Propane Emissions
Also known as liquefied petroleum gas (LPG), propane can be used to replace gasoline in light-duty vehicles (LDVs) or diesel in heavy-duty vehicles (HDVs). Because availability of propane vehicles is limited, most are conventional vehicles converted to run on LPG. Converted vehicles, however, may emit more emissions than manufactured propane vehicles if conversions are not properly designed and installed. For this reason, this page will consider conversions and manufactured propane vehicles separately.
Conversions
The Argonne National Laboratory (ANL) report, A Full Fuel-Cycle Analysis of Energy Emissions Impacts of Transportation Fuels Produced from Natural Gas, combined emissions data from three primary studies to determine the effects of propane on LDV emissions. The combined results were used in ANL's GREET-based analysis and compared to LDVs running on reformulated gasoline in the table below.
| Pollutant | Percent Reduced |
|---|---|
| Volatile Organic Compounds (VOC) | 0% |
| Carbon Monoxide (CO) | 20% to 40% |
| Oxides of Nitrogen (NOx) | 0% |
| Particulate Matter (PM) | 80% |
| Methane | 10% increase |
Manufactured Propane Vehicles
One propane-powered LDV and several HDV propane engines and fueling systems are available by U.S. original equipment manufacturers today. Because other countries offer more options in propane vehicles, thorough testing has been conducted on these vehicles in Europe. Two of these tests were combined in The Report of Alternative Fuels Group of the Cleaner Vehicles Task Force, as represented in the table below.
| Pollutant | Percent Reduction: LDV | Percent Reduction: HDV |
|---|---|---|
| Total Hydrocarbons (THC) | >40% | >80% |
| CO | >20% | >90% |
| NOx | >30% | ~60% |
| PM | ~100% | ~100% |
Source: The Report of Alternative Fuels Group of the Cleaner Vehicles Task Force
Comparing the two tables suggests that manufactured propane vehicles are generally cleaner-burning than conversions because the systems can be optimized. However, the California Energy Commission found VOC emissions from propane vehicles to be much higher than those of gasoline largely due to venting losses from vehicle storage tanks.
To augment LPG's generally cleaner combustion properties, propane engines can be calibrated to choose between pollutants, making the engine additionally useful in achieving pollution-reduction targets. A rich calibration reduces NOx at the expense of increasing CO and nonmethane hydrocarbons and a lean calibration does just the opposite.
Small Engines
Propane is also frequently used to replace gasoline in smaller applications, such as forklifts and lawn mowers. Due to a lack of regulation, these applications emit a substantial amount of pollution. Therefore, emissions reductions are substantial when one of these engines is replaced by propane. However, there have not been any studies quantifying these reductions.
Lifecycle GHG Emissions and Petroleum Use
Unlike natural gas, propane is not a greenhouse gas (GHG) when released directly into the atmosphere. When considering the entire lifecycle of propane used in converted LDVs, ANL found that propane reduced GHG emissions by 21% to 24% and petroleum use by 98% to 99%.
The Propane Education and Research Council also compares GHG emissions from forklifts, buses, and light-duty trucks operating on various fuels in its report Propane Reduces Greenhouse Gas Emissions: A Comparative Analysis.