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Energy & Fuels 1997, 11, 716-723
Laboratory Study of the CO/NH3/NO/O2 System: Implications for Hybrid Reburn/SNCR Strategies Maria U. Alzueta,*,† Hanne Røjel, Per G. Kristensen,‡ Peter Glarborg, and Kim Dam-Johansen Department of Chemical Engineering, Technical University of Denmark, 2800 Lyngby, Denmark Received September 4, 1996X
A laboratory study of the hybrid reburning/SNCR technique has been performed under flow reactor conditions. This technique involves the use of a selective reducing agent in the presence of combustibles (mainly CO) arising from the reburning zone. The impact of CO and O2 concentrations, NH3/NO ratio, as well as the presence of HCN has been investigated as a function of the temperature in the range 700-1300 K. The results show that CO in concentrations typical of the rich/lean transition in reburning shift the regime for NO reduction in the SNCR process to temperatures below 1000 K and cause a narrowing of the temperature window. The NO reduction potential is largely unaffected compared to conditions with low CO, and the effect varying the O2 concentration in the range 0.5-4.0% as well as of adding of HCN is found to be insignificant. Synergistic effects between reburning and SNCR are only observed in a narrow range of operating conditions with very low concentrations of CO and O2. Model predictions using a detailed reaction mechanism generally compare favorably with the experimental results. The present results indicate that a reduction of the CO level from the reburn zone is required before the SNCR chemistry is initiated. This can be obtained either by staging the burnout air or by injecting the N-agent in an aqueous solution to delay reaction.
Introduction As legislation on nitrogen oxide emissions becomes increasingly stringent, it may be necessary to either employ catalytic flue gas cleaning or improve the NOx reduction potential of combustion modification technologies such as reburning. Currently it seems that the only way to obtain NOx reduction in the 80-90% range by noncatalytic means is to use hybrid processes. The advanced reburning concept developed by EER1-4 where natural gas reburning is combined with selective reducing agent injection is a promising approach. Both reburning5-11 and selective noncatalytic reduction (SNCR)12-17 have been studied extensively and have also been implemented successfully in large-scale installations.13,18,19 However, while these technologies † Permanent address: Department of Chemical and Environmental Engineering, Faculty of Science, University of Zaragoza, Spain. ‡ Present address: Danish Gas Technology Centre, Hørsholm, Denmark. X Abstract published in Advance ACS Abstracts, March 15, 1997. (1) Chen, S. L.; Cole, J. A.; Heap, M. P.; Kramlich, J. C.; McCarthy, J. M.; Pershing, D. W. Advanced NOx Reduction Processes using -NH and -CN Compounds in Conjunction with Staged Air Addition. TwentySecond Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; pp 1135-1145. (2) Chen, S. L.; Kramlich, J. C.; Seeker, W. R.; Pershing, D. W. J. Air Waste Manage. Assoc. 1989, 39, 1375-1379. (3) Pont, J. H.; Evans, A. B.; England, G. C.; Lyon, R. K.; Seeker, W. R. Environ. Prog. 1993, 12, 140-145. (4) Zamansky, W. M.; Ho, L.; Maly, P. M.; Seeker, W. R. Reburning Promoted by Nitrogen and Sodium-Containing Compounds. TwentySixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, in press. (5) Chen, S. L.; McCarthy, J. M.; Clark, W. D.; Heap, M. P.; Seeker, W. R.; Pershing, D. W. Bench and Pilot Scale Process Evaluation of Reburning for In-Furnace NOx Reduction. Twenty-First Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1986; pp 1159-1169. (6) Mechenbier, R.; Kremer, H. VDI-Ber. 1987, 645, 87-98.
S0887-0624(96)00140-5 CCC: $14.00
separately offer NOx reductions on the order of 50%,2 pilot scale studies2-4 report efficiencies for the hybrid process of 80-85%. The choice of location for injection of the SNCR agent (or N-agent) as well as the possibility of staging the burnout air opens up a number of possibilities for the hybrid reburning/SNCR process. The following configurations have been considered: (7) Mereb, J. B.; Wendt, J. O. L. Reburning Mechanisms in a Pulverized Coal Combustor. Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; pp 1273-1279,. (8) Kicherer, A.; Spliethoff, H.; Maier, H.; Hein, K. R. G. Fuel 1994, 73, 1443-1446. (9) Bilbao, R.; Millera, A.; Alzueta, M. U. Ind. Eng. Chem. Res. 1994, 33, 2846-2852. (10) Bilbao, R.; Alzueta, M. U.; Millera, A. Ind. Eng. Chem. Res. 1995, 34, 4531-4539. (11) Alzueta, M. U.; Glarborg, P.; Dam-Johansen, K. Low-Temperature Interactions between Hydrocarbons and Nitric Oxide: An Experimental Study. Combust. Flame, in press. (12) Lyon, R. K. U.S. Patent 3,900,554, 1975. (13) Lyon, R. K.; Hardy, J. E. Ind. Eng. Chem. Fundam. 1986, 25, 19-24. (14) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287-338. (15) Duo, W.; Dam-Johansen, K; Østergaard, K. Widening the Temperature Range of the Thermal DeNOx Process. An Experimental Investigation. Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; pp 297-303. (16) Kasuya, F.; Glarborg, P.; Johnsson, J. E.; Dam-Johansen, K. Chem. Eng. Sci. 1995, 50, 1445-1446. (17) Caton, J. A.; Narney, J. K.; Cariappa, H. C.; Laster, W. R. Can. J. Chem. Eng. 1995, 73, 345-350. (18) Glarborg, P.; Karll, B.; Pratapas, J. Review of Natural Gas Reburning. Initial Full Scale Results. In Proceedings of the Nineteenth World Gas Conference, Milano, 1994. (19) Ballester, J.; Fueyo, N.; Dopazo, C.; Hernandez, M.; Vidal, P. J. Influence of Operational Parameters on the Results of Reburning in Coal Combustion. In Proceedings of the 3rd International Conference on Combustion Technologies for a Clean Environment, Lisbon, 1995; pp 27.3.17-27.3.25.
© 1997 American Chemical Society
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Figure 1. Selected configurations for hybrid reburn/SNCR corresponding to (a) configuration 3, (b) configuration 4, and (c) configuration 5, discussed in the text.
(1) N-agent injection with the reburn fuel.20-22 This corresponds to using a reburn fuel with bound nitrogen, such as coal or biomass. (2) N-agent injection in the reducing zone, but with a delay time after addition of reburn fuel.4 (3) N-agent injection with the burnout air at the rich/ lean transition. This is shown as configuration (a) in Figure 1. (4) Staging of the burnout air into two streams, where the first stream increases the overall excess air ratio to a value close to stoichiometric (0.99-1.03), and the N-agent is injected with the second burnout air.3 This is shown as configuration (b) in Figure 1. (5) Staging of the burnout air as above; injection of the N-agent between the two air addition locations.2 This is shown as configuration (c) in Figure 1. The choice of configuration will depend on the targeted NOx reduction as well as restrictions of the given boiler. Given the proper configuration, synergistic effects can be obtained when combining reburning and SNCR,2-4 thereby leading to appreciable additional NO reduction. The simultaneous injection of reburn fuel and N-agent (configuration 1) using for instance natural gas and ammonia may improve reburn performance at high initial NO concentrations, provided proper reaction conditions.21,22 However, for low primary NO levels, the N-agent injection can have an adverse effect on NO reduction efficiency.20,23 Injection of the N-agent down(20) Mulholland, J. A.; Hall, R. E. The Effect of Fuel Nitrogen in Reburning Application to a Firetube Boiler. In Proceedings of the 1985 Joint Symposium on Stationary Combustion NOx Control; EPA-EPRI: Washington, DC, 1985. (21) Kolb, T.; Jansohn, P.; Leuckel, W. Reduction of NOx Emission in Turbulent Combustion by FuelsStaging/Effects of Mixing and Stoichiometry in the Reduction Zone. Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; pp 1193-1203. (22) Spliethoff, H. Large Scale Trials and Development of Fuel Staging in a 160 MW Coal Fired Boiler. In Proceedings of the Joint Symposium of Stationary Combustion NOx Control; EPA-EPRI: Washington, DC, 1991; pp 6B,. (23) McCarthy, J. M.; Chen, S. L.; Seeker, W. R.; Pershing, D. W. Pilot Scale Studies on the Application of Reburning for NOx Control. In Proceedings of the 1987 Joint Symposium on Stationary Combustion NOx Control; EPA-EPRI: Washington, DC, 1987.
stream in the reducing zone (configuration 2) shows greater promise but may require the addition of a promoter agent such as a sodium salt in order to achieve high NOx reduction efficiencies.4 Since alkali metals are known to enhance slagging and corrosion problems, the need for a promoting agent may limit the feasibility of this configuration. In configurations 3, 4, and 5, which are the interest of the present study, the N-agent is injected under nearstoichiometric or lean conditions. The best results are obtained, if the CO and O2 concentrations at the N-agent injection point are tailored for promoting the SNCR performance. The presence of CO enhances the SNCR chemistry by replenishing the radical pool and shifts the low-temperature boundary for the process toward lower temperatures. Bench scale tests of configurations 4 and 5, both involving splitting of the burnout air, have shown high NOx reduction efficiencies.1,3 However, if it is competitive in NOx reduction, configuration 3 with simultaneous injection of burnout air and SNCR agent is more attractive for retrofit application. Assessment of the NO reduction potential of configurations 3, 4, and 5 requires a systematic investigation of the SNCR performance in the presence of combustibles such as CO. Previous studies of the CO/NH3/NO/ O2 system2,13,17,24-26 have provided significant insight into the effect of CO on the SNCR performance. However, the data are generally obtained under conditions with low CO and relatively high O2. Such conditions apply to some extent to configuration 4, but not to 3 or 5. Furthermore, most of the data are obtained under experimental conditions, which are less well-defined than those of the present study. The objective of the present work is to assess the potential of the hybrid reburning/SNCR technique, emphasizing particularly configuration 3, from experimental results obtained under well-controlled, isother(24) Kasaoka, S.; Sasaoka, E; Ikoma, M. Chem. Soc. Jpn. 1980, 8, 1282-1290 (in Japanese). (25) Kasaoka, S.; Sasaoka, E; Ikoma, M. Chem. Soc. Jpn. 1981, 4, 597-604 (in Japanese). (26) Lodders, P.; Lefers, J. B. Chem. Eng. J. 1985, 30, 161-167.
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Alzueta et al. and water vapor, is preheated in the reactor and mixed with the reactant gases at the inlet of the reaction zone. The reactant mixture is quenched efficiently at the outlet of the reaction zone by means of an external addition of cooling air. The analysis of the products at the outlet of the reaction zone, after drying and conditioning, is performed using different methods. The concentrations of CO, CO2, and NO are measured by means of UV and IR continuous analyzers, and O2 is measured paramagnetically. The uncertainty of these measurements is (3%, but not less than 5 ppm. Hydrogen cyanide, NH3, and N2O are determined using a Fouriertransformed infrared (FTIR) analyzer. An estimated uncertainty for these measurements is (10%, but not less than 10 ppm. The flow system and the reactor temperature are controlled automatically by a PC system also used for data acquisition. The reactor temperature is measured with a type K thermocouple. Pressure is measured in the main stream prior to the reaction inlet with an absolute pressure transmitter. The experiments are carried out with a simulated flue gas mixture containing varying amounts of CO, O2, H2O, NO, and NH3. To ensure isothermal conditions in the reaction zone, the reactant gas is highly diluted in N2. The reaction takes place at atmospheric pressure. The flow rate, around 1050 normal mL/min (1 atm and 273 K), is kept constant during an experiment, providing different residence times depending on the temperature in the reaction zone.
Numerical Procedure Figure 2. Schematic of the quartz flow reactor: (1) gas injectors, (2) main flow, (3) mixing area; (4) reaction zone, (5) gas outlet, and (6) cooling air inlet.
mal flow reactor conditions. Furthermore, the performance of a recently published chemical kinetic model27 in describing the process is evaluated. The effect of CO on the SNCR process is studied systematically over a wide range of conditions, using ammonia as reducing agent. The variables considered are the temperature, the concentration of CO, the stoichiometry (both fuellean and fuel-rich conditions are considered), the NH3/ NO ratio, and the presence of HCN.
The model calculations were performed using Senkin,31 a plug-flow code that runs in conjunction with the Chemkin library.32 The thermodynamic data were taken from the Sandia Thermodynamic Database.33 The chemical kinetic model used in this work consists of a mechanism for the thermal DeNOx process,27 together with a reaction subset describing the moist oxidation of CO.34 In selected calculations, an HCN oxidation subset was included.35 For a detailed description of the reaction subsets used, the reader should address the individual references. Results and Discussion
Experimental Method The experimental setup consists basically of a gas feeding system, a reaction system, and a gas analysis system. This experimental installation has previously been used and described in a number of studies,15,28-30 and only a brief explanation is presented here. Pure gases from gas cylinders are dosed and controlled by mass flow controllers, and water is added by saturation of a gaseous nitrogen stream at a well-defined temperature. The reaction system includes a quartz flow reactor (Figure 2), designed to minimize the axial dispersion of the reactants.15,30 The reactor tube has an inner diameter of 0.9 cm and a length of 19 cm. The reactor is placed in an electrically heated threezone oven, with individual control of the temperature in each zone. This allows reactor temperatures up to 1300 K with a uniform temperature profile throughout the reaction zone within (7 K. The gas feeding to the reactor is done by means of four injectors: a main stream and three additional side streams. The main stream, which contains nitrogen, oxygen, (27) Miller, J. A.; Glarborg, P. Springer Ser. Chem. Phys. 1996, 61, 318-333. (28) Hulgaard, T.; Dam-Johansen, K. AIChE J. 1993, 39, 13421354. (29) Glarborg, P.; Kristensen, P. G.; Jensen, S. H.; Dam-Johansen, K. Combust. Flame 1994, 98, 241-258. (30) Kristensen, P. G.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1996, 107, 211-222.
A parametric study of the reduction of NO by NH3 in the presence of 0-2% CO and temperatures between 700 and 1300 K has been conducted. Levels of 0-100 ppm CO are characteristic of the conventional SNCR process, while 2% CO corresponds to the exit level from a reducing zone operated at a stoichiometry of 0.95. At 2% CO, the oxygen concentration was varied in the range 0.5-4.0%, thereby covering stoichiometries from fuel-rich to -lean. The influence of the NH3/NO ratio has been studied by varying the NH3 concentration (approximately 900, 300, and 150 ppm), keeping the NO concentration around 300 ppm. This value is representative of the NO levels found in boilers where NO is (31) Lutz, A. E.; Kee, R. J.; Miller, J. A. Senkin: A Fortran Program for Predicting Homogeneous Gas-Phase Chemical Kinetics with Sensitivity Analysis. Sandia Report SAND87-8248, Sandia National Laboratories, Livermore, CA, 1990. (32) 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. (33) Kee, R. J.; Rupley, F. M.; Miller, J. A. The Chemkin Thermodynamic Database. Sandia Report SAND87-8215, 1991 update, Sandia National Laboratories, Livermore, CA, 1991. (34) Glarborg, P.; Kubel, D.; Kristensen, P. G.; Hansen, J.; DamJohansen, K. Combust. Sci. Technol. 1995, 110-111, 461-485. (35) Glarborg, P.; Miller, J. A. Combust. Flame 1994, 99, 523-532.
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experimental results indicate that the presence of CO affects the radical pool but that CO does not contribute to reducing NO. This observation is in agreement with reported data on CO/NOx interactions under reducing and oxidizing conditions11,34 and corroborated by the model calculations. As discussed in detail elsewhere (e.g., ref 14), the NO concentration results from competition between consumption and production reactions of NO during the NH3 oxidation, which in the low-temperature part of the regime for NO reduction involves mainly the following (simplified) reaction sequence:
Figure 3. Comparison of experimental data and model predictions for reduction of NO by NH3: effect of CO concentration. Symbols denote experimental data, while solid lines denote model predictions. Inlet concentrations: 300 ppm NH3, 300 ppm NO, 4.0% O2, 4.5% H2O; balance N2. Pressure is 1.05 atm. Residence time at 1200 K (constant molar rate) is about 150 ms.
already controlled by gas reburning. In selected experiments, HCN was added with a concentration of 100 ppm. In all the experiments, approximately 4.5% of water was added and the residence time was roughly 150 ms at 1200 K. The effect of CO on the reduction of NO by NH3 under fuel-lean conditions is shown in Figure 3. The oxygen level is 4%, corresponding to N-agent injection with the final burnout air (configurations 3 and 4). Experimental data for NO and NH3 are shown as symbols, model predictions as lines. The concentration of N2O was in all experiments lower than 20 ppm, i.e., close to the estimated experimental uncertainty, and therefore, results for this species are not shown. Consistent with previous studies of the SNCR process, a minimum in NO is observed, and this minimum is shifted toward lower temperatures as the CO concentration increases. The temperature for onset of NO reduction coincides with the initiation temperature for NH3 consumption. Varying the concentration of CO from 0 ppm to 2% produces a shift of approximately 250 K in the low-temperature boundary for the process. The NO reduction potential, 65-70%, and the shape of the concentration profiles are roughly independent of the CO concentration. However, the temperature window for NO reduction is narrowed as the CO concentration increases. In general, a good agreement between experimental results and model calculations is observed. The model predicts reasonably well the onset of NH3 consumption and NO reduction, as well as the shape of the concentration profiles, even though the NO concentration is slightly underpredicted for high CO concentrations. The
NH3 + OH h NH2 + H2O
(1)
NH2 + NO h N2 + H + OH
(2a)
NH2 + NO h N2 + H2O
(2b)
H + O2 h O + OH
(3)
O + H2O h OH + OH
(4)
Below roughly 1050 K,14,27 this sequence of reactions is chain terminating due to the low branching fraction of reaction 2.27 As a consequence, little or no reaction takes place and the concentrations of NO and NH3 remain at their initial levels. As the temperature is raised, the branching fraction for the NH2 + NO reaction increases and the above reaction sequence becomes chain branching. Since the key amine radical, NH2, reacts almost solely with NO, a significant reduction in NO can be obtained. However, a further increase in temperature beyond an optimum value promotes oxidation of NHi radicals to NO, resulting in an increase in NO. In this way, the selectivity for NO or N2 depends strongly on the reaction temperature. A detailed discussion of the SNCR chemistry can be found elsewhere.14,27 The decrease in the optimum temperature as the CO concentration increases can be attributed to the branching behavior of CO oxidation. While in the presence of NO, NH3 is oxidized through a series of reactions which under present conditions is only slightly chain branching, the moist oxidation of CO involves a strongly chain branching reaction sequence:
CO + OH h CO2 + H
(5)
H + O2 h O + OH
(3)
O + H2O h OH + OH
(4)
corresponding to the net reaction
CO + O2 + H2O f CO2 + 2OH Both CO and NH3 are consumed mainly by reaction with OH. The NH3 + OH reaction is about 1 order of magnitude faster than the CO + OH reaction in the present temperature range, and addition of NH3 acts to delay or inhibit CO oxidation. However, the CO consumed promotes the SNCR chemistry at lower temperatures due to the enhanced production of radicals. In this way, the presence of CO shifts the regime for NO reduction toward lower temperatures.
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Figure 5. Comparison of experimental data and model predictions for reduction of NO by NH3 at 2% CO: effect of NH3/NO ratio. Symbols denote experimental data, while solid lines denote model predictions. Inlet concentrations: 300 ppm NO, 2.0% CO, 4.0% O2, 4.5% H2O; balance N2. Pressure is 1.05 atm. Residence time at 1200 K (constant molar rate) is about 150 ms.
Figure 4. Comparison of experimental data and model predictions for reduction of NO by NH3 at 2% CO: effect of HCN addition. Symbols denote experimental data, while solid lines denote model predictions. Inlet concentrations: 300 ppm NH3, 300 ppm NO, 100 ppm HCN, 2.0% CO, 4.0% O2, 4.5% H2O; balance N2. Pressure is 1.05 atm. Residence time at 1200 K (constant molar rate) is about 150 ms.
The results of Figure 3 agree with results previously reported for the lean CO/NH3/NO/O2 system.2,13,17,25,26 These studies cover CO and O2 concentrations of 0-1% and 1-15%, respectively, but each over a fairly narrow range of conditions. In agreement with the present results, it is found that the presence of CO shifts the window for NO reduction toward lower temperatures, and the maximum NO reduction is constant or decreased slightly with increasing CO. These effects have been observed also for other combustible additives13-15 and for the HCN/NH3/NO system.30 The influence of presence of HCN (100 ppm) has been investigated for selected conditions. Figure 4 shows results for NO, NH3, and HCN obtained in experiments with CO concentrations of 0 and 2%, respectively, and 4% O2. The results do not show any appreciable influence of HCN. As expected, a slightly higher concentration of N2O was detected in the experiments with HCN; HCN is known to be a better precursor of N2O than is NH3.28,30 However, even though the N2O formation can be attributed to NO-removing reactions, mainly NCO + NO,29,30 no additional NO reduction due to the presence of HCN was observed, except for a minor difference at higher temperatures. Consequently, it appears that under the present conditions, even in
presence of HCN, the onset and extent of NO reduction is controlled by the NH3/NO/O2 system and the level of CO. Model calculations predict well the onset for NO reduction under the conditions of Figure 4 but overestimate the NO concentration at high temperatures. The results are similar to our recent study of the influence of CO on HCN/NH3/NO conversion in the burnout zone,30 even though the nitrogen species partitioning is different. Figure 5 shows NO and NH3 concentrations as function of temperature for values of the NH3/NO ratio of 0.5, 1, and 3. The conditions with 2% CO/4% O2 correspond to configuration 3. Increasing the NH3/NO ratio results in higher NO reductions, but also in a significantly higher ammonia slip at the optimum temperature. Furthermore, a slight shift in the optimum temperature for NO reduction toward higher values, accompanied by a narrower temperature window, is observed when the NH3/NO ratio is increased. As the temperature increases beyond the optimum value, the amount of NH3 oxidized to NO increases considerably with the NH3/NO ratio. At these high temperatures, the quantity of ammonia oxidized corresponds approximately to the ammonia slip obtained for optimal NO reduction conditions. Model calculations reproduce the main trends observed in the experiments of Figure 5, and the optimum temperature for NO reduction is reasonably well predicted. However, consistent with the results of Figures 3 and 4, the model somewhat overpredicts the NO reduction at and above the optimum temperature. Figure 6 shows the effect of varying the concentration of oxygen under conditions with high CO concentrations.
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Figure 7. Comparison of experimental data and model predictions for reduction of NO by NH3 at near-stoichiometric conditions and low CO: effect of O2 concentration. Symbols denote experimental data while solid lines denote model predictions. Inlet concentrations: 400 ppm NH3, 400 ppm NO, 1400 ppm CO, 2.0% H2O; balance N2. Pressure is 1.1 atm. Residence time at 1200 K (constant molar rate) is about 100 ms.
Figure 6. Comparison of experimental data and model predictions for reduction of NO by NH3 at 2% CO: effect of O2 concentration. Symbols denote experimental data while solid lines denote model predictions. Inlet concentrations: 300 ppm NH3, 300 ppm NO, 2.0% CO, 4.5% H2O; balance N2. Pressure is 1.05 atm. Residence time at 1200 K (constant molar rate) is about 150 ms.
A change in stoichiometry toward fuel-rich conditions does not have any significant effect on the NO reduction level or the temperature window for reduction. However, the increase in NO at the right branch of the window is less pronounced as the oxygen concentration diminishes. Because NH3 competes favorably with CO for OH radicals, as discussed above, the NH3 consumption profile is largely unaffected by the oxygen availability, while oxidation of CO is inhibited at low O2 concentrations. The model predicts the experimental trends, but some discrepancy is observed for NO and NH3 at the lowest oxygen concentration. The present results as well as those previously reported for the NH3/HCN/NO system30 suggest that further work is needed on the model in order to obtain a satisfactory description of reactive nitrogen conversion under fuel-rich conditions. Figure 7 shows results for near-stoichiometric conditions with low CO and O2 concentrations, i.e., close to the conditions recommended in configuration 5.1 Several interesting characteristics are worth noting. Compared to the SNCR performance under conditions with higher oxygen levels, the shape of the NO profile is changed significantly. The results show a sharp distinction between a very slow reaction regime at lower
temperatures and a rapid oxidation regime at higher temperatures. Compared to experiments with the same level of CO but higher O2 concentration, the onset of NO reduction is shifted toward higher temperatures. The low-temperature boundary is very sensitive to small variations in the O2 concentration. The minimum in NO is reached at temperatures only slightly higher than the low boundary value, and NO increases only slowly with increasing temperature. The model describes these trends qualitatively correctly, but the location of the low-temperature boundary is not predicted well. The discrepancy is attributed to inaccuracies in the NH2 + NO branching fraction in the mechanism; at very low O2 concentrations, the model predictions are extremely sensitive to this parameter, while other parameters in the mechanism are less significant.27 The issue of the NH2 + NO branching fraction is currently investigated, using a.o. the data of Figure 7.36 The results of Figure 7 are not directly comparable to the data for configuration 5 reported by Chen et al.,1 which were obtained in a bench scale setup with temperature gradients and presumably somewhat higher CO and O2 concentrations. However, comparison of the results of Figure 7 with those of Figure 3 (lean, no CO) tend to support the observation of Chen and co-workers, that near-stoichiometric conditions with low O2 levels broaden the temperature window and may increase the NO reduction potential. Practical Implications Hybrid processes offer a possibility to significantly enhance the reburn potential. Pilot scale results2-4 suggest that the reburn/SNCR concept offers NOx removal efficiencies comparable to catalytic methods. (36) Glarborg, P.; Kristensen, P. G.; Dam-Johansen, K.; Miller, J. A. J. Phys. Chem. Accepted for publication.
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However, a better understanding of the chemistry of this and similar hybrid processes is essential in further refining the technology. On the basis of the present results, the potentials of configurations 3-5 (Figure 1) can be assessed. According to EER,2,3 the best results are obtained if the reducing agent injection is combined with a staged addition of burnout air. The first burnout air addition increases the excess air ratio from the reburn zone stoichiometry (typically 0.9) to values close to stoichiometric (0.99-1.03). In this way, a lower CO concentration at the N-agent injection point is obtained. The O2 concentration will depend on whether the SNCR agent is added prior to (configuration 5, Figure 1c) or together with (configuration 4, Figure 1b) the final air. The present results largely support the findings of EER. The idea of configuration 5 is to employ a low oxygen concentration to suppress NO formation, while using CO to shift the NO reduction regime to lower temperatures. The results of Figure 7 confirm the potential of this approach. The SNCR temperature window is broadened under these conditions, and a comparatively high NO reduction can be achieved. A major disadvantage, however, is the difficulty of applying a complex staged injection for burnout air and SNCR agent, particularly considering the need to control reaction conditions fairly accurately. Our work indicates that the low-temperature boundary for the SNCR process is very sensitive to small variations in CO or O2 concentrations as well as the local NH3/NO ratio under the conditions of this configuration. To obtain the required process conditions would be a major challenge considering the difficulties of mixing comparatively small amounts of reagents into a large furnace, even if the SNCR agent is injected together with recirculated flue gas (FGR) to obtain sufficient momentum. Configuration 4 involves a two-stage injection of reactants. This configuration, which compared to 5 is easier to implement and control, was preferred by Pont et al.3 The results of Figure 3 suggest, in agreement with pilot scale results,3 that this scheme has a significant NO reduction potential. The comparatively low CO concentration at the N-agent injection location shifts the regime for NO reduction to slightly lower temperatures but does not have any adverse effects on the NO reduction potential or the width of the temperature window. However, unless a shift in optimum temperature is warranted, the CO present does not promote the SNCR performance, i.e., there is not a synergistic effect between the two processes. The major disadvantage of this configuration is the need to split the burnout air, which in some boilers may prove difficult. Configuration 3 (Figure 1a) is the most simple and therefore commercially most attractive; it requires only minor modifications and investment costs. This scheme involves the injection of the selective reducing agent together with the burnout air in a single stage at the rich/lean transition. The concentration of CO at the rich/lean transition is high (around 2% or higher), and a large shift in stoichiometries is produced when the selective reducing agent and the burnout air are added. As shown in Figures 3-5, this results in an optimum temperature for NO reduction below 1000 K. Addition of the final burnout air at such low temperatures may cause considerable concern about unburnt CO and
Alzueta et al.
carbon in ash. Furthermore, the temperature window for NO reduction becomes very narrow. Under the conditions of our experiments significant NO reductions are obtained only for an interval of approximately 30 K around the optimum temperature, and no reduction at all is achieved except for a window of approximately 100 K. Therefore, this configuration seems to be effective only under very critical operating conditions. To make this configuration feasible, it is necessary to delay the SNCR reactions until most of the CO present at the rich/ lean transition is consumed. This can be done by injecting the SNCR agent in an aqueous solution.4 By adjusting the droplet size, it can be secured that CO is largely oxidized prior to the evaporation of the agent. The performance of this configuration will then be similar to that of configuration 4. As stated above, the choice of configuration will depend on the targeted NOx reduction as well as restrictions of the given boiler. However, any staging of reactants (air, N-agent) may prove difficult in retrofit applications. Incomplete mixing may affect the SNCR performance as well as burnout. This problem becomes more severe as the amount of burnout air is reduced in the injection streams when the burnout air is staged. The use of FGR in the case where the amount of air to be injected is very small has been recommended.22,37 A better solution, as suggested above, is to avoid burnout air staging and inject the SNCR agent in an aqueous solution with the burnout air. Independent of configuration, the NH3/NO ratio is an important parameter in the process, for both the NO reduction efficiency and the ammonia slip. Increasing this ratio results in an appreciable improvement of NO reduction but may also lead to significant increases in ammonia slip. The use of a high NH3/NO ratio thus requires considerable control of the operating conditions, a difficult task as discussed above. Conclusions A parametric study of the reduction of NO by ammonia under the presence of CO (hybrid reburning/ SNCR conditions) has been performed in a flow reactor under controlled conditions and compared with model predictions with a detailed chemical kinetic model. The work emphasizes the effect of a high CO concentration on the SNCR performance, corresponding to a hybrid reburn/SNCR configuration with N-agent injection with the burnout air at the rich/lean transition, but the potential of burnout air staging is also evaluated. The results obtained show that the effect of CO on the SNCR process is mainly to shift the regime for NO reduction toward lower temperatures. The NO reduction potential is largely unaffected by the presence of CO, but in higher concentrations, CO has an adverse effect on the width of the temperature regime for NO reduction. At the high CO level (2%), the effect of oxygen concentration on SNCR performance is insignificant in the range studied (0.5-4.0%). The SNCR process is also largely unaffected by the presence of HCN carried over from the reducing zone in reburning. An increase in the NH3/ NO ratio up to a value of three results in enhanced NO reduction at the optimum temperature, but also in(37) Chen, S. L.; Lyon, R. K.; Seeker, R. W. Environ. Prog. 1991, 10, 182-185.
Laboratory Study of the CO/NH3/NO/O2 System
creased NH3 slip. As the temperature increases above the optimum temperature for reduction, the NH3 slip diminishes, but the NO level increases considerably with the NH3/NO ratio. Calculations with the chemical kinetic model are generally in good agreement with the experimental data, indicating that the model can be used with some confidence to evaluate process performance. However, care must be taken under fuel-rich conditions, where model predictions are less accurate. Generally, our results indicate little synergistic effect between reburn and SNCR. Only in a narrow range of operating conditions, close to stoichiometric and with low concentrations of CO and O2, do we observe an enhanced SNCR performance. Here the temperature window for the SNCR process is broadened and the NO reduction potential high. However, such operating conditions may be difficult to obtain in retrofit applications. The practical implications for advanced reburning configurations have been discussed. The most simple configuration, where the selective reducing agent is injected together with the burnout air, is not expected
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to be effective, unless the N-agent is injected in form of an aqueous solution. This ensures that the CO is partially consumed when the SNCR reactions take place. Other more complicated configurations might be more efficient but involve splitting of the burnout air, which for practical and economical reasons may be undesirable. Acknowledgment. This work was supported by the Physical Sciences Department of the Gas Research Institute, Chicago, and the Danish Gas Technology Center. The work is part of the research program CHEC (Combustion and Harmful Emission Control), which is cofunded by the Danish Technical Research Council, Elsam (the Jutland-Funen Electricity Consortium), Elkraft (the Zealand Electricity Consortium), and the Danish Ministry of Energy. M.U.A. expresses her gratitude to CAI-CONSID for the “Ayuda Europa” awarded. EF960140N
