Effects of Various Factors on the Conversion Efficiency of Urea

Aug 6, 2014 - In a urea selective catalytic reduction (SCR) system, the urea ... marine diesel engine is very low when compared to that of a diesel ve...
0 downloads 0 Views 7MB Size
Download PDF
Article pubs.acs.org/EF

Effects of Various Factors on the Conversion Efficiency of Urea Solution in a Urea Selective Catalytic Reduction System Kun Woo Ku,† Jung Goo Hong,*,† Cheol Woo Park,† Kyung Yul Chung,‡ and Sang Ho Sohn‡ †

Department of Mechanical Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 702-701, Republic of Korea Korea Institute of Machinery & Materials 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea

‡

ABSTRACT: In a urea selective catalytic reduction (SCR) system, the urea solution is injected into hot exhaust gas, after which the urea solution becomes ammonia that acts as a reducing agent for de-NOx through evaporation, thermolysis, and hydrolysis. The formation of the reducing agent from urea decomposition is closely connected with thermofluid dynamics as well as various chemical reactions. An experimental study was performed to investigate urea decomposition in a low-temperature environment that is similar to the emission gas temperature of a large marine diesel engine. Also, this study investigated urea decomposition in conjunction with thermofluid dynamics related to the urea SCR system driving conditions. The modeled exhaust pipe was designed to control the inflow gas temperature and velocity. The urea solution injector was chosen to obtain almost identical spray performance, regardless of the urea solution flow rate, to exclude the effect of the spray on urea decomposition. A multicomponent Fourier transform infrared spectroscopy gas analyzer was used to measure the concentrations of ammonia and isocyanic acid (HNCO) in the modeled exhaust pipe. This study showed that the conversion efficiencies of ammonia and HNCO were different under the experimental conditions of this study, although there is no difference between the conversion efficiencies of ammonia and HNCO in theoretical urea thermolysis. Also, it showed that there is no need for a long residence time to improve the total conversion efficiency at a low temperature.

1. INTRODUCTION Many researchers have studied aftertreatment systems to reduce NOx emissions, such as NOx trap, selective non-catalytic reduction (SNCR), selective catalytic reduction (SCR), etc. Among the deNOx aftertreatment technologies, urea SCR is currently regarded as the most promising technology.1 SCR was introduced for deNOx treatments of power plants in the 1970s. SCR technology is being applied widely in many industrial fields, and thus, many results about it have been reported. A SCR system uses ammonia as the reducing agent and converts NOx into N2 by a reduction reaction through catalysis. Generally, it is accepted that the global SCR reactions to reduce NOx emissions are as follows: standard SCR reaction 4NO + 4NH3 + O2 → 4N2 + 6H 2O

isocyanic acid (HNCO) hydrolysis HNCO + H 2O → NH3 + CO2

Many research works have described SCR in the mobile field, where the environmental regulations are relatively strict. Birkhold et al.3,4 reported the conversion efficiency from an aqueous urea solution injected into a diesel car exhaust pipe to ammonia by numerical simulations. Dong et al.5 mentioned that deposit formation from undesirable urea decomposition is related to the quality of the spray of the injected urea solution. Furthermore, many works on methods to increase de-NOx efficiency in SCR systems for diesel vehicles have been reported.6−12 Many results on urea decomposition have been reported by chemical engineers because of the nature of urea SCR systems that the aqueous urea solution is injected directly into the exhaust pipe to yield the reducing agent from urea thermolysis and isocyanic acid hydrolysis, as shown in reactions 4 and 5. Also, these studies have focused on the formation of deposits caused by biuret, cyanuric acid (CYA), ammelide, ammeline, and melamine, which are created by side reactions that do not follow reactions 4 and 5 during the urea decomposition process. Lundström et al.13 reported that the CYA and biuret generated by these side reactions can be converted into the reducing agent for a SCR by hydrolysis catalysis. Urea decomposition and deposit formation have been investigated with a micro reactor by Bernhard.14 He reported that the side reactions begin with condensed urea, which is not completely decomposed during the urea decomposition process, and that HNCO plays a significant

(1)

fast SCR reaction NO + NO2 + 2NH3 → 2N2 + 3H 2O

(2)

slow reaction 6NO2 + 8NH3 → 7N2 + 12H 2O

(3)

However, from the viewpoint of storage, ammonia has disadvantages in terms of being used as the reducing agent for SCR systems because it is corrosive as well as strongly toxic. Because of these disadvantages, aqueous urea solutions are widely used as ammonia storage media for SCR processes. Urea is inexpensive, nontoxic, and decomposes according to reactions 4 and 5 to yield the actual reducing agent NH3.2 urea thermolysis CO(NH 2)2 → NH3(g) + HNCO(g) © 2014 American Chemical Society

(5)

Received: June 18, 2014 Revised: August 5, 2014 Published: August 6, 2014

(4) 5959

dx.doi.org/10.1021/ef501358j | Energy Fuels 2014, 28, 5959−5967

Energy & Fuels

Article

Figure 1. Schematic diagram of the experimental setup to measure the spray performance.

Figure 2. Schematic diagram of the experimental setup for measuring urea decomposition.

role in deposit formation. Yim et al.15 found that the hydrolysis of HNCO is possible even in the absence of a catalyst, although the rate was slow when the reaction temperature was sufficiently high (above 400 °C). The evaporation and decomposition of particles containing an aqueous urea solution were investigated by Kontin et al.16 They reported that the effect of a solid crust has to be considered during the evaporation of an aqueous urea solution under low temperatures. Urea decomposition under relatively high-temperature conditions was observed by Alzueta et al.17,18 Recently, there has been rising interest in applying urea SCR systems to large marine diesel engines because the International

Maritime Organization (IMO) has decided to enforce NOx reduction regulations.19 The exhaust gas temperature for a large marine diesel engine is very low when compared to that of a diesel vehicle, and the quantity of NOx in the exhaust is much greater than in that of a diesel vehicle. Thus, it is difficult to apply the present research on urea SCR systems, which have focused almost entirely on diesel vehicles, to large marine diesel engines. Moreover, the studies on urea decomposition have been conducted primarily to investigate the chemical kinetics using micro reactors. Therefore, it is quite difficult to obtain information on urea decomposition with consideration of the driving conditions for urea SCR systems, in which the 5960

dx.doi.org/10.1021/ef501358j | Energy Fuels 2014, 28, 5959−5967

Energy & Fuels

Article

multiphase flow caused by the interaction between the injected urea solution droplets and the exhaust gas flow appear. For these reasons, the practical information necessary to design urea SCR systems is insufficient, although the interest in such systems for large marine diesel engines is gradually increasing. Through the above-mentioned literature, it was found that the most important factors pertaining to the design of a urea SCR system are the urea decomposition in the reactor, catalyst, spray injector, system control algorism, and mixer. Among these factors, this study is concerned with the urea decomposition characteristics under low-temperature conditions that are similar to the exhaust gas temperature of a large marine diesel engine. Furthermore, this work investigates the differences between the conversion efficiencies of ammonia and HNCO as well as the total conversion efficiency of the reducing agent considered with the aqueous urea solution flow rate and the exhaust gas temperature and velocity corresponding with the urea SCR driving conditions given the modeled exhaust pipe.

Table 1. Experimental Conditions urea solution injector urea solution concentration (wt %) urea solution flow rate (g/min) inflow gas composition (%) inflow gas velocity (m/s) inflow gas temperature (°C)

air assist type 40 4, 5.5, and 7 15% O2 in N2 6, 8, and 12 210 and 250

2. EXPERIMENTAL SECTION Figure 1 illustrates the experimental setup to investigate the spray characteristics of the urea solution. The urea solution injector was an air assist type that is, in general, used for the urea SCR systems of large marine diesel engines. The spray experiment was carried out under atmospheric conditions. The experimental equipment consisted of a fluid supplying device, a visualization system, and measurement units. The fluid supplying device consisted of a liquid pump (2, Longer Precision Pump, BT50-1J) that supplied the urea solution from the storage tank to the injector (6) and an air assist supply device for atomization. The urea solution flow rate was controlled by revolutions per minute (rpm) of the liquid pump. The air compressor (1) and regulator (3) were used for making the pressurized assist air, and the volume flow rate of the assist air supplied to the injector was controlled by the needle valve (4) that was positioned after the air regulator. The volume flow rate of the assist air was measured by an air flow meter (5, SMC, PF2A750). The pressure and temperature of the assist air were measured by a pressure transducer (12, Sensys, PSH series) and a K-type thermocouple (13). The volume flow rate, pressure, and temperature signals were acquired by a data acquisition board and monitored in real time. The spray images were obtained using a charge-coupled device (CCD) camera (8, Vieworks, VM-2 M 35) and Nd:YAG laser (9, New Wave, Solo II) as a light source. The shutter signal of the CCD and Nd:YAG laser signal became synchronized by the delay generator (7, Qunatum, model 9514). The macroscopic characteristics of the injected urea solution were analyzed using the spray images. The Sauter mean diameter (SMD) and the volume-based size distribution with the urea solution flow rate conditions for this study were measured using the laser diffraction method based on a measurement technique reported by Suzuki et al.20,21 The modeled exhaust pipe was designed to investigate urea decomposition, as shown in Figure 2. The diameter of the modeled exhaust pipe was 66 mm, and the length from the urea solution nozzle to the final measurement position for urea decomposition was 7700 mm. The urea solution injector was installed in parallel to the inflow gas in the center of the modeled exhaust pipe. A multicomponent Fourier transform infrared spectroscopy (FTIR) gas analyzer (Gasmet, DX-4000) was used for analyzing the urea decomposition. The FTIR gas analyzer for this study was operated using the application method of the FTIR gas analyzer proposed by Kim et al.,22 who investigated the conversion efficiency of ammonia in the exhaust pipe. The sampling part of FTIR was purged using nitrogen gas every 5 min to minimize its contamination by impurities, such as residues and byproducts, during urea decomposition.22 At each sampling position, the gas sampling probe had five holes from the bottom of the modeled pipe to the top to avoid the difference of spatial distributions. The first gas sampling position for FTIR was 2500 mm from the urea solution injector, and then FTIR five gas sampling

Figure 3. Spray images for various flow rates of the injected urea solution. positions were placed along the modeled exhaust pipe. This experimental device can monitor urea decomposition for a minimum of 0.21 s and a maximum of 1.3 s under the inflow gas conditions. The 5961

dx.doi.org/10.1021/ef501358j | Energy Fuels 2014, 28, 5959−5967

Energy & Fuels

Article

Figure 4. SMD and volume-based size distributions for the flow rates of the urea solution. inflow gas was heated by an electronic heater (4, Orem Sylvania, Superheat MAX F074733), and the velocity of the inflow gas was controlled by the inverter synchronized with the gas blower (1). The inflow gas velocity and temperature were measured by a micro manometer (5, Furness controls, FCO-12) located in front of the urea solution injector and a K-type thermocouple (7). The model gas composition of this study was based on results by Wakatsuki et al.,23 who investigated the SCR performance of the marine diesel engine. The model gas composition was controlled by the evaporator (4) and liquid nitrogen (3). The model gas composition was 15% O2 in balance N2. Also, the inflow gas temperature was measured at the same position as the urea solution injector and at the final gas sampling position. The temperature difference between measuring positions was a maximum of 20 °C. The experimental conditions are given in Table 1. The concentration of urea solution used for this study was set at 40 wt % because it has been used for urea SCR systems of large marine diesel engines. The flow rates of the injected urea solution and the inflow gas velocities were selected to investigate urea decomposition based on increasing the quantity of urea solution injected and the reaction time, as shown in Table 1. In this study, the minimum flow rate of the injected urea solution together with the inflow gas conditions corresponded with almost 900 ppm of NOx.

used for the urea solution injector in this study. The laser diffraction method was used to measure the SMD and volumebased size distributions. Figure 4 shows the SMDs and volumebased size distributions for the three flow rates of the injected urea solution. The measurement position was at 20 mm of the axial distance from the nozzle exit. Also, the spray velocity was measured by a laser doppler velocimetry (LDV) system at the spray center of this position. The SMDs and volume-based size distributions were almost identical, although the flow rates of the injected urea solution differed. A summary of the spray performance for the nozzle is shown in Table 2. As shown in Table 2. Summary of Spray Performance

3. RESULTS AND DISCUSSION 3.1. Spray Characteristics with the Urea Solution Flow Rate. The spray characteristics of the injected urea solution influence the urea decomposition in a urea SCR system.5 This study was conducted to investigate the atomization characteristics of the urea solution injector that was being tested. Images of the droplets emanating from the test nozzle are presented in Figure 3. The spray angles as macroscopic atomization characteristics were measured from these images. The spray angle was defined by a vertical angle made from the injector exit and outer boundary of spray in images of Figure 3. As shown in Figure 3, there was no significant difference of the spray shapes with increasing flow rates of the injected urea solution. Lefebvre24 found that the spray angle from the external mixing air assist nozzle was almost identical, regardless of the flow rate injected from the nozzle. The reason why there was no significant difference of the spray shapes can be explained by the nature of the external mixing air assist nozzle, which was

urea solution flow rate (g/min)

assist air flow rate (L/min)

SMD (μm)

spray velocity (m/s)

spray angle (deg)

4 5.5 7

6 7 7

149 147 146

7.3 7.9 7.8

39 40 41

Table 2, the spray performance and flow rate of the assist air for the atomization were almost identical under the flow rate conditions of the injected urea solution. This means that the effect of the spray on the urea solution at the given flow rates was minimized for the decomposition of urea. 3.2. Effect of the Driving Conditions on the Total Conversion Efficiency of the Reducing Agent. From the viewpoint of urea SCR system size, the conversion efficiency of the reducing agent through the urea decomposition process is one of the important design factors. The conversion efficiencies of the reducing agent along with the residence times were measured for the injected urea solution flow rates as well as the inflow gas temperatures and velocities. Figure 5 shows the conversion efficiencies of the reducing agent with the residence times. The hollow symbols in Figure 5 are for 250 °C inflow gas, and the solid symbols are for 210 °C. As mentioned above, 1 mol of the urea solution injected into the exhaust pipe was decomposed into 1 mol of ammonia as the reducing agent and 5962

dx.doi.org/10.1021/ef501358j | Energy Fuels 2014, 28, 5959−5967

Energy & Fuels

Article

Figure 6. Total conversion efficiencies of reducing agent with the urea solution flow rate.

the urea SCR system. We defined the total conversion efficiency as described in eq 6. total conversion efficiency (%) = (ammonia moleobserved + HNCO moleobserved) /(ammonia molestoichiometric + HNCO molestoichiometric) × 100

(6)

The total conversion efficiency depends upon the ratio of the moles of stoichiometric ammonia and HNCO from the injected urea solution to the moles of ammonia and HNCO observed in reality. The residence times in Figure 5 were defined as the amounts of time from when the inflow gas passes from the position of the urea solution injector to each gas sampling position. The residence time is the time the injected urea solution can be decomposed by inflow gas. In this study, the comparison of total conversion efficiency at each experimental condition was made at a given residence time. As shown in Figure 5, the total conversion efficiency increased dramatically for relatively short residence times under all of the experimental

Figure 5. Total conversion efficiencies of reducing agent with residence times.

1 mol of HNCO by the thermolysis reaction. After that, HNCO was also decomposed to ammonia through HNCO hydrolysis. Therefore, HNCO can potentially be a reducing agent. In this study, HNCO is treated as a reducing agent for 5963

dx.doi.org/10.1021/ef501358j | Energy Fuels 2014, 28, 5959−5967

Energy & Fuels

Article

Figure 7. Reaction network for urea decomposition with byproduct formation and decomposition.14

As shown in Figure 6, in the case of 250 °C of inflow gas temperature, the total conversion efficiency decreased as the urea solution flow rate increased, regardless of inflow gas conditions, such as velocity and temperature. The evaporation of the urea solution is essential for it to be decomposed into the reducing agent in a urea SCR system. If the urea solution flow rate increases under the same inflow gas conditions, the latent heat of the urea solution increases proportionally. At the same time, the quantity of urea to be thermolyzed becomes larger. These processes play roles in increasing the heat loss of the inflow gas; therefore, the temperature of the mixture of the urea solution and inflow gas drops. For these reasons, it can be deduced that the total conversion efficiency decreases with an increasing urea solution flow rate. However, in the case of 210 °C of inflow gas temperature, the decrease of total conversion efficiency was insignificant with increasing the urea solution flow rate. 3.3. Effect of the Driving Conditions on Urea Decomposition. Kim et al.22 investigated the conversion efficiency of ammonia from the injected urea solution. The conversion efficiency that they calculated was based on an assumption that is identical with the conversion efficiency of ammonia and HNCO. Bernhard14 described the reaction network for urea decomposition from comprehensive studies by many researchers, as shown in Figure 7. He stated that HNCO is consumed during the reaction to form byproducts, such as biuret, CYA, ammelide, etc. Schaber et al.25 reported that biuret and CYA are largely formed at temperatures below 300 °C. Also, Eichelbaum et al.26 asserted that the performance of SCR becomes worse because of deposit formation from CYA and ammelide at temperatures below 300 °C. From these results, it can be concluded that the byproducts formed by consuming HNCO during urea decomposition have considerable influence on SCR performance under low-temperature conditions. In this study, the conversion efficiencies for both ammonia and HNCO were measured to ascertain the consumption of HNCO during the urea decomposition process,

conditions. It is thought that the relatively small droplets of the injected urea solution, as shown in Figure 4, were evaporated rapidly in the initial stage of mixing of the urea solution and inflow gas. After that, the total conversion efficiency for the 250 °C inflow gas increased at a slow rate as the residence time increased, and for the 210 °C inflow gas, it was almost identical, regardless of the residence time. Generally, the urea thermolysis reaction is known as being an endothermic reaction, and therefore, the temperature for the mixture of the urea solution and inflow gas becomes lower than that of the inflow gas during the process of urea solution evaporation and thermolysis.3,22 This means that, as the urea solution evaporates and thermolysis progresses, the energy in the inflow gas becomes dissipated. For this reason, it seems that the urea solution evaporation and decomposition are suppressed with increasing residence time. From these results, in the case of applying a urea SCR system at a low temperature, it is thought that there is no need for a long residence time to improve the total conversion efficiency. It is known from the general information in the previous literature that the total conversion efficiency increases with the temperature of the inflow gas. The total conversion efficiency was not significantly different from the inflow gas velocity. The total conversion efficiency at 250 °C was 0.39−0.45 with the urea solution flow rate at 1.3 s of residence time, and the total conversion efficiency at 210 °C was 0.28−0.31. The flow rate of the urea solution injected into the exhaust pipe is proportional to the NOx quantity. There are large differences in the NOx quantities coming from different combustors based on combustor design and combustion conditions, and there are instances in which the quantity of the urea solution injected is larger than the stoichiometric quantity of urea solution necessary for NOx conversion because urea is not completely decomposed into the reducing agent. This study investigated the effect of the urea solution flow rate on the total conversion efficiency. Figure 6 shows the total conversion efficiencies with the urea solution flow rates. Figure 6a is for 6 m/s inflow gas velocity, and Figure 6b is for 8 m/s. 5964

dx.doi.org/10.1021/ef501358j | Energy Fuels 2014, 28, 5959−5967

Energy & Fuels

Article

unlike the study by Kim et al.22 We defined the conversion efficiency of ammonia and HNCO as described in eq 7. conversion efficiency of ammonia (%) (ammonia moleobserved) = × 100 (ammonia molestoichiometric) conversion efficiency of HNCO (%) (HNCO moleobserved) = × 100 (HNCO molestoichiometric)

(7)

The conversion efficiency of ammonia depends upon the ratio of the moles of stoichiometric ammonia from the injected urea solution to the moles of ammonia observed in reality. Also, the conversion efficiency of HNCO was calculated in the same manner. Figure 8 show the conversion efficiency of ammonia

Figure 9. Relative difference indices with inflow gas temperature and urea solution flow rate.

total conversion efficiency using the right y axis. As shown in Figure 8, the conversion efficiency of ammonia was higher than that of HNCO, regardless of the inflow gas temperature, although there is no difference between the conversion efficiencies of ammonia and HNCO in the theoretical urea thermolysis described by reaction 4. This means that simultaneous measurements of ammonia and HNCO are needed to verify the conversion efficiency of the reducing agent in a urea SCR system because the difference between the conversion efficiencies of ammonia and HNCO influence the total conversion efficiency. Also, it is reasonable to assert that the side reactions of HNCO are more activated to form byproducts during the urea decomposition process when compared to ammonia. Equation 8 was used to describe the relative difference between the conversion efficiencies of ammonia and HNCO compared to the total conversion efficiency. The values of the relative differences between the conversion efficiencies of ammonia and HNCO were averaged with the residence time under the same driving conditions

Figure 8. Conversion efficiencies of ammonia and HNCO at 6 m/s inflow gas velocity and 4 g/min urea solution flow rate. Inflow gas temperature: (a) 210 °C and (b) 250 °C.

and HNCO at an inflow gas velocity of 6 m/s and a urea solution flow rate of 4 g/min. Figure 8a is for an inflow gas temperature of 210 °C, and Figure 8b is for 250 °C. The bars represent the conversion efficiencies of ammonia and HNCO using the left y axis in Figure 8, and the symbol line is the 5965

dx.doi.org/10.1021/ef501358j | Energy Fuels 2014, 28, 5959−5967

Energy & Fuels

Article

most significantly influences the Reynolds number because the density and viscosity of the inflow gas are identical. Therefore, the convective heat and mass transfer rates between the urea solution and the inflow gas increase as the inflow gas velocity increases. From the results of Figure 10, it is thought that the relative difference index is affected by the convective heat and mass transfer rates and also that the side reactions that form byproducts can be inhibited by increasing the convective heat and mass transfer rates.

because there is little difference in these values for different residence times. relative difference index = (conversion efficiency of ammonia − conversion efficiency of HNCO) /total conversion efficiency

(8)

The relative difference index as defined in this study is the index used to express the consumption rate of HNCO by the side reactions compared to the total conversion efficiency: a value of 1 means that all HNCO from the urea decomposition is consumed by the side reactions to form byproducts, and a value of 0 means the opposite. In other words, this means that the side reactions are more activated as the relative difference index increases. Figure 9 shows the relative difference indices with inflow gas temperatures and flow rates of the injected urea solution. Figure 9a is for 6 m/s of inflow gas temperature, and Figure 9b is for 8 m/s. As shown in Figure 9, the relative difference index decreased as the inflow gas temperature increased. Because this result is similar to the above-mentioned literature,25,26 it is confirmed that the reaction temperature plays a role in the side reactions that form the byproducts. The flow rates of the injected urea solution also influenced the relative difference indices. The relative difference index increased as the flow rate of the injected urea solution increased, regardless of the inflow gas temperature and velocity. As mentioned above, it can be deduced that the increases in the relative difference index were caused by decreasing the temperature of the mixture of the urea solution and inflow gas. The relative difference index was affected by the inflow gas velocity, as illustrated in Figure 10.

4. CONCLUSION Urea decomposition was studied by experiments under lowtemperature conditions that were similar to the exhaust gas temperatures of large marine diesel engines. Also, the urea decomposition process considering the urea SCR driving conditions was investigated with a modeled exhaust pipe. The remarkable conclusions of this study are as follows: (1) There is no need for a long residence time to improve the total conversion efficiency at low temperatures because the urea thermolysis reaction is inactivated with longer residence times. The urea themolysis characteristics under low-temperature conditions should be considered when designing the optimum distance between the position of the urea solution injector and the catalyst. (2) The total conversion efficiency is affected by the flow rate of the injected urea solution and it decreases with increasing the flow rate under the same inflow gas conditions of inflow gas. (3) The conversion efficiency of ammonia is larger than that of HNCO. The difference between these conversion efficiencies is affected by the aqueous urea solution flow rate, inflow gas temperature, and inflow gas velocity. (4) On the basis of the experimental conditions of this study, it can be deduced that the side reactions caused by the consumption of HNCO are inhibited with increases of the inflow gas velocity and temperature as well as minimizing the flow rate of the urea solution injected into the exhaust pipe.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: 82-53-950-6570. Fax: 82-53-950-6550. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This research was supported by the Kyungpook National University Research Fund, 2013(2014). REFERENCES

(1) Ciardelli, C.; Nova, I.; Tronconi, E.; Chatterjee, B.; BandlKonrad, B.; Weibel, M.; Krutzsch, B. Appl. Catal., B 2007, 70, 80−90. (2) Bernhard, A. M.; Peitz, D.; Elsener, M.; Wokaun, A.; Kröcher, O. Appl. Catal., B 2012, 115−116, 129−137. (3) Birkhold, F.; Meingast, U.; Wassermann, P. Appl. Catal., B 2007, 70, 119−127. (4) Birkhold, F.; Meingast, U.; Wassermann, P. SAE Tech. Pap. Ser. 2006, DOI: 10.4271/2006-01-0643. (5) Dong, H.; Shuai, S.; Wang, J. SAE Tech. Pap. Ser. 2008, DOI: 10.4271/2008-01-1544. (6) De Rudder, K. Proceedings of the Conference on Thermo and Fluid Dynamics Processes in Direct Injection Engines; Valencia, Spain, Sept 11− 14, 2012 (7) De Rudder, K. SAE Tech. Pap. Ser. 2012, DOI: 10.4271/2012-011087.

Figure 10. Relative difference indices with the inflow gas velocity at an inflow gas temperature of 250 °C.

The relative difference index decreased with increasing inflow gas velocity. The heat and mass transfer between the injected urea solution and the inflow gas are governed by convection.3 The convective heat and mass transfer rates are known to be proportional to the Reynolds number in a turbulent flow.27 It can be deduced that the flow field in this study is turbulent because the minimum Reynolds number is almost 9500 under the experimental conditions of this study. In the case of the same temperature of the inflow gas, the inflow gas velocity 5966

dx.doi.org/10.1021/ef501358j | Energy Fuels 2014, 28, 5959−5967

Energy & Fuels

Article

(8) Bonfils, A.; Creff, Y.; Lepreux, O.; Petit, N. J. Process Control 2014, 24, 368−378. (9) Willems, F.; Cloudt, R.; van den Eijnden, E.; van Genderen, M.; Verbeek, R.; de Jager, B.; Boomsma, W.; van Heuvel, I. SAE Tech. Pap. Ser. 2007, DOI: 10.4271/2007-01-1574. (10) Acharya, R.; Alam, M.; Boehman, A. SAE Tech. Pap. Ser. 2006, DOI: 10.4271/2006-01-0638. (11) van Helden, R.; Verbeek, R.; Willems, F.; van der Welle, R. SAE Tech. Pap. Ser. 2004, DOI: 10.4271/2004-01-0154. (12) van Helden, R.; van Genderen, M.; van Aken, M.; Verbeek, R.; Patchett, J.; Kruithof, J.; Straten, T.; De Saluneaux, C. G. SAE Tech. Pap. Ser. 2002, DOI: 10.4271/2002-01-0286. (13) Lundström, A.; Snelling, T.; Morsing, P.; Gabrielsson, P.; Senar, E.; Olsson, L. Appl. Catal., B 2011, 106, 273−279. (14) Bernhard, A. M. Catalytic urea decomposition, side-reactions and urea evaporation in the selective catalytic reduction of NOx. Ph.D. Dissertation, ETH Zurich, Zurich, Switzerland, 2012; Thesis 20813. (15) Yim, S. D.; Kim, S. J.; Baik, J. H.; Nam, I. S.; Mok, Y. S.; Lee, J. H.; Cho, B. K.; Oh, S. H. Ind. Eng. Chem. Res. 2004, 43, 4856−4863. (16) Kontin, S.; Höfler, A.; Koch, R.; Bauer, H. J. Proceedings of the 23th Annual Conference on Liquid Atomization and Spray Systems; Brno, Czech Republic, Sept 6−9, 2010. (17) Alzueta, M. U.; Bilbao, R.; Millera, A.; Oliva, M.; Ibanez, J. C. Energy Fuels 1999, 13 (2), 541. (18) Alzueta, M. U.; Bilbao, R.; Millera, A.; Oliva, M.; Ibanez, J. C. Energy Fuels 2000, 14 (2), 509−510. (19) Azzara, A.; Rutherford, D.; Wang, H. Feasibility of IMO Annex VI Tier III Implementation Using Selective Catalytic Reduction; International Council on Clean Transportation (ICCT): Washington, D.C., 2014; Working Paper 2014-4. (20) Suzuki, T.; Saito, A.; Fujimatsu, T.; Hayashida, K. J. ILASS-Jpn. 2007, 16, 34−46. (21) Suzuki, T.; Saito, A.; Fujimatsu, T.; Hayashida, K. J. ILASS-Jpn. 2008, 17, 44−51. (22) Kim, J. Y.; Ryu, S. H.; Ha, J. S. Proceedings of the 2004 Fall Technical Conference of the ASME Internal Combustion Engine Division; Long Beach, CA, Oct 24−27, 2004; pp 165−170. (23) Wakatsuki, Y.; Misawa, K. Proceedings of the 9th International Symposium on Marine Engineering; Kobe, Japan, Oct 17−21, 2011; C62. (24) Lefebvre, A. H. Textbook of Atomization and Sprays; Hemisphere Publishing Corporation: Bristol, PA, 1989; pp 136−140. (25) Schaber, P. A.; Colson, J.; Higgins, S.; Thielen, D.; Anspach, B.; Brauer, J. Thermochim. Acta 2004, 424, 131−142. (26) Eichelbaum, M.; Siemer, A. B.; Farrauto, R. J.; Castaldi, M. J. Appl. Catal., B 2010, 97, 98−107. (27) Incropera, F. P.; DeWitt, D. P.; Bergman, T. L.; Lavine, A. S. Textbook of Fundamentals of Heat and Mass Transfer; John Wiley and Sons, Inc.: Hoboken, NJ, 2005; pp 486−533.

5967

dx.doi.org/10.1021/ef501358j | Energy Fuels 2014, 28, 5959−5967