Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

In 2006, approximately 43,000
medium- and heavy-duty CNG
and LNG vehicles were in
operation across the country.
OEM natural gas engines are more
common in the medium- and
heavy-duty market, with four
models available for purchase in
2008 for various applications,
including refuse haulers, buses,
and line-haul trucks. Five certified
retrofit kits also were available for
these vehicle types as well
(NGVAmerica, 2009). The
transportation sector currently
consumes about 25 billion cubic
feet of natural gas per year, or
about 0.1 percent of total national
consumption of 23 trillion cubic
feet. 10
For rail transport, LNG is more
applicable for long-haul railroad
operations because the fuel
density is five times greater than
CNG, reducing the space
requirement and the required
frequency of refueling. LNG can
be stored in a separate tender car
that is constructed as a double
walled stainless shell similar in
design to a thermos bottle, and
can keep the LNG adequately cold
Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
for up to 14 days before gas venting is needed. During operation a heat exchanger
converts the LNG back to a gaseous state which is piped to the engine. Dual LNG/diesel
engines have been developed and used by Burlington Northern Santa Fe (BNSF) for more
than a decade, using diesel fuel as an ignition source for LNG combustion, as well as a
backup fuel if there is a failure in the LNG fuel supply. It is possible that LNG may be
released from storage tanks if pressure exceeds the tank’s design strength. Little data
available on LNG venting currently is available, though the practice is not required as long
as the tank pressure is maintained at a safe level by keeping the fuel at or below -160 C.
10 AEO 2009, Table 13.
Biogas
Anaerobic digestion of organic matter produces a mixture of
gases consisting primarily of methane and CO2 called biogas,
also known as landfill or digester gas. Sources include sewage
sludge, agricultural and industrial wastes, animal by-products,
and municipal solid wastes (AFDC, xxxx). Biogas typically has
somewhat lower methane content compared to natural gas,
which typically is more than 70 percent methane. The
remaining content of natural gas is other hydrocarbons, while
biogas contains 20 to 50 percent CO2. Biogas must be treated to
increase the proportion of methane and decrease the CO2
content and other contaminants (Persson et al, 2006).
After purification, the gas must be transported. Although the
purified gas can be distributed using the conventional natural
gas infrastructure, further technology development and testing
is needed before biogas distribution via the natural gas
pipeline grid may become common practice.
Purified biogas can substitute for conventional CNG or LNG as
a vehicle fuel. An estimated 12,000 vehicles worldwide, mostly
in Europe, are fueled with upgraded biogas. Conversion of
landfill gas to transportation fuel has the added benefit of
reducing landfill methane emissions.
Research and development projects are underway to reduce
costs of producing and purifying biogas as well as to evaluate
the performance of vehicles using biogas. However, even
allowing for cost-effective recovery, processing, and delivery
strategies, the ultimate amount of biogas available for
transportation use is quite limited. Assuming all of the
methane from landfills, wastewater treatment facilities, and
composting could be recovered for transportation use, this
would translate to only about 2 percent of current gasoline
consumption in the U.S. (based on the 2007 EPA Sources and
Sinks value of 158.9 Tg CO2e emissions, equating to ~ 3 billion
gge/yr ).
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