Cold Temperature and Biodiesel Fuel Effects on ... - ACS Publications

Nov 13, 2014 - Protection Agency, Research Triangle Park, North Carolina 27711, United States. •S Supporting Information. ABSTRACT: Speciated volati...
0 downloads 0 Views 1MB Size
Subscriber access provided by Library, Special Collections and Museums, University of Aberdeen

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,

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

Environmental Science & Technology

Page 8 of 30

110

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

Page 9 of 30

Environmental Science & Technology

133

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

9 ACS Paragon Plus Environment

Environmental Science & Technology

Page 10 of 30

155

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

Page 11 of 30

Environmental Science & Technology

177

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