Well-to-wheels (WTW) diagram of the use of natural gas as a transportation fuel (CNG and LNG pathways only). The bounds of the WVU Pump-to-Wheels (PTW) study are shown by the dashed boundary line, which includes the stations and the vehicles. The study included emissions from the point that tankers or pipelines crossed station property lines through to the end use in the vehicle. The gray boxes indicate the sources of methane emissions that are possible. Credit: ACS, Clark et al. Click to enlarge.

The WVU team developed a a novel measurement system was developed to quantify methane leaks and losses. They characterized engine-related emissions from 22 natural-gas-fueled transit buses, refuse trucks, and over-the-road (OTR) tractors. Losses from six LNG and eight CNG stations were also characterized during compression, fuel delivery, storage, and from leaks.

The researchers characterized cryogenic boil-off pressure rise and pressure control venting from LNG storage tanks using theoretical and empirical modeling. Field and laboratory observations of LNG storage tanks were used for model development and evaluation.

The team combined the pump-to-wheel (PTW) emissions with a specific scenario to view emissions as a percent of throughput. The researchers found that vehicle tailpipe and crankcase emissions were the highest sources of methane. The authors are applying the data from this research to develop models to forecast methane emissions from the future HD transportation sector.

Absolute and relative contribution of methane emissions by component for the base case scenario. Numbers represent the average methane loss per unit of fuel used (g/kg). The percentage values reflect the contribution of each source to the total PTW emissions. Credit: ACS, Clark et al. Click to enlarge.

Natural gas vehicle and dispensing technology has evolved steadily. We characterized methane emissions from real-world operations to support well-informed projections of future pump-to-wheels contributions from heavy-duty vehicle use.

—Lead author Nigel Clark, George Berry Chair of Engineering and professor of mechanical and aerospace engineering

The WVU team collaborated on the study with Environmental Defense Fund and a group of industry leaders including: the American Gas Association, Chart, Clean Energy, Cummins, Cummins Westport, International Council on Clean Transportation, PepsiCo, Shell, Volvo Group, Waste Management, and Westport Innovations. Sponsors provided access to vehicles or facilities. Equipment for testing was also provided by a number of other industry participants or rented.

A Scientific Advisory Panel comprised of academic experts in the fields relevant to the study served as independent advisors, reviewing the appropriateness of the methodologies, results and statistical methods.

Karen Hamberg, Vice-President of Natural Gas Industry and Government Relations, Westport Fuel Systems, said that the study provides a critical baseline by which ongoing product and technology enhancements can be measured, as it represents the first significant effort to quantify actual in-use methane emissions from natural gas filling stations and heavy-duty vehicles.

Natural gas-fueled vehicles are expected to play a greater role in future transportation to meet the global regulatory trend for more stringent greenhouse gas emission reductions. The natural gas vehicle industry has already implemented technology solutions to dramatically minimize, or in some cases, eliminate the largest sources of methane emissions from vehicle tailpipe, crankcase ventilation, and dynamic venting that were identified in the study.

—Karen Hamberg

Resources

• Nigel N. Clark, David L. McKain, Derek R. Johnson, W. Scott Wayne, Hailin Li, Vyacheslav Akkerman, Cesar Sandoval, April N. Covington, Ronald A. Mongold, John T. Hailer, and Orlando J. Ugarte (2017) “Pump-to-Wheels Methane Emissions from the Heavy-Duty Transportation Sector” Environmental Science & Technology doi: 10.1021/acs.est.5b06059