Interactions between Nitric Oxide and Urea under Flow Reactor

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Energy & Fuels 1998, 12, 1001-1007

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Interactions between Nitric Oxide and Urea under Flow Reactor Conditions M. U. Alzueta,* R. Bilbao, A. Millera, M. Oliva, and J. C. Iban˜ez Department of Chemical and Environmental Engineering, University of Zaragoza, 50015 Zaragoza, Spain Received March 18, 1998

An experimental and theoretical study of the interactions between urea and NO under lean selective noncatalytic reduction conditions has been performed. The experiments were conducted in an isothermal quartz flow reactor at atmospheric pressure in the temperature range 7001500 K. The influence of the temperature, oxygen concentration, and urea/NO ratio on the NO reduction has been analyzed. A reaction mechanism including literature NH3, HNCO, and moist CO oxidation subsets as well as their interactions with NO and reactions describing urea thermal decomposition have been used for calculations. The results show that urea is effective in reducing NO in a given temperature window, accompanied by the formation of an appreciable amount of N2O, which reaches its maximum value for the higher NO reduction. The impact of oxygen concentration in the 1-10% range is appreciable, and lower O2 concentrations shift the reduction regime toward higher temperatures, the higher N2O formation being observed for the richer environment. Using urea, the onset of NO reduction is shifted to higher temperatures compared to the use of ammonia, even though the effective temperature window for NO reduction roughly coincides for both selective reduction agents. The efficiency of NO reduction and the NO to N2O conversion increase as the urea/NO ratio increases, even though at high temperatures the excess of urea can be oxidized to NO. Model predictions are in good agreement with the experimental results and indicate that the classical reaction pathway for urea decomposition (i.e., H2N-CONH2 f NH3 + HNCO) is not able alone to reproduce the experimental findings obtained in the upper temperature range. This is attributed either to uncertainties of the HNCO oxidation mechanism or to the fact that other decomposition channels are likely to be produced. Results of this work as well as other literature data suggest that the method chosen for urea injection is important with respect to the N2O emissions attained. Adding urea at least partially decomposed prior to its interaction with NO results in similar NO reduction efficiencies but in considerably lower N2O concentrations.

Introduction Selective noncatalytic reduction, SNCR, is a wellknown technique to reduce NOx emissions in combustion processes. Since 1975, when Lyon patented the process called Thermal DeNOx1 in which ammonia is used as a reducing agent of NO under lean conditions, many works have dealt with the study of such technique in both laboratory, pilot, and full scale plants.2-5 The different combustion devices that exist (utility boilers, waste incinerators, and other stationary combustors), the various nitrogen reducing agents that are used (ammonia, urea, cyanuric acid, etc.), as well as the possible modifications that can be necessary when implementing the SNCR technique result in a variety of cases and operating conditions which need individual and careful analysis. The SNCR process using ammonia has been extensively studied from both experimental and kinetic modeling points of view, and the main issues are fairly well characterized.2,4,6-8 Attempts have also been per* Author to whom correspondence should be sent. Fax: +34 976 761861. E-mail: [email protected]. (1) Lyon, R. K. U.S. Patent 3, 900, 554, 1975.

formed when using other nitrogen reduction agents such as urea,9-14 cyanuric acid,15-17 and others.4,9 The use of reducing agents different from ammonia can be of interest mainly for reasons of storage and handling. (2) Lyon, R. K.; Hardy, J. E. Ind. Eng. Chem. Fundam. 1989, 25, 19-24. (3) Lucas, D.; Brown, N. J. Combust. Flame 1992, 47, 219-243. (4) Duo, W.; Dam-Johansen, K.; Østergaard, K. Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; pp 297-303. (5) Caton, J. A.; Narney, J. K.; Cariappa, H. C.; Lester, W. R. Can. J. Chem. Eng. 1995, 73, 345-350. (6) Miller, J. A.; Glarborg, P. Springer Ser. Chem. Phys. 1996, 61, 318-333. (7) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287-313. (8) Hulgaard, T.; Dam-Johansen, K. AIChE J. 1993, 39, 1342-1354. (9) Muzio, L. J.; Arand, J. K.; Teixeira, D. P. Sixteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1977; pp 199-208. (10) Jødal, M.; Nielsen, C.; Hulgaard, T.; Dam-Johansen, K. TwentyThird Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; pp 237-243. (11) Arand, J. K.; Muzio, L. J.; Sotter, J. G. U.S. Patent 4, 208, 386, 1990. (12) Sun, W. H.; Stamakis, P.; Hofman, J. E. ACS Symp. Ser. 1993, 38, 734-739. (13) Brouwer, J.; Heap, M. P.; Pershing, D. W.; Smith, P. J. TwentySixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; pp 2117-2124.

S0887-0624(98)00055-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/05/1998

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Figure 1. Schematic of experimental installation. 1: Gas cylinders. 2: Mass flow controllers. 3: Urea solution addition. 4: Electric oven. 5: Reactor. 6: Oven control. 7: Flow measurements. 8: Vent. 9: FTIR analyzer + PC. 10: Cooling air addition.

In this work, urea has been chosen as the SNCR reducing agent. Muzio et al.9 first demonstrated the effectiveness of urea in reducing NOx in pilot scale, adding urea either as a solid or as a water solution, and the process was patented by Arand. 18 Additional studies of SNCR with urea under lean conditions were performed by Nalco Tech,12,19,20 who called the process NOxOUT. Sun et al.20 performed experiments in a pilot scale combustor, injecting an atomized urea solution together with air as carrier gas, and they evaluated the process performance under different conditions. In the past recent years, additional studies of urea-SNCR have been performed in pilot scale,10 in full scale,21 as well as involving hybrid strategies for NOx reduction.22 In general, urea has been seen to be effective in reducing NO in a specific temperature window which is shifted toward higher temperatures compared to the use of ammonia, even though significant amounts of N2O can be formed. However, the various results (14) Itaya, Y.; Deguchi, S.; Takei, M.; Yoshima, M.; Matsuda, H.; Hasatani, M. Fourth International Conference for Technologies and Combustion for a Clean Environment, Lisbon, 1997; Vol. II, pp 34.2/ 7-12. (15) Perry, R. A. U.S. Patent 4, 731, 231, 1988. (16) Caton, J. A.; Siebers, D. L. Combust. Sci. Technol. 1989, 65, 277-293. (17) Lyon, R. K.; Cole, J. A. Combust. Flame 1990, 82, 435-443. (18) Arand, J. K. U.S. Patent 4, 325, 924, 1982. (19) Sun, W. H.; Michels, W. F.; Stamakis, P.; Comparato, J. R.; Hofman, J. E. AFRC Fall International Symposium, Cambridge, MA, 1992. (20) Sun, W. H.; Hofman, J. E.; Pachaly, R. Seventh Annual International Pittsburgh Coal Conference, Pittsburgh, PA, 1990. (21) Karll, B.; Gustaffson, P. Å. NOx Reduction by Injection of Natural Gas Above the Grate in Combination with Urea Injection in the Furnace. Nordic Gas Technology Centre Report, Hørsholm, 1994. (22) Zamansky, V. M.; Ho, L.; Maly, P.; Seeker, W. R. Fourth International Conference for Technologies and Combustion for a Clean Environment, Lisbon, 1997; Vol. II, pp 10.1/1-9.

obtained in the investigations differ significantly, in relation to the optimum temperature for NO reduction, the formation of N2O, as well as the efficiencies obtained. This may be due to the different operating conditions used by the different investigators in experimental equipment, in particular to the method chosen for urea injection. To our knowledge, the only laboratory study of the urea/NO system has been performed by Itaya et al.,14 who investigated the main characteristics of NO reduction by adding a urea solution in a tubular reactor in the temperature range 850-1250 K. They pointed out interesting facts concerning the products distribution of the thermal decomposition of urea, which has been traditionally considered to give NH3 and HNCO.13,16 The aim of this work is to carry out a systematic study of the interactions between urea and nitric oxide in a flow reactor in the temperature range 700-1500 K, to obtain a wider and more detailed knowledge of such interactions, and to identify the main characteristics of the urea-SNCR process. This is done by means of both experiments and kinetic modeling, and the analysis of the influence of the main variables affecting the process, such as temperature, oxygen concentration, and urea/ NO ratio, is performed. Experimental Method The experimental installation is shown in Figure 1 and consists basically of a gas feeding system, a gas reaction system, and a gas analysis system. Pure gases from gas cylinders are dosed with mass flow controllers and are fed into the reactor. The addition of urea is done by passing a nitrogen stream through a saturated urea solution at a given temperature (∼330 K). Logically, water is contained in such a stream

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Numerical Procedure The model calculations were carried out using Senkin,25 a plug-flow code that runs in conjunction with the Chemkin library.26 Thermodynamic data were taken from the Sandia Thermodynamic Database,27 except for a few species that were taken from the same sources as the mechanism used for calculations. The chemical kinetic model used in this work consists of a mechanism for the interactions between NO and NH36 together with reaction subsets describing HNCO28 and HCN oxidation.29 The reader should address the individual references for a detailed description of the reaction subsets. This kinetic model has been previously used with significant success in predicting different experimental data under flow reactor conditions.6,28-30 Furthermore, to account for urea decomposition, in this work two different product channels have been considered, as discussed later. Results and Discussion

Figure 2. Schematic of the quartz flow reactor.23 1: Main flow injector (usually containing N2, water vapor, and urea). 2: Side injectors (containing the rest of reactants, i.e., NO and O2). 3: Mixing zone. 4: Reaction zone. 5: Reactor outlet. 6: Cooling air entrance. as well. The reaction system includes a quartz flow reactor located inside an electrical oven that allows us to reach temperatures up to 1500 K. The reactor has a reaction zone of 8.7 mm inside diameter and 200 mm in length and has been constructed following a similar design of the “CHEC flow reactor”,23,24 Figure 2. Reaction conditions are carefully controlled. The reactants being led separately to the reactor are quickly and efficiently mixed at the entrance of the reaction zone, and the temperature is kept uniform along the reaction zone within 10 K. The gas entering into the reaction zone consists of a diluted mixture, in order to keep isothermal conditions, of NO, urea, O2, H2O, and N2 to balance. Reaction is frozen at the reactor outlet using an external addition of air. The analysis of the products (NO, N2O, NO2, NH3, CO, and CO2), after conditioning, is done using a FTIR (Fourier transform infrared) analyzer coupled with an appropriate gas cell and the necessary calibration setup. An estimated uncertainty for the measurements is 5% but not less than 10 ppm. The reactor temperature is measured with a K-type thermocouple, and the pressure with an absolute pressure transducer. Data acquisition is performed by a personal computer system. (23) Kristensen, P. G.; Glarborg, P.; Dam-Johansen, K. 1992 International Gas Research Conference; Goverment Institutes, Inc.: Chicago, 1993; pp 2437-2446. (24) Duo, W. Kinetic Studies of the Reactions Involved in Selective Non-Catalytic Reduction of Nitric Oxide. Ph.D. Thesis, Technical University of Denmark, Lyngby, 1990.

A parametric study of the reduction of NO by urea in a flow reactor was performed, and the influence of temperature, oxygen concentration, and urea/NO ratio has been analyzed. Temperature is varied in the 7001500 K range, and the oxygen concentration is changed between 0 and 10%. The inlet NO concentration ranges between 100 and 1200 ppm, even though in most of the experiments it is kept around 300 ppm. The concentration of urea is determined through a balance of carbon and nitrogen atoms in the total oxidation of urea in the presence of water and excess oxygen at a temperature of 1500 K and after a long residence time (i.e., ∼500 ms). The concentration of urea was always kept at a level of approximately 150 ppm. In all the experiments, the total flow rate is approximately 1000 N mL/min, and thus the residence time in the reaction zone, which is dependent on temperature, is about 200 ms at 1000 K. Figure 3 shows the experimental results of NO and N2O concentrations when the effect of the oxygen concentration is considered on the reduction of NO by action of urea. For each oxygen level studied, a minimum in NO concentration is observed. These results coincide with the well-known behavior of the SNCR process. At low temperatures, little or no NO reduction is produced, due to both the low reaction rates and to the low levels of the radical pool. At high temperatures, the formation of NO from the oxidation of the SNCR agent is favored compared to its destruction. These facts result in an effective temperature window for NO reduction by action of urea, which is located in the range (25) Lutz, A.; Kee, R. J.; Miller, J. A. Senkin: A Fortran Program for Predicting Homogeneous Gas-Phase Chemical Kinetics with Sensitivity Analysis. Sandia National Laboratories Report SAND87-8248, 1990. (26) 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 National Laboratories Report SAND89-8009, 1989. (27) Kee, R. J.; Rupley, F. M.; Miller, J. A. The Chemkin Thermodynamic Data Base. Sandia National Laboratories Report SAND878215, 1991 update, 1991. (28) Glarborg, P.; Kristensen, P. G.; Jensen, S. H.; Dam-Johansen, K. Combust. Flame 1994, 98, 241-258. (29) Glarborg, P.; Miller, J. A. Combust. Flame 1994, 99, 523-532. (30) Alzueta, M. U.; Røjel, H.; Kristensen, P. G.; Glarborg, P.; DamJohansen, K. Energy Fuels 1997, 11, 716-723.

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observed in pilot scale applications.10 However, the low N2O levels obtained in the present work are in agreement with previous results obtained in pilot scale when urea is added at least partially decomposed to the reaction zone.10 Little is known about the mechanism of thermal decomposition of urea. Traditionally, NH3 and HNCO have been considered as primary decomposition products of urea.13,16 At temperatures up to 593 K, the main route for thermal decomposition of urea seems to proceed through10

H2N-CO-NH2 f NH3 + HNCO

(1)

However, other reaction channels for urea decomposition have been proposed for high temperatures. Jødal et al.10 suggest that at higher temperatures NH2 can be removed instead of NH3, being followed by the formation of NCO, e.g.:

H2N-CO-NH2 f NH2 + H2NCO

(2)

H2NCO f H2 + NCO

(3)

Also, the direct elimination of NH2 and H radicals from urea has been proposed,32 i.e.:

H2N-CO-NH2 f NH2 + H + HNCO Figure 3. Experimental NO and N2O concentrations versus temperature for different oxygen concentrations: b, 0.5% O2; O, 1.0% O2; ), 4.0% O2; 4, 10% O2. (Inlet concentrations: ∼300 ppm NO, ∼150 ppm urea, ∼4% H2O, N2 to balance).

1200-1350 K as seen in Figure 3. The level of NO reduction increases as the oxygen concentration diminishes, the effect being more significant as the oxygen concentration diminishes. When using ammonia as the SNCR agent, the temperature window is seen to shift significantly toward higher temperatures as the oxygen concentration diminishes.5,30,31 This is due to the fact that low oxygen levels are not able, at low temperatures, to replenish the radical pool which interacts with NH3 to produce the active NH2 radicals that react with NO. The results shown in Figure 3 indicate that when urea is used, the effect of the oxygen concentration on the onset of NO reduction is appreciable, in particular for the lowest oxygen value studied, i.e., 0.5%, for which the onset of NO reduction is significantly shifted toward higher temperatures. For all the oxygen levels considered, the temperature for which maximum N2O concentrations are attained (up to 40 ppm) coincides approximately with the temperature at which the minimum NO concentration is obtained; the major N2O levels being obtained for the lowest oxygen concentrations. The formation of significant amounts of N2O coinciding with the maximum NO reduction can be attributed to the presence of HNCO being formed in the decomposition of urea, since HNCO is known to result in higher N2O concentrations compared to ammonia.28 It is interesting to note that the concentrations of N2O obtained in this laboratory work are significantly lower than those (31) Kasuya, F.; Glarborg, P.; Dam-Johansen, K. Chem. Eng. Sci. 1995, 50, 1445-1466.

(4)

Itaya et al.14 performed thermogravimetric analysis of both solid urea and a solution of urea and observed that the thermal decomposition of urea proceeds in two steps: one at approximately 500 K which accounts for around 65% of the total urea decomposition and a second one at around 620 K which accounts for approximately 30%. In any case, no urea is present at all for temperatures higher than 700 K. Furthermore, they observed that thermal decomposition of urea yielded at least NH3 and HNCO as previously mentioned, even though other products are likely to be produced. In presence of water, urea has been suggested to react as follows

H2N-CO-NH2 + H2O f 2NH3 + CO2

(5)

which is completed after 100 ms.14 However, Itaya et al.14 obtained significantly less ammonia than the amount predicted theoretically by reaction 5, when performing thermal decomposition experiments of a urea solution. Therefore, the results concerning the products formed from urea do not seem to be conclusive. Since the feeding of urea in our experimental system is done from a solution of urea in water by saturating a nitrogen stream, we analyzed the content of ammonia in the stream containing nitrogen, water vapor, and urea, and no appreciable amounts of ammonia were detected. This confirms that the decomposition of urea is accomplished inside the reactor and not previously. As a first approximation for calculations, we considered the thermal decomposition of urea to proceed in a rapid step giving NH3 and HNCO. An example of the results obtained is shown in Figure 4 as short-dashed (32) Bilbao, R.; Oliva, M.; Iban˜ez, J. C.; Zapater, A.; Millera, A.; Alzueta, M. U. 9th International Conference on Coal Science, Essen, 1997; pp 1863-1866.

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Figure 4. Experimental and calculated results of NO and N2O concentrations versus temperature. Solid lines: calculations made with H2N-CO-NH2 f NH2 + H2 + NCO. Short-dashed lines: calculations made with H2N-CO-NH2 f NH3 + HNCO. Long-dashed lines calculations made with H2N-CONH2 f NH2 + H + HNCO. (Inlet concentrations: 100 ppm NO, 150 ppm urea, 4% O2, 4% H2O, N2 to balance).

lines. The results show a fairly good agreement between experiments and calculations at low temperatures. However, discrepancies increase for temperatures above 1300 K. Following the suggestion of Jødal et al.,10 another reaction channel for urea decomposition has been included, accounting for the above-mentioned reactions 2 and 3. This decomposition pathway for urea has been included in the mechanism as a global reaction step, H2N-CO-NH2 f H2 + NH2 + NCO, with an estimated reaction constant of 1015 exp (-65000/RT) s-1. The inclusion of this reaction is seen to bring the model predictions in a closer agreement with the experimental results in all the temperature range studied, as shown in the solid lines in Figure 4. This is the approach that will be used for the rest of the model calculations in this work. Similar modeling results are observed with the reaction channel proposed by Bilbao et al.,32 reaction 4, for which the same reaction constant has been considered, shown as long-dashed lines in Figure 4. The results obtained indicate that, assuming the oxidation mechanism of HNCO to be reliable,28 it is necessary to consider a high-temperature channel for urea decomposition. However, it should be stated that the HNCO oxidation mechanism resulted in the highest discrepancies for the high-temperatures range.28 Further progress in the HNCO chemistry and an exact determination of the products formed from urea at high temperatures and the reaction rates are desirable. The comparison of the results of NO and N2O for similar experiments performed, respectively, with urea and ammonia are shown in Figure 5. Symbols denote experimental results and lines model calculations. Using urea, the onset for NO reduction is shifted slightly toward higher temperatures (about 50 K), but both the minimum in NO concentration and the NO reduction level are similar when using either urea or ammonia. Higher N2O concentrations are obtained with urea.

Figure 5. Experimental and calculated results of NO and N2O concentrations versus temperature, when using either urea (O) or ammonia (b) as reducing agent. Solid lines denote calculations for urea and dashed lines for ammonia. (Inlet concentrations: 300 ppm NO, 150 ppm urea, 300 ppm NH3, ∼4% O2, 4% H2O, N2 to balance).

Under the conditions of this work, at the left branch of the window, the NO reduction is mainly produced by reaction of NO with NH2 radicals following the simplified reaction sequence:

NH3 + OH T NH2 + H2O

(6)

NH2 + NO T N2 + H + OH

(7)

NH2 + NO T N2 + H2O

(8)

H + O2 T O + OH

(9)

O + H2O T OH + OH

(10)

Even though both NH3 and HNCO are formed directly and quickly from urea, the onset of NO reduction when using urea is shifted toward higher temperatures compared to that in the absence of HNCO. Calculations show that at the left branch of the window the presence of HNCO formed from urea slightly inhibits NH3 oxidation, mainly through the consumption of OH radicals by

HNCO + OH T NCO + H2O

(11)

This is an important chain-terminating reaction. At high temperatures and using urea, reaction 11 is part of the mechanism which also contributes to the reduction of NO and the formation of N2O28 by action of NCO:

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Figure 6. Experimental and calculated results of NO and N2O concentrations versus temperature for different urea/NO ratios. The urea/NO ratio is varied changing the NO concentration (i.e., 100, 300, and 1200 ppm) for a given urea concentration (150 ppm). Symbols denote experimental results, and lines model calculations. (O, solid lines: 100 ppm NO. ), short-dashed lines: 300 ppm NO. 0, long-dashed lines: 1200 ppm NO.) (Inlet concentrations: ∼4% O2, ∼150 ppm urea, ∼4% H2O, N2 to balance).

NCO + NO T N2O + CO

(12)

NCO + NO T N2 + CO2

(13)

NCO + O T NO + CO

(14)

N2O + M T N2 + O + M

(15)

Increasing temperature results in increasing the level of radicals, up to values that are high enough to keep both mechanisms proceeding, i.e., NH3 and HNCO oxidation and the further interaction of NH2 and NCO with NO, which globally results in the minimum NO and maximum N2O concentrations observed. A further increase in temperature results, to the right of the window, in increased NO and decreased N2O concentrations. The oxidation of NHi and NCO radicals to NO is favored instead of their reaction with NO. The influence of the ratio urea/NO has also been considered in this work by varying the NO concentration, i.e., 100, 300, and 1200 ppm, for a given urea concentration of approximately 150 ppm, and the results are shown in Figure 6. Symbols denote experimental results and lines model calculations. Similar maximum reduction efficiencies are obtained for the inlet NO concentrations of 100 and 300 ppm, while a significantly lower NO conversion is seen for the initial NO concentration of 1200 ppm. This is logical, since for the inlet

urea concentration of 150 ppm the maximum amount of N-species formed from urea is 300 ppm which is considerably lower than the amount of NO present. From the results obtained, it seems that for molar urea/ NO ratios greater than or equal to 0.5, the reduction efficiency is roughly similar up to temperatures of 1200 K. A further raise in temperature results in an increase in NO concentration, which is more significant as the inlet NO concentration is lower. For the conditions of Figure 6 and the inlet NO concentration of 100 ppm, most of the NO has reacted at lower temperatures with NH2 radicals, and for temperatures higher than 1200 K, the oxidation reactions of the products formed in urea decomposition, which have not interacted with NO, are favored compared to their conversion to N2. For the rest of inlet NO concentrations, the outlet NO level remains approximately equal to the initial value. With respect to N2O concentration, the major conversion of NO to N2O is obtained for the lowest NO value studied, i.e., the higher N2O/(NO)inlet ratio even though the N2O concentration attained increases as the initial NO concentration increases. From the results obtained in this work and from the analysis of other literature results, we believe that the method used for urea injection may be a key factor for the results obtained. When urea is added in the form of a vapor stream, as in this work, the temperature window for NO reduction is slightly shifted toward higher temperatures, compared to that obtained by the use of ammonia, but in lower quantity than previously reported when the injection of urea was performed either in an aqueous solution or in solid form. Also, the addition of urea, when at least partially decomposed into the reaction zone as is presumably done in the present work, results in a much lower N2O concentration.10 This can be attributed to the residence time available for evaporating water, for allowing the interaction between urea and NO, and for the conversion of NO once formed. In pilot and full scale plants, residence time is presumably limited in most of the cases, and when injecting urea as a solid or as a solution, part of the residence time would be consumed, respectively, in heating and decomposing urea or evaporating and decomposing urea. In these conditions, the interaction between urea decomposition products and NO would be delayed, and the time necessary for NO reduction and N2O thermal dissociation would be limited. This is interesting because it indicates that a simple modification of the injection method chosen for urea addition, leading to the prior decomposition of urea in the carrier gas stream, would result in similar NO reduction efficiencies compared to the classical urea injection methods, but in considerably lower N2O concentrations which is one of the major concerns when using urea in SNCR strategies. Conclusions A flow reactor study of the interactions between urea and NO under SNCR conditions has been performed at atmospheric pressure in the temperature range 7001500 K. The use of urea as a SNCR agent succeeds in reducing NO (up to around 80%, depending on the operating conditions) in a specific temperature window

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located around 1200-1350 K, even though appreciable amounts of N2O are obtained. Compared to that when ammonia was used, the onset for NO reduction using urea is shifted to slightly higher temperatures, which is due to the presence of HNCO, formed in the thermal decomposition of urea, that inhibits the oxidation of NH3, also formed from urea. However, the minimum in NO concentration is attained at the same temperature, and similar NO reduction efficiencies are seen with both NH3 and urea. The efficiency of the process increases as the urea/NO ratio increases, even though ratios higher than 0.5 can result in an increase of NO at high temperatures. The results have been modeled with a literature mechanism together with reactions for describing the thermal decomposition of urea. Model predictions agree fairly well with experimental results and indicate that at low temperatures urea decomposition can be described as H2N-CO-NH2 f NH3 + HNCO. However, at high temperatures discrepancies are appreciable. These discrepancies can be attributed to uncertainties in the HNCO oxidation mechanism or the significance of other product channels for urea

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decomposition, e.g., H2N-CO-NH2 f NH2 + H2NCO, followed by H2N-CO f NCO + H2, or H2N-CO-NH2 f NH2 + H + HNCO. Finally, the results obtained in this work as well as other literature pilot scale results indicate that the method chosen for urea injection may be an important factor for process performance. Adding urea presumably decomposed prior to the reaction zone results in similar NO reduction efficiencies and lower N2O concentrations compared to those from the injection of solid urea or a solution of urea. Acknowledgment. This work was carried out as a part of the combustion research program of the Department of Chemical and Environmental Engineering of the University of Zaragoza. Financial support from the EU through a subcontract of the EU project JOR3CT960059 and from CICYT, project AMB97-0852CE, is acknowledged. M. U. Alzueta acknowledges Dr. Peter Glarborg for helpful discussions about the HNCO chemistry. EF980055A