Passive Ammonia-SCR Catalyst System for NOx Abatement from Lean-burn Gasoline Engines
Passive Ammonia-SCR Catalyst System for NOx Abatement from Lean-burn Gasoline Engines

Author: Vitaly Y. Prikhodko

Publisher:

Published: 2018

Total Pages: 135

ISBN-13:

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This dissertation summarizes experimental and computational observations from investigations of a selective catalytic reduction (SCR) system for reducing nitrogen oxides (NOx) in lean gasoline engine exhaust based on utilizing ammonia (NH3) generated by a three-way catalyst (TWC) during brief periods of fuel-rich engine operation. NH3 released from the TWC is stored and available to reduce NOx on a downstream SCR catalyst during subsequent periods of lean engine operation. The experimental results include high-speed measurements of transient NH3 formation on the TWC monolith catalysts, as the catalysts were exposed to lean gasoline engine exhaust from a commercial engine. In addition to the experimental investigations, dynamic computational simulations of NH3 generation on the TWC catalyst were implemented to provide more detailed information about NH3 generation on TWCs based on available reaction kinetic mechanisms. Based on the experimental and computational results, estimates of the potential fuel efficiency gains and emissions relevant to simulated drive cycles indicate that passive SCR can potentially achieve significant fuel efficiency benefits while still meeting regulated NOx emissions limits for vehicles powered by lean gasoline engines. However, optimal performance of the system will most likely require development of emission control methods that include accurate models for SCR catalyst NH3 storage and reaction under realistic drive-cycle transients.

Ammonia Generation and Utilization in a Passive SCR (TWC+SCR) System on Lean Gasoline Engine
Ammonia Generation and Utilization in a Passive SCR (TWC+SCR) System on Lean Gasoline Engine

Author:

Publisher:

Published: 2016

Total Pages: 7

ISBN-13:

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Lean gasoline engines offer greater fuel economy than the common stoichiometric gasoline engine, but the current three way catalyst (TWC) on stoichiometric engines is unable to control nitrogen oxide (NOX) emissions in oxidizing exhaust. For these lean gasoline engines, lean NOX emission control is required to meet existing Tier 2 and upcoming Tier 3 emission regulations set by the U.S. Environmental Protection Agency (EPA). While urea-based selective catalytic reduction (SCR) has proven effective in controlling NOX from diesel engines, the urea storage and delivery components can add significant size and cost. As such, onboard NH3 production via a passive SCR approach is of interest. In a passive SCR system, NH3 is generated over a close-coupled TWC during periodic slightly rich engine operation and subsequently stored on an underfloor SCR catalyst. Upon switching to lean operation, NOX passes through the TWC and is reduced by the stored NH3 on the SCR catalyst. In this work, a passive SCR system was evaluated on a 2.0-liter BMW lean burn gasoline direct injection engine to assess NH3 generation over a Pd-only TWC and utilization over a Cu-based SCR catalyst. System NOX reduction efficiency and fuel efficiency improvement compared to stoichiometric engine operation were measured. A feedback control strategy based on cumulative NH3 produced by the TWC during rich operation and NOX emissions during lean operation was implemented on the engine to control lean/rich cycle timing. At an SCR average inlet temperature of 350 °C, an NH3:NOX ratio of 1.15:1 (achieved through longer rich cycle timing) resulted in 99.7 % NOX conversion. Increasing NH3 generation further resulted in even higher NOX conversion; however, tailpipe NH3 emissions resulted. At higher underfloor temperatures, NH3 oxidation over the SCR limited NH3 availability for NOX reduction. At the engine conditions studied, greater than 99 % NOX conversion was achieved with passive SCR while delivering fuel efficiency benefits ranging between 6-11 % compared with stoichiometric operation.

Passive SCR for Lean Gasoline NOX Control
Passive SCR for Lean Gasoline NOX Control

Author:

Publisher:

Published: 2016

Total Pages: 8

ISBN-13:

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Lean gasoline engines offer greater fuel economy than common stoichiometric gasoline engines. However, excess oxygen prevents the use of the current three-way catalyst (TWC) to control nitrogen oxide (NOX) emissions in lean exhaust. A passive SCR concept, introduced by General Motors Global R & D, makes use of a TWC that is already onboard to generate NH3 under slightly rich conditions, which is stored on the downstream SCR. The stored NH3 is then used to reduce NOX emissions when the engine switches to lean operation. In this work, the effect of engine parameters, such as air-fuel equivalence ratio and spark timing, on NH3 generation over a commercial Pd-only TWC with no dedicated oxygen storage component was evaluated on a 2.0-liter BMW lean burn gasoline direct injection engine. NOX reduction, NH3 formation, and reductant utilization processes were evaluated, and fuel efficiency was assessed and compared to the stoichiometric engine operation case. We found air-fuel equivalence ratio to be one of the most important parameters in controlling the NH3 production; however, the rich operation necessary for NH3 production results in a fuel consumption penalty. The fuel penalty can be minimized by adjusting spark timing to increase rich-phase engine out NOX emissions and, thereby, NH3 levels. Additionally, higher engine out NOX during engine load increase to simulate acceleration resulted in additional fuel savings. Ultimately, a 10% fuel consumption benefit was achieved with the passive SCR approach by optimizing rich air-fuel equivalence ratio and spark timing while also utilizing acceleration load conditions.

Selective Catalytic Reduction of NOx
Selective Catalytic Reduction of NOx

Author: Oliver Kröcher

Publisher:

Published: 2018

Total Pages:

ISBN-13: 9783038973652

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The most efficient process to reduce NOx emissions from lean exhaust gases, selective catalytic reduction (SCR) with ammonia, has undergone tremendous development over the past decades. Originally only applied in stationary power plants and industrial installations, SCR systems are now installed in millions of mobile diesel engines, ranging from off-road machineries, to heavy-duty and light-duty trucks and passenger cars, to locomotives and ships. All of these applications involve specific challenges due to tighter emission limits, new internal combustion engine technologies, or alternative fuels. Three review articles and 14 research articles in this book describe recent results and research trends of various aspects of the SCR process. Reaction engineering aspects, such as the proper dosage of ammonia or urea, respectively, are as important as further developments of the different SCR catalysts, by deepening the understanding of their functionality or by systematic improvements of their properties, such as low-temperature activity, selectivity, or poisoning-resistance. Another covered aspect is cost reduction through the use of cheaper base materials for the production is active and stable SCR catalysts. Finally, research efforts are reported to develop SCR processes with different reducing agents, which would open doors to new applications in the future. The range of topics addressed in this book will stimulate the reader's interest as well as provide a valuable source of information for researchers in academia and industry.

Lean NOx Trap Catalysis for Lean Burn Natural Gas Engines
Lean NOx Trap Catalysis for Lean Burn Natural Gas Engines

Author:

Publisher:

Published: 2004

Total Pages:

ISBN-13:

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As the nation's demand for energy grows along with concern for the environment, there is a pressing need for cleaner, more efficient forms of energy. The internal combustion engine is well established as one of the most reliable forms of power production. They are commercially available in power ranges from 0.5 kW to 6.5 MW, which make them suitable for a wide range of distributed power applications from small scale residential to large scale industrial. In addition, alternative fuels with domestic abundance, such as natural gas, can play a key role in weaning our nations dependence on foreign oil. Lean burn natural gas engines can achieve high efficiencies and can be conveniently placed anywhere natural gas supplies are available. However, the aftertreatment of Nox emissions presents a challenge in lean exhaust conditions. Unlike carbon monoxide and hydrocarbons, which can be catalytically reduced in lean exhaust, NOx emissions require a net reducing atmosphere for catalytic reduction. Unless this challenge of NOx reduction can be met, emissions regulations may restrict the implementation of highly efficient lean burn natural gas engines for stationary power applications. While the typical three-way catalyst is ineffective for NOx reduction under lean exhaust conditions, several emerging catalyst technologies have demonstrated potential. The three leading contenders for lean burn engine de-NOx are the Lean NOx Catalyst (LNC), Selective Catalytic Reduction (SCR) and the Lean Nox Trap (LNT). Similar to the principles of SCR, an LNT catalyst has the ability to store NOx under lean engine operation. Then, an intermittent rich condition is created causing the stored NOx to be released and subsequently reduced. However, unlike SCR, which uses urea injection to create the reducing atmosphere, the LNT can use the same fuel supplied to the engine as the reductant. LNT technology has demonstrated high reduction efficiencies in diesel applications where diesel fuel is the reducing agent. The premise of this research is to explore the application of Lean NOx Trap technology to a lean burn natural gas engine where natural gas is the reducing agent. Natural gas is primarily composed of methane, a highly stable hydrocarbon. The two primary challenges addressed by this research are the performance of the LNT in the temperature ranges experienced from lean natural gas combustion and the utilization of the highly stable methane as the reducing agent. The project used an 8.3 liter lean burn natural gas engine on a dynamometer to generate the lean exhaust conditions. The catalysts were packaged in a dual path aftertreatment system, and a set of valves were used to control the flow of exhaust to either leg during adsorption and regeneration.

Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts
Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts

Author: Isabella Nova

Publisher: Springer Science & Business Media

Published: 2014-03-14

Total Pages: 715

ISBN-13: 1489980717

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Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts presents a complete overview of the selective catalytic reduction of NOx by ammonia/urea. The book starts with an illustration of the technology in the framework of the current context (legislation, market, system configurations), covers the fundamental aspects of the SCR process (catalysts, chemistry, mechanism, kinetics) and analyzes its application to useful topics such as modeling of full scale monolith catalysts, control aspects, ammonia injections systems and integration with other devices for combined removal of pollutants.

Multi-Stage Selective Catalytic Reduction of NOx in Lean-Burn Engine Exhaust
Multi-Stage Selective Catalytic Reduction of NOx in Lean-Burn Engine Exhaust

Author:

Publisher:

Published: 1997

Total Pages: 0

ISBN-13:

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Recent Studies suggest that the conversion of NO to NO2 is an important intermediate step in the selective catalytic reduction (SCR) of NOx to N2. These studies have prompted the development of schemes that use an oxidation catalyst to convert NO to NO2, followed by a reduction catalyst to convert NO2 to N2. Multi-stage SCR offers high NOx reduction efficiency from catalysts that, separately, are not very active for reduction of NO, and alleviates the problem of selectivity between NO reduction and hydrocarbon oxidation. A plasma can also be used to oxidize NO to NO2. This paper compares the multi-stage catalytic scheme with the plasma-assisted catalytic scheme for reduction of NOx in lean-burn engine exhausts. The advantages of plasma oxidation over catalytic oxidation are presented.

Investigation of the Kinetics of No Reduction by Ammonia on an Automotive Catalyst
Investigation of the Kinetics of No Reduction by Ammonia on an Automotive Catalyst

Author:

Publisher:

Published: 2005

Total Pages:

ISBN-13:

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As lean-burn engines are being introduced in the United States, both advantages and disadvantages arise. Lean-burn engines can operate at a high efficiency, and are developed for a wide range of power supplies. Unfortunately, due to the low temperature at which these engines operate, NO[subscript x] formation becomes an issue. Forthcoming legislation pertaining to heavy-duty lean-burn engines aimed at reducing both particulate matter emissions and emissions of nitric oxides has brought about a need for a better method for reducing NO[subscript x] from lean exhaust gases at moderate temperatures. It is generally accepted that current fuel treatment processes alone will be unable to accommodate emission standards proposed for upcoming years. While the current 3-way catalyst is ineffective in reducing NO[subscript x] under lean conditions, many new strategies are being developed. The Lean NO[subscript x] Catalyst (LNC), Lean NO[subscript x] Trap (LNT), and Selective Catalytic Reduction (SCR) catalyst are all viable methods with research underway. Currently, the selective catalytic reduction (SCR) of nitrogen oxides by N-containing reducing agents is one of the most powerful methods for accomplishing the removal of NO[subscript x] from an exhaust stream. This technology has been in place in steady state power plants, but has yet to be fully implemented in mobile engines. This is due in part to the problems encountered in the automated control of ammonia addition to the exhaust gas. In steady state operation, a relatively constant amount of NO[subscript x] is produced over a given amount of time. Thus, to provide a stoichiometric amount of ammonia only the steady state concentration of NO[subscript x] must be known. In an automotive application the NO[subscript x] produced is not constant and the addition of ammonia must vary accordingly. The purpose of this thesis is to explore the SCR process of the reaction between NO and NH3 through an experimental matrix and also through a kinetic study extracted from the results. These results are used in a simple theoretical model of the SCR reaction. The use of NO as the only form of NO[subscript x] allows for the kinetics of the NO reaction to be studied separately from the NO2 kinetics. This will be a first step in understanding the overall SCR process involving both NO and NO2. The SCR process for the reaction between NO and NH3, while understood on a global scale, is still under debate at the elementary level. It is currently thought that the reaction occurs according to an Eley-Rideal mechanism, where strongly absorbed ammonia reacts with weakly absorbed or gas phase NO to produce nitrogen and water. It is generally accepted that this reaction proceeds in first order with respect to nitric oxide and zero order with respect to ammonia and oxygen.