In Dallas, grease is good. And not just any grease, but the grease used to cook French fries, onion rings and yes, even pickles. In an effort to save money on fuel and reduce its carbon footprint, the city's school district is converting all of its 1,700 buses to run on a special engine that runs on both biodiesel and recycled vegetable oil donated by local restaurants. In 2009, the first bus, the Fryer Flyer, took to the road. Officials expect to save $400,000 each year by running its entire bus fleet on the biofuel [source: NPR].

For years, politicians, journalists and scientists have touted biofuels -- fuels made from plants -- as a way to decrease the world's consumption of fossil fuels, especially oil. These so-called "energy crops" include wheat, corn, soybeans and sugarcane [source: Walker]. Biofuels burn cleaner than fossil fuels, releasing fewer pollutants and greenhouse gases, such as carbon dioxide, into the atmosphere. They are sustainable, and energy companies often mix biofuels with gasoline. In other words, unlike oil, coal or natural gas, biofuels won't run out.

Biofuels fall into two main categories: bioalcohol and biodiesel. To create bioalcohol, such as ethanol, engineers use yeast and bacteria to break down the starch in corn and other plants. To create biodiesel, refineries use the oil already found in crops such as soybeans [source: Thomas]. These vegetable oils are treated with alcohol and turned into biodiesel.

There are, however, some drawbacks to these miracle fuels. Among other things, some energy crops compete with food crops for land, creating problems like higher food prices and deforestation. In addition, the costs of converting some energy crops, as well as retrofitting cars and power plants to run on biofuel, can be pricey [source: Brune]. Find out what else there is to consider about the top 10 biofuel crops.

In the world of ethanol, corn is king. Turning sugar-rich corn into ethanol is much like brewing beer. Workers first grind the golden kernels and mix them with warm water, and next add yeast. The yeast causes the slurry to ferment, or turn into energy-producing alcohol. Refineries blend ethanol with gasoline for use in existing car engines. Ethanol, whether it is made from corn, wheat or sugarcane, releases less carbon monoxide, nitrogen oxide and sulfur into the atmosphere than gasoline. Ethanol also reduces smog, which can minimize health problems for people, especially those living in cities.

Using corn kernels in the fermentation process is cheaper than using the entire corn plant. The sugar in the stalks and leaves of corn plants play hide and seek in a substance known as cellulose. Cellulose is difficult and expensive to break down. However, researchers are trying to make that process more cost effective. In addition, scientists at Michigan State University have developed a strain of corn that contains special enzymes that turn the stubborn cellulose into sugar, which engineers can ferment into ethanol. Scientists at Michigan State say their new strain of corn, Spartan Corn, will make the production of ethanol from plant waste cheaper and less time consuming [source: Science Daily].

People have been using rapeseed oil to cook food and illuminate homes for centuries. Today, rapeseed oil is an important form of biodiesel fuel. Perhaps the most important type of rapeseed oil comes from canola, a type of rapeseed. Unlike other strains of rapeseed, canola is low in eurcic acid, which makes it healthier for animals and humans to eat.

Generally, biodiesel made from vegetable oil does not perform well in cold climates. Why is that? Since most vegetable oil is high in saturated fat, ice crystals tend to form in the biodiesel causing a car's engine to seize. However, since canola oil is low in saturated fat, it is harder for ice to materialize in frigid temperatures [source: University of Connecticut].

Also, canola and other types of rapeseed have a higher oil content than other vegetable plants, which means canola and rapeseed can generate more energy when burned. Biodiesel made from rapeseed and canola oils releases less carbon monoxide than diesel fuel. Biodiesel made from rapeseed and canola is very efficient in powering heavy machinery and other vehicles. In general, engines that run on biodiesel are more efficient than gasoline-powered engines.

If there is one country that has done the most to wean itself from oil and increase its use of biofuel, it is Brazil. The South American country started going green after the 1973 Middle East oil embargo reduced shipments of oil worldwide. When the price of oil climbed, the Brazilian government encouraged its farmers to plant more sugarcane. They then processed that sugarcane into ethanol. Brazil invested billions of dollars to make the transition and eventually sugarcane ethanol became less expensive than gasoline. By the mid-1980s, every driver in Brazil was driving a vehicle that was ethanol-powered. Today, almost all cars in Brazil have "flexible" fuel engines that can run on either gasoline or ethanol [source: Walker].

Brazil's climate is well suited to growing sugarcane. However, producing ethanol from sugarcane is six times less expensive than producing ethanol from corn. Growing sugarcane requires fewer chemicals, including pesticides and fertilizers [source: Cox]. But when harvesting sugarcane, farmers must burn their fields, which spews massive amounts of greenhouse gases into the atmosphere.

If there is one country that has done the most to wean itself from oil and increase its use of biofuel, it is Brazil. The South American country started going green after the 1973 Middle East oil embargo reduced shipments of oil worldwide. When the price of oil climbed, the Brazilian government encouraged its farmers to plant more sugarcane. They then processed that sugarcane into ethanol. Brazil invested billions of dollars to make the transition and eventually sugarcane ethanol became less expensive than gasoline. By the mid-1980s, every driver in Brazil was driving a vehicle that was ethanol-powered. Today, almost all cars in Brazil have "flexible" fuel engines that can run on either gasoline or ethanol [source: Walker].

Brazil's climate is well suited to growing sugarcane. However, producing ethanol from sugarcane is six times less expensive than producing ethanol from corn. Growing sugarcane requires fewer chemicals, including pesticides and fertilizers [source: Cox]. But when harvesting sugarcane, farmers must burn their fields, which spews massive amounts of greenhouse gases into the atmosphere.

Whether it is tofu or tacos, crayons or shampoos, candles or pens, products made from soybeans line store shelves. Now the ubiquitous soybean can add green energy to its resume. In fact, most of the biodiesel in use in the United States comes from soybean oil. Motor vehicles, especially heavy equipment and buses, can run on pure soybean biodiesel, or a blend of biodiesel and diesel. According to the National Academy of Sciences, soybean biodiesel is more environmentally sound and yields more energy than corn ethanol [source: Barrioneuvo]. Researchers studied the amount of energy needed for growing corn and soybeans along with the amount of energy to convert those crops into biofuels and found that soybeans use less fertilizers and pesticides, and released less pollutants, including greenhouse gases, into the environment [source: Mongabay].

Plus, one bushel of soybeans yields 1.5 gallons (5.68 liters) of biodiesel. In addition, the amount of land devoted to soybean production is much greater than the amount of acreage devoted to other oilseed crops, which leads to greater biodiesel production. However, the oil content of soybeans (20 percent) is lower than the oil content of canola (40 percent) and sunflower seeds (43 percent) [source: University of Connecticut].

Sunflowers are big and beautiful, and baseball players love to chew the seeds. Those seeds are rich in oil, which makes the sunflower a popular biofuel crop. Refineries process the oil into biodiesel, or use the plant waste as biomass, which can fuel factories and power plants. According to the National Sunflower Association, 1 acre (.4 hectares) of sunflowers can produce 600 pounds (272.1 kilograms) of oil [source: National Sunflower Association].

Sunflowers are big business in Dove Creek, Colo. Many farmers began planting acres of sunflowers a few years ago turning the flowers' oil into biodiesel to power their farm equipment. By 2008, thousands of acres around the town were yellow with sunflowers. Farmers sold some of the seeds to a bioenergy company that turned the oilseed into biofuel and pressed the flowers' plant waste into tiny fuel pellets. Workers converted the pellets into gas, which the company used to fuel its electric generator [source: Burke].

Forget corn. The heck with soybeans. Don't even mention palm oil. If anyone wants to take a look at the wonder plant that has the potential to reduce the world's appetite for oil and save the planet from global warming, drive through the Great Plains and look out the window. What you will see is grass -- switchgrass.

Unlike corn, using the cellulose in switchgrass to make ethanol requires less energy from fossil fuels. Furthermore, cellulose ethanol contains more energy than corn ethanol, so there's a dramatic reduction in greenhouse gas emissions. Although there are no great switchgrass plantations or farms, scientists are currently working on various methods to make switchgrass the energy of the future. For example, researchers at Auburn University in Alabama grew test plots of switchgrass that produced 15 tons of biomass per acre. The scientists say that each acre can also produce 1,150 gallons of ethanol each year [source: Oak Ridge National Laboratory].

When farmers begin growing switchgrass as an energy crop, they will use less fertilizer, pesticides and water. The neat thing about switchgrass is that as a perennial, farmers only have to plant it once. Scientists say switchgrass provides five times as much energy as it takes to grow, and it grows on marginal land not used for farming. Therefore, farmers are not taking cropland away from food production [source: Biello].