A recent study from Carnegie Mellon University has revealed the uncertainty and variability of the potential life cycle greenhouse gas (GHG) emissions from using natural gas to power light-duty vehicles.
The study contributes to the library of studies conducting life cycle analysis of GHG emissions from using natural gas in the transportation sector through multiple pathways.
“The analysis of life cycle GHG emissions from transportation is a very complicated process and is subject to intense scrutiny by vested stakeholders,” acknowledged John Eichberger, executive director of the Fuels Institute. “Determining what sources to leverage for certain values and which assumptions to use when developing a model are critical ingredients that affect the ultimate outcome. Carnegie Mellon researchers have done a fantastic job developing an analysis that contributes directly to the scientific body of work evaluating the long-term effects of using natural gas to fuel our transportation sector. We are proud to have been a part of this work and hope it elevates the discussion on the topic and leads to broader understanding of the potential viability of natural gas-based fuels.”
The researchers evaluated the use of natural gas used in transportation as compressed natural gas (CNG) and as a feedstock to produce electricity, hydrogen, liquid fuels (via the Fischer-Tropsch method), methanol and ethanol. These fuels were then modeled for use in spark-ignited internal combustion engines, compression-ignited engines, flex fuel vehicles, hybrid, plug-in and battery-electric vehicles and fuel cell vehicles. The analysis evaluated the life cycle emissions for each pathway, including vehicle manufacturing and fugitive emissions of natural gas. In addition, the researchers evaluated the global warming potential of natural gas-powered light duty vehicles over a period of both 20 years and 100 years.
In general, the study found that the greatest GHG emissions benefit is derived from battery electric vehicles charged with electricity solely from advanced combined cycle natural gas powered generating facilities. Hydrogen fuel cell and CNG vehicles were found to have comparable life cycle emissions with conventional gasoline. Other pathways found emissions increased compared with conventional gasoline.
But the researchers acknowledged that the uncertainty and variability in life cycle GHG emissions of natural gas pathways must be taken into consideration when contemplating the implications of such studies and, in particular, when policymakers are setting emissions-based policy goals. For this evaluation, the authors used a mean average.
“An accurate evaluation of the environmental effects of any fuel, including natural gas, requires a comprehensive assessment,” Eichberger cautioned. “This paper by Carnegie Mellon is a valuable addition to the information available and will help advance our efforts to understand the role natural gas may play in the transportation sector.”
The low-cost and abundant supply of shale gas in the U.S. has increased the interest in using natural gas for transportation. The paper, “Comparison of Life Cycle Greenhouse Gases from Natural Gas Pathways for Light-Duty Vehicles,” is the second in a series of studies sponsored by the Fuels Institute and NATSO Foundation evaluating the use of natural gas as a transportation fuel.
As the authors mention in the conclusion of this study, additional work in this series will look at the potential for natural-gas based fuels to reduce other air pollutants and what effect such fuels might have on consumers, specifically related to cost, performance, convenience and safety. Carnegie Mellon is also evaluating the feasibility of developing an infrastructure system to support a natural gas transportation economy.
Each publication prepared by Carnegie Mellon University within the series is intended to be published in a peer reviewed scientific journal and will contribute directly to the overall understanding of the developing natural gas market.