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Computational Model for NOx Reduction by Advanced Reburning H. Xu and L. D. Smoot* Chemical Engineering Department, Brigham Young University, Provo, Utah 84602
S. C. Hill Advanced Combustion Engineering Research Center, Brigham Young University, Provo, Utah 84602 Received April 21, 1998
Advanced reburning is a NOx reduction process wherein injection of a hydrocarbon fuel such as natural gas downstream of the combustion zone is followed by injection of a nitrogen-containing species such as ammonia. The authors recently reported a seven-step, 11-species reduced mechanism for NO reduction by advanced reburning processes. However, inclusion of even a seven-step reduced mechanism into a CFD code for turbulent combustion leads to substantial computational demands. In this work, the authors have further simplified the kinetic mechanism. A simpler four-step, eight-species reduced mechanism for NO reduction by advanced reburning has been developed from a 312-step, 50-species full mechanism through the use of a systematic reduction method. The four-step reduced mechanism is in good agreement with the full mechanism for most laminar flow cases. It also agrees qualitatively with three sets of experimental data, which show the influences of temperature, CO concentration, O2 concentration, and the ratio (NH3/NO)in. It can be applied for coal-, gas-, and oil-fired combustion. The four-step reaction sequence has been integrated into a comprehensive CFD combustion code for turbulent combustion, PCGC-3. The method of integration is described. Several computations are reported with the combined code to demonstrate the predictive behavior of the advanced reburning mechanism in turbulent, pulverized coal combustion. The model calculations show the effects of temperature and concentrations of CO, O2, and NH3 on NO reduction.
Introduction Nitrogen oxides play an important role in photochemical smog, the formation of acid-rain precursors, the destruction of ozone in the stratosphere, and possibly in global warming. Approximately 70% of the global NOx emissions is believed to be anthropogenic in nature, and various combustion sources make up to 90% of these emissions. Increasingly stringent standards imposed on NOx emissions from combustion sources have resulted in a greater effort to develop novel reduction approaches. Among the most recent developments for reducing NOx emissions from coal combustion systems are (1) the reburning technologies, wherein gaseous, liquid or solid hydrocarbon fuels are injected downstream of the main combustion zone to react with NO and produce HCN1-3 and (2) the advanced reburning technologies, wherein ammonia, urea, or similar substances are injected after hydrocarbon injection to further reduce NOx species,4 (1) Wendt, J. O. L.; Mereb, J. B. DOE Final Report, DE-AC2287PC79850; University of Arizona: Tucson, AZ, September, 1991. (2) Boardman, R. D.; Smoot, L. D. Fundamentals of Coal Combustion; Smoot, L. D., Ed.; Elsevier: The Netherlands, 1993. (3) Folsom, B. A.; Sommer, T.; Ritz, H.; Pratapas, J.; Bautista, P.; Facchiano, T. EPRI/EPA 1995 Joint Symposium on Stationary Combustion NOx Control; Kansas City, MO, May 19, 1995. (4) Folsom, B. A.; Payne, R.; Moyeda, D.; Zamansky, V.; Golden, J. EPRI/EPA 1995 Joint Symposium on Stationary Combustion NOx Control; Kansas City, MO, May 19, 1995.
Figure 1. Schematic of advanced reburning process (adapted from Folsom et al.4).
as illustrated in Figure 1. Up to 85-95% reduction of NOx can be achieved by using reburning and advanced reburning technologies together,5 and at the same time the problems of carbon loss, slagging, and tube wastage may be avoided.4 As Folsom et al.6 pointed out, there are two new advanced reburning approaches referred to as (1) non(5) Zamansky, V. M.; Ho, L.; Maly, P. M.; Seeker, W. R. 26th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; pp 2075-2082. (6) Folsom, B. A.; Sommer, T. M.; Latham, C. E.; Moyeda, D. K.; Gaufillet, G. D.; Janik, G. S.; Whelan, M. P. 1997 Joint Power Generation Conference, Nov. 3-6, 1997.
10.1021/ef980090h CCC: $18.00 © 1999 American Chemical Society Published on Web 01/08/1999
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synergistic advanced reburning which involves reburning in series with selective noncatalytic reduction (SNCR) and (2) synergistic advanced reburning. While nonsynergistic advanced reburning reduces NO emissions, the N-agent suffers from the usual SNCR problems including a narrow temperature window and ammonia slip. In the synergistic advanced reburning approach, the reburning zone is adjusted from the normal fuel-rich conditions to near-stoichiometric conditions. This results in high CO concentrations that improve N-agent effectiveness for removing NO by widening the temperature window. The combination of reburning and advanced reburning makes the NO reduction mechanisms more complex, and there are no clear answers to many questions for advanced reburning processes, such as effects of CO species, shift of the temperature window, the use of mixtures of NH3 and flue gas, or effects of nitrogen agent phases.7 The objective of this work has been to develop a predictive model for the advanced reburning process. This has required development of an advanced reburning submodel for integration into comprehensive CFD combustion codes (e.g., PCGC-38). Previous Work The main reactions of advanced reburning are
NH3 + OH, O, H f NH2 + ... NH2 + NO f N2 + H2O From comparisons with a full mechanism, Zamansky et al.5 indicated that published full mechanisms still do not quantitatively describe the experimental results due to insufficient understanding of reactions of NO and carbon-containing species. Zamansky et al.9 experimentally investigated six strategies, including nonsynergistic and synergistic advanced reburning, to create a family of highly efficient NO control technologies. Brouwer et al.10 developed a reduced mechanism for the closely related SNCR process in practical systems, including the influence of CO. This simplified SNCR mechanism was derived from the Miller and Bowman mechanism11 through sensitivity analysis and curve fitting. The formation and reduction of NO and NH3 were globally expressed as k ) ATbe-E/T, where A, b, and E were given. This global mechanism was integrated into a comprehensive CFD code. To account for the chemical effects of CO, these authors used an empirical effective temperature rather than the predicted temperature in the rate expressions, where Teff ) T + S(CO), and where S(CO) ) 17.5 × ln([CO]) 68.0. These expressions were deduced from curvingfitting to match CHEMKIN12 predictions with the full mechanism. However, this model may be too specific to (7) Zamansky, V. M. Personal Communication at the 1997 Fall Meeting of the Western States Section of the Combustion Institute: Diamond Bar, CA, Oct. 23-24, 1997. (8) Hill, S. C.; Smoot L. D. Energy Fuels 1993, 7, 874-883. (9) Zamansky, V. M.; Maly, P. M.; Ho, L. 1997 Joint Power Generation Conference 1997; EC-Vol. 5, pp 107-113. (10) Brouwer, J.; Heap, M. P.; Pershing, D. W.; Smith, P. J. 26th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; pp 2117-2124. (11) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287-338.
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generally consider important factors such as the effects of O2, CO, and H2O concentrations. For example, the CO concentration affects not only the temperature shift, but also the reactions which remove NO as experimentally suggested by Suhlmann and Rotzoll.13 Very recently, Han et al.14 applied a two-stage Lagrangian (TSL) model to simulate the basic and advanced reburning processes in a practical furnace. The TSL model retains the full chemical kinetic elementary mechanisms and accounts for effects of turbulence on reactions. The important characteristics of the natural gas and N-agent injections are included using experimental information for a description of the mixing to account for both the flow and chemistry effects with minimal computational time. Model computations generally follow the trends observed in the experiments. However, since the equations of motion are not solved, the model is not generally applicable to the many possible advanced reburning process configurations and conditions. Xu et al.15 postulated a seven-step, 11-species reduced mechanism for the prediction of nitric oxide concentrations for advanced reburning, derived from a 62-step, 20-species skeletal mechanism, which itself was based on the 312-step, 50-species GRI-B9616,17 full mechanism. The derivation of this reduced mechanism was described in detail, including the selection of the full mechanism, the development of the skeletal mechanism, and selection of the steady-state species. The nine steady-state radical species were O, HO2, N, NH, NH2, NNH, HNO, H, and N2H2, and the 11 non-steady-state species were H2O, CO2, N2, O2, CO, NO, NH3, H2, NO2, N2O, and OH, which represented the independent variables. The seven reactions used in the mechanism were:
I: H2 + O2 f 2OH II: O2 + 2CO f 2CO2 III: H2 + CO2 f CO + H2O IV: O2 + N2 f 2NO V: H2 + O2 +CO2 + NO f 2OH + CO + NO2 VI: H2 + O2 + CO2 + N2 f 2OH + CO + N2O VII: OH + 2CO + 3NO + NH3 f 2H2 + O2 + 2CO2 + 2N2 The predictions of the seven-step reduced mechanism for laminar systems were in good agreement with those of the full mechanism over a wide range of parameters applicable to coal-based, gas-based, and oil-based combustion cases. (12) Kee, R. F.; Rupley, F. M.; Miller, J. A. Chemkin II; Sandia Report SAND 89-8009; Sandia National Laboratories: Livermore, CA, 1989. (13) Suhlmann, J.; Rotzoll, G. Fuel 1993, 72, 175-179. (14) Han, D.; Mungal, M. G.; Zamansky, V. M.; Tyson, T. J. 1998 Fall Meeting of the Western States Section of the Combustion Institute; Seattle, WA, Oct. 26-27, 1998. (15) Xu, H.; Smoot, L. D.; Hill, S. C. Energy Fuels 1998, 12, 1278. (16) Bowman, C. T.; Hanson, R. K.; Davidson, D. F.; Gardiner, J. W. C.; Lassianski, V.; Smith, G. P.; Golden, D. M.; Frenklach, M.; Wang, H.; Goldenberg, M. 1995, http://www.gri.org. (17) Bowman, C. T. Physical and Chemical Aspects of Combustion: A Tribute to Irvin Glassman; Dryer, F. L., Sawyer, R. F., Eds.; Gordon and Breach: Newark, NJ, 1996.
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Table 1. Reaction Steps and Rate Expressions for the Four-Step Reduced Mechanism Expressions for the Steady-State Species L([O]) ) 0 ) -wl - w2 - w3 + w4 + w9 + w13 + w17 + w18 - w21 - w22 + w27 - w32 - w33 - w39 - w41 - w47 - w48 L([H2]) ) 0 ) - w1 + w10 - w12 + w24 + w29 + w34 + w42 + w45 + w48 + w58 L([H]) ) 0 ) w1 - w5 - w6 - w7 - w8 - w9 - w10 - w11 + w12 + w15 + w19 - w23 - w24 + w25 + w31 + w33 - w34 + w36 + w37 - w40 - w42 - w45 + w54 + w55 L([HO2]) ) 0 ) -w2 + w5 + w6 + w7 + w8 - w10 - w11 - w14 - w16 - w20 + w38 + w44 - w62 L([N]) ) 0 ) - w17 - w18 - w19 + w24 + w26 L([NO2]) ) 0 ) w20 + w21 - w22 - w23 - w60 L([NH])) 0 ) -w24 - w25 - w26 - w27 - w28 - w29 - w30 - w31 + w32 + w34 + w35 + w39 - w54 + w61 L([HNO]) ) 0 ) w25 + w27 + w29 + w33 + w40 - w41 - w42 - w43 - w44 + w52 - w53 + w59 L([N2O]) ) 0 ) w31 +w57 + w60 L([NH2]) )0 ) -w32 - w33 - w34 - w35 + w45 + w46 + w47 - w48 - w49 - w50 - w51 - w53 - w54 + w57 - 2 w58 w59 - w60 - 2 w61 - w62 L([NNH]) )0 ) - w36 - w37 - w38 - w39 + w49 - w52 + w55 + w56 L([N2H2]) )0 ) w54 - w55 - w56 - w57 + w58 where L(X) ) dX/dt I: O2 + CO + H2O f 2 OH + CO2 II: O2 + 2 CO f 2CO2
Reduced Reaction Mechanism III: O2 + N2 f 2NO IV: OH + 3NO + NH3 f O2 + 2H2O + 2N2
Reaction Rates for the Non Steady-State Species L([OH]) ) 2wI - wIV L([H2O]) ) -wI + 2wIV L([O2]) ) -wI - wII - wIII + wIV L([N2]) ) -wIII + 2wIV L([CO]) ) -wI - 2wII L([NO]) ) 2wIII - 3wIV L([CO2]) ) wI + 2wII L([NH3]) ) -wIV where: wI ) w1 + w2 - w5 - w6 - w7 - w8 + w11 - w13 + w16 - w21 + w22 + w23 + w28 + w30 w34 - w35 + w36 + w37 - w40 + w41 + w47 + w55 - w58 - w61 wII ) w3 + w5 + w6 + w7 + w8 + w21 - w36 - w37 + w40 - w55 wIII ) w18 + w19 + w28 + w32 + w33 + w34 + w35 + w36 + w37 + w38 + w39 - w40 + w41 + w42 + w43 + w44 + 2 w48 + w50 + w51 + w53 + w55 + w56 + w58 + w59 + w60 + w61 wIV ) w45 + w46 + w47 - w53 - w61 - w62
For the comprehensive combustion code, PCGC-3,8 major equilibrium species include H2O, CO2, N2, O2, and CO. Beyond these five species, six more non-steady-state species are included in the reduced mechanism of Xu et al.:15 NO, NH3, H2, NO2, N2O, and OH. Among them, the very dilute species NO and NH3 are solved with Favre-averaged species continuity equations in PCGC-3 through an uncoupled postprocessor. A different approach must be used to track the concentrations of H2, NO2, N2O, and OH. It has been shown that partial equilibrium or steady-state assumptions for H2, NO2, N2O, or OH species are not adequate.15 Also, the partial equilibrium assumptions for CO and O2 at low concentrations may cause errors. Although it is possible to solve a species-continuity equation to track the concentrations of H2, NO2, N2O, OH, CO, and O2, this approach would add considerable computational time to the calculation. In this study, a simpler advanced reburning submodel was sought for integration into comprehensive combustion codes. The seven-step reaction sequence is still useful for advanced reburning when the computational resources are available to rigorously treat the 11 non-steady-state species but is too computationally timeconsuming for many turbulent flame applications.
NO2, and OH) could be assumed to be steady-state species. The others are either major species encountered in fossil fuel combustion or pollutant species that are already calculated in the PCGC-3 pollutant model (i.e., CO2, H2O, N2, O2, NO, NH3, and CO). Among the four species, OH is believed to be a non-steady-state species in most combustion flames, H2 is present in low concentrations in fossil-fuel combustion and may be chosen as a steady-state species, and N2O and NO2 contribute to the formation and destruction of NO according to Miller and Bowman,11 so they may or may not be steadystate species. A trial-and-error approach has been used to determine if the steady-state assumption is applicable for H2, N2O, and NO2. The 20 species in the skeletal mechanism were divided into 12 steady-state species and eight non-steady-state species, compared to nine steady-state and 11 non-steady-state species in the seven-step mechanism. The eight non-steady-state species are CO2, H2O, N2, O2, NO, NH3, CO, and OH. The 12 steady-state species and their corresponding rates to be eliminated are listed as follows:
Four-Step Reduced Submodel Development
where wi (e.g., w10) refers to the ith reaction in the skeletal mechanism.15 The RedMech code19 was then used to obtain the chemical reaction rate expressions for the four-step mechanism, as shown in Table 1. Compared with the seven-step mechanism,15 three species, H2, N2O, and NO2, were added into the steady-
al.15
Xu et show the derivation of the seven-step, 11species reduced mechanism in detail, including selection of the full mechanism, development of the skeletal mechanism, selection of steady-state species, and use of Peters’18 systematic reduction method. The same method and the same 62-step, 20-species skeletal mechanism were applied to development of the simpler reduced mechanism herein. Close inspection of the 11 non-steady-state species in the seven-step mechanism shows that only four additional species (i.e., N2O, H2,
HO2, w10; HNO, w25; N, wl7; NH, w24; NH2, w49; NNH, w52; N2H2, w57; H,w9; O,w4; N2O, w31; H2,w12; NO2,w20
(18) Peters, N. In Reduced Kinetic Mechanisms and Asymptotic Approximations for Methane-Air Flames; Smooke, M. D., Ed.; SpringerVerlag: Heidelberg, Germany, 1991; Chapter 3. (19) Go¨ttgens, J.; Terhoeven, P. Reduced Kinetic Mechanisms for Application in Combustion Systems; Peters, N., Rogg, B., Eds.; Springer-Verlag: Heidelberg, Germany, 1993; pp 345-349.
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Figure 2. Comparisons of the seven-step reduced mechanism with the full mechanism (GRI-B96) for the 25 experiments of Dill and Sowa.20
Figure 4. Comparisons of the four-step mechanism (rate of NO ) RNO - 0.5|RNO|) with full mechanism (GRI-B96) for the 25 experiments of Dill and Sowa.20
Figure 3. Comparisons of the four-step reduced mechanism with the full mechanism (GRI-B96) for the 25 experiments of Dill and Sowa.20
Figure 5. Comparisons of the four-step mechanisms (Rate of NO (+) ) RNO - 0.5|RNO| and rate of NO ([) ) RNO 0.89|RNO|) with data from Dill and Sowa.20
state species for the four-step reduced mechanism. Further simplifications, such as a steady-state assumption for OH, resulted in significant errors in the predicted concentrations, when compared to the full or skeletal mechanisms. As discussed by Xu et al.,15 predictions of the sevenstep reduced mechanism were in good agreement with the full mechanism, GRI-B96,16,17 applied to the 25 experimental cases from Dill and Sowa,20 as shown in Figure 2. Figure 3 shows that the trends for the predicted values from the four-step reduced mechanism are reasonably consistent with those from the full mechanism, except for cases 10 and 16. Significant deviation in case 10 is caused by the steady-state assumption of NO2, while deviation in case 16 is caused by the steady-state assumptions of H2 and N2O. Most (20) Dill, J. W.; Sowa, W. A. 1992 Fall Meeting of the Western States Section of the Combustion Institute; Berkeley, CA, Oct. 12-13, 1992.
of the NO conversion values (l - NOout/NOin) predicted by the four-step reduced mechanism are lower than those predicted by the full mechanism. This means that the rate of NO reduction, RNO, in the four-step reduced mechanism is somewhat lower (i.e., ) 2wIII - 3wIV, is usually negative), and a correction is made to increase this rate. If the rate of NO formation and reduction (RNO - 0.5|RNO|) rather than the original rate (i.e., RNO) is used, predictions from the revised four-step mechanism show significant improvement over the original fourstep reduced mechanism. This is shown in Figure 4, in which all values except for case 16 are close to the diagonal line. A revised rate for NO reactions (RNO 0.89|RNO|) is obtained if the experimental data from Dill and Sowa20 instead of predictions from the full mechanism are used to calibrate the model. Comparisons of the predicted and measured concentrations using this rate are shown in Figure 5. The value of (RNO 0.5|RNO|) is not nearly as good as (RNO - 0.89|RNO|)
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Figure 6. Effects of CO concentration on the conversions of NH3, NO, and CO at (a) 1048 and (b) 1123 K: (s) Four-step model, (- - -) seven-step model. Data from Suhlmann and Rotzoll:13 (0) NO measured, (9) NH3 measured, (+) CO measured.
Figure 7. Effects of temperature on NH3 and NO in (a) CO-free and (b) CO-containing systems. Main component is N2. Data from Suhlmann and Rotzoll.13 Symbols as in Figure 6.
when compared with Dill and Sowa’s20 data. The revised four-step reduced mechanism with the NO rate (RNO 0.89|RNO|) will be used in the discussions which follow. Four-Step Reduced Mechanism Evaluations Independent experimental data from Suhlmann and Rotzoll13 were used to evaluate the four-step reduced mechanism. These data reflect the effects of temperature, CO, O2, and NSR (i.e., NH3in/NOin). Predicted results from the full mechanism (GRI-B96) and the seven-step reduced mechanism were consistent and represented the complex trends observed in the data reasonably well.15 Therefore, the seven-step mechanism is used as a reference for further evaluation of the fourstep reduced mechanism. These calculations were done with the CHEMKIN-PaSR code.21 In Figure 6a, predictions from the four-step mechanism show better agreement with the experimental data than the seven-step mechanism in terms of reaction (21) Correa, S. M.; Braaten, M. E. Combust. Flame 1993, 94, 469486.
onset and NO conversion. Figure 6b shows that at higher temperatures, the predicted values of NO conversion from the four-step mechanism show better agreement with experimental data than the seven-step reduced mechanism. Further, the four-step reduced mechanism correctly predicts the influence of CO concentration on NO, such that the CO concentration for initiating reactions decreases with temperature and NO conversion decreases with increasing CO concentration after it reaches the peak value. In Figure 7, the four-step reduced mechanism predicts the profiles of NO, NH3, and CO conversion including the locations where the reactions are initiated and where the maximum values occur; however, when the temperature is increased in both CO-free and CO-seeded systems, NO conversion decreases somewhat more rapidly than predicted by the seven-step mechanism or observed experimentally. This is not caused by use of the NO rate correction (i.e., RNO - 0.89|RNO|) in the fourstep reduced mechanism but by the steady-state assumptions of H2, N2O, and NO2. Possibly, a temperature correction could be included to further improve the four-
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Figure 8. Effects of O2 Concentration on NH3, NO and CO at (a) 1073 and (b) 1123 K. The main component is N2. Data from Suhlmann and Rotzoll.13 Symbols as in Figure 6.
Figure 9. Effects of NSR on NO. Experimental data from Caton and Siebers.22 The main component is N2.
step reduced mechanism, since NO conversion drops too rapidly with increasing temperature (see Figure 7). The effects of temperature change on NO, NH3, and CO conversions are predicted correctly by the four-step reduced mechanism, including the shift of the temperature window and reduction of NO conversion due to CO addition. In Figure 8a and b, the effects of O2 concentration were investigated at different temperatures. Increasing the temperature has three impacts: a lower O2 concentration is required to initiate the reaction, the peak value of NO fractional conversion becomes higher, and the decline in NO fractional conversion with increased O2 concentration is more substantial. At the higher temperature, the NO fractional conversion changes from nearly 100% to 50% experimentally, compared to a decline from 82% to 25% in the calculations. The NO values predicted from the four-step reduced mechanism show better agreement with the experimental data than the seven-step reduced mechanism, especially at the lower temperature. Experimentally, as shown in Figure 9, NO reduction increases as NSR (NH3in/NOin) increases, up to values
Figure 10. Temperature window for NO reduction. Data from Caton and Siebers.22 The main component is N2.
of 3.0 with H2O and CO and 1.8 without H2O and CO. Increasing the NSR above these values resulted in no additional NO reduction. From the predictions with the seven-step reduced mechanism, the ratio is 3.0 with H2O and CO and 1.5 without H2O and CO. With the fourstep reduced mechanism, the ratio without H2O and CO is about 1 and with H2O and CO, NO reduction increases as NSR increases. With respect to NSR, the discrepancy between the seven-step mechanism and the four-step reduced mechanism is acceptable. A comparison of the four-step and seven-step mechanisms with experimental data22 for NO reduction using SNCR is shown in Figure 10. Experimentally, NO reduction starts to occur until the temperature is above about 1050 K, reaches its maximum at about 1175 K, and decreases with further increases in temperature. The seven-step and four-step mechanisms basically follow this pattern, but predicted NO reduction decreases more rapidly after reaching the peak values. The four-step reduced mechanism also shifts the optimum reduction to somewhat lower values, compared to the (22) Caton, J. A.; Siebers, D. L. Combust. Sci. Technol. 1989, 65, 277-293.
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Table 2. Species Concentrations in Various Flue Gases Compared to the Application Window of the Four-Step Reduced Mechanism suitable window coal-based gas gas-based gas oil-based gas
CO2
H2O
N2
O2
0-15% ∼14% ∼8% ∼13.5%
0-16% ∼7% ∼15% ∼11%
70-100% ∼76% ∼75% ∼74.5%
