The Damage Done — Natural Gas Vehicles, Cleaner and Greener?

Natural gas vehicle fueling stationNatural gas is often suggested as a cleaner path to a renewable world — that is, cleaner than coal for electrical generation and cleaner than oil for transportation. In the United States, some advocates are pushing to increase the use of compressed natural gas (CNG) for motor vehicles as a means for the country to gain energy independence.

(Photo: CNG fueling station, California. Credit: pgegreenenergy, CC BY 2.0.)

Financier T. Boone Pickens is one such advocate. He has introduced what he calls the Pickens Plan, under which the U.S. would undertake a massive effort to convert vehicle fleets over to natural gas. According to Pickens, this would not be so hard to do, as the technology is all available “off-the-shelf” right now. The Pickens Plan website explains:

Natural gas already has a tremendous advantage, particularly when used for trucks and fleet vehicles. Most trucking today is round-trip, one-tank routes. There are approximately 1.5 million miles of gas pipe and distribution lines crisscrossing the country, making natural gas available on nearly every street and community in America today.

According to the Natural Gas Supply Association:

Natural gas is the cleanest of all the fossil fuels … [Compared with coal and fuel oil,] combustion of natural gas … releases very small amounts of sulfur dioxide and nitrogen oxides, virtually no ash or particulate matter, and lower levels of carbon dioxide, carbon monoxide, and other reactive hydrocarbons.

Well, that all sounds pretty rosy, doesn’t it? And after all, natural gas is “natural,” right?
Natural gas platform in Louisiana
Well, we aren’t necessarily persuaded by pronouncements from partisans, the misinformed, or those with skin in the game. So, in the tradition of “The Damage Done,” let’s examine the external environmental effects, or “externalities,” of natural gas as a transportation fuel compared with other energy sources, both green and conventional.

(To review the “Damage Done” articles devoted to the environmental effects of electrical generation energy, please see the summary article, “The Damage Done, Part 10 — Are Renewables Really Better for the Environment Than Fossil Fuels?“)

(Photo: Natural gas platform, Louisiana. Credit: Daniel Foster, CC BY-SA 2.0.)

As you can see from the chart below, natural gas accounts for only a small percentage of transportation energy in the U.S. — only 2.5 percent (too small to merit a label on this Google Docs pie chart), compared with a whopping 93.2 percent for petroleum fuels. Natural gas accounts for a much greater percentage of electrical-generation energy, as you can see from the right-hand pie chart.

Pie charts showing US 2010 consumption of energy for transportation and electricity

However, that doesn’t mean that our investigation into the externalities of natural gas for transportation is meaningless. If anything like the Pickens Plan comes to fruition, natural gas’s percentage could increase dramatically in the coming decades — and its environmental impacts would become increasingly important as well.

Tailpipe Emissions: How Does Natural Gas Stack Up Against Gasoline?

Researchers have produced various reports on the environmental effects of natural gas consumption. For our purposes, the most useful one I’ve found is Hidden Costs of Energy: Unpriced Consequences of Energy Production and Use, a 2010 report from the National Academy of Sciences (NAS). The NAS has examined environmental externalities of all major energy sources, which allows a side-by-side comparison.

NAS finds that natural gas has lower exhaust emissions than gasoline or diesel when it comes to CO2, carbon monoxide (CO), nonmethane volatile organic compounds (VOCs), and nitrogen oxides (NOx). Natural gas vehicles (NGVs) do emit unburned methane, which is a greenhouse gas (GHG). The damages NAS calculates in its Hidden Costs study are based on health and other environmental effects of these kinds of emissions.

The fuel supply pathway can make a big difference in the fuel’s GHG emissions, says the report:

If non-North American natural gas is imported as LNG via ocean tanker and then regasified and compressed to produce CNG [compressed natural gas], for example, CNG reduces life-cycle GHG emissions by only 5 percent compared with gasoline. If domestic gas is used, life-cycle GHG emissions are reduced by 15 percent. If gas that otherwise would be flared or landfill gas is used as the feedstock, net GHG emissions can be negative.

The following table shows overall 2005 emissions-related damages estimated for compressed natural gas versus conventional gasoline light-duty trucks and automobiles:

Mean health and other non-GHG damages in dollars per vehicle mile traveled (VMT) Mean health and non-GHG damages per gasoline gallon equivalent (gge) Carbon footprint in CO2 equivalents in grams/VMT
Gasoline vehicle




Compressed natural gas vehicle




You can see from these figures that natural gas vehicles are somewhat less damaging than gasoline — 91 percent as damaging in health and other environmental effects per VMT and 78 percent per gasoline gallon equivalent. Natural gas has 89 percent the carbon footprint of gasoline in transportation, an 11 percent reduction.

So, we find a reasonable argument that natural gas is a cleaner substitute for gasoline for autos and trucks.

A report from the U.S. Department of Energy (DOE) cites other studies comparing the emissions of natural gas vehicles to those of gasoline and diesel vehicles. A study of light-duty vehicles produced the following results:

Pollutant Percent reduction in light-duty CNG vehicles over gasoline
Volatile Organic Compounds (VOCs)


Carbon Monoxide (CO)

20% – 40%

Nitrogen Oxides (NOx)


Particulate Matter (PM)



-400% (increase)

These figures for the most part correlate with the NAS findings, i.e., that natural gas vehicles offer environmental benefits over gasoline, except in terms of methane emissions. With regard to methane, the NAS study makes the point that, while “methane emissions for CNG vehicles are greater than those for gasoline, … CO2 emissions are much lower, yielding a net decrease in CO2-equivalent emissions for CNG vehicles.”

The DOE also cites a study of heavy-duty vehicles comparing emissions of NGVs with diesel vehicles, with the following results:

Reduction in Emissions by Natural-Gas Vehicles Over Diesel (NSS = Not statistically significant)

Mail delivery trucks Buses Semi trucks Refuse trucks Refuse trucks
Particulate Matter (PM) reduction






Nitrogen oxide (NOx) reduction






Non-methane Volatile Organic Compounds reduction (VOC or NMHC)






Carbon Monoxide (CO) reduction






The general picture here is that the environmental impacts of natural gas come in significantly lower than those of diesel by most measures. Possibly the largest and most definite improvement for both cars and trucks is in the area of particulate matter (PM), where reductions range from 80 percent to 96 percent for CNG over gasoline. The nitrogen oxide and VOC figures for heavy-duty vehicles suggest that CNG is better in these areas also.

Photo of taxi in Brazil with CNG tanks in trunkThe carbon monoxide (CO) figures yield ambiguous results. Why should natural gas produce far lower CO emissions in cars, delivery trucks, and buses, but far more CO emissions in semis and garbage trucks? After much careful consideration and analysis, my answer is I have no idea and apparently neither does DOE. The original reports “do not offer insight to this wide variance,” according to the agency.

It’s worth noting that the various classes of vehicles studied all come from different fleets; discrepancies could have arisen from different methods of measurement employed by the various owners (UPS, Dallas Area Rapid Transit, Raleys, Waste Management, and the City of Los Angeles).

(Photo: Hybrid taxi with CNG tanks in the trunk, Brazil. Credit: Mariordo, CC BY-SA 3.0.)

Besides those non-GHG emissions, the DOE report also finds a reduction in GHGs of 21 percent to 26 percent for light-duty vehicles. This reduction range is quite a bit higher than that calculated in the earlier-cited NAS study, which found only an 11 percent improvement in carbon footprint.

Natural Gas: Universally Loved for Its Greenness?

As with all energy sources, natural gas shows up on our doorstep with its own unique load of environmental baggage.

The current boom in natural gas production comes thanks to a new set of methods and technologies that allow extraction of shale gas. Shale is a very common sedimentary rock containing natural gas formed by organic material. Starting in the late 1990s, horizontal drilling, hydraulic fracturing (“fracking”), three-dimensional mapping, and seismic imaging made shale gas extraction economical.

Fracking is too new to have been studied thoroughly, but the practice has raised environmental concerns, especially over potential contamination of groundwater. Tracey Schelmatic has written on Green & Clean:

Fracking fluid can be made up of any combination of substances in liquid, gel, or foam form, with any number of chemical ingredients. Among the chemicals used in fracturing fluid is a cocktail of nasty substances that include known carcinogens, skin irritants, and endocrine disruptors – chemicals that affect the healthy function of human adrenal glands that govern development, growth, reproduction, and behavior in people and animals.

(Photo: Natural gas drilling in an environmentally sensitive area, Dudley Bluffs, Colorado. Credit: U.S. Fish and Wildlife Service, CC BY 2.0.)

Natural gas drilling, Colorado

A draft report from the U.S. Environmental Protection Agency (EPA) in December 2011 raised concerns over the effects of fracking on drinking water in a community in Wyoming. The agency’s announcement about its research makes clear:

EPA’s analysis of samples taken from the Agency’s deep monitoring wells in the aquifer indicates detection of synthetic chemicals, like glycols and alcohols consistent with gas production and hydraulic fracturing fluids, benzene concentrations well above Safe Drinking Water Act standards and high methane levels. Given the area’s complex geology and the proximity of drinking water wells to ground water contamination, EPA is concerned about the movement of contaminants within the aquifer and the safety of drinking water wells over time.

A more recent announcement stresses that more testing and monitoring will be required:

Together with the Tribes, the EPA and the State will convene a group of stakeholders and experts to develop and carry out a plan for further investigation of the Pavillion gas field to identify potential risks to drinking water, including possible sources and pathways for the migration of contaminants.

The Energy Institute of the University of Texas at Austin has released a preliminary report about environmental concerns over shale gas development. The institute’s scientists report that their research finds “no evidence of aquifer contamination from hydraulic fracturing chemicals in the subsurface by fracturing operations” and “no leakage from hydraulic fracturing at depth.” Groundwater contamination can occur in conventional oil and gas development and is “not unique to hydraulic fracturing.” Researchers think that surface spills of fracking fluids actually pose a greater environmental risk than fracking itself.

For more discussion of the environmental risks of shale gas extraction, see my previous article for ThomasNet’s Industry Market Trends, “Is Fracking Environmentally Sound?



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