Diesel Particle Filter and Fuel Effects on Heavy-Duty Diesel Engine

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Environ. Sci. Technol. 2010, 44, 8343–8349

Diesel Particle Filter and Fuel Effects on Heavy-Duty Diesel Engine Emissions M A T T H E W A . R A T C L I F F , * ,† A. JOHN DANE,‡ AARON WILLIAMS,† JOHN IRELAND,† JON LUECKE,† ROBERT L. MCCORMICK,† AND K E N T J . V O O R H E E S * ,‡ National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado, and Colorado School of Mines, Golden, Colorado

Received March 25, 2010. Revised manuscript received August 20, 2010. Accepted September 9, 2010.

The impacts of biodiesel and a continuously regenerated (catalyzed) diesel particle filter (DPF) on the emissions of volatile unburned hydrocarbons, carbonyls, and particle associated polycyclic aromatic hydrocarbons (PAH) and nitro-PAH, were investigated. Experiments were conducted on a 5.9 L Cummins ISB, heavy-duty diesel engine using certification ultra-low-sulfur diesel (ULSD, S e 15 ppm), soy biodiesel (B100), and a 20% blend thereof (B20). Against the ULSD baseline, B20 and B100 reduced engine-out emissions of measured unburned volatile hydrocarbonsandPMassociatedPAHandnitro-PAHbysignificant percentages (40% or more for B20 and higher percentage for B100). However, emissions of benzene were unaffected by the presence of biodiesel and emissions of naphthalene actually increased for B100. This suggests that the unsaturated FAME in soy-biodiesel can react to form aromatic rings in the diesel combustion environment. Methyl acrylate and methyl 3-butanoate were observed as significant species in the exhaust for B20 and B100 and may serve as markers of the presence of biodiesel in the fuel. The DPF was highly effective at converting gaseous hydrocarbons and PM associated PAH and total nitro-PAH. However, conversion of 1-nitropyrene by the DPF was less than 50% for all fuels. Blending of biodiesel caused a slight reduction in engine-out emissions of acrolein, but otherwise had little effect on carbonyl emissions. The DPF was highly effective for conversion of carbonyls, with the exception of formaldehyde. Formaldehyde emissions were increased by the DPF for ULSD and B20.

Introduction Diesel engines fueled with biodiesel, either neat or in blends with petroleum diesel, generally produce lower emissions of unburned hydrocarbons, carbon monoxide (CO), and particulate matter (PM) (1). Consistent with the hydrocarbon trends, total aldehyde emissions typically decrease as biodiesel is added to the fuel (2). Polycyclic aromatic hydrocarbons (PAH) emissions are reduced by incorporating biodiesel into the fuel because key PAH precursors, such as alkylbenzenes and alkylnaphthalenes, are replaced by fatty acid methyl * Corresponding author e-mail: [email protected] (M.R.), [email protected] (K.J.V.); telephone: 303-275-4438 (M.R.), 303273-3616 (K.J.V.). † National Renewable Energy Laboratory. ‡ Colorado School of Mines. 10.1021/es1008032

 2010 American Chemical Society

Published on Web 10/01/2010

esters (FAME) (2, 3). Nitro-PAHs are formed during combustion by electrophilic aromatic substitution reactions between nitrating agents such as nitrogen dioxide (NO2) and the parent PAHs (4). Therefore, biodiesel blended fuels are expected to produce lower nitro-PAH emissions because of lower availability of PAH precursors (5). Emissions of nitro-PAH compounds from diesel engines are typically at least an order of magnitude lower than rates for PAH emissions. However, many nitro-PAHs are highly mutagenic and carcinogenic (6); consequently, there is continuing interest in characterizing and minimizing their emissions. NO2 can react with PAH to form nitro-PAH in the atmosphere (7, 8). PAH nitration reactions with NO2, nitrous acid (HNO2), and nitric acid (HNO3) can also occur in the exhaust system as declining temperatures lead to aerosol formation and adsorption onto PM (9). The incorporation of a diesel particle filter (DPF) into a heavy-duty diesel exhaust system presents an additional opportunity to form nitroPAH (10, 11). A diesel oxidation catalyst (DOC) located upstream of the DPF can intentionally have the function of converting NO to NO2 to accelerate oxidation of accumulated PM (including adsorbed PAH and other organics) in the DPF. Nitration of PAH in a DPF using fuel borne catalysts for regeneration has been documented (10). Tests using a wide range of DPF designs showed that certain nitro-PAH can be formed in a DPF, while others are converted (11). Biodiesel has been shown to slightly increase NOx emissions in many cases, while also reducing PM (12). Furthermore, the PM produced from biodiesel (13, 14) and other oxygenates (15) is significantly more reactive in DPF regeneration than PM from conventional diesel fuel. Thus, a DPF may produce different nitro-PAH and other unregulated species when the engine is operated on biodiesel rather than conventional diesel. The overall goal of this research was therefore to measure gas phase and particle associated nonregulated pollutant emissions from a heavy-duty engine operating both with and without a catalyzed DPF (intended for passive regeneration at moderate temperatures), and operating on biodiesel fuel. Emissions were measured from three fuels: certification ultra-low-sulfur diesel (ULSD), neat soy biodiesel (B100), and a 20% blend (B20).

Experimental Section Engine Testing Procedure. The engine test setup consisted of a 2002 model year 5.9 L 300 hp Cummins ISB, retrofitted with a diesel particulate filter. Specifications for the test engine are in the Supporting Information Table S1. The engine was equipped with cooled high-pressure exhaust gas recirculation (EGR), a variable geometry turbocharger, and high-pressure common rail direct fuel injection. It was engineered and calibrated to meet the 2004 U.S. heavy-duty emissions standards. Biodiesel effects on regulated pollutant emissions from this engine have been previously reported (16). The DPF (12 L capacity) was a catalyzed continuously regenerating technology (CCRT) manufactured by Johnson Matthey. This DPF included a diesel oxidation catalyst to convert NO to NO2, followed by a catalyzed wall-flow soot filter (17). It is intended for applications with exhaust temperatures as low as 200-250 °C. The DPF was mounted 152 cm from the engine turbo flange outlet. Three fuels were used in this study. The baseline diesel fuel was a 2007 certification ULSD supplied by Haltermann Products (Channelview, Texas). This fuel was used for baseline comparison and as the diesel blend stock for the biodiesel blend. A soy methyl ester supplied by Agland Inc. (Eaton, Colorado) was tested in both a 20% biodiesel blend VOL. 44, NO. 21, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Hydrocarbons in 8-mode composite samples of raw engine-out exhaust.

FIGURE 2. Gas chromatogram of B100 raw engine-out exhaust. and as a neat biodiesel. A thorough fuel swap procedure was carried out between experiments with each test fuel, which flushed three times the volumetric capacity of the entire fueling system, including running the engine at 1500 rpm at 50% throttle. As a check, the fuel density measured by the fuel meter provided an indication of the fuel type being delivered to the engine. The dynamometer test cell conforms to the requirements of the Code of Federal Regulations (CFR) 40, part 86, subpart N. Engine intake air temperature, pressure and humidity were controlled. Fuel consumption was measured with a Pierburg fuel meter, which measured volumetric fuel flow 8344

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and density with an accuracy of (0.5%. Engine testing was conducted over an 8-mode test cycle that captures a wide range of operating conditions. The test cycle was based on the AVL 8-mode test, a steady-state test procedure designed to closely correlate with the exhaust emission results obtained from the U.S. Federal Test Procedure heavy-duty engine transient cycle (18). Some modes were slightly modified for greater test repeatability with this engine. The eight test modes are detailed in the Supporting Information Table S2. The individual mode times reflect the weightings applied to regulated emissions concentration measurements in the original AVL 8-mode test, and sum to 30 min. The PM,

FIGURE 3. Comparison of major engine-out carbonyls in 8-mode composite diluted exhaust samples. Error bars are standard deviations of four replicate measurements (three for acrolein). unburned hydrocarbon, and carbonyl samples were collected as time-weighted composites of the eight steady-state modes, in accordance with the protocol for weighting the continuous emissions measurements. Details of sample collection, workup and analysis are provided in the Supporting Information. Engine-out refers to samples obtained either without an installed DPF or from a location upstream of an installed DPF. DPF-out refers to samples obtained downstream of an installed DPF. Nitro-PAH Analysis. Over the last 15 years, electron monochromator mass spectrometry (EM-MS) has gained considerable interest for detecting electrophilic compounds because it offers the ionization selectivity of electron capture detectors (ECD) and the mass spectral information produced by pseudonegative-ion chemical ionization (NCI)-MS (19). Furthermore, EM-MS is capable of controlling ionization fragmentation that may occur, which adds a degree of selectivity and specificity that is not realized by either ECD or NCI-MS. This is achieved by precisely controlling the ionizing electron beam energy to within (0.1 eV, with beam intensities only slightly below those observed for conventional electron ionization (EI) sources (19). Analysis of nitro-PAH was enabled by electron monochromator mass spectrometry (EM-MS), which has the attribute of selectively ionizing the nitro-compounds within a complex sample matrix. Two types of electron capture processes were used. The first ionization process is described as pure electron capture (electron energy is ∼0 eV), forming molecular ions. The second ionization process is called dissociative electron capture utilizing electron energies ranging from 0.1 to 10 eV, producing the characteristic m/z 46 ion from nitro-PAHs (20). This ion is unique in negative ion analysis and allows the differentiation of nitrocontaining compounds from the sample matrix (21).

Results and Discussion Unburned Hydrocarbon Emissions. GCMS analysis of the 8-mode composite raw exhaust samples showed that the DPF was highly effective in oxidizing C3-C11 hydrocarbons, as most were below detection limits in the DPF-out samples. Quantitation results for representative engine-out hydro-

carbons in the 8-mode composite exhaust samples are compared in Figure 1. The data are averages of triplicate engine tests and the error bars are (1 standard deviation. Anticipated reductions in the emissions of unburned diesel fuel hydrocarbons (e.g., decane and undecane) are clearly evident as more biodiesel was incorporated into the fuel. Products of incomplete combustion also decreased with increasing biodiesel concentration, represented here by isobutylene and toluene. However, within the error of these measurements, benzene emissions appeared to be insensitive to the changes in fuel chemistry. These observations are consistent with measurements made by Sharp et al. (2) using a similar engine tested with the transient heavy-duty Federal Test Procedure. Since there are no aromatics in biodiesel, explanations for benzene emission (as well as PAH and nitro-PAH discussed below) may involve pyrosynthetic reactions of the unsaturated FAME in the soy biodiesel or of their decomposition products. Soy biodiesel contains high concentrations of oleic, linoleic, and linolenic acid methyl esters containing single, double, and triple unsaturations, respectively (22). Above 250-300 °C, thermal polymerization of unsaturated FAME occurs via isomerization of the bis-allylic chains to conjugated structures, followed by Diels-Alder reaction linking chains through a cyclohexene ring (23). Therefore, it seems reasonable to speculate that FAME pyrolysis occurring within the diffusion flame of a diesel engine may produce these cyclohexene structures. Stable aromatics could subsequently form after hydrogen abstractions from the cyclohexene rings by O2, OH, or OOH. Alternatively, gaseous olefinic fragments (e.g., acetylene, propargyl, and 1, 3 butadiene) from pyrolysis of unsaturated FAME may react to form aromatics (24). Other organic species were identified in the engine-out exhaust gases with the NIST mass spectral library (25) but were not quantified because reliable standards were not available. Products of incomplete combustion unique to biodiesel were identified as FAME fragments, for example, methyl acrylate, the most intense peak in the B100 exhaust chromatogram shown in Figure 2 and methyl 3-butenoate. Methyl acrylate was also found in the exhaust from B20 tests VOL. 44, NO. 21, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Formaldehyde in 8-mode composite diluted exhaust samples, before and after DPF.

FIGURE 5. Overlaid nitro (m/z 46) chromatograms from engine-out PM samples. but was not detected from ULSD tests; thus, its presence in hydrogen atoms (27). We propose that methyl acrylate the exhaust is indicative of FAME in the fuel. The relatively formation occurs from FAME during combustion by a twohigh levels of methyl acrylate in B100 exhaust (estimated at step reaction mechanism. An alpha-carbon hydrogen is first 100-200 ng/L, comparable to benzene) may be explained abstracted by hydroxyl radical (or O2). This is followed by beta-scission, yielding methyl acrylate and an alkyl radical by its conjugated structure, which resists further oxidation R• (or an alkenyl radical if the FAME is unsaturated). Similar by hydroxyl radicals. Supporting evidence for its refractory reactions at the carbon beta to the carbonyl would lead to nature comes from methyl acrylate’s autoignition tempermethyl 3-butenoate and an R-1• alkyl or alkenyl radical. This ature (26) of 468 °C, which far exceeds the 200-220 °C mechanism is consistent with the FAME combustion kinetic autoignition temperature range for n-alkanes in typical diesel modeling efforts of Pitz et al. (28). This proposed mechanism fuel. may also explain the general observation that biodiesel The most labile hydrogen atoms in saturated FAME are reduces total hydrocarbon emissions from diesel engines; on the carbon alpha to the carbonyl, having bond energies i.e., relative to petroleum diesel less energy is required to of 394 kJ/mol compared to 413-423 kJ/mol for other 8346

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TABLE 1. Engine-out Nitro-PAH Emissions from 8-Mode Composite PM Samples

1-nitronaphthalene 2-nitronaphthalene 9-nitroanthracene 9-nitrophenanthrene 3-nitrophenanthrene 3-nitrofluoranthene 1-nitropyrene total nitro-PAH

ULSD (pg/bhp h)

B20 (pg/bhp h)

B100 (pg/bhp h)

ratio ULSD/B20

ratio ULSD/B100

24.5 111.9 777.8 304.9 62.3 63.0 3188.6 4533

12.7 77.8 480.6 224.5 39.0 29.2 1563.3 2427

5.9 18.9 190.4 133.9 26.6 17.1 692.5 1085

1.9 1.4 1.6 1.4 1.6 2.2 2.0 1.9

4.2 5.9 4.1 2.3 2.3 3.7 4.6 4.2

TABLE 2. Engine-out PAH Emissions from 8-Mode Composite PM Samples

naphthalene biphenyl acenaphthylene fluorene phenanthrene anthracene fluoranthene pyrene benzo[c]-phenanthrene chrysene total PAH

ULSD (pg/bhp h)

B20 (pg/bhp h)

B100 (pg/bhp h)

ratio ULSD/B20

ratio ULSD/B100

2137.0 423.8 51.7 333.3 23 018.1 1491.0 21 242.9 13 863.0 341.1 496.1 63 398

1612.4 428.9 49.1 217.1 13 692.5 917.3 15 372.1 8578.8 232.6 457.4 41 558

4421.2 565.9 46.5 69.8 2310.1 51.7 4,829.5 3379.8 108.5 237.7 16 021

1.3 1.0 1.1 1.5 1.7 1.6 1.4 1.6 1.5 1.1 1.5

0.48 0.75 1.1 4.8 10 29 4.4 4.1 3.1 2.1 4.0

TABLE 3. Comparison of PM Associated 1-Nitropyrene Emission Rates before and after the DPF

ULSD B20 B100

engine-out (pg/bhp h)

DPF-out (pg/bhp h)

% destruction

3189 1563 693

2075 1382 388

35 12 44

form hydrocarbon radicals from FAME, allowing their formation at lower temperatures. Aldehyde and Ketone Emissions. Figure 3 compares the five highest concentration carbonyl compounds in the 8-mode composite, diluted exhaust samples, accounting for ∼80% of the total aldehydes measured. Slight reductions in aldehyde emissions, particularly acrolein, were measured from the B20 and B100 fuels, but overall the effect of biodiesel chemistry on the engine-out carbonyl species was minimal. This contrasts with the findings of Sharp et al. (2), wherein addition of biodiesel significantly reduced aldehyde emissions. The apparent insensitivity of carbonyl emissions to biodiesel in the present study may result from the use of a more modern engine with EGR that reduces combustion temperature. Alternatively, the extended periods of low combustion temperature operation in the test procedure (i.e., the low-speed, low-load modes 1 and 2; see Supporting Information Table S2) may have been a dominant factor for aldehyde emissions. Modes 1 and 2 were also associated with the lowest exhaust gas temperatures and highest total hydrocarbon emissions, indicative of low combustion temperature and efficiency.

The DPF effectively oxidized most carbonyl compounds, with the notable exception of formaldehyde; the net combined carbonyl conversions were 25% for ULSD, 36% for B20, and 62% for B100. Interestingly, net increases in formaldehyde of 59% from ULSD and ∼30% from B20 were measured exiting the DPF, as shown in Figure 4. Yet in the case of B100, formaldehyde decreased by ∼24%. A comprehensive explanation for these observations requires further study. However, a reasonable hypothesis is that formaldehyde was an intermediate oxidation product from other aldehydes and hydrocarbons within the DPF and that, under the conditions in the DPF imposed by the test cycle, the oxidation of formaldehyde was inhibited. PM Associated Nitro-PAH Emissions. The engine-out PM sample extracts were analyzed for nitro compounds utilizing GC/EM-MS. The m/z 46 SIM chromatograms for the ULSD, B20, and B100 samples are overlaid for qualitative comparison in Figure 5. Many of the nitro compounds were common across the fuel types, but concentrations of those compounds varied widely. Some unidentified volatile nitro-compounds eluting in the 3-7 min range appear to increase from B100 compared to B20 and ULSD. However, these are exceptions to the general trend of reduced nitro-PAH emissions for the biodiesel fuels. Mass emission rates of the major nitro-PAHs measured in the engine-out PM samples show clearly that biodiesel leads to a significant decrease of combustion or postcombustion originated nitro-PAH (see Table 1). Most notably, the emission rate of 1-nitropyrene decreased 51% and 78% by fueling with B20 and B100, respectively. Other metrics for nitro-PAH dependency on fuel composition are the fuel

TABLE 4. Comparison of PM-Associated PAH Emission Rates before and after the DPF phenanthrene (pg/bhp h)

ULSD B20 B100

fluoranthene (pg/bhp h)

pyrene (pg/bhp h)

engine-out

DPF-out (% destruction)

engine-out

DPF-out (% destruction)

engine-out

DPF-out (% destruction)

23 018 13 693 2310

41 (99.8%) 44 (99.7%) 297 (87.1%)

21 243 15 372 4830

54 (99.7%) 47 (99.7%) 649 (86.6%)

13 863 8579 3380

26 (99.8%) 7 (99.9%) 390 (88.5%)

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source ratios of nitro-PAH reported in Table 1. Comparing neat fuels, the ULSD/B100 nitro-PAH ratio ranges from 2.3 for the nitrophenanthrenes to 5.9 in the case of 2-nitronaphthalene; the ratio of the total measured nitro-PAHs is 4.2. The ULSD/B20 ratios indicate a significant biodiesel dosing effect to reduce nitro-PAH emissions, for example, 20% biodiesel reduced 1-nitronaphthalene, 3-nitrofluoranthene, and 1-nitropyrene emissions by approximately 50%. NitroPAH was also present at lower concentration on the collected PM when biodiesel was a fuel component. This is demonstrated in Table S4 (Supporting Information), which compares masses of nitro-PAHs per mg of PM for the same samples represented in Table 1, and the resulting ULSD/B100 and ULSD/B20 ratios. The observed decreases in nitro-PAH from biodiesel occurred despite a general trend to higher engineout NOx as the biodiesel fuel fraction increased (see Supporting Information Table S5). PM Associated PAH Emissions. Analogous to nitro-PAH, the masses of PAH per brake horsepower-hour and the fuel source PAH ratios indicate a strong trend toward lower PAH emissions with increasing biodiesel in the fuel (see Table 2). The effect was strongest for phenanthrene and anthracene, which were emitted from ULSD combustion at rates 10 and 29 times higher than from B100, respectively. Phenanthrene was also the most abundant PAH emitted from ULSD. Fluoranthene and pyrene were next most abundant and were reduced by factors of 4.4 and 4.1 from B100, respectively. The substantial decreases in emitted 3-ring and 4-ring PAH from B100 combustion may be explained by the absence of aromatics in the fuel, which can act as templates for the formation of larger PAH through reactions with olefin gases produced during combustion (3). PAH emissions from B100 were not zero, however, and measured naphthalene was the second highest PAH from B100; at double the rate from ULSD. While recoveries for naphthalene from PM were low (see Supporting Information Table S3) as a result of its high vapor pressure relative to the other PAHs, the data show that aromatization reactions leading to naphthalene formation can occur during B100 combustion and may increase naphthalene emissions relative to ULSD. DPF Effects on PAH and Nitro-PAH Emissions. The DPF caused >90% conversion of most measured nitro-PAH except for 1-nitropyrene (see Supporting Information Table S7), which was the most abundant nitro-PAH in all tests. Its notably low oxidation rate in the DPF is demonstrated in Table 3 which compares 1-nitropyrene before and after the DPF. In contrast, DPF oxidation of pyrene exceeded 99% for ULSD and B20 (see Table 4). In a previous study with the same engine and DPF the raw engine-out and raw DPF-out NO2 and NOx emissions were measured at six of the same steady-state modes utilized here (29). On average the DOC/ DPF increased NO2 from 7% to 34% of NOx, but in low exhaust temperature modes 5 and 6 NO2 was over 60%. Thus, high concentrations of NO2 in the DPF could cause nitration of some of the available pyrene, forming 1-nitropyrene. However, this is not consistent with the very efficient conversion of pyrene, nor with the high conversion of other nitro-PAH species in the DPF.

Acknowledgments This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-99GO10337 with the National Renewable Energy Laboratory.

Supporting Information Available Additional experimental details on sampling and analyses and Tables S1-S9 summarize engine specifications, test cycle modes, PM internal standard recovery rates, and emissions data. This information is available free of charge via the Internet at http://pubs.acs.org/. 8348

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