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Impact of Second NH 3 Storage Site on SCR NO x Conversion in an Ultra-Low NO x Aftertreatment System
Abstract
div class="section abstract"> Typical two-site storage-based SCR plant models in literature consider NH3 stored in the first site to participate in NH3 storage, NO x conversion and second site to only participate in NH3 storage passively. This paper focuses on quantifying the impact of stored NH3 in the second site on the overall NO x conversion for an ultra-low NO x system due to intra site NH3 mass transfer. Accounting for this intra site mass transfer leads to better prediction of SCR out NH3 thus ensuring compliance with NH3 coverage targets and improved dosing characteristics of the controller that is critical to achieving ultra-low NO x standard. The stored NH3 in the second site undergoes mass transfer to the first site during temperature ramps encountered in a transient cycle that leads to increased NO x conversion in conditions where the dosing is switched off. The resultant NH3 coverage fraction prediction is critical in dosing control of SCR. This phenomenon is evaluated and quantified with different aging conditions, where the increased second site storage and reduced standard SCR activity due to hydrothermal aging leads to further increase in the reported phenomena. Although this phenomenon was observed for both light-off SCR (Lo-SCR) and downstream SCR based on analysis of the data, the impact on Lo-SCR performance was found to be higher compared to the downstream system due to the transient thermal conditions and higher temperatures experienced by the Lo-SCR system. This mass transfer mechanism also plays a role in determining NH3 slip characteristics of Lo-SCR for real world conditions where the gradual transfer of NH3 in the axial direction leads to NH3 slip. This phenomenon is demonstrated using experimental data collected on a production engine for a set of HFTP, CFTP, RMC and LLC cycles
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div class="section abstract"> The heavy-duty low NOx program funded by EMA at Southwest Research Institute (SwRI) evaluates a combination of engine and advanced aftertreatment systems to achieve a 0.035 g/bhp-hr tailpipe NOx standard. This work emphasizes improvements to the light-off SCR (LO SCR) model used for low NOx controls. Two key mechanisms drive these improvements: the first is a real-time feedback system that utilizes the LO SCR outlet NOx sensor for short-term corrections to the model state, and the second involves adjustments to the dosing mechanism based on long-term trends in dosing signals compared to predicted NH3 consumption, derived from LO SCR inlet and outlet NOx sensors, referred to as long-term trim. An algorithm is incorporated to differentiate the LO SCR outlet NOx sensor readings into NOx and NH3 components based on cross-correlation between inlet and out NO x sensors termed as speciation. The integration of this speciation algorithm with both short-term and long-term trim mechanisms significantly enhances the accuracy of the model estimated NH3 storage state, as well as the prediction of outlet NOx, and NH3 levels under various transient conditions, including CFTP, HFTP, RMC, and LLC cycles. This improved accuracy in the LO SCR observer model enables more precise control of transient tailpipe NOx in the system.
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div class="section abstract"> Diesel aftertreatment (AT) systems are critical for controlling emissions of CO, HC, NOX, and PM in the on-road transportation sector. Ensuring compliance with regulatory standards throughout the AT system's lifespan requires precise prediction of various degradation mechanisms under real-world operating conditions and mitigating their impact through proper catalyst sizing and advanced controls. In the SwRI A2CAT-II consortium, a medium-duty diesel engine production aftertreatment system was subjected to full useful life aging, involving chemical poisoning with phosphorus (P) and sulfur (S) species, along with hydrothermal aging following the DAAAC protocol. This study was aimed to model and predict the aging trajectory of this production AT system thereby capturing changes in system dynamics under both steady-state and transient conditions. The system, designed to meet the 0.2 g/bhp-hr standard, comprised a Diesel Oxidation Catalyst (DOC), Diesel Particulate Filter (DPF), Selective Catalytic Reduction (SCR), and Ammonia Slip Catalyst (ASC). It was aged to 3600 hours equivalent to the DAAAC protocol and tested across a series of steady-state and regulatory cycles (HFTP, RMC, and LLC) at degreened, 33%, 66%, and 100% aging points. A linear decrease in system NOx conversion was observed between 0% to 66% aging, followed by a nonlinear drop from 66% to 100%. The cumulative decline in NOx conversion was 0.8%, which could significantly impact systems designed to meet a 0.05 g/bhp-hr target.
IntroductionAftertreatment durability demonstration is a mandatory validation exercise for on-road medium and heavy-duty diesel engine certification. This requirement ensures that the emission compliance can be achieved for the intended full useful life (FUL) of the engine system or multiple vehicle families. Conventional Deterioration Factor (DF) approach considers linear effect of sulfur and phosphorous poisoning on the catalysts over the entire useful life based on performance from 33% of aging the catalysts and extrapolating them to FUL. However, based on various studies, the impact of fuel derived Sulfur on diesel aftertreatment components was found to be exponentially significant particularly on the SCR due to the sulfur poisoning effect which require an active means to liberate sulfur to maintain appropriate SCR NOx conversion performance. Phosphorous from lubricating oil is known to adversely affect the activity of the oxidation catalyst in a catalyzed DPF thereby reducing the passive regenerative performance of DPF. For these reasons, an extensive understanding of chemical poisoning (particularly Sulfur and Phosphoros) and hydrothermal aging are warranted to design, validate and demonstrate the durability of aftertreatment components that are subjected to the prolonged chemical exposure.
To address the extensive challenges in durability demonstration, the Diesel Aftertreatment Accelerated Aging Protocol, or DAAAC, was developed by Southwest Research Institute as a part of consortium effort that includes input from diesel engine manufacturers [ 1 , 2 , 3 ]. DAAAC is an accelerated aging cycle developed for each application based on the available field data. It includes the exposure from hydrothermal aging, sulfur and lubricant derived poison at accelerated rates. The protocol also requires the entire aftertreatment system to be aged as a complete system, since the upstream components, such as DOC, can impact the chemical makeup of sulfur derived constituents.
The protocol does not introduce chemical constituents not normally observed in the field. The accelerated chemical exposure rate is limited only to a degree that has previously demonstrated successful correlation to normal, unaccelerated aging. The Protocol also requires that chemical aging mechanisms are to be introduced and / or consumed in a manner representative of the engine’s defined consumption pathways. Examples include sulfur exposure via high sulfur fuel or gaseous SO2 and oil consumption via pre-combustion / post-combustion pathways. The Protocol also does not introduce chemical components that are not normally present in oil or fuel (other than doping the fuel with higher concentrations of sulfur, but this amount is relatively small). A comprehensive step by step breakdown of DAAAC protocol is presented elsewhere [ 2 ].
The primary objective of this paper is to disseminate the observation of long-term impacts of chemical poisoning and hydrothermal aging on a production aftertreatment catalyst subjected to FUL DAAAC protocol. The results of this experimental campaign were used to develop and validate a model capable of predicting hydrothermal and chemical aging mechanisms of conventional diesel aftertreatment to optimize long term emissions reduction performance. The paper is divided into following sections: 1 Introduction: An overview of the system
2 Background: A review of literature in the field of diesel AT aging
3 Test Campaign: Description of experimental setup used for collection of both steady state and transient data.
4 Experimental setup: Description of burner stand used for data collection.
5 Results and Discussion: Description of experimental results followed by discussion about underlying degradation mechanism identified through simulation work.
6 Summary and conclusion
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In this study, we present the first of two parts in the development and validation of a two-site detailed global kinetic model for NH3 Selective Catalytic Reduction (SCR) over a well-characterized Cu-SSZ-13 catalyst. Based on fundamental literature studies and experimental data for two distinct hydrothermal aging conditions, it was observed that at least two distinct sites were necessary to describe the storage, oxidation, and SCR behavior for this catalyst. The site definitions were allocated based on analysis of simulated net desorption rates during the NH3-TPD experiment. The S1 sites were associated with extra framework copper ions stabilized by -OH ligands (ZCuOH), likely located near the eight-membered ring CHA cages, along with Brønsted acid sites, while S2 sites were associated with copper ions attached directly to the repeating units (Z2Cu) near the six-membered rings, along with physisorbed NH3 sites and low temperature transient copper dimers. The selective NH3 oxidation and NO oxidation reactions were only modeled over S1, in line with the expected catalytic sites for these reactions. Standard, fast, and NO2 SCR reactions were modeled on both sites, with different activation energies. Finally, noticeable nitrate-based hysteresis effects were observed, in both N2O concentrations and fast SCR NO conversions. These were accounted for by explicitly modeling nitrate formation, titration by NO, and thermal decomposition to N2O. The developed SCR model was validated with additional reactor data at nominal inlet NH3-to-NOx ratios (ANRs) of 0.8 and 1.2. In general, the model showed good predictability in the temperature range of 150–550 °C for both hydrothermal ageing conditions and space velocities. Further full-scale engine dynamometer validation and development of a downstream NH3 slip catalyst (ASC) reaction-diffusion model will be reported in the second part of the paper.
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Selective catalytic reduction (SCR) of NOx with NH3 is a widely used after-treatment technology for reducing the NOx emissions from diesel engines. Mathematical models play an important role in analyzing and optimizing SCR reactors and also in controller design. Detailed mathematical models of SCR reactors consist of a number of coupled differential and algebraic equations, which can only be solved numerically. Due to the limited computational capability of engine control unit (ECU) and hardware-in-loop (HIL) systems, it is not practical to solve detailed model equations on these systems in real time, and hence, there is a need for reduced-order models. In this work, we provide a systematic procedure for deriving reduced-order models from detailed models of a SCR reactor. This systematic procedure consists of making reasonable assumptions, good input signal design, and system identification procedure. The reduced-order model consists of non-stiff system of equations which can be solved with an explicit solver, is two to three orders of magnitude faster than the detailed model, and has same level of accuracy as detailed model. The proposed model remains fundamental-based, with model parameters directly related to physical and chemical properties of the catalyst, and can be used on ECU and HIL systems to predict ammonia storage, NOx conversion efficiency, and ammonia slip during transient operating conditions. Moreover, it is shown that the model equations can be easily linearized around various operating points, thus allowing the development of advanced control strategies based on linear control theory. Although mainly demonstrated in the context of SCR reactors, the procedures can be applied to other monolith reactors as well.
Selective catalytic reduction (SCR) is a promising diesel after treatment technology that enables low nitrogen oxides (NOx) tailpipe emissions with relatively low fuel consumption. Future emission legislation is pushing the boundaries for SCR control systems to achieve high NOx conversion within a tailpipe ammonia (NH3) slip constraint, and to provide robustness to meet in-use compliance requirements. This work presents a new adaptive control strategy that uses an ammonia feedback sensor and an online ammonia storage model. Experimental validation on a 12-liter heavy-duty diesel engine with a 34-liter Zeolite SCR catalyst shows good performance and robustness against urea under- and over-dosage for both the European steady-state and transient test cycles. The new strategy is compared with a NOx sensor-based control strategy with cross-sensitivity compensation. It proved to be superior in terms of transient adaptation and taking an NH3 slip constraint into account.
Selective catalytic reduction (SCR) is well known for exhaust gas aftertreatment in power plant applications. In the near future, it will be introduced in Europe in mobile heavy-duty diesel applications. Stringent specifications and the dynamic operation mode of such applications demand advanced control strategies. Such a control strategy is presented here. It incorporates a model-based feedforward controller (FFC) which handles the known dynamics of the plant. It further contains a feedback controller (FBC) to compensate for disturbances and slowly varying parameters. The realization of the feedback control loop is possible only due to the use of a nitrogen oxide (NOx) sensor which is cross-sensitive against ammonia (NH3), and the implementation of a procedure separating the detection of NOx and NH3.
The goal of a control system for a SCR catalyst on a DPF (SCR-F) is to minimize the fuel consumption due to back pressure while maximizing the NOx conversion across the SCR-F. To meet these goals, prediction of the internal states of the 2D spatial distribution of temperature, PM mass retained and NH 3 coverage fraction in an SCR-F is required. To predict these internal states, a state estimator capable of simulating the PM oxidation rate, SCR reactions, filtration efficiency and pressure drop across the SCR-F based on inlet conditions and outlet sensor data is required. A 2D model of the SCR-F capable of predicting the SCR-F internal states was developed and validated using experimental data from a Johnson Matthey SCRF® and Cummins 2013 ISB engine. To reduce the computational cost, an estimator model with a coarser mesh and a quasi-steady state solution for the chemical species and energy conservation equations was developed which was faster than real time. The model was combined with the outlet thermocouple, pressure drop and NO x sensor data from the engine control unit using an extended Kalman filter to create the SCR-F state estimator. The estimator was able to predict the target internal states to within 5% of the experimental data. The estimator was able to compensate for calibration parameter errors by up to 5% using the sensor data along with filtering of the sensor noise. A DOC estimator was used to obtain the SCR-F inlet NO x concentrations and temperatures.
The 2010 Environmental Protection Agency (EPA) Emission Standard for heavy-duty engines required 0.2 g/bhp-hr over certification cycles (cold and hot Federal Test Procedure [FTP]), and the California Air Resources Board (CARB) standards require upto 90% reduction of overall oxides of nitrogen (NOx) emissions. Similar reductions may be considered by the EPA through its Cleaner Trucks Initiative program. In this article, aftertreatment system components consisting of a diesel oxidation catalyst (DOC); a selective catalytic reduction catalyst on a diesel particulate filter (DPF), or SCR-F; a second DOC (DOC2); and a SCR along with two urea injectors have been analyzed, which could be part of an aftertreatment system that can achieve the 0.02 g/bhp-hr standard. The system performance was evaluated using validated one-dimensional (1D) DOC, two-dimensional (2D) SCR-F, and 1D SCR models at various combinations of inlet ammonia (NH3)-to-NOx ratio (ANR) values for the SCR-F and the SCR to determine the injection rates required to achieve an optimum nitrogen dioxide (NO2)/NOx ratio at the inlets of both the SCR-F and the SCR. A strategy was developed that yielded 99.5% NOx conversion at inlet temperatures from 203° to 450°C, while maximizing particulate matter (PM) oxidation rate in the SCR-F and minimizing the urea consumption rate. These system components have the potential to be robust to variations in the inlet NOx and NH3 concentrations and the NOx conversion performance of the system components. NOx conversions greater than 95% in the SCR-F and SCR were determined to be primarily due to the fast SCR reaction. The two urea injectors were used to maximize NOx reduction in both devices and SCR-F PM oxidation. For the case with ANR1 = 0, a 90%-100% increase in NO2-assisted PM oxidation in the SCR-F was determined compared to a system without the second DOC and urea injector. Further development of the system components should be pursued in terms of catalyst type, catalyst loading, and external heating along with a close-coupled SCR/DOC or passive NOx adsorbers (PNA) to reduce the light-off time for cold-start emissions control.
Dynamic changes in the state of a commercial Cu-SSZ-13 catalyst as a function of hydrothermal aging are explained through a unified and quantitative theoretical model. NH3 adsorption and desorption rate constants are identified on individual active sites, utilizing NH3-temperature programmed desorption (TPD) experiments on a commercial Cu-SSZ-13 and a model H-SSZ-13 catalyst. NH3 adsorption on Brønsted acid sites is described by a Type-II BET isotherm model to account for NH3 hydrogen bonded to a tetrahedral NH4⁺ ion. NH3 adsorption on different types of Copper sites is modeled with identical energetics, utilizing a Temkin isotherm model to account for minor site heterogeneity and lateral interactions between adsorbates. In the model, NH3 storage at low temperatures (<200 °C) is captured by a Physisorbed site to account for NH3 bound to extra-framework Al species and additional low temperature adsorption on Copper sites and Brønsted acid sites. The adsorption enthalpies and entropic penalties on individual sites in the kinetic model are consistent with the binding energies and entropies reported from first principles density functional theory (DFT) calculations by Paolucci et al., 2016, and the site-specific storage dynamics follow reported spectroscopic characterizations (Giordanino et al., 2014). Changes in NH3-temperature programmed desorption (TPD) peaks are then used as a probe to identify the transformation of individual active sites as a function of hydrothermal aging time and temperature, assuming fixed site-specific turnover rates (mean field approximation). An Arrhenius correlation is developed for the loss of Brønsted acid sites upon hydrothermal aging, yielding a similar activation energy for the aging process as reported by Luo et al., 2018. The quantification of different types of Copper sites is hypothesized, and the limitations of the mean field approximation at extreme aging temperatures are discussed. The systematic quantification of active sites as a function of hydrothermal age provides a foundation for improved understanding and modeling of the SCR reaction mechanism, and serves as a guide to better catalyst design for stricter durability requirements and lower NOx emissions.
In the recent years, engine developer attention is focused on the application of advanced control strategies for the SCR system, considered as the leading technology for the abatement of NOX emissions in Diesel engines. The most challenging task in SCR control is the optimization of the urea dosing strategy while reaching the balance between high NOX reduction efficiency and low NH3 slip in the tailpipe, especially during transients. In order to accomplish this task, an Extended Kalman Filter (EKF) estimator is developed, to be integrated in the closed loop control of SCR urea dosing. The EKF estimator is applied to a one-state model, in which the ammonia coverage ratio θ is the only dynamic process taken into account. To improve the model accuracy, an additional dynamics, related to total ammonia adsorption capacity Ω, is considered while conceiving the EKF. The recursive EKF algorithm allows achieving enhanced estimation accuracy with a slight increase of the computational burden that is still suitable with on-board application.
Different aftertreatment systems consisting of a combination of selective catalytic reduction (SCR) and SCR catalyst on a diesel particulate filter (DPF) (SCR-F) are being developed to meet future oxides of nitrogen (NOx) emissions standards being set by the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB). One such system consisting of a SCRF® with a downstream SCR was used in this research to determine the system NOx reduction performance using experimental data from a 2013 Cummins 6.7L ISB (Interact System B) diesel engine and model data. The contribution of the three SCR reactions on NOx reduction performance in the SCR-F and the SCR was determined based on the modeling work. The performance of a SCR was simulated with a one-dimensional (1D) SCR model. A NO2/NOx ratio of 0.5 was found to be optimum for maximizing the NOx reduction and minimizing NH3 slip for the SCR for a given value of ammonia-to-NOx ratio (ANR). The SCRF® + SCR system was simulated using the 2D SCR-F + 1D SCR system model. For all the test conditions, the NO2/NOx ratio downstream of the SCRF® was found to be 0 due to NO2 consumption by the NO2 assisted particulate matter (PM) oxidation and the SCR reactions in the SCRF®. Due to this low NO2/NOx ratio, the NOx conversion performance of the downstream SCR was limited to a maximum of 70% and the system performance to a maximum of 97%. The low SCR performance is due to low fast SCR (<10%) and high standard SCR reaction (>85%) rates in the downstream SCR. Also, high NH3 slip due to lower utilization by the SCR reactions was observed from the SCR. Improved NO2/NOx ratio at the SCRF® inlet results in NO2 slip at the SCRF® outlet, which then leads to better NOx reduction performance of this system.
The most recent 2010 emissions standards for heavy-duty engines have established a tailpipe limit of oxides of nitrogen (NOX) emissions of 0.20 g/bhp-hr. However, it is projected that even when the entire on-road fleet of heavy-duty vehicles operating in California is compliant with 2010 emission standards, the National Ambient Air Quality Standards (NAAQS) requirement for ambient particulate matter and Ozone will not be achieved without further reduction in NOX emissions. The California Air Resources Board (CARB) funded a research program to explore the feasibility of achieving 0.02 g/bhp-hr NOX emissions. This paper details the thermal management strategies employed by the engine and supplemental exhaust heat addition device as was needed to achieve Ultra-Low NOX levels on a heavy-duty diesel engine with an advanced technology aftertreatment solution Further development is necessary for optimizing vocational test cycle emissions, but the results presented here demonstrate a potential pathway to achieving ultra-low NOX emissions on future heavy duty vehicles.
In this paper, a model-based linear estimator and a non-linear control law for an Fe-zeolite urea-selective catalytic reduction (SCR) catalyst for heavy duty diesel engine applications is presented. The novel aspect of this work is that the relevant species, NO, NO 2 and NH 3 are estimated and controlled independently. The ability to target NH 3 slip is important not only to minimize urea consumption, but also to reduce this unregulated emission. Being able to discriminate between NO and NO 2 is important for two reasons. First, recent Fe-zeolite catalyst studies suggest that NO x reduction is highly favored by the NO 2 based reactions. Second, NO 2 is more toxic than NO to both the environment and human health. The estimator and control law are based on a 4-state model of the urea- SCR plant. A linearized version of the model is used for state estimation while the full nonlinear model is used for control design. An experimentally validated, higher or- der simulation is used to evaluate the performance of the closed loop system. For the cases considered, the control strategy uses less urea, produces less NH 3 slip, and less tailpipe NO x than a similar strategy where NO and NO 2 are assumed as all NO during estimation and control law implementation.
This paper presents a model-based control system for SCR urea dosing employing an embedded real-time SCR chemistry model and a NH3 sensor. The control algorithm consists of a number of control features designed to enhance ammonia storage control and closed-loop compensation using the mid-brick NH3 sensor. An adaptive control algorithm is developed to demonstrate robustness of the feedback control system to compensate for catalyst aging, urea injection malfunction, or dosing fluid concentration variation. Simulation and engine dynamometer testing following ESC and FTP emission cycles are used to demonstrate the advantages of this control approach for meeting both NOx emission requirements and NH3 slip targets. Furthermore this paper demonstrates potential of a NH3 sensor for on-board diagnostics. Additionally the feasibility of implementing model based algorithms in a 32-bit floating-point environment with an automotive controller is examined.
Selective catalytic reduction with NH3 (NH3-SCR technology), based on V2O5/WO3/TiO2 catalysts, has been previously commercialized for abating NOx emissions from various stationary and mobile lean-burn or diesel engines. However, meeting the uniquely stringent US EPA 2010 regulations for diesel engines required introduction of a new class of SCR catalysts, based on Cu- or Fe-exchanged zeolites. While remarkably active and stable, these new materials proved substantially more difficult than vanadia-based catalysts to operate transiently on the road, due to their much higher NH3 storage. The objective of this work was to develop a concise experimental protocol, elucidating multiple catalytic functions, steady-state and transient, of practical relevance to the mobile SCR applications. This paper provides a comprehensive overview of such functions, using select data from various representative Cu- and Fe-zeolite catalysts. While the bulk of the reported results originated directly from the developed protocol, additional experiments, validating the assumptions or clarifying unexpected experimental observations, are included.
This paper presents a diesel engine selective catalytic reduction (SCR) ammonia surface coverage ratio profile control design for a two-cell SCR system, which enables a staircase am- monia coverage ratio profile for simultaneously achieving high NO conversion efficiency and low ammonia slip. Such a novel two-cell SCR architecture greatly increases the control system complexity. A nonlinear backstepping-based control law is pro- posed to regulate the ammonia coverage ratio of the upstream cell to a desired (high) value in order to maintain high-SCR NO conversion efficiency, and to keep the ammonia coverage ratio of the downstream cell below a low upper limit for high-ammonia adsorption capability. The control law thus possesses the advan- tages of being able to simultaneously maintain SCR high-NO conversion efficiency and guarantee low ammonia slip, as well as being robust to the SCR ammonia storage capacity uncertainty. TheoreticalanalysesandFTP75drivingcyclesimulationsbasedon an experimentally validated full-vehicle simulator were conducted to evaluate the effectiveness of the proposed control strategy. Comparisons with conventional SCR controllers are presented to show the advantages of the proposed two-cell backstepping-based SCR control strategy.