Natural Gas Emissions

Natural gas is stored on vehicles in two forms: compressed and liquefied. Tailpipe emissions are the same for either form of natural gas in light-duty vehicles (LDVs), and evaporative emissions are negligible for both forms since the fuel systems in natural gas vehicles were built to accommodate their extremely low evaporation temperature and pressure.

Light-Duty Vehicles

Compressed natural gas (CNG) and liquefied natural gas (LNG) can be used as a replacement for gasoline in light-duty vehicles (LDVs). The Argonne National Laboratory (ANL) report, A Full Fuel-Cycle Analysis of Energy and Emissions Impacts of Transportation Fuels Produced from Natural Gas, combined emissions data from 14 primary studies to determine the effects of natural gas fuels on LDV tailpipe emissions. The weighted results were used in their GREET-based analysis and are illustrated in the table below.

Fleet Studies

A National Renewable Energy Laboratory (NREL) fleet study of 13 vans in the Denver Super Shuttle fleet found that dedicated CNG vans have significantly lower emissions of nonmethane hydrocarbons (NMHC), CO, and NO_x than gasoline or bi-fuel vans operated on CNG or gasoline. Another NREL study of 20 taxicabs also found that CNG reduced NMHC and CO but did not find any reduction in NO_x.

Heavy-Duty Vehicles

CNG can also replace diesel fuel in heavy-duty vehicles (HDVs). Due to the variety of HDVs and drive cycles that natural gas can be used for, emissions benefits must be assessed for multiple working fleets. NREL and the West Virginia University (WVU) studied numerous fleets as part of its Alternative Fuel Truck Evaluation Project. The following table lists the natural gas vehicle emissions reductions as a percentage below the emissions of comparable diesel vehicles.

This table shows that PM is heavily reduced by natural gas—reduced to undetectable levels in two tests and by at least 85% in the other three. NO_x is reduced by 17% to 80% from diesel. The reduction of NMHCs varies widely, but all tests show a reduction. Two of the tests compare NMHCs from natural gas to the total hydrocarbons (including methane) from diesel, and both show the NMHCs to be 60% lower. The CO reduction varies widely from a near 100% reduction to a 260% increase, but the reports do not offer insight to this wide variance.

Lifecycle GHG Emissions and Petroleum Use

ANL's GREET model estimates the lifecycle petroleum use and greenhouse gas (GHG) emissions from CNG and LNG LDVs. Throughout its lifecycle, natural gas emits a much smaller amount of GHGs than gasoline. CNG uses less petroleum and emits fewer GHGs than LNG because the compression takes less energy than liquefaction. Many of the GHGs emitted from the lifecycle of natural gas fuels result from leakage. Natural gas is largely comprised of methane, which is 23 times more potent as a GHG than carbon dioxide.

In 2007, the California Energy Commission (CEC) found that CNG reduces GHG emissions by 30% in cars and 23% in buses compared to their gasoline and diesel counterparts. LNG reduced GHG emissions by 16% in buses compared to its diesel counterpart. These counterparts, however, are cleaner in California than in the rest of the nation, therefore understating the benefits of natural gas vehicles.

The ANL and CEC studies both consider the petroleum reductions natural gas provides over its lifetime and find that CNG reduces petroleum consumption almost 100% from the level of gasoline. LNG reductions are slightly less because it requires more energy to process.