Urea Water Solution Research Articles

Computational Study of Urea–Water Solution Sprays for the Analysis of the Injection Process in SCR-like Conditions

Exhaust after-treatment devices for NOx reduction have become mandatory for achieving strict diesel emission standards. The Selective Catalytic Reduction (SCR) method has proven to be efficient in ...

Reduced simulation of the evaporation and decomposition of droplets and films of urea-water solution in exhaust gas environment

Reduced models for the evaporation and decomposition of urea-water solution in exhaust gas systems are developed to improve computational efficiency for detailed simulations. The models describe a droplet or wall film of urea water solution in hot exhaust gas, which is simulated in detail including gas phase chemistry and an evaporation model for the urea decomposition. The time scales of all involved processes are analyzed and it is found that only during the phase of urea decomposition transport and chemistry are coupled, while there is no significant chemistry during the preceding phase of water evaporation and no significant transport after complete evaporation and decomposition. Based on these results reduced models for all phases are suggested and a two-dimensional tabulated model for the processes in the gas phase and the urea decomposition phase is developed, which accurately reproduces the results of detailed simulations.

Simulation Study on Urea Thermal Decomposition for SCR Process of Thermal Power Plant

Urea was used for SCR reducer in many power plants, but its pyrolysis process was very complex, and this process was showed by the TGA-DSC experimental results. An effective method for urea-water solution thermal decomposition was investigated in this paper. The factors of temperature, O2 concentration and catalyst had been mainly studied and analyzed. It can be concluded that in range of 200-650 °C, with temperature increasing, the NH3 conversion rate increased, HNCO could be hydrolyzed to produce NH3 and CO2 when temperature was higher than 400 °C, and the NH3 conversion rate was higher than 50%, and the maximal NH3 conversion rate was 85.1% when temperature was 650 °C; NH3 conversion rate decreased with O2 concentration in carrier gas increasing. More NH3 was produced by hydrolysis of HNCO on SCR catalyst below 400 °C, because the hydrolysis of HNCO took place at temperature lower than 400 °C mainly, however, the promotion of catalyst for hydrolysis of HNCO was not observed at higher temperature.

Theoretical study of the gas-phase thermal decomposition of urea

Urea is used in Selective Catalytic and Non Catalytic Reduction (SCR and SNCR) methods for NOx abatement in post-combustion processes. The decomposition of urea-water solution in exhaust systems leads to the formation of NH3, which is the NOx reducing agent. However, the decomposition of urea can also lead to the formation of unwanted pyrolysis by-products, particularly in region where the temperature and pressure conditions are not optimal or via wall-spray interactions. In this study, a gas phase kinetic model for urea pyrolysis is proposed to explain the growth of pyrolysis by-products of urea, and in particular, the route of formation of the major product: isocyanuric acid. Systematic theoretical calculations, using electronic structure calculations and transition state theory, were performed to explore all the possible unimolecular and bimolecular reactions of the initial species. Kinetic modeling was used to select the relevant reaction pathways at each growing step, and their rate constants were refined using CCSD(T)/CBS//B2PLYP-D3/cc-pvTZ calculations. Quantum calculations showed that the postulated growing schemes of the literature, based on the successive urea + HNCO ⇆ biuret, biuret + HNCO ⇆ triuret followed by the cyclization of triuret into isocyanuric acid are not energetically favored. It is observed that the most favored reaction route to isocyanuric acid involves carbamimidic acid, the urea tautomer, that first yields biuret in a reaction with HNCO and then is involved in the formation of triuret by reacting with a decomposition product of biuret. Isocyanuric acid is produced from the reaction between HNCO and the same decomposition product of biuret. This new mechanism of formation of isocyanuric acid is tested at different conditions to explore its most favored conditions of formation in the gas phase. Exploratory simulations of a pseudo condensed phase are also performed to qualitatively simulate condensed phase experiments and explain the observed pyrolysis yields.

Modeling of Urea Decomposition in Selective Catalytic Reduction (SCR) for Systems of Diesel Exhaust Gases Aftertreatment by Finite Volume Method

This study presents modeling of selective catalytic reduction (SCR) for systems of diesel exhaust gases aftertreatment. The main purpose of this work is to develop the modeling approach that allows accurate prediction of urea–water solution behavior (UWS) in the real diesel exhausts in temperature range 373 K to 873 K. The UWS is a key element of catalytic reduction of diesel NOx which utilizes ammonia as reducing specie. The finite volume method (FVM) extended by the nonrandom two liquids (NRTL) phase equilibrium model was used to perform the calculations. The results obtained were verified with experimental measurements. The comparison show that the NRTL extension introduced in this work allows reproducing the actual process conditions in the diesel exhaust environment. The accuracy of the results permits the model to be used for the design purposes and simulation approaches as well.

Investigation into Low-Temperature Urea-Water Solution Decomposition by Addition of Titanium-Based Isocyanic Acid Hydrolysis Catalyst and Surfactant

<div class="section abstract"><div class="htmlview paragraph">Mitigation of urea deposit formation and improved ammonia production at low exhaust temperatures continues to be one of the most significant challenges for current generation selective catalytic reduction (SCR) aftertreatment systems. Various technologies have been devised to alleviate these issues including: use of alternative reductant sources, and thermal treatment of the urea-water solution (UWS) pre-injection. The objective of this work was to expand the knowledge base of a potential third option, which entails chemical modification of UWS by addition of a titanium-based urea/isocyanic acid (HNCO) decomposition catalysts and/or surfactant to the fluid. Physical solid mixtures of urea with varying concentrations of ammonium titanyl oxalate (ATO), oxalic acid, and titanium dioxide (TiO<sub>2</sub>) were generated, and the differences in NH<sub>3</sub> and CO<sub>2</sub> produced upon thermal decomposition were quantified. It was found that addition of 2.0 mol % ATO to urea increased CO<sub>2</sub> production by 821 % and NH<sub>3</sub> production by 96 % at temperatures ≤ 215 °C, indicating significantly enhanced hydrolysis of HNCO. Conversely, it was demonstrated that addition of oxalic acid or TiO<sub>2</sub> to urea exhibited little effect on NH<sub>3</sub> and CO<sub>2</sub> production, indicating both the importance of titanium in the mixture and adequate catalyst-substrate contact. Previous work by the authors demonstrated that addition of ATO to UWS did indeed result in decreased deposit formation in the exhaust system when compared to conventional UWS, and the extent of deposit reduction could be further enhanced by addition of surfactant to the solution. In this work, six distinct surfactants were added to UWS both with and without ATO, and the effect on deposit formation was quantified. In all cases, addition of both surfactant and ATO resulted in superior deposit mitigation when compared to UWS treated with ATO or surfactant individually. Finally, various ammonium peroxo-hydroxo titanium coordination complexes were synthesized and evaluated for catalytic urea decomposition using TGA-FTIR and compared against the observed activity of ATO.</div></div>

A Fast Modeling Approach for the Numerical Prediction of Urea Deposit Formation

<div class="section abstract"><div class="htmlview paragraph">The permanently tightening emission regulations for NOx pollutants force further development of automotive exhaust aftertreatment systems with selective catalytic reduction (SCR). Of particular interest is the long-term reliability of SCR systems with regard to unfavorable operating conditions, such as high injection rates of urea water solution (UWS) or a low exhaust gas temperature. Both of them may lead to formation of solid deposits which increase backpressure and impair ammonia uniformity.</div><div class="htmlview paragraph">A fast modeling approach for numerical prediction of deposit formation in urea SCR systems is desired for optimization of system design. This paper presents a modified methodology for the modeling of deposit formation risk. A new determination of the initial footprint of the spray, where the deposit formation is inhibited, is proposed. The threshold values for the evaluation of the film transport were validated based on experimental results. To achieve a more realistic simulation in terms of wall wetting and cooling, the impingement heat transfer as well as the impingement model were modified based on optical investigations.</div><div class="htmlview paragraph">In order to accomplish the modeling of deposit formation with typical time ranges of several minutes, a recently developed injection source approach was applied. The substitution of the Lagrange-particles with source terms of mass, momentum and energy allowed to reduce simulation time by a factor of 30. The presented modeling approach was validated against both, the experimental data from an optical box and an exhaust aftertreatment system. The comparison of measured and simulated results shows the capability of the presented modeling approach to predict the position and the severity of solid deposits.</div></div>

Spray evaporation of urea–water solution: Experiments and modelling

Selective catalytic reduction is a key technology for reducing nitrogen oxide (NOx) emissions of diesel engines, which requires the injection of urea–water solution (UWS) into the hot exhaust gas. The main objective of this paper is the development of a methodology for predicting the evaporation of urea-water sprays. For this reason a generic experiment was conducted in order to provide validation data under realistic operating conditions. The configuration is based on a twin-fluid atomiser, which is used to inject UWS into a hot pipe flow. Microscopic imaging was applied in conjunction with particle tracking velocimetry to record droplet size and velocity distribution profiles. Using this approach, droplets of a diameter as small as 4 μm were successfully detected at operating conditions of the gaseous phase of up to 773 K and 0.24 MPa. The analysis of droplet data at different positions downstream of the atomiser revealed details of the evaporation characteristics of UWS. In particular, a characteristic droplet size is proposed, which may serve to characterise the overall progress of UWS spray evaporation. Using this criterion, the comparison of experimental results to numerical predictions is straightforward. A numerical model, which is based on one-dimensional droplet kinematics as well as a rapid mixing evaporation model, was developed to describe the most important physical processes of the experimental configuration. A distinct advantage of the experimental methodology is that the velocity of the gas phase can be approximated by the velocity of the smallest detectable droplets with sufficient accuracy. This information was used to calculate the relative velocity between larger droplets and the hot gas, which is an important parameter considering convective heat and mass transfer in the evaporation model. The results obtained via the numerical model were found to agree very well with experimentally obtained evaporation characteristics.

Experimental study and numerical simulation of impact factors on spray impingement of urea water solution

The impingement behavior is inevitable during the injection process of urea water solution in selective catalytic reduction aftertreatment system of diesel engine. The impingement contributes to the formation of liquid film on the pipe wall and mixer, and it will block the pipe and reduce the urea water solution decomposition efficiency. In this work, the morphological changes of the spray during the impingement process were captured by the CCD camera. The liquid film development under different injection flow rates, compressed air pressures, injection heights, injection angles and metal wall temperatures were simulated and analyzed by AVL_FIRE. A good agreement with the experiment results is achieved. The results indicate that the liquid film area increases with higher injection flow rate, compressed air pressure, injection height and lower injection angle. According to Kuhnke model, the behaviors of impingement droplets are dominated by the sub-velocity of droplets which is affected by the injection flow rate and injection angle. Higher metal wall temperature can promote the evaporation of the liquid film and droplets, which reduces the liquid film area. It is an effective way to prevent the liquid film formation through rising the metal wall temperature.

Synergistic effect of mixer and mixing chamber on flow mixing and NOx reduction in a marine urea-SCR system

For marine diesel engines, the selective catalytic reduction (SCR) system can be tailored to ensure high nitrogen oxides (NOx) reduction performance, low ammonia (NH3) slip, and low urea consumption. A sufficient mixing zone of urea water solution (UWS) and exhaust gas are required because of the limited installation space. In this study, therefore, mixing units such as a swirl-type mixer and an additional mixing chamber were adopted to improve the flow mixing performance with a low-pressure drop in a marine urea-SCR system. The application of mixing units gave a positive influence on the flow turbulent characteristics and urea decomposition into NH3. The mixer caused flow recirculation and increased turbulent intensity. Moreover, the additional mixing chamber was able to achieve an enhanced residence time for the UWS injections. When mixing units both were combined, it showed the highest conversion efficiency (urea to NH3). In comparison to the SCR system without any mixing units, improvement in the efficiency of deNOx was observed of 61.3 % by the application of the mixing chamber, 76.2 % by the application of the mixer, and 86.3 % by the application of the mixer and the mixing chamber combined.

A Holistic View on Urea Injection for NOx Emission Control: Impingement, Re-atomization, and Deposit Formation

The injection of urea-water solution (UWS) sprays into engine-close selective catalytic reduction (SCR) systems faces developers with complex challenges regarding two-phase flow and deposit formation. Potential spray impact on the static mixing elements inside the exhaust duct can lead to accumulation of liquid film and solid deposits, which may result in detachment of large UWS droplets at the trailing edges of the mixer blades. Present work focuses on the mechanisms of film and deposit formation within the scope of a joint study on spray preparation in the mixing section of SCR systems. A production type SCR exhaust system is used for optical investigations on gas flow and UWS droplets at the mixing element using Laser Doppler Anemometry (LDA) and Long Distance Microscopy (LDM) with a high-speed camera. Measured shear flows inside the mixing element are up to three times higher than the flow velocity upstream the mixer. Here, a decrease of droplet diameters of detached droplets is revealed with increased shear flow velocity. Characteristic urea deposits are created in a laboratory test bench and their surface structure is measured by use of Confocal Microscopy. Results show highly rough surface structures, which are used as input parameters together with the flow measurements to define the computational domain at the trailing edge of the mixer blades. Smoothed Particle Hydrodynamics (SPH) method is adopted for the numerical simulation of the transient two-phase flow and validated by LDM high-speed recordings of droplet detachment. Comparisons reveal a distinct influence of solid depositions on the re-atomization of the liquid film at the rear edge of the mixer blade. By interface resolving simulations with a phase-field method, the wall impingement of a sequence of large secondary droplets and associated film formation are studied. The results indicate that UWS film formation at the exhaust pipe wall may be reduced by hydrophobic surfaces and high gas velocities near the wall.

Deposit Formation from Urea Injection: a Comprehensive Modeling Approach

Long-term reliability is one of the major requirements for automotive exhaust aftertreatment systems with selective catalytic reduction (SCR) using urea water solution (UWS) as NH3 carrier fluid. A high injection rate of UWS or unfavorable operating conditions may lead to formation of solid deposits, which decrease system efficiency by increasing backpressure and impairing ammonia uniformity. A reliable numerical prediction of deposit formation in urea SCR systems is desired for optimization of system design. However, comprehensive modeling of physical and chemical processes in the tailpipe as well as different time scale phenomena represents a challenging task. This study presents a comprehensive approach for modeling UWS injection, droplet impingement, liquid film and deposit formation based on CFD-simulation. An existing kinetic model for urea decomposition is integrated into the CFD code to predict solid by-product formation from wall films. Physical simulation time is extensively increased by substituting the Lagrange-particles with source terms of mass, momentum and energy reducing simulation time by a factor of 20. The comparison of measured and simulated results shows the capability of the presented modeling approach to predict position and chemical composition of solid deposits.

Study on the thermal decomposition rate of ammonium carbamate for a diesel NOx reducing agent-generating system

Ammonium carbamate (AC) is a substance that exists in solid state at room temperature and decomposes into ammonia and carbon dioxide upon heating. In this study, AC was thermally decomposed to produce ammonia gas, which is used as a reducing agent for reduction of NOx emitted from a diesel engine. Currently, most diesel engines utilize a UWS (urea-water solution) as a reducing agent. However, the need for significant heat energy for evaporation and thermal decomposition is one of the reasons driving low NOx reduction efficiency at low exhaust gas temperature conditions. On the other hand, the alternative method of supplying gaseous ammonia by thermal decomposition of solid AC has an advantage of relatively high NOx reduction efficiency at low exhaust gas temperature conditions. Recently, there have been studies on material properties of AC and system feasibility in order to use the AC as a diesel NOx reducing agent. In this study, a simple system for generating diesel NOx reducing agent upon thermal decomposition of AC was mathematically modeled. In addition, the thermal decomposition rate of the AC was expressed via an Arrhenius equation through TGA (thermo-gravimetric analysis) experiments, and the activation energy and pre-exponential factor of the Arrhenius equation were determined. Using the mathematical model and the Arrhenius equation of the thermal decomposition of AC, the effect of various parameters required for designing a reactor and system operation were investigated for a heavy-duty diesel engine with a displacement of 3.9L.

Characteristics of Deposit Formation in SCR System for Varying Temperature and Flow Rate of Exhaust Gases a Study using XRD

To mitigate environmental pollution due to NOx emitted from exhaust of diesel engines, the automobile manufacturers are adopting technique called “Selective Catalytic Reduction (SCR) System” for modern automobiles. The reducing agent NH3 is generated by spraying Urea-Water Solution (UWS). When exhaust temperature is low, the overdosing of UWS is necessary; but it results in deposits of urea and its by-products. The temperature and Reynolds number have significant effects on ammonia generation and urea deposits. In the present work, investigation is done for deposit formation considering temperature and flow rate as variables. The results show that the urea deposits obtained at open reactor vessel and in hot air test bench are different with their crystallanity, quantity and type. However, EDAX indicates urea as a major constituent in the deposits at low exhaust temperatures.

Experimental analysis of urea water solution spray behavior injected by Urea SCR injector

Experimental analysis of urea water solution spray behavior injected by Urea SCR injector

Effect of cosolvents in the preferential binding affinity of water in aqueous solutions of amino acids and amides

Effects of two naturally occurring osmolytes, urea and trimethylamine-N-oxide (TMAO) on the solvation structure of hydrophobic moiety of alanine, glycine, N-methylacetamide and acetamide are investigated by classical molecular dynamics simulations. Our results are analysed in terms of site-site radial distribution functions (RDF), spatial distribution functions (SDF), number of hydrogen bonds, orientation profile, KB integrals, preferential binding coefficient and hydrogen bond dynamics. RDF and SDF showed presence of an extra hydration shell near the hydrophobic unit when TMAO is present in the solution. This hydration shell mainly consists of broken hydrogen bonds. In urea-water solution, intramolecular association is favoured compared to intermolecular association: which is in contrast to the TMAO-water solution. Alanine, glycine, NMA and acetamide showed preferred interactions with the water molecules in presence of TMAO compared to urea. Urea and TMAO both are found to be excluded from the alanine, glycine, NMA and acetamide surface but presence of urea was slightly favoured at higher distances in case of NMA and acetamide. The strong hydrogen bond between TMAO-water increases the hydrogen bond lifetime of other hydrogen bonds in the system. The preferential binding affinity of water with the protein molecules and strong hydrogen bonds are found to be the key reasons for stability in presence of TMAO.

Improved urea-water solution spray model for simulations of selective catalytic reduction systems

Successive diesel engine emissions’ regulations impose increasingly tighter limits on the automotive industry. A well-established method to meet nitrogen oxides emissions’ requirements is selective catalytic reduction with a urea-water solution injection. State-of-the-art designs are based on close-coupled exhaust architecture which suffers from a lack of space for a proper decomposition of a urea-water solution. The optimization of such systems is very challenging and is always intensively supported by numerical simulations. Within this work a two-zone spray representation for a urea-water solution injection is proposed. The two-zone approach captures a non-uniform droplet distribution inside the spray cone while maintaining a reasonable calculation time, since it is still based on a Lagrangian method. The proposed model was applied into exhaust system simulations and tested against a standard spray representation. The results revealed that uniform spray representation led to a smaller wall film formation and different mass balance. Ammonia distribution at the catalyst inlet was not directly affected significantly; however, in real systems one could expect catalyst clogging due to wall film formation, followed by a decrease of ammonia distribution uniformity. The results clearly show that uniform spray representation may lead to misestimation of the selective catalytic reduction system’s performance and the novel two-zone approach should be used instead.

Application of electrostatic force for the atomization improvement of urea-water sprays in diesel SCR systems

Urea-based selective-catalytic-reduction (SCR) system is one of the techniques applied to reduce NOx emission in diesel engines. Due to the application of low injection pressure to prevent the wall-wetting of the urea-water solution to the exhaust pipe, the spray atomization, which is important to increase the NOx conversion efficiency, is still unsatisfactory. In this study, we pay attention to the electrostatic force as a source to improve the spray atomization of the urea-water solution. An electric voltage up to 7 kV was charged to a single-hole nozzle injector and various injection pressures up to 7 MPa were applied to investigate how the effect of electric charge on spray atomization appears differently in different flow regimes. High-speed visualization was conducted to observe the macroscopic spray structure. The microscopic spray characteristics were analyzed by a laser diffraction technique. The results showed that the electrostatic force effect, which results in finer droplet size and wider spray coverage area, is obtained especially at higher applied voltage in two different manners depending on the injection pressure, i.e. electrostatic induced jet breakup as a dominant effect at low injection pressures and electrostatic repulsion force-induced spray expansion as a dominant effect at high injection pressures.

Experimental investigation of the evaporation behavior of urea‐water‐solution droplets exposed to a hot air stream

AbstractThe behavior of droplets of urea‐water‐solution (UWS) evaporating under the influence of a hot stream of air was investigated experimentally, under temperatures ranging from 100°C to 400°C. The droplets were suspended on a glass microfiber to minimize the influence of heat conduction, through the fiber, on the evaporation rate of the droplet. The flow rate of air, under all experimental conditions, was measured and these data were used to estimate the average velocity of air around the droplet. Experiments were also conducted on droplets of pure water and the results were compared. The initial mass fraction of urea, in the solution, did not appear to have a significant effect on the evaporation constant, but it did affect a few essential aspects of the evaporation behavior. The evaporation of water droplets was in accordance with the d2 law at all temperatures, whereas the evaporation of UWS droplets was ambient‐temperature dependent.

SCR 시스템의 믹서 및 요소 수용액 분사각도가 유동 혼합에 미치는 영향

For efficient reduction of nitrogen oxides in selective catalytic reduction systems, the injected urea water solution must be converted almost 100% to ammonia for a short time. This paper presented the effects of mixer configuration on the urea water solution evaporation and mixing process in a selective catalytic reduction system, using numerical analysis tool of ANSYS FLUENT R15.0. The results of numerical analysis were compared to experimental results for verifying the numerical reliability. Compared to the case without the mixer, in case of the vane type mixer was installed, the conversion rate of NH₃ increased by 21.4% and uniformity by 48.9%. In addition, when the injection angle of the urea-water-solution was 45° based on the exhaust gas flow direction, the conversion rate of NH3 was highest as 89.0%, and the uniformity was highest as 52.5% at 15°. In this study, proposed a method to convert urea solution rapidly, and it is expected that this results could be used to reduce ammonia slip in SCR systems.