Comparison of Life Cycle Greenhouse Gases from Natural Gas Pathways for Light-Duty Vehicles

Low prices and abundant
resources open new opportunities for using
natural gas, one of which is the production of transportation fuels.
In this study, we use a Monte Carlo analysis combined with a life
cycle analysis framework to assess the greenhouse gas (GHG) implications
of a transition to natural gas-powered vehicles. We consider six different
natural gas fuel pathways in two representative light-duty vehicles:
a passenger vehicle and a sport utility vehicle. We find that a battery
electric vehicle (BEV) powered with natural gas-based electricity
achieves around 40% life cycle emissions reductions when compared
to conventional gasoline. Gaseous hydrogen fuel cell electric vehicles
(FCEVs) and compressed natural gas (CNG) vehicles have comparable
life cycle emissions with conventional gasoline, offering limited
reductions with 100-year global warming potential (GWP) yet leading
to increases with 20-year GWP. Other liquid fuel pathways (methanol,
ethanol, and Fischer–Tropsch liquids) have larger GHG emissions
than conventional gasoline even when carbon capture and storage technologies
are available. Life cycle GHG emissions of natural gas pathways are
sensitive to the vehicle fuel efficiency, to the methane leakage rates
of natural gas systems, and to the GWP assumed. With the current vehicle
technologies, the break-even methane leakage rates of CNG, gaseous
hydrogen FCEV, and BEV are 0.9%/2.3%, 1.2%/2.8%, and 4.5%/10.8% (20-year
GWP/100-year GWP). If the actual methane leakage rate is lower than
the break-even rate of a specific natural gas pathway, that natural
gas pathway reduces GHG emissions compared to conventional gasoline;
otherwise, it leads to an increase in emissions.