NOx Conversion Efficiency Research Articles

Tracking control of dynamic ammonia coverage ratio references based on DC-SMC under a novel SCR-DeNOX structure

In response to the growing challenges of nitrogen oxides (NOX) emission control, NH3-based selective catalytic reduction (SCR) technology has been widely adopted to reduce NOX emission from marine diesel engine exhaust. In this study, the SCR mechanism model is based on a refined eight-state model. A novel structure of SCR-DeNOX system is established, which incorporates a by-pass valve and pipe. Unlike existing parallel and series SCR-DeNOX platforms, the novel structure shows a certain advantage in reducing NH3 and maintains a relatively high NOX conversion efficiency. Adaptive sliding mode observers (ASMO) are designed to generate dynamic ammonia coverage ratio references, replacing traditional fixed references. In addition, they also address the variations in the SCR-DeNOX model. The sliding mode control (SMC) is used to track the ammonia coverage ratio reference in upper cell by controlling ammonia injection. The decoupling with SMC (DC-SMC) is proposed to track the ammonia coverage ratio reference in the lower cell by controlling split ratio. The simulation results show the system with DC-SMC shows a 15.46% improvement in NOX conversion efficiency compared to that with SMC. Additionally, it shows a lower NH3 cost.

Synergistic approaches to NOx reduction: Fuel innovations and engine modifications in biomass waste-fueled CI engines

Achieving low nitric oxide emissions in biomass fuel combustion remains a challenge. This study aims at enhancement of NOx reduction by employing a sequence of strategies to arrive at the optimal combination of techniques. This work explores the role of nano particle emulsion, change in profile of combustion chamber geometry, exhaust gas recirculation rates and contribution of selective catalytic reduction (SCR) in the combustion and emission phenomena of CI engine. Cerium oxide and zinc oxide nanoparticles are added in emulsion form at 50 ppm and 100 ppm dosage to grapeseed oil biodiesel. Furthermore, combustion chamber geometry is modified to shallow depth and toroidal shape and tested with ZnO 100 nano blend of grapeseed oil biodiesel (GSBD). EGR at 5 %, 10 %, 15 % and 20 % rates are included in the toroidal shape after concluding that profile as the best performing geometry. A customized model of SCR is implemented to the above combination of ZnO fuel blend, toroidal combustion chamber geometry and 5 % EGR to further reduce NOx emission. SCR device is fabricated with suitable flow and injection parameters with dual type mixers to enhance its NOx conversion efficiency. Overall, this research work gives useful insights for development of NOx reduction strategies in biomass fuel combustion in CI engines. A comprehensive reduction of NOx from 9.3 g/kWh to 2.05 g/kWh is noticed at the end of cumulative set of experiments.

Effects of MOx modification (M = Mn, Ce, and Fe) on the SCR catalytic properties of the CrOx-WOx mixed oxide catalysts

In this study, a series of MOx-modified (M = Mn, Fe, and Ce) CrOx-WOx mixed oxides were synthesized via the sol-gel technique for the selective catalytic reduction of NOx with NH3. Given the addition of MnOx or CeOx, the availability of the catalyst surface acid sites tended to decline, which negatively affected the proceeding of the deNOx process. Moreover, the retention of oxidative ability by introducing MnOx also favored the over-oxidation of NH3, which accelerated the additional formation of NOx and created a crucial barrier for the increment in the higher-temperature deNOx activity. Apparently, <80 % of the NOx could be effectively eliminated from the working gas within the whole temperature range for the CrCeW and CrMnW samples. By contrast, doping FeOx was able to introduce ample acid sites onto the catalyst surfaces, which was beneficial for the adsorption of NH3 and subsequently the facilitation of the progression of the deNOx process. Furthermore, the weakened oxidizability suppressed the over-oxidation of NH3. Consequently, a broad operating temperature window was obtained for the FeOx-doped catalyst; >90 % NOx conversion efficiency was achieved in the temperature range of 230–450 °C. During the deNOx process, the Eley-Rideal mechanism dominated over the four investigated samples.

Experimental study of ammonia storage characteristics of selective catalytic reduction for diesel engine based on Cu-based catalysts

In this paper, the ammonia storage characteristics of Cu-based zeolite catalysts at different exhaust temperatures in heavy-duty diesel engines were experimentally investigated. The data such as exhaust temperature, rotational speed, torque, urea consumption, NOx concentration at selective catalytic reduction (SCR) inlet and outlet, ammonia storage, and NH3 leakage of the diesel engine were recorded respectively. The trend of ammonia storage characteristics of the SCR system was analyzed using the urea injection stopping time as the cut-off point. Both the SCR catalyst ammonia storage volume and the time to saturation of ammonia storage volume decreased with the increase in exhaust temperature. The maximum value of ammonia storage was 9.58 g at 193 °C and 1.08 g at 376 °C. The saturation time decreased from 466.4 s to 96.8 s. The increase in catalyst activity at high temperatures increased the rate of denitrification reaction, but it was observed that the ammonia slippage occurred earlier from 450 s to 100 s, which was mainly due to a more active ammonia desorption reaction at high temperatures. The larger ammonia storage at low temperatures made the effect on NOx conversion efficiency more significant, and the effect of exhaust temperature gradually increased when the temperature was further increased.

Simulation and experiment on improving NOX conversion efficiency of ship selective catalytic reduction system

Selective catalytic reduction (SCR) technology is the main means of reducing NOX emissions. Urea injection structure parameters (i.e., the hole number in a nozzle, spray angle, the distance between the nozzle and the front section of the catalyst) and operating parameters (i.e., exhaust gas temperature, gas hourly space velocity, ammonia-to-nitrogen ratio (ANR)) of the SCR systems have a significant impact on denitrification efficiency (deNOX). A three-dimensional simulation model is developed to investigate the impact of diverse parameters on deNOX. The influence of temperature, ANR and space velocity on the deNOX of SCR system is studied by using the control variable method. The simulation model is modified according to the SCR catalyst gas distribution test results. A diesel bench experiment is conducted to test the deNOX of the SCR system of D2 cycle and two additional operating points to verify the accuracy of the numerical simulation model. Based on this model, the influence of key factors on the deNOX of SCR system is studied. In SCR system, the optimal urea injection angle is about 75°, the optimal number of nozzle holes is 4, the distance between the optimal urea injection point and the front section of the catalyst is 8 times of the pipe diameter, and the optimal operating temperature is 300 ∼ 420℃. The influence trend of ANR on deNOX at different temperatures is obtained. This provides a theoretical support for the realization of SCR system in the future.

Ammonia coverage ratio control of a novel two-cell SCR-DeNOX system configuration based on IA-ISMC

To respond the China’s “14th Five-Year Plan” on the control of nitrogen oxides (NOX) emission, urea-based selective catalytic reduction (SCR) system has been widely applied to reduce the NOX emission from the diesel engine. NO2 is no longer a factor to be ignored while refining the SCR-DeNOX system model because of the ship diesel engine's long-term low temperature and oxygen-rich condition, which provides abundant conditions for the creation of NO2. Therefore, a refined eight-state two-cell SCR-DeNOX model is established in this paper. Meanwhile, the trade-off between NOX emission and tailpipe ammonia (NH3) slip is one of the major challenges which SCR-DeNOX system is facing. While meeting NOX emission standards, the requirement of limiting NH3 slip to prevent secondary pollution is becoming increasingly urgent, which needs a better structure or controller. Therefore, a two-cell SCR-DeNOX system with a by-pass valve in front of the NH3 injection is designed, integral sliding mode control based on inversion algorithm (IA_ISMC) is proposed for each cell. Based on the FTP-75 test cycle, there are three points of view are presented. Firstly, compared with the traditional six-state model, simulation results from the refined eight-state SCR-DeNOX model are closer to the experimental data. Secondly, the introduction of the by-pass structure has a mild impact on the NOX conversion efficiency, but can largely limit the NH3 slip. Finally, the controller based on IA_ISMC effectively improves the tracking performance of NH3 coverage ratio and reduces the cost of NH3 injection. To sum up, the refined two-cell SCR-DeNOX system with a by-pass valve based on IA_ISMC can enhance SCR performance and lower costs.

Multi-objective optimization of Fe-based SCR catalyst on the NOx conversion efficiency for a diesel engine based on FGRA-ANN/RF

With the development of industrial level and the increasingly strict emission regulations, selective catalytic reduction (SCR) system is one of the important ways to control NOx emissions from diesel engines. In this study, the effects of intake pressure on pressure and NOx conversion efficiency before and after catalyst were investigated at different loads. In order to improve the SCR efficiency, a multi-objective prediction method based on FGRA-ANN/RF was developed for SCR systems, where the predicted output parameters (the front and back section temperatures of the catalyst, the pressure difference, and the instantaneous catalytic efficiency) and the decision variables (the engine operating parameters). Firstly, the sensitivity analysis of the multi-objective inputs obtained from the ETC condition experiments is performed by the fuzzy grey relation method (FGRA) to select the high sensitivity parameters. Then, the prediction was carried out by artificial neural network (ANN) as well as the improved random forest (RF). The experimental results showed that the evaluation index R2 of RF was generally greater than 0.92 and the improved random forest method had high accuracy and robustness. It is valuable for solving SCR industrial simulation and reducing the hysteresis of ammonia injection response.

New insights into the various SCR performances of the CeOx-FeOx mixed-oxide catalysts prepared using different iron precursors

This article illustrated the effects of iron precursors on the deNOx performance of the CeOx-FeOx catalysts. With regard to the catalyst prepared using Fe(NO3)3 as the iron precursor (CeFe–N), relatively poor deNOx performance was observed; <80% NOx conversion was obtained at 130–450 °C. While for the samples synthesized using FeCl3 and Fe2(SO4)3 as the iron sources (CeFe–C and CeFe–S), >90% of the NOx was effectively removed from the working gas in the temperature range of 230–350 °C and >270 °C, respectively, signifying the broadened operational temperature range. The superior oxidative ability of metal cations primarily contributed to a low activation energy, which was favorable for the activation of the reactants and the acceleration of the reaction via the "fast SCR" pathway at lower temperatures. Meanwhile, a comparatively low availability of the surface acid sites signified the limited adsorption of NH3, which thus endowed CeFe–N with a low pre-exponential factor and primarily caused the restrained proceeding of the SCR reaction. In contrast, the presence of ample acid sites and the weakened oxidative ability of metal cations successively explained the increased pre-exponential factor and activation energy of CeFe–C and CeFe–S. The synergistic effects of the previously indicated two factors enabled these two catalysts to have a higher or similar low-temperature NOx conversion compared with CeFe–N. With increasing the temperature, the additional generation of NOx was favored over CeFe–N, causing a decline in its NOx conversion efficiency. On the contrary, the inhibited proceeding of NH3 over-oxidation ensured most NH3 to serve as the reductant to reduce NOx on CeFe–C and CeFe–S, which mainly explained their comparatively broad operational temperature window.

Investigation of the effect of different structure parameters and operating factors on the integrated exhaust aftertreatment system for diesel engines and parameter importance analysis

With diesel emission regulations growing more stringent, it is important to study the effect of different structure parameters and operating factors on the integrated aftertreatment system consisting of diesel oxidation catalyst (DOC), catalytic diesel particulate filter (CDPF) and selective catalytic reduction (SCR) during the design and optimization of the aftertreatment system. In this paper, a simulation model capable of exploring the interaction between the aftertreatment components was developed based on some relevant reaction mechanism, and the model accuracy was verified well using the experiment data. Then the effect of different parameters on the overall performance of the aftertreatment system was analyzed qualitatively. Finally, the importance score of each parameter was determined quantitatively based on Latin Hypercubic Sampling method and Random Forests algorithm. The results showed that larger diameter or larger cell density in DOC could improve the aftertreatment performance effectively. The increase in the wall thickness of each component had a negative impact on the aftertreatment system, except for CDPF wall thickness, which improved the filtration efficiency for particular matter (PM). When total aftertreatment length was fixed, high conversion efficiencies for carbon monoxide (CO) and total hydrocarbons (THC) were distributed in the region with large DOC length ratio, and NOx conversion efficiency was affected simultaneously by the length ratio of DOC, CDPF and SCR. An increase in CDPF length ratio and a decrease in DOC and SCR length ratios could contribute to the reduction of total pressure drop. Finally, Random Forests algorithm provided a good identification of the importance score of each parameter. DOC length and diameter played a dominant role in CO and THC conversion. The aftertreatment diameter had higher effect on NOx conversion efficiency than SCR length and PM filtration efficiency was mainly affected by DOC and CDPF length. Based on these qualitative and quantitative results, a design strategy for the integrated aftertreatment system was proposed, which provided some ideas for improving aftertreatment performance.

The effect of hydrothermal aging on Mn2O3/Cu-SSZ-39 composite catalysts for selective catalytic reduction of NOx by NH3

The latest generation of fuel-efficient engines produces exhaust at cooler temperatures, highlighting a need for enhancement in the low-temperature effectiveness of Cu-zeolite catalysts. The metal oxide/Cu-zeolite composite catalysts have demonstrated superior deNOx performance at lower temperatures compared to Cu-zeolite alone. However, the hydrothermal stability of these composite catalysts remains unexplored. In this study, we focus on the Mn2O3/Cu-SSZ-39 catalyst, chosen for its exceptional activity at low temperatures. The synergy between the nitrate groups on Mn2O3 and NH4+ ions on the Brønsted acid sites of Cu-SSZ-39 enhances NOx conversion efficiency. Nonetheless, the efficiency of Mn2O3/Cu-SSZ-39 diminishes following exposure to hydrothermal conditions at various temperatures (700 °C, 750 °C, and 800 °C), with the degradation degree intensifying at higher temperatures. Characterization studies revealed that hydrothermal treatment leads to the recrystallization of Mn2O3 particles, which reduces the content of active adsorbed oxygen. This reduction impedes the formation of active bridged nitrates in the aged catalysts and disrupts the SCR pathway through the interaction between bridged nitrates and NH4+ on the composite catalysts. Despite these challenges, some adsorbed oxygen remains on the Mn2O3/Cu-SSZ-39 after aging, ensuring that the aged composite catalyst still exhibits higher activity than the fresh Cu-SSZ-39 catalyst. This observation suggests that there is potential for the application of Mn2O3/Cu-SSZ-39 in practical scenarios.

Electric field-assisted NSR process for lean NOx reduction at low temperatures.

Lean-burn engines are gaining attention for their lower CO2 emissions, higher thermal efficiency, and improved fuel economy compared to traditional combustion engines. However, they present some difficulty for reducing nitrogen oxides (NOx) because of residual oxygen. To address this difficulty, NOx storage reduction (NSR) system, which combines noble metals and NOx adsorbents, is developed as a viable approach. But it requires cyclic operation, which adversely affects fuel efficiency. A novel approach proposed in this work is electric field-assisted lean NOx reduction, which applies an electric field to the NSR catalyst during lean conditions. This innovation uses surplus vehicle electricity for exhaust purification, enhances hydrogen transfer, and improves NOx reduction, even at low temperatures. Tests with a 3 wt% Pt-16 wt% BaO/CeO2 catalyst demonstrate markedly higher NOx conversion to N2 (13.1% vs. 2.9% without an electric field). This process is effective with extended electric field exposure, doubling the conversion rate. Electric field-assisted lean NOx reduction, by improving NSR technology, can enhance NOx conversion efficiency, reduce emissions, and optimize fuel efficiency in lean-burn engines.

Data-driven machine learning model of a Selective Catalytic Reduction on Filter (SCRF) in a heavy-duty diesel engine: A comparison of Artificial Neural Network with Tree-based algorithms

The Selective Catalytic Reduction on Filter (SCRF) system is yet to be deployed in current heavy-duty diesel engine aftertreatment system. Due to the thermal, space and cost benefits of the SCRF, it could become a useful component of the after-treatment system of a heavy-duty diesel engine, as regulators continue to demand an even cleaner environment. Ammonia cross-sensitivity of NOx sensors at the post-SCRF location poses challenges in measuring NOx emission accurately at this location and in turn affects the NOx conversion efficiency calculations. Developing a model that could replace the NOx sensor helps to mitigate the ammonia cross-sensitivity challenge as well as provides a medium to measure post-SCRF NOx concentration and NOx conversion efficiency while saving the cost on NOx sensors. This work focuses on a data-driven approach to developing a model for predicting NOx conversion efficiency across the SCRF using Artificial Neural Network, Bootstrap Forest, and Boosted Tree methods. Further, the three modeling techniques were also compared for accuracy and computation cost.

Study on Control Strategies on NOX Emissions to Meet Real Driving Emissions.

The aim of this study was to fulfill the NOx emissions standards for a light-duty diesel vehicle under real driving emissions (RDE) testing conditions by implementing various control strategies. In this study, RDE tests were performed by adjusting the air mass quantity and postinjection quantity in order to analyze engine-out and tail-pipe nitrogen oxides (NOx) emissions for different phases of RDE. The results showed that reducing in air mass quantity enabled the engine to operate in higher exhaust gas recirculation (EGR) rate regions, resulting in a 32.5% reduction in engine-out NOx emissions and an 80.4% decrease in tail-pipe NOx emissions. Increasing the postinjection quantity primarily enhanced the NOx conversion efficiency for the urban phase by 7.5%, leading to a 22.6% reduction in tail-pipe NOx emissions. By employing both strategies, vehicles can comfortably meet the CN6b emission regulations by a substantial margin.

Accelerated aging for after-treatment devices of diesel engine: Method, emission characteristics, and equivalence

After-treatment devices of diesel engines effectively reduce exhaust emissions but deteriorate over time. Consequently, the China-VI emission regulation mandates a useful life of seven years or 700,000 km for after-treatment devices. Conventional engine bench aging methods, being costly and time-consuming, are inapplicable for long-term emission durability verification. This paper introduces an accelerated aging method based on the thermal accumulation and Arrhenius equation. Accelerated aging of four after-treatment devices of the same model at different temperatures and durations was carried out. Results revealed that barring the CDPF's PN filtration efficiency, the efficiencies of the DOC, SCR, and ASC catalysts declined with accelerated aging, especially under lower temperatures. Deterioration in conversion efficiencies of catalysts increased the NOx, CO, THC, and NH3 emissions under cold- and warm-WHTC tests. Considering that the equivalent durability mileage of the after-treatment devices under 150 h 650 °C accelerated aging condition is approximately 870,000 km, while the CO, THC, PN, NOx, and NH3 emission factors are 0.05 g/kWh, 0.21 g/kWh, 2.14 × 1010 #/kWh, 0.44 g/kWh, and 3.48 ppm, respectively, so the durability of the tested after-treatment devices could comply with the China-VI emission regulation. Although accelerated aging did not significantly affect PN emissions, it increased the balanced point temperature of CDPF and the proportion of sub-23 nm particles. If emission regulations further reduce the lower limit of PN measurement to 10 nm, it could negatively impact the emission durability of CDPF. In addition, comparing the 1000 h of conventional aging showed similar characteristics and results in NOx conversion efficiency and NOx emission, suggesting that the accelerated aging method is reliable. In conclusion, the accelerated aging method reduces the durability verification duration, providing a cost-effective regulatory tool for environmental protection authorities and guidance for manufacturers to optimize device performance and durability.

Novel insights into the NOx emissions characteristics in PEMS tests of a heavy-duty vehicle under different payloads

Real Drive Emission (RDE) test with Portable Emission Measurement System (PEMS) is a widely adopted way to assess vehicle emission compliance. However, the current NOx emissions calculation method stipulated in the China VI emission standard easily ignores the NOx emissions during cold start and low-power operation. To study the effect of cold start and low-power operation on the calculation of NOx emissions in the PEMS test, in this study, a China VI Heavy-Duty Vehicle (HDV) for urban use was used to conduct PEMS tests under various vehicle payload conditions. The data analysis results show that the increase in vehicle payload is beneficial to reducing the specific NOx emissions and passing the NOx emission compliance test because the increased payload improves the NOx conversion efficiency of the SCR system. Cold start duration has no obvious relationship with vehicle payload, accounting for only about 4∼6% in each test, but contributing more than 30% of NOx emissions. Due to the effect of the power threshold and the 90th cumulative percentile, the cold start data has little influence on the result of the NOx emissions assessment and the maximum variation of the NOx emissions result in this study is an 8% rise. For the HDV for urban use, the variation of the power threshold resulting from vehicle payload is small, no more than 2% in this study. The presence of the power threshold makes almost only the low-power operation in the second half of urban driving have an impact on the NOx emissions calculation, which may make more than 50% of NOx emissions in the PEMS test be neglected. The impact of the low-power operation on NOx emissions calculation result will be significantly enhanced if all windows are considered in the Moving Average Window (MAW) method. In the meantime, the degree of variation is closely related to the NOx emissions level during the first half of urban driving. The maximum deterioration of NOx emission assessment result can be more than 90% in this study.

Experimental investigation on the characteristics of ammonia storage for heavy-duty diesel engine SCR catalyst

The ammonia storage characteristics were studied experimentally in two stages before and after shutting off the urea injection on a full-size Cu-based zeolite SCR catalyst. The experiment was conducted under the engine operating conditions with exhaust temperatures of 180, 220, 260, 300, 340, and 380 ℃. The variations of the ammonia storage amount, NOx conversion efficiency, and ammonia slip were separately investigated within the period of ammonia storage and consumption process. The results indicate that the maximum ammonia storage is only 10 g at 180 ℃, but around 43 g at 220 ℃. The maximum ammonia storage decreases with the increase of exhaust temperature over 220 ℃, which drops from 35 g at 260 ℃ down to below 13 g at 380 ℃. The most extended duration of 8082 s on the ammonia storage process occurred at the exhaust temperature of 220 ℃, mainly due to the deposit formation. The time width of the ammonia storage process is shortened with the exhaust temperature increasing over 220 ℃. The maximum NOx conversion efficiency is 80% at the temperature of 180 ℃ and 100% when the temperature rises to 220 ℃ or above. The higher temperature can improve the catalyst activity to supply more activated ammonia reacting with NOx. However, it promotes further ammonia slip. The ammonia storage and exhaust temperature are the main factors in the NOx conversion performance in the SCR catalyst. When the temperature is below 220 ℃, the NOx conversion efficiency strongly depends on the ammonia storage and increases with its amount. With the temperature further rising to 220℃ and above, the impact of the exhaust temperature on NOx conversion efficiency gradually increases.

The impact of the variation in driving conditions on the NOx emissions characteristics in PEMS test for heavy-duty vehicle

The real driving emission test using a portable emission measurement system is the advanced solution to assess NOx emissions from heavy-duty vehicles in recent years. To reflect the NOx emissions of heavy-duty vehicles more realistically, the restrictions on driving conditions will likely be relaxed in the next emission standard for heavy-duty vehicles. In this study, several tests with different payloads were conducted on a China VI heavy-duty vehicle under two different driving conditions. The test results show that some NOx emissions “spikes” may occur in any driving type during the tests of heavy-duty vehicles, and they are mainly related to the NOx conversion efficiency of the SCR system. In general, there is no obvious relevance between the NOx emission rate and the driving type, but the increase of the vehicle speed is beneficial to maintain the temperature of the SCR system in the high-efficiency zone, thus reducing the NOx emission rate. The variation of driving conditions will cause a change in the valid windows of the test, which will lead to a change in the evaluation results of NOx emission compliance. Adding motorway driving to the test route significantly increases the power threshold of the valid windows and reduces specific NOx emissions of the valid windows, with a minimum reduction of more than 30% and a maximum reduction of nearly 50% for “SENOx-90” in this study. The variation in driving conditions can severely affect the true NOx emissions assessment, especially for low-payload tests. In this study, the difference in NOx emissions assessed due to variation in driving conditions reached 1/3 of the total NOx emissions and the difference will gradually decrease as the vehicle load increases.

Key design and layout factors influencing performance of three-way catalytic converters: Experimental and semidecoupled numerical study under real-life driving conditions

The optimization of three-way catalyst (TWC) design and layout is important for reducing mobile source emissions and requires numerical modeling. To date, the high computational cost of accurate models has inspired the development of numerous tradeoff approaches; however, none of them is fully satisfactory. To address this gap, we herein theoretically and experimentally explore the key design and layout factors influencing TWC performance under transient driving conditions, starting with the optimization and verification of a simplified reaction mechanism. This mechanism is used to construct a semidecoupled posttreatment GT-Power model featuring boundary conditions derived from experimental and upstream engine modeling results, and experiments on a real engine are carried out to evaluate the model accuracy. The performance of three posttreatment systems is analyzed to identify the key performance-affecting parameters, and the following conclusions are drawn. (1) For a single reactor, cell density is the most important factor influencing pressure loss.(2) The exhaust temperature rapidly decreases in the longitudinal direction of the reactor in the initial start stage, although the reaction rate always decreases in this direction during the whole cycle.(3) One-dimensional plug-flow reactor simulations do not account for inhomogeneity and local turbulence, thus overestimating the performance of large reactors.(4) Under the employed conditions, the exhaust temperature is sufficiently high for pollutant conversion, although the efficiency of this conversion often decreases in the high-speed stage. Moreover, we show that high-speed adaptation can be enhanced by increasing the crosswise size of the reactor and propose a new reactor structure. Compared with our model, this structure is characterized by a lower catalyst loading and NOx conversion efficiency but achieves higher hydrocarbon and CO conversion efficiencies. Thus, our study provides a useful and effective method for evaluating the design/layout of posttreatment devices and facilitates the development of more efficient TWCs, contributing to the establishment of a more ecofriendly society.

A review of solid SCR systems as an alternative for NOx reduction from diesel engines

Several NOx reduction technologies have been developed to alleviate the impact of NOx emissions on the environment and meet the more stringent upcoming emission legislation. Although UWS is the most commonly used NOx reduction technology, it has various issues listed in this article. Several alternative technologies based on solid materials as a gaseous ammonia source are developed to eradicate these issues. This article reviewed the solid SCR systems currently being developed as an alternative to UWS for deNOx applications. Furthermore, the chemistry of the various ammonia storage materials used in these systems and their potential to produce gaseous ammonia for use in SCR have been evaluated. Furthermore, the deNOx performance of ammonium salts-based and metal ammine chlorides-based solid SCR systems have been evaluated. Besides, we have discussed various numerical modeling and simulation approaches applied to develop these solid SCR systems. Due to gaseous ammonia injection solid SCR systems offer better NOx conversion efficiency compared with UWS systems, especially at low exhaust gas temperatures. The authors believe this article will be helpful for future studies in this field to further develop and establish these solid SCR systems as a viable alternative deNOx technology.

Effect of migration of ZCu(OH) to Z2Cu upon hydrothermal aging on the NH3-SCR reaction mechanism over a Cu-CHA catalyst

In this study, the experiments combined with activity tests and characterizations were conducted to investigate the influence of hydrothermal aging (HTA) on the reactivity of NH3-SCR over a Cu-CHA catalyst. The results showed that the NOx conversion efficiency can reach more than 80% below 225 ℃ in the reaction between NH3 and NO+O2 after hydrothermal aging, but it increases the N2O production. The wave spectrum of in-situ DRIFTs confirmed the notable differences in the NH3-SCR mechanism between Cu species of ZCu(OH) and Z2Cu at the same reaction conditions. The NH3-SCR reaction occurring on ZCu(OH) followed the ‘Langmuir-Hinshelwood’ (L-H) mechanism, where NH3 combined with products that oxidize NO to NO2 via dimerization and disproportionation reactions to produce the key intermediates as NH4NO2 and HONO. However, the entire NH3-SCR on Z2Cu followed the ‘Eley-Rideal’ (E-R) mechanism, in which adsorbed NH3 effectively reacted with gas-phase NO to form H2NNO and then produced N2.