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Article
Cold temperature and biodiesel fuel effects on speciated emissions of volatile organic compounds from diesel trucks Ingrid George, Michael D. Hays, Richard Snow, James Faircloth, Barbara Jane George, Thomas Long, and Richard Baldauf Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es502949a • Publication Date (Web): 13 Nov 2014 Downloaded from http://pubs.acs.org on November 17, 2014
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 30
Environmental Science & Technology
Cold temperature and biodiesel fuel effects on speciated emissions of volatile organic compounds from diesel trucks Ingrid J. George,1 Michael D. Hays,1,* Richard Snow,1 James Faircloth,1 Barbara J. George,2 Thomas Long1 and Richard W. Baldauf1 1
Office of Research and Development, National Risk Management Research Laboratory, United States Environmental Protection Agency, Research Triangle Park, NC 27711. 2
Office of Research and Development, National Health and Environmental Effects Research Laboratory, United States Environmental Protection Agency, Research Triangle Park, NC 27711.
*Corresponding Author: E-mail: [email protected]; phone: +1 919-541-3984; fax: +1 919685-3346) Keywords: dynamometer, diesel exhaust, volatile organic compounds, carbonyls, mobile source air toxics, biodiesel
TOC/Abstract Art
1 ACS Paragon Plus Environment
Environmental Science & Technology
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2 ACS Paragon Plus Environment
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1
Environmental Science & Technology
ABSTRACT
2
Speciated volatile organic compounds (VOCs) were measured in diesel exhaust from three
3
heavy-duty trucks equipped with modern aftertreatment technologies. Emissions testing was
4
conducted on a chassis dynamometer at two ambient temperatures (-7 °C and 22 °C) operating
5
on two fuels (ultra-low sulfur diesel and 20 % soy biodiesel blend) over three driving cycles:
6
cold start, warm start and heavy-duty urban dynamometer driving cycle. VOCs were measured
7
separately for each drive cycle. Carbonyls such as formaldehyde and acetaldehyde dominated
8
VOC emissions, making up ~72 % of the sum of the speciated VOC emissions (ΣVOCs) overall.
9
Biodiesel use led to minor reductions in aromatics and variable changes in carbonyls. Cold
10
temperature and cold start conditions caused dramatic enhancements in VOC emissions, mostly
11
carbonyls, compared to the warmer temperature and other drive cycles, respectively. Different
12
2007+ aftertreatment technologies involving catalyst regeneration led to significant
13
modifications of VOC emissions that were compound-specific and highly dependent on test
14
conditions. A comparison of this work with emission rates from different diesel engines under
15
various test conditions showed that these newer technologies resulted in lower emission rates of
16
aromatic compounds. However, emissions of other toxic partial combustion products such as
17
carbonyls were not reduced in the modern diesel vehicles tested.
18
3 ACS Paragon Plus Environment
Environmental Science & Technology
19
Page 4 of 30
INTRODUCTION
20
Vehicle exhaust emissions have substantial negative impacts on air quality, human health and
21
climate. Primary pollutants that are directly emitted from mobile sources include NOx, volatile
22
organic compounds (VOCs), CO, CO2 and particulate matter (PM). Atmospheric chemistry of
23
VOCs and NOx from vehicle emissions in the presence of sunlight creates secondary pollutants
24
leading to enhanced ozone and secondary organic aerosol (SOA) formation, which contribute to
25
photochemical smog.1 A subset of VOCs from vehicle exhaust emissions, termed mobile source
26
air toxics (MSATs) that include some aromatics and carbonyls, are of particular concern because
27
they are carcinogenic, mutagenic or are otherwise suspected to cause serious health effects.2 An
28
accurate account of detailed mobile source emission profiles is vital information for local- and
29
regional-scale air quality models to predict the environmental and health impacts of vehicle
30
emissions.
31
Diesel combustion, especially from heavy-duty vehicles, has become subject to progressively
32
more stringent regulatory oversight in recent years due to the fact that diesel exhaust was
33
emitting disproportionately more NOx and PM than gasoline combustion.3 Furthermore, diesel
34
exhaust and specific components within that exhaust have been associated with acute and
35
chronic adverse health effects.4,5 The 2007 Heavy-Duty Highway Rule was enacted by the U.S.
36
EPA to reduce diesel exhaust emissions for NOx and PM over 90 % below previous standard
37
levels, with full compliance required for model year 2010.6 The fuel standard for sulfur content
38
in highway diesel fuel was reduced to 15 ppm, referred to as ultra-low sulfur diesel (ULSD), to
39
allow for the employment of more sophisticated diesel exhaust aftertreatment technologies that
40
are required to reach the new emission standards. Therefore, the implementation of these diesel
4 ACS Paragon Plus Environment
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Environmental Science & Technology
41
emission standards resulted in a need to fully characterize the impact of the new aftertreatment
42
technologies on diesel exhaust emissions.7-10
43
There is a growing interest in the use of renewable biofuels, such as biodiesel, to replace
44
petroleum-based transportation fuels to reduce dependence on limited fossil fuel resources and to
45
address climate change. Biodiesel consists of fatty acid alkyl esters that are produced from
46
transesterification of vegetable oils or animal fats with an alcohol. Biodiesel can be used directly
47
in diesel engines as blends with petroleum diesel up to 20% by volume biodiesel without engine
48
modification. After corn ethanol, biodiesel is the second most widely used biofuel in the U.S and
49
its consumption has increased by approximately a factor of 10 since 2005.11 The U.S. Energy
50
Independence and Security Act (EISA) was enacted with the ultimate goals of increasing
51
domestic energy independence while reducing negative climate impacts of the transportation
52
sector through greenhouse gas reductions.12 The legislation requires the implementation of
53
renewable fuel standards with an increase in the future usage of renewable fuels as transportation
54
fuels from 9 billion gallons in 2008 to 36 billion gallons per year by 2022. The European
55
Parliament set forth analogous legislation recently requiring 10 % (by energy) of transportation
56
fuel as renewable fuels by 2020.13
57
As a result of the growing interest in biofuel use, there has been a dramatic escalation of
58
scientific research in the past few years to quantify the environmental impacts of diesel engine
59
exhaust using biodiesel.14 However, the majority of the research has focused on characterizing
60
emissions of criteria pollutants, i.e., NOx, CO, total hydrocarbons (THC) and PM, from biodiesel
61
combustion in diesel engines.14-17 In general, the addition of biodiesel to diesel fuel as blends
62
(e.g., B20 for 20 % biodiesel blend) or as a replacement (i.e., B100 for 100 % biodiesel) has
63
consistently been shown to reduce PM, CO and THC emissions significantly and lead to modest
5 ACS Paragon Plus Environment
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64
increase in NOx emissions in diesel engine exhaust as compared to conventional diesel fuel
65
combustion. However, there is considerable variability in the pollutant emission rates due to
66
various experimental conditions, including quality and characteristics of biodiesel feedstock,
67
driving cycle, aftertreatment technologies, model year, vehicle test weight (VTW), engine type
68
and vehicle age.
69
Few studies have assessed the impact of biodiesel application for unregulated emissions,
70
such as chemically speciated VOCs, and in particular MSATs. Although the effects of biodiesel
71
on MSATs have been reviewed,14-16 a number of more recent studies have also reported biodiesel
72
effects on carbonyl emissions18-25 and other MSAT emissions26-32 in diesel exhaust. While the
73
majority of these studies reported decreases in aromatics and increases in carbonyls from the use
74
of biodiesel, there is still no clear consensus on the impact of biodiesel due to conflicting trends
75
observed in literature. As noted above, many factors can impact the diesel exhaust emissions,
76
which may perhaps explain some of the inconsistencies in the literature on the effect of biodiesel
77
on MSATs. Although ambient temperatures below freezing may have significant adverse effects
78
on unregulated vehicle emissions,33 the effect of cold temperature on MSATs in diesel exhaust
79
with biodiesel use is unknown.
80
In this study, the MSAT emissions from soy B20 biodiesel blended with ULSD was
81
compared to emissions from ULSD in three heavy-duty trucks equipped with aftertreatment
82
technologies designed to meet EPA 2010 model year diesel emissions standards. Emission rates
83
were measured using two chassis dynamometers, where one of which has been uniquely set up to
84
test the vehicles under cold ambient temperature conditions. To our knowledge, this is the first
85
study to characterize the unregulated emissions from biodiesel blend under cold temperature
86
conditions. A complete set of emission rates for all conditions for MSATs is reported in the
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87
supporting information to facilitate their utilization in air quality modeling efforts and
88
improvement of emissions inventories. However, the major focus of this work is to examine the
89
influence of various experimental conditions on the VOC emissions of modern heavy-duty diesel
90
vehicles. To that end, a systematic approach was taken to investigate the effects of operating
91
conditions, including ambient temperature, driving cycle, fuel type and VTW.
92 93
EXPERIMENTAL METHODS
94
Dynamometer testing
95
Emissions testing was conducted on three heavy-duty diesel trucks. Most of the testing
96
occurred on a chassis dynamometer (#1) enclosed within a climate-controlled chamber that
97
permitted low ambient temperature testing. Additional testing was performed using a second
98
chassis dynamometer (#2) without low temperature capabilities, in cases where the anticipated
99
simulated VTW exceeded the capacity of dynamometer #1 (i.e. ~5500 kg). Dynamometer #1 was a
100
48-inch roll MIM 4800 dynamometer (Burke E. Porter Machinery Co., Grand Rapids, MI, U.S.),
101
and dynamometer #2 was a 72-inch roll Emission Truck Chassis Dynamometer (Renk AG,
102
Augsburg, Germany). The three test vehicles named Vehicle 1, 2 and 3 (V1, V2, V3) were all
103
model year 2011 trucks in U.S. EPA heavy-duty (HD) truck classes HDV2B, HDV5 and HDV6,
104
respectively. Odometer readings (converted to km) at the beginning of the study were 35,498 km
105
for V1, 4,333 km for V2 and 5,850 km for V3. Further characteristics of the test vehicles are
106
summarized in Table S2 in the supporting information. The stock exhaust aftertreatment systems
107
for all three trucks were designed to meet the EPA 2010 model year PM and NOx emission
108
standards for HD diesel trucks. All three trucks utilized a Diesel Particulate Filter (DPF) to control
109
particulate emissions by trapping exhaust particles during vehicle operation. PM collected on the 7 ACS Paragon Plus Environment
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DPF was periodically removed (i.e., oxidized) typically by injection of fuel into the exhaust
111
stream, resulting in an increase in exhaust and DPF temperatures. This process is referred to as
112
active DPF regeneration. For NOx control, V1 used a NOx Adsorber Catalyst (NAC), which
113
chemically stores NOx emissions. Once saturated, the NAC catalyst undergoes regeneration,
114
releasing NOx under fuel rich conditions and reducing it to N2. Both V2 and V3 trucks utilized
115
Selective Catalytic Reduction (SCR) for controlling NOx emissions, which combine NOx with urea
116
over a catalyst to produce N2 and CO2. All three trucks included a Diesel Oxidation Catalyst
117
(DOC) for the removal of hydrocarbons and CO.
118
The test matrix shown in Table S3 in the supporting information summarizes the conditions for
119
each test that included VOC sampling and observed fuel consumption. For each test condition, one
120
vehicle preconditioning test with no sampling was conducted, followed by three replicates with
121
sampling. Three driving cycles were performed for each test day. The speed vs. time trace for all
122
three cycles is shown in Figure S1 in the supporting information. Two cycles were identical to the
123
lower-speed transient mode of the CARB Medium Heavy Duty Truck three-mode test
124
(MHDTLO).34 This cycle was performed twice for each test to simulate cold start and warm start
125
driving conditions. The Federal Heavy-Duty Urban Dynamometer Driving Schedule (HD-UDDS)
126
(Code of Federal Regulations (CFR), Title 40, Part 86.1216-85) was performed between the cold
127
start and warm start driving cycles. A twenty-minute engine off period occurred between driving
128
cycles. Note that both DPF and NAC regenerations occurred only during HD-UDDS cycles, but
129
not simultaneously. During the testing, it was observed that the NAC of V1 underwent
130
regenerations during every HD-UDDS cycle at 22 °C unless an active DPF regeneration occurred.
131
NAC regenerations were considered as part of normal operation, but active DPF regeneration
132
events were not. When a DPF regeneration occurred on a particular day, the test was repeated the 8 ACS Paragon Plus Environment
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following day to obtain triplicate measurements under normal operation mode (i.e. in absence of
134
DPF regenerations).
135
Two test fuels used in this study were Ultra Low Sulfur Diesel (ULSD) and 20 % soybean oil-
136
based biodiesel (B20) that was splash-blended in the same ULSD used in the pure ULSD tests,
137
which met ASTM D7467 specifications. Both fuels were obtained from Gage Products Co.
138
(Ferndale, MI, U.S.). A fuel change procedure, as detailed in the supporting information, was
139
performed with each change in fuel type in order to minimize residual effects from the previous
140
fuel on the fuel and aftertreatment systems. The measured chemical and physical properties of the
141
test fuels are listed in Table S4 in the supporting information. Testing was conducted for unladen
142
(i.e., vehicle curb weight plus 68 kg; UNL) and laden (i.e., at 90 % of gross vehicle weight rating;
143
LAD) VTW conditions for all vehicles. VOC sampling was undertaken for all conditions except
144
under laden VTW conditions for V2 because the VOC sampling setup was not available for
145
dynamometer #2 during the V2 testing period.
146
Dynamometer #1 was housed inside a custom made temperature-controlled chamber (Luwa-
147
Environmental Specialties, Raleigh, NC, U.S.) that enabled chassis dynamometer emissions testing
148
to be conducted at ambient temperatures in the range of -30 to 45 °C. The chamber was designed to
149
maintain temperatures within ±2 °C of setpoint temperature with up to a maximum applied heat
150
load of 80,000 BTU. In this work, exhaust emissions were sampled under two ambient temperature
151
conditions T = -7 °C and 22 °C for V1 (UNL and LAD) and V2 (UNL only) with dynamometer
152
#1. Chamber temperature was monitored approximately 30 cm from the test vehicle’s air intake,
153
where temperatures within ±2 °C of setpoint for cold and warm start cycles and within ±3 °C for
154
HD-UDDS were measured. Prior to testing, the vehicles were conditioned for 12 hours at the test
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temperature. Because V3 exceeded weight limits of dynamometer #1, testing for V3 was
156
conducted only at ~22 °C on dynamometer #2.
157
Sampling methods
158
A schematic of the setup for the constant volume sampler (CVS) dilution tunnel is included in
159
the supporting information (Figure S2). In this study, both time-integrated and real-time
160
measurements were made for the gaseous regulated pollutants (i.e., CO2, CO, NOx, CH4 and THC).
161
Particulate mass was determined gravimetrically, and particles were further characterized by
162
particle size, number and chemical composition. This paper focuses on measurements of speciated
163
VOC and carbonyls emissions while the other gaseous and particulate measurements in this study
164
will be covered elsewhere. The dilution flow in the CVS dilution tunnel consisted of ambient
165
laboratory air that was passed through a charcoal bed to reduce volatile organics, then a HEPA
166
filter to remove particles. Typical THC background values were in the range of 2.2-3.4 ppmC, of
167
which ~80 % was methane. Dilution flow was pulled through the tunnel with a turbine (Spencer
168
Turbine Company, Model 2025-H-SPEC, Windsor, CT, U.S.). The dilution air temperature was
169
equilibrated with the laboratory room air temperature (~21 °C). The total flow through the dilution
170
tunnel was maintained within ±2 % of the set flow rate of 29.7 standard m3 min-1 that was
171
monitored and controlled by a critical flow venturi (CFV, Horiba Instruments Inc., CVS-48M, Ann
172
Arbor, MI, U.S.).
173
VOCs were sampled in SUMMA canisters and analyzed by gas chromatography/mass
174
spectrometry (GC/MS) in accordance with U.S. EPA TO-15 Method. Carbonyls were sampled
175
with dinitrophenylhydrazine (DNPH)-coated cartridges (Sigma-Aldrich Corp., St. Louis, MO,
176
U.S., LpDNPH H30), and the hydrazones in acetonitrile extracts were analyzed by high 10 ACS Paragon Plus Environment
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performance liquid chromatography (HPLC) according to U.S. EPA Method TO-11A. Detailed
178
descriptions of the TO-15 and TO-11A sampling and analytical procedures along with THC and
179
methane measurements are provided in the supporting information. Before the test driving cycles
180
commenced, one background sample of the dilution air was taken each day. Time-integrated
181
samples of each medium were taken over each driving cycle: cold start, HD-UDDS and warm
182
start. Background samples were analyzed to determine background-corrected VOC concentrations,
183
which were used to estimate emission rates as described in the supporting information. One blank
184
sample was also taken for each test condition to confirm that media were contamination-free.
185
Blank samples were handled identically to other samples but with no flow passing into the
186
sampling media (canister or cartridge). For TO-15 analysis of VOCs, method detection limits
187
(MDLs) ranged from 15 to 186 ppt but were mostly
Article
Cold temperature and biodiesel fuel effects on speciated emissions of volatile organic compounds from diesel trucks Ingrid George, Michael D. Hays, Richard Snow, James Faircloth, Barbara Jane George, Thomas Long, and Richard Baldauf Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es502949a • Publication Date (Web): 13 Nov 2014 Downloaded from http://pubs.acs.org on November 17, 2014
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 30
Environmental Science & Technology
Cold temperature and biodiesel fuel effects on speciated emissions of volatile organic compounds from diesel trucks Ingrid J. George,1 Michael D. Hays,1,* Richard Snow,1 James Faircloth,1 Barbara J. George,2 Thomas Long1 and Richard W. Baldauf1 1
Office of Research and Development, National Risk Management Research Laboratory, United States Environmental Protection Agency, Research Triangle Park, NC 27711. 2
Office of Research and Development, National Health and Environmental Effects Research Laboratory, United States Environmental Protection Agency, Research Triangle Park, NC 27711.
*Corresponding Author: E-mail: [email protected]; phone: +1 919-541-3984; fax: +1 919685-3346) Keywords: dynamometer, diesel exhaust, volatile organic compounds, carbonyls, mobile source air toxics, biodiesel
TOC/Abstract Art
1 ACS Paragon Plus Environment
Environmental Science & Technology
Page 2 of 30
2 ACS Paragon Plus Environment
Page 3 of 30
1
Environmental Science & Technology
ABSTRACT
2
Speciated volatile organic compounds (VOCs) were measured in diesel exhaust from three
3
heavy-duty trucks equipped with modern aftertreatment technologies. Emissions testing was
4
conducted on a chassis dynamometer at two ambient temperatures (-7 °C and 22 °C) operating
5
on two fuels (ultra-low sulfur diesel and 20 % soy biodiesel blend) over three driving cycles:
6
cold start, warm start and heavy-duty urban dynamometer driving cycle. VOCs were measured
7
separately for each drive cycle. Carbonyls such as formaldehyde and acetaldehyde dominated
8
VOC emissions, making up ~72 % of the sum of the speciated VOC emissions (ΣVOCs) overall.
9
Biodiesel use led to minor reductions in aromatics and variable changes in carbonyls. Cold
10
temperature and cold start conditions caused dramatic enhancements in VOC emissions, mostly
11
carbonyls, compared to the warmer temperature and other drive cycles, respectively. Different
12
2007+ aftertreatment technologies involving catalyst regeneration led to significant
13
modifications of VOC emissions that were compound-specific and highly dependent on test
14
conditions. A comparison of this work with emission rates from different diesel engines under
15
various test conditions showed that these newer technologies resulted in lower emission rates of
16
aromatic compounds. However, emissions of other toxic partial combustion products such as
17
carbonyls were not reduced in the modern diesel vehicles tested.
18
3 ACS Paragon Plus Environment
Environmental Science & Technology
19
Page 4 of 30
INTRODUCTION
20
Vehicle exhaust emissions have substantial negative impacts on air quality, human health and
21
climate. Primary pollutants that are directly emitted from mobile sources include NOx, volatile
22
organic compounds (VOCs), CO, CO2 and particulate matter (PM). Atmospheric chemistry of
23
VOCs and NOx from vehicle emissions in the presence of sunlight creates secondary pollutants
24
leading to enhanced ozone and secondary organic aerosol (SOA) formation, which contribute to
25
photochemical smog.1 A subset of VOCs from vehicle exhaust emissions, termed mobile source
26
air toxics (MSATs) that include some aromatics and carbonyls, are of particular concern because
27
they are carcinogenic, mutagenic or are otherwise suspected to cause serious health effects.2 An
28
accurate account of detailed mobile source emission profiles is vital information for local- and
29
regional-scale air quality models to predict the environmental and health impacts of vehicle
30
emissions.
31
Diesel combustion, especially from heavy-duty vehicles, has become subject to progressively
32
more stringent regulatory oversight in recent years due to the fact that diesel exhaust was
33
emitting disproportionately more NOx and PM than gasoline combustion.3 Furthermore, diesel
34
exhaust and specific components within that exhaust have been associated with acute and
35
chronic adverse health effects.4,5 The 2007 Heavy-Duty Highway Rule was enacted by the U.S.
36
EPA to reduce diesel exhaust emissions for NOx and PM over 90 % below previous standard
37
levels, with full compliance required for model year 2010.6 The fuel standard for sulfur content
38
in highway diesel fuel was reduced to 15 ppm, referred to as ultra-low sulfur diesel (ULSD), to
39
allow for the employment of more sophisticated diesel exhaust aftertreatment technologies that
40
are required to reach the new emission standards. Therefore, the implementation of these diesel
4 ACS Paragon Plus Environment
Page 5 of 30
Environmental Science & Technology
41
emission standards resulted in a need to fully characterize the impact of the new aftertreatment
42
technologies on diesel exhaust emissions.7-10
43
There is a growing interest in the use of renewable biofuels, such as biodiesel, to replace
44
petroleum-based transportation fuels to reduce dependence on limited fossil fuel resources and to
45
address climate change. Biodiesel consists of fatty acid alkyl esters that are produced from
46
transesterification of vegetable oils or animal fats with an alcohol. Biodiesel can be used directly
47
in diesel engines as blends with petroleum diesel up to 20% by volume biodiesel without engine
48
modification. After corn ethanol, biodiesel is the second most widely used biofuel in the U.S and
49
its consumption has increased by approximately a factor of 10 since 2005.11 The U.S. Energy
50
Independence and Security Act (EISA) was enacted with the ultimate goals of increasing
51
domestic energy independence while reducing negative climate impacts of the transportation
52
sector through greenhouse gas reductions.12 The legislation requires the implementation of
53
renewable fuel standards with an increase in the future usage of renewable fuels as transportation
54
fuels from 9 billion gallons in 2008 to 36 billion gallons per year by 2022. The European
55
Parliament set forth analogous legislation recently requiring 10 % (by energy) of transportation
56
fuel as renewable fuels by 2020.13
57
As a result of the growing interest in biofuel use, there has been a dramatic escalation of
58
scientific research in the past few years to quantify the environmental impacts of diesel engine
59
exhaust using biodiesel.14 However, the majority of the research has focused on characterizing
60
emissions of criteria pollutants, i.e., NOx, CO, total hydrocarbons (THC) and PM, from biodiesel
61
combustion in diesel engines.14-17 In general, the addition of biodiesel to diesel fuel as blends
62
(e.g., B20 for 20 % biodiesel blend) or as a replacement (i.e., B100 for 100 % biodiesel) has
63
consistently been shown to reduce PM, CO and THC emissions significantly and lead to modest
5 ACS Paragon Plus Environment
Environmental Science & Technology
Page 6 of 30
64
increase in NOx emissions in diesel engine exhaust as compared to conventional diesel fuel
65
combustion. However, there is considerable variability in the pollutant emission rates due to
66
various experimental conditions, including quality and characteristics of biodiesel feedstock,
67
driving cycle, aftertreatment technologies, model year, vehicle test weight (VTW), engine type
68
and vehicle age.
69
Few studies have assessed the impact of biodiesel application for unregulated emissions,
70
such as chemically speciated VOCs, and in particular MSATs. Although the effects of biodiesel
71
on MSATs have been reviewed,14-16 a number of more recent studies have also reported biodiesel
72
effects on carbonyl emissions18-25 and other MSAT emissions26-32 in diesel exhaust. While the
73
majority of these studies reported decreases in aromatics and increases in carbonyls from the use
74
of biodiesel, there is still no clear consensus on the impact of biodiesel due to conflicting trends
75
observed in literature. As noted above, many factors can impact the diesel exhaust emissions,
76
which may perhaps explain some of the inconsistencies in the literature on the effect of biodiesel
77
on MSATs. Although ambient temperatures below freezing may have significant adverse effects
78
on unregulated vehicle emissions,33 the effect of cold temperature on MSATs in diesel exhaust
79
with biodiesel use is unknown.
80
In this study, the MSAT emissions from soy B20 biodiesel blended with ULSD was
81
compared to emissions from ULSD in three heavy-duty trucks equipped with aftertreatment
82
technologies designed to meet EPA 2010 model year diesel emissions standards. Emission rates
83
were measured using two chassis dynamometers, where one of which has been uniquely set up to
84
test the vehicles under cold ambient temperature conditions. To our knowledge, this is the first
85
study to characterize the unregulated emissions from biodiesel blend under cold temperature
86
conditions. A complete set of emission rates for all conditions for MSATs is reported in the
6 ACS Paragon Plus Environment
Page 7 of 30
Environmental Science & Technology
87
supporting information to facilitate their utilization in air quality modeling efforts and
88
improvement of emissions inventories. However, the major focus of this work is to examine the
89
influence of various experimental conditions on the VOC emissions of modern heavy-duty diesel
90
vehicles. To that end, a systematic approach was taken to investigate the effects of operating
91
conditions, including ambient temperature, driving cycle, fuel type and VTW.
92 93
EXPERIMENTAL METHODS
94
Dynamometer testing
95
Emissions testing was conducted on three heavy-duty diesel trucks. Most of the testing
96
occurred on a chassis dynamometer (#1) enclosed within a climate-controlled chamber that
97
permitted low ambient temperature testing. Additional testing was performed using a second
98
chassis dynamometer (#2) without low temperature capabilities, in cases where the anticipated
99
simulated VTW exceeded the capacity of dynamometer #1 (i.e. ~5500 kg). Dynamometer #1 was a
100
48-inch roll MIM 4800 dynamometer (Burke E. Porter Machinery Co., Grand Rapids, MI, U.S.),
101
and dynamometer #2 was a 72-inch roll Emission Truck Chassis Dynamometer (Renk AG,
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Augsburg, Germany). The three test vehicles named Vehicle 1, 2 and 3 (V1, V2, V3) were all
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model year 2011 trucks in U.S. EPA heavy-duty (HD) truck classes HDV2B, HDV5 and HDV6,
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respectively. Odometer readings (converted to km) at the beginning of the study were 35,498 km
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for V1, 4,333 km for V2 and 5,850 km for V3. Further characteristics of the test vehicles are
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summarized in Table S2 in the supporting information. The stock exhaust aftertreatment systems
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for all three trucks were designed to meet the EPA 2010 model year PM and NOx emission
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standards for HD diesel trucks. All three trucks utilized a Diesel Particulate Filter (DPF) to control
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DPF was periodically removed (i.e., oxidized) typically by injection of fuel into the exhaust
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stream, resulting in an increase in exhaust and DPF temperatures. This process is referred to as
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active DPF regeneration. For NOx control, V1 used a NOx Adsorber Catalyst (NAC), which
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chemically stores NOx emissions. Once saturated, the NAC catalyst undergoes regeneration,
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releasing NOx under fuel rich conditions and reducing it to N2. Both V2 and V3 trucks utilized
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Selective Catalytic Reduction (SCR) for controlling NOx emissions, which combine NOx with urea
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over a catalyst to produce N2 and CO2. All three trucks included a Diesel Oxidation Catalyst
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(DOC) for the removal of hydrocarbons and CO.
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The test matrix shown in Table S3 in the supporting information summarizes the conditions for
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each test that included VOC sampling and observed fuel consumption. For each test condition, one
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vehicle preconditioning test with no sampling was conducted, followed by three replicates with
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sampling. Three driving cycles were performed for each test day. The speed vs. time trace for all
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three cycles is shown in Figure S1 in the supporting information. Two cycles were identical to the
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lower-speed transient mode of the CARB Medium Heavy Duty Truck three-mode test
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(MHDTLO).34 This cycle was performed twice for each test to simulate cold start and warm start
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driving conditions. The Federal Heavy-Duty Urban Dynamometer Driving Schedule (HD-UDDS)
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(Code of Federal Regulations (CFR), Title 40, Part 86.1216-85) was performed between the cold
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start and warm start driving cycles. A twenty-minute engine off period occurred between driving
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cycles. Note that both DPF and NAC regenerations occurred only during HD-UDDS cycles, but
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not simultaneously. During the testing, it was observed that the NAC of V1 underwent
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regenerations during every HD-UDDS cycle at 22 °C unless an active DPF regeneration occurred.
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NAC regenerations were considered as part of normal operation, but active DPF regeneration
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events were not. When a DPF regeneration occurred on a particular day, the test was repeated the 8 ACS Paragon Plus Environment
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following day to obtain triplicate measurements under normal operation mode (i.e. in absence of
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DPF regenerations).
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Two test fuels used in this study were Ultra Low Sulfur Diesel (ULSD) and 20 % soybean oil-
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based biodiesel (B20) that was splash-blended in the same ULSD used in the pure ULSD tests,
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which met ASTM D7467 specifications. Both fuels were obtained from Gage Products Co.
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(Ferndale, MI, U.S.). A fuel change procedure, as detailed in the supporting information, was
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performed with each change in fuel type in order to minimize residual effects from the previous
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fuel on the fuel and aftertreatment systems. The measured chemical and physical properties of the
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test fuels are listed in Table S4 in the supporting information. Testing was conducted for unladen
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(i.e., vehicle curb weight plus 68 kg; UNL) and laden (i.e., at 90 % of gross vehicle weight rating;
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LAD) VTW conditions for all vehicles. VOC sampling was undertaken for all conditions except
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under laden VTW conditions for V2 because the VOC sampling setup was not available for
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dynamometer #2 during the V2 testing period.
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Dynamometer #1 was housed inside a custom made temperature-controlled chamber (Luwa-
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Environmental Specialties, Raleigh, NC, U.S.) that enabled chassis dynamometer emissions testing
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to be conducted at ambient temperatures in the range of -30 to 45 °C. The chamber was designed to
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maintain temperatures within ±2 °C of setpoint temperature with up to a maximum applied heat
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load of 80,000 BTU. In this work, exhaust emissions were sampled under two ambient temperature
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conditions T = -7 °C and 22 °C for V1 (UNL and LAD) and V2 (UNL only) with dynamometer
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#1. Chamber temperature was monitored approximately 30 cm from the test vehicle’s air intake,
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where temperatures within ±2 °C of setpoint for cold and warm start cycles and within ±3 °C for
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HD-UDDS were measured. Prior to testing, the vehicles were conditioned for 12 hours at the test
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temperature. Because V3 exceeded weight limits of dynamometer #1, testing for V3 was
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conducted only at ~22 °C on dynamometer #2.
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Sampling methods
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A schematic of the setup for the constant volume sampler (CVS) dilution tunnel is included in
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the supporting information (Figure S2). In this study, both time-integrated and real-time
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measurements were made for the gaseous regulated pollutants (i.e., CO2, CO, NOx, CH4 and THC).
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Particulate mass was determined gravimetrically, and particles were further characterized by
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particle size, number and chemical composition. This paper focuses on measurements of speciated
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VOC and carbonyls emissions while the other gaseous and particulate measurements in this study
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will be covered elsewhere. The dilution flow in the CVS dilution tunnel consisted of ambient
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laboratory air that was passed through a charcoal bed to reduce volatile organics, then a HEPA
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filter to remove particles. Typical THC background values were in the range of 2.2-3.4 ppmC, of
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which ~80 % was methane. Dilution flow was pulled through the tunnel with a turbine (Spencer
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Turbine Company, Model 2025-H-SPEC, Windsor, CT, U.S.). The dilution air temperature was
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equilibrated with the laboratory room air temperature (~21 °C). The total flow through the dilution
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tunnel was maintained within ±2 % of the set flow rate of 29.7 standard m3 min-1 that was
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monitored and controlled by a critical flow venturi (CFV, Horiba Instruments Inc., CVS-48M, Ann
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Arbor, MI, U.S.).
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VOCs were sampled in SUMMA canisters and analyzed by gas chromatography/mass
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spectrometry (GC/MS) in accordance with U.S. EPA TO-15 Method. Carbonyls were sampled
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with dinitrophenylhydrazine (DNPH)-coated cartridges (Sigma-Aldrich Corp., St. Louis, MO,
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U.S., LpDNPH H30), and the hydrazones in acetonitrile extracts were analyzed by high 10 ACS Paragon Plus Environment
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performance liquid chromatography (HPLC) according to U.S. EPA Method TO-11A. Detailed
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descriptions of the TO-15 and TO-11A sampling and analytical procedures along with THC and
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methane measurements are provided in the supporting information. Before the test driving cycles
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commenced, one background sample of the dilution air was taken each day. Time-integrated
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samples of each medium were taken over each driving cycle: cold start, HD-UDDS and warm
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start. Background samples were analyzed to determine background-corrected VOC concentrations,
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which were used to estimate emission rates as described in the supporting information. One blank
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sample was also taken for each test condition to confirm that media were contamination-free.
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Blank samples were handled identically to other samples but with no flow passing into the
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sampling media (canister or cartridge). For TO-15 analysis of VOCs, method detection limits
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(MDLs) ranged from 15 to 186 ppt but were mostly
