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Biodiesel is a promising alternative to traditional diesel fuel due to its similar combustion properties to diesel and lower carbon emissions on a well-to-wheel basis. However, combusting biodiesel still generates hydrocarbon (HC), CO, NOx and particulate matter (PM) emissions, similar to those from traditional diesel fuel usage. Therefore, aftertreatment systems will be required to reduce these emissions to meet increasingly stringent emission regulations to minimize the impact to the environment. Diesel oxidation catalysts (DOC) are widely used in modern aftertreatment systems to convert unburned HC and CO, to partially convert NO to NO2 to enhance downstream selective catalytic reaction (SCR) catalyst efficiency via fast SCR and to periodically clean-up DPF via controlled soot oxidation. In this work, we focus on the performance difference between biodiesel and diesel over a commercial DOC catalyst to identify the knowledge gap during the transition from diesel fuel to biodiesel.

Diesel oxidation catalyst (DOC) is one of the critical catalyst components in modern diesel aftertreatment systems. It mainly converts unburned hydrocarbon (HC) and CO to CO2 and H2O before they are released to the environment. In addition, it also oxidizes a portion of NO to NO2, which improves the NOx conversion efficiency via fast SCR over the downstream selective catalytic reduction (SCR) catalyst. HC light-off tests, with or without the presence of NOx, has been typically used for DOC evaluation in laboratory. In this work, we aim to understand the influences of DOC light-off experimental conditions, such as initial temperature, initial holding time, HC species, with or without the presence of NOx, on the DOC HC light-off behavior. The results indicate that light-off test with lower initial temperature and longer initial holding time (at its initial temperature) leads to higher DOC light-off temperature.

Selective catalytic reduction (SCR) of oxides of nitrogen (NOx) with gaseous ammonia is the leading technology used to meet on- and off-highway NOx emission standards across the world. In typical SCR systems, a low-pressure injector introduces a solution of urea and water (UWS) into hot exhaust gases leading to atomization and subsequent spray processes that finally lead to production of gaseous ammonia. Through their synergetic effect, the UWS injector and mixing enhancement devices (such as static mixers or baffles) help deliver a uniform mixture of ammonia and NOx to the SCR catalyst with minimal urea-derived solid deposits. To develop an efficient and robust aftertreatment system, it is essential to have experimental and simulation capabilities to assess the behavior of sprays under flow conditions representative of engine exhaust.

Cu-SSZ-13 catalysts are widely used for diesel aftertreatment applications for NOx (NO and NO2) abatement via selective catalytic reaction (SCR) due to their high conversion efficiency and excellent hydrothermal stability. Diesel engine exhaust contains small amounts of SOx due to the combustion of sulfur compounds in diesel fuel. The engine out SOx level mainly depends on the sulfur content in the diesel fuel. The presence of SOx from engine exhaust can deteriorate the SCR performance of Cu-SSZ-13 catalysts in real-world applications. This work is focused on the sulfur-induced deactivation process of a Cu-SSZ-13 catalyst under a range of simulated diesel engine operating conditions. Two catalyst deactivation modes, namely chemical poisoning and physical poisoning, are identified, primarily depending on the operating temperature. Chemical poisoning mainly results from the interaction between SOx and Cu species within the zeolite framework.

Cu-SSZ-13 selective catalytic reduction (SCR) catalysts are broadly applied in diesel aftertreatment systems for the catalytic conversion of oxides of nitrogen (NO + NO2). Diesel exhaust contains a wide range of water vapor concentrations depending on the operating condition. In this study, we evaluate the impact of water vapor on the relevant SCR catalytic functions including NOx conversion, NO oxidation, NH3 oxidation, and N2O formation under both standard and fast SCR conditions. Reactor-based experiments are conducted in the presence and absence of water vapor. Results indicate that water vapor can have both a positive and negative impact on low temperature NOx conversion for standard SCR reaction. At low inlet NOx concentrations, the presence of water vapor has a negative effect on NOx conversion, whereas, at high inlet NO concentrations, water vapor improves NOx conversion.

To meet increasingly stringent diesel engine emission regulations, diesel engines are required to use ultra-low sulfur diesel (ULSD) and are equipped with advanced aftertreatment systems. Cu-SSZ-13 zeolite catalysts are widely used as selective catalytic reduction (SCR) catalysts due to their high NOx reduction and excellent hydrothermal stability. However, active Cu sites of Cu-SSZ-13 catalysts can be poisoned by exposure to engine exhaust sulfur species. This poison effect can be mitigated with the use of ULSD and high temperature exposure from engine operation. On the other hand, ULSD is still not universally available where regulations require it, and vehicles may inadvertently operate with high sulfur diesel fuel (HSD) in some locations. The high concentration of exhaust sulfur species resulting from HSD combustion may rapidly poison the Cu-SSZ-13 SCR catalyst. In this study, the catalytic performance of a sulfur poisoned Cu-SSZ-13 SCR catalyst is analyzed.

Worldwide, regulations continue to drive reductions in brake-specific emissions of nitric oxide (NO) and nitrogen dioxide (NO2) from on-highway and nonroad diesel engines. NOx, formed as a byproduct of the combustion of fossil fuels (e.g., natural gas, gasoline, diesel, etc.), can be converted to dinitrogen (N2) through ammonia (NH3) selective catalytic reduction (SCR). In this study, we closely examine the low-temperature storage, isothermal desorption, reactive consumption, and thermal release of NH3 on commercial Cu-SSZ-13 and V2O5-WO3/TiO2 SCR catalysts. Catalyst core-reactor, N2 adsorption (BET) surface area, and in-situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) experiments are utilized to investigate the fundamental chemical processes relevant to low-temperature (T < 250°C) NH3 SCR.

Cummins has recently launched next-generation aftertreatment technology, the Single ModuleTM aftertreatment system, for medium-duty and heavy-duty engines used in on-highway and off-highway applications. Besides meeting EPA 2010+ and Euro VI regulations, the Single ModuleTM aftertreatment system offers 60% volume and 40% weight reductions compared to current aftertreatment systems. In this work, we present model-based approaches that were systematically adopted in the design and development of the Cummins Single ModuleTM aftertreatment system. Particularly, a variety of analytical and experimental component-level and system-level validation tools have been used to optimize DOC, DPF, SCR/ASC, as well as the DEF decomposition device.

Natural gas-powered engines are widely used due to their low fuel cost and in general their lower emissions than conventional diesel engines. In order to comply with emissions regulations, an aftertreatment system is utilized to treat exhaust from natural gas engines. Stoichiometric burn natural gas engines use three-way catalyst (TWC) technology to simultaneously remove NOx, CO, and hydrocarbon (HC). Removal of methane, one of the major HC emissions from natural gas engines, is difficult due to its high stability, posing a challenge for existing TWC technologies. In this work, degreened (DG), standard bench cycle (SBC)-aged TWC catalysts and a DG Pd-based oxidation catalyst (OC) were evaluated and compared under a variety of lean/rich gas cycling conditions, simulating stoichiometric natural gas engine emissions.

Nitrogen dioxide (NO2) is known to pose a risk to human health and contributes to the formation of ground level ozone. In recognition of its human health implications, the American Conference of Governmental Industrial Hygienists (ACGIH) set a Threshold Limit Value (TLV) of 0.2 ppmv NO2 in 2012. For mobile sources, NO2 is regulated as a component of NOx (NO + NO2). In addition, the European Commission has indicated it is considering separate Euro 6 light-duty diesel and Euro VI heavy-duty diesel NO2 emissions limits likely to mitigate the formation of ground level ozone in urban areas. In this study, we conduct component-level reactor-based experiments to understand the effects that various aftertreatment catalyst technologies including diesel oxidation catalyst (DOC), diesel particulate filter (DPF), selective catalytic reduction (SCR) catalyst and ammonia oxidation (AMOX) catalyst have on the formation and mitigation of NO2 emissions.

Low-temperature (T ≤ 200°C) NOx conversion is receiving increasing research attention due to continued potential reductions in regulated NOx emissions from diesel engines. At these temperatures, ammonium salts (e.g., ammonium nitrate, ammonium (bi)sulfate, etc.) can form as a result of interactions between NH3 and NOx or SOx, respectively. The formation of these salts can reduce the availability of NH3 for NOx conversion, block active catalyst sites, and result in the formation of N2O, a regulated Greenhouse Gas (GHG). In this study, we investigate the effect of hydrothermal aging on the formation and decomposition of ammonium nitrate on a state-of-the-art Cu/zeolite selective catalytic reduction (SCR) catalyst. Reactor-based constant-temperature ammonium nitrate formation, temperature programmed oxidation (TPO), and NO titration experiments are used to characterize the effect of hydrothermal aging from 600 to 950°C.

Nitrous oxide (N2O), with a global warming potential (GWP) of 297 and an average atmospheric residence time of over 100 years, is an important greenhouse gas (GHG). In recognition of this, N2O emissions from on-highway medium- and heavy-duty diesel engines were recently regulated by the US Environmental Protection Agency (EPA) and National Highway Traffic Safety Administration’s (NHTSA) GHG Emission Standards. Unlike NO and NO2, collectively referred to as NOx, N2O is not a major byproduct of diesel combustion. However, N2O can be formed as a result of unselective catalytic reactions in diesel aftertreatment systems, and the mitigation of this unintended N2O formation is a topic of active research. In this study, a nonroad Tier 4 Final/Stage IV engine was equipped with a vanadium-based selective catalytic reduction (SCR) aftertreatment system. Experiments were conducted over nonroad steady and both cold and hot transient cycles (NRSC and NRTC, respectively).

Diesel exhaust fluid, DEF, (32.5 wt.% urea aqueous solution) is widely used as the NH3 source for selective catalytic reduction (SCR) of NOx in diesel aftertreatment systems. The transformation of sprayed liquid phase DEF droplets to gas phase NH3 is a complex physical and chemical process. Briefly, it experiences water vaporization, urea thermolysis/decomposition and hydrolysis. Depending on the DEF doser, decomposition reaction tube (DRT) design and operating conditions, incomplete decomposition of injected urea could lead to solid urea deposit formation in the diesel aftertreatment system. The formed deposits could lead to engine back pressure increase and DeNOx performance deterioration etc. The formed urea deposits could be further transformed to chemically more stable substances upon exposure to hot exhaust gas, therefore it is critical to understand this transformation process.

Natural gas powered vehicles are attractive in certain applications due to their lower emissions in general than conventional diesel engines and the low cost of natural gas. For stoichiometric natural gas engines, the aftertreatment system typically consists only of a three-way catalyst (TWC). However, increasingly stringent NOx and methane regulations challenge current TWC technologies. In this work, a catalyst reactor system with variable lean/rich switching capability was developed for evaluating TWCs for stoichiometric natural gas engines. The effect of varying frequency and duty-cycle during lean/rich gas switching experiments was measured with a hot-wire anemometer (HWA) due to its high sensitivity to gas thermal properties. A theoretical reactor gas dispersion model was then developed and validated with the HWA measurements. The model is capable of predicting the actual lean/rich gas exposure to the TWC under different testing conditions.

The oxidation of short-chain alkanes, such as methane, ethane, and propane, from the exhaust of lean-burn natural gas and lean-burn dual-fuel (natural gas and diesel) engines poses a unique challenge to the exhaust aftertreatment community. Emissions of these species are currently regulated by the US Environmental Protection Agency (EPA) as either methane (Greenhouse Gas Emissions Standards) or non-methane hydrocarbon (NMHC). However, the complete catalytic oxidation of short-chain alkanes is challenging due to their thermodynamic stability. The present study focuses on the oxidation of short-chain alkanes by vanadium-based and Cu/zeolite selective catalytic reduction (SCR) catalysts, generally utilized to control NOx emissions from lean-burn engines. Results reveal that these catalysts are active for short-chain alkane oxidation, albeit, at conversions lower than those generally reported in the literature for Pd-based catalysts, typically used for short-chain alkane conversion.

Regulations on methane emissions from lean-burn natural gas (NG) and lean-burn dual fuel (natural gas and diesel) engines are becoming more stringent due to methane’s strong greenhouse effect. Palladium-based oxidation catalysts are typically used for methane reduction due to their relative high reactivity under lean conditions. However, the catalytic activity of these catalysts is inhibited by the water vapor in exhaust and decreases over time from exposure to trace amounts of sulfur. The reduction of deactivated catalysts in a net rich environment is known to be able to regenerate the catalyst. In this work, a multicycle methane light-off & extinction test protocol was first developed to probe the catalyst reactivity and stability under simulated exhaust conditions. Then, the effect of two different regeneration gas compositions, denoted as regen-A and regen-B, was evaluated on a degreened catalyst and a catalyst previously tested on a natural gas engine.

Introduction of modern diesel aftertreatment, primarily selective catalytic reduction (SCR) designed to reduced NOx, has increased the presence of urea decomposition byproducts, mainly ammonia, in the aftertreatment system. This increase in ammonia has been shown to lead to particle formation in the aftertreatment system. In this study, a state of the art diesel exhaust fluid (DEF)-SCR system was investigated in order to determine the influence of DEF dosing on solid particle count. Post diesel particulate filter (DPF) particle count (> 23 nm) is shown to increase by over 400% during the World Harmonized Transient Cycle (WHTC) due to DEF dosing. This increase in tailpipe particle count warranted a detailed parametric study of DEF dosing parameters effect on tailpipe particle count. Global ammonia to NOx ratio, DEF droplet residence time, and SCR catalyst inlet temperature were found to be significant factors in post-DPF DEF based particle formation.

U.S. and European nonroad diesel emissions regulations have led to the implementation of various exhaust aftertreatment solutions. One approved configuration, a vanadium-based selective catalytic reduction catalyst followed by an ammonia oxidation catalyst (V-SCR + AMOX), does not require the use of a diesel oxidation catalyst (DOC) or diesel particulate filter (DPF). While certification testing has shown the V-SCR + AMOX system to be capable of meeting the nitrogen oxides, carbon monoxide, and particulate matter requirements, open questions remain regarding the efficacy of this aftertreatment for volatile and nonvolatile organic emissions removal, especially since the removal of this class of compounds is generally attributed to both the DOC and DPF.

Lean-burn natural gas (NG) engines are used world-wide for both stationary power generation and mobile applications ranging from passenger cars to Class 8 line-haul trucks. With the recent introduction of hydraulic fracturing gas extraction technology and increasing availability of natural gas, these engines are receiving more attention. However, the reduction of unburned hydrocarbon emissions from lean-burn NG and dual-fuel (diesel and natural gas) engines is particularly challenging due to the stability of the predominant short-chain alkane species released (e.g., methane, ethane, and propane). Supported Pd-based oxidation catalysts are generally considered the most active materials for the complete oxidation of low molecular weight alkanes at temperatures typical of lean-burn NG exhaust. However, these catalysts rapidly degrade under realistic exhaust conditions with high water vapor concentrations and traces of sulfur.

Selective catalytic reduction (SCR) is a promising technology for meeting the stringent requirements pertaining to NOx emissions. One of the most important requirements to achieve high DeNOx performance is to have a high uniformity of ammonia to NOx ratio (ANR) at the SCR catalyst inlet. Steady state 3D computational fluid dynamics (CFD) models are frequently used for predicting ANR spatial distribution but are not feasible for running a transient cycle like Federal Test Procedure (FTP). On the other hand, 1D kinetic models run in real time and can predict transient SCR performance but do not typically capture the effect of non-axial non-uniformities. In this work, two 3D to 1D coupling methods have been developed to predict transient SCR system performance, taking the effect of ANR non-uniformity into account. First is a probability density function (PDF) based approach and the second is a geometrical sector based approach.