Browse Topic: Selective catalytic reduction (SCR)
Fe/zeolite selective catalytic reduction (SCR) catalysts are commercially used for NOx emissions reduction from diesel engines. In comparison to Cu/zeolite, these catalysts are widely reported to form less N2O as a byproduct of the SCR reactions. However, Fe/zeolite SCR is less active than Cu/zeolite for low temperature NOx conversion under standard SCR conditions. In this study, a state-of-the-art Fe/zeolite SCR catalyst is probed with a combination of N2 physisorption, SEM/EDX, reactor-based performance and active site quantification. Measurements investigate the impact of degreening, mild and extreme hydrothermal aging. In a degreened condition, the impact of water vapor on standard and fast SCR and isothermal desorption of NH3 is assessed. The Fe/zeolite catalyst’s hydrothermal durability is studied following hydrothermal aging at temperatures from 550°C up to 950°C. NH3 adsorption and temperature programmed desorption (TPD) and NO2 adsorption and TPD experiments are used to quantify the surface acidity and active Fe sites of the catalysts, respectively. Kinetic analysis of the standard SCR data is conducted to elucidate the mechanisms responsible for SCR activity loss upon hydrothermal aging. The authors believe the results presented herein can support the industry wide efforts to continue to improve diesel emissions control.
As hydrogen internal combustion engines (H2-ICE) gain traction, optimizing exhaust aftertreatment technologies for nitrogen oxide (NOx) control has become increasingly critical. While selective catalytic reduction (SCR) systems remain the primary approach for NOx mitigation, oxidation catalysts are also being explored to facilitate hydrogen oxidation and improve overall exhaust treatment efficiency. This work presents a multifunctional catalyst (MFC) concept that combines supported Pd and Cu-zeolite to enable simultaneous NOx reduction and hydrogen oxidation within a single catalytic unit. Preliminary results show that hydrogen oxidation on supported Pd occurs above 300 °C, while Cu-zeolite achieves nearly complete NOx conversion. Experiments on individual components indicate that supported Pd initiates ammonia oxidation only after hydrogen is depleted. In the presence of hydrogen, ammonia conversion remains below 20%, indicating that hydrogen availability suppresses ammonia oxidation, which is favorable for SCR operation. The MFC can be configured either on a substrate or as an on-filter catalyst (MFCoF), providing simultaneous chemical conversion and urea-derived particulate filtration. In addition to hydrogen engines, the MFCoF concept can be applied to diesel and biodiesel engines, enabling effective filtration of soot particles while maintaining NOx reduction performance along with hydrocarbon oxidation capability. By combining NOx reduction, hydrogen/hydrocarbon oxidation, and particulate filtration in one unit, the MFCoF approach provides a promising pathway for next-generation exhaust systems across diverse engine platforms.
In the pursuit of achieving stringent BS VI emission standards, maintaining the efficiency of Selective Catalytic Reduction (SCR) systems is paramount, especially in vehicles operating under low duty cycles. A significant concern in such scenarios is the accumulation of urea deposits within the SCR, which can lead to detrimental push-out effects and compromised catalyst performance. This issue is particularly prevalent during low-temperature operations, where the conditions are less favorable for the effective conversion of nitrogen oxides (NOx). To address this challenge, an innovative software control system has been developed to monitor operating conditions and detect potential urea deposit faults. The software continuously evaluates parameters such as temperature and vehicle duty cycle, identifying conditions that may lead to urea crystallization within the SCR system. When unfavorable conditions are detected, the software triggers a fault alert that activates a regeneration process aimed at dissolving the accumulated urea deposits. This proactive approach not only prevents the adverse effects of urea buildup but also ensures the continued efficiency of the SCR system in reducing NOx emissions. By facilitating timely regeneration, the software enhances the operational reliability of the SCR catalyst, thereby supporting compliance with regulatory standards.
The study emphasizes on development of Diesel Exhaust Fluid (DEF) dosing system specifically used in Selective Catalytic Reduction (SCR) of diesel engine for emission control, where a low pressure pumpless DEF dosing system is developed, utilizing compressed air for pressurizing the DEF tank and discharging DEF through air assisted DEF injection nozzle. SCR systems utilize Diesel Exhaust Fluid (DEF) to convert harmful NOx emissions from diesel engines into harmless nitrogen and water vapor. Factors such as improper storage, handling, or refilling practices can lead to DEF contamination which pose significant operational challenges for SCR systems. Traditional piston-type, diaphragm-type, or gear-type pumps in DEF dosing systems are prone to mechanical failures leading to frequent maintenance, repairs, and costly downtimes for vehicles. To overcome the existing challenges and to create a more reliable and simple DEF delivery mechanism the pumpless DEF Dosing system is developed. The system includes a completely sealed DEF tank, pressurized to a calibrated system pressure, by connecting the tank entry line to compressed air. Signal from the Engine Control Unit is provided to the metering valve positioned in the delivery line which controls the quantity to be dosed. Pressure reducing valve, quick relief valve and pressure sensor for feedback are integrated, with the tank and the metering valve to develop a completely reliable DEF dosing system. The system has been modelled in MATLAB and tested for different operating pressure, height and volume of the tank and by varying the signals provided to the metering unit. Overall, we can conclude that the pumpless DEF dosing system is a simplified version among the existing DEF dosing practices. When implemented it provides significant reduction in cost, complexity, repair and maintenance of the system eliminating the challenges and failures posed by the existing pump-based dosing systems.
This paper is to introduce a new catalyst family in gasoline aftertreatment. The very well-known three-way catalysts effectively reduce the main emission components resulting from the combustion process in the engine, namely THC, CO, and NOx. The reduction of these harmful emissions is the main goal of emission legislation such as Bharat VI to increase air quality significantly, especially in urban areas. Indeed, it has been shown that under certain operating conditions, three-way catalysts may produce toxic NH3 and the greenhouse gas N2O, which are both very unwanted emissions. In a self-committed approach, OEMs could want to minimize these noxious pollutants, especially if this can be done with no architecture change, namely without additional underfloor catalyst. In most Bharat VI gasoline aftertreatment system architectures, significant amounts of NH3 occur in two phases of vehicle driving: situations with the catalyst temperature below light-off, which appear after cold start or at low-speed urban driving and hot, high mass flow phases. In this paper, we will compare several approaches to reduce NH3 starting with an existing gasoline technology, diesel technologies modified to gasoline conditions and the especially developed novel gasoline Secondary Emission Treatment (SET) catalyst, providing both ammonia abatement and underfloor three-way functionality. SET is the combination addressing both the cold start phase and hot driving conditions. In addition, it fulfills the role of an underfloor three-way catalyst, responsible for CO and NOx hot phase treatment.
Hydrogen internal combustion engines (H2-ICE) do not emit any fuel-borne carbon emission species. Nitrogen oxides are the remaining raw emission species at significant levels. However, the exhaust aftertreatment system is exposed to a different exhaust matrix, including unburned hydrogen. This raises the question of the role of hydrogen emissions for the aftertreatment system. Extensive synthetic gas bench (SGB) test campaigns address the role of hydrogen in several production catalyst components. Starting with selective catalytic reduction (SCR) systems, a systematic variation of the hydrogen concentration shows rather small effects on the NOX reduction performance. A change in selectivity results in increased secondary N2O emissions for a copper-zeolite system, whereas a vanadium-based SCR catalyst is unaffected. However, both SCR types are highly sensitive to the NO2/NOX ratio in the raw emission. Therefore, an upstream oxidation catalyst remains important for low temperature performance. Investigations of oxidation catalysts with varying platinum loadings show increased oxidation performance with higher hydrogen content. This effect is attributed to the accelerated heating of the catalytic centers due to the exothermic hydrogen reaction. In parallel however, secondary N2O emissions increase during light-off, speaking against a post-oxidation-based catalyst heating strategy. The strongest sensitivity to hydrogen is found in lambda sensors. Fast hydrogen diffusion through the zirconia distorts the signal towards rich mixtures. Overall, the results emphasize the important role of hydrogen, especially with respect to secondary N2O emissions, requesting H2-ICE-specific operating strategies to achieve zero-impact tailpipe emissions.
This paper investigates heated and cold Diesel Exhaust Fluid (DEF) sprays with the aim of establishing the effect of temperature on the resulting spray characteristics. The work is motivated by the need to optimize active Selective Catalytic Reduction (SCR) systems to meet more stringent nitrogen oxide (NOx) emission regulations for internal combustion engines. Pre-heating DEF has the potential to improve evaporation of the injected fluid, increasing the NOx conversion efficiency of the SCR at low exhaust temperatures. Experiments are carried out using the MAHLE SmartHeat fluid heater and mounted atop a DEF injector, with an incorporated thermocouple for fluid temperature. The fluid temperature established by the heater in this configuration was about 130 °C. The fluid is injected into an atmospheric environment and Schlieren imaging is used to visualize the spray evolution. CFD simulations are also carried out to validate the experimental observations and further shed light on the associated velocity field. With respect to the spray characteristics, the results show that the most significant difference between cold and heated DEF sprays occurs when the heated fluid experiences flash boiling prior to injection. This flash boiling is thought to arise from occasional overheating near the heating surfaces. Under conditions of normal hot liquid DEF injection, only a slight improvement in the atomization pattern is observed. The merit of the heating is expected to be more pronounced in the subsequent evaporation of the droplets. With respect to the simulations, it was found that the predicted penetration depths agree with the experimental observations. The observed pronounced differences between the characteristics of the cold and hot spray with flash boiling as well as the limited improvements of the hot liquid spray characteristics are explainable by considering their Reynolds and Weber numbers.
Cu/zeolite selective catalytic reduction (SCR) catalysts are used globally to reduce NOx emissions from diesel engines. These catalysts can achieve high NOx conversion efficiency, and they are hydrothermally durable under real world diesel exhaust environments. However, Cu/zeolite catalysts are susceptible to sulfur poisoning and require some type of sulfur management even when used with ultra-low sulfur diesel (ULSD). In the present study, the authors seek to better illuminate the chemical processes responsible for ammonium sulfate formation and decomposition occurring in Cu/zeolite SCR catalysts. Reactor-based experiments are first conducted with a real-world concentration of SO2 (0.5 ppmv) and a typical diesel exhaust water vapor concentration (7 vol.%) to quantify progressive effects of ammonium sulfate formation. A second group of experiments probe the chemical decomposition of ammonium sulfate via NO titration. The “movement” of sulfate species during this process is monitored with temperature programmed desorption experiments. Finally, the effect of NO2 on ammonium sulfate is investigated via either co-feeding during ammonium sulfate formation or post-feeding following ammonium sulfate formation, since it is expected that NH3, SO2, and NO2 will all be present at the catalyst surface during real-world operation. The authors believe the results presented herein can support the broad research effort to continue to improve low temperature NOx conversion which is notably degraded by the formation of ammonium sulfate species.
Selective Catalytic Reduction (SCR) is an optimized technology developed to encounter current BS6 Emission Regulations. AdBlue is the reductant used in the SCR Dosing system to eliminate NOx in the Exhaust gas. In order to ensure engine emissions compliance, insufficient or improper reductant in tank required to be detected. The right AdBlue concentration of 32.5% is highly necessary to attain the higher NOX conversion efficiency. Low concentration of the reductant will drastically reduce the NOx conversion in the system. Hence monitoring the AdBlue concentration in the tank itself is more important as per the OBD legislation. This implies on a physical quality sensor in the tank for detecting the reductant concentration. The functionalities of the quality sensor can also be instituted via a virtual software modelling called Improper Reductant Detection (IRD). IRD logic is highly robust and work competently to meet the BS6 stage 2 legislation’s NOx target. The reductant is suspected after each tank refill event, automatically IRD logic runs in the software in general, but the routine is started only if the NOx conversion efficiency is getting worse as well. The approach is to increase the AdBlue dosing rate if there is a suspicion of an improper reductant. The comparison of the NOx conversion efficiency before and after increasing dosing determines whether the reductant is good or not. Henceforth, the component failure of high-cost Quality sensor can be prevented and improvised through a software model that promises a low-cost solution to the product which in turn benefits the customer.
Urea-NH3 dosed Selective Catalytic Reduction is a powerful reaction system to ensure NOx reduction in the exhaust gases by minimizing ammonia slip. When the dosed ammonia exceeds the actual request than the required, NH3 to NOx ratio is potentially high, the unused ammonia is limited to 10ppm corresponding to experimental result of every World Harmonic Transient Cycle. The dosage estimation depends on the NOx sensors which has this drawback of high cross-sensitivity to ammonia that can affect the measurement of NOx and compromise the SCR-ASC closed loop strategies. This paper aims to resolve the complexity in prediction of ammonia slip to resolve the cross-sensitivity of tailpipe NOx sensor in the SCR system by a closed loop estimation of NOx and ammonia slip to ensure high NOx conversion efficiency. The focus is to develop a simplified model-based solution for estimating ammonia slip, because of the limitations in the real drive conditions in SCR system. This model approach is designed in such a way that it predicts the NOx and NH3 emissions after the SCR catalyst and calculates even in the failure conditions of closed loop feedback of urea dosing. A Filtering solution that combines the signals from inlet NOx sensor, tailpipe NOX sensor and the total system efficiency to provide a reliable estimation of NH3 slip.
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