Mechanisms of Increased Particle and VOC Emissions during DPF

Article Science & Mechanisms Environmental of Increased Technology is published by the American Chemical Particle and VOC Society. 1155 Sixteenth Street N.W., Washington, Emissions DPF of DC 20036 by University Subscriber access during provided Published by American Colorado Boulder Chemical Society. Copyright © American Chemical Society. However,

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Hiroyuki Yamada, Satoshi Inomata, and Hiroshi Tanimoto Environmental Science &

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DPF Regeneration

Epractical& (x)Technology = r(x) × Enorm(x) Page Environmental 1>23 ofnm40 Science Particles Soot due to lowered filtering efficiency.

r(x) :the Regeneration Correction Factor of species x

VOCs Smaller species

Emission(x)

Regeneration Interval Particles < 23 nm H2SO4/SO3 trapped by Cat.

Epractical(x): Average during regeneration interval

Regeneration Enorm(x) Car Millage

ACS Larger Paragon Plus Environment Species r(PM) = 1.2 to 1.5 Time

r(PN>23nm) = 3.9 to 57.4 r(PN > 2.5nm) = 9.7 to 137.4

Environmental Science & Technology

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Mechanisms of Increased Particle and VOC Emissions during DPF Active Regeneration and Practical Emissions Considering Regeneration Hiroyuki Yamada,a,* Satoshi Inomata, b Hiroshi Tanimotob a

Environmental Research Division, National Traffic Safety and Environment Laboratory, 7-42-

27, Jindaiji-higashimachi, Chofu, Tokyo 182-0012, Japan b

Center for Global Environmental Research, National Institute for Environmental Studies, 16-2

Onogawa, Tsukuba, Ibaraki 305-8506, Japan *

Corresponding author. E-mail: [email protected]; Tel: +81-422-41-3220; Fax: +81-422-76-

8604.

ACS Paragon Plus Environment

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ABSTRACT

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Mechanisms involved in increased particle and volatile organic compound (VOC)

4

emissions during active and parked active regenerations of a diesel particulate filter (DPF) were

5

investigated using heavy-duty trucks equipped with both a urea selective catalytic reduction

6

system and a DPF (SCR + DPF) and a DPF-only. Particle emissions increased in the later part of

7

the regeneration period but the mechanisms were different above and below 23 nm. Particles

8

above 23 nm were emitted due to the lower filtering efficiency of the DPF because of the

9

decreasing amount of soot trapped during regeneration. Small particles below 23 nm were

10

thought to be mainly sulfuric acid particles produced from SO2 trapped by the catalyst, being

11

released and oxidized during regeneration. Contrary to the particle emissions, VOCs increased in

12

the earlier part of the regeneration period. The mean molecular weights of the VOCs increased

13

gradually as the regeneration proceeded. To evaluate “practical emissions” in which increased

14

emissions during the regeneration were considered, a Regeneration Correction Factor (RCF),

15

which is the average emission during one cycle of regeneration/emission in normal operation,

16

was adopted. The RCFs of PM and VOCs were 1.1 to 1.5, and those of PNs were as high as 3 to

17

140, although they were estimated from a limited number of observations.

18 19

KEYWORDS Active Regeneration, Diesel Particulate Filter, Volatile Organic Compounds,

20

Particle Numbers, Regeneration Correction Factor.


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1. Introduction

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Emissions from diesel engines have been recognized as one of the sources causing air

23

pollution [1], and great efforts have been made to reduce such emissions. In fact, the regulation

24

limit of particulate matter (PM) for heavy-duty engines in Japan was lowered to 0.01 g/kWh in

25

2010 from the previous limit of 0.7 g/kWh in 1994 [2]. This trend is not particular to Japan, and

26

the situations in the US and Europe are similar. A key technology enabling lower particle

27

emissions from diesel engines was the development of the diesel particulate filter (DPF) [3-5].

28

The DPF is an aftertreatment device whose filtering efficiency is over 90 % [6, 7]. Indeed, PM

29

emitted from DPFs is sometimes too low to be detected with the traditional filter weighing

30

method, and the particle number (PN) counting method is often used because it is considered to

31

have higher sensitivity [8, 9].

32

The particles trapped by a DPF must be burnt up during the so-called regeneration period.

33

Our previous study [10] revealed that PN emissions during regeneration that occurred once every

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20 tests are thousands of times higher than those in normal operation (in a test without active

35

regeneration). And PN in just one test accompanying the regeneration accounted for 81 % of the

36

total emission in 20 tests, suggesting the importance of evaluating emissions during regeneration.

37

It is known that there are three types of regeneration process, called passive regeneration,

38

active regeneration, and parked active regeneration. The passive regeneration process is an

39

uncontrolled process in which the trapped particles start to be oxidized automatically as a result

40

of the increased exhaust gas temperature during a high-load operation. Retrofitted DPFs perform

41

only this type of regeneration. Active regeneration is regeneration that is controlled by the engine

42

computer while driving. In this process, the exhaust gas temperature is increased by injecting

43

fuel into the engine exhaust, inducing oxidation. Parked active regeneration is a driver-controlled


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process carried out when requested by the engine computer. This occurs in cases where the DPF

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is almost filled with particles by continued low-load operation in which the exhaust temperature

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is too low to start the active regeneration. In such a situation, the engine computer lights an

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indicator showing that manual regeneration should be performed. The driver, upon noticing the

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indicator, puts the vehicle in the idling state and starts parked active regeneration by pushing the

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regeneration start button.

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Some studies have examined the performance of DPFs for treating exhaust emissions,

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including the regeneration period [11-15]. Dwyer et al. [15] observed increased emissions of

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particles below 20 nm during regeneration in a light-duty vehicle called the “Golden Vehicle”,

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which was used for a test sequence for introducing the PN counting method into certification for

54

light-duty diesel vehicles in Europe [8]. Similar emissions of small particles (below 20 nm),

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which are peculiar to regeneration, from the passive regeneration of retrofitted catalytic DPFs

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were observed in several studies [16-18]. These findings suggest that the particles were semi-

57

volatile sulfate particles because these emissions were affected by the dilution factor. However,

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these studies concluded that further information is needed to fully understand the nature of the

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small particles. Small particles were also observed during active and parked active regenerations

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[19, 20], but the nature of the particles was not reported. A recent study reported that these small

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particles could be sulfuric acid [21], and more detailed investigation of such small particles has

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been an emerging issue. As for emissions other than particles, volatile organic compounds

63

(VOCs) have also been paid attention because of their direct adverse effects on health and

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conversion to ozone [22]. Some studies reported increased VOC emissions during regeneration

65

[10, 19], but no discussions were made about their composition and how they are emitted.


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In the process of certification, increased emissions during the regeneration mentioned above

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have been considered. “Practical emissions”, which are average emissions that take account of

68

the appearance of DPF regeneration, have been used for the certification [23, 24]. Important

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factors in estimating the practical emissions are the regeneration interval, which is how far the

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vehicle can run between two active regenerations, and the increased amount of emissions in the

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test with the regeneration relative to normal operation. These data are supplied from the

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manufacturer in the process of certification, but are not available to the public. Furthermore,

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there has been no study that considers these factors in discussing the actual impact of emissions

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from vehicles equipped with DPFs on the atmosphere.

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This study focuses on the mechanism of increased PM, PNs above and below 23 nm, VOCs

76

and regulated gas emissions during active and parked active regenerations in heavy-duty vehicles

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that comply with the 2010 Japanese regulations (PM: 0.01 g/kWh, NOx: 0.7 g/kWh) with real-

78

time PN and VOC analysis. Then, we discuss the increased emissions by considering the

79

appearance of regenerations and by introducing a Regeneration Correction Factor (RCF) which

80

is equivalent to the adjustment coefficients Ki and Kr used in certification [23, 24], to estimate

81

practical emissions from the vehicles with DPF by multiplying this factor by the exhaust

82

emissions in normal operation.

83 84

2. Experimental methods

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2.1. Outline of chassis dynamometer tests

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In this study, using a chassis dynamometer, we observed exhaust emissions from three

87

heavy-duty trucks, two of which were equipped with a urea selective catalytic reduction (SCR)

88

system and DPF (SCR+DPF). We repeated the mode tests a total of 50 times (43 normal


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operation and 7 active regenerations) and performed a single parked active regeneration test. The

90

diesel fuel used in this study was obtained from the Japanese market and its sulfur content was

91

below 10 ppmV.

92

PN based on the Particle Measurement Programme (PMP) methodology [23, 24] and

93

alternative counters, PM, regulated gases, SO2 and VOCs were measured during the regeneration

94

period. Details are shown in Fig. 1. The tested trucks were set on the chassis dynamometer, and

95

all of the exhaust gas was introduced into a constant-volume sampler (CVS), where it was

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diluted with filtered air. Non-methane hydrocarbon (NMHC), CO, NOx, and CO2 were measured

97

using a MEXA-7500DEGR (Horiba Inc., Kyoto, Japan). PM was trapped by a glass fiber filter

98

(TX40HI20WW47mm, Pall, USA) and weighed. These measurement systems meet the

99

requirements of certification in Japan. For the measurement of SO2, we used MEXA-4000FT

100

(Horiba Inc., Kyoto, Japan), which is based on Fourier Transform Infrared Spectroscopy (FT-

101

IR). For the measurement of SO2, direct exhaust gas was measured instead of diluted exhaust

102

using the CVS. Ion chromatography (ICS-1000, Dionex, USA) was used to perform composition

103

analysis of the PM and the analytical method was based on a Japanese guideline for PM2.5

104

composition analysis proposed by the Japanese Ministry of the Environment [25]. A detailed

105

explanation can be found in Section S1 of the supporting information. Observed ions were Cl-,

106

NO2-, NO3-, SO42-, Na+, NH4+, K+, Mg2+ and Ca2+. In some cases, real-time emissions of alkanes,

107

alkenes, aromatics and di-enes were measured using a proton transfer reaction plus switchable

108

reagent ion mass spectrometry (PTR+SRI-MS). Detailed information can be found in Section S2

109

of the supporting information and elsewhere [26, 27].

110 111

2.2. PN Measurement


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The PNs in different size ranges were measured simultaneously with a PN measurement

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system (AVL Particle Counter; APC, AVL Inc., Austria) that meets the requirements of the PMP

114

[23, 24] and two additional condensation particle counters (CPCs). Details are described in our

115

previous study [28]. Particles were sampled downstream of the CVS. In the APC, particles with

116

relatively high volatility were removed by a Volatile Particle Remover (VPR), and mainly solid

117

particles were counted by a Particle Number Counter (PNC) whose 50% detection efficiency

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particle diameter (D50) is 23 nm. In this study, we named the PN observed with this PNC PN_23,

119

which indicates the number of low-volatility particles above 23 nm. In addition to the normal

120

PNC, to measure the particles below 23 nm, we used two CPCs (CPC 3776, TSI Inc.; CPC3772,

121

TSI Inc.) whose D50 values were 2.5 nm and 10 nm, respectively downstream of the VPR. We

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named the PN obtained by these counters PN_2.5 and PN_10, indicating the numbers of particles

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above 2.5 nm and 10 nm, respectively, with volatility low enough to survive under the conditions

124

in the VPR. Thus, we could measure particles in three different size ranges simultaneously, and

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the difference between PN_23 and PN_2.5 indicates PNs in the size range 2.5–23 nm. Similarly,

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the difference between PN_23 and PN_10 corresponds to particles in the size range 10–23 nm.

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The PMP methodology was established to measure particles above 23 nm; hence, some

128

verifications are needed before it can be applied to particles below 23 nm. Particle loss in the

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VPR was corrected based on the PMP methodology using the average loss at particle sizes of 30,

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50 and 100 nm. Our previous study revealed that the loss at 15 nm was almost 30% [28], which

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is higher than the average value used in the PMP methodology. Thus, the PNs below 23 nm

132

discussed in this study might be underestimated by up to 50 %. The removal efficiency of

133

volatile particles below 23 nm at the VPR will be higher than the PMP requirement (over 99 %)

134

because smaller particles can be evaporated easier. Formation of particles below 23 nm at the


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VPR should be evaluated in detail. However, the observed small particles below 23 nm in this

136

study exhibited similar features to those observed in other studies using particle counters without

137

the VPR, suggesting that the formation of particles below 23 nm was negligible.

138

The approach adopted in this study is similar to former studies [16, 18] in which it was

139

reported that small particles around 10 nm were semi-volatile particles, because these particles

140

decreased when the dilution factor at the VPR was increased from 110 to 1500. To minimize the

141

effect of these semi-volatile particles, we used a dilution factor of 3000.

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2.3. Tested vehicles, mode and occurrence of regeneration

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We named the trucks tested in this study “DPFt”, “SCRta” and “SCRtb”. As listed in Table

145

1, all of the trucks were equipped with a catalyzed DPF and meet the 2010 Japanese regulations.

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DPFt was only equipped with a diesel oxidation catalyst (DOC) followed by a catalyzed DPF.

147

SCRta and SCRtb had a SCR + DPF to reduce both PM and NOx. The order of devices is: DOC,

148

catalyzed DPF, SCR system, and DOC for an ammonia slip. The test mode used in this study

149

was the JE05 mode (duration: 1830 s, distance: 13.9 km), which is used in the Japanese

150

certification for heavy-duty engines. In the certification, a preconditioning cycle is performed

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before the emissions tests. In this study, however, we repeated the emission tests without the

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preconditioning cycle. When the former test ended, we shut down the engine for 10 minutes and

153

then started a new test. The test matrix in this study is listed in table S1 in the Supporting

154

Information. During the test sequence, we observed three active regenerations with DPFt, two

155

active regenerations with SCRta, and two active regenerations with SCRtb. We named these

156

active regenerations after the name of the truck followed by a hyphen and an individual number


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(DPFt-1, SCRta-2 etc.). We also performed manually started parked active regeneration with

158

SCRtb and named this “SCRtb-M”.

159

Table 2 summarizes detailed information about the seven observed active regenerations and

160

the single parked active regeneration. In the observed active regenerations, only SCRtb-1

161

appeared in the later part of the test mode and continued to the end of the test mode. Thus, we

162

shut down the engine although regeneration still continued. Therefore, SCRtb-1 appeared only in

163

the earlier part of regeneration, and the DPF was not cleaned completely. In the case of parked

164

active regeneration, the truck was left idling, and the exhaust emissions were measured for 1830

165

s, which is equal to the duration of the JE05 mode.

166

The regeneration interval varied depending on the test vehicles. The regeneration interval of

167

DPFt was 100 km, estimated from the odometer mileage between DPFt-1 and DPFt-2. We did

168

not adopt the mileage from DPFt-2 to DPFt-3, because some regenerations took placed between

169

DPFt-2 and DPFt-3. On the other hand, the regeneration interval of SCRta was almost 1100 km,

170

which is 10-times longer than that of DPFt. We could not estimate the regeneration interval of

171

SCRtb because the DPF was not fully refreshed in SCRtb-1 for the reason described above. Thus,

172

we assumed that the regeneration interval of SCRtb was similar to that of SCRta (1100 km)

173

because they had similar aftertreatment systems and were made by the same manufacturer.

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3. Results and Discussion

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3.1. Practical emissions and comparisons with normal operation

177

The results for PN_23, PN_2.5 and PN_10 regulated emissions in all active regenerations

178

and normal operation (no regeneration) are summarized in Table 3. The tests numbers for normal

179

operation are also listed in Table 3. Most of the emissions in the regenerations were higher than


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those in normal operation. The emissions of NOx and CO2 observed in some tests showed

181

correlation to the duration of the regeneration periods. For example, DPFt-1 continued longer

182

than DPFt-2 and DPFt-3 (see Table 2), and NOx and CO2 emissions in DPFt-1 were higher than

183

the others. This is because commonly the injection of fuel to increase the exhaust gas

184

temperature to cause regeneration resulted in increased NOx and CO2 emissions. The same

185

feature was also observed in the results of SCRta and SCRtb, except for the NOx level of SRCtb.

186

The duration of SCRtb-1 was shorter than the other regenerations of SCRta and SCRtb because it

187

appeared in the later part of test mode and had terminated by the end of test mode, resulting in

188

slight increases compared with the normal operation.

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PN_23, PN_10 and PN_2.5 emissions in the regeneration periods of DPFt, SCRta SCRtb

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except SCRtb-1 were almost 500-times higher than those in normal operation. The PNs in

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SCRtb-1 were similar to those in normal operation. This is due to not only the short duration

192

discussed above but also the fact that only the earlier part of the regeneration appeared in SCRtb-

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1. As discussed in detail in Sections 3.2 and 3.3, PN emissions increased only in the later part of

194

the regeneration period.

195 196

Effects of increased emissions during the regeneration on practical emissions will be discussed in Section 4 by considering the frequency of appearance of the active regenerations.

197 198

3.2. Emission profiles of DPF-only truck (DPFt).

199

PN_23 and PN_2.5 profiles of all three active regenerations with the DPF-only truck (DPFt)

200

are shown in Fig. 2. The SO2 profile in DPFt-3 is also shown in this figure. All regenerations

201

exhibited the common features described below. PN_23 gradually increased in the regeneration

202

period, and it remained at a relatively high level even after the regeneration. This is in agreement


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with the feature observed in our previous study [10], and it can be explained by the amount of

204

soot in the DPF. It is known that the filtering efficiency is affected by the amount of trapped soot.

205

The DPF shows a higher efficiency when it is filled with a lot of soot, and the efficiency falls

206

with less soot. During the regeneration period, the loaded soot was oxidized and removed. Then

207

PN emission increased due to the falling DPF filtering efficiency. After the regeneration period,

208

the amount of loaded soot was still low, and high emissions continued for a while.

209

PN_2.5 (number of particles above 2.5 nm) increased rapidly also in the later part but

210

earlier than the increases of PN_23 (number of particles above 23 nm). As described above, the

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difference between PN_23 and PN2.5 indicates PNs in range 2.5–23 nm (indicated by “2.5

Mechanisms of Increased Particle and VOC Emissions during DPF

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