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Modeling analysis of urea direct injection on the NOx emission reduction of biodiesel fueled diesel engines
Energy Conversion and Management 101 (2015) 442–449
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Modeling analysis of urea direct injection on the NOx emission reduction of biodiesel fueled diesel engines H. An, W.M. Yang ⇑, J. Li, D.Z. Zhou Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore, Singapore
a r t i c l e
i n f o
Article history: Received 7 April 2015 Accepted 3 June 2015 Available online 15 June 2015 Keywords: Urea injection Nitrogen oxides (NOx) Diesel engine Biodiesel Emissions
a b s t r a c t In this paper, a numerical simulation study was conducted to explore the possibility of an alternative approach: direct aqueous urea solution injection on the reduction of NOx emissions of a biodiesel fueled diesel engine. Simulation studies were performed using the 3D CFD simulation software KIVA4 coupled with CHEMKIN II code for pure biodiesel combustion under realistic engine operating conditions of 2400 rpm and 100% load. The chemical behaviors of the NOx formation and urea/NOx interaction processes were modeled by a modified extended Zeldovich mechanism and urea/NO interaction sub-mechanism. To ensure an efficient NOx reduction process, various aqueous urea injection strategies in terms of post injection timing, injection angle, and injection rate and urea mass fraction were carefully examined. The simulation results revealed that among all the four post injection timings (10 °ATDC, 15 °ATDC, 20 °ATDC and 25 °ATDC) that were evaluated, 15 °ATDC post injection timing consistently demonstrated a lower NO emission level. The orientation of the aqueous urea injection was also shown to play a critical role in determining the NOx removal efficiency, and 50 degrees injection angle was determined to be the optimal injection orientation which gave the most NOx reduction. In addition, both the urea/water ratio and aqueous urea injection rate demonstrated important roles which affected the thermal decomposition of urea into ammonia and the subsequent NOx removal process, and it was suggested that 50% urea mass fraction and 40% injection rate presented the lowest NO emission levels. At last, with the optimized injection strategy, the engine presented significantly reduced NO emission from 131 ppm to 55 ppm compared to the case of pure biodiesel combustion, achieving a reduction efficiency of 58%, suggesting that urea direct injection could be an effective method to reduce the NOx emissions of a biodiesel fueled diesel engine. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Among various types of internal or external combustion engines, diesel engines play a major role in many automotive and industrial applications, owning to their superior thermal efficiency and low exhaust emissions of carbon monoxide (CO) and unburned hydro-carbon (HC). However, there are still some obstacles that need to be conquered in diesel engine’s research and development activities, due to the large nitrogen oxides (NOx) and particulate matter (PM) emissions and the trade-off between these two pollutants. Generally, a high peak cylinder temperature in the combustion chamber can help improve the oxidation of soot particles but leading to significantly increased NOx emission level, and on the contrary, lowering the peak cylinder temperature can tangibly suppress NOx formations but tend to produce large soot ⇑ Corresponding author. Tel.: +65 6516 6481; fax: +65 6779 1459. E-mail address: [email protected] (W.M. Yang). http://dx.doi.org/10.1016/j.enconman.2015.06.008 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
emissions and reduced engine thermal efficiency. Although it could be theoretically possible to achieve near zero NOx and PM emissions, in practice, it could only be achieved by a combination of close control of the combustion process and good aftertreatment systems [1]. To optimize the combustion process of conventional fossil fuel, the use of oxygenated fuels such as biodiesel has been on the spotlights as a promising additive which can significantly enhance the combustion efficiency by replenishing oxygen in the fuel rich zones due to the release of oxygen atoms from its chemical structure, thereby reducing the PM, CO and HC emissions [2–4]. However, the drawback followed would be the slightly elevated NOx emissions as reported by many researchers [5–9]. In scientific research and practical applications, there are many NOx removal technologies: exhaust gas recirculation (EGR), selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), miller cycle, emulsion technology and engine performance optimization, just to name a few. For example, Saleh [10] conducted an experimental study on a two-cylinder direct injection
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diesel engine fueled by jojoba methyl ester (JME) biodiesel to explore the effectiveness of EGR on NOx reduction. It was found that EGR was indeed an effective technique for NOx removal, however, with greater EGR rate (above 40%), the combustion process deteriorates, leading to reduced power output, increased brake specific fuel consumption (BSFC, up to 17%), and substantially increased CO and HC emissions. To better make use of the EGR technique, Park and Bae [11] proposed the simultaneous utilization of both low pressure exhaust gas recirculation (LP EGR) and high pressure exhaust gas recirculation (HP EGR), and investigated the effects of the proportion between LP EGR and HP EGR on the engine’s combustion, emission and fuel consumption characteristics. It was identified that as the LP EGR portion increased, the intake manifold temperature decreased, significantly suppressing the NOx formations, however, the CO emissions showed an opposite tread due to its incomplete combustion. In general, the tradeoff between NOx emissions and BSFC could be optimized by adjusting the LP EGR rate under different engine operating conditions. Although EGR technique has been demonstrated to be an effective engine modification technique for reducing the NOx emissions, it still leads to the increased CO and HC emissions, as well as BSFC especially when large EGR rate is used. Hence, from energy saving point of view, a second approach based on NOx aftertreatment technology is suggested to be a better choice. Dong et al. [12] evaluated the application of ethanol over Ag/Al2O3 catalyst on a high speed direct injection diesel engine. They observed that the NOx conversion efficiency went up as the ethanol dosage increased, but caused a great increase on the CO and HC emissions. To overcome the inevitable increase of the CO and HC emissions, the application of Ag/Al2O3 + Cu/TiO2 + Pt/TiO2 catalysts was introduced together with the close control of ethanol dosing, and it was claimed that based on the ESC (European Stationary Cycle) test cycle, the engine emissions could completely meet the EURO III emission regulations. A similar study was also carried out by Herreros et al. [13] who focused on the effects of alternative reactants/promoters such as gas to liquid (GTL), butanol and hydrogen on an Ag/Al2O3 catalyst. Their results indicated that hydrocarbon injection promoted the Ag/Al2O3 catalyst activity in reducing the NOx emissions, and the increased NOx conversion was dependent on the HC:NOx ratio, showing that a lower HC:NOx ratio was required at low temperatures and a higher HC was required as temperature increased. To overcome the relatively high cost of aftertreatment systems and sulfur poisoning, the application of Miller cycle combustion strategy has been reintroduced to abate NOx emissions. Gonca et al. [14] manufactured three different camshafts to provide 5, 10 and 15 degrees crank angle (°CA) late intake value closing (IVC) timing as compared to the original camshaft on a single cylinder, naturally aspirated, direct injection diesel engine. Experimental results indicated that 5 °CA would be the optimum retarding angle in terms of NOx reduction, and simulation results further revealed that at this condition, the NO emissions decreased by 30% with a 2.5% power loss and negligible change on the CO and HC emissions. Verschaeren et al. [15] quantified the effects of early IVC (Miller cycle) together with EGR on a heavy duty diesel engine. It was shown that up to 70% NOx reduction could be attained at different engine loads, and the greatest NOx reduction could be achieved with Miller cycle due to expansion cooling. Another approach to provide cooling effect is through water injection or emulsion technology by taking advantage of the high latent of vaporization of water. Tauiza et al. [16] compared the effects of inlet manifold water injection and EGR on the combustion and pollutant emissions of a common rail fuel injection diesel engine. It was shown that a large reduction of NOx emission could be achieved with high water injection rates under both low and high engine load conditions. Compared to EGR, water injection presented clear advantage in terms of NOx reduction at high engine
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loads, whereas at low load condition, ERG seemed to be a better choice to reduce the NOx emissions without significantly increasing the PM emissions. Koc and Abdullah [17] used biodiesel nanoemulsions containing 5%, 10% and 15% water on a 4 cylinder diesel engine and showed that biodiesel nanoemulsions produced less NOx emissions and soot opacity than those of B5 (5 vol% of biodiesel and 95% diesel), B20 and NO. 2 diesel fuels. It was further concluded that the biodiesel nanoemulsion with 5% water concentration produced similar engine power and torque to that of B5 fuel, suggesting that emulsion technology could be a promising strategy to reduce engine harmful emissions. Besides the above technologies, there are also some researchers [18,19] looking into engine optimization strategies to reduce the NOx emissions. For example, Al-Dawody and Bhatti [18] attempted different strategies to eliminate biodiesel NOx effect on the B20 blend of soybean methyl ester (SME). They discovered that cooling the intake air temperature from 55 to 15 °C could reduce NOx, air pollutant emissions (SE), bosch smoke number and BSFC by 10.53%, 17.63%, 24.35% and 6.2% respectively with respect to the base line operation of B20 SME. Furthermore, deeper piston bowl with small diameter could also give a significant reduction in the NOx emission, but lead to slightly increased BSFC. Even though there are many NOx removal technologies, the trade-off still exists in many strategies which would lead to excessive sacrifice on the CO, HC emissions or fuel penalties (BSFC), especially when large NOx conversion efficiency is expected. Therefore, the objective of this study is to numerically explore the possibility of an alternative approach: direct aqueous urea solution injection during the expansion stroke to secure the normal NOx removal rate without severe impact on the CO emissions and BSFC. 2. Numerical methodology The idea of direct aqueous urea solution injection is very similar to that of direct water injection (DWI) which has been investigated by many researchers experimentally and numerically [20–23]. However, unlike direct water injection system whose water is often injected through the intake manifold or directly into the combustion chamber before actual fuel injection, the aqueous urea solution is proposed to be injected after the fuel injection process during the expansion stroke by a dedicated nozzle or a separate water injector. For the purpose of this simulation, the same fuel injector with two injection nozzles (one for fuel and the other one for water) closely packed to each other was assumed, and the same nozzle parameters (nozzle diameter, location of injector nozzle, cone angle) except for fuel injection orientation were applied so that the urea/water solution could be effectively sprayed into the high temperature flame zones where most of the NOx emissions are formed. To ensure an effective and efficient NOx removal process, the injection of urea/water solution should be carefully examined and controlled in terms of injection timing, injection angle, injection rate, and urea mass fraction. 2.1. Numerical models Simulation studies were carried out using multi-dimensional software KIVA4 code specially developed for transient, two or three dimensional, chemically reactive fluid flows with sprays inside a combustion chamber. The KIVA4 code uses the finite volume scheme (arbitrary Lagrangian–Eulerian) to solve the conservation of mass, momentum and energy equations, and it also accounts for the turbulence effect using the RNG k-epsilon model. For fuel spray modeling, various sub-models are employed to describe the fuel spray, breakup, collision and coalescence, and multi-component fuel evaporation processes. Vaporized fuel
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species are considered to be ideal gas mixtures which will then undergo a series of complex chemical reactions solved by CHEMKIN II code as the chemistry solver. To improve the overall prediction accuracy, the Kelvin–Helmholtz and Rayleigh–Taylor (KH–RT) spray break up model was implemented in the KIVA code in conjunction with the blob-injection model for the atomization process of the injected spray. The blob-injection model assumes that during the injection process, there are continuously added large drops (blobs) injected from the nozzle hole with a diameter equal to the effective nozzle diameter. Immediately after the injection, the Kelvin–Helmholtz model is used to break up the jet, which is suitable for both the primary and secondary breakup. The Rayleigh–Taylor model is only used together with the KH model after a certain break up length to describe the secondary break up. The KIVA4 code was further modified to accommodate multiple injections, different fuel types and different injection orientations. A 60-degree sector mesh was generated based on the bowl geometry of the 2KD-FTV Toyota car engine by taking advantage of the symmetric distribution of the injector nozzle holes (6 holes) as shown in Fig. 1. Detailed engine specifications are listed in Table 1 and extensive mesh independence test can be found in our earlier works [24–26].
(MD) was chosen to represent the saturated methyl esters, and Methyl-9-decenoate (MD9D) was chosen to represent the unsaturated methyl esters because its double bond is located at the same position as the one in methyl oleate and at the same position as the first double bond in methyl linoleate and methyl linolenate. The detailed reaction mechanism comprises 3299 species and 10,806 reactions. However, to save computational time, a skeletal reaction mechanism [28] developed from the detailed reaction mechanism was adopted, which consists of 112 species and 498 reactions with CO, NOx and soot formation mechanisms embedded. Additionally, to further preserve the computational power and for the interest of this study, the skeletal reaction mechanism was modified by removing the species and reactions that are associated with soot formation and oxidation processes, resulting to a skeletal mechanism with 95 species and 438 elementary reactions. The NOx formation process was simulated using a modified extended Zeldovich mechanism by Tao et al. [29] as shown in Table 2, and the NOx/urea interaction reactions were emulated by a model described in [30] (see Table 3) and combined with the skeletal biodiesel reaction mechanism. The final mechanism includes 99 species and 454 reactions. 2.3. Operating conditions
2.2. Chemical kinetics Biodiesel derived from vegetable oil or animal fats, typically contains five major methyl esters having the molecular structure of RA(C@O)AOAR0 with 0, 1, 2 or 3 double bonds. Compared to typical diesel surrogates such as n-heptane, the chemical structure of biodiesel is much longer and more complex, which poses great challenges on the development of a reliable chemical kinetics that could accurately mimic biodiesel’s combustion and emission characteristics. To take into account the effects of double bonds, a detailed biodiesel blend surrogate mechanism [27] was developed to be representative of biodiesel fuels, in which, Methyl decanoate
Although aqueous urea has been extensively used in some NOx removal technologies, a considerable drawback that limits its practical application is its narrow temperature range within which the NOx reduction process is effective. Earlier study has reported that the effective temperature range of urea DeNOx process is from 1200 to 1400 K depending on its operating conditions [31]. Hence, to ensure sufficient residence time and effective temperature range for the NOx reduction process, a comprehensive study should be conducted to obtain the optimal injection timing, injection orientation (injection angle), the amount of urea/water solution injection (injection rate) as well as the quality of the reducing agent (urea mass fraction). For the present investigation, a simulation study was first conducted for pure biodiesel combustion without post urea/water solution injection at 2400 rpm and 100% engine load conditions on a 2KD FTV Toyota car engine for comparison purposes, and subsequent simulation cases were conducted for biodiesel combustion with urea/water post injection at 4 different injection timings (10 °ATDC, 15 °ATDC, 20 °ATDC and 25 °ATDC), 3 injection orientations (50°, 60° and 70°), 3 urea mass factions (20%, 50% and 80%), and 3 injection rates (10%, 20% and 40%). Here injection orientation refers to the angle between the urea/water solution injection direction (injector nozzle hole) with the downward vertical axis, and injection rate is expressed as the mass ratio between the total urea/water solution and biodiesel fuel. Table 4 summaries the detailed injection parameters that have been used in this study.
Table 2 Elementary reactions for the NOx mechanism. Fig. 1. The 60-degrees sector mesh shown at TDC.
Table 1 Engine specifications. Engine type Bore Stroke Connecting rod Compression ratio Rated power Charging Fuel injection system
Four stroke, DI, 4 cylinder inline 92 93.8 mm 158.5 mm 18.5:1 75 kW at 3600 rpm Turbocharged Common rail, Denso (6 Holes)
N/S
Reaction
A
b
E
1 2 3 4 5 6 7 8 9 10 11 12
N + NO = N2 + O N + O2 = NO + O N + OH = NO + H N2O + O = N2 + O2 N2O + O = NO + NO N2O + H = N2 + OH N2O + OH = N2 + HO2 N2O (+M) = N2 + O (+M) HO2 + NO = NO2 + OH NO + O + M = NO2 + M NO2 + O = NO + O2 NO2 + H = NO + OH
2.700E+13 9.000E+09 3.360E+13 1.400E+12 2.900E+13 3.870E+14 2.000E+12 7.910E+10 2.110E+12 1.060E+20 3.900E+12 1.320E+14
0.00 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.41 0.00 0.00
355.0 6500.0 385.0 10810.0 23150.0 18880.0 21060.0 56020.0 480.0 0.0 240.0 360.0
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H. An et al. / Energy Conversion and Management 101 (2015) 442–449 Table 3 NOx/urea interaction mechanism. N/S
Reaction
A
b
E
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
NH2CONH2 + H2O = NH3 + NH3 + CO2 NH3 + M = NH2 + H + M NH3 + H = NH2 + H2 NH3 + O = NH2 + OH NH3 + OH = NH2 + H2O NH2 + N = N2 + H + H NH2 + NO = N2 + H2O NH2 + H = NH + H2 NH2 + O = NH + OH NH2 + OH = NH + H2O NH2 + HO2 = NH3 + O2 NH + O2 = NO + OH NH + NH = N2 + H + H NH + N = N2 + H NH + NO = N2 + OH NH + NO = N2O + H N2O(+M) = N2 + O(+M)
6.13E+10 2.20E+16 6.36E+05 9.40E+06 2.00E+06 7.20E+13 1.26E+16 4.00E+13 6.80E+12 4.00E+06 1.00E+13 1.30E+06 2.50E+13 3.00E+13 2.16E+13 2.90E+14 7.910E+10
0.00 0.0 2.39 1.94 2.04 0.0 1.25 0.0 0.0 2.0 0.0 1.5 0.0 0.0 0.23 0.4 0.00
20980.0 93470.0 10171.0 6460.0 566.0 0.0 0.0 3650.0 0.0 1000.0 0.0 100.0 0.0 0.0 0.0 0.0 56020.0
Table 4 Injection parameters for different simulation cases. Simulation cases
Injection timing
Injection orientation
Urea mass fraction
Injection rate (% of diesel injection) (%)
Pure biodiesel combustion
No urea injection
NA
NA
0
Effects of injection timing
10 °ATDC 15 °ATDC 20 °ATDC 25 °ATDC
60°
20
20
Effects of injection angle
15 °ATDC 20 °ATDC 25 °ATDC
50° 60° 70°
20
20
Effects of urea mass fraction
15 °ATDC 20 °ATDC 25 °ATDC
60°
20 50 80
20
Effects of total mass injection
15 °ATDC
60°
50
10 20 40
3. Results and discussions 3.1. Mechanism validation Developed biodiesel + urea reaction mechanism was first validated against the experimental results conducted on a light duty 2KD FTV Toyota car engine in terms of in-cylinder pressure, heat release rate, and exhaust emissions of NOx and CO. The test engine is a four-cylinder, four-stroke, turbocharged, direct injection diesel engine with a rated engine power of 75 kW and rated engine speed of 3600 rpm. Detailed engine specifications are listed in Table 1. For the purpose of this study, simulation models were validated based on the experimental results of pure biodiesel (derived from waste cooking oil) combustion at a medium engine speed of 2400 rpm under full load conditions. Detailed descriptions on the experimental set-up, engine testing procedures, emission analyzer specifications and biodiesel fuel properties can be found in [32]. Fig. 2 compares the predicted and experimental results of the in-cylinder pressure and apparent heat release rate curves. It can be seen that the simulation results agree well with the experimental results, although slightly higher peak cylinder pressure and heat release rate are seen for the computational results due to the slightly over predicted combustion rate during the premixed combustion phase by the biodiesel reaction mechanism. Furthermore, the simulated and experimental results of NOx and CO emissions were normalized to the indicated power, and expressed and
Fig. 2. In-cylinder pressure and heat release rate validations of pure biodiesel combustion at 2400 rpm and 100% load.
compared in terms of indicated specific exhaust emissions as shown in Fig. 3. As seen, the predicted indicated specific NOx emission is about 15 ppm/kW and gives rise to about 7% deviation from the experimental results, while the predicted and experimentally determined CO emissions are 0.41 ppm/kW and 0 ppm/kW respectively, suggesting that the NOx and CO emission sub-models are
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Fig. 3. Emissions validation of pure biodiesel combustion at 2400 rpm and 100% load.
Fig. 5. Effects of urea/water solution injection angle on NO emission.
very accurate to emulate the NOx and CO formation and oxidation processes. 3.2. Effects of injection timing The efficiency of aqueous urea NOx removal process highly depends on its operating conditions such as reaction temperature and residence time, which further requires an accurate control and optimization on the post injection timing of the urea/water solution during the expansion stroke. Early injection timing would give us a longer residence time and higher combustion temperature, while late injection timing would lead to a shorter residence time and lower effective temperature range. However, for extreme cases when the injection timing is too close to the main injection of biodiesel fuel, the NOx reducing agents would be sprayed into the high temperature flame zones, leading to an undesirable oxidation of NH3 and elevated NOx emissions. Fig. 4 shows the effects of different aqueous urea injection timings on the total in-cylinder NO formation process (based on one cylinder, same for subsequent sections) with an injection angle of 60° (medium injection angle), urea mass faction of 50% (medium mass fraction) and injection rate of 20% (medium injection rate). As can be seen, for the case of pure biodiesel combustion without urea/water injection, the NO concentration increases rapidly after the top dead center, and reaches a peak value of 132 ppm without significant further oxidation
Fig. 6. Effects of urea mass fraction on NO emission.
Fig. 7. Effects of total urea/water solution on NO emission.
Fig. 4. Effects of urea/water solution injection timing on in-cylinder NO formation process.
processes. However, for the cases with urea/water post injection, both the peak and final NO concentrations have reduced significantly except for the case with post injection timing of 10 °ATDC. As expected, with posting injection timing immediately after the
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top dead center (10 °ATDC in this case), the predicted average cylinder temperature is as high as 1450 K which is sufficiently high to result to the oxidation of NH3 and adversely increased NO emissions. However, with further retarded injection timings, the introduction of urea/water solution effectively reduces the NO emissions, with 15 °ATDC post injection timing being the most effective.
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3.3. Effects of injection angle Besides injection timing, the orientation of the aqueous urea injection also plays a vital role in determining its efficiency by affecting the amount of effective NOx removing agents that are sprayed into the high temperature combustion regions where most of the NOx emissions are formed. Fig. 5 compares the predicted NO emission with three different injection angles of 50°, 60° and 70°, urea mass faction of 50% and injection rate of 20% under three different injection timings of 15 °ATDC, 20 °ATDC and 25 °ATDC conditions. It can be seen that under all the 3 different injection timings, the 50 degrees injection angle consistently demonstrates a lower NO formation level, indicating that 50 degrees injection angle seems to be an optimal injection orientation which gives the most effective NO removal process. However, this simulation result is not conclusive, and optimization study should be conducted based on the engine piston design, injector specifications, injection strategies and etc. 3.4. Effects of urea mass fraction and total mass injection In the present study, the thermal decomposition of urea into ammonia was assumed by a one-step gas phase hydrolysis reaction as shown in Table 3 (reaction 1). From reaction 1, it can be seen that although larger aqueous urea injection rate is expected for a more complete NOx removal process, the urea/water ratio also plays an important role in determining its efficiency on the urea
Fig. 8. Injection strategy of urea/water solution on NO emission.
NO Concentration Contours Crank Angle Without Urea Injection
With Urea Injection
15 °ATDC
25 °ATDC
50 °ATDC
Fig. 9. Spatial plots of the in-cylinder NO concentrations at 15 °ATDC, 30 °ATDC and 50 °ATDC of pure biodiesel combustion with and without urea injection.
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decomposition process. In order to highlight the importance of urea mass fraction on the urea hydrolysis rate and hence the subsequent NH3 production and NOx removal process, Fig. 6 presents the final NO emission levels when aqueous urea is being injected at an injection angle of 60° (medium injection angle), injection rate of 20% (medium injection rate), urea mass factions of 20%, 50% and 80% under three different injection timings of 15, 20 and 25 °ATDC conditions. It appears evident that with 15 and 20 °ATDC post injection timings, 50% urea mass fraction presents the lowest NO emission levels. In addition, although a linearly reduced NO emission level is observed for the 25 °ATDC post injection timing case, no tangible improvement on the NO removal efficiency is demonstrated from 50% to 80% urea mass fraction, suggesting that 50% mass fraction could be the optimal mass fraction under all the three mass fractions that are being considered in this study. Besides urea mass fraction, Fig. 7 further examines the effects of total urea/water solution on the NOx removal efficiency. The three different cases were simulated with an injection angle of 60° (medium injection angle), urea mass fraction of 50% (medium mass fraction), injection timing of 15 °ATDC and three injection rates of 10%, 20% and 40%. As expected, the NO emission level reduces almost proportionally with the increase of total urea/water solution. However, with further increase in the aqueous urea injection rate, the NOx removal effect may become less evident and simultaneously result to significant amount of urea emissions, which will be discussed in the next section. 3.5. Injection strategy optimization From the above analysis, it suggests that the optimal injection timing, injection angle, urea mass fraction and total urea injection rate are 15 °ATDC, 50°, 50% and 40% respectively. To further optimize the urea NOx removal process, another three simulation runs were conducted with the injection timing of 15 °ATDC, injection angle of 50°, urea mass fraction of 50% and three injection rates of 20%, 40% and 50%. It is found that the NO emission level has reduced from 68 ppm to 55 ppm when the total urea/water solution injection rate increases from 20% to 40% as shown in Fig. 8. However, when the aqueous urea increases from 40% to 50%, only marginally reduced NO emission is found with severe sacrifice on the urea emissions. Comparing with the case of pure biodiesel combustion without post aqueous urea injection, the injection of aqueous urea with 40% injection rate significantly reduces the NO emission from 131 ppm to 55 ppm, achieving a reduction efficiency of 58%. This is further proved in Fig. 9 which compares the spatial distribution of the in-cylinder NO concentration at 15 °ATDC, 30 °ATDC and 50 °ATDC of pure biodiesel combustion with and without post aqueous urea injection. As seen, a remarkable reduction on the local NO concentration is observed when the aqueous urea is being injected into the combustion chamber and reacted with the surrounding NO emissions especially at 50 °ATDC. In addition, it is also discerned in this study that with the aqueous urea injection, the predicted indicated power and CO emissions have changed from 9.4 kW and 4 ppm respectively to 10 kW and 13 ppm, indicating that no severe impacts on the CO emissions and BSFC are found. Hence, it can be concluded that urea direct injection can be an effective method to reduce the NOx emission of a biodiesel fueled diesel engine. 4. Conclusions Direct aqueous urea solution injection strategy was successfully investigated on a four cylinder, turbocharged, direct injection diesel engine to secure the normal NOx removal rate without severe impacts on the CO emissions and BSFC. Simulation studies were
conducted based on pure biodiesel combustion at an engine speed of 2400 rpm under full load conditions with and without aqueous urea injection. The aqueous urea solution was proposed to be injected after the fuel injection process during the expansion stroke, and to ensure an effective and efficient NOx removal process, various injection timings, injection angles, injection rates, and urea mass fractions were carefully controlled and examined. It was suggested that, the introduction of urea/water solution effectively reduced the NO emissions with 15 °ATDC post injection timing being the most effective. However, with the post injection timing too close to the end of main fuel injection such as 10 °ATDC, the predicted average cylinder temperature was too high to result to the oxidation of NH3 and adversely increased NO emissions. Among all the three injection angles that were studied, 50 degrees injection angle consistently demonstrated a lower NO formation level. In addition, the urea/water ratio was also found to play an important role in determining the efficiency of the urea decomposition process, and 50% mass fraction was found to be the optimal mass fraction. Furthermore, the NO emission level reduced almost proportionally with the increase of total urea/water solution. The final suggested injection strategies were 15 °ATDC injection timing, 50 degrees injection angle, 50% urea mass fraction and 40% total urea injection rate. Comparing with the case of pure biodiesel combustion without post aqueous urea injection, the optimized injection strategy significantly reduced the NO emission from 131 ppm to 55 ppm, achieving a reduction efficiency of 58% without severe impacts on the CO emissions and BSFC for a biodiesel fueled diesel engine. Acknowledgment This work is supported ‘‘R-265-000-800-733’’.
by
the
MOE
research
grant
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Modeling analysis of urea direct injection on the NOx emission reduction of biodiesel fueled diesel engines H. An, W.M. Yang ⇑, J. Li, D.Z. Zhou Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore, Singapore
a r t i c l e
i n f o
Article history: Received 7 April 2015 Accepted 3 June 2015 Available online 15 June 2015 Keywords: Urea injection Nitrogen oxides (NOx) Diesel engine Biodiesel Emissions
a b s t r a c t In this paper, a numerical simulation study was conducted to explore the possibility of an alternative approach: direct aqueous urea solution injection on the reduction of NOx emissions of a biodiesel fueled diesel engine. Simulation studies were performed using the 3D CFD simulation software KIVA4 coupled with CHEMKIN II code for pure biodiesel combustion under realistic engine operating conditions of 2400 rpm and 100% load. The chemical behaviors of the NOx formation and urea/NOx interaction processes were modeled by a modified extended Zeldovich mechanism and urea/NO interaction sub-mechanism. To ensure an efficient NOx reduction process, various aqueous urea injection strategies in terms of post injection timing, injection angle, and injection rate and urea mass fraction were carefully examined. The simulation results revealed that among all the four post injection timings (10 °ATDC, 15 °ATDC, 20 °ATDC and 25 °ATDC) that were evaluated, 15 °ATDC post injection timing consistently demonstrated a lower NO emission level. The orientation of the aqueous urea injection was also shown to play a critical role in determining the NOx removal efficiency, and 50 degrees injection angle was determined to be the optimal injection orientation which gave the most NOx reduction. In addition, both the urea/water ratio and aqueous urea injection rate demonstrated important roles which affected the thermal decomposition of urea into ammonia and the subsequent NOx removal process, and it was suggested that 50% urea mass fraction and 40% injection rate presented the lowest NO emission levels. At last, with the optimized injection strategy, the engine presented significantly reduced NO emission from 131 ppm to 55 ppm compared to the case of pure biodiesel combustion, achieving a reduction efficiency of 58%, suggesting that urea direct injection could be an effective method to reduce the NOx emissions of a biodiesel fueled diesel engine. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Among various types of internal or external combustion engines, diesel engines play a major role in many automotive and industrial applications, owning to their superior thermal efficiency and low exhaust emissions of carbon monoxide (CO) and unburned hydro-carbon (HC). However, there are still some obstacles that need to be conquered in diesel engine’s research and development activities, due to the large nitrogen oxides (NOx) and particulate matter (PM) emissions and the trade-off between these two pollutants. Generally, a high peak cylinder temperature in the combustion chamber can help improve the oxidation of soot particles but leading to significantly increased NOx emission level, and on the contrary, lowering the peak cylinder temperature can tangibly suppress NOx formations but tend to produce large soot ⇑ Corresponding author. Tel.: +65 6516 6481; fax: +65 6779 1459. E-mail address: [email protected] (W.M. Yang). http://dx.doi.org/10.1016/j.enconman.2015.06.008 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
emissions and reduced engine thermal efficiency. Although it could be theoretically possible to achieve near zero NOx and PM emissions, in practice, it could only be achieved by a combination of close control of the combustion process and good aftertreatment systems [1]. To optimize the combustion process of conventional fossil fuel, the use of oxygenated fuels such as biodiesel has been on the spotlights as a promising additive which can significantly enhance the combustion efficiency by replenishing oxygen in the fuel rich zones due to the release of oxygen atoms from its chemical structure, thereby reducing the PM, CO and HC emissions [2–4]. However, the drawback followed would be the slightly elevated NOx emissions as reported by many researchers [5–9]. In scientific research and practical applications, there are many NOx removal technologies: exhaust gas recirculation (EGR), selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), miller cycle, emulsion technology and engine performance optimization, just to name a few. For example, Saleh [10] conducted an experimental study on a two-cylinder direct injection
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diesel engine fueled by jojoba methyl ester (JME) biodiesel to explore the effectiveness of EGR on NOx reduction. It was found that EGR was indeed an effective technique for NOx removal, however, with greater EGR rate (above 40%), the combustion process deteriorates, leading to reduced power output, increased brake specific fuel consumption (BSFC, up to 17%), and substantially increased CO and HC emissions. To better make use of the EGR technique, Park and Bae [11] proposed the simultaneous utilization of both low pressure exhaust gas recirculation (LP EGR) and high pressure exhaust gas recirculation (HP EGR), and investigated the effects of the proportion between LP EGR and HP EGR on the engine’s combustion, emission and fuel consumption characteristics. It was identified that as the LP EGR portion increased, the intake manifold temperature decreased, significantly suppressing the NOx formations, however, the CO emissions showed an opposite tread due to its incomplete combustion. In general, the tradeoff between NOx emissions and BSFC could be optimized by adjusting the LP EGR rate under different engine operating conditions. Although EGR technique has been demonstrated to be an effective engine modification technique for reducing the NOx emissions, it still leads to the increased CO and HC emissions, as well as BSFC especially when large EGR rate is used. Hence, from energy saving point of view, a second approach based on NOx aftertreatment technology is suggested to be a better choice. Dong et al. [12] evaluated the application of ethanol over Ag/Al2O3 catalyst on a high speed direct injection diesel engine. They observed that the NOx conversion efficiency went up as the ethanol dosage increased, but caused a great increase on the CO and HC emissions. To overcome the inevitable increase of the CO and HC emissions, the application of Ag/Al2O3 + Cu/TiO2 + Pt/TiO2 catalysts was introduced together with the close control of ethanol dosing, and it was claimed that based on the ESC (European Stationary Cycle) test cycle, the engine emissions could completely meet the EURO III emission regulations. A similar study was also carried out by Herreros et al. [13] who focused on the effects of alternative reactants/promoters such as gas to liquid (GTL), butanol and hydrogen on an Ag/Al2O3 catalyst. Their results indicated that hydrocarbon injection promoted the Ag/Al2O3 catalyst activity in reducing the NOx emissions, and the increased NOx conversion was dependent on the HC:NOx ratio, showing that a lower HC:NOx ratio was required at low temperatures and a higher HC was required as temperature increased. To overcome the relatively high cost of aftertreatment systems and sulfur poisoning, the application of Miller cycle combustion strategy has been reintroduced to abate NOx emissions. Gonca et al. [14] manufactured three different camshafts to provide 5, 10 and 15 degrees crank angle (°CA) late intake value closing (IVC) timing as compared to the original camshaft on a single cylinder, naturally aspirated, direct injection diesel engine. Experimental results indicated that 5 °CA would be the optimum retarding angle in terms of NOx reduction, and simulation results further revealed that at this condition, the NO emissions decreased by 30% with a 2.5% power loss and negligible change on the CO and HC emissions. Verschaeren et al. [15] quantified the effects of early IVC (Miller cycle) together with EGR on a heavy duty diesel engine. It was shown that up to 70% NOx reduction could be attained at different engine loads, and the greatest NOx reduction could be achieved with Miller cycle due to expansion cooling. Another approach to provide cooling effect is through water injection or emulsion technology by taking advantage of the high latent of vaporization of water. Tauiza et al. [16] compared the effects of inlet manifold water injection and EGR on the combustion and pollutant emissions of a common rail fuel injection diesel engine. It was shown that a large reduction of NOx emission could be achieved with high water injection rates under both low and high engine load conditions. Compared to EGR, water injection presented clear advantage in terms of NOx reduction at high engine
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loads, whereas at low load condition, ERG seemed to be a better choice to reduce the NOx emissions without significantly increasing the PM emissions. Koc and Abdullah [17] used biodiesel nanoemulsions containing 5%, 10% and 15% water on a 4 cylinder diesel engine and showed that biodiesel nanoemulsions produced less NOx emissions and soot opacity than those of B5 (5 vol% of biodiesel and 95% diesel), B20 and NO. 2 diesel fuels. It was further concluded that the biodiesel nanoemulsion with 5% water concentration produced similar engine power and torque to that of B5 fuel, suggesting that emulsion technology could be a promising strategy to reduce engine harmful emissions. Besides the above technologies, there are also some researchers [18,19] looking into engine optimization strategies to reduce the NOx emissions. For example, Al-Dawody and Bhatti [18] attempted different strategies to eliminate biodiesel NOx effect on the B20 blend of soybean methyl ester (SME). They discovered that cooling the intake air temperature from 55 to 15 °C could reduce NOx, air pollutant emissions (SE), bosch smoke number and BSFC by 10.53%, 17.63%, 24.35% and 6.2% respectively with respect to the base line operation of B20 SME. Furthermore, deeper piston bowl with small diameter could also give a significant reduction in the NOx emission, but lead to slightly increased BSFC. Even though there are many NOx removal technologies, the trade-off still exists in many strategies which would lead to excessive sacrifice on the CO, HC emissions or fuel penalties (BSFC), especially when large NOx conversion efficiency is expected. Therefore, the objective of this study is to numerically explore the possibility of an alternative approach: direct aqueous urea solution injection during the expansion stroke to secure the normal NOx removal rate without severe impact on the CO emissions and BSFC. 2. Numerical methodology The idea of direct aqueous urea solution injection is very similar to that of direct water injection (DWI) which has been investigated by many researchers experimentally and numerically [20–23]. However, unlike direct water injection system whose water is often injected through the intake manifold or directly into the combustion chamber before actual fuel injection, the aqueous urea solution is proposed to be injected after the fuel injection process during the expansion stroke by a dedicated nozzle or a separate water injector. For the purpose of this simulation, the same fuel injector with two injection nozzles (one for fuel and the other one for water) closely packed to each other was assumed, and the same nozzle parameters (nozzle diameter, location of injector nozzle, cone angle) except for fuel injection orientation were applied so that the urea/water solution could be effectively sprayed into the high temperature flame zones where most of the NOx emissions are formed. To ensure an effective and efficient NOx removal process, the injection of urea/water solution should be carefully examined and controlled in terms of injection timing, injection angle, injection rate, and urea mass fraction. 2.1. Numerical models Simulation studies were carried out using multi-dimensional software KIVA4 code specially developed for transient, two or three dimensional, chemically reactive fluid flows with sprays inside a combustion chamber. The KIVA4 code uses the finite volume scheme (arbitrary Lagrangian–Eulerian) to solve the conservation of mass, momentum and energy equations, and it also accounts for the turbulence effect using the RNG k-epsilon model. For fuel spray modeling, various sub-models are employed to describe the fuel spray, breakup, collision and coalescence, and multi-component fuel evaporation processes. Vaporized fuel
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species are considered to be ideal gas mixtures which will then undergo a series of complex chemical reactions solved by CHEMKIN II code as the chemistry solver. To improve the overall prediction accuracy, the Kelvin–Helmholtz and Rayleigh–Taylor (KH–RT) spray break up model was implemented in the KIVA code in conjunction with the blob-injection model for the atomization process of the injected spray. The blob-injection model assumes that during the injection process, there are continuously added large drops (blobs) injected from the nozzle hole with a diameter equal to the effective nozzle diameter. Immediately after the injection, the Kelvin–Helmholtz model is used to break up the jet, which is suitable for both the primary and secondary breakup. The Rayleigh–Taylor model is only used together with the KH model after a certain break up length to describe the secondary break up. The KIVA4 code was further modified to accommodate multiple injections, different fuel types and different injection orientations. A 60-degree sector mesh was generated based on the bowl geometry of the 2KD-FTV Toyota car engine by taking advantage of the symmetric distribution of the injector nozzle holes (6 holes) as shown in Fig. 1. Detailed engine specifications are listed in Table 1 and extensive mesh independence test can be found in our earlier works [24–26].
(MD) was chosen to represent the saturated methyl esters, and Methyl-9-decenoate (MD9D) was chosen to represent the unsaturated methyl esters because its double bond is located at the same position as the one in methyl oleate and at the same position as the first double bond in methyl linoleate and methyl linolenate. The detailed reaction mechanism comprises 3299 species and 10,806 reactions. However, to save computational time, a skeletal reaction mechanism [28] developed from the detailed reaction mechanism was adopted, which consists of 112 species and 498 reactions with CO, NOx and soot formation mechanisms embedded. Additionally, to further preserve the computational power and for the interest of this study, the skeletal reaction mechanism was modified by removing the species and reactions that are associated with soot formation and oxidation processes, resulting to a skeletal mechanism with 95 species and 438 elementary reactions. The NOx formation process was simulated using a modified extended Zeldovich mechanism by Tao et al. [29] as shown in Table 2, and the NOx/urea interaction reactions were emulated by a model described in [30] (see Table 3) and combined with the skeletal biodiesel reaction mechanism. The final mechanism includes 99 species and 454 reactions. 2.3. Operating conditions
2.2. Chemical kinetics Biodiesel derived from vegetable oil or animal fats, typically contains five major methyl esters having the molecular structure of RA(C@O)AOAR0 with 0, 1, 2 or 3 double bonds. Compared to typical diesel surrogates such as n-heptane, the chemical structure of biodiesel is much longer and more complex, which poses great challenges on the development of a reliable chemical kinetics that could accurately mimic biodiesel’s combustion and emission characteristics. To take into account the effects of double bonds, a detailed biodiesel blend surrogate mechanism [27] was developed to be representative of biodiesel fuels, in which, Methyl decanoate
Although aqueous urea has been extensively used in some NOx removal technologies, a considerable drawback that limits its practical application is its narrow temperature range within which the NOx reduction process is effective. Earlier study has reported that the effective temperature range of urea DeNOx process is from 1200 to 1400 K depending on its operating conditions [31]. Hence, to ensure sufficient residence time and effective temperature range for the NOx reduction process, a comprehensive study should be conducted to obtain the optimal injection timing, injection orientation (injection angle), the amount of urea/water solution injection (injection rate) as well as the quality of the reducing agent (urea mass fraction). For the present investigation, a simulation study was first conducted for pure biodiesel combustion without post urea/water solution injection at 2400 rpm and 100% engine load conditions on a 2KD FTV Toyota car engine for comparison purposes, and subsequent simulation cases were conducted for biodiesel combustion with urea/water post injection at 4 different injection timings (10 °ATDC, 15 °ATDC, 20 °ATDC and 25 °ATDC), 3 injection orientations (50°, 60° and 70°), 3 urea mass factions (20%, 50% and 80%), and 3 injection rates (10%, 20% and 40%). Here injection orientation refers to the angle between the urea/water solution injection direction (injector nozzle hole) with the downward vertical axis, and injection rate is expressed as the mass ratio between the total urea/water solution and biodiesel fuel. Table 4 summaries the detailed injection parameters that have been used in this study.
Table 2 Elementary reactions for the NOx mechanism. Fig. 1. The 60-degrees sector mesh shown at TDC.
Table 1 Engine specifications. Engine type Bore Stroke Connecting rod Compression ratio Rated power Charging Fuel injection system
Four stroke, DI, 4 cylinder inline 92 93.8 mm 158.5 mm 18.5:1 75 kW at 3600 rpm Turbocharged Common rail, Denso (6 Holes)
N/S
Reaction
A
b
E
1 2 3 4 5 6 7 8 9 10 11 12
N + NO = N2 + O N + O2 = NO + O N + OH = NO + H N2O + O = N2 + O2 N2O + O = NO + NO N2O + H = N2 + OH N2O + OH = N2 + HO2 N2O (+M) = N2 + O (+M) HO2 + NO = NO2 + OH NO + O + M = NO2 + M NO2 + O = NO + O2 NO2 + H = NO + OH
2.700E+13 9.000E+09 3.360E+13 1.400E+12 2.900E+13 3.870E+14 2.000E+12 7.910E+10 2.110E+12 1.060E+20 3.900E+12 1.320E+14
0.00 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.41 0.00 0.00
355.0 6500.0 385.0 10810.0 23150.0 18880.0 21060.0 56020.0 480.0 0.0 240.0 360.0
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H. An et al. / Energy Conversion and Management 101 (2015) 442–449 Table 3 NOx/urea interaction mechanism. N/S
Reaction
A
b
E
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
NH2CONH2 + H2O = NH3 + NH3 + CO2 NH3 + M = NH2 + H + M NH3 + H = NH2 + H2 NH3 + O = NH2 + OH NH3 + OH = NH2 + H2O NH2 + N = N2 + H + H NH2 + NO = N2 + H2O NH2 + H = NH + H2 NH2 + O = NH + OH NH2 + OH = NH + H2O NH2 + HO2 = NH3 + O2 NH + O2 = NO + OH NH + NH = N2 + H + H NH + N = N2 + H NH + NO = N2 + OH NH + NO = N2O + H N2O(+M) = N2 + O(+M)
6.13E+10 2.20E+16 6.36E+05 9.40E+06 2.00E+06 7.20E+13 1.26E+16 4.00E+13 6.80E+12 4.00E+06 1.00E+13 1.30E+06 2.50E+13 3.00E+13 2.16E+13 2.90E+14 7.910E+10
0.00 0.0 2.39 1.94 2.04 0.0 1.25 0.0 0.0 2.0 0.0 1.5 0.0 0.0 0.23 0.4 0.00
20980.0 93470.0 10171.0 6460.0 566.0 0.0 0.0 3650.0 0.0 1000.0 0.0 100.0 0.0 0.0 0.0 0.0 56020.0
Table 4 Injection parameters for different simulation cases. Simulation cases
Injection timing
Injection orientation
Urea mass fraction
Injection rate (% of diesel injection) (%)
Pure biodiesel combustion
No urea injection
NA
NA
0
Effects of injection timing
10 °ATDC 15 °ATDC 20 °ATDC 25 °ATDC
60°
20
20
Effects of injection angle
15 °ATDC 20 °ATDC 25 °ATDC
50° 60° 70°
20
20
Effects of urea mass fraction
15 °ATDC 20 °ATDC 25 °ATDC
60°
20 50 80
20
Effects of total mass injection
15 °ATDC
60°
50
10 20 40
3. Results and discussions 3.1. Mechanism validation Developed biodiesel + urea reaction mechanism was first validated against the experimental results conducted on a light duty 2KD FTV Toyota car engine in terms of in-cylinder pressure, heat release rate, and exhaust emissions of NOx and CO. The test engine is a four-cylinder, four-stroke, turbocharged, direct injection diesel engine with a rated engine power of 75 kW and rated engine speed of 3600 rpm. Detailed engine specifications are listed in Table 1. For the purpose of this study, simulation models were validated based on the experimental results of pure biodiesel (derived from waste cooking oil) combustion at a medium engine speed of 2400 rpm under full load conditions. Detailed descriptions on the experimental set-up, engine testing procedures, emission analyzer specifications and biodiesel fuel properties can be found in [32]. Fig. 2 compares the predicted and experimental results of the in-cylinder pressure and apparent heat release rate curves. It can be seen that the simulation results agree well with the experimental results, although slightly higher peak cylinder pressure and heat release rate are seen for the computational results due to the slightly over predicted combustion rate during the premixed combustion phase by the biodiesel reaction mechanism. Furthermore, the simulated and experimental results of NOx and CO emissions were normalized to the indicated power, and expressed and
Fig. 2. In-cylinder pressure and heat release rate validations of pure biodiesel combustion at 2400 rpm and 100% load.
compared in terms of indicated specific exhaust emissions as shown in Fig. 3. As seen, the predicted indicated specific NOx emission is about 15 ppm/kW and gives rise to about 7% deviation from the experimental results, while the predicted and experimentally determined CO emissions are 0.41 ppm/kW and 0 ppm/kW respectively, suggesting that the NOx and CO emission sub-models are
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Fig. 3. Emissions validation of pure biodiesel combustion at 2400 rpm and 100% load.
Fig. 5. Effects of urea/water solution injection angle on NO emission.
very accurate to emulate the NOx and CO formation and oxidation processes. 3.2. Effects of injection timing The efficiency of aqueous urea NOx removal process highly depends on its operating conditions such as reaction temperature and residence time, which further requires an accurate control and optimization on the post injection timing of the urea/water solution during the expansion stroke. Early injection timing would give us a longer residence time and higher combustion temperature, while late injection timing would lead to a shorter residence time and lower effective temperature range. However, for extreme cases when the injection timing is too close to the main injection of biodiesel fuel, the NOx reducing agents would be sprayed into the high temperature flame zones, leading to an undesirable oxidation of NH3 and elevated NOx emissions. Fig. 4 shows the effects of different aqueous urea injection timings on the total in-cylinder NO formation process (based on one cylinder, same for subsequent sections) with an injection angle of 60° (medium injection angle), urea mass faction of 50% (medium mass fraction) and injection rate of 20% (medium injection rate). As can be seen, for the case of pure biodiesel combustion without urea/water injection, the NO concentration increases rapidly after the top dead center, and reaches a peak value of 132 ppm without significant further oxidation
Fig. 6. Effects of urea mass fraction on NO emission.
Fig. 7. Effects of total urea/water solution on NO emission.
Fig. 4. Effects of urea/water solution injection timing on in-cylinder NO formation process.
processes. However, for the cases with urea/water post injection, both the peak and final NO concentrations have reduced significantly except for the case with post injection timing of 10 °ATDC. As expected, with posting injection timing immediately after the
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top dead center (10 °ATDC in this case), the predicted average cylinder temperature is as high as 1450 K which is sufficiently high to result to the oxidation of NH3 and adversely increased NO emissions. However, with further retarded injection timings, the introduction of urea/water solution effectively reduces the NO emissions, with 15 °ATDC post injection timing being the most effective.
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3.3. Effects of injection angle Besides injection timing, the orientation of the aqueous urea injection also plays a vital role in determining its efficiency by affecting the amount of effective NOx removing agents that are sprayed into the high temperature combustion regions where most of the NOx emissions are formed. Fig. 5 compares the predicted NO emission with three different injection angles of 50°, 60° and 70°, urea mass faction of 50% and injection rate of 20% under three different injection timings of 15 °ATDC, 20 °ATDC and 25 °ATDC conditions. It can be seen that under all the 3 different injection timings, the 50 degrees injection angle consistently demonstrates a lower NO formation level, indicating that 50 degrees injection angle seems to be an optimal injection orientation which gives the most effective NO removal process. However, this simulation result is not conclusive, and optimization study should be conducted based on the engine piston design, injector specifications, injection strategies and etc. 3.4. Effects of urea mass fraction and total mass injection In the present study, the thermal decomposition of urea into ammonia was assumed by a one-step gas phase hydrolysis reaction as shown in Table 3 (reaction 1). From reaction 1, it can be seen that although larger aqueous urea injection rate is expected for a more complete NOx removal process, the urea/water ratio also plays an important role in determining its efficiency on the urea
Fig. 8. Injection strategy of urea/water solution on NO emission.
NO Concentration Contours Crank Angle Without Urea Injection
With Urea Injection
15 °ATDC
25 °ATDC
50 °ATDC
Fig. 9. Spatial plots of the in-cylinder NO concentrations at 15 °ATDC, 30 °ATDC and 50 °ATDC of pure biodiesel combustion with and without urea injection.
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decomposition process. In order to highlight the importance of urea mass fraction on the urea hydrolysis rate and hence the subsequent NH3 production and NOx removal process, Fig. 6 presents the final NO emission levels when aqueous urea is being injected at an injection angle of 60° (medium injection angle), injection rate of 20% (medium injection rate), urea mass factions of 20%, 50% and 80% under three different injection timings of 15, 20 and 25 °ATDC conditions. It appears evident that with 15 and 20 °ATDC post injection timings, 50% urea mass fraction presents the lowest NO emission levels. In addition, although a linearly reduced NO emission level is observed for the 25 °ATDC post injection timing case, no tangible improvement on the NO removal efficiency is demonstrated from 50% to 80% urea mass fraction, suggesting that 50% mass fraction could be the optimal mass fraction under all the three mass fractions that are being considered in this study. Besides urea mass fraction, Fig. 7 further examines the effects of total urea/water solution on the NOx removal efficiency. The three different cases were simulated with an injection angle of 60° (medium injection angle), urea mass fraction of 50% (medium mass fraction), injection timing of 15 °ATDC and three injection rates of 10%, 20% and 40%. As expected, the NO emission level reduces almost proportionally with the increase of total urea/water solution. However, with further increase in the aqueous urea injection rate, the NOx removal effect may become less evident and simultaneously result to significant amount of urea emissions, which will be discussed in the next section. 3.5. Injection strategy optimization From the above analysis, it suggests that the optimal injection timing, injection angle, urea mass fraction and total urea injection rate are 15 °ATDC, 50°, 50% and 40% respectively. To further optimize the urea NOx removal process, another three simulation runs were conducted with the injection timing of 15 °ATDC, injection angle of 50°, urea mass fraction of 50% and three injection rates of 20%, 40% and 50%. It is found that the NO emission level has reduced from 68 ppm to 55 ppm when the total urea/water solution injection rate increases from 20% to 40% as shown in Fig. 8. However, when the aqueous urea increases from 40% to 50%, only marginally reduced NO emission is found with severe sacrifice on the urea emissions. Comparing with the case of pure biodiesel combustion without post aqueous urea injection, the injection of aqueous urea with 40% injection rate significantly reduces the NO emission from 131 ppm to 55 ppm, achieving a reduction efficiency of 58%. This is further proved in Fig. 9 which compares the spatial distribution of the in-cylinder NO concentration at 15 °ATDC, 30 °ATDC and 50 °ATDC of pure biodiesel combustion with and without post aqueous urea injection. As seen, a remarkable reduction on the local NO concentration is observed when the aqueous urea is being injected into the combustion chamber and reacted with the surrounding NO emissions especially at 50 °ATDC. In addition, it is also discerned in this study that with the aqueous urea injection, the predicted indicated power and CO emissions have changed from 9.4 kW and 4 ppm respectively to 10 kW and 13 ppm, indicating that no severe impacts on the CO emissions and BSFC are found. Hence, it can be concluded that urea direct injection can be an effective method to reduce the NOx emission of a biodiesel fueled diesel engine. 4. Conclusions Direct aqueous urea solution injection strategy was successfully investigated on a four cylinder, turbocharged, direct injection diesel engine to secure the normal NOx removal rate without severe impacts on the CO emissions and BSFC. Simulation studies were
conducted based on pure biodiesel combustion at an engine speed of 2400 rpm under full load conditions with and without aqueous urea injection. The aqueous urea solution was proposed to be injected after the fuel injection process during the expansion stroke, and to ensure an effective and efficient NOx removal process, various injection timings, injection angles, injection rates, and urea mass fractions were carefully controlled and examined. It was suggested that, the introduction of urea/water solution effectively reduced the NO emissions with 15 °ATDC post injection timing being the most effective. However, with the post injection timing too close to the end of main fuel injection such as 10 °ATDC, the predicted average cylinder temperature was too high to result to the oxidation of NH3 and adversely increased NO emissions. Among all the three injection angles that were studied, 50 degrees injection angle consistently demonstrated a lower NO formation level. In addition, the urea/water ratio was also found to play an important role in determining the efficiency of the urea decomposition process, and 50% mass fraction was found to be the optimal mass fraction. Furthermore, the NO emission level reduced almost proportionally with the increase of total urea/water solution. The final suggested injection strategies were 15 °ATDC injection timing, 50 degrees injection angle, 50% urea mass fraction and 40% total urea injection rate. Comparing with the case of pure biodiesel combustion without post aqueous urea injection, the optimized injection strategy significantly reduced the NO emission from 131 ppm to 55 ppm, achieving a reduction efficiency of 58% without severe impacts on the CO emissions and BSFC for a biodiesel fueled diesel engine. Acknowledgment This work is supported ‘‘R-265-000-800-733’’.
by
the
MOE
research
grant
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