Direct Numerical Simulation of Ozone Injection ... - ACS Publications

into a 2D reaction jet flow and handled by Chemkin. The code was parallelized and run on a cluster system. The reactions between O3 and NOx are relati...
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Direct Numerical Simulation of Ozone Injection Technology for NOx Control in Flue Gas Zhihua Wang, Junhu Zhou,* Jianren Fan, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang UniVersity, Hangzhou 310027, Zhejiang, China ReceiVed July 13, 2006. ReVised Manuscript ReceiVed August 31, 2006

The ozone injection process for NO control was studied using an ozone-NOx reaction coflow jet by direct numerical simulation. A 65-step detailed kinetic mechanism for the O3-NOx reaction was developed and verified by our experiment and another published experiment. The detailed kinetic mechanism was incorporated into a 2D reaction jet flow and handled by Chemkin. The code was parallelized and run on a cluster system. The reactions between O3 and NOx are relatively slow compared to the turbulence time scale. The chemical reactions occur mostly in the core of all kinds of vortexes. The dynamics of vortexes make a great contribution to the mixing of the reactants and reactions. The vortexes begin to roll up alternately at about 7d from the root of the jet. The order of the two-line vortexes become disordered after 10d. NO2 is the main product of the O3-NO reactions, NO3 and N2O5 are the minority products. Favre averaged results show that about 45.44% of NO can be oxidized by O3 at the 30d outlet of the computational domain.

1. Introduction Turbulent reacting flows occur during the chemical reaction process and energy utilization process in a wide variety, such as combustion phenomena, relative pollution emission, and emission control. With the continuous improvement of the computational ability, especially the appearance of parallel cluster system, it is possible for common users to apply the direct numerical simulation (DNS) method to more complex reaction flows.1-3 DNS is undoubtedly the most precise numerical method.4,5 In DNS, the exact governing equations are solved directly without any turbulence or combustion models; it therefore becomes a standard analysis tool as experiments. DNS is usually used for the insight investigation of turbulence, combustion phenomena, and the development or examination of the turbulence closure models. During the past few decades, DNS has been successfully used in premixed and non-premixed hydrogen and methane combustion simulations.2,6-11 From the one-step, irreversible reaction to the detailed kinetic mechanism, more complex finite-rate chemistry was studied and incorporated into DNS.8,12-14 The ignition, microscale flame structure, and * Corresponding author. Telephone: +86-571-87951668. Fax: +86571-87951616. E-mail: [email protected]. (1) Zhou, X. Z. S.; Brenner, G.; Durst, F. Int. J. Heat Mass Transfer 2000, 43, 2075-2088. (2) Hsu, J. S. M. Combust. Theory Modell. 2003, 7, 365-382. (3) Lange, M. Lect. Notes Comput. Sci. 2003, 2565, 24-38. (4) Vervisch, L. T. P. Annu. ReV. Fluid Mech. 1998, 30, 655-691. (5) Poinsot, T. S. C.; Trouve, A. Prog. Energy Combust. Sci. 1996, 21, 531-576. (6) Gokula, H. K. T. E. Combust. Flame 2006, 146, 155-167. (7) Hong, G.; Im, J. H. C. Combust. Flame 2002, 131, 246-258. (8) Shankar, M.; Chen, J. H.; Vervisch, L. Combust. Flame 1995, 102, 285-297. (9) Hilbert, R. D. T. Combust. Flame 2002, 128, 22-37. (10) Evatt, R.; Hawkes, J. H. C. Combust. Flame 2004, 138, 242-258. (11) Chen, J. H. E. R. H.; Sankaran, R.; Mason, S. D.; Im, H. G. Combust. Flame 2006, 145, 128-144. (12) Baum, M. T. P.; Haworth, D. C.; Darabiha, N. J. Fluid Mech. 1994, 281, 1-32. (13) James, S. F. A. J. Combust. Flame 2000, 123, 465-487.

turbulence chemistry interaction are widely studied.9,11,15,16 But these studies are still limited to the simple hydrocarbon combustion such as H2 and CH4, even as C7H16,17 although there are various reaction flows that need to be deeply investigated by DNS, such as pollution formation and all kinds of control processes. With the development of society and industry, the pollution emitted from coal combustion is receiving much more attention by researchers and the public. The most abundant air pollution emitted from stacks are sulfur dioxide (SO2) and nitrogen oxides (NOx) during coal combustion. For the reduction of SO2 and NOx in coal-fired boilers, combustion modification technologies such as low NOx burner, over fire air, reburning, sorbent injection, etc., and even choosing a low content of nitrogen and sulfur coals was first considered.18-21 With the further limitation of environmental legislation, substantial reduction can be accomplished by wet flue gas desulfurization (WFGD) and selective catalytic reduction (SCR) technology. However, the combined application of WFGD with SCR results in expensive investments and operating costs.22 Therefore, many new technologies have been updated for the high-efficiency, low-cost, simultaneous removal of SO2 and NOx.23-26 (14) Mantel, T. J. M. S. Combust. Flame 1999, 118, 557-582. (15) Echekki, T. J. H. C. Combust. Flame 2003, 134, 169-191. (16) Jiang, X. K. H. L. Int. J. Heat Fluid Flow 2001, 22, 633-642. (17) Viggiano, A. V. M. Combust. Flame 2004, 137, 432-443. (18) Jost, O. L.; Wendt, W. P. L.; Groff, P. W.; Srivastava, R. K. AIChE J. 2000, 47 (11), 2603-2617. (19) Zabetta, E. C.; Hupa, M.; Saviharju, K. Ind. Eng. Chem. Res. 2005, 44, 4552-4561. (20) Zhi-hua, W. Z. J.-h.;Yan-wei, Z.; Zhi-min, L.; Jian-ren, F.; Ke-fa, C. J. Zhejiang UniV. Sci. 2005, 6B (3), 187-194. (21) Hampartsoumian, E. O. O. F.; Nimmo, W.; Gibbs, B. M. Fuel 2003, 82, 373-384. (22) Lani, B. W.; Feeley, T. J.; Murphy, J.; Green, L. A ReView of DOE/ NETL’s AdVanced NOx Control Technology R& D Program for Coal-Fired Power Plants; National Energy Technology Laboratory of the U.S. Department of Energy: Pittsburgh, PA, March, 2005. (23) Bueno-Lopez, A.; Garca-Garca, A. Fuel Process. Technol. 2005, 86, (16), 1745-1759.

10.1021/ef0603176 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/10/2006

DNS of Ozone Injection Technology for NOx Control

Energy & Fuels, Vol. 20, No. 6, 2006 2433

Table 1. Detailed O3-NOx Reaction Mechanism β

no.

reaction

A

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 R29 R30 R31 R32 R33

O3 + H ) O2 + OH O3 + H ) O + HO2 O3 + OH ) O2 + HO2 O3 + H2O ) O2 + H2O2 O3 + HO2 ) 2O2 + OH O3 + N ) O2 + NO O3 + NO ) NO2 + O2 O3 + NO2 ) O2 + NO3 O3 ) O2 + O O3 + O ) 2O2 H + O2 + M ) HO2 + M H + H + M ) H2 + M H + H + H2 ) H2 + H2 H + H + H2O ) H2 + H2O H + OH + M ) H2O + M H + O + M ) OH + M O + O + M ) O2 + M H2O2 + M ) OH + OH + M H2 + O2 ) 2OH OH + H2 ) H2O + H O + OH ) O2 + H O + H2 ) OH + H O + HO2 ) O2 + OH 2OH ) O + H2O H + HO2 ) H2 + O2 H2O2 + H ) HO2 + H2 N + O2 ) NO + O N + OH ) NO + H NO + M ) N + O + M N + HO2 ) NO + OH NO+NdN2+O NO + O(+M) ) NO2(+M) NO + OH ) HNO2

1.64 × 4.52 × 1011 1.15 × 1012 6.62 × 101 1.19 × 108 6.00 × 107 1.08E+10 8.40 × 1010 4.30 × 1014 1.34 × 1012 3.61 × 1017 1.00 × 1018 9.20 × 1016 6.00 × 1019 1.60 × 1022 6.20 × 1016 1.89 × 1013 1.30 × 1017 1.70 × 1013 1.17 × 109 3.61 × 1014 5.06 × 104 1.40 × 1013 6.00 × 108 1.25 × 1013 1.60 × 1012 6.40 × 109 3.80 × 1013 9.64 × 1014 1.00 × 1013 2.04 × 1013 1.30 × 1015 5.45 × 1017 1013

0.8 0 0 0 4.6 0 0 0 0 0.8 -0.7 -1 -0.6 -1.3 -2 -0.6 0 0 0 1.3 -0.5 2.7 0 1.3 0 0 1 0 0 0 0 -0.8 0

E 750 0 1990 0 1380 0 0 4910 22260 3140 0 0 0 0 0 0 -1788 45500 47780 3626 0 6290 1073 0 0 3800 6280 0 620910 8390 50 0 0

no.

reaction

A

R34 R35 R36 R37 R38 R39 R40 R41 R42 R43 R44 R45 R46 R47 R48 R49 R50 R51 R52 R53 R54 R55 R56 R57 R58 R59 R60 R61 R62 R63 R64 R65

O + HNO2 ) NO2 + OH H + HNO2 ) HNO + OH H + HNO2 ) NO2 + H2 H + HNO2 ) NO + H2O O3 + HNO2 ) O2 + HNO3 HNO3 + O ) NO3 + OH HNO3 + H ) NO3 + H2 HNO3 + H ) NO2 + H2O HNO3 + NO ) NO2 + HNO2 HNO3 + OH ) NO3 + H2O HNO3 + OH ) NO2 + H2O2 HNO3 ) NO2 + OH HNO + O ) NO + OH HNO + HNO ) N2O + H2O HNO + H ) NO + H2 HNO + NO2 ) NO + HNO2 HNO + OH ) NO + H2O H + HO2 ) OH + OH NO2 + HO2 ) HNO2 + O2 NO + HO2 ) HNO + O2 NO + HO2 ) NO2 + OH NO + HO2 ) HNO3 H2O + HO2 ) H2O2 + OH H2O2 + HO2 ) O2 + H 2O + OH OH + HO2 ) O2 + H2O HO2 + HO2 ) H2O2 + O2 HO2 ) H + O2 NO2 + NO3 ) N2O5 N + NO2 ) O + O + N2 NO2 + H ) NO + OH NO + NO ) O2 + N2 NO + N2O ) NO2 + N2

1.21 × 7.57 × 1012 1.37 × 1012 3.85 × 1011 3.01 × 105 1.81 × 107 3.40 × 1012 8.39 × 109 4.48 × 1003 4.82 × 1008 4.82 × 1008 6.90 × 1017 2.29 × 103 2.55 × 107 2.70 × 1013 6.03 × 1011 4.82 × 1013 8.22 × 1012 2.20 × 10-1 5.84 × 105 6.32 × 1011 3.47 × 1012 2.80 × 1013 6.03 × 1010 4.28 × 1013 1.87 × 1012 1.45 × 1016 7.98 × 1017 1.30 × 10-01 2.41 × 1014 3.10 × 1013 1.73 × 1011 1013

β

E

0 0.9 1.6 1.9 0 0 1.5 3.3 0 0 0 0 0 4 0.7 0 0 0.8 0 0 0.6 0 0 0 -0.2 0 -1.2 -3.9 0 0 0 2.2

5938 4914 6592 3840 0 0 16332 6255 0 0 0 45730 0 1188 651 1980 990 0 0 5600 1430 -5720 32790 0 110 1540 48490 0 0 680 63190 46300

Reaction rate constants expressed as k ) ATβexp(-E/RT) (cal, cm, mol, s).

Ozone injection technology is an attractive method that is much more energy efficient than the typical nonthermal plasma and electron beam process directly applied to the exhaust gas for the oxidization of NO.27-29 The lifetime of O3 is longer compared to the kinetic reactions under traditional flue gas conditions. So, it is possible to discharge only small amounts of pure O2 or air to produce O3 and then inject them into the flue gas. The solubilities of NO2, NO3, and N2O5 are much higher than that of NO, which is the predominant species of NOx in flue gas. And the SO2 is soluble in water and alkaline solution. Therefore, it is possible to remove NOx and SO2 in one scrubber simultaneously by converting NO into NO2, NO3, or N2O5. The detailed experimental procedures and results can be found in our earlier articles,30 in which more than 80% of NOx and nearly 100% of SO2 can be removed simultaneously. The key step in this kind of technology is the oxidization property of NO in flue gas. Besides kinetics, mixing is another important factor for the realization and application of this technology. A deep understanding of the small-scale ozone-

NOx reaction jet is fundamental to the design and optimization of this technology. Due to a lack of detailed experimental results about the O3NO reaction jet, like all kinds of H2 or methane flames, the selection of traditional turbulence and combustion models is arbitrary or even wrong. Therefore, this paper tries to use an experiential independent DNS numerical method to understand and give an insight to the O3/NOx reaction process. Here, a nonpremixed ozone-NOx 2D reaction jet flow is considered. The reaction properties, spatial structures, and turbulence reaction interactions are investigated with the DNS results. In this paper, a new 20-species, 65-step detailed kinetic mechanism between ozone and NOx was developed and discussed. The numerical method and the boundary conditions are then introduced here in detail. Finally, the DNS results are explained, including the O3-NOx reaction properties, the vortex chemical interactions, and the chemical turbulence interactions.

(24) Huang, L. W. H. M. AIChE J. 2004, 50 (11), 2676-2681. (25) Nimmo, W. A. A. P.; Hampartsoumian, E.; Gibbs, B. M.; Fairweather, M.; Williams, P. T. Fuel 2004, 83, 1143-1150. (26) Haddad, E. J. R.; Gree, G.; Castagnero, S. Full-Scale Evaluation of a Multi-Pollutant Reduction Technology: SO2, Hg, and NOx. Presented at the Combined Power Plant Air Pollutant Control Mega Symposium, Washington, DC, May 19-22, 2003. (27) Jarvis, J. B.; Naresh, A. T. D.; Suchak, J. LoTOxTM Process Flexibility and Multi-Pollutant Control Capability. Presented at the Combined Power Plant Air Pollutant Control Mega Symposium, Washington, DC, May 19-22, 2003. (28) Yan Fu, U. M. D. AdV. EnViron. Res. 2003, 8, 173-196. (29) Young Sun Mok, H.-J. L. Fuel Process. Technol. 2006, 87, 591597. (30) Zhi-hua, W. Z. J.-h.; Lin-sheng, W.; Zheng-cheng, W.; Ke-fa, C. Proc. Chin. Soc. Electr. Eng. 2006, accepted for publication.

The reactions between O3 and NOx include the decomposition of ozone as well as the reactions between O3 and NO and subsequent conversion of the intermediate nitrogenous species. Young has listed a 12-step mechanism between O3 and NO, but the species of H2O, N2O, and HNO3, which are important species in flue gas, are not taken into consideration.29 Therefore, a new 20-species, 65-step detailed kinetic mechanism is developed and listed in Table 1. All the elemental reactions are reversible. Part of the rate constants in Table 1 were obtained from the National Institute for Standards and Technology (NIST) kinetic database31 and the others from GRI-Mech3.0.32 The elemental reactions and

2. Kinetic Mechanism of O3-NOx

(31) NIST, http://kinetics.nist.gov/index.php. (32) GRI-Mech, http://www.me.berkeley.edu/gri-mech/.

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Figure 1. Comparison simulation results with Young’s experiment, T ) 443 K.

Figure 2. Comparison simulation results with our experiment.

Figure 4. Change in NOx composition with ozone added at a molar ratio of O3:NO ) 1.5.

kinetic parameters that relate to H2, H2O, O2, and NOx are derived from the GRI-Mech3.0. The decomposition of ozone and reactions between ozone and NOx are derived from the NIST kinetic database. Kinetic calculations were performed by the Senkin model in Chemkin III with the new mechanism, and the results were verified by our experimental results and Young’s.29 Figure 1 shows the comparison between the simulation and Young’s experiment results at 443 K and 2.93 s, with initial NO concentration 280 ppm. As can be seen, the calculation results agree well with those of the experiment. The NO mainly converts to NO2 with the addition of ozone, which means reaction R7 is the dominant step under this condition. O3 + NO ) NO2 + O2

Figure 3. Dynamic profile of NOx conversion with ozone added in the system at a molar ratio of O3:NO ) 1.5.

(R7)

N2O5 can be ignored, and NO3 appeared only when O3:NO > 0.9. Figure 2 displays the comparison between simulation and our experimental results from 373 to 673 K. The detailed experimental results can be found in our previous paper,30 and here are just given a brief introduction. The experiment was carried out in a plug flow quartz reactor with 215 ( 15 ppm NO in flue gas and a 0.05-0.09 s resident time. The concentrations of ozone and NOx were monitored by continuous emissions monitors (CEMs; In2000 ozone analyzer, In USA Co., Rosemount NGA2000, Emerson Process Management Co. Ltd.). Fortunately, the simulation fits quite well with the experimental results. The above comparisons of Young’s and our former experiments declare that this mechanism is suitable and accurate for the O3-NOx reactions. For further investigation of the ozone-NOx reaction system, the condition at a molar ratio of O3:NO ) 1.5 was calculated and is shown in Figures 3 and 4. Figure 3 shows the dynamic conversion of NO, NO2, NO3, and N2O5 at a temperature of 423 K and initial NO concentration of 200 ppm. In the first 0.02 s, the concentration of NO decreased dramatically, along with the consumption of O3. But the curve slope of NO is much sharper than the O3, that is to

say, apart from the consumption of NO by reaction R7, reactions between NO and other radicals such as N/O/OH/M also play an important role, which cannot be ignored, according to elemental reactions of R29, and R31-R33 NO + M ) N + O + M

(R29)

NO + N ) N2 + O

(R31)

NO + O(+M) ) NO2(+M)

(R32)

NO + OH ) HNO2

(R33)

Figure 4 shows the corresponding mole fraction changes and the profile of NO conversion. In the first stage, the line of NO2 and the NO conversion coincide, which means NO2 is the primary product in the first stage. More than 90% of NO can be oxidized before 0.05 s. Because the ozone is overdosed for the oxidization of NO from reaction R7, the further conversion of nitrogenous species should be taken into account. Reactions R8 and R61 imply the deeper conversion of NO2 into NO3 and N2O5. But the results from Figures 3 and 4 show that reactions R8 and R61 are much slower than reaction R7. The predominant products of the ozoneNOx reaction are NO2, NO3, and N2O5, and the formation of NO3 and N2O5 need a longer reaction time than NO2. O3 + NO2 ) NO3 + O2

(R8)

NO2 + NO3 ) N2O5

(R61)

3. Computational Procedures 3.1. Governing Equations. The 2D flow configuration consists of a jet with diluted O3 gas into a coflow flue gas stream. The governing equations are the continuity, the momentum, the energy,

DNS of Ozone Injection Technology for NOx Control

Energy & Fuels, Vol. 20, No. 6, 2006 2435

and the species transport equations. The original equations are expressed as ∂F ∂Fui )0 + ∂t ∂Fxi

(1)

∂Fuj ∂Fujui ∂τij ) + ∂t ∂Fxi ∂xi

(2)

∂Fet ∂Fetui ∂qi ∂τjtuj ) + - qr + ∂t ∂xi ∂xi ∂xi

(3)

∂FYs ∂FYsui ∂YsVsi ) + ω˘ s + ∂t ∂xi ∂xi

(4)

where F is the mixture density, µi is the ith component of fluid velocity, et is the total energy, Ys is the mass fraction of species s (s ) 1,2, ..., N). The stress tensor, diffusion velocity, and the chemical production rate are τij, Vsi, and ω˘ s. In the energy equation, N qi is the heat flux, and qr ) ∑s)1 ω˘ shs0 is the chemical reaction heat release. They are described as follows Ys

N

R ) R0/M h ) R0

∑W s)1

(

N

s

)

1

p

(6)

(7)

hs ) h0s + cpsT

(8)

N ∂T qi ) -k + F hsYsVsi ∂xi s)1

∑

(9)

DsN∂Ts Ys ∂xi

(10)

k k

s)1

Vsi ) -

Where Ws is the molar weight of species s, h0s and cps are the standard formation enthalpy and specific heat at constant pressure of species s, respectively, and DsN is the binary diffusion coefficient of species s to N2, which can be described as13,15 DsN )

T k k , ) 2.58 × 10-5 FcpLes cp 298

0.7

( )

(11)

The Lewis numbers for O2, O, H, OH, H2O, and H2O2 are 1.11, 0.7, 0.18,0.73,0.83, and 1.12, respectively,15 and the others are fixed to 1.0. The specific heat and viscosity of the mixture vary, depending on the local area mixture components cp ) N N Yscps(T), µ ) ∑s)1 ∑s)1 Ysµs(T). The cps(T) is curve-fitted as a function of temperature. Here, we rely on the library functions CKCPMS of Chemkin III to do it.33 The viscosity of species µs(T) is determined by the n exponent equations µs µs0

≈

() T T0

∂xi

(12)

∂Fuj ∂Fujui ∂F 1 ∂τij )- + + ∂t ∂Fxi ∂xi Re ∂xj

(13)

(γ - 1)tr ∂F γk ∂2T γ - 1 φ+ ui ) + ∂xi ∂xi RePr ∂x2 Re pr

∂uj

+ γp

i

N

∑h

ω˘ s

0 s

s)1

(14)

2

∂FYs ∂FYsui tr k ∂ Ys 1 + ) + ω˘ (s ) 1,2, ..., N) (15) ∂t ∂xi PrRe cpLes ∂x 2 FrYsr s i

∑h Y - F + 2 u u s s

∂F

∂F ∂Fui + )0 ∂t ∂Fxi

(5)

∂ui ∂uj 2 ∂uk τij ) - pδij + µ + - δ ∂xj ∂xi 3 ij∂xk et )

quantities used are ur ) U1 - U2, the reference of velocity is the velocity difference of the jet and coflow; lr ) d, diameter of the jet; tr ) lr/ur, reference time; pr ) Frur2, reference pressure; Fr, kr, µr, cpr are the reference density, thermal conductivity, viscosity, and specific heat and refer to the properties of the jet flow at inlet conditions; Ysr is the reference of species mass fraction at the inlet boundary. The body force and radiation are neglected here. According to the mentioned reference quantities, the final normalized equations used in this paper can be written as

n

For the detailed chemical mechanism, the reaction rate and the species production rate ω˘ s are more difficult to solve with the increasing of elemental equations. Chemkin III is well-known and effective software for solving these kinds of kinetic problems. In this paper, the ω˘ s values were calculated by library functions of Chemkin dynamically in every time step. The conservation equations should be non-dimensionalized before they can be used in the final calculations. The major reference (33) Application Programming Interface Manual for Chemkin Software; Reaction Design: San Diego, CA, 2004.

where γ ) cp/cV, φ ) τij∂ui/∂xj. The energy equation is written in the form of p according to the relationship p ) FRT for an ideal gas. 3.2. Numerical Method. The computational domain is a 2D square. The diameter of the jet is d ) 0.974 mm and the simulation domain is L×R ) 30d × 18d with a grid resolution of 467 × 303. In order to understand the ozone injection process, the computational configuration is kept consistent with the real conditions, which is a cold ozone gas injection into a hot flue gas. The velocity of the jet is U1 ) 50 m/s, the temperature is 298 K, and the composition is 3000 ppm O3 gas with air as dilute medium. The coflow has traditional flue gas composition during coal combustion; for improving the calculation efficiency, the CO2, SO2, and H2O are replaced by N2 and the others are O2 ) 3.1%, NO ) 200 ppm. The velocity of the coflow is U2 ) 2.5 m/s and the temperature is 423 K. On the basis of the velocity difference of U1 - U2, a jet diameter of d, and inlet jet gas properties, the Reynold’s number of the flow is 3000, Pr ) 0.71. The equations are solved using a fourth-order accurate compact finite difference scheme for the evaluation of the spatial derivatives in the two directions. A fourth-order fully explicit compact-storage Runge-Kutta scheme is used to advance the equations in time. For the spatial DNS of jet reaction flows, the boundary conditions should be carefully treated. In the simulations, the specification of boundary conditions is performed by NSCBC (Navier-Stokes characteristic boundary conditions) derived by Poinsot and Lele.34,35 Besides that, a PML (perfectly matched layer) buffer zone was attached to outlet and tow-sides of boundaries by 16 grids with a 5% stretching ratio.36-39 On the basis of the Courant FriedrichsLewy (CFL) condition for stability, the time step is 0.1985 µs with CFL ) 0.15 in this paper. The velocity, temperature, and gas composition are imposed on the inflow boundary with a top-hat transition scheme.38,40 The initial conditions of the flow field are mapped from inlet to outlet with the inflow conditions. The mismatch between the imposed initial conditions and solutions will raise the initial flow vortexes. So, no external perturbations are imposed at the inflow conditions. Because of the heavy task of (34) Poinsot, T. J.; Lele, S. K. J. Comput. Phys. 1992, 101, 104-129. (35) Baum, M. T. P.; Thevenint, D. J. Comput. Phys. 1994, 116, 247261. (36) Hu, F. Q. J. Comput. Phys. 2001, 173, 455-480. (37) Stanley, S.; Sarkar, S. AIAA Journal 2000, 38, 1615-1623. (38) Stanley, S.; Sarkar, S.; Mellado, J. J. Fluid Mech. 2002, 450, 377407. (39) Fan, J. K. L.; Ha, M. Y.; Cen, K. Phys. ReV. E 2004, 70, 026303. (40) Stanley, S. A. Ph.D. Thesis, University of California, San Diego, 1998.

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Figure 5. Species mass fraction profile at time step of 41599, 8.26 ms: (a) O3, (b) O, (c) NO, (d) NO2, (e) NO3, and (f) N2O5.

calculation with a 65-step kinetic mechanism, parallel computing technology of MPI (message passing interface) was used and the simulations were performed on a 32 CPU cluster system with Myrinet network and Intel 2.8 G XEONs.

4. Results and Discussion 4.1. Instantaneous Flame Structure. Figure 5 draws the instantaneous images of the contour plot during the evolution

DNS of Ozone Injection Technology for NOx Control

of the reaction jet. Figure 5a-f represents the mass fractions of O3, O atom, NO, NO2, NO3, and N2O5, respectively. The simulation was run for approximately 10tc, where tc is the time unit of convection periods on the basis of the velocity and domain length scale tc ) 2L/(U1 + U2) ) 1.11 ms. There are 5714 time steps in each one tc on the basis of the time march step of 0.1985 µs. The snapshots in Figure 5 are the instantaneous species profile at time step of 41599 at 8.26 ms and 7.28tc. From Figure 5, the reactions process between O3 and NOx are clearly expressed on the basis of reactions R7, R8, and R61. Panels a and c of Figure 5 are the reactants of O3 and NO. NO2, NO3, and N2O5 are the reaction products at different reaction degrees. The concentrated O3-rich gas was emitted from the center jet nozzle with a higher speed and lower temperature. And the coflow gases outside the jet nozzle are filled with NO, O2, and N2 at a relatively lower speed and higher temperature. The two corresponding shear layers have complex interactions during the evolution of the reaction jet. There are various scale vortexes in the field, which play a key role for the mixing of reactants O3 and NO. The vortexes can be doubtlessly identified by the species profiles of O3 and NO. The vortexes that appear in the two sides of the jet begin to roll up alternately in the flow direction of 7d. With the growing of the vortex, the jet starts to swing around the center line unsteadily. The sequence of the vortex begins to disrupt at approximately 10d, with the attraction of the same rotation direction vortex and repulsion of different rotation direction vortex. Large scales of vortexes were formed downstream of the jet by continuous entrainment of the flue gas and interaction among the vortexes. Unlike the combustion of hydrocarbons, the reactions about NOx are usually relatively slow compared to the turbulence time scale. And so the reactions go deeply in the core of all kinds of vortexes, where the reactants mix well and have a relatively longer resident time in the flow field. The consumption rate of NO in Figure 5c interprets this kind of phenomenon well. The appearance situations of NO2 are significantly coupled with NO and O3. The quantities of NO2 are on the same order of magnitude with NO, that is, the NO is chiefly oxidized into NO2 by O3. Large amounts of NO2 appear in 15d downstream of the jet flow. The high concentration areas of NO3 coincid with NO2 but mostly appear later than 20d. In comparison with NO3 and NO2, the amount of N2O5 is quite small and the appearance areas shrink quite a bit and barely exist after 25d. The instantaneous image of O3 in Figure 5a also shows that there is still a lot of O3 remaining, especially in the center of the vortex, whereas the amounts of NO3 and N2O5 are still much lower; we can interpret this to mean that the reactants of NO2 and O3 coexist in the small gas pockets. This phenomenon fits well with the results in a mechanism study that NO2 is the dominant product and NO3 and N2O5 are the minority, although O3 is overdosed. Figure 5b shows that the O atom distributes in the flow field. Unlike OH as a key species in the H2 flame, the O atom acts as a kind of middle species for ozone decomposition and other elemental reactions that cannot be represented in the flame sheet. The O atom mainly appears at the edge of the vortex structure. The reason may be that the edge of the vortex is at the O3-air and NO-flue gas interface, where there are higher concentration gradients of O2 (from 21% to 3.1%), O3, and NO; the O atom may be formed according to reactions R9 and R29. But the detailed influences of temperature and species gradients on turbulence and the chemical reaction are still unclear and need further investigation. 4.2. Vortex Dynamic and Flow Field. Figure 6 shows the corresponding vorticity, density, pressure, and temperature field

Energy & Fuels, Vol. 20, No. 6, 2006 2437

of the reaction jet flow at 41599, 8.26 ms, and 7.28tc. Figure 6a is the vortex distribution in the flow field. There are two kinds of vortexes, for clockwise and counterclockwise, at the two sides of the jet. The positive value is for the clockwise eddies and the negative value is for the anticlockwise eddies. The absolute value represents the intensity of the vortex. The evolution and interaction of the vortex can be observed in Figure 6a much clearer than in Figure 5a. Before 10d, the two kinds of vortexes exist alternately and orderly at the two sides of the jet. With rolling up the vortex, two types of vortexes begin to interact with each other. At the same time, the velocity of the former vortex begins to decrease with the continuous entrainment of fresh gas. Consequently, the fresh vortex may catch up with the former vortex in the same side. It is known that vortexes with the same rolling direction will attract each other and exclude dissimilar ones. The fresh vortex catching up with the former will combine together later. The two lines of the vortexes will move to the opposite direction and mix with each other, with the complex interaction of the vortex downstream of the jet, which may serve as a great contributor to the mixing process. Figure 6b is the corresponding normalized pressure field and the reference value is pr ) 2658.15 pa. The pressure holes in the field imply the position of the vortex core. The biggest pressure hole at (-4,24) is the result of the combination of eddies, which can be found in the vorticity field. The density and temperature greatly depend on the gas composition, chemical reaction, and heat transfer in the local area. For the weak heat release of the reaction, the distribution of density and temperature are mainly determined by the mixing process, which are similar to the profile of O3 as shown in panels c and d of Figure 6. 4.3. Time-Averaged Statistics. The purpose of this paper is to investigate the ozone injection process for the oxidization of NO in flue gas. The overall conversion of NO along the jet flow was shown in Figure 7. The results were calculated from the integral value of NO in the Favre average species profiles along the jet flow. Figure 7 tells that the NO conversion rate slowly increases along with the evolution of the jet before 10d, which corresponds to the vortex dynamics for early-stage generation and development. The mixing of reactants is mostly controlled by the vortex behaviors. After 15d, the conversion rates of NO are quickly increased and reach 45.44% at the 30d outlet of the computational domain. The increase in NO conversion is the result of the vortex development and increasing resident time in the flow field, which improves the mixing and chemical reaction degrees. The flow time is 11.69 ms on the basis of the coflow velocity at the 30d outlet in this simulation. The range of the jet influence and the conversion rate of NO could be taken into consideration in future engineering design. 5. Conclusions The structure and reaction properties of the ozone injection process with a reaction jet by direct numerical simulations (DNS) were investigated. A 65-step mechanism between O3 and NOx was developed, and the kinetic calculation results showed good agreement with the public experiment results. The developed detailed kinetic mechanism was incorporated into a 2D DNS jet flow. The detailed mechanism was handled by Chemkin library and was running on the clusters of CEU (State Key Laboratory of Clean Energy Utilization in China). The results imply that the reaction of O3-NOx is relatively slow compared to the turbulence time scale; therefore, the chemical reactions go deeper in the core of vortexes with longer resident time and good mixing. The dynamics of vortexes make a great

2438 Energy & Fuels, Vol. 20, No. 6, 2006

Wang et al.

Figure 6. Vortex and flow field variables at time step of 41599, 8.26 ms. (a) vorticity; (b) normalized pressure p/pr, pr ) 2658.15 pa; (c) normalized density, F/Fr Fr ) 1.178 kg/m3; and (d) temperature, K.

disordered by the interaction of vortexes. NO2 is the main product of O3-NO reactions; it appears from the root of the jet and expands gradually after 15d. The production of NO3 and N2O5 is minor, despite overdosed O3, and they appear after 20d and 25d, respectively. The overall conversion rates of NO can reach 45.44% at the 30d outlet of the computational domain by the Favre average species profile along the flow direction. The simulation results can improve the understanding of the O3 injection process for NOx reduction both controlled by kinetic and mixing. The O3-NOx reaction jet property can be considered in future engineering design.

Figure 7. Overall NO conversion rate along the jet by transverse integral.

contribution to the mixing of the reactants and reactions. The vortexes begin to roll up alternately at about 7d from the root of jet. After 10d, the order of the two-line vortexes become

Acknowledgment. The authors acknowledge the financial support of the National Natural Science Foundation of China (50476059), Key Project of the Chinese National Programs for Fundamental Research and Development (2006CB200303), the National Science Foundation for Distinguished Young Scholars (50525620), and the Zhejiang provincial Natural Science Foundation of China (Z104314). EF0603176