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Investigation of PCDD/F Emissions from Mobile Source Diesel Engines: Impact of Copper Zeolite SCR Catalysts and Exhaust Aftertreatment Configurations Z. Gerald Liu,* John C. Wall, Patrick Barge, Melissa E. Dettmann, and Nathan A. Ottinger Cummins Inc., 1801 U.S. Highway 51, Stoughton, Wisconsin 53589, United States ABSTRACT: This study investigated the impact of copper zeolite selective catalytic reduction (SCR) catalysts and exhaust aftertreatment configurations on the emissions of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) from mobile source diesel engines. Emissions of PCDD/Fs, reported as the weighted sum of 17 congeners called the toxic equivalency quotient (TEQ), were measured using a modified EPA Method 0023A in the absence and presence of exhaust aftertreatment. Engine-out emissions were measured as a reference, while aftertreatment configurations included various combinations of diesel oxidation catalyst (DOC), diesel particulate filter (DPF), Cu-zeolite SCR, Fe-zeolite SCR, ammonia oxidation catalyst (AMOX), and aqueous urea dosing. In addition, different chlorine concentrations were evaluated. Results showed that all aftertreatment configurations reduced PCDD/F emissions in comparison to the engine-out reference, consistent with reduction mechanisms such as thermal decomposition or combined trapping and hydrogenolysis reported in the literature. Similarly low PCDD/F emissions from the DOC-DPF and the DOC-DPF-SCR configurations indicated that PCDD/F reduction primarily occurred in the DOC-DPF with no noticeable contribution from either the Cu- or Fe-zeolite SCR systems. Furthermore, experiments performed with high chlorine concentration provided no evidence that chlorine content has an impact on the catalytic synthesis of PCDD/Fs for the chlorine levels investigated in this study.
’ INTRODUCTION Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), collectively denoted as PCDD/ Fs in this paper, have been identified to pose a toxicological hazard for human health.1,2 World-wide organizations such as the U.S. Environmental Protection Agency (EPA) and the World Health Organization (WHO) have addressed the potential health risks of PCDD/Fs by issuing maximum daily exposure guidelines. The EPA has specified a virtually safe dose of 0.006 pg TEQ kg-1 d-13 and estimated the exposure levels of multiple PCDD/F congeners in the Mobile Source Air Toxics (MSAT) list,4 while the WHO has issued a Tolerable Daily Intake (TDI) of 10 pg 2,3,7,8-TCDD kg-1 d-1.5 Although inapplicable to diesel engines, EPA regulations exist for municipal waste combustors, hazardous and medical waste incinerators, pulp and paper mills, and secondary aluminum smelters.6,7 As a result, PCDD/F emissions have decreased from certain anthropological sources from 1987 to 2000. For example, municipal waste incinerator emissions decreased from 63.8 to 5.9% of total PCDD/F emissions according to a 2006 EPA survey.7 In the same survey, heavy-duty diesel engines accounted for the sixth largest source of PCDD/Fs in 2000, producing 4.6% of the total PCDD/F emissions. The emergence of diesel engines as a non-negligible source of PCDD/F emissions has motivated research into the effect of modern engine and aftertreatment technology on PCDD/F emissions. r 2011 American Chemical Society
Designed to meet increasingly stringent worldwide particulate matter (PM) and oxides of nitrogen (NOx) regulations, modern aftertreatment technology may include urea selective catalytic reduction (SCR) catalysts, one of the most promising NOx reduction technologies which has already appeared in production vehicles. Urea SCRs reduce NOx through a series of reactions in which NOx reacts with NH3 to form N2 and H2O.8,9 Due to their high temperature stability, zeolite-based urea SCR catalysts are being used for NOx reduction in applications which require active diesel particulate filter (DPF) regeneration.10,11 Although both Cu- and Fe-zeolite SCR catalysts possess high thermal durability, Cu-zeolite displays better low temperature NOx conversion and less sensitivity to NO/NO2 ratio. However, concern exists that copper may catalyze the synthesis of PCDD/Fs, whose optimal formation temperature overlaps with typical SCR operating temperatures. For example, Heeb et al.12 and Mayer et al.13 have reported that copper fuel additives can significantly increase the formation of PCDD/Fs for diesel fuels doped to high levels of chlorine, in contrast to the negligible effect of iron and cerium additives. To investigate the impact of Cu-zeolite SCR catalysts and exhaust aftertreatment configurations on PCDD/F emissions, Received: November 23, 2010 Accepted: February 3, 2011 Revised: January 28, 2011 Published: February 28, 2011 2965
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Environmental Science & Technology this study measured dioxin levels with a modified EPA method 0023A from two modern diesel engines in the absence and presence of aftertreatment. Engine-out emissions were measured as a reference, while aftertreatment configurations included various combinations of diesel oxidation catalyst (DOC), DPF, Cu-zeolite SCR, Fe-zeolite SCR, ammonia oxidation catalyst (AMOX), and aqueous urea dosing. In addition, different chlorine concentrations were evaluated. The TEQ concentrations of PCDD/Fs emitted by engine A were first compared between engine-out and various aftertreatment configurations to determine the impact of aftertreatment components on PCDD/F formation, especially with respect to the Cu-zeolite SCR catalyst. Further analysis compared the PCDD/F emissions from engines A and B using the same aftertreatment configuration but different Cu-zeolite formulations to verify the cross-platform validity of the experimental data. Finally, all PCDD/F emissions were compared to EPA limits for two industrial sources to assess the relative magnitude of PCDD/F concentrations measured in this study and to provide updated information for determining the associated atmospheric implications of these emissions.
’ PROCESSES OF PCDD/F FORMATION AND DESTRUCTION The characteristic dioxin fingerprint for diesel engine emissions resembles unleaded gasoline vehicles.7 In diesel exhaust systems, the formation and destruction of PCDD/Fs may occur simultaneously. PCDD/Fs can form through the de novo, precursor, or gas phase mechanisms, while PCDD/F destruction can occur through dechlorination, oxidation, or pyrolysis. Both of the heterogeneous processes of de novo and precursor formation share a temperature regime of 200-400 °C, while homogeneous formation proceeds uncatalyzed in the gaseous phase at 350-800 °C. In de novo formation, the carbonaceous matrix undergoes a series of oxidation and chlorination reactions to produce PCDD/ Fs.14-17 De novo formation may involve the direct transformation of pre-existing source structures in the carbonaceous matrix or alternatively the condensation reactions of aromatics originating from the macromolecular carbonaceous matrix but released during combustion. Gullett et al.16 proposed three steps for the latter case: the metal catalyzed formation of Cl2, chlorination of aromatic rings through substitution reactions, and the formation of dual ring PCDD/F structures by a second metal catalyzed reaction. However, other research has indicated the limited role of this process.14,18 Recent studies have also focused on the individual ingredients required for de novo formation. For example, Stanmore et al.14 found that free chlorine above 1 wt % and oxygen below 2 mol % may control the rate of de novo formation under certain conditions, while the absence of gaseous oxygen immobilizes PCDD/F formation. In contrast to de novo formation, which demolishes the macromolecular carbon matrix, precursor formation synthesizes PCDD/Fs through condensation reactions of chemically similar molecules non-native to the macromolecular carbon matrix.18,19 Possibly adsorbed directly onto the macromolecular carbon matrix from ambient air, formed as products of incomplete combustion, or produced by postcombustion heterogeneously catalyzed reactions, common precursor molecules include chlorinated benzenes and phenols which react through catalyzed Ullman type reactions. In one mechanism proposed by Lomnicki and Dellinger,19 chlorophenols combine in surface-mediated, radical-radical and radical-molecule reactions to form PCDDs and
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PCDFs by Eley-Rideal and Langmuir-Hinshelwood mechanisms, respectively. The formation of PCDD/Fs by condensation reactions of chlorophenol precursors also describes homogeneous formation, which proceeds uncatalyzed in the gaseous phase. Though trivialized for several years in comparison to de novo and precursor formation,20 more recent studies have demonstrated the nonnegligible contribution of homogeneous formation in combustion processes.21,22 As proposed by Khachatryan et al.,22 homogeneous formation proceeds through both radical-radical and radical-molecule reactions. Copper, primarily in the form of Copper(II), may participate in the formation of PCDD/Fs in de novo and precursor formation, catalyzing oxidation, chlorination, and Ullman type reactions. Some studies have cited copper as the most active catalyst for PCDD/F formation in municipal solid waste fly ash.14-16 Copper shuttles chlorine from the gas to solid phases, possibly through a CuCl2 complex that facilitates carbon-chlorine bonds with a simultaneous change of oxidation state. In addition to chlorine, the copper catalyst also facilitates the incorporation of gaseous oxygen into the carbon matrix. Isotope studies with 18O have revealed a correlation in which gaseous and bonded oxygen correspond to the higher and lower chlorinated congeners, respectively.14 However, the formation of PCDD/Fs represents an equilibrium process, in which certain conditions promote the PCDD/F destruction pathways of dechlorination, oxidation, and pyrolysis.23-26 Some studies have indicated that copper may participate in the first two processes: Weber et al.23 found that Cu2O and elemental Cu more effectively dechlorinated PCDD/F from a model fly ash system at 260 °C than either Fe2O3 or SnO, while Visez et al.24 stated that CuCl2 or CuCl highly accelerated the oxidative destruction of phenols. Under low-oxygen pyrolysis conditions at 450-700 °C, PCDD/F destruction proceeds either through unimolecular decomposition or hydrogenolysis reactions. Since pyrolysis of dibenzo-p-dioxin produces dibenzofuran, and due to the lower relative toxicity of the latter type of compounds, Altarawneh et al.15 have identified pyrolysis as a promising option for PCDD/F abatement in municipal waste incinerators. In regard to diesel exhaust systems, some chemicals present in the combustion fluids additionally hinder PCDD/F formation: sulfur may interfere with active copper sites,26-38 while urea and its decomposition products may participate in inhibition reactions.28
’ EXPERIMENTAL SECTION Sampling Procedure. This study adopted a modified EPA method 0023A for PCDD/F measurement, described in detail by Liu et al.,29 with the corresponding apparatus depicted in Figure 1. In brief, the engine-out or aftertreatment exhaust flowed through a sampling section and into a critical flow venturi-constant volume system (CFV-CVS). Monitored by a pitot tube, a sample of the undiluted exhaust was isokinetically drawn into a sampling probe, through a quartz particulate prefilter (maintained at 120 ( 14 °C), a condenser, and a XAD absorbent to retain PCDD/Fs. The sample then proceeded through four glass impingers in series to desiccate the sample and to determine the amount of H2O in the exhaust: the first impinger remained empty, the second and third contained 100 mL of water, and the fourth contained 200 g of silica. The sample flow rate was controlled by an orifice and continuously 2966
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Figure 1. Schematic of the modified method 0023A apparatus.29
Table 1. Engine Operating Points and Aftertreatment Configurations aftertreatment configuration
engine
engine point
SCR
chlorine (ppm)
engine out
A
1
-
0.6
engine out
A
2
-
0.6
DOC-DPF DOC-SCR
A A
1 1
Cu-Z
DOC-DPF-SCR
A
1
DOC-DPF-SCR
A
1
DOC-DPF-SCR
A
DOC-DPF-SCR DOC-DPF-SCR
RPM
torque (N 3 m)
-
1000
529
-
-
1100
624
-
0.6 0.6
on
1000 1000
529 529
300 300
Cu-Z
0.6
on
1000
529
300
Cu-Z
8.4
on
1000
529
300
2
Fe-Z
0.6
on
1100
624
350
A
1
Cu-Z
0.6
off
1000
529
300
B
3
Cu-Z
0.6
on
1100
739
300
measured with a dry gas meter. To maintain an isolated test cell during experiments, remote cameras monitored the XAD coolant level, the urea dosing system, the aftertreatment components, and the oil pressure gauge. Supply and return lines of the pumping system extended from the impinger ice bath to a cooler located outside the test cell, while pumps in both locations maintained the circulation of cold water to achieve rigorous control of the XAD gas inlet temperature (below 20 °C). To withstand engine vibrations during testing, a titanium sampling probe was used instead of borosilicate glass. Test Configurations. The experiments were performed on two model year 2010 engines from different manufacturers: Engine A, an 8.9 L engine rated for 246 kW (330 hp) at 2000 rpm, and Engine B, a 12.9 L engine rated for 358 kW (480 hp) at 1900 rpm. Controlled with a General Electric direct current dynamometer, the engines were fueled with Chevron Phillips ultralow sulfur diesel with a sulfur content of 9.7 ppm. Neutron activation analysis was used to determine the chlorine concentrations of fuel (0.2 ppm), oil (77.60 ppm), urea (0.13 ppm), and coolant (5.92 ppm), while NIOSH method 6011 assessed the chlorine concentration of intake air (nondetected
urea
SCR temperature (°C)
with a detection limit of 0.0024 ppm). As determined by the consumption of each chlorine source during a testing cycle, the majority of chlorine present throughout combustion was derived from the fuel. To simulate chlorine from all probable sources in a controlled manner, the fuel was doped with 1-chlorohexadecane. Although more dopant was required to achieve the same chlorine concentration than with the other possible dopant of dichlorohexane, 1-chlorohexadecane was chosen because its carbon chain length is more representative of diesel fuel. Both 1-chlorohexadecane and dichlorohexane are fully soluble in diesel fuel and have been used in other studies. A concentration of 0.6 ppm chlorine was selected because this is equivalent to the highest possible chlorine concentration in current U.S. diesel fuels as determined by an EPA survey of various fuel samples. On the other hand, a chlorine concentration of 10 ppm was chosen to determine the impact of high chlorine content on PCDD/F formation, though a final concentration of 8.4 ppm was actually achieved. Table 1 describes the engine conditions and aftertreatment configurations for the PCDD/F emissions tests conducted in this study. Aftertreatment components included a DOC consisting of 2967
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Environmental Science & Technology precious metals for the oxidation of CO, unburned hydrocarbons, NO, and the soluble organic fraction (SOF) of soot; a catalyzed cordierite wall-flow DPF for the reduction of PM; and Cu- and Fe-zeolite SCR systems composed of two SCR catalysts followed by an AMOX catalyst for NOx and NH3 reduction, respectively.30 Urea was injected in all aftertreatment configurations with an SCR component except for three experiments with the DOC-DPF-SCR on engine A which were carried out in order to assess the effect of urea on PCDD/F formation. Steady-state engine operating points (shown in Table 1) were used for PCDD/F measurements and were selected based on the following criteria: SCR bed temperatures of 300 and 350 °C for Cu- and Fe-zeolite SCR catalysts, respectively, to obtain worst-case scenarios for PCDD/F formation;14,15 lowest air to fuel ratio possible to maximize fuel and chlorine consumption; and within typical engine operating range. During experiments without aftertreatment, a butterfly valve was used to maintain the same backpressure as the aftertreatment systems in order to simulate identical engine operating conditions. DPF regenerations were managed by the aftertreatment control module, and testing was not interrupted during these events. Sample Processing and Analysis. Prior to sampling, the filters and the XAD absorbent were precleaned through a series of Soxhlet extractions: 16 h with toluene for the filter; and 8 h with water, 22 h with methanol, 22 h with methylene chloride, and 22 h with toluene for the XAD. Both the filters and the XAD absorbent were dried under a stream of inert gas and received aliquots of sampling standard solutions for later use in determining PCDD/F concentrations. Glassware cleaning included soaking three times with deionized water, baking at 400 °C for 2 h, and rinsing three times each with methylene chloride and toluene. Prebaked aluminum foil covered the exposed surfaces on the glassware until the sampling day when one additional rinse each with acetone and methylene chloride completed the sampling preparations. Upon completion of each experiment, extraction of the modified EPA method 0023A sampling train consisted of three rinses each with acetone, methylene chloride, and toluene applied to the probe and all glassware upstream of the XAD cartridge. The solvent rinses, along with the XAD absorbent trap and prefilter, were analyzed using EPA method 8290 with high resolution gas chromatography (HRGC) and high resolution mass spectrometry (HRMS). The HRGC/HRMS analysis relied on several types of sample spikes: 1) a sampling standard solution, four 13 C12-labeled standards plus a 37Cl4 2,3,7,8-TCDD standard added to the XAD absorbent trap prior to sampling, 2) an extraction standard solution, 17 toxic PCDD/F isomers labeled with 13C12 added to the combined XAD absorbent trap, prefilter, and solvent rinses after sampling but before extraction, and 3) an injection standard solution, two 13C12-labeled standards including an optional standard of 13C12 1,2,3,7,8,9-HxCDD added to the combined samples just before injection into the HRGC/ HRMS. PCDD/F Toxic Equivalency Quotient Calculation. The toxic equivalency quotient (TEQ) is a measure of the aggregate toxicity of the PCDD/F congeners. Since the toxicity of PCDD/F congeners depends on their chemical structure, the toxic equivalent factor (TEF) scales the relative toxicity of each congener compared to the most toxic congener, 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (TCDD). Computed from eq 1, the TEQ is calculated by multiplying the TEF for each congener by either the congener’s mass or detection limit, depending on the TEQ
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scheme, and summing over the 17 toxic PCDD/F congeners. This study reported four TEQ values using two TEFs to reflect a variety of governmental agency and industrial preferences: ND=0, ND=1/2 3 DL, ND=DLWHO 1998, and ND=DLWHO 2005, where ND stands for nondetect and DL stands for detection limit. Detection limits were calculated with eq 2, where HN is the height of the noise, QIS is the amount of extraction standard added to the sample before extraction, HIS is the peak height of the extraction standard, W is the weight of the sample (1.0 for method 0023A), and RRF is the calculated relative response factor for the analyte TEQ ¼ DL ¼
∑i TEFi 3 massi
ð1Þ
2:5HN 3 QIS HIS 3 W 3 RRF
ð2Þ
’ RESULTS AND DISCUSSION For all engine-out cases and aftertreatment configurations, replicate experiments were performed to determine measurement repeatability and sample variation. The average values along with the corresponding standard errors are reported. Due to the ultralow concentrations of PCDD/Fs, this study strived to achieve less than 25% relative standard error for each test configuration. Typically three repeats were performed, but additional runs were required for certain test configurations in order to achieve this objective. The HRGC/HRMS analyses determined the masses in picograms of the 17 toxic PCDD/F congeners as well as the total amounts of the tetra- to octa-isomeric groups which collectively encompass 210 PCDD/F congeners. Possible qualifiers to the data reported for each detected analyte include the following: below the calibration range (J), detected in the laboratory blank (B), and outside the theoretical range (estimated maximum possible concentration, EMPC). When provided, EMPC values were always included in this study to obtain worst-case TEQ emissions of PCDD/Fs. The results were calculated and reported in units of pg/dscm and pg/bhp 3 hr (pg/kW 3 hr) to facilitate comparisons with other mobile and industrial sources. Comparison of Engine-Out and Aftertreatment Systems. The TEQ concentrations of PCDD/Fs emitted by engine A were first compared between engine-out and various aftertreatment configurations. An example of the data collected for each type of engine and aftertreatment configuration in the present study appears in Table 2 for the experiments performed on engine A at engine-out point 1 and the DOC-DPF-SCR configuration with urea dosing and fuel doped to 0.6 ppm Cl. Average concentrations of toxic and total PCDD/Fs, detection limits, calculated TEQs, and percent reductions resulting from the DOC-DPFSCR aftertreatment are all reported. If average detection limits are reported in Table 2, a congener was not detected in at least one of the replicate experiments performed with this testing condition. The tabulated results show that an ND=0 TEQ of 0.31 pg/bhp 3 hr was measured for the engine-out, whereas an ND=0 TEQ of 0.12 pg/bhp 3 hr was measured for the DOC-DPF-SCR case, corresponding to a greater than 60% reduction in PCDD/F emissions. Individual PCDD/F congeners also experienced similar reductions between the engine-out cases and the aftertreatment configurations. For example, engine-out point 1 emitted 2.95 pg/bhp 3 hr (3.96 pg/kW 3 hr) of 2,3,7,8-TCDF 2968
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Table 2. Average PCDD/F Congener Emissions, Reductions with Aftertreatment, and Detection Limits for Engine A Point 1 Experiments without Aftertreatment and with the DOC-DPF-SCR Configuration with Urea Dosing and Fuel Doped to 0.6 ppm Cl concentration engine out compound
pg/dscm
pg/bhp 3 hr
detection limit
DOCþDPFþSCR pg/dscm
pg/bhp 3 hr
engine out % reduction
DOCþDPFþSCR
pg/dscm
pg/bhp 3 hr
pg/dscm
pg/bhp 3 hr 0.45
2,3,7,8-TCDD
ND
ND
ND
ND
-
0.19
0.75
0.11
1,2,3,7,8-PeCDD
ND
ND
ND
ND
-
0.45
1.74
0.20
0.79
1,2,3,4,7,8-HxCDD 1,2,3,6,7,8-HxCDD
ND ND
ND ND
ND ND
ND ND
-
0.38 0.38
1.46 1.48
0.16 0.16
0.63 0.64
1,2,3,7,8,9-HxCDD
ND
ND
ND
ND
-
0.40
1.54
0.19
0.75
1,2,3,4,6,7,8-HpCDD
0.69
2.68
0.29
1.14
57%
0.66
2.56
0.17
0.68
OCDD
3.06
11.92
1.44
5.74
52%
-
-
-
-
2,3,7,8-TCDF
0.76
2.95
0.25
1.01
66%
-
-
-
-
1,2,3,7,8-PeCDF
ND
ND
ND
ND
-
0.60
2.31
0.12
0.49
2,3,4,7,8-PeCDF
ND
ND
ND
ND
-
0.54
2.11
0.12
0.47
1,2,3,4,7,8-HxCDF 1,2,3,6,7,8-HxCDF
ND ND
ND ND
0.09 ND
0.37 ND
-
0.35 0.34
1.38 1.35
0.10 0.10
0.39 0.38
2,3,4,6,7,8-HxCDF
ND
ND
ND
ND
-
0.35
1.36
0.09
0.37
1,2,3,7,8,9-HxCDF
ND
ND
ND
ND
-
0.18
0.71
0.12
0.47
1,2,3,4,6,7,8-HpCDF
ND
ND
0.10
0.40
-
0.53
2.03
0.10
0.40
1,2,3,4,7,8,9-HpCDF
ND
ND
ND
ND
-
0.17
0.66
0.11
0.44
OCDF
ND
ND
ND
ND
-
1.14
4.39
0.39
1.55
Total TCDD
0.98
3.78
0.38
1.50
60%
-
-
-
-
Total PeCDD Total HxCDD
0.29 0.64
1.13 2.49
ND 0.20
ND 0.78
69%
0.38 0.52
1.50 2.02
0.20 0.19
0.79 0.78
Total HpCDD
1.15
4.46
0.56
2.23
50%
-
-
-
-
Total TCDF
4.10
15.93
1.67
6.68
58%
-
-
-
-
Total PeCDF
0.55
2.14
0.17
0.67
69%
0.40
1.58
0.09
0.36
Total HxCDF
0.37
1.42
0.15
0.59
59%
0.26
1.02
0.08
0.34
Total HpCDF
0.44
1.71
0.16
0.62
64%
0.24
0.93
0.09
0.37
TEQ (ND=0 WHO 1998)
0.08
0.31
0.03
0.12
61%
TEQ (ND=DL/2 WHO 1998) TEQ (ND=DL WHO 1998)
0.66 1.27
2.56 4.94
0.26 0.50
1.05 1.98
59% 60%
TEQ (ND=DL WHO 2005)
1.15
4.47
0.47
1.88
58%
while the DOC-DPF-SCR with urea dosing and fuel doped to 0.6 ppm chlorine emitted 1.01 pg/bhp 3 hr (1.35 pg/kW 3 hr), a reduction of approximately 66% by the aftertreatment. Overall, only five of the 17 toxic PCDD/F congeners were detected for these two testing conditions. While data from other experiments are not included in Table 2 for brevity, results from all engine-out and aftertreatment configurations are presented in Figure 2, which includes ND=0 and ND=DL WHO 1998 TEQ values in pg/bhp 3 hr as well as standard errors. Similar to the results presented in Table 2, all configurations with aftertreatment reduced PCDD/Fs below engine-out levels. TEQ concentrations of PCDD/Fs were compared between engine A point 1 and point 2 without aftertreatment to examine the effect of engine operating conditions. Figure 2 shows that the results were comparable between the two engine-out cases. Interestingly, point 1 produced a higher ND=DL TEQ while point 2 produced a higher ND=0 TEQ. This discrepancy is accounted for by the variation of detection limits which ranged from 1 to 12 pg/sample in this study and have a large impact on ND=1/2 3 DL and ND=DL TEQ values. Other researchers have experienced similar detection limit variation in their results.31 Therefore, further comparisons will use ND=0 TEQ values, as
ND=1/2 3 DL and ND=DL have been provided only to estimate worst-case emissions. The following section will rationalize the possible mechanisms accounting for the above results to better understand the fundamental effects of the aftertreatment components. Effects of Aftertreatment Components. The reductions in PCDD/F emissions achieved by the aftertreatment configurations may have resulted from several processes present in the aftertreatment components. In a DOC-DPF-SCR system, opportunities exist for removing some of the materials required for PCDD/F formation prior to the SCR catalyst. In addition, some considerations specific to the Cu-zeolite SCR system may limit the PCDD/F formation observed in this study. DPFs trap particulate matter, substantially reducing the amount of particle-phase compounds available for PCDD/F formation. Liu et al.32 have shown that DPFs often achieve above 90% gravimetric and fractional efficiencies with respect to particle mass- and number-based measurements. As a result, DPFs can trap particle-phase PCDD/Fs along with the soot, fly ash, and related materials that constitute the carbonaceous matrix required for de novo formation. In addition to trapping, catalyzed DPFs are also capable of participating in the chemical 2969
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Figure 2. Average ND=0 (a) and ND=DL (b) WHO 1998 TEQ values for all engine and aftertreatment configurations. (Unless specified, all SCR components are Cu-zeolite and fuel was doped to 0.6 ppm Cl.) Different y-axis scales are used for clarity.
mechanisms of oxidation, hydrogenolysis, and thermal decomposition, especially during regeneration when the temperature can exceed 500 °C.23-26 Similarly, the DOC may reduce the amount of material available for precursor formation of PCDD/Fs. Studies have shown that DOCs can reduce hydrocarbon emissions by 5288% and PM emissions by 23-29% for transient FTP cycles33 through oxidation of SOF which co-condenses with soot particles.34 Many SOF compounds, including aromatics and polyaromatic hydrocarbons (PAH), may participate in precursor formation. Dyke et al.35 have conducted experiments with a DOC only aftertreatment system and concluded that a DOC can reduce PCDD/F emissions by up to 70% in comparison to emissions without aftertreatment. In addition to the presence of upstream aftertreatment components, several factors may influence the extent of PCDD/F emissions from the Cu-zeolite SCR system. Possible conditions required for PCDD/F formation that differ from other sources may include the catalyst surface area and microstructure, residence time, amount and carbonaceous matrix composition of hydrocarbons and soot, and chlorine content. For example, exhaust gas in a Cu-zeolite SCR system typically has a short residence time compared to an electrostatic precipitator and a small contact area compared to copper fuel borne catalysts. The low concentration of chlorine and the presence of chemical inhibitors such as urea and sulfur can discourage PCDD/F formation in the Cu-zeolite SCR system. Furthermore, the AMOX catalyst in the
Cu-zeolite SCR system serves a similar purpose as a DOC with the net effect of reducing PCDD/F emissions. Effect of a Cu-Zeolite SCR. Two comparisons assessed the effect of a Cu-zeolite SCR on the experimental system. The DOC-DPF configuration was first compared to the DOC-DPFSCR configuration in order to determine whether or not the Cuzeolite SCR actively promoted the formation of PCDD/Fs. As seen in Figure 2, similar results from these two configurations failed to indicate the downstream formation of PCDD/Fs in the Cu-zeolite SCR system. A second analysis then compared the DOC-DPF and DOC-SCR configurations. Similar PCDD/F levels from these configurations indicate that the two systems achieved comparable reductions in engine-out PCDD/F emissions. These results may be attributed to several factors discussed in the previous section. Since previous work has demonstrated that ammonia suppresses the production of PCDD/Fs by inhibiting the catalytic activity of fly ash,28 this study tested the DOC-DPF-SCR configuration in the presence and absence of urea, and the resulting ND=0 TEQs differed by only 0.05 pg/bhp 3 hr (0.07 pg/kW 3 hr). The ultralow concentrations of PCDD/Fs in this study may have limited the perceived effect of urea dosing. Additionally, the DOC-DPF-SCR configuration with a Cuzeolite SCR was compared to the same configuration with an Fe-zeolite SCR. The measured emissions of PCDD/Fs from these two zeolite SCR systems differed by only 0.01 pg/bhp 3 hr (ND=0 TEQ), implying that PCDD/F formation and 2970
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Figure 3. Comparison of average ND=0 and ND=DL TEQ values from all engine and aftertreatment configurations studied and two EPA limits for industrial sources.7 (Unless specified, all SCR components are Cu-zeolite and fuel was doped to 0.6 ppm Cl.)
destruction mechanisms are unaffected by SCR formulation for the simulated worst-case PCDD/F formation temperatures used in this study. Another set of experiments with a DOC-DPF-SCR configuration and 8.4 ppm chlorine doping was performed to determine the impact of high chlorine concentration. As seen in Figure 2, no effect on PCDD/F emissions was observed as a result of this increase in chlorine concentration. This may be a result of the nonoptimal formation conditions encountered in the SCR system, as already discussed in the previous sections. To verify the cross-platform validity of the experimental data, further analysis compared the PCDD/F emissions between engines A and B using the DOC-DPF-SCR configuration but different Cu-zeolite formulations. As shown in Figure 2, PCDD/ F emissions from engine B equipped with a DOC-DPF-SCR were also low, and the marginal increase in comparison to engine A with a similar aftertreatment configuration likely results from the larger engine displacement and higher torque of engine B. Consistent with the above results, recent studies by Laroo36 and Hovemann et al.37 have also shown that there was no risk of elevated PCDD/F emissions from Cu-zeolite SCR systems for the various engines, operating conditions, and aftertreatment configurations in their respective studies. Comparison with Stationary Sources. Finally, all PCDD/F emissions were compared to EPA limits for industrial sources to assess the relative magnitude of PCDD/F concentrations and to provide updated information on the associated atmospheric implications of these toxic emissions from various sources. Figure 3 compares the average ND=0 and ND=DL WHO 1998 TEQ concentrations for all test configurations from both engines to new facility limits for a medium or large medical waste incinerator (MWI) and a cement kiln with temperature control to less than 400 °F at the air pollution control device inlet.7 Error bars shown in the figure represent standard errors. All data referred to in the following discussion is calculated based on the ND=0 TEQ scheme. The TEQ limits for the two industrial sources are defined with the international TEF scheme (I-TEQ)
adopted by the EPA in 1989, which differs from the WHO 1998 scheme used in this study for only three congeners. The results of this study show that PCDD/F emissions per dry standard cubic meter of exhaust from engines A and B with aftertreatment are lower than EPA emissions limits for MWIs by a factor of 1000 or more. Cumulatively, the approximately 1000 MWIs operating in the US in 2000 emitted 378 g TEQ/yr (WHO 1998).7 In comparison, the same report estimated the annual PCDD/F release from diesel on-road (Trucks) to be only 65.4 g TEQ/yr. However, this value was based on a PCDD/F emission rate of 182 pg TEQ/km obtained from tunnel measurements of untreated diesel emissions.38 In contrast, the PCDD/F emissions from the DOC-DPF-SCR configuration with urea and fuel doped to 0.6 ppm Cl (0.28 pg TEQ/km) are more than 2 orders of magnitude lower than that reported in the EPA’s 2006 inventory. This estimate is based on the EPA established useful life of 435,000 miles over 22,000 h for heavy heavy-duty diesel engines such as urban buses. The results presented above clearly show that the Cu-zeolite SCR systems analyzed in this study do not actively participate in the catalytic synthesis of PCDD/Fs. In fact, the modern diesel aftertreatment systems reduced PCDD/F emissions, consistent with reduction mechanisms such as thermal decomposition or combined trapping and hydrogenolysis. Over the range of analyzed values, both urea and chlorine had a negligible impact on PCDD/F mitigation or formation. When compared to published TEQ emission rates and limits from the EPA’s 2006 inventory, measured PCDD/Fs from the aftertreatment configurations in this study were at least 2 orders of magnitude lower than the rate used to estimate emissions from diesel on-road trucks in the year 2000 and at least a factor of 1000 lower than limits for industrial sources.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
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’ ACKNOWLEDGMENT The authors of this study thank Dr. Norbert Heeb of the Swiss Federal Laboratories for Materials Testing and Research, Prof. James Schauer of the University of Wisconsin-Madison, as well as Joseph McDonald, Charles Schenk, Christopher Laroo, Jeffrey Ryan, and Michael Hays of the U.S. EPA for their invaluable advice. The authors also appreciate the program support provided by Wayne Eckerle, Steve Charlton, Sean Milloy, Jeffrey Weikert, and Bryan Blackwell of Cummins, the testing and sample collection carried out by Christopher Cremeens, Niklas Schmidt, Ryan Foley, Renato Yapaulo, Jack Oberrath Jr., Michael Robinson, and Robert Holt of Cummins Emission Solutions, and the sample analysis performed by Analytical Perspectives Laboratory. ’ REFERENCES (1) Federal Register. Control of emissions of air pollution from nonroad diesel engines and fuel, 2004, 38957-39273. (2) Federal Register. Control of air pollution from new motor vehicles: Heavy-duty engine and vehicle standards and highway diesel fuel sulfur control requirements, 2003, 5001-5193. (3) Travis, C. C.; Nixon, A. G. Human exposure to dioxin. The Royal Society of Chemistry 1996. (4) EPA. Final rule: Control of hazardous air pollutants from mobile sources. EPA-420-F-07-017. (5) Harrison, N. Ch. 8: Environmental organic contaminants in food. Food Chemical Safety, Vol. 1. Foods Standards Agency. (6) EPA. Exposure and human health reassessment of 2,3,7,8tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. 2003, EPA/600/P-00/001Cb. (7) EPA. An inventory of sources and environmental releases of dioxin-like compounds in the United States for the years 1987, 1995, and 2000. 2006, EPA/600/P-03/002F. (8) Tronconi, E.; Nova, I.; Ciardelli, C.; Chatterjee, D.; Weibel, M. Redox features in the catalytic mechanism of the “standard” and “fast” NH3-SCR of NOx over a V-based catalyst investigated by dynamic methods. J. Catal. 2007, 245, 1–10. (9) Grossale, A.; Nova, I.; Tronconi, E.; Chatterjee, D.; Weibel, M. The Chemistry of the NO/NO2-NH3 “fast” SCR reaction over Fe-ZSM5 investigated by transient reaction analysis. J. Catal. 2008, 256, 312–322. (10) Fedeyko, J. M.; Chen, H.; Ballinger, T. H.; Weigert, E. C.; Chang, H.; Cox, J. P.; Andersen, P. J. Development of thermally durable Cu/SCR catalysts. SAE 2009, 2009-01-0899. (11) Cavataio, G.; Jen, H.-W.; Warner, J. R.; Girard, J. W.; Kim, J. Y.; Lambert, C. K. Enhanced durability of a Cu/Zeolite-based SCR catalyst. SAE 2008, 2008- 01-1025. (12) Heeb, N. V.; Zennegg, M.; Gujer, E.; Honegger, P.; Zeyer, K.; Gfeller, U.; Wichser, A.; Kohler, M.; Schmid, P.; Emmenegger, L.; Ulrich, A.; Wenger, D.; Petermann, J. L.; Czerwinski, J.; Mosimann, T.; Kasper, M.; Mayer, A. Secondary effects of catalytic diesel particulate filters: Copper-induced formation of PCDD/Fs. Environ. Sci. Technol. 2007, 41, 5789–5794. (13) Mayer, A.; Heeb, N.; Czerwinski, J.; Wyser, M. Secondary emissions from catalytic active particle filter systems. SAE 2003, 2003-01-0291. (14) Stanmore, B. R. The formation of dioxins in combustion systems. Combust. Flame 2004, 136, 398–427. (15) Altarawneh, M.; Dlugorgorski, B. Z.; Kennedy, E. M.; Mackie, J. C. Mechanisms for formation, chlorination, dechlorination and destruction of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Prog. Energy Combust. Sci. 2009, 35, 398–427. (16) Gullett, B.; Bruce, K.; Beach, L.; Drago, A. Mechanistic steps in the production of PCDD and PCDF during waste combustion. Chemosphere 1992, 25, 1387–1392. (17) Huang, H.; Beukens, A. De Novo synthesis of polychlorinated dibenzo-p-dioxins and dibenzofurans: Proposal of a mechanistic scheme. Sci. Total Environ. 1996, 193, 121–141.
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(18) Tuppurainen, K.; Halonen, I.; Ruokojarvi, P.; Tarhanen, J.; Ruuskanen, J. Formation of PCDDs and PCDFs in municipal waste incineration and its inhibition mechanisms: A review. Chemosphere 1998, 36, 1493–1511. (19) Lomnicki, S.; Dellinger, B. A Detailed Mechanism of the SurfaceMediated Formation of PCDD/F from the oxidation of 2-chlorophenol on a CuO/Silica Surface. J. Phys. Chem. A 2003, 107, 4387–4395. (20) Shaub, W.; Tsang, W. Dioxin Formation in Incinerators. Environ. Sci. Technol. 1983, 17, 721–730. (21) Babushok, V. I.; Tsang, W. Gas-phase mechanism for dioxin formation. Chemosphere 2003, 51, 1023–1029. (22) Khachatryan, L.; Burcat, A.; Dellinger, B. An elementary reaction-kinetic model for the gas-phase formation of 1,3,6,8- and 1,3,7,9-tetrachlorinated dibenzo-p-dioxins from 2,4,6-trichlorophenol. Combust. Flame 2003, 132, 406–421. (23) Weber, R.; Nagai, K.; Nishino, J.; Shiraishi, H.; Ishida, M.; Takasuga, T.; Konndo, K.; Hiraoka, M. Effects of selected metal oxides on the dechlorination and destruction of PCDD and PCDF. Chemosphere 2002, 46, 1247–1253. (24) Visez, N.; Sawerysyn, J.-P. Thermal degradation of 2-chlorophenol promoted by CuCl2 or CuCl: Formation and destruction of PCDD/Fs. Chemosphere 2007, 67, S144–S149. (25) Boos, R.; Budin, R.; Hartl, H.; Stock, M.; Wurst, F. PCDD- and PCDF-destruction by a SCR-unit in a municipal waste incinerator. Chemosphere 1992, 25, 375–382. (26) Buekens, A.; Huang, H. Comparative evaluation of techniques for controlling the formation and emission of chlorinated dioxins/furans in municipal waste incineration. J. Hazard. Mater. 1998, 62, 1–33. (27) Gullett, B. K.; Bruce, K. R.; Beach, L. O. Effect of sulphur dioxide on the formation mechanism of polychlorinated dibenzodioxin and dibenzofuran in municipal waste combustors. Environ. Sci. Technol. 1992, 26, 1938–1943. (28) Ruokojarvi, P. H.; Asikainen, A. H.; Tuppurainen, K. A.; Ruuskanen, J. Chemical inhibition of PCDD/F formation in incineration processes. Sci. Total Environ. 2004, 325, 83–94. (29) Liu, Z. G.; Dettmann, M. E.; McNeilly, A. M.; Schmidt, N. M.; Ottinger, N. A.; Delaney, M. J. Development and validation of PCDD/F testing approaches for mobile source engines over transient and steadystate cycles. SAE Int. J. Fuels Lubr. 2010, 3, 745–754. (30) Eastwood, P. Critical topics in exhaust gas aftertreatment; Research Studies Press LTD: Philadelphia, PA, 2000. (31) Geueke, K. J.; Gessner, A.; Quass, U.; Broker, G.; Hiester, E. PCDD/F Emissions from heavy duty vehicle diesel engines. Chemosphere 1999, 38, 2791–2806. (32) Liu, Z. G.; Skemp, M. D.; Lincoln, J. C. Diesel particulate filters: Trends and implications of particle size distribution measurements. SAE 2003, 2003-01-0046. (33) Lloyd, A. C.; Cackette, T. C. Diesel Engines: Environmental Impact and Control. J. Air Waste Manage. Assoc. 2001, 51, 809–847. (34) Hawker, P. N. Diesel Emission Control Technology. Platinum Metals Rev. 1995, 39, 2–8. (35) Dyke, P. H.; Sutton, M.; Wood, D.; Marshall, J. Investigations on the effect of chlorine in lubricating oil and the presence of a diesel oxidation catalyst on PCDD/F releases from an internal combustion engine. Chemosphere 2007, 67, 1275–1286. (36) Laroo, C. Dioxin, Furan, PCB, and PAH emissions from a compression ignition engine utilizing CuZ SCR. SAE 2010, World Congress Presentation. (37) Hovemann, A.; Steeg, W.; Rabl, H. P.; Bock, M.; Helm, M.; Pfaffinger, A. PCDD/PCDF Emissions of Modern Diesel Direct Injection Powertrains and their Aftertreatment System. The 30th International Symposium on Halogenated Persistent Organic Pollutants 2010. (38) Gertler, A.; Sagebiel, J.; Dippel, W. Measurement of dioxin and furan emissions factors from heavy duty diesel vehicles. J. Air Waste Manage. Assoc. 1998, 48, 276–278.
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