Influence of Reactant Mixing in a Laminar Flow Reactor: The Case of

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Influence of Reactant Mixing in a Laminar Flow Reactor: The Case of Gas Reburning. 1. Experimental Study F. Liesa, M. 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

An experimental work of the reburn process in a laminar flow reactor with two coaxial feeding streams has been performed, with the objective of analyzing the influence of reactant mixing in this reactive system. The ways of reaching different mixing conditions have been diverse: by means of changing the velocity ratio among the two streams through which reactants are fed into the reactor, and by introducing the reactants into the reactor through different conducts (fuel introduced through the inner stream and oxidant introduced through the outer stream and vice versa). Moreover, the influence of the main variables of the reburn processs average residence time, temperature, and stoichiometryshas been studied under laminar flow conditions. In addition, the results achieved in the present work have been compared with data from the literature obtained from experiments conducted under ideal plug-flow reactor conditions. The results show an important influence of the reactant injection mode, which affects the reaction conditions. That influence is mostly attributed to the combination of the effects of the local stoichiometry conditions within the reaction zone and the residence time distribution. Introduction In many real gas-phase processes, reactants are fed into the reaction system in a non-premixed manner. The description of these reactive systems usually requires information about the way in which reactant mixing occurs to know the specific reaction conditions. Different situations can be assumed, depending on the magnitude of the temporal scales of the reactant mixing and the chemical reaction. If the chemical reaction is very fast (i.e., short reaction times), compared to the mixing process (i.e., long mixing times), the chemical reaction can be considered to be confined to a flame sheet and chemical equilibrium can be assumed, with negligible error.1 Thus, mixing among reactants controls the entire process. When the mixing time is very low, compared to the reaction time, then the chemical reaction controls the entire process. However, for many real gas-phase processes, the reaction and mixing times are of the same order of magnitude and, therefore, the previously mentioned simplifications cannot be made. In this latter case, both factors are important and may determine the extent of the reactions and the selectivity achieved. Therefore, both factors must be addressed to achieve an appropriate treatment of the system. The study of gas reactions on a laboratory scale usually considers that reactant mixing occurs before the reaction proceeds (i.e., using premixed conditions or assuming that mixing time is very little, compared to reaction time). In addition, the plug-flow reactor (PFR) model, in which all molecules are assumed to have the same residence time, is usually considered to describe these systems theoretically. Therefore, the reactant mixing process in these systems is * To whom correspondence should be addressed. Tel.: +34 976 761876. Fax: +34 976 761879. E-mail: [email protected].

usually very different, compared to real applications in which, as mentioned previously, the process occurs under non-premixed conditions and, moreover, there is a residence time distribution of the molecules. In this context, the main objective of the present work is to analyze the influence of mixing on a non-premixed chemically reacting system, i.e., how mixing effects affect the reaction process. This study can be helpful to understand and evaluate the effect of mixing in reactive systems. To do this, we have chosen a very well-known chemical reaction process in which mixing may be critical: gas reburn. Reburning is a commercial technology that, by introducing fuel as a reducing agent, removes NOx from the combustion products coming from primary combustion. Primary combustion products and reburning fuel are not premixed before entering into the combustion chamber. Therefore, both the mixing process and the chemical reaction may be important to describe the process properly. The efficiency of NOx reduction is dependent on many factors, such as fuel type, temperature, residence time, stoichiometry, initial NOx concentration, and mixing. The choice of this process is made because of different reasons: (a) despite its complexity, it is a very well-known chemical reaction that has been extensively studied in detail under well-controlled laboratory conditions recently, both experimental (see, for example, refs 2-7) and theoretically (see, for example, refs 7-13); (b) it is a process of real practical interest, because of its high potential for reducing NOx in several installations; and (c) there is, in particular, a significant interest in the simultaneous effects of mixing and chemical reactions for the performance of the reburn process in real applications, as shown in the literature.14-16 In fact, many authors (e.g., refs 17-23) have suggested the mixing between primary combustion products and reburning fuel to be

10.1021/ie060943q CCC: $37.00 © 2007 American Chemical Society Published on Web 04/10/2007

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one of the most important factors that affects NOx reduction through reburning. Specifically, in the present work, we analyze the influence of reactant mixing on hydrocarbon oxidation and nitrogen compound evolution under reburn conditions. As in real application systems, in the present experimental study, the reactants are not premixed and therefore mixing occurs in the reactor. A recent laboratory study of reburn under turbulent flow conditions16 showed that the mixing time, in that case, was less than that in full-scale applications. For this reason, laminar flow conditions, implying higher mixing times, are used, with the objective of simulating the high mixing times, corresponding to real-scale processes. Moreover, we must take into account that, under laminar flow conditions, there is a velocity profile in the radial direction, which implies a residence time distribution of the molecules in the reactor, which may also affect the reburn process. In addition, the influence of other key variables for the reburning process (residence time, temperature, and stoichiometry) will be analyzed under laminar flow conditions. Experimental Section The experimental laboratory installation used in the present work is described elsewhere.24-26 For this study, we have used a new reactor that has been specifically designed for the present experiments. Briefly, the experimental installation includes a gas feeding system from gas cylinders, a reaction system constituted by the hereafter-named “laminar flow reactor” and an electric oven that reaches temperatures up to 1500 K in the experiments, and a gas analysis system (including continuous analyzers for CO, CO2, NO, and NO2, and a Fourier transform infrared (FTIR) spectrometer that has been used to quantify CH4, C2H6, HCN, and NH3. The concentration of both NO2 and NH3 were negligible under the experimental conditions used in this work. The reactor used for the present experiments is shown in Figure 1, together with its main dimensions. It is composed of quartz and includes two separate co-flowing streams (labeled “1” and “2” in Figure 1), which are fed into the reaction zone coaxially. Both streams flow without mixing through the heating zone (labeled as “4” in Figure 1). After the streams penetrate in the region labeled as “5” in Figure 1, they start to mix with each other, mainly by diffusion, and react. In this reactor, as a difference with ideal laboratory reactors, the mixing time is not short enough to be considered negligible, compared to the reaction time; therefore, both mixing and reaction occur simultaneously. After the gases arrive at the end of the reaction zone, they are conducted through the outlet stream (labeled as “7” in Figure 1). A cooling system (labeled as “6” in Figure 1) is necessary to freeze the reactions and prevent them for continuing outside the reaction zone. In addition, a thermocouple 1 mm in diameter is located inside the reactor (labeled as “3” in Figure 1). The experiments were performed under highly diluted conditions, using nitrogen as a carrier gas. Two main streams were fed through different inlets: one mixture of H2O, O2, and NO, as a representative of the products exiting the primary combustion zone of interest in a reburn configuration, and a second mixture that contained the reburn fuel, which is a synthetic mixture prepared with ∼90% CH4 and ∼10% C2H6, as a representative of the natural gas composition. Table 1 shows the conditions of the experiments performed. The values of the concentrations of the reactants correspond to global values, by considering the total amount of gas introduced through both inlets.

Figure 1. Detail of the laminar flow reactor. Dimensions are given in millimeters.

The concentration of hydrocarbons used has been 800 ( 25 ppm of CH4 and 95 ( 5 ppm of C2H6, which yield to stoichiometry values of λ ) 0.57-1.98. The NO concentration was maintained constant for every set of experiments that was used, to study the influence of a specific variable (800 ( 25 ppm for experiments 1-15, 490 ( 10 ppm for experiments 16-19; see Table 1). Water concentration was set between 1% and 2% in the experiments. The influence of water concentration between these values was analyzed and important differences were not observed for the reburn experiments.27 To obtain different mixing conditions in the reactor, the velocity ratio (R) between the streams fed into the reactor was changed. The values of R correspond to the ratio between the

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Table 1. Experimental Matrix set

inner tube

O2 (ppm)

λ

total flow rate (NmL/min)

tra (s)

R

set

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

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

1070 1080 1080 1155 1100 1094 1149 1094 1099 1081 1174 1925 3789 3747 1136 1140 1154 1148 1151

0.57 0.58 0.57 0.60 0.60 0.57 0.60 0.60 0.58 0.57 0.60 1.00 1.97 1,98 0.60 0.59 0.60 0.59 0.59

2665 3946 4622 7034 2785 4545 7042 2795 3925 4567 6986 4616 4618 4669 7073 7080 7091 7004 6966

2133/T[K] 1440/T[K] 1230/T[K] 808/T[K] 2041/T[K] 1251/T[K] 807/T[K] 2034/T[K] 1448/T[K] 1245/T[K] 814/T[K] 1231/T[K] 1231/T[K] 1217/T[K] 804/T[K] 803/T[K] 802/T[K] 811/T[K] 816/T[K]

1 1 1 1 1.9 1.9 2 7.9 7.7 8.3 8 1 1 8 2.8 1 2.9 4.9 7.9

1A

a The average residence time (t ) is dependent on the temperature, as r listed. The temperature range of study is 900-1500 K.

average velocity of the inner flow and the average velocity of the outer flow. This variation was achieved by means of changing the amount of carrier gas introduced through each stream, which produces a modification on reactant concentrations in each stream. However, global concentration conditions were kept constant by maintaining the total amount of carrier gas. Therefore, by changing the velocity ratio (R), a modification of the conditions within the reactor is obtained (mainly a change in the stoichiometry values along the radius). Moreover, experiments are performed in two different ways: (a) the H2O/O2/NO mixture is fed through the outer tube and the reburn mixture through the inner tube, and (b) the H2O/ O2/NO mixture is fed through the inner tube while the reburn mixture is added through the outer tube. These two different feeding ways result in radial zones with significantly different stoichiometries, which, furthermore, have a different residence time distribution inside the reactor. For example, when the stream that contains oxygen is fed through the outer tube, the most oxidizing areas are present close to the reactor walls. Near the reactor walls, the flow is slower, because of the laminar flow inside the reactor, leading, therefore, to a higher residence time of those molecules. Thus, these different ways of feeding the reactants (procedures (a) and (b)) may be responsible for a different behavior of the reaction process. The parametric experimental study performed under laminar flow conditions includes the study of the influence of important variables for the reburn process, other than mixing, such as residence time, temperature, and stoichiometry. The influence of these variables was studied for different values of R. All the experiments that have been performed allow us to study of the influence of temperature, because, in each experiment listed in Table 1, the temperature was varied from 900 K to 1500 K, in intervals of 25 K. The results of experiments 1-11 allow us to study the influence of the average residence time (tr) for three different R values. The average residence time, tr, is changed by varying the total flow rate and is calculated as function of the temperature:

tr )

reactor volume total flow rate

Despite the variation of the total flow rate (Table 1), laminar

a

fuel CH4/C2H6 concentration (ppm)

O2 (ppm)

λ

NO (ppm)

H2O (ppm)

residence time, tra (s)

2770/260

4885

0.76

850

20000

170/T[K]

The residence time (tr) is dependent on the temperature, as listed.

flow conditions are assumed in all the experiments, because the highest Reynolds number was 800 ms. This average residence time needed is much higher than that in PFR experiments.6 For these high temperatures, mixing and reaction times are of the same order of magnitude and, thus, mixing effects become important, as is indicated in a parallel modeling study.32 It is worth mentioning that residence times needed to obtain the highest NO reduction under laminar flow conditions are similar to those needed in pilot-plant and realscale installations.29-31 According to the average residence time, total flow rates in the range of 4000-7000 NmL/min were used to study the influence of the other variables to ensure that higher average residence times (i.e., lower flow rates) do not have a major influence on the total outlet concentration of species. The influence of another key variable for reburnings temperatureshas also been studied. Figure 3 shows an example of the outlet CO2 evolution divided by the inlet amount of carbon (Cin, defined as methane plus ethane, given in ppm), and nitrogen species (NOin, defined as the NO and HCN outlet concentration

divided by the initial concentration of NO), as a function of temperature. Results shown are obtained in the two different reactor configurations: the “laminar flow reactor” of the present work (experiment 3 in Table 1) and the PFR of Alzueta et al.6 (experiment 1A in Table 2). Because the two types of datasets are taken from different reaction systems, the experimental conditions are not equal. Specifically, the absolute levels of hydrocarbons are different, but the global stoichiometry (λ) is maintained fuel rich in both cases (see Tables 1 and 2). Figure 3 is useful to show some differences that mixing implies, such as, for example, the different evolution of the concentrations of the species, depending on the temperature. In particular, for the conditions of Figure 3, the experiments performed under laminar mixing conditions show that the onset of CO2 formation occurs at a lower temperature, compared to the PFR results (premixed conditions), despite the globally lessreducing conditions of the PFR experimental data. In the laminar flow reactor, the non-premixed conditions are responsible for the existence, within the reactor, of oxidizing zones (despite the globally rich conditions). Hydrocarbons, which are fed through the inner stream, when diffusing, encounter an environment that is more concentrated, in regard to oxygen, near the reactor wall and, thus, the appearance of CO2 is actually possible at low temperatures. In regard to NO, the onset of conversion of this compound also occurs at lower temperatures for laminar flow conditions than for PFR conditions. This effect could be a consequence of the lower temperature of the hydrocarbon oxidation onset. For the temperatures at which the conversion onset is produced, below ∼1300 K, the reaction time is greater than the mixing time. In addition, although the laminar results exhibit a progressive evolution of the species considered as a function of the reaction temperature, the PFR results show a sharp increase in the CO2 concentration, accompanied by a sharp decrease in NO at ∼1300 K. For higher temperatures, species concentrations do not vary appreciably in the PFR configuration, whereas in the laminar mixing experiments, the CO2 concentration and NO reduction continue to increase in the temperature range studied. This is attributed to the fact that, in the PFR configuration, the stoichiometry conditions are equal for a given residence time, and, therefore, the NO reduction proceeds as soon as the hydrocarbon radical pool is built up. In the laminar mixing reactor configuration, the stoichiometry conditions vary along the radius of the reactor, thus chain carriers of the reaction diffuse in the system, and, therefore, for a given residence time, the reactant molecules are not present in the same conditions of stoichiometry. There is a two-dimensional (axial and radial) distribution of the stoichiometry conditions in the reactor and a residence time distribution for these local conditions. This fact, which occurs under laminar mixing, is responsible for the progressive increase in CO2 and the progressive decrease in NO. Stoichiometry is a parameter of high importance in the process studied. Reburning studies conducted under PFR conditions show an optimum for NO reduction for λ values between 0.533 and 0.7-0.9,6 so that, generally, fuel-rich conditions are needed to obtain an optimum reburn performance. We studied the influence of the stoichiometry for λ values of ∼0.6-2 in the laminar flow reactor. Note that this value of λ logically corresponds to the stoichiometry in the experiments calculated considering the total amount of fuel and oxygen entering into the reactor in the present reburn experiments. This is the usual way to express stoichiometry in laboratory studies, and this value may be significantly different, compared to the global stoichiometry values referenced in boilers of practical installations.

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Figure 5. Influence of the velocity ratio (R) on NO concentration for different temperatures. Reburn fuel fed through the inner tube (experiments 3, 6, and 10; see Table 1).

Figure 6. Influence of the velocity ratio (R) on NO concentration for different temperatures. Reburn fuel fed through the outer tube (experiments 16, 17, 18, and 19; see Table 1).

Figure 4. Influence of stoichiometry (λ) on the CO, CO2, and NO concentrations in the laminar flow reactor (experiments 3, 12, and 13; see Table 1).

Moreover, the influence of the stoichiometry was studied for two different velocity ratios: R ) 1 (results shown in Figure 4) and R ≈ 8 (not shown). In both cases, similar trends of the influence of stoichiometry were observed. Figure 4 shows the CO and CO2 concentrations and NO reduction, as a function of temperature obtained in the laminar flow reactor for R ) 1. It is observed that the formation of CO and CO2sand, therefore, the oxidation of the methane/ethane mixture, as well as the onset of NO reductionsoccur at lower temperatures as the stoichiometry becomes leaner. This trend is also found under PFR conditions8 and may be attributed to the more-oxidizing environment that exists in the reactor as λ increases. In the non-premixed laminar flow experiments, the λ values at which the highest NO reduction is attained are dependent on temperature. For temperatures of >1350 K, the highest NO reduction is attained for fuel-rich conditions (λ ) 0.6). For temperatures of 1175-1350 K, the highest reduction is obtained for λ ) 1, and for lower temperatures, the highest reduction is obtained for fuel-lean conditions (λ ) 2). It is worth emphasizing that certain NO reduction by reburn reactions is obtained even in the fuel-lean non-premixed laminar flow experiments (as mentioned in the Experimental Section, no appreciable conversion of NO to NO2 was observed). This can mean that a delayed mixing may increase NO reduction under stoichiometric or lean conditions. Miller et al.34 mentioned similar observations in a pilot plant, in which they obtained a

higher reburn performance in a globally lean atmosphere while optimum reburn conditions are usually referred to as being fuelrich. They already noted that the explanation for those results could rely on mixing effects. Thus, Figure 4 may confirm that a delayed mixing can improve the NO reduction under stoichiometric and oxidant conditions. It can be observed that, for temperatures of >1300 K, NO reduction for λ ) 1 seems to be limited, because an asymptotic behavior is observed. To summarize, we can conclude that results and conclusions on the influence of stoichiometry obtained from ideal reactors should not be directly extrapolated to real scale, because mixing conditions have an important role in the global process. Moreover, although the reburn process under full-scale operation has been mainly applied under globally fuel-rich conditions,34 the use of a globally fuel-lean environment may be of interest, because of the minimization of the unburned products and the minimization of the burnout zone that can be achieved.34,35 In addition, the feeding of the reburn fuel and the combustion air in any real boiler is made under non-premixed conditions, which are ultimately responsible for the mixing effects that happen. To analyze the influence of different mixing conditions under laminar flow, we have varied the flow velocities of the inner and outer streams for a given total flow rate (i.e., R). In these experiments, the only variation made is the amount of nitrogen that is added through each reactor tube, keeping constant the amount of reactants added and, thus, the global conditions. This issue results in two different effects. On the one hand, when the amount of nitrogen introduced through each tube is varied, the initial velocity profile at the entrance of the reactor is

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changed and, therefore, the velocity profile and mixing conditions inside the reactor, at the outlet of the inner tube, are changed. Previous studies27 showed that the impact of such influence is limited. On the other hand, when varying the added amount of nitrogen, the reactant concentrations in each stream also change, although the absolute amount of reactants remains the same. This results in a change of the local concentration conditions inside the reactor, thus modifying the local stoichiometry along the radius of the reactor, keeping the same global stoichiometry. Figure 5 shows the influence of the velocity ratio (R) on the concentration of NO, as a function of temperature, for a total flow rate of ∼4600 NmL/min. A slight difference between the NO concentrations obtained at the outlet of the reactor is observed for the different velocity ratios studied. For the highest value of R studied, R ) 8.3, most of the nitrogen is added through the inner stream. Therefore, the outer stream that contains O2 and NO enters into the reactor very concentrated. As a result of this, when the R value increases, the local stoichiometry is leaner, because the position is closer to the wall. These more-oxidizing conditions near the reactor walls favor the oxidation of hydrocarbons, and this could be the reason for the process to be initiated at lower temperatures, which is consequent with the results of Faravelli et al.36 For higher temperatures, the oxidation of hydrocarbons and the reduction of NO proceed faster, and this happens for the three different R values considered. Because the local conditions are more fuelrich, when diminishing R, a higher NO reduction is achieved for lower velocity ratios. Similar results are observed for different total flow rates of ∼2700, ∼4600, and ∼7000 NmL/ min (not shown in this work). To test the influence of the different mixing conditions for the reburn results further, we have varied the tube of the reactor through which the reactants are fed. Several experiments (experiments 15-19; see Table 1) were performed with this other configuration in which NO and O2 are introduced through the inner tube, while the reburn mixture is injected through the outer one. This change involves fuel-rich conditions being found near the wall, where molecules have a longer residence time, whereas fuel-lean conditions are found in the axis of the reactor, where the molecules have a shorter residence time. In Figure 6, the influence of R, when the H2O/O2/NO mixture is fed through the inner tube and the reburn mixture is added through the outer tube, is shown. We can observe that, in this case, the influence of R is opposite to that observed in Figure 5, in which the contrary feeding system configuration (the H2O/O2/NO mixture is fed through the outer tube and the reburn mixture through the inner tube) is used. This is logical because, in Figure 6, by increasing the value of R (i.e., increasing inner flow rate and decreasing the outer flow rate of carrier gas), the concentration of O2 in the inner stream decreases, which is just the opposite of what happened in the experiments presented in Figure 5. Thus, the analysis of these results is consistent with the previous results shown in Figure 5. Figure 7 shows the evolution of CH4, CO, CO2, and NO concentrations, as a function of temperature for two experiments (experiments 11 and 15; see Table 1) in which the same global concentrations of all the reactants are used, although, in experiment 11, the reburn fuel is added through the inner tube and, in experiment 15, it is added through the outer tube. The dilution level of both streams was kept equal in both experiments; therefore, at the entrance of the reaction zone, the stream that contained oxygen and NO had the same concentration level in both experiments. In a similar way, the concentration of the reburn fuel at the entrance was also constant in both experiments.

Figure 7. Influence of two reactant feeding configurations on the CH4, CO, and CO2 concentrations, and the NOout/NOin ratio, for different temperatures.

The results obtained indicate the strong effect of changing the feeding tube on the experimental results of CH4 conversion. For the lowest temperatures studied, below ∼1325 K, the CH4 conversion is favored when the reburn fuel is fed through the inner tube (experiment 11; see Table 1). For higher temperatures, the contrary effect is found. Similar observations can be made for CO, CO2, and NO. The onset of the formation of CO and CO2, together with the onset for NO reduction, occur at lower temperatures when the reburn fuel is fed through the inner tube, whereas, for high temperatures (higher than ∼1325-1350 K), the CO and CO2 levels, as well as the NO reduction, are higher when the reburn fuel is injected through the outer tube. It is interesting to note that the influence of mixing in Figures 5 and 6 was basically due to changes in the local stoichiometry

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coming from the different dilution of the streams fed into the reactor. Nevertheless, in Figure 7, the concentration of reactants prior to mixing is equal in both experiments, but the change of the fuel injection tube leads to very different local stoichiometry conditions along the radius in each experiment. The combination of oxidant conditions and a higher residence time near the wall lead to a decrease in the reaction onset temperature when the reburn fuel is injected through the inner stream. As a result, for low temperatures, the NO reduction process is favored when the reburn fuel is injected through the inner stream. For high temperatures, oxidation reactions occur, although conditions are fuel-rich. In this case, NO reduction is favored when reburn fuel is injected through the outer stream, i.e., richer conditions and longer residence time to interact with NO. Conclusions With the objective of analyzing the influence of mixing of the reactants in a chemically reactive system, an experimental study of the reburn process under laminar flow conditions has been performed. The results obtained in the present work have been compared with experimental data from the literature obtained under ideal plug-flow reactor (PFR) conditions. The results show an important influence of the manner in which reactants are injected (and, therefore, mixed) on the hydrocarbon oxidation process, on the NO reduction, and, consequently, on the efficiency of the reburn process. The average residence time needed to achieve a reduction of NO, independent of a further increase of the residence time, becomes shorter as the temperature increases, under nonpremixed laminar conditions, and it is much longer, compared to premixed conditions (i.e., PFR conditions), because a certain residence time is necessary for mixing. In regard to stoichiometry, certain NO reduction is obtained in laminar flow, even for fuel-lean conditions. Moreover, for low temperatures, the highest NO reduction is obtained under fuel-lean conditions. This may be possible because of the nonpremixed conditions used in the non-premixed laminar flow experiments, which imply the existence of locally fuel-rich conditions within the reactor, despite the globally fuel-lean conditions. Mixing effects may be responsible for this behavior. The influence of mixing for a non-premixed system under laminar conditions seems to be important, as a combination of two different aspects: (i) the influence due to the change in local stoichiometry, as a function of the radius; and (ii) the influence attributed to the residence time distribution of the reactants. It seems that the NO reduction is promoted at lower temperatures when, near the wall (longer residence times), locally leaner conditions are observed. This is due to the fact that lean conditions and longer residence times allow the onset of oxidation reactions and, thus, the formation of radicals that are able to interact and destroy NO, even at low temperatures. For higher temperatures, the oxidation process and radical formation proceed quickly, even under fuel-rich conditions, and, thus, the process is favored under locally reducing conditions near the wall, where the molecules have a longer residence time to interact with NO. Finally, it should be mentioned that the results and conclusions of the influence of certain variables (e.g., residence time, stoichiometry, temperature) obtained from ideal reactors should not be directly extrapolated to real scale, because mixing conditions have an important role in the process.

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ReceiVed for reView July 20, 2006 ReVised manuscript receiVed February 12, 2007 Accepted March 6, 2007 IE060943Q