Urea in Water-in-Oil Emulsions for Ammonia Production

54 International Journal of Plasma Environmental Science and Technology Vol.3, No.1, MARCH 2009Urea in Water-in-Oil Emulsions for Ammonia ProductionG. Prieto 1, 2 , Y. Iitsuka 1 , H. Yamauchi 1 , A. Mizuno 1 , O. Prieto 2 , and C. R. Gay 21 Department of Ecological Engineering, Toyohashi University of Technology, Japan2 Department of Chemical Engineering, National University of Tucumán, Argentine RepublicAbstract—Most Selective Catalytic Reduction (SCR) systems to remove NO x from diesel exhaust use urea solutions toprovide the reducing agent ammonia. The use of urea solutions avoids the storage of liquid ammonia on-board the vehicle,but has some drawbacks as freezing problems, and limited dosing capability in exhaust at temperatures below 200 °C. Asan alternative method for ammonia production, a dielectric barrier discharge (DBD) plasma reactor packed with Al 2 O 3 orTiO 2 was studied. The reactor was driven by a positive pulse discharge at a constant temperature of 100 °C. Ammoniaproduction was compared for a) urea powder mixed with pellets of Al 2 O 3 , and humid nitrogen gas flow b) Al 2 O 3 pellets,dry nitrogen gas flow, and urea-water-in-oil emulsion, and c) TiO 2 pellets, dry nitrogen gas flow, and urea-water-in-oilemulsion. The experimental results demonstrated that the highest concentration of ammonia were obtained with ureawater-in-oilemulsion, when the reactor was packed with TiO 2 , for the highest applied voltage (15 kV), at the highestfrequency evaluated (5 kHz), and with a low carbon dioxide production.Keywords—Ammonia production, Urea pyrolysis, Urea emulsion, DBD reactor, Non-thermal plasmaI. INTRODUCTIONNitrogen oxides (NOx) are common byproducts ofinternal combustion engines. Because NOx contributes tosmog, ozone at ground level, and acid rain, many nationsare gradually strengthening their regulations about NOxemissions. The United States for example is going toprohibit the circulation of cars and light trucks (gasolineand diesel) emitting more than 0.07 gram of NOx permile. Also, very large diesel engines are being used moreand more for generating electrical power in remotelocations such as along pipelines and in oil fields. Theyare also being used for load leveling of the power used inurban areas. These stationary engines, as well aslocomotive engines, will also come under more stringentemissions control requirements in the near future [1-3].Most combustion processes use air as the oxidant.Due to the high temperatures involved in the exothermicreactions with air, and with nitrogen and oxygenavailable, nitric oxides are produced. Nitric oxide (NO)and nitrogen dioxide (NO 2 ) are two of the major pollutantgases produced by combustion processes.Since the early 1970s, the importance of SelectiveCatalytic Reduction (SCR) of oxides of nitrogen (NOx)by ammonia (NH 3 ) has grown in the economical andtechnological aspects, being nowadays the leading aftertreatmenttechnology for NOx removal process in dieselengines for stationary and mobile applications. It iscalled “Selective” because it does not reduce the excessoxygen which is typical to a lean exhaust in a dieselengine.In an SCR system, an aqueous urea solution isinjected into the exhaust where it evaporates anddecomposes to ammonia and mixes with the exhaustCorresponding author: G. Prietoe-mail address: gprieto@herrera.unt.edu.arAccepted; March 25, 2009before passing through a catalyst. For the emergingautomotive SCR systems, the catalyst has to operateunder much wider temperature window than in stationaryapplications, and the dosage system has to deliver to thecatalytic converter the required amount of urea solutiondependent on the actual transient duty cycles.SCR is the process in which nitrogen oxides in theexhaust gas of diesel engine are reduced to nitrogen andwater by reacting with ammonia in the presence ofoxygen:4NO + 4NH 3 + O 2 → 4N 2 + 6H 2 O (1)The general reducing agent for SCR process isammonia (NH 3 ). Handling anhydrous NH 3 requiresextreme caution, and would not be practical forautomotive applications, although it is used in somestationary applications. Urea is a crystalline powder thatis usually introduced to the exhaust stream as a solution32.5 % by weight with water. Since this technology onlyrequires enough reductant to treat the NOx beingproduced, the amount of urea injected into the exhauststream is very small. Injecting a solution of urea in wateris, at present, more convenient than measuring the solidform [3-9].Other methods for nitric oxides reduction for the postcombustion gas treatment are: selective non-catalyticreduction (SNCR) [10], non-selective catalytic reduction(NSCR), scrubbing methods, and techniques usingelectric fields [11]. Other possibilities include combiningSNCR and SCR techniques.In general, the SNCR techniques involve theinjection of one or more chemical agents downstream ofthe primary combustion zone. The reduction of nitricoxide by these processes is a complex set of chemicalreactions between the radicals produced by the injectedchemicals and the products of combustion. A necessaryprerequisite for an effective chemical agent is thepresence of at least one nitrogen atom in its molecularstructure. The chemistry is affected by the specific agent,

Prieto et al. 55temperature, pressure, concentrations of species, andresidence time [12].There are three main SNCR processes currentlybeing implemented: (1) the thermal DeNOx processwhich uses the injection of ammonia (NH 3 ) into the hotexhaust gas. The form of the ammonia may be asaqueous (with water) solution or as anhydrous (no water).This process is used in exhaust streams such as fromfurnaces and boilers [13, 14]; (2) the RAPRENOxprocess which uses solid cyanuric acid ((HNCO) 3 )injected into the hot exhaust gases. The solid cyanuricacid sublimates at about 650 K to give gaseous isocyanicacid (HNCO). The isocyanic acid initiates a series ofreactions in appropriate conditions which result in theremoval of nitric oxide [15]; and (3) the NOxOUTprocess which uses urea (NH 2 CONH 2 ). In this process,typically either solid urea or a urea solution (with water)is injected into the hot exhaust gases. Urea maydecompose into HNCO and NH 3 under certain conditions[16, 17].For conventional SNCR processes, either ammoniaor urea have been proposed as the reducing agent.Ammonia has been widely used in SNCR processes, butin some forms it is both corrosive and toxic. Ureacontinues to be considered as an alternative to ammonia.The use of urea, therefore, is of interest. The two mostcommon methods for introducing urea into the exhaustgases is by injection of dry urea, or injection of a ureawatersolution. Decomposition of dry urea in nitrogentakes place at temperatures of about 720 K, and the ureadecomposes into roughly equal amounts of ammonia(NH 3 ) and isocyanic acid (HNCO) [16]:NH 2 CONH 2 → NH 3 + + HNCO (2)Urea-water solutions have a different decompositionpath than dry urea, producing ammonia as the mainproduct [5, 18, 19]:NH 2 CONH 2 + H 2 O → 2NH 3 + +CO 2 (3)Ammonia (NH 3 ) as well as aqueous solutions of urea((NH 2 ) 2 CO) are the most widely used reducing agents forSCR reduction of NOx. Urea may be considered as aNH 3 storage solid compound which is producedcommercially by reacting ammonia with carbon dioxide.Adoption of urea-SCR over NH 3 -SCR could bepreferable due to a number of drawbacks associated withthe use of ammonia. Ammonia is corrosive and toxic innature. It is also a primary as well as a secondarypollutant. Use of ammonia entails careful and accuratehandling requiring pressure. In order to introduce NH 3into the exhaust gas stream, accurate dosage controlmechanism is required which can be expensive. Urea onthe other hand is non-toxic being extensively used inagriculture as well as in the industry and it is available ina number of quality grades.Among many metal oxide supports, TiO 2 support hasbeen found to be highly effective for the selectivecatalytic reduction of NOx with NH 3 because of itsdurability to sulfur compounds. The action of sulfur onthe TiO 2 supported catalyst has been found to evenenhance the level of NO removal. Hence, due to its highactivity and durability to sulfur compounds, V 2 O 5supported on TiO 2 is well known to be the most effectiveand widely used commercial catalyst for the SCRprocesses. A ternary catalyst, V 2 O 5 -WO 3 /TiO 2 , was alsorecently introduced. Promoters such as WO 3 , SiO 2 andMoO 3 are usually added to V 2 O 5 /TiO 2 catalyst in order toenhance the catalyst activity. The promoters lead to theformation of acid sites on the surface of the catalyst, andthe catalysts exhibit a higher catalytic activity than thatof a single TiO 2 supported vanadium catalyst. Theaddition of WO 3 provides some poison resistance andimproves NH 3 oxidation [20].Urea decomposition does not reach completion in thegas phase at temperatures that are prevalent during alight-duty diesel engine operation (below 300 ºC).Stradella et al. [21] studied the thermal decomposition ofurea and related compounds. Their investigations werebased on TGA and DSC, together with the evolved gasanalysis (EGA) with the help of an FTIR spectrometer[21, 22]. They were able to identify biuret, ammelide andcyanuric acid as intermediate compounds. Schaber et al.[23] found that the urea is melted at 133 ºC andprecipitates start to form prior to 225 ºC.It is generally accepted that urea decomposes in twosteps that are physically separated in time and space: thethermolysis of finely sprayed urea into ammonia andisocyanic acid (HNCO), which would occur along theexhaust pipe prior to the catalyst, and the hydrolysis ofisocyanic acid, which would occur on the catalyst surface.These two steps correspond to the overall ureadecomposition(NH 2 ) 2 CO + H 2 O → 2NH 3 + CO 2 (4)in which 1 mol of urea would generate 2 mol of ammonia.The NH 3 :NO stoichiometric molar ratio in typical SCRreactions is 1:2. The thermal decomposition of urea canform stable decomposition products. Some of these stableproducts do not further decompose to produce ammonia[20].In the pyrolysis of urea, no significant reaction isobserved when heating from room temperature up to133 °C (urea melting point). Urea typically melts withdifficulty, and usually a complete melt is not achieveduntil 135 °C. Mass loss begins about 140 °C and isprimarily associated with urea vaporization.(NH 2 ) 2 CO (m) + heat → (NH 2 ) 2 CO (g) (5)At 152 °C, urea decomposition begins, as noted byvigorous gas evolution from the melt, giving ammonia(NH 3 ) and isocyanic acid (HNCO),(NH 2 ) 2 CO (m) + heat → NH 3 (g) + HNCO (g) (6)Biuret ((NH 2 ) 2 NH(CO) 2 ) is also produced at thistemperature,

56 International Journal of Plasma Environmental Science and Technology Vol.3, No.1, MARCH 2009(NH 2 ) 2 CO (m) + HNCO (g) → (NH 2 ) 2 NH(CO) 2 (s) (7)At temperatures above 190 °C, other solidscompounds are produced primarily from biuret and inparallel reactions: cyanuric acid ((HNCO) 2 ), and thetriazides ammelide (C 3 H 4 N 4 O 2 ), and ammeline(C 3 H 5 N 5 O). The biuret itself is a result of prior reactionof HNCO with urea. The HNCO is a result of ureadecomposition at temperatures over 152 °C. Cyanuricacid and ammelide first appear at about 175 °C, but thereaction rate is very slow based on the amounts formed.At temperatures above 190 °C, several parallel processescompete for the generation of products. These routes,where direct biuret decomposition is considered, areconsistent with the copious evolution of gases observedfrom the melt between 190 and 210 °C. As thetemperature exceeds 210 °C, a white precipitate begins toform a sticky solid matrix. Production of cyanuric acidand ammelide continues, although reaction rates decreaseas the solid matrix forms. Production of cyanuric acidappears complete at 275 °C, after which sublimation andeventual decomposition occur. Ammelide and ammelinecontinue to increase in mass up to 350 °C. Attemperatures above 360 °C, sublimation anddecomposition processes begin to dominate [18].From the literature two different ways for thethermal decomposition can be derived: (a) evaporation ofmolten/solid urea to gaseous urea, which decomposes inthe gas phase into NH 3 and HNCO or (b) the direct wayfrom molten/solid urea to gaseous NH 3 and HNCO.These two ways differ mainly in the way how the heat ofthe reaction is provided. The results reveal that thethermal decomposition of urea is limited by kinetics [20].Fang et al. [20] also investigated the effect ofmoisture on urea decomposition process and found thatthe moisture could assist the hydrolysis of HNCO only inthe temperature region below the first decompositionstage (below 250 o C). The DRIFTS measurementsshowed that the final brown color product formed at 450o C could be a chemical complex of polymeric melamineswith high molecular weights which might actually blockthe active sites on the catalyst surface. Their studyshowed that urea thermolysis exhibits two decompositionstages, involving ammonia generation and consumptionrespectively. Decomposition occurring after the secondstage leads to the production of melamine complexes,that hinder the overall performance of the catalyst. Theyasserted that polymeric melamine complexes can beformed both with and without the catalyst and they donot undergo further decomposition (at least up to 320 o C).There is not complete consensus among the authorsabout in which state of aggregation urea appears after theevaporation of water and during the thermaldecomposition: Yim et al. [24] indicated liquid orgaseous urea; Schaber et al. [23] reported that moltenurea evaporates to gaseous urea at temperatures above413 K, but mainly decompose directly to NH 3 andHNCO above 425 K; Koebel et al. [25] revealed thatatomization of urea-water-solution (UWS) in hot exhauststream yields to solid or molten urea.For automotive applications, Birkhold et al. [19]claim that the urea-water-solution based SCR is apromising method for control of NOx emissions. UWScontaining 32.5 wt.% urea is sprayed into the hot exhauststream, for the subsequent generation of NH 3 in the hotexhaust gas. As the evaporation and spatial distributionof the reducing agent upstream the catalyst are crucialfactors for the conversion of NOx, the dosing system hasto ensure the proper preparation of the reducing agent atall operating conditions.Specific concerns with the ammonia process includethe storage, handling, and delivery of the ammonia. Also,any ammonia not consumed in the process may beemitted (“ammonia slip”) as a result of this process. Forthese and other reasons, alternative agents have beenproposed over the years. Two of these that have receivedsignificant interest include cyanuric acid ((HNCO) 3 ) andurea ((NH 2 ) 2 CO) [12].The main goal of the present research is to developan alternative method for ammonia production, using adielectric barrier discharge (DBD) plasma reactor packedwith Al 2 O 3 or TiO 2 . The reactor is driven by a positivepulse discharge at a constant temperature of 100 ºC.Ammonia production is evaluated for a) urea powdermixed with pellets of Al 2 O 3 , and a humid nitrogen gasflow b) Al 2 O 3 pellets, a dry nitrogen gas flow, and ureawater-in-oilemulsion fed with a micropump, and c) TiO 2pellets, a dry nitrogen gas flow, and urea-water-in-oilemulsion fed with a micropump. Urea emulsions presentgreat advantages: are easy to be prepared, require lowamount of water, easy for handling, easy for feedingwith a micropump, not freezable and not stackable intothe piping.II. EXPERIMENTAL SETUPThe diagram of the experimental setup is depicted inFig. 1. The reactor is a concentric dielectric barrierdischarge (DBD) packed with catalyst pellets of γ-Al 2 O 3 ,or TiO 2 , 2.5-4 mm in diameter. The high voltageelectrode is a stainless steel rod of 25 mm in length and11 mm in outer diameter. The reactor, depicted in Fig. 2,is placed in a quartz glass tube of 50 mm in length andinner diameter of 19 mm. A stainless steel sheet wrappedon the outer surface of the quartz glass tube is the groundelectrode.The reactor was fed with nitrogen, as a carrier gas, ata constant flow rate of 2000 mL/min, controlled with amass flow controller. For the experiments with ureapowder, humid gas was obtained by bubbling in deionizedwater at room temperature, and the water contentin the gas stream was measured using a gas-detectingtube (Gastec Co).In the case of Urea Powder, 1 g of urea powder and 3g of hexadecane were packed in the reactor mixed withthe catalyst pellets. In the case of Urea Emulsion, 0.1 g ofurea water-in-hexadecane was fed with a micropump at amass flow rate of 0.1 g/min. The urea water-in-oilemulsion was prepared by using emulsifiers “Tween80”

Prieto et al. 59NH3 concentration (ppm)15000100005000Hold ValuesT 13554153 10Frequency (kHz) 25 Voltage (kV)Fig. 3. Urea powder (Packed-Bed Al 2 O 3 ).Surface plot of NH 3 concentration (ppm) vs Frequency (kHz);Voltage (kV), for a Temperature of 135 o C.NH3 concentration (ppm)12000100008000600040002000010 11 12 13 14 15 16NH3 Urea powder (Packed-Bed Al2O3)NH3 Urea emulsion (Packed-Bed TiO2)Applied Voltage (kV)NH3 Urea emulsion (Packed-Bed Al2O3)Fig. 6. Ammonia concentration (ppm) as a function of the AppliedVoltage (kV) for Urea powder (Packed-Bed Al 2 O 3 ); Urea emulsion(Packed-Bed Al 2 O 3 ); Urea emulsion (Packed-Bed TiO 2 ).NH3 concentration (ppm)600040002000054 14312Frequency (kHz) 210 Voltage (kV)Fig. 4. Urea emulsion (Packed-Bed Al 2 O 3 ).Surface plot of NH 3 concentration (ppm) vs Frequency (kHz);Voltage (kV)NH3 concentration (ppm)1000050000543Frequency (kHz) 21210experiments with urea powder, or from the emulsion inthe case of the experiments with urea emulsion, makesthe iso-cyanic acid undergo in a second reaction with onemolecule of water (hydrolysis) giving one molecule ofNH 3 and one molecule of CO 2 .The CO 2 produced experiments the same sensibilityto the variables tested, as the NH 3 produced. For theexperiments carried out with Urea Powder, the highestconcentrations of CO 2 (at 135 o C), was about 6000 ppmFor the experiments carried out with Urea Emulsion andTiO 2 (at 100 o C), the highest concentrations of CO 2 wereabout 2800 ppm, and with Urea Emulsion and Al 2 O 3 (at100 o C), the highest concentrations of CO 2 were about2000 ppm. Fig. 7 displays the carbon dioxideconcentration as a function of the applied voltage for thethree cases studied, at a temperature of 100 o C for urea14Voltage (kV)Fig. 5. Urea emulsion (Packed-Bed TiO 2 ).Surface plot of NH 3 concentration (ppm) vs Frequency (kHz);Voltage (kV)CO2 concentration (ppm)600050004000300020001000010 11 12 13 14 15 16CO2 Urea powder (Packed-Bed Al2O3)CO2 Urea emulsion (Packed-Bed TiO2)Applied Voltage (kV)CO2 Urea emulsion (Packed-Bed Al2O3)Fig. 7. Carbon Dioxide concentration (ppm) as a function of theApplied Voltage (kV) for Urea powder (Packed-Bed Al 2 O 3 ); Ureaemulsion (Packed-Bed Al 2 O 3 ); Urea emulsion (Packed-Bed TiO 2 ).emulsion, and at 135o C for urea powder. Thetemperature of 135 o C was selected in order to comparethe behavior with urea powder and with urea emulsionfor the different packed-bed materials.The whole evaluation of the experiments carried out,besides to demonstrate the feasibility of using solid ureain a water-in-oil emulsion, which makes this process acandidate for in situ application, has the advantage ofusing an emulsion with a very low content of water, notfreezable and with constant ammonia provision, besidesto generate not a high concentration of carbon dioxide.IV. CONCLUSIONA new method for ammonia production in situ wassuccessfully developed. A dielectric barrier discharge(DBD) packed with catalyst pellets of γ-Al 2 O 3 , or TiO 2and solid urea in the form of powder or in a water-in-oilemulsion were used to study the ammonia production,under experimental design methodology. The plasmareactor was driven by positive pulsed high voltagegenerated by a pulse power generator. By applying highvoltage, an intense pulse discharge plasma is generated atthe contact points of the catalyst pellets without arcing.The intense electric field produced is high enough formelting the urea powder or urea emulsion giving place tothe reaction of urea vaporized to form ammonia and isocyanicacid which in turn, due to the presence of water

60 International Journal of Plasma Environmental Science and Technology Vol.3, No.1, MARCH 2009from the humid carrier gas in the case of solid urea orwater from the emulsion in the case of urea emulsion,undergo in a secondary reaction in gas phase to give onemolecule of ammonia and one molecule of carbondioxide. The maximum ammonia concentrations obtainedper gram of solid urea fed, were about 14000 ppm forurea powder on γ-Al 2 O 3 (at 135 ºC), about 9000 ppm forurea emulsion on TiO 2 (at 100 ºC), and about 5000 ppmfor urea emulsion on γ-Al 2 O 3 (at 100 ºC).The whole evaluation of the experiments carried out,besides to demonstrate the feasibility of using solid ureain a water-in-oil emulsion, which makes this process acandidate for in situ application, has the advantage ofusing an emulsion with a very low content of water, notfreezable and with a constant ammonia provision. 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