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Published: May 18, 2011 r2011 American Chemical Society 8285 dx.doi.org/10.1021/ie102313k | Ind. Eng. Chem. Res. 2011, 50, 82858294 ARTICLE pubs.acs.org/IECR Experimental Study on Evaporation Characteristics of Ammonium FormateUreaWater Solution Droplet for Selective Catalytic Reduction Applications Seung Yeol Lee* and Seung Wook Baek Propulsion and Combustion Laboratory, School of Mechanical, Aerospace and Systems Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea ABSTRACT: The evaporation behavior of ammonium formateureawater solution (AFUWS) droplet was studied for applications to selective catalytic reduction (SCR) systems. A number of experiments were performed with single AFUWS droplet suspended on the tip of a ne quartz ber. To cover the temperature range of real diesel exhausts, droplet ambient temperature was regulated from 373 to 873 K using an electrical furnace. As a result of this investigation, AFUWS droplet revealed dierent evaporation characteristics depending on its ambient temperature. As ambient temperature increases, in addition to evaporation of water content, thermal decompositions of ammonium formate and urea were additionally observed. At high temperatures, it showed quite complicated behaviors such as bubble formation, distortion, and partial rupture after a linear D 2 -law period. However, as temperature decreases, these phenomena became weak and nally disappeared. Also, droplet evaporation coecients were retrieved from transient evaporation histories for various ambient temperatures, which yield a quantitative evaluation on evaporation characteristics of AFUWS droplet as well as provide valuable empirical data required for modeling or simulation works on SCR systems. 1. INTRODUCTION Among various techniques of reducing diesel NO x to levels required by strict worldwide emission regulations, the most realistic solution is ammonia-selective catalytic reduction (SCR), which uses ammonia as a reducing agent. 1 However, due to its inherent toxicity and handling problems with pure ammonia, aqueous urea is now practically utilized as an alternative to the direct use of ammonia for mobile SCR applications. Once ureawatersolution (UWS) spray is injected into hot exhaust gas stream prior to reaching SCR catalyst, water content rst evaporates from UWS. Next, ammonia is formed by both thermal decomposition of urea and hydrolysis of isocyanic acid as follows: 24 ðNH 2 Þ 2 CO f NH 3 þ HNCO ð1Þ HNCO þ H 2 O f NH 3 þ CO 2 ð2Þ By collectively reviewing the literature, 58 it is found that the state of aggregation urea is not clear during the evaporation of UWS; rather it can be varied among solid, molten, and gas phases depending on thermo-physical conditions. However, in ureaSCR systems, several negative eects are possibly derived due to the use of UWS. First is the relatively high freezing point (11 °C for AdBlue, which contains 32.5% urea by weight), which makes it dicult to handle, transport, store, and use in cold temperature. This requires additional heating systems to prevent freezing of UWS. Furthermore, the amount of ammonia stored in AdBlue is rather small with approximately 0.2 kg NH 3 /kg solution, 8,9 which means that unnecessary weight is transported in the vehicle application. Moreover, with using UWS, there is always a risk of deposits especially when the urea solution gets in contact with cold spots. 9 Various urea decomposition products (i.e., biuret, cyanuric acid (CYA), ammelide, ammeline, melamine, etc.) and their poly- meric complexes involving hydrogen bondings can be generated in ureaSCR systems. 7,8 Fang and DaCosta 8 reported that urea thermolysis involves two decomposition stages; one is the ammo- nia generation stage, which is desirable for reducing NO x , whereas the other is the ammonia consumption stage, which promotes the formation of undesirable species. In particular, consecutive decompositions after ammonia consumption stage lead to the production of melamine complexes, (HNCdNH) x (HNCO) y , which are considered as a major source in hindering the perfor- mance of catalyst by not only consuming a part of ammonia produced during urea thermolysis but also degrading the struc- tural and thermal properties of the catalyst surface. From a thermo-chemical calculation by Koebel and Strutz, 10 it was reported that heat required to thermally decompose UWS is 144.8 kJ/mol for 76.93% urea solution, 263.1 kJ/mol for 50% urea solution, and 444.6 kJ/mol for 32.5% urea solution at 500 K. It is seen that with increasing water content the heat requirement for thermal decomposition of UWS increases. Thus, reducing agents with low water contents are favorable not only due to the avoidance of extra weight but also due to energetic consideration. Received: November 16, 2010 Accepted: May 18, 2011 Revised: May 15, 2011
Transcript
Page 1: Experimental Study on Evaporation Characteristics of ...procom.kaist.ac.kr/Download/IJP/122.pdf · Experimental Study on Evaporation Characteristics of Ammonium Formate Urea Water

Published: May 18, 2011

r 2011 American Chemical Society 8285 dx.doi.org/10.1021/ie102313k | Ind. Eng. Chem. Res. 2011, 50, 8285–8294

ARTICLE

pubs.acs.org/IECR

Experimental Study on Evaporation Characteristics of AmmoniumFormate�Urea�Water Solution Droplet for Selective CatalyticReduction ApplicationsSeung Yeol Lee* and Seung Wook Baek

Propulsion and Combustion Laboratory, School of Mechanical, Aerospace and Systems Engineering, Korea Advanced Institute ofScience and Technology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea

ABSTRACT: The evaporation behavior of ammonium formate�urea�water solution (AFUWS) droplet was studied forapplications to selective catalytic reduction (SCR) systems. A number of experiments were performed with single AFUWS dropletsuspended on the tip of a fine quartz fiber. To cover the temperature range of real diesel exhausts, droplet ambient temperature wasregulated from 373 to 873 K using an electrical furnace. As a result of this investigation, AFUWS droplet revealed differentevaporation characteristics depending on its ambient temperature. As ambient temperature increases, in addition to evaporation ofwater content, thermal decompositions of ammonium formate and urea were additionally observed. At high temperatures, it showedquite complicated behaviors such as bubble formation, distortion, and partial rupture after a linear D2-law period. However, astemperature decreases, these phenomena became weak and finally disappeared. Also, droplet evaporation coefficients were retrievedfrom transient evaporation histories for various ambient temperatures, which yield a quantitative evaluation on evaporationcharacteristics of AFUWS droplet as well as provide valuable empirical data required for modeling or simulation works on SCRsystems.

1. INTRODUCTION

Among various techniques of reducing diesel NOx to levelsrequired by strict worldwide emission regulations, themost realisticsolution is ammonia-selective catalytic reduction (SCR), whichuses ammonia as a reducing agent.1 However, due to its inherenttoxicity and handling problems with pure ammonia, aqueousurea is now practically utilized as an alternative to the direct use ofammonia for mobile SCR applications.

Once urea�water�solution (UWS) spray is injected into hotexhaust gas stream prior to reaching SCR catalyst, water contentfirst evaporates from UWS. Next, ammonia is formed by boththermal decomposition of urea and hydrolysis of isocyanic acid asfollows:2�4

ðNH2Þ2CO f NH3 þHNCO ð1Þ

HNCOþH2O f NH3 þ CO2 ð2Þ

By collectively reviewing the literature,5�8 it is found that thestate of aggregation urea is not clear during the evaporation ofUWS; rather it can be varied among solid, molten, and gas phasesdepending on thermo-physical conditions.

However, in urea�SCR systems, several negative effects arepossibly derived due to the use of UWS. First is the relatively highfreezing point (�11 �C for AdBlue, which contains 32.5% ureaby weight), which makes it difficult to handle, transport, store,and use in cold temperature. This requires additional heatingsystems to prevent freezing of UWS. Furthermore, the amountof ammonia stored in AdBlue is rather small with approximately

0.2 kg NH3/kg solution,8,9 which means that unnecessary weight

is transported in the vehicle application.Moreover, with using UWS, there is always a risk of deposits

especially when the urea solution gets in contact with cold spots.9

Various urea decomposition products (i.e., biuret, cyanuric acid(CYA), ammelide, ammeline, melamine, etc.) and their poly-meric complexes involving hydrogen bondings can be generatedin urea�SCR systems.7,8 Fang and DaCosta8 reported that ureathermolysis involves two decomposition stages; one is the ammo-nia generation stage, which is desirable for reducing NOx, whereasthe other is the ammonia consumption stage, which promotes theformation of undesirable species. In particular, consecutivedecompositions after ammonia consumption stage lead to theproduction of melamine complexes, (HNCdNH)x(HNCO)y,which are considered as a major source in hindering the perfor-mance of catalyst by not only consuming a part of ammoniaproduced during urea thermolysis but also degrading the struc-tural and thermal properties of the catalyst surface.

From a thermo-chemical calculation by Koebel and Strutz,10 itwas reported that heat required to thermally decompose UWSis 144.8 kJ/mol for 76.93% urea solution, 263.1 kJ/mol for50% urea solution, and 444.6 kJ/mol for 32.5% urea solutionat 500 K. It is seen that with increasing water content the heatrequirement for thermal decomposition of UWS increases. Thus,reducing agents with low water contents are favorable not onlydue to the avoidance of extra weight but also due to energeticconsideration.

Received: November 16, 2010Accepted: May 18, 2011Revised: May 15, 2011

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The effort to find new SCR reducing agents as alternatives forUWS has been exercised in various experiments. Among others,Kr€ocher et al.9,11 proposed various ammonia precursor com-pounds as new SCR reducing agents such as ammonium formate,methanamide, and guanidinium formate. The main advantage ofthese alternative reducing agents resides in their much lowerfreezing point, whichmake a heating system unnecessary. Amongthese agents, ammonium formate turned out to be an additive forurea�water solution, which significantly lowers the freezingpoint of the solution. Denoxium-30, a solution of 26% ammo-nuim formate and 20%urea inwater, has a freezing point of�30 �Cand improves in the DeNOx activity in the low temperatureoperation. Also, Denoxium solution showed a similar reductioncapacity comparable to standard UWS.12

In the first study of Koebel and Elsener,13 ammonium formatefirst dissociates in diesel exhaust gas to ammonia and formic acid,which further decomposes to water and CO at higher tempera-ture (above 573 K) as follows:11,13

HCOONH4 f NH3 þHCOOH ð3Þ

HCOOH f COþH2O ð4ÞAfter that, when using Denoxium-30 as a reducing agent,

ammonia produced by the above reactions 1, 2, and 3 takes partin various deNOx reactions as a reductant. Main deNOx reactionsunder a typical diesel exhaust environment can be summarized asfollows:5,14

4NH3 þ 4NOþO2 f 4N2 þ 6H2O ð5Þ

4NH3 þ 2NOþ 2NO2 f 4N2 þ 6H2O ð6Þ

4NH3 þ 3NO2 f 7=2N2 þ 6H2O ð7Þ

4NH3 þ 4NOþ 3O2 f 4N2Oþ 6H2O ð8ÞA sufficient understanding on the behavior of ammonium

formate�urea�water solution (AFUWS) spray is a prerequisitefor controlling the formation of various products derived by urea.However, there are lots of complexities involved in the evapora-tion process of AFUWS at a heated environment. Dissolvedammonium formate and urea influence the evaporation of waterfrom AFUWS, and simultaneously ammonium formate and ureathemselves undergo thermal decomposition as well as react toproduce various species. Because of these difficulties, the evapora-tion behavior of AFUWS has not been clearly understood yet.

In our previous study by Wang et al.,15 the evaporationbehavior of UWS droplet with AdBlue was investigated usingsuspended droplet experiment. Through a number of repeatedmeasurements, the evaporation characteristics of UWS dropletwere categorized into several groups according to ambienttemperature, and microexplosion phenomenon was observedabove 573 K. Also, after the complete depletion of liquid compo-nent constituting UWS droplet, solidified deposit was observedto remain at temperatures below 773 K, and its amount wasreduced with increase in ambient temperature, while there wasalmost no deposit remaining at temperatures above 823 K.

In this study, the evaporation behavior of AFUWS droplet wasexperimentally observed over the temperature range of actualdiesel exhaust. Measurements were conducted by changing thedroplet ambient temperature by 50 K from 373 to 873 K. Themain objectives of this study are to understand the evapora-tion characteristics of AFUWS droplet in various ambienttemperatures and also to provide empirical evaporation coeffi-cients, which can be utilized as reference data in modeling orsimulation works on AFUWS spray. So far, to our knowledge,there have been few useful experimental data capable of quantify-ing the evaporation of AFUWS droplet over a wide range oftemperatures.

The initial diameter of AFUWS droplets used in this experi-ment ranges from about 650 to 1150 μm. However, the initialSauter mean diameter (SMD) of UWS spray applied in practicalsystems ranges between 20 and 150 μm.16 Therefore, in thisstudy, for an extension of the results to much smaller-sizeddroplets, the effect of initial droplet size on its evaporation wasinvestigated through a number of experiments under variousinitial diameters and ambient temperatures.

As typically appeared in the evaporation process of a multi-component fuel droplet, the phenomenon of violent dropletfragmentation, which is often referred to as microexplosion, is alsoobserved during the evaporation of AFUWS droplets at elevatedtemperatures in this study. The basic mechanism responsiblefor microexplosion can be best explained through the diffusionlimit model.17,18 More volatile components trapped inside thedroplet due to diffusional resistance can be heated beyond thelocal boiling point and hence undergo internal superheating. Thismay eventually lead to the onset of homogeneous nucleation,while an extremely rapid rate of gasification causes intenseinternal pressure build-up, thereby leading to disruptive dropletfragmentation. Therefore, it was also discussed how microexplo-sion affects both the evaporation of AFUWS droplet and theformation of urea-derived deposits under various ambienttemperatures.

Figure 1. A schematic picture of the experimental apparatus. (1)Cylindrical vessel, (2) guide bar, (3) furnace bottom hole, (4) electricfurnace, (5) quartz glass window on furnace, (6) temperature controller,(7) furnace lever, (8) air vessel, (9) quartz glass window on pressurevessel, (10) backlight source, (11) quartz fiber, (12) droplet, (13) shockabsorber, (14) droplet generator, (15) droplet lever, (16) plungermicropump, (17) high-speed CCD camera.

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2. EXPERIMENTS

2.1. Procedures and Facilities. In the current study, Denox-ium-30 (contains 26.2% ammonium formate and 20.1% urea byweight) is used as a representative AFUWS. A schematic pictureof the experimental apparatus is illustrated in Figure 1. In thissection, experimental procedures and facilities are described, andfurther details can be found in Ghassemi et al.19,20

First, an electrical furnace is lifted up away from a dropletsuspension system, and its interior temperature is regulated to anintended value using a controller and thermocouple. At the sametime, the inside of a cylindrical vessel (height = 0.8 m, innerdiameter = 0.15 m) is refreshed by the outside air at roomtemperature.Once the control of droplet ambient temperature is com-

pleted, then a single AFUWS droplet is suspended around a beadthat is placed at the tip of a quartz fiber (diameter = 0.125 mm)using a droplet generator whose end part is equipped with ahollow stainless-steel needle (outer diameter = 0.21 mm). Themovement of this droplet generator is adjusted by handling alever, which causes liquid AFUWS to be transported to the beadwhose diameter is approximately 0.25 mm, thereby generating adroplet. Here, for continuous monitoring of droplet ambienttemperature, a K-type thermocouple was placed about 30 mmbeside the suspended droplet.The next step is to drop the electrical furnace to the position as

shown by the dotted line in Figure 1. This then leads the suspendedAFUWS droplet to be exposed to high temperature, whicheventually initiates droplet evaporation. Glass window enablesus to observe the evaporation process of droplet using a high-speed charge-coupled device (CCD) camera, and its transientvariation of images is recorded on a computer.It should be noted that there are several error sources in

measuring the evaporation rate of AFUWS droplet under aconstant temperature condition. Despite these errors, a certaindegree of measurement accuracy may be conserved in this study.

For detailed information of experimental error sources, pleaserefer to our previous study on the evaporation of UWS droplet.15

2.2. Retrieval of Effective Droplet Diameter and Evapora-tion Coefficient. Figure 2 shows a sample image of AFUWSdroplet that was captured during the evaporation process. Here,the needle beside the droplet-suspended fiber provides a refer-ence scale of known diameter (0.21 mm). To extract dropletdiameter from the photograph, a flexible image-processing pro-gram was developed using Visual Basic language. In this program,the number of pixels corresponding to the diameter of referenceneedle is first calculated. Next, an imaginary square of known areais drawn around droplet as shown in Figure 2. After that, thenumber of pixels that constitute droplet is calculated on the basisof intensity difference from surroundings. Finally, the circulararea having the same number of pixels is calculated, which in turnsgives an effective droplet diameter from the law of proportion withreference needle. This procedure is iteratively executed for eachimage file, which yields a temporal variation of droplet diameterduring evaporation. Note that the shape of suspended droplet isnot an exactly circular shape due to both gravitation and dro-plet�fiber interaction. This eccentricity becomes severe as initialdroplet size decreases, which may lead to some measurementerrors.In general, the early lifetime of evaporating droplet shows a

nonlinear behavior due to transient droplet heat-up by hotenvironment and subsequent thermal expansion. As the tempera-ture of the droplet surface increases, evaporation starts beforereaching boiling temperature. After that, a balance betweenthermal expansion and evaporation determines the size of dropletat that time. When the temperature inside the droplet reaches aquasi-steady state, only evaporation is effective on determinationof droplet size. Starting from this stage, an evaporation processaccording to the D2-law becomes valid. In droplet evaporationstudies, the initial heat-up period does not have any importance

Figure 2. A sample photograph of evaporating AFUWS droplet (initialdroplet diameter: 1.156 mm).

Figure 3. Normalized temporal histories of the diameter squared ofevaporating AFUWS droplets with ambient temperature. Initial dropletdiameter: 0.930 mm at 373 K, 0.917 mm at 423 K, 0.909 mm at 473 K,0.927 mm at 523 K, 0.899 mm at 573 K, 0.925 mm at 623 K, 0.868 mmat 673 K, 0.929 mm at 723 K, 0.936 mm at 773 K, 0.889 mm at 823 K,0.941 mm at 873 K.

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because evaporation rate is generally evaluated by its subsequentperiod. Moreover, as initial droplet size becomes smaller, thisinitial heat-up period becomes negligible.19 Regarding the linearD2-law part, droplet evaporation is quantitatively characterizedby the following evaporation coefficient, Ke.

Ke ¼ dðD2Þdt

ð9Þ

This evaporation coefficient is extracted from the temporalvariation of squared droplet diameter by measuring the slope ofits linear regression part. In this study, the above procedure is alsoused to obtain the evaporation coefficient of AFUWS droplet.

3. RESULTS AND DISCUSSION

3.1. Evaporation Characteristics of AFUWS Droplet withVarious Ambient Temperatures. Figure 3 displays normalizedtemporal histories of the diameter squared of AFUWS dropletsevaporating at ambient temperatures ranging from 373 to 873 K.Note that, to minimize the initial droplet size effect on itsevaporation, the droplets having similar initial diameters (from0.868 to 0.941 mm) were selected from a number of experi-mental data. As is clearly shown in the figure, the evaporationbehaviors of AFUWS droplets are evidently dependent on theirambient temperature. On the basis of this observation, theevaporation characteristics of AFUWS droplet can be classifiedinto different cases in the following according to the variation ofambient temperature.Also, Figure 3 reveals that droplet microexplosion begins to

obviously appear at 623 K after approximately 15 s/mm2 with afluctuating behavior, and it becomes more intense as the tempera-ture increases. Figure 4 shows some sequential photographs ofevaporating AFUWS droplet at 873 K, and microexplosionphenomenon is clearly observed here. As shown in the figure,the droplet seems to expand due to bubble formation inside it.Also, droplet distortion and fragmentation are seen. Note thatthese irregular behaviors are typically observed in the evapora-tion process of the multicomponent droplet.19

Now it is worthwhile to compare current results to our previousresults on microexplosion of the UWS droplet.15 Microexplosion

of the AFUWS droplet begins at higher ambient temperaturethan that of the UWS droplet. In the case of the UWS droplet,microexplosion was observed at the temperature above 573 K.Also, the microexplosion intensity of AFUWS droplet is gen-erally weaker than that of UWS droplet. These observations arebecause the amount of less volatile component (urea) inside theAFUWS droplet (containing 20.1% urea by weight) is smallerthan that inside the UWS droplet (containing 32.5% urea byweight) so that diffusional resistance to more volatile componenttrapped inside AFUWS droplet becomes smaller than that ofUWS droplet. Therefore, in the case of AFUWS droplet, the

Figure 4. Some sequential photographs of evaporating AFUWS droplets at 873 K, which show microexplosion. Initial droplet diameter: 1.063 mm.Camera speed: 30 fps.

Figure 5. Normalized temporal variations of the diameter squared ofevaporating AFUWS droplets with each least-squares fit to the linearwater evaporation part (first stage) at both 373 and 423 K and to thelinear ammonium formate thermal decomposition part (second stage)at 423 K. For 373 K, camera speed, 10 fps; ambient temperature rangemeasured during evaporation, 367�377 K. For 423 K, camera speed,10 fps; ambient temperature range measured during evaporation,417�430 K.

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bubble formation is weak, and microexplosion is incomplete ascompared to the case of the UWS droplet.19

3.2. For Ambient Temperatures of 373 and 423 K. Figure 5presents the whole droplet lifetime history for each AFUWSdroplet evaporating at 373 and 423 K, respectively. Here, eachevaporation history of normalized droplet diameter squared isdisplayed up to 0.676 mm2/mm2 for 373 K and up to 0.525mm2/mm2 for 423 K. This is because liquid component con-stituting AFUWS droplets is completely evaporated at thesepoints, and hereafter almost no more reduction in droplet overallsize is observed.Themelting points of ammonium formate and urea are known

to be about 38921 and 406 K,22 respectively. Therefore, at ambienttemperature of 373 K, only water content will evaporate from theAFUWS droplet. Accordingly, the AFUWS droplet evaporating at373 K exhibits almost linear history as if it consists only of a singleingredient. On the other hand, at 423 K, ammonium formate andurea near droplet surface are expected to melt and thermallydecompose into ammonia and formic acid according to reaction(3) and into ammonia and isocyanic acid according to reaction(1). Therefore, the evaporation history at 423 K seems toindicate three stages: vaporization of water, thermal decomposi-tions of ammonium formate, and then urea. However, as shownin the figure, the evaporation history at this temperature can bedivided into two distinguished branches with an obvious differ-ence in their diminishing rates. During the first stage of theevaporation period, the water component first evaporates fromAFUWS droplet. Musa et al.23 compared the evaporation behaviorof the urea solution droplet to that of distilled water, and thereinalmost no difference in their first-stage evaporation was observed.During the second stage of evaporation period, the gasificationof ammonium formate may be a dominant mechanism respon-sible for diminishment in AFUWS droplet size. This view isconsistent with the result produced by Solla et al.12 who reportedthat ammonia gas evolution from ammonium formate starts atabout 388 K. After the second stage of evaporation period, there isalmost no more change in AFUWS droplet size. Hence, it isexpected that urea gasification does not occur at 423K in this study.This observation can be expected from studies of Solla et al.12 andSchaber et al.7 who reported that ammonia gas evolution from

solid urea starts about 468 K and that a vigorous gas evolutionfrom molten urea commences at 425 K, respectively. Also, in ourprevious study on evaporation of UWS droplet, gasification of ureawas not observed at this temperature.15

To estimate the evaporation coefficient of AFUWS droplet,the least-squares regression method was used with the datacorresponding to the linear part of evaporation history except forboth initial heat-up and final solidification periods. The estimatedevaporation coefficients were given with actual measurement

Figure 6. Sequential photographs of evaporating AFUWS droplet at 373 K. Initial droplet diameter: 0.930 mm. Camera speed: 10 fps.

Figure 7. Normalized temporal variations of the diameter squared ofevaporating AFUWS droplets with each least-squares fit to the linearwater evaporation part (first stage), to the linear ammonium formatethermal decomposition part (second stage), and to the linear ureathermal decomposition part (third stage) at 473, 523, and 573 K. For473 K, camera speed, 10 fps; ambient temperature range measuredduring evaporation, 470�475 K. For 523 K, camera speed, 10 fps;ambient temperature range measured during evaporation, 520�527 K.For 573 K, camera speed, 15 fps; ambient temperature range measuredduring evaporation, 571�757 K.

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data in Figure 5. Note that, in the case of 423 K, two slope data aregiven for two stages of AFUWS droplet evaporation.It must be noted that solidified deposit remains after the com-

plete depletion of liquid component evaporation from AFUWSdroplets for both 373 and 423 K. Sequential photographs includingevaporation and solidification processes of AFUWS droplet at373 K are displayed in Figure 6. Close observations after 70 sindicate that solidified deposit undergoes no change until theend of camera capture. The deposit produced at 373 K may becomposed of solidified ammonium formate and urea, because bothspecies do not melt at this ambient temperature during the wholeevaporation period. On the other hand, at 423 K, the deposit mayconsist of only solidified urea because the first urea-derivedspecies, biuret, begins to be produced from the reaction ofisocyanic acid with intact urea at ∼433 K.7 When compared tothe amount of solidified deposit of UWS droplet at 373 K15 afterwater content was completely evaporated, the remaining overallsize of the AFUWS droplet (about 0.676 mm2/mm2) was largerthan that of the UWS droplet (about 0.617 mm2/mm2). Thisobservation may be because the amount of ammonium formateand urea forming the AFUWS droplet is more than that of theUWS droplet.3.3. For Ambient Temperatures of 473, 523, and 573 K.

Figure 7 plots normalized temporal variations of the diametersquared of evaporating AFUWS droplets for ambient gas tem-peratrure of 473, 523, and 573 K. As shown in the figure,evaporation histories in this temperature range can be divided intothree distinguished stages due to the apparent difference in theirdiminishment rates.During the first and second stages of evaporation periods,

water content evaporates completely, and the ammonium for-mate thermally decomposes subsequently as in the previous case of423 K. The most distinguished feature at these ambient tempera-tures is that thermal decomposition of urea occurs during the thirdstage of evaporation period. Note that these multistage evapora-tion characteristics observed at these ambient temperatures sup-port the plausible employment of the rapid mixing model forestimating evaporation rate of UWS droplet as in Birkhold et al.16

Previously, at ambient temperature of 423 K, urea gasificationseemed not to occur. However, at ambient temperature of473�573 K, it was clearly observed after the complete depletion

of water content and the thermal decomposition of ammoniumformate component. This is because the decomposition of urea israpidly enhanced at temperature above 425 K.7 Therefore, attemperatures of 473�573 K, the reduction rate of AFUWSdroplet behavior should be respectively determined for each ofthree stages. Here, the least-squares method was also utilized tofind the slope of each stage. A comparison of three evaporationhistories presented in Figure 7 indicates that the differencesamong the slope of each stage become smaller as ambienttemperature increases.

Figure 8. Sequential photographs of evaporating AFUWS droplet at 473 K. Initial droplet diameter: 0.909 mm. Camera speed: 10 fps.

Figure 9. Normalized temporal variations of the diameter squared ofevaporating AFUWS droplets with each least-squares fit to the linearwater evaporation part (first stage) and two-point estimation to themicroexplosion part (second stage) at 623, 673, and 723 K. For 623 K,camera speed, 15 fps; ambient temperature range measured duringevaporation, 620�623 K. For 673 K, camera speed, 20 fps; ambienttemperature range measured during evaporation, 670�675 K. For 723 K,camera speed, 20 fps; ambient temperature range measured duringevaporation, 719�725 K.

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Figure 8 illustrates sequential photographs of evaporatingAFUWS droplet at 473 K, and there seems to be almost nodifference in apparent evaporation behavior for each stage exceptthe magnitude of diminishment rate. Also, at these temperatures,solidified deposit was observed to remain in the final stage as inthe previous results for the cases with 373 and 423 K. However,the size of deposit remained was significantly reduced, and itsconstituents may be different. Referring to the pyrolysis experi-ment conducted by Schaber et al.,7 the deposits generated at 473,523, and 573 K probably consist of several urea-derived productssuch as biuret, CYA, ammelide, and ammiline with undecom-posed urea itself.3.4. For Ambient Temperatures of 623, 673, and 723 K.

Figure 9 shows normalized evaporation histories for ambienttempeartures of 623, 673, and 723 K. The most prominentcharacteristic of evaporating AFUWS droplets at these ambienttemperatures is the onset of microexplosion. In Figure 9, after thelinear variation of droplet diameter squared during water eva-poration period, the droplet re-expansion is observed with afluctuating behavior. The reason for this fluctuation is that, aspreviously mentioned, the more volatile component trapped insideAFUWS droplet due to diffusional resistance is heated, whichcauses homogeneous nucleation, and thereby droplet internalpressure builds up. Here, the rates of both nucleation anddiffusion compete against each other so that they influencedroplet expansion characteristics. A comparison of three eva-poration histories reveals that more intense fluctuation occurs athigher ambient temperature. A fast internal pressure build-upcaused by rapid nucleation at higher temperature results in moreaggressive evaporation behaviors such as bigger expansion, moresevere bubble formation, and more disruptive fragmentation.Another feature to be noted at these ambient temperatures

is that two-stage type of evaporation characteristics is observedunlike the previous three-stage type for ambient temperature of473�573 K. It is because microexplosion occurs after the firstlinear evaporation period. Therefore, as shown in Figure 9,evaporation histories at 623�723 K can be divided into twodistinguished stages.To obtain the evaporation coefficient for the first stage, the

least-squares method was also applied to the linear part ofexperimental data. However, for the next evaporation stage, theuse of the least-squares method is not justified due to dropletexpansion and fluctuation so that its diminishment rate is simplyestimated using the slope determined with two end points. Notethat, in Figure 9, the difference in diminishment rates for both thefirst and the second stages becomes smaller as ambient tempera-ture increases. Particularly, for ambient temperature of 723 K, themagnitude of the first stage evaporation rate is almost equal tothat of the second stage rate as can be seen in Figure 9.

Photographs of solidified deposits remaining after the com-plete depletion of liquid component at temperatures from 623 to873 K are displayed in Figure 10. As shown in the pictures,the amount of deposit remained was considerably reduced asambient temperature rises from 623 to 673 K. This may beattributed to the fact that the decomposition of CYA is com-pleted between 648 and 653 K and the sublimation and decom-position of ammelide significantly occurs at temperatures from623 to 673 K.7

3.5. For Ambient Temperatures of 773, 823, and 873 K.Figure 11 presents normalized evaporation histories of AFUWSdroplets for ambient temperature of 773, 823, and 873 K. In thistemperature range, the second stage diminishment rate becomesfaster than the first stage rate. This is primarily because dropletmicroexplosion occurs more aggressively as compared to the casefor 623�723 K. Figure 11 reveals more intense second-stageevaporation behaviors than does Figure 9. Note that the differ-ence in diminishment rates of both the first and the second stagesbecomes larger as ambient temperature rises from 773 to 873 K.

Figure 10. Photographs of solidified deposits remaining after the complete evaporation of AFUWS droplets for various ambient temperatures. Initialdroplet diameter: 0.925 mm at 623 K, 0.868 mm at 673 K, 0.929 mm at 723 K, 0.936 mm at 773 K, 0.889 mm at 823 K, 0.941 mm at 873 K.

Figure 11. Normalized temporal variations of the diameter squared ofevaporating AFUWS droplets with each least-squares fit to the linearwater evaporation part (first stage) and two-point estimation to themicroexplosion part (second stage) at 773, 823, and 873 K. For 773 K,camera speed, 30 fps; ambient temperature range measured duringevaporation, 767�779 K. For 823 K, camera speed, 30 fps; ambienttemperature range measured during evaporation, 821�828 K. For 873 K,camera speed, 30 fps; ambient temperature range measured duringevaporation, 880�868 K.

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In this experiment, solidified deposits were still observedto remain up to 773 K, while almost no deposit was found forambient temperatures of 823 and 873 K. It is known that ammelideis completely eliminated at about 873 K, while ammeline requirestemperatures over 973 K.7 Therefore, it can be deduced that quitea tiny amount of ammeline is produced during the evaporation ofAFUWS droplet. If ammeline was produced considerably, then acertain amount of deposit would be observed even at tempera-tures exceeding 823 K. Also, it is known that CYA is complete-ly eliminated below 673 K.7 As a consequence, the depositsremaining at temperatures from 673 to 773 K are mainlycomposed of ammelide.

3.6. Initial Diameter Effect on the Evaporation of AFUWSDroplet. Figures 12�14, respectively, present each stage dimin-ishment rate coefficient for various AFUWS droplet sizes eva-porating at temperatures from 373 to 873 K in increments of50 K. It should be noted that the range of ambient temperaturemeasured during evaporation is somewhat different in eachfigure. Averaged lower and upper bounds for ambient tempera-ture and their standard deviations are given in Table 1.In general, as shown in Figures 12�14, it is evident that at

lower ambient temperature, the effect of initial droplet diameteron evaporation coefficient is almost negligible. Yet at higherambient temperature, the effect of initial droplet diameter ismore pronounced, and evaporation coefficients increase withinitial droplet diameter. In addition, the least-squares fits indicatethat the increment of each coefficient with initial dropletdiameter becomes larger as ambient temperature increases. Thisobservation is consistent with the result studied by Xu et al.24

Figure 12. Evaporation coefficients of the water evaporation part forvarious initial diameters of AFUWS droplets and ambient temperatures.

Figure 14. Evaporation coefficients of the urea thermal decompositionand microexplosion parts for various initial diameters of AFUWSdroplets and ambient temperatures.

Figure 13. Evaporation coefficients of the ammonium formate thermaldecomposition part for various initial diameters of AFUWS droplets andambient temperatures.

Table 1. Range of Ambient Temperature Measured duringEvaporation and Standard Deviations (Presented inParentheses)

temperature setting, K temperature variation range, K

373 367.9(1.3)�377.6(1.3)

423 416.5(3.1)�428.3(3.6)

473 467.9(2.9)�480.9(4.0)

523 521.0(1.0)�527.8(2.1)

573 571.9(1.5)�575.3(1.3)

623 620.8(0.9)�624.3(1.0)

673 668.9(0.8)�674.8(1.4)

723 716.9(2.2)�725.8(1.6)

773 765.6(3.1)�777.9(1.6)

823 816.2(3.3)�828.8(1.9)

873 864.7(3.5)�878.8(2.3)

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who reported that the evaporation coefficient increased in hotambiences with raising initial droplet diameter and that theevaporation coefficient increase with increasing the initial dropletdiameter was larger at higher temperatures.Also, Figure 15 shows a comparison of evaporation coefficients

of urea thermal decomposition part for UWS droplets in ourprevious study15 and AFUWS droplets in current study at tem-peratures from 473 to 673 K. At ambient temperatures of 473�573 K, both rate coefficients for UWS and AFUWS droplets arevery similar. However, as shown in the figure, there is somedifference between diminishment rates for UWS and AFUWSdroplets at temperatures above 623 K. The reason for thisdistinction is that, as previously mentioned, microexplosion in-tensity of AFUWS droplet is generally weaker than that of theUWS droplet. Therefore, in the case of AFUWS droplet,evaporation coefficients were smaller than that for the UWSdroplet at temperatures above 623 K.

4. CONCLUSIONS

In this study, the evaporation behavior of AFUWS droplet forapplication to SCR is investigated using suspended droplet experi-ment. Through a number of repeated measurements, evaporationrates were extracted for various initial droplet diameters andambient temperatures. As a consequence, the current study helpsto understand the evaporation characteristic of AFUWS dropletquantitatively as well as qualitatively. Furthermore, this studyprovides some empirical data required in modeling or simulationworks on SCR system using ammonium formate�urea�watersolution as a reducing agent. A summary of the major results is asfollows.(1) On the basis of the current observations, the evaporation

behavior of the AFUWS droplet can be categorized intodifferent cases according to ambient temperature. Atambient temperature of 373 K, the AFUWS dropletexhibits almost linear history. At 423 K, the UWS dropletis divided into two distinct stages such as evaporation ofwater content and thermal decomposition of ammonium

formate. At 473�573 K, thermal decomposition of ureastarts to take place so that the three-stage type of evapora-tion characteristics is observed with obvious differentevaporation rates. At 623�723 K, weak microexplosionappears, while the first-stage rate exceeds the second-stagerate. At 773�873 K, strong microexplosion happens,while the second-stage rate exceeds the first-stage rate.

(2) After the complete depletion of liquid componentconstituting the AFUWS droplet, solidified deposit isobserved to remain at temperatures below 773 K. Itsamount is reduced with increase in ambient temperature,while there is almost no deposit remaining at tempera-tures above 823 K.

(3) The diminishment rate of each evaporation stage in-creases as the initial size of AFUWS droplet increases.Also, the increment of each evaporation coefficient withinitial droplet size becomes larger as ambient temperatureincreases.

’AUTHOR INFORMATION

Corresponding Author*Tel.:þ82-42-350-5754. Fax:þ82-42-350-3710. E-mail: [email protected].

’ACKNOWLEDGMENT

This work was supported by the Midcareer ResearcherProgram through the NRF grant funded by MEST (2010-0000353).

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