NOx Conversion Efficiency Research Articles

Effect of Zirconia on Hydrothermally Synthesized Co3O4/TiO2 Catalyst for NOx Reduction from Engine Emissions

Effect of zirconia on the 6 wt.% Co3O4/TiO2 catalyst for NOx reduction is investigated in this paper. Co3O4/TiO2 catalyst was prepared by using hydrothermal method and then was promoted with zirconia by impregnation to get 8% wt. ZrO2-Co3O4/TiO2 catalyst. Catalysts were characterized by using XRD, SEM, and TGA. Catalysts real time activity was tested by coating them on stainless steel wire meshes, containing them in a mild steel shell and mounting them at the exhaust tailpipe of a 72 cm3 motorcycle engine. Zirconia promoted catalyst showed higher conversion efficiency of NOX than the simple Co3O4/TiO2 catalyst due to small crystalline size, fouling inhibition and thermal stability.

Experimental Study on Solid SCR Technology to Reduce NOx Emissions from Diesel Engines

To solve the problems of low exhaust temperature NO x conversion efficiency of urea Selective Catalytic Reduction (SCR) and easy production of urea crystals during actual vehicle operation, this paper studies the effect of solid SCR on NO x emission on the engine bench. The experimental results show that for a solid SCR carrying the same reducing agent, its volume is only 1/3 the volume of urea SCR. When the exhaust temperature is 160°C, the NO x conversion efficiency of the solid SCR system can reach 40%. Based on the same ammonia-nitrogen ratio setting, the World Harmonized Steady Cycle (WHSC) NO x conversion efficiency is improved by 3.3%, and the World Harmonized Transient Cycle (WHTC) NO x conversion efficiency is increased by 4.5%. When the solid SCR injection temperature is reduced to 160°C, the NO x conversion efficiency is significantly improved, which is 9.7% and 15.5% higher than that of the 200°C solid SCR system and the urea SCR system, respectively, and the number of power base windows is between [0 20]. The NO x emission of diesel vehicles matching the urea SCR system is significantly higher, reaching 2.38 and 1.73 times that of the solid SCR system with a starting temperature of 160°C and 200°C, respectively.

Highly dispersed MnOx–FeOx supported by silicalite-1 for the selective catalytic reduction of NOx with NH3 at low temperatures

MnOx–FeOx-Loaded silicalite-1 catalysts exhibit high NOx conversion at low temperatures.

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.

Simulation of the flow field and the chemical reaction coupling of selective catalytic reduction (SCR) system using an orthogonal experiment.

It is difficult to simulate both the flow field and the chemical reaction using, respectively, the flow state and kinetics calculations and actually reflect the influence of the gas flow state on the chemical change in a selective catalytic reduction (SCR) system. In this study, the flow field and the chemical reaction were therefore coupled to simulate a full Cu-Zeolite SCR system and the boundary conditions of the simulation were set by a relevant diesel engine bench test which included the exhaust temperature, the mass flow, and the exhaust pressure. Then, the influence of the gas flow state on the NOx conversion efficiency was investigated. Specifically, an orthogonal experimental design was used to study the influence of the injection parameters (position, angle, and speed) on the NH3 distribution by establishing the NH3 uniformity coefficient γ at the SCR catalyst capture surface in the flow field simulation. Then, the velocity capture surface of the SCR catalyst front section was sliced into coupled data transfer interfaces to study the effects of exhaust temperature, ammonia to NOx ratio (ANR), and the NO2/NOx on the NOx conversion efficiency. This was used as guidelines to optimize the SCR system control strategy. The results showed that a 1150 mm injection position, a 45°injection angle, and a 23 m/s injection velocity provided the most uniform NH3 distribution on the SCR catalyst capture surface. For constant injection parameters, the NOx conversion efficiency was the highest when the exhaust temperature was 200°C—400°C, the ANR was 1.1, and NO2/NOx was 0.5.

An analysis of SCR reactor deactivation impact on NOx emissions from a compression ignition engine

Catalytic exhaust gas aftertreatment devices fitted to combustion engines are susceptible to partial deactivation as their operating time progresses. This includes selective catalytic reduction (SCR) reactors meant for NOx emission control. There are several known deactivation mechanisms of SCR reactors already analyzed in detail in the literature. This paper, however, approaches the analysis of reactor deactivation by comparison of exhaust gas characteristics over repeatable cycle for fresh and aged samples of a SCR reactor for non-road mobile machinery. The research aims to outline which parameters describing the SCR reactor’s performance are most affected by its ageing. In order to do that, fresh and aged samples of the reactor were tested under the Non Road Steady Cycle. The acquired emission results, including concentration traces of particular compounds were analyzed. The specific NOx emission of the aged reactor was significantly higher than that of the fresh one. The NOx conversion efficiency of both reactors was found similar at periods of steady engine operation. It was recognized that during transient conditions the NOx conversion efficiency of the aged reactor was decreased. It was found that the main factor contributing to that phenomenon is the drop in the ammonia storage capacity of the aged SCR sample.

A NOx Emission Model Incorporating Temperature for Heavy-Duty Diesel Vehicles with Urea-SCR Systems Based on Field Operating Modes

The selective catalytic reduction (SCR) is the most commonly used technique for decreasing the emissions of nitrogen oxides (NOx) from heavy-duty diesel vehicles (HDDVs). However, the same injection strategy in the SCR system shows significant variations in NOx emissions even at the same operating mode. This kind of heterogeneity poses challenges to the development of emission inventories and to the assessment of emission reductions. Existing studies indicate that these differences are related to the exhaust temperature. In this study, an emission model is established for different source types of HDDVs based on the real-time data of operating modes. Firstly, the initial NOx emission rates (ERs) model is established using the field vehicle emission data. Secondly, a temperature model of the vehicle exhaust based on the vehicle specific power (VSP) and the heat loss coefficient is established by analyzing the influencing factors of the NOx conversion efficiency. Thirdly, the models of NOx emissions and the urea consumption are developed based on the chemical reaction in the SCR system. Finally, the NOx emissions are compared with the real-world emissions and the estimations by the proposed model and the Motor Vehicle Emission Simulator (MOVES). This indicates that the relative error by the proposed method is 12.5% lower than those calculated by MOVES. The characteristics of NOx emissions under different operating modes are analyzed through the proposed model. The results indicate that the NOx conversion rate of heavy-duty diesel trucks (HDDTs) is 39.2% higher than that of urban diesel transit buses (UDTBs).

Manganese oxide catalysts supported on zinc oxide nanorod arrays: A new composite for selective catalytic reduction of NOx with NH3 at low temperature

Abstract In this study, a new composite of low-temperature selective catalytic reduction (LT-SCR) catalyst is synthesized by impregnating MnOx onto ZnO nanorod arrays integrated with cordierite ceramic substrate (MnOx-ZnO/CC), which exhibits improved activities at 100–250 °C compared to its analogue that fabricated from TiO2 nanorod arrays (MnOx-TiO2/CC). By optimizing the synthesis conditions, the MnOx-ZnO/CC catalyst obtains a maximum NOx conversion efficiency of 96.8% at 200 °C. The introduction of ZnO nanorod arrays not only provides a high surface area, but also results in the formation of amorphous MnOx. A higher content of Mn4+, proper ratio of surface-adsorbed oxygen (Oα), and more Lewis acid sites are found on the surface of the MnOx-ZnO/CC catalyst than that on the MnOx-TiO2/CC counterpart. Additional information about the reaction mechanism is systematically investigated via in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), the results of which demonstrate that the LT-SCR reactions over the MnOx-ZnO/CC catalyst generally occur between NOx and NH3 species in the adsorption status, following the Langmuir-Hinshelwood pathway. The enhanced activities can be attributed to the generation of abundant NH4NO3 originating from NO2 in the “Fast SCR” route, because the synergistic effect between MnOx and ZnO facilitates the oxidation of NO.

Research on Optimization Design of SCR Nozzle for National VI Heavy Duty Diesel Engine

For the National VI heavy-duty diesel vehicles, NOx emission regulations are becoming more and more stringent, and the selective catalytic reduction (SCR) system has become a necessary device. The design of the adblue nozzle in the SCR system is especially critical, directly affecting the NOx conversion efficiency and deposit formation. According to the structure of a National VI diesel engine exhaust pipe and SCR system, the nozzle is optimized by computational fluid dynamics (CFD) method to avoid the collision between the urea droplets and the exhaust pipe wall, to ensure that the exhaust gas and the urea droplets are as much as possible in full contact to ensure a sufficient urea pyrolysis. With the optimized nozzle, the NH3 distribution uniformity of the inlet face of the SCR catalyst can increase from 0.58 to 0.92. Additionally, test verifications are implemented based on the spray particle size test and the engine bench tests; the results show that the Sauter mean diameter of the optimized nozzle is more decreased than the initial nozzle and that the NOx conversion efficiency of the World Harmonized Transient Cycle (WHTC) and World Harmonized Stationary Cycle (WHSC) cycle improves by nearly 3%; additionally, it can also avoid deposit formation.

Study on the denitrification performance of FexLayOz/activated coke for NH3-SCR and the effect of CO escaped from activated coke at mid-high temperature on catalytic activity.

Currently, activated coke is widely used in the removal of multiple pollutants from industrial flue gas. In this paper, a series of novel FexLayOz/AC catalysts was prepared by the incipient wetness impregnation for NH3-SCR denitrification reaction. The introduction of Fe-La bimetal oxides significantly improved the denitrification performance of activated coke at mid-high temperature, and 4% Fe0.3La0.7O1.5/AC exhibited a superior NOx conversion efficiency of 90.1% at 400°C. The catalysts were further characterized by BET, SEM, XRD, Raman, EPR, XPS, FTIR, NH3-TPD, H2-TPR, et al., whose results showed that the perovskite-type oxide of LaFeO3 and oxygen vacancies were produced on the catalysts' surfaces during roasting. Fe-La doping enhanced the amount of acid sites (mainly Lewis and other stronger acid sites) and the content of multifarious oxygen species, which were beneficial for NOx removal at mid-high temperature. Moreover, it was investigated that the effect of released CO from activated coke at mid-high temperature on the NOx removal through the lifetime test, in which it was found that a large amount of CO produced by pyrolysis of activated coke could promote the NOx removal, and long-term escaping of CO on the activated coke carrier did not have a significant negative impact on catalytic performance. The results of the TG-IR test showed that volatile matter is released from the activated coke while TG results showed that the weight loss rate of 4% Fe0.3La0.7O1.5/AC only was 0.0015~0.007%/min at 300-400°C. Hence, 4% Fe0.3La0.7O1.5/AC had excellent thermal stability and denitrification performance to be continuously used at mid-high temperature. Finally, the mechanisms were proposed on the basis of experiments and characterization results.

Durability of V2O5-WO3/TiO2 selective catalytic reduction catalysts for heavy-duty diesel engines using B20 blend fuel

The durability of V2O5-WO3/TiO2 selective catalytic reduction (SCR) catalysts for heavy-duty diesel engines was evaluated based on 500 h SCR durability tests conducted using B20 fuel (20% v/v biodiesel + 80% v/v petroleum diesel). The fresh and deteriorated SCR catalysts were characterized by X-ray fluorescence, X-ray diffraction, and Brunauer-Emmett-Teller surface area measurements to investigate the deterioration mechanism of the catalysts. The results show that the SCR de-NOx performance is degraded after SCR durability testing, particularly at high engine speeds. The NOx conversion efficiency of the deteriorated SCR catalysts decreases at temperatures above 400 °C but remains high at temperatures between 250 and 400 °C. The catalyst characterization results reveal that the deterioration in the de-NOx performance is not owing to anatase-to-rutile transformation of TiO2, but because of a decrease in the specific surface area and the loss of the catalyst components, which are greater in the front cross section of the SCR than in the rear cross section. For a given catalyst cross section, the decrease in the specific surface area exhibits a positive correlation with the flow rate of the exhaust gas.

Global kinetic modeling of rapidly pulsed reductants for lean NOx traps: Frequency domain analysis and impact of mass transfer

In the Rapidly Pulsed Reductants (RPR) process, also known as Di-Air (Bisaiji et al., 2011 [1]), hydrocarbons are injected in rapid pulses ahead of a lean NOx trap (LNT) in order to improve its performance at higher temperatures and space velocities while maintaining a reasonable fuel penalty associated with injecting reductants.In this study, a 23-step global kinetic model is developed to predict the performance and product selectivity of the RPR process using propylene as the reductant over a wide range of pulsing frequencies, particularly for the temperature range of 450–600 °C. Some of the previous studies, (Bisaiji et al., 2012 [2], Perng et al., 2014 [3]) have suggested a reaction mechanism based on the formation of stable isocyanate (NCO) intermediates at high temperatures which promote the NOx conversion efficiency. However, in this study, we are checking whether a global kinetic model based on the conventional (low frequency) NOx storage and reduction mechanism is able to predict the NOx conversion improvements at higher pulsing frequencies at high temperatures, where the stability and role of previously proposed intermediates for NOx reduction is more questionable.A wide range of low frequency switching experiments were used to fit the kinetic parameters to the low frequency response of the RPR process. This model was then used to explore the high frequency performance of the RPR process. The kinetic model can predict the increase in NOx and propylene conversion as well as the change in selectivity of the reaction products, such as NH3 and CO, when the pulsing frequency is increased from conventional LNT to optimal RPR frequencies near 1 Hz corresponding to maximum NOx conversion (Reihani et al., 2018 [4]). However, in order to predict the behavior of RPR above the optimal frequency, it was required to add a model of axial pulse mixing to the kinetic model.The model results indicate that the main mechanism for improvement in NOx conversion at high frequencies and high temperatures is the more efficient utilization of storage sites. Also, the drop in NOx conversion at frequencies higher than the optimal frequency is caused by axial mixing of the reductant pulses. In addition, the model and experimental results provide insights into the importance of washcoat diffusion resistance. Particularly, the NOx reduction step with propylene becomes increasingly limited by washcoat diffusion at high temperatures and pulsing frequencies. The agreement between the model and the experimental results provides strong support for using this model to describe the RPR process.

Exhaust emissions and conversion efficiency of catalytic converter for an ethanol‐fueled spark ignition engine

AbstractThe effects of the air‐fuel (A/F) ratio and spark timing on exhaust emissions and thermal efficiency were analyzed under part load conditions using an ethanol‐fueled spark ignition (SI) engine. The conversion efficiency of exhaust emissions after the application of a three‐way catalyst (TWC) to a gasoline engine was also evaluated. The cycle‐resolved measurement of hydrocarbon (HC) and NOx emissions was attempted, to investigate phenomena in the exhaust port with a fast‐response exhaust gas analyzer, and this was compared under different operating conditions. The different HC emission trends for the relative A/F ratios, as measured by a conventional exhaust gas analyzer and by the fast response analyzer, are thought to be due to differences in measuring location and the oxidation reaction of HC in the exhaust system. To optimize the ethanol A/F ratio to improve the conversion efficiency of TWC, it is thought that a slightly richer or leaner A/F ratio than stoichiometry would be better depending on the operating conditions and the levels of HC, CO, and NOx emissions, because the conversion efficiency of HC and CO is better with a leaner A/F ratio and the conversion efficiency of NOx is increased with a richer A/F ratio. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd

2.2 L 직접분사 디젤 엔진에서 LNT 흡장률이 NOx 정화 효율에 미치는 영향

2.2 L 직접분사 디젤 엔진에서 LNT 흡장률이 NOx 정화 효율에 미치는 영향

An investigation on the catalytic characteristic of NO reduction in SCR systems

Abstract The Selective catalytic reduction (SCR) technology is an effective and economical method for reducing NOx emissions from the heavy duty diesel engine. To ensure a reasonable design of the SCR system, it is critical to understand the effects of space velocity (GHSV), NO2/NOx ratio, NH3/NOx ratio and structural parameters of the SCR reactor on the NOx conversion efficiency. Using the experimental method to conduct all the investigations though the entire development cycle of SCR system is expensive and would cost a long time. Numerical method as a complementary tool to bench tests can help to optimize the design and shorten the development cycle of SCR system. In this paper, a mathematical model is developed to predict the NOx conversion efficiency over a Cu catalyst. The model is calculated in the MATLAB environment. The simulation results agree well with the experimental data, which indicates that the mathematical model is reliable and can effectively predict the performance of the SCR catalytic system. Then the effects of the parameters, such as space velocity, NO2/NOx ratio, NH3/NOx ratio, length, diameter and number of channels per square inch (CPSI) of the SCR reactor on the NOx conversion efficiency were studied.

Simultaneous optimization of urea dosing and ammonia coverage ratio of selective catalytic reduction system in diesel engine by using physico-chemical model based NSGA-II algorithm

The selective catalytic reduction (SCR) technology shall achieve a high NOx conversion efficiency but a limited reductant slip to satisfy the increasingly stringent Euro VI regulations. The physico-chemical model based non-dominated sorting genetic algorithm (NSGA-II) was applied here to gain the desired urea dosage feed ratio and achieve the optimal trade-off between NOx reduction and NH3 slip so that some benefits such as fuel and urea efficiency gains may be materialized; moreover, the dependence of the optimized solution on dynamic model was illustrated based on a sensitivity study and a reduced order one-state model was utilized to capture essential behaviors of the system under low computational burdens. Two different steady operating conditions were selected to validate optimal results from the model based NSGA-II method and conclude that they shall be in good agreement with acceptable errors. The optimization results were demonstrated by means of several typical cases where effects of the temperature and the space velocity on ammonia coverage ratio and NOx conversion efficiency were analyzed. In addition, the full load optimal maps for the engine were obtained based on our method to provide a reference or benchmark on the control system.

Selective catalytic reduction of NOx emissions by hydrocarbons over Ag–Pt/Al2O3 catalyst in diesel engine

Selective catalytic reduction is an application used to control NOx pollutants in diesel engines. Aqueous urea solution, commercially called AdBlue and obtained by mixing pure water and NH3, is the most commonly used reductant while the V2O5–WO3/TiO2 structure has a widespread use as catalyst in SCR technology. However, the SCR systems included AdBlue and V2O5–WO3/TiO2 structure have low NOx conversion efficiency at low exhaust gas temperatures. The use of hydrocarbons as reductant and catalysts containing silver improves the conversion efficiency of selective catalytic reduction systems at low exhaust temperatures. In this work, selective catalytic reduction of NOx emissions from diesel engines in the presence of hydrocarbons has been studied. While ethanol and biodiesel mixtures were used as hydrocarbons, the Ag–Pt/Al2O3 structure was preferred as the catalyst. Scanning electron microscope and X-ray fluorescence analyses of the catalyst produced by the impregnation method were carried out. In order to determine the NOx conversion efficiency of ethanol–biodiesel mixtures in the selective catalytic reduction system, tests were carried out at different engine loads and different exhaust gas temperatures under actual operating conditions. As a result of the tests carried out, it was concluded that the reductant, which consists of 15% biodiesel and 85% ethanol, has the highest conversion performance.

Structure Optimization for SCR Reactor Based on RSM Method

Selective Catalytic Reduction (SCR) system provides an effective approach to removing NOx emission, to meet stringent emission legislation of NOx generated by internal combustion engine. However, the oversized SCR system would increase its operation costs and occupy more space, which restricts its widespread application to a certain extent. Therefore, the compact structure design of SCR system is worthy of research considering the limited installation space. In the present study, one-dimensional SCR model is established, and the effect of structural parameters on the SCR performance has been investigated. Then, based on the Response Surface Methodology (RSM), the coupling relationship among these structural parameters is explored. The optimal structural parameter values of SCR are calculated through the multi-objective optimization function. The SCR volume of the optimal structural parameters is reduced by 23.82% and the pressure drop generated by SCR reactor is reduced by 10.38%. Meanwhile, the decrease of NOx conversion efficiency could be negligible under the same condition.

Effects of insulation on exhaust temperature and subsequent SCR efficiency of a heavy-duty diesel engine

Ammonia/urea selective catalytic reduction (SCR) is an efficient technology to control NOx emission from diesel engines. One of the critical challenges of SCR systems is its lack of catalytic activities in low temperature conditions such as cold start or warm-up periods. In this study, the effect of pipe insulation on the reduction of exhaust temperature drop was experimentally investigated based on dynamometer tests with a heavy-duty non-road diesel engine. The temperature drop of exhaust gas was measured with and without applying pipe insulation. Measurement results revealed that the effect of insulation on the mitigation of temperature loss becomes significant as engine brake power decreases. Thus, it is expected that the need for pipe insulation would be increased to keep SCR deNOx efficiency high, especially at low temperature conditions. Also, this study carried out numerical simulations to quantify the impact of pipe insulation on NOx conversion efficiency of a commercial SCR catalyst over a scheduled non-road transient cycle (NRTC) mode. Under the current heavy-duty non-road engine and SCR conditions, the application of pipe insulation would reduce 19.5 % of total cumulative tail-out NOx emission over NRTC hot mode. These results led to the cycle-averaged SCR efficiency of 98.2 % for ‘with insulation’, while it was predicted to be lowered to 97.6 % for ‘without insulation’.

The alkali resistance of CuNbTi catalyst for selective reduction of NO by NH3: A comparative investigation with VWTi catalyst

The alkali metal poisoned CuNbTi and VWTi catalysts were prepared by wetness impregnation method. Poisoning effects of various K2O mass ratios (0.1%–2%) on CuNbTi and VWTi catalysts were studied, respectively. CuNbTi exhibited an excellent alkali metal resistance that 2%K2O-CuNbTi still remained 80% NOx conversion efficiency and 98% N2 selectivity while 2%K2O-VWTi was completely deactivated. Moreover, different alkali metals (K, Na, Ca, Mg) with the same molar ratio were doped on CuNbTi. The poisoned degree followed the order: K2O > Na2O > CaO > MgO. Then 2%K2O-CuNbTi and 2%K2O-VWTi were selected for follow-up study. To understand the poisoning mechanism, further investigations were performed by SEM, XRD, N2-physisorption, XPS, EPR, NH3-TPD, Py-IR, H2-TPR, in situ DRIFTS characterizations and DFT calculations. The particle of CuNbTi and VWTi agglomerated and the surface area decreased after 2%K2O loading. Loss of acid sites and drop of reducibility resulted in the deactivation of 2%K2O-VWTi. By contrast, experimental and computational results indicated that the alkali resistance of 2%K2O-CuNbTi was mainly due to the interaction between Ti2NbOx support and K atoms that K atoms were preferentially bound to NbOH and NbO with a lower bonding energy of −2.33 eV–−2.83 eV when Cu atoms were coordinated to TiO with a binding energy of −1.54 eV. This protected the active copper species from linking to K2O and the weak acid sites were preserved with the increasing isolated Cu2+. Ti2NbOx weakened the impact of potassium on NH3 adsorbing over the catalyst while the preserved copper species provided adsorption sites and redox ability for NH3-SCR reaction. Hence, the synergetic effect of copper and niobium species contributed to the alkali metal resistance. Ti2NbOx trapped the potassium and retained active copper species over 2%K2O-CuNbTi catalyst while potassium deactivated both TiO2 and active vanadium species on 2%K2O-VWTi catalyst. Meanwhile, both Eley-Rideal (E-R) and Langmuir-Hinshelwood (H-L) mechanisms with adsorbed NH3 coordinated to the Lewis acid sites and bidentate nitrate as the dominating intermediate species existed during the NH3-SCR reaction procedure over 2%K2O-CuNbTi at 225 °C.