Natural gas has a limited range of flammability; it will not burn in concentrations below 5 percent or above 15 percent when mixed with air. Gasoline and diesel burn at much lower concentrations and ignite at lower temperatures. Although it takes very little energy to ignite a flammable mixture of air and natural gas, gasoline, or diesel, natural gas burns at a somewhat lower temperature. Natural gas can also be blended with hydrogen.

Propane
Propane is produced as a by-product of natural gas processing and crude oil refining. Approximately 85 percent of propane used in this country is generated from domestic sources. Propane, or liquefied petroleum gas (LPG), is a popular alternative fuel choice for vehicles; more than 200,000 vehicles, mostly in fleets, are operating by propane power. There are more than 3,000 propane fueling stations in the United States. Besides being accessible to the general public, LPG produces fewer vehicle emissions than gasoline. Tests on light-duty, bi-fuel vehicles have demonstrated a 98 percent reduction in the emissions of toxics, including benzene, 1,3-butadiene, formaldehyde and acetaldehyde.

Hydrogen
Most hydrogen is produced in centralized natural gas steam reforming processes. Hydrogen can be produced from a variety of feedstocks using an assortment of process technologies. Additional sources of feedstock are fossil resources such as coal and petroleum, and renewable resources such as biomass, sunlight and wind. Process technologies include thermochemical water-splitting using nuclear and solar heat, biological (algae and bacteria), electrolytic and photolytic.

Hydrogen is transported from the point of production to the point of use as a compressed gas or cryogenic liquid via pipeline, tanker trucks, tube trailers or railcars or barges. It must also be handled and moved within refueling stations or stationary power facilities. Hydrogen compression, liquefaction and handling systems require further efficiency and cost improvements to compete with the delivery costs of conventional fuels.

Hydrogen has been used effectively as pure hydrogen mixed with natural gas in a number of internal combustion engine vehicles. In addition, hydrogen is used in a growing number of demonstration fuel cell vehicles. Hydrogen and oxygen from air fed into a proton exchange membrane (PEM) fuel cell "stack" produce enough electricity to power an electric automobile, without producing harmful emissions.

Ethanol
Ethanol is an alcohol-based fuel produced by fermenting and distilling starch crops that have been converted into simple sugars. Feedstocks for this fuel include corn, barley, and wheat. A bushel of field corn will yield 1.6 pounds of corn oil which can be converted to 2.7 gallons of ethanol.

Ethanol can be blended with gasoline to create E85, a blend of 85 percent ethanol and 15 percent gasoline. E85 and blends with higher concentrations of ethanol, E95, for example, qualify as alternative fuels under the Energy Policy Act of 1992 (EPAct).Vehicles that run on E85 are called flexible fuel vehicles (FFVs) and E85 is approved by all flexible fuel vehicle manufacturers. Click here to learn more about clean vehicles. E85 burns cleaner compared to conventional gasoline by reducing ozone-forming volatile organic compounds by 15 percent, carbon monoxide by 40 percent, particulate emissions by 20 percent, nitrogen oxide emissions by 10 percent and greenhouse gas emissions by nearly 30 percent. E85 has a 105 octane rating and provides a boost in horsepower. It burns cooler than gasoline and keeps car engines clean.

In some areas of the United States, lower concentrations of ethanol are blended with gasoline. The most common low concentration blend is E10 (10 percent ethanol and 90 percent gasoline). While it reduces emissions, E10 is not considered an alternative fuel under EPAct regulations.

Electricity
Electricity can be used as a transportation fuel to power battery electric and fuel cell vehicles. When used to power electric vehicles (EVs), electricity is stored in an energy storage device such as a battery. EV batteries have a limited storage capacity and their electricity must be replenished by plugging the vehicle into an electrical source. The electricity for recharging the batteries can come from an existing power grid or from distributed renewable sources such as solar or wind energy.

Fuel cell vehicles use electricity produced from an electrochemical reaction that takes place when hydrogen and oxygen are combined in the fuel cell "stack." Vehicles that run on electricity have no tailpipe emissions. The production of electricity-using fuel cells takes place without combustion or pollution and leaves only two byproducts, heat and water. Emissions that can be attributed to EVs are generated in the electricity production process at the power plant.

EVs have lower fuel and maintenance costs than gasoline-powered vehicles. The cost of an equivalent amount of fuel for EVs is less than the price of gasoline. Also, maintenance for EVs is less—EVs have fewer moving parts to service and replace, although the batteries must be replaced every three to six years.

Biodiesel
Biodiesel is a domestically produced, renewable fuel that can be manufactured from vegetable oils and animal fats. Biodiesel is safe, biodegradable and releases fewer serious air pollutants than diesel. Biodiesel contains no petroleum, but can be blended at any level with petroleum diesel to create a biodiesel blend. The use of B20, which is a blend of 20 percent biodiesel with 80 percent petroleum diesel, results in 20 percent reduction of unburned hydrocarbons, 12 percent reduction of carbon monoxide and 12 percent reduction in particulate matter compared to emissions from diesel fuel. Research is underway to explore additives that will reduce emissions of nitrogen oxides.

B20 provides similar horsepower, torque and mileage as diesel. Biodiesel has a solvent effect that may release deposits accumulated on tank walls and pipes from previous diesel fuel storage. The release of deposits may clog fuel filters upon the initial use and should be monitored closely. Blends can generally be used in unmodified diesel engines; however, users should consult their Original Equipment Manufacturers (OEM) and engine warranty statement. Biodiesel can also be used in its pure form (B100), but it may require certain engine modifications to avoid maintenance and performance problems and may not be suitable for wintertime use. Users should consult their engine warranty statement.

To ensure that biodiesel is produced and maintained at the industry standard the fuel should meet the American Society for Testing and Materials (ASTM D-6751) quality specification.

For more information about clean fuels, visit http://www.eere.energy.gov/afdc/altfuel.html