2 2 a2 md;k Bv2 exp 2 : RTk RTk ð37Þ The rate of mass reduction of daf coal can be given as     E E m _ d;k ¼ 2md;k Bv1 exp 2 v1 2 md;k Bv2 exp 2 v2 : ð38Þ RTk RTk

2.8.3. Char reaction rate On the surface of coal particles, three kinds of heterogeneous char reaction are assumed as follows: C þ O2 ! CO2 ;

ðAÞ

2C þ O2 ! 2CO;

ðBÞ

C þ CO2 ! 2CO:

ðCÞ

The overall reaction rate is simulated by a diffusion –kinetic model [15]. The heterogeneous char reaction rate is assumed to be of first-order in oxygen concentration and carbon dioxide concentration. The char reaction rates for

the above three kinds of surface reactions can then be written as   1 EA A 2 pd r Y B exp 2 ; ð39Þ m _ c;k ¼ 2 bCA k s O2 ;s A RTk   1 E pd 2 r Y B exp 2 B ; ð40Þ m _ Bc;k ¼ 2 bCB k s O2 ;s B RTk   1 E pdk2 rs YCO2 ;s BC exp 2 C : ð41Þ m _ Cc;k ¼ 2 bCC RTk The total species reaction rate are then obtained as m _ c;k ¼ m _A _ Bc;k þ m _ Cc;k ; c;k þ m m _ O2 ;k ¼

bCA m _A c;k

þ

bCB m _ Bc;k ;

ð42Þ ð43Þ

m _ CO2 ;k ¼ 2ð1 þ bCA Þm _A _ Cc;k ; c;k þ bCC m

ð44Þ

m _ CO;k ¼ 2ð1 þ bCB Þm _ Bc;k 2 ð1 þ bCC Þm _ Cc;k :

ð45Þ

The oxygen and carbon dioxide concentrations YO2 ;s and YCO2 ;s at the coal particle surface can be calculated using a stagnant film theory, by solving the species diffusion equations near the surface of the coal particles and obtained as   m _ O ;k m _ O2 ;k YO2 ;s ¼ 2 2 þ YO2 ;g þ ð46Þ expð2Bk Þ; m _k m _k   m _ CO2 ;k m _ CO2 ;k YCO2 ;s ¼ 2 þ YCO2 ;g þ ð47Þ expð2Bk Þ; m _k m _k where YO2 ;g and YCO2 ;g are mass fractions of oxygen and carbon dioxide surrounding the coal particles which are equal to the oxygen and carbon dioxide concentration at the grid. As mass transfer rates of m _ k; m _ c;k ; m _ O2 ;k and m _ CO2 ;k are initially not known, an iterative process is needed to determine the char combustion rate. The iterative procedure is described as follows 1. Initial values of YO2 ;s and YCO2 ;s are assumed. 2. Values of YO2 ;s and YCO2 ;s are substituted into Eqs. (39)– (41) to obtain the char reaction rates m _A _ Bc;k ; m _ Cc;k : c;k ; m 3. From Eqs. (42)–(45) and Eq. (32), the terms m _ O2 ;k ; m _ CO2 ;k ; m _ k and transfer number Bk are obtained. 4. These values of m _ O2 ;k ; m _ CO2 ;k ; m _ k and Bk are substituted into Eqs. (46) and (47) to obtain the new values of YO2 ;s and YCO2 ;s : 5. The steps (2)–(4) are repeated until converged solution is obtained, and the convergence criterions are taken as new old lYOnew 2 YOold2 ;s l , 1% and lYCO 2 YCO l , 1%: 2 ;s 2 ;s 2 ;s 3. Auxiliary expressions and model constants 3.1. Contributions of coal particle mass transfer to gas phase species In this paper, five gas phase species transport equations are solved including volatile fuel (CH4), oxygen (O2), water vapor (H2O), carbon dioxide (CO2) and carbon monoxide

Y.C. Guo et al. / Fuel 82 (2003) 893–907 Table 1 Gas phase mixture characteristics auxiliary expressions



Gas mixture density

Table 3 Reaction kinetics constants

P X Ys RT s Ms

Chemical or physical process

Specific heat capacity

Cp ¼ 0:106T þ 1173:0

Viscosity

pffiffi m ¼ 0:1672 £ 1025 T 2 1:058 £ 1025

Diffusivity



Thermal conductivity

l ¼ 5:526 £ 1025 T þ 0:01155

l rCp

CH4 þ 2O2 ! CO2 þ 2H2O 2CO þ O2 ! 2CO2 Moisture evaporation Devolatilization reaction (1) Devolatilization reaction (2) Char surface reaction A Char surface reaction B Char surface reaction C

 E  K ¼ B exp 2 RT B

Unit

E (J/mol)

1.6 £ 1010 7.0 £ 104 8.32 £ 105 3.7 £ 105 1.46 £ 1013 1.225 £ 103 1.813 £ 103 7.351 £ 103

m3 kg21 s21 m3 kg21 s21

1.081 £ 105 6.651 £ 104 4.228 £ 104 7.366 £ 104 2.511 £ 105 9.977 £ 104 1.089 £ 105 1.380 £ 105

s21 s21 m s21 m s21 m s21

3.4. Reaction kinetic constants

1 Mm ¼ X Ys s Ms

Mixture molecular weight

899

(CO). In the gas phase P species mass fraction equation, there is a source term 2as k nk m _ k ; due to the change of coal particle mass. Similar source terms for the five species transport equations are given as follows X SCH4 ¼ 2 nk m _ vk ; ð48Þ k X _ wk ; ð49Þ SH 2 O ¼ 2 nk m X k SO 2 ¼ nk m _ O2 ;k ; ð50Þ k

X SCO2 ¼ nk m _ CO2 ;k ; Xk SCO ¼ nk m _ CO;k :

ð51Þ ð52Þ

k

3.2. Gas phase mixture characteristics The gas phase mixture characteristics such as specific heat capacity, viscosity, diffusivity and thermal conductivity are given in Table 1. The gas mixture density is calculated using the ideal gas state equation. 3.3. Model constants The values of the empirical constants in the turbulence model, turbulent combustion model and particle phase turbulence model are given by Launder and Spalding [11], Liao et al. [12] and Zhou and Huang [17] and specified in Table 2. Table 2 Model constants

The reaction kinetic constants for the gas phase species of CH4 and CO, together with kinetic constants of coal particle moisture evaporation, devolatilization and char surface reactions are given in Table 3. The reaction kinetic constants of methane and carbon monoxide combustion, moisture evaporation, and char surface reactions of A, B and C are taken from Zhou [18]. The reaction kinetic constants of methane and carbon monoxide do not differ with coal type, as they are gas phase reaction kinetic constants. However, the mass of volatile matter would be different for coals of different grade. Reaction kinetic constants of char surface reactions of A, B and C vary with coal type and the data in Table 3 apply to bituminous coal only. For other coal types, such as lignite coal and low-rank coal, the reaction kinetic constants of char surface reactions of A, B and C are different. The kinetic constants of moisture evaporation do not vary with coal type as the moisture evaporation process is assumed to be similar to water droplet evaporation. Kinetic constants of devolatilization (1) and (2) are taken from Ubhayakar et al. [15]. These constants are only valid for high volatile coals, such as lignite coal and bituminous coal. The heat generation for each reaction, latent heat of moisture evaporation and devolatilization heat absorption of coal particles are given in Table 4.

4. Numerical solution procedure Simulation of the pulverized coal combustion process involves modeling a number of complex inter-relating Table 4 Heat generation and absorption of physical and chemical process

C1 1.44

C2 1.92

Cm 0.09

sk 1.0

s1 1.3

CEBU 1.0

QCH4 (J/kg) 5:0 £ 107

QCO (J/kg) 1:0 £ 107

LW (J/kg) 22:257 £ 106

QV1 (J/kg) 21:675 £ 106

sY 1.0

sh 1.0

sp 0.35

Cpk 0.75

Cmp 0.0064

skp 1.0

QV2 (J/kg) 28:37 £ 105

QA (J/kg) 1:287 £ 107

QB (J/kg) 7:656 £ 106

QC (J/kg) 23:682 £ 106

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Fig. 2. Sub-models and interaction correlations.

processes. The comprehensive model needs to account for the turbulent gas-particle flow, gas phase combustion, coal particle mass transfer due to moisture evaporation, devolatilization, char combustion and radiation heat transfer. Fig. 2 outlines the coupling and correlation of the sub-models for turbulence, radiation heat transfer, turbulent combustion and coal particle phase characteristics. With the pure multi-fluid model, both gas phase and particle phase conservation equations in Eulerian coordinates are integrated in the computational cell to obtain the finite-difference equations. The gas phase equations are solved by means of the SIMPLE algorithm of Patankar [19], i.e. p –v corrections with TDMA line-by-line iterations and a staggered grid system, in which an upwind scheme is used. A similar procedure is used for the particle phase, but without p –v corrections. Multiple iterations between the gas phase and the particle phase are adopted based on a twoway coupling (Eulerian gas –Eulerian particle) to achieve convergence of the two phases.

the friction effect between the coal particles and the wall of inner tube at the exit. Oxygen is injected into the combustor through an annulus between the two tubes with a high velocity and low temperature, having a self-cool effect on the whole lance, which would prolong the lifespan of the lance. Along the divergent exit of the inner tube, the large velocity difference between the primary flow and the slanted oxygen jet causes reversed flow near the oxygen –coal lance exit and enhances mixing of the coal particles and oxygen. 5.2. Inlet conditions Numerical simulations of isothermal gas-particle flows and pulverized coal combustion are carried out in an oxygen – coal tubular combustor. Parameters for the isothermal and pulverized coal reacting flow are shown in Table 5. In simulating coal combustion, the type of pulverized coal burnt in the combustor is bituminous coal

5. Results and discussion 5.1. Configuration of the oxygen – coal combustor A new type of oxygen – coal tubular combustor is shown schematically in Fig. 3. The combustor is characterized by co-axial tubes with a slanted divergent exit in the inner tube, so that the exit of the coal injection is enlarged and the primary injection velocity of coal is reduced. This helps to lengthen the coal particle residence time in the tubular combustor as well as to reduce

Fig. 3. Schematic diagram of the slanted oxygen jet tubular coal combustor.

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Table 5 Inlet flow parameters Isothermal flows Substitutive flow material Main flow velocity Vmain Oxygen jet velocity Vjet Primary jet velocity Vprimary Oxygen jet angle Flow temperature Particles mean diameter Particle rate/primary air rate Pressure at the combustor

Air 10.0; 10.5 ms21 13.6; 13.8; 15 ms21 3.3; 5.2 ms21 208 300 K 70 mm 0.75 0.1 MPa

Pulverized coal combusting flows Hot blast velocity Oxygen jet velocity Primary coal jet velocity Oxygen jet angle Blast temperature Oxygen jet temperature Primary coal jet temperature Coal rate Pressure at the combustor

164 ms21 100 ms21 11 ms21 208 1300 K 300 K 300 K 500 kg/hr 0.26 MPa

and its proximate analysis is shown in Table 6. The computational domain is taken from the exit of lance to the exit of the combustor and a grid system of 50 £ 14 is adopted. 5.3. Isothermal gas-particle flow Initially, mathematical model is used to simulate isothermal flows in the slanted combustor. Fig. 4 shows the predicted gas phase axial velocity distributions together with the experimental results of Wang et al. [20] under three different flow conditions. In Wang et al.’s experiment, alumina powder of mean diameter of 70 mm was used. The agreement between the experimental and predicted gas phase axial velocity is good and the central recirculation zone is well predicted. It also correctly predicts that this recirculation zone becomes larger as the velocity of the oxygen jet increases, while it becomes smaller as the velocity of the primary jet increases. This re-circulation helps to lengthen the coal particle residence time and enhances the mixing between oxygen and coal particles. Fig. 5 shows the predicted particle phase bulk density distributions together with the experimental data in isothermal flow. The predicted particle phase concentration has Table 6 Proximate analysis of coal Moisture (%) Volatile matter (%) Fixed carbon (%) Ash (%)

1.5 25.4 44.2 28.9

Fig. 4. Gas phase axial velocity distribution in isothermal flow (a) Vmain ¼ 10:0 m=s; Vjet ¼ 13:8 m=s; Vprimary ¼ 3:3 m=s (b) Vmain ¼ 10:5 m=s; Vjet ¼ 15:0 m=s; Vprimary ¼ 5:2 m=s (c) Vmain ¼ 10:5 m=s; Vjet ¼ 13:6 m=s;  Vprimary ¼ 5:2 m=s:

a peak in the central region of the tubular combustor and is in good agreement with the experimental results. The high concentration of particle also enhances the mixing of coal and oxygen and increases the combustion efficiency in the blast furnace. 5.4. Reacting gas-particle flow Flow pattern affects the mixing process, heat and mass transfer, and subsequently local combustion conditions. Fig. 6 shows the axial velocity distributions of both gas and particle phases for different coal particle sizes in the reacting flow. For 10 mm particles, due to coal evaporation, devolatilization and char combustion, the coal particles reduce in mass and result in smaller velocity slip between the gas phase and particle phase. Also, devolatilization takes place rapidly, and there exists volatile combustion. This results in gas phase expansion and the acceleration of both gas and particle phases in the central region is high. At

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Fig. 5. Particle phase bulk density distribution in isothermal flow.

the exit of the tubular combustor, velocities of gas and particle phases near the centre reach 130 and 128 ms21, respectively. For the 40 and 70 mm particles, the velocity slip is larger than that of the 10 mm particles. Combustion does not take place in the combustor and the velocities of both phases in the center remain relatively low. At the exit of the combustor, the particle phase velocities are 79 and 71 ms21 for 40 and 70 mm coal particles, respectively. Fig. 7 shows the gas phase velocity field in the region near the oxygen – coal lance exit for injecting different coal particles in the reacting flow. For the injection of 10 mm coal particles, due to the high heating rate of coal particles and gas phase combustion, the effect of gas phase expansion eliminates the central reversed flow. For the injection of 40 and 70 mm coal particles, reversed flow exists near the center of the lance exit of the tubular combustor. This recirculation zone enhances mixing between the surrounded oxygen and the primary jet.

5.5. Gas phase temperature distributions In a blast furnace, hot blast constitutes the main mechanism for coal particle heating. The mixing of primary stream of pulverized coal and hot blast is identified by the gas phase temperature distribution. Fig. 8 shows the temperature distributions of the gas phase for different coal particles in the reacting flow. When 10 mm pulverized coal particles are injected, the temperature of the gas phase rises rapidly from 300 to 2000 K along the axis of the tubular combustor. The temperature of the hot blast remains at 1300 K. As the velocity of the hot blast is 164 ms21 and the residence time is only about 2.78 ms in the tubular combustor, the heating effect of combustion near the centre to the blast is negligible. When 40 or 70 mm pulverized coal particles are injected, the primary jet and the hot blast mix without any combustion. The carrier gas of the primary jet is only heated to 800 and 760 K, respectively, at the exit of the combustor. In addition, the temperature is relatively low in the oxygen jet dominated region. This helps to keep the lance cool, thus prolonging the lifespan of the lance.

Fig. 6. Axial velocity distributions of gas and particle phase in reacting flow (a) injecting 10 mm coal particles (b) injecting 40 mm coal particles (c) injecting 70 mm coal particles.

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Fig. 8. Gas phase temperature distribution (a) injecting 10 mm coal particles (b) injecting 40 mm coal particles (c) injecting 70 mm coal particle.

5.6. Heating rate of coal particles

Fig. 7. Gas phase velocity field (a) injecting 10 mm coal particles (b) injecting 40 mm coal particles (c) injecting 70 mm coal particles.

Devolatilization and char surface reaction are strongly affected by the coal particle-heating rate. Fig. 9 shows the temperature distributions of particle phase for different coal particles. In general, the coal particle phase temperature distribution is similar to that of the gas phase. When 10 mm pulverized coal particles are injected, temperature of the particle phase is lower than that of gas phase. This indicates that the main combustion mechanism in the tubular oxygen –coal combustor is gas phase combustion, and the direction of heat transfer is from the gas phase to the coal particles. Compared with gas combustion, the heterogeneous char combustion is relatively weak. From the simulation, residence time at the axis reaches 6.5 ms. As the coal particle temperature changes from 300 K at the exit of

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the oxygen–coal combustor, the heating rate of coal particles is reduced. Predicted results of heating rates are lower than the values obtained by Jamaluddin et al. [8] and Suzuki et al. [3].

5.7. Mixing of coal and oxygen As the coal particle residence time is too short in the blowpipe of a blast furnace, main combustion and gasification take place in the raceway of the blast furnace. Under the blast furnace operating condition, the gas phase is in a reduced oxygen environment and the efficiency of coal particle combustion and gasification depends strongly on the mixing of oxygen and coal. If mixing of oxygen and coal is adequate, the combustion of the injected coal particles with oxygen will take place. Thus, sufficient

Fig. 9. Coal particle phase temperature distribution (a) injecting 10 mm coal particles (b) injecting 40 mm coal particles (c) injecting 70 mm coal particles.

the lance to 1970 K at the exit of the combustor, the mean particles heating rate is 2.57 £ 105 K s21. When 40 and 70 mm pulverized coal particles are injected, coal particle temperature at the exit of the tubular combustor rises to 750 and 670 K, respectively. For 40 mm coal particles, at the center, the coal particles residence time is 9.07 ms and the mean heating rate of the coal particles is 4.96 £ 104 K s21, which is lower than when 10 mm coal particles are injected. For 70 mm coal particles, the coal particle residence time is 9.98 ms along the center and the mean heating rate is only 3.71 £ 104 K s21, which is again lower than when 10 and 40 mm pulverized coal particles are injected. The heating rate of coal particles is mainly dependent on the mixing between the carrier gas and the hot blast. As the slanted oxygen jet has a cooling effect in the hot blast of

Fig. 10. Oxygen mass fraction distribution (a) injecting 10 mm coal particle (b) injecting 40 mm coal particle (c) injecting 70 mm coal particle.

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mixing ensures high efficiency of injected coal combustion and gasification in the raceway of blast furnace. In this paper, oxygen mass fraction and coal particle phase bulk density distributions are shown in Figs. 10 and 11. Coal particle phase concentration remains high in the central region of the tubular combustor and concentration of oxygen in this region is also high. The rate of turbulent mixing is high due to the high velocity difference between the primary jet and the oxygen jet at the exit of the lance. When 10 mm coal particles are injected, the oxygen–coal mixing process is almost complete at an axial location of 50 mm from the exit of the lance. At the central region behind the lance, the oxygen mass fraction reaches 60%, and with volatile and char combustion, the oxygen concentration is gradually reduced. When 40 and 70 mm coal particles are injected, the oxygen mass fraction

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Fig. 12. Volatile (CH4) mass fraction distribution for the injection of 10 mm coal particles.

reaches a high level of about 60% near the primary jet exit due to the presence of the central reversed flow. Although there is no devolatilization and combustion in the tubular combustor due to the low heating rates in these two cases. In the raceway, the efficiency of coal combustion and gasification remains high due to adequate mixing of coal and oxygen. 5.8. Devolatilization and combustion of small coal particles Even when there is sufficient mixing and heating for coal particles larger than 40 mm, combustion does not take place in the combustor. However, due to high heating rate of the small coal particles of 10 mm, devolatilization and combustion takes place. Fig. 12 shows the volatile (CH4) concentration distribution when 10 mm coal particles are injected. With coal devolatilization, the volatile concentration rises gradually and reaches a maximum value of 25%. Volatile combustion begins at an axial location of about 200 mm from the exit of lance, where there is a high temperature gradient as showed in Fig. 8(a). Due to the short residence time, volatile combustion is not complete and the concentration of volatiles is still high at the exit of the tubular combustor. Fig. 13 gives carbon monoxide (CO) mass fraction distribution. With the coal particle temperature rising at the end of coal devolatilization, char combustion exists at the surface of coal particles. Concentration of carbon monoxide rises along the axis in the central zone of the tubular

Fig. 11. Coal particle phase bulk density distribution (a) injecting 10 mm coal particle (b) injecting 40 mm coal particle (c) injecting 70 mm coal particle.

Fig. 13. Carbon monoxide (CO) mass fraction distribution for the injection of 10 mm coal particles.

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Fig. 14. Coal particle mass and temperature profile along the axis for the injection of 10 mm coal particles (a) Coal particle mass change along axis (b) Coal particle temperature change along axis.

combustor. From the axial location of 300 mm to the exit of the combustor, coal particle temperature is higher than 1600 K, and the char reaction of 2C þ O2 ! 2CO is the main reaction. Even though the coal particle temperature is high, the residence time is too short to sustain char combustion due to the high velocity. Fig. 14 shows coal particle mass and temperature distributions along the axis of the tubular combustor when 10 mm coal particles are injected. Initially, the coal particles are heated before the axial location of 150 mm. Rapid devolatilization occurs at the axial location of between 150 and 250 mm, resulting in rapid mass reduction. Subsequently, the reduction of coal particle mass is caused by char surface reaction, and this reaction is much slower than the devolatilization process. Coal devolatilization is therefore the dominant process in the tubular oxygen–coal combustor.

6. Conclusions A mathematical model for pulverized coal combustion is developed based on a pure Eulerian model. This model is applied to the simulation of isothermal gas flow, isothermal gas-particle flow and pulverized coal combustion in a tubular oxygen – coal combustor. Simulated results show good agreement with experimental data in isothermal flow. It shows that there is a central

recirculation zone in front of the oxygen –coal lance under isothermal flow conditions, and particle phase concentration in this region is high. This recirculation zone enhances the mixing of coal and oxygen, lengths the coal particle residence time and enhances pulverized coal combustion efficiency. Simulations of gas-particle flows and pulverized coal combustion in a real blowpipe condition of a blast furnace show that the particle residence time is about 6.5– 10 ms in the combustor. The coal particle size has remarkable effects on coal devolatilization and char combustion. Small particles have a higher heating rate, resulting in more rapid devolatilization, and volatile combustion takes place. Due to the short residence time, volatile combustion is not complete, char combustion is rather weak and the coal devolatilization process is the dominant process in the tubular combustor. For large coal particles, only mixing and heating occur in the combustor. The slanted oxygen jet produces a central recirculation zone, which enhances the mixing of oxygen and coal. The high coal particle phase concentration and oxygen mass fraction in the central coal – oxygen mixing region enhance coal combustion and gasification in the raceway of the blast furnace. In addition, the temperature in the slanted oxygen jet dominated region is relatively low. This keeps the lance cool and thus prolongs the lifespan of the lance.

Acknowledgements This work was partially supported by Beijing Natural Science Foundation (No. 3982008) and the Research Committee of The Hong Kong Polytechnic University (Project Account Code G-YD39)

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