Mitigation of PAH and Nitro-PAH Emissions from Nonroad Diesel

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Mitigation of PAH and Nitro-PAH Emissions from Nonroad Diesel Engines Z. Gerald Liu, John C Wall, Nathan Ottinger, and Dana McGuffin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505434r • Publication Date (Web): 10 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Environmental Science & Technology

Mitigation of PAH and Nitro-PAH Emissions from Nonroad Diesel Engines

Z. Gerald Liu,* John C. Wall, Nathan A. Ottinger, Dana McGuffin Cummins Inc. 1801 U.S. Highway 51, Stoughton, WI 53589, U.S.A. KEYWORDS: Nonroad diesel exhaust emissions, Polycyclic aromatic hydrocarbon, nitro-PAH, Diesel exhaust aftertreatment, DOC, DPF, SCR.

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ABSTRACT: More stringent emission requirements for nonroad diesel engines introduced with U.S. Tier 4 Final and Euro

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Stage IV and V regulations have spurred the development of exhaust aftertreatment technologies. In this study, several

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aftertreatment configurations consisting of diesel oxidation catalysts (DOC), diesel particulate filters (DPF), Cu zeolite- and

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vanadium-based selective catalytic reduction (SCR) catalysts, and ammonia oxidation (AMOX) catalysts are evaluated using

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both Nonroad Transient (NRTC) and Steady (8-mode NRSC) Cycles in order to understand both component and system-

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level effects of diesel aftertreatment on emissions of polycyclic aromatic hydrocarbons (PAH) and their nitrated derivatives

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(nitro-PAH). Emissions are reported for four configurations including engine-out, DOC+CuZ-SCR+AMOX, V-SCR+AMOX,

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and DOC+DPF+CuZ-SCR+AMOX. Mechanisms responsible for the reduction, and, in some cases, the formation of PAH

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and nitro-PAH compounds are discussed in detail, and suggestions are provided to minimize the formation of nitro-PAH

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compounds through aftertreatment design optimizations. Potency equivalency factors (PEFs) developed by the California

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Environmental Protection Agency are then applied to determine the impact of aftertreatment on PAH-derived exhaust

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toxicity. Finally, a comprehensive set of exhaust emissions including criteria pollutants, NO2, total hydrocarbons (THC), n-

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alkanes, branched alkanes, saturated cycloalkanes, aromatics, aldehydes, hopanes and steranes, and metals is provided, and

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the overall efficacy of the aftertreatment configurations is described. This detailed summary of emissions from a current

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nonroad diesel engine equipped with advanced aftertreatment can be used to more accurately model the impact of anthro-

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pogenic emissions on the atmosphere.

ACS Paragon Plus Environment


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Diesel exhaust is a complex mixture of gas- and particle-phase chemical compounds, which, in its untreated state, has

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been found to be both toxic to humans and detrimental to environmental systems. While there is no single source of nascent

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diesel exhaust’s toxicity, polycyclic aromatic hydrocarbons (PAH) and their derivatives contribute significantly.1,2

Introduction

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PAHs and their derivatives are volatile and semi-volatile hydrocarbons with two or more benzene rings and a boiling

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point of at least 200°C. They occur naturally in hydrocarbon deposits and are also a byproduct of the combustion of hydro-

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carbon-based fuels. There is general agreement that many of these compounds are highly toxic atmospheric pollutants.

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Benzo(a)pyrene (BaP), listed by the International Agency for Research on Cancer (IARC) as a Group 1 human carcinogen,

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is the most extensively studied of the thousands of PAH compounds in existence.3 Several other PAH compounds are listed

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by the IARC and U.S. Environmental Protection Agency (EPA) as probable human carcinogens.2 Nitro-PAHs are generally

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more toxic than their parent PAHs because of the resulting localized electron displacement.4-6 Additionally, PAHs with

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more benzene rings tend to negatively affect human health and the environment more than lower molecular weight PAHs

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with only two benzene rings.5 To calculate a cumulative toxicity from PAH concentrations in the environment, Collins et

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al. and the California EPA defined potency equivalency factors (PEFs) which can be applied to emissions of individual PAH

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and nitro-PAH compounds.7 All 19 of the PAHs that have assigned PEFs contain at least four benzene rings, as do the

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majority of nitro-PAH compounds with PEFs.

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The effect of diesel aftertreatment on PAH and nitro-PAH emissions has previously been studied. Heeb et al. investigated

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the formation and removal of nitro-PAHs in a diesel engine operated with a diesel particulate filter (DPF). The analysis

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showed an overall decrease in diesel exhaust toxicity, but selected PAHs were nitrated, forming primarily 2- and 3-ring

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nitro-PAHs.8 Another study on PAH and nitro-PAH emissions by Carrara et al. utilized a Euro IV heavy duty engine and

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diesel oxidation catalyst (DOC)+DPF system.9 Out of the 11 nitro-PAH compounds investigated by the authors, five had

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increased emissions after the DPF, with the low temperature B25 condition being significantly more prone to nitro-PAH

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formation than the high temperature C100 condition. Additionally, the Advanced Collaborative Emissions Study (ACES)

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reported a reduction of unregulated emissions in their first phase using 2007 on-highway engines equipped with a

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DOC+DPF system. A reduction of 79% and 81% was reported from 2004 engine technology for PAH and nitro-PAH emis-

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sions, respectively.10

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Previous work, cited above, has focused on the combined ability of a DOC+DPF system to reduce emissions because on-

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highway aftertreatment systems for EPA 2007 and 2010 required DPF technology to meet particulate matter (PM) regula-

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tions. Several studies have shown that DPFs reduce semi-volatile and particle-phase organics and consume NO2 via passive


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soot oxidation.10,11 However, some nonroad aftertreatment architectures that meet emissions standards for criteria pollu-

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tants do not contain a DPF, raising questions about the efficacy of these systems for PAH and nitro-PAH emissions reduc-

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tion.

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In this study, a source dilution sampling system was implemented to characterize PAH and nitro-PAH emissions in the

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exhaust of a nonroad, Tier 4 Final diesel engine. Various aftertreatment systems, including DOC+copper zeolite selective

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catalytic reduction (CuZ-SCR) catalyst+ammoxia oxidation (AMOX) catalyst, vanadium-based SCR (V-SCR) cata-

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lyst+AMOX, and DOC+DPF+CuZ-SCR+AMOX, were evaluated. Experiments were performed with both the Nonroad Tran-

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sient Cycle (NRTC) and the 8-mode Nonroad Steady Cycle (NRSC). Emission rates of more than 135 PAH and nitro-PAH

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compounds are reported, and the effect of aftertreatment components and system-level interactions on the formation and

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destruction of these compounds is discussed in detail. Potency equivalency factors (PEFs) defined by the California EPA are

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then applied to relevant brake-specific PAH and nitro-PAH emissions in order to gauge emissions toxicity and discern the

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impact of the various aftertreatment configurations. Finally, a comprehensive set of exhaust emissions including NO2, NOx,

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PM, total hydrocarbons (THC), CO, n-alkanes, branched alkanes, saturated cycloalkanes, aromatics, aldehydes, hopanes

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and steranes, and metals is provided, and the overall efficacy of the aftertreatment configurations is described.

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PAH and nitro-PAH emissions cannot be explained by a single mechanism because of the possibility of both formation

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and destruction in diesel engine exhaust,12 diesel aftertreatment systems,8,9,13 and in the atmosphere.6,14,15 In an engine, nitro-

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PAHs along with parent PAHs are formed de novo during combustion. During combustion’s extreme conditions, low mo-

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lecular weight hydrocarbon radicals form and participate in ring-forming reactions with small, reactive radicals like acety-

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lene (C2H2) and propargyl (C3H3).16 Aromatics continue to grow by combining with other radicals either present in fuel or

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formed by combustion. In comparison to the extensive research on in-cylinder PAH formation, significantly less infor-

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mation on the in-cylinder formation of PAH derivatives is available. Aromatic compounds and nitrogen components may

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participate in the growth of nitro-PAHs.17,18

Formation & Reduction Mechanisms of PAHs and their Derivatives

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Post-combustion, in the engine and aftertreatment system, nitro-PAHs can be formed de novo and through electrophilic

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substitution reactions. De novo formation is possible with a combination of high temperature, nitro-PAH precursors, and

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long residence times.8 Thus, de novo formation may proceed more favorably in aftertreatment systems that contain a DPF

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which significantly increases the residence time of particulate matter. In addition, PAHs in particle- and liquid-phases may

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react with NO or NO2 ions or acids (HNO2, HNO3), and gas-phase PAHs may react with electrophiles such as NO2, NO3,

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N2O5, or OH found in diesel exhaust to form nitro-PAHs through electrophilic substitution.8 There are several reaction

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paths through which a nitro-PAH compound may be formed via the electrophilic substitution of a parent PAH, and these


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follow a few general mechanistic schemes. First, the PAH is activated when a charged species (NO2, NO3, or OH) disrupts

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the aromaticity of a benzene ring. A nitrogen-carbon bond is formed, creating a reactive intermediate. Then, an elimination

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step takes place so that the hydrogen on the substituted carbon is abstracted by a base, making the ring a conjugated system

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again. Another mechanism involves two substitution steps by electrophilic nitrogen radicals or ions. In this case, one of the

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substituents is removed by a base (NO3+, H2O, or O2) to produce the respective PAH derivative.19,20

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Electrophilic substitution of aromatic compounds with hydrocarbon substituents, like PAHs, typically occurs at either

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the ortho or para positions, also known as the α- and γ-carbons. In general, the position that results in an intermediate with

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the most aromaticity or largest conjugated system is selectively nitrated.19 For example, naphthalene can be nitrated in

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either the α- or β-position to produce 1-nitronaphthalene or 2-nitronaphthalene, respectively. Since the α-intermediate has

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two transition states with intact aromaticity and the β-intermediate has only one, the yield of 1-nitronaphthalene is higher

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than 2-nitronaphthalene.

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Diesel aftertreatment systems also actively remove PAH and nitro-PAH compounds through oxidation. In addition to

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hydrocarbon oxidation by oxygen, oxidation by NO2 is also possible.21 Diehl et al. investigated the oxidation of polycyclic

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aromatic hydrocarbons over a Pt/Al2O3 catalyst, similar to the DOC substrate used in the present study.22 The researchers

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measured T50 temperatures—the temperature at which 50% of a compound is oxidized—in air for naphthalene, flourene,

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acenaphthene, and acenaphthylene of 240, 220, 243, and 255°C, respectively. Partial oxidation to oxygenated PAHs is pos-

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sible at lower temperatures. In addition, multiple studies previously mentioned have shown that a DPF can efficiently re-

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move PAHs by filtration mechanisms.8-11

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Engine and Aftertreatment

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charge air cooled and was equipped with electronically controlled high pressure common rail fuel injection and cooled

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exhaust gas recirculation. Additional engine details are located in supplemental information Table S.1. This is an industrial

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nonroad U.S. Tier 4 Final/ Euro Stage IV engine rated for use with ultra-low-sulfur diesel (ULSD). The engine was operated

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Experimental An 8.9 liter engine rated for 380 HP (283 kW) at 2,100 rpm was used for this study. The engine was turbocharged and

with BP #2 ULSD fuel with 6 ppm of sulfur and Valvoline Premium Blue 15W-40 lubrication oil.

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Three aftertreatment systems were evaluated in this study. The first consisted of a DOC designed to reduce CO, THC,

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and the soluble organic fraction of PM, as well as convert NO to NO2 for optimal reduction of total NOx, a CuZ-SCR catalyst

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for NOx emission control, and an AMOX catalyst for NH3 reduction. This configuration is referred to as “DOC+SCR”

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throughout this study. The second configuration consisted of a V-SCR catalyst followed by the same AMOX catalyst as used

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in the first configuration, referred to as “V-SCR.” The final configuration consisted of the same catalysts as the first, with an


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additional catalyzed DPF retrofitted upstream of the CuZ-SCR, referred to as “DOC+DPF+SCR.” All catalysts evaluated in

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this study are state-of-the-art and their sizes are typical of commercial components for this engine with a maximum SCR

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gas hourly space velocity in this study of approximately 80,000 hr-1. The engine was equipped with an open crankcase

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ventilation system.

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Sampling Apparatus

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constant volume system (CFV-CVS) for primary dilution, a residence time chamber (RTC) used to simulate atmospheric

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aging of exhaust samples, isokinetic sampling probes, and multiple mass flow controlled sample trains for filter- and car-

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tridge-based sample collection. Dilution air was humidity and temperature controlled, and high-efficiency particulate air

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(HEPA) and activated carbon filters removed background particles and organic compounds.Error! Reference source not

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found. Figure S.1 (supplemental information) shows a schematic of the RTC portion of the SDS system, and a detailed

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discussion of the design, operating principles, quality assurance and quality control (QA/QC) procedures for the SDS sys-

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tem is provided elsewhere by Liu et al.23

Engine exhaust samples were collected with a source dilution sampling (SDS) system composed of a critical flow venturi-

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The CVS system was operated at a nominal flow rate of 2250 standard cubic feet per minute (SCFM) (64.7 sm3/min),

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which resulted in average primary dilution ratios ranging from 4.3 to 5.3, depending on the test cycle. Gas- and particle-

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phase criteria pollutants and THC were quantified, per Title 40, Part 1065 of the U.S. Code of Federal Regulations, using a

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Horiba MEXA 7200 DEGR gas analyzer bench and an AEI PM sampler. An MKS HS 2030 Fourier transform infrared spec-

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trometer (FTIR) was incorporated for NH3, N2O, and NO2 measurements.

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The RTC was equipped with multiple sampling ports, each containing upstream PM2.5 cyclone separators and collection

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media specific to the target compounds. Samples for elemental and organic carbon (EC/OC) and organic species were col-

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lected on pre-baked quartz filters, ion and trace metals samples on acid-treated polytetrafluoroethylene (PTFE) filters, and

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soluble organic fraction samples using PTFE filters. Aldehyde and ketone emissions were measured with a sample train

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consisting of a PTFE pre-filter for PM removal, a potassium iodide ozone scrubber (Waters Corp., WAT054420), and a 2,4-

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dinitrophenylhydrazine (DNPH) cartridge (Waters Corp., WAT047205). Finally, emissions of PAHs and nitro-PAHs were

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collected with a high-volume sampler. A 90 mm PTFE impregnated glass fiber (TIGF) filter was used to trap PM-phase

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emissions and was located upstream of an Amberlite XAD-4 polymeric adsorbent cartridge (Sigma-Aldrich Corp.) which

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trapped volatile and semi-volatile species that were not PM-bound in the sampling apparatus.

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Prior to each day of testing, the components of all filter holders in contact with sampling media were cleaned with nitric

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acid (metals sample train) or with acetone and hexane (all other sample trains). Dilution air samples were then taken, and

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this data was utilized to background correct emissions from all configurations. Handling, storage, and transportation blanks


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were analyzed for every batch of sample media, and the RTC was purged after each experiment to ensure that subsequent

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tests were not affected by residual exhaust in the sampling apparatus.

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Test Cycles

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engine-out measurements were conducted to attain a reference level of engine emissions. During experiments without

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aftertreatment, an electronic exhaust throttling valve was used to simulate the backpressure of the aftertreatment system

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in order to maintain consistency of performance of the internal combustion system across all exhaust aftertreatment con-

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figurations.

Four configurations were tested during this study. In addition to the three aftertreatment configurations discussed above,

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Tests were conducted utilizing both the transient and steady-state engine dynamometer test-cycles required by U.S. EPA

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Tier 4 Final standards for nonroad compression-ignition engines. The NRTC “provides a representation of a broad range of

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nonroad diesel equipment activity” and is a composite of various duty cycles based on seven different nonroad applica-

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tions.24 The ISO 8178 test cycle, in its 8-mode, C1 schedule, applies to nonroad vehicles and industrial equipment and sim-

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ulates intermittent engine use at various steady-state operating conditions between idle and rated power. This cycle is

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referred to as the NRSC throughout this study. The NRTC was run four times in succession for each sample, beginning with

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a cold start. This resulted in 82 min of sample time and a cold start weighting factor of 25%. In contrast, cold start weighting

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factors of 5% (U.S. Tier 4 Final) and 10% (Euro Stage IV) are stipulated for criteria pollutant reporting. A 20 min hot soak

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was performed after the first cold cycle during which the engine was shut off and sampling was suspended. In addition, the

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engine was shutdown overnight to achieve an initial cold start system temperature between 20 and 25°C. NRSC experiments

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had a duration of 40 min, and each experiment was preceded by a NRTC cycle to bring the engine and aftertreatment system

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to a repeatable starting temperature.

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Analysis

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Toledo Ultra-microbalance with anti-static kit which weighed each filter with three repetitions under controlled tempera-

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ture and humidity. Samples collected with the RTC were analyzed at the Wisconsin State Laboratory of Hygiene (WSLH)

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and Desert Research Institute (DRI). WSLH analyzed media for EC/OC with National Institute for Occupational Safety and

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Health (NIOSH) Method 5040. The analytical methods utilized by WSLH for the quantification of aldehydes, metals, and

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organic pollution tracers which include n-alkanes, branched alkanes, saturated cycloalkanes, aromatics, hopanes, and ster-

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anes have been previously detailed.11

PM mass emissions were determined from pre- and post-test weights of Teflon filters measured with a XP2U Mettler

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Sampling media for PAH and nitro-PAH compounds was prepared and analyzed by DRI. Prior to sampling, the XAD pre-

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filter was sonicated for 10 minutes twice in both dichloromethane (DCM) and methanol and was then dried in a vacuum

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oven at 50°C for a minimum of 24 hrs. The XAD-4 resin was washed with liquinox soap and rinsed with hot water, deionized


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water, and methanol before being extracted with DCM and then acetone at 1500 pounds per square inch (psi) and 80°C.

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The XAD-4 resin was then dried in a vacuum oven at 50°C and finally loaded in a glass sampling cartridge. After sampling,

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the TIGF pre-filter and XAD-4 resin were spiked with deuterated PAH and nitro-PAH standards and then extracted sepa-

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rately with DCM followed by acetone at 1500 psi and 80°C. The full method is explained in detail by Zielinska et al.16 The

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PAH and nitro-PAH compounds of interest were isolated with the use of a solid-phase extraction aminopropyl Sep-Pak

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cartridge (Waters). PAH compounds were either measured with a Varian 4000 gas chromatograph/mass spectrometer

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(GC/MS) or a Scion 456 GC coupled to a Scion triple quadrupole (TQ) MS for XAD-4 and pre-filter extracts, respectively.

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Nitro-PAHs were analyzed by negative ion chemical ionization mass spectrometry (NICI-MS) with methane as the reagent

175

gas.25 A 6-level calibration was performed for all analytes, and a mid-level standard was run at least once every 10 samples.

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For all configurations, at least three replicate experiments were performed to determine measurement repeatability. All

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results reported are average background corrected values with their corresponding standard error.

178 179 180

PAH and Nitro-PAH Emissions Overview

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ous, relevant aftertreatment configurations. Oxygenated PAHs are primarily reported with the group of unsubstituted

182

PAHs, but since some of these species are mutagenic,26 their emissions are reported separately in the discussion of PEF

183

adjusted emissions. Table S.2 (supplemental information) summarizes the NRTC emissions from all aftertreatment config-

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urations for all measured compounds. All PAH and nitro-PAH values reported in this article include particle- and gas-phase

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emissions, as discussed above. The standard error is provided, and the detection limit (DL) is given for all non-detect (ND)

186

species. Because reported emissions are background corrected, it is possible to have an emitted value lower than the detec-

187

tion limit. When this occurred, the emitted value was still used for all subsequent calculations.

Results and Discussion Emissions of more than 100 PAH and 30 nitro-PAH compounds were measured in order to understand the effect of vari-

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Naphthalene, 1-methylnapthalene, 2-methylnapthalene, and dimethylnapthalenes were the largest contributors to en-

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gine-out PAH emissions, while 1-nitronaphthalene, 2-nitronaphthalene, and 1-methyl-6-nitronaphthalene were the largest

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contributors to engine-out nitro-PAH emissions. The engine-out PAH emissions measured in this study are similar to pre-

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viously published emissions from diesel engines.11,27-30 Lea-Langton et al., in their study of PAH formation during diesel

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combustion, showed that PAH emission levels decreased monotonically as the temperature of combustion and the molec-

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ular weight of PAH compounds increased.28 This trend was observed for engine-out measurements with 14% higher PAH

194

emissions during the lower temperature NRTC and with 45 and 59% of total PAH emissions accounted for by two-ring

195

naphthalene and its methylated derivatives in the NRSC and NRTC cycles, respectively. In contrast to results for PAH com-

196

pounds, nitro-PAH emissions are orders of magnitude lower and appear to vary depending on the engine and operating

197

conditions. For example, Khalek et al. did not report significant emissions of nitronaphthalene isomers from either 2000 or


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2007 engine technologies.10 However, similar to the present study, Heeb et al. measured higher emissions of 1-nitronaph-

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thalene than other reported nitro-PAH compounds.8 Engine and combustion parameters clearly have an effect on the spe-

200

ciation of engine-out nitro-PAHs. Additional work outside the scope of this study is needed to understand and model in

201

cylinder PAH and nitro-PAH formation and destruction.

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The various diesel aftertreatment configurations achieved large reductions in PAH compounds and the majority of nitro-

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PAH compounds shown in Table S.1, but an increase above engine out levels is noted for some of the nitro-PAH compounds.

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Total PAH and nitro-PAH emissions are shown in Figure 1. All aftertreatment configurations reduce PAH emissions by two

205

orders of magnitude over both nonroad cycles in comparison to engine-out. A similar PAH reduction was reported by Liu

206

et al. for DOC+DPF and SCR+AMOX architectures.31 The authors showed that these aftertreatment systems reduce PAH

207

emissions by greater than 85% in comparison to engine-out for an on-highway diesel engine operating under Federal Test

208

Procedure (FTP) cycles. In the present study, PAH reductions of 99% were measured for similar aftertreatment configura-

209

tions with current catalyst and filter technologies and a nonroad engine operating over nonroad drive cycles. In addition,

210

the V-SCR configuration, which approximates Euro IV and V on-highway and U.S. and Euro nonroad architectures, also

211

reduced PAH emissions by 99%. Total nitro-PAH emissions measured in this study are reduced by all aftertreatment archi-

212

tectures in the NRSC (396°C average DOC temperature), but an increase in emissions is seen during the NRTC (330°C

213

average DOC temperature) for the DOC+SCR and DOC+DPF+SCR configurations. Similarly, Carrara et al. have observed

214

higher nitro-PAH emissions from a DOC+DPF at B25 (300°C) in comparison to C100 (440°C).9 Interestingly, the V-SCR

215

aftertreatment reduced NRTC nitro-PAH emissions by 87%. All architectures reduced combined PAH and nitro-PAH emis-

216

sions (not shown) by two orders of magnitude in comparison to engine-out emissions.


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PAH and Nitro-PAH Emissions [g/bhp·hr]

1.E-02 Engine Out DOC+SCR V-SCR DOC+DPF+SCR

1.E-03

1.E-04

1.E-05

1.E-06

1.E-07

1.E-08

217

PAH NRTC

Nitro-PAH NRTC

PAH NRSC

Nitro-PAH NRSC

218 219 220

Formation of Nitro-PAH Compounds

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the exception of 4-nitrophenanthrene (3-ring), and all of these compounds are derivatives of five parent PAHs: acenaph-

222

thene, biphenyl, naphthalene, 1-methylnaphthalene, and phenanthrene. This increase in nitro-PAH emissions is likely due

223

to electrophilic aromatic substitution reactions between parent PAH species and nitrogen containing cations such as NO2+.

224

Engine-out emissions of all parent PAHs, shown in Figure S.2 (supplemental information), are at least two orders of mag-

225

nitude higher than their respective nitro-PAH emissions, indicating that these PAHs are available for nitration. Further-

226

more, emissions of these parent PAH compounds are reduced significantly with the addition of aftertreatment. As expected,

227

formation of 1-nitronaphthalene (nitrated in the higher electron density α-position) is greater than 2-nitronaphthalene

228

(nitrated in the β-position). Other nitro-PAH compounds with increased emissions were also nitrated to a greater extent

229

at the structural location most favorable for such substitution reactions (e.g., nitrobiphenyl).

Figure 1: Total emissions of PAH and nitro-PAH compounds for all aftertreatment configurations

The 11 nitro-PAH compounds with increased NRTC emissions are shown in Figure 2. These all have 2 benzene rings, with


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(a)

(b)

230 231 232 233

Figure 2: Emissions of nitro-PAH compounds which increased as a result of diesel aftertreatment during NRTC (a) and NRSC (b) testing

234

Carrara et al., in their study of PAH emissions from a Euro IV engine, reported that a DPF increased nitro-PAH emissions

235

of 2-nitronaphthalene, 1- and 6-nitrobenzo[a]pyrene, and 1,6-dinitropyrene.9 Hu et al., in their study of retrofit aftertreat-

236

ment technologies, found that a DPF increased particle-phase emissions of 3- and 9-nitrophenanthrene.29 Finally, Heeb et

237

al. has reported that a DPF can increase nitro-PAH emissions of 1- and 2-nitronaphthalene, 3-nitrophenanthrene, 9-nitro-

238

anthracene, and 1-nitropyrene.32 The authors showed that DPFs with low oxidation activity—unable to oxidize CO to CO2—

239

were more likely to increase nitro-PAH emissions in comparison to DPFs with high oxidation activity. Similar to these

240

previous studies, emissions of 1- and 2-nitronaphthalene were increased by both DOC+SCR and DOC+DPF+SCR aftertreat-

241

ment in the present study. But, the increase occurred to the same degree in both configurations, indicating that the DPF

Several recent studies have also reported the selective nitration of some PAH compounds by aftertreatment systems.8,9,29,32


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has negligible impact on the formation of these nitro-PAH compounds under the conditions studied. In fact, the addition

243

of a DPF has only a minor impact on nitro-PAH emissions in this study (Figure 1). While the DPF reduced some nitro-PAH

244

compounds such as dinitronaphthalenes, it also increased the concentration of others such as 1-methyl-nitronaphthalenes.

245

The DPF is likely ineffective at reducing nitro-PAH compounds in this study because of the relatively low molecular weight

246

and high vapor pressure of the predominant 2- and 3-ring nitro-PAHs. To illustrate this, NRTC emissions are presented as

247

a function of benzene ring # in Figure 3. While the benefit of a DPF is minimal for 2- to 4-ring PAH species, it is effective at

248

reducing 4-ring nitro-PAH species and PAH compounds with more than 4-rings. In agreement with the previous work of

249

Heeb et al.,32 the DPF used in this study, which is catalyzed and has high oxidation activity (i.e., converts CO to CO2), does

250

not significantly increase the formation of nitro-PAH compounds.

251

Without a DPF, the DOC+SCR aftertreatment evaluated in this study increased nitro-PAH emissions in comparison to

252

engine-out. However, it is unlikely that the CuZ-SCR component promotes the formation of nitro-PAHs. Both the V-SCR

253

and CuZ-SCR catalysts significantly reduce exhaust NO2 concentrations through the fast SCR reaction at temperatures

254

above 200ºC when diesel exhaust fluid (DEF) dosing generally begins, minimizing the availability of NO2 for PAH nitration.

255

In addition, while the small pore zeolite of this CuZ-SCR permits molecules such as NOx and NH3 to diffuse into the zeolite

256

framework, larger molecules such as PAHs are excluded, reducing their residence time in the post-DOC, high-NO2 portion

257

of the diesel aftertreatment system.33 Furthermore, Hu et al. evaluated the impact of V-SCR and CuZ-SCR catalysts placed

258

behind a DOC+DPF.29 The authors found that the addition of a CuZ-SCR to a DOC+DPF aftertreatment reduced nitro-PAH

259

emissions, and that this combined aftertreatment had the lowest nitro-PAH emissions measured.

260

The predominantly 2-ring nitro-PAH compounds with increased emissions are most likely formed as a result of the DOC

261

catalyst. While DOC catalysts contribute to the removal of PAH and nitro-PAH species,9 they also oxidize NO to NO2. NO2

262

is beneficial for both SCR NOx conversion and DPF soot oxidation, so it plays a critical role in today’s aftertreatment systems.

263

However, high NO2 concentrations are also known to increase the rate of nitro-PAH formation. Carrara et al. have studied

264

the impact of NO2 concentration on the formation rate of both 1-nitropyrene and 6-nitrobenzo(a)pyrene.13 The authors

265

could not observe 1-nitropyrene formation at a NO2 concentration of 0.11 parts per million (ppm), but a rate of 0.0011 ng1-

266

nitropyrene

267

reported for the formation rate of 6-nitrobenzo(a)pyrene. In the lower temperature NRTC cycle, it appears that high NO2

268

concentrations, resulting from the DOC, increase the nitration rates of some predominantly gas-phase PAH compounds

269

such as naphthalene and biphenyl. It is likely that the DOC is still participating in the catalytic oxidation of these same

270

compounds as reported by Diehl,22 but that the rate of decomposition is slower than the nitration rate in high-NO2 condi-

271

tions. In contrast to these results, a previous study by Liu et al. on a DOC+DPF aftertreatment system operated over the

/(mgsoot·min) was observed at a NO2 concentration of 4 ppm. A similar dependence on NO2 concentration was


11


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272

FTP cycle did not find increased nitro-PAH emissions.31 However, while the DOC catalysts used in these two studies are

273

similar, the engine-out NOx levels differ significantly. The previous work of Liu et al. was conducted on a 2007 on-highway

274

engine tuned for low NOx emissions (< 1.2 g NOx/bhp·hr), whereas the engine out NOx of this nonroad engine tuned for

275

low PM exceeds 3 g NOx/bhp·hr for both nonroad cycles. The higher NOx levels of the nonroad engine substantially increase

276

both engine-out and post-DOC NO2 concentrations available for PAH nitration.

277

Total NRSC nitro-PAH emissions were reduced by all aftertreatment configurations as seen in Figure 1, indicating that

278

the DOC is more effective at removal of nitro-PAH compounds at the higher temperatures of the NRSC cycle. Even so,

279

emissions of some compounds are increased above engine-out levels. Emissions of 1-nitronaphthalene, 4-nitrobiphenyl, 1,5-

280

dinitronaphthalene, and 1,8-dinitronaphthalene all increased as a result of the DOC+SCR aftertreatment. The lack of ni-

281

trated methylnaphthalenes during the NRSC cycle is likely due to the decomposition of 1-methylnaphthalene to naphtha-

282

lene at the higher temperatures of this cycle.34

283 284

V-SCR

285

does not contain an upstream DOC catalyst, and, thus, the average NO2 concentration is significantly lower than in the

286

other 2 aftertreatment configurations. Engine-out NO2 is rapidly reduced by the fast SCR reaction in the V-SCR catalyst. In

287

addition to significantly lower NO2 concentrations in the V-SCR configuration, it is also possible that the V-SCR catalyst

288

participates in the reduction of PAH and nitro-PAH species. V-SCR catalysts are active for hydrocarbon conversion includ-

289

ing aromatics such as benzene,35 but the authors are not aware of a study that has evaluated PAH conversion and nitration

290

over these catalysts, a significant gap in the literature since V-SCRs are used in the majority of the world for NOx removal

291

from diesel exhaust. In contrast to the DOC catalyst in the DOC+SCR configuration, the downstream AMOX catalyst (the

292

only oxidation catalyst in the V-SCR configuration) likely contributes to nitro-PAH reduction since post-SCR NOx concen-

293

trations are significantly lower than they are in the case of a pre-SCR DOC.

The V-SCR configuration is the only one to achieve a net reduction in NRTC nitro-PAH emissions. This configuration

294

Even with an overall reduction in nitro-PAH emissions, the V-SCR configuration still promotes the formation of 5-nitro-

295

acenaphthene and 1,5-dinitronaphthalene as seen in Figure 3. In fact, this is the only configuration with higher emissions

296

of 5-nitroacenaphthene; the other configurations reduced this compound by at least 85%. Although, it should be noted

297

that this compound was only detected in one of the three V-SCR NRTC runs, so additional investigation into the formation

298

of this Group 2B carcinogen is needed. Furthermore, juxtaposing the V-SCR configurations ability to reduce 2-ring nitro-

299

PAH compounds (Figure 3), this configuration is less active for the reduction of PAH compounds with 3-rings or more. The

300

lack of a DOC leads to lower 2-ring nitro-PAH emissions but increased high molecular weight PAH emissions.


12


1.E-02

Engine Out DOC+SCR V-SCR DOC+DPF+SCR

PAH Emissions [g/bhp·hr]

1.E-03 1.E-04 1.E-05

Nitro-PAH Emissions

Page 13 of 19

1.E-05 1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 2

3

4

Number of Benzene Rings 1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 2

3

4

5

6

Number of Benzene Rings

301 302 303 304

Potency Equivalency Factor Emissions

305

the toxicity of these compounds ranges from non-toxic to carcinogenic, it is also important to consider the impact of after-

306

treatment on emissions toxicity. To calculate PAH and nitro-PAH derived emissions toxicity, the potency equivalency fac-

307

tors (PEFs) originally proposed by the California Air Resources Board (CARB) in 1994 and later modified by Collins et al.

308

(1998) and the California EPA (2005) were used.7 The PEFs used in this study are shown in Table S.3 (supplemental infor-

309

mation), along with all IARC classifications for the compounds evaluated in this study. As is evident from the table, there

310

is not complete agreement on the toxicity and carcinogenicity of these compounds, and this underscores the fact that

311

toxicity assessments are based on the best available information and are subject to change as new information becomes

312

available. Per all PEF classifications for PAH and nitro-PAH compounds, BaP is assigned a PEF of 1.0, and all other com-

313

pounds are scored relative to BaP.

Figure 3: NRTC emissions of PAH and nitro-PAH compounds by number of benzene rings

All aftertreatment configurations reduce combined PAH and nitro-PAH emissions by two orders of magnitude, but since

314

Figure 4 presents the NRTC PEF adjusted emissions of aromatics, PAH, nitro-PAH, and oxygenated-PAH for all after-

315

treatment configurations. Engine-out PEF adjusted emissions are three orders of magnitude lower than brake-specific PAH

316

emissions (Figure 1). In comparison to engine-out, aftertreatment has only a minimal impact on PEF adjusted emissions of

317

PAHs. This is because PEF adjusted emissions for engine-out, DOC+SCR, and DOC+DPF+SCR are all dominated by 7,12-

318

dimethylbenz(a)anthracene (DMBA), an immunosuppressor used in laboratory studies to induce tumor formation. Even


13


Page 14 of 19

319

though this compound makes up less than 0.5% of total PAH emissions, 52% of NRTC PEF adjusted emissions from the

320

DOC+SCR configuration are a result of DMBA. While the V-SCR reduced emissions of DMBA by more than one order of

321

magnitude, V-SCR PEF adjusted emissions are dominated by dibenzo(a,e)pyrene, a PAH only found in V-SCR experiments.

322

Nitro-PAH PEF adjusted emissions are at least an order of magnitude lower than engine-out regardless of aftertreatment

323

configuration. Thus, while emissions of some nitro-PAHs are increased due to aftertreatment, the overall PEF-based toxicity

324

of nitro-PAH emissions is substantially reduced by all aftertreatment configurations. The DOC+SCR and V-SCR configura-

325

tions reduced PEF adjusted emissions by nearly two orders of magnitude, and the addition of a DPF further reduced PEF

326

adjusted emissions. The nitro-PAH compounds with higher emissions as a result of aftertreatment (shown in Figure 2) are

327

primarily 2-ring nitronaphthalene and nitrobiphenyl congeners which do not have associated PEFs. On the other hand,

328

engine-out nitro-PAH PEF adjusted emissions are dominated by 1-nitropyrene and 6-nitrochrysene with PEFs of 0.1 and 10,

329

respectively. All aftertreatment configurations reduced 1-nitropyrene emissions by at least one order of magnitude, and 6-

330

nitrochrysene emissions were reduced below the detection limit by all aftertreatment systems.

PEF Adjusted Emissions [g/bhp-hr]

1.E-06 Engine Out DOC+SCR V-SCR DOC+DPF+SCR

1.E-07

1.E-08

1.E-09

1.E-10

1.E-11

331

Aromatics

PAH

Nitro-PAH

Oxy-PAH

332 333 334

Optimizing Diesel Exhaust Aftertreatment

335

favor the formation of PAH and nitro-PAH compounds while others promote the destruction of those same compounds. A

336

clear understanding of these parameters can enable the cautious design and integration of aftertreatment that minimizes

337

these emissions. As with the majority of chemical processes, temperature plays a key role in the formation and destruction

338

of PAH and nitro-PAH compounds. At higher temperatures, such as the NRSC cycle in this study, aftertreatment is suc-

339

cessful at reducing nitro-PAH emissions. While exhaust gas temperature is ultimately determined by the engine operating

Figure 4: NRTC PEF adjusted emissions based on California EPA PEF assignments.7

From both the current study and the body of literature already available on this subject, it is clear that certain conditions


14

Page 15 of 19


340

condition, catalyst temperatures can be influenced with state-of-the-art aftertreatment packaging designed to minimize

341

external surface area, insulation, and by positioning catalysts within close proximity of the turbine outlet. Minimizing high

342

concentrations and the co-location of precursor molecules (i.e., PAH compounds and electrophiles such as NO2) is another

343

method to control the formation of nitro-PAHs. For example, while NO2 is beneficial for certain catalytic processes (e.g.,

344

passive soot oxidation and NOx conversion via the fast SCR reaction), aftertreatment systems and engine calibrations can

345

be designed that minimize the requirement for NO2. By eliminating the upstream DOC, the V-SCR configuration in this

346

study successfully controls the NO2 concentration, and, as a result, nitro-PAH emissions for this configuration are signifi-

347

cantly lower than the other configurations. The combined oxidation activity of the V-SCR and AMOX catalysts in the V-

348

SCR configuration is still adequate to reduce CO and THC emissions. Furthermore, advances in SCR technology continue

349

to improve low temperature NOx conversion of the standard SCR reaction, which does not depend on NO2. Additional

350

research in this area is needed to further minimize the NO2 requirement of SCR catalysts. High capacity and low restriction

351

DPFs can also help by extending the intervals between active DPF regenerations and reducing the need for NO2-dependent

352

passive soot oxidation. Finally, since the substitution reactions that increase nitro-PAH emissions occur at a finite rate,

353

reducing exhaust residence time can decrease nitro-PAH formation. This is especially true if the NO2 exhaust concentration

354

is at engine-out levels or higher. A reduction in residence time can be achieved through technologies such as selective

355

catalytic reduction on filter (SCRF) and through designs that are compact and/or close-couple aftertreatment components

356

to the engine.

357 358

Comprehensive Emissions from a Nonroad Diesel Engine with Aftertreatment

359

benefits and trade-offs of various nonroad aftertreatment architectures. These emissions, in addition to other relevant emis-

360

sions such as criteria pollutants, THC, etc., must be considered in order to find the best available technology for each

361

aftertreatment application. Percent reductions of NO2, NOx, PM, THC, CO, n-alkanes, branched alkanes, saturated cyclo-

362

alkanes, aromatics, aldehydes, PAH and derivatives, hopanes and steranes, and metals by the three aftertreatment config-

363

urations are presented for both test cycles in Figure 5. Details on the individual species included in these categorized emis-

364

sions have been previously published.11,36 The DOC+SCR has the lowest NO2 reduction, but this configuration still reduces

365

engine-out NO2 by 35% over the NRTC. Both the DOC+DPF+SCR and the V-SCR configuration reduce NO2 by more than

366

90%; passive soot oxidation over the DPF contributes to this reduction. NOx conversion is slightly lower for the V-SCR due

367

to the lower activity of the V-SCR catalyst evaluated in this study in comparison to the CuZ-SCR. As expected, the DPF

368

configuration reduces greater than 99% of PM mass and 92% of metal emissions, while the other configurations have lower

369

reductions of PM mass and metals, a constituent of PM. On the other hand, the addition of a DPF increases fuel consump-

370

tion and CO2 emissions (not shown). It should be noted that sulfur is not included in metal emissions in this study, and the

The combined brake-specific and PEF adjusted PAH and nitro-PAH emissions reported above provide insight into the


15


Page 16 of 19

371

release of residual sulfur—present as a result of manufacturing—as sulfate may contribute to the negative PM reduction

372

reported for the V-SCR configuration.”. In addition to the large reduction in total and PEF adjusted emissions of PAH and

373

derivatives (Figures 1 and 4, respectively), other organic species including aromatics, aldehydes, hopanes, and steranes are

374

reduced to a significant extent by all aftertreatment configurations. Similarly, THC emissions are also reduced by greater

375

than 90% for all configurations. This is of particular interest for the V-SCR configuration which does not contain a DOC.

376

As mentioned previously, the combined oxidation capability of the V-SCR and AMOX enable this configuration to reduce

377

hydrocarbons, including aromatics and PAH and derivatives.

Percent Reduction

100%

(a)

80% 60% DOC+SCR 40%

VSCR DOC+DPF+SCR

20% 0% 100%

378

(b)

Percent Reduction

80% 60% 40% 20% 0%

380 381

Metals

Hopanes/Steranes

PAH and Derivs

Aldehydes

Aromatics

Sat. Cycloalkanes

n-Alkanes

CO

THC

Branched Alkanes

379

PM

NOx

NO2

-20%

Figure 5: Percent reductions of NO2, NOx, PM, THC, CO, n-alkanes, branched alkanes, saturated cycloalkanes, aromatics, aldehydes, PAH and derivatives, hopanes and steranes, and metals for NRTC (a) and NRSC (b) cycles.

382 383

AUTHOR INFORMATION ACS Paragon Plus Environment

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Page 17 of 19


384 385 386 387 388 389

Corresponding Author

390 391 392 393 394 395 396 397 398 399

ACKNOWLEDGMENTS

400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445

REFERENCES

Correspondence should be addressed to: Dr. Z.G. Liu at Cummins Inc., 1801 U.S. Highway 51-138, Stoughton, WI 53589, [email protected].

Author Contributions The manuscript was written through contributions of all authors, and all authors have given approval to the final version of the manuscript.

The authors of this study thank Dr. Michael Hays, Dr. David DeMarini, and Dr. Matthew Spears of the U.S. EPA as well as Dr. Alberto Ayala and Dr. Shaohua Hu of CARB for their invaluable advice. The authors also appreciate the program support of Dr. Wayne Eckerle, Ken Federle, Jennifer Rumsey, Chris Cremeens, Annamarie Murray, Niklas Schmidt and Brad Nolley of Cummins, and the sample preparation and analysis performed by the Wisconsin State Laboratory of Hygiene and the Desert Research Institute.

SUPPORTING INFORMATION AVAILABLE This information is available free of charge via the Internet at http://pubs.acs.org/ .

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9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

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TOC ART

Nonroad Emissions of PAH and Derivatives Engine Out

V-SCR DOC+SCR DOC+DPF+SCR

NRTC NRSC

474 475


19

Mitigation of PAH and Nitro-PAH Emissions from Nonroad Diesel

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