Natural Gas Vehicle Emissions
Natural gas burns cleaner than conventional gasoline or diesel due to its lower carbon content. When used as a vehicle fuel, it can offer life cycle greenhouse gas (GHG) emissions benefits over conventional fuels, depending on vehicle type, drive cycle, and engine calibration. In addition, using natural gas may reduce some types of tailpipe emissions.
Tailpipe emissions result from fuel combustion in a vehicle's engine and are emitted from its exhaust system. The emissions of primary concern include the regulated emissions of hydrocarbons, oxides of nitrogen (NOx), carbon monoxide (CO), as well as carbon dioxide (CO2). Due to increasingly stringent emissions regulations, the gap between tailpipe emissions benefits from natural gas vehicles (NGVs) and conventional vehicles with modern emissions controls has narrowed. That's because the U.S. Environmental Protection Agency (EPA) is holding all fuels and vehicle types accountable to the same levels of air pollutants emitted from vehicle combustion. Still, NGVs continue to provide emissions benefits—especially when replacing older conventional vehicles or when considering life cycle emissions.
Natural gas is frequently used to replace gasoline in smaller applications, such as forklifts and commercial lawn equipment. Because natural gas is a low-carbon, clean-burning fuel, a switch to natural gas in these applications can result in substantial reductions of hydrocarbon, carbon monoxide, oxides of nitrogen, and greenhouse gas emissions. In addition, natural gas is nontoxic, so it isn't harmful to soil or water.
Life Cycle Emissions and Petroleum Use
Argonne National Laboratory's GREET model estimates the life cycle petroleum use and greenhouse gas (GHG) emissions of light-duty vehicles running on compressed natural gas (CNG) and liquefied natural gas (LNG). Based on this model, natural gas emits approximately 6% to 11% lower levels of GHGs than gasoline throughout the fuel life cycle. The GHG emissions impacting the CNG and LNG life cycle are predominately the result of production-phase fuel 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 fewer GHGs than LNG, because compressing natural gas requires less energy than liquefying it.
In 2007, a study for the California Energy Commission (CEC) found that both CNG and LNG reduce life cycle GHG emissions in both light- and heavy-duty vehicles compared to their gasoline and diesel counterparts. Again, this is primarily due to the low petroleum usage in the production phase and the low-carbon intensity of the fuel during use.
An Argonne National Laboratory analysis and the CEC study both consider the petroleum-use reductions natural gas provides over its life cycle. Both studies find that CNG uses far less petroleum over its life cycle – in fact, there is a greater than 90% reduction in petroleum use for CNG compared to gasoline over its life cycle. LNG's petroleum-use reductions are slightly less, because the fuel requires more petroleum to process than CNG.
Renewable Natural Gas (RNG), also known as biogas, digester gas or landfill gas, refers to natural gas from unconventional sources where biological processes (like anaerobic digestion) produce biomethane from organic matter. Natural gas derived in this fashion is considered a renewable fuel because the original source of the carbon, in the decomposed waste, can be traced back to the organic source that replenished it. Until recently RNG was either vented to the atmosphere or captured and burned off in a collection system. Because it is chemically identical to fossil natural gas, yet produces far fewer GHG emissions, the blending of relatively small quantities of RNG with fossil natural gas can provide significant life cycle GHG benefits. In a 2011 study of RNG production pathways, Argonne National Laboratory concluded that all RNG pathways show significantly less GHG emissions and fossil fuel consumption than conventional fossil fuel natural gas and gasoline.
Overall, CNG and LNG are both clean-burning fuels and perform well against current vehicle emissions standards.
Converted Vehicles
Converting conventional vehicles to run on natural gas is a good option for incorporating alternative fuels into light- and heavy-duty fleet operations. EPA's emissions requirements and regulations apply to vehicles converted to run on CNG or LNG.
EPA requires conversion system manufacturers to demonstrate that converted vehicles or engines meet or exceed the same emissions standards as the original vehicle or engine. Therefore, it's important that conversions be performed by careful and reputable qualified system retrofitters, because converted vehicles could produce higher emissions levels than manufactured natural gas vehicles if conversions are not properly installed and calibrated.
