A Numerical Analysis of Pulverized Coal Combustion in a Multiburner

A three-dimensional numerical simulation is applied to a pulverized coal combustion field in a furnace equipped ... is clearly the dominant market for...
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1950

Energy & Fuels 2007, 21, 1950-1958

A Numerical Analysis of Pulverized Coal Combustion in a Multiburner Furnace Nozomu Hashimoto,*,† Ryoichi Kurose,‡ Hirofumi Tsuji,† and Hiromi Shirai† Energy Engineering Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196, Japan, and Department of Mechanical Engineering and Science, Kyoto UniVersity, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan ReceiVed March 27, 2007. ReVised Manuscript ReceiVed May 22, 2007

A three-dimensional numerical simulation is applied to a pulverized coal combustion field in a furnace equipped with three burners, and the trajectories of the coal particles with respect to each burner, which are hardly obtained experimentally, are also investigated in detail. Simulation results are compared with experimental results. The results show that the numerical and experimental results are consistent generally. Also, the examination of the particle trajectories shows that most of the unburned carbon originates from the upperstage burner. This result suggests that the overall unburned fraction can be reduced by supplying coal with a low combustibility to lower- or middle-stage burners and supplying coal with a high combustibility to the upper-stage burner.

Introduction Coal is an important energy resource from the viewpoint of energy security, since coal quarries and mines exist in various regions all over the world and the number of coal deposits is much greater than those of other fossil fuels. According to IOE2006,1 it is speculated that world coal consumption will be nearly doubled from 2003 to 2030. Since a large part of coal is consumed by a method of pulverized coal combustion, the beneficial uses of the fly ash collected from the electrostatic precipitator of pulverized-coalfired thermal power plants are increasingly important. Most of the fly ash is utilized as a cement or concrete additive, and this is clearly the dominant market for the ash.2 The utilization of fly ash in concrete is beneficial to the properties of the concrete due to the pozzolanic nature of the fly ash.3,4 However, when fly ash contains a large amount of unburned carbon, the adsorption of the air-entraining admixtures, which are special surfactants used to stabilize air bubbles in concrete mixtures, destroys the ability of the concrete to hold the required air.5-7 Recently, combustion techniques for reducing NOx emissions, such as staged combustion, have been utilized in the pulverizedcoal-fired plant. However, the staged combustion tends to increase the residual carbon levels in fly ash. For the promotion of fly ash utilization, therefore, the reduction of unburned carbon * Corresponding author. Phone: +81-46-856-2121. Fax: +81-46-8575829. E-mail: [email protected]. † CRIEPI. ‡ Kyoto University. (1) International Energy Outlook 2006; Energy Information Administration: Washington, DC, 2006; pp 51-61. (2) Manz, O. E. Fuel 1997, 76, 691-696. (3) Yamamoto, T.; Kanazu, T.; Nambu, M.; Tanosaki, T. Fuel 2006, 85, 2345-2351. (4) Rixom, R.; Mailvaganam, P. Chemical Admixtures for Concrete, 3rd ed.;. E.&F.N. Spon, Ltd: London, 1999. (5) Ku¨laots, I.; Hsu, A.; Hurt, R. H.; Suuberg, E. M. Cem. Concr. Res. 2003, 33, 2091-2099. (6) Ku¨laots, I.; Hurt, R. H.; Suuberg, E. M. Fuel 2004, 83, 223-230. (7) Freeman, E.; Gao, Y. M.; Hurt, R.; Suuberg, E. Fuel 1997, 76, 761765.

in fly ash without any increase in NOx emission under the conditions of staged combustion is desired. This requires the promotion of the combustion reaction in the region upstream of the second-stage combustion region, where a reduced atmosphere is formed. The development of the combustion technology meeting this demand has been conducted at the Central Research Institute of Electric Power Industry (CRIEPI).8 To make a further improvement, an understanding of the flow field and coal particle behaviors in the furnace is necessary. However, it is difficult to obtain such information solely on the basis of experiments. To understand and predict the detailed characteristics of the pulverized coal combustion field in furnaces, a computational fluid dynamics simulation of pulverized coal combustion has been performed recently.9-22 We have performed numerical simulations of pulverized coal combustion fields in a singleburner furnace by duplicating the complex shape of the burner (8) Makino, H.; Matsuda, H. CRIEPI ReV. 2002, 46, 1-122. (9) Kurose, R.; Makino, H.; Suzuki, A. Fuel 2004, 83, 693-703. (10) Kurose, R.; Ikeda, M.; Makino, H. Fuel 2001, 80, 1447-1455. (11) Kurose, R.; Tsuji, H.; Makino, H. Fuel 2001, 80, 1457-1465. (12) Kurose, R.; Ikeda, M.; Makino, H.; Kimoto, M.; Miyazaki, T. Fuel 2004, 83, 1177-1785. (13) Eaton, A. M.; Smoot, L. D.; Hill, S. C.; Eatough, C. N. Prog. Energy Combust. Sci. 1999, 25, 387-436. (14) Abbas, T.; Costa, M.; Costen, P.; Godoy, S.; Lockwood, F. C.; Ou, J. J.; Romo-Millares, C.; Zhou, J. Fuel 1994, 73, 1423-1436. (15) Bosoga, A.; Panoiu, N.; Mihaescu, L.; Backreedy, R. I.; Ma, L.; Pourkashanian, M.; Williams, A. Fuel 2006, 85, 1591-1598. (16) Williams, A.; Backreedy, R.; Habib, R.; Jones, J. M.; Pourkashanian, M. Fuel 2002, 81, 605-618. (17) Asotani, T.; Tominaga, H. J. Jpn. Inst. Energy 2004, 83, 329-334 (in Japanese). (18) Backreedy, R. I.; Jones, J. M.; Ma, L.; Poukashanian, M.; Williams, A.; Arennillas, A.; Arias, B.; Pis, J. J.; Rubiera, F. Fuel 2005, 84, 21962203. (19) Zhou, L. X.; Li, L.; Li, R. X.; Zhang, J. Powder Technol. 2002, 125, 226-233. (20) Pallares, J.; Arauzo, I.; Diez, L. I. Fuel 2005, 84, 2364-2371. (21) Backreedy, R. I.; Jones, J. M.; Pourkashanian, M.; Williams, A. Fuel 2003, 82, 2097-2105. (22) Xu, M.; Azevedo, J. L. T.; Carvalho, M. G. Comput. Methods Appl. Mech. Eng. 2001, 190, 3581-3590.

10.1021/ef070151o CCC: $37.00 © 2007 American Chemical Society Published on Web 06/29/2007

Numerical Analysis of PulVerized Coal Combustion

Energy & Fuels, Vol. 21, No. 4, 2007 1951

Figure 1. Schematic diagram of test furnace.

it is released from the outlet or completely burned. In processing the particle tracking data statistically, the unburned fraction for each stage burner, which can hardly be obtained experimentally, is estimated. Furthermore, suggestions on the operating method for reducing the total unburned fraction of a multiburner furnace are given. Numerical Simulation

Figure 2. Schematic of computational domain. Table 1. Coal Properties coal

Senakin

Enshu

Newlands

Proximate Analysis [wt %] (4.2) (2.8) 43.1 36.3 43.2 52.2 13.7 11.5 1.00 1.44

(1.9) 28.5 57.2 14.3 2.01

Ultimate Analysisb [wt %] carbon 70.0 70.8 hydrogen 5.6 4.8 nitrogen 1.40 1.34 oxygen 8.7 11.4 combustible sulfur 0.59 0.18 heating value (low)b [MJ/kg] 27.7 28.4

73.3 4.4 1.54 6.2 0.29 28.5

moisturea volatile matterb fixed carbonb ashb fuel ratio [-]

a

As received. b Dry basis.

and investigated the effects of coal properties on the combustion characteristics in detail.9-12 In this study, a numerical simulation is applied to a pulverized coal combustion field in a multiburner furnace, and the trajectories of coal particles with respect to each burner are investigated in detail. Some researchers have performed numerical simulations for large-scale furnaces with a multiburner system.17-22 As far as we know, however, most of them did not discuss the combustion characteristics for each burner. In this study, on the other hand, the combustion characteristics for each burner are evaluated by tracking each coal particle until

Computational Domain and Conditions. The furnace studied here is that at the Energy Engineering Research Laboratory of CRIEPI, in which three advanced low-NOx burners (CI-R burner9), each with a coal combustion capacity of about 100 kg/h, are vertically installed (Figure 1). The furnace is a water-cooled furnace made of steel in which refractory materials are placed on the inside wall. The height, horizontal width, and depth of this furnace are 11, 0.9, and 1.9 m, respectively. The configuration of the computational domain is shown in Figure 2. This was designed to faithfully match the actual configuration. Combustion air was injected into the furnace through the burner and staged combustion air ports located 2.0 m downstream from the upper-stage burner outlet. The air passing through the burner was divided into primary, secondary, and tertiary air. Primary air, which carries pulverized coal, had a straight motion, and secondary and tertiary air had strong swirling motions. The swirl vane angles for secondary and tertiary air were set at 72° and 63°, respectively, which are optimum values for bituminous coal (these values are zero when the swirl force is zero). The operating conditions of the furnace in the simulation were given to correspond with those in our experiment.23 The thermal input of the coal combustion test furnace was 8.62 × 106 kJ/h (the coal feed rate for each burner was approximately 100 kg/h). The air ratio was 1.24, and the excess O2 concentration at the furnace outlet was 4.0%. The staged combustion air ratio was set at 30%. The mass ratio of the pulverized coal (dry base) to the primary air was 1:2.2, and the mass ratio of secondary air to tertiary air was 1:6. Three bituminous coals with fuel ratios of 1.0, 1.4, and 2.0 were chosen for the test fuels. The properties of the test fuels are listed in Table 1. Mathematical Models and Numerical Method. The simulation was performed using the STAR-CD code with a NO reduction model incorporated as a user subroutine. The mathematical models and numerical method employed here are as follows.9-11 The gas-phase turbulence was represented by the renormalization group k- model,24,25 which is believed to give more accurate results (23) Tsuji, H.; Kotsuji, T.; Ikeda, M.; Sirai, H. CRIEPI report 2006, M05006 (in Japanese). (24) Yakhot, V.; Orszag, S. A. J. Sci. Comput. 1986, 1, 3-51. (25) Yakhot, V.; Orszag, S. A.; Thangam, S.; Gatski, T. B.; Speziale, C. G. Phys. Fluids 1992, 4, 1510-1520.

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for swirl and other highly strained flows than the generally used k- two-equation model.26 The gas-phase time-averaged continuity equation and conservation equations of momentum, turbulent kinetic energy, dissipation, enthalpy, and species are ∂ (FUi) ) 0 ∂xi

(1)

( )

(2)

∂ ∂φ ∂ (FUiφ) ) Γ + Sφ + Spφ ∂xi ∂xi φ ∂xi

where φ denotes the generalized variables expressing fluid velocity components Ui, the turbulent kinetic energy k, the rate of eddy dissipation , the fluid enthalpy h, and the mass fractions of chemical species Yi. Γφ denotes the turbulent exchange coefficient, and Sφ and Spφ represent the gas-phase source terms that are in addition to the convection and diffusion terms and the particlephase source terms, respectively. The actual forms of these terms are provided in other papers.13,24-26 The continuity and momentum equations were solved using the PISO algorithm.27,28 The nonlinear terms in the conservation equations were approximated by a firstorder upwind scheme, and a second-order central difference scheme was employed for other derivatives. The equation of motion for the representative coal particles is given by dUpi 1 mp ) Cd FpAp|Uf j - Upj|(Ufi - Upi) dt 2 24(1 + 0.15Rep0.687) Cd ) Rep Rep )

Dp|Ufi - Upi| ν

(3)

(4)

(5)

where mp, Upi, Fp, Ap, and Dp are the particle mass (kg), velocity components (m/s), density (kg/m3), projected area (m2), and diameter (m), respectively. Cd and Rep represent the particle drag coefficient (-) and particle Reynolds number (-), respectively, where ν is the kinematic viscosity of the fluid (m2/s). The effect of the turbulence of the gas phase on particle motion was modeled by a stochastic method.29 The particle temperature Tp (K) was calculated by considering the heat transfer due to convection, radiation, heat loss due to devolatilization in coal, and heat gain due to char combustion, using the following equation: mp

d(cp,pTp) dmp ) -Asqp′′ + Aspσ(ΘR4 - Tp4) + hfg + Qp dt dt

(6)

where As, Cp,p, σ, hfg, p, and Qp are the particle surface area (m2), specific heat (J/kg K), Stefan-Boltzmann constant () 5.670 × 10-8 J/m2 s K4), latent heat of the phase change of moisture (J/kg; this term is negligible in this study), absorptivity of coal particles (-), and heat gain due to the char combustion (J/s). qp′′ is the surface heat flux defined by qp′′ ) h(Tp - Tg), where h, Tp, and Tg are the heat transfer coefficient (J/m2 s K) and particle and gas temperatures (K), respectively. ΘR was calculated as ΘR ) (I/4σ)1/4, where I is the radiant intensity (J/m2 s). The radiant intensity I was calculated by the discrete transfer radiation method of Lockwood and Shah,30 which simulates the radiative heat transfer among the gas, particles, and wall. In this study, the absorptivities of the coal particles and wall are assumed to be 0.85 and 0.4, respectively. Also, the (26) Launder, B. E.; Spalding, D. B. Comput. Methods Appl. Mech. Eng. 1974, 3, 269-289. (27) Issa, R. J. Comput. Phys. 1986, 62, 40-65. (28) Issa, R.; Gosman, A.; Watkins, A. J. Comput. Phys. 1986, 62, 6682. (29) Gosman, A. D.; Ioannides, E. AIAA Pap. 1981, 81-0323. (30) Lockwood, F. C.; Shah, N. G. Proc. Combust. Inst. 1981, 18, 14051414.

Figure 3. Concept of pulverized coal combustion. Table 2. Parameters in Eq 8 Obtained Using the FLASHCHAIN Model

coal

particle diameter (µm)

temperature of DTF (°C)

AV (1/s)

EV (J/mol)

Newlands Enshu Senakin

38.6 42.5 45.6

1400 1400 1400

9.78 × 104 5.75 × 104 1.50 × 105

4.66 × 104 3.88 × 104 5.06 × 104

absorption coefficient of the gas was set at 0.075 m-1. The interaction of the conserved properties between the gas phase and coal particles was calculated by the particle-source-in-cell technique.31 The schematic diagram of the pulverized coal combustion simplified for this calculation is shown in Figure 3. Coal devolatilization was simulated by a first-order single reaction model (arrow 1 in Figure 3): dV ) KV(V* - V) dt

( )

KV ) AV exp -

EV RTp

(7)

(8)

where V* and V indicate the total volatile matter content in the coal (kg) and volatile mass released from the coal (kg), respectively, and R is the universal gas constant () 8.31 J/mol K). The values of pre-exponential factor AV (1/s) and activation energy EV (J/mol) in eq 8 strongly affect the simulation results.32 It is known that appropriate values for AV and EV are largely dependent on coal properties.33 In this study, these values for each coal were obtained by pyrolysis simulation using a FLASHCHAIN model,34 which can simulate the volatile matter evolution process of coal in a drop tube furnace (DTF). The obtained values are listed in Table 2. The mass median particle diameter measured in the experiment was employed as the diameter of a coal particle for the pyrolysis simulation. The temperature of DTF was set at 1400 °C for all cases. Gaseous combustion between the volatilized fuel and air was calculated using the combined model of kinetics and eddy dissipation models35 (arrows 3-5 in Figure 3). The chemical mechanism consists of two global reactions: CaHbOc + O2 f RCO + βH2O (31) Crowe, C. T.; Sharma, N. P.; Stock, D. E. Trans. ASME J. Fluids Eng. 1977, 99, 325-332. (32) Gera, D.; Mathur, M.; Freeman, M. Energy Fuels 2003, 17, 794795. (33) Niksa, S.; Kerstein, A. R. Energy Fuels 1991, 5, 647-665.

Numerical Analysis of PulVerized Coal Combustion CO + O2 f CO2

Energy & Fuels, Vol. 21, No. 4, 2007 1953 (9)

where a, b, c, R, and β are governed by the coal constituents given in Table 1. Regarding the kinetics, the rate of reaction for reactants such as CaHbOc and CO is given as an Arrhenius expression:

( )

Eg Rg ) Ag exp [Reactant]d[O2]e RTg

(

)

Kd )

(

)

5.06 × 10-7 Tp + Tg Dp 2

( )

Kc ) Ac exp -

Ec RTp

Ag [1/s]

Eg [J/mol]

d [-]

e [-]

CaHbOc CO

2.8 × 9.0 × 1013

2.023 × 1.676 × 105

-0.3 1

1.3 0.25

109

105

Table 4. Initial Diameter, Mass Fraction, and Number of Representative Coal Particles

(10)

where [ψ] means the mol fractions of chemical species ψ. The values of the pre-exponential factor Ag (1/s), activation energy Eg (J/mol), and orders d and e are listed36 in Table 3. The char burning rate was calculated using Field’s model37 (arrow 2 in Figure 3): KcKd dC P πD 2 )dt Kc + K d g p

Table 3. Parameters in Eq 10 reactant

(11)

0.75

(12)

(13)

where C is the char mass (kg), Kc and Kd are the surface reaction rate coefficient and diffusional reaction rate coefficient [(kg/m2 s)/ Pa], respectively, and Pg is the partial pressure of oxygen in the bulk gas (Pa). This model is obtained under the assumption that the char burning rate is controlled by both the chemical reaction rate and the rate of diffusion of oxygen to the surface of the char particle. The values of the kinetic parameters, namely, the preexponential factor Ac [(kg/m2 s)/(N/m2)] and activation energy Ec (J/mol), in eq 13 were determined in such a manner that the predicted and experimental unburned fractions for Newlands coal are consistent, which are Ac ) 1.1 × 10-2 (kg/m2 s)/(N/m2) and Ec ) 5.0 × 104 J/mol, respectively. The NOx formation model was typically employed in a “postprocessing” fashion, where a converged combustion flow field solution is first obtained before performing the NO prediction.13 For the formation of NOx, three different mechanisms were employed, namely, Zeldovich NOx, prompt NOx, and fuel NOx formation mechanisms indicated by arrows 6-9 in Figure 3, respectively. Zeldovich NOx and prompt NOx are classified as thermal NOx. Here, only the production of NO was taken into account because the NOx emitted to the atmosphere from combusting fuels consists mostly of NO, with much lower concentrations of NO2 and N2O. Zeldovich NO (arrow 6 in Figure 3) was evaluated by applying the quasi-steady-state approximation for N species to the extended Zeldovich mechanisms (N2 + O f NO + N, N + O2 f NO + O, and N + OH f NO + H) with the rate constants used by Baulch et al.38 The O concentration required is given by Westenberg’s expression.39 Prompt NO (arrow 7 in Figure 3) and fuel NO (arrows 8 and 9 in Figure 3) were both calculated using De Soete’s models.40,41 Fuel NO originating from volatile N was assumed to be formed through instantaneous evolution in the form of HCN, and the minor evolution of NH3 was neglected. (34) Niksa, S. Combust. Flame 1995, 100, 384-394. (35) Magnussen, B. F.; Hjertager, B. W. Proc. Combust. Inst. 1976, 16, 719-729. (36) Borman, G. L.; Ragland, K. W. Combustion Engineering; McGrawHill: New York, 1998; pp 120-122. (37) Field, M. A. Combust. Flame 1969, 13, 237-252. (38) Baulch, D. L.; Drysdall, D. D.; Horne, D. G.; Lloyd, A. C. EValuated Kinetic Data for High Temperature Reactions; Butterworths: Markham, ON, Canada, 1973. (39) Westenberg, A. A. Combust. Sci. Technol. 1971, 4, 59-64. (40) De Soete, G. G. Proc. Combust. Inst. 1975, 15, 1093-1102. (41) De Soete, G. G. Proc. Combust. Inst. 1990, 23, 1257-1264.

coal

Dp [µm]

MFp [%]

Np [-]

Newlands (mass-based mean diameter: 38.6 mm)

10 20 40 60 80 100 10 20 40 60 80 100 10 20 40 60 80 100

10.62 15.12 25.76 17.39 23.13 7.98 4.63 12.59 29.29 24.14 22.56 6.79 0.99 9.12 30.99 27.62 21.35 9.93

200 280 400 320 400 160 120 200 400 360 320 120 40 160 400 400 320 160

Enshu (mass-based mean diameter: 42.5 mm)

Senakin (mass-based mean diameter: 45.6 mm)

The reduction of NO by char was considered in this study. The NO-char reaction has been investigated by various researchers42 and has been believed to have a significant effect on NO reduction in a pulverized coal combustion environment.43 The rate of NO reduction by char is expressed as follows:44

( )

Ere d [NO] ) Are exp A P dt RT E NO

(14)

where [NO] is the NO mole fraction (mol), AE is the external surface area of char in a unit volume (m2/m3), and PNO is the NO partial pressure (atm). The pre-exponential factor Are and the activation energy Ere were set at Are ) 1.1 × 10-2 (m mol)/(s atm) and Ere ) 1.45 × 105 J/mol,45 respectively (this NO reduction model is incorporated into the original STAR-CD code as a user subroutine). It was assumed that the pulverized coal particles consisted of particles with diameters Dp of 10, 20, 40, 60, 80, and 100 µm. The mass fractions MFp of coal particles and their representative coal particle numbers Np are shown in Table 4. The temperature and velocity of the particles at the burner inlet were equal to those of primary air. The temperature of primary combustion air and coal particles was set to be 353 K, and that of secondary and tertiary combustion air was 623 K. Regarding the boundary condition on the wall, the temperature outside the furnace was assumed to be 308 K, and thermal resistance was set at 0.04 m2 s K/J. The number of fluid cells was about 300 000, and the cells near the burners were refined. The CPU time required for this simulation was about 500 h.

Results and Discussion Comparison with Experiments. Figure 4 shows the predicted gas velocity vectors in the case of Newlands coal. The colors of the vectors indicate the magnitude of the absolute velocity. It is found that recirculation flows are formed in the exit area of each burner (region A in the figure). The recirculation zones are created by the strong swirling flow of the (42) Aarna, I.; Suuberg, E. M. Fuel 1997, 76, 475-491. (43) Hahn, W.; Shadman, F. Combust. Sci. Technol. 1983, 30, 89-104. (44) Scho¨nenbeck, C.; Gadiou, R.; Schwartz, D. Fuel 2004, 83, 443450. (45) Levy, J. M.; Chan, L. K.; Beer, J. M. Proc. Combust. Inst. 1981, 18, 111-120.

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Figure 4. Gas velocity vectors (simulation result for Newlands coal).

Figure 5. Trajectories of coal particles (simulation result for Newlands coal).

secondary and tertiary burner air. Another recirculation flow is also observed in the region above the upper-stage burner (arrow B in Figure 4). It is considered that this recirculation flow is induced by the main gas stream, which flows along the wall opposite the burners (arrow C in Figure 4). Figure 5 shows the trajectories of the coal particles. A total of 180 particles are randomly chosen for display. The colors of the particles indicate the temperature of the particles. Here, only the coal particles containing unburned carbon are plotted, and the particles containing ash only are not shown. The highest particle temperature is observed in the region between the upperstage burner and middle-stage burner. In the region downstream of the staged combustion air ports, coal particles tend to flow outward on the side opposite the burners (right-hand side in the figure). This is because most particles are captured by the main gas stream along the wall opposite the burners (arrow C in Figure 4). Figure 6 shows comparisons of contour plots of gas temperature and chemical species mole fractions between simulations and experiments.23 On the whole, simulated and experimental contour plots are consistent qualitatively, although there are some quantitative discrepancies. In the temperature contour plot (Figure 6a), for example, there is a high-temperature region between the upper-stage and middle-stage burners, and the temperature on the right-hand side is higher than that on the left-hand side in the region downstream of the staged combustion air ports, for both experimental and simulated results. The quantitative difference in peak temperature is considered to be due to the radiative heat loss from the thermocouples46 used for temperature measurement in the experiments.

In the O2 mole fraction contour plot (Figure 6b), the O2 mole fraction is almost 0% near the wall opposite the burners (righthand wall) in the region upstream of the staged combustion air ports. The O2 mole fraction on the right-hand side is lower than that on the left-hand side in the region downstream of the staged combustion air ports. This means that second-stage combustion mainly occurs on the right-hand side of the furnace and the oxygen consumption on the right-hand side is lager than that on the left-hand side. This is confirmed by the fact that, in the region downstream of the staged combustion air ports, coal particles concentrate on the right-hand side, in Figure 5, and the fact that the temperature on the right-hand side is higher than that on the left-hand side, in Figure 6a, because of the heat released by the combustion reaction. In the NO mole fraction contour plot (Figure 6c), it is observed that the NO produced by the combustion in the burner zone is reduced gradually in the region upstream of the staged combustion air ports. In the region downstream of the ports, NO is reproduced by the second-stage combustion. As discussed above, simulated and experimental contour plots are consistent qualitatively for Newlands coal. Unfortunately, since the experimental contour plots for Enshu coal and Senakin coal were not obtained, no comparisons were able to be performed. However, the trends of the combustion fields for Enshu coal and Senakin coal were generally similar to those for Newlands coal. The overall unburned fractions Uc*, at the furnace outlet derived from simulations and experiments are compared in Figure 7. The definition of Uc* is expressed as

(46) Bardley, D.; Matthews, K. J. J. Mech. Eng. Sci. 1968, 10, 299305.

Uc* ) 100 - η )

Uc Cash × × 100 (15) 100 - Uc 100 - Cash

Numerical Analysis of PulVerized Coal Combustion

Energy & Fuels, Vol. 21, No. 4, 2007 1955

Figure 6. Comparisons of contour distributions of gas temperature and O2 and NO mole fractions between simulations and experiments (Newlands coal).

Figure 7. Comparison of overall unburned fractions at the furnace outlet between simulations and experiments (FR: fuel ratio).

where Uc* (%) is the unburned fraction, η (%) is the combustion efficiency, Uc (%) is the unburned carbon concentration in fly ash, and Cash (%) is the ash content in coal. It is well-known that there is a rough trend that Uc* generally increases with the fuel ratio.47 Simulation results in Figure 7 indicate this trend well; that is, Uc* from simulations increases with the fuel ratio. However, the Uc* of Enshu coal from experiments is low compared with that from simulations. This is considered to be due to the fact that the ash content for Enshu coal is lower than that for Newlands coal. Ash in a coal particle can inhibit char combustion by occupying some volume within the particle, thereby reducing the carbon mass and carbon surface available per unit particle volume.48,49 However, the ash inhibition of char combustion is not considered in the current study. As mentioned earlier, kinetic parameters for the char combustion model were determined in such a manner that the Uc* values from experiments and simulations are consistent for Newlands coal (high ash content). Because of this, the simulation underestimated the char combustion rate for Enshu coal (low ash content). Figure 8 shows the comparison of NO mole fractions at the furnace outlet between simulations and experiments. It is found that the simulated and measured NO mole fractions at the furnace outlet are consistent for all cases. (47) Makino, H.; Sato, M.; Kimoto, M. J. Jpn. Inst. Energy 1994, 73, 906-913 (in Japanese). (48) Vleeskens, J. M.; Nandi, B. N. Fuel 1986, 65, 797-802. (49) Hurt, R. H.; Davis, K. A. Proc. Combust. Inst. 1994, 25, 561-568.

Figure 8. Comparison of NO mole fractions at the furnace outlet between simulations and experiments (FR: fuel ratio).

Figure 9. Unburned fraction for each stage burner (simulation result).

Combustion Characteristics for Each Burner. It is considered that the unburned fractions Uc*, differ depending on which stage burner the coal particles are issued from, since their residence times in the furnace differ. Figure 9 shows Uc* from each stage burner. The coal particles were tracked until they were released from the outlet boundary or completely burned. It should be noted that these data can hardly be obtained experimentally but are very useful for examining the new combustion technology. It is found that Uc* for the upper-stage burner is higher than that for the middle- or lower-stage burner, for all cases. This is caused by the difference in particle residence time in the high-temperature region. As seen in Figure 6a, the highest-temperature region is formed between the upper-

1956 Energy & Fuels, Vol. 21, No. 4, 2007

and middle-stage burners. The coal particles supplied from the middle- or lower-stage burner pass through the highesttemperature region, spending a long residence time there. On the other hand, the coal particles supplied from the upper-stage burner leave the high-temperature region immediately, and therefore, the residence time in the high-temperature zone is short. As a result, the combustion reaction of the coal particles supplied from the upper-stage burner is slower than those of coal particles from the other stage burners. Because of this, Uc* for the upper-stage burner is higher than that for the middle- or lower-stage burner. In recent years, coal blending has frequently been conducted in coal-fired plants, to widen the usage capability of the coal brands. There are two methods of coal blending, that is, line blending (where the blending is carried out in the coal banker) and in-furnace blending (where the blending is carried out in the furnace). In the case of line blending, the blending ratio of coal brands is the same for all stage burners. In the case of in-furnace blending, on the other hand, the coal brands are completely different between burners, or the blending ratios of coal brands are different between burners. From the discussion above, it is considered that the overall Uc* of coal-fired plants can be reduced by employing an appropriate in-furnace blend, that is, supplying coal with a low combustibility (for example, high-fuel-ratio coal) to the lower- or middle-stage burner and supplying coal with a high combustibility (low-fuel-ratio coal) to the upper-stage burner. Recently, Ikeda et al.50 have conducted an in-furnace blend experiment using the MARINE furnace. The results show that Uc* for the case of supplying coal with a low combustibility to the lower- and middle-stage burners is lower than Uc* for the case of supplying the coal to the upperstage burner. The current study supports their experimental results. Figure 10 shows the effect of initial particle diameter on the ejection rate of unburned carbon (kg/s) for different stage burners. It is seen that coal particles with initial diameters of 10-40 µm are almost completely burned. Therefore, it is considered that the carbon in ash mostly originates from coal particles with initial diameters greater than or equal to 60 µm. Particularly, the ejection rate for particles with initial diameters of 80 and 100 µm supplied from the upper-stage burner is higher than that from the middle- or lower-stage burner. From the results, it is suggested that the overall Uc* of coal-fired plants can be effectively reduced by preferentially crushing the coal supplied from the upper-stage burner into finer particles. Figure 11 shows the contribution of each stage burner to the total unburned fraction. The contribution of the upper-stage burner is found to be higher than that of the middle- or lowerstage burner for all coal brands. However, the contribution of the upper-stage burner decreases with decreasing fuel ratio. This is considered to be due to the difference in residence time for the coal particles supplied from the upper-stage burner. Figure 12 shows the residence time in the furnace for the coal particles supplied from the upper-stage burner. It can be seen that the residence time for low-fuel-ratio coal brands is longer than that for high-fuel-ratio coal brands. This is caused by the difference in the coal particle trajectory, as discussed below. To clarify the effect of the fuel ratio on the residence time of the particles supplied from the upper-stage burner, the gas velocity vectors and coal particle trajectories for Newlands coal (fuel ratio ) 2.0) and Senakin coal (fuel ratio ) 1.0) are compared. The comparisons of gas velocity vectors and coal (50) Ikeda, M.; Hashimoto, N.; Takahama, H.; Shirai, H. CRIEPI Rep. 2007 (submitted, in Japanese).

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Figure 10. Effect of initial particle diameter on ejection rate of unburned carbon for different stage burners (simulation result).

Figure 11. Contribution of each stage burner to total unburned fraction (simulation result).

Figure 12. Mean residence time in the furnace for the coal particles supplied from the upper stage burner (simulation result).

particle trajectories in the region near the upper-stage burner are shown in Figures 13 and 14, respectively. The trajectories for 10 typical coal particles are shown in Figure 14. In Figure 13, it is seen that the recirculation flow formed above the upper-

Numerical Analysis of PulVerized Coal Combustion

Energy & Fuels, Vol. 21, No. 4, 2007 1957

Figure 13. Effect of coal properties on flow field above the upper-stage burner (simulation result).

Figure 14. Effect of coal properties on particle behavior above the upper-stage burner (simulation result).

stage burner for Senakin coal is larger than that for Newlands coal. This is considered to be caused by the difference in the main gas stream, which flows along the wall opposite the burners. The velocity magnitude of the main gas stream for Senakin coal is larger than that for Newlands coal. As a result, a larger recirculation flow is induced for Senakin coal than for Newlands coal. Because the coal particles supplied from the upper-stage burner pass the region around the recirculation flow, the trajectories of the coal particles are strongly affected by the recirculation flow. The larger the recirculation flow, the greater the number of particles captured by the recirculation flow. Therefore, the number of coal particles captured by the recirculation flow for Senakin coal is larger than that for Newlands coal. As a consequence, the residence time of the coal particles supplied from the upper-stage burner for Senakin coal is longer than that for Newlands coal (Figure 12), and the combustion reaction is promoted. This is the reason why the contribution of the upper-stage burner to the total unburned fraction for Senakin coal is low (Figure 11). The discussion above indicates that the total unburned fraction can be decreased by making the residence time of the coal particles supplied from the upper-stage burner longer.

Conclusion In this study, a numerical analysis in the pulverized coal combustion field in a multiburner furnace was conducted. The accuracy of the numerical analysis was evaluated by comparing simulation and experimental results. The combustion characteristics for each stage burner, which can hardly be obtained experimentally, were also investigated. The principal findings are as follows. Although there are some discrepancies in absolute value, the temperature and chemical species contours from the numerical and experimental results are consistent, which means that the present numerical method is of sufficient accuracy to predict the general characteristics of the pulverized coal combustion field in furnaces with multiburner systems. The unburned fraction for the upper-stage burner is higher than that for the middle- or lower-stage burner. This is due to the fact that the residence time in the high-temperature region of the coal particles supplied from the upper-stage burner is shorter than that in the case of the middle- or lower-stage burner. This result suggests that the overall Uc* of coal-fired plants can be reduced by supplying coal with a low combustibility to the

1958 Energy & Fuels, Vol. 21, No. 4, 2007

middle- or lower-stage burner and supplying coal with a high combustibility to the upper-stage burner. The contribution of the upper-stage burner to the total unburned fraction decreases with decreasing fuel ratio. This is because the residence time of the coal particles supplied from the upper-stage burner for low-fuel-ratio coal is longer than that for high-fuel-ratio coal. The difference in residence time is

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caused by the difference in the size of the recirculation flow above the upper-stage burner induced by the main gas flow. Acknowledgment. The authors are indebted to Dr. M. Ikeda of CRIEPI for assistance in providing the experimental data and for helpful discussions. EF070151O