a comparison of air emissions from natural g as p athways for road transportation

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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al., 2011; NRC, 2013) or comprehensive but outdated (Wang et al., 2000; NRC, 2010a).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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Vehicle Specifications

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Results - Passenger Vehicles

-100 -50 0 50 100 150 2000

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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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-100 -50 0 50 100 150 2000

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0.2

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0.5

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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

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0.2

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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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-400 -200 0 200 400 600 800 10000

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0.2

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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

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akag

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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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Thank you!

[email protected]

2013-14 Northrop Grumman Fellowship

Supported by

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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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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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a comparison of air emissions from natural g as p athways for road transportation

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