New Study Improves Understanding of Natural Gas Vehicle Methane Emissions

By Joe Rudek and Jason Mathers, Environmental Defense Fund.

Many commercial fleet operators have considered switching their fleet vehicles from diesel to natural gas to take advantage of the growing abundance of natural gas and reduced emissions. Natural gas trucks have the potential to reduce nitrogen oxides emissions (NOx) from freight trucks and buses.

Yet, adopting the emission reduction technologies and practices needed to curb the methane escaping during the production, transport and delivery of natural gas is critical to unlock the full environmental potential of these vehicles. Methane, the main component of natural gas, is a potent greenhouse gas released to the atmosphere at every step from production wells to the vehicle fuel tanks. Even small amounts of methane emitted across the natural gas supply chain can undermine the climate benefit of fuel-switching vehicles to natural gas for some period of time, as EDF research has shown.

A newly published scientific study, led by researchers with West Virginia University at the Center for Alternative Fuels, Engines and Emissions, measured methane emissions from heavy-duty natural gas-powered vehicles and refueling stations, and is greatly expanding what we know about emissions from natural gas-fueled vehicles. The study is the first project in EDF’s coordinated methane research series to analyze where and by how much methane emissions occur during natural gas end uses.

The WVU study found that emissions from the vehicle tailpipe and engine crankcase were the highest methane sources, representing roughly 30 and 39% (respectively) of total pump to wheels (PTW) emissions. Fortunately, engines with closed crankcases have recently been certified by EPA, avoiding the single largest source of methane emissions from these vehicles.

Fueling station methane emissions were reported to be relatively low, representing about 12% of total PTW emissions. WVU researchers based the fueling station emission estimates on the assumption that liquefied natural gas (LNG) stations have sufficient sales volume to effectively manage boil off gases, or the fuel lost as vapors when the LNG heats above its boiling point. Without alternative methods to manage boil off gas, low sales volume risks large methane releases.