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

Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
Table 2.6 Emissions for Natural Gas (CNG) versus
Conventional Gasoline
Pollutant Percent Change versus Light-Duty Gasoline
VOC -45%
CO 0%
NOx -20%
PM10
PM2.5
2-34
-9%
-20%
SOx +11%
In addition, about 90 percent of all natural gas consumed in the United States is
domestically produced, thereby improving energy security relative to current petroleumbased
fuels.
Feasibility
As discussed above, CNG LDVs have relatively high incremental vehicle costs and limited
fueling infrastructure. In addition, CNG LDVs also have significantly decreased
operational range compared to comparable gasoline vehicles, due to the lower energy
density of natural gas, even when under pressure. For example, the dedicated CNG
Honda Civic can only accommodate a 7.2 gge storage tank, limiting its operating range to
170 miles (Honda, 2009). Operating range can be extended, especially when converting
light trucks and SUVs, by increasing tank capacity, but at the cost of lost storage and
hauling capacity (NGVAmerica, 2009).
CNG LDVs have excellent safety records on the whole. CNG tanks are made to withstand
very high pressures and temperatures to minimize safety problems. The odorant ethyl
mercaptan also is added to natural gas to aid in leak detection for CNG, although odorants
are not added to LNG. In addition, engine maintenance requirements appear to decline in
natural gas engines relative to conventional engines, due to the very clean burning
properties of the fuel, which may in turn result in extended engine life.
Despite the acceptance of LNG as a transportation fuel, there are a few safety concerns
associated with its use. Some of the potential dangers include cryogenic burns, lung
damage, and asphyxiation. LNG also can vaporize to the gaseous phase leading to
extremely high pressure buildup; hence, pressure release valves are used to prevent