Critical perspective of using natural gas for vehicles

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Critical perspective of using natural gas for vehicles
Jeremy Horne, Ph.D.

Current technology in context

Two factors have spurred the manufacturer of compressed natural gas (CNG) and liquefied
natural gas (LNG) vehicles – fuel economy and environmental factors. A frequent confusion
needs to be cleared up, the difference between the two forms of the gas. CNG is similar to
LPG, in that it too is a fossil fuel and is methane. However, the central difference is that the
former is a highly compressed gas – signified by the “C”, and the latter is an uncompressed
very cold liquid, as the “L” means.

When not transported by pipeline, LNG (CH4, or methane), is liquefied as a result of extreme
compression. (25 kPa/3.6 psi) to about 1/600 of its natural volume and is stored in thick-
walled cylinders at about −162 °C (−260 °F). Contaminants, such as dirt, helium, heavy
hydrocarbons, gases with acid, and water are removed in the processing. Odorants are added
to this otherwise odorless, non-toxic, and colorless gas so as to make it detectable [1]. In
that form it is safe, but when the gas is let out or escapes, it expands and is subject to
ignition at a 5-15 percent gas-to-air concentration. While the storage cylinders are very
efficiently insulated, there is a tendency for the gas to warm. Temperatures are kept at
storage levels by a small continuous boil-off called “cryogenic boiling”. This purposeful leaking
subtracts heat from the remaining gas, in a similar manner that heat passes from water to
create steam.

Natural gas, of course, is a fossil fuel, a hydrocarbon, subject to the same restraints as
regular gasoline, but it does burn much cleaner, hence the original and primary motivation to
switch to it several decades ago. However, it has been found that methane is a greenhouse
gas, more threatening than carbon dioxide, but is there is less natural gas emitted into the
atmosphere at present. On the other hand, natural gas has 20 times the ability to create
radiative energy than carbon dioxide. A ton of methane can capture just as much radiation as
carbon dioxide, though it stays in the atmosphere from eight to 40 times shorter [2]. As the
permafrost warms up huge quantities of methane are released, and there is concern that this
could accelerate global warming. In the event of spillage, natural gas will disperse more

1
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quickly than gasoline or diesel fuel. LNG comes primarily from Trinidad, Tobago, Algeria,
Indonesia, Malaysia, and Qatar. Iran and Russia are potentially large sources, as well. The
cost of LNG plants is high due to lacked of skilled persons construct them, material costs, and
general rising prices of petrochemicals.

CNG is less than 1% of its original volume in a free state at standard pressure and
temperature and about 42% of the volume occupied by LPG. It is stored in thick walled
cylinders or spheres at a pressure of 200–248 bar (2900–3600 psi. Because there is no
cryogenic storage as with LPG, CNG costs less to keep on site. An alternative storage method
is adsorbed natural gas (ANG) with pressures around 500 psi, the same as in a natural gas
pipeline. The storage tanks contain spongy materials like activated carbon and matrices of
substances composed of metals combined with organic materials – organic metal frameworks
(MOF). Hence, vehicles can be refueled at sites that don’t need to compress the gas further.
As a result of the lower storage pressures, the vehicle tank can be of lighter material, and the
walls do not have to be as thick. The storage containers have thicker walls than those of LPG.
It is CNG that is primarily used as a vehicle fuel, there being some 12.6 million worldwide in
2010 [3]. Many of these vehicles are bi-fueled, i.e., having the capability of running either on
gasoline or CNG. This is to say that the vehicle’s combustive apparatus – injectors, cylinders,
emissions control, and so forth are used for either fuel.

An advantage of CNG is that it can be mixed with biofuels, such as methane produced from
landfills and animal waste. Like LNG, CNG does not foul spark plugs as readily and
contaminants don’t befoul crankcase oil. There is 1.6 times the amount of energy in standard
gasoline [4]. Like LPG, CNG is in closed spaces, thus doing away with spillage or evaporation
as with conventional liquid petrochemicals. However, because of its storage requirements,
CNG-powered vehicles need storage space – lots of it – for the tanks, which, at the same
time, are much heavier than conventional tanks used for regular liquid fuels.

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Typical CNG storage [5]

Even with a redesign of the tank, such as flattening it out and storing it on the roof or
underneath the vehicle, the vertical profile of the vehicle, perforce, will be higher. While the
gas is the same, methane, CNG is the method of choice for vehicle, as opposed to LNG
because there are no cryogenic storage requirements for the former.

Now, for the vehicles
Although many cars and trucks can be and are run on propane (liquefied propane gas, or
LPG) our concern is with natural gas. Propane (C3H8) needs be refined from natural gas, as is
methane, but there is less of it than methane. However, the heat content of propane is
higher, about twice that of methane. Yet, there is more natural gas available. LPG is sold in
gallons, while CNG is in cubic feet.

As to the mechanics, the engine for both conventional liquid fuels, such as gasoline and
diesel, and natural gas is basically the same, with the driver simply being able to turn a
switch to use either the gasoline or natural gas.

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CNG fueled engine [6]

However, you simply can’t fill up the tank with natural gas and drive away. There are
numerous after-market conversion kits to make a vehicle into a duel use one – both for
propane and natural gas. Installation cannot be done on any car (some cars cannot
accommodate the conversion), there should be a qualified technician to do the work (this is
not shade tree work), conversions in the U.S. must be in according with Federal
Environmental Protection Act (EPA) law, and generally only newer cars can be candidates [7].
These are high pressure systems and incompetent installation can result in fatalities.
Conversion costs about $3,000 to $5,000, and one must assess the overall savings in price
per gallon and engine maintenance balanced against this [8].

After-market CNG conversion kit [9]

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A drawback to any duel-fueled car – with both natural gas and gasoline is the need for two
tanks, one for each fuel. A natural gas tank can add a good 100 pounds, and the space must
be found to accommodate it, usually in the car’s trunk. As natural gas uses more space than
gasoline, the range of such a vehicle is not as great [10]. Especially for smaller cars, where
there is not as much room for the required tanks for natural gas, the range on all these cars
is less, thus requiring more frequent fill-ups and supporting infrastructure to do so. This is to
say that for long trips across country, as in the case of the U.S., there will be need to be
natural gas filling stations, many more so than now. Other maintenance factors should be
considered in a CNG powered car. A sobering cost factor is that one major brand of car
requires the replacement of its low pressure filter every 10,000 miles at a cost of $122.00 –
excluding labor! Every 30,000 the high pressure filter needs replacing, and that is about
$52.00 [11].

Refilling the natural gas part of the car requires an infrastructure, and currently, there are
two competing ones, the one for natural gas and the other for propane. In essence, there will
need to be three types of refueling options – gasoline, propane, and natural gas. Propane
requires a smaller jet than natural gas, so the two gases are not interchangeable in a vehicle.
However, there is “propane air”, which the manufacturer claims is interchangeable with
natural gas. However, this is for non-vehicle use [12].

Environmental impact of CNG and LNG
Energy is required in compressing gas, manufacturing cylinders, etc. No energy is needed in
extracting it, as it emerges naturally from the ground. Both the facilities required to produce
gasoline by fractional distillation and those for compressing gas involve energy expenditure,
and to assess the overall net energy gain, the complete production cycle needs to be
assessed. There is much carbon dioxide emission involved with gas compression and
transport. However, in transport there are fewer LNG carbon dioxide emissions than those
from pipelines per kilometer [13].

Petroleum is petroleum is petroleum, no matter whether it is LNG, CNG, propane, diesel, or
gasoline. Continued extraction of natural gas or petroleum faces increasing demand and the
supply may be a constraint on further development of petroleum and gas powered vehicles,

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despite industry claims to the contrary [14]. The Argonne National Laboratory’s Energy
Systems Division states that “…the issue of natural gas availability in the United States is not
a near-term concern [15]”, although there are no sources given, other than the U.S. Energy
Information Administration (U.S. EIA), itself [16], which cites industry sources, such as the
National Petroleum Council and the Potential Gas Committee. The National Petroleum Council,
which, it can be expected, is going to paint a rosy picture of the situation [17] as well as the
Potential Gas Committee, largely made up of industry representatives [18]. Even the
proponents of natural gas are self contradictory in their statements. For example, the website
NaturalGas.org states, “…no one really knows exactly how much natural gas exists until it is
extracted”, but then says, “A common misconception about natural gas is that we are running
out, and quickly. However, this couldn't be further from the truth.” And, then, “…analysts use
different methodologies and systems of classification in various estimates.” For sources of its
data, this website refers to the U.S. EIA [19].“ The point is one must not simply accept
figures, especially from industry sources that have an “axe to grind”, because they are simply
issued but be critically thoughtful and ask who is using the information and why. Often, there
is a middle ground between opposing sides.

Even if there is an ample supply of fossil-based natural gas, how is all this natural gas to be
extracted? As each year passes, the amount of effort to extract fossil fuels increases. A major
controversy has been with hydraulic fracturing, or “fracking”, a method of extracting natural
gas by injecting and pressuring a fluid through sedimentary layers, causing new rock
fractures through which the gas can be released. A common problem is that that injected
substances used to force the gas out will emerge in the water table, along with the gas and
toxic metals, such as arsenic, and subsequently be discharged through plumbing. There are
numerous cases of petrochemicals actually coming out of water faucets in areas where this
fracturing takes place [20].

Other than fossil-fuel based natural gas, the renewable potential is with biogas, or methane
generated such as that from animal wastes and landfills. This offers a realistic transition from
hydrocarbon fuels to something even more renewable, such as solar power. At this point,
however, storage technology has not been developed satisfactorily to make electricity a viable
option for long-distance travel or heavy vehicle use.

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The future of CNG and LNG-powered vehicles
There are some 12 million natural gas-powered vehicles in the world today [21], a paltry
number, compared to the some billion conventionally-fueled ones. The use of natural gas as a
fuel will rise, but there is the reality of peak oil and the advent of electric-powered vehicles.
Natural gas has been and will continue to be a seemingly desirable partial solution to the air
pollution problem but only in a limited sense. While it is true that engine life can be extended
by this cleaner burning fuel and there are fewer contaminants, what needs to be determined
is the overall energy output required to produce the gas as balanced against the output. We
rarely see discussions along these lines. That is, what is the nature of the complete energy
cycle involved the production of alternate forms of powers, such as natural gas in its diverse
forms of distribution, as balanced against the energy output of that fuel? There is the issue of
the infrastructure of filling stations, as well as an assembly-line production of these vehicles.

In December 2011, Obama signed a tax bill that gives a $30,000 credit to gasoline filling
stations for adding a natural gas pump [22], and this very well may spur growth in natural
gas usage. Numerous automobile manufacturers are beginning to produce hybrid gasoline-
natural gas vehicles [23]. By the time – and even if – the infrastructure is finally put into
place, the looming question still remains of how available fossil fuels will be. Of course,
electric cars have their limitations, not the least of which is limited range and power storage
issues, i.e., battery technology. Electric vehicles very well may be the future, but in the
interim, one may expect electric-gas hybrids, another step away from gasoline powered
vehicles. With electric cars, too, one has to compute how much energy it requires to generate
that electricity (for recharging) and the associated environmental costs, given that most
power generating plants still run on fossil fuels. No comprehensive study has been done
comparing the cost, vehicle needs, required and existing infrastructure, and environmental
issues [24]. What is the end-to-end cost to produce various fuels as opposed to benefit
derived?

We cannot delude ourselves and jump from fad to fad, such as LNG or electric cars, without
appreciating the overall picture. It is clear that alternatives to the present reliance on fossil
fuels need to be found, not only because of their dwindling supply but because of global
warming. Many persons discussing the “wow” of natural gas vehicles fail to think critically and
account for the wider context of the alternative power sources both of electric and fossil fuel
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powered cars. One simply has to separate out the marketing hype from the reality in asking
what really is “green”. If we don’t think critically about all this we will be deluding only
ourselves.

Another constraint is the use of private vehicles, themselves. As traffic congestion grows, it
has to be asked how many more individual vehicles can be added, to say nothing of the
added environmental contamination. That 20% of the world’s population consumes 80% of
the world’s resources [25], the stark and non-economic reality is that there is going to be a
limit to how many vehicles can be produced. Just because of the congestion and
environmental constraints not everyone may be able to have a personal occupancy vehicle,
just because, alone, the planet will not support it. Rather, alternatively powered vehicles
more likely will be oriented towards mass transit. So, even if there were an unlimited reserve
of gas, these two factors alone - strain or the Earth’s resources and congestion - will radically
shape the direction of any-powered vehicles.

Want to learn more about Natural Gas Vehicles?
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References(Subject is indicated by URL – accessed 18 September 2011)
https://docs.google.com/viewer?url=http://www.controlsouthern.com/documents/regulators/
Literature/Bulletins/D103095X012.pdf&embedded=true&chrome=true
http://en.wikipedia.org/wiki/Methane, http://en.wikipedia.org/wiki/Natural_gas
http://en.wikipedia.org/wiki/Compressed_natural_gas
http://en.wikipedia.org/wiki/File:Carroagas.jpg
http://www.cngnow.com/EN-US/Vehicles/Pages/ConverttoCNG.aspx
http://cngchat.com/forum/showthread.php?1767-quot-Can-I-convert-my-____-to-run-on-
CNG-quot old 6
http://www.cartalk.com/content/eco/fringe.html
http://www.ewsews.com/cnghome.html
http://stevemark122000.hubpages.com/hub/Pros-and-Cons-of-Natural-Gas-Cars
http://www.cngcivic.com/CIVIC_GX_CNG_Filters.php
https://docs.google.com/viewer?url=http://standby.com/propane/pdf/ixpv_05172005.pdf&e
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mbedded=true&chrome=true
http://www.ipcc.ch/pdf/special-reports/spm/sres-en.pdf
http://www.theoildrum.com/node/5247,
http://www.pennenergy.com/index/articles/display/0663918147/articles/pennenergy/microbl
ogs/rafael-sandrea/natural-gas-supply--potential-setbacks.html,
http://ideas.repec.org/a/eap/articl/v39y2009i2p255-270.html ,
http://www.ourbusinessnews.com/natural-gas-supplies-within-5-year-average ,
https://docs.google.com/viewer?url=http://www.postcarbon.org/reports/PCI-report-nat-gas-
future-plain.pdf&embedded=true&chrome=true , and
http://environmentalheadlines.com/ct/2011/05/12/the-truth-about-natural-gas-supply-costs-
environmental-impact/ , as examples
http://www.ngv-
conference.com/uploadedFiles/EventRedesign/Germany/2011/December/12116003/Assets/N
atural-Gas-Vehicles-Status-Barriers-and-Opportunities.pdf, p. 20
http://www.eia.gov/dnav/ng/ng_enr_sum_dcu_NUS_a.htm
http://www.naturalgas.org/overview/resources.asp
http://www.earthisland.org/journal/index.php/elist/eListRead/how_neutral_is_the_potential_g
as_committee/
http://naturalgas.org/overview/resources.asp
http://en.wikipedia.org/wiki/Hydraulic_fracturing ,
http://www.youtube.com/watch?v=dEB_Wwe-uBM , http://what-the-
frack.org/2011/08/20/chip-northrup-speaks-about-the-dangers-of-hydrofracking/ ,
http://www.eeb.cornell.edu/howarth/Marcellus.html , and
http://www.fedflyfishers.org/LinkClick.aspx?fileticket=iHCRIpvS7W8%3D&tabid=5095&mid=4
733 , http://www.cfg.cornell.edu/
http://www.cnbc.com/id/40794709/Natural_Gas_Vehicles_On_The_Road_To_Acceptance
Ibid.
http://www.greencarcongress.com/natural_gas/
http://www.greencarcongress.com/2011/06/mit-and-iea-reports-take-different-views-of-the-
future-of-natural-gas-in-transportation.html#more
http://www.sustainablesettlement.co.za/issues/resources.html ,
https://docs.google.com/viewer?url=http://www.worldresourcesforum.org/files/fischer-
kowalski.pdf&embedded=true&chrome=true , http://articles.cnn.com/1999-10-
12/us/9910_12_population.cosumption_1_global-population-worlds-scientists?_s=PM:US

Resources(Subject is indicated by URL – accessed 18 September 2011)

http://en.wikipedia.org/wiki/Natural_gas_vehicle


http://en.wikipedia.org/wiki/Liquefied_natural_gas

http://www.lngpollutes.org/article.php?list=type&type=13

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http://www.naturalgas.org/lng/lng.asp

http://ferc.gov/industries/gas/indus-act/lng.asp

http://www.beg.utexas.edu/energyecon/lng/

http://www.beg.utexas.edu/energyecon/lng/documents/CEE_INTRODUCTION_TO_LNG_FINAL
.pdf

http://www.beg.utexas.edu/energyecon/lng/documents/NGC_Interchangeability_Paper.pdf

http://www.beg.utexas.edu/energyecon/lng/documents/CEE_Role_of_LNG_in_Nat_Gas_Suppl
y_Demand_Final.pdf

http://www.propane.pro/lpg/15890127/

http://en.wikipedia.org/wiki/Natural_gas

http://en.wikipedia.org/wiki/Special_Report_on_Emissions_Scenarios#B2

http://www.grida.no/publications/other/ipcc_sr/?src=/climate/ipcc/emission/

http://www.cnbc.com/id/40794709/Natural_Gas_Vehicles_On_The_Road_To_Acceptance

http://www.nextgreencar.com/lpg-cng.php

https://docs.google.com/viewer?url=http://www.norriseal.com/files/comm_id_47/Norriseal_6
300_tech_brochure.pdf&embedded=true&chrome=true

http://www.ngvaeurope.eu/members/weekly-
news/brochure_NAtural_Gas_Vehicle_conference_22_08_2011.pdf

http://www.ngv-
conference.com/uploadedFiles/EventRedesign/Germany/2011/December/12116003/Assets/IS
SUES-RELATED-TO-THE-TRANSITION-TO-NATURAL-GAS-UTILIZATION.pdf

http://www.ngv-
atural-Gas-Vehicles-Status-Barriers-and-Opportunities.pdf

http://www.ngv-
atural-Gas-Vehicles-Fueling-Infrastructure-and-Economics.pdf

https://docs.google.com/viewer?url=http://www.cleanvehicle.org/committee/technical/PDFs/
Conversion_Press_Release.pdf&embedded=true&chrome=true

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http://www.racq.com.au/motoring/cars/car_advice/car_fact_sheets/lp_gas

http://www.youtube.com/watch?v=W-uhCIpPhws

http://www.fueleconomy.gov/feg/bifueltech.shtml

http://www.greencarcongress.com/natural_gas/ .

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