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A Comparison of Air Emissions from Natural Gas Pathways for Road Transportation
Fan Tong, Paulina Jaramillo, Ines Azevedo Department of Engineering and Public PolicyCarnegie Mellon University
2013-14 Northrop Grumman Fellowship
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• Potential benefits: cost savings, energy security, and cleaner combustion.
• Barriers: lack of fueling infrastructure, high upfront cost.
Natural Gas Use in the Transportation Sector
Both figures are drawn with data from EIA’s website
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• What are the life-cycle greenhouse gas emissions of natural gas pathways?
• Which pathway or which vehicle application provides the largest greenhouse gas emission reduction compared to conventional liquid pathways?
• How does methane leakage affect the life-cycle greenhouse gas emissions of natural gas pathways?
• What are the key parameters/stages to reduce life-cycle greenhouse gas emissions of natural gas pathways?
Research Questions
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• Limitations of existing studies• Hard to compare the results because studies tend to use different
assumptions and system boundaries. (Wang et al., 2002; Jaramillo et al., 2008; Samaras et al., 2008; Sioshansi et al., 2009; Michalek et al., 2011).
• There are a few natural gas-centered studies on light-duty vehicles (LDVs), but they are either limited in pathways considered (Venkatesh et
al., 2011; NRC, 2013) or comprehensive but outdated (Wang et al., 2000; NRC, 2010a).
• There is relatively few existing studies on air emissions from alternative fuels for heavy-duty vehicles except for transit buses. (Beer et al., 2002; Ally, et al., 2007; Clark et al., 2007; Graham et al., 2008; Hesterberg, et al., 2013; Weigel, 2009; Krupnick, 2010; NRC, 2010b & 2014; EPA, 2011; Meyer et al., 2011; Meier, et al., 2013; MJB&A, 2014)
• Few studies treated uncertainty and variability explicitly (Venkatesh et al., 2011).
• Estimates of natural gas upstream GHG emissions have been controversial. However, new on-site measurements of natural gas upstream emissions (Allen et al., 2013; EPA GHGRP 2013) are available.
Research Gap
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Greenhouse gases: CO2, CH4, N2OGlobal warming potential: IPCC (2013)
Functional unit: km
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8
9
10
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Vehicle Specifications
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Results - Passenger Vehicles
-100 -50 0 50 100 150 2000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
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Life cycle GHG emission changes compared to gasoline (Unit: g CO2-eq/km)
Cum
ulat
ive
dist
ribu
tion
Conventional gasoline
Emission reduction Emission increase
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Results - Passenger Vehicles
-100 -50 0 50 100 150 2000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Life cycle GHG emission changes compared to gasoline (Unit: g CO2-eq/km)
Cum
ulat
ive
dist
ribu
tion
BEV130
PHEV30
GH2c
GH2d
FTG w/o CCS
HEV
PHEV60
CNG
LH2c
E85
M85
FTG w/ CCS
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Results – Line-haul Tractor Trailers
-400 -200 0 200 400 600 800 10000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Life cycle GHG emission changes compared to diesel (Unit: g CO2-eq/km)
Cum
ulat
ive
dist
ribu
tion
Emissionreduction
Emission increase
Conventional diesel
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Results – Line-haul Tractor Trailers
-400 -200 0 200 400 600 800 10000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Life cycle GHG emission changes compared to diesel (Unit: g CO2-eq/km)
Cum
ulat
ive
dist
ribu
tion
Diesel-HEV
CNG
LNG-SI
FTD w/ CCS
FTD w/o CCS
LNG-CI
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What roles do leakage rate and fuel economy play for CNG and LNG pathways?
70% 80% 90% 100% 110% 120%0%
1%
2%
3%
4%
5%
6%
7%
8%
9%
10%
Relative fuel economy of natural gas vehicles
Bre
akev
en le
akag
e ra
te
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Break-even leakage rate is a linear function of relative fuel economy of NGVs
70% 80% 90% 100% 110% 120%0%
1%
2%
3%
4%
5%
6%
7%
8%
9%
10%
Relative fuel economy of natural gas vehicles
Bre
akev
en le
akag
e ra
te
Gasoline
Diesel
100-year GWP
20-year GWP
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• Not all natural gas pathways achieve GHG emission reductions compared to existing petroleum pathways.
• Indirect use of natural gas to produce electricity utilized in BEVs achieves significant reductions in all applicable vehicle segments.
• E85, M85, and Fischer-Tropsch liquids are very unlikely to achieve emission reductions while hydrogen fuel cell electric vehicles, CNG and LNG pathways are possible (to a varying extent).
• Emission reduction potentials of CNG and LNG depend on two key parameters, life-cycle methane leakage rate and relative fuel economy of natural gas vehicles.
• Assuming a 90% relative fuel economy, the break-even leakage rate is around 1.2% or around 3.0% for 20-year and 100-year GWP.
• An efficiency-increasing technology, such as hybridization or electrification, allows higher leakage rate to achieve emission reductions.
Conclusions
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Results - Passenger Vehicles
0
100
200
300
400
500
600
700
69 61 50
183
117 96 105159
255
336
259
119169
204
265 262
189
265
265
222
265
241 0
0
0
110 66 0
69 61 50
264
199154
110
216
288
387
297
133
192235
266 262
189
266
266
229266
242
0
0
0
110
66 0
Gasoline Diesel
Gasoline-HEVFTG w/o CCS
FTG w/ CCS CNG E85 M85GH2 central
LH2 central
GHG2 distributedPHEV30
PHEV60BEV130
Lif
e cy
cle
GH
G e
mis
sion
s (U
nit:
g C
O 2-eq/
km)
UpstreamTailpipe
20-year GWP
100-year GWP
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Results – Line-haul Tractor Trailers
0
500
1000
1500
2000
2500
3000
3500
4000
365 344
1029
660 584 775 694
15691426
1569
1569
1335
13971299
365 344
1485
1119 932
1122 998
15691426
1569
1569
1379
1536
1420
DieselDiesel-HEV
FTG w/o CCSFTG w/ CCS CNG
LNG-SILNG-CI
Lif
e cy
cle
GH
G e
mis
sion
s (U
nit:
g C
O 2-eq/
km)
UpstreamTailpipe
100-year GWP
20-year GWP
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Results – Transit Buses
0
1000
2000
3000
4000
5000
6000
7000
593 498
1673
1072 9491263
2267 2475
2548
2124
2548
2548
2168
2269 0
593 498
2414
18191515
1830
2604 2475
2548
2124
2548
2548
2241
2495
0
DieselDiesel-HEV
FTG w/o CCSFTG w/ CCS CNG
LNG-SIBEV-NGCC
BEV-Grid
Lif
e cy
cle
GH
G e
mis
sion
s (U
nit:
g C
O 2-eq/
km)
UpstreamTailpipe
20-year GWP
100-year GWP
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Natural gas upstream GHG emissions
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Natural gas upstream GHG emissions
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Natural gas upstream GHG emissions
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Natural gas upstream GHG emissions