For the study, the team used a Monte Carlo analysis combined with a full lifecycle analysis framework to assess the greenhouse gas (GHG) implications of a transition to light-duty natural gas-powered vehicles. The researchers considered six different types of transportation fuels—CNG; natural gas-based electricity; natural gas-based hydrogen; natural gas-based Fischer−Tropsch liquids (gasoline and diesel); natural gas-based methanol; and ethane-based ethanol—in two representative light-duty vehicles: a passenger vehicle and a sport utility vehicle.

They evaluated six vehicle technologies: a spark ignition internal combustion engine vehicle (SI-ICEV); a flex fuel vehicle (FFV); a compression ignition internal combustion engine vehicle (CI-ICEV); a hybrid electric vehicle (HEV); a plug-in hybrid electric vehicle (PHEV); a battery electric vehicle (BEV); and a fuel cell electric vehicle (FCEV). For both passenger vehicles and SUVs, the functional unit was one vehicle kilometer traveled.

The study on light-duty vehicles is a companion piece to a study published earlier this year assessing the GHG implications of the same transition to natural-gas powered medium- and heavy-duty vehicles. (Earlier post.)

Life cycle GHG emissions of natural gas pathways for passenger vehicles (top panels) and SUVs (bottom panels) in gCO2-equiv/km. Here we assume both baseline (left panels) and pessimistic estimates (right panels) of methane emissions from natural gas systems. Error bars represent the 95% confidence interval of life cycle GHG emissions, which comprise three sources: vehicle manufacturing; upstream (well-to-pump); and tailpipe (pump-to-well) emissions. Upstream emissions include all use-related emissions from primary energy extraction to dispensing the fuel into vehicles. Tailpipe emissions include all use-related emissions from vehicle operation. Estimates using both 100-year GWPs (left bars) and 20-year GWPs (right bars) are presented side by side for each pathway. Pathways are are sorted based on life cycle emissions with 100-year GWP. Data labels represent mean life cycle GHG emissions. Credit: ACS, Tong et al. Click to enlarge.

In addition to the findings outlined above, the researchers also determined that SUVs have larger life cycle GHG emissions than passenger cars for all natural gas pathways. For example, a hydrogen-powered fuel-cell SUV has life cycle GHG emissions that are at least 19% higher than those of a fuel-cell passenger vehicle, while a battery electric SUV has life cycle GHG emissions that are 41% higher than a battery-electric passenger vehicle.

We find that the benefits of natural gas pathways largely depend on two factors, vehicle fuel efficiency and carbon intensity of the fuel (cradle-to-gate GHG emissions per MJ of fuel delivered; see the Supporting Information for our estimates). Pathways that run on electric vehicles (except liquid hydrogen) have smaller emissions than pathways that run on ICEVs. Within each vehicle technology group, if vehicles’ fuel efficiencies are comparable, pathways with higher supply chain efficiency emit less than pathways with lower supply chain efficiency (e.g., CNG vs methanol or gaseous hydrogen vs liquid hydrogen). To assess the contribution of these two effects, we developed a break-even analysis that shows the trade-offs between vehicle fuel efficiency and methane leakage rate that influence the carbon intensity of a natural gas-based fuel.

—Tong et al.

The break-even rate is the methane leakage rate at which life cycle GHG emissions from a natural gas pathway equals that of conventional gasoline. If the actual methane leakage rate from the natural gas systems is lower than the calculated break-even rate of a specific pathway, that pathway has lower life cycle emissions than conventional gasoline. The higher the break-even rates, the larger the emissions reduction coming from that pathway.

They performed the break-even analysis on the CNG, gaseous hydrogen fuel cell vehicle and battery-electric vehicle pathways because those emissions are highly dependent on fugitive methane emissions. Further, these pathways are the closest to commercial deployment.

They found that CNG vehicles offer emissions reductions if the lifecycle methane leakage rate is lower than 2.3% (using the 100-year GWP) or 0.9% (using the 20-year GWP). A shorter time horizon (such as 20 years), which considers a higher warming potential of methane, requires a lower break-even rate than a longer time horizon (such as 100 years). Current FCEVs offer emission reductions if the life cycle methane leakage rate is lower than 2.8% (100 years) or 1.2% (20 years). Of the three pathways considered, BEVs have largest break-even rates with current vehicle technologies, 10.8% (100 years) and 4.5% (20 years).

Their baseline estimate of methane leakage was 1.3%, with a pessimistic estimate of 2.0%.

Increasing vehicle fuel efficiency and reducing methane emissions from the natural gas system are promising strategies to reduce GHG emissions from using natural gas for road transportation. For the same mobility service, a higher vehicle fuel efficiency leads to lower life cycle emissions or translates into a higher allowable break-even methane leakage rate. Recent studies find evidence of “super emitters” in natural gas systems: a small number of emission sources that lead to a significant share of methane emissions. There are cost-effective technologies to reduce these emissions that would reduce the methane leakage rate and provide better opportunities for reducing GHG emissions from light-duty vehicles by using natural gas as a transportation fuel.

—Tong et al.

Resources

• Fan Tong, Paulina Jaramillo, and Inês M. L. Azevedo (2015) “Comparison of Life Cycle Greenhouse Gases from Natural Gas Pathways for Light-Duty Vehicles” Energy & Fuels doi: 10.1021/acs.energyfuels.5b01063