Natural Gas Vehicle Emissions
When used as a vehicle fuel, natural gas can offer life cycle greenhouse gas (GHG) emissions benefits over conventional fuels, depending on vehicle type, duty cycle, and engine calibration. In addition, natural gas reduces some engine emissions.
Tailpipe emissions result from fuel combustion in a vehicle's engine. The emissions of primary concern include the regulated emissions of hydrocarbons, oxides of nitrogen (NOx), carbon monoxide (CO), as well as carbon dioxide (CO2). The U.S. Environmental Protection Agency (EPA) requires all fuels and vehicle types to meet increasingly lower, near zero, thresholds for tailpipe emissions of air pollutants and particulate matter. One advantage to natural gas vehicles (NGVs) is their ability to meet these stringent standards with less complicated emissions controls. NGVs continue to provide life cycle emissions benefits—especially when replacing older conventional vehicles. NGVs with ultra-low NOx engines can produce near-zero NOx emissions, which meets the California Air Resources Board’s optional near-zero emission standard of 0.02 NOx.
Natural gas is increasingly used to replace gasoline in smaller applications, such as forklifts and commercial lawn equipment. Because natural gas is a low-carbon fuel, a switch to natural gas in these applications can result in reductions of hydrocarbon, CO, NOx, and GHG emissions.
Life Cycle Emissions and Petroleum Use
Argonne National Laboratory's GREET model estimates the life cycle petroleum use and GHG emissions of vehicles running on compressed natural gas (CNG) and liquefied natural gas (LNG). Based on this model, heavy-duty natural gas vehicles can provide small to moderate GHG reduction benefits compared to diesel throughout the fuel life cycle. Actual reductions will depend on both the relative fuel economy, as well as upstream and vehicle methane leakage. When comparing the life cycle emissions of the two types of natural gas, CNG and LNG are nearly identical. CNG production uses less petroleum and emits slightly fewer GHGs than LNG because compressing natural gas requires less energy than liquefying it.
Because renewable natural gas (RNG), also known as biomethane, is chemically identical to fossil natural gas, yet yields far fewer life cycle GHG emissions, the blending of even relatively small quantities of RNG with fossil natural gas can provide significant life cycle GHG emission benefits. In a 2011 study of RNG production methods, Argonne National Laboratory concluded that all RNG methods show significantly less GHG emissions and fossil fuel consumption than conventional fossil natural gas and gasoline.
Overall, CNG and LNG are both clean-burning fuels and perform well against current vehicle emissions standards.
Converting conventional vehicles to operate on natural gas is a good option for incorporating alternative fuels into fleet operations. EPA's emissions requirements and regulations apply to vehicles converted to CNG and LNG fuel systems.
EPA requires conversion system manufacturers to demonstrate that converted vehicles or engines meet the same emissions standards as the original vehicle or engine. For this and many other reasons, it's important that conversions be performed by reputable and well-qualified system retrofitters.