Fuels used in transportation are the largest source of greenhouse gas emissions in the United States. The Environmental Protection Agency reports that transportation accounts for 28% of greenhouse gas emissions by economic sector, surpassing the emissions of electrical generation, industry and other sources of these emissions. The US Energy Information Agency (EIA) reported that in 2019 about 142.17 billion gallons were consumed in the United States, an average of about 389.51 million gallons per day. The EIA also reported that during the same year the US transportation sector used about 47.2 billion gallon of diesel fuel.
Gasoline and diesel engines have been vital to the US economy and much research has gone into developing alternatives to those fossil fuels to enable those engines to continue to operate with lower emissions. Alternative fuels such as biodiesel and ethanol can be used in diesel powered or gasoline powered vehicles respectively. It is useful to note there are differences in the environmental aspects of different alternative fuels.
There is frequent misunderstanding of the terms used regarding alternative fuels. In this document the following definitions are used:
Alternative Fuel: A non traditional, typically non-petroleum fuel; i. e. not gasoline or diesel
Renewable Fuel: A fuel whose feedstock can be replenished; i.e., ethanol and biodiesel
Sustainable Fuel: A renewable fuel whose lifecycle, including production and use, can be perpetuated; i.e., ethanol from wood waste
Alternative Fuels Used for Transportation:
Biodiesel: Biodiesel production uses feedstocks of vegetable oils, animal fats and used cooking oils and greases. These are typically reacted with an alcohol in the presence of a catalyst in a process called transesterification to form biodiesel. Biodiesel is typically used as a blend with petroleum diesel. A commonly used blend is B20 which is 20% biodiesel and 80% petroleum diesel.
Electricity: Electric vehicles initially were light duty vehicles such as passenger cars. Recently several companies have announced electric pick up trucks and even large Class 8 over the road trucks. Electric vehicles may utilize either batteries or fuel cells for their energy storage.
Ethanol: Ethanol is typically made from corn or other plant materials through a fermentation process. Most gasoline in the US has up to a 10% blend of ethanol and 90% gasoline, called E10. Flexfuel vehicles can use a higher level of ethanol blends.
Hydrogen: Hydrogen is used in fuel cell vehicles to produce electricity. Fuel cell vehicles can be refilled quickly, usually in about 10-15 minutes. There is interest in both light and heavy duty fuel cell vehicles. Hydrogen filling station infrastructure is being developed.
Natural Gas: Methane, or natural gas, is domestically produced. It is stored either under pressure or as a liquid. Smaller vehicles, such as light duty cars and trucks typically utilize pressurized natural gas where as some larger commercial trucks use liquid natural gas for longer intervals between refilling. Liquified Natural Gas is also being introduced as a fuel in commercial marine vessels due to its clean burning properties.
Propane: Also called propane Autogas, this high octane, high energy and clean burning fuel has been used in vehicles for many years. It is stored under pressure and then becomes a vapor as the pressure is released and it is injected into the engine.
There has been much interest in producing alternative fuels from renewable feedstocks, thereby greatly lowering their life cycle greenhouse gas emissions. The following are examples of renewable alternative fuels:
Renewable Diesel: Renewable diesel is typically made from used oils and greases. It is different than biodiesel as renewable diesel meets the specifications of standard diesel fuel.
Renewable Natural Gas: Renewable natural gas is produced by anerobic processes, such as those that occur at sanitary landfills and wastewater treatment plants. The raw gas is collected from those facilities and contaminants removed to meet natural gas specifications.
Renewable Propane: Renewable propane is also typically made from used oils and greases and can be a byproduct of the renewable diesel process.
The Department of Energy's Clean Cities Program:
The Department of Energy’s Clean Cities program was initiated in 1993 to promote the nation’s economic, environmental, and energy security by working locally to advance affordable, domestic transportation fuels, energy efficient mobility systems, and other fuel-saving technologies and practices. There are 13 Clean Cities coalitions in the southeast they serve as the foundation of Clean Cities by implementing transportation projects in their respective communities. These public-private partnerships are comprised of businesses, fuel providers, vehicle fleets, state and local government agencies, and community organizations. Nationally, the Clean Cities program has achieved a cumulative impact in energy use equal to nearly 8 billion gasoline gallon equivalents through the implementation of diverse transportation projects. The Clean City programs in the southeast are listed below along with their respective website.