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Ind. Eng. Chem. Res. 2004, 43, 4856-4863
Decomposition of Urea into NH3 for the SCR Process Sung Dae Yim, Soo Jean Kim, Joon Hyun Baik, and In-Sik Nam* Department of Chemical Engineering/School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), san 31 Hyoja-dong, Pohang 790-784, Korea
Young Sun Mok Department of Chemical Engineering, Cheju National University, Jeju 690-756, Korea
Jong-Hwan Lee, Byong K. Cho, and Se H. Oh General Motors R&D and Planning Center, Warren, Michigan 48090-9055
The thermal and catalytic decomposition of urea over a fixed-bed flow reactor system has been examined for the selective catalytic reduction (SCR) of NOx from mobile sources. The conversion of urea into NH3 and HNCO, the two major products from the thermal decomposition of urea, increased with the reaction temperature and the reactor space time. Urea was completely decomposed into NH3 and HNCO at 350 °C when the residence time was longer than 0.1 s. As the reaction temperature increased to 400 °C or higher, complete decomposition of urea was possible at a much shorter residence time of the feed gas stream. The simultaneous thermal and catalytic decomposition of urea was also examined in a dual-reactor system in which the first reactor was for thermal decomposition and the second was for catalytic decomposition, specifically over copper exchanged ZSM5 catalyst. The role of the catalyst in the decomposition of urea into NH3 and HNCO was negligible; urea decomposition occurs mainly by the thermal reaction. However, the catalyst was able to rapidly hydrolyze HNCO to NH3 even at a temperature as low as 150 °C. The catalyst also oxidizes NH3 to N2 in the presence of oxygen at reaction temperatures above 250 °C. A kinetic model describing all of the major reactions involved in urea decomposition over an SCR reactor has been developed. The model adequately predicts the general trend of variations of the urea, NH3, and HNCO concentrations with the reaction conditions. Introduction Selective catalytic reduction (SCR) of NO by NH3 is a well-developed technique and the most common technology for the control of nitrogen oxides emitted from stationary sources.1 However, NH3 might not be a suitable reducing agent for mobile sources mainly because of difficulties in its storage, handling, and transportation. As an alternative to overcome these problems associated with NH3, urea is currently being considered as an NH3 carrier for automotive emission control applications. According to previous studies,2-4 the SCR by urea has been reported to be the most promising method for the control of NOx emissions from heavy-duty diesel engines. It is generally accepted that urea decomposes into NH3 as follows:2 When urea solution is atomized into a hot exhaust gas stream, the primary step for the decomposition is the evaporation of water from the droplets of urea solution
NH2-CO-NH2(aq) f NH2-CO-NH2(l or g) + xH2O(g) (1) Pure urea is then thermally decomposed into ammonia and isocyanic acid (HNCO)
NH2-CO-NH2(l or g) f NH3(g) + HNCO(g) (2) * To whom correspondence should be addressed. Tel. : 8254-279-2264. Fax: 82-54-279-8299. E-mail:
[email protected] Isocyanic acid formed by reaction 2 is quite stable in the gas phase, but it easily hydrolyzes on the surface of metal oxides, producing ammonia and carbon dioxide5
HNCO(g) + H2O(g) f NH3(g) + CO2(g)
(3)
Consequently, 1 mol of urea produces 2 mol of ammonia and 1 mol of carbon dioxide. Alzueta et al.6,7 investigated the thermal decomposition of urea under selective noncatalytic reduction conditions covering high temperatures from 700 to 1500 K, and they confirmed that urea decomposition occurs primarily via reactions 1-3, particularly at low reaction temperatures below 1300 K, by comparing model predictions with experimental results for NOx removal performance. The hydrolysis of isocyanic acid on various oxide catalysts in the temperature range 140-475 °C was studied by Kleemann et al.,5 who reported that the overall hydrolysis rate of HNCO on SCR catalysts is very high (about 2 orders of magnitude higher than the SCR reaction rate). In addition, Smeets et al.8 calculated the theoretical reactor length in which the urea solution can completely be decomposed into HNCO and NH3. Although these previous studies employed a variety of approaches, they led to the same conclusion that urea fed to an SCR or SNCR process does not directly react with NO but rather NH3 formed as a result of the decomposition of urea serves as the reductant for NO reduction. Therefore, in such processes, the effective decomposition of urea into NH3 is critical from the practical point of view of the technology application. Nevertheless, most previous studies on urea SCR or
10.1021/ie034052j CCC: $27.50 © 2004 American Chemical Society Published on Web 07/02/2004
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4857
Figure 1. Schematic flow diagram for urea decomposition reactor system.
urea SNCR have been focused on the NOx removal performance itself, bypassing the initial reaction step producing NH3, a critical reactant for the SCR of NO by urea. In the present study, the decomposition of urea to produce NH3 for the SCR process was systematically investigated. First, the thermal decomposition of urea into NH3 and HNCO was examined at a variety of reaction temperatures and reactor residence times, and then the combined thermal and catalytic decomposition over the CuZSM5 catalyst bed was investigated. The thermal decomposition was conducted in a fixed-bed reactor filled with glass beads. For the simultaneous thermal and catalytic decomposition, a catalytic reactor containing CuZSM5 catalyst was employed along with the thermal decomposition reactor. On the basis of the reaction mechanisms determined experimentally, a unified power-law kinetic model was developed for the simultaneous thermal and catalytic decomposition of urea. The model derived in the present study can provide a practical guideline for the design of a commercial urea SCR process. Experimental Section Catalyst Preparation. The CuZSM5 catalyst was prepared by the wet ion-exchange method using 0.01 M of Cu(CH3COO)2‚H2O (Merck) solution at room temperature to obtain a Cu content of 2.9 wt. %, as extensively described elsewhere.9,10 ZSM5 zeolites with a Si/Al ratio of 14 were obtained from Tosoh (HSZ830NHA). The catalyst prepared was dried at 110 °C for 12 h and then calcined in air at 500 °C for 5 h. The catalysts were again pretreated in air at 500 °C for 1 h before use. Reaction System and Experimental Procedure. The experimental apparatus depicted in Figure 1 consists of three main parts, namely, the urea injection system; the reactor; and the analysis train for NH3, urea, and HNCO.11 A desired amount of 1.0 M urea
solution was continuously fed into the reactor system using a homemade atomizing nozzle connected to an injection pump and was mixed with the main gas stream containing 5% O2, 2% H2O, and 93% N2 to yield. A concentration of urea of 250 ppm was maintained throughout the present study, unless otherwise specified. The injection tube of urea solution and its mixing chamber were well insulated and maintained the temperature at ∼120 °C to prevent the condensation of urea solution for the eventual formation of high-molecularweight products. In addition, the feed concentration of 1.0 M in the urea solution employed in the present study might also reduce the possibility of the formation such products during the course of injection, mixing, and reaction.13 In Figure 1, the first horizontal reactor is for urea thermal decomposition and consists of a 7.5-mm-i.d. aluminum tube filled with 3 mL of 2-mm-diameter glass beads. The reactor residence time is defined as the ratio of the volume of the reactor including the glass beads to the void volume where the thermal decomposition temperature of urea solution is controlled by the flow rate of the feed gas stream. The second vertical reactor containing catalyst in Figure 1, which is identical to the first horizontal reactor, allows one to independently control the reactor temperature for the study of further the thermal and catalytic decomposition of urea. For the simultaneous thermal and catalytic decomposition study, a dual-reactor system was employed as shown in Figure 1. First, urea introduced into the horizontal reactor is thermally decomposed, and then the thermal decomposition products such as unreacted urea, NH3, and HNCO are directed to the second vertical rector packed with 0.1-1.0 g of CuZSM5 pellets, at 20/30 mesh size to minimize the effect of mass transfer and pressure drop, for the catalytic reaction. Note that the changes in the operating conditions of the first reactor are tantamount to changes in the composition of the input to the second catalytic reactor. As
4858 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 Table 1. Product Distributions for the Thermal Decomposition of Urea Solutiona flow rate (L/min)
urea (ppm)
NH3 (ppm)
1.6 2.4 3.3 4.0
123 160 170 187
115 90 70 50
1.6 2.4 3.3 4.0
17 57 87 110
1.6 2.4 3.3 4.0
0 0 15 45
a
HNCO (ppm)
N balance (%)
urea (ppm)
NH3 (ppm)
HNCO (ppm)
N balance (%)
150 °C 123 75 65 50
97 97 95 95
55 95 115 140
187 152 125 100
250 °C 177 147 110 95
95 98 93 95
220 173 151 123
300 °C 205 160 143 120
92 89 94 93
0 30 55 92
255 212 180 155
350 °C 215 190 170 152
94 92 92 98
283 250 227 190
400 °C 197 217 195 175
96 93 90 91
0 0 0 30
321 313 270 220
450 °C 155 170 205 227
95 97 95 100
Feed: 250 ppm urea, 5% O2, 2% H2O, 93% N2.
mentioned, in the second catalytic reactor, the thermal decomposition of unreacted urea from the first reactor occurs simultaneously with the catalytic reaction including the hydrolysis of HNCO and the oxidation of NH3. The flow rate of the feed gas was varied in the range of 1.6-4.0 L/min (STP), giving a standardized residence time at STP (defined as the ratio of the volume of catalyst bed to the total gas flow rate) of 0.113-0.045 s. The reaction temperature was varied from 150 to 450 °C. NH3, one of the major products of urea decomposition, was analyzed with an on-line NDIR-type NH3 analyzer (Rosemount Analytical, model 880A). To analyze the urea remaining unconverted and the HNCO produced during the decomposition reaction, part of the reacted gas was passed through a series of absorption bottles each containing 200 mL of deionized water. Three absorbing bottles were employed on the basis of experimental observations of the urea and HNCO concentrations in the bottles. The concentrations of urea and HNCO absorbed into the water were then analyzed by HPLC with UV detection (Younglin UV730D), as also reported by Koebel and Elsener.14 Results Thermal Decomposition of Urea. Table 1 presents the results of the urea thermal decomposition experiments at various combinations of reaction temperature and flow rate (or residence time) of the feed gas. Throughout the experiments, a nitrogen balance based on urea, HNCO, and NH3 of higher than 90% was always maintained, and the formation of cyanuric acid and other higher-molecular-weight compounds during the course of the urea decomposition was observed to be negligible, as determined by the HPLC chromatogram. As shown in Table 1, the extents of urea thermal decomposition and of ammonia formation strongly depended on the reaction temperature and the residence time in the reactor. The amounts of urea decomposed and ammonia formed became greater as the reaction temperature increased and the flow rate decreased. At a reaction temperature of 350 °C, complete decomposition of urea was achieved at a flow rate of 1.6 L/min (0.113-s residence time at STP). When the reaction temperature was elevated to 450 °C, complete conversion of urea was possible at higher flow rates up to 3.3 L/min (0.055-s
Table 2. Product Distributions for Urea Decomposition over CuZSM5 Catalyst flow catalyst rate (g) (L/min)
SV (h-1)
N temp NH3 urea HNCO balance (°C) (ppm) (ppm) (ppm) (%)
Thermal Decomposition Reactor, 250 °C (Feed: 115 ppm Urea, 125 ppm NH3, 110 ppm HNCO) 0.1 3.3 990 000 150 195 110 45 250 220 106 24 280 152 105 20 310 105 102 15 350 75 100 12 450 65 86 5
92.0 91.2 76.4 64.8 57.4 48.4
Thermal Decomposition Reactor, 350 °C (Feed: 55 ppm Urea, 180 ppm NH3, 170 ppm HNCO) 0.1 3.3 990 000 150 285 52 68 250 310 49 35 350 95 46 17 450 75 40 7
91.4 88.6 40.8 32.4
Thermal Decomposition Reactor, 250 °C (Feed: 115 ppm Urea, 125 ppm NH3, 110 ppm HNCO) 1.0 3.3 99 000 150 295 82 5 250 274 60 0 350 50 35 0 450 41 0 0
92.8 78.8 24.0 8.2
residence time at STP). Obviously, complete decomposition of urea can be obtained at a given temperature by controlling the flow rate (or the residence time), and vice versa. The experimental observations in Table 1 confirm that urea is decomposed primarily into HNCO and NH3 by thermal reaction, as is generally recognized.2 Moreover, these results also reveal that the hydrolysis of HNCO to produce additional NH3, although not significant, can occur without catalyst if the reaction temperature is sufficiently high (g400 °C).7,12 According to reaction 2, identical amounts of NH3 and HNCO should be produced unless the subsequent hydrolysis reaction, reaction 3, occurs. However, Table 1 shows that the ratio of NH3 to HNCO increases with increasing temperature, thus indicating hydrolysis of HNCO. This might be due to heterogeneous catalytic reaction on the reactor walls (Al2O3 formed on the aluminum tube) or to homogeneous reaction in the gas phase. Simultaneous Thermal and Catalytic Decomposition of Urea. Table 2 reports the product distributions obtained using the dual-reactor system shown in Figure 1 for the decomposition of urea at a variety of reaction conditions. The variation of the reaction temperature in the first thermal decomposition reactor
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4859
produced different concentrations of urea, HNCO, and NH3, which were introduced into the second vertical reactor containing CuZSM5 as a probe catalyst among the catalytic systems, including V2O5-WO3/TiO2 and Pt/ Al2O3, examined in the present study for the catalytic decomposition of urea. Indeed, the CuZSM5 catalyst revealed the highest NO removal activity in the SCR by NH3 and/or urea.15,16 In the catalytic reactor, the amount of catalyst and the total flow rate of feed gas were changed from 0.1 to 1.0 g and 2.4 to 3.3 L/min, respectively, to vary the reactor space velocity from 99 000 to 990 000 h-1. The high end of the space velocity range is useful for HNCO measurements at high reaction temperatures, because complete conversion of HNCO is achieved at a low space velocity. As observed in Table 2, the concentrations of urea and HNCO decreased monotonically with increasing reaction temperature, whereas the NH3 concentration increased up to 250 °C and then decreased sharply when the reaction temperature further increased. A N balance of greater than 90% was maintained at reaction temperatures below 250 °C, but the higher reaction temperatures resulted in much poorer N balances as a result of the catalytic oxidation of NH3 to N2.17 These results imply that three reactions, including the decomposition of urea into HNCO and NH3, the catalytic hydrolysis of HNCO, and the catalytic oxidation of NH3, occur simultaneously in the catalytic reactor. In the case of catalytic decomposition experiments conducted without O2 (results not shown here), the concentration of NH3 produced increased monotonically with increasing reaction temperature up to 450 °C, contrary to the case in the presence of O2 as listed in Table 2. This result suggests that NH3 produced from urea by the thermal and catalytic reactions can be oxidized in the catalyst bed, particularly at higher reaction temperatures above 250 °C. This finding is consistent with a recent study by Long and Yang,17 who reported that NH3 is easily oxidized to N2 on the CuZSM5 catalyst in the presence of O2. Despite the ammonia oxidation, oxygen was included in the feed gas to realize the actual composition of exhaust gas. The results in Table 2 emphasize that the catalytic hydrolysis of HNCO occurs rapidly even at a temperature as low as 150 °C when the reactor space velocity is 99 000 h-1. Such a rapid catalytic hydrolysis of HNCO to NH3 enables urea to be utilized as an effective reducing agent for urea SCR technology.5 Kinetic Model. The decomposition of urea can be expressed as a series of reactions including the conversion of urea into HNCO and NH3, followed by the hydrolysis of HNCO to produce additional NH3 and CO2 and by the oxidation of NH3 to N2
Thermal decomposition of urea k1
A 98 B + C
(4)
Hydrolysis of isocyanic acid k2
B + W 98 C + D
(5)
Catalytic oxidation of ammonia k3
C 98 products
(6)
where ki (i ) 1, 2, 3) stands for the respective apparent
reaction rate constants of reactions 4-6 and the symbols for A, B, C, D, and W represent urea, HNCO, NH3, CO2, and water, respectively. It should be noted that the effect of the water and O2 contents on the hydrolysis of isocyanic acid and the oxidation of NH3 has been ignored in reactions 5 and 6, respectively, because the former exist in stoichiometric excess compared to HNCO and NH3 (2% water vs 250 ppm HNCO and 5% O2 vs 500 ppm NH3). On the basis of these three reactions, reactions 4-6, a simple power-law kinetic model was developed. On the assumption that reaction 4 is first-order with respect to the concentration of urea, the decomposition rate can be described as
RA ) k1CA
(7)
Similarly, the rates of HNCO hydrolysis and NH3 oxidation are given by
RB ) k2CBCW
(8)
RC ) k3CC
(9)
Assuming plug flow, the steady-state mass balance for urea over a fixed-bed reactor can be expressed as
dCA ) -k1CA dτ
(10)
where τ is the reactor space time (residence time) of the feed gas stream. Likewise, the mass balances for the other two components, isocyanic acid and ammonia, are as follows
dCB ) k1CA - k2CBCW dτ
(11)
dCC ) k1CA + k2CBCW - k3CC dτ
(12)
Equations 10-12 above were derived under the assumption that they are elementary reactions and obey first-order reaction kinetics. The validity of such assumptions was experimentally confirmed in the present reactor system containing thermal and catalytic reactors in series. The conversions of urea, HNCO, and NH3 were observed to be virtually independent of their feed concentrations into the reactor for the simultaneous thermal and catalytic decomposition reactions. For the thermal reactor, various amounts of 1 M urea solution were fed into the thermal decomposition reactor maintained under different reaction conditions for incomplete and complete conversions of urea in the reactor. The conversions of urea and HNCO by the thermal reactor were found to be independent of the feed concentrations of urea and HNCO to the reactor. For the catalytic decomposition of urea, it was confirmed that the conversions of urea, HNCO, and NH3 over the CuZSM5 catalyst were also essentially independent of the feed concentration of each compound, which indicates that reactions 4-6 in the present system can be described by first-order kinetics with respect to the urea, HNCO, and ammonia concentrations.
4860 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004
Figure 2. Temperature dependence of the rate constants for urea decomposition and HNCO hydrolysis in the thermal decomposition reactor.
Equation 10 is readily solved by integration to give
CA ) CA0 exp(-k1τ)
(13)
Substituting eq 13 into eq 11 for CA leads to a nonhomogeneous linear first-order differential equation with respect to the concentration of HNCO (CB), with its concentration at the reactor inlet (τ ) 0) given as CB0. Thus, the solution of eq 11 becomes
CB )
k1CA0 [exp(-k1τ) - exp(-k2CWτ)] + k2CW - k1 CB0 exp(-k2CWτ) (14)
The content of water (CW) is assumed to be constant because of the high concentration of water (2 vol %) in the feed gas stream. Similarly, substituting eqs 13 and 14 for CA and CB into eq 12 leads to a nonhomogeneous linear first-order differential equation for CC as well, and its solution is given by
k1CA0 [exp(-k1τ) -k1 + k3 k1k2CA0CW exp(-k1τ) - exp(-k3τ) exp(-k3τ)] + k2CW - k1 -k1 + k3 exp(-k2CWτ) - exp(-k3τ) + -k2CW + k3 k2CWCB0 [exp(-k2CWτ) - exp(-k3τ)] (15) -k2CW + k3
CC ) CC0 exp(-k3τ) +
[
]
where CC ) CC0 at t ) 0. Discussion Thermal Decomposition of Urea. The results in Tables 1 and 2 suggest that reaction 6 can occur only in the presence of catalyst. Thus, k3 was set to be zero when only the thermal decomposition was considered. Rate constants k1 and k2 can be obtained from eqs 1315 using the experimental data in Table 1. In the case of thermal decomposition, CB0 in eq 14 should be zero because only urea was introduced into the reactor system. Figure 2 shows the Arrhenius plot of the rate constants obtained at the temperatures of interest in the present study. The hydrolysis of HNCO by reaction
Figure 3. Comparison of the measured and calculated concentrations of urea at the outlet of the thermal decomposition reactor.
5 did not occur significantly at temperatures below 300 °C, i.e., k2 at such temperatures was estimated to be zero. As observed in Figure 2, the rate constants for both reactions 4 and 5 increased with increasing reaction temperature. The dependency of k2 on the reaction temperature was particularly high, indicating that the activation energy for reaction 5 is large and a high reaction temperature is necessary to induce the reaction successfully. The temperature dependence of rate constants k1 and k2 can be characterized by Arrhenius equations as
k1 ) 4.9 × 103 exp(-5505/RT)
(16)
k2′ ) 2.5 × 105 exp(-14 861/RT)
(17)
where k2′ is the product of k2 and CW. Because the concentration of water vapor (CW) can be treated as a constant, k2′ is also a constant that depends on temperature only. In determining the values of k20 and E2 by linear regression, the data in the temperature range from 150 and 250 °C were excluded because of uncertainties associated with measurements of the low reaction rates at these temperatures (see Figure 2). To verify the validity of the rate constants determined in the present study, the experimental data for the thermal decomposition of urea in Table 1 were compared with the results calculated by the model. Figures 3-5 compare the measured and calculated concentrations of the residual urea, isocyanic acid, and ammonia as functions of residence time and reaction temperature. The validity of the model and the rate constants can be confirmed by the good agreements of the calculated results with the experimental data. Simultaneous Thermal and Catalytic Decomposition of Urea. In the analysis of the simultaneous thermal and catalytic decomposition occurring in the catalytic reactor, eq 15 as well as eqs 13 and 14 were used to determine the relevant rate constants. Note that the inlet composition of the second catalytic reactor corresponds to the outlet composition of the first thermal reactor, i.e., the concentrations at the inlet of the second reactor, CA0, CB0, and CC0, are given by the concentrations at the outlet of the first reactor. First, rate constant k1 should be determined by applying the appropriate set of experimental data to eq 13, and then k2 and k3 can be determined from eqs 14
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4861
Figure 4. Comparison of the measured and calculated concentrations of isocyanic acid at the outlet of the thermal decomposition reactor.
Figure 6. Temperature dependence of the rate constants for urea decomposition, HNCO hydrolysis, and NH3 oxidation in the catalytic reactor.
Figure 6. The resulting rate constants in the form of Arrhenius equation were found to be
Figure 5. Comparison of the measured and calculated concentrations of NH3 at the outlet of the thermal decomposition reactor.
and 15, respectively. To determine the rate constants using eqs 13-15 only, the experimental data obtained with 0.1 g of catalyst presented in Table 2 were employed because the use of 0.5 or 1.0 g of catalyst commonly employed in the present study led to the complete conversion of HNCO, making it impossible to estimate the rate constants. The channeling problem often encountered with the use of very small amounts of catalyst was not observed during the course of the reaction, which was confirmed by independent experiments where the amount of the catalyst packed in the reactor was systematically varied. Figure 6 presents the Arrhenius plot of the determined values of rate constants k1-k3. Rate constant k1 for reaction 4 is almost identical to the value in Figure 2 over the reaction temperature range covered in the present study, which suggests that the presence of catalyst does not affect the decomposition of urea into HNCO and NH3. (That is, urea decomposition occurs via the thermal reaction pathway.) On the other hand, k2′ ()k2CW) in Figure 6 is much larger than the value in Figure 2, indicating that the hydrolysis of HNCO can be greatly accelerated by the catalyst.18 Although not significant below 250 °C, the ammonia oxidation became important at higher temperatures, as evidenced by the rapid increase of rate constant k3 with the reaction temperature. The frequency factor and activation energy (E/R) can be obtained from the three straight lines in
k1 ) 4.5 × 103 exp(-5405/RT)
(18)
k2′ ) k2CW ) 3.1 × 104 exp(-3780/RT)
(19)
The activation energy for the thermal decomposition of urea in the catalytic reactor, 5.4 kcal/mol, is quite close to the value observed for the thermal decomposition reactor (5.5 kcal/mol) as given in eq 16. On the other hand, the CuZSM5 catalyst greatly reduced the activation energy for the HNCO hydrolysis reaction from 14.9 to 3.8 kcal/mol. It is noteworthy that a similar low activation energy of 3.1 kcal/mol for the HNCO hydrolysis over a V2O5-WO3/TiO2 catalyst was reported by Kleemann et al.5 Unlike k1 and k2, the rate constants for ammonia oxidation (k3) did not fall on a straight line. However, the trend of the temperature dependence of k3 can be reasonably expressed as follows
k3 ) 1.1 × 108 exp(-15 218/RT)
(20)
To verify the validity of the model developed in the present study, the concentrations of urea, isocyanic acid, and ammonia calculated by eqs 13-15 were compared with the experimental data listed in Table 2. As can be seen in Figures 7 and 8, the calculated results at high reactor space velocities employing 0.1 g of catalyst adequately describe the concentration variations of urea, HNCO, and NH3. This is to be expected, given that the kinetic parameters were determined with the data obtained at high reactor space velocities. It should be noted that reasonably good agreement was also obtained between the measured and calculated values at lower reactor space velocities (198 000 and 720 000 h-1). However, the calculated concentrations of NH3 exhibited some deviation from the experimental data at high reaction temperatures, although relatively good agreement was observed at low temperatures (see Figure 9). In Figure 9, the model predictions at low reactor space velocity underestimated the concentration of ammonia in the high-temperature region. That is, the rate of NH3 oxidation was overestimated in the region of high reaction temperatures. This estimation of the NH3 oxidation rate might arise from the somewhat inaccurate estimation of the kinetic parameters as observed in Figure 6. The effect of the mass-transfer resistance
4862 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004
as observed in Figure 6. When the ammonia oxidation is not significant at low temperatures, the main reactions are reactions 4 and 5, both of which are independent of NH3 oxidation. As shown in Figure 6, the estimation of the kinetic parameters for reactions 4 and 5 is expected to be quite reliable, whereas that for reaction 6 is not. At low temperatures, the accuracy of the kinetic parameters for reaction 6 would not be critical because the rate is so low. This might be the reason for the good agreement between the calculated concentrations of ammonia in the low-temperature region below 250 °C and the experimental data. However, the model still reasonably describes the trend of the oxidation of NH3 during the course of the decomposition of urea. Figure 7. Observed and calculated product distributions for urea decomposition over CuZSM5 catalyst at 990 000 h-1 (thermal decomposition reactor at 250 °C; 115 ppm urea, 125 ppm NH3, 110 ppm HNCO).
Figure 8. Observed and calculated product distributions for urea decomposition over CuZSM5 catalyst at 990 000 h-1 (thermal decomposition reactor at 350 °C; 55 ppm urea, 180 ppm NH3, 170 ppm HNCO).
Conclusions The major products from the thermal decomposition of urea are NH3 and HNCO. The disappearance of urea and the formation of NH3 and HNCO strongly depend on the reaction conditions, including the reaction temperature and the residence time of the feed gas stream. At a reaction temperature of 350 °C and a residence time of 0.113 s, urea was completely decomposed into NH3 and HNCO via the homogeneous thermal reaction. As the reaction temperature increased, complete decomposition of urea was achieved at shorter residence times. When the reaction temperature was sufficiently high (above 400 °C), the hydrolysis of HNCO was possible even in the absence of catalyst although the rate was slow. The kinetic model for the thermal decomposition of urea described well the experimentally observed concentration variations of urea, isocyanic acid, and ammonia. The CuZSM5 catalyst significantly enhanced the hydrolysis rate of HNCO, producing NH3. The hydrolysis of HNCO over the catalyst was very fast, yielding higher than 95% HNCO conversion into NH3 at a temperature as low as 150 °C. In the presence of O2 in the feed gas stream, the NH3 produced by the urea decomposition was rapidly oxidized to N2 over CuZSM5 catalyst at the reaction temperatures above 250 °C. The model based on three main reactions including the thermal decomposition of urea, the catalytic hydrolysis of HNCO, and the catalytic oxidation of ammonia during the course of the decomposition of urea adequately describes the experimental data for the concentrations of urea, NH3 and HNCO, for all of the reactor operating conditions considered. The results suggest that the decomposition of urea into HNCO and NH3 occurs via the thermal decomposition route, whereas the subsequent hydrolysis of HNCO and the oxidation of ammonia are mainly caused by catalytic reaction. Literature Cited
Figure 9. Observed and calculated product distributions for urea decomposition over CuZSM5 catalyst at 99 000 h-1 (thermal decomposition reactor at 250 °C; 115 ppm urea, 125 ppm NH3, 110 ppm HNCO).
of the catalyst on the oxidation of NH3, the feature that was not considered for the kinetic modeling in the present study, cannot be excluded, particularly at high reaction temperature. The reason for the relatively good agreement in the low-temperature region below 250 °C can be explained by the low oxidation rate of ammonia,
(1) Bosch, H.; Janssen, F. Catalytic Reduction of Nitrogen Oxides. Catal. Today 1988, 2, 369. (2) Koebel, M.; Elsener, M.; Kleemann, M. Urea SCR: A Promising Technique to Reduce NOx Emissions from Automotive Diesel Engines. Catal. Today 2000, 59, 335. (3) Hug, H. T.; Mayer, A.; Hartenstein, A. Off-Highway Exhaust Gas After-Treatment: Combining Urea SCR, Oxidation Catalysis and Traps; SAE Paper No. 930363; SAE International: Warrendale, PA, 1993. (4) Miller, W. R.; Klein, J. T.; Mueller, R.; Doelling, W.; Zuerbig, J. The Development of Urea SCR Technology for U.S. Heavy Duty Trucks; SAE Paper No. 2000-01-0190; SAE International: Warrendale, PA, 2000.
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4863 (5) Kleemann, M.; Elsener, M.; Koebel, M.; Wokaun, A. Hydrolysis of Isocyanic Acid on SCR Catalysts. Ind. Eng. Chem. Res. 2000, 39, 4120. (6) Alzueta, M. U.; Bilbao, R.; Millera, A.; Oliva, M.; Ibanez, J. C. Interactions between Nitric Oxide and Urea under Flow Reactor Conditions. Energy Fuels 1998, 12, 1001. (7) Alzueta, M. U.; Bilbao, R.; Millera, A.; Oliva, M.; Ibanez, J. C. Impact of New Findings Concerning Urea Thermal Decomposition on the Modeling of the Urea SNCR Process. Energy Fuels 2000, 14, 509. (8) Smeets, R. G. H.; Calis, H. P.; Lugt, P. M.; van den Bleek, C. M. Catalytic Removal of NOx from Total Energy Installation Flue-gases: Process Design and Development. Catal. Today 1996, 29, 133. (9) Kim, M. H.; Nam, I.-S.; Kim, Y. G. Selective Catalytic Reduction of Nitrogen Oxide by Hydrocarbons over Mordenite Type Zeolite Catalysts. Appl. Catal. B 1995, 6, 297. (10) Nam, I.-S.; Yim, S. D.; Baik, J. H.; Oh, S. H.; Cho, B. K. Cu/Zeolite catalyst for removal of nitrogen oxides and process of preparing same. Korean Patent Application 2003-0012400, 2003. (11) Baik, J. H.; Kim, S. J.; Yim, S. D.; Nam, I.-S.; Lee, J.-H.; Cho, B. K.; Oh, S. H. 2nd Annual Report for GM R&D; POSTECH: Pohang, Korea, 2002. (12) Koebel, M.; Elsener, M.; Marti, T. NOx-Reduction in Diesel Exhaust Gas with Urea and Selective Catalytic Reduction. Combust. Sci. Technol. 1996, 121, 85.
(13) Koebel, M.; Elsener, M. Determination of Urea and Its Thermal Decomposition Compounds by High-Performance Liquid Chromatography. J. Chromatogr. A 1995, 689, 164. (14) Komatsu, T.; Nunokawa, M.; Moon, I. S.; Takahara, T.; Namba, S.; Yashima, T. Kinetic Studies of Reduction of Nitric Oxide with Ammonia on Cu2+-Exchanged Zeolites. J. Catal. 1994, 148, 427. (15) Baik, J. H.; Yim, S. D.; Nam, I.-S.; Lee, J.-H.; Cho, B. K.; Oh, S. H. Control of NOx Emissions from Diesel Engine by Selective Catalytic Reduction (SCR) with Urea. Top. Catal. 2004, 30-31, 37-41. (16) Baik, J. H.; Yim, S. D.; Nam, I.-S.; Lee, J.-H.; Cho, B. K.; Oh, S. H. Selective Catalytic Reduction of Diesel Engine NOx Emissions with NH3/Urea. Presented at the 18th North American Catalysis Society Meeting (NAM), Cancun, Mexico, Jun 1-6, 2003. (17) Long, R. Q.; Yang, R. T. Superior ion-exchanged ZSM-5 catalysts for selective catalytic oxidation of ammonia to nitrogen. Chem. Commun. 2000, 1651-1652. (18) Tachi, T.; Kato, A.; Yamashita, H. Reaction Mechanism of Catalytic Reduction of NO by Urea. Nippon Kagaku Kaishi 1992, 812-816.
Resubmitted for review February 10, 2004 Revised manuscript received April 6, 2004 Accepted April 30, 2004 IE034052J
