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Computerized Analysis of Low NOx W-Shaped Coal-Fired Furnaces J. R. Fan,* X. D. Zha, and K. F. Cen Institute of Thermal Power Engineering and CE&EE, Zhejiang University, Hangzhou, 310027, People's Republic of China Received February 4, 2000. Revised Manuscript Received February 27, 2001
Approaches to controlling NOx emission have been investigated worldwide. Numerical simulation models describing gas-particle flow, heat transfer, and combustion processes are presented in this paper to provide reasonably accurate predictions of the flow field, temperature field, and the distribution of chemical species in a W-shaped furnace. The predictions indicate that the temperature field of model II is more uniform and the contour is lower due to the complement of nozzles. It results in the decrease of NOx formation. Comparison about NOx emission of two furnace models at different temperatures has been made, it shows that the furnace of model II is better in reducing the emission of NOx.
1. Introduction Boilers of W-shape are often used to burn anthracite and low volatile matter coals. They have higher combustion efficiencies than tangentially fired boilers because of longer flame lengths and high coal-to-air ratios for the primary mixtures feeding the second combustion stage. The combustion of pulverized coal in utility boilers leads to the formations of nitrogen oxides (NOx). The emissions of NOx contribute to the formation of acid rain and the production of photochemical smog. The need to control pollutant emissions from coal-fired plants has been recognized internationally. Many studies have appeared and several reviews have been presented.1,2 Concentration levels of NOx depend on a number of factors, e.g., rank of coal, burner type, flame temperature, mixing intensity with air, firing intensity, coal particle size, and functional forms of nitrogen in the coal. The prediction of NOx formation in turbulent, pulverized coal systems is very complex because of the large number of reactions that are required to describe this process and the turbulent flow in practical systems. The modeling of the NOx formation from coal flames is usually based on the mechanism proposed by De Soete for the nitrogen from volatiles.3 Several investigators have attempted to use a correlation-type approach to predict NOx levels from furnace operational and coal parameters.4 Smouse et al.5 used a neural network * Author to whom correspondence should be addressed. (1) McConville, A. Proceedings of the 22nd International Technical Conference on Coal Utilization and Fuel Systems, Sakkestad, B. A., Ed.; Coal and Slurry Technology Association: Washington, DC, 1997; pp 1-12. (2) Smart, J. P.; Nakamura, T. J. Inst. Energy 1993, 66, 99-105. (3) De Soete, G. G. 15th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1975; pp 1093-1192. (4) Pohl, J. H.; Chen, S. L.; Heap, M. P.; Pershing, D. W. Proc. 1982 Joint Symp. Stationary NOx Control, 1983. (5) Smouse, S. M.; Wildman, K. J.; Mcllvreid, T. S.; Harding, N. S. Proc. 1993 Joint EPA/EPRI Symp. Stationary Combustion NOx Control, 1994.
approach on a set of 69 tests in tangentially fired furnaces. Although an alternative NOx modeling approach, in which the full set of nitrogen-chemistry equations contains on the order of 100 identified elementary reactions, has been used to predict NOx levels, it is not practical to solve this set combined with a complex gas-particle fluid dynamics model.6 Therefore, the simplified approach uses a postprocessor NOx chemistry model in conjunction with a gas-particle fluid dynamics model.7-10 Many attempts have been made to predict NOx formation by numerical simulations. The purpose of our research is to investigate numerically the mechanism of NOx formation in W-shaped furnaces. We compare the NOx formation of two models and provide the difference between them. 2. Mathematical Models Mathematical models involve two stages of analysis. In the first, we describe the gas-phase flow in Eulerian coordinates. Numerical solution methods for these have been developed by Patankar and Spalding.11 In the second stage, we describe coal particle behavior governed by the mass, momentum and heat balance equations in the flow field as estimated in the first stage. 2.1. Gas-Phase Models. The Eulerian equations for the gas phase may be described as the standard form:
div(Fvφ) - div(Γgradφ) ) Sφ
(1)
(6) Lockwood, F. C.; Romo-Millares, C. A. Combust. Sci. Technol. 1994, 102, 57-80. (7) Fiveland, W. A.; Wessel, R. A. J. Inst. Energy, 1991, 64, 41-54. (8) Lockwood, F. C.; Romo-Millares, C. A. J. Inst. Energy 1992, 65, 144-152. (9) Fiveland, W. A.; Latham, C. E. Combust. Sci. Technol. 1993, 93, 53-72. (10) Coimbra, C. F. M.; Azevedo, J. L. T.; Carvalho, M. G. Fuel 1994, 73, 1128-1134. (11) Patankar, S. V.; Spalding, D. B. Int. J. Heat Mass Transfer 1972, 15, 1787.
10.1021/ef0000173 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/10/2001
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Table 1. Eulerian Conservation Equations and Identification of Terms in Eq 1
where φ is the particle-dependent variable of immediate interest, v the velocity vector, F the fluid mass density, Γ the exchange coefficient in the transport law, and Sφ the generalized source term for φ which depends on the geometry, transport coefficients, and other dependent variables. Table 1 identifies the Eulerian or gas-phase variables, together with the corresponding expressions for the exchange coefficients Γ and the source term Sφ used in eq 1. 2.2. Particle Phase Models. The particles are treated in a Lagrangian framework in which the particle field is represented as a series of trajectories through the gas continuum. The Lagrangian equations of continuity, momentum, and energy are used to calculate the particle properties and trajectories. Particle trajectories are tracked throughout the computational domain, and interactions between the particles and gas are incorporated by an exchange of source terms for mass, momentum, and energy. Particles are assumed to be composed of raw coal, char, ash, and moisture. Coal devolatilization and char-oxidation steps are included. Coal devolatilization is modeled by a two-step mechanism. The gases from coal devolatilization are converted into particulate phase by chemical gaseous reactions followed by homogeneous nucleations. The char reacts heterogeneously with various oxidizers that diffuse to the particle surface. The coal reaction rate is of the first order with fixed activation energy.
The conservation equations for particles are different in form from eq 1 and will now be summarized. The particle mass-conservation equation is
dmp/dt ) -rp
(2)
The equation of motion of a particle is
1 mp(dvp/dt) ) FCD(vg - vp)|vg - vp|Ap + mpg (3) 2 The conservation of particle energy is
mp(dhp/dt) ) Qc + Qr + rwLw + rv∆hv + rhQh (4) Diffusion-limited vaporization of moisture from the coal particle is described by
rw ) MwNumCgDwmAp(Xwp - Xwg)/dp(1 - Xwprp/rw) (5) The formation of volatile matter from coal dust is described as follows:
dV/dt ) (V′daf - V)K exp(E/RT)
(6)
V′daf ) QVdaf
(7)
Char is produced in competition with volatiles pro-
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duction as expressed by
rhm ) rv(1 - Ym)/Ym
(8)
Char is assumed to be oxidized heterogeneously by a gaseous oxidizer that diffuses to the particle, is adsorbed, reacts with carbon, and is then described as CO. The char oxidizing rate is
rhl ) (Apnp)2Mhpmgφ1Kcp1Kp1ξpCogCg/ [MgApnpCg(ξpKp1 + Kcp1) + rp] (9) The total reaction rate for the coal particle is
rp ) rv + rhl + rw
(10)
The carbon reaction rate is
rh ) rhm - rhl
(11)
The convection heat transfer rate to a spherical particle is quantified by the Nusselt number
Nu ) Qcdp/ApKg(Tg - Tp)
(12)
where Nu ) 2 + 0.654Re0.5Pr1/3. The radiative heat transfer to a particle of diameter dp, is given by
Qr ) πd2p(I - σT4p)
(13)
Computations of char-oxidation rates require oxidizer mass transfer rates from the bulk gas to the particle surface. The mass transfer coefficient to spheres in the absence of surface samples transpiration effects is
Sh ) 2 + 0.654Re0.5Sc1/3
(14)
A simple model for nitrogen chemistry is included in the mathematical model. Both the thermal and fuel mechanisms are taken into account for the formation of NOx. The prompt NOx is neglected because it was shown to form in insignificant amounts. Thermal NOx was assumed to form by a Zeldovich mechanism as shown in reactions as follows: k1
N2 + O 98 NO + N k2
O2 + N 98 NO + O
(Reaction 1) (Reaction 2)
The formation rate of NO from atmospheric nitrogen is 0.5 RN2fNO ) 3 × 1014 F1.5 g YN2YO2 × 0.5 ) exp(-65300/Tg)MNO/(MN2MO 2
(15)
The nitrogen in the coal is considered to partition between the volatiles and char such that its concentration in the volatiles is identical to that in the dry, ashfree parent coal.12 Fuel NO formed in the gas phase results from the oxidation of devolatilized nitrogen constituents, and generally accounts for 60-80% of the total NO formed. (12) Wand, W.; Brown, S. D.; Hindmarsh, C. J.; Thomas, K. M. Fuel 1994, 73, 1381-1388.
A simplified reaction mechanism has been used to model the rate of the fuel-nitrogen,8 in which contributions from the nitrogen contents of the volatiles and the char are distinguished. Volatile-NO is formed through the reaction of the HCN with O2 and reduced by the reaction of HCN with NO, its formation rate is given by
SHCN ) SVmN,V/MN
(16)
where SV is the release rate of the volatiles, mN,V, the nitrogen mass fraction in the volatile, and MN, the molecular weight of nitrogen. Conversion rates for the gas-phase reactions are as follows: b exp(-67000/RT) RHCNfNO ) 0.4e11XHCNXO 2
(17)
RNOfN2 ) 3e12XHCNXNO exp(-60000/RT)
(18)
where Xi is a mole fraction, T the temperature (K), and R the universal gas constant. The order of reaction b varies according to the oxygen mole fraction.3 Char-NO is mainly released as a desorption product from oxidized char-nitrogen atoms.13 As a result, the fraction of char nitrogen converted to NO is roughly proportional to the carbon burnout, which is expressed by the rate of formation of NO from char as
SNO,CH ) SCmN,CHFcη
(19)
where SNO,CH is the rate of formation of NO from charnitrogen, SC is the mass combustion rate of char, which is known from the calculations of the parent code, mN,CH is the mass fraction of nitrogen in the char, Fc is the degree of char burnout, and η is the coefficient of NO yield taken from De Soete13 for char from high-volatile bituminous coal. NO may also be reduced by the char.14 The rate of NO reduction is described as follows:
W ) 4.18 × 107 RpnpAEFNO exp(-34.7/RT) (20) A simplest production mechanism of CO and CO2 is as follows:15 (1) Reaction of carbon dioxide with an active site on the solid surface, yielding a molecule of CO and an adsorbed oxygen atom:
CO2 + () T CO + (O)
(reaction 1)
(2) Reaction of the surface-bound oxygen with another surface carbon atom, followed by desorption of the CO, leaving a vacant active site:
(O) + C* f CO + ()
(reaction 2)
and we have chosen to express the rate of C*-CO2 (13) De Soete, G. G. 23rd Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; p 1257. (14) Levy, J. M.; Chan, L. K.; Beer, J. M. 18th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1981; p 1207. (15) Smoot, L. D.; Pratt, D. T. Pulverized-Coal Combustion and Gasification; Plenum Press: New York, 1979.
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Figure 2. The flow field of gas phase of two different furnace models (a) Model I; (b) Model II. Table 3. Coal Properties ultimate analysggis (%) proximate analysis (%) MS
ash
VM
FC
C
H
O
N
S
6.00 21.82 16.72 55.46 64.89 2.83 2.4 0.98 1.08
Figure 1. A schematic view of the two models (a) Model I; (b) Model II; (1-burners; 2-tertiary air; 3-staged air). Table 2. Operational Parameters and Nozzle Sizes parameter
nozzle size (mm)
temperature (K)
velocity (m/s)
primary air of burners inner secondary air outer secondary air 1 staged air tertiary air coal particle
550 × 14 951 × 8 164 × 6 382 × 6 482 × 6 see Table 4
488 614 614 614 363 303
17.0 20.2 41.1 38.6 22.3 9.8
reaction in the form
r ) kpn
(21)
k ) A exp(-E/RT)
(22)
where
The rate constant in all these cases is based on a unit external surface area of the char. 3. Results and Discussion The model described is applied to two different models of full-scale furnace chamber, which are parts of 300 MW (electrical output) W-shaped utility boiler. A schematic view of the two models is given in Figure 1. We can see from the figure that the difference between model I and model II is that a row of burners and staged air is added to model II on the basis of model I. This improvement is designed for reduction of NOx emission. The geometric parameters of the burners and the operating parameters are listed in Table 2. The analysis of the coal used is presented in Table 3. A particle size
heating value (MJ/kg) 28.27
distribution, in the range of 5 and 140 µm, for raw coal particles is presented in Table 4. Our numerical results were obtained using 68 × 86 × 50 ()292 400) nonuniform grids. Grid-dependence tests were conducted. The specified grid is fine enough to give grid-independent solutions. The calculation begins by solving the gas flow field equations assuming that the particles are absent. Using the flow field, particle trajectories, their temperatures, and burnout histories are determined. The mass, momentum, and energy source terms for each cell are calculated. The source terms are included in the gas-phase equations and the flow field is then recalculated. The process is repeated until further repetition fails to change the solution. Thus, the mutual interaction of the gas and particles is accounted for. A five-stage iteration method is used in integrating the conventional difference equations for the particle. This method reduces the solution time by a factor of 4 compared to the four-stage RungeKutta method. 3.1. The Flow Field of Gas Phase and Coal Particles. Figure 2 presents the flow fields of gas phase of two different furnace models. We can see it from the figure that the primary and secondary air mix with tertiary and staged air in the middle of the lower part of the furnace. The gas flow does not penetrate far before turning upward, and the W-shaped flow pattern is finally formed. We can also see that the primary air penetrating distance of model II is longer than that of model I, this is because the air from the added burner and the supplemented staged air below of model II is beneficial to the increase of penetrating distance. And also the flow field of model II is better than that of model I. Figure 3 displays the trajectories of coal particles. It indicates that the coal particles of two models are ejected along with the primary and secondary air from the burner to the furnace and penetrate downward. Then particles turned upward along with gas flows after they penetrated downward to a certain depth. The penetrating distance of model II is longer than that of model I, which is same as the results of Figure 2. And
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Table 4. The Percentages of Various Size Classes of Pulverized Coal particle diameter (µm) percentage (%) particle diameter (µm) percentage (%)
5 20.22 160 1.42
20 28.04 180 0. 94
40 17.95 200 0. 62
60 11.61 220 0. 41
80 7.58 240 0. 27
100 4.96 260 0. 18
120 3.27 280 0. 12
140 2.16 300 0. 25
Figure 5. The contours of temperature of two furnace models (a) Model I; (b) Model II. Table 5. The Sizes of Coal Flame (m)
Figure 3. The trajectories of coal particles (a) Model I; (b) Model II).
Figure 4. The sketch map of coal flame description.
the particles are not entirely coincident with the gas flow because of the fluctuation of the turbulence. 3.2. The Performance of the Coal Flame. Figure 4 is the sketch map of coal flame description. The symbols H, L in the figure are the structure size of furnace, h and R are the penetrating depth and distance of flame, and l is the width of flame which is at the position of h/2. The values of h and R relate to the ratio of the primary air momentum and the secondary air
furnace model
H
L
I
R
Model I Model II
11.2 11.2
15.6 15.6
10.2 10.8
9.8 11.4
momentum. They are also related with the jet angle and momentum of staged air. The performance of coal flame is shown in Table 5. We can see from the table that the value of H, L is same because the two models are the same at the exterior structure size. The difference between the value l of two models is small. But the difference between the R values is big. The values h and R of model II are bigger than that of model I because the primary and secondary air momentum of model II furnace is bigger. The flame penetrating distance is neither too long nor too short. If it is too long, the flame and gases will sweep the furnace hopper. And if it is too short, the flame will not burn the coal particles completely. Also, if the value l is too big, the flame will sweep and burn the furnace wall. Contrarily, if the value is too small, the distribution of radiative heat in the furnace will be uneven. The uneven distribution of radiative heat is harmful to heat transfer. Obviously, the degree of admission of model II is better than model I. 3.3. The Contours of Temperature Field. Figure 5 presents the contours of the temperature in the central vertical plane. As expected, the higher temperature regions appears under the arches and the temperatures along the central line is higher than other regions. Comparing Figure 5b with Figure 5a, we can see that the highest temperature in the furnace of model II is lower than the highest temperature in the furnace of model I. But the distribution of the temperature field
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Figure 6. Comparison of the numerical simulation results and the experimental data (∆, Model I experimental data; O, Model II experimental data; s, Numerical result).
Figure 8. The concentration of CO2 mass fraction (a) Model I; (b) Model II.
Figure 7. The concentration of CO mass fraction of two furnace models (a) Model I; (b) Model II.
of model II is more uniform than that of model I. The flame permeation degree of model II is better than that of model I. The addition of tertiary and staged air by the complemented nozzles contributes it. Because NOx formation is highly temperature dependent and even a little decrease of temperature will result in a significant corresponding drop on NOx formation, the furnace of model II is better in reducing the NOx emission of coal combustion. Figure 6 is the comparison of our numerical simulation results and the experimental data of the temperature along the height of the furnace. From the figure, we can conclude that the simulation is coincident with
the experiment. It demonstrates that the simulation is valid and accurate. 3.4. The Concentration of Gas Species. Figure 7 shows the concentration distributions of CO mass fraction at different height positions of two furnace models. It can be seen from the figure that the concentration of CO in the upper furnace part is lower than that in the lower furnace part. The distribution of CO concentration is uneven. The concentration of two sides is high, whereas that of the middle part is lower. This is because the concentration of coal particles under the furnace arch is high. The volatiles of coal particles are separated out immediately after the coal particles are ejected into furnace. The massive CO gas is formed and cannot be burned out in time because of lack of oxygen in this local position. So the concentration is higher than the other place. We can see by comparing Figure 7a and Figure 7b that the CO concentration of model II is lower than that of model I in the corresponding positions. This is because the secondary and tertiary air from the added nozzles in model II can sufficiently supplement the oxygen, which is lacking in this location. So the CO gas can be burned out more completely. And also from the overall view, the CO emission is reduced in model II. Figure 8 shows the concentration of CO2 mass fraction. It shows that the distribution of CO2 concentration is even along the width direction and the concentration of central part is a bit higher. This is because the temperature is higher and the oxygen is more abundant due to the strong mixture and combustion of gas and coal particles. The distributions of CO2 of two models are almost similar.
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Nomenclature
Figure 9. The concentration of NO mass fraction (a) Model I; (b) Model II).
Figure 9 shows the concentration of NO mass fraction. Since the fuel-NOx formation is strongly influenced by oxygen concentration, this species is mostly formed within the envelope of the flames. Coal volatiles are formed in the near-burner region. Both char and volatiles release HCN. Therefore, in the fuel-rich region of the flames NOx is abundantly formed around the burners and reaches to a maximum value of 874 ppm of the model I and 628 ppm of the model II. The oxygen concentration is lower in the center of the flame where the temperature is very high, but the NOx concentration is not the highest. For model II due to the increase of excess air compared with the model I, the NOx formed from volatiles increases while the contribution from char is less. Comparing model II with model I, the NOx concentration decreases due to the effective air supply. For model II the highest NOx mass fraction was 628 ppm and was reduced to 320 ppm at the exit. As a whole, the NOx concentration of model II is lower than that of model I. This indicates that furnace model II can effectively reduce the NOx emission. Because the experimental data of the NOx emission of the W-shaped furnace is not available, we cannot make comparison to our numerical simulation now. Conclusion We have discussed the distributions of flow field, temperature contour, CO and CO2 concentration, and the NOx formation of different furnaces at different temperature. It can be concluded that the modified model II can reduce the NOx emission because its complemented burners made the flow field and temperature field more uniform and the temperature contours lower. The NOx formation is highly temperature dependent. The decrease of temperature, the completeness of combustion, and the sufficiency of air supply can reduce the NOx emission. It also suggests that the numerical simulation method may be used with some confidence to design and optimize the operation of W-shaped boiler furnaces. Acknowledgment. This project is supported by the Special Funds for Major State Basic Research Project of P. R. China.
AE ) Arrhenius coefficient, dimensionless Ap ) coal-particle area, m2 CD ) drag coefficient, dimensionless Cg ) gas molar concentration, mol/L Cµ, C1, C2 ) empirically determined constants, dimensionless Dw ) binary diffusivity, m2 s-1 dp ) particle diameter, m E ) activation energy, kJ mol-1 g ) gravitational vector, m/s2 h ) enthalpy, J kg-1 I ) radiation intensity, J m-2 s-1 K ) reaction-rate coefficient, dimensionless Kc ) mass-transfer coefficient, dimensionless Kg ) thermal conductivity, J m-1 s-1 K-1 k ) mean turbulent kinetic energy, J L ) latent heat, J M ) molecular weight, dimensionless mp ) particle mass, kg Nu ) Nusselt number, dimensionless np ) particle number density Pr ) Prandtl number, dimensionless p ) sticking probability, dimensionless Q ) particle-to-gas heat-transfer rate, J s-1 R ) gas constant, J kmol-1 K-1 Re ) Reynolds number, dimensionless r ) particle reaction rate, kmol m-3 s-1 S ) formation rate of chemical species, kmol m-3 s-1 Sc ) Schmidt number, dimensionless Sφ ) gas source term for variable φ Sφ,p ) particle source term for variable φ t ) time, s T ) temperature, K u ) velocity along x direction, m s-1 ν ) velocity along y direction, m s-1 v ) velocity vector, m s-1 V ) concentration of volatiles, kmol m-3 w ) velocity along z direction, m s-1 X ) mole fraction x,y,z ) Cartesian coordinate Y ) pyrolysis coefficient, dimensionless Yk ) concentration of the kth chemical species, kmol m-3 Greek Symbols R′kj,R′′kj ) stoichiometric coefficient of species k in reaction j Φ ) general variables ) emissivity, or dissipation rate of turbulence η ) coefficient of NO yield µ ) viscosity, kg m-1 s-1 ν ) kinetic viscosity, m2 s-1 F ) density, kg m-3 σk, σ ) turbulent Prandtl and Schmidt numbers Subscripts c ) convection, or coal daf ) daf coal eff ) effective g ) gas h ) char l ) char oxidation reaction m ) mean p ) particle r ) radiation v ) volatiles w ) moisture EF0000173
