DENVER -- The City of Denver will begin using biodiesel, an alternative fuel, in 60 city vehicles as part of a pilot project unveiled this year on Earth Day, according to the Rocky Mountain News newspaper. Mayor John Hickenlooper said he expects the city to use about 50,000 gallons of clean-burning B20 biodiesel by the end of the year. Biodiesel is produced from renewable resources, such as soybeans and mustard seed. It can be used in a pure form or blended with petroleum diesel. Denver's project will use B20 biodiesel, which is a blend of 20 percent biodiesel and 80 percent petroleum diesel. Biodiesel has been shown to emit lower levels of carbon monoxide, unburned hydrocarbons, and particulate matter than regular petroleum diesel.