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Communications Impact of New Findings Concerning Urea Thermal Decomposition on the Modeling of the Urea-SNCR Process 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 September 14, 1999 The interest in selective non-catalytic reduction (SNCR) applications has motivated the study of a number of possible configurations for NOx reduction. Among those, the use of urea as selective non-catalytic agent appears to be interesting.1,2 Urea seems to be suitable because of handling and storage reasons, compared to other selective NOx reduction agents such as ammonia. A number of investigations have been carried out concerning the use of urea in the SNCR process during the past years, both experimentally on different scales,2-4 and from a kinetic modeling point of view.3,5 While the effectiveness in the process is well demonstrated experimentally through the different investigations, the kinetic modeling of the process presented some uncertainties mainly due to the behavior of urea under high temperature conditions. Urea has been traditionally considered to be decomposed into NH3 and HNCO at high temperatures,6,7 even though other decomposition paths have been proposed.5,8 The agreement between experiments and calculations using those different mechanisms for urea decomposition is reasonably good, but a reliable determination of urea thermal decomposition was needed. Recent experimental results have appeared concerning the measurement of the reaction rate for the thermal urea decomposition reaction under conditions applicable to SNCR conditions.9 * Corresponding author. Fax: +34 976 761879. E-mail:
[email protected]. (1) Muzio, L. J.; Arand, J. K.; Teixeira, D. P. Sixteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1977; pp 199-208. (2) Sun, W. H.; Stamakis, P.; Hofman, J. E. ACS Symp. Series 1993, 38, 734-739. (3) Alzueta, M. U.; Bilbao, R.; Millera, A.; Oliva, M.; Iban˜ez, J. C. Energy Fuels 1998, 12, 1001-1007. (4) Karll, B.; Gustaffson, P. A° . 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. (5) Jødal, M.; Nielsen, C.; Hulgaard, T.; Dam-Johansen, K. TwentyThird Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; pp 237-243. (6) 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. (7) Caton, J. A.; Siebers, D. L. Combust. Sci. Technol. 1989, 65, 277293. (8) Bilbao, R.; Oliva, M.; Iban˜ez, J. C.; Zapater, A.; Millera, A.; Alzueta, M. U. 9th International Conference on Coal Science, Essen, 1997; pp 18631866. (9) Aoki, H.; Fujiwara, T.; Morozumi, Y.; Miura, T. Fifth International Conference on Technologies and Combustion for a Clean Environment, Lisbon, 1999; pp 115-118.
Figure 1. Experimental and calculated results of NO and N2O concentrations vs temperature. Solid lines: calculations made assuming ideal mixing. Dashed lines: calculations made assuming a mixing time of 10 ms. (Inlet concentrations: 100 ppm NO, 150 ppm urea, 4% O2, 4% H2O, N2 to balance). Residence time(s) ) 200/T(K).
In this communication, we want to show the impact of the new recent results by Aoki et al. 9 in the modeling of the SNCR process using urea. Aoki et al. 9 determined the reaction rate and products distribution of the thermal decomposition of urea. They obtained the following rates and product channels for the decomposition of urea at high temperatures when it is fed as an aqueous solution:
(NH2)2CO f NH3 + HNCO k1 ) 1.2676 × 104 exp(-15540/[cal/mol]RT) (NH2)2CO + H2O f 2 NH3 + CO2 k2 ) 9.925 × 103 exp(-20980/[cal/mol]RT) We have included those reactions in the mechanism we used previously for studying the urea-SNCR process,3 and the results are in good agreement with the previous
10.1021/ef990187j CCC: $19.00 © 2000 American Chemical Society Published on Web 01/29/2000
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calculations in which we used a rate constant for urea decomposition estimated from the experimental findings. An example of the new calculations including the rate constants from Aoki et al. 9 are shown in Figure 1 as solid lines. Calculations for the rest of the experimental results of previous work 3 also yield good agreement. The discrepancy observed at high temperatures, in which the model calculations underpredict the NO concentration are attributed to mixing effects in the experimental results, which are described elsewhere.10 To analyze this, we have considered a simple mixing approach for the urea stream with the rest of reactants based on the Zwietering approximation,11 and following the procedure of Alzueta et al.12 Using a mixing time of 10 ms for the urea stream entrained by the rest of the flow, the calculated results
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obtained are shown as dashed lines in Figure 1. It is seen that the model calculations approach the experimental results much more closely at high temperatures, indicating that urea is partially oxidized to NO resulting in a net increase of the concentration of NO at high temperatures. The results presented in this work indicate the importance and validity of the recent determination of the rate of thermal decomposition of urea, resulting in a more accurate knowledge of the chemistry involved in the ureaSNCR process. Acknowledgment. Financial support from CICYT, project AMB97-0852-CE, is acknowledged. EF990187J
(10) Oliva, M.; Alzueta, M. U.; Millera, A.; Iba´n˜ez, J. C.; Bilbao, R. In Biomass: A Growth Opportunity in Green Energy and Value-Added Products; Overend, R., Chornet, E., Eds.; 1999; pp 1315-1320. (11) Zwietering, T. N. Chem. Eng. Sci. 1959, 11, 1-15.
(12) Alzueta, M. U.; Bilbao, R.; Millera, A.; Glarborg, P.; Østberg, M.; Dam-Johansen, K. Energy Fuels 1998, 12, 329-338.
