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A combined experimental and computational fluid dynamics (CFD) modeling study of the reburn process under turbulent mixing conditions on a laboratory ...
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Energy & Fuels 2005, 19, 833-841

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An Experimental and Computational Fluid Dynamics (CFD) Simulation Study of Reburning under Laboratory Turbulent Mixing Conditions F. Liesa, Marı´a U. Alzueta,* A. Millera, and R. Bilbao Arago´ n Institute of Engineering Research, Department of Chemical and Environmental Engineering, University of Zaragoza, Campus Rı´o Ebro, 50018 Zaragoza, Spain Received September 22, 2004. Revised Manuscript Received January 18, 2005

A combined experimental and computational fluid dynamics (CFD) modeling study of the reburn process under turbulent mixing conditions on a laboratory scale has been performed. The experimental work includes novel laboratory results of reburn under different mixing conditions, and intends to cover the gap between the studies of the reburn process made in ideal reactors and the studies conducted in pilot-scale and full-scale plants that are available in the literature. A model has been developed using the AIOLOS software, together with a reduced description of the chemistry taken from literature, and it allows us to describe the reburn process, reproducing the main features that occur in the experimental system, and thus it can be used as a useful tool for prediction and optimization of the NO reduction by reburn. The results obtained indicate the necessity for considering a detailed description of mixing, because they indicate that simple modeling approaches, such as the plug flow reactor and the Zwietering configuration descriptions, fail to describe, in detail, what occurs experimentally in well-controlled laboratory experiments in which turbulent mixing occurs. In addition, the main effect of mixing between the reburn fuel and the primary combustion gases has been determined to be the modification of the local stoichiometry, which is ultimately responsible for the variation of the NO reduction. A significant synergy between the variables that are influencing the reburn process and the mixing conditions is observed, and this indicates the necessity of not neglecting any of the variables for the assessment of the NO reduction and for optimization of the reduction process.

Introduction The reburning technology has been largely studied in recent years. Since the first studies of Wendt et al.,1 based on the Myerson et al.2 finding that the interaction between hydrocarbons and NO resulted in an effective decrease of NO, many experimental laboratory-scale, pilot-plant, and modeling studies have been performed. Furthermore, demonstration of the process currently is observed in many achievements, and even several power plants all over the world have also installed reburn techniques in their boilers. Therefore, it is evident that the reburn technology is of high interest, and that the studies performed so far may be used for optimization of the full-scale applications. In this context, the models applicable for reburning, based on experimental work, are a very valuable tool. Reburn studies existing in the literature can be divided into (a) basic knowledge studies, such as those experimental works performed in laboratory reactors (plug flow and perfect stirred reactors, and flames) under a variety of conditions,3-6 and the kinetic modeling of the process, through detailed reaction schemes * Author to whom correspondence should be addressed. Telephone: +34976761876. Fax: +34976761879. E-mail address: [email protected]. (1) Wendt, J. O. L.; Sterling, C. V.; Matovich, M. A. Proc. Combust. Inst. 1972, 14, 897-904. (2) Myerson, A. L.; Taylor, F. R.; Faunce, B. G. Proc. Combust. Inst. 1956, 6, 154-163.

that describe the reburn chemistry,6-9 or through moresimplified kinetic mechanisms;10-14 and (b) more-applied works, such as pilot-plant experimental works,1,15-19 (3) Bilbao, R.; Alzueta, M. U.; Millera, A. Ind. Eng. Chem. Res. 1995, 34, 4531-4539. (4) Alzueta, M. U.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1997, 109, 25-36. (5) Dagaut, P.; Lecomte, F.; Chevallier, S.; Cathonnet, M. Combust. Sci. Technol. 1998, 139, 329-363. (6) Kilpinen, P.; Glarborg, P.; Hupa, M. Ind. Eng. Chem. Res. 1992, 31, 1477-1490. (7) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. Combust. Flame 1998, 115, 1-27. (8) Smith, G. P.; Golden, D. M.; Frenklach, M.; Moriarty, N. W.; Eiteneer, B.; Goldenberg, M.; Bowman, C. T.; Hansson, R. K.; Song, S.; Gardiner, W. C.; Lissianski, V. V.; Qin, Z. GRI-Mech3.0. (Available via the Internet at http://www.me.berkeley.edu/gri_mech/1999.) (9) Frassoldati, A.; Faravelli, T.; Ranzi, E. Combust. Flame 2003, 135, 97-112. (10) Glarborg, P.; Lilleheie, N. I.; Byggstoyl, S.; Magnussen, F.; Kilpinen, P.; Hupa, M. Proc. Combust. Inst. 1992, 24, 889-895. (11) Bilbao, R.; Alzueta, M. U.; Millera, A.; Duarte, M. Ind. Eng. Chem. Res. 1995, 34, 4540-4548. (12) Hewson, J. C.; Bollig, M.; Byggstoyl, S.; Magnussen, F.; Kilpinen, P.; Hupa, M. Proc. Combust. Inst. 1996, 26, 2171-2179. (13) Giral, I.; Alzueta, M. U. Fuel 2002, 81, 2263-2275. (14) Han, X.; Ru¨ckert, F.; Schnell, U.; Hein, K. R. G.; Koger, S.; Bockhorn, H. Combust. Sci. Technol. 2003, 175, 523-544. (15) Spliethoff, H.; Greul, U.; Rudiger, H.; Hein, K. R. G. Fuel 1996, 75, 560-564. (16) Yang, Y. B.; Naja, T. A.; Gibbs, B. M.; Hampartsoumian, E. J. Inst. Energy 1997, 70, 9-16. (17) Miller, C. A.; Touati, A. D.; Becker, J.; Wendt, J. O. L. Proc. Combust. Inst. 1998, 27, 3189-3195. (18) Maly, P. M.; Zamansky, V. M.; Payne, R. L. H. Fuel 1999, 78, 327-334.

10.1021/ef049757s CCC: $30.25 © 2005 American Chemical Society Published on Web 03/12/2005

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Figure 1. Scheme of the experimental reactor and mobile sampling probe. Dimensions given in millimeters.

and integral modeling works devoted to include the effects of both chemistry and mixing.14,20-24 However, note that there is a significant distance between experimental laboratory studies, usually performed under mixing conditions, which can be considered as ideal, and real installations, in which turbulent mixing occurs. In this later case, the mixing process that occurs between the reburn fuel and the gases coming from the primary combustion zone may significantly affect the efficiency of the reburn process. Mixing conditions determine the local stoichiometries in the different areas of the reburn zone and, therefore, the oxidation process of the reburn fuel, thereby determining the level of NO reduction. Thus, mixing conditions may generate modification of the effect of other important variables, such as temperature, residence time, etc., together with synergic effects, which may result in consequences that are not always easy to predict. In this context, the present work is intended to contribute to cover the gap that exists between the available laboratory experimental studies on reburn, performed mostly in plug flow and perfect stirred reactors, and the pilot-scale plant results, in which control of the individual variables affecting the reburn process is difficult. To our knowledge, in the literature, no experimental study of the influence of the mixing process under turbulent conditions in well-controlled laboratory reburn conditions is available. To palliate this lack, we have conducted reburn experiments under a variety of operating conditions in a laboratory experimental installation in which turbulent mixing occurs, and we have analyzed the influence of important variables for the reburn performance, such as temperature and stoichiometry, and their synergy with the mixing conditions. The experimental conditions have been simulated using a computational fluid dynamics (CFD) code, together with a simplified description of the chemistry, taken from the literature, and the experimental results obtained have been satisfactorily predicted. Experimental Section The experimental laboratory installation used in the present work is described elsewhere,25-27 although slight modifications have been performed for the present study. Specifically, we have used a new reactor that has been designed and built for (19) Nazeer, W. A.; Jackson, R. E.; Peart, J. A.; Tree, D. R. Fuel 1999, 78, 689-699. (20) Alzueta, M. U.; Bilbao, R.; Millera, A.; Glarborg, P.; Østberg, M.; Dam-Johansen, K. Energy Fuels 1998, 12, 329-338. (21) Han, D.; Mungal, M. G.; Zamansky, V. M.; Tyson, T. J. Combust. Flame 1999, 119, 483-493. (22) Cha, C. M.; Kramlich, J. C. Combust. Flame 2000, 122, 151164.

the present experiments. In brief, the experimental installation includes a gas feeding system from gas cylinders, a reaction system constituted by the hereafter-named “turbulent reactor”, an electric oven that allows us to attain temperatures up to 1800 K, and a gas analysis system including continuous analyzers for CO, CO2, and NO, and a Fourier transform infrared (FTIR) spectrometer, which has been used to quantify the amount of CH4, C2H6, HCN, and NH3. The experimental uncertainty is estimated to be 2% for the different analyzers, but not less than (5 ppm. The reactor used for the present experiments was designed and constructed for the specific purpose of analyzing the effect of mixing under turbulent conditions, and it is shown in Figure 1, together with its main dimensions. It is composed of quartz and includes two separate co-flowing streams that are fed into the reaction zone coaxially. The inner stream flows through the hereafter-called “jet tube” at high velocity and mixes at the outlet of the jet tube with the stream that is entering through the outer coaxial tube under turbulent conditions. A mobile sampling probe was incorporated to the reactor, also shown in Figure 1, to take gas samples and measure the temperature in the reactor. The probe is formed by three concentric tubes. The sample is extracted through the internal tube of the probe with an inside diameter of 4 mm. The other two tubes in the probe correspond to the cooling system, with internal diameters of 8 and 22 mm, respectively. This cooling system is necessary to freeze reactions to prevent them from continuing inside the probe, and consists basically of the addition of cooling air. In addition, a thermocouple with a diameter of of 1 mm is located inside the probe. The experiments were performed under highly diluted conditions, using nitrogen as the carrier gas. A mixture of H2O, O2, and NO, which was representative of the main combustion products exiting the primary combustion zone in a reburn configuration, constituted one of the streams, whereas the second stream was formed by the reburn fuel mixture, which is a synthetic mixture prepared with ∼90% CH4 and ∼10% C2H6, which was representative of the composition of natural gas. The different mixing conditions are achieved by varying the amount of carrier gas, i.e., N2, in each stream, taking into account that mixing is dependent on the jet momentum and the relative mass flow of both streams. In all experiments, a globally fuel-rich stoichiometry (λ ) 0.8) was used, because it corresponds to optimum values for the reburn stoichiometry, using natural gas as a reburn fuel.4 In this context, the parametric experimental study performed includes the study of the influence of important variables for the reburn process, such as temperature and stoichiometry, under different mixing conditions. The different mixing conditions are achieved by means of the variation of (23) Lissianski, V. V.; Zamansky, V. M.; Maly, P. M.; Sheldon, M. S. Combust. Flame 2001, 125, 1310-1319. (24) Zarnescu, V.; Pisupati, S. V. Energy Fuels 2001, 15, 363-371. (25) Alzueta, M. U.; Bilbao, R.; Finestra, M. Energy Fuels 2001, 15, 724-729. (26) Alzueta, M. U.; Bilbao, R.; Glarborg, P. Combust. Flame 2001, 127, 2234-2251. (27) Lumbreras, M.; Alzueta, M. U.; Millera, A.; Bilbao, R. Combust. Sci. Technol. 2001, 172, 1-18.

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Table 1. Experimental Matrixa

exptb 1 2 3 4 5 6 7 8 9 10 11 12 13 14

oven total temp flow rate flow rate (K) (NL/min) ratio, ωc 1300 1300 1300 1400 1500 1550 1675 1675 1675 1700 1700 1700 1700 1700

25 35 50 50 50 50 50 25 50 50 50 50 50 50

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2

Modeling Section

inner stream

outer stream

CH4, C2H6 CH4, C2H6 CH4, C2H6 CH4, C2H6 CH4, C2H6 CH4, C2H6 CH4, C2H6 CH4, C2H6 NO, O2 CH4, C2H6 NO, O2 NO, O2 NO, O2 CH4, C2H6

NO, O2, H2O NO, O2, H2O NO, O2, H2O NO, O2, H2O NO, O2, H2O NO, O2, H2O NO, O2, H2O NO, O2, H2O CH4, C2H6, H2O NO, O2, H2O CH4, C2H6, H2O CH4, C2H6, H2O CH4, C2H6, H2O NO, O2, H2O

a Constant experimental conditions: 900 ppm CH , 100 ppm 4 C2H6, 1720 ppm O2, 800 ppm NO, 10 000 ppm H2O, and balance N2; λ ) 0.8. Average residence times up to 240 ms (Qtot ) 50 NL/ min), 340 ms (Qtot ) 35 NL/min), 440 ms (Qtot ) 25 NL/min). b Experiments 1-3: analysis of the influence of the degree of turbulence; experiments 3-7 and 10: analysis of temperature influence; experiments 7 and 9: analysis of the influence of the tube through which the reburn fuel is introduced; experiments 10-14: analysis of the influence of the flow rate ratio and the tube through which the fuel is added. c ω ) Qin/Qtot.

the flow rate ratio between the two streams (ω ) Qin/Qtot, where Qin is the jet flow rate and Qtot the total flow rate) flowing through the reactor, and by changing the tube through which each stream is introduced (i.e., the inner tube or the outer tube). Note that, basically, because of the laboratory experimental limitations and the scaling factor, the mixing time in the present work is less, compared to the time necessary for the macroscopic mixing to occur (the macromixing time) in real full-scale installations. Nevertheless, despite the differences in conditions corresponding to the different systems (the full-scale and the laboratory conditions), and despite the nondirect extrapolation adequacy of the present experimental results, they contribute new information that is related to the reburn process, under nonideal mixing conditions. Table 1 lists the experimental matrix. The experimental system was carefully characterized by means of the temperature profiles inside the reactor, as well as by evaluating the reproducibility of the experiments. It was also proved that the time necessary for a given species to reach the average concentration in the axis of the reactor tube, i.e., mixing time, increased when the flow rate is decreased (experiments 1-3 in Table 1), indicating that an increase of the total flow rate results in an effective increase of turbulence, thereby favoring the mixing process. After the influence of the turbulence conditions on the results was evaluated, the impact of temperature level was analyzed, by means of experiments 3-7 and 10 in Table 1. Increasing the temperature results in a higher conversion of hydrocarbons. The influence of the impact of introducing the reburn fuel by either the inner or outer tubes of the reactor was analyzed by means of experiments 7 and 9. Taking into account the results attained, an oven temperature of 1700 K and a total flow rate of 50 NL/min were selected to investigate the influence of ω and the influence of the tube through which the reburn fuel is added (experiments 10-14 in Table 1). Note that the temperature profiles are taken into consideration for simulations and that the reproducibility of experiments (experiments 12 and 13 in Table 1) was determined to be fairly good. For reasons of space, those results on reproducibility are not shown but can be found elsewhere.28 (28) Liesa, F. Influence of Mixing on Reburn Process (in Sp.), Ph.D. Thesis, University of Zaragoza, Zaragoza, Spain, 2004.

To simulate the conditions of the laboratory-scale experiments, a CFD program called AIOLOS (developed in the Institute of Process Engineering and Power Plant Technology (IVD) at the University of Stuttgart) has been used. The AIOLOS code is based on conservative finite-volume formulation, using the SIMPLEC (or SIMPLE) method for velocity-pressure coupling and the standard k- model or differential Reynolds stress model for turbulence. Further description of the code can be observed in the works of Schnell29 and Knaus et al.30 To account for the reburn chemistry, together with the fluid dynamics, we have used the augmented reduced mechanism (ARM) of Giral and Alzueta,13 which was developed from the GADM detailed mechanism for reburning that was reported by Glarborg et al.7 and validated under different reburn conditions.13 This ARM, which includes 19 chemical species and 15 lumped reaction steps, has been implemented in AIOLOS, partially modifying the source code related to this part. The interactions between chemistry and turbulence are modeled in AIOLOS using the eddy dissipation concept (EDC),31,32 which has been successfully used in previous applications for reburn and hybrid reburn/ selective noncatalytic reduction (reburn/SNCR) pilotplant results.14,15,33 The EDC model postulates that the turbulent flame space is divided into two parts: the socalled “fine structures” and the “surrounding fluid”. Homogeneous reactions that have more than one reactant are assumed to occur only in the fine structures, which are treated as locally well-stirred reactors, which transfer mass to the surrounding fluid. The mean residence time of the reactants in a fine structure (τ*) is determined by the dissipation of turbulent kinetic energy () and the dynamic viscosity (ν):

(ν)

τ* ) 0.41

0.5

(1)

The fraction of “fine structures” in the entire flow (γ*) is modeled as a mass fraction by34

[ ( ) ]

γ* ) 2.13

ν κ2

0.25 2

(2)

where κ is the turbulent kinetic energy. The reaction rate of each species k is calculated from the mass balance in the fine structure:35

Mkω/k )

F* j /k) (w/ - w τ*(1 - γ*) k

(3)

where ω/k is the molar rate of formation of the kth species in the fine structure, Mk the molecular weight, w/k the mass fraction of species k in the fine structure, (29) Schnell, U. Prog. Comput. Fluid Dynamics 2001, 4, 208-218. (30) Knaus, H.; Schnell, U.; Hein, K. R. G. Prog. Comput. Fluid Dynamics 2001, 1, 62-69. (31) Magnussen, B. F.; Hjertager, B. H. Proc. Combust. Inst. 1976, 16, 719-729. (32) Magnussen, B. F. The Eddy Dissipation Concept, XI Task Leaders MeetingsEnergy Conversion in Combustion, IEA, 1989.

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Figure 2. Two-dimensional grid of the reactor used for simulations.

Figure 4. Temperature and concentration profiles of CO, CO2, and NO along the axial positions in the reactor for different flow-rate ratios (ω): experiment 10 (ω ) 0.3) and experiment 14 (ω ) 0.2) from Table 1.

Figure 3. Temperature and concentration profiles of different species along the axial positions in the reactor for different oven temperatures and the conditions of experiments 6, 7, and 10 described in Table 1.

w j /k the mass fraction of species k in the surrounding fluid, and F* the density of the fine structure. The molar rate of formation of the kth species is given by

ω/k )

∑i (vi,k(r) - vi,k(f) )(K(f)i ∏l Clv

(f) i,l

- K(r) i

∏l Clv

i,l

(r)

) (4)

in which Ki is the rate constant of the ith reaction, vi,k is the stoichiometric coefficient of the kth species in the ith reaction, Cl is the molar concentration of the lth (33) Han, X.; Wei, X.; Schnell, U.; Hein, K. R. G. Combust. Flame 2003, 132, 374-386. (34) Magel, H. C.; Schnell, U.; Hein, K. R. G. Proc. Combust. Inst. 1996, 26, 67-74. (35) Fo¨rtsch, D.; Kluger, F.; Schnell, U.; Spliethoff, H.; Hein, K. R. G. Proc. Combust. Inst. 1998, 27, 3037-3044.

species, and the superscripts “(f)” and “(r)” indicate forward or reverse. In practice, and considering the reactor layout, axial symmetry has been assumed, to minimize the computing time. To assess the two-dimensional (2D) resultant system, a 2D grid, including 25 cells in the radial direction and 120 cells in the axial direction, was used (see Figure 2). Apart from the description of the mixing and reaction zones, the grid also includes a section of 30 cm upwind, because the results were determined to be dependent on the velocity profiles at the entrance of the mixing and reaction zones. Therefore, the simulation of the flow pattern upwind was necessary to have a more representative velocity profile in the simulations, compared to the experimental profile. For the simulations, the measured temperature profile in the axis is introduced in AIOLOS, with the assumption that the temperature profile in the radial direction, if any, is negligible. We need to assume that, because we cannot experimentally measure those radial profiles by means of our measuring probe; however, we do not expect them to be very important, because of the size and characteristics of the experimental system. However, to take this uncertainty into consideration, together with other uncertainties related to both the mixing description and the simplified kinetic scheme used, we have made simulations not only with the nominal temperature of the experiments, but also with this nominal temperature, (25 K (T ( 25 K) and -50 K (T - 50 K). In addition, we have made simulations for the conditions of experiments listed in Table 1 assuming instantaneous mixing by means of a plug-flow reactor configuration, to have a reference for comparisons, and also

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Figure 5. Flow pattern in the reactor for a total flow rate of 50 NL/min: (a) ω ) 0.3 and (b) ω ) 0.2. Position zero corresponds to the jet entrance.

using the simple approximation of Zwietering36 to account for mixing. This latter approach has been previously used in the literature with significant success to simulate the reburning results of pilot-scale plants.20 In the Zwietering approach, the influence of macromixing is represented through a mixing time derived either experimentally or through fluid-dynamic models, such as those present in AIOLOS. In this work, the mixing time used, together with the Zwietering approach, was determined from the experimental results. It was identified as the average residence time for which the NO level measured in the axis of the reactor corresponded to the global NO concentration. Following this procedure, we estimated mixing times of ∼35 and ∼55 ms for jet flow rates of 15 and 10 NL/min respectively, with a total flow rate of 50 NL/min. Because the concentration was measured every 5 cm with the mobile probe within the reactor, we estimate a maximum uncertainty in our estimated mixing time of (5 ms. Further analysis on the sensitivity of the Zwietering approach within this uncertainty did not reveal an important influence. Results and Discussion The experiments are analyzed through the evolution of the main species considered, i.e., CH4, C2H6, CO, CO2, and NO. As an example, Figure 3 shows the concentrations of those species, together with temperature in the reactor axis, for different oven temperatures (1550, 1675, and 1700 K) and as a function of the length in the reactor. In those experiments, the reburn fuel diluted with nitrogen with a flow rate of 15 NL/min is introduced through the inner tube, whereas the O2 and (36) Zwietering, Th. N. Chem. Eng. Sci. 1959, 11, 1-15.

NO mixture, which also is diluted in nitrogen, is added through the outer tube, at a total flow rate of 35 NL/ min. The high concentrations of CH4 and C2H6 and their fast decrease for reactor positions lower than 10 cm along the axis is due to the fact that the reburn fuel is introduced through the jet, and, for the closest positions to the jet, the mixing at the outlet is still very poor, resulting therefore in CH4 and C2H6 levels that are higher than the global levels, because the probe takes samples from the axis of the reactor. For reactor positions higher than 10 cm, the mixing process is almost fully completed and the CH4 and C2H6 concentrations decrease as the temperature is increased or the gases progress farther down the reactor. However, for the conditions of Figure 3, the total conversion of the reburn fuel is not achieved, because of a limitation in the reaction time. The partial oxidation of the methane-ethane mixture results in the formation of CO and CO2. The concentrations of CO and CO2 are very low for the oven temperature of 1550 K, which corresponds, according to the measured temperature profiles, to a maximum reactor temperature of ∼1350 K. As temperature increases, the concentrations of CO and CO2 increase, which is attributed to the major conversion of the reburn fuelsin particular, of methanesas the temperature is increased. The concentration of CO2 is always found to be higher than the CO concentration. This effect can be attributed to the fact that, even though the stoichiometry is globally fuel-rich (λ ) 0.8) within the reactor, areas with stoichiometries that are locally fuel-lean may exist, as a function of the mixing extent, thereby contributing to favor the evolution of CO to CO2. With respect to the NO concentration, the NO levels measured at the

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Figure 6. Stoichiometry (λ) profiles within the reaction zone: (a) experiment 10 from Table 1 and (b) experiment 14 from Table 1. Position zero corresponds to the jet entrance.

different oven temperatures considered and at the different location positions in the reactor are very similar (see Figure 3). A slight effect of reburn temperature is observed in the studied range, with a certain decrease of the NO concentration as temperature increases. A noticeable low NO reduction is also achieved, ranging from 3% to 13% under the conditions of Figure 3, which is due to the specific stoichiometry and mixing conditions considered, which are imposed principally by the characteristics and limitations of the experimental installation. Working under turbulent conditions in a laboratory installation is not easy to handle; therefore, there are some restrictions in relation with the intervals in which the variables studied can be modified. The low levels of NO for reactor positions closer than 10 cm to the entrance are not representative, because those are due to the fact that the reactant streams are not fully mixed yet, and because the NO is added through the outer tube while the probe is located in the axis of the reactor. As mentioned previously, to vary the mixing conditions, the flow-rate ratio between both streams (ω) has been varied. To do this, the amount of carrier gas (i.e., nitrogen) added to the reaction zone, both through the jet and the outer stream, is varied for a constant total flow rate. This variation results in a modification of the concentration of the species in each stream, thus changing local stoichiometries, while maintaining the same global stoichiometry of λ ) 0.8. This is the way to analyze how mixing may affect the reburn performance.

Figure 4 shows the results of temperature, CO, CO2, and NO obtained for two different experiments, in which, for a total flow rate of 50 NL/min, the jet contained 10 or 15 NL/min (experiments 14 and 10, respectively, in Table 1). The change from 10 NL/min to 15 NL/min represents an increment of the jet velocity of a 50%, forcing the mixing to occur more rapidly. Both experiments are performed, keeping the same initial global concentrations for reactants, but, logically, the local conditions in the 15 NL/min flow rate will be more fuel-lean, compared to using a flow rate of 10 NL/min. As observed, the temperature profiles for the two different experiments are very similar. Slightly higher concentrations of CO and CO2 are obtained when a higher flow rate is introduced through the jet stream. Experimentally, one can observe that the mixing time is higher when a flow rate of 10 NL/min is introduced in the jet, compared to the mixing time of a 15 NL/min jet, for a given total flow rate of 50 NL/min and an oven temperature of 1700 K. To support and explain the present results, Figure 5 shows the calculated velocity pattern for the two jet flow rates studied (ω ) 0.3 and 0.2). The different mixing conditions imply different results of the reburn process, and the changes in the local stoichiometry are usually believed to be the consequence of the mixing effects. Nevertheless, there is no evidence for this in the literature, and we have used AIOLOS to quantify the differences in the local stoichiometries in our reactor. Figure 6 shows the local

Turbulent Reburn

stoichiometry (λ) pattern, calculated with AIOLOS, inside the reactor for the same conditions of Figures 4 and 5. The results of Figure 5 show that the center of the eddy formed appears farther away as the jet flow rate decreases (ω ) 0.2), and that the radial velocities also are lower. This is due to the fact that, as the flow rate increases, the jet exhibits a higher intensity and, thus, macro-mixing may occur in a shorter time. This behavior is in agreement with the results experimentally observed, which indicate that the distance for which the concentration in the axis of the reactor is representative of the global concentration increases as the jet flow rate diminishes. With respect to the stoichiometry pattern, the only difference between the two parts of Figures 5 and 6 is the ω value, which is 0.3 in panel a and 0.2 in panel b, which implies a faster mixing for the conditions of Figures 5a and 6a, compared to the conditions of Figures 5b and 6b. The faster mixing for the conditions of Figure 6a, together with the transition from fuel-lean to fuelrich conditions occurring in a comparatively narrower area of the reactor, can be the reasons for the present results, i.e., the slightly lower NO concentrations for reactor positions farther than 10 cm down. Despite the fact that the differences in NO levels for both cases (experiments 10 and 14 in Table 1) are low, we have realized that, as mentioned previously, the reproducibility of the experiments is very good and, in addition, the small differences experimentally observed, in all the conditions studied, are captured by the model, as described below. Thus, we attribute the higher NO reduction for the conditions of Figure 6a (experiment 10), compared to Figure 6b (experiment 14), to the higher dispersion of the reburn fuel in the reaction zone, which, in the one hand, minimizes the mixing time and, on the other hand, makes the concentrations more homogeneous, compared to that obtained using a jet flow of 10 NL/min (Figure 6b). This is in agreement with literature studies (see, for example, Zarnescu and Pisupati24 and Chen et al.37) in which it is observed that, in the presence of globally reducing conditions, a major dispersion of the reburn fuel results into a major NO reduction. We have also analyzed the impact of introducing the reburn fuel through each of the two coaxial tubes. Because the flow rates flowing through each tube are different, both in amount and composition, their exchange results in different mixing conditions. An example of the evolution of temperature along the axis of the reactor, relative to the CO, CO2, and NO concentrations, as a function of the length in the reactor is shown in Figure 7, for a given total flow rate of 50 NL/min and for two ratios of the inlet to total flow rates, i.e., an inlet flow-rate of 15 NL/min corresponding to ω ) 0.3 (see Figure 7a, and experiments 10 and 11 in Table 1) and an inlet flow rate of 10 NL/min, corresponding to ω ) 0.2 (see Figure 7b, and experiments 13 and 14 in Table 1). The formation of CO and CO2 seems to be favored when the reburn fuel is introduced through the outer tube (see experiments 11 and 13 in Table 1). This is due to the fact that the reburn fuel is more diluted when it (37) Chen, S. L.; McCarthy, J. M.; Clark, W. D.; Heap, M. P.; Seeker, W. R.; Pershing, D. W. Proc. Combust. Inst. 1986, 21, 1159-1169.

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Figure 7. Temperature and concentration profiles of CO, CO2, and NO along the axial positions in the reactor for different reburning fuel stream introduction (experiments 10, 11, 13, and 14 from Table 1): (a) ω ) 0.3 and (b) ω ) 0.2. In experiments 10 and 14, CH4 and C2H6 were introduced through the jet, whereas in experiments 11 and 13, CH4 and C2H6 were introduced through the outer stream.

enters through the outer tube, because the global concentration is constant while the outer stream has a higher flow rate. Furthermore, when a jet at high speed enters into a flow with a lower speed, a net flow of mass is entrained into the jet, because of the lower pressure of the high-speed stream. Therefore, when the reburn fuel is introduced through the outer stream, it is entrained into the jet containing oxygen; thus, the stoichiometry conditions are favorable for the hydrocarbons to be oxidized. In contrast, when the reburn fuel is injected in the jet (experiments 10 and 14), the oxygen entrained into the jet is not able to oxidize the hydrocarbons, because the jet environment is fuel-rich and, therefore, oxidation is limited. The NO concentration decreases when the reburn fuel is added through the outer tube, which is produced because of the major conversion degree of hydrocarbons active in NO reduction. Furthermore, as can be seen, a lower flow rate through the jet stream (Figure 7b) implies lower CO and CO2 concentrations, compared to the case in which a higher flow rate is introduced (Figure 7a). A shorter mixing time, i.e., fast mixing, allows the process to have a higher residence time available for reaction. As mentioned previously, we have used AIOLOS together with a simplified kinetic scheme13 for the chemistry to simulate what happens in our reactor. For the simulations, as mentioned previously, we have made simulations not only with the nominal temperature measured in the axis of the reactor in the experiments, but also with this nominal temperature, (25 K (T ( 25 K) and -50 K (T - 50 K). Figure 8 shows an example

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Figure 8. Comparison between experimental and calculated results for the conditions of experiment 10 (Table 1) with AIOLOS. Symbols denote experimental data and lines represent model calculations.

of the comparison between experimental results and CFD simulations for the conditions of experiment 10 in Table 1. As seen, when the measured temperature profile is introduced in AIOLOS, although the NO concentration is well-represented, the CO and CO2 levels are significantly overpredicted. As mentioned previously, because the calculations were determined to be very sensitive to temperature, we made new calculations with a temperature profile of T ( 25 K, respectively, as shown in the Figure 8. Although the T + 25 K predictions are worse, the T - 25 K calculations are observed to be more similar to the CO and CO2 experimental data. An additional AIOLOS calculation for temperatures corresponding to T - 50 K results in a significantly better approach to the experimental results for CO, CO2, and, more importantly, the NO concentration. This sensitivity to temperature may be attributed to both the reaction mechanism and the fluid-dynamics description and resolution in the CFD code. Note that the model approaches very well all the experiments made, with the 50 K shift in the temperature. To test the utility of other approximations for mixing, as has been mentioned previously, we also tested the performance of the plug-flow reactor configuration and the Zwietering approach for the present results. The Zwietering approach should represent an improvement, compared to plug-flow simulation results, and has been used with significant success in given simulations of pilot-plant results (see, for example, Alzueta et al.20). The comparison of the experimental results of experi-

Liesa et al.

Figure 9. Comparison between experimental and calculated results for the conditions of experiment 10 (Table 1) with AIOLOS, the plug flow configuration (PFR), and using the Zwietering approach (FPR-ZM). Symbols denote experimental data and lines represent model calculations.

ment 10 in Table 1 with the modeling results obtained with the AIOLOS model and the plug-flow reactor and the Zwietering approach models are shown in Figure 9. It is observed that neither the plug-flow approach nor the Zwietering configuration are able to describe, with detail, the experimental results, because the CO and CO2 levels and the NO reduction are significantly overpredicted, even for the temperature of T - 50 K. These discrepancies can be attributed to giving no consideration of mixing at all in the plug flow configuration or to the important simplification of the mixing description that is done in the Zwietering approach, which only considers a distributed macroscopic mixing in the axial direction, assuming an instantaneous molecular mixing and constant concentration profiles in the radial direction. Therefore, this fact implies that simple mixing approaches for describing experimental results in which mixing is present should be cautiously used, and that those simple approaches may not always be suitable for prediction in pilot-scale and full-scale plants. Figure 10 shows an example of the comparison between experimental and calculated results with AIOLOS for the two different alternatives of introduction conduct of the reburning fuel, using a temperature profile of T - 50 K. The model is able to reproduce the observed experimental trends, as observed in Figure 10 for the concentration of NO, both when the NO is added through the outer stream (experiment 10) and through

Turbulent Reburn

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Figure 10. Experimental and calculated NO concentrations with AIOLOS for different mixing conditions for experiments 10 and 11 from Table 1. In experiment 10, CH4 and C2H6 were introduced through the jet, whereas in experiment 11, CH4 and C2H6 were introduced through the outer stream. Symbols denote experimental data and lines represent model calculations.

the inner jet (experiment 11). Interestingly, note that, despite the slight differences of the results obtained in both cases, the model very well predicts the NO levels that have been obtained experimentally. Conclusions A combined experimental and computational fluid dynamics (CFD) modeling study of the reburn process under turbulent mixing conditions has been performed. The experimental work, which was conducted in a laboratory setup in which turbulent mixing occurs, can contribute to covering the gap between the existing ideal

reactor studies of the reburn process and the studies performed in pilot-scale and full-scale plants. The results obtained indicate that the use of a plug-flow approximation or simple mixing approaches such as the Zwietering configuration are not able to reproduce the experimental findings obtained under turbulent mixing conditions, indicating the necessity of a more-detailed description of the mixing in many cases. In addition, the main effect of mixing between the reburn fuel and the primary combustion gases seems to be the modification of the local stoichiometry, which is responsible for the variation of the NO reduction. A significant synergy between the variables influencing the reburn process and mixing conditions is observed. The developed CFD model, including a simplified scheme of the chemistry, allows us to describe the reburn process by reproducing the main features, and, thus, it can be used as a useful tool for prediction and optimization of the NO reduction by reburn. Acknowledgment. The authors express their gratitude to the European Union (EU) (under Contract No. ENK5-CT-2000-00324) and the MCYT (under Project No. PPQ2001-4913-E) for funding, and the IVD group for providing us the AIOLOS code for its use in the frame of the previously mentioned EU program. In addition, Dr. Uwe Schnell and Mr. Jorge Santamarı´a are gratefully acknowledged for their help with the use of the code. EF049757S