Focus on Fuels: Developing Sustainable Biofuel Crops

Fuels developed from sugar, grain crops, or algae could be used more widely in the future.


Sweet sorghum also is easier to process into fuel than corn.

Ethanol is produced from corn and other grain crops in basically two steps: First the starch is broken down into sugars, and then the sugars are fermented into ethanol by yeast, just as beer is fermented.

When juiced, crops such as sugarcane and sweet sorghum yield sugars instead of starch, thus eliminating the first steps of the process for corn-based ethanol.

"Even after you squeeze the stalks and get the juice out, there's a lot of biomass that's still available," said Riley. "That could be used as animal feed as a way to supplement what a grower would be able to get."

Don Slack, a professor in the department of agricultural and biosystems engineering, has studied the irrigation needs of sweet sorghum. "It's known as a drought-tolerant, salt-tolerant, tough crop," said Slack. "Sorghum would be a preferred crop in a hot, arid region."

Sweet sorghum needs little water, and since it's not intended for food it can be irrigated with treated wastewater, which provides many of the nutrients the crop needs.

It also has very low nitrogen requirements, according to a study by Mike Ottman, an agronomist in the plant sciences department.

Said Ottman: "You wouldn't think about it but nitrogen fertilizer is made using fossil fuels. In the case of corn, maybe one-third to half of the energy that goes into growing the crop is the energy required to make the fertilizer."

"Sweet sorghum's not part of the food chain, so if there's more sweet sorghum that's going toward producing biofuels it's not taking away corn that's used to feed animals or people," added Riley.

The researchers are trying to develop a way to plant and harvest effectively two crops in one year.

"You could effectively double the growth, double the amount of ethanol per year, just because our growing seasons are much longer than other parts of the country," said Riley. "It's really taking advantage of our location."

The right crop: the search for sustainability

The scientists are testing and developing several other crops, including three types of switchgrass and three varieties of perennial grasses in the genus Panicum.

These grasses produce cellulose instead of starch. Like starch, cellulose is made up of chains of sugar molecules held together by what are called glycosidic bonds. However, the orientation of the bonds in a cellulose molecule creates a more rigid structure, making cellulose harder to break down.

"The conversion of cellulose to sugar is a little more difficult than the conversion of starch to sugar," said Ottman. "You can't just do straight chemical hydrolysis. You need enzymes, and these enzymes are expensive."

For now, most of the ethanol production plants in the U.S. process grain crops. But that could change: The cellulosic process should start taking off by 2015, according to the target date of our energy policy, said Ottman.

And perennial grasses as biofuel crops have important benefits. Cellulose is one of the most abundant organic molecules on Earth and could yield an equally abundant source of fuel on the future.

Numerous research questions remain for each crop, including the amount of water and fertilizers needed, when to plant and when to harvest, how far apart to plant the seeds to optimize the harvest and how best to store and transport the fuel once it's processed. Said Ray: "These are all little things that make a big difference."

"The goal of the project, of all these projects, is to have a commercial product," said Ray. "The end goal is to give back to folks who have given to us through funding and in other ways."

The agricultural methods developed in Arizona would work in many arid or semi-arid environments, said Ray, "which actually applies to much of the Earth. So what we do here is transferrable. The desert plants don't know if they're in Chihuahua or Texas. Borders mean nothing to them. It has to do with the environment."

"One hundred and fifty years ago petroleum was used for kerosene for lamps in the Northeast to replace whale oil. In 150 years you wouldn't recognize what we're doing with biofuels now," said Ray. "We're gaining knowledge so that we understand what's needed and how this whole process works."