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

A theoretical study of the influence of mixing on reburning under laminar flow non-premixed conditions has been performed. Two models with a different...
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Influence of Reactant Mixing in a Laminar Flow Reactor: The Case of Gas Reburning. 2. Modelling 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

A theoretical study of the influence of mixing on reburning under laminar flow non-premixed conditions has been performed. Two models with a different level of detail of the fluid dynamics description have been developed and adapted. Models have been validated against experimental results. The models are able to reproduce the experimental trends of the influence of the studied variables: temperature, stoichiometry, velocity ratio between the streams, and reactant feeding configuration. Modeling and experimental results have been compared with plug-flow reactor (PFR) results, showing significant differences. Reactant injection mode involves a change in local stoichiometry and residence time distribution in the reactor, which seems to be the main influencing factor. For low temperatures, a higher NO reduction is obtained when local fuel-lean conditions have a longer residence time. For high temperatures, a higher NO reduction is obtained when local fuel-rich conditions have a longer residence time. Introduction Reburning is a NOx control technology that involves the injection of a secondary fuel as a reducing agent (i.e., reburning fuel) into the so-called “reburning zone” downstream of the main combustion chamber of the furnace. The hydrocarbon radicals generated in the reburning zone provide the chemical pathways to reduce NOx to N2. The application of reburning process as a NOx control technology has been successfully demonstrated at full scale; therefore, it is a valuable technology to be further studied and optimized. In this process, reactants are not premixed and, in most cases, mixing and reaction times are of the same order of magnitude.1 Therefore, reburning is a process influenced by mixing effects. In fact, several authors2-8 have suggested that mixing between primary combustion products and reburning fuel is one of the main factors that influence NOx reduction performance through reburning. Thus, despite the valuable results obtained by means of reactors in which mixing effects are not considered, the performance of experiments to study the influence of mixing and the development of models to simulate mixing conditions are required. Most of the developed models include the influence of mixing by means of one of these two different approximations. Reburning fuel is entrained into the bulk flow coming from primary combustion, direct mixing,3,9,10 or the opposite situation, inverse mixing1,7-9,11,12 (i.e., the main flow is entrained into the reburning fuel jet). Both of these considerations improve the description of pilot- and real-scale reburn performance, compared to models that do not take into account mixing effects. Among these considerations, inverse mixing, which is a morerealistic description of physical processes, gives modeling results more approximated to experimental data.9,12 Despite the simplicity of this approximation, it takes into account the segregation of the reactants in the mixing area of the reactor, which may be the reason for its success.7 * To whom correspondence should be addressed. Tel.: +34 976 761876. Fax: +34 976 761879. E-mail: [email protected].

To describe the entrainment of the bulk flow into the reburning fuel jet, several approximations have been used, including instantaneous mixing, but with a distributed addition of the reactants in the reaction zone during a fixed period of time1,3,7,9-11 (mixing time), and the application of the two-stage Lagrangian (TSL) model.8,12 Using this model, the mixing time can be determined within the model itself. Moreover, it includes the reaction process, which occurs in an additional reaction zone (the flame sheet). The flame sheet temperature and composition can be different from the main reactor temperature and composition and, therefore, the reaction rate can also be different. When using a distributed addition of the reactants, mixing time is determined either empirically, or by means of the use of detailed fluid flow field calculation (e.g., by means of CFD calculations, or entrainment relations). Experimental work under pilot- and full-scale conditions have contributed to obtain interesting conclusions about reburning process. However, in these experimental facilities, the control and the study of how each variable (i.e., temperature, stoichiometry, reburning fuel, residence time, residence time distribution) influence the entire process, independent of the other variables, is quite complex. Moreover, the manner in which mixing occurs affects the local conditions in the reaction zones mainly, temperature, local stoichiometry, and residence time distributionsand affect the entire process. Because of the complexity of these reactive systems, a detailed description of both fluid dynamics and reaction processes is too computationally costly and most of the time, it is not feasible. Thus, the realization of a study of reburning process at laboratory scale, under well-controlled conditions to get a better understanding on how mixing affects the process, was considered interesting. Therefore, an experimental and modeling study of the influence of mixing under turbulent conditions was performed.13 In that study, it was stated that mixing has an effect on the local stoichiometry, which influences NOx reduction. Moreover, it was shown that simplified mixing models, as an inverse mixing model with a distributed addition of the reactants, did not reproduce the experimental behavior.

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Mixing times (35-55 ms) in the experiments under turbulent conditions were shorter than in several pilot-scale7,14,15 and fullscale facilities. That is the reason why it was thought that the influence of mixing could be visualized much better by working under laminar flow non-premixed conditions, because the mixing times under these conditions are longer. Therefore, an experimental study of the influence of the different variables under non-premixed laminar conditions was performed.16 In this work, additionally, the result tendencies obtained from the experimental installation were compared with others obtained under plug-flow reactor (PFR) conditions. To analyze the results of those experiments,16 several models have been used and adapted. Similarly, these models allow us to study the influence of the mixing process between the reburning fuel and the primary combustion gases in the NO reduction through reburning from a theoretical viewpoint. The models used here do not require adjustable parameters. Modelling of Reburning Process under Laminar Mixing Conditions The theoretical description of the reburning process requires modeling of the chemical processes and mixing conditions that are occurring in the reactive system. Chemical processes are modeled by means of reaction mechanisms, and mixing conditions are determined by the fluid dynamics in the reactor as well as the conditions of the reactants at the entrance (premixed or non-premixed). Regarding the chemical kinetics of the reburning process, mechanisms with a different level of complexity can be found in the bibliography. The level of detail in the kinetic description of the reburning process determines the existence of reaction mechanisms that are classified as detailed mechanisms,17-21 simplified mechanisms,22-24 and reduced mechanisms.22,25,26 The different level of complexity of the mechanisms determines not only the accuracy of the results, but also their applicability to the modeling of systems, which are, more or less, complex, from the fluid-dynamics viewpoint. Thus, detailed mechanisms have a very wide application range, with regard to operational conditions (mainly, temperature and stoichiometry). Nevertheless, the computational cost requirements in complex systems are high and this significantly limits the use of the detailed schemes. Reduced mechanisms have a greater applicability, because of their simplicity and the low computational cost, compared with detailed mechanisms. However, in most cases, their application is restricted to a reduced interval of operational conditions. Because of computational advances, the reduced mechanisms currently developed27,28 incorporate a greater number of reactions and chemical species, because their application to fluid-dynamics complex systems without an excessive computational time is possible. These mechanisms, the so-called “augmented reduced mechanisms”, have an interval of validity of the variables that is higher, compared to the reduced mechanisms, which represents a significant advantage. In this work, the Augmented Reduced Mechanism (ARM) of Giral and Alzueta28 has been used to describe the kinetics of the process. The results obtained with this model for a wide interval of operation conditions were compared with those obtained from the detailed mechanism of Glarborg et al.20 and similar tendencies and results were obtained. The complexity of the fluid dynamics of the reactive system allows the use of this model with a reasonable computational cost, which makes possible to get trustworthy results, from the kinetic viewpoint. The ARM28 consists of 15 lumped reactions and 19 chemical species, whereas the initial mechanism20 consists of 438

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

reactions and 65 chemical species. It is noteworthy to note that the used ARM only describes NO reduction through reburn reactions, because a parallel experimental work16 showed that this was the only significant mechanism for NO decrease, under the conditions studied. The fluid dynamics of the process is determined by the characteristics of the reactor and the operational conditions of the system. The reactor used in the experiments16 is presented in Figure 1, together with its dimensions. It includes two separate co-flowing streams (labeled as “1” and “2” in Figure 1) that are fed into the reaction zone coaxially. Both streams flow without mixing through the heating zone (labeled as “4” in Figure 1). Therefore, the reactor operates in non-premixed conditions. After the streams penetrate in the feature labeled as “5” in Figure 1 they start to mix to each other, mainly by diffusion, and react. In this reactor, to be different from ideal

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Figure 2. Physical scheme and boundaries representation of the modeled system. Table 1. Experiments of Liesa et al.16 set

inner tube

O2 (ppm)

λ

total flow rate (NmL/min)

tra (s)

R

1 2 3 4 5 6 7 8

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

1174 1136 1080 1925 3789 1081 1140 1151

0.60 0.60 0.57 1.00 1.97 0.57 0.59 0.59

6986 7073 4622 4616 4618 4567 7080 6966

814/T[K] 804/T[K] 1230/T[K] 1231/T[K] 1231/T[K] 1245/T[K] 803/T[K] 816/T[K]

8 2.8 1 1 1 8.3 1 7.9

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

laboratory reactors, the mixing time is not short enough to be considered negligible, compared with the reaction time; thus, both mixing and reaction occur simultaneously and must be taken into account to describe the process theoretically. The experiments were performed under highly diluted conditions, using nitrogen as a carrier gas. The streams fed through the two different inlets consist of H2O, O2, and NO, to be representative of the products exiting the primary combustion zone of interest in a reburn configuration for one of the streams, and the reburn fuel, which is a synthetic mixture prepared with ∼90% CH4 and ∼10% C2H6, to be representative of a natural gas composition for the second stream. The global concentration of hydrocarbons diluted in N2 has been 800 ( 25 ppm of CH4 and 95 ( 5 ppm of C2H6 and the stoichiometry values (λ) used were in the range of 0.57-1.98. The temperatures used in the experiments were in the range of 1000-1500 K. The maximum total flow rate used in the experiments was ∼7000 NmL/min and the minimum was 2600 NmL/min that implies average residence times of 0.5-2 s. The specific experimental conditions considered in the present work are summarized in Table 1. Because of the experimental conditions used, the following approximations can be adopted: (1) The system can be considered axisymmetric, and, therefore, bidimensional. Taking into account this fact, the modeled system and its boundaries, where the boundary conditions are applied, is presented in Figure 2. (2) The reactor works under stationary conditions. (3) The reactor works under laminar flow conditions, because the Reynolds number is 1325 K, a greater NO reduction is obtained with the BLM and LMM models than with the PFR Model. For fuellean conditions (λ ) 2), this behavior is shifted to lower temperatures (∼1250 K); above this temperature, a greater reduction of NO is obtained with the LMM and BLM models, compared to the PFR model. This behavior may be attributed to the effect of non-premixed conditions that are modeled by LMM and BLM and not in PFR. Non-premixed conditions lead to the existence of fuel-rich zones within the reactor, despite the globally fuel-lean conditions. These fuel-rich zones, which do not exist under PFR conditions, favor NO reduction. Under fuel-lean and PFR conditions, the hydrocarbons oxidation process is too fast and does not allow hydrocarbon radicals to interact with NO effectively. Besides the differences previously explained, there is another difference between LMM, BLM, and PFR predictions for all the values of λ studied. There is a maximum of NO reduction with temperature that has been predicted by the PFR model that is not observed in any of the following cases: BLM model, LMM model, and experiments. Figures 5 and 7 show that the LMM and BLM models describe the experimental behavior better than the PFR model for fuel-rich, stoichiometric, and fuel-lean conditions in a wide range of temperatures. Moreover, results obtained with the LMM and BLM models are very similar to each other. As an example of the influence of stoichiometry under different mixing conditions, in Figure 8, there is a comparison of NO reduction modeling results obtained at 1425 K with PFR and LMM models for different λ values. Under PFR conditions, the evolution in the NO reduction, as the value of λ increases, is sharper than under non-premixed laminar conditions (LMM).

It can be observed that under non-premixed conditions and laminar mixing (LMM model), the maximum of NO reduction is shifted to leaner conditions, compared to PFR. Moreover, for λ > 0.7, a higher reduction of NO is obtained under laminar mixing conditions. This is a probe of evidence that a delayed mixing may result in an improvement of reburning performance. Miller et al.43 mentioned similar observations in a pilot plant, in which they obtained a higher reburn performance in a lean atmosphere while optimum reburn conditions are usually considered to be fuel-rich. They already noted that the explanation for those results could rely on the mixing effects. Moreover, Glickert and Houy44 quantified significant NO reductions under fuel-lean reburning conditions in a real installation. Despite the globally fuel-lean conditions in all cases, because of nonpremixed reactant feeding, there are fuel-rich zones within the reactor that make it possible to obtain a significant NO reduction. To obtain different local stoichiometries under laminar flow while maintaining constant global conditions, the amount of nitrogen added through the inner and outer streams has been varied, but the total flow rate introduced in the reactor is maintained constant. Thus, the local concentration of the reactants added change and, therefore, the local stoichiometry also changes. Moreover, the variation of the nitrogen added through each stream modifies the initial velocity profile at the entrance of the reactor and, therefore, the velocity flow field in the reactor also changes. To take into account the nitrogen introduced through each stream, the velocity ratio (R) has been defined as follows:

R)

velocity flow of the inner stream velocity flow of the outer stream

Figure 9 shows the theoretical and experimental influence of the velocity ratio (R) on NO reduction, as a function of temperature, when reburning fuel is added through the inner stream and NO and O2 through the outer stream. An example of the concentration field in the reactor for this configuration is shown in Figure 3. It is worth mentioning that, applying the PFR model, the simulation of both velocity ratios is indeed the same case of study and, consequently, the same results are obtained. The BLM and LMM models show slight differences between the results obtained for both velocity ratios, which may indicate that the influence of the initial velocity profile, which is only taken into account by the BLM model to calculate the velocity flow field in the reactor (velocity flow field is fixed in LMM), has a low impact on the results.

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Figure 9. Experimental and theoretical influence of the velocity ratio (R). Reburning fuel is introduced through the inner tube (experiments 3 and 6, in Table 1).

Despite the differences being small, a greater NO reduction is obtained for R ) 1 and high temperatures. These differences are even lower at temperatures of 0.7, a greater reduction of NO is obtained under laminar mixing conditions. This is a probe of evidence that a delayed mixing may result in an improvement of reburning performance. Moreover, it is worth mentioning that, under PFR conditions, the evolution in the NO reduction, as a function of the global value of λ, is sharper than that under non-premixed laminar conditions. Stoichiometry and residence time distribution in the reactor have an important role in reburning performance, even with the same global stoichiometry. For low temperatures, a higher NO reduction is obtained when local fuel-lean conditions have a longer residence time (reburning fuel introduced through the inner stream); however, for high temperatures, a higher NO reduction is obtained when local fuel-rich conditions have a longer residence time (reburning fuel introduced through the outer stream). LMM and BLM follow these trends, which are also shown by the experimental results. Acknowledgment The authors express their gratitude to the EU (Contract No. ENK5-CT-2000-00324) and the MCYT (Project No. PPQ20014913-E) for financial support, and to Mr. Jose´ Manuel Martos and Tinus Cording for their cooperation in the early parts of the present investigations. Nomenclature Cl ) molar concentration of the lth species (kmol/m3) Dkm ) diffusion coefficient of species k in the mixture (kg m-1 s-1) ki ) rate constant of the ith reaction MWk ) molecular weight of species k (kg/kmol) p ) pressure (N/m2) R ) radius (m) Vx ) velocity in the x-direction (m/s) Yk ) mass fraction of species k in the mixture Greek Symbols µ ) viscosity (kg m-1 s-1)

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ReceiVed for reView July 20, 2006 ReVised manuscript receiVed February 12, 2007 Accepted March 6, 2007 IE060944I