NGVs and the Environment

Natural gas is the cleanest burning alternative transportation fuel available today that can economically power light-, medium-, and heavy-duty vehicle applications as well as many non-road applications, such as rail and marine vehicles. Whether in the form of compressed natural gas (CNG) or liquefied natural gas (LNG), natural gas is a proven alternative fuel that significantly improves local air quality and reduces greenhouse gases (GHG).

Overview

  • Natural gas vehicles generally emit 13–21 percent fewer GHG emissions than comparable gasoline and diesel vehicles on a well-to-wheels basis.
  • Medium and heavy duty natural gas engines were the first engines to satisfy U.S. Environmental Protection Agency's (EPA) demanding 2010 emission standards for nitrogen oxides (NOx).
  • The light-duty Honda Civic Natural Gas held the American Council for An Energy-Efficient Economy’s (ACEEE) title of “Greenest Vehicle” for eight consecutive years. 
  • Natural gas primarily consists of methane (around 90 percent), with small amounts of ethane, propane, and other gases. Methane is lighter than air and burns almost completely, creating carbon dioxide and water as byproducts.
  • Natural gas rises and disperses quickly, so in the event of a leak or emergency venting, surrounding ecosystems and water systems are not threatened.

Emission Benefits
Today, natural gas vehicles (NGV), as well as diesel and gasoline vehicles, are delivering superior emissions compared to what was achievable just a few years ago. An NGV that has continued to deliver improved emissions throughout the years is the Honda Civic Natural Gas, the longest-consecutively produced OEM NGV. The CNG-powered Civic was once characterized by EPA as the cleanest commercially available internal-combustion vehicle on earth. Compared to its gasoline-burning counterpart, the 2013 version of the Civic Natural Gas produces 80 percent fewer emissions of non-methane hydrocarbons and 50 percent fewer emissions of NOx, which contribute to ozone depletion. It also produces 67 percent less carbon monoxide than its gasoline counterpart. Most available light-duty NGV models have been certified to meet the Federal Tier 2, Bin 2 standard (only Bin 1, which requires zero emissions, is more demanding).  For 2014, the bi-fuel GM Impala joins the Honda Civic Natural Gas as the only other OEM production NGV passenger car.   

Beyond light-duty passenger cars, there are many options to choose from when considering an NGV. Around 50 different manufacturers in the U.S. produce 100 models of light-, medium-, and heavy-duty natural gas vehicles and engines. These vehicles include refuse trucks, transit buses, shuttle vans, and a variety of vocational work trucks. These larger vehicles are often placed in service in fleets that consume a lot of fuel and accumulate more miles than the average consumer vehicle. This means these vehicles are reducing even more pollution than if they were used in applications that accumulate fewer miles and use less fuel.

The emission charts below provide a comparison of the relative emission benefits of natural gas versus gasoline and diesel fuel looking at both new and older vehicles. The primary benefit when it comes to “new” vehicles is a reduction in NOx emissions and a reduction in greenhouse gas emissions.  Depending on the particular applications and vehicles involved, the benefits may be greater or less for some pollutants.  Another major advantage of NGVs is using them to replace older, dirtier trucks in urban areas where their contribution to pollution is often disproportionately high.  Replacing these older trucks in urban areas can be a very cost-effective way to reduce pollution since operating natural gas trucks is more economical than operating today’s diesel or gasoline vehicles and much more economical than continuing to operate older, less efficient vehicles.  

Greenhouse Gas Emissions
Natural gas contains less carbon than any other fossil fuel and thus produces fewer carbon dioxide (CO2) emissions when burned. While NGVs do emit methane, another principle greenhouse gas, the increase in methane emissions is more than offset by a substantial reduction in CO2 emissions. The California Air Resources Board (CARB) has conducted extensive analysis on this issue. CARB concludes that a CNG fueled vehicle emits 20 to 29 percent fewer GHG emissions than a comparable gasoline or diesel fueled vehicle on a well-to-wheel basis. For natural gas vehicles that run on biomethane, the GHG emissions reduction approaches 90 percent.

CARB LCFS Carbon Intensity Reductions for Natural Gas

Light-Duty Vehicles

Carbon Intensity
of Fuel
(gC02e/MJ)

EER

Total

Reductions
Relative to
Gasoline

Gasoline
(Baseline—CA RFG with EtOH mix)

95.86

1

95.9

NA

CNG

68

1

68

29%

Biomethane CNG

11.26

1

11.26

89%

CNG 20% Biomethane

56.602

1

56.6

41%

Heavy-Duty Vehicles

Carbon Intensity
of Fuel
(gC02e/MJ)

EER

Total

Reductions
Relative to
Gasoline

Diesel Fuel

94.71

1

94.7

NA

CNG

68

0.9

75.6

20%

Biomethane CNG

11.26

0.9

12.38

87%

CNG 20% Biomethane

56.602

0.9

62.9

34%

More recent studies indicate that these benefits may be somewhat reduced by higher levels of fugitive methane emissions occurring in the upstream production and distribution of natural gas.  According to the latest analysis that factors in these higher emissions, NGVs still produce about 13–21 percent fewer GHG emissions than comparable gasoline and diesel vehicles. This range depends of the type of vehicle and its particular application. The tables below are based on the Argonne National Labs complete full fuel cycle analysis using the latest U.S. EPA figures. The first table is a comparison of new vehicles, while the second table looks at the environmental benefits of replacing older in-use vehicles with new NGVs.

Emissions Reductions (%) of New NGVs Compared to New Gasoline and Diesel Vehicles (2012)

LD Car

LD Truck

School Bus

Heavy Duty Trucks (v. Diesel)

CNG v. Gasoline

CNG v. Diesel

CNG

LNG

LNG Dual Fuel

GHG

13

14

13

13

13

21

NOx

16

16

16

40

40

40

Emissions Reductions (%) of New NGVs Compared to In-Use Gasoline and Diesel Vehicles

LD Car

LD Truck

School Bus

Heavy Duty Truck

CNG v. Gasoline

CNG v. Diesel

CNG v. Diesel

2002

2007

2002

2007

2002

2007

2002

2007

GHG

18

18

25

25

25

25

25

25

NOx

91

34

 97

91

92

76

95

88

PM10

50

0

98

12

98

21

98

22

Fugitive methane remains an important issue that industry continues to address to comply with strict state regulations. Natural gas producers and distributors are motivated to meet and exceed regulations for both environmental and economic reasons—natural gas lost is money lost. According to U.S. EPA’s most recent analysis of methane emissions, approximately 1.4 percent of the natural gas that is produced is emitted into the atmosphere during the various processes involved in extracting, processing, and distributing the natural gas to end-use customers. This estimate is down from much higher estimates previously made by the EPA. At these revised levels, NGVs provide the 13–21 percent reduction in total greenhouse gas emissions (well-to-wheels) compared to new diesel and gasoline vehicles, as shown in the charts above.