Modeling Low-Temperature Gas Reburning. NOx Reduction Potential

Energy & Fuels 1998, 12, 329-338

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Modeling Low-Temperature Gas Reburning. NOx Reduction Potential and Effects of Mixing Marıá U. Alzueta,*,† Rafael Bilbao, and Angela Millera Department of Chemical and Environmental Engineering, University of Zaragoza, 50015-Zaragoza, Spain

Peter Glarborg,‡ Martin Østberg, and Kim Dam-Johansen Department of Chemical Engineering, Technical University of Denmark, Building 229, 2800 Lyngby, Denmark Received July 28, 1997

A model for simulating reburning in semi-industrial scale has been developed. It consists of a recently developed reaction mechanism for reburning with C1 and C2 hydrocarbons, in combination with ideal reactor modeling and a simplified mixing approach. The reaction mechanism as well as the mixing model has been validated separately against experimental data from laboratory and pilot scale tests. Modeling predictions have been compared with experimental data from a number of pilot scale studies of gas reburning with good results. The large differences in reburn efficiency reported in different low-temperature pilot scale experiments are reconciled in terms of the different operating conditions used. The model has been used to assess the potential of the reburn process at low temperatures, and recommendations for process optimization are provided. Results show that the low-temperature gas reburn process has a significant potential for NO reduction and that both the reburn and burnout regions are important in process optimization.

Introduction Natural gas reburning is a well-known technique to reduce NOx emissions in different combustion systems. The reburning concept includes the addition of a reburn fuel downstream the main combustion zone in order to create a reducing environment where the NOx are partially destroyed. Afterwards, air is injected in order to obtain complete combustion. This technology has matured through a number of pilot and bench scale studies,1-5 and recent full scale tests have shown that reburn efficiencies of 50-60% are usually obtained in high-temperature applications.6 Presently, research and development efforts are undertaken in order to assess the reburn process potential at low temperatures, i.e., below 1500 K. The assessment of the potential of low-temperature gas reburn is of interest for two main reasons: the fact that low Fax: +34 976 761861. E-mail: [email protected]. Fax: +45 45882258. E-mail: [email protected]. (1) Kolb, T.; Jansohn, P.; Leuckel, W. Twenty-Second Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; pp 1193-1203. (2) Burch, T. E.; Tillman, F. R.; Chen, W. Y.; Lester, T. W.; Conway, R. N. Energy Fuels 1991, 5, 231-237. (3) Mereb, J. B.; Wendt, J. O. L. Fuel 1994, 73, 1020-1026. (4) Bilbao, R.; Millera, A.; Alzueta, M. U. Ind. Eng. Chem. Res. 1994, 33, 2486-2852. (5) Bilbao, R.; Alzueta, M. U.; Millera, A. Ind. Eng. Chem. Res. 1995, 34, 4531-4539. (6) Glarborg, P.; Karll, B.; Pratapas, J. M. Proc. Nineteenth World Gas Conf., Milan 1994. † ‡

temperatures are favorable for minimizing NO formation in the burnout region,7 and the emerging lowtemperature applications such as municipal waste incinerators.6 Bench and pilot scale results8-14 are not conclusive, reporting NOx reductions ranging from below 40%10 to 80%.9 The discrepancies observed can be attributed to the differences in experimental setups and operating conditions used, but trends are not clear. The first full scale test at an MSW facility15 shows that this is a demanding application of the reburn technology, and more work is needed in order to implement and optimize the process under these conditions. (7) Kristensen, P. G.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1996, 107, 211-222. (8) Lanier, W. S.; Mulholland, J. A.; Beard, J. T. Twenty-First Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1986; pp 1171-1179. (9) Mechenbier, R.; Kremer, H. VDI Ber. 1987, 645, 87-92. (10) Chen, S. L.; Kramlich, J. C.; Seeker, W. R.; Pershing, D. W. J. Air Manag. Assoc. 1989, 89, 1375-1379. (11) Alzueta, M. U. Reduccioń de Emisiones de NOx mediante Reburning con Gas Natural. Estudio Experimental y Modelado. Ph.D. Thesis. University of Zaragoza, Zaragoza, 1994 (in Spanish). (12) Alzueta, M. U.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1997, 109, 25-36. (13) Bilbao, R.; Alzueta, M. U.; Millera, A.; Prada, L. Ind. Eng. Chem. Res. 1997, 36, 2440-2444. (14) Bilbao, R.; Millera, A.; Alzueta, M. U.; Prada, L. Fuel 1997, 76, 1401-1407. (15) Bergstro¨m, J. “NOx Reduction using Reburning with Natural Gas”. Final Report from Full-Scale Trial at SYSAV's Waste Incineration Plant in Malmo¨. Nordic Gas Technology Centre, Hørsholm, Denmark, 1993.

S0887-0624(97)00129-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/17/1998

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Recent experimental and theoretical work under flow reactor conditions4,5,7,11-14,16-18 has improved our understanding of the reburn chemistry. Temperature, stoichiometry, and reaction time are the key intrinsical process parameters. The destruction of NO in the reburn zone appears to be enhanced at high temperatures,4,17 while in the burnout region the formation of N2 from other nitrogenous species is favored at low temperatures.7,18 A certain temperature, the initiation temperature, is required in the reducing zone in order to have reaction under flow reactor conditions.12 Above this initiation temperature, the NO reduction level is roughly independent of temperature up to 1500 K. The initiation temperature increases with the reburn fuel in the sequence C2H2 < C2H4 < C2H6 < CH4/C2H6 mixtures < CH4,16 with decreasing air excess ratio, λ2,12 decreasing reactant concentration,12,13 and decreasing reaction time.12 Based on maximum NO reduction or minimum TFN concentration, optimum values for the reburn stoichiometry using natural gas as reburn fuel range from values below 0.513,14 to 0.7-0.9.12 The stoichiometry is also important for the distribution of products attained at the outlet of the reburn zone, as the HCN/NO ratio increases as the environment becomes more fuel rich.5,12 The residence time is particularly important under low-temperature conditions, because it can be a limiting factor in case of retrofit applications, due to space limitations. As the temperature diminishes, the induction time required to build up the radical pool before fuel oxidation and NO reduction can take place increases. Induction times of the order of 100-200 ms have been reported12,16 for natural gas reburning at 1300-1400 K. The experimental and theoretical work on reburning under idealized reaction conditions cannot be extrapolated directly to practical applications. To do this, mixing effects need to be taken into consideration. The mixing process may change the selectivity of reactions both at the reburn fuel injection and at the rich-lean transition. Furthermore, at low temperatures the delay caused by mixing in combination with a comparatively slow reaction may have an adverse effect on the reburn efficiency. In the present work, a recently developed reaction mechanism for reburning with C1-C2 hydrocarbons,16 together with ideal reactor modeling (plug flow) and a simplified mixing approach based on Zwietering,20 is used to evaluate the influence of key variables on low-temperature gas reburn. The specific aims of this work have been to reconcile the lowtemperature pilot scale data available in the literature in terms of detailed kinetic modeling, to assess the potential of the reburn process under low-temperature (16) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. Kinetic Modelling of Hydrocarbon/Nitric Oxide Interactions in a Flow Reactor. Combust. Flame, in press. Presently available as: LowTemperature Nitrogen Chemistry. Report GRI-97/0130. Gas Research Institute, Chicago, IL, 1997. (17) Kilpinen, P.; Glarborg, P.; Hupa, M. Ind. Eng. Chem. Res. 1992, 31, 1477-1490. (18) Bilbao, R.; Alzueta, M. U.; Millera, A.; Cantıń, V. Chem. Eng. Sci. 1995, 33, 2846-2852. (19) Chen, S. L.; McCarthy, J. M.; Clark, W. D.; Heap, M. P.; Seeker, W. R.; Pershing, D. W. Twenty-First Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1986; pp 1159-1169. (20) Zwietering, T.N. Chem. Eng. Sci. 1959, 11, 1-15.

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conditions, and to provide indications for process optimization under the conditions studied (i.e., below 1500 K). Numerical Procedure The model calculations have been performed using Senkin,21 a plug-flow code that runs in conjunction with the Chemkin library.22 The thermodynamic data were taken from the Sandia Thermodynamic Database,23 as well as from Glarborg et al.16 The chemical kinetic model used in this work, which consists of oxidation mechanisms for moist CO, C1-C2 hydrocarbons, HCN, and NH3, together with a reaction subset describing interactions between hydrocarbons and nitrogenous species, was adopted without changes from Glarborg et al.16 A detailed description of the reaction mechanism can be found here in ref 16 and can be obtained from the authors. The kinetic model provides a reasonably good description of flow reactor data obtained under very dilute conditions.12,16 In the present work it is further validated against flow reactor experiments5 performed with more realistic reactants concentrations. For calculations, natural gas, which is used as reburn fuel in most of the pilot scale experiments considered, is approximated as 90% CH4 and 10% C2H6. The simulation of experimental results from bench and pilot scale setups not following the plug-flow reactor approximation implies an additional complexity for modeling. The ideal procedure for such simulation would involve the use of a CFD (computational fluid dynamics) description coupled with detailed chemical kinetics. This procedure is extremely CPU-time expensive, and few attempts at following this approach have been reported. Ballester et al.24 developed a combined CFD-detailed chemistry approach that was applied to a pilot scale combustor of 0.5 MWt with acceptable results. However, even in this study, important simplifications were done, such as using simplified reaction schemes applicable only in a narrow range of operating conditions for which the simplification was done. In this work a simple approach for the mixing process of a jet (reburn fuel) with a cross flow (the bulk flow coming from the primary combustion zone) has been employed, based on the work of Zwietering.20 In the present approach, the reburn fuel jet is gradually diluted by the bulk gas. It is assumed that the mass flow of the cross flow entrained into the jet at a given time is

dmjet/dt ) kmbulk

(1)

With this assumption, which implies an exponential mixing rate, the mass flow of the jet gas at a given time is expressed by the following equation: (21) Lutz, A., Kee, R. J.; Miller, J. A. Senkin: A Fortran Program for Predicting Homogenous Gas-Phase Chemical Kinetics with Sensitivity Analysis. Sandia Report SAND87-8248. Sandia National Laboratories, Livermore, CA, 1990. (22) Kee, R. J.; Rupley, F. M.; Miller, J. A. Chemkin-II: A Fortran Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics. Sandia Report SAND89-8009. Sandia National Laboratories, Livermore, CA, 1989. (23) Kee, R. J.; Rupley, F. M.; Miller, J. A. The Chemkin Thermodynamic Data Base, Sandia Report SAND87-8215, 1991 update. Sandia National Laboratories, Livermore, CA, 1991. (24) Ballester, J.; Fueyo, N.; Dopazo, C. Proc. Int. Gas Reburn Technol. Workshop, Malmo¨ (Sweden) 1995, D-187-218.

Low-Temperature Gas Reburning

mjet(t) ) mjeto + mbulko(1 - exp(-kt))

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(2)

The constant k is calculated on the basis of an experimentally determined or estimated mixing time, according to

k ) ln(m ˘ bulk,0/m ˘ bulk,t)/t

(3)

In the present work the mixing time has been taken as the time for which 90% of the cross flow is mixed with the fuel jet, i.e., k ) ln 10/t90. The approach does not physically describe the actions that take place in the mixing region. However, it does capture an essential feature of the mixing, i.e., that the reburn fuel reacts initially under conditions going from very fuel rich (early stages of entrainment) toward less fuel rich (full mixing). We assume in the modeling that the fuel jet is heated rapidly by penetration of the hot bulk fluid. An important advantage of the present approach is that it is easy to incorporate in the Chemkin Software Package22 as additional first-order reactions for the reactants constituents of the bulkflow. The reverse approach, entrainment of the reburn fuel jet into the cross flow, has also been considered and an example of the results obtained is shown and discussed below. This mixing approach has been used previously in different reburn modeling studies.17,25,28 Results and Discussion The analysis of different experimental studies of the gas reburn process under low-temperature conditions is performed in terms of detailed chemical kinetic modeling. Reburn has been shown to be a very complex process, involving interactions among a number of operational parameters. This complexity may explain the variety of bench and pilot scale results reported, which seem different or even contradictory at a first sight. The use of low temperatures for reburn purposes may be responsible for an increased sensitivity of the main parameters controlling reburn performance (e.g. temperature, stoichiometry, residence time, reburn fuel type), compared to the high-temperature applications. Validation of the Reaction Mechanism. The reaction mechanism of Glarborg et al.16 for hydrocarbon/ NO interactions used in this work for model calculations has previously been validated against flow reactor results, showing a reasonably good agreement.16 In order to further evaluate the performance of the reaction mechanism, model predictions have been compared with additional experimental results from Alzueta et al.12 and Bilbao et al.4 The experimental results of Alzueta et al.12 were obtained in a plug-flow reactor in which very diluted mixtures of reactants were preheated separately and mixed at the entrance of the reaction zone. Figure 1 compares data and model calculations for reburning with a methane/ethane mixture at 1328 K, an excess air ratio, λ2, of 0.76, and varying inlet NO concentration, (NO)p. A good agreement for NO, HCN, and TFN is observed. An increase in inlet NO concentration produces an increase in both NO and HCN outlet concentrations. However, the selectivity to HCN is favored for (25) Payne, R.; Moyeda, D. Proc. Int. Power Generation Conf., ASME 1993.

Figure 1. Comparison between experimental data (Alzueta et al.12) and model predictions for NO, HCN, and TFN using simulated natural gas (CH4/C2H6 mixture) as reburn fuel as function of inlet NO concentration at T ) 1328 K. Symbols denote experimental results and lines model calculations. Experimental conditions: ∼2950 ppm CH4, ∼300 ppm C2H6, ∼5320 ppm O2, ∼2% H2O, tr ) 0.136 s.

low initial NO values, due to the high hydrocarbon/NO ratio in the initial stage of reaction, when most of the NO conversion takes place. In order to test model performance under more realistic conditions, i.e., at higher fuel concentrations and with natural gas as reburn fuel, modeling predictions have also been compared with the experimental results of Bilbao et al.4 These data were obtained in a flow reactor with feeding preheated up to the nominal reaction temperature. Figure 2 compares experimental data and model calculations as function of the natural gas concentration for a temperature of 1573 K. The experimental trends are well predicted by the model, even though NO conversion to HCN is slightly overpredicted. In their kinetic study of low-temperature reburning under flow reactor conditions, Glarborg et al.16 found that NO was primarily reduced by reaction with CH3 and HCCO radicals. This is also observed for the conditions of Figures 1 and 2. The reduction of NO by smaller hydrocarbon radicals (CH2, CH, C), which is the major pathway at high temperatures,11,17,26,27 is not important under low-temperature conditions. The importance of CH3 and HCCO for reducing NO depends on operating conditions, primarily temperature and stoichiometry, as well as on reburn fuel type and fuel conversion. Under the conditions of Figure 2, with higher temperatures and higher reactant concentrations, HCCO is most important for removing NO, even though also CH3 and CH2 are active. In general, the results obtained by Glarborg et al.16 and in the present work show that the reaction mechanism for hydrocarbon/NO interactions provides a good description of reburning with small hydrocarbons in the (26) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287-313. (27) Bilbao, R.; Alzueta, M. U.; Millera, A.; Duarte, M. Ind. Eng. Chem. Res. 1995, 34, 4540-4548. (28) Rota, R.; Bonini, F.; Servida, A.; Morbidelli, M.; Carra´, S. Combust. Sci. Technol. 1997, 123, 83-105.


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Figure 3. Comparison between experimental (Kolb et al.1) and calculated results of NO, HCN, NH3, and TFN for a fast mixing regime (tmix ) 10 ms). Symbols denote experimental results and lines calculations. T ) 1600 K.

Figure 2. Comparison between experimental data (Bilbao et al.4) and model predictions for NO, HCN, TFN, CO, and CO2 using natural gas as reburn fuel as function of stoichiometry. Symbols denote experimental results and lines model calculations. Experimental conditions: 1573 K, 2% O2, 6% H2O, 900 ppm NO, tr ) 0.165 s. Natural gas composition: 90.5% CH4, 8.5% C2H6, 0.5% C3H8, 0.1% higher hydrocarbons, 0.4% N2. In the modeling, a natural gas composition of 90.5% CH4 and 9.5% C2H6 was assumed.

temperature range of interest in the present study. For this reason, the model can be considered useful as a starting point in the simulation of more practical reburn systems. Validation of the Mixing Approach. Kolb et al.1 performed an experimental study of reburning in a 0.35 MWt combustor, where they considered the effects of mixing and stoichiometry in the reducing zone. Because the data cover different mixing conditions, they can be used for testing the mixing approach together with detailed modeling for the reburn chemistry. Kolb et al.1 used natural gas doped with ammonia as both primary and reburn fuels. Their results are interesting because the experimental conditions are well characterized. Variation of the jet momentum, through the addition of variable amounts of N2 as diluting agent, results in slow and fast mixing conditions, respectively, of the reburn fuel with the cross flow coming from the primary combustion zone. High jet fuel momentum causes faster entrainment and improved mixing with the primary gas products. In the model calculations of the reducing zone, Senkin is used together with the detailed kinetic model and the mixing approach described above. The following assumptions were made: 1. The gases coming from the primary zone are the products of complete combustion (i.e., CO2, H2O, and N2, with NO specified) in that zone for a given stoichiometry of λ1 ) 1.1.

Figure 4. Comparison between experimental (Kolb et al.1) and calculated results of NO, HCN, NH3, and TFN for a slow mixing regime (tmix ) 100 ms). Symbols denote experimental results and lines calculations. T ) 1600 K.

2. The dilution with nitrogen that Kolb et al.1 used to vary independently stoichiometry and momentum is neglected. 3. A constant and average temperature in the reducing zone of 1600 K is chosen for calculations. Kolb et al.1 reported temperatures in the range 1550-1650 K. 4. The constant k appearing in eq 1 has been calculated from the characteristic mixing times of 10 ms for the fast mixing regime and 100 ms for the slow mixing regime obtained experimentally by Kolb et al.,1 assuming that these values corresponded to the time for which 90% of the cross flow is entrained in the fuel jet. The implications of these simplifying assumptions are discussed below. Figures 3 and 4 compare modeling predictions with the experimental results of Kolb et al.1 for NO, HCN, NH3, and TFN as function of stoichiometry (λ2) for fast and slow mixing regimes, respectively. Despite some



Figure 5. Comparison between experimental NO concentrations (Kolb et al.1) and calculated results with different mixing approaches for: (a) high jet fuel momentum, i.e., fast mixing; and (b) low jet fuel momentum, i.e., slow mixing. Symbols denote experimental results and lines calculations. T ) 1600 K.

discrepancy, the results are encouraging since the main trends are well represented. For the fast mixing regime (Figure 3), the TFN concentration is well predicted for stoichiometries higher than 0.9, while the NO profile and the NH3 decay are fairly well described over the whole range. However, the HCN concentration is overestimated for low λ2 values, resulting in a sharp minimum for the calculated TFN concentration, which is not as pronounced for the experimental data. For the slow mixing regime (Figure 4), the model predicts a wider minimum in TFN concentration than that observed experimentally, due mainly to a rapid decay of the NH3 present in the reburn fuel. Again, the NO profile is well predicted by the model, and the HCN concentration is overpredicted. For both slow and fast mixing regimes, model calculations agree fairly well with experimental data for stoichiometries higher than 0.9. The discrepancies observed under rich conditions are primarily attributed to inadequacies in the reaction mechanism used, which has not been validated for as complex a reburn fuel as a CH4/NH3 mixture. To illustrate the importance of mixing approach in the calculations, Figure 5 compares the experimental results for NO for both the high and low fuel jet momentum with plug flow calculations performed with three mixing approaches: (i) instantaneous mixing; (ii) fuel jet entrainment, i.e., the reburn fuel jet is gradually entrained into the bulk gas coming from the primary zone; and (iii) bulk flow entrainment, i.e., the bulk flow is gradually entrained into the reburn fuel jet. The same mixing times are used in cases (ii) and (iii), resulting in values for k of 230 and 23 s-1 for the high jet momentum (fast mixing) and the low jet momentum

Figure 6. Comparison between experimental TFN concentrations (Kolb et al.1) and calculated results when the assumptions done for calculations are varied, i.e., (a) assumption 1 (composition coming from the primary zone); (b) assumption 2 (dilution level); and (c) assumption 3 (reburn temperature). In all cases, the results correspond to the fast mixing regime.

(slow mixing), respectively. The mixing approach is seen to be important for the modeling predictions, both for the fast and the slow mixing regime. This can be attributed to differences in the local air/fuel ratio in the first instants of reaction. The oxygen availability for the initial reaction increases in the sequence bulk flow entrainment, instantaneous mixing, and fuel jet entrainment. In the fast mixing regime (Figure 5a), the assumption of instantaneous mixing provides a satisfactory description of the experimental results, but in the slow mixing regime (Figure 5b), the agreement is poor. The best results are obtained with the bulk flow entraining into the jet, which is the approach used in the rest of the model calculations shown in this work. Entrainment of the fuel jet into the bulk flow, which is the approach taken in previous studies of mixing in reburn systems,17,25,28 describes qualitatively the effect of mixing, but quantitatively the prediction is less accurate. In addition to mixing, we have evaluated the sensitivity of the modeling predictions to assumptions 1-3 described above. As shown in Figure 6, which compares experimental TFN results and model calculations, each


of these assumptions influences somewhat the calculated results, although the main trends are maintained. In Figure 6a the sensitivity of modeling predictions to the assumption of complete combustion in the primary zone is investigated. The gas composition coming from the primary combustion zone may contain CO and unburnt hydrocarbons in the case of incomplete combustion. Calculations are performed with 0 and 2% CO, respectively, varying the O2 concentration to maintain stoichiometry. The profile shape for TFN in the presence of 2% CO is quite similar to that obtained in the absence of CO, indicating little sensitivity of the modeling predictions to assumption 1. Dilution of the fuel in order to enhance the jet momentum is not important for the conditions studied, as observed in Figure 6b. In this figure, the ratio outlet TFN/inlet TFN values has been used as ordinate instead of TFN mole fraction, in order to avoid the variation in absolute concentrations due to N2 dilution. However, the modeling results are shown to be quite sensitive to variation in the reaction temperature from 1550 to 1650 K (Figure 6c). Both an increase and a decrease in temperature compared to the average value selected for calculations result in a shift of the minimum in TFN concentration to leaner stoichiometries. Furthermore, calculations show that the temperature affects the products distribution, in particular the HCN concentration, which diminishes drastically as the temperature increases, resulting in an increase in NO concentration. Decreasing temperature results in a lower conversion of the initial amount of NO entering the reburn zone. Even though both details in the kinetic model, the mixing approach, and the validity of other model assumptions remain uncertain, the model provides a surprisingly good prediction of the pilot plant data of Kolb et al.;1 in particular, the effect of mixing is well described. For this reason, the model can be considered as a useful tool for analyzing gas reburn performance under pilot scale conditions. Modeling of Low-Temperature Gas Reburn Data. In order to further validate the reburn model and to reconcile low-temperature reburn data reported, using natural gas and methane in pilot scale installations, the results of Lanier et al.,8 Mechenbier and Kremer,9 and Chen et al.10 have been selected. These experiments were chosen because they were performed at relatively low temperatures under well characterized conditions. In all of these studies, care was taken to obtain rapid mixing between reburn gas and combustion products, but no mixing times are reported. Mechenbier and Kremer9 report instantaneous mixing, while Chen et al.10 state that under their experimental conditions NO-CHi contact is not rate limiting. By analogy with the measurements of Kolb et al.1 for the fast mixing regime, we apply a mixing time of 10 ms in the calculations of all the selected pilot scale experiments. In general, the value chosen for the mixing time within the fast mixing regime had little impact on the modeling predictions. Figures 7 and 8 compare modeling predictions with the data of Lanier et al.8 They performed experiments in a 0.88 MWt combustor using natural gas as reburn fuel, varying both stoichiometry and initial NO concentration in the reducing zone while maintaining a

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Figure 7. Comparison between experimental (Lanier et al.8) and calculated results of NO, HCN, and TFN versus residence time for different stoichiometries. Symbols denote experimental results and lines calculations. T ) 1400 K; tmix ) 10 ms.

primary zone stoichiometry of 1.07. An average temperature of 1400 K was used in the calculations. Figure 7 shows data for NO, HCN, and TFN concentrations versus residence time for different stoichiometries. A good agreement is observed, even though NO is generally overpredicted. The differences between experimental and calculated results increase slightly as the stoichiometry becomes more fuel rich, as was seen also in the simulation of the data of Kolb et al.1 Lanier et al.8 report that approximately 50 ms is sufficient to complete reaction. This observation is partially confirmed by our calculations since TFN concentrations remain approximately constant. However, modeling predictions show a progressive conversion of NO to HCN as the reaction time increases, and only for the leanest stoichiometry (i.e., λ2 ) 0.97) the N-species concentration remains constant for residence times higher than around 40 ms. Figure 8 shows the effect of inlet NO concentration, (NO)p. The main trends are represented by calculations even though some discrepancy is observed. An interesting issue is that, for the lowest (NO)p concentration, experimental results indicate an appreciable formation of TFN with an important contribution of HCN. The authors8 attribute this increment in TFN to the destruction of N2, following similar reaction mechanisms to prompt NO formation, but this is not confirmed by the calculations.



2

2

Figure 9. Comparison between experimental (Mechenbier and Kremer9) and calculated results of NO versus residence time for different stoichiometries. Symbols denote experimental results and lines calculations. T ) 1423 K; tmix ) 10 ms. Table 1. Experimental and Calculated Results for Different Stoichiometries and an Inlet NO Concentration of 600 ppm (Chen et al.10) (Temperature ) 1560 K) experimental results (ppm)

calculated results (ppm)

NO

HCN

NH3

TFN

NO

HCN

NH3

TFN

137 180 378

300 108 30

60 40 10

492 328 418

98 106 347

365 256 0

0 0 0

463 362 347

Figure 8. Comparison between experimental (Lanier et al.8) and calculated results of NO, HCN, and TFN versus residence time for different primary NO concentrations. Symbols denote experimental results and lines calculations. T ) 1400 K; tmix ) 10 ms.

λ2 ) 0.7 λ2 ) 0.9 λ2 ) 1.0

Mechenbier and Kremer9 performed reburn experiments in an electrically heated cylindrical furnace with plug flow behavior. Bituminous coal was used as primary fuel and methane as reburn fuel. They varied in their experiments parameters such as wall temperature and stoichiometry in both the primary and the reburn zone. The data chosen for calculations correspond to a primary stoichiometry of 1.1, with two different reburn stoichiometries: 0.7 and 0.9. Residence time in the reburn zone was 360 ms and the wall temperature was 1373 K. A gas temperature of 1423 K, i.e., 50 K higher than the wall temperature, was used in the calculations. Figure 9 shows good agreement between the experimental and calculated results for NO for both stoichiometries, in particular for residence times higher than around 150 ms. It is noteworthy that both experiments and model show a remarkably high NO reduction for the richer stoichiometry of 0.7, compared to the results shown for natural gas in Figures 7 and 8. The differences are mainly due to the reaction temperature and to the use of methane in the experiments of Mechenbier and Kremer. Model calculations show that under the conditions of Figure 9, the radical CH3 is present in high concentrations and is responsible for most of the conversion of NO.

Chen et al.10 performed a parametric study of reburning in a 25 kW furnace with different combustibles as reburning fuel. In the present work, the experiments performed with natural gas both as primary and reburn fuel with a primary stoichiometry of 1.1 and an initial NO concentration of 600 or 240 ppm have been chosen for simulation. In the calculations the effect of dilution of the reburn fuel with argon is not taken into account. Table 1 compares data obtained at different reburn zone stoichiometries at a temperature of 1560 K and a residence time of 400 ms. Because of the limited amount of data, for simplicity they have not been plotted in a figure. Experimental results and modeling predictions show similar trends, even though in general the calculated concentrations for the reactive nitrogen species are higher than the experimental ones. A noticeable difference is that, although experimentally NH3 amounts up to 60 ppm are found, the model calculations do not predict any NH3 at all. Figure 10 compares data showing the effect of temperature for two different (NO)p concentrations. The main trends are well reproduced by the model and a fairly good quantitative agreement for NO is observed, in particular for the lowest temperatures represented. As the temperature increases the discrepancies become larger. The model underpredicts


Alzueta et al.

Figure 11. Model calculations of NO concentration at the outlet of both the reburn and burnout zones versus reburn temperature for a reburn stoichiometry: λ2 ) 0.7. The burnout stoichiometry is λ3 ) 1.2, and the burnout temperature is 100 K lower than the reburn temperature. Solid lines denote fast mixing regime (i.e., tmix ) 10 ms), and dashed lines slow mixing regime (tmix ) 100 ms). Figure 10. Comparison between experimental (Chen et al.10) and calculated results of NO versus temperature for different primary NO concentrations. Symbols denote experimental results and lines calculations. Reburn stoichiometry: λ2 ) 0.9; tmix ) 10 ms.

the NO concentration in the 1300-1600 K range, whereas for the highest temperatures the model overpredicts NO. Considering the uncertainties in the model, both in the reaction mechanism and in the mixing approach, as well as the uncertainties inherent in the pilot scale results, the general agreement between model and experimental data has been surprisingly good. The results show that differences in the pilot scale data can be explained in terms of differences in reburn fuel and process conditions and they augment the importance of these process parameters for obtaining a high NO reduction. The impact of process conditions and mixing is discussed further below for natural gas as reburn fuel. Assessment of Low-Temperature Gas Reburning under Practical Conditions The results presented in this work show that gas reburning has a considerable NO reduction potential, also for low-temperature applications, provided that the operating conditions chosen are adequate. Because of the good results obtained with the model in simulation of the pilot scale data, we have used it to assess the effect of operating conditions in low-temperature reburning. Calculations are performed for both the reburn and burnout zones, with temperatures in the reburn zone between 1300 and 1600 K, and 100 K lower in the burnout zone. A primary stoichiometry of λ1 ) 1.1 and a primary NO concentration of 500 ppm have been chosen, together with stoichiometries in the reburn zone of 0.7 and 0.9 and a value of 1.2 for the stoichiometry in the burnout zone. The natural gas used as

Figure 12. Model calculations of NO concentration at the outlet of both the reburn and burnout zones versus reburn temperature for a reburn stoichiometry: λ2 ) 0.9. The burnout stoichiometry is λ3 ) 1.2, and the burnout temperature is 100 K lower than the reburn temperature. Solid lines denote fast mixing regime (i.e., tmix ) 10 ms) and dashed lines slow mixing regime (tmix ) 100 ms).

reburn fuel is approximated as 90% CH4 and 10% C2H6. Residence time in both the reburn and the burnout zone is 300 ms, and mixing is simulated following our adaptation of the Zwietering approach, with entrainment of the bulk flow into the fuel jet. Both a fast and a slow mixing regime have been considered in the reburn zone, with mixing times of 10 and 100 ms, respectively. Mixing of the burnout air with the bulk gas coming from the reburn zone is assumed to occur during 20 ms by adding equal amounts of air each 2 ms, i.e., 10 times. Figures 11 and 12 show the simulated results of NO concentration versus temperature at the outlet of the reburn and burnout zones for both slow and fast mixing


regimes and at two different stoichiometries. Some interesting points can be made from these results. The results show that the effect of process parameters such as mixing and temperature depends on the reburn zone stoichiometry. At more fuel-rich conditions (Figure 11), the NO reduction is less sensitive to mixing and temperature than closer to stoichiometric conditions (Figure 12). However, it is noteworthy that for most process conditions a reduction of NO in the range 45-55% is predicted, almost independent of process parameters. The exception is for high temperatures and conditions close to stoichiometric, where calculations indicate a higher reduction potential in the case of delayed mixing. The reburn zone stoichiometry affects both the NO reduction level and the product distribution at the richlean transition. For a stoichiometry of 0.7 (Figure 11), the NO concentration at the outlet of the reburn zone decreases drastically as the temperature increases. However, NO is mainly converted to HCN; when the burnout air is added, a significant fraction of the HCN is recycled to NO. For this reason, the NO at the outlet of the burnout zone is not a strong function of temperature but decreases only moderately as the temperature increases. While the impact of mixing is small under the conditions of Figure 11, i.e., for the stoichiometry of 0.7, appreciable differences are obtained between mixing regimes for the stoichiometry of 0.9 (Figure 12). For this stoichiometry, the concentration of NO exiting the reducing zone exhibits a minimum for a temperature of around 1500 K for both mixing regimes. However, this minimum is not found at the exit of the burnout zone. For the fast mixing regime, the NO reduction is fairly independent of temperature between 1300 and 1600 K, while for the delayed mixing the final NO decreases strongly with increasing temperature. At this stoichiometry, delayed mixing promotes the reburn efficiency at high temperatures by limiting the oxygen availability in the initial stage of reaction. At lower temperatures, reaction times are longer and the mixing process is less significant. Similarly to the effect in the reducing zone, a longer mixing time in the burnout zone enhances the conversion of reactive nitrogen to N2 at high temperatures. From the results obtained in the simulations, some advice can be given for the implementation of reburn strategies in low-temperature combustion applications. In general, reburn stoichiometries as close as possible to stoichiometric are desired both for the economy of the process (less amount of reburn fuel to be added) and for minimizing corrosion problems in the combustion chamber due to a reducing environment. The present results indicate that a reburn zone stoichiometry of 0.9 is sufficient; no further reduction of NO is obtained by increasing the amount of reburn fuel. This observation is in agreement with a number of pilot scale studies for different reburn fuels under high-temperature conditions. However, the use of a more reducing environment in the reburn zone, e.g. a value of λ2 of 0.7, may offer a higher NO reduction potential, provided the conditions in the burnout zone are optimized. The combination of a high temperature at the reburn fuel injection point and a low temperature at the rich-lean transition would


enhance reburn efficiency, but such conditions are difficult to apply in retrofit applications without adverse effects on fuel burnout. In the present study, residence times in the reburn zone and in the burnout zone were restricted to 300 ms, typical of a retrofit application. If longer residence times are available, it opens up possibilities for further process enhancement. Longer reaction times in the reducing zone would increase the reburn efficiency at low temperatures by enhancing conversion of HCN to N2. Furthermore, sufficient residence time may allow the use of additional control strategies in the burnout zone. An improvement of the global efficiency of reburn performance could be accomplished by coupling reburn with burnout air staging7 or selective noncatalytic reduction of NO.29 The mixing issue is of special interest. In retrofit applications of reburn the mixing system for the reburn fuel is usually designed to obtain fast and complete mixing with the bulk flow. In large boilers the ability to obtain a good coverage with the reburn fuel is often critical for the reburn efficiency. In addition to the coverage, the available residence time, which includes the contribution of mixing and reaction times, can be a limiting factor. Often, techniques such as flue gas recirculation are necessary to improve mixing. The large scale systems are characterized by slow mixing regimes in comparison with the fast mixing that can be obtained in a bench scale facility. The present results indicate that the slow or delayed mixing in the large systems can be beneficial for the reburn efficiency, provided the temperature is sufficiently high. In industrial systems such as glass melting processes, which are a potential application of gas reburning, dimensions are usually much smaller and a fast mixing regime may be obtainable. In these systems, it may be worthwhile to consider different mixing regimes during process optimization. Conclusions A model for simulating reburning in semiindustrial scale has been developed. The model consists of a detailed reaction mechanism for reburning with C1 and C2 hydrocarbons, in combination with ideal reactor modeling and a simplified mixing approach, which is an adaptation of the work of Zwietering. The reaction mechanism and the mixing model have been validated separately against experimental data from laboratory and pilot scale tests and they have been shown to provide a good description of the reburn chemistry as well as the effect of mixing. Modeling predictions have been compared to results from a number of pilot scale studies of gas reburning. The results confirm the applicability of the modeling approach for simulating the pilot scale data, and the apparently contradictory results from the different pilot scale experiments are reconciled by means of the different operating conditions used in the various studies. In order to assess the NO reduction potential of the gas reburn technique at low temperatures and to provide guidelines for process optimization, a parametric modeling study has been (29) Alzueta, M. U.; Røjel, H.; Kristensen, P.; Glarborg, P.; DamJohansen, K. Energy Fuels 1997, 11, 716-723.


carried out, investigating the effect of temperature, stoichiometry, and mixing regime. Both experimental data and model calculations show a significant potential of the gas reburn process at temperatures as low as 1300 K, even though the reburn potential in general increases with temperature. Acknowledgment. This work was carried out as a part of the research combustion programs of the Department of Chemical and Environmental Engineering of the University of Zaragoza (financial support from the EU through subcontract of the EU project JOR3CT960059, and from CICYT, project AMB97-0852CE is acknowledged); and of the Department of Chemical Engineering of the Technical University of Denmark: CHEC research program, which is cofunded by the Danish Technical Research Council, Elsam, Elkraft, and the Danish Ministry of Energy. The authors express

Alzueta et al.

their gratitude to Mr. Lars S. Pedersen for useful discussions about the mixing approach. Glossary TFN k m t

total fixed nitrogen (i.e., NO + HCN + NH3) rate constant flow rate time

Greek Symbols λ

air excess ratio

Subscripts o p 1 2 3

initial primary primary zone reburn zone burnout zone EF9701293

Modeling Low-Temperature Gas Reburning. NOx Reduction Potential

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