A Reduced Kinetic Model for NOx Reduction by Advanced Reburning

1278

Energy & Fuels 1998, 12, 1278-1289

A Reduced Kinetic 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 17, 1998

Advanced reburning technology, which makes use of natural gas injection followed by ammonia injection, has proven to be an effective method for the removal of up to 85-95% of the NO in pulverized, coal-fired furnaces. This paper reports the development of a seven-step, 11-species reduced mechanism for the prediction of nitric oxide concentrations using advanced reburning from a 312-step, 50-species full mechanism. The derivation of the reduced mechanism is described, including the selection of the full mechanism, the development of the skeletal mechanism, and the selection of steady-state species. The predictions of the seven-step reduced mechanism are 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. Comparisons with three independent sets of experimental laminar data indicate that the reduced mechanism correctly predicts the observed trends, including the effects of temperature, the ratio of (NH3/NO)in, and concentrations of CO, CO2, O2, and H2O on NO reduction. The observed effects of CO on NH3 slip were also reliably predicted. Mechanistic considerations provide an explanation for the roles of the important radicals and species. Also, parametric studies of the effects of CO2 and H2O have been performed with the reduced mechanism. A maximum in NO reduction exists, which strongly depends on the concentrations of NOin, CO, and O2, the ratio of (NH3/NO)in, and temperature.

Introduction Nitrogen oxides (NOx) have been recognized as acidrain precursors that impose a significant threat to the environment. Coal combustion is a major anthropogenic source of NOx. To meet the requirements of the Clean Air Act Amendments of 1990 (CAA) within budget limitations and scheduling constraints, electric utilities have considered a variety of novel approaches to reduce NOx emissions from powerplants. 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 intermediate HCN,1,2,3 which is subsequently reduced to N2, and (2) the advanced reburning technologies wherein ammonia, urea, or similar substances are injected after hydrocarbon injection for reburning to further reduce NOx species.4 (1) Wendt, J. O. L.; Mereb, J. B. DOE Final Report, DE-AC2287PC79850; University of Arizona: Tucson, AZ, 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, Missouri, 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, Missouri, May 19, 1995.

The concept of advanced reburning combines reburning with concepts from selective noncatalytic reduction (SNCR). In advanced reburning, NH3 injection follows hydrocarbon fuel injection (to create a slightly fuel-rich zone), combined with or followed by injection of additional air. Advanced reburning technology is attractive since it has been reported that up to 85-95% NOx reduction can be achieved,5 and the problems of carbon loss, slagging, and tube wastage may be avoided.3 However, unreacted ammonia (i.e., ammonia slip) is an issue with advanced reburning. In SNCR, NH3 is injected into the postcombustion zone, at lower temperatures within a fuel-lean region. However, the reaction proceeds within a narrow temperature range and small quantities of NH3 have to be rapidly mixed into the flue gas stream. The applicable temperature range of SNCR can be shifted or widened by additives such as H2, H2O, CO, or natural gas.6 CO is of special interest since it can be produced in situ by fuel-rich combustion, such as fuel reburning. The main reactions of the advanced reburning5 are NH3 + OH, O, H f NH2 + H2O, OH, H2, and NH2 + NO f N2 + H2O. With increased CO present due to upstream reburning fuel injection, a (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) Suhlmann, J.; Gerd Rotzoll Fuel 1993, 72, 175-179.

10.1021/ef980084l CCC: $15.00 © 1998 American Chemical Society Published on Web 10/17/1998

A Reduced Kinetic Model for NOx Reduction

Energy & Fuels, Vol. 12, No. 6, 1998 1279

Table 1. Input Test Parameters and Experimental Data of Dill & Sowa13 in a Plug-Flow Reactora inlet

outlet

case

temp (°C)

NH3/NO

NO (ppm)

CO (ppm)

NO (ppm)

CO (ppm)

NH3 (ppm)

NO (out/in)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

927 927 927 927 927 927 927 927 1010 843 843 1010 927 927 927 927 1010 843 843 1010 1010 843 843 1010 927

2 1 2 1 2 1 2 1 2 1 2 1 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

450 450 450 450 800 100 100 800 450 450 450 450 800 100 100 800 450 450 450 450 800 100 800 100 450

800 100 100 800 450 450 450 450 450 450 450 450 800 100 800 100 800 100 800 100 450 450 450 450 450

100 145 6.6 215 46 98 88 195 104 38 130 170 107 18 103 16.5 168 265 58 40 115 43 630 107 54

82.4 82.4 90.3 71.8 79.7 77.1 85.0 87.7 58.7 223.2 423.3 58.7 71.8 56.0 58.7 69.2 69.2 108.8 108.8 63.9 58.7 58.7 447.9 56 58.7

2.2 1.2 89.5 1.7 3.0 1.9 1.0 1.1 1.4 17.9 356.0 1.0 1.5 2.0 0.8 19.8 2.3 427 2.9 0.9 1.2 2.8 860.3 1.1 3.0

22.2% 32.2% 1.50% 47.8% 5.80% 98.0% 88.0% 24.4% 23.1% 8.40% 28.9% 37.8% 13.4% 18.0% 103% 2.10% 37.3% 58.9% 12.9% 8.90% 14.4% 43.0% 78.8% 107% 12.0%

a

Initial conditions are 2.8% O2, 14.1% CO2, 76.1% N2, and 6% water vapor for 0.5 s residence time.

larger temperature window for the NOx reduction reactions and reduction of NH3 slip can be achieved. The first observation of advanced reburning data was reported by Chen et al.7 As Chen et al.8 noted, more CO provides additional OH to initiate NH3 oxidation. Several full elementary mechanisms for hydrocarbon flames and NH3 oxidation are available for description of advanced reburning. However, unacceptable computer times result when large kinetic schemes are used with turbulent combustion models. Brouwer et al.9 developed a global model for predicting SNCR which includes the influence of CO through an empirical adjustment to temperatures. However, that model is unable to describe more complex impacts of CO and O2 concentrations, including Suhlmann and Rotzoll’s6 observation that only small amounts of CO or O2 are needed to initiate the reaction of NO reduction at low temperatures. This paper reports a reduced mechanism to predict NOx reduction using advanced reburning. The tasks identified to develop the mechanism were to (1) select a suitable full mechanism, (2) identify a skeletal mechanism, and (3) derive and verify a reduced mechanism. Model Development Full Mechanism. Full mechanisms available to describe hydrocarbon flames with ammonia addition include GRI2.11,10 MB89A&B,11 and B96.12 The GRI2.11 mechanism is applied to hydrocarbon flames, B96 and (7) Chen, S. L.; Cole, J. A.; Heap, M. P.; Kreamlich, J. C.; McCarthy, J. M.; Pershing, D. W. 22rd Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; pp 1135-1145. (8) Chen, S. L.; Lyon, R. K.; Seeker, W. R. Environ. Prog. 1991, 10, 182-185. (9) Brouwer, J.; Heap, M. P.; Pershing, D. W.; Smith, P. J. 26th Symposium (Intl.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; pp 2117-2124. (10) 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.

MB89B are used only for SNCR, and MB89A can be used for both. Several combinations of these mechanisms were generated herein (i.e., GRI-B96, GRIMB89B, MB89A, and MB89A-B96) for comparisons with the data of Dill and Sowa13 for ammonia injection with CO, as given in Table 1. Reactions for each of these mechanisms are summarized by Xu.14 Twenty-five sets of data from a plug-flow reactor included changes of NSR (i.e., ratio of NH3in/NOin, 1, 1.5, and 2), initial concentrations of NO and CO (100, 450, and 800 ppm), and temperature (843, 927, and 1010 °C). Coal-air flue gas was simulated as 2.8% O2, 14.1% CO2, 76.1% N2, and 6% H2O. The PaSR code in the CHEMKIN package15 was used for the reacting flow simulation with the various full mechanisms in the plug-flow reactor. The partially stirred reactor (PaSR) model was developed by Correa16 and Chen17 to study the effects of turbulence-chemistry interactions. A Monte Carlo method was used in the PaSR that allows for influences of finite-rate mixing and subsequently provides a more realistic model. The chemical composition in the PaSR is represented by an ensemble of fluid elements or particles. A plug-flow reactor can be simulated when a single particle is used.18 (11) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287-338. (12) 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. (13) Dill, J. W.; Sowa, W. A. 1992 Fall Meeting of the Western States Section of the Combustion Institute; Berkeley, CA, Oct. 12-13, 1992. (14) Xu, H. Ph.D. Dissertation (in preparation), Department of Chemical Engineering, Brigham Young University, Provo, UT, 1998. (15) Kee, R. F.; Rupley, F. M.; Miller, J. A. Chemkin II; Sandia Report SAND 89-8009; Sandia National Laboratories: Livermore, CA, 1989. (16) Correa, S. M. Combust. Flame 1993, 93, 41-60. (17) Chen, J.-Y. Western States Section of the Combustion Institute; Spring Meeting, 1994; Paper WSS93-071. (18) Correa, S. M.; Braaten, M. E. Combust. Flame 1993, 94, 469486.

1280 Energy & Fuels, Vol. 12, No. 6, 1998

Xu et al.

Figure 1. Comparisons of data from Dill and Sowa13 with various full mechanisms.

Comparisons of laboratory data13 with full mechanism predictions are shown in Figure 1. The curve-fitting values determined from the calculated data indicate the slope (X) and its variance (R). Calculated NOout/NOin less than the measured value gives X < 1. Both variables were equal to one on the diagonal line if fullmechanism predictions agree exactly with the data. None of the full mechanisms was highly reliable. The X and R values from GRI-B96, as shown in Figure 1b, predicted values with the least variance of any of the full mechanisms tested. Table 2 shows the qualitative comparisons of experimental data with the three full mechanisms, tested for effects of NSR, COin, NOin, and (NO/CO)in on NOout. The second column shows the experimental trends, while the third column shows the calculated values. The symbol “x” means the calculations have the same trends as experimental data, and “×” means opposite trends were observed. Basically,

the three full mechanisms (GRI-B96, MB89A, and MB89-B96) correctly predict the four key trends, missing only 2 out of 25 cases for GRI-B96, 2 for MB89A, and 5 for MB89-B96. In addition, it was also observed (not shown in the paper) that the COout values calculated from each full mechanism were always lower than the experimental data. NH3 slip (unreacted NH3) occurred experimentally for cases 3, 10, 11, 16, 18, and 23; the predictions of GRI-B96 show this same trend except for case 10. MB89A-B96’s showed this same trend except for cases 2 and 10. The MB89A mechanism showed this same trend except for cases 5 and 10. According to the above quantitative comparisons, qualitative trend, and NH3 slip, the GRI-B96 mechanism appears to be the best among these full mechanisms evaluated. Table 3 shows the effect of CO on NH3 slip corresponding to Dill and Sowa’s 25 experimental cases. The



Table 2. Qualitative Comparisons of Data13 with Three Full Mechanismsa

a In the table, NSR ) NH /NO , π ) NO /NO . Case numbers correspond to those in Table 1. Other observations: (1) CO calculated 3in in out in is always lower than experimental data. (2) NH3 slip (unreacted NH3) occurs for experimental cases 3, 10, 11, 16, 18, 23; for GRI-B96 cases 3, 11, 16, 18, 23; for MB89A cases 3, 5, 11, 16, 18, 23; for MB89A-B96 cases 2, 3, 10, 11, 16, 18, 23.

last three columns denote measured NH3out, NH3out calculated with CO present, and NH3out calculated without CO present. It was seen that the cases with NH3 slip can be predicted correctly by using GRI-B96, though some NH3out values were not in good agreement with the experimental data. According to the calculations, the number of cases with NH3 slip increased dramatically without CO compared with those with CO. This observation indicates that the existence of CO helps to substantially reduce the NH3 slip because CO enhances oxidation of NH3 to NO, which is consistent with other researchers’ observations.6,19 Skeletal Mechanism. The GRI-B96 mechanism containing 312 reactions and 50 species, given in detail in ref 10, was used as the basis for the balance of the present work. In Peters’ systematic reduction method,20 development of a reduced mechanism can be divided into two tasks: (a) establish a skeletal or “short” mechanism and (b) develop a reduced mechanism by applying steady-state and partial-equilibrium assumptions. A skeletal mechanism is derived from the full reaction mechanism through sensitivity analysis, covering only the key variables in our range of interest. With coal as the primary fuel, a typical application window (19) Lodder, P.; Lefers, J. B. Chem. Eng. J. 1985, 30, 161-167. (20) Peters, N. Reduced Kinetic Mechanisms and Asymptotic Approximations for Methane-Air Flames; Smooke, M. D., Ed.; SpringerVerlag: Heidelberg, Germany, 1991; Chapter 3.

Table 3. Comparisons of Predicted and Measured Effects of CO Concentration on NH3 Slip (Data from Dill and Sowa13)a NH3 out NH3 out NH3 out NOin NSR NH3 in (expl) GRI-B96(w/CO) GRI-B96(w/o CO) 450 450 450 450 800 100 100 800 450 450 450 450 800 100 100 800 450 450 450 450 800 100 800 100 450

2 1 2 1 2 1 2 1 2 1 2 1 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

900 450 900 450 1600 100 200 800 900 450 900 450 1200 150 150 1200 675 675 675 675 1200 150 1200 150 675

0 0 89.5 0 0 0 0 0 0 17.9 356 0 0 0 0 19.8 0 427 0 0 0 0 860 0 0

0 0 127 0 0 0 0 0 0 0 867 0 0 0 0 846 0 667 0 0 0 0 1183 0 0

640 127 640 120 138 29.9 99 625 0 445 892 0 1004 61 61 1001 39.4 443 669 0 0 147 1191 3.65 0

a Note: All cases correspond to Dill and Sowa’s cases (Table 1). GRI-B96(w/o CO) means that there is no CO included and the rest of minor species are the same as that of Dill and Sowa.13


Xu et al.

Table 4. Sixty-Two-Step 20-Species Skeletal Mechanism from the GRI-B96 for Advanced Reburninga 1. NO 8. CO2 15. NNH

2. NH3 9. NH 16. NO2

3. CO 10. NH2 17. N2O

Species Considered 4. OH 11. N 18. HNO

5. O2 12. H2O 19. N2H2

6. H 13. H2 20. HO2

7. O 14. N2

Reactions Considered 1. O + H2 h H + OH 2. O + HO2 h OH + O2 3. O + CO + M h CO2 + Mb 4. O2 + CO h O + CO2 5. H + O2 + M h HO2 + Mb 6. H + 2O2 h HO2 + O2 7. H + O2 + H2O h HO2 + H2O 8. H + O2 + N2 h HO2 + N2 9. H + O2 h O + OH 10. H + HO2 h O2 + H2 11. H + HO2 h 2OH 12. OH + H2 h H + H2O 13. 2OH h O + H2O 14. OH + HO2 h O2 + H2O 15. OH + CO h H + CO2 16. HO2 + CO h OH + CO2 17. N + NO h N2 + O 18. N + O2 h NO + O 19. N + OH h NO + H 20. HO2 + NO h NO2 + OH 21. NO + O + M h NO2 + Mb 22. NO2 + O h NO + O2 23. NO2 + H h NO + OH 24. NH + H h N + H2 25. NH + OH h HNO + H 26. NH + OH h N + H2O 27. NH + O2 h HNO + O 28. NH + O2 h NO + OH 29. NH + H2O h HNO + H2 30. NH + NO h N2 + OH 31. NH + NO h N2O + H 32. NH2 + O h OH + NH 33. NH2 + O h H + HNO 34. NH2 + H h NH + H2 35. NH2 + OH h NH + H2O 36. NNH h N2 + H 37. NNH + M h N2 + H + Mb 38. NNH + O2 h HO2 + N2 39. NNH + O h NH + NO 40. H + NO + M h HNO + Mb 41. HNO + O h NO + OH 42. HNO + H h H2 + NO 43. HNO + OH h NO + H2O 44. HNO + O2 h HO2 + NO 45. NH3 + H h NH2 + H2 46. NH3 + OH h NH2 + H2O 47. NH3 + O h NH2 + OH 48. NH2 + O h NO + H2 49. NH2 + NO h NNH + OH 50. NH2 + NO h N2 + H2Oc 51. NH2 + NO h N2 + H2Oc 52. NNH + NO h N2 + HNO 53. HNO + NH2 h NH3 + NO 54. NH2 + NH h N2H2 + H 55. N2H2 + M h NNH + H + Mb 56. N2H2 + OH h NNH + H2O 57. N2H2 + NO h N2O + NH2 58. NH2 + NH2 h N2H2 + H2 59. NH2 + O2 h HNO + OH 60. NH2 + NO2 h N2O + H2O 61. NH2 + NH2 h NH + NH3 62. NH2 + HO2 h NH3 + O2

A

b

E

5.00E+04 2.00E+13 6.02E+14 2.50E+12 2.80E+18 3.00E+20 9.38E+18 3.75E+20 8.30E+13 2.80E+13 1.34E+14 2.16E+08 3.57E+04 2.90E+13 4.76E+07 1.50E+14 3.50E+13 2.65E+12 7.33E+13 2.11E+12 1.06E+20 3.90E+12 1.32E+14 3.20E+13 2.00E+13 2.00E+09 4.61E+05 1.28E+06 2.00E+13 2.16E+13 4.16E+14 7.00E+12 4.60E+13 4.00E+13 9.00E+07 3.30E+08 1.30E+14 5.00E+12 7.00E+13 8.95E+19 2.50E+13 4.50E+11 1.30E+07 1.00E+13 5.40E+05 5.00E+07 9.40E+06 5.00E+12 2.80E+13 1.30E+16 -2.8E+13 5.00E+13 2.00E+13 1.50E+15 5.00E+16 1.00E+13 3.00E+12 5.00E+11 4.50E+12 2.84E+18 5.00E+13 4.30E+13

2.7 0.0 0.0 0.0 -0.9 -1.7 -0.8 -1.7 0.0 0.0 0.0 1.5 2.4 0.0 1.2 0.0 0.0 0.0 0.0 0.0 -1.4 0.0 0.0 0.0 0.0 1.2 2.0 1.5 0.0 -0.2 -0.5 0.0 0.0 0.0 1.5 0.0 -0.1 0.0 0.0 -1.3 0.0 0.7 1.9 0.0 2.4 1.6 1.9 0.0 -0.6 -1.3 -0.55 0.0 0.0 -0.5 0.0 0.0 0.0 0.0 0.0 -2.2 0.0 0.0

6290.0 0.0 3000.0 47 800.0 0.0 0.0 0.0 0.0 14 413.0 1068.0 635.0 3430.0 -2110.0 -500.0 70.0 23 600.0 330.0 6400.0 1120.0 -480.0 0.0 -240.0 360.0 330.0 0.0 0.0 6500.0 100.0 13 850.0 0.0 0.0 0.0 0.0 3650.0 -460.0 0.0 4980.0 0.0 0.0 740.0 0.0 660.0 -950.0 13 000.0 9915.0 955.0 6460.0 0.0 0.0 0.0 0.0 0.0 1000.0 0.0 50 000.0 1000.0 0.0 0.0 25 000.0 0.0 10 000.0 0.0

a k ) AT be-E/RT; A units of mol cm s K; E units of cal/mol. b Third body [M] collision efficiency values. (3) H /2.0, O /6.0, H O/6.0, 2 2 2 CO/1.5, CO2/3.5. (5) O2/0, H2O/0, CO/0.75, CO2/1.5, N2/0. (21) H2/2.0, H2O/6.0, CO/1.5, CO2/2.0. (37) H2/2.0, H2O/6.0, CO/1.5, CO2/2.0. (40) c H2/2.0, H2O/6.0, CO/1.5, CO2/2.0. (55) H2O/15.0, O2/2.0, N2/2.0, H2/2.0. Both reactions competitively included in reaction mechanism.

is temperature ) 1000-1400 K; residence time ) 0.5-5 ms; NSR ) 0.8-2.5; CO2 z 15%, N2 z 75%, H2O z 7%, O2 < 3%, CO ) 0-2%, NO ) 150-1000 ppm. Gas

species compositions in the post-flame zone are typically in the range noted above. The skeletal mechanism was developed by neglecting reactions that (1) involve the



Next, a reaction rate was chosen to be eliminated corresponding to each steady-state species. The rate for each species should be the fastest by which the species is consumed. The choice is, to some extent, arbitrary but sometimes has an effect on the stiffness of the resultant reduced mechanism.22 A sensitivity analysis was used to evaluate the rate parameters to determine the reaction rates to be eliminated. The reaction rates selected in the present work were: 〈HO2, w10〉, 〈HNO, w25〉, 〈N, w17〉, 〈NH, w24〉, 〈NH2, w49〉, 〈NNH, w52〉, 〈N2H2, w57〉, 〈H, w9〉, 〈O, w4〉, where wi refers to the ith reaction in the skeletal mechanism shown in Table 4. This information, when processed by RedMech (a code for automatic reduction of chemical reaction),23 provided the following seven-step, 11-species reduced mechanism:

I: H2 + O2 f 2OH II: O2 + 2 CO f 2CO2 III: H2 + CO2 f CO + H2O IV: O2 + N2 f 2 NO Figure 2. Comparison of the 62-step skeletal mechanism and seven-step reduced mechanism with the GRI-B96 312-step full mechanism for conditions corresponding to 25 cases of Dill and Sowa’s data.13

CiHj species, given that no CiHj species were involved in this process, (2) have particularly large activation energies (g90 kJ/mol), and (3) contain minor species such as HCCO, HCCOH, HCNN, HCNO, H2O2, and H2CO, because of their very low exit mole fractions (e.g., 1 × 10-25). The sensitivity analysis based on the CHEMKIN-PSR (perfectly stirred reactor)21 calculations over the specific application window was performed using the first-order sensitivity coefficients of temperature and species mass fractions with respect to the reaction rate coefficients. The reactions having small sensitivity coefficients to all species considered were deleted. A 62-step, 20-species skeletal mechanism was thus derived from the GRI-B96, as shown in Table 4. Under the same input conditions as those 25 cases from Dill and Sowa,13 the skeletal mechanism was in good agreement with the GRI-B96 full mechanism within the application window, as shown in Figure 2. Seven-Step Reduced Mechanism. A reduced mechanism with fewer steps has been developed from the skeletal mechanism employing steady-state assumptions. For a steady-state species, the consumption reactions are typically very fast and rapidly consume all of a species as it is formed; consequently, its concentration remains much smaller than those of the initial reactants and final products.20 Thus, the 20 species included in this skeletal mechanism were divided into steady-state species and non-steady-state species as follows:

9 steady-state species: HO2, HNO, N, NH, NH2, NNH, N2H2, H, O 11 non-steady-state species: CO2, H2O, N2, O2, NO, CO, NH3, OH, H2, N2O, NO2

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 detailed information about the seven-step reduced mechanism is given in Table 5, where the reactions, in general, are not fundamental but are rather combined or overall reactions. Net rates of formation of the 11 non-steady-state species can, thus, be solved implicitly, as shown in Table 5. Reduced Mechanism Evaluation Comparison of Skeletal and Reduced Mechanisms. The seven-step reduced mechanism has been tested against the full and skeletal mechanisms and by comparison with laboratory data.13 Calculations for the plug-flow reactor of Dill and Sowa13 were performed with the PaSR code and the GRI2.11 database for transport and thermochemical properties.10 Minor modification of the ckwyp.f file in the CHEMKIN package15 was needed to allow modeling with this reduced mechanism, including the addition of (1) the expressions for steady-state species based on L([X]) ) 0, (2) forward and backward reaction rates of the skeletal mechanism based on the forward rate constants and thermal data file for the reverse rate through the chemical equilibrium constant,15 and (3) global non-steady-state species production rates for particular reduced mechanisms. The concentrations of the steady-state species were solved iteratively without truncation. (21) Glarborg, P.; Kee, R. J.; Grcar, J. F.; Miller, J. A. Sandia Report SAND86-8209; Sandia National Laboratories: Livermore, CA, 1992. (22) Leung, K. M.; Lindstedt, R. P.; Jones, W. P. Reduced Kinetic Mechanisms for Application in Combustion Systems; Peters, N., Rogg, B., Eds.; Springer-Verlag: Heidelberg, Germany, 1993; pp 259-283. (23) 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.


Xu et al.

Table 5. Detailed Equations for the Seven-Step Reduced Mechanisms Steady-State Species L([O]) ) 0 ) -w1 - w2 - w3 + w4 +w9 + w13 + w17 + w18 - w21 - w22 + w27 - w32 - w33 - w39 - w41 - w47 - w48 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([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([NH2]) ) 0 ) -w32 - w33 - w34 - w35 + w45 + w46 + w47 - w48 - w49 - w50 - w51 - w53 - w54 + w57 - 2w58 - w59 - w60 - 2w61 - w62 L([NNH]) ) 0 ) -w36 - w37 - w38 - w39 + w49 - w52 + w55 + w56 L([N2H2]) ) 0 ) w54 - w55 - w56 - w57 + w58 I II III IV V VI VII

Global Reactions H2 + O2 f 2OH O2 + 2CO f 2CO2 H2 + CO2 f CO + H2O O2 + N2 f 2NO H2 + O2 + CO2 + NO f 2OH + CO + NO2 H2 + O2 + CO2 + N2 f 2OH + CO + N2O OH + 2CO + 3NO + NH3 f 2H2 + O2 + 2CO2 + 2N2

L([H2]) ) - wI - wIII - wV - wVI + 2wVII L([OH]) ) 2wI + 2wV + 2wVI - wVII L([O2]) ) - wI - wII - wIV - wV - wVI + wVII L([CO]) ) - 2wII + wIII + wV + wVI - 2wVII L([CO2]) ) 2wII - wIII - wV - wVI + 2wVII L([H2O]) ) wIII L([N2]) ) - wIV - wVI + 2wVII L([NO]) ) 2wIV - wV - 3wVII L([NO2]) ) wV L([N2O]) ) wVI L([NH3]) ) -wVII

Species Balance Equations

Global Rates 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 ) w12 + w13 + w14 + w26 - w29 + w35 + w43 + w46 + w50 + w51 + w56 + w60 wIV ) w18 + w19 + w28 + w32 + w33 + w34 + w35 + w36 + w37 + w38w39 - w40 + w41 + w42 + w43 + w44 + 2w48 + w50 + w51 + w53 + w55 + w56 + w58 + w59 + w60 + w61 wV ) w20 + w21 - w22 - w23 - w60 wVI ) w31 + w54 - w55 - w56 + w58 + w60 wVII ) w45 + w46 + w47 - w53 - w61 - w62

The seven-step reduced mechanism generally compares well with the skeletal mechanism and the full mechanism (Figure 2) and usually works well within the application window for which it was derived. However, for practical use, it was desired to extend the model to wider ranges: (1) to make use of various compositions of flue gas, such as natural gas, (2) to expand the temperature window that could result from turbulence effects or load-changing, and (3) to apply to diffusion flames which typically have greater spacial variations in temperature and composition. For example, the compositions of flue gas from coal-fired and natural gasfired combustors are typically different:

Coal: 12-15% CO2, 5-10% H2O, 0-3% O2, 72-78% N2 Gas: 7-9% CO2, 14-17% H2O, 0-3% O2, 72-78% N2 To determine if the seven-step reduced mechanism is applicable to natural gas combustion, cases were identified analogous to Dill and Sowa’s 25 cases13 with changes in the concentrations of the major species, such as CO2, H2O, O2, N2, from a coal-fired to gas-fired basis while retaining the same concentrations of minor species, such as NO, CO, and NH3. The seven-step reduced

mechanism is also in good agreement with the skeletal and full mechanisms under these simulated, gas-fired conditions14 (not shown in the paper). Similar work was done to investigate the temperature effects on the reduced mechanism. This result showed that the model can be used from 900 to 1550 K without significant deviation from the full mechanism. Effects of flue gas composition on NO emissions were obtained by comparing the 25-case calculations for both the gas-fired and coal-fired cases, as shown in Figure 3. In some of cases, the NOout values are different for these two different flue gas compositions, though the NOin, NH3in, and COin concentrations and temperatures are the same. In particular, the NOout values in cases 3, 5, 16, and 22 were significantly different for unidentified reasons. Thus, flue gas composition seems to have some effect on the advanced reburning NO concentration, possibly through changes in the mole fractions of CO2 and H2O, whose compositions differ significantly in flue gases from coal-fired to natural-gas-fired cases, as noted above. This result suggests that an advanced reburning mechanism needs to include these species. Comparisons of Reduced Mechanism with Laboratory Data. Suhlmann and Rotzoll6 tested the effects of CO, temperature, NSR, and O2 on SNCR processes in a reaction system NO-NH3-O2-CO. The seven-step reduced mechanism has been verified further by com-


Figure 3. Effect of composition of flue gas on NO emissions (the input concentrations of minor species NH3, NO, CO are the same as those of 25 cases from Dill and Sowa13).

paring these data for the following range of variables: temperatures ) 970-1250 K, residence time ) 0.2-1.4 s, NSR ) 0.5-3.7, O2 ) 0.1-9%, CO ) 0.01-0.7%, NO ) 1000 ppm, and N2 ) balance. CO2 and H2O were not included in this test gas; thus, gas composition differs from the application window that was used to derive the reduced mechanism. Therefore, it was necessary to compare the model with data and also with the full mechanism for these conditions. All calculations were done with the PaSR code, which only approximates the reactor of this experiment. Influence of CO Concentration. The amount of CO in the gas had a marked effect on the reaction process (Figure 4). Experimentally, at 1048 K, a very small increase to 0.4% CO causes the fractional conversion (i.e., 1 - speciesout/speciesin) to jump to high values in a steplike manner. At 1123 K, high fractional conversions are obtained at about 0.15% CO with a somewhat less steep slope. Once the reaction starts, conversion of NH3 quickly reaches 100% while the CO concentration is still increasing and the NO concentration passes through a maximum. The calculations from both the reduced mechanism and the full GRI-B96 mechanism are in good qualitative agreement with these sharp trends, except that the former predicts the reaction increase at 0.3% CO while the latter is at 0.25% CO. At 1048 K, a predicted small increase of CO concentration at 0.3% CO compared to 0.4% CO experimentally initiates the reaction. At 1123 K, high conversions are also obtained at about 0.1% CO compared to 0.13% CO experimentally and the predicted conversion is less steep. In both cases, the predicted and measured patterns of the NH3 fractional conversion are similar, the predicted CO conversion is more rapid and occurs earlier than measured, and the predicted NO conversion declines more quickly after peaking, compared to data. Influence of Temperature. Temperature was the most sensitive parameter. As shown in Figure 5a, in the COfree system, the fractional conversion of NO strongly


increases with temperature at ∼1125 K and reaches a maximum value of about 100% at 1200 K experimentally. In the calculation, the conversion of NO starts increasing at ∼1150 K and reaches a maximum value near 100% at 1175 K. From Figure 5b, when CO is added, the sharp increase in NO fractional conversion occurs at 1050 K experimentally and at 1025 K theoretically and the maximum conversion of NO is lower in both experiments (from 1 to 0.75) and calculations (from 1 to 0.68) at the higher temperatures. The overall effects of CO addition are (1) a shift of the temperature window by ∼75 K experimentally (∼150 K in other experiments19) and by 125 K theoretically to lower values and (2) a reduction of NO fractional conversion. The reduced, seven-step kinetic mechanism predicts these complex trends correctly, and values are quite close to the data. Also, the reduced mechanism shows no significant deviation from the full mechanism. Influence of O2. Increasing O2 concentration is important in initiating the reaction (Figure 6). At 1073 K, ∼0.4% O2 is experimentally sufficient to start the reaction compared to 0.5% O2 theoretically. Additional O2 concentration up to 5.5% leads to only a slight decrease of NO conversion from 80% to 70% experimentally compared to a decline from 74% to 40% theoretically. Increasing temperature has three impacts: (1) a lower O2 concentration (i.e.,

A Reduced Kinetic Model for NOx Reduction by Advanced Reburning

Recommend Documents